This work has been carried out in collaboration with the University of Birmingham

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(1)This work has been carried out in collaboration with the University of Birmingham. Supervisors: Dr Neil Rowson Dr Mark Simmon. Academic Year 2007/2008.

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(3) Un professore di filosofia iniziò la lezione introduttiva del corso affrontando gli studenti senza dire niente: prese un grosso barattolo vuoto e lo riempì di pietre del diametro di circa 5 cm. Quindi chiese agli studenti se il barattolo fosse pieno. Loro risposero che lo era. Il professore prese allora alcuni sassolini e li versò nello stesso barattolo. Scuotendo leggermente il contenitore, questi andarono a infilarsi tra le pietre. Chiese nuovamente se il barattolo ora fosse pieno. Di nuovo la risposta fu affermativa. A questo punto egli prese la sabbia e la versò nel vasetto. Naturalmente la sabbia riempì gli spazi vuoti. Chiese quindi un’altra volta se il barattolo fosse pieno. Gli studenti risposero in modo unanime di si. Il professore spiegò: “Vorrei che capiste che questo barattolo rappresenta la vostra vita. Le pietre sono le cose davvero importanti: gli amici, la salute, la famiglia, cose che, se qualsiasi altra cosa fosse perduta, continuerebbero ad essere fondamentali e a riempire la vostra vita. I sassi più piccoli sono cose importanti, ma un po’ meno, il lavoro, la casa…la sabbia è tutto il resto, le cose minori. Ora, se mettete, anteponete, la sabbia nel barattolo, non rimane posto per le pietre e i sassi. Se anteponete le cose meno importanti a quelle via via più fondamentali, non avrete energia, tempo, spazio per quelle davvero necessarie. Fate attenzione per prima cosa alle pietre importanti. Decidete le vostre priorità. Il resto è solo sabbia.”. Ringrazio tutte le mie pietre..

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(5) Table of Content. TABLE OF CONTENT. TABLE OF CO TE T....................................................................................................... i. OME CLATURE............................................................................................................ v LIST OF FIGURES .......................................................................................................... vii LIST OF TABLES ............................................................................................................. ix ABSTRACT........................................................................................................................ xi Chapter 1 I TRODUCTIO ....................................................................................... 1 1.1 Environmental Impact of Mine Waste .................................................................. 2 1.2 Acid Mine Drainage Formation ............................................................................ 3 1.3 Case Study: Wheal Jane Mine .............................................................................. 5 1.4 Environmental Quality Standard (EQS) and Consent Limits ............................... 6 1.5 Motivation for Research ....................................................................................... 6 Chapter 2 LITERATURE REVIEW .......................................................................... 9 2.1 Heavy Metals ........................................................................................................ 9 2.1.1 Metal Precipitation and pH ......................................................................... 10 2.2 Zeolites................................................................................................................ 11 2.2.1 Framework Structure .................................................................................. 12 2.2.2 Sorption....................................................................................................... 14 2.2.3 Ion Exchange .............................................................................................. 15 2.2.4 Clinoptilolite ............................................................................................... 16 2.2.5 Synthetic Zeolite ......................................................................................... 17 2.3 Heavy Metal Removal Studies Using Clinoptilolite........................................... 20 2.3.1 Effect of Surface Dust and Particle Size..................................................... 20 2.3.2 Effect of Zeolite Amount ............................................................................ 22 2.3.3 Effect of Initial Ions Concentration ............................................................ 22 2.3.4 Equilibrium Time........................................................................................ 22 2.3.5 Effect of pH................................................................................................. 23 i.

(6) Table of Content 2.3.6 Effect of Agitation Speed............................................................................ 24 2.3.7 Effect of Conditioning ................................................................................ 25 2.3.8 Effect of Presence of Anions ...................................................................... 27 2.3.9 Investigation of Sorption Capacity of Clinoptilolite................................... 28 2.3.10 Selectivity of Clinoptilolite for Different Metal Cations............................ 28 2.3.11 Possible Mechanisms Involved in Heavy Metal Solutions-Clinoptilolite Interaction ................................................................................................................... 29 2.4 Heavy Metal Removal Studies Using A4 ........................................................... 31 2.5 Sorption Modeling .............................................................................................. 31 2.5.1 Adsorption isotherms .................................................................................. 33 2.5.2 Adsorption Kinetics .................................................................................... 35 2.6 Disposal of Metal Saturated Zeolites.................................................................. 37 2.6.1 Effect of Ionic Strength on the Desorption ................................................. 38 2.6.2 Effect of pH on the Desorption................................................................... 39 2.6.3 Effect of Temperature on the Desorption ................................................... 39 2.6.4 Effect of Particle Size of Zeolite and Agitation Rate ................................. 39 2.6.5 Effect of Organic Acid on the Desorption .................................................. 39 Chapter 3 MATERIALS A D METHODS ............................................................. 42 3.1 Chemicals............................................................................................................ 42 3.2 Sorbent ................................................................................................................ 43 3.3 Vessel.................................................................................................................. 45 3.4 Atomic Adsorption Spectrometry (AAS) ........................................................... 47 3.5 Positron Emission Particle Tracking (PEPT)...................................................... 48 3.6 Sample Preparation and Experimental Methods................................................. 48 3.7 Preliminary Experiment ...................................................................................... 49 3.7.1 Concentration, Size and Kind of Zeolite..................................................... 49 3.7.2 Impeller Speed ............................................................................................ 50 3.7.3 Concentration of Metals Ions...................................................................... 51 3.7.4 Initial pH of Solution .................................................................................. 52 3.8 Equilibrium Studies ............................................................................................ 53 3.9 Matematical Modeling ........................................................................................ 53 Chapter 4 RESULTS A D DISCUSSIO S ............................................................. 55 4.1 Soil Characteristic............................................................................................... 55 4.1.1 Synthetic Zeolite ......................................................................................... 57 4.1.2 Natural Zeolite ............................................................................................ 57 4.2 Fluidynamics Studies by PEPT........................................................................... 59 4.3 Effect of the Zeolite Amount .............................................................................. 59 4.3.1 Synthetic Zeolite ......................................................................................... 59 4.3.2 Natural Zeolite ............................................................................................ 61 4.4 Effect of Speed Impeller ..................................................................................... 63 4.4.1 Synthetic Zeolite ......................................................................................... 64 4.4.2 Natural Zeolite ............................................................................................ 65 4.5 Effect of Concentration of Metal Ions ................................................................ 67 4.5.1 Synthetic Zeolite ......................................................................................... 67 4.5.2 Natural Zeolite ............................................................................................ 69 4.6 Effect of initial pH .............................................................................................. 70 4.6.1 Synthetic Zeolite ......................................................................................... 71 ii.

(7) Table of Content 4.6.2 Natural Zeolite ............................................................................................ 71 4.7 Conclusion .......................................................................................................... 74 Chapter 5 EQUILIBRIUM STUDIES A D MATHEMATICAL MODELI G . 76 5.1 Isotherm Modeling.............................................................................................. 77 5.1.1 Langmuir Model ......................................................................................... 77 Synthetic Zeolite ......................................................................................................... 78 Natural Zeolite ............................................................................................................ 81 5.1.2 Freundlich Model........................................................................................ 83 Synthetic Zeolite ......................................................................................................... 85 Natural Zeolite ............................................................................................................ 86 5.1.3 Dubnin-Kaganer-Radushkevich (DKR) Model .......................................... 87 Synthetic Zeolite ......................................................................................................... 88 Natural Zeolite ............................................................................................................ 89 5.2 Kinetic Modeling ................................................................................................ 91 5.2.1 First Order Model ....................................................................................... 91 Synthetic Zeolite ......................................................................................................... 93 Natural Zeolite ............................................................................................................ 95 5.2.2 Pseudo-First Order Model........................................................................... 96 Synthetic Zeolite ......................................................................................................... 97 Natural Zeolite ............................................................................................................ 98 5.2.3 Pseudo-Second Order Model ...................................................................... 99 Synthetic Zeolite ....................................................................................................... 100 Natural Zeolite .......................................................................................................... 101 5.2.4 Intra Particle Diffusion Model .................................................................. 103 Synthetic Zeolite ....................................................................................................... 103 Natural Zeolite .......................................................................................................... 106 5.3 Conclusion ........................................................................................................ 109 5.3.1 Isotherm Modeling.................................................................................... 109 5.3.2 Kinetic Modeling ...................................................................................... 110 Chapter 6 CO CLUSIO S ..................................................................................... 111 Chapter 7 RECOMME DATIO S........................................................................ 114 REFERE CES................................................................................................................ 116 APPE DICES ................................................................................................................. 125 A. Calibration AAS.................................................................................................... 125 B. Effect of Amount of Zeolite and Time Contact .................................................... 128 Synthetic Zeolite ....................................................................................................... 128 Natural Zeolite .......................................................................................................... 130 C. Effect of Agitation Speed...................................................................................... 132 Synthetic Zeolite ....................................................................................................... 132 Natural Zeolite .......................................................................................................... 134 D. Effect of Initial Metal Ions Concentration ............................................................ 136 Synthetic Zeolite ....................................................................................................... 136 Natural Zeolite .......................................................................................................... 138 E. Effect of the pH..................................................................................................... 140 Synthetic Zeolite ....................................................................................................... 140 Natural Zeolite .......................................................................................................... 142 F. Isotherm Modeling................................................................................................ 144 iii.

(8) Table of Content. iv.

(9) Nomenclature. NOMENCLATURE. Angstrom 10-10m Interchannel cations Atomic Absorption Spectrometry Acid Mine Drainage Activity coefficient Tetrahedral coordinated trivalent cations in zeolite framework Bt Mathematical function C Clearance [mm] C0 Initial concentration of ions in solution [mg/l] CA Concentration of solute in solution [mg/g] Ce Equilibrium concentration of AMD in solution [mg/l] CB Concentration of solute on the sorbent [mg/g] CC Concentration of the conditioning agent Ct Concentration of ions during the time [mg/l] D Diameter of impeller [mm] DKR Dubinin-Kaganer-Radushkevich ε Polanyi potential E Free energy of adsorption[kJ/mol] EDS Energy Dispersive Spectroscopy EDTA Ethylenediamine tetraacetic disodium acid EK Electrokinetic EQS Environmental Quality Standard F Fraction of solute adsorbed h Initial sorption rate Å A AAS AMD β B. H IEP k1 k2 K KC kd KL L1 L2 M n N NZ PEPT ppm Q0 qe qt qtmax r rpm R R2 SAPO SEM S/L SZ. v. Liquid height [mm] Isoelectric Point Rate constant of pseudo-first order adsorption [h-1] Rate constant of pseudo-second order [g·mg-1·h] Freundlich constant Equilibrium constant Rate constant of pore diffusion Langmuir constant Side of rectangular base of vessel [mm] Side of rectangular base of vessel [mm] Molarity Freundlich constant Normality Natural Zeolite Positron Emission Particle Tracking Parts per Million Monolayer adsorption capacity Equilibrium adsorbed amount of AMD on adsorbent [mg/g] Adsorption capacity [mg/g] Maximum adsorption capacity Constant separation factor Revolution per Minute Percentage rate Correlation coefficient Silicoaluminophosphate Scanning Electron Microscope Solid/Liquid ratio Synthetic Zeolite.

(10) Nomenclature T T tC TCEC U(t) V Vb W. x. Average side of base of vessel [mm] Temperature Contact Time Theoretical Cation Exchange Capacities Fractional attainment of equilibrium Volume of the solution [l] Volume of NaCl solution in batch mode applications Weight of the dry adsorbent [g]. XA XAe XRD XRF y z ZPC. vi. Stoichiometric coefficient for trivalent cations Fractional conversion Fractional conversion at equilibrium X-Ray Diffraction X-Ray Fluorescence Stoichiometric coefficient for trivalent cations Charge on the interchannel cations Zero Point Charge.

(11) List of Figures. LIST OF FIGURES. Fig. 1.1: Location of Wheal Jane Mine................................................................................. 5 Fig. 2.1: General Framework Structure of Zeolites ............................................................ 13 Fig. 2.2: The shape of para-xylene means that it can diffuse freely in the channels of silicalite ....................................................................................................................... 14 Fig. 2.3: Representation of ion exchange between sodium ions and calcium ions............. 16 Fig. 2.4: pH dependent surface charge on clinoptilolite ..................................................... 24 Fig. 3.1 Kind and size of zeolites used: A) NZ [3÷5 mm]; B) NZ [1÷3 mm]; B) NZ [<63 µm]: D) SZ [<45 µm].................................................................................................. 43 Fig. 3.2: SEM image: A) NZ WD 7.8 mm; B) NZ WD 7.9 mm; C) SZ top view; D) SZ cross-section view....................................................................................................... 45 Fig. 3.3: Schematic diagram of a baffled tank (left) with a Rushton turbine impeller (right) ..................................................................................................................................... 47 Fig. 4.1: SEM pictures of Synthetic Zeolite by zoom of 20µm (A) and 2µm (B) .............. 56 Fig. 4.2: Elementary analysis of all elements of synthetic zeolite...................................... 57 Fig. 4.3: Elementary analysis of all elements of natural zeolite. ........................................ 57 Fig. 4.4: SEM pictures of Natural Zeolite by zoom of 50µm (A) and 2µm (B) ................. 58 Fig. 4.5: Effect of synthetic zeolite amount on the removal of heavy metal ions. ............. 60 Fig. 4.6: Effect of natural zeolite amount on the removal of heavy metal ions.................. 63 Fig. 4.7: Effect of speed impeller on the removal of heavy metal ions on synthetic zeolite. ..................................................................................................................................... 65 Fig. 4.8: Effect of speed impeller on the removal of heavy metal ions on natural zeolite. 66 Fig. 4.9: Comparison of adsorption capacity at constant synthetic zeolite-pollution concentration rapport. ................................................................................................. 68 Fig. 4.10: Comparison of adsorption capacity at constant natural zeolite-pollution concentration rapport. ................................................................................................. 69 Fig. 4.11: Effect of the pH on the removal of heavy metal ions on synthetic zeolite......... 71 vii.

