Preparation Of Straw Carbon Film Electrode And Electrodynamic Coupling With Acid Washing For Remediation Of Cadmium Contaminated Soil

Cadmium contamination of soil has become a serious environmental problem due to the rapid development of industrialization and agriculture in modern society.This has caused widespread concern in society.According to survey results,the concentration of cadmium in about 7%of Chinese soils exceeds the limit of environmental standards.Long-term exposure of soil to cadmium not only affects its own fertility but also enters the human body through the diet of crops such as rice,wheat,and vegetables,ultimately posing a threat to human health.Therefore,effective remediation of heavy metal cadmium-contaminated soils is an urgent task to ensure food safety and human health.In recent years,many soil remediation technologies have been effectively developed for heavy metal soil contamination.Among them,electrokinetic remediation technology and chemical leaching technology are two widely recognized remediation technologies.However,both technologies have limitations respectively.For example,electrokinetic remediation technology has the disadvantages of small applicable area and high energy consumption,while chemical drenching technology needs to consume a large amount of drenching solution and will cause damage to the soil’s own physicochemical properties after drenching.To address the problems in soil remediation technology,this paper adopts a combined electrokinetic remediation-chemical drenching approach to remediate heavy metal cadmium-contaminated soil.Firstly,biochar made from agricultural waste rice straw was used as raw material to develop straw carbon film electrode materials.By exploring the effects of different parameters on the removal of cadmium by straw carbon film electrodes,and studying the surface morphology,functional group types,and sediment morphology of straw carbon film electrodes before and after the removal of cadmium,the feasibility of electrochemical removal of cadmium by this electrode material was verified.Secondly,the mechanism of electrokinetic remediation-chemical leaching combined technology for repairing cadmium-contaminated soil was expounded.Finally,a pot experiment of Chinese cabbage was used to evaluate the effect of repairing cadmium-contaminated soil.The main research conclusions of this paper are as follows:（1）Biochar was prepared with different temperatures（400°C,600°C,800°C,960°C）using agricultural waste straw as raw material.The results showed that pyrolysis carbonization temperature was the key factor affecting electrical conductivity.With increasing pyrolysis carbonization temperature,biochar started to appear graphite-like structure,carbon layer arrangement region was ordered,and electrical conductivity improved.The resistivity of 960℃biochar was only 7.60×10^-3Ω·m,and electrical conductivity was good,which could be used to prepare excellent electrode materials.Ethyl cellulose as a film-forming agent has good film-forming properties.Together with previously prepared high-temperature conductive biochar,a new electrode material with independent molding,low cost and low environmental pollution can be prepared.（2）To verify the removal effect of straw carbon film electrode on heavy metal cadmium,we used it as cathode and graphite electrode as anode,and carried out an experiment of cadmium removal effect under the action of external DC power supply in combination with the principle of electrokinetic repair.The results showed that p H was the key factor affecting the removal effect of cadmium.The removal efficiency of cadmium at p H=3,4,7,8 and 9 were 27.4%,55.2%,62.2%,69.1%and 88.1%respectively,with the highest removal rate at p H=9 having the best removal effect.The removal process of cadmium by straw carbon film electrode can be described by Lagergren equation,and by fitting the primary and secondary kinetic equations,it was found that the equilibrium adsorption amount reached 8.86 mg/g at the optimum p H=9,and the adsorption equilibrium time was10 h.The correlation coefficient R²=0.998 under the quasi-secondary equation indicates that the quasi-secondary equation can better describe the process of cadmium removal by straw carbon film electrode.The process of cadmium removal and its reaction process rate of cadmium removal was mainly controlled by the chemical phase.（3）The straw fermentation solution was used as an eco-friendly drenching agent in combination with electrokinetic remediation technology to remediate heavy metal Cd-contaminated soil.The results showed that the extraction concentrations of cadmium were6.614 mg/kg,7.337 mg/kg and 8.000 mg/kg for different applied drencher:soil mass ratios of 1:1,1.5:1 and 2:1,respectively.The extraction rates of cadmium were 48.7%,54.0%and58.9%for 1h as the saturation time of drenching.Moreover,the p H of the restored soil remained basically the same as that of the unrestored soil,and the physicochemical properties of the soil did not change,proving that the straw fermentation solution is an eco-friendly drenching agent.The best drenching technology is the drenching solution:soil mass ratio of 2:1 and the drenching time of 1h.In the experiment of using electrochemical-chemical washing to remove cadmium from soil,the research results showed that the removal rates of cadmium in different washing solutions with soil quality ratios of 1:1,1.5:1 and 2:1 were 67.1%,85.4%and 91.0%respectively when the processing equilibrium time was 8 hours.Among them,the washing solution:soil quality ratio of 2:1 and processing time of 8 hours was the best combined treatment process.（4）The characterization tests（SEM-EDS,FTIR,XRD）conducted on straw carbon film electrodes before and after remediation of cadmium contaminated soil showed that the removal of cadmium from soil by electrochemical-chemical drenching technique was the result of a combination of mechanisms.（1）SEM-EDS spectroscopy demonstrated that Cd²⁺was reduced to cadmium monomers and deposited on the straw carbon film electrode under the action of electric field forces.（2）Complexation with small molecule organic acids（glycolic acid,acetic acid,etc.）contained in the straw fermentation drench solution can form stable complexes（C₂H₂Cd O₄,C₂Cd O₄,etc.）with cadmium.In addition,the oxygen-containing functional groups（hydroxyl,carboxyl,carbonyl,etc.）introduced on the surface of biochar can form stable complexes（R-OCd⁺,R-COOCd⁺）with cadmium by complexation reaction.（3）Co-precipitation occurs when silicates carried in the original soil and carbonates and hydroxides carried in biochar react with cadmium to form precipitates（Cd Si O₃,Cd CO₃,Cd（OH）₂）.（4）Finally,cation-πbonding occurs when the highly graphitized structure unique to high-temperature biochar containsπ-πconjugated structures that also form cation-πbonds with cadmium.（5）The effect of remediation of heavy metal Cd contaminated soil was evaluated by a small cabbage pot experiment.The results of the cabbage pot experiment showed that the soil remediation effect was best with the drench solution:soil ratio of 2:1.Compared with the pre-remediation soil,the post-remediation soil cabbage performed better in all indicators,including height,dry matter accumulation and total cadmium content,The height,dry matter accumulation and total cadmium content of cabbage were 12.6 cm,5.3g and 0.16 mg/kg,respectively,compared with 5.8 cm,1.8 g and 1.71 mg/kg of cabbage plant height,dry matter accumulation and total cadmium content in the pre-remediation soil.As far as the above-ground part is concerned,the Cd content of Brassica napus in the restored soil is only 0.05 mg/kg,which is lower than the national standard(national standard GB 2762-2017,Cd≤0.2 mg·kg^-1 for leafy vegetables).In summary,the restored soil is beneficial to the growth and development of crops.The above research provides theoretical guidance and new ideas for the resource utilization of agricultural waste and the management of heavy metal contaminated soil.