Study On Adsorption Of Sulfamethoxazole In Water By G-C₃N₄ Composite Chitosan-based Hydrogel

The wide application of pharmaceuticals and personal care products（PPCPs）makes them continuously detected in the water environment,posing a threat to the aquatic environment and human health.The adsorption method is simple,high-efficiency,low-cost and recyclable,which is favored by researchers.In this paper,chitosan-based hydrogel beads were used as adsorbents to remove sulfamethoxazole（SMX）in water.In view of the difficulty of separating the adsorbent in the water and the unknown safety performance,the preparation of chitosan-based hydrogel microspheres was simple and easy to control,and the adsorbent was easy to separate from water.At the same time,chitosan had good biocompatibility and biodegradability.However,the adsorption performance and mechanical strength of chitosan were still insufficient.In this paper,polyvinyl alcohol（PVA）was selected as the blended polymer of CS to prepare CS/PVA hydrogels,where PVA could improve the mechanical stability and at the same time improve the selective adsorption performance of the material by introducing additional hydroxyl functional groups.In order to further solve the problem of high cost and secondary pollution caused by the desorption and regeneration of existing adsorbents using alkali,alcohol,etc.,g-C₃N₄ was introduced to prepare g-C₃N₄/CS/PVA hydrogel,which was degraded by photodegradation to achieve regeneration.The two prepared adsorbents were characterized and used to adsorb sulfamethoxazole（SMX）.First,the influences of some operational conditions and water matrices including p H,adsorbent dosages,and ionic strength were investigated.Then the adsorption kinetics were disclosed,and the adsorption mechanism was further analyzed based on adsorption kinetics,adsorption isotherms and thermodynamics.Furthermore,the application potential of the g-C₃N₄/CS/PVA hydrogel bead on the removal of SMX was evaluated through studying its reusability by photodegradation.The main conclusions were as follows:（1）The synthesized CS/PVA hydrogel was a mesoporous adsorption material with a particle diameter of about 3 mm.The maximum adsorption capacity was 8.23 mg/g when the p H was 6.The adsorption force of the hydrogel on SMX was that the-OH,-NH₂ on the hydrogel interact with the-NH₂ or H-bonding acceptors（N and O）on the SMX to form a hydrogen bond and the electrostatic attraction between the positively charged hydrogel and the anionic form of SMX.The low concentration of humic acid（HA）could promote the adsorption of SMX on the CS/PVA hydrogel,and the high concentration would inhibit the adsorption of SMX and reduced the adsorption capacity.The adsorption capacity of CS/PVA hydrogel on SMX gradually decreased with the increase of the concentration of competing anions.Among them,SO₄^2-had the greatest competitive influence on the adsorbent.The order of the influence of the four coexisting anions on the adsorption capacity of SMX was roughly SO₄^2->HCO₃^->NO₃^->Cl^-.The adsorption kinetics followed the pseudo-second-order（PSO）model and the intraparticle diffusion （IPD）model,indicating that IPD model was a rate-limiting step in the adsorption process.Therefore,increasing the porosity of the hydrogel might be beneficial to increase the adsorption rate of SMX.At three temperatures of 25,35,and 45℃,the Sips model could fit the data well,and the adsorption thermodynamics showed that the adsorption followed the self-discharge heat process.（2）The synthesized g-C₃N₄/CS/PVA hydrogel was a small sphere with a particle diameter of about 3 mm.It was a mesoporous adsorption material.When the p H was 5,the adsorption capacity reached the maximum value of 5.8 mg/g.The adsorption force of hydrogel to SMX were hydrogen bonding and electrostatic attraction.As the concentration of competing anions increased,the adsorption capacity of the hydrogel on SMX gradually decreased.Among them,SO₄^2-had the greatest competitive impact on the adsorbent.The order of influence of four kinds of coexisting anions on the adsorption capacity was roughly SO₄^2->HCO₃^->NO₃^->Cl^-.The adsorption capacity of g-C₃N₄/CS/PVA hydrogel to SMX increased firstly and then decreased with the increase of HA concentration.The adsorption kinetics followed PSO model and IPD model,showing that IPD model was one of the rate-limiting steps of the adsorption process.At 25,35 and 45℃,Sips model fitted the data well.The adsorption thermodynamics showed that the adsorption followed a spontaneous exothermic process.（3）g-C₃N₄/CS/PVA hydrogel was regenerated by ultraviolet radiation for 5 cycles.After the first and second cycles,the adsorption capacity of SMX was reduced to 72% and 66% of the initial adsorption capacity,respectively.Starting from the third cycle,the adsorption capacity of the subsequent regeneration cycle tended to be stable,about 55%of the initial adsorption capacity,indicating that the adsorbent had a certain degree of reusability and would not cause secondary pollution.After regeneration,the adsorption capacity was reduced because the internal pores of the hydrogel beads were not straight channels through the pellets,and light could not enter the interior of the beads.Therefore,the contaminants adsorbed inside the hydrogel could not be degraded by UV radiation,and were still adsorbed on the hydrogel,resulting in reduced adsorption sites and reduced adsorption capacity of the regenerated hydrogel.