(12) List of Figures Fig. 4.12: Effect of the pH on the removal of heavy metal ions on natural zeolite. ........... 73 Fig. 5.1: Langmuir plots for metal ions adsorption onto synthetic zeolite. ........................ 79 Fig. 5.2: Operation line and Langmuir isotherm for the adsorption of zinc on synthetic zeolite to predict the final equilibrium solution concentration. .................................. 80 Fig. 5.3: Langmuir plots for metal ions adsorption onto natural zeolite............................. 82 Fig. 5.4: Operation line and Langmuir isotherm for the adsorption of copper on natural zeolite to predict the final equilibrium solution concentration. .................................. 84 Fig. 5.5: Freundlich plots for metal ions adsorption onto synthetic zeolite........................ 85 Fig. 5.6: Freundlich plots for metal ions adsorption onto natural zeolite. .......................... 87 Fig. 5.7: DKR isotherm plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto synthetic zeolite A................................................................. 89 Fig. 5.8: DKR isotherm plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto natural zeolite. ....................................................................... 90 Fig. 5.9: First order kinetic plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto synthetic zeolite..................................................................... 94 Fig. 5.10: First order kinetic plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto natural zeolite. ....................................................................... 95 Fig. 5.11: Pseudo-first order kinetic plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto synthetic zeolite. ................................................... 97 Fig. 5.12: Pseudo-first order kinetic plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto natural zeolite........................................................ 99 Fig. 5.13: Pseudo-second order kinetic plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto synthetic zeolite. ................................................. 101 Fig. 5.14: Pseudo-second order kinetic plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto natural zeolite...................................................... 102 Fig. 5.15: Intra-particle diffusion plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto synthetic zeolite. ......................................................... 104 Fig. 5.16: Corelationship between Bt and t plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto synthetic zeolite. ......................................... 106 Fig. 5.17: Intra-particle diffusion plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto natural zeolite. ............................................................ 107 Fig. 5.18: Corelationship between Bt and t plots for the sorption of Cu2+, Mn2+, Fe3+ and Zn2+ ions from aqueous solutions onto natural zeolite. ............................................ 108. viii.

(13) List of Tables. LIST OF TABLES. Tab. 1.1 Wheal Jane mine water treatment plant water quality requirements ...................... 6 Tab. 1.2 Environmental Quality Standard (EQS) and Wheal Jane discharge consent limit. 7 Tab. 2.1 Physical characteristics of some naturally occurring zeolites .............................. 12 Tab. 2.2 Various batch mode pretreatment conditions given in the literature .................... 26 Tab. 2.3 Some examples for selectivity series obtained in different studies ...................... 28 Tab. 3.1 Characteristics of synthetic solutions ................................................................... 42 Tab. 3.2 Chemical composition of natural zeolite (XRF)................................................... 44 Tab. 3.3 Mineralogical Analysis XRD (wt %) ................................................................... 44 Tab. 3.4Particle size by Sedigraph and Fraunhoffer (wt %)............................................... 45 Tab. 3.5 Structural characteristic and distribution of the volume of pores for the zeolite 4A ..................................................................................................................................... 46 Tab. 3.6 Distribution of pores volume depending on radius size ....................................... 46 Tab. 3.7 Parameter of vessel and impeller.......................................................................... 47 Tab. 3.8 Initial experiment conditions for the determination of optimum concentration of zeolite.......................................................................................................................... 50 Tab. 3.9 Initial experiment conditions for the determination of optimum speed impeller . 51 Tab. 3.10 Initial experiment conditions for the equilibrium studies................................... 52 Tab. 3.11 Initial experiment conditions for the dependence from pH ................................ 53 Tab. 4.1: Selectivity sequences for the natural and synthetic zeolite obtained from the experiment and by the property of ions. ..................................................................... 60 Tab. 4.2: Concentration of metal ions and zeolite used for the effect of concentration of metal ions. ................................................................................................................... 67 Tab. 4.3: Optimal condition of exercise to adsorb the best amount of metal ions using natural and synthetic zeolite........................................................................................ 75 Tab. 5.1: Concentration of metal ions used for the analysis of kinetics and equilibrium study............................................................................................................................ 77 ix.

(14) List of Tables Tab. 5.2: Characteristic parameters and determination coefficients of the experimental data according to Langmuir equation on synthetic zeolite. ................................................ 80 Tab. 5.3: Comparison of Qe and Qm and Ce and Cm with relative error for zinc adsorption on synthetic zeolite. .................................................................................................... 81 Tab. 5.4: Characteristic parameters and determination coefficients of the experimental data according to Langmuir equation on natural zeolite. ................................................... 82 Tab. 5.5: Comparison of Qe and Qm and Ce and Cm with relative error for copper adsorption on natural zeolite....................................................................................... 83 Tab. 5.6: Freundlich adsorption constants for metal ions on synthetic zeolite................... 86 Tab. 5.7: Freundlich adsorption constants for metal ions on natural zeolite. ..................... 86 Tab. 5.8: DKR isotherm parameters of Cu2+, Mn2+, Fe3+ and Zn2+ sorbed onto synthetic zeolite A. ..................................................................................................................... 90 Tab. 5.9: DKR isotherm parameters of Cu2+, Mn2+, Fe3+ and Zn2+ sorbed onto natural zeolite.......................................................................................................................... 91 Tab. 5.10: Experimental data of concentration of metal ions during the adsorption tests.. 93 Tab. 5.11: First order kinetic parameters of Cu2+, Mn2+, Fe3+ and Zn2+ sorbed onto synthetic zeolite. ......................................................................................................... 94 Tab. 5.12: First order kinetic parameters of Cu2+, Mn2+, Fe3+ and Zn2+ sorbed onto natural zeolite.......................................................................................................................... 96 Tab. 5.13: Pseudo-first order rate constant of Lagergren plots for heavy metal ions onto synthetic zeolite. ......................................................................................................... 97 Tab. 5.14: Pseudo-first order rate constant of Lagergren plots for heavy metal ions onto natural zeolite.............................................................................................................. 98 Tab. 5.15: Pseudo-second order rate constant for heavy metal ions onto synthetic zeolite. ................................................................................................................................... 100 Tab. 5.16: Pseudo-second order rate constant for heavy metal ions onto natural zeolite. 102 Tab. 5.17: Intra-particle diffusion parameters of Cu2+, Mn2+, Fe3+ and Zn2+ sorbed onto synthetic zeolite. ....................................................................................................... 104 Tab. 5.18: Boyd parameters of Cu2+, Mn2+, Fe3+ and Zn2+ sorbed onto synthetic zeolite.105 Tab. 5.19: Intra-particle diffusion parameters of Cu2+, Mn2+, Fe3+ and Zn2+ sorbed onto natural zeolite............................................................................................................ 107 Tab. 5.20: Boyd parameters of Cu2+, Mn2+, Fe3+ and Zn2+ sorbed onto natural zeolite. .. 108 Tab. 5.21: Results of non-linear regression of Langmuir, Freundlich and DKR models for each equilibrium studyof Cu2+, Mn2+, Fe3+ and Zn2+ sorbed onto synthetic and natural zeolite........................................................................................................................ 109 Tab. 5.22: Results of kinetic models for each equilibrium studyof Cu2+, Mn2+, Fe3+ and Zn2+ sorbed onto synthetic and natural zeolite. ........................................................ 110. x.

(15) Abstract. ABSTRACT. In this study, the removal of heavy metal ions from acid mine drainage (AMD) using natural zeolite (clinoptilolite), obtained from the Gordes region of Turkey, and synthetic zeolite (Na-4A) under different experimental conditions was investigated in an agitated vessel. The efficiency of zeolite as an adsorbent for the removal of heavy metals such as Zn (II), Cu (II), Mn(II) and Fe(III) from AMD has been determined at the different initial concentrations, zeolite amount, speed impeller and pH. Adsorption data have been interpreted in terms of Langmuir, Freundlich and Dubinin-Kaganer-Radushkevich (DKR) equations. The results straightforwardly showed that the adsorption phenomena of heavy metals into clinoptilolite and synthetic zeolite are well described by the Langmuir isotherm in most of the cases. The agitated batch adsorber method was employed in order to find a correlation wherein the adsorption percentage was function of experimental conditions. The selectivity of the studied metals was determined as Fe>Mn>Zn>Cu using synthetic zeolite and Cu>Fe>Mn>Zn using natural zeolite. Synthetic zeolite exhibited about 10 times greater sorption capacities than natural zeolite. The sorption kinetics was tested for the first order reaction, intra-particle diffusion, pseudo-first order, and pseudo-second order reaction at different experimental conditions. The rate constants of sorption for all these kinetic models were calculated. Good correlation coefficients were obtained for the. xi.

