Catalytic Degradation Of Polyvinyl Alcohol By Transitional Metal Oxides/nano ?-Al₂O₃ Composites

The water-soluble polymer polyvinyl alcohol?PVA?is widely used in chemical,textile and paper manufacture with rich hydroxyl groups as polymerization aid,slurry,coating,etc.Besides,PVA fibers show a prospect in disposable clothing manufacture for better barrier property,acid or alkali resistance and degradable ability,when compared with other chemical fibers such as polyester.However,the PVA wastewater or PVA waste fabrics,produced by the production and utilization of various water-soluble PVA products will be a serious potential threat to environment,if not treated properly before emission.It seems feasible to treat PVA pollutants with a pretreatment of advanced oxidation in aqueous solutions,which could promote the subsequent biochemical process and hence level up the efficiency of the whole treating procedure.In this study,Mn_xO_y/?-Al₂O₃,Cu_x O_y/?-Al₂O₃,Fe_xO_y/?-Al₂O₃ and Fe_xCu_y Mn_zO_w/?-Al₂O₃particles were firstly prepared by a wet impregnation-calcination method and characterized by X-ray diffraction?XRD?,N₂ adsorption-desorption,scanning electron microscope?SEM?and Energy Dispersive Spectrometer?EDS?.Results showed that the crystalline phase of?-Al₂O₃was almost intact after the doping of mental oxides via impregnation and calcination.Besides,the accumulation and agglomeration of these mental reactive oxides on?-Al₂O₃ would change the surface morphology of?-Al₂O₃,which altered from a mesoporous structure with dense holes to a relatively rougher one.It was also noted that the mesoporous structure of virgin?-Al₂O₃ was kept intact after the loading of mental oxides,with only a slight decrease in specific surface area,pore volume and pore size.The most suitable calcination temperature of catalyst mentioned above in this paper was 450°C,which would promote the excellent distribution of reactive sites and inhibit the significant agglomeration of internal voids brought by higher calcination temperature.Besides,the doped mental oxides could be steadily bound with?-Al₂O₃ by chemical bonds with an appropriate calcination at 450°C,when the mesoporous structure of support was well maintained at the same time.Here the BBD response surface optimization method was also adopted to investigate the degradation of water-soluble PVA fabrics in aqueous solution,under the circumstance of four prepared catalyst and H₂O₂.And it turned out that there was a significant linear relationship between the PVA removal efficiency and the three independent variables,which including reaction temperature,H₂O₂ dosage and catalyst dosage.In addition,the influence of three independent variables on the removal of PVA could be ranked as follows:reaction temperature>H₂O₂ dosage>catalyst dosage.And there was also an interaction effect from H₂O₂ and catalyst dosage on the the removal of PVA.In this paper,the effect of initial solution pH,H₂O₂ dosage,catalyst dosage and initial PVA concentration on the removal of PVA were studied respectively.The reusability of four prepared particles was also studied.Results showed that the initial solution pH could obviously effect the performance of Mn_x O_y/?-Al₂O₃,which meant the degradation of PVA would be considerably inhibited when the initial pH was above 3.On the other hand,the catalytic abilities of Fe_xO_y/?-Al₂O₃ and Fe_xMn_yCu_zO_w/?-Al₂O₃ could remove PVA in aqueous solution with high efficiency and reduce the viscosity-average molecular weight of final degradation products from 100773 to below 2000 when the initial pH was from1 to 6.However,it also turned out that when used in neutral conditions,where the initial pH=7,the catalytic performance of Cu_x O_y/?-Al₂O₃ was better than Mn_xO_y/?-Al₂O₃,Fe_xO_y/?-Al₂O₃ and Fe_xMn_yCu_zO_w/?