Participation of soil active components in the reduction of Cr(VI) by biochar: Differing effects of iron mineral alone and its combination with organic acid.

Biochar as a soil amendment could be involved in redox process of elements which would be affected by soil-redox-active components including minerals and organic acids. This study evaluated the effects of Fe mineral and lactate on reducing capacity of biochar for Cr(VI) reduction and the underlying mechanisms. Fe minerals inhibited the reduction of Cr(VI) by biochar, with the decrease of Cr(VI) reduction rate constant obtained by pseudo first-order reaction model from 2.18 to 2.47 × 10-2 h-1 to 0.71-1.51 × 10-2 h-1. The decrease of reduction rate constant was because (1) the loss of electron donating moieties in biochar; and (2) inhibition of electron transfer between biochar and Cr(VI) due to surface coverage by biochar-Fe complex. However, the coexistence of Fe minerals with lactate enhanced the reduction of Cr(VI) by biochar, with the rate constant increasing from 2.58 to 3.05 × 10-2 h-1 to 2.91-27.2 × 10-2 h-1. The positive effect was also attributed to two reasons: (1) lactate can decrease the surface Fe-coverage of biochar through chelating process; (2) electron from lactate can be shuttled by Fe(II) and thus enhancing the Cr(VI) reduction. Our results revealed that different soil redox-active components could have varying effects on biochar amendment for Cr(VI) reduction, which should be further considered during the application of biochar.