Subscribe Now Subscribe Today
Research Article
 

Reduction Remediation of Hexavalent Chromium by Pyrite in the Aqueous Phase



Zaidi Houda, Qian Wang , Yanjun Wu and Xinhua Xu
 
Facebook Twitter Digg Reddit Linkedin StumbleUpon E-mail
ABSTRACT

The objective of this study was to investigate the reduction remediation of hexavalent chromium by pyrite in the aqueous phase, batch experiment indicated that: (1) the rate of chromate removal was dependent on the initial pyrite concentration; (2) the Cr (VI) reduction rate increased with an decrease in initial Cr (VI) concentration; (3) the Cr (VI) reduction was found to be pH dependent, the high reduction of Cr (VI) under acidic conditions; (4) the rate of chromium removal increased with increased temperature; (5) the Cr (VI) reduction depends on the pyrite type. The effective removal of Cr (VI) by pyrite suggests that the use of pyrite for the treatment of wastewater containing Cr (VI) is an innovative method that constitutes a simple, effective and economical means for wastewater treatment.

Services
Related Articles in ASCI
Similar Articles in this Journal
Search in Google Scholar
View Citation
Report Citation

 
  How to cite this article:

Zaidi Houda, Qian Wang , Yanjun Wu and Xinhua Xu , 2007. Reduction Remediation of Hexavalent Chromium by Pyrite in the Aqueous Phase. Journal of Applied Sciences, 7: 1522-1527.

DOI: 10.3923/jas.2007.1522.1527

URL: https://scialert.net/abstract/?doi=jas.2007.1522.1527

INTRODUCTION

Chromium, with its great economic importance in industrial use, is a major metal pollutant of the environment (Konovalova et al., 2003). Chromium pollution as a consequence of effluent discharge from tanneries and other industries, which include metal plating, manufacturing industries and ferrochrome production (Shrivastava et al., 2003). Chromium is found in the environment predominantly as Cr (VI) and Cr (III) (Chirwa and Wang, 2000). Hexavalent chromium (Cr (VI)) is mobile in the environment, Subject to biological uptake and is highly toxic (Guha et al., 2003). Furthermore, they are widely distributed as an anthropogenic pollutant (Vainshtein et al., 2003). Trivalent chromium (Cr (VI)) is less toxic than Cr (VI) and is less susceptible to biological uptake (Guha, 2004).

Chromium (VI) can be reduced by biological and chemical means. There have been several studies on the methods and possible mechanisms of reduction of hexavalent chromium (Stollenwerk and Grove, 1985; Siegel and Clifford, 1988; Palmer and Wittbrodt, 1991; Anderson et al., 1994; Deng and Stone, 1996; Wittbrodt and Palmer, 1996; Vitale, 1997; Beukes et al., 1999; Puls et al., 1999; Ponder et al., 2000; Wielinga et al., 2001; Alowitz and Scherer, 2002; Daulton et al., 2002; Hansel et al., 2003; Lee et al., 2003; Vainshtein et al., 2003; Bojic et al., 2004; Tora et al., 2004; Xu et al., 2005; Lee et al., 2006; Lo et al., 2006). Numerous observations indicate that ferrous iron [Fe (II)] could be an important of Cr (VI) in natural waters (Sedlak and Chan, 1997; Pettine et al., 1998; Schlautman and Han, 2001; Hwang et al., 2002; Nunez et al., 2003; Tzou et al., 2003; Erdema and Tumen, 2004.

Iron minerals are ubiquitous in nature and play a critical role in the geochemical cycling of trace elements. In particular, the Fe (III)-Fe (II) redox couple is an important electron-transfer mediator for many biological and chemical species. Pyrite and other reactive iron-sulfide minerals are important to sedimentary trace element behaviour (Morse and Luther, 1999). Pyrite, an extremely cheap and readily available waste material, may be suitable for the removal of hexavalent chromium (Zouboulis et al., 1995). Furthermore, Pyrite was found to act as an efficient Cr (VI) reducing agent (Doyle et al., 2004).

Studies have demonstrated the heterogeneous reduction of Cr (VI) by Fe (II) in biotite, vermiculite, illite, smectites, chlorite, magnetite, ilmenite, Fe (II) hematite, Fe (II)-goethite and sulfides (Ilton and Veblen, 1994; White et al., 1996; Patterson and Fendorf, 1997; Kendelewicz et al., 1999; Kendelewicz et al., 2000; Mullet et al., 2004; He et al., 2005). In comparison, fewer studies have been carried out on aqueous Cr (VI) reduction by pyrite (Zouboulis et al., 1995; Benincasa et al., 2002; Kim et al., 2002; Doyle et al., 2004; Chon et al., 2006).

