Sorption Behavior Of Organophosphate Flame Retardants On Montmorillonite And Kaolinite

Organophosphorus flame retardants（OPFRs）,a new kind of environmental persistent organic pollutants,have been detected in a variety of environmental media.The interfacial behavior of OPFRs on soil greatly affects their environmental fate,while studies on sorption behavior are still limited.Triphenylphosphine oxide（TPPO）and triphenyl phosphate（TPhP）are selected to explore their sorption behaviors on clay minerals before and after modification.The effects of pH,inorganic ions,natural organic matter（NOM）,and temperature on sorption of OPFRs are investigated,and sorption mechanisms are also discussed.Sorption of OPFRs on montmorillonite and kaolin is well fitted by the pseudo-second-order kinetic model.Membrane diffusion and intra-particle diffusion are the sorption controlling steps.The isothermal study shows that sorption can be well described by the Freundlich model（R²>0.98）.Sorption is inhibited when the temperature rises.The process is spontaneous and exothermic,with the disorder of the solid-liquid interface decreasing.The pH has little effect on sorption apart from pH<2,the adsorption capacity decreased slightly due to the protonation of OPFRs.The coexistence of Na Cl and Ca Cl₂ in the solution can promote sorption,and the effect of Ca Cl₂ is more remarkable than Na Cl.Natural organic matter can also facilitate the sorption process.Hydrophobic and van der Waals force are involved in the sorption process and hydrophobic is the major driving force.Sorption capacities of montmorillonite and kaolin on OPFRs are significantly enhanced when they are modified by cetyltrimethylammonium bromide（HDTMA）.The distance between montmorillonite layers is increased from 1.55 nm to 1.90 nm after modification,indicating the long alkyl chain entered into the interlayer of montmorillonite with good thermal stability.The specific surface area of kaolin increased by 40%after modification.Sorption capacities of TPPO by the 0.4 CEC HDTMA-MMT and 0.4 CEC HDTMA-KAO increased from 5.06μg/g and 2.21μg/g to 87.70μg/g and 68.73μg/g,respectively;Sorption capacities of TPhP increased from 89.71μg/g and 69.04μg/g to 140.48μg/g and 86.27μg/g,respectively.The adsorption kinetics of OPFRs by organic montmorillonite and organic kaolin is best fitted by the pseudo-second-order kinetic model,and intra-particle diffusion is not the only controlling step.The adsorption isotherm is approximately a straight line,and the linear correlation coefficient is greater than 0.96.Sorption of OPFRs by modified clay minerals is less affected by pH.The coexistence of Na Cl and Ca Cl₂ in the solution can improve sorption capacities,and the effect of Ca Cl₂ is more remarkable than Na Cl.The presence of natural organic matter in the solution can slightly increase the adsorption capacity of OPFRs.Low temperature is conducive to the adsorption of OPFRs on organic montmorillonite and organic kaolin.The adsorption reaction is a spontaneous exothermic reaction,and the degree of freedom decreases during the adsorption process.Sorption is controlled dominantly by partition.Organic montmorillonite and organic kaolin were regenerated successfully five times with ethanol and reused 5 times with stable adsorption capacity,indicating good application prospect in the removal of OPFRs from water.