关键词: 聚乙烯醇, 六价铬, 过硫酸盐, 印染废水, 协同处理

Abstract:

Objective Polyvinyl alcohol (PVA), a major sizing agent used in textile weaving, is eluted in the pretreatment of textile wet processing and becomes the main source of organic pollutants in textile wastewater. Meanwhile, textile industries sometimes adopt chromium-containing developers for the printing screen, leading to Cr(VI) pollution in the textile wastewater. The simultaneous treatment technology of high concentration of PVA and highly toxic Cr(VI) in printing and dyeing wastewater is of great concern. Persulfates can initiate the free radical crosslinking of the polymeric chain, which is considered as one of the solutions for the crosslinking-induced precipitation of PVA. Additionally, under high temperature and acidic conditions, PVA can reduce Cr(VI) to Cr(III) which is less toxic. Thus, in the thermally activated persulfate system, with the rapid precipitation of high concentration of PVA, the effective reduction of Cr(VI) becomes possible.

Method To achieve the simultaneous removal of PVA and Cr(VI) from wastewater, persulfate (K₂S₂O₈) was chosen as free radical crossling initiator. Simulative wastewater containing the high concentrations of PVA (10 g/L) and the coexisting Cr(VI) was the object of treatment. The thermal activation was used to activate K₂S₂O₈ because desizing wastewater is often processed at high temperatures (70-80 °C) and Cr(VI) is more easily reduced under high temperature. The remoal performance of PVA and chemical oxygen demand(COD) and the reductive efficiency of Cr(VI) were studied, with specific attention paid to the performance of free radical-induced crosslinking of PVA and the production of free radicals and explored the critical factors controlling their efficacy by electron spin-resonance spectroscopy (ESR). In addition, the possible underlying mechanism was studied using X-ray photoelectron spectroscopy (XPS), gel permeation chromatography (GPC), and gas chromatography-mass spectrophotometry (GC-MS).

Results The K₂S₂O₈ dosage, reaction temperature, and pH value of the solution were the key factors affecting the removal efficiency of PVA and Cr(VI). When the concentration of K₂S₂O₈ was 8.0 g/L, the reaction temperature was 70 °C and the pH value of wastewater was less than 6, the COD value of simulated printing and dyeing wastewater were reduced from 18 000 to 1 458 mg/L with a removal rate of 91.9%, the removal rate of PVA could reach 98.0% and the reduction rate of Cr(VI) was 94.3%. As a crosslinking agent, K₂S₂O₈ can induce the generation of PVA carbon radicals and promote the effective crosslinking. Meanwhile, the oxidation of —OH groups of the PVA polymer could enhance the production of $\mathrm{O}_{2}^{.-}$ radicals. This property which is analogous to catalyst could effectively promote the crosslinking of PVA. In addition to the reduction property of PVA itself, the reactive oxygen species(ROS) such as $\mathrm{O}_{2}^{.-}$ produced from the process of inducing radical cross-linking of PVA can also reduce Cr(VI) to Cr(III).

Conclusion Based on the free radical cross-linking technologies for polymers, a synergistic treatment technique for PVA and Cr(VI) pollutants removal using thermally activated persulfate system was proposed. The radical induced cross-linking and the reducibility of PVA under high temperature is found to be the main reason for efficient precipitation of PVA and synergistic reduction of Cr(VI), which has a certain application prospect in the treatment of textile wastewater. The advantages of this technology are as follows. 1) The thermally activated K₂S₂O₈ can efficiently induce the radical based crosslinking of PVA and promoted the precipitation. 2) As a strong oxidant, K₂S₂O₈ can oxidize the hydroxyl groups in PVA to promote the crosslinking efficiency. 3) The oxidation of —OH groups of the PVA polymer can enhance the production of ROS, which facilitates the simultaneous reduction of coexisting Cr(VI). Under optimal conditions, the maximum removal efficiencies of PVA and COD reached 98.0% and 91.9%, and the reduction rate of Cr(VI) was 94.3%. This process is technically highly efficient and cost effective, and provides new insights for the simultaneous treatment technology of high concentration of PVA and highly toxic Cr(VI) in printing and dyeing wastewater.

Key words: polyvinyl alcohol, Cr(VI), persulfate, dyeing and printing wastewater, synergistic removal