Research On The Residual Heavy Metal Removal From The MSWI Fly Ash By Electrokinetic Remediation Enhancement Technology

MSWI fly ash collected from the bags after the incineration of the precursors including of the municipal,industrial and agricultural wastes is rich in heavy metal elements.The current electrokinetics technology for the treatment of MSWI fly ash is relatively single,insufficient in the supporting integration of the remediation systems.The EKR treated samples only achieve low heavy metal removal efficiencies and still are considered to be environmentally harmful.The research has mainly explored on the focusing phenomenon of HM distribution in the electrokinetic process and investigated the electromotive migration,transformation and enrichment of HM elements in the electrolyser during a complete processing time based on the physicochemical characteristics and the environmental activity of the fly ash samples to summarize the main factors influencing the remedial efficacy for the current EKR system.In this study,we further optimized the EKR process by controlling pH in the sample area,changing the stacking modes of the samples,activating the matrices using ultrasonication,coupling permeable activated charcoal reactive barrier in order to improve remedial efficacy and significantly reduce the environmental hazards of the samples.MSWI fly ash ranging from 7?m to 750?m is mainly composed of the sodium chloride,silver chloride sodium,potassium chloride,calcium carbonate and silicon tetrachloride and has rough surface and polycrystalline melting morphology.The leaching toxicities of Zn,Pb and Cd in the raw fly ash are all higher than the standard values specified in the Pollution Control of Municipal Solid Waste Landfill.The MSWI fly ash has low residual moiety,and is ranked as the high-risk pollutants.The concentration focusing phenomena of HMs were observed in S2 region of the sample area at 4-6d during the EKR processes.There existed a significant pH gradient between S1 and S2 in the electrolyser at the period of 3-12 d.The conductivity increased in the initial and middle stage and then decreased over the time with the maximum value being obtained on the fifteenth day.The removal efficiencies of HMs in the MSWI fly ashes can be effectively elevated by scooping out the pulp matrix in the S2 region of the electrolyser during the electrokinetic experiment.The remediation systems of SD-EKR-S2 and SD-EKR-S25 can effectively avoid the near-anode concentration effect happening in the sample area during the EKR process for the MSWI fly ashes.The changes of the concentration ratios of HM elements in the anode normalized distance of 0-0.6 was stable at 6-18 d,and the concentration values were maintained within a relatively low range.The removal efficiencies of HMs obtained in the PC-EKR-W system was the most significant under 1: 2.5 solid-liquid ratio washing pretreatment.The best remedial efficacy for the treatment of the MSWI fly ashes and lower risk of the treated samples to the aquatic environment were observed in the optimized system of PC-EKR-O with the factors of the voltage gradient,the repairing time and the nitric acid concentration all being chosen to be the L3 level.The combined pretreatment of acidification and ultrasonication intensified the extraction of HMs from the MSWI fly ashes in the sample area,enhanced the migration of ions and improved the enrichment condition during the AUS-EKR process.In the ultrasonic activation experiment,the voltage gradient had the greatest influence on the AUS-EKR system when the combination of operating parameters was A3B3C2.The fitting equations in the AUS-EKR system confirmed the effect of the protonic attack and the ultrasonic cavitation on the transformation of the physicochemical characteristics of MSWI fly ash samples.The removal efficiencies of residual HMs from the MSWI fly ashes in the coupling system from were significantly higher than that of single motorized system,and the addition of oxalic acid at the location of the permeable reaction wall of the coupling system was beneficial to further enhance the removal efficiencies of HMs.Generally,the design,assembly and optimization of the electrokinetic remediation systems for the MSWI fly-ash treatment are improved by adjusting the stacking modes of the fly ashes in the sample area,recycling the cationic buffer solution,affiliating the pretreatments of water washing,acidification and ultrasonication,and coupling with the permeable reaction barrier from the aspects of process adjustment,pH control,improvement of substrate and recovery of heavy metals according to the electrokinetic characteristics of MSWI fly ashes and the exploration of concentration-focusing effect of heavy metals near the anode of the electrolyser,which provides the theoretical support and technical guidance for the large-scale popularization of MSWI fly ash-electrokinetic remediation.