Chromium Reduction by Zero-valent Iron and Bacteria

A study of alternative chromium reduction methods was performed for wastewater or contaminated water. Two existing chromium reduction treatment methods were tested on the wastewater samples: treatment with (1) zero-valent iron, and (2) bacteria cultures. We found that both methods can reduce chrome. However, the chrome reduction process using bacteria is considerably slower than that using iron powder. When iron powder was used to reduce hexavalent chromium to trivalent chromium, we found that (a) when the sample containing low levels of total chromium (< 20 mg/liter) was mixed properly with a sufficient amount of iron powder at a pH of 4 to 8, 70% to 90% of total chromium could be reduced in 2 to 4 hours; and (b) the chrome adsorption-reduction efficiency is limited by the transport process of Cr(VI) from the bulk of the wastewater to the surface of the iron powder. We also found that both pH and conductivity of the sample gradually increased while the chromium concentration decreased. In a leaching test of used iron powder filtered from the treated wastewater, we found that most of the chromium adsorbed on the iron powder was very stable, and only low levels of Cr(VI) were detected in the leachate. When hexavalent chrome samples were treated with mixed bacteria cultures (methanol-enriched) or the Bacillus cereus strain, we found that (a) the presence of low chromium concentrations (between 0.6 and 3.1 mg/liter) did not inhibit bacterial growth, and (b) the initial chromium reduction rates for the mixed cultures and Bacillus cereus were comparable (0.05 ± 0.03 mg/liter/hr for the first 24 hours). However, when the sample containing higher chromium concentrations (e.g., 8.9 mg/liter) was treated using the mixed bacteria culture, the initial bacterial growth was inhibited, but was not completely suppressed, by the chromium. INTRODUCTION Chromates (CrO4) or chromic acid are commonly used in plating and metal surface cleaning processes and are also used frequently in cooling towers as a corrosion inhibitor. Since hexavalent chromium, or Cr(VI), is very soluble in water and highly toxic, wastewater generated from these facilities containing chromates or groundwater contaminated with chromates near some industrial sites must be treated before discharge or reuse. The conventional treatment process generally involves two steps: (a) a detoxification process that reduces Cr(VI) to Cr(III), and (b) precipitation or removal of chromium or other heavy metals from the contaminated water. Under oxic conditions, Cr(VI) can exist as bichromate (HCrO4) or chromate (CrO4), depending on the pH of the aqueous system. These oxyanions are readily reduced to trivalent forms when the electron donors (reducing reagents) exist in aqueous solution. The reduction rate increases as the pH WM’04 Conference, Tucson AZ February 29 March 4, 2004 WM4126 decreases. Trivalent chromium, Cr(III), species are very stable with respect to the oxidationreduction potential. They can react with particulates and form metal salts in the form of Cr(OH)3 that are less soluble in water when the pH is 8 to 10 or higher. Methods of treating water contaminated by chromium or chromates have been studied extensively. For example, chromium reduction can be coupled with oxidation of various reducing reagents, including sodium metabisulfite [1], hydrogen sulfide [2], divalent iron [1, 3, 5–8], Fe(II)-bearing minerals [4], zero-valent iron [1, 9–13], organic compounds (e.g., thios or thiols) [14, 15], or via biotic pathways with certain bacteria species [7, 16]. After chromium reduction, it is essential to follow certain precipitation procedures to destabilize and remove the metal complex from the water. In aqueous solution, free metal ions are coordinated with water molecules (hydrolysis) containing free pairs of electrons to achieve stabilization [17]. Thus the complexes would remain in suspension in solution. When some chelating ligands, such as humic acid or ethylenediaminetetraacetic acid, are present in the solution, a ligand with multiple electron-donor groups will attach to a metal ion and form a stable chelated metal complex [18, 19]. In order to precipitate metal ions as salts from contaminated water, the metal complexes must be destabilized. Factors influencing destabilization include type and dosages of coprecipitant and flocculant, pH, chemical composition of the water, and mixing conditions of the treatment process [20–22]. REVIEW OF CHROME REDUCTION METHODS In this study, we focus on the reduction reaction of Cr(VI) with zero-valent iron powder and with bacteria cultures for wastewater samples collected from a metal surface cleaning facility. We examine the reduction efficiencies of these alternative treatment methods. A brief review of these abiotic or biotic methods is presented below. 1. Chrome or chromate reduction by zero-valent iron (Fe). Lately there has been considerable interest in using zero-valent iron to treat halogenated organic chemicals or inorganic chemicals [e.g., 23, 24] in contaminated aqueous solutions. Many types of reactive barriers or permeable walls have been constructed for groundwater remediation sites or landfill leachate treatments [9–13]. Laboratory and field tests for the iron-filling treatment process have proved to be effective. The thermodynamic instability of iron can drive redox reactions when an electron acceptor is present in the aqueous system. For water contaminated with chromate, iron is the electron donor and chromate is the electron acceptor. In an aqueous solution containing chromate, several reactions can occur: Fe + CrO4 + 4H2O Fe(OH)3 + Cr(OH)3 + 2OH (Eq. 1) (1–x)Fe(OH)3 + xCr(OH)3 (CrxFe1–x)(OH)3 (Eq. 2) WM’04 Conference, Tucson AZ February 29 March 4, 2004 WM4126 Fe + xH2O Fe(H2O)x + 2 e Fe(H2O)x + e (Eq. 3) 3Fe + Cr 3Fe + Cr (Eq. 4) Reactions (1) and (2) mainly occur on the solid surface of iron powder. As reactions (1) or (2) suggest, when reduced, chromium can be precipitated by adjusting the pH as chromium– iron hydroxide solid solution. Earlier studies [9–11] indicated that the removal of Cr(VI) by Fe is through chrome reduction and the subsequent precipitation of the Fe(III)-Cr(III) hydroxide, (oxy)hydroxide phase, or (CrxFe1–x)(OH)3. Compared to reactions (1) or (2), reaction (3) may be slower or insignificant but is a continuous process when the water is in the aerobic system and when the solution pH is increased [3]. Although ferrous ions can also reduce Cr(VI) to Cr(III), our previous studies [1] have indicated that reaction (1) or (2) dominates the overall process of chrome reductionabsorption, and reaction (3) or (4) is secondary. 