Preparation Of Sludge-Based In Situ Magnetic Biochar And Application To Industrial Wastewater Treatment

With the development of water treatment industry in China,a large amount of municipal sludge has been continuously produced.However,sludge contains toxic substances such as pathogens,heavy metals and some organic pollutants,which cannot be dumped or landfilled directly.Therefore,it is especially important to find a suitable treatment method for sludge.Considering that thermal treatment can prepare sludge into carbon-based materials with well-developed pores and abundant functional groups,it has good treatment effect on industrial wastewater.In this experiment,the residual sludge from wastewater treatment plants supplemented with red mud were used as the main raw materials,while KOH and（NH₄）₂C₂O₄ were used as activators,during which the effects of factors such as ratio,temperature and retention time on the performance of the preparation process were investigated.The final sludge-based in situ magnetic biochar（SRMBC-750）was prepared.Its adsorption performance on Cu²⁺in simulated wastewater and its adsorption and catalytic performance on tetracycline hydrochloride were investigated,and the adsorption and catalytic mechanisms of the reaction system were explored.The details of the study and the conclusions are as follows.（1）The adsorption performance of simulated wastewater containing Cu²⁺was used to adjust the ratio of municipal sludge to red mud,pyrolysis temperature,retention time,and the dose of KOH and（NH₄）₂C₂O₄.The results showed that the overall adsorption performance of different ratios of biochar increased with the rising of the pyrolysis temperature.The maximum adsorption capacity of 4:1 biochar was 81.2 mg/g when the pyrolysis temperature reached 750℃.The maximum adsorption capacity of Cu²⁺was found to exist for the biochar retained for 1 h at 750℃for 0.5,1 and 1.5 h,respectively.Among them,KOH had the greatest influence on the adsorption performance of this biochar with the best dose of 5 g.Similarly,（NH₄）₂C₂O₄ could effectively increase the adsorption capacity of the biochar and reached the maximum adsorption capacity of146.385 mg/g with 4:1 biochar at a dose of 3 g.（2）Characterization by SEM,BET,XRD,FTIR and XPS showed that the surface of the biochar pyrolyzed at 750°C was rougher and had a rich void structure compared to those pyrolyzed at 450,600 and 900°C,respectively.Among them,the minerals in the red mud were uniformly loaded on the surface of SRMBC-750 and generated characteristic peaks of Fe-O functional groups,Fe₃O₄ and even Fe⁰ with abundant surface functional groups.VSM analysis indicated that the pyrolyzed biochar formed magnetic properties spontaneously,which contributed to the later solid-liquid separation.（3）Adsorption experiments of Cu²⁺in simulated wastewater using SRMBC-750biochar.The effects of different adsorption factors on the adsorption performance of the biochar were investigated.The results showed that the initial solution p H was 5.0 and the corresponding dosage of Cu²⁺solution of 300 mg/L was 2 g/L,at which the maximum adsorption amount of 303 K was 152.63 mg/g and the removal rate was 96.03%.From the analysis of adsorption kinetics and isotherm models,it is concluded that the adsorption process conforms to the pseudo-second-order kinetics and Freundlich isotherm models,respectively,and is a heterogeneous multilayer adsorption dominated by physical adsorption.The potential mechanisms involved in the adsorption were electrostatic attraction,complexation,cation exchange and"cation-π"interactions.（4）Adsorption experiments on tetracycline hydrochloride using SRMBC-750 biochar.The effects of different adsorption factors on the adsorption of tetracycline hydrochloride on this biochar were investigated.The results showed that the initial solution p H was 6.0and the corresponding dosage of 50 mg/L of tetracycline hydrochloride solution was 0.5g/L,at which the maximum adsorption of 303 K was 85.88 mg/g and the removal rate was85.89%.From the analysis of adsorption kinetics and isotherm models,it was found that the adsorption process was consistent with the pseudo-second-order kinetics and Langmuir isotherm models,respectively,for monolayer chemisorption.In addition,the adsorption process was found to be spontaneous and disorderly increasing by the thermodynamic model.The potential mechanisms involved in the adsorption wereπ-πinteractions,hydrogen bonding and metal complexation.（5）The SRMBC-750/PDS reaction system was established for the degradation and removal of tetracycline hydrochloride using SRMBC-750 biochar with potassium persulfate,and the active species involved in the degradation of tetracycline hydrochloride were identified by bursting experiments.The results showed that a p H of 3.0 and doses of15 mg and 1 m M of catalyst and persulfate,respectively,had good catalytic degradation effects.It was worth mentioning that the temperature did not make this removal efficiency significantly higher.The radicals that produce an effect during degradation were O₂^-and were generated by the activation of Fe²⁺and Fe⁰.