Oxidation mechanisms and chemical bioavailability of chromium in agricultural soil - pH as the master variable

Chromium (Cr) is a heavy metal that exists in soils in two stable oxidation states, +III and +VI. The trivalent species is an essential nutrient, whereas the hexavalent species is highly toxic. This study investigated the environmental impact of Cr-III potentially released into soil from wastes and various materials by determining the risk of oxidation of initially soluble inorganic Cr-III into hazardous Cr-VI. The principal aim was to describe the pH-dependent mechanisms that regulate 1) the formation of Cr-VI from the easily soluble Cr-III and 2) the potential bioavailability of Cr-III and that of Cr-VI species produced in the oxidation of Cr-III in agricultural soil (fine sand, organic carbon 3.2%). The amount of Cr-VI formed in oxic soil conditions was regulated by two counteracting reactions: 1) oxidation of Cr-III into Cr-VI by manganese oxide ((MnO2)-O-IV) and 2) the subsequent reduction of Cr-VI by organic matter back to Cr-III. The effect of pH on this net-oxidation of Cr-III and on the chemical availability of both Cr-III and Cr-VI species was investigated in soil samples incubated with or without excessive amounts of synthetic MnO2, over the chemically adjusted pH range of 3.9-6.3 (broken vertical bar 22 degrees C, 47 d). In soil subsamples without added MnO2, the net-oxidation of Cr-III into Cr-VI (1 mM CrCl3 in soil suspensions, 1: 10 w/V) was negligible. As for the MnO2-treated soils, at maximum only 4.7% of added Cr-III was oxidized - regardless of the high oxidation potential of these subsamples. The lowest production of Cr-VI was observed under acidic soil conditions at pH similar to 4. At low pH, the net-oxidation diminished as result of enhanced reduction of Cr-VI back to Cr-III. At higher pHs, the oxidation was limited by enhanced precipitation (or adsorption) of Cr-III, which lowered the overall amount of Cr-III susceptible for oxidation. Moreover, the oxidation reactions by MnO2 were inhibited by formation of Cr(OH)(3) coverage on its surface. The pH-dependent chemical bioavailability of added Cr-III differed from that of the Cr-VI formed. At elevated pHs the chemical availability of Cr-III decreased, whereas that of Cr-VI produced increased. However, the risk of Cr-VI formation through oxidation of the easily soluble inorganic Cr-III was considered to be low in agricultural soils high in organic matter and low in innate MnO2. (C) 2016 Elsevier Ltd. All rights reserved.