The primary objective of this study is to prepare pyrite particles for degradation of Cr (VI). The specific objectives are to (1) characterize the pyrite particles with environmental scanning electron microscope (ESEM); (2) quantify the effect of pyrite particles dosage, Cr (VI) concentration, initial pH and temperature on the rate of Cr (VI) reduction; (3) compare the effectiveness of Cr (VI) reduction by different type of pyrite particles.

MATERIALS AND METHODS

Chemicals: The pyrite used in this experiment was obtained from Hangzhou weimin geologic sample factor. Pyrite particle size range was separated and used in the experiments, having a mean particle size of around (<100 mesh). Cr (VI) stock solutions were prepared by dissolving a weighed amount of dried K2Cr2O7 in distilled deionized water. The pH of the samples was previously adjusted with small amount of dilute HCl or NaOH solutions in order to have the desired final pH after the addition of Cr (VI).

Batch experiments: Batch kinetic experiments were performed to evaluate the removal rate of Cr (VI) in the presence of pyrite particles.

A pyrite particle was added to 1000 mL flasks filled with 500 mL of K2Cr2O7 solution. The solution was continuously stirred at constant temperature (25±0.5°C). Oxygen was removed from the solution by continuous sparging with water-saturated nitrogen before and during the reactions. The samples were filtered through a 0.45 μm filter.

A series of batch experiments was used to study the reduction kinetics of Cr (VI) with pyrite under conditions of (1) different pyrite concentrations, (2) different initial Cr (VI) concentrations and (3) different pH values, (4) different temperatures and (5) different size pyrite particles.

The pH value was initially 5.5 (the pH value of deionized water) and not controlled during the experiment.

Characterization and analytical methods: The morphology of the metal particles was observed under an XL30-ESEM. Aqueous concentrations of Cr (VI) were determined by a diphenylcarbazide procedure at 540 nm using UV-VIS spectrophotometer (TU-1800PC, Beijing, China).

RESULTS AND DISCUSSION

Characterization of pyrite: Figure 1 compares the SEM image of the pyrite particles before or after reaction. Single crystals were observed in the both micrographs. The platy morphology of the crystals suggests that most of the FeS2 may be present as marcasite (a dimorph of pyrite). While pyrite is typically present as cubes or octahedra, marcasite often assumes a tabular morphology (Rakovan et al., 1995).

Fig. 1: ESEM image of pyrite particles (a) before reaction and (b) after reaction

Figure 1a is shown that the surface morphology of the crystals was characterized by some cracks. Figure 1b is shown that the surface of crystals deposit was smooth uniform coating, there was no evidence of any cracking or significant grain structure, this is due to the precipitation of the hydroxide solid (Cr, Fe)(OH)3.

Effect of pyrite concentration: The effect of initial pyrite particles concentration on the reduction of the Cr (VI) was investigated. The reduction rats depending on the initial pyrite concentration are shown in Fig. 2. At the same initial Cr (VI) concentration, the reduction ratio increased with the increase of initial pyrite concentration. When the initial pyrite concentration was 28 g L-1, the Cr (VI) reduction rate reaches 98.18% at time of 30 min and when the initial pyrite concentration was 4 g L-1, the Cr (VI) reduction rate reaches only 12.73% at the time of 30 min as a consequence, it can be concluded that the higher initial concentration ratio of pyrite resulted in higher reduction ratio.

Fig. 2: Cr (VI) concentration vs. time at different pyrite concentration Ccr (VI) = 10 mg L–1, pH = 5.5, T = 25°C, ω = 500 r min–1

This is probably because that there were more amounts of Fe (II), thus helping the Cr (VI) reduction.

The oxidation of pyrite is generally described by (Chon et al., 2006):

(1)

(2)

(3)

The reduction of Cr (VI) to Cr(III) involves the oxidation of Fe(II) and S22¯ on the pyrite surfaces, following the reaction (5) (Benincasa et al., 2002).

