Functional Modification And Application Of PAA-based Hydrogel Composites

Polyacrylic acid(PAA)based hydrogel,is synthesized with acrylic acid(AA)as the main monomer and crosslinked into a network after graft polymerization.It is a kind of common synthetic organic polymer hydrogel with abundant functional groups and stable network structure.Traditional PAA-based hydrogels have the characteristics of hydrophilic ionic hydrogels due to their composition and structure,such as good pH responsiveness,ion responsiveness,water absorption and adsorption properties.How to functionalize PAA-based hydrogels on this basis,improve and enrich the material properties,and expand its application is the research focus of this paper.A variety of organic polymers and inorganic nanomaterials can participate in the composite modification of PAA-based hydrogels through graft copolymerization or in situ precipitation.Based on the molecular design of materials,and taking full advantage of the structure and performance advantages of PAA-based hydrogels,different components introduced in appropriate ways often exhibit significantly synergistic effects in the composites.In this paper,according to the current research progress,organic(feather keratin CFP)and inorganic components(g-C3N4,GO,Fe0,CdS)were introduced into PAA hydrogel matrix to conduct multi-functional modification.Through sample characterization and performance test,the application of PAA-based hydrogel composites in slow-release of fertilizer,degradation of antibiotics and adsorption of heavy metals in water were explored.This research was mainly carried out from the following aspects:(1)The CFP-PAA/PVA/NP semi IPN as nitrogen and phosphorus slow-release materials with good water retention and biodegradability were prepared by introducing feather keratin CFP through graft polymerization with AA,semi-interpenetration with linear polyvinyl alcohol and addition of urea and dipotassium hydrogen phosphate.According to the Box Behnken model of response surface methodology(RSM),the optimal preparation conditions were determined through ANOVA based on the response values-swelling capacity:ND=68.69%,mAA:mCFP=1 1.74,mMBA:mAA=0.32%and mPVA:mCFP=2.15.The characterization results confirmed the porous network structure.It can be concluded that nitrogen and phosphorous have been introduced into the polymer network through condensation reaction,electrostatic adsorption and embedding.The N and P slow-release kinetic studies in soil showed that the release process of both of them conformed to the Ritger-Peppas release model.And the release mechanism can be explained as the combination of Fickian diffusion and Case II diffusion.The slow-release of N and P could be affected by sample particle size,soil moisture and soil salt content.The larger the particle size,the lower the water content and the higher the salt content resulted in the lower the release rate.The soil solution pH brought different impacts on the release of N and P:low pH was beneficial to the release of nitrogen while hindered the release of phosphorous.The slow-release of N\P could be achieved with the aid of pH and ion responses.In addition,the results of biodegradation experiment confirmed that the material were biodegradable.Aspergillus Niger culture experiment confirmed that the samples could provide carbon source for the growth and reproduction of mold,so as to achieve the degradation caused by the collapse of the microstructure.After 120 days of soil burial,the weightlessness rate of the tested samples reached over 60%.Finally,the important role of the composite in soil water retention was confirmed by comparing the evaporation rates of soil with and without the addition of CFP-PAA/PVA/NP semi IPN.