Determination of the aluminium complexing capacity of fulvic acids and natural waters, with examples from five New Zealand rivers

Abstract

An FIA technique with 7 s reaction time was used to analyse free plus labile Al in fulvic acid (FA) solutions and natural waters at pH 4.7, without the need for separation procedures. Titrations of these solutions using incremental pH or total Al were used to determine pH binding curves or estimates of the 'kinetic' Al complexation capacity (Al-CC_k) respectively. The operational definition of Al-CC_k relates to the capacity of a humic substance or natural water to bind Al through a 7-s FIA reaction time under defined experimental conditions of chromophore (CAS) concentration, ionic strength, and pH. Both Al binding strength and complexation capacity were greater than the corresponding Cu-CC (ISE) values. The Al-CCk measurements at pH 4.7 were 710 μmol Al g^-1 v. 590 μmol Cu g^-1. Al-CC_k results (pH 4.7) were higher for soil FA (710 μmol g^-1) than for aquatic FA (390 μmol g^-1). Al-CC_k results (pH 4.7) for five unfiltered river waters from different catchments gave results in the range 6.5-9.8 μmol Al L^-1. The differences between total (natural) Al in the samples and Al-CC_k were between 2.7 μM and 8.6 μM. Filtration experiments identified fractionation patterns between total (natural) Al and the fraction of Al-CC_k not utilized. The Al titration of alginate, another component of natural organic matter, is reported.