Chromatographic techniques for the separation of Al and associated organic ligands present in soil solution

Abstract

Organic acids including humic, fulvic, aliphatic and aromatic acids comprise part of the dissolved organic carbon (DOC) present in soil solution. They act as ligands for trace metals and are effective detoxifiers of monomeric aluminium (Al). Solid phase extraction (SPE) techniques permit fractionation of the DOC into organic classes but yield no information on the pre-existing Al/organic acid complexes. Aliphatic and aromatic acids may be separated and determined by High Performance Liquid Chromatography (HPLC); however, the conditions used dissociate the organic acid Al complexes. Humic and fulvic acids are of a variable and ill-defined nature and only limited information exists regarding their binding of Al. This paper reports on fractionation studies of soil solutions, using both SPE and molecular weight cut-off filters, to characterise the DOC components and on the subsequent development of a size exclusion chromatography (SEC) system for the separation of organically complexed Al into different species using a low ionic strength mobile phase at pH 4.2. Model complexes of Al and Cu citrate were used to evaluate chromatographic performance of a Fractogel TSK HW-40(S) column (1×30 cm). For soil solution samples, the column eluate, after passing through a UV detector, was directly coupled to an Inductively Coupled Plasma Atomic Emission Spectrometer (ICPAES) for on-line multi-element detection to characterise DOC and trace metal distribution.

Fractionation studies revealed that polysaccharides constituted the major proportion of the DOC which passed the 10000 dalton molecular weight cut-off filter. Analysis of soil solutions from an organically amended soil by the SEC-ICPAES system showed that Al, Fe and Mn eluted as multiple peaks prior to the bed volume, indicating their presence as complexes with organic ligands.