Transforming contaminant ligands at water-solid interfaces via trivalent metal coordination

Abstract

In environmental matrices, the migration and distribution of contaminants at water-solid interfaces play a crucial role in their capture or dissemination. Scientists working in environmental remediation and wastewater treatment are increasingly aware of metal-contaminant coordination; however, interfacial behaviors remain underexplored. Here, we show that trivalent metal ions (e.g. Al³⁺ and Fe³⁺) mediate the migration of pollutant ligands (e.g. tetracycline (TC) and ofloxacin) to the organic solid interface. In the absence of Al³⁺, humic acid (HA) colloids (50 mg/L) capture 26.1 % of the TC in water (initial concentration: 10 mg/L) via weak intermolecular interactions (binding energy: -5.71 kcal/mol). Adding Al³⁺ (2.5 mg/L) significantly enhances the binding of TC to an impressive 94.2 % via Al³⁺ mediated coordination (binding energy: -84.89 kcal/mol). The significant increase in binding energy results in superior interfacial immobilization. However, excess free Al³⁺ competes for TC binding via direct binary coordination, as confirmed based on the unique fluorescence of Al³⁺-TC complexes. Density functional theory calculations reveal the intricate process of HA-Al³⁺ binding via carboxyl and phenolic hydroxyl sites. The HA-Al³⁺ flocs then leverage the remaining coordination capacity of Al³⁺ to chelate with TC. As well as providing insights into the pivotal role of metal ion on the self-purification of natural water bodies, our findings on the interfacial behavior of metal-contaminant coordination will propel coagulation technology to the capture of microscale pollutants.

Keywords: Coagulation; Contaminant ligands; Density functional theory; Humic acid; Interfacial complex; Trivalent metal ions.