Metal Bioavailability and Trivalent Chromium Removal in ABR

Trace metal bioavailability and chromium(III) removal were investigated in an anaerobic baffled reactor (ABR) treating a synthetic waste [COD = 4,000 mg/L, COD:SO2 - 4 ratio 8:1 and 40:1, Cr(III) = 50 mg/L]. Sulfide precipitation reduced nutrient bioavailability as follows (most deficient first): Fe > Co > Ni. However, the metals had the following affinity for complexation: Ni > Co >> Fe; complexation potential was greatest at the front and rear of the reactor. At the front, it was hypothesized that high concentrations of soluble microbial products provided an excess of potential ligands, which may have sequestered the metals. However, in the last compartment, a higher pH would have altered the competition between protons and metal cations for complexing sites in favor of the metal cations. The complexing potential in the ABR increased with sulfide levels, and this coincided with higher residual COD production, probably to provide a diffusion barrier to the harsh environmental conditions. Although the addition of chromium (50 mg/L) had no observable effect on reactor performance/stability, its removal was controlled by the following factors (most influential first): solubility > complexation > absorption > physical adsorption.