-Al₂O₃.The viscosity-average molecular weight of degradation product could be as low as 1637 when Mn_xO_y/?-Al₂O₃ dose was 1 g/L,while it basically decreasing with the dosage of Cu_xO_y/?-Al₂O₃,Fe_xO_y/?-Al₂O₃ and Fe_xMn_yCu_zO_w/?-Al₂O₃.Results indicated that the increase of H₂O₂ usage would lead to a lower average-viscosity molecular weight of final PVA degradation products,when applying Cu_xO_y/?-Al₂O₃ and Fe_xCu_yMn_zO_w/?-Al₂O₃as catalyst.However,the degradation of PVA was inhibited when the dose of H₂O₂ was up to 240 mL/L,taking Mn_xO_y/?-Al₂O₃ or Fe_xO_y/?-Al₂O₃as catalyst.And this phenomenon ascribed to the scavenging effect of excess H₂O₂ on hydroxyl radicals,which consequently rose up the average-viscosity molecular weight of degradation products.In addition,the average-viscosity molecular weight of final degradation products under the circumstance of different catalysts changed obviously,when the initial concentration of PVA solution raised up.In terms of the final degradation ratio of PVA,the employment of Mn_xO_y/?-Al₂O₃ was proved to be feasible in treating PVA solutions of different initial concentrations,while the catalytic performances of Cu_xO_y/?-Al₂O₃,Fe_xO_y/?-Al₂O₃ and Fe_xCu_yMn_zO_w/?-Al₂O₃ particles were inhibited due to the increase of PVA concentration.Given the PVA solution with concentration below 2.5%,it turned out that the average-viscosity molecular weight of degradation products was below 5000,with a molecule decreasing rate at about 95%,when using Cu_xO_y/?-Al₂O₃,Fe_xO_y/?-Al₂O₃and Fe_xCu_yMn_zO_w/?-Al₂O₃particles as catalysts.It seemed that Mn_xO_y/?-Al₂O₃,Fe₂O₃/?-Al₂O₃ and Fe_xCu_yMn_zO_w/?-Al₂O₃ particles Mn_xO_y/?-Al₂O₃ could be reused and maintained favorable catalytic ability in four times of utilization.Although there was a slight loss of reactive mental oxides from?-Al₂O₃ after several times of stir,filtration and washing processes,which consequently inhibited the catalytic performance of prepared catalysts.The removal ratios of PVA during three utilization was all above 90%,when it was under the circumstance of Fe_xCu_y Mn_zO_w/?-Al₂O₃Particles.And it still reached to about 80%at the forth run.According to the analysis of GC-MS,the mechanism of hydroxyl radicals produced by four kinds of prepared catalysts and H₂O₂ was also discussed.During the utilization of Fe_xCu_yMn_zO_w/?-Al₂O₃,it was noted that Mn²⁺and Cu²⁺would not only react with H₂O₂,directly producing hydroxyl radicals for PVA degradation,but also promote the formation of HO₂^?/O₂^?-,hence accelerating the reduction of Fe³⁺to Fe²⁺and preventing the scavenging effect of hydroxyl radicals on H₂O₂.Aside from that,degradation mechanism of PVA when oxidized by hydroxyl radicals was proposed.As the large amount of hydroxyl radicals produced by prepared catalyst particles successively oxidized the part of high density distribution,it led to a dehydrogennation of PVA molecules as well as a formation of olefinic bonds.With oxidation of olefinic bonds by hydroxyl radicals,the produced intermediates with aldehyde or enol groups could be further oxidized until the formation of carboxyl groups.And therefore,it finally led to the breakage of backbone chains of PVA molecules and unsaturated products with lower molecular weight,such as ketones,aldehydes and carboxylic acids.According to the result in this paper,it made sense to adopt Fe_xCu_yMn_zO_w/?-Al₂O₃particles as catalyst in the process of degradation of PVA.More than 95%of final degradation ratio was achieved at an initial PVA concentration of 10 g/L,reaction temperature of 60 ^oC,initial pH=3,H₂O₂ and catalyst dose of 60 mL/L and 0.5 g/L respectively.Besides,PVA macro-molecule could be oxidized into products of lower viscosity-average molecular weight,with a molecule decreasing rate of nearly 99%,from 100773 to 1395,generating unsaturated products like butanones,benzaldehydes,hexanals and acetic acids.