3. Factors affecting phosphate adsorption in soils

#### 3.1 Time

Adsorption of phosphate by soils increases with increasing reaction time, first rapidly and then slowly, but without reaching a true equilibrium [18]. Increasing phosphate concentration increases with the equilibrium time.

The slow reaction of phosphate with oxides has been attributed to formation of iron phosphate, with a surface coating on the oxides [19]. The porous structure often observed in goethite may similarly not only account for slow phosphate adsorption but also for slow desorption and thus irreversibility [20]. Accordingly, adsorption of phosphate by well crystallised goethite having few pores was complete after 3 days and remained constant up to 260 days [21]. According to Schwertmann [22], aluminium substituted goethite crystals are generally smaller and less porous than non-substituted goethite crystals therefore the former should adsorb phosphate faster and reach equilibrium faster than the latter. Furthermore,

self-aggregation (clustering) and porosity seem to be important factors in controlling adsorption/desorption (irreversibility) of phosphate by iron oxides and thus by soils. Formation of iron phosphate coatings has, however, been rejected by others [9], who considered migration (diffusion) of phosphate into aggregated iron oxides, particularly ferrihydrite, to cause the slow reaction.
