Arsenic adsorption by carboxylate and amino modified polystyrene micro- and nanoplastics: kinetics and mechanisms

We investigated adsorption characteristics of As(III) and As(V) onto two different functionalized polystyrene (PS) microplastics (MPs). Our results show that there is the potential for PS MPs to adsorb both As(III) and As(V). Using a particle size of 80-82 nm, maximum As(III) and As(V) adsorption capacities of 0.57 mg/g and 0.37 mg/g were obtained by PS-COOH MPs. These capacities were markedly higher than those for PS-NH₂ MPs, which were 0.41 mg/g and 0.27 mg/g, respectively. The pseudo-second-order adsorption kinetic model was found to effectively describe the sorption kinetics of As(III)/As(V) on two different functionalized PS MPs. Langmuir isotherms better represented the equilibrium adsorption results. The kinetic models, XPS, and FTIR results indicate that hydrogen bonding, hydroxyl complexation, and outer-sphere surface complexation may have partly contributed to adsorption of As onto PS MPs. Adsorption capacity markedly decreases with increasing salinity or presence of humic acids (HA), suggesting an inhibiting effect of salinity and HA through outer-sphere complexation. These findings confirm that microplastics have great potential to adsorb As and hence are ultimately highly likely to affect the environmental behavior of As in an ecosystem.