Effects of redox-modified biochar on mercury reduction and methylation on electron transfer in Geobacter sulfurreducens PCA

Abstract

Geobacter plays a key role in mercury (Hg) methylation and reduction in rice fields. Biochar boosts microbial electron transfer, but its impact on Hg reduction and methylation remains unclear. This study investigates how oxidative (OBC) and reductive (RBC) biochar influence Hg reduction and methylation by Geobacter sulfurreducens PCA. OBC increased electron donating capacity (EDC) but reduced electron accepting capacity (EAC), while RBC decreased electric resistance. Correlation analysis revealed biochar’s electron exchange capacity (EEC) positively correlated with –OH (r = 0.73*), O-CO (r = 0.81*), COO/CO (r = 0.73*), –NH (r = 0.67*), and electron transfer number (n) (r = 0.99**), but negatively with –CH (r = -0.70*) and –NH₃ (r = -0.80**). Both total Hg (THg) and methylmercury (MeHg) negatively correlated with EEC (THg:r = -0.99**, MeHg: r = -0.92**), EDC (THg:r = -0.99**,MeHg:r = -1.00**), and n (THg:r = -1.00**,MeHg: r = -0.85**), but positively with ΔIp (THg:r = 1.00**,MeHg:r = 0.80**). These findings suggest biochar with higher EEC, EDC, and electron transfer capacity enhances Hg²⁺ reduction and inhibits methylation, highlighting its potential for Hg pollution control.