Effects and mechanisms of the Bt Cry1Ab protein on the adsorption of Cu(II) in red and fluvo-aquic soils: insights from adsorption kinetics, isotherms, thermodynamics, and functional group characterization

• In recent years, the copper pollution in farmlands has considerably increased. • The planting area of Bt crops is constantly expanding. • The interaction between Bt protein and Cu(II) has become inevitable. • Bt protein enhanced the adsorption capacity of Cu(II) in red and fluvo-aquic soil. • Bt proteins released by Bt crops may affect the environmental behavior of Cu(II). In recent years, copper contamination in agricultural soils has exhibited a consistent upward trend. With the widespread cultivation of Bt crops, the influence of Bt insecticidal proteins on Cu(II) adsorption behavior has emerged as a potentially significant factor. However, this issue has received limited attention. This study investigated the effects and underlying mechanisms of Bt protein on Cu(II) adsorption in red and fluvo-aquic soils, employing kinetic, thermodynamic, and isothermal analyses. The adsorption of Cu(II) in both soil types conformed to the pseudo-second-order and Elovich models. The Langmuir, D-R, Freundlich, and Temkin models fit the adsorption data well. Bt protein enhanced Cu(II) adsorption within the soil. In terms of the magnitude of adsorption energy, red and fluvo-aquic soils exhibit different Cu(II) adsorption mechanisms: red soil mainly adopts ion exchange adsorption, while fluvo-aquic soil mainly adopts chemical adsorption. As suggested by our thermodynamic analysis, Cu(II) adsorption in soils represented a spontaneous (Δ G 0 <0), endothermic (Δ H 0 >0) process, and the spontaneity improved as the Bt protein concentration increased. Positive values of Δ S ° for Cu(II) adsorption in soils indicated that entropy was the driving force for adsorption. FTIR and XPS analyses indicated that Bt protein enhanced the chemisorption of Cu(II), primarily through bridging interactions and complexation between Cu(II) and the adsorbed Bt protein. These findings suggest that Bt proteins released by Bt crops may influence the environmental behavior of Cu(II).