Cadmium immobilization in river sediments using nano-chlorapatite modified biochars: Effects on prokaryotic communities

In the face of worsening cadmium (Cd) pollution in river sediments, nano-chlorapatite (nClAP) modified biochars exhibit significant immobilization potential, but little is known about the ecological response of prokaryotic communities after remediation. In this study, nClAP-modified biochars derived from rice and wheat husk residues were used to immobilize Cd and evaluate their effects on α-diversity, co-occurrence network, and Cd-resistant genes of prokaryotic communities. Results revealed that pristine biochars converted exchangeable Cd to oxidizable and residual states via complexation, ion-exchange, and precipitation, while nClAP-modified biochars further facilitated the precipitation of Cd₅(PO₄)₃Cl, Cd₃(PO₄)₂, and Cd₄P₂O₉. Consequently, the concentrations of Cd associated with toxicity characteristic leaching procedure (TCLP), physiologically based extraction test (PBET), overlying water, and pore water notably decreased, indicating a substantial reduction in Cd mobility and bio-accessibility. Benefiting from effective Cd stabilization and abundant nutrient inputs including carbon and phosphorus, the prokaryotic Species richness and Shannon diversity increased from 5762 to 6064–6443 and from 7.36 to 7.37–7.51, respectively. Furthermore, co-occurrence network analysis highlighted the keystone species responsible for nutrient cycling and Cd resistance. Prediction of Cd-resistance genes suggested that the abundance of Cd efflux genes (czcABCD) and Cd binding genes (dsbAB) decreased, attributing to the alleviated Cd toxic stress on prokaryotic cells after remediation. This study emphasizes the exceptional immobilization efficiency of nClAP-modified biochars and demonstrates their remarkable ecological benefits for microbes.