Immobilization Effect And Mechanism Of Modified Waste Wood Ear Mushroom Sticks Biochar On Soil Cadmium-Lead Pollution

Due to mining activities and the widespread use of sludge,fertilizers,and pesticides in agriculture,heavy metal pollution has become a global environmental issue.The"National Soil Pollution Status Survey Bulletin"shows that the total number of sites with heavy metal exceedances in soil is 16.1%,with particularly severe contamination by Cd and Pb.Immobilization remediation is currently an efficient,simple,and rapid method of remediation,which is of great practical significance for reducing environmental heavy metal pollution.China is a major producer of edible fungi,and the waste substrate from the cultivation of edible fungi after harvesting is gradually becoming a new type of agricultural waste in China,with discarded Auricularia auricula wood ear mushroom sticks being the most common source.Hydroxyapatite（HAP）is naturally present in the bones and teeth of humans and animals.Research has shown that HAP can adsorption heavy metals through mechanisms such as ion exchange and complexation.Alginate is derived from the extracellular walls of brown algae（Phaeophyta）,easily obtained from natural resources,and exhibits strong affinity for heavy metal ions.Taking advantage of the above materials,this study first prepared waste Auricularia auricula wood ear mushroom stick-based biochar（WMBC）at different pyrolysis temperatures and selected the optimum preparation temperature through adsorption experiments on Cd²⁺and Pb²⁺.Subsequently,a new modified material,calcium alginate-modified hydroxyapatite Auricularia auricula wood ear mushroom sticks biochar sphere(CA-HAP_MB),was prepared using biochar as a substrate,and its adsorption effects and characteristics on Cd²⁺and Pb²⁺pollution were explored through adsorption experiments.Soil cultivation experiments were conducted to study its effects on the physicochemical properties and Cd²⁺and Pb²⁺remediation of acidic soil in the South and alkaline soil in the North.Furthermore,the characterization analysis of the materials before and after remediation was conducted to clarify their remediation mechanisms for heavy metals.Additionally,high-throughput sequencing was used to analyze the microbial community structure and diversity in soil treated with different levels of CA-HAP_MB,and pot experiments were conducted to discuss changes in crop absorption of heavy metals and physiological,biochemical,and nutritional indicators.The main results are as follows:（1）Study on the adsorption of Cd²⁺and Pb²⁺in water by WMBC.Different pyrolysis temperatures were used to prepare biochar materials from waste Auricularia auricula wood ear mushroom sticks,and water adsorption experiments were conducted to explore the adsorption characteristics of WMBC on Cd²⁺and Pb²⁺single and combined pollution.Characterization analysis found that as the pyrolysis temperature of WMBC increased from 250°C to 650°C,the specific surface area and pore volume gradually increased,with WMBC（650°C）exhibiting the highest adsorption capacity for Cd²⁺(46.16 mg·g^-1)and Pb²⁺(234.2 mg·g^-1).This indicates that the increase in pyrolysis temperature significantly affects the adsorption capacity and stability of biochar for heavy metals.Competitive adsorption inhibited the adsorption of Cd²⁺on biochar,while the adsorption of Pb²⁺was not significantly affected.（2）Study on the adsorption of Cd²⁺and Pb²⁺in water by CA-HAP_MB.The newly prepared material CA-HAP_MB showed significantly improved adsorption effects on Cd²⁺and Pb²⁺compared to WMBC materials,with maximum adsorption capacities reaching 302.2 mg·g^-1 and 564.5 mg·g^-1,respectively（based on the Langmuir model）.Adsorption experiments revealed the presence of physical adsorption,negative charge attraction,ion exchange,complexation,and precipitation throughout the adsorption system.Among them,chelation and precipitation adsorption were the main mechanisms for CA-HAP_MB to remove pollutants.Additionally,the modified material was easier to collect compared to biochar,overcoming the limitations of WMBC separation.（3）Study on the adsorption effects and characteristics of CA-HAP_MB on Cd and Pb in soil.Polluted farmland soils were collected,and immobilization materials were applied at 1%to 3%.Through a 150-day soil cultivation experiment,the effects of CA-HAP_MB on the physicochemical properties,effective content,and forms changes of heavy metals in polluted farmland soils were explored.The results showed that CA-HAP_MB has the characteristics of p H regulator and immobilization materials,reducing the mobility of heavy metals.The content of effective Cd²⁺and Pb²⁺in Chenzhou decreased by 30.85%to 69.26%and 31.91%to 78.55%,respectively,while in Shenyang,the content of Cd and Pb decreased by 27.49%to 53.73%and 26.35%to62.33%,respectively.Immobilization materials transformed Cd and Pb in soil into more stable forms.Characterization analysis of CA-HAP_MB extracted from soil revealed changes in the number of functional groups,proving that the formation of precipitation and complexation is the main reason for the immobilization of Cd and Pb by CA-HAP_MB.Additionally,ion exchange,electrostatic attraction,and physical adsorption also participated in the remediation process.（4）Influence of CA-HAP_MB on the soil microbial community structure and function.Samples collected after soil cultivation experiments were subjected to high-throughput sequencing of 16S r RNA to investigate the effects of modified materials on bacterial communities in two types of polluted soil samples.The results showed that applying 1%to 3%CA-HAP_MB increased the relative abundance of Proteobacteria,Gemmatimonadota,and Acidobacteria in both types of soil,which is beneficial for fixing heavy metals,and maintaining the balance and stability of soil ecosystems.CA-HAP_MB reduced the relative abundance of sensitive bacteria and resistant bacteria under soil Cd and Pb stress.（5）The influence of CA-HAP_MB on the absorption of heavy metals and antioxidant stress by crops.Pot experiments were conducted with high and low-accumulating varieties of Chinese cabbage（Brassica chinensis L.）in cadmium-lead polluted farmland soils by applying 1%to 3%of immobilization materials.The effects of CA-HAP_MB on the physiological functions and quality of Brassica chinensis L.were investigated.Application rates of 1%to 2%could reduce the Cd content in low-accumulating varieties in both types of soil,and Pb content in low-accumulating varieties in both types of soil could meet the food safety standards（GB 2762-2022）at application rates of 2%to 3%.CA-HAP_MB could reduce heavy metal toxicity,improve soil fertility,and significantly increase the biomass of Brassica chinensis L.（p<0.05）.CA-HAP_MB could reduce the proportion of heavy metals in organelles,promote the physiological metabolic functions of Brassica chinensis L.,and alleviate oxidative damage caused by heavy metals.The contents of soluble sugars,ascorbic acid,and soluble proteins in plants were significantly increased（p<0.05）.Through the evaluation of Brassica chinensis L.quality and human health risks,simultaneous application of CA-HAP_MB and planting of low-accumulating varieties（Huo qing 91-5C）were found to be the optimal choice for achieving safe production in polluted farmland soils in this experiment.The CA-HAP_MB prepared in this study combined the advantages of various basic materials and applied them in water,soil cultivation,and soil-crop experiments.The results demonstrated that the adsorption performance and stability of the modified materials were significantly improved.In soil,they could transform the forms of heavy metals,reducing their mobility and toxicity to Brassica chinensis L.,and had no adverse effects on native soil microorganisms.By enhancing soil fertility and Brassica chinensis L.quality,the combined application of CA-HAP_MB and low-accumulating varieties of Brassica chinensis L.can achieve safe production.This provides reference for the application of CA-HAP_MB in the treatment of cadmium-lead pollution in farmland soils in China and the safe production of Brassica chinensis L..