Adsorption kinetics of organic phosphates on goethite and aluminium oxide: The equation used to describe the reaction

Adsorption kinetics of three organic phosphate compounds (OPs) with varying molecular sizes and structures and inorganic phosphate (Pi) were investigated on α-Al₂O₃ and poorly crystalline goethite. The organic phosphates were inositol hexaphosphate (IHP), glycerol phosphate (GlyP) and glucose-6-phosphate (G6P), and the inorganic phosphate was KH₂PO₄. Batch adsorption experiments were performed at 25°C. We tested the sorption kinetic data using various non-linear models/equations and on their transformed linear forms by applying appropriate statistics. Besides, we also used a modified non-linear equation having four parameters to this effect. Data were found to fit best with the modified equation and described the whole sorption process satisfactorily. For sorption of compounds on to the surface of these minerals, the equation with four parameters may be used in contrast with many standard equations applied for kinetic studies in soils. Sorption was described to take place in two processes: a fast one that takes place in less than 45 min and a slow one that takes place in several hours or more. The rate of the slow process did not depend directly on the concentration of phosphate compounds in solution, but depended linearly on the amount of phosphate that was adsorbed during the fast process. These initially adsorbed ions carrying some amount of negative charge likely hindered the movement of subsequent adsorbate ions to the solid surface due to decreased surface potential. This caused the variation in fast and slow sorption rate constants. Sorption densities increased in the order, Pi >Gly P >G6P >IHP, which revealed that the sorption density and initial sorption rate of OPs decreased with increasing molecular weights of OPs.