Phosphate Sorption Speciation and Precipitation Mechanisms on Amorphous Aluminum Hydroxide

Abstract

Aluminum (Al) oxides are important adsorbents for phosphate in soils and sediments, and significantly limit Phosphate (P) mobility and bioavailability, but the speciation of surface-adsorbed phosphate on Al oxides remains poorly understood. Here, phosphate sorption speciation on amorphous Al hydroxide (AAH) was determined under pH 3–8 and P concentration of 0.03 mM–15 mM using various spectroscopic approaches, and phosphate precipitation mechanisms were discussed as well. AAH exhibits an extremely high phosphate sorption capacity, increasing from 3.80 mmol/g at pH 7 to 4.63 mmol/g at pH 3. Regardless of reaction pH, with increasing P sorption loading, the sorption mechanism transits from bidentate binuclear (BB) surface complexation with dP-Al of 3.12 Å to surface precipitation of analogous amorphous AlPO4 (AAP), possibly with ternary complexes, such as (≡Al-O)2-PO2-Al, as intermediate products. Additionally, the percentage of precipitated phosphate occurring in AAP linearly and positively correlates with P sorption loading. Compared to phosphate reaction with ferrihydrite, phosphate adsorbs and precipitates more readily on AAH due to the higher solubility product (Ksp) of AAH. The formation of AAP particles involves AlIII release, which is promoted by phosphate adsorption, and its subsequent precipitation with phosphate at AAH surfaces or in the bulk solution.

Phosphorus adsorption characteristics in forested and managed podzolic soils

“…(2001) reported that oxalate‐extractable Al and Fe were responsible for P adsorption in 35‐ to 40‐yr‐old agricultural soils. Also, Podzols, which are rich in amorphous Al hydroxide, are accountable for fixing P via surface complexation and precipitation reaction (Wang et al., 2019). We may conclude that high P adsorption in the tested forested and managed Podzol horizons is mainly influenced by the combined effect of SOM, Al, and Fe.…”

“…A combination of high SOM in SJRDC forested B horizon and higher Al in SJRDC forested BC horizon might explain the apparently similar q max values (Table 3). The amorphous Al minerals in Podzols form a firm surface complexation or precipitation with P added as fertilizer (Wang et al., 2019; Weihrauch & Opp, 2018). Also, organically bound Al has been reported to be responsible for 60% of P retention, of which about 70% is nonlabile P (Villapando & Graetz, 2001) but with the potential of being released in low‐P soils (Zhou et al., 1997).…”

“…Usually, a small proportion of the P added to the soil in the form of mineral or organic fertilizer is expected to remain soluble in the soil solution and to be accessible to plant roots (Shen et al., 2011). However, depending on the soil properties, a significant amount of P can be fixed by sesquioxides of aluminum (Al) and iron (Fe), clay minerals, soil organic matter (SOM) (Debicka et al., 2015; Eriksson et al., 2016; Luo et al., 2017; Read & Campbell, 1981; Roy et al., 2017; Wang et al., 2019), and poorly crystalline Al and Fe oxyhydroxides (Ayenew et al., 2018). Most P fertilizer thus ends up in chemically fixed forms and stored in the soil (Roy et al., 2017).…”

Atomic Level Understanding of Orthophosphate Adsorption by Magnesium Aluminum-Layered Double Hydroxides—A Multitechnique Study

“…50 Guo et al assigned the two dominating resonances at δ iso ( 31 P) = -14.5 and -7.5 ppm in the 31 P MAS NMR spectrum to phosphate associated with the LDH. 49 However, δ iso ( 31 P) = -14.5 ppm are in the same range as aluminum phosphate synthesized under acid condition 36,39,60 and δ iso ( 31 P) = -7.5 ppm match newberyite. 27,61,69 Both minerals are also clearly identified by PXRD.…”

Sequestration of P fractions in the soils of an incipient ferralisation chronosequence on a humid tropical volcanic island

“…2 ). These organic subfractions of P therefore appear to be sorbed onto and incorporated into complexes of organic matter and amorphous sesquioxides, especially Al o (Wang et al 2019 ). This P is more available than that associated with crystallising sesquioxides, but its recycling depends on the dynamics of the complexes, and the rate at which the organic components are decomposed (Barthes et al 2008 ; Hernandez-Soriano 2012 ).…”