| Treatment of coking wastewater with high concentrations of organic pollutants, complex composition and high toxicity is a difficulty for domestic and foreign industrial wastewater treatments. Conventional biological treatment methods of coking wastewater were limited in organics removal ability, especially low removal efficiencies for nitrogen heterocyclic compound(NHCs) and polycyclic aromatic hydrocarbons(PAHs) etc, often resulting in effluent COD failing to meet the discharge standard, and it still need to combine other treatment technology as pretreatment method. Selection and optimization of pretreatment methods for refractory wastewater with high concentration are directly related to the efficiency of subsequent biochemical treatment. In recent years, with continuous development and improvement of wastewater treatment technology, the advanced oxidation processes(AOPs) has gradually been known. Due to increase in variety and amount of harmful pollutants and refractory organic pollutants in wastewater, electro-Fenton process as one of the advanced oxidation processes(AOPs) has been widely used to treat high-concentration organic wastewater. However, there were still no related reports on the study on utilizing electro-Fenton as a pretreatment technology for high-concentration coking wastewater.In the study, the feasibility of the pretreatment for raw wastewater from a coke plant in Shanxi was investigated, and process optimization and removal characteristics of organic compounds were studied by electro-Fenton technology. The single-factor experiment was used to investigate the effects of initial pH value, current density and reaction time on the removal efficiency of COD by electro-Fenton treatment. By applying response surface method(RSM) with COD removal efficiency of coking wastewater as response value, process parameters of electro-Fenton oxidation process were optimized. Combined with UV-visible spectra analysis(UV-Vis), three-dimensional fluorescence spectrum analysis(3D-EEM), fourier transform infrared spectroscopy(FTIR) and gas chromatography-mass spectrometry(GC-MS) and other characterization methods, the changes of type and quantity of organic matters in the coking wastewater before and after by electro-Fenton treatment were analyzed, and removed kinetics of the organics and improvement of wastewater biodegradability were studied under different influent COD concentrations.The findings will provide theoretical and scientific basis for process design and operation optimization of the pretreatment of coking wastewater by electro-Fenton.On the basis of single-factor experiment, second-order polynomial model of COD removal efficiency of wastewater was established by response surface methodology. The model results showed that significant order of the three factors that affect COD removal were Initial pH value(X3) > Reaction time(X2) > Current density(X1). The optimal reaction conditions were 2 h reaction time, pH 2.26 and 3.17 mA/cm2 of current density achieving the highest COD removal efficiency with 70.02% and the relative error between experimental and prediction results was only1.17%.UV-Vis results showed that concentration of organic compounds of wastewater was decreased gradually with the increase of reaction time in the pretreatment of coking wastewater by electro-Fenton oxidation technology. The content of micromolecule organics in the effluent of 2 h was increased, and humification degree was the lowest, implying the biodegradability of coking wastewater was improved. 3D-EEM results showed that a series of soluble microbial by-product-like, aromatic protein and fulvic acid-like were in the coking wastewater. The fluorescence intensity of Peak A、Peak B and Peak C decreased 27.94%, 17.75% and 26.79%, respectively, achieving the purpose of degradation of organic pollutants from coking wastewater to some degree. FTIR results showed that OH-stretching, C ≡ N stretching vibration, aromatic annularity C=C stretching vibration, C=S stretching vibration and hydrocarbon stretching vibration were discovered both in raw and effluent of coking wastewater, which implied double-bond compounds with unsaturated structure and arometic compounds as well as methyl- and alkyl- compounds existed in coking wastewater. GC-MS analyses showed that most of the organics can be removed or converted by the electro-Fenton oxidation technique. The types of organics were reduced from 23 kinds and 216 species of raw water to 14 kinds and 49 species. In particular, complex molecular structure and refractory organics such as phenols, organic cyanide, polycyclic aromatic hydrocarbons(PAHs), nitrogen-containing heterocycles, oxygen heterocycles and a small amount of esters, alkanes and halogenated organics had good removal efficiencies by electro-Fenton. The degradation process for COD was well fitted third-second kinetics model and the value of B/C ratio from 0.12 to 0.41 was achieved under the best parameter condition.The study demonstrated that electro-Fenton technology was able to effectively reduce concentration and toxicity of organic compounds from coking wastewater and improve the biodegradability of wastewater, which were beneficial to improve oxidative and degraded capability of the subsequent biochemical treatment units. Electro-Fenton process is an efficient, cost-effective and promising pretreatment technology with application prospects. |
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