Degradation Of CV In Aqueous Solution By Non-thermal Plasma/BiPO₄ And Non-thermal Plasma/persulfate

Crystal violet ?CV?, a typical type of triphenylmethane dye, has been widely used in the textile, paper, leather, cosmetics and pharmaceutical industries. CV is easily dissolved in water during use, and large amounts of dye-contaminated effluents are produced. The presence of CV in drinking water poses a serious threat to human health because CV exhibits toxicity to mammalian cells as a mutagen and mitotic poison.Therefore, it is critical to develop effective methods that can remove CV from aqueous solutions prior to discharge to the environment. Dielectric barrier discharge ?DBD? is one of the most common ways to produce non-thermal plasma. There are many different kinds of active species existing in DBD-induced non-thermal plasma. Compared with usual chemical reaction, they are more active, easier to react with the organic contaminants. The non-thermal plasma has the advantages of high efficiency, simple operation and wide range of application compared with the traditional method. In this paper, the degradation of CV in water was studied by using non-thermal plasma combined with BiPO₄, and non-thermal plasma combined with Fe²⁺ activated persulfate,respectively. The main operating conditions of CV degradation were investigated. The mechanism of CV synergetic degradation process was proposed. At the same time, the artificial neural network models of non-thermal plasma combined with BiPO₄, and non-thermal plasma combined with Fe²⁺ activated persulfate were established,respectively. The degradation efficiency of CV under continuous changed multi-factors was simulated.Firstly,degradation of crystal violet ?CV? in aqueous solution by non-thermal plasma combined with BiPO₄ was investigated. BiPO₄ was synthesized by the hydro-thermal method and characterized by X-ray diffraction, X-ray fluorescence, scanning electron microscopy and Brunauer-Emmett-Teller surface area. With the addition of BiPO₄ in the non-thermal plasma system,a 28% enhancement in the CV degradation rate was observed compared with the single non-thermal plasma system. The effects of discharge power, initial concentration of CV, initial pH and air flow rate on the degradation of CV were reported. The degradation of CV by non-thermal plasma and BiPO₄ followed pseudo-first-order reaction kinetics. The concentrations of generated H₂O₂, O₃ and OH in the non-thermal plasma combined with the BiPO₄ system increased with increasing discharge time, which eventually achieved 0.43 mg/L, 1.52 mg/L and 5.2×10^-6-6 mol/L in CV solution, respectively. In the non-thermal plasma combined with the BiPO₄ system,the absorbance of the aqueous solution at 584 nm declined from 0.63 to 0.02, the pH values of the aqueous CV solution decreased from 3.55 to 2.93, and the conductivity of the CV aqueous solution steadily increased from 10^-62 to 203 ?S/cm. Good stability of the synthetic BiPO₄ was demonstrated. Gas chromatography-mass spectrometry was used to determine the degradation intermediates of CV, and the degradation pathway of CV was proposed according to the detected intermediates, which were produced mainly by conjugated structure destruction, demethylation, hydroxylation, carboxylation and the ringopening reactions. The acute toxicity of the treated CV solution on luminescent bacteria exhibited a stably declining trend with increasing discharge time.Secondly, degradation of CV in aqueous solution by non-thermal plasma combined with Fe²⁺ activated persulfate was investigated. Compared with single Fe²⁺ activated persulfate systerm and single non-thermal plasma systerm, 54% and 42% enhancement were observed in the non-thermal plasma combined with Fe²⁺ activated persulfate systerm, respectively. The effects of discharge power, initial pH, persulfate concentration and Fe²⁺ addition on the degradation of CV were reported. The degradation of CV by non-thermal plasma and Fe²⁺ activated persulfate followed pseudo-first-order reaction kinetics. The results showed that the highest degradation efficiency were obtained when the discharge power was 40 W, neutral pH and the ratio of persulfate to Fe²⁺ concentration was 4:3. The concentrations of generated H₂O₂, O₃ and ·OH in the non-thermal plasma combined with Fe²⁺ activated persulfate system achieved 0.36 mg/L, 1.36 mg/L and 3.2×10^-6-6 mol/L at the first minute, respectively; and they could not be detected at the tenth minute. In the non-thermal plasma combined with the Fell activated persulfate system, the absorbance of the aqueous solution at 584 nm declined from 1.97 to 0.11. The pH value and TOC decreased with increasing discharge time. Intermediate products and the degradation mechanism of CV were proposed according to the analysis of GC-MS. The acute toxicity of the solution showed a decreasing trend as a whole with the reaction time during CV degradation.Finally, the artificial neural network models of non-thermal plasma combined with BiPO₄ and non-thermal plasma combined with Fe²⁺ activated persulfate were established, respectively. The experimental data were used as training samples and testing samples. The trainlm algorithm in artificial neural network BP model was choosen to train. The comparison between the calculated values and the experimental values showed that the maximum error of the two models was less than ±5%. The ANN model could simulate the CV degradation process in the two degradation ways effectively. The discharge power was the most important operating condition in the process of non-thermal plasma combined with BiPO₄. In the process of non-thermal plasma combined with Fe²⁺ activated persulfate degradation, pH value had the greatest effect on degradation efficiency of CV.