Phosphate Sorption Speciation and Precipitation Mechanisms on Amorphous Aluminum Hydroxide

Xiaoming Wang,Mengqiang Zhu,Brian Phillips,Peng Yang,Xionghan Feng,Zhen Hu,Yongfeng Hu,Jean-François Boily,Wenqian Xu

doi:10.3390/soilsystems3010020

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.

Highlights

IntroductionTo maintain or increase the crop productivity, massive phosphorus (P) fertilizers are applied into soils, while the utilization efficiency of P is low, usually less than 20% [1], since the majority of P was strongly sorbed by soil minerals, e.g., clay minerals, aluminum (Al) and iron (Fe) oxides [2,3,4,5]
Phosphorus (P) is one of the most limiting soil nutrients for agricultural and ecosystem productivity.To maintain or increase the crop productivity, massive phosphorus (P) fertilizers are applied into soils, while the utilization efficiency of P is low, usually less than 20% [1], since the majority of P was strongly sorbed by soil minerals, e.g., clay minerals, aluminum (Al) and iron (Fe) oxides [2,3,4,5]
P fate and bioavailability require the detailed knowledge of P speciation on mineral surface, which still remains poorly understood for phosphate sorption on Al oxides, a common mineral in acidic soils

Summary

Introduction

To maintain or increase the crop productivity, massive phosphorus (P) fertilizers are applied into soils, while the utilization efficiency of P is low, usually less than 20% [1], since the majority of P was strongly sorbed by soil minerals, e.g., clay minerals, aluminum (Al) and iron (Fe) oxides [2,3,4,5]. This excess P could be a potential risk to the surface water quality due to transportation of P-sorbed colloidal particles by surface runoff and soil erosion [6]. It remains unclear how the binding geometries of phosphate on Al minerals change with solution conditions

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