Kinetic characterization of hexavalent chromium stabilization in contaminated soils amended with cocopeat

Cocopeat is a cheap and easily available organic product. In this study, kinetics of hexavalent chromium [Cr(VI)] stabilization by cocopeat in three soils with different pH and organic carbon content, and the effects of influencing factors including ionic strength (0.03, 0.1 and 0.3 M KCl), temperature (10, 25 and 35 °C) and pH (5.2, 6.2 and 7.2) were investigated in a set of batch experiments by varying Cr(VI) and cocopeat concentrations. In this research, there was no possibility of spiking the soils with the same range of initial Cr(VI) concentration. Addition of Cr(VI) in higher levels masked the effect of soil on Cr(VI) reduction in a calcareous soil, while lower levels resulted in complete stabilization of Cr(VI) in a slightly acid soil. Cocopeat efficiently stabilized Cr(VI) in the soil suspensions within a few days. The time trend of Cr(VI) reduction was different in the three soil samples. The rate of Cr(VI) stabilization strongly increased with decreasing pH and increasing temperature, while remained unchanged with changes in ionic strength. Finally, the overall empirical pseudo-first order rate equation was determined to describe the reduction rate of Cr(VI) by cocopeat in soils.