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

Biochar is used to remove ammonium (NH) from wastewater, where microplastics are emerging pollutants. However, whether microplastics can adsorb NH or how they will affect the sorption of NH by biochars have not been studied. Here, batch sorption kinetics and isotherm experiments were conducted to elucidate the sorption of NH 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 = 3.29 mg g) but a faster adsorption rate (k = 0.08 g (mg min)) for NH than biochars (Q = 5.67 ~ 20.54 mg g; k = 0.02 ~ 0.04 g (mg min)). When PE and biochars coexisted in an aqueous solution, the NH 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 NH; moreover, they can enhance the NH sorption capacity of biochars. Therefore, when biochar is used for NH removal from wastewater, the interaction of biochar and microplastics needs to be considered.