Dechlorination And Resource Recovery Of Typical Chlorinated Organic Waste In Subcritical Water

Organic wastes with high chlorine content,such as polyvinyl chloride（PVC）waste plastics and chlorinated paraffins（CPs）waste,have received widespread attention because of their potential toxicity and difficulty in natural degradation.Because of the large amount of chlorine and plasticizers,these two wastes cannot be treated by traditional methods such as incineration and landfills,which are prone to produce highly toxic substances such as chlorinated dioxins,while landfills can lead to soil and groundwater contamination by chlorinated substances,plasticizers and other toxic substances.Safe and efficient dechlorination is a key link in the environmentally sound and resourceful treatment of both PVC and CPs wastes.In this paper,we studied the dechlorination behavior of PVC waste and the decomposition mechanism of plasticizer DEHP in subcritical water-ammonia system by using PVC waste rich in diisooctyl phthalate（DEHP）plasticizer and chlorinated paraffin（52#CPs）with 52%chlorine content,and comparatively studied the dechlorination characteristics of PVC waste in three systems of subcritical water,subcritical methanol and subcritical water-ammonia and the residue The adsorption behavior of carbon materials on hexavalent chromium ions was studied,and the effects of different alkalis in the subcritical water-alkali system on the dechlorination behavior of 52#CPs and the dechlorination pathway were investigated.The main results are as follows.（1）Dechlorination behavior of PVC waste in subcritical water-ammonia system and the decomposition mechanism of plasticizer DEHP.The increase of subcritical water-ammonia temperature and residence time were both beneficial to improve the dechlorination rate.250℃,60 min,5%ammonia concentration,PVC dechlorination efficiency reached 94.8%.The dechlorination path was direct dechlorination of HCl,and the solid phase product was polyene structure.At 250℃,the subcritical water-ammonia condition significantly promoted the hydrolysis of DEHP and led to the oil phase product containing 86.12%of the plasticizer synthesis monomer 2-ethyl-1-hexanol with high recovery added value.When the temperature was increased to 400°C,the concentration of 2-ethyl-1-hexanol in the oil phase product decreased to 10.16%,and the concentration of benzaldehyde and acetophenone increased to20.48%and 26.63%,respectively.Benzaldehyde and acetophenone originated from the further cyclization,decomposition,and partial oxidation of the polyene structure of PVC dechlorination products.Therefore,two-stage temperature-based PVC waste treatment in subcritical water-ammonia media can achieve efficient recovery of high-value-added chemicals such as 2-ethyl-1-hexanol,benzaldehyde,and acetophenone.（2）A comparative study of the dechlorination characteristics of PVC waste in three systems:subcritical water.subcritical methanol and subcritical water-ammonia and the adsorption behavior of residual carbon material on Cr（VI）ions after dechlorination.The solid residues obtained under different reaction conditions of the above three systems were used for the adsorption of Cr（VI）ions,respectively,and the PVC dechlorination reaction conditions with the best adsorption performance residues were preferably selected as follows:Subcritical water system（R₁:300℃,60 min,1:30 g/m L）,dechlorination rate close to 100%;subcritical methanol system（R₂:200℃,30 min,1:30 g/m L）,dechlorination rate of 44.9%;subcritical ammonia system（R₃:250℃,60 min,1:30 g/m L）,dechlorination rate close to 100%.PVC was dechlorinated in subcritical water,subcritical methanol and subcritical water-ammonia systems to obtain three residues,R₁（subcritical water）,R₂（subcritical methanol）and R₃（subcritical water-ammonia）.Cr（VI）adsorption experiments showed that R₁and R₃had the best adsorption effect at p H 2,with adsorption amounts of 9.41 mg/g and 7.59 mg/g,respectively.R₂adsorbed at a maximum of 7.17 mg/g at p H 10.R₁and R₃mainly adsorbed in monolayer,and R₂mainly adsorbed in multilayer.ΔH<0 for R₁and R₂adsorption processes,andΔH>0 for R₃adsorption process.The solid residue of PVC waste after dechlorination in low temperature subcritical water,methanol and ammonia system can be used as adsorbent material for the adsorption removal of hexavalent chromium ions.（3）Effects of subcritical water and subcritical water-alkali system on the dechlorination behavior and dechlorination path of 52#CPs.Complete dechlorination of 52#CPs could be achieved under the condition of 300℃and 45 min of subcritical water reaction.In the subcritical water-sodium hydroxide system,complete dechlorination of 52#CPs was achieved at 250℃,5 min and 2 m L（0.05 mol/l）Na OH.alkaline additives such as KOH,Na₂CO₃,Na HCO₃and NH₃·H₂O could effectively enhance the dechlorination of 52#CPs by subcritical water and reduce the treatment temperature and treatment time.Under the reaction conditions of 250℃,5 min,and 1:5 m L/m L（52#CPs/（2 m L alkaline additives+H₂O）m L/m L）,the dechlorination of 52#CPs by alkaline additives was found to be nearly complete.At 250℃,5 min,1:15（52#CPs/（2 m L alkaline additive+H₂O）m L/m L）reaction conditions,the best dechlorination effect of Na OH was found to be 97%,followed by Na₂CO₃with65.4%dechlorination,and finally Na HCO₃,NH₃·H₂O,and KOH in order,which proved that Na₂CO₃as a strong alkali alternative has certain application prospects.The subcritical water dechlorination of 52#CPs was mainly based on the hydroxyl substitution path,and the products were mainly polyol compounds such as 1,2,6-hexanetriol.After the introduction of Na OH enhancer,the main reaction path of 52#CPs is dechlorination of hydrogen chloride,and the products are mainly unsaturated hydrocarbons such as 2,4-hexadiyne.All the chlorine removed from 52#CPs was recovered in the form of inorganic chlorine transferred to the aqueous phase.