Simultaneously immobilization of Cd and Pb in paddy soil by magnetic modified biochar based on textile dyeing sludge: metal speciation and soil microbial community evolution

Purpose The preparation of biochar from textile dyeing sludge (TDS) not only solves the problem of sludge reuse problem but also provides a feasible solution for soil heavy metals remediation. In this study, the primary biochar (TDSBC) and magnetically modified biochar (MTDSBC) were prepared from TDS and characterized. Combined with the change of the paddy soil physicochemical properties, the immobilization mechanism of Cd and Pb were explored from the perspective of metal speciation and soil microbial community evolution. Methods The diethylenetriaminepentaacetic acid (DTPA) extraction and modified sequential speciation extraction method recommended by the European Community Bureau of Reference (BCR) were used to extract the available content and speciation of Cd and Pb, and 16S rRNA sequencing was performed to analyze soil microbial community. Results and conclusions The characterization results revealed that MTDSBC had a smaller specific surface area compared to TDSBC, and the iron oxides were successfully immobilized on the MTDSBC. The incubation experiments showed that both biochars reduced the available and acid-soluble states content of Cd and Pb, increased soil pH and TOC. Furthermore, the addition of TDSBC transformed more metals from the acid-soluble state to residual state than MTDSBC, indicating that TDSBC had better immobilization effect on Cd and Pb than MTDSBC. The addition of both biochars increased soil TOC and provided additional carbon source for microorganisms, which affected microbial community diversity. Specifically, compared with the Blank group, the addition of both biochars increased Actinobacteriota and Thiobacillus and decreased Bacteroidota in the soil. DTPA-Cd and DTPA-Pb were significantly correlated with these microorganisms, both of which reflecting the reduction of Cd and Pb pollution.

Automated summary

This study investigates the immobilization mechanism of Cd and Pb by preparing biochar from textile dyeing sludge. The results show that the addition of biochar reduces the available and acid-soluble states content of Cd and Pb, increases soil pH and organic carbon content. Microbial community analysis reveals that the addition of biochar increases Actinobacteriota and Thiobacillus and decreases Bacteroidota in the soil.