(16) Abstract pseudo-second order kinetic model showing that metal ions uptake processes followed the pseudo-second order rate expression. The batch sorption model, based on the assumption of a pseudo-second order mechanism, has been developed to predict the rate constant of sorption and the equilibrium capacity with the effect of initial metal ions concentration, mass of zeolite used, speed impeller and initial solution pH.. xii.

(17) Introduction. Chapter 1 INTRODUCTION. Metals are extracted from minerals through a series of mining and metallurgical processes. These mining and extractive processes vary from one place to the other depending on the geology of the site and the nature of the valuable metal. Minerals occurring closer to the surface of the earth are mined by the open-pit method whilst the ones embedded deep in the earth crust are mined via the underground method, by a series of tunnels in a network. The network is accessed via shaft or ramp, and used to transport both labor and ore to the surface. Nevertheless, the two processes are similar in principle by exposing the ore-body for removal for subsequent processing. Mining, by definition, involves making large holes in the ground and producing a large volume of waste material [1] since cut-off grades for some mineral deposits are in the order of low percentages or parts per million (ppm). Therefore for both open-pit and underground mining, a lot of overburden is moved in order to access the mineral, this ratio of waste rock to ore (called stripping ratio) is higher for open pits than underground operations. The overburden can be either acid producing or acid consuming in nature. This. 1.

(18) Introduction property affects the environment of the mining site greatly, due to the nature of liquor that is produced when water and oxygen comes into contact with such waste material. The products (usually, tailings) from the mineral processing and extractive metallurgy unit operations can also be either acidic or acid producing in nature. Such a product which is acidic in nature is termed acid mine drainage. The main sources of heavy-metal pollution are mining, milling and surface finishing industries, discharging a variety of toxic metals such as Cd, Cu, Ni, Co, Zn and Pb into the environment [2]. This puts a lot environmental constraints on the mining and mineral processing companies to either prevent or minimize the potential pollution from getting into the environment.. 1.1 Environmental Impact of Mine Waste Acid mine drainage (AMD) is a growing world-wide problem for both working and abandoned mines due to the highly acidic media generated in both underground and surface mining. Acid mine drainage (AMD) is the natural process of generating contaminated water through exposure of sulphide-bearing minerals to water and oxygen, leading to elevation of acidity, and concentration of heavy metals and sulphates in mining environment. Sulphide-bearing minerals are ubiquitous, therefore AMD is a worldwide problem that is encountered in all mining situations: active or abandoned mines, open-pit or underground mine working. Unreclaimed colliery and pyretic heap spoils, quarries and mine tailing dams are other sources of such contaminated mine water [3, 4, 5, 6]. Hence, a multitude of diffuse sources in mining districts can lead to water contamination in such catchments areas. These ions leached into streams (especially, hydrogen ions) cause the water to turn acidic, normally with a pH of less than 3. Apart from the iron ions, other heavy metals like cadmium (Cd), copper (Cu), manganese (Mn), aluminium (Al) and arsenic (As), are also leached into the mine drainage, usually in concentrations far above permissible legal levels. This has been a problem for the mining and metal surface finishing industries especially in the less developed countries because “municipal water treatment facilities in most of the developing countries, at present, are not equipped to remove traces of heavy metals,. 2.

(19) Introduction consequently exposing every consumer to unknown quantities of pollutants in the water they consume” [2]. On contact of AMD with saline or alkaline water, the color of the recipient system usually changes to brownish-orange due to the formation of ochre [Fe(OH )3 ] and other iron complexes that precipitate out of the solution [7, 8, 9]. The most important environmental problem posed by acid mine drainage is acidity. This has two major sources namely proton acidity (associated with pH) and mineral acidity (associated with dissolved metals) whilst the third potential source of acidity due to dissolved organic compounds is too insignificant to be reflected in AMD [5]. The acidity destroys the natural bicarbonate buffer system, an essential feature of natural waters, impacting therefore directly on the ecosystems. The environmental impact of acid mine drainage water can be significant because of the various metal ions dissolved in solution. It is well known that heavy metals can be extremely toxic, as high levels can damage nerves, liver and bones, and also block functional groups of vital enzymes [10, 11]. Both the acidic nature of AMD water and the heavy metals present disrupt the growth and reproduction of aquatic plants and animals. It has been generally accepted that an increase in acidity reduces the species diversity in an aquatic ecosystem and elevated concentrations of zinc and cadmium in aqueous environments result in a decrease in species diversity and ambulance [12].. 1.2 Acid Mine Drainage Formation Acid mine drainage (AMD) forms when sulfide minerals in rocks are exposed to oxidizing conditions in coal and metal mining, highway construction, and other large-scale excavations. There are many types of sulfide minerals, but iron sulfides common in coal regions, pyrite and marcasite. (FeS 2 ) ,. are the predominant AMD producers. Upon. exposure to water and oxygen, pyritic minerals oxidize to form acidic, iron and sulfate-rich drainage. The drainage quality emanating from underground mines or backfills of surface mines is dependent on the acid-producing (sulfide) and alkaline (carbonate) minerals contained in the disturbed rock. In general, sulfide-rich and carbonate-poor materials are expected to produce acidic drainage. In contrast, alkaline-rich materials, even with significant sulfide concentrations, often produce alkaline conditions in water. 3.

(20) Introduction A complex series of chemical weathering reactions are spontaneously initiated when surface mining activities expose spoil materials to an oxidizing environment. The mineral assemblages contained in the spoil are not in equilibrium with the oxidizing environment and almost immediately begin weathering and mineral transformations. The reactions are analogous to "geologic weathering" which takes place over extended periods of time (i.e., hundreds to thousands of years) but the rates of reaction are orders of magnitude greater than in "natural" weathering systems. The accelerated reaction rates can release damaging quantities of acidity, metals, and other soluble components into the environment. For purposes of this description, the term "pyrite" is used to collectively refer to all iron disulfide minerals [13]. The following equations show the generally accepted sequence of pyrite reactions: 2 FeS 2 + 7O2 + 2 H 2 O → 2 Fe 2+ + 4SO42− + 4 H +. (1). 4 FeS 2 + O 2 + 4 H + → 4 Fe 3+ + 2 H 2 O. (2). 4 Fe 3+ + 12 H 2 O → 4 Fe(OH )3 + 12 H +. (3). FeS 2 + 14 Fe 3+ + 8 H 2 O → 15 Fe 2+ + 2 SO42− + 16 H +. (4). In the initial step (Eq. 1), pyrite reacts with oxygen and water to produce ferrous iron, sulfate and acidity. The second step (Eq. 2) involves the conversion of ferrous iron to ferric iron. This second reaction has been termed the "rate determining" step for the overall sequence. The third step (Eq. 3) involves the hydrolysis of ferric iron with water to form the solid ferric hydroxide (ferrihydrite) and the release of additional acidity. This third reaction is pH dependent. Under very acid conditions of less than about pH 3.5, the solid mineral does not form and ferric iron remains in solution. At higher pH values, a precipitate forms, commonly referred to as "yellowboy." The fourth step (Eq. 4) involves the oxidation of additional pyrite by ferric iron. The ferric iron is generated by the initial oxidation reactions in steps one and two. This cyclic propagation of acid generation by iron takes place very rapidly and continues until the supply of ferric iron or pyrite is exhausted. Oxygen is not required for the fourth reaction to occur.. 4.