(4)

Initial Cr (VI) concentration effect: Figure 3 shows the removal of Cr (VI) when contacted with pyrite particles. When Cr (VI) concentrations were less than 5 mg L-1 all Cr (VI) were removed within 30 min however, the required reaction times for the complete removal of Cr (VI) increased substantially as initial Cr (VI) concentrations increased up to 10 mg L-1. for example, approximately 2 h was needed to remove Cr (VI) when the initial Cr (VI) was 10 mg L-1. the model fits in Fig. 3 which indicates that the Cr (VI) reduction in this research follow a first-order reaction.

Effect of pH: Chon et al. (2006) demonstrated the importance of solution pH on reaction rates: the reaction rate of chromate reduction by pyrite was faster at pH 3 than pH 4.

The removal rate of hexavalent chromium at a fixed Cr (VI) concentration (10 mg L-1) as a function of pH is shown in Fig. 4. The reduction of the Cr (VI) was found to depend on the pH of the solution. Reduction capacity was found to decrease with an increase in the pH, it is evident that the reduction rate of Cr (VI) was higher at lower pH (pH = 3) and the removal percentage made up to 100% in 30 min with the pH increased, the removal of Cr (VI) decreased.

Fig. 3: Cr (VI) concentration vs. time at different initial Cr (VI) concentrations CFe = 20 g L–1, pH = 5.5, T = 25°C, ω = 500 r min–1

Fig. 4: Cr (VI) concentration vs. time at different initial pH values CFe = 20 g L–1, Ccr (VI) = 10 mg L–1, T = 25°C, ω = 500 r min–1

Iron exists in the ferric (Fe3+) or ferrous (Fe2+) form, depending upon the pH and dissolved oxygen concentration (Mohan and Chander, 2006). At neutral pH and in presence of oxygen, soluble Fe2+ is oxidized to Fe3+, which readily hydrolyzes to ferric hydroxide that is insoluble in water (Mohan and Chander, 2006). In most of the surface waters, Fe3+ predominates (Mohan and Chander, 2006). Ferrous (Fe2+) on the other hand is soluble and dominates under anaerobic conditions (Mohan and Chander, 2006). The increase in the solubility and the surface changes of the minerals at lower pH accelerate the reduction of chromate by the Fe (II) ions and structural Fe (II) and therefore, results in rapid reduction in more acidic conditions (Chon et al., 2006). Above pH 9 soluble Cr (OH)¯4 species are formed, having as a consequence the lowering of respective Cr (VI) removals (Zouboulis et al., 1995).

Temperature effect: The effect of temperature on the Cr (VI) removal was investigated at 25 and 37°C. the reduction rats at different temperatures indicated that there are significant changes in the Cr (VI) removal.

Fig. 5: Cr (VI) concentration vs. time at different temperatures CFe = 20 g L–1, CCr (VI) =10 mg L–1, pH = 5.5, ω = 500 r min–1

Fig. 6: Cr (VI) concentration vs. time at different pyrite type CFe = 20 g L–1, CCr (VI) =10 mg L–1, pH = 5.5, ω = 500 r min–1

The highest Cr (VI) reduction was observed at 37°C. the model fits in Fig. 5 which indicates that the Cr (VI) reduction in this research was dependent on temperature. The effect of temperature on the observed reaction rates can be explained by considering how the speciation of Fe (II) changes as a function of temperature (Sedlak and Chan, 1997).

Comparison of different pyrite type: The experiments about studying the effect of pyrite particles type on Cr (VI) reduction was carried out in the presence of three different pyrite particles 40-100, 40-30, <100 mesh. The Cr (VI) reduction and the change of pyrite particles type are presented in Fig. 6. As shown in Fig. 6, the particle size of around (<100 mesh) acquired the best results of Cr (VI) reduction, with the reduction ratio of 91.81% being obtained at time of 2 h. And the reduction of Cr (VI) in this pyrite particles type was faster than the other two. Compared with the particle size of around (40-30 mesh), whose reduction ratio was only 22.73% at time of 2 h. The pyrite particle size of around (<100 mesh) exhibited higher removal efficiency because particle as a good dispersant.

CONCLUSION

Commercial processes and unregulated disposal of the chromium containing effluent has led to the contamination of surface and ground waters. Chromium (VI) is highly toxic and carcinogenic even when present in very low concentration in water.

