Comparative sorption of chromium species as influenced by pH, surface charge and organic matter content in contaminated soils

Excessive use of chromium (Cr) in several industrial applications has a significant impact on soil biota. Chromium when present in soil and water occurs in two forms [Cr(III) and Cr(VI)] which exhibit contrasting characteristics and therefore retention of these two species is of prime importance to arrest Cr contamination in the environment. In this study, the effect of lime, elemental sulfur (to change pH), Fe(III) oxide (for surface charge) and cow manure (organic matter content) was tested on the retention capacity of Cr(III) and Cr(VI) in contaminated soils. The results showed that the addition of lime increased Cr(III) retention while elemental sulfur (S^o), Fe(III) oxide and cow manure increased Cr(VI) retention. The effect of S^o on Cr(VI) sorption is mediated through a decrease in soil pH, which facilitated an increased retention of Cr(VI) as measured by Freundlich sorption coefficient from 0.079 (in pristine soil, pH 7.15) to 21.06 L/kg (in S^o amended soil, pH 4.08). Also, Fe(III) oxide addition at 5% has favoured for an increase in the retention of Cr(VI) (K_f = 91.15 L/kg). Cow manure promoted Cr(VI) reduction, possibly due to the presence of functional groups that are present along with dissolved organic carbon and also by increased microbial activity. It is concluded that the mitigation of Cr toxicity is brought about by the addition of amendments which manipulate the properties of soil to increase retention of Cr(III) and Cr(VI).