Repurposing aluminum-based water treatment residual (WTR) as a filtration medium for microplastic removal:...

Valorizing water treatment residual (WTR) aligns with circular-economy objectives by transforming water treatment sludge waste into value-added products. For the first time, we evaluated the performance and mechanisms of aluminum-based WTR as a cost-effective and environmentally benign filtration medium for MP removal. Our column experiments removed up to 97.47% of 10-µm carboxylated polypropylene microplastics (PP-COOH MPs) from synthetic water. MP removal increased with ionic strength and the presence of mono/multivalent cations (Na⁺, Ca²⁺), primarily through charge neutralization, double-layer compression, and pore straining. WTR-based filtration also effectively removed MPs from the primary influent (95.6%) and final effluent (86.9%) of a full-scale wastewater treatment plant, as well as from the influent of a full-scale water treatment plant (97.1%). In addition, dissolved Al leached from WTR enhanced electrostatic attraction and MP aggregation, thereby promoting MP deposition on WTR surfaces. Overall, these findings demonstrate that WTR can be repurposed as a valuable material to control MP pollution, offering attractive opportunities to turn this waste into a beneficial resource. Given its potential to remove MP and other pollutants, WTR can be used as a filter medium to polish WWTP effluent, improve stormwater quality, or serve as a functional layer in landfill sites to mitigate leachate pollution.