Batch experiments characterizing the reduction of chromium(VI) using suboxic material from a mildly reducing sand and gravel aquifer

Batch experiments were conducted with sand collected from a shallow sand and gravel aquifer to identify the principal chemical reactions influencing the reduction of Cr(VI), so that field-observed Cr(V1) reduction could be described. The reduction appeared to be heterogeneous and occurred primarily on Fe(I1)-bearing minerals. At only 1 wt % , the fine fraction (<64 μm diameter) of the sediments dominated the amount of aqueous Cr(V1) reduction because of its greater reactivity and surface area. Although reduction of Cr(V1) increased with decreasing pH, small variations in the abundance of fine fraction among the replicate samples obscured pH trends in the batch experiments. Consistent results could only be obtained by separating the fine material from the sand and running parallel experiments on each fraction. As pH decreased (6.4 to 4.5), Cr(V1) reduction increased from 30 to 50 nmol/m² for the sand fraction (64-1000 μm) and from 130 to 200 nmol/m² for the fine fraction. The amount of Cr(V1) reduced in both the sand-sized and fine material increased from 35 to 80 and from 130 to 1000 nmol/m², respectively, for a 10-fold increase in Cr(VI)_initial. A consistent description of the rate data was achieved by assuming that intraparticle diffusion limited the observed rate of reduction.