Etude de la complexation des ions aluminium par des molécules organiques: