Insights into aqueous reduction of Cr(VI) by biochar and its iron-modified counterpart in the presence of organic acids

Chromium (Cr) pollution in water has become an environmental and social problem because of the highly toxic nature of Cr(VI). Biochar has been widely used in Cr-containing wastewater treatment due to its adsorption advantage and intrinsic electron-donating ability. In this paper, Cr(VI) was taken as the target pollutant, and corn-straw derived biochar (BC) and its iron-modified counterpart (BC–Fe) were taken as the main adsorbents. The effects of fulvic acid (FA) and lactic acid (LA) on the adsorption efficiency of BC and BC-Fe in aqueous solution were discussed, and the internal reaction mechanism was revealed by SEM, FTIR, XPS, and Zeta potential analysis. The results showed that the BC-Fe pyrolyzed at 600 °C (i.e., BC-Fe600) had good magnetic property and adsorption effect across a wide pH range (pH 3–9) (the maximum removal efficiency was 96%). At the same time, LA had a concentration-dependent promoting effect on Cr(VI) adsorption in the BC600. However, the addition of FA and LA both inhibited the adsorption of Cr(VI) by BC-Fe600 at pH = 5 and 7, with LA showing a more inhibiting effect on Cr(VI) removal (decreased by 16.09% at pH 5) than FA (decreased by 2.09% at pH 5). The addition of FA and LA caused the surface potential of BC-Fe600 to drop, resulting in an increasing electrostatic repulsion between Cr(VI) and the material. However, LA increased the reduction of Cr(VI) on BC-Fe600, possibly through the combined effects of the electron-donating ability of LA and the photolysis of Fe(III)-lactate complexes.