Study On The Reductive Treatment Of Cis-Dichloroethylene With Sulfide Catalyzed By Alfalfa-Based Biochar

Perchlorinated alkenes,such as Tetrachloroethylene（PCE）and Trichloroethylene（TCE）,are very common volatile organic pollutants in groundwater and soil,and are reduced to dechlorination under anoxic/anaerobic conditions,but incomplete reduced dechlorination of Perchlorinated alkenes leads to the accumulation of intermediate Cis-Dichloroethylene（Cis-DCE）and vinyl chloride（VC）.The lower the content of chloro-substituted groups in Chlorinated alkenes（PCE>TCE>Cis-DCE>VC）,the more difficult the reduction and dechlorination is.At the same time,Cis-DCE and VC have stronger toxicity and durability compared with the parent pollutants of Perchlorinated alkenes,which brings great difficulties to the environmental remediation work.Up to now,reductive dechlorination methods of Cis-DCE mainly fall into two categories,namely microbial methods and chemical methods.In the chemical reduction of repair method,the main use of raw material is ferrous metal materials,including zero-valent iron,iron pyrite,sulfur modified materials,however,compared with iron-based raw material,non-metallic sulfides(including H₂S,HS^-,S^2-,S_x^2-,etc.)is an another kind of reductive reagent widespread in the environment,which have better solubility in water,migration,reductive and nucleophilic in alkaline conditions.Therefore,from the perspective of thermodynamics,the reduction of chlorinated alkenes pollutants by sulfides is theoretically feasible,but the kinetics is extremely slow.However some catalysts are expected to accelerate the degradation of chlorinated alkenes.To verify the scientific hypothesis,this paper intends to use biochar as catalytic materials to accelerate the sulfide reduction degradation of Cis-DCE.The main research contents include:（1）the alfalfa straw which is agricultural and forestal waste was used as raw materials,by high-temperature anaerobic treatment,and with nitric acid on the modified treatment,according to different anaerobic pyrolysis temperature,the three biochar prepared were named as MXBC-400,MXBC-600 and MXBC-800.（2）Scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,nitrogen adsorption and desorption experiments（BJH,BET）,photoelectron spectroscopy（XPS）,Fourier infrared spectroscopy（FTIR）and other techniques were used to characterize the three kinds of MXBC,and the physical and chemical properties of the materials were obtained.（3）The optimal catalyst was determined by comparing the adsorption and degradation capacities of three kinds of MXBC for Cis-DCE.（4）Examines the MXBC-800 dosing quantity,concentration of sulfide（Na₂S）,initial p H value,reaction temperature and Cis-DCE initial concentration on Cis-DCE removal effect.（5）On the GC-MS test of system intermediate,the reaction pathway and mechanism was explored.（6）The effects of some common anions and dissolved humic acid in groundwater on the removal efficiency of Cis-DCE were investigated.At the same time,its catalytic stability was verified according to the recycling experiment of the material.The results showed that the three materials all had certain adsorption effect on Cis-DCE,but only MXBC-600 and MXBC-800 could catalyze the reduction of Cis-DCE.The biochar prepared under 800℃had the best catalytic performance,which could completely degrade CIS-DCE into acetylene.When the dosage of MXBC-800 is 500mg/L,the sulfide concentration is 10m M,the reaction temperature is 80℃,and the reaction time is 72 h,the removal rate of Cis-DCE（5-20ppm）at a certain initial concentration can reach more than 95%.In a certain range,the increase of material dosage,sulfide concentration,reaction temperature and initial p H of solution can promote the reaction.The main reaction of Cis-DCE degradation isβ-elimination reaction（elimination of hydrogen chloride molecule）,and the reaction chain is Cis-Dichloroethylene→Chloroacetylene→Acetylene;the presence of dissolved humic acid will inhibit the removal effect of Cis-DCE in the system,and the presence of anions will slightly inhibit the removal effect of Cis-DCE in the system.The results of reusability experiments show that MXBC-800 has certain catalytic stability.The experimental study in this paper proves that the new Cis-DCE treatment technology based on alfa-biochar/sulfide system can not only realize the green and harmless treatment of Cis-DCE,but also be less affected by common components in groundwater.The research content can provide reference for the treatment of Cis-DCE contaminated groundwater in the actual site.