Coexistence of polyethylene microplastics and biochar increases ammonium sorption in an aqueous solution

Biochar is used to remove ammonium (NH₄⁺) from wastewater, where microplastics are emerging pollutants. However, whether microplastics can adsorb NH₄⁺ or how they will affect the sorption of NH₄⁺ by biochars have not been studied. Here, batch sorption kinetics and isotherm experiments were conducted to elucidate the sorption of NH₄⁺ on a manure biochar (MBC), a straw biochar (SBC), a wood sawdust biochar (WBC), a polyethylene microplastic (PE), and their combination. The results showed that PE had a smaller sorption capacity (Q_max = 3.29 mg g^-1) but a faster adsorption rate (k_s = 0.08 g (mg min)^-1) for NH₄⁺ than biochars (Q_max = 5.67 ~ 20.54 mg g^-1; k_s = 0.02 ~ 0.04 g (mg min)^-1). When PE and biochars coexisted in an aqueous solution, the NH₄⁺ sorption capacity was increased by 17.0% in PE+SBC, 7.1% in PE+MBC, and 8.6% in PE+WBC, which likely due to the deprotonation of functional groups and the decreases in small molecular-size dissolved organic carbon. We conclude that microplastics can adsorb NH₄⁺; moreover, they can enhance the NH₄⁺ sorption capacity of biochars. Therefore, when biochar is used for NH₄⁺ removal from wastewater, the interaction of biochar and microplastics needs to be considered.