Effect of Temperature, Ionic Strength, Background Electrolytes, and Fe(III) on the Reduction of Hexavalent Chromium by Soil Humic Substances

Abstract

The rate of hexavalent chromium reduction by soil humic substances (SHSs) was investigated in aqueous solutions where the temperature, ionic strength, background electrolyte, [Fe(III)], and [Cr(III)] were independently varied. Rate experiments were conducted with an excess of SHS over Cr(VI). An Arrenhius plot for the reduction of Cr(VI) by a soil fulvic acid and a soil humic acid indicates that the activation enthalpies for oxidation of these substances are nearly the same (63 ± 1 and 61 ± 3 kJ mol-1, respectively) and the activation entropies are significantly different (−160 ± 5 and −203 ± 9 J mol-1 K-1, respectively). Rates of reduction are not significantly altered due to changes in either background electrolyte or ionic strength. The presence of Cr(III) slightly inhibits the rate of reduction by soil humic acid, but not that of soil fulvic acid. Ferric iron increases the rate of Cr(VI) reduction, even when only a small amount of Fe(III) is added to the system. Fe may enhance the reduction of Cr(VI) by being alternately reduced by the SHS and then oxidized by the Cr(VI) as part of a redox cycle. The reduction of FeCrO4 + complexes via a parallel reaction pathway may also enhance Cr(VI) reduction in the Cr−Fe−SHS system.

Effects of liming on Cr(VI) reduction and Cr phytotoxicity in Cr(VI)-contaminated soils

“…These results are similar to those found in many other studies (Zhilin et al 2004;Deiana et al 2007), indicating that reductions of Cr(VI) by organic materials can be accelerated at low pH. As the pH of Pinchen soils increased to 5.0 and 6.2 from 4.5 by the adding of CaCO 3 , this probably took them out of the most favorable pH range for Cr(VI) reduction (Wittbrodt and Palmer 1996;Schlautman and Han 2001). However, for the Neipu soils, there was no dependence of pH on the extent of Cr(VI) reduction.…”

“…The most favorable solution pH for Cr (VI) reduction by organic materials ranges from 1 to 4 (Wittbrodt and Palmer 1996). Zhilin et al (2004) showed that Cr(VI) was reduced by peat and coal humic substances at pH 5.4, but not at pH 9.2.…”

Effect of Organic Matter on Cr(VI) Removal from Groundwaters by Fe(II) Reductive Precipitation for Groundwater Treatment

“…The kinetics of a combined reduction by the co-presence of Fe(II) and HAs are hereby different from the kinetics of experiments where only Fe(II) or only HAs were available [22,23]. It is claimed that this effect is dependent on the specific type of humic substance [22] and it is also limited by the lower solubility of Fe(III) at higher pH values, which would lead to a precipitation of ferric hydroxide(s), so that no further recycling of Fe can take place [24]. Therefore, there is a need for further investigation, regarding the effect of HAs or of natural organic matter on Cr(VI) and on total chromium removal by the application of Fe(II) reductive precipitation.…”

“…However, at the tested pH range, Fe(III) is practically insoluble (with the respective Ksp = 6 × 10 −38 [8]) and is therefore removed from the solution, indicating that redox cycling might not take place in this case [24].…”

Metal content and distribution in surface sediments in an industrial region

“…According to the data, Cr and OM appearance at the same statistical analysis factor (Table V) proves a Cr preferred association to organic fraction of the sequential extraction procedure (step 3). The preferential association of Cr to the oxidizable phase indicated that most of that metal could be retained in the sediments in the trivalent state, less toxic, and bound to organic matter (Armienta et al 1996, Wittbrodt andPalmer 1996). From the Portão stream, we extracted 1,039 mg/kg in the oxidizable step, equivalent to 81% of the total concentration of this metal.…”