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UNIVERSITÀ DEGLI STUDI DI NAPOLI FEDERICO II

SCUOLA POLITECNICA E DELLE SCIENZE DI BASE D

IPARTIMENTO

D

I

I

NGEGNERIA

C

IVILE

, E

DILE E

A

MBIENTALE

STEVENS INSTITUTE OF TECHNOLOGY

D

EPARTMENT

O

F

C

IVIL,

E

NVIRONMENTAL &

O

CEAN

E

NGINEERING

Master’s thesis in

Environmental

Engineering

Comparative evaluation of chromium removal from water by zero valent iron (ZVI) and ZVI in combination

with sulfide materials

Supervisor Candidate Prof. Massimiliano Fabbricino Antonio Siesto Matr. M67/441 Co-supervisor

Prof. Xiaoguang Meng

ACADEMIC YEAR 2018/2019

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ABSTRACT

Environmental pollutants can be widely dispersed in the biosphere from pond to ocean, grassland to mountain, troposphere to exosphere, natural to built-up ecosystem. Pollutants can cause drastic changes in the physical, chemical and mechanical properties of the abiotic components resulting in numerous changes to the biodiversity.

Continuous contamination of the environment by heavy metals has become a significant and well recognized problem of the modern world. Metals are particularly problematic contaminants because unlike most organic compounds, they are not biodegradable and they can accumulate in living tissues, thus becoming concentrated throughout the food chain, causing various diseases and disorders in living organisms.

Chromium (Cr) is one of the highest priority pollutants in surface water and groundwater released mainly from anthropogenic source and this work is focused on its removal from contaminated water.

Cr has two different forms, hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)).

Cr(VI) is highly toxic due to its high solubility in water; it is considered as a common teratogenic, mutagenic, and carcinogenic chemical. On the other hand, Cr(III) is an essential nutrient in humans and it is insoluble thus easily to precipitate and remove.

Hence Cr(VI) reduction to Cr(III) can be beneficial because a more mobile and more toxic chromium species is converted to a less mobile and less toxic form.

There are different technologies developed over the years for its removal, that have shown their effectiveness. During the last two decades, there has been important interest in using zero valent iron (ZVI) as a Cr(VI) reducing agent. A considerable volume of research has been carried out in order to investigate the mechanism and kinetics of Cr(VI) reduction with ZVI.

The aim of this work is to study the effect of sulphur compounds on the chromium removal by Zero Valent Iron (ZVI), in particular sulphide (Na2S) and pyrite (FeS2) have been used.

Literature studies compared the behaviour of ZVI and sulfidated ZVI (S-ZVI) made through a sulfidation process. Sulfidation involves addition of reduced sulphur to ZVI

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in a way that the passive layer on the particles becomes predominantly iron sulphides, instead of the iron oxide passive film that controls the reactivity of ZVI.

However, in this work batch experiments were conducted in order to report the effect of ZVI, S-ZVI and synergetic ZVI + pyrite effect on chromium removal from water.

In the beginning it has been noted a higher efficiency of S-ZVI compared to ZVI.

More interesting results have been obtained using ZVI + pyrite thus several tests were conducted using this combination. The synergetic effect between pyrite and ZVI was found to contribute to enhanced Cr(VI) removal. Moreover, the influence of aerobic and anaerobic conditions, pyrite dosage and initial pH on the removal efficiency were evaluated.

This research showed the combination of ZVI and sulphide material could significantly improve the effectiveness of chromium removal from water.

Figure 1 Cr removal trend under aerobic condition using 0.3 g of ZVI, S-ZVI ratio 7 and S-ZVI ratio 14 and 100 ml of solution mixing for 6 hours at 130 rpm.

It is evident that the sulfidation increases the reactivity of ZVI. In particular, decreasing the Fe/S mass ratio resulted in a faster chromium removal, which confirm the importance of the sulfide.

S-ZVI has not been used anymore both for the complexity of sulfidation process and for the instability of the treated iron which change its properties depending on storage conditions. Thus, experiments have been carried out by adding a natural material like pyrite to 3 g/L of ZVI.

