Conversion mechanism of pyrolysis humic substances of cotton stalks and carbide slag and its excellent repair performance in Cd-contaminated soil

Unlike traditional biomass composting humification, pyrolysis humification can rapidly convert waste biomass into humic substances (HSs) at lower temperatures and in an alkaline environment for soil remediation. This study utilized the strong alkaline solid waste carbide slag (CARBIDE) and cotton stalk (CS) for pyrolysis humification. The pyrolysis humification mechanism and effect of CARBIDE and CS were investigated using FT-IR, NMR, and Py-GC/MS. Furthermore, the ability of HSs to remediate cadmium-contaminated soil (Cd-soil) was assessed through soil property analysis, pot tests, and microbial diversity studies. The results showed that humic acid (HA) and fulvic acid (FA) had the highest yield of 19 % and 5 %, respectively, among the humic substances (HS-CS&CA) produced under the conditions of 250 ℃, reaction time of 2 h and mass ratio of CS to CARBIDE of 5: 1. At 250 ℃, CARBIDE changed the heat transfer mechanism of biomass, promoted the decomposition of more hemicellulose and protein, promoted the Deamination, Ketonization and Maillard reaction, and synthesized more HAs. HS-CS&CA had a significant remediation effect on Cd-soil. The immobilization rate of Cd was as high as 23 %, and the accumulation of Cd in maize was reduced by 65 %. At the same time, the relative abundance of beneficial bacteria such as Xanthomonadaceae and Bacillaceae for immobilizing Cd was increased, and the cycling effect of N and P was also enhanced. This study provides an innovative technical route for the rapid humification of CARBIDE and CS and its application in Cd-soil remediation.