Comprehensive chemical passivation of active-state heavy metals in relation to soil biological activity in Taojia River basin

Rapid global industrialization has resulted in irreversible environmental degradation, with one pressing issue being the escalating problem of land contamination induced by heavy metals (HMs). This phenomenon not only inflicts lasting harm on plants and microorganisms within the affected regions but also imposes significant losses on various industrial chains, particularly in the realm of agriculture. A noteworthy illustration is the soil contamination in the Taojia River Basin, Hunan Province, China, attributed to mining activities. Therefore, this study is dedicated to examining the passivation of active-state lead (Pb) and cadmium (Cd) in the cultivated fields of the Taojia River Basin. To achieve this, two passivating agents, namely bentonite (referred to as passivator-P) and an acidifying soil agent known as “Shi Di Jia” (referred to as passivator-S), were employed, along with a control group (CK). The passivation of Pb primarily relies on cation exchange adsorbed on the passivator's surface, while the passivation of Cd is intricately linked to total nitrogen content (TN). Indoor incubation tests were conducted to assess Pb and Cd in the soil. Additionally, an analysis of various soil health indicators and their relationships with microbial dynamics and soil enzymes was undertaken. The findings revealed that passivator-P exhibited greater efficacy in the passivation of Pb at concentrations of 3% and 5%, while higher concentrations of passivator-S demonstrated increased effectiveness in passivating Cd. Concerning bacterial diversity, the soil pH post-passivation showed a positive correlation with the Observed_species index, whereas Pb and Cd exhibited negative associations with the Chao1 index. Regarding fungal diversity, soil TN post-passivation displayed a positive correlation with the Simpson index. It is concluded that adding passivator-P at a 3% concentration proved to be the optimal passivating agent for Pb-contaminated soils in the Taojia River Basin. Although passivator-S demonstrated lesser efficacy in passivating Pb, it proved to be suitable for Cd passivation. These findings offer a crucial reference for passivation passivation remediation in the Taojia River area and other sites with similar characteristics, shedding light on the impacts on soil organisms. The implications underscore the necessity for site-specific remediation strategies and a holistic approach to addressing HMs contamination.