Efficiency And Mechanism Of Wormwood-based Biochar Catalyzing Sulfide Degradation Of Perchloroethylene In Groundwater

Perchloroethylene(PCE)is widely distributed in the environment,mainly from industrial processes,dry cleaning,textile processing and metal degreasing.Although PCE is currently banned by the dry cleaning industry,due to its good volatility and stability,it is easy to migrate over long distances in the environment and exists stably for a long time.It is still one of the groundwater pollutants with high concern and belongs to 2A carcinogens.Long-term human exposure to PCE can cause a series of health problems,so it is necessary to develop an inexpensive and harmless technique to remedy PCE.Sulfide is a reductant and nucleophile abundant in natural environments,such as marine sediments and anaerobic environments rich in sulfate-reducing bacteria.However,it is difficult for sulfides to reduce pollutants alone,and the activation energy of the reaction needs to be reduced by the addition of catalyst.Studies have shown that carbon materials as catalysts can promote the reduction of nitrobenzenes,organochlorine pesticides,dyes and other pollutants by sulfides,but the current research on the catalytic reduction of chlorinated hydrocarbon pollutants is not comprehensive,the mechanism of the reduction process and the active groups catalyzed by carbon materials are not clear.In addition,the technology is mainly applied to polluted groundwater,and there is little research on the impact of underground environmental conditions on the remediation effect.The carbon material selected in this paper is biochar,which is the easiest to achieve large-scale application among many carbon materials due to its low cost,easy preparation,and harmlessness.The characteristics of large specific surface area and abundant surface functional groups make it have certain application potential in the fields of adsorption and catalysis.In addition,converting solid waste into biochar is an effective strategy to reduce the global waste carbon footprint,achieving carbon neutrality is a common global mission.The preparation of biochar achieves carbon sequestration,which is conducive to promoting the development of circular economy.Therefore,in this paper,the biochars prepared from wormwood were used as carbon catalysts to study the system of reducing PCE by sulfide,and the following results were obtained:(1)The pyrolysis temperature setting in the preparation process affected the remediation effect of biochar,and 700-800?was the optimal pyrolysis temperature range for effective materials.This paper successfully prepared the carbon materials that can catalyze sulfide to completely harmlessly treat PCE.The adsorption process of PCE by biochars conformed to the pseudo-second-order adsorption kinetic process,and the thermodynamics conformed to the Freundlich model.The adsorption equilibrium can be reached in about 24 h after the reaction.The maximum adsorption capacity of PCE was 71.44 mg/g,with fast surface adsorption and slow intrapore diffusion processes.The catalytic process fitted pseudo-first-order kinetics.PCE with a concentration of 20mg/L can be completely removed within 5 days and completely transformed within 7days in the system containing 100 mg/L biochar and 5 mmol/L sulfide.The catalytic reduction effect of biochar was better than that of commercially synthesized graphitic carbon materials and mesoporous carbon materials,and the final PCE removal rate of nitrogen-doped ordered mesoporous carbon(N-CMK-3)was only 49.5%in the same time.(2)Based on PCE reduction product analysis and mass balance calculation,the study clarified the transformation pathway of PCE in the biochar-sulfide system.The main products of PCE catalytic reduction were acetylene and Cl^-.The final conversion rate of acetylene was up to 99%after mass conservation calculation,which can achieve complete harmless removal.Through product analysis results,monitoring environmental parameters(pH,ORP)and a series of material characterization methods,the nucleophilic sulfur-containing structure formed by the combination of carbon atoms adjacent to the pyridine nitrogen and the sulfide was speculated,leading to the main mechanism of the catalytic reaction.After the biochars undergoed the catalytic reaction,the sulfide in the solution bonded to carbon material in the form of C-S bonds.The dechlorination process of PCE was mainly combined action of?-elimination and hydrogenolysis.The reduction process can be divided into two degradation pathways,the main pathway products were acetylene,chloroacetylene and Cl^-,and the secondary pathway products were TCE,acetylene and Cl^-,chlorine-containing by-products were converted to acetylene,and the final products are only acetylene and Cl^-.(3)Five biochar catalysts with different surface properties were successfully prepared by using different concentrations of HNO₃(WW-n H)and KOH+H₂O₂(WW-K-HO),and the reaction mechanism was further explored by comparing the material properties and catalytic properties.The results showed that the concentration and type of modifier can affect the performance of catalysts.The optimal concentration of modifier HNO₃ was 2 M-4 M,and the catalytic effect of acid-modified carbon materials were better than that of alkali-modified carbon material.Combined with the adsorption performance and catalytic performance of different carbon materials,the important properties of carbon materials in the catalytic process were analyzed,which provided a theoretical basis for the future directional preparation of effective carbon materials.The results showed that the specific surface area and the composition of functional groups affected the catalytic performance.The increase of specific surface area improved the adsorption performance of PCE and thus affected the catalytic performance.Combined with the characterization phenomenon,the functional group removal experiment and the density functional calculation of the atomic dipole correction Hirshfeld charge and potential energy,it was found that carboxyl group and pyridine nitrogen were active groups in the catalytic reduction system,which increased the catalytic active site by promoting the formation of reductive nucleophilic structures between the carbon atoms adjacent to the functional groups and sulfide.(4)A series of influencing factors in the catalytic system are explored to provide theoretical reference for application conditions.The slopes obtained by fitting the reductant,catalyst dosage and k_obs were 4.16×10^-3 and 1.35×10^-3,respectively,indicating that the reductant dosage had a greater impact on the efficiency of the catalytic reaction than the catalyst dosage,because the nucleophilic sulfur-containing structure formed by the reductant was the limiting factor of the dechlorination process,rather than the adsorption efficiency.The pH of the environment is an important factor,since the reductant sulfide has different forms in the solution with the change of pH.The catalysts showed a good catalytic effect in the pH range of 7-11,and more than 90%of PCE can be removed within 7 d,while the removal of PCE under acidic conditions mainly depends on the adsorption process.When the initial concentration of PCE was10-20 mg/L,it had the fastest catalytic effect,and the order of reaction rate constant was 20,10,30,40,and 5 mg/L.(5)The effects of underground chemical composition,temperature,dissolved oxygen(DO),and humic acid concentration were explored to clarify the underground environment for which the biochar-sulfide system was suitable.It was found that the addition of Cl^-and Ca²⁺increased the catalytic rate,SO₄^2-,Fe³⁺,NO₃^-and Mg²⁺inhibited the catalytic performance,in which SO₄^2-and Fe³⁺had a greater impact,and HCO₃^-had barely obvious effect.The remediation was still effective at low temperature;the reaction rate constant was 4.60×10^-3 h^-1 under the condition of underground ambient temperature(10?).The catalytic reaction did not require severe anaerobic conditions,the carbon materials can completely remove PCE under the condition of DO value below 6.60 mg/L.The increase of humic acid concentration inhibited the catalytic effect,which may be due to the simultaneous consumption of sulfide and adsorption of PCE by humic acid.The simulation column experimental results showed that the biochar-sulfide system can be effectively applied to the adsorption control and reductive removal of PCE-contaminated groundwater.For the simulated groundwater with an initial concentration of 5 mg/L PCE pollution,at a flow rate of 0.61 m/d,when the height of the filler exceeded 170.4 cm,the effluent concentration can reach the Class III groundwater standard(<40?g/L).In conclusion,the above conclusions for the first time indicated that remediation of PCE contaminated groundwater with biochar-sulfide system was effective and feasible.