Stabilisation Of Humic Composites On Cadmium And Lead In Baiyin Agricultural Soil

Soil heavy metals pose a serious threat to ecosystem safety and human health due to their long residual cycles,irreversibility,easy migration,high toxicity,and tendency to enter the food chain.As a result,investigating and assessing,remediating and treating,and ecologically controlling soil heavy metals have become issues of great concern for governments and scientists.In this paper,humic acid-goethite（Ge-HA）and humic acid-biochar（BC-HA）composites were selected for the remediation of Cd and Pb contaminated farmland soil in Baiyin,Gansu Province.The available content,toxic leaching rate and geochemical speciation of Cd and Pb before and after the addition of the composites were analysed by DTPA leaching,toxicity characteristics leaching procedure,improved BCR sequential extraction,characterization analysis,batch adsorption experiments and high-throughput sequencing.The stabilizing effect of the composites on Cd and Pb in soils,the feedback of soil physicochemical properties,the relationship between the amount of composites added and the stabilizing effect,the surface properties of Ge-HA,the adsorption performance on Cd and Pb in aqueous solution,the changes of microbial community in the inter-root soil and the physiological and biochemical responses of maize seedlings were investigated.The stabilizing effect of two humic acid materials were compared.The mechanism of the binding of heavy metals by Ge-HA and the environmental impact on contaminated soils have been revealed.The main findings of the study are as follows:（1）Although Ge-HA was more successful in stabilizing soil Cd and Pb,both BC-HA and Ge-HA had a sizable stabilizing impact on soil Cd and Pb.The Cd and Pb contents of the test soils were 5.76 and 117.91 mg·kg^-1,respectively.Both humic acid composites decreased the leaching toxicity and accessible content of the heavy metals in soil.In the soil,weak acid extraction of Cd and Pb was decreased by 33.2%and95.2%,respectively with Ge-HA-40.It also decreased the Cd and Pb migration indices in soil.The findings of the correlation study demonstrated that humic acid material addition had a significant impact on soil p H,CEC,and SOM,which changed the heavy metal contents that were readily accessible in the soil.（2）Excellent surface characteristics of Ge-HA make heavy metal binding easier.After HA compounding,Ge-HA’s surface has more oxygen-containing functional groups,according to FTIR analysis.The surface of Ge-HA is rough and mostly made up of Fe,O,and C components,according to SEM-EDX data.The Ge crystal structure is altered by HA loading,according to XRD data,and the material’s crystallinity diminishes as HA loading increases.（3）The Freundlich and the pseudo-second-order kinetic model were used to explain the adsorption of Cd and Pb by Ge-HA-40.Chemisorption and multilayer adsorption are the dominant adsorption processes.The formation of complexes with heavy metals by functional groups like carboxyl groups,hydroxyl groups,and lattice oxygen on the surface of Ge-HA-40 causes it to bind Cd and Pb,according to XPS and FTIR results.This process is also accompanied by the ion exchange of Na⁺with Cd²⁺and Pb²⁺.（4）Ge-HA-40 composites boosted soil enzyme activity and microbial diversity while enhancing the physicochemical characteristics of polluted soils.Addition of Ge-HA to polluted soils greatly enhanced the SOM and rapid potassium content as well as the activities of the catalase,urease,sucrase,and alkaline phosphatase in soils.Without negatively influencing soil microbial diversity,Ge-HA stabilization enhanced soil bacteria and improved soil microbial abundance.（5）The development of maize seedlings was positively impacted by the addition of Ge-HA-40 to polluted soil,which lessened the stress brought on by soil Cd and Pb.Plant height,root length,biomass,and chlorophyll content were all considerably raised by the addition of Ge-HA-40 by 24.00%,28.57%,42.86%,and 28.07%,respectively whereas Cd and Pb accumulation in maize seedlings were lessened by 52.51%and50.49%,respectively.