Study Of Oxidation And Reductionpretreatment Technologies Of Herbicide Wastewater

Each year, less than10%of pesticide wastewater was treated and discharged intorivers, which makes pesticide pollution of surface water, soil, groundwater more andmore serious. Pesticide wastewater from a fomesafen producing plant was pretreatedby six processes. Batch experiments were employed to study influencing factors,dynamics and degradation of pollutants. Results achieved can provide theoretical andengineering basis for pretreatment of herbicide wastewater. Results are as follows:（1） Fenton oxidation of pollutants was slightly influenced by temperature. Undernormal temperature conditions （20°C-25°C）, Fenton oxidation could achieve arelatively high pollutants removal efficiency. Removal efficiencies of pollutants withnano zero-valent iron and potassium permanganate increased along with an increaseof temperature. Oxidation capacity of potassium ferrate was the highest whentemperature was25°C.（2） Optimum pH of Fenton oxidation was3-4. Oxidant potassiumpermanganate, potassium ferrate and ozone perform better under acidic conditions.The lower pH is, the higher pollutants removal efficiency will be achieved. Acidicenvironment was also preferred for nano zero-valent iron reduction.（3） Pollutants removal efficiency could be enhanced via increasing dosages ofpotassium permanganate, O3, O3/H₂O₂Fenton and nano zero-valent iron, respectively.With H₂O₂dosage larger than threshold, Fenton oxidation of organic pesticidewastewater will be inhibited. Excessive H₂O₂will lead to a mitigation of increasingtrend of CODCrremoval or a decrease of removal efficiency. Meanwhile, excessivedosage of catalyst Fe²⁺led to a decrease of pollutants removal efficiency. Over dosageof O3and H₂O₂will both lead to decreases of pollutant removal efficiencies.Increased dosage of ceric sulfate will cause CODCrremoval efficiency decrease firstand increase subsequently. The highest CODCrremoval efficiency was achieved with0g/L of ceric sulfate.（4） Extending reaction time can improve pollutants removal efficiency. Oxidants oxidation and nano zero-valent iron reduction of pesticide wastewater were allcompleted within1h. However, nano zero-valent iron reduction of3,4-dichloro-benzotrifluoride was basically completed within14hs.（5） Through oxidation of substances as dimethyl sulfoxide, chlorinatedhydrocarbons, BTEX（benzene, toluene, xylene） or degradation of substances aschlorophenol, trichlorethylene, tetrachlorethylene, those five pretreatment processescould ameliorate the biodegradability of pesticide wastewater.3,4-DCBTE was firstlydegraded to1-chloro-4-trifluoromethyl toluene or3-chloro benzotrifluoride;ring-opening reaction of complete dechlorination product benzotrifluoride took placeconsequently and chain product of trifluorine-methyl butylacetate was generated.（6） Pretreatment reactions of potassium permanganate and Fenton oxidation,nano zero-valent iron reduction and nano zero-valent iron-Fenton comprehensivetreatment of pollutants all followed the pseudo-first order kinetics equations.Activation energies were6.48kJ/mol、7.55kJ/mol、63.1kJ/mol and21.7kJ/mol,respectively. Activation energy of Fenton oxidation of hydroxybenzoic acid was11.12kJ/mol. Activation energy of nanoiron dechlorination of3,4-DCBTE was54.8kJ/mol.（7） Nano zero-valent iron can enhance water quality and biodegradability ofwastewater and Fenton oxidation could effectively remove organics of pesticidewastewater. Nano zero-valent iron-Fenton oxidation pretreatment technology cancombine advantages of both processes. Simultaneously, process combination canshorten the reaction half-life time. This conclusion proves technical feasibility ofcombination of reduction and oxidation processes to enhance wastewater pretreatmentefficiency.