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Figure 2 Cr removal trend under aerobic condition using 3 g/l of ZVI and different amounts of pyrite

The removal efficiency is higher with the increase of pyrite dosage; in particular the Cr removal trend shows that there was no significant difference between 3 g/L ZVI + 0.5 g/L pyrite and 3 g/L ZVI + 1 g/L pyrite, thus 3 g/L ZVI + 0.5 g/L pyrite was chosen to conduct experiments on chromium removal.

Several tests under different initial pH conditions have been performed using 3 g/L ZVI + 0.5 g/L pyrite.

Figure 3 Cr removal under aerobic conditions using 0.6 g of ZVI and 0.1 g of pyrite in 200 ml of solution at different initial pH

The rate of Cr(VI) reduction by pyrite is strongly dependent on pH. Chromium removal at initial pH 4 and 7 is comparable. Being difficult to fix pH to 4, experiments

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were conducted adjusting initial pH to 7 and the amount of Cr(III) and Cr(VI) was also determined as shows the table below.

Table 1 Cr tot, Cr(VI) and Cr(III) values

In this study aerobic and anaerobic conditions have been also evaluated in Cr removal by ZVI/pyrite mixture. The results of the experiments conducted in closed test tube under anaerobic conditions using 3 g/L of ZVI and 0.5 g/L of pyrite at different initial pH are reported in the graph below.

Figure 4 Cr removal under anaerobic conditions using 0.15 g of ZVI and 0.025 g of pyrite in a closed 50 ml test tube at different initial pH

Unlike the aerobic conditions, under anaerobic conditions pH does not affect the Cr removal much.

Time

(min) Cr (tot)

(ppm) Cr (VI)

(ppm) Cr (III) (ppm)

0 9.7 9.6 0

10 2 1.6 0.5

20 1 0.7 0.3

30 0.2 0.1 0.1

40 0.2 0 0.2

60 0.2 0 0.2

120 0.1 0 0.1

180 0.1 0 0.1

240 0 0 0

300 0 0 0

360 0 0 0

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Experiments by using pyrite only have been carried out for more than 24 hours at initial pH 7. Aerobic and anaerobic conditions were performed and solution was mixed for 28 hours. For both condition Cr(VI) reduction was fast during the first 10 minutes.

Figure 5Cr removal using 0.5 g/L of pyrite only under aerobic and anaerobic conditions at initial pH 7

After 8 hours Cr concentration is constant. It means that chromate was not completely removed from solution at the time scale of the experiments, maybe because all dissolved iron existed in the ferric form and ferrous form was not available to reduce Cr(VI). On other hand, under aerobic condition, after initial Cr concentration dropped an increase of Cr concentration was noted. After 28 hours of observation, Cr concentration reached almost the same initial Cr concentration. It means that after an initial adsorption, under aerobic condition there is a release of Cr.

The tests that have been carried out have shown that zero valent iron is an efficient and fast solution but the surface become covered by Cr(III) and the reaction is stopped. The main advantage of this system is that it doesn't require high costs for energy and chemicals. Furthermore, the use of S-ZVI has revealed its superior properties respect to the ZVI in the tests performed because sulfidation form a very porous layer on ZVI surface to prevent passivation; moreover the porous layer can transfer electrons. After 2 hours chromium removal is of 99% by using S-ZVI.

Nevertheless, S-ZVI is not a reliable material because after time it loses its characteristic and the efficiency decreases; in addition sulfidation is a complex

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process because we need to use chemical material, it is expensive and there is problem of smell cause by the use of Na2S. The best results was observed using ZVI + pyrite.

The advantages are low costs and the absence of a process as a sulfidation to prepare S-ZVI because pyrite exists in nature and both ZVI and pyrite are cheap. Moreover in this system pyrite may behaves as a catalyst regenerating Fe2+ ions that becomes available for Cr(VI) reduction; ongoing studies are currently being conducted about this system.

More interesting is also the behaviour of pyrite that produces a Cr release after an initial removal.

Riferimenti

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