Removal of Cr(VI) from Cr-contaminated groundwater through electrochemical addition of Fe(II)

doi:10.1016/j.jenvman.2005.12.005

Journal of Environmental Management

Volume 82, Issue 1, January 2007, Pages 66-76

https://doi.org/10.1016/j.jenvman.2005.12.005 Get rights and content

Abstract

The conventional chemical reduction–precipitation technique in the removal of Cr(VI) from contaminated groundwater involves a two-step process whereby Cr(VI) is first reduced to Cr(III) at an acidic pH by a reducing agent and in a subsequent step, Cr(III) is precipitated as insoluble hydroxide at an alkaline pH. In a variation of this method, Fe(II) is added electrochemically to the Cr(VI) containing water. From a pure iron electrode, Fe²⁺ ions are released into the solution and bring forth the reduction of Cr(VI). At the cathode, H₂O is reduced whereby the OH⁻ ions entering the solution keep the pH of the solution in the alkaline range. This latter fact greatly facilitates simultaneous reduction of Cr(VI) to Cr(III) and co-precipitation of hydroxides of trivalent Cr and Fe. On the basis of a set of experimental data, it is shown that this process is both thermodynamically and kinetically efficient, meaning, with the electrochemical method, rapid and nearly complete removal of Cr(VI) from a groundwater source with both high and low levels of Cr-contamination can be achieved. These factors make the electrochemical process superior to the conventional chemical process in remediation of Cr-contaminated groundwater.

Introduction

Removal of Cr from certain contaminated groundwater remains an important issue in environmental remediation because of the mutagenic and carcinogenic characters of various chromium (Cr) compounds (Nriagu and Nieboer, 1988). Contamination of groundwater by Cr at numerous localities primarily resulted from uncontrolled or accidental release of Cr-bearing solutions, used in various industrial applications, into the subsurface environment. Cr in such solutions mostly occurs as oxyacids and oxyanions of Cr(VI). It is this oxidation state in which Cr is highly soluble, mobile, and toxic.

Hexavalent Cr can be removed from an aqueous solution by a variety of techniques, such as chemical reduction–precipitation, ion exchange, reverse osmosis, and adsorption by granular activated carbon (Frank and McMullen, 1996). Of these, chemical reduction–precipitation whereby Cr(VI) is reduced to Cr (III) with subsequent precipitation of Cr(III) as chromic hydroxide [Cr(OH)₃(s)] is the most widely used method for removal of Cr(VI) from waste streams (Eckenfelder, 1989). The reducing agent in this process is chiefly a salt of either S(IV) or Fe(II). The method is a two-step process in which reduction and precipitation at highly acidic and alkaline pH conditions, respectively, occur in succession. A variation of this method that emerged in the past decade is the reduction and precipitation of Cr(VI) from the contaminated water stream in a single step by electrochemical addition of Fe(II). A formal description of this technology, the controlling factors, its merits and limitations are lacking in the literature. Eary and Rai (1988) cited several patented studies of Cr(VI) reduction by Fe(II) and conducted their own study using synthetic Cr(VI) solutions and salts of Fe(II) to examine the stoichiometry of the Cr(VI)–Fe(II) reaction as a function of dissolved oxygen concentration, pH, and ionic composition with the aim of optimizing experimental conditions for Cr(VI) removal from aqueous wastes. Sengupta (1995) commented that the chemical reduction–precipitation method of removal of Cr(VI) from a waste stream is inefficient both thermodynamically and kinetically, especially when Cr(VI) occurs in water at low concentration levels (e.g. <10 mg/L).

In this paper, we present the results of a laboratory investigation, carefully designed, to show that removal of high as well as low levels of Cr(VI) from groundwater, contaminated by anthropogenic release of Cr-bearing aqueous solutions, by electrochemical addition of Fe(II) is an efficacious method, both thermodynamically and kinetically.

Section snippets

The groundwater source

One notable area where from Cr-contaminated groundwater has been identified at several localities is the city of Odessa, Ector County, Texas (Fig. 1). This widespread contamination resulted from several industrial sources, including machine and chrome plating shops that were operational in the past (e.g., see Henderson, 1994). The present investigation is based upon groundwater from the Precision Machine and Supply Superfund site (Precision site; Latitude: 31°50′29″N, Longitude: 102°21′48″W).

