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SOIL CHEMISTRY

Modeling Phosphate Adsorption by Agricultural and Natural Soils

^a National Land and Water Information Service, Agriculture and Agri-Food Canada, P.O. Box 20280, 850 Lincoln Rd., Fredericton, NB, Canada E3B 4Z7
^b Dep. of Natural Resource Sciences, McGill Univ., Ste. Anne de Bellevue, QC, Canada H9X 3V9

^* Corresponding author (william.hendershot{at}mcgill.ca).

The nonideal competitive adsorption model (NICA) was developed from the Langmuir adsorption theory and the Freundlich empirical equation. It is well suited to describing ion adsorption in complex systems like soils, which have multiple ions, highly heterogeneous surfaces, and a variety of adsorption sites on the particle surfaces. It has been successfully used to model the binding of protons and metal ions to humic substances, but its application for the adsorption of anions to heterogeneous surfaces has not been documented. The purpose of this study was to adapt the NICA model to describe hydroxyl and phosphate adsorption. Results show that by considering two types of surface sites, the NICA model can provide an excellent fit (R² > 0.99) of the hydroxyl adsorption data obtained from –11 to –4 of log[OH], which corresponds to soil pH from 3 to 10. By using the parameters generated from hydroxyl adsorption, including adsorption maxima (Q_max,OH), binding strength (K_OH), and nonideality (m), the NICA model gave a remarkable goodness of fit (R² > 0.98) for the phosphate adsorption data obtained at different pH values. The model sensitivity test showed that the Type 1 surface (SOH₂⁺) has up to 100 times greater contribution to phosphate adsorption than the Type 2 surface (SOH). Thus, the model may be simplified into a three-parameter model by only considering the Type 1 surface for phosphate adsorption in acidic soils. The adapted NICA model can thus describe phosphate adsorption combined with hydroxyl adsorption and the parameters (n_PO4,1/n_OH,1 < 1, where n_PO4,1 accounts for nonideality of PO₄^3– and n_OH,1 accounts for nonideality of OH^–, both on a Type 1 surface) reveal the multidentate binding of phosphate. It provides a promising tool for analyzing competitive anion adsorption processes in soils.

Abbreviations: NICA, nonideal competitive adsorption