This report is one of a series of technical reports prepared by staff of the British Geological Survey in support of the Overseas Development Administration (ODA) funded Technology Development and Research (TDR) Project No. 92/10 (R5557), Geochemistry and toxicity of naturally occurring aluminium.
A primary aim of the project is to develop methodologies to predict the vulnerability of highly weathered tropical terrain to aluminium toxicity by combining geochemical mapping and modelling techniques. A pre-requisite of the study is a simple and rugged analytical methodology suitable for the quantification and speciation (i.e. the physio-chemical nature and chemical associations) of aqueous aluminium (Al) under hydrochemical environments typical of surficial and shallow groundwaters associated with tropical terrain. The report describes the refinement and validation of techniques developed by (Dougan and Wilson, 1974) for the colorimetric determination of Al using pyrocatechol violet (PCV) and for the speciation of Al (Driscoll, 1984). In the later methodology, speciation of aqueous Al is accomplished by determining the amount of aluminium present in each of three broadly defined forms: acid reactive aluminium (i.e. that liberated by the addition of acid), total monomeric aluminium (e.g. total Al3+) and non-labile monomeric aluminium (e.g. Al present as Al3+ but strongly bound to other inorganic/organic ligands). From these measurements, three distinct fractions consisting of non-labile (non-reactive) monomeric organic aluminium; labile (reactive) monomeric inorganic aluminium; and monomeric hydroxide, fluoride and sulphate complexes can be identified. Determination of the labile form of Al is most important since this fraction includes the more toxic aluminium forms (e.g. free Al3+, Al2+OH and Al+(OH)2) which are able to reduce levels of available phosphate within soils and are potentially toxic to fish and mammals.
The methods have been refined to enable the accurate detection and measurement of Al in waters rich in dissolved iron and relatively high concentrations of fluoride. Investigations into the use of measured Fe concentrations to correct low level Al measurements and/or simultaneous equations to correct for Fe interference during Al measurement have been undertaken and validated. Detailed analytical methodologies and Al speciation procedures are given in sections 3 and 4 of the report to faciliate implimentation of the method in geochemical laboratories. Preliminary results and conclusions of the present investigations are given in sections 5 and 6 of the report.
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