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

Abstract

We examined P sorption characteristics in Al-based water treatment residuals (Al-WTR) generated from slightly alkaline surface water and in an organic residual composite (WW-Al/O-WTR), produced by using the Al-WTR to treat organic-rich and high P concentration dairy wastewater. Solids from both residuals were examined using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD), and exposed to P additions of 0 to 4000 mg L in a sorption experiment. The Al-WTR removed ∼97% of the added P, whereas WW-Al/O-WTR removed only 78% of the added P in the addition range of 0 to 100 mg P L. With P additions of ≥100 mg L, the removal rate declined to <38% by Al-WTR and to 16% by WW-Al/O-WTR, possibly implying a change in sorption mechanisms. Analysis by XRD indicated that the major mineral was calcite, with some silica and poorly crystalline Al hydroxides. Analysis by SEM-EDS, which used three-element overlay maps of the residual surfaces, indicated that P was sparsely sorbed on both calcic and Al (hydr)oxide surfaces, along with a few clusters, even at low P concentrations of the treated waters. Ternary clusters of P, Al, and Ca were more abundant on the WW-Al/O-WTR. Carbon distribution suggested that organic substances covered Al surfaces. Sorption of P onto WW-Al/O-WTR may be reversible due to relatively weak Ca-P and Al-P bonds induced by the slight alkaline nature and in the presence of organic moieties, enhancing the WW-Al/O-WTR potential to act as a P source, rather than a P sink, in agricultural applications.

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

“…This finding is consistent with XRD analyses reported by Zohar et al (2018), which indicated the presence of calcite in the WTR; calcite can act as an adsorbent for P (Sø et al, 2011). Additionally, SEM‐EDS analyses reported by Zohar et al (2018) indicated that P was associated both with Al‐ and Ca‐rich surfaces of WTR particles. The known XANES spectra used to fit the unknown spectra in the WTR samples were from adsorbed P, supporting the contention that adsorption was a dominant retention mechanism.…”

“…As noted by Zohar et al (2018), and given the high organic matter content of the WTR (Table 1), organic P species are almost certainly present in the WW‐Al/O‐WTR. Therefore, it may seem curious that no explicitly organic P species were detected using P K ‐edge XANES spectroscopy.…”

“…Apatite‐like P grains were found in both the Al‐WTR and WW‐Al/O‐WTR (Fig. 1), although no crystalline P species were identified via XRD by Zohar et al (2018). The P K ‐edge XANES spectrum of apatite is similar to that of weathered and hydrated Ca phosphate species such as octocalcium phosphate (Ingall et al, 2011; Giguet‐Covex et al, 2013), so the precise speciation of these Ca‐P solids is difficult to determine via P K ‐edge XANES spectroscopy.…”

“…Phosphorus that sorbed to the WTR on exposure to wastewater appeared to preferentially associate with Al, although Ca‐associated P was still a substantial portion of the P in the WW‐Al/O‐WTR (Table 2, Fig. 5, and Zohar et al, 2018). Discrepancies between studies could be attributable to differences in the WTR material, experimental conditions, or limitations in analytical techniques.…”

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.…”