Extent of chromium contamination

The maximum contaminant level (MCL) for Cr in drinking water is 0.1 mg/L according to the United States Environmental Protection Agency and 0.05 mg/L according to World Health Organization, European Economic Community, and Health and Welfare Canada (Cotruvo and Craig, 1990). Cr concentrations in groundwater from the Precision site far exceed such MCL limits. In general, the shallow aquifer is contaminated with dissolved Cr reaching concentrations as high as 118 mg/L, whereas the extent of

Chemical calculations

Equilibrium distributions of various aqueous species resulting from the chemical reactions discussed in this study were calculated by the PHREEQC computer program (Parkhurst and Appelo, 1999). These results were used in subsequent thermodynamic calculations outside the PHREEQC environment. The thermodynamic data used in this study are those given in PHREEQC database. However, this database does not contain the thermodynamic data on aqueous Cr species. These were incorporated in the database

Aqueous species of Cr(VI) and Cr(III) in the groundwater source

Various authors used different conventions in describing aqueous speciation of Cr(VI) and Cr(III) as a function of both Eh and pH (e.g., Deltombe et al., 1966; Robertson, 1975; Richard and Bourg, 1991; Deutsch, 1997). For the sake of consistency, the speciation models of the groundwater sources are presented below.

In pure chromate solutions, the critical Cr(VI) bearing aqueous species are the oxyanions or oxyacids, namely ${HCrO}_{4}^{-}$ (bichromate/hydrogen chromate ion), ${CrO}_{4}^{2 -}$ (chromate ion), ${Cr}_{2} O_{7}^{2 -}$

The electrochemical process

The electrochemical process relies upon the redox reactions taking place at the surface of the conductive electrodes immersed into water. With an applied electrical potential difference through a direct current source, the simultaneous reactions are as follows:Oxidation reaction at the anode:Fe→Fe²⁺ (aq)+2e⁻Reduction reaction at the cathode:2H₂O+2e⁻ →H₂↑+2OH⁻(aq).

The amount of Fe²⁺ released primarily depends upon the applied current and the duration of the passage of the current through the

Reaction stoichiometry

In the conventional chemical reduction–precipitation process, reduction reactions are normally carried out at an acidic pH condition, usually in the range between 2 and 3 (Eckenfelder, 1989). In contrast, the electrochemical reduction–precipitation can be carried out at a near neutral pH. The pH values typically found in groundwater vary between 7 and 8. Thus, the predominant Cr(VI) species involved in the redox reaction is ${CrO}_{4}^{2 -}$ . The redox equilibrium describing the transformation of the ${CrO}_{4}^{2}$

Experimental methods

Cr concentration and pH as measured after collection of the groundwater samples (Table 1) could change over time. For this reason, immediately prior to the electrochemical experiments, the initial concentration of Cr(VI), the pH, and the conductivity were measured for each of the two water streams. These initial conditions were used to determine operational parameters such as the electrochemical Fe(II)-dose and necessary pH adjustment for optimal condition of hydroxide precipitation. For the

Experimental results

The experimental results are summarized in Table 4. Due to differences in the methods of determination of Cr_T and Cr(VI), some of the Cr (VI) concentrations are higher than the corresponding values of Cr_T. In general, compared to the ICP spectroscopy, the colorimetric method is less accurate. For this reason, the Cr_T values are more reliable than the corresponding Cr(VI) values reported in Table 4. All of the experiments yielded an effluent containing less than 0.1 mg/L of Cr_T (Table 4). The

Thermodynamics of Cr(VI) reduction and hydroxides precipitation

As noted earlier, the predominant Cr(VI) species in the near neutral pH condition and given Cr_T concentration, in both MW-15 and MW-30 source waters, is ${CrO}_{4}^{2 -}$ . Thus, the thermodynamics of the redox reaction (3) is examined to determine the relative merits of pure chemical versus electrochemical addition of Fe(II) in causing the reduction of aqueous Cr(VI).

The standard free energy change (ΔG⁰) of reaction (3) is −213.51 kJ/mol. The corresponding equilibrium constant, $K = 2.55 \times 1 0^{37}$ . Due to this

Conclusions

The chemical principle involved in the electrochemical method is essentially the same as that of the chemical reduction–precipitation method. However, the difference in these two methods lies in the manners in which the reductant is introduced and precipitation is carried out. In the electrochemical process, ferrous iron as a reducing agent is contributed directly into the source water from an iron electrode rather than in the form of a ferrous salt such as FeSO₄ or FeCl₂ used in conventional

Acknowledgements

This study was undertaken by the authors as part of a project conducted by Ecology and Environment, Inc. (E&E) under the Superfund Engineering and Remediation Contract (no. 4300093000) with the Texas Natural Resource Conservation Commission (TNRCC). Thus, the data presented in this paper are from the reports submitted by E&E to TNRCC. The ideas and interpretations presented here are those of the authors and the authors are solely responsible for any error or omission. The authors are thankful

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¹: Present address: Walter P Moore and Associates, Inc. 3131 Eastside, Houston, TX 77098-1919, USA.

²: Present address: URS Corporation, 282 Delaware Ave. Buffalo, NY 14202, USA.

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