(21) Introduction The overall pyrite reaction series is among the most acid-producing of all weathering processes in nature. The pyrite weathering process is a series of chemical reactions, but also has an important microbiological component. The conversion of ferrous to ferric iron in the overall pyrite reaction sequence has been described as the "rate determining step" [14]. This conversion can be greatly accelerated by a species of bacteria, Thiobacillus ferroxidans. This bacterium and several other species thought to be involved in pyrite weathering are widespread in the environment. Thiobacillus ferroxidans has been shown to increase the iron conversion reaction rate by a factor of hundreds to as much as one million times [14, 13]. The activity of these bacteria is pH dependent with optimal conditions in the range of pH 2 to 3. Thus, once pyrite oxidation and acid production has begun, conditions are favorable for bacteria to further accelerate the reaction rate. At pH values of about 6 and above, bacterial activity is thought to be insignificant or comparable to abiotic reaction rates. The catalyzing effect of the bacteria effectively removes constraints on pyrite weathering and allows the reactions to proceed rapidly.. 1.3 Case Study: Wheal Jane Mine The Wheal Jane tin mine, located in Cornwall, England (Fig. 1.1), closed in 1991 following several hundred years of continual mining operations in the vicinity. In 1992, the failure of an adit plug led to the catastrophic release of acidic, heavy-metal laden water (often referred to as acid mine drainage, ‘AMD’) that had flooded the closed mine. About 50000m3 of the acidic. iron-rich. contained Fig. 1.1: Location of Wheal Jane Mine. water  200 mg  , l  . significant. zinc 100 mg  , l  . 5. cadmium. which. also. concentrations. of.  20.1 mg  l  . and.

(22) Introduction. Parameter. Unit. pH Total As Total Al Total Cd Total Cu Total Fe Total Mn Total Ni Total Zn Total Pb. mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L. Influent Ave Max 3.5 3.85 3 16 23 40 0.056 0.149 0.8 7.65 206 402 6 19.2 0.55 1.2 51 176 0.15 0.6. Short-term 6÷10 0.5 13 0.04 0.3 5 7 1 20 –. Consent Long-term 6.5÷10 0.1 10 0.04 0.08 5 1 1 2.5 –. Tab. 1.1 Wheal Jane mine water treatment plant water quality requirements. arsenic  6 mg  , spilled out into l   The Carnon river and into the Fal estuary, leaving behind an iron-hydroxide coating on the floor of the estuary [15, 16]. Although the impact of this disaster was mainly visual, similar incidents at the Aznacollar mine complex in Spain and Baia Mare gold mine in Romania have had a much more negative environmental impact on the water bodies that they have polluted and also on the surrounding land. Tab. 1.1 shows the range of chemical properties of a particular acidic minewater from Wheal Jane Mine which is the reference site for this research work.. 1.4 Environmental Quality Standard (EQS) and Consent Limits Mine discharges rather than watercourses may not have to meet environmental quality standard (EQS) [17, 19] as such metal and other mineral mines discharges are dealt on a case-by-case basis [20], by issuance of consent limits by regulatory bodies such as the Environmental Agency and monitored to ensure compliance. Tab. 1.2 list the EQS values for discharges into freshwater and compares these to maximum consent discharge limits for Wheal Jane minewater active treatment plant.. 1.5 Motivation for Research Pollution of ground water due to leaching of heavy metals from exposed ores in mine works is a major environmental problem which can cause severe damage to vegetation and wildlife. Conventional treatment of this acid mine drainage (AMD) involves precipitation 6.

(23) Introduction. EQS Value Determinand. Wheal Jane Maximum Consent Limit. Unit. SEPA, 2002; Bone, 2003; Bone, 2003; Morgan, 2005 SEPA, 2004 pH 6.0 9.0 ≤10.0 Al mg/L 0.01 0.025 10.0 As mg/L 0.05 0.1 Cd mg/L 0.005 0.04 Cu mg/L 0.001 0.028* 0.08 Fe mg/L 1.0 5.0 Mn mg/L 0.03 1.0 Ni mg/L 0.05 0.2* 0.5 Zn mg/L 0.008 0.5*ª 2.5 Note: * means the value depends on the water CaCo3 content value ª covers most sensitive and less sensitive aquatic life Tab. 1.2 Environmental Quality Standard (EQS) and Wheal Jane discharge consent limit. of the metal salts in large open lagoons; a process which is not only slow and inefficient, but highly unsightly. Removal of heavy metal cations from aqueous solutions can be achieved by several processes such as chemical precipitation, oxidation, ultrafiltration, reverse osmosis, electrodialysis or ion exchange [21]. Among these techniques chemical precipitation is the mostly widely applied method. However, production of significant amounts of sludge, which is difficult to dewater and requires careful, frequently, expensive disposal techniques, appears as the major disadvantage of this process [22]. Therefore, ion exchange arises as an attractive one because of the relative simplicity and safety of application as well as recovery potential of both the adsorbent and heavy metals [23, 24]. Ion exchange can also be cost effective especially when a low cost ion exchanger is used [25]. Zeolites have been recognized for more than 200 years, and are used in various technological areas, but they have found extensive attention in the field of wastewater treatment in recent years [26]. There are more than 30 distinct species of zeolite that occur in nature. However, only seven of them, mordenite, clinoptilolite, ferrierite, chabazite, erionite, philipsite and analcime occur in sufficient quantities to be considered as viable mineral resources [27]. Among these species, clinoptilolite is one of the most important. 7.

(24) Introduction natural zeolite, since it is found in extensive deposits worldwide; it has a stable structure and shows high selectivity for various cations [28]. Natural zeolites are materials which can meet these requirements and the purpose of this project is to determine their effectiveness for the treatment of a number of synthetic AMD solutions in batch. Also of considerable importance is the ability to regenerate the zeolite once loaded with heavy metals, which will be performed using a stirred tank for the solid-liquid reaction. The major problem in all studies is that they are limited to equilibrium or batch contact systems. Moreover, in the literature there are only a few examples about the mathematic modelling of adsorption of heavy metal using natural and synthetic zeolite. Optimization of the process by experimental methods is expensive and time-consuming. It would be much cheaper and faster to use mathematical modelling to predict how many cycles clinoptilolite will do before regeneration becomes necessary. On the other hand, for effective removal of heavy metals from water streams it is essential to understand the ionexchange properties including not only the equilibrium but also the kinetics and dynamics in the vessel. The modelling of ion-exchange and adsorption vessel by advanced mathematical modelling could not only be very helpful in understanding the process mechanism but also in widening the practical application of natural zeolite by reduction of costs of scaling-up the process from laboratory to industrial scale. There are several problems with the use of the model because one model can be not good for all cases. This difference can result from different experimental conditions, the difference between the chemical and mineralogical compositions of zeolitic, or from the fact that calculations based on empirical correlations are not always adequate to the process mechanism. Parameters to be investigated in batch are the influence of the particle size distribution of the zeolite and the initial concentration of metal ions, both individually and in combination, upon the maximum loading of ions onto the zeolite. Moreover it will be investigated the influence of the speed of impeller and the time contact sorbant-sorbent. The aim is to develop correlation that allows having a capacity of adsorption of the natural and synthetic zeolita in relation of the parameters investigates.. 8.

(25) Literature Review. Chapter 2 LITERATURE REVIEW. 2.1 Heavy Metals Heavy metals are natural constituents of the Earth's crust and are present in varying concentrations in all ecosystems. Human activity has drastically changed the biogeochemical cycles and balance of some heavy metals. Heavy metals are stable and persistent environmental contaminants since they cannot be degraded or destroyed. Therefore, they tend to accumulate in the soils, seawater, freshwater, and sediments. Excessive levels of metals in the marine environment can affect marine biota and pose risk to human consumers of seafood. Heavy metals are also known to have adverse effects on the environment and human health. The main anthropogenic sources of heavy metals are various industrial sources, including present and former mining activities, foundries and smelters, and diffuse sources such as piping, constituents of products, combustion by-products, traffic and metal plating facilities, petroleum refining and mining operations [29]. Relatively volatile heavy metals and those that become attached to airborne particles can be widely dispersed on very large scales. Heavy metals conveyed in aqueous and sedimentary transport enter the normal. 9.

