Study On Degradation Of Pva In Desizing Wastewater Of Dyeing Industry

Polyvinyl alcohol(PVA) is a kind of water-soluble synthetic polymer. It has been widely used in the textile as a sizing reagent because of its high water-solubility, film-forming and adhesiveness. But it is diffcult to be degraded by common microorganism. So the desizing wastewater of dyeing industry containing massive PVA results in seriously environment pollution. The treatment result is unsatisfactory by using traditional biochemical process, since PVA is non-biodegradable. On the other hand, convenient physicochemical technology only contributes a little to remove PVA in the wastewater. In this dissertation, PVA was treated by several Advanced Oxidation Technologies. The main factors influencing PVA degradation were studied in detail and liquid degradation products of SCWO were analyzed by GC/MS. The reaction path and mechanism of PVA degradation in SCWO processes were evaluated. The results obtained are shown as follows.(1) PVA was treated by analogous Fenton reagent respectively with Fe²⁺, Fe³⁺, Cu²⁺, Mn²⁺, Zn₂₊ or Ni²⁺ as catalyst and 30% H₂O₂ as oxidation agent. Effects of initial pH of PVA solution, reaction temperature, the quantity of metal ions and H₂O₂ and reaction time were studied and an optimal operation condition was obtained.(2) Heterogeneous catalytic degradation of PVA solution in the presence of H₂O₂ with Mn₂O₃ as catalysts. Effects of initial pH of PVA solution, reaction temperature, the quantity of catalyst and H₂O₂ and reaction time were studied respectively The experimental results indicated that this was a practicable catalytic system.(3) Degradation of PVA solution with an initial COD_Cr = 9183.33mg/L was performed in an intermittent flow reactor of SCWO and influential factors including reaction temperature, reaction pressure, reaction time and oxygen excess degree on COD elimination of PVA were studied. Higher temperature,pressure, oxygen excess degree and longer reaction time can promote COD elimination. The optimal operation condition was obtained by an orthogonal test. 99.03% COD removal was obtained when the temperature was 440℃, pressure was 28MPa, n=4 and reaction time was 40min, COD of the effluent were COD_Cr=89.09mg/L less than the national discharge standard. Among these factors, the temperature has greater influence on the COD elimination and oxygen excess degree,reaction time and pressure's influence were weakest comparatively.(4)The products of PVA degradation by SCWO under different conditions were analyzed and identified systematically by GC-MS. The possible reaction mechanism of PVA degradation reaction in SCWO process were evaluated. Firstly, PVA molecule was collisioned by·OH, the long-chain of PVA was broken and C=C bond was formed, and then some olefine, ketone and organic acid were formed. Finally, these molecules were completely degraded to CO₂, H₂O and other smaller molecules.