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Coagulation performance and mechanism of different hydrolyzed aluminum species for the removal of composite pollutants of polyethylene and humic acid
Summary
Researchers examined how different forms of hydrolyzed aluminum coagulants remove polyethylene microplastics and humic acid from water. The study found that when humic acid adsorbs onto microplastic surfaces, it changes the removal dynamics, and polyaluminum chloride with hexagonal clusters achieved the best microplastic removal through adsorption bridging and sweeping mechanisms.
Microplastics (MPs) and natural organic matter (NOM) composite pollutants have become emerging contaminants with potential threats. Coagulation has been widely used to remove MPs and NOM, but the underlying mechanisms for the removal of MPs-NOM composite pollutants by hydrolyzed Al species remain unclear. Therefore, the coagulation performance and mechanism of AlCl, polyaluminum chloride with basicity of 2.2 (PAC), and PAC in treating polyethylene (PE), humic acid (HA), and PE-HA composite systems were systematically investigated. The results showed that in the single PE system, PAC with hexagonal clusters achieved the maximum removal (68.09 %) (pH: 5, dosage: 0.5 mM) since adsorption bridging and sweeping effect were the main mechanisms for PE removal. The adsorption of HA on the PE surface enhanced its hydrophilicity and electrostatic repulsion, resulting in decreased PE removal. In the AlCl-PE-HA system, the oligomeric Al first interacted with the -COOH and C-OH of HA through complexation, followed by the meso- and polymers of Al interacted with PE by electrostatic adsorption. The pre-formed medium polymeric Al species (Al) and colloidal or solid Al species (Al) in PAC and PAC formed complexes with the -OH and -COOH groups of HA, respectively, and then removed PE by adsorption bridging and sweeping effect.