Study On Removal Efficiency And Mechanism Of Pollutants In Wastewater By Sinusoidal Alternating Current Electrochemical Technology With Iron Electrodes

Phosphorus,heavy metals and antibiotics are the typical pollutants which are widely concerned by researchers.These pollutants have seriously affected the ecological environment.Therefore,how to effectively remove them from wastewater has become the focus of wastewater treatment.Traditional electrocoagulation technology has been applied in wastewater treatment for many years,but it shows disadvantages of large electrode consumption,easy passivation of electrode and large power consumption.In this paper,wastewater containing phosphorus or heavy metals was treated by sinusoidal alternating current coagulation（SACC）,and wastewater containing tetracycline was treated by sinusoidal alternating electro-Fenton（SAEF）.The influence of concentration（c₀）,p H（pH₀）,current density（j）,conductivity（σ）,H₂O₂ dosage（c[30%H₂O₂]）and reaction duration（t）were studied.Furthermore,the adsorption kinetics and thermodynamics were also studied.Moreover,the morphology,composition,functional groups and crystal structure of the produced flocs were characterized with SEM,EDS,XRD,FT-IR and XPS.The reaction process was suggested by UV-Vis,LC-MSMS and GC-MS.The optimum reaction conditions were obtained,and possible mechanism was proposed as following:（1）Phosphorus wastewater was treated with SACC.At the condition of pH₀=7,σ=800μS·cm^-1,c（P）=5 mg·dm^-3,j=2.12 m A·cm^-2,f=50 Hz,t=60 min,phosphorus removal rate reaches 90.9%.Compared with direct current coagulation（DCC）,SACC exhibits the higher removal efficiency of phosphorus due to the stronger adsorption of the produced flocs.The adsorption of iron sol to phosphorus was an endothermic and spontaneous process,and its adsorption behavior can be characterized with Langmuir model,Redlich Peterson model and pseudo-second-order kinetic model.（2）The Zn²⁺ removal from wastewater was applied by SACC.Box-Behnken design response surface methodology was used to obtain the optimal reaction conditions.The results showed that the Re(Zn²⁺)can reach 98.80% and the EEC=1.47 k Wh·m^-3 at the condition of t=21.3 min,j=0.31 A·m^-2,f=50 Hz,pH₀=7,c(Zn²⁺)=120 mg·dm^-3.Fe₅O₇OH·₄H₂O（HFO）was produced in SACC process.Zn²⁺ was adsorbed and precipitated by HFO.The adsorption of Zn²⁺ conformed to Freundlich adsorption isotherm model and pseudo-second-order kinetic model.The adsorption process of Zn²⁺ by HFO is a spontaneous endothermic reaction.（3）Nano-iron sol was prepared by SACC and used as adsorbent in heavy metal wastewater.The results showed that reduction reaction of heavy metal ions(Ni²⁺,Zn²⁺,Cu²⁺)at the electrode（cathode）surface was the main reaction.Meanwhile,precipitation of Ni（OH）₂,Zn（OH）₂ and Cu（OH）₂ was also occurred.The results showed that the adsorption selectivity was Cu²⁺>Ni²⁺>Zn²⁺.The maximum adsorption capacity of Ni²⁺,Zn²⁺ and Cu²⁺ in single-metal system were 56.3 mg·g^-1,52.5 mg·g^-1and 60 mg·g^-1,respectively.The maximum adsorption capacity of Ni²⁺,Zn²⁺ and Cu²⁺ in multi-metal system were 43.3 mg·g^-1,35.6 mg·g^-1and 52.1 mg·g^-1,respectively.In the multi-metal system,some adsorption sites occupied by Zn²⁺ were replaced by Cu²⁺.The competitive adsorption of Ni²⁺,Zn²⁺ and Cu²⁺ in multi-metal system conformed to LOGISTIC model.（4）Tetracycline（TC）wastewater was treated by SAEF.The results showed that the removal rates of TC and chemical oxygen demand（COD）were 94.87% and 82.42%,respectively,under the optimal conditions(p H = 3.0,conductivity（κ）=1075 μS·cm^-1,current density（j）=0.694 m A · cm^-2,initial c（TC）=100 mg · dm^-3,c[30%H₂O₂] =1.17 cm³ · dm^-3,frequency（f）=50 Hz,t=120 min).It was found that the conjugated structure of TC was destroyed to generate intermediate products,and then most of them were gradually mineralized into inorganic materials in the SAEF process.Comparing with direct electro-Fenton（DEF）or SACC,SAEF was a highly effective method with low-cost for the treatment of TC wastewater.Under the optimal conditions,the TC degradation process conformed to the second-order kinetic model.