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

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.