Theelectrolyticdissolutionofthealuminumanodeproducesthe cationic monomeric species such as Al3+ and Al(OH)2+ at low pH, which at appropriate
M.Yousuf A. Mollah et al./Journal of Hazardous Materials B84 (2001) 29–41 37
pH values are transformed initially into Al(OH)3 and finally polymerized to Aln(OH)3n according to the following reactions: Al →Al3+ (aq) +3e− (1) Al3+ (aq) +3H2O→Al(OH)3 +3H+ (aq) (2) nAl(OH)3 →Aln(OH)3n (3) However,dependingonthepHoftheaqueousmediumotherionicspecies,suchasAl(OH)2+, Al2(OH)24+ and Al(OH)4− may also be present in the system. Examination of the pE–pH equilibrium diagram reveals that under appropriate conditions various forms of charged multimeric hydroxo Al3+ species may be formed. For example, the structures of dimeric and polymeric Al3+ hydroxo complexes are shown below: