Reduction and adsorption capacities of soils for Cr( <scp>vi</scp> ) and quantitative contributions of key influencing factors

The saturated sorption capacity of Cr(VI) in soil is determined by reduction and adsorption processes. Red soil, paddy soil, black soil, and fluvo-aquic soil were selected for repeated sorption experiments in this study to determine Cr(VI) saturated sorption capacity as well as reduction/adsorption capacities and the contributions of key factors were analyzed through regulating soil properties. The results showed that the saturated sorption capacities of the four soils were 972, 589, 551, and 76 mg kg^-1. Per pH unit increase, saturated sorption capacity decreased in the order: red soil (142 mg kg^-1) > paddy soil (134 mg kg^-1) > black soil (132 mg kg^-1) > fluvo-aquic soil (25 mg kg^-1). Specifically, red soil showed a significantly greater drop in adsorption capacity (50 mg kg^-1) than the other three soils; paddy soil and black soil had over 27% decrease in reduction capacity, while fluvo-aquic soil exhibited no significant change. Organic matter removal decreased reduction capacities by over 60% but increased adsorption capacities by over 20%. Removing Fe and Al oxides significantly reduced adsorption capacities by over 50%; Mn oxide removal had minor impact. Correlation analysis and random forest modelling identified pH as the primary factor influencing soil Cr(VI) sorption, contributing 27.8%, 45.4%, and 28.0% respectively to saturated sorption capacity, reduction capacity, and adsorption capacity. Adsorption capacity was mainly affected by Fe, Al and Mn oxides, collectively contributing 70.5%, while organic matter mainly affected reduction capacity, contributing 26.6%. This initial quantitative analysis provides new insights into the fate of Cr(VI) in soil environments.