Phosphorus Sorption to Aluminum‐based Water Treatment Residuals Reacted with Dairy Wastewater: 2. X‐Ray Absorption Spectroscopy

Phosphorus capture from wastewater can decrease water pollution and provide a P‐rich fertilizer alternative for use in agricultural production. This study was conducted to elucidate P retention mechanisms in Al‐based water treatment residuals (Al‐WTR) to gain insight regarding P sorption and the potential for P release from Al‐WTR after reaction with dairy wastewater. Synchrotron‐based microfocused X‐ray fluorescence (micro‐XRF) spectrometry, bulk P K‐edge X‐ray absorption near edge structure spectroscopy (XANES), and P K‐edge micro‐XANES spectroscopy were used to determine P distribution and speciation within the Al‐WTR materials. Bulk XANES analyses indicated a shift from ∼56 P atom % Ca‐associated P in the initial Al‐WTR to ∼32% P atom % Ca‐associated P after reaction with wastewater; Al‐associated P made up the remainder of the P species. According to XANES analyses, adsorption appeared to be the primary P retention mechanism in the Al‐WTR materials. However, micro‐XANES analyses depicted a more complicated picture of P retention mechanisms, with regions of primarily Al‐associated P, regions of primarily Ca‐associated P, regions of mixed Al‐ and Ca‐associated P, and distinct apatite‐ or octocalcium phosphate‐like P grains. Synchrotron micro‐XRF mapping further suggested that exposure of the aggregate exteriors to wastewater caused P to diffuse into the porous Al‐WTR aggregates. Organic P species were not explicitly identified via P K‐edge XANES despite high organic matter content, suggesting that organic P may have been predominantly associated with mineral surfaces. Although diffusion and sorption to Al may decrease P bioavailability, Ca‐associated P may increase P bioavailability from Al‐WTR that is reused as a soil amendment. CORE IDEAS: XAS provided unambiguous evidence of P associations with Al and Ca in water treatment residuals. Phosphorus adsorbed to Al and Ca surfaces in Al‐based water treatment residuals. Grains of apatite‐ or octocalcium phosphate‐like P were found in water treatment residuals. Phosphorus diffused into aggregates on reaction with P‐rich wastewater. Evidence from electron microscopy and XRD complemented these findings.