Effects of soil environmental factors and UV aging on Cu²⁺ adsorption on microplastics

Microplastics (MPs) in natural environments have attracted lots of attention. Although the quantity of MPs present in terrene is much higher than that in aquatic environment, few studies have investigated the chemical behavior of MPs in terrestrial environment. This study investigate the Cu²⁺ (as a model heavy metal) adsorption capacity of six kinds of MPs (polyamide-6 (PA), polyethylene (PE), polystyrene (PS), polyethylene terephthalate (PET), polyvinyl chloride (PVC), polymethyl methacrylate (PMMA)) in batch adsorption experiments and the effects of different soil environmental factors, including pH and the presence of cations and low-molecular-weight organic acids (LMWOAs), as well as ultraviolet (UV) aging. The Cu²⁺ adsorption capacities of PA and PMMA were higher than those of other MPs and their maximum equilibrium adsorption capacities (estimated by the Langmuir adsorption equation) were 323.6 μg/g ± 38.2 and 41.03 ± 1.78 μg/g, respectively. The Cu²⁺ adsorption on MPs was affected by pH, and the greatest amount of Cu²⁺ adsorbed on PA and PMMA was observed at pH = 6 and pH = 7, respectively. The presence of Ca²⁺ or Mg²⁺ inhibited Cu²⁺ adsorption by MPs, due to competition for the adsorption sites. Moreover, Cu²⁺ adsorption by MPs was affected by various types of LMWOAs. The Cu²⁺ adsorption on PA was significantly reduced by citric acid, followed by oxalic acid, and oxalic acid was particularly evident for Cu²⁺ adsorption on PMMA. UV aging (200 h) had different effect on Cu²⁺ adsorption on MPs and it depends on the change of carbonyl index. Results demonstrate that soil environmental factors can change the ability of different MPs to adsorb Cu²⁺ and affect the transport of pollutants as carriers.