Direct Effects and Interactions Involving Iron and Humic Acid During Formation of Colloidal Phosphorus

Abstract

Precipitation and complexation reactions involving factorial treatment combinations of iron (Fe), phosphorus (P), and humic acid (HA) in aerobic, slightly alkaline (pH 8.65) media permitted differentiation of direct effects and interactions among these factors, which result in formation of colloidal (nondialyzable) P. The range of concentrations studied were 0.05 to 0.2 mg total P/1, 0.1 to 100 mg total Fe/1, and 0.05 to 50.0 mg HA (Aldrich)/1. The data showed that HA, in contrast to Fe, had no capacity to bind inorganic P directly to form a colloidal phase. However, the extent of formation of colloidal P by Fe varied inversely with the amount of HA and inorganic P added to the experimental systems. Adsorption isotherms grouped according to the Fe/HA ratio (w/w) of the treatment combinations indicated occurrence of a maximum adsorption density of 0.1 mg P/mg Fe at a ratio of approximately 0.2:1. The results suggested that HA in increasing amounts decreased the polynuclear nature of the colloidal Fe and, coincidently, changed the number of sites for adsorption of P.