The Effect of Different Ways and Amount of Straw Returning on Cd Sorption for Different Soils.

Straw return, as one of the measures to utilize the agricultural byproduct worldwide, can not only improve soil fertility and crop yield, but also sequestrate carbon emission and remediate soil heavy metal contamination. To further explore the influence of straw return on the sorption of heavy metals by soils and make a foundation for the remediation of soil heavy metal pollution with straw return, four typical soilsblack soil (BS), paddy soil (PS), latosol (LS), and red soil (RS) were selected to conduct isothermal adsorption experiment to investigate the influence of different ways of straw return on Cd²⁺ adsorption by soils, including crushed straw, burned straw, and fermented straw. In this study, the Langmuir fitting results indicate that the adsorption capacity of Cd²⁺ by soils was in the order of black soil (4 703-10 598 mg·kg^-1) > paddy soil (2 804-4 100 mg·kg^-1) > latosol (2 387-3 906 mg·kg^-1). The n value of Freundlich fitting results show an ascending order of latosol (0.13-0.19) < paddy soil (0.28-0.43) < black soil (0.27-0.65) < red soil (0.91-2.74), indicating that the affinity of Cd²⁺ to four soils gradually decreased. For three different ways of straw return, the adsorption capacity for control (i. e., without addition of straw) was the highest, 10 598 mg·kg^-1, on black soil. For paddy soil and latosol, the addition of burned straw significantly increased the adsorption capacity (3 109-4 100 mg·kg^-1), compared with the addition of other straws (2 387-3 290 mg·kg^-1) and controls (2 444-2 872 mg·kg^-1). Similarly, the adsorption energy of the addition of burned straw was the minimum (-6.40--5.01 kJ·mol^-1) on black soil. On the contrary, for paddy soil, latosol and red soil, the adsorption energy was in the following order with the addition of different ways of straw returning, burned straw (-16.67--3.13 kJ·mol^-1) > control (-10.54--2.35 kJ·mol^-1) > other straws (-10.66--2.17 kJ·mol^-1). The addition of burned straw significantly increased pH in favor of heavy metal adsorption. Besides, the inorganic mineral composition in ash played an important role in Cd²⁺ adsorption. For tested soils, straw returning is not suitable for black soil with high content of organic matter, but more suitable for soil with low content of organic matter. Generally speaking, on the same type soil, burned straw ash has positive significance on the adsorption capacity and affinity of Cd²⁺ to soils.