Speciation of aluminium in environmental systems
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis is directed towards the development of a speciation protocol for Al in natural waters and soil solutions. Factors that might influence the accuracy of the speciation results are investigated. The equilibrium reactions between aluminium(III) and (i) pyrocatechol violet (PCV) and (ii) 4-nitrocatechol have been studied by potentiometric titration in aqueous solution, I=0.10 M K(CI), 25.0 dc. The adsorption behaviour of pyrocatechol violet on the surface of the aluminium-oxide hydroxide boehmite [α- AlO(OH)] has been investigated. In this work, the experimental data for surface complexation were evaluated on the basis of the electrostatic constant capacitance model. The acidity constants of the surface hydroxy groups at the boehmite-solution interface were evaluated as a prerequisite to studying the ligand adsorption behaviour. This work and the equilibrium reactions between aluminium(III) and pyrocatechol violet (PCV) were performed at Umea University, Sweden. A comparison of the chromophores chrome azurol S (CAS), eriochrome cyanine R (ECR) and pyrocatechol violet (PC V) for the determination of Al was made. These studies were performed using computer modelling calculations. The pH and mass action effects were calculated for the representative 'interferents' citrate, oxalate, salicylate and fluoride. The effects of complexation kinetics on the use of PCV for the determination of 'labile' Al in the competitive systems: 'PCV- Al ³+-citrate 1 and 'PCV-Al³+-oxalate' was investigated. These studies also validated the 'H+ -Al ³+ -pyrocatechol violet' equilibrium data for typical analytical conditions. A preliminary survey was made of the electrochemical properties of a series of Al ³+-ligand systems using cyclic voltammetry. This was to identify redox-active ligands which may be used as electrochemical probes for Al. Flow injection analysis (FIA) methods for the determination of Al which exploit the indirect electrochemical detection of AI using the ligands pyrocatechol violet and tetrahydroxy-l,4-quinone were investigated. The development of an Al ³+ speciation procedure is described. It is based on a 2-s reaction with oxine-derivatised Fractogel positioned in a 22 μL column reactor in an FIA manifold. A1³+ (pre )concentrated on the column from a 650 J.LL sample was selectively eluted (via the rapid quantitative conversion of resin-bound Al ³+ to the Al (OH)4- ion) with 150-250 μL of 0.02 mol L-1 NaOH and detected spectrophotometric ally as the Al -CAS (chrome azurol S) complex at pH 5.0. The resultant FIA method was tested for (i) the selective separation of reactive Al from Fe, (ii) the sequestering of Al from its citrate, oxalate, malonate and fluoride complexes and (iii) retention of Al -hydroxy polymers [e.g. Al13(OH)327+] by the adsorbent. The method was applied to humic waters and soil solutions and the results for 'free' Al ³+ were compared with those obtained by the previously published 7-s CAS method. The method's detection limit and linear working range were evaluated. The potential for further development of this Al speciation technique is discussed.