Enhanced retention of small-sized microplastics by iron-containing sand filtration system: Effectiveness and mechanisms.

This study explores the enhanced retention of small-sized microplastics (MPs) in sand filtration systems using iron-loaded sand (IS). Conventional sand filtration has limited efficiency in removing MPs smaller than the pore size of the filter media, which presents a significant challenge for drinking water treatment plants (DWTPs). To address this issue, quartz sand was modified with iron (hydro)oxide coatings to alter the surface charge, enabling a strong electrostatic attraction with negatively charged MPs. In a 10 mmol/L NaCl solution at a flow rate of 1.5 mL/min, the effluent mass percentages (M) of polystyrene 200 nm MPs (MP200) and 1000 nm MPs (MP1000) decreased from 53.52% and 39.40% in bare sand (BS) to 0.79% and 2.81%, respectively. Additionally, IS maintained complete retention of MPs at various ionic strengths and valences. In binary system, while competitive attachment caused an 8.42% increase in the M of MP1000 in IS, series-connected columns achieved complete retention. Operational stability tests under realistic conditions, such as variable flow rates, 24-hour interruptions, and back-flushing cycles, demonstrated that IS consistently outperformed BS, with minimal MPs release. Moreover, IS achieved near-total MPs removal in the presence of humic acid and natural pond water, emphasizing its durability under complex environmental conditions. By addressing the challenges of limited retention in traditional systems and competitive attachment in mixed MPs systems, this study highlights IS as a scalable and effective solution for mitigating MPs pollution in drinking water. These findings offer crucial insights into enhancing filtration efficiency across a range of environmental and operational scenarios.