Competitive mechanism of microplastic addition for the adsorption of arsenic from an aqueous solution by magnetic biochar-supported layered double hydroxide

This study explores the competitive mechanisms of different types of microplastics (MPs) on pentavalent arsenic (As(V)) adsorption by magnetic biochar-supported layered double hydroxide composite (MBC@LDH). The effects of the solution pH, ionic strength, temperature, material dosage, and MP concentration on As(V) adsorption were investigated in co-existing MPs and MBC@LDH systems. Results revealed MPs competitively occupied As(V) adsorption sites on MBC@LDH, with varying inhibition efficiencies: polyvinyl chloride (PVC) provided 45.44 % of the competitive adsorption, representing a significant reduction in adsorption affinity especially in acidic environments and higher ionic strength. The reduced percentage of adsorption capacity of MBC@LDH for As(V) induced by polystyrene (PS) was 35.55 %, and that for polyethylene (PE) was also just between PVC and PS, especially in acidic environments and higher ionic strength. Further exploration manifested that the presence of MPs decreased the crystallinity of the CaMgAl LDH in MBC@LDH, which disrupted the surface complexation and hydrogen bonding between MBC-LDH and As. In addition, the strengths of the As-O bonds in MBC@LDH were more significantly reduced by PVC with its entering into the binary system rather than PS and PE, which was ascribable to the strong negative charge and hydrophobic separation properties of PVC. Interestingly, PVC promoted the conversion of As(V) to As(III) on MBC@LDH but not when PE and PS were added, which might be related to the surface chemical bonding and polarity of the different MPs. This study provides a theoretical reference for MPs' competitive adsorption mechanisms of As on porous materials in aqueous environments.