Microplastics (MPs), act as vectors of heavy metal pollutants in the environment, is of practical significance to study the adsorption process and mechanism on heavy metals. In this study, polystyrene microplastics (PSMPs) were used as model MPs to study the adsorption of Pb2+ on PSMPs and the effects of humic acid (HA) on the adsorption process. The results showed that HA promoted the adsorption of Pb2+ on PSMPs, and the higher the concentration of HA, the greater the adsorption of Pb2+. With the increase of pH value and decrease of ionic strength, the adsorption capacity of PSMPs for Pb2+ increased. The scanning electron microscope equipped with the energy dispersive spectroscope (SEM–EDS), fourier transform-infrared spectra (FT-IR) and X-ray photoelectron spectroscopy (XPS) analysis showed that Pb2+ could be adsorbed directly onto PSMPs and also indirectly by HA. The higher KSV values in the PSMPs-HA-Pb2+ system than PSMPs-HA system by fluorescence analysis of HA suggested that HA acted as a bridging role in the adsorption of Pb2+ on PSMPs. The site energy distribution analysis further revealed that HA increased the average site energy μ(E*) and its standard deviation σe* of PSMPs by introducing more adsorption sites, thus enhanced the adsorption affinity of PSMPs. This study provided more thoughts and insights into the adsorption behavior and mechanism of MPs for Pb2+ in aquatic environments.
Microplastics (MPs) have a great potential to adsorb heavy metal pollutants such as Pb2+ and the dissolved organic matter(DOM) in the aquatic environment will affect this adsorption behavior. In this study, batch experiments were performed to investigate the adsorption characteristics of Pb2+ onto polystyrene microplastics (PSMPs) in the presence and absence of HA(a kind of representative DOM). The adsorption kinetics of Pb2+ onto PSMPs conformed to the pseudo-second order model, and the adsorption isotherms were well fitted by the Langmuir model. With the increase of HA concentration, the Pb2+ adsorption onto PSMPs increased. Site energy distribution analysis showed that the presence of HA increased the adsorption site energy of PSMPs, thus enhancing the adsorption capacity for Pb2+. The fluorescence quenching analysis of HA further indicated that part of HA were adsorbed onto PSMPs, which increased additional binding sites on the surface of PSMPs. This was attributed to the abundant functional groups that could react with Pb2+ of HA. The pH and ionic strength of solution changed the structure of HA and the adsorption sites of PSMPs, which influenced the adsorption capacity of PSMPs for Pb2+. This study illustrated the effect of HA on the process and mechanism of Pb2+ adsorption onto PSMPs, and provided more information for the evaluation of environmental behavior and toxicological effects of microplastics in aquatic environments.
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