Preparation Of Fenton Sludge-based Biochar And Its Mechanism Of Enhanced Contaminants Removal

Fenton oxidation technology,as the most widely used advanced oxidation technology,has become an effective treatment process for refractory wastewater due to its strong oxidation capacity,non selectivity,fast reaction speed,simple operation,simple equipment and low cost.However,the wastewater after Fenton oxidation treatment needs to be neutralized and flocculated to balance the p H of the effluent.In this stage,a large amount of iron-containing sludge will be generated,also known as Fenton sludge.It is difficult to dispose the huge amount of Fenton sludge.The traditional landfill method or incineration method is expensive,which may cause secondary pollution and waste Fenton sludge as a potential resource.Therefore,there is an urgent need for a new green technology for effective disposal and resource recovery of Fenton sludge.Biochar is a kind of carbon-rich material obtained from biomass after high-temperature heat treatment,which has rich surface functional groups and high specific surface area.It is widely used as a catalyst or adsorbent in water treatment process.In this paper,biochar was derived from Fenton sludge according to its characteristics.Its feasibility of resource utilization for Fenton sludge in the field of pollutant removal was explored.Its adsorption capacity for heavy metal ions,enhanced degradation and decolorization of azo dyes in anaerobic biological systems,activation mechanism of persulfate and degradation patyway of nitrogenous heterocyclic compounds were studied,providing a new way and method for resource recovery of Fenton sludge.In view of the limitation of traditional adsorbent materials for heavy metal ions removal from wastewater,Fenton sludge was prepared into magnetic aminated hydrochar（AHFS）by one-step ammoniation hydrothermal method.The saturated adsorption capacity of AHFS for heavy metal ion Pb²⁺in water is up to 359.83 mg g^-1,and the adsorption capacity is increased by 39.23%due to the amine group loading.It is confirmed by XRD that AHFS is mainly composed of hydrochar sheet,γ-Fe₂O₃ particles and Ca CO₃ crystal.The adsorption process was simulated by adsorption kinetics,adsorption isotherm and thermodynamic fitting calculation.The results showed that the adsorption of Pb²⁺by AHFS was dominated by chemisorption between homogeneous monolayer surfaces and assisted by diffusion in particles.The adsorption of AHFS on Pb²⁺has good reusability.After five times of adsorption-desorption,the adsorption rate of AHFS on Pb²⁺still maintains 78.5%.In addition,the adsorption mechanism was studied.AHFS could adsorb Pb²⁺through various mechanisms such as cation exchange,surface complexation of basic amine groups and acidic oxygen-containing groups,electrostatic attraction and physical adsorption.The contribution of different mechanisms to the adsorption effect can be inferred by quantitative experiments as follows:Cation exchange（43.15%）>Surface complexation with basic amine group（28.17%）>Surface complexation with oxygen-containing acid group（24.06%）>Other mechanisms（electrostatic attraction,physical adsorption,etc.）（4.62%）.In view of the insufficient response of anaerobic digestion to azo dyes degradation,magnetic composite hydrochar was prepared by hydrothermal method using Fenton sludge and sewage sludge as raw materials.Magnetic composite hydrochar was used as a redox mediator in the anaerobic decolorization and degradation process of azo dye AO7,which showed significant enhancement effect.The optimal preparation ratio of magnetic composite hydrochar（HC-1:3）can increase the anaerobic decolorization efficiency of AO7 by 1.91 times compared with traditional anaerobic digestion,and has a good recycling effect.The addition of HC-1:3significantly increased the electrical conductivity,electron transport system activity and azo reductase activity of the sludge in the anaerobic system.After 120 days of continuous UASB operation,the addition of HC-1:3 significantly improved the diversity of microbial communities in the system.At the same time,the abundance of AO7 degradation and decolorization related bacteria,dissimilated iron reducing bacteria,anaerobic fermentative bacteria and electrochemical active bacteria was significantly increased,and the ecological stability of the anaerobic system was enhanced.Besides,three electron transfer pathways that promote the degradation and decolorization of AO7 in anaerobic system by HC-1:3 were proposed:（1）Electrons were transferred through the graphite structure and conductive structure of HC-1:3;（2）Electrons are stored and released by the redox of Fe（Ⅲ）/Fe（Ⅱ）maintained by the dissimilar iron-reducing bacteria;（3）Electrons are directly transferred between cells through redox protein contained in EPS,conductive pili of electrochemically active bacteria and external conductive particles.In view of the outstanding problem that refractory chemical pollutants are difficult to degrade,magnetic composite biochar was prepared by mixing Fenton sludge and sewage sludge after hydrothermal and high temperature pyrolysis.Magnetic composite biochar was used as catalyst to activate persulfate,which showed a good effect on the oxidative degradation of 1-hydro-1,2,4-triazole（TZ）.The response surface method was used to optimize the optimal material preparation and experimental operating conditions.The results showed that when the dry weight ratio of sewage sludge to Fenton sludge was 5:1,catalyst dosage was 0.9 g L^-1,and the persulfate concentration was 0.5 g L^-1,the removal rate of TZ could reach 97.6%.After 60minutes of reaction,the removal rate of TOC could reach 80.33%,and the conversion rate of organic nitrogen to inorganic nitrogen could reach 100%.Simultaneously,it showed preferable recycling catalytic utilization,and had good tolerance to temperature（15-45℃）,p H（2-12）,co-existing anions and natural organic substances,indicating its practicability in actual wastewater treatment.Through quenching experiment,electron spin resonance analysis and electrochemical experiment,it was confirmed that magnetic composite biochar could activate persulfate through its own surface oxygen-containing functional groups,defect structure,Fe²⁺released and surface shuttle electrons.The oxidative degradation of TZ can be attributed to both free radical and non-free radical pathways.The contribution order of reactive oxygen species involved in the oxidation of TZ were as follows:SO₄^?->?OH>O₂^?->¹O₂.The possible degradation pathway of TZ was inferred through DFT calculation and LC-MS results.