Aluminium ions organic molecules complexation: Formation constants and stoichiometry. Application to drinking water production

Aluminium salts are commonly used as reactants for coagulation-flocculation in the treatment of drinking water supplies. Powdered activated carbon (PAC) is also sometimes used to decrease accidental or chronic organic pollution. If organic matter has to be removed, powdered activated carbon and aluminium salts can be used simultaneously. The aluminium salts and natural organic matter such as fulvic acids will be competitive adsorbates on PAC. There could, furthermore, be complexation of organic matter and Al3+ ions. For this present study, phenol, pyrocatechol, phthalic, benzoic, salicyclic and tannic acids are generally considered to be simple models of humic matter. The complexation of these molecules with aluminium ions was investigated and, if possible, the stoichiometry of the complexes and the values of the apparent complex formation constant were determined. All the solutions were prepared with distilled water and filtered through a 0.2 mu m cellulose filter. The pH was adjusted to 4.6 with HCl or NaOH solutions. Measurements were performed by UV spectrophotometry methods, using a Schimadzu UV 160 A apparatus. The spectra of phenol (269 nm) and pyrocatechol (275 nm) were the same with or without Al3+ ions indicating that they probably did not complex (at pH 4.6) with Al3+ ions. At pH 4.6 there was a shift of the maximum adsorption of the spectrum (specific peak) for salicylic and benzoic acids (297 and 227 nm, respectively), but it was small enough to allow measurements. This shift revealed complexation of the acids and Al3+ ions, even though the ionization of these molecules depends on their pKa. (C) 1997 Elsevier Science Ltd.