In this study, higher concentrations of pyrite increased the Cr (VI) reduction. Cr (VI) reduction rates increased with decreasing initial Cr (VI) concentration. A result shows that pH significantly affects the rate of Cr (VI) reduction, with more rapid reduction occurring at pH 3. Cr (VI) reduction rates were dependent on temperature and pyrite particles type. This study has shown that the use of pyrite for the removal of chromate through reduction reaction is applicable to Cr (VI) contaminated solutions; these data also facilitate the understanding of pyrite interaction in natural communities exposed to hexavalent Cr. The rapid reduction of chromate suggests that abiotic reduction of chromate may be an important transformation process in natural systems and engineered remediation technologies based on iron metal (Fe (II)). The chemical reduction of Cr (VI) to Cr (III) by pyrite may provide a less costly approach remediation.

REFERENCES
1:  Alowitz, M.J. and M.M. Scherer, 2002. Kinetics of nitrate, nitrite and Cr (VI) reduction by iron metal. Environ. Sci. Technol., 36: 299-306.
Direct Link  |  

2:  Anderson, L.D., D.B. Kent and J.A. Davis, 1994. Batch experiments characterizing the reduction of Cr (VI) using suboxic material from a mildly reducing sand and graver aquifer. Environ. Sci. Technol., 28: 178-185.

3:  Benincasa, E., M.F. Brigatti, G. Franchini, D. Malferrari, L. Medici, L. Poppi and M. Tonelli, 2002. Reactions between Cr (VI) solutions and pyrite: Chemical and surface studies. Geologica Carpathica, 53: 79-85.
Direct Link  |  

4:  Beukes, J.P., J.J. Pienaar, G. Lachmann and E.W. Giesekke, 1999. The reduction of hexavalent chromium by sulphite in wastewater. Water SA., 25: 363-370.
Direct Link  |  

5:  Bojic, A.L., M. Purenovic and D. Bojic, 2004. Removal of chromium (VI) from water by micro-alloyed aluminium composite (MAlC) under flow conditions. Water SA., 30: 353-360.
Direct Link  |  

6:  Chirwa, E.N. and Y.T. Wang, 2000. Simultaneous chromium (VI) reduction and phenol degradation in an anaerobic consortium of bacteria.Water. Water Res., 34: 2376-2384.
Direct Link  |  

7:  Chon, C.M., J.G. Kim and H.S. Moon, 2006. Kinetics of chromate reduction by pyrite and biotite under acidic conditions. Applied Geochem., 21: 1469-1481.
Direct Link  |  

8:  Daulton, T.L., B.J. Little, J.W. Kim, S. Newell and K. Lowe et al., 2002. Quantitative environmental cell-transmission electron microscopy: Studies of microbial cr (vi) and fe (iii) reduction. JEOL News, 37: 6-13.
Direct Link  |  

9:  Deng, B. and A.T. Stone, 1996. Surface-catalyzed chromium (VI) reduction: Reactivity comparisons of different organic reductants and different oxide surfaces. Environ. Sci. Technol., 30: 2484-2494.
CrossRef  |  Direct Link  |  

10:  Doyle, C.S., T. Kendelewicz, B.C. Bostick and G.E. Brown, Jr, 2004. Soft X-Ray spectroscopic studies of the reaction of fractured pyrite surfaces with Cr (VI)-containing aqueous solutions. Geochim. Cosmochim. Acta, 68: 4287-4299.
Direct Link  |  

11:  Erdema, M. and F. Tumen, 2004. Chromium removal from aqueous solution by the ferrite process. J. Hazardous Mater., 109: 71-77.
Direct Link  |  

12:  Guha, H., K. Jayachandran and F. Maurrasse, 2003. Microbiological reduction of chromium (VI) in presence of pyrolusite-coated sand by Shewanella alga simidu atcc 55627 in laboratory column experiments. Chemosphere, 52: 175-183.
Direct Link  |  

13:  Guha, H., 2004. Biogeochemical influence on transport of chromium in manganese sediments: Experimental and modeling approaches. J. Contaminant Hydrol., 70: 1-36.
Direct Link  |  

14:  Hansel, C.M., B.W. Wielinga and S. Fendorf, 2003. Structural and compositional evolution of Cr/Fe solids after indirect chromate reduction by dissimilatory iron-reducing bacteria. Geochim. Cosmochim. Acta, 67: 401-412.
Direct Link  |  

15:  HE, T.Y., J.M. Bigham and S.J. Traina, 2005. Biotite dissolution and Cr (VI) reduction at elevated pH and ionic strength. Geochim. Cosmochim. Acta, 69: 3791-3800.
Direct Link  |  

16:  Hwang, I., B. Batchelor, M.A. Schlautman and R. Wang, 2002. Effects of ferrous iron and molecular oxygen on chromium (VI) redox kinetics in the presence of aquifer solids. J. Hazardous Mater., 92: 143-159.
Direct Link  |  

17:  Ilton, E.S. and D.R. Veblen, 1994. Chromium sorption by phlogopite and biotite in acidic solutions at 25°C: Insights from X-Ray photoelectron spectroscopy and electron microscopy. Geochim. Cosmochim. Acta, 58: 2777-2788.

