Removal Of Thallium And Arsenic From Wastewater By Biochar-supported Copper Ferrite Activated Persulfate

Thallium and arsenic are two metal(loid)s with variable valence states,which seriously threaten the ecological environment and human health due to their high toxicity and bioaccumulation.The polymetallic complex pollution in water caused by mining has become a growing environmental problem.Rare researches have focused on the treatment of Tl and As in wastewater with binary pollution.Advanced oxidation technologies based on persulfate activation have become a hotspot on account of their high oxidizing ability,fast reaction rate and no secondary pollution.Persulphates can be activated in many ways,among which metal oxides are the main catalysts for multiphase oxidation and can also be used as adsorbents for thallium and arsenic removal.The copper ferrate(CuFe₂O₄)has been widely used in wastewater treatment due to its high catalytic efficiency,easy magnetic separation and environmental friendliness.However,CuFe₂O₄with interparticle magnetism is vulnerable to aggregate.In order to prevent the agglomeration,CuFe₂O₄doping on the surface of biochar was considered to make a composite material(CuFe₂O₄@BC)with higher stability and catalytic efficiency.It was used to activate persulfate to generate free radicals,so that Tl(I)was oxidized into Tl(?)with lower mobility.At the same time,As(?)was oxidized to As(V)with lower toxicity.The influences of different initial pH,equilibrium pH,catalyst dosage,persulfate dosage,initial concentration of pollutants,reaction time,co-existing ions and co-existing organic compounds on its removal performance and mechanism were comprehensively investigated.Firstly,CuFe₂O₄(CF)and CuFe₂O₄@BC(CFBC)were prepared by co-precipitation method for activating persulfate(PS).The performance and mechanism of removing Tl(I)containing wastewater were investigated.Results showed that both CF-PS and CFBC-PS systems can effectively remove thallium from water.Tl concentration was 10 mg/L with PS concentration of 1 m M,the maximum removal rate of Tl reached above 99%(dosage:1 g/L,reaction time:10 min,initial pH:4-12 and equilibrium pH:10).By means of scanning electron microscopy(SEM),transmission electron microscope(TEM),X-ray diffractometer(XRD),X-ray photoelectron spectroscopy(XPS),infrared spectroscopy(FTIR),hysteresis loop(VSM)and electron spin resonance spectroscopy(ESR),it was found that the main radical species generated during the activation of PS by CF were sulfate radicals(SO₄^·-).The reaction is mainly accomplished in the form of non-free radicals in the CFBC-PS system.The main removal mechanisms of Tl(I)were oxidative,precipitation,surface complexation and electrostatic attraction in both system.Secondly,the performance and mechanism of removing As(?)in wastewater in CFBC-PS system were explored.The results showed that Tl concentration was 10 mg/L with PS concentration of 2 m M,the maximum removal rate of Tl reached above 98%(dosage:1g/L,reaction time:60 min,initial pH:acidic and neutral 2-6 and equilibrium pH:10).In the presence of high concentration of coexisting ions and organic matter,it was observed with slight inhibition of arsenic removal.CFBC-PS has a great adsorption capacity for arsenic(88.9 mg/g),indicating that CFBC is an ideal arsenic removal material.Hydroxyl free radical(?OH)played a key role in the removal of As.The removal mechanisms of As(?)are oxidative and complexation.Finally,the performance and mechanism of simultaneous removal of binary thallium-arsenic wastewater in CFBC-PS system were investigated.Meanwhile,the interaction between binary pollutants were discussed.The results showed that the simultaneous removal of thallium-arsenic was the most efficient only when the initial pH is acidic(pH=2).The removal rate of thallium was much lower than arsenic under other pH conditions.Arsenic can inhibit the removal of thallium via inhibiting the oxidation of Tl(I).The reaction order and rate are As>Tl.In this system,hydroxyl free radical(·OH)played an important role.The main removal mechanisms of Tl(I)were surface complexation and electrostatic attraction.The main removal mechanisms of As(?)were oxidative and hydroxyl complexation.