Adsorption Behavior And Mechanism Of Oxytracycline And Heavy Metals By Nano-hydroxyapatite

In recent decades,Tetracycline antibiotics?TCs?have been extensively used as growth hormone in animal husbandary and prophylactic agent in modern medicine.During the production,application and disposal processes,oxytracycline?OTC?and their secondary metabolite could penetrate into the surrounding water and soil.After that,it could complex with diverse metal ions to form OTC-metal ion pollutants and pose a threat to human health and ecological system.Owning to good solubility and high persistence,it takes more time and cost to remove co-pollutants from wastewater through traditional wastewater treatment.Thus,this paper aims to develop a green functional material that could efficiently remove heavy metals and antibiotics in aqueous solution.Nano-hydroxyapatite?Ca₅?PO₄?₃OH,nHAP?,a secondary metallic mineral,has been applied as mature stabilizer in the field of pollution control of heavy metals.Based on the complexation between metal ions and OTC,it is unknown whether nHAP has the potentiality to simultaneously treat these co-pollutants in pollution issue.This paper not only studied the influences of external factors?pH,co-existing and temperature?on OTC adnsrotion onto nHAP,but also discussed the complexation mechanism between OTC and metal ions.Meanwhile,the co-adsorption behavior and mechanism of OTC and heavy metals onto nHAP was also explored.The results show that pH and metal ions have regulatory effects on nHAP adsorption for OTC.The amount of sorbed OTC under four pH values were in the order of pH 8.0>pH 10.0>pH 5.5>pH 3.0,implying that adsorption affinity of these species onto nHAP followed the order of OTC^->OTC^2->OTC^±>OTC⁺.At pH 8.0,the maximum Freundlich parameter K_f was 139.38 mmol^1-nLⁿ/kg.When metal ions presented in the system,the distribution coefficient?K_d?of OTC increased,following the order of Fe³⁺>Cu²⁺>Pb²⁺>Cd²⁺=Ca²⁺=Na⁺,because metal ions could interact with OTC and nHAP to form ternary complex via bridging effect.In OTC-Cu²⁺systems,OTC adsorption was significantly promoted with the co-existence of 0.25mmol/L Cu²⁺,while Cu²⁺adsorption was slightly enhanced with the co-existence of 0.25mmol/L OTC.In OTC-Pb²⁺systems,the co-existence of 0.25 mmol/L Pb²⁺also led to the improvement of OTC adsorption,conversely,Pb²⁺adsorption was depressed with the co-existence of 0.25 mmol/L OTC while Pb²⁺adsorption was enhanced with the co-existence of0.1 mmol/L OTC.Furthermore,co-adsorption behaviors of OTC and Cd²⁺showed unapparent variation in the binary systems,resulting from the smaller complexing constant of them.Finally,the reaction mechanisms among nHAP,metal ions and OTC were analyzed by XPS,FTIR and XRD.The bridging effect involving metal ions,N-and O-containing groups in OTC molecules,and?CaOH or?POH sites of nHAP resulted in the formation of ternary complexes which were responsible for the promotion of their adsorption,while dissolution-precipitation was another key mechanism with the co-existence of Pb²⁺.