Dynamics of Aluminum Complexation in Multiple Ligand Systems
- Michael A. Anderson and
- Paul M. Bertsch
The role of complexing ligands in attenuating aluminum (Al) toxicity and in influencing Al transport in soils and waters has been recognized for some time though the kinetics of such complexation reactions within complicated multi-ligand aqueous solutions have not been previously evaluated. As a result, this study was undertaken to examine the dynamics of Al complexation in the presence of multiple ligands, specifically fluoride (F), oxalate (Ox), and citrate (Cit), utilizing an ion chromatographic speciation method. The rate constant for formation (kf) of AlF2+ under the experimental conditions used in this investigation was found to be 12.3 ± 1.1 M−1s−1 in the absence of competing ligands, but was observed to increase to 23.4 ± 0.6 and 52.0 ± 5.7 M−1s−1 in the presence of Ox and Cit, respectively. The rate of formation of the AlOx+ complex was more rapid than that of AlF2+ (kf ≃ 100 M−1s−1) and was unaffected by the presence of F. Citrate complexed with Al sufficiently rapid that kf could not be determined. That kf for AlF2+ increased in the presence of competing ligands contradicts the accepted dogma on the kinetics of concurrent reactions, where reactions are envisioned to proceed independently and in relative isolation. This aberrance can be explained by one of two possible mechanisms: (i) the rapidly complexing Ox and Cit form reduced charge Al-species with resultant increased rates of water exchange (k); the increased kH2O exceeds the reduction in the extent of outer sphere Al-F association, thus creating a condition favorable for inner sphere penetration of F through displacement of a coordinated water molecule with ejection of inner sphere Ox or Cit; or (ii) F may exchange directly with inner sphere Ox or Cit, perhaps through HF. Additionally, AlCit formation in the presence of F was found to exceed the thermodynamically imposed equilibrium level and necessarily reequilibrated to a lower solution concentration as the slower Al-F complexation reaction proceeded, though no such behavior was witnessed for AlOx+ formation in the presence of F.
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