Abstract
The associations of elements upon a heterogeneous surface may control nutrients or pollutants sorption and release, having agricultural and environmental implications. This chemical behavior can be elucidated by spatial spectroscopy, followed by image analysis. The purpose of this paper is to present a working procedure for image analysis using the free program ImageJ that can be applied for dot maps of three or more elements produced by solid-state spectroscopy. Detailed step-by-step instructions lead to visual and quantitative information regarding elements associations. The working procedure was demonstrated for P, Al and Ca dot maps produced by scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) for surfaces of Al-based water treatment residuals (Al-WTRs), a by-product of drinking water pretreatment with alum coagulant. Al-WTR was reused to adsorb the macro-nutrient P from polluted soil leach and dairy wastewater (WW). Surficial P onto Al-WTR, SL-Al/O-WTR, and WW-Al/O-WTR (0.56, 0.93, and 2.15%, respectively) displayed sorption dynamics, mostly with Al and Ca. Quantification of the spatial proportions of individual elements and their associations indicated P-Al pool > P-Ca pool (45–24% and 17–7%, respectively). Upon introducing P-rich dairy wastewater, the behavior of P sorption by Al and Ca changed and became more clustered. A ternary phase of P-Al-Ca covered 38% of the area with signal, compared to 4.3 and 4.6% of the area in Al-WTR and SL-Al-WTR, where it was limited to particles edges only. Thus, the presented protocol may promote employing image analysis for geochemical applications, elucidating chemical behavior and affinities. Advantages and pitfalls are discussed.
Highlights
Phosphorus (P) might be an environmental threat for pollution and eutrophication in aquatic settings when impacted by P-rich waste streams (Sims and Sharply, 2005)
Aluminum water treatment residuals (Al-WTR) recovered phosphorus by shaking a mixture containing a ratio of 9 g to 1 L waste liquid (SL or WW) for three days to ensure solid-solution equilibrium; the enriched solid products of this process, namely soil leach (SL)-Al/O-WTR and WW-Al/O-WTR, were each separated from the solution by centrifugation; P recovery level was about 95% (Zohar et al, 2017)
The main P pools on the Al-WTR surface, before and after reaction with SL or with dairy WW, were Al oxides and Ca minerals (Zohar et al, 2020). These pools compete for P sorption when Al-WTR is mixed with P-rich streams
Summary
Phosphorus (P) might be an environmental threat for pollution and eutrophication in aquatic settings when impacted by P-rich waste streams (Sims and Sharply, 2005). Those streams can be used as P sources, as P is an essential nutrient for plant growth. Identifying major P-binding pools in heterogeneous materials, such as WTRs (Zohar et al, 2020; Zohar et al, 2018) may facilitate its effective recovery. It is important to thoroughly examine P dynamics with major pools on top of the materials, as these dynamics have an essential role in P immediate sequestration and partitioning out of the solution
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