A novel Zr-P-modified nanomagnetic herbal biochar immobilized Cd and Pb in water and soil and enhanced the relative abundance of metal-resistant bacteria: Biogeochemical and spectroscopic investigations to identify the governing factors and potential mechanisms

Remediation of toxic metals contaminated water and soil using engineered biochar is of great concern. This study introduces a novel zirconium-phosphate-modified nanomagnetic biochar (ZMBC) synthesized from traditional Chinese herbal residues as a potential adsorbent for remediation of Cd2+ and Pb2+ contaminated water and soil. The impact of co-existing ions, ionic strength, metal concentrations, pH, and contact time on the adsorption/ immobilization efficiency/mechanisms of ZMBC for Cd2+ and Pb2+ in water and soil were investigated together with the induced changes in soil bacterial community for the first time. The governing factors and potential mechanisms for the interaction between metals and ZMBC have been investigated using biogeochemical, microscopic, and spectroscopic techniques. The adsorption capacity of ZMBC for Pb2+ (357.1 mg g(-1)) and Cd2+ (136.2 mg g(-1)) was significantly higher than that of pristine biochar (BC) and nanomagnetic biochar (MBC) (<5 mg g(-1)). The adsorption behavior for Pb2+ and Cd2+ followed the pseudo-second-order, intra-particle diffusion and Langmuir model, and was minimally affected by ionic strength. Co-existence of Ca2+, Cu2+ and Zn2+ ions at lower concentrations (<10 mg L-1) promoted the adsorption of Pb2+ but inhibited it at higher concentrations (50-200 mg L-1). Conversely, the co-existence of those ions at 10-200 mg L-1, particularly Cu2+ and Zn2+ significantly inhibited Cd2+ adsorption. Adsorption of Pb2+ and Cd2+ on ZMBC was dominated by functional group complexation, co-precipitation, metal-pi electronic coordination, and ion exchange. Addition of ZMBC to the contaminated soil altered the mobile fraction of Cd2+ and Pb2+ to the residual fractions and thus significantly reduced the DTPA-extractable Pb2+ by 51.3% and Cd2+ by 56.1%, and also reduced the Cd and Pb in the wheat roots (by 64.0% and 36.1%, respectively) and shoots (by 73.5% and 62.2%, respectively), as compared to the untreated soil. The analysis of the soil 16S rDNA gene showed that the addition of ZMBC increased soil bacteria suitable for bioremediation of Pb2+ and Cd2+ contaminated soil. Those findings conclude that ZMBC can be used as a promising amendment for remediation of water and soils co-polluted with Cd and Pb.