PH effect on kinetics of adsorption of organic and inorganic phosphates by short-range ordered aluminum and iron precipitates

Abstract

pH is an important factor affecting the adsorption of inorganic and organic phosphate compounds by soils and their components. The effects of solution pH on the kinetics of adsorption of orthophosphate (Pi), inositol hexaphosphate (IHP) and glucose 6-phosphate (G6P) by short-range ordered Al and Fe precipitates were investigated under pH-stat conditions. The adsorption reaction was monitored from 0.083 to 6 h at pH 4.5, 5.5 and 6.5 for the Al precipitate system, and 4.0, 5.0 and 6.0 for the Fe precipitate system, respectively. The adsorption reaction was described by first-order kinetics and divided into two stages, namely, fast adsorption before 0.5 h and slow adsorption after 0.5 h. Although the rate of adsorption of both organic and inorganic phosphates by Al and Fe precipitates generally decreased as pH increased, the individual reaction systems responded differently with pH changes. The pH effect was evident in the adsorption of Pi by both precipitates and IHP by Al precipitate. There was little or null pH effect on the adsorption of G6P by both precipitates. The proportion of P adsorbed over 6 h that was adsorbed at the end of 0.083 h decreased with increasing pH, suggesting that pH had more influence on the adsorption reaction at the beginning than at the later stage. The relatively low exchange ratio of OH to P for the Pi adsorption compared with the adsorption of organic phosphates was attributed to the dissociation of more protons from Pi ions than from IHP and G6P. The information on release kinetics of the surface hydroxyl groups strongly indicated that aquo groups on the surfaces were more reactive than hydroxo groups to phosphates and dominated the fast adsorption of phosphates.