The co-application of bacteria with humic acid, hydroxyapatite, silicon-doped ferrihydrite, and biochar: An exploration of remediation strategies for Cd-contaminated soil

In this work, cadmium (Cd) tolerant bacteria ( Cupriavidus sp. H200) and silicon-doped ferrihydrite (SiFh) were used for the first time to remediate Cd-contaminated soil, and the remediation effects of each strategy were evaluated by comparing the different substrate materials (humic acid, hydroxyapatite and biochar). The results demonstrated that SiFh + Cupriavidus sp. H200 had the best remediation effect. Compared with the blank treatment group, SiFh + Cupriavidus sp. H200 increased the content of residual Cd in soil (1.007 mg·kg −1 ) and reduced the concentration of Cd in the Diffusive Gradients in Thin-films (DGT) sampling (4.55 μg·L −1 ). The DGT Induced Fluxes in Sediments (DIFS) model revealed that Cupriavidus sp. H200 + remediation materials significantly inhibited the resupply capacity of Cd in the soil solid phase , diminished each of the partition coefficient , adsorption rate constant and desorption rate constant of Cd. SiFh + Cupriavidus sp. H200 significantly reduced the bioavailability of Cd to pakchoi (50.01 %) and increased or maintained the activities of sucrase , alkaline phosphatase , urease , and catalase in the soil. The potential explanation for the experimental results was that SiFh reacted with the secretion of Cupriavidus sp. H200 to form microsilicate, which reduced the mobility of Cd in soil. Moreover, Cupriavidus sp. H200 + SiFh could also reduce the bioavailability of Cd by regulating the expression of Cd transport genes in pakchoi. This study offered a novel solution for the combined mineral-biological remediation of Cd-contaminated soil. • Combined application of Cupriavidus sp. H200 and silicon-doped ferrihydrite can effectively convert Cd in the soil to a residue state. • Cupriavidus sp. H200 greatly prolonged the characteristic time of Cd adsorption on the DGT device and significantly reduced the partition coefficient , adsorption rate constant, and desorption rate constant of Cd on the DGT device. • Silicon-doped ferrihydrite significantly reduced the Cd content in pakchoi and Cupriavidus sp. H200 further enhanced the effect of silicon-doped ferrihydrite. • Cupriavidus sp. H200 combined with silicon-doped ferrihydrite enhanced or maintained sucrase, alkaline phosphatase , urease , and peroxidase activities in soil.