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Environmental fate & pathways

Adsorption / desorption

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adsorption / desorption
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
weight of evidence
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Well documented non-GLP study
Principles of method if other than guideline:
Long-term transformation characteristics of biogenic Ca-P were examined using anthropogenic soils along a chronosequence from centennial to millennial time scales.
GLP compliance:
Remarks on result:
not measured/tested

Changes in bioavailability of phosphorus (P) during pedogenesis and ecosystem development have been shown for geogenic calcium phosphate (Ca-P). However, very little is known about long-term changes of biogenic Ca-P in soil. Materials and methods Long-term transformation characteristics of biogenic Ca-P were examined using anthropogenic soils along a chronosequence from centennial to millennial time scales. Results and discussion Phosphorus fractionation of Anthrosols resulted in overall consistency with the Walker and Syers model of geogenic Ca-P transformation during pedogenesis. The biogenic Ca-P (e.g., animal and fish bones) disappeared to 3% of total P within the first ca. 2,000 years of soil development. This change concurred with increases in P adsorbed on metal-oxides surfaces, organic P, and occluded P at different pedogenic time. Phosphorus K-edge X-ray absorption near-edge structure (XANES) spectroscopy revealed that the crystalline and therefore thermodynamically most stable biogenic Ca-P was transformed into more soluble forms of Ca-P over time. While crystalline hydroxyapatite (34% of total P) dominated Ca-P species after about 600–1,000 years, β-tricalcium phosphate increased to 16% of total P after 900–1,100 years, after which both Ca-P species disappeared. Iron-associated P was observable concurrently with Ca-P disappearance. Soluble P and organic P determined by XANES maintained relatively constant (58–65%) across the time scale studied. Conclusions Disappearance of crystalline biogenic Ca-P on a time scale of a few thousand years appears to be ten times faster than that of geogenic Ca-P.

Description of key information

Key value for chemical safety assessment

Additional information

Experimental data on the adsorption/desorption of tricalcium bis(orthophosphate) are not available. Testing the adsorption/desorption behaviour according to OECD Guideline 121 is not feasible as the test method is not validated for inorganic substances. A batch equilibrium study according to OECD Guideline 106 was not conducted since analysis of the test material may not be possible due to interference from the soil extracts that may leach into the aqueous media during the test. This would prevent quantification of the test material.

With exception of dissolved calcium that generally has a high mobility and occurs in soil solutions as Ca2+, Calcium is expected to adsorb to clay and organic matter in soil and thus to be relatively immobile in natural soils. However, the mobility strongly depends on the cation-exchange capacity of the soil. The availability of free calcium will increase with soil pH.

Phosphate adsorption is affected by numerous factors, e.g. pH, type and concentration of electrolytes, clay content, Al 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). Sato et al. (2009) observed that phosphorus released from calciumphosphate was adsorbed to aluminium and iron-oxyhydroxides. Basically, phosphate adsorption dominates in mineral soil with a low pH.



De Vos W. and Tarvainen T. (2006), Eds., Geochemical Atlas of Europe Part 2: Interpretation of Geochemical Mops, Additional Tables, Figures, Maps and Related Publications, EuroGeosurveys & Foregs, Espoo, Finland, 2006.

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.

Sato et al. (2009) Biogenic calcium phosphate transformation in soils over millennial time scales. Jorunal of Soils Sediments (2009) 9:194–205