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Aluminium tris(dihydrogen phosphate) (CAS 13530-50-2) is well soluble and dissociates to dihydrogen phosphate and aluminium ions in aqueous and biological systems.

The mobility of aluminium species is very complex and difficult to predict in areas affected by a number of geochemical factors which can affect the formation of speciation of aluminium and adsorption and desorption of aluminium on suspended particles. The relative mobility of aluminium species depends on many factors, especially pH, alkalinity, temperature, dissolved organic carbon, dissolved inorganic carbon and anion concentration. At low pH (less than 5.5), aluminium is mainly present as highly mobile Al3+ species. As pH increases above 5.5, aluminium-hydroxide complexes and organically complexed aluminium are formed by hydrolysis and combination with organic matter. Above pH 7, anionic aluminium hydroxide dominates. Kaplan (2005) published several Kd values based on measured Al concentration in groundwater and sediment and suggested Al Kd values for soil, grout, gravel, clay to be 3700 mL/g and 5000 mL/g for concrete, respectively. Furthermore, aluminium species play also an important role in the adsorption of other trace metal or anions in soils and sediments.

Phosphate adsorption is affected by numerous factors, e.g. pH, type and concentration of electrolytes, clay content, A1 and Fe oxides, and organic matter Content (Razaq, 1989). At normal soil pH values of 4.5 - 6.2 the dominant species are H2PO4 - and HPO4 -2. These ions can be present in soil water and also absorb onto the surface (or adsorb into) solid matter in soil. Two types of inorganic reactions control the concentration of phosphate ions in solution; these are precipitation-dissolution and sorption-desorption processes. Precipitation-dissolution reactions involve the formation and dissolving of precipitates, which is significantly pH depended. Sorption-desorption reactions involve sorption and desorption of ions and molecules from the surfaces of mineral particles, such as Al/Fe oxides/hydroxides, clay minerals and carbonates (Parker et al., 1998). Basically, phosphate adsorption dominates in mineral soil with a low pH. 



Kaplan D.I. (2005) Recommended Distribution Coefficients, Kd Values, for Special Analysis Risk Calculations Related to Waste Disposal and Tank Closure on the Savannah River Site (U), Westinghouse Savannah River Company Savannah River Site,


Parker J.E., Robertson J., Wansbrough H. (1998) Chemical Processes in New Zealand, Bd 2. New Zealand Institute of Chemistry, 01.01.1998.

Razaq, Ibrahim Bakry Abdul (1989) Effect of pH and exchangeable metals on phosphate adsorption by soils. Retrospective Theses and Dissertations, Paper 9170.