Advanced Treatment Of Biologically Pretreated Coal Chemical Industry Wastewater By Three-dimensional Electro-fenton With SAC-Fe Catalytic Particle Electrodes

As the coal conversion industry,coal chemical industry has been playing vital roles in the production of clean energy and improving the capacity of energy security in our country.Coal chemical industry is of important strategic significance.However,effectiveness of the traditional treatment system is unsatisfactory due to the enormous amount of wastewater from coal chemical industry with the characteristics of high contaminants and toxicity.The biologically pretreated coal chemical industry wastewater still contains a large number of toxic and refractory compounds as well as their derivatives,with lower biodegradability than the raw wastewater.Under the background of water resource scarcity and deteriorating water environment in our country,the state has promulgated the strict regulation of “approximated zero liquid discharge”.Therefore,searching for advanced treatment options with high efficiency and low cost is the key to achieve “approximated zero liquid discharge”.In this study,in order to solve the problems of unsatisfactory treatment effect,low current efficiency and space utilization as well as other issues in traditional electro-Fenton technology,sewage sludge and waste iron sludge were used as raw materials to synthesize catalytic particle electrodes.Three-dimensional electro-Fenton system was established with the prepared catalytic particle electrodes to treat biologically pretreated coal chemical industry wastewater,aiming at degradations of the residual contaminants and further purification of wastewater.In addition,the prepared catalytic particle electrodes were employed as carriers to set up biological activated carbon?BAC?.Three-dimensional electro-Fenton was introduced to enhance the performance of BAC to strengthen the removal of contaminants,providing reliable quality influent for subsequent reuse water treatment system.The study focused on the investigation of treatment effectiveness,dynamic analysis of pollutants removal,dynamic model of H₂O₂ generation and the discussion of catalytic mechanism in three-dimensional electro-Fenton,laying the foundation for applying three-dimensional electro-Fenton to the advanced treatment of coal chemical industry wastewater.Sewage sludge and waste iron sludge were used as raw materials to synthesize catalytic particle electrodes?named as SAC-Fe and SAC?via a one-step method combining the carbonization and Fe-loading process.The analysis and optimization of preparation parameters were conducted with orthogonal test.SAC-Fe and SAC had high specific surface area and wonderful pore development.Fe element in SAC-Fe was mainly from iron sludge and C and O were the dominant elements of SAC-Fe.The main active component in S AC-Fe existed in the form of spinel Fe₃O₄.Fe₃O₄ particles embedded and encapsulated in the porous carbonaceous support with high decentrality and bonded together with chemical bonds,enhancing the stability and resistance to corrosion.Heavy metal dissolved little from SAC-Fe in leaching test,without secondary pollution to environment.Compared to Fe₃O₄ MNPs,SAC-Fe and SAC exhibited hither adsorption performance.The maximum adsorption content for COD of SAC,SAC-Fe and Fe₃O₄ MNPs were 112.6,101.1 and 33.4 mg/g respectively by calculating with Langmuir model.Three-dimensional electro-Fenton system was established by employing the prepared catalytic particle electrodes?SAC-Fe?.The three-dimensional electro-Fenton system exerted excellent performance in the treatment of the wastewater.The response surface methodology coupled with central composite design was applied to optimize the operational parameters.The optimal parameters were determined as p H 3.95,13.96 mmol/L of [Fe] and 16.78 mA/cm² of current density,with the predicted and observed COD removal efficiency of 75.74% and 75.89%.COD,total phenols,TOC and COLOR in the effluent of three-dimensional electro-Fenton met the class I criteria of the Integrated Wastewater Discharge Standard.Moreover,biodegradability of the treated wastewater was improved remarkably and toxicity was reduced significantly.SAC-Fe exhibited superior stability with little Fe leaching.Regeneration rate of SAC-Fe reached 94.33% with electrochemical method.TOC removal in three-dimensional electro-Fenton conformed to L-H dynamic model.Adsorption effect in the L-H dynamic model could not be ignored to describe pollutants degradation in the reaction system.The apparent rate constant of TOC removal was determined to be 1.33×10^-2 min^-1,and the apparent Ea of TOC removal reaction in three-dimensional electro-Fenton system was calculated to be 33.67 k J/mol.Interface control mechanism was deduced.H₂O₂ generation dynamic model was established according to the?elec tro?-chemical reactions referring to the generation and consumption of H₂O₂.H₂O₂ in three-dimensional electro-Fenton system accumulated along with the reaction proceeds and then H₂O₂ concentration stabilized.The maximum accumulation H₂O₂ concentration related to current density,dissolved oxygen concentration and [Fe].Amount of generated H₂O₂ increased significantly with the addition of SAC-Fe.The maximum accumulation H₂O₂ concentration in two-dimensional and three-dimensional electro-Fenton systems were 433.3 ?mol/L and 741.6 ?mol/L respectively.Reactive species produced in three-dimensional electro-Fenton system in acidic solution varied from that in weakly alkaline solution.The main reactive species produced in three-dimensional electro-Fenton system under acidic condition was ·OH,following Haber-Weiss mechanism.While under weakly alkaline condition,the oxidants generated from the decomposition of H₂O₂ were mainly ·O₂^-/ HO₂· and to lesser extent were ·OH,involving the formation and deactivation of H₂O₂ complex.SAC-Fe offered support and place for electrochemical reactions and pollutants degradation reactions.Catalytic reactions and pollutants degradation reactions occurred on SAC-Fe surface and the immediate vicinity.Contaminants were enriched on SAC-Fe resulting from adsorption and maintained at high concentration,these pollutants in situ captured radicals and improved radicals utilization.SAC-Fe served both as the particle electrodes and the heterogeneous catalyst of Fenton reaction,participating in the generation and decomposition of H₂O₂.Abundant iron species embedded in porous carbonaceous support with high exposure endowed more catalytic active sites,facilitating the high catalytic activity.After treated by three-dimensional electro-Fenton,the kinds of organic pollutant composition reduced from 45 to 34.Phenolic compounds were mostly removed.Pollutants residual in the treated wastewater were N-heterocyclic compounds,long-chain hydrocarbons and a small part of phenolic compounds.The degradation properties of pyridine,quinoline and indole were different from each other.The developed quantitative structure-activity relationship?QSAR?model described the relationship between the structure and activity appropriately.BAC was set up with the prepared SAC as carriers to strengthen the pollutants removal.The bioactivity of biofilm in the bottom of BAC was higher,with the average SOUR of 24.6×10^-2 mgO₂/?gSAC·h?.Three-dimensional electro-Fenton enhanced the performance of BAC dramatically.COD,total phenols,TOC and COLOR of treated wastewater by three-dimensional electro-Fenton-BAC integrated process were 40.24,0.42,13.15 mg/L and 20 times,guaranteeing influent wastewater quality for subsequent reuse water treatment system.The HRT of both parts were shortened in three-dimensional electro-Fenton-BAC integrated process.Pollutants were removed efficiently at a much shorter retention time.Three-dimensional electro-Fenton system and integrated process represented excellent performance in trea tment of real wastewater.The study has certain guiding significance for the employment of three-dimensional electro-Fenton in actual application.