Comment on 'arsenite and arsenate adsorption on ferrihydrite: Surface charge reduction and net OH- release stoichiometry' (multiple letters)

10.1021/es990573fEnvironmental Science and Technology33203695-3696ESTH

Organic ligand-enhanced photochemical reduction and immobilization of chromium(VI) on TlO2 particles in acidic aqueous media

Chromium(VI) is both toxic and mutagenic; Cr(III), on the other hand, is less toxic and essential for human nutrition. Thus, the reduction of Cr(VI) to less toxic Cr(III) is extremely important to decrease the potential hazard of Cr(VI) to the ecosystem. The reduction of Cr(VI) by organics alone is a slow proces; however, the simultaneous applications of light energy and activated surface sites may lead to the enhancement of Cr(VI) reduction by organic compounds. The objective of the study was to evaluate the effects of different intensities of photon flux on Cr(VI) reduction catalyzed by titanium dioxide (TiO2), with or without the presence of organic compounds. The results show that less than 5% of 40 muM Cr(VI) was removed by 2 g L-1 TiO2 in the dark. The application of fluorescence lights, however, could catalyze Cr(VI) reduction in the TiO2 suspensions, and 40 muM Cr(VI) would be reduced upon exposure to growth chamber light at pH 2.5 within 2 days. In the absence of organics, photolysis of water molecules provided electrons for Cr(VI) reduction with the release of oxygen. Because organic compounds served as better reductants than water, light-induced TiO2-catalyzed Cr(VI) reduction was further enhanced. For instance, 40 muM Cr(VI) was reduced within 100 minutes in the presence of 0.2 mM HEDTA exposed to growth chamber light at pH 2.5. Therefore, to accelerate Cr(VI) reduction, lower pH (i.e., pH 2.5), stronger light intensity (i.e., growth chamber light), and decomposable organic compounds should be involved in the TiO2 system

Effects of phosphate, HEDTA, and light sources on Cr(VI) retention by goethite

Iron hydrous hydro(oxide) has been regarded as an important sorbent for Cr(VI) in soil systems due to its wide distribution. However, many factors, such as phosphate (P), organic ligands, and light sources, could influence Cr(VI) retention by the soil components. The existence of inorganic or organic ligands not only competes with solution Cr(VI) for surface sites, but also results in releasing sorbed Cr(VI). Although organic matter can reduce Cr(VI) to less toxic Cr(111), the reduction rate is extremely slow. The objective of this study was to evaluate the influence of P on Cr(VI) sorption by goethite. The reduction of Cr(VI) by N-hydroxyethyl-ethylenediamine-triacetic acid (HEDTA) and goethite under different intensity of light was also investigated. Competitive sorption experiment indicated that P had lower inhibition of Cr(VI) sorption when the initial Cr(VI) concentration was higher than P. Goethite suspensions could catalyze Cr(VI) reduction under growth chamber light. Goethite accompanied with light could also accelerate Cr(VI) reduction by HEDTA. This phenomenon could be evidenced by the formation of Cr(III) and decreasing desorption of retained Cr(VI) by P

Sorption of phosphate and Cr(VI) by Fe(III) and Cr(III) hydroxides

Understanding the chemical behavior and interactions of Cr(VI) (e.g., HCrO4-) and other anions, such as orthophosphate (P) with insoluble metal hydroxides (i.e., Cr[III] and Fe[III]) in disposal landfills or in chromite ore processing residue (CORP)-enriched soil is very important in predicting the movement and the fate of Cr(VI). This study evaluates the sorption behavior of P and Cr(VI) by Fe(III) (i.e., ferrihydrite), Cr(III) (i.e., Cr[OH](3)), and coprecipitated Fe(III)/Cr(III) hydroxides. These metal hydroxide sorbents were synthesized, and sorption of P and Cr(VI) were conducted at different pH using a batch technology. Our results show that P and Cr(VI) sorption by metal hydroxides decreased with increasing suspension pH. Greater decrease in P sorption was observed when Cr(III) was present in the structures of hydroxides. Following the sorption of low concentration of P (i.e., 0.5 mM), the sorption of subsequently added Cr(VI) by hydroxides was less influenced. However, Cr(VI) sorption was greatly inhibited when high concentration of P (i.e., 10 mM) prereacted with hydroxides, particularly in Fe(III) hydroxide system. Results also indicated that high concentration of Cr(VI) (10 mM) could dissolve Cr(III) hydroxide at pH 3 and reprecipitate as an amorphous form of Cr(VI) and Cr(III) compound at pH about 6.5. Although coprecipitation of Cr(VI) with Cr(III) can inhibit Cr(VI) movement through soil profiles, the inhibition seems to be low due to the gradual release of Cr(VI) with increasing pH

Effect of N-hydroxyethyl-ethylenediamine-triacetic acid (HEDTA) on Cr(VI) reduction by Fe(II)

The complexation of Fe(II) with organic ligand results in the decrease of redox potential, and enhances the reduction ability of Fe(II). An important example is the use of Fe(II)-organic complexes to accelerate Cr(VI) reduction. Dissolved O-2 and light can potentially affect Cr(VI) reduction; however, these two factors have not been adequately evaluated. A batch technique was used to investigate the Cr(VI) reduction as influenced by the light and dissolved O-2 using N-hydroxyethyl-ethylenediamine-triacetic acid (HEDTA) and Fe(II) solutions. The oxidation of Fe(II) by dissolved O-2 was rapid in the presence of HEDTA at low pH; nonetheless, the oxidation proceeded slowly when HEDTA was absent. Although Cr(VI) could be reduced by free Fe(II) at low pH, the reaction was considerably slower than that of systems involving HEDTA. The enhancement of Cr(VI) reduction by Fe(II) in the presence of high concentrations of HEDTA was achieved as a result of two processes. First, HEDTA acted as a ligand for expediting electron transfer between Fe(II) and Cr(VI). Secondly, HEDTA served as a reductant for Cr(VI) under illumination. (C) 2003 Elsevier Science Ltd. All rights reserved

Effect of organic complexing ligands on Cr(III) oxidation by MnOx

Although Cr(III) is considered a Cr species with low mobility and toxicity, the potential oxidation of Cr(III) to Cr(VI) in soils containing Mn requires further scientific attention because of the acute toxicity of Cr(VI). This study evaluates the inhibition or enhancement of Cr(III) oxidation by Mn oxide in the presence of various organic ligands and concentrations. Batch experiments indicate that 200 muM of organic ligands inhibited 20 muM Cr(III) oxidation by Mn oxide at pH 4 because of the occurrence of reductive dissolution of Mn oxide. Although oxalate did not show strong inhibition of Cr(III) oxidation at pH 10, nevertheless, citrate and N-hydroxyethyl-ethylenediamine-triacetic acid (HEDTA) ligands did retard Cr(III) oxidation. It is possible that the co-precipitation of Cr(III) and organic ligand, which has a low dissociation constant, results in low release of Cr(III) and subsequent oxidation. The current results imply that the management of Cr(III)-containing waste is very important to prevent potential oxidation of Cr(III) by Mn oxides, particularly at low pH. However, the presence of organic ligands in the field may lead to low Cr(III) oxidation where Cr(III) waste is disposed