Effects of humic acid on Pb(2+) adsorption onto polystyrene microplastics from spectroscopic analysis and site energy distribution analysis

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 Pb(2+) on PSMPs and the effects of humic acid (HA) on the adsorption process. The results showed that HA promoted the adsorption of Pb(2+) on PSMPs, and the higher the concentration of HA, the greater the adsorption of Pb(2+). With the increase of pH value and decrease of ionic strength, the adsorption capacity of PSMPs for Pb(2+) 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 Pb(2+) could be adsorbed directly onto PSMPs and also indirectly by HA. The higher K(SV) values in the PSMPs-HA-Pb(2+) system than PSMPs-HA system by fluorescence analysis of HA suggested that HA acted as a bridging role in the adsorption of Pb(2+) on PSMPs. The site energy distribution analysis further revealed that HA increased the average site energy μ(E(*)) and its standard deviation σ(e)(*) 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 Pb(2+) in aquatic environments.