Abstract 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.
Understanding the interactions between heterogeneous dissolved organic matter (DOM) and nonylphenols (NPs) is essential for predicting their behavior and fate in the environment. Herein, we firstly obtained different MW-fractionated humic acids (HAs) using the ultrafiltration method. Afterward, the molecular weight (MW)-dependent interactions of HAs with 4-nonylphenol (4-NP) were analysed by excitation emission matrix (EEM) fluorescence spectroscopy, fluorescence quenching, UV-vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and principal component analysis (PCA). EEM spectra indicated that the quenching mechanism was static. In the binding process, the higher MW fractions showed stronger interaction with 4-NP than the lower MW counterparts, exhibiting a clear MW-dependent interaction heterogeneity. The interaction constants for the 4-NP-HAs system were suppressed as the ionic strength decreased and pH increased, which was especially obvious in the binding of 4-NP to the lower MW-fractionated HAs. The FTIR spectra revealed that hydroxyl and aromatics were involved in the interaction process of HA fractions with 4-NP. It was also found from 1H NMR that π-π interactions between aromatic rings of 4-NP and MW-fractionated HAs were responsible for the complexation. The correlation analysis and PCA results indicated that aromaticity and MW play important roles in the interaction process and confirmed an obvious interaction heterogeneity among MW-fractionated HAs samples. This work highlighted MW-dependent interaction heterogeneities of HA, which suggested that heterogeneity in MW distribution should be taken into consideration when exploring the fate and biogeochemistry cycling of 4-NP from contaminated environments.
Abstract Microplastics (MPs) have a great potential to adsorb heavy metal pollutants such as Pb 2+ 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 Pb 2+ onto polystyrene microplastics (PSMPs) in the presence and absence of HA(a kind of representative DOM). The adsorption kinetics of Pb 2+ 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 Pb 2+ 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 Pb 2+ . 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 Pb 2+ 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 Pb 2+ . This study illustrated the effect of HA on the process and mechanism of Pb 2+ 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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