18:  Kendelewicz, T., P. Liu, C.S. Doyle, G.E. Brown, Jr., E.J. Nelson and S.A. Chambers, 1999. Chambers c. X-ray absorption and photoemission study of the adsorption of aqueous Cr (VI) on single crystal hematite and magnetite surfaces. Surf. Sci., 424: 219-231.
Direct Link  |  

19:  Kendelewicz, T., P, Liu., C.S, Doyle and G.E. Brown Jr, 2000. Spectroscopic study of the reaction of aqueous Cr (vi) with Fe3o4 (111) surfaces. Surface Sci., 469: 144-163.
Direct Link  |  

20:  Kim, J., P.K. Jung, H.S. Moon and C.M. Chon, 2002. Reduction of hexavalent chromium by pyrite-rich andesite in different anionic solutions. Environ. Geol., 42: 642-648.
Direct Link  |  

21:  Konovalova, V.V., G.M. Dmytrenko, R.R. Nigmatullin, M.T. Bryk and P.I. Gvozdyak, 2003. Chromium (VI) reduction in a membrane bioreactor with immobilized pseudomonas cells. Enzy. Microbial. Technol., 33: 899-907.
Direct Link  |  

22:  Lo, I.M.C., C.S.C. Lam and K.C.K. Lai, 2006. Hardness and carbonate effects on the reactivity of zero-valent iron for Cr (VI) removal. Water Res., 40: 595-605.
Direct Link  |  

23:  Lee, T., H. Lim., Y. Lee and J.W. Park, 2003. Use of waste iron metal for removal of Cr (VI) from water. Chemosphere, 53: 479-485.
PubMed  |  Direct Link  |  

24:  Lee, S.E., J.U. Lee, J.S. Lee and H.T. Chon, 2006. Effects of indigenous bacteria on Cr (VI) reduction in Cr-contaminated sediment with industrial wastes. J. Geochem. Explorat., 88: 41-44.
Direct Link  |  

25:  Mohan, D., S. Chander, 2006. Removal and recovery of metal ions from acid mine drainage using lignite-a low cost sorbent. J. Hazardous Mater., 137: 1545-1553.
Direct Link  |  

26:  Morse, J.W. and G.W. Luther, 1999. Chemical influences on trace metal-sulfide interactions in anoxic sediments. Geochim. Cosmochim. Acta, 63: 3373-3378.
Direct Link  |  

27:  Mullet, M., S. Boursiquot and J.J. Ehrhardt, 2004. Removal of hexavalent chromium from solutions by mackinawite, tetragonal FeS. Colloids and Surfaces A: Physicochem. Eng. Aspects, 244: 77-85.
Direct Link  |  

28:  Nunez, F.U., P.D. Jimenez, C.B. Diaz., M.R. Romo and M.P. Pardave, 2003. Gamma radiation-induced polymerization of Fe (II) and Fe (III) methacrylates for Cr (VI) removal from wastewater. Radiat. Phys. Chem., 68: 819-825.
Direct Link  |  

29:  Rakovan, J., M.A.A. Schoonen, R.J. Reeder, P. Tyrna and D.O. Nelson, 1995. Epitaxial overgrowths of marcasite on pyrite from the tunnel and reservoir project, Chicago, Illinois, USA: Implications for marcasite growth. Geochimi. Cosmochim. Acta, 59: 343-346.
Direct Link  |  

30:  Palmer, C.D. and P.R. Wittbrodt, 1991. Processes affecting the remediation of chromium-contaminated sites. Environ. Health Perspect., 92: 25-40.
Direct Link  |  

31:  Patterson, R.R. and S. Fendorf, 1997. Reduction of hexavalent chromium by amorphous iron sulfide. Environ. Sci. Technol., 31: 2039-2044.
CrossRef  |  Direct Link  |  

