Competitive and Synergistic Effects in pH Dependent Phosphate Adsorption in Soils: LCD Modeling

• Published in: Environmental science & technology Vol. 45; no. 19; pp. 8420 - 8428
• Main Authors: Weng, Liping, Vega, Flora Alonso, Van Riemsdijk, Willem H
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.10.2011
• Subjects: Adsorption; Applied sciences; binding; Biological and physicochemical properties of pollutants. Interaction in the soil; Carbon - analysis; charge-distribution; Chemical elements; Chemical precipitation; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental Modeling; Exact sciences and technology; goethite; humic substances; Hydrogen-Ion Concentration; induced chemical-changes; ion adsorption; Ions; Ligands; Minerals - isolation & purification; Models, Chemical; Organic Chemicals - isolation & purification; Oxides - chemistry; parameters; Phosphates; Phosphates - isolation & purification; phosphorus; Phosphorus - analysis; Pollution; Pollution, environment geology; Soil - chemistry; Soil adsorption; Soil and sediments pollution; Solubility; solution interface; surface; Phosphates; Adsorption; Competitive reaction; pH; Environmental factor; Numerical simulation; Phosphorus compound; Agricultural soil; Soil pollution; Synergism
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es201844d

The pH dependency of soluble phosphate in soil was measured for six agricultural soils over a pH range of 3–10. A mechanistic model, the LCD (ligand charge distribution) model, was used to simulate this change, which considers phosphate adsorption to metal (hydr)oxides in soils under the influence of natural organic matter (NOM) and polyvalent cations (Ca2+, Al3+, and Fe3+). For all soils except one, the description in the normal pH range 5–8 is good. For some soils at more extreme pH values (for low P-loading soils at low pH and for high P-loading soils at high pH), the model over predicts soluble P. The calculation shows that adsorption is the major mechanism controlling phosphate solubility in soils, except at high pH in high P-loading soils where precipitation of calcium phosphate may take place. NOM and polyvalent cations have a very strong effect on the concentration level of P. The pattern of pH dependency of soluble P in soils differs greatly from the pH effects on phosphate adsorption to synthetic metal (hydr)oxides in a monocomponent system. According to the LCD model, the pH dependency in soil is mainly caused by the synergistic effects of Ca2+ adsorption to oxides. Adsorption of Al3+ to NOM adsorbed plays an important role only at a pH < 4.5. Presence of NOM coating strongly competes with phosphate for the adsorption and is an important factor to consider in modeling phosphate adsorption in natural samples.