The effects of pH and drying on adsorption of phosphate by aluminium‐organic matter associations

Abstract

SUMMARYThe effects of pH value and of drying on phosphate adsorption by Al‐peat and Al‐humate associations were investigated. Raising the pH of the Al‐organic matter associations from 3 to 7, by addition of Ca(OH)2, greatly increased their phosphate adsorption capacity. There was a pronounced increase in phosphate adsorption as a result of drying the limed, Al‐organic matter associations, but when equilibration time was increased phosphate adsorption by moist Al‐peat samples approached that of the dried samples, indicating that the drying effect was at least partially caused by incomplete equilibration of P in the moist samples. In contrast, increasing the equilibration time did not alter the magnitude of the pH effect.A detailed study was made of the adsorption of phosphate both by Al‐peat limed to pH 7.0, and by its constituent components Ca‐peat (pH 7.0) and hydroxy‐Al (pH 7.0), synthesized separately then mixed together. For limed Al‐peat, drying increased phosphate adsorption, whereas for the individually synthesized components in combination the reverse was true. Thus, the limed Al‐peat behaved differently to its components.The increasing adsorption of phosphate by Al‐organic matter complexes with rising pH was attributed to increasing hydrolysis and polymerization of hydroxy‐Al associated with the organic matter. However, the interaction of organic matter with hydroxy‐Al would block many phosphate adsorption sites on the hydroxy‐Al. Upon drying, it is suggested that the organic matter condenses and the accessibility of phosphate to adsorption sites on the hydroxy‐Al is increased. Thus, drying resulted in increased phosphate adsorption.