32:  Pettine, M., L. D'Ottone, L. Campanella, F.J. Millero and R. Passino, 1998. The reduction of chromium (VI) by iron (II) in aqueous solutions. Geochim. Cosmochim. Acta, 62: 1509-1519.
Direct Link  |  

33:  Puls, R.W., C.J. Paul and R.M. Powell, 1999. The application of in situ permeable reactive (Zero-Valent Iron) barrier technology for the remediation of chromate-contaminated groundwater: A field test. Applied Geochem., 14: 989-1000.
Direct Link  |  

34:  Ponder, S.M., J.G. Darab and T.E. Mallouk, 2000. Remediation of Cr(VI) and Pb(II) aqueous solutions using supported, nanoscale zero-valent iron. Environ. Sci. Technol., 34: 2564-2569.
CrossRef  |  Direct Link  |  

35:  Schlautman, M.A. and I. Han, 2001. Effects of pH and dissolved oxygen on the reduction of hexavalent chromium by dissolved ferrous iron in poorly buffered aqueous systems. Water Res., 35: 1534-1546.
Direct Link  |  

36:  Sedlak, D.L. and P.G. Chan, 1997. Reduction of hexavalent chromium by ferrous iron. Geochim. Cosmochim. Acta, 61: 2185-2192.
Direct Link  |  

37:  Shrivastava, R., R.K. Upreti and U.C. Chaturvedi, 2003. Various cells of the immune system and intestine differ in their capacity to reduce hexavalent chromium. FEMS Immunol. Med. Microbiol., 38: 65-70.
CrossRef  |  Direct Link  |  

38:  Siegel, S.K. and D.A. Clifford, 1988. Removal of chromium from ion exchange regenerant solution. US Government Printing Office.

39:  Stollenwerk, K.G. and D.B. Grove, 1985. Reduction of hexavalent chromium in water samples acidified for preservation. J. Envirom. Qual., 14: 396-399.

40:  Tora, A., T. Buyukerkek, Y. Cengeloglu and M. Ersoz, 2005. Simultaneous recovery of Cr (III) and Cr (VI) from the aqueous phase with ion-exchange membranes. Desalination, 171: 233-241.
Direct Link  |  

41:  Tzou, Y.M., M.K. Wang, R.H. Loeppert, 2003. Effect of N-hydroxyethyl ethylenediamine-triacetic acid (HEDTA) on Cr (VI) reduction by Fe (II). Chemosphere, 51: 993-1000.
Direct Link  |  

42:  Vainshtein, M., P. Kuschk, J. Mattusch, A. Vatsourina and A. Wiessner, 2003. Model experiments on the microbial removal of chromium from contaminated groundwater. Water Res., 37: 1401-1405.
PubMed  |  Direct Link  |  

43:  Vitale, R.J., G.R. Mussoline and K.A. Rinehimer, 1997. Extraction of sparingly soluble chromate from soils: Evaluation of methods and Eh-pH effects. Environ. Sci. Technol., 31: 390-394.
CrossRef  |  Direct Link  |  

44:  White, A.F., M. Peterson and L. Peterson, 1996. Reduction of aqueous transition metal species on the surfaces of Fe (II)-containing oxides. Geochim. Cosmochim. Acta, 60: 3799-3814.
Direct Link  |  

45:  Wielinga, B., M.M. Mizuba, C.M. Hansel and S. Fendorf, 2001. Iron promoted reduction of chromate by dissimilatory iron-reducing bacteria. Environ. Sci. Technol., 35: 522-527.
CrossRef  |  Direct Link  |  

46:  Wittbrodt, P.R. and C.D. Palmer, 1996. Effect of temperature, ionic strength, background electrolytes and Fe (III) on the reduction of hexavalent chromium by soil humic substances. Environ. Sci. Technol., 30: 2470-2477.
CrossRef  |  Direct Link  |  

47:  Xu, W., Y. Liu, G. Zeng, X. Li, C. Tang and X. Yuan, 2005. Enhancing effect of iron on chromate reduction by Cellulomonas flavigena. J. Hazardous Mater., 126: 17-22.
Direct Link  |  

48:  Zouboulis, A.I., K.A. Kydros and K.A. Matis, 1995. Removal of hexavalent chromium anions from solutions by pyrite fines. Water Res., 29: 1755-1760.
Direct Link  |  

©  2021 Science Alert. All Rights Reserved