Sorption Of Polycyclic Aromatic Hydrocarbons In Microplastic Polystyrene

Marine microplastics become a new type of marine pollutants,which contains toxic additives and serves as a carrier of organic contaminants in marine environment.As a result,it is important to study the sorption behaviors of organic pollutants on microplastics for ecological risk assessment of microplastic,transport and accumulation of organic pollutants in marine environment.In the present study,typical microplastic polystyrene?PS?of two sizes?250?m and 900?m?were used with polycyclic aromatic hydrocarbons?PAHs?as target compounds to examine the sorption behavors of organic pollutants on microplastics.Four different spiking systems,such as direct spiking methods under the shaking and agitation conditions,flow-through system and the spiking with dialysis tubing under the shaking condition,were used to determine the PS-water partition coefficients(K_psw)for PAHs.Low density polyethylene?LDPE?membrane was used as a reference sorbent for examing the feasibility of spiking systems in determining the K_psw values of PAHs.The adsorption isotherms of PAHs on PS were examined at different spiking concentrations of PAHs.In addition,to investigate the possible effects of the structure of microplastics on adsorption ability,LDPE membrane and PS were analyzed by X-ray diffractometer?XRD?for their crystallinity measurements.The results showed that the immiscible microparticles in water by direct spiking methods may affect the determination of dissolve concentrations of PAHs.Thereby,the direct spiking methods may not be suitable for determination of K_psw for high molecular PAHs with low water solubility.The flow-through dynamic system is a semi-sealed water circulation system,which has the characteristics of large water volume,long processing time and water running path.These factors might affect the determination of the concentration of PAHs in the aqueous phase and the adsorbed phase.Determination of PAHs through dialysis tube spiking method is of plausible feasibility.In addition,the PAHs loss due to the sorption on glass bottle wall and volatilization were observed in the four spiking systems.This result indicated that the concentrations of organic pollutants on the microplastics may not be derived from the mass balance method,which assumed that the spiked amount of a target compound was the sum of its amounts in the aqueous phase and microplastics.As a result,it is critical to measure the concentrations of organic pollutants on microplastics directly for dertermination of their partition coefficients between microplastics and water.On the other hand,the K_psw values of phenanthrene,anthracene and fluorene determined in two direct spiking methods were different and decreased with their concentrations in water increasing.The concentrations of PAHs on the 900?m PS were much lower than that on the250?m PS,suggesting that PAHs sorption amount on PS was size-dependent.The coefficients of adsorption isotherms for PAHs on the 900?m PS were dertermined by the dialysis tubing spiking system with different PAHs concentrations?0.5?g,1?g and 2?g?under the shaking condition.The coefficient values were less than 0.7,suggesting a nonlinear adsorption of PAHs on PS.In addition,the partitioning coefficients of low molecular PAHs between LDPE membrane and water were found to be well consistent in four spiking systems and greater than those K_psw values.This finding suggested that the sorption capacity of PAHs on LDPE membrane was greater than on PS.XRD analysis of LDPE and PS showed that the peak half width of LDPE was smaller than PS,suggesting that the crystallinity of LDPE was greater than that of PS.The sorption capacity of organic contaminants may be related to the crystallinity of polymer.