(26) Literature Review coastal biogeochemical cycle and are largely retained within near-shore and shelf regions. Therefore, wastewaters containing heavy metals are required to be treated prior to discharge into receiving environments. Removal of heavy metals from aqueous wastes is a challenging task for the correct management of waste disposal. Among various treatment processes available, ion exchange has several advantages over other processes such as simplicity and safety of operation and recovery potential of both the sorbent and precious heavy metals. Activated carbon is considered to be a particularly competitive and effective sorbent for the removal of heavy metals, however; the use of activated carbon may not be suitable due to high costs associated with production and regeneration of spent carbon [30]. Therefore, investigation of low cost sorbents such as peat, clay, lignin, chitosan, fly ash and zeolites in removal heavy metals has arisen as a popular issue in recent years [31]. Among the given low cost sorbents, natural zeolites have been proposed as one of the most promising alternative to the activated carbon [32] due to their abundance in nature and high selectivity for certain heavy metal cations.. 2.1.1 Metal Precipitation and pH Enough alkalinity must be added to raise water pH and supply hydroxides (OH-) so dissolved metals in the water will form insoluble metal hydroxides and settle out of the water. The pH required to precipitate most metals from water ranges from pH 6 to 9 (except ferric iron which precipitates at about pH 3.5). The types and amounts of metals in the water therefore heavily influence the selection of an AMD treatment system. Ferrous iron converts to a solid bluish-green ferrous hydroxide at pH >8.5. In the presence of oxygen, ferrous iron oxidizes to ferric iron, and ferric hydroxide forms a yellowish-orange solid (commonly called yellow boy), which precipitates at pH >3.5. In oxygen-poor AMD where iron is primarily in the ferrous form, enough alkalinity must be added to raise the solution pH to 8.5 before ferrous hydroxide precipitates. A more efficient way of treating high ferrous AMD is to first aerate the water (also outgassing CO2), causing the iron to convert from ferrous to ferric, and then adding a neutralizing chemical to raise the pH to 6 or 7 to form ferric hydroxide. Aeration after chemical addition is also beneficial. Aeration before and after treatment usually reduce the amount of neutralizing reagent necessary to precipitate iron from AMD. Manganese (Mn) precipitation is variable due to its many. 10.

(27) Literature Review oxidation states, but will generally precipitate at a pH of 9.0 to 9.5. Sometimes, however, a pH of 10.5 is necessary for complete removal of manganese.The appropriate chemical treatment can depend on both the oxidation state and concentrations of metals in the AMD (U.S. Environmental Protection Agency 1983). Interactions among metals also influence the rate and degree to which metals precipitate. For example, iron precipitation will largely remove manganese from the water at pH 8 due to co-precipitation, but only if the iron concentration in the water is much greater than the manganese content (about 4 times more or greater). If the iron concentration in the AMD is less than four times the manganese content, manganese may not be removed by co-precipitation and a solution at pH>9 is necessary to remove the manganese. Because AMD contains multiple combinations of acidity and metals, each AMD is unique and its treatment by these chemicals varies widely from site to site. For example, the AMD from one site may be completely neutralized and contain no dissolved metals at a pH of 8.0, while another site may still have metal concentrations that do not meet effluent limits even after the pH has been raised to 10.. 2.2 Zeolites Identification of zeolite as a mineral goes back to 1756, when a Swedish mineralogist, Fredrich Cronstet, began collecting some wellformed crystals from a copper mine in Sweden. They were named “Zeolite” from the Greek words meaning “boiling stones”, that is, because of ability to froth when heated to about 200°C. After their discovery, zeolites were considered as minerals found in volcanic rocks for a period of two hundred years. Fortunately in 1950s, they were rediscovered and reported to exist on all the continents. In the world, their commercial production and use started in 1960s, but in Turkey they were first discovered in 1971. There are more than 30 distinct species of zeolite that occur in nature. However, only seven of them, namely mordenite, clinoptilolite, heulandite, chabecime, erionite, philipsite and analcime occur in sufficient quantity to be considered as viable mineral resources [24, 34] (Tab. 2.1). Zeolites are a family of crystalline aluminosilicate minerals [35]. About fifty different natural zeolites are now known and more than one hundred and fifty have been synthesised for specific applications such as industrial catalysis or as detergent builders.. 11.

(28) Literature Review. Zeolite. Analcime Chabecime Clinoptilolite Erionite Heulandite Mordenite Philipsite. Porosity [%]. Heat Stability. 18 47 34 35 39 28 31. high high high high low high moderate. Ion Exchange Capacity [meq/g] 4.54 3.84 2.16 3.21 2.91 4.29 3.31. Specificy Gravity [g/cm3] 2.24-2.29 2.05-2.10 2.15-2.25 2.02-2.08 2.18-2.20 2.12-2.15 2.15-2.20. Bulk Densituy [g/cm3] 1.85 1.45 1.15 1.51 1.69 1.70 1.58. Tab. 2.1 Physical characteristics of some naturally occurring zeolites. In practice, both synthetic and natural zeolites are utilized for pollution control. With the discovery of their exciting surface and structural properties, zeolites have been exploited in industrial, agricultural, and biological technology for more than 40 years. However, recently they have been utilizing in environmental technologies. Some applications of zeolites in environmental technology can be classified as; removal of ammonium from drinking and municipal wastewater, extraction of Cs2+ and Sr2+ from nuclear wastes and the mitigation of radioactive fallout, removal of odor and removal of heavy metals from wastewaters [36] Most common zeolites are formed by alteration of glass-rich volcanic rocks with fresh water in playa lakes or by seawater [24, 37]. Natural zeolites are further able to lose and gain water reversibly and to exchange extraframework cations, both without change of crystal structure [36].. 2.2.1 Framework Structure Zeolites are crystalline, naturally occurring hydrated aluminosilicate minerals of alkali and alkaline earth cations in particular Na+, K+, Ca2+, Mg2+, Sr2+ and Ba2+. Structurally, zeolites consist of a framework of aluminosilicates which is based on an infinite three dimensional structure of SiO4 and AlO4 tetrahedral molecules linked to each other by shared oxygen. They belong to the class of minerals known as “tectosilicates”. A defining feature of zeolites is that their frameworks are made up of 4-connected networks of atoms. One way of thinking about this is in terms of tetrahedra, with a silicon atom in the middle and oxygen atoms at the corners. These tetrahedra can then link together by their corners (Fig. 2.1) to a rich variety of beautiful structures. The pore structure is characterized by cages approximately 12Å in diameter, which are interlinked through channels about 8Å in diameter, composed of rings of 12 linked tetrahedrons [38].. 12.

