Interactions of aluminium and fulvic acid in moderately acid solutions:: stoichiometry of the H+/Al3+ exchange

Complexation with organic matter controls the activity of dissolved Al3+ in many soils. The buffering intensity of these soils is largely dependent on the H+/Al3+ exchange ratio, i.e. the number of protons consumed by the solid phase when one Al3+ is released. Here, the H+/Al3+ exchange ratio was determined from batch titrations using solutions of fulvic acid (FA) as a model for soil organic matter. Aluminium was added, from 1.04 to 6.29 mmol Al per g FA, which is within the range of humus-bound Al found in the upper B horizon of podzolized soils. Furthermore, pH was varied with NaOH to give values between 3.5 and 5.0. The H+/Al3+ exchange ratio ranged between 1.49 and 2.23 with a mean of 1.94. It correlated positively with pH and the total concentration of Al present. Theoretically, this can be explained with a partial hydrolysis of bound Al. The slope of logAl (log(10) of Al3+ activity) against pH generally underestimated the actual exchange ratio, which can partly be attributed to the systems being diluted (100 mg FA l(-1)). However, where 4 mmol Al or more had been added per g FA, the logAl slope gradually approached -3 between pH 4.5 and 5.0. This might be the result of a shift from Al3+ activity control by humus complexation to control by Al(OH)(3)(s).