Study On Construction Of Polyvinyl Alcohol Based Three-Dimensional Porous Hydrogel And Its Application In Catalytic Degradation Of Tetracycline

As the representative of antibiotics in water environment,the residues and pollution of tetracycline（TC）pose a serious threat to the ecological environment and human health.It is urgent to develop a new material which can respond to visible light,be recycled and recovered easily to purify the residual TC in water.Therefore,a three-dimensional polyvinyl alcohol-based hydrogel with visible light induced activity is proposed.Polyvinyl alcohol,sodium alginate and semiconductor catalyst bismuth tungstate were utilized to construct polyvinyl alcohol/sodium alginate-bismuth tungstate hydrogel（PVA/SA-BWO）through physical crosslinking method.Polyvinyl alcohol/sodium alginate hydrogel was constructed by physical crosslinking using the hydroxyl of polyvinyl alcohol molecules and hydroxyl and carboxyl groups of sodium alginate molecules,providing good mechanical properties of the materials.The photoelectron hole generated by BWO under photoexcitation and redox reaction could provide the material the ability to degrade TC;Boric acid and ferric chloride were introduced into polyvinyl alcohol and sodium alginate to construct polyvinyl alcohol/sodium alginate-ferric chloride hydrogel（PVA/SA-FeCl₃）.Polyvinyl alcohol was reacted with boric acid through chemical crosslinking and sodium alginate was complexed with trivalent metal ferric ions by an ion exchange.Hydrogen peroxide（H₂O₂）was involved into the catalytic process and TC was purified through the ability of Fe³⁺and H₂O₂ producing strong oxidizing hydroxyl radical（·OH）;On the basis of the above step,tannic acid was introduced and through the hydrogen bonding effect between tannic acid and polyvinyl alcohol and the complexation of tannic acid and ferric ions,the polyvinyl alcohol/tannic acid-hydroxyl iron oxide hydrogel（PVA/TA-β-FeOOH）was constructed by in-situ preparation to improve the stability of the material.The main research contents and results are as follows:（1）A series of PVA/SA-BWO hydrogels were constructed through freezing-thawing method using PVA and SA as substrate and BWO as photocatalyst.The mechanical properties,structure and morphology of the hydrogel were characterized by compression test,X ray diffraction（XRD）analysis,scanning electron microscopy（SEM）observation,X ray photoelectron spectroscopy（XPS）analysis and ultraviolet visible diffuse reflectance spectroscopy（UV-vis）technology.The results show that the constructed hydrogel has a compressive strength of 2.2 MPa,possessing three-dimensional porous network structure and the band gap of the hydrogel is 2.55 e V which can respond to visible light below 485 nm.The effect of BWO content on the degradation of TC was investigated and the possible degradation pathway of TC was analyzed according to the TC degradation intermediates.The results show that the degradation efficiency of TC over the PVA/SA-BWO hydrogel increases first and then decreases with the rise of BWO content.When the mass ratio of PVA and BWO is 1:2,the PVA/SA-BWO hydrogel exhibits the optimum photocatalytic degradation ability,which the degradation rate of TC is 91.0%and the mineralization rate is44.70%.·O₂^-is the main active species during the process of photocatalytic degradation.And TC forms small molecules gradually through the chain breakage and ring opening reaction.The results of cyclic experiment show that the degradation rate of TC over the PVA/SA-BWO hydrogel remains about 80%after five repeated use,and the porous and crystal structure of the PVA/SA-BWO hydrogel remains intact.These results lay a good foundation for the recycling of the materials.（2）Polyvinyl alcohol/sodium alginate-ferric chloride hydrogel（PVA/SA-FeCl₃）was constructed by chemical crosslinking method using boric acid and ferric chloride as crosslinking agents.The morphology,structure,chemical composition and optical properties of PVA/SA-FeCl₃ hydrogel were analyzed by means of high-speed stirring at room temperature,SEM,Fu Liye transform infrared spectroscopy（FT-IR）,XPS and UV-vis techniques.The results show that the constructed PVA/SA-FeCl₃hydrogel has good mechanical property（the retention rate of hydrogel remains 94%after stirring for 15 h under the condition of 2000 r/min rate）,shows the porous structure（average pore size of about 1.16μm）and possesses a light response in the range of 200～500 nm.The effect of different H₂O₂ dosage on the photo-Fenton degradation of TC over PVA/SA-FeCl₃ hydrogel during the process was investigated.The results show that the degradation of TC over the hydrogel increases first and then decreases when H₂O₂ concentration increases from 2 m M to 10 m M.When the H₂O₂concentration is 6 m M,the degradation rate and the mineralization efficiency of TC over the PVA/SA-FeCl₃ hydrogel are 90.5%and 28.6%,respectively,after 60 min illumination.The results of active species capture and electron paramagnetic spectrum tests（EPR）show that·OH,h⁺and·O₂^-are all involved in the degradation of TC.The results of cyclic tests show that the PVA/SA-FeCl₃ hydrogel could degrade TC during reusing five times.However,the PVA/SA-FeCl₃ hydrogel generates a great amount of loss of ferric ions,which needs to be solved.（3）In order to solve the problem of the loss of ferric ions,polyvinyl alcohol/tannic acid-ferric hydroxide hydrogel（PVA/TA-β-FeOOH）was constructed through the in-situ preparation ofβ-FeOOH utilizing the hydrogen bonding effect of tannic acid and polyvinyl alcohol,the complexation of tannic acid and ferric ions.The morphology,structure and optical properties of the hydrogels were analyzed by SEM,FT-IR,XRD,XPS,nitrogen adsorption desorption curves and UV-vis technology.The results show that the PVA/TA-β-FeOOH hydrogel has porous structure and theβ-FeOOH particles are dispersed evenly on the hydrogel matrix.The specific surface area of the hydrogel increases from 16.8 m²g^-1 to 45.8 m²g^-1 after the introduction ofβ-FeOOH.The effect of PVA/TA-β-FeOOH hydrogel constructed in different reaction time on the photo-Fenton catalytic degradation of TC.The results show that photo-Fenton catalytic degradation rate of TC over the PVA/TA-β-FeOOH hydrogel constructed under the reaction of 12 h is the fastest(0.145 min^-1)and the degradation efficiency of TC over the PVA/TA-β-FeOOH hydrogel is better than that of TC overβ-FeOOH powder.The degradation rate and mineralization efficiency of TC over the PVA/TA-β-FeOOH hydrogel are 85.5%and 42.3%,respectively.·OH,h⁺and·O₂^-are all the active components in the photo-Fenton catalytic process.After reusing five times for PVA/TA-β-FeOOH,the loss of ferric ions is reduced by～19%compared with PVA/SA-FeCl₃ hydrogel,indicating that the PVA/TA-β-FeOOH hydrogel shows good stability,which provides a new way to solve the massive loss of ferric ions during the photo-Fenton process.The result of photo-Fenton catalytic degradation in dynamic system shows that PVA/TA-β-FeOOH hydrogel exhibits good degradation efficiency of TC through the synergistic effect of adsorption and photo-Fenton.And the degradation rate of TC over PVA/TA-β-FeOOH hydrogel is 41.2%.