Organic matter from water treatment residuals stabilizes amorphous iron to maintain phosphorus adsorption by iron hydroxides

Functional materials enriched with amorphous iron are commonly used for environmental remediation. However, their performance is often susceptible to aging and reductive dissolution effects. Amorphous iron in drinking water treatment residuals (DWTRs) primarily exists in organic matter-bound forms, possessing a stable phosphorus adsorption capability. In this study, in-depth investigations were conducted to evaluate the effect of organic matter extracted from DWTRs on amorphous iron stability in iron hydroxides by using incubation tests for 180 days. The results showed that organic matter from different DWTRs exhibited similar FTIR and XPS patterns; whereas, the solid-state carbon-13 NMR analysis and the carbon/nitrogen atom ratios indicated variations in structures and origins. Notably, organic matter addition increased the 0.01 and 0.005 M-oxalic acidammonium oxalate extractable iron, especially the later one which was greater than the control by approximately 24 times, inhibiting iron aging in iron hydroxides. The addition also promoted the transformation of iron from acid-soluble and reducible to oxidizable and non-European Community Bureau of Reference methodextractable forms, reducing the iron release potential. This effect was intensified when organic matter contained relatively low levels of carboxylic carbon and nitrogen/oxygen-alkyl carbon while high levels of carbon=oxygen/ketone carbon. The inhibited iron aging further led to stronger phosphorus adsorption capabilities of iron hydroxides, and the reduced iron release potential caused the adsorbed phosphorus to exist in more stable forms. Therefore, organic matter from DWTRs stabilized amorphous iron to maintain phosphorus adsorption capabilities of iron hydroxides. These mechanisms show potential for application in the development of ironbased materials.