(29) Literature Review The pores are interconnected and form long wide channels of varying sizes depending on the mineral. These channels allow the. easy. movement. of. the. resident ions and molecules into and out of the structure. Zeolites Fig. 2.1: General Framework Structure of Zeolites. have large vacant spaces or cages within and resemble honeycomb. or cage like structures. The presence of aluminium results in a negative charge, which is balanced by positively charged cations. This fact means the molar ratio Al2O3: (Ca, Sr, Ba, Na2, K2) O = 1 and that O: (Si + Al) = 2 in the empirical formula. Another characteristic feature of zeolites is the potential for reversible lowtemperature dehydration, the ability of dehydration and reversibly absorption of other molecules. In most cases Ca2+, Na+ or K+ and less frequently Li+, Mg2+, Sr2+ and Ba2+ are situated in cavities within the framework structures. This feature can also be observed in feldspar and feldspathoid minerals. But in contrast to this feldspar and feldspathoid minerals the zeolite aluminosilicate framework contain open cavities and open channels (i.e. they have lower densities) through which ions can be either extracted or introduced ions [39]. Their compositions are represented by the structural formula:. ( A + z ) y z (B + 3) y(Si )xO2 (x + y )nH 2 O Where A represent interchannel cations (such as Na+, K+, Ca2+, Ba2+, Sr2+, Mg2+ and Fe2+), B are tetrahedral coordinated trivalent cations in the zeolite framework (Al3+ and Fe3+), z is the charge on the interchannel cations, n is the number of moles of interchannel molecular water, and x and y are the stoichiometric coefficients for trivalent cations and Si4+ in tetrahedral sites, respectively. The quantities y/z and 2(x+y) represent the stoichiometries of the interchannel cations and framework oxygens, respectively, necessary for maintaining charge balance in the tectosilicate lattices of zeolites. In all, over 130 different framework structures are now known. In addition to having silicon or aluminium as the tetrahedral atom, other compositions have also been 13.

(30) Literature Review synthesised, including the growing category of microporous aluminophosphates, known as ALPOs. An additional feature, which differentiated the zeolites still further from the feldspar and feldspathoid minerals, is the presence of water molecules within the structural channels. These are relatively loosely bound to the framework and cations, and like the cations they can be removed and replaced without disrupting framework bonds [40].. 2.2.2 Sorption The three properties of zeolites which have industrial potential are their capacities to sorb gases, vapours and liquids; to catalyse reactions; and to act as cation exchangers. The property of zeolites was considered to be due to the crystalline nature of the sorbents [41]. The porous crystals were thought to be permeated by channel networks, the free dimensions of which were determined by the positions of the framework oxygens just as exactly as the rest of the crystal lattice was defined by the positions of its constituent atoms. The zeolite were considered as rigid, crystalline sponges capable of imbibing large amounts of molecules small enough or of the right shapes to pass through their surfaces and enter the intercrystalline pores, but unable to sorb molecules having the wrong sizes or shapes (Fig. 2.2). The cations are present in the same channels and voids as the guest species. The cations may sometimes occupy sites adjacent to the windows or apertures leading from one void to the next. Those cations so located may according to their sizes and numbers act as sentinels, effectively barring passage of a larger molecule while allowing passage of a smaller one, in situations where if there had been no cations in or near the windows, both molecules would have passed through. The effect of the cations on the molecule sieving can be very sensitive therefore to the size of the ions relative to the free dimensions of the windows, and also to the number of the ions in or near windows sites. These numbers vary with Fig. 2.2: The shape of para-xylene means that it can diffuse freely in the channels of silicalite. (. ). ion valence 2 !a + ↔ Ca 2 + and with ion type. A. 14.

(31) Literature Review zeolite usually provides several kinds of crystallographically distinct exchange site and the distributions of cations of a given type among these sites may change according to the ion.. 2.2.3 Ion Exchange Zeolite with a negative charge provides an ideal trap for positive cations such as sodium, potassium; barium and calcium, and positively charged groups such as water and ammonia. Both carbonate and nitrate ions are attracted by the negative charge within zeolites. Therefore, alkali and soil alkali metallic cations are attracted in the same way and water can be absorbed by zeolites [34]. Absorbed cations are relatively mobile due to their weak attraction, and can be replaced using the standard ion exchange techniques, making zeolites good ion exchangers Ion exchange has proved to be of major significance in regulating the mole sieving behavior and selectivity of zeolite sorbents; and in the preparation of zeolite-based catalysts. In addition the high selectivity shown by particular zeolites for certain cations [42] can assist in the concentration and isolation of such ions. Nevertheless ion exchange processes in their own right have not been developed as extensively as those based on molecule-sieving, selective sorption and catalysis. Zeolites can exhibit ion sieving properties just as they possess molecule sieving characteristics. When the zeolite framework is more compact (analcime) or the intercrystalline voids less accessible (sodalite hydrate) ion sieving has been demonstrated for inorganic cations of different sizes. Thus Ag-analcime or Ag-sodalite hydrate will readily exchange Ag+ by Na+ but not by Cs+. Accordingly CsCl solutions with NaCl impurity may easily be freed of the NaCl by the reaction: !aCl + Ag − Zeolite → !a − Zeolite + AgCl ↓ If CsCl alone is contacted with the silver form of either analcime or sodalite hydrate the CsCl can be hydrolysed: CsCl + 2 H 2 O + Ag − Zeolite → CsOH + H 3 O − Zeolite + AgCl ↓ The hydrolysis can be demonstrated by the rise in pH of the aqueous solution. The same behaviour is shown with tetramethylammonium chloride and the silver forms of zeolite A or chabazite, or indeed of any zeolite from which the tetramethylammonium ion is excluded by sieve action.. 15.

(32) Literature Review. Fig. 2.3: Representation of ion exchange between sodium ions and calcium ions. Due to the presence of alumina, zeolites exhibit a negatively charged framework, which is counter-balanced by positive cations resulting in a strong electrostatic field on the internal surface. These cations can be exchanged to fine-tune the pore size or the adsorption characteristics. For instance, the sodium form of zeolite A has a pore opening of approximately 4 Å (4 x 10–10 m), called 4A molecular sieve. If the sodium ion is exchanged with the larger potassium ion, the pore opening is reduced to approximately 3 Å (3A molecular sieve). On ion exchange with calcium, one calcium ion replaces two sodium ions. Thus, the pore opening increases to approximately 5 Å (5A molecular sieve) (Fig. 2.3). Ion exchange with other cations is sometimes used for particular separation purposes. 2.2.4 Clinoptilolite Clinoptilolite originally received its Greek name, meaning "oblique feather stone" because it was thought to be the monoclinic phase of the mineral ptilolite (as in "oblique ptilolite") but it was later found the earlier named mineral was mordenite; consequently the name ptilolite, is no longer in use. Clinoptilolite, one of the most useful naturally occurring zeolites, is applied as a chemical sieve, feed and food additive, as well as gas and odour absorber. Suitability for such applications is due to its large amount of pore spaces, a high resistance to extreme temperatures and chemically neutral basic structure. Clinoptilolite can easily absorb ammonia and other toxic gases from air and water and thus can be used in filters, both for health reasons and odour removal. The properties such as high absorption level, ion exchange capacity, catalysis, dehydration activity and easily shapeable features make clinoptilolite important in plant. 16.

