Enhancement of Cr(VI) decontamination by irradiated sludge biochar in neutral conditions: Evidence of a possible role of persistent free radicals

The effect of photo-irradiation on the removal of Cr(VI) by sludge biochar in neutral conditions is here investigated. The results indicate that photo-irradiation can significantly enhance the removal of aqueous Cr(VI) by sludge biochar. The apparent removal rate constants under ultraviolet (0.61 h−1) and visible-light (0.27 h−1) irradiation are more than 5 and 2 times higher, respectively, than that in the dark (0.11 h−1). Cr(VI) reduction plays a predominant role in Cr removal, accounting respectively for 71.8% (dark), 71.9% (visible light irradiation), and 92.3% (ultraviolet irradiation) of total removal. The abundance of persistent free radicals (PFRs) in biochar (produced hydrothermally at a temperature of 220 °C, a reaction time of 2 h, and a solid weight ratio of 40 %w) can reach up to 4.72 × 1016 spins/g, and their EPR signal intensity remains almost unchanged within 720 min in the dark. Ultraviolet irradiation may promote PFRs production in biochar, and PFRs (especially the oxygen-centered ones) act as electron donors to transform Cr(VI) into Cr(III), thereby contributing to Cr(VI) reduction. Our findings shed new light on the role of irradiation in enhancing the removal of Cr(VI) by sludge biochar under neutral conditions, which can be an interesting technique in the field of environmental remediation.