2. Biotic Chromate Reduction. Biological reduction of Cr(VI) can occur under anaerobic or aerobic conditions. Several bacterial species have been studied for Cr(VI) reduction [7, 16, 25–28]. They include the Bacillus strain, sulfate-reducing bacteria, the Shewanella alga strain, the Pseudomonas strain, and some other unspecified anaerobic or aerobic bacteria identified from contaminated and noncontaminated sites. Earlier studies [16, 25] using a Cr(VI)-reducing species, Bacillus sp., in various reactors found that the Bacillus sp. cells may produce a soluble reductase that complexes with Cr(VI) in an enzymatic reaction in which hexavalent chromium receives electrons to form trivalent chromium. In other studies of chromium-reducing anaerobes isolated from contaminated and noncontaminated sites [26], bacterial growth inhibition by Cr(VI) was observed for all anaerobic bacteria cultures. However, Cr(VI) did not completely arrest the growth, and Cr(VI) reduction correlated with bacterial growth. This study also demonstrated the advantages of selecting indigenous bacteria from the contaminated zone or media for bioremediation rather than using pure cultures. Several other bacteria strains were also studied for their ability to reduce Cr(VI) [27]. Chromate reduction via a biotic-abiotic pathway under iron-reducing conditions was also examined [7]. The results indicated that direct bacterial reduction is considerably slower than reduction by ferrous ions. EXPERIMENTAL MATERIALS AND METHODS Wastewater samples were prepared by diluting the solution taken directly from the chromatesolution tank of the metal cleaning facility with water or media. The solution used in the process tank normally contains 30 g/liter of chromate and trace amounts of several other heavy metals (mainly copper, nickel, and zinc). The initial pH of our samples was between either 4 and 5 or 7 and 8. WM’04 Conference, Tucson AZ February 29 March 4, 2004 WM4126 Iron powders were obtained from the machine shop next to the surface cleaning facility of the research institute. The sizes of these powders were about 1 mm by 3 mm. Treatment tests of wastewater samples were conducted in 500 ml glass beakers. The tests were always conducted at room temperature in aerobic conditions. When iron powder was used, a shaker or a manual mixer was used at a speed equivalent to 100 to 400 rpm. Biological reduction tests were always performed with duplicates. Wastewater samples were prepared by diluting the stock solution containing 30 g/liter of chromic acid with deionized water and media. Each sample flask contained either 100 or 200 ml of liquid sample. Two types of bacteria cultures were used: mixed bacteria enriched by methanol, and the Bacillus cereus strain. Approximately 3 to 5 ml of bacteria cultures was added to each flask. Each flask containing the methanol-enriched mixed bacteria was fed with 0.2 to 0.3 ml of methanol (as a carbon source). When the Bacillus cereus strain was used in the test, the media also contained glucose and yeast, and methanol was not added. Wastewater samples were mixed on a stirrer plate with stir bars at 200 to 350 rpm. The tests were conducted at room temperature in aerobic conditions. Media Preparation: The media used for the methanol-enriched mixed cultures contained the following: 2,000 mL distilled water; KH2PO4, 2.00 g; Na2HPO4, 1.72 g; NH4Cl, 2.00 g; MgSO47H2O, 0.24 g; CaCl22H2O, 0.12 g. The media used for the Bacillus cereus culture consisted of the following: K2HPO4, 60 mM; KH2PO4, 45 mM; (NH4)2SO4, 15 mM; MgSO4, 10 mM; Nacitrate, 2.4 mM; glucose, 0.2% (w/v); and yeast extract, 0.2% (w/v). Analysis: Wastewater samples were analyzed before and after treatment using a HACH DR/4000 spectrophotometer. Several batches of samples were also analyzed before and after treatment by a commercial analytical laboratory, following standard EPA methods (EPA 7196 for dissolved hexavalent chromium and EPA 6010 for total chromium). A gas chromatograph (HP6890N) with a flame ionization detector was used for organic chemical analysis (methanol). Bacteria growth was measured as optical density or light absorbance with a spectrometer (Perkin Elmar EZ210). RESULTS AND DISCUSSIONS Tests with zero-valent iron powder. Several sets of tests were performed for samples with a wide range of chromium concentrations, using various amounts of iron powder. The first set of samples, containing 5.6 ± 1.0 mg/liter of total chromium (120 ml per sample; original pH ~8.5), was treated with 1 gram of iron powder. The sample flasks were sealed and placed on a shaker set at 200 rpm. Within 5 to 6 hours, the chromium concentration gradually decreased to 2.9 ± 0.6 mg/liter. (The initial reduction was ~50%.) When the iron powder was left in the sample for 24 hours (on the shaker), the total chromium concentration was further reduced to 1.2 mg/liter. Figure 1 shows the results of this test. WM’04 Conference, Tucson AZ February 29 March 4, 2004 WM4126 Reducing Cr with Fe Powder (1 gram)