Rapid reduction of aqueous Cr(VI) by oxalic acid on N-doped lignin charcoal: A significant contribution of structural defects and electronic shuttle effect

Rapid reduction of aqueous Cr(VI) to Cr(III) with waste biomass N-doped lignin charcoals (PLCNx) as a catalyst via mediate oxalic acid (OA) for hexavalent chromium wastewater. The experiment results showed that the PLCN20 composite had the greatest capacity for removing Cr(VI), and the removal rate of Cr(VI) kept higher than 98% when the addition of 5 mM OA in the system. The bubble-shaped structures of PLCNx could act as an electron shuttle mediating the electron transfer and participate in the reduction of Cr(VI), and the N vacancies defects and graphitic N on its surface were a premise for initiating the Cr(VI) reduction process. More importantly, the formation of the Cr(VI)-OA complex facilitated electron transfer as well, which significantly enhanced Cr(VI) reduction. The combined effects of the electron-donating ability of OA and the catalysis of Cr(VI)-OA complex, lead to the enhancement of Cr(VI) reduction on the PLCNx. This work not only offers novel insights into the synthesis of a superior high-efficiency heterostructure for catalyst but also proposes one new mechanism of PLCNx activate OA enhanced catalysis capacity, which helps design and optimize the catalyst process.