Probing the Mechanism of Selective Phosphate Adsorption from Wastewater using Aqueous and Synchrotron X-ray Characterization

01 April 2025, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Ion exchange shows promise for recovering phosphate from wastewater as value-added products, but requires high phosphate selectivity to compete with conventional treatment. Hybrid anion exchange (HAIX) resins, which contain non-selective basic functional groups and selective iron oxide nanoparticles (FeOnp), can effectively remove phosphate from wastewater. However, knowledge gaps remain regarding the mechanisms of phosphate selectivity and influence of competing ions, hindering needed efforts to model adsorption dynamics and design scalable adsorption processes for varying wastewaters. To address these gaps, we integrated aqueous-phase adsorption analysis with solid-phase, synchrotron-based X-ray characterization; this integration facilitated elucidation of the distribution and speciation of iron, phosphate, and competing anions on HAIX resins. We compared a quaternary ammonium-functionalized HAIX resin (SBA) to a tertiary amine version (WBA) to determine the role of functional groups. X-ray radiography revealed differences in FeOnp speciation (goethite vs. ferrihydrite) and distribution (peripheral vs. homogeneous) between the resins, resulting in varied phosphate affinity and intraparticle diffusion resistance. Using micro-X-ray fluorescence (μ-XRF) and micro-X-ray absorption near-edge structure (μ-XANES) spectroscopy, we identified differences in where and how phosphate binds across resin types and wastewaters. Across wastewater compositions, FeOnp sites in WBA contribute more to phosphate adsorption than in SBA, possibly due to variations in Fe distribution and speciation. Phosphate adsorption densities calculated from quantitative μ-XRF maps matched those from aqueous analysis, demonstrating the effectiveness of this integrated approach. Overall, results demonstrate the use of synchrotron-based X-ray characterization for investigating adsorption mechanisms and advance HAIX as a phosphate recovery technology from complex wastewaters.

Keywords

Nutrient recovery
phosphate recovery
adsorption
ion exchange
resins
synchrotron
radiography
μ-XRF
μ-XANES

Supplementary materials

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Supporting information including data and methodology
Description
Supporting information includes historical trends of global phosphorus mining, phosphate speciation as a function of pH, speciation of hydrated iron oxides as a function of pH, composition of wastewaters, difference in phosphate adsorption performance between resins, scanning electron microscopy (SEM) images of resins, phosphate selectivity analyses, quantitative analysis of sulfate adsorption from XRF mapping, and kinetic profiles for phosphate, sulfate, and chloride adsorption.
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