Contrasting effects of three aging processes on arsenic immobilization in red versus black soils amended by cerium-manganese modified biochar: the unique role of freeze–thaw cycling in governing arsenic fate at micro/nano interfaces

Abstract The aging mechanisms of modified biochar for arsenic (As) immobilization at micro/nano-interfacial scales in diverse soils remain poorly understood. Herein, we employed three aging treatments, including natural aging (NA), freeze–thaw cycles (FT), and dry–wet alternation (DW), to simulate the aging behavior of cerium-manganese modified biochar (CMBC) in two As-contaminated field soils. Results indicated that CMBC amendment significantly reduced soil pH by 7.5–16.7%, while simultaneously increasing dissolved organic carbon contents by 10–45%, available phosphorus levels by 11–43%, and the activities of four soil enzymes by 30–320% in comparison to unamended soils. These improvements proved to be most effective under FT-aging, followed by DW-aging and NA-aging. FT-aging also led to the most pronounced reduction in water-soluble As concentrations ranging from 94 to 99%, as well as a decrease in As mobilization coefficients of 38% to 59% in CMBC-amended soils when compared to DW-aging and NA-aging. The superior As immobilization under FT-aging can be attributed to adhesion mediated by Ce–Si crystal nano-bridge between soil microparticles and CMBC matrix, whereas such adhesion was not observed in NA/DW-aged samples. This unique interfacial configuration promoted Ca/Fe-oxide intercalation and amorphous Ce-oxides formation within CMBC, which facilitated the development of As–Fe/Ce crystalline phases. Meanwhile, the synergistic enrichment of metallic and oxygen-containing groups on FT-aged CMBC surface induced the formation of stable As–Ce/Fe–O species and triggered dual redox transformations: (1) Ce/Mn reduction drove bulk As(III) oxidation to As(V), and (2) Fe(0) oxidation mediated partial reduction of As(V)/As(III) to inert As(0). Notably, CMBC-amended red soil exhibited preferential As immobilization during aging due to the tighter adhesion between nano-CMBC and soil colloids. This enhanced adhesion strengthened the bonding of Ce/Fe-oxides with As and intensified the oxidation of As(III) to As(V) through increased Ce/Mn reduction. This study provides innovative microscale mechanistic insights into the aging behavior of modified biochar for remediating diverse soils contaminated with potentially toxic elements. Graphical Abstract