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

Abstract

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 -edge X-ray absorption near edge structure spectroscopy (XANES), and P -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 -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.

Phosphorus Sorption Characteristics in Aluminum‐based Water Treatment Residuals Reacted with Dairy Wastewater: 1. Isotherms, XRD, and SEM‐EDS Analysis

“…In the current publication, we present the detection of P distribution onto the materials and identification of its associations with the major constituents (Al, Ca, C, and Si) using advanced SEM‐EDS. In the accompanying publication by Massey et al (2018), we present specific forms of P bound to Al and Ca, complementing the SEM‐EDS findings.…”

“…The SEM‐EDS results in the current study indicated that the usually scattered P had high affinity for Al surfaces but also demonstrated scattered P associated with Ca areas. Phosphorus adsorption to Al and to Ca in both the Al‐WTR and the WW‐Al/O‐WTR was identified by the micro‐X‐ray fluorescence (XRF) P and P K‐edge X‐ray absorption near‐edge structure (XANES) analyses (Massey et al, 2018). These findings are in contrast with those of Bertrand et al (2001) and Makris et al (2004), which suggested that P adsorption is exclusive to Al surfaces.…”

“…The three‐element SEM‐EDS overlay maps indicated particle‐shaped “clusters” of P on Al and Ca areas in both the Al‐WTR and the WW‐Al/O‐WTR and the P minerals identified by P K‐edge XANES in the samples (Massey et al, 2018). This suggests P sorption onto Al‐WTR involving other mechanisms in addition to adsorption, which is commonly addressed (Makris et al, 2004; Dayton and Basta, 2005; Babatunde and Zhao, 2009; Ippolito et al, 2011).…”

X‐Ray Spectroscopic Quantification of Struvite and Dittmarite Recovered from Wastewater

“…Ajiboye et al (2007) used NMR spectroscopy, chemical extractions, and P K ‐edge XANES analysis to quantify P speciation in organic soil amendments such as manure and biosolids. Similarly, Zohar et al (2018) and Massey et al (2018) combined chemical extractions, scanning electron microscopy with energy dispersive X‐ray spectroscopy (SEM‐EDS), and XANES spectroscopy to quantify speciation in P recovered from wastewater using a mixed Al oxide–calcite sorbent.…”

Mechanisms Responsible for Soil Phosphorus Availability Differences between Sprinkler and Furrow Irrigation

“…Data were collected and analyzed as described above for soils from the producer fields. Since bulk XANES spectroscopy may not always detect minor components, as demonstrated in Massey et al (2018), microfocused X‐ray fluorescence mapping and microfocused P K ‐edge XANES (micro‐XANES) were also performed at Stanford Synchrotron Radiation Lightsource Beamline 14‐3. Samples were encased in optical epoxy (Epotek 301‐2FL), cut to expose soil grains, and lightly polished before analysis.…”