(33) Literature Review production. Pure or composite clinoptilolite added to soil improves its physical and chemical characteristics. Clinoptilolite is a zeolite species with a relatively ‘open’ structure, a total pore volume of approximately 35% [43] and a chemical formula of (Na4K4)(Al8Si4O96) 24H2O [44]. It is further characterized by having a Si/Al ratio greater than 4 [45]. Culfaz and Yagiz [46] reported a Si/Al ratio between 4 and 5.5 for clinoptilolite. Furthermore, its threedimensional aluminosilicate framework contains narrow four- and five- membered rings constituting intra-framework micro pores or channels capable of hosting extraframework/exchangeable cations (e.g. Na+, K+, Ca2+) in association with mobile H2O molecules [43]. The crystals of clinoptilolite form two different systems of micro pores interconnected within the lattice, the first with both eight- and ten-member rings forming A- and B-type channels (3.3x4.6 and 3.0x7.6 Å , respectively), and the second with eightmember rings forming C-type channels (2.6x4. Å). These channels give rise to the wellknown ‘‘molecular sieving’’ property of zeolites. Molecules having effective crosssectional diameters small enough to fit through the channels are readily adsorbed on the inner surfaces. Molecules which are too large to fit through the channels are excluded and pass around the outside of the zeolite particle [44]. Specifically adsorbed within the channels are mobile water molecules and exchangeable cations such as NH4+ , Na+, K+ and Ca2+ and gases such H2S, SO2, CO,N2 and CH4 [47]. The differences in chemical compositions also result in Si/Al ratios and in theoretical cation exchange capacities (TCEC) varying in a considerably wide range of values which are considered as to be the most significant chemical properties of clinoptilolites. Oxides of Si4+, Al3+, Na+, K+, Ca2+, Mg2+, Fe2+ and Fe3+ are common compounds in clinoptilolite and it also contains Ba2+, Ti2+, Mn2+, S6+, P5+ and Sr2+ oxides, generally in trace amounts. Since these oxides are found in trace amounts, their identification strongly depends on the method employed for the determination of chemical composition. In the literature, various methods are reported for this purpose; however, XRF, SEM/EDS and classical silicate analysis arise as the ones that are most widely used.. 2.2.5 Synthetic Zeolite During the 1930's, R. Barrer and J. Sameshima did extensive work in zeolite synthesis. In 1948, Richard Barrer first produced a synthetic zeolite that did not have a. 17.

(34) Literature Review natural counterpart. At approximately the same time, Milton made the first materials that had no natural counterpart such as zeolite A. In 1953, Linde Type A zeolite became the first synthetic zeolite to be commercialized as an adsorbent to remove oxygen impurity from argon at a Union Carbide plant [48]. Synthetic zeolites were introduced by Union Carbide as a new class of industrial adsorbents in 1954 and as hydrocarbon-conversion catalysts in 1959. New zeolites and new uses appeared steadily through the 1960s. An explosion of new molecular sieve structures and compositions occurred in the 1980s and 1990s from the aluminosilicate zeolites to the microporous silica polymorphs to the microporous aluminophosphate-based polymorphs and metallo-silicate compositions [49]. Molecular sieves now serve the petroleum refining, petrochemical, and chemical process industries as selective catalysts, adsorbents, and ion exchangers. Many zeolites can be synthesized with SiO2 higher or lower than in nature for the same framework type. Higher SiO2 generally gives greater hydrothermal stability, stronger-acid catalytic activity, and greater hydrophobicity as adsorbents. Conversely, lower SiO2 gives greater cation exchange capacity and higher adsorbance for polar molecules. Controlling the synthesis process optimizes a zeolite for different applications. Many synthetic zeolites have framework topologies not found to date among the natural zeolites. The natural zeolite faujasite has the same framework (FAU) and similar framework composition to the Type Y synthetic zeolite but is rare in nature. Where both natural and synthetic forms of the same zeolite are available in commercial quantity, the variable phase purity of the natural zeolite and the chemical impurities, which are costly to remove, can make the synthetic zeolite more attractive for specific applications. Conversely, where uniformity and purity are not important, the cheapness of a natural zeolite may favor its use. Hence, natural and synthetic zeolites seldom compete for the same applications. Variants involve Ge substitution for Si in the framework or involve substitution of Fe, Co, Mn, Zn, Ti, or Mg for Al. In the related aluminophosphates (AlPO4), each negatively charged AlO4 tetrahedron is balanced by a positively charged PO4 tetrahedron, and nonframework cations are not needed. Still other variants include the silicoaluminophosphate (SAPO) structures in which Si substitutes some P in the AlPO4. 18.

(35) Literature Review framework; each added Si needs a nonframework cation to balance the charge on the framework. The pore geometry and volume in a specific microporous oxide are determined by the specific topology of the particular 3D framework. The lower the T-atom density per volume of the zeolite crystal is the higher the void fraction inside the crystal. The void fraction is 50% for NaX and 47% for NaA. The size of the largest pore in a zeolite is determined by the number of oxygen ions rimming the pore and its shape; e.g. a planar, circular eight-ring (8R) pore rimmed by eight oxygen ions has a diameter of 4.1 Å, as in Linde Type A zeolite, whereas the elliptical 8R pore of NaP zeolite (GIS) is 4.5 × 3.1 Å. Applications in separation and purification processes often used the ability of zeolites and other molecular sieves to exclude molecules too large to enter the pores and admit smaller ones. Similarly, shape-selective catalysis takes advantage of the ability of the pores to favor the admission of smaller reactant molecules, the release of smaller reaction product molecules, or the restriction of the size of transition-state complexes inside the micropores of the zeolite [50]. The simplest synthetic zeolite is the zeolite A with a molecular ratio of one silica to one alumina to one sodium cation. The zeolite A synthesis produces precisely duplicated sodalite units which have 47% open space, ion exchangeable sodium, water of hydration and electronically charged pores. The biggest differences between natural and synthetic are: •. Synthetics are manufactured from energy consuming chemicals and naturals are processed from natural ore bodies;. •. Synthetic zeolites have a silica to alumina ratio of 1 to 1 and clinoptilolite (clino) zeolites have a 5 to 1 ratio;. •. Clino natural zeolites do not break down in a mildly acid environment, where synthetic zeolites do. The natural zeolite structure has more acid resistant silica to hold its structure together. The clino natural zeolite is broadly accepted for use in the agricultural industry as a soil amendment and as a feed additive.. The sorption capacities of synthetic zeolite are much higher than those of natural zeolite (about 10 times greater) in accord with its higher H+ exchange capacity and its higher cation exchange capacity, consequence of its lower Si:Al ratio (Si/AlClinoptilolite=4.8. 19.

(36) Literature Review and Si/AlNa4A=1.7). Greater sorption capacities are always found for zeolites with lower Si:Al ratios [51, 52]. The sorption capacity values shown by synthetic zeolite are high enough to consider its use to purify metal finishing waste waters. The affinity of metal cations for synthetic zeolite is very high (much higher than that shown for natural zeolite), which means that cations are almost completely sorbed from diluted solutions.. 2.3 Heavy Metal Removal Studies Using Clinoptilolite The prediction of adsorbate uptake rates by adsorbent pellets is important for adsorption calculations. For the transport of adsorbates from the bulk of the fluid phase to the interior of a pellet before adsorption takes place, the following mass-transfer processes may be present: film mass transfer and intraparticle mass transfer. Two intraparticle diffusion mechanisms are involved in the adsorption rate (a) diffusion within the pore volume known as pore diffusion, and (b) diffusion along the surface of pores known as surface diffusion [53]. Some of the investigators have applied the pore diffusion model with [54, 55] and without film resistance [54, 56]. The importance of obtaining isotherms and kinetics curves lies in developing a model which accurately represents the results obtained and could be used for design purposes. There are several factors affecting heavy metal removal both in batch and continuous mode applications. Presence of surface dust, particle size, contact time, pH, agitation speed, conditioning, presence of anions, initial metal concentration, flow rate, flow mode and column dimensions can be given as some examples for the most significant factors affecting removal of heavy metals using clinoptilolite.. 2.3.1 Effect of Surface Dust and Particle Size Metal uptake by clinoptilolite takes place at sites on the exterior surface of the particle as well as sites within the particle. However, only a fraction of the internal sites is accessible to metal ions. The reason for this partial accessibility of internal sites may be attributed to the presence of fine particles (surface dust) and pore diffusion resistance [57]. Thus, increasing the external surface area by decreasing particle size and removing the. 20.

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