Study On Heavy Metal Migration And Product Characteristics During Hydrothermal Liquefaction Of Remediation Plants

In the face of heavy metal pollution in soil,phytoremediation has become the most feasible treatment method due to its advantages such as low cost,large area planting,relatively permanent control effect and no impact on the surrounding environment.However,the treatment of repair plants has become a difficult problem to be solved in the future.Hydrothermal liquefaction takes water as the solvent,does not need to dry the repair plant,and the reaction temperature is much lower than the boiling point of heavy metals,which can eliminate the shortcomings of air pollution caused by the volatilization of heavy metals.Therefore,hydrothermal liquefaction technology is the most feasible way to repair plants.In this paper,two restorative plants,Solanum nigrum and Sedum alfredii,were used as research objects to explore the distribution of heavy metals in hydrothermal liquefaction,and to further promote the directional transfer of heavy metals to biochar by adding different admixtures（catalysts and additives）.At 300 ^oC and 15 min,the recovery rate of heavy metal Mn in biochar was increased by 46.81%and 63.16%by adding 15%zeolite and trisodium phosphate,respectively.The addition of zeolite and trisodium phosphate promoted esterification.The main components of bio-oil were esters and phenols.Metal oxides,hydroxides and carbonates were detected in both biochar,and phosphorous related substances were also added under the action of trisodium phosphate.The hydrothermal liquefaction of glucose,xylose and guaiacol was carried out under different admixtures with single metal and mixed solution as the solution.The results showed that the recovery rate of Mn²⁺in biochar increased by 48.38%and42.57%with the addition of zeolite and trisodium phosphate.Combined with the FT-IR and XRD results of biochar,the mechanism of action between zeolite and trisodium phosphate was explained.The addition of zeolite enhanced the electrostatic adsorption and ion exchange capacity of biochar,and the hydroxide and phosphate provided by the hydrolysis of trisodium phosphate promoted the increase of heavy metal precipitation.The yield of bio-oil prepared by mixed ionic hydrothermal liquefaction was 17.78%.According to the results of GC-MS,the influence path of the presence of heavy metal ions on bio-oil was proposed.The presence of heavy metal ions promoted the decarboxylate hydrogenation of 5-hydroxymethylfurfural to form lipids,the side chain group fracture of aromatic derivatives to form ethers and alcohols,and inhibited the alkylation of phenol to form hydrocarbons.The biochar prepared by hydrothermal liquefaction was used for the adsorption of Pb²⁺.The results showed that when 80 mg of biochar was added,the removal rate could reach 97.78%,and the corresponding adsorption capacity was 22.1 mg/g.After6 hours of adsorption,it basically entered the equilibrium stage,and the adsorption capacity was 37.2 mg/g at equilibrium.According to the fitting parameter R² of biochar,which is greater than 0.9,the adsorption process more follows the quasi-second-order adsorption kinetics and Langmuir adsorption model,which belongs to the single-layer adsorption dominated by chemical adsorption.Based on the research on the migration and transformation of heavy metals in the hydrothermal liquefication process of restorative plants,this paper analyzed the mechanism of the transfer of heavy metals to biochar promoted by admixtures such as zeolite and trisodium phosphate,and discussed the effects of the inherent metals in restorative plants on the properties of the products,which enriched the relevant studies on hydrothermal liquefication of heavy metal pollution restorative plants,and also provided reference for the resource utilization of restorative plants.