Interaction between hexavalent chromium and biologically formed iron mineral-biochar composites: Kinetics, products and mechanisms

Highlights: • Reductive Cr(VI) transformation by Fe(II) contributed to Cr(VI) removal by IMBC. • Cr(VI) removal followed pseudo-second-order kinetic and Langmuir isotherm models. • Biochar improved Fe(II) yields but decreased Fe(II) reactivity in Fe(III) bioreduction. • Mechanisms of biochar-influenced Fe–Cr coprecipitation were investigated. Biogenic Fe(II) is a dominant natural reductant to convert carcinogenic Cr(VI) to less toxic Cr(III). Field-applied biochar could promote microbial production of Fe(II) and form iron-biochar composites. Although there have been mounting research on the interactions of biochar or Fe(II) with Cr(VI), their coupling effects on Cr(VI) immobilization have been largely neglected. Here, iron mineral-biochar composite (IMBC) was prepared via biochar-mediated dissimilatory reduction of ferrihydrite or goethite by Shewanella oneidensis MR-1, and its reaction with Cr(VI) was investigated. IMBC was able to effectively remove aqueous Cr(VI) via reductive transformation by adsorbed Fe(II). The removal process nicely followed pseudo-second-order kinetics and Langmuir isotherm model. The removal ability of IMBC decreased with increasing pH (5.5–8.0) but was independent of ionic strength changes (0–100 mM). After reaction, the Fe–Cr coprecipitates formed on IMBC exhibited slightly higher Fe/Cr ratios (0.93–0.96) than those on corresponding iron mineral controls (0.88–0.94). For IMBC, while the presence of biochar decreased the reactivity of adsorbed Fe(II), their removal capacities were ~30% higher than those of iron minerals alone, due to the enhanced yields of adsorbed Fe(II). These findings improved our knowledge of interactions among biochar, iron mineral and iron-reducing bacteria and their contribution to chromium immobilization.