(2)The PAA/g-C3N4-Fe0 was prepared by graft polymerization of g-C3N4 nanosheets into PAA hydrogel matrix,and then Fe0 was introduced by in-situ precipitation method,which could be used as a photocatalyst to remove sulfadimidine(SM2)from wastewater under visible light.Taking the degradation efficiency as the evaluation index,the dosage of the main components,g-C3N4 dispersion(C=1 g/L)and Fe2+,were optimized as follows:V(g-C3N4)=10 mL,C(Fe2+)=400 mg/L.According to the experimental results,the photocatalytic performance of PAA/g-C3N4-Fe0 was closely related to the contribution of each component in the composite:PAA-based hydrogels promoted the adsorption of SM2 and thus enhanced the photocatalytic degradation rate of SM2;PAA-based hydrogels promoted the uniform distribution of g-C3N4 and Fe0 nanomaterials;as a photoresponsive component,g-C3N4 generated photoelectrons and holes under visible light;the Fe0 nanoparticlees effectively inhibited the photon-hole recombination and promoted the generation of free radicals.According to the investigation of degradation mechanism,·O2-generated by photoelectrons could further produce OH through a series of reactions,and then both of them participated in the degradation of SM2.Several solution parameters were investigated,including solution pH,anions(Cl-,NO3-,HCO3-,H2PO4-)and humic acid(HA).The results showed that the highest degradation rate(80%)of SM2 was obtained at solution pH=4.Cl-and NO3-had little effect on the degradation of SM2,while the ionic strength of HCO3-and H2PO4-showed obvious negative effects on the degradation process.Low concentration of humic acid promoted the SM2 degradation,while high concentration of humic acid showed inhibition.According to the DFT calculation results based on B3LYP/6-31 1G(d,p)basis,it can be concluded that N-11 atom in SM2 with the maximum f0 was extremely vulnerable to free radical attack.The recycle experiment proved that the material showed excellent stability and can be efficiently reused through in-situ reduction.(3)The GO/PAA hydrogel composite was prepared by graft polymerization of GO and AA,which can be used as highly efficient adsorbent for Cd2+removal from water.Taking the equilibrium adsorption capacity as the evaluation standard,the optimal concentration of GO dispersion and the valume of AA were determined as follows:cGo=2.0 mg/L and VAA=10 mL.The maximum adsorption capacity of GO/PAA-2-10 samples prepared under the optimal synthesis conditions was up to 302.6 mg/g at 20?.The results indicated that the adsorption process could be well fitted with pseudo-second-order kinetics model and the intra-particle diffusion model,and the Langmuir isotherm model could better depict the adsorption process.Combined with the characterization results,it was inferred that the adsorption mechanism was the chelation coordination between oxygen-containing functional groups in the polymer(-COO-)and Cd2+.The solution pH has a significant effect on the adsorption,and the highest adsorption capacity was obtained at pH=6-7.Meanwhile,we focused on the reuse of adsorbents:GO/PAA-CdS was prepared through in-situ precipitation of S2-which can effectively decolorize and degrade methylene blue(MB)by photocatalysis.Although GO contributed less to the adsorption of Cd2+ compared with PAA compound,it could significantly improve the photocatalytic performance after being reused,which was speculated to be mainly due to the effective transport of photogenerated electrons along GO nanosheets reducing photocorrosion.(4)The GO/PAA-CdS was prepared by graft polymerization of GO nanosheets into PAA hydrogel matrix,and then CdS was introduced by in-situ precipitation method,which can be used as photocatalyst to remove chlortetracycline(CTC)from wastewater under visible light.Taking the degradation efficiency as the evaluation index,the optimal preparation conditions of CTC were confirmed as follows:cGo=2 g/L,nCd:ns=1:2,mMBA:mAA=0.4%.Through the comparison and analysis of sample characterization and kinetic fitting studies,it can be concluded that the excellent photocatalytic performance of GO/PAA-CdS was inseparable from the cooperation among the main components:PAA-based hydrogels promoted the adsorption of CTC and thus enhanced the photocatalytic degradation rate of CTC;PAA-based hydrogels promoted the uniform distribution of GO and CdS nanomaterials;as a photoresponsive component,CdS generated photoelectrons and holes under visible light;the GO nanosheets effectively inhibited the photon-hole recombination and promoted the generation of free radicals.When the solution pH value was sat at 6-7,the highest removal rate of CTC could reach 85%.According to the DFT calculation results based on B3LYP/6-31G(d)basis,the f0 value of C1-25 and O-24 on CTC molecules ranked the top two,indicating the vulnerable sites for free radical attack.In addition,GO/PAA-CdS showed good stability and remained highly active after ten cycles.