Comparison of phosphate adsorption on clay minerals for soilless root media

Publisher Website
Google Scholar
Abstract
The greenhouse industry aims to decrease phosphate discharge to help reduce eutrophication of surface waters, to reduce fertilizer consumption, and to maintain a more constant level of plant‐available phosphate. Iron and aluminum oxides and some aluminosilicate minerals are efficient sorbents for phosphate. The phosphate adsorption characteristics of synthetic hematite (α‐Fe2O3), goethite (α‐FeOOH), and allophane (Si3Al4O12 nH2O), and a commercial alumina (A12O3) were evaluated to determine their potential for reducing phosphate leaching from soilless root media. The pH dependence of phosphate adsorption and maximum adsorption capacities were determined by reacting each mineral with various levels of phosphate between pH 4.0 and 9.0 in a 10 mM potassium chloride (KCl) background solution. Adsorbed phosphate was determined by loss from solution. Adsorption envelopes (adsorbed phosphate versus pH) showed a decrease in phosphate adsorption with increasing pH, particularly for alumina and allophane, and at greater added phosphate concentrations. The maximum adsorption capacities per unit mass of the minerals at pH 5.4 decreased in the order allophane > alumina ? goethite > hematite. When expressed on a surface area basis, the order of maximum adsorption capacity remains the same except that alumina exceeded that of goethite. The allophane, goethite, and alumina sorbed enough phosphate that 3 to 9 g of these minerals would retain the amount of phosphate required for a high nutrient element requiring plant such as chrysanthemum.