Effects Of Humic Acids From Bog Soil On The Adsorption And Photocatalytic Degradation Of Oxytetracycline On TiO₂ Nanoparticles In Pastoral Area

TiO₂ nanoparticles（TiO₂ NPs）are wordwidely used nanomaterials.Large amounts of TiO₂ NPs have entered the environment,posing a potential threat to human health and ecosystems.Humic acids（HA）are organic macromolecular compounds widely exist in soil,especially in bog soil in pastoral area.HA can combine with TiO₂ NPs in the environment.Thereby,properties of TiO₂ NPs and interaction mechanisms between coexisting contaminants and TiO₂ NPs can be changed by HA.Oxytetracycline（OTC）is used as a veterinary medicine in pastoral areas,which can be easily adsorbed by soil then stay on the soil surface.In this paper,OTC was used as a representative environmental contaminant to investigate the effect mechanisms of humic acids from pastoral marsh soil on the adsorption and photocatalytic degradation of OTC on TiO₂ NPs.The main results are as follows.（1）HA from bog soil in Zoige plateau were isolated according to IHSS（International Humic Substances Society）method,and the complexes（TiO₂ NPs-HA,THAs）were TiO₂NPs coated with HA.Element,UV-Vis,UV-Vis DRS,FT-IR,XRD,SEM,and EPFRs analyses were conducted to characterize HA and THAs.The results showed that there were block structure and porous sponge structure in HA.In addition,the aromatization and humification degree of HA was high,and the molecular structure of HA was complex.There were nitrogenous compounds（amino acid,aromatic,lipid compounds,etc.）,oxygen-containing functional groups（phenolic hydroxyl,carboxyl functional groups,etc.）,and quinone/semi-quinone persistent free radicals in HA.TiO₂ NPs and HA were chemically bounded.HA enhanced the optical activity in visible region,reduced the band gap,relieved the aggregation,and increased the content of persistent free radicals of TiO₂ NPs.（2）Effects of HA on the adsorption of OTC on TiO₂ NPs were investigated by batch sorption experiments.The results showed that the adsorption capacity of OTC on THAs was lower than that on TiO₂ NPs.In addition,the higher HA content in the THAs was more capable of lowering adsorption capacity of OTC.The equilibrium adsorption capacity of OTC on THAs was reduced from about 3.36μg/mg to 2.26μg/mg when the initial concentration of HA had increased from 100 mg/L to 1000 mg/L during coating,and it was decreased by up to 44.1%compared with that on TiO₂ NPs.The main reason was the competitive adsorption of OTC and HA on TiO₂NPs.Meanwhile,HA increased the adsorption capacity of OTC on TiO₂ NPs during slow adsorption phase（6 h～48 h）,and made the isotherms of OTC on TiO₂ NPs more linear.（3）Effects of HA on the photocatalytic degradation of OTC on TiO₂ NPs were investigated under simulated solar light irradiation.The results showed that OTC could be easily photolyzed especially in alkaline solution.As reflected by the quenching experiments,hydroxyl radicals and superoxide radicals were the active species responsible for OTC degradation.The photocatalytic degradation of OTC was facilitated by THAs when the initial concentration of HA had been 100 mg/L and 250 mg/L during coating,with the degradation rates raised by 5.5%and 9.9%.Owing to narrower band gap,better utilization of visible light and lighter aggregation of THAs.However,Owing to the yield of superoxide radicals decreased,the photocatalytic degradation of OTC was restrained by THAs when the initial concentration of HA had been 500 mg/L and 1000 mg/L during coating.In this paper,the effects of HA from bog soil on the adsorption and photocatalytic degradation of OTC on TiO₂ NPs in pastoral area were investigated systematically,and the actions of HA on the adsorption and photocatalytic degradation of OTC on TiO₂ NPs were clarified.Overall,methodological and theoretical support for studies about the interaction between nanoparticles coated soil organic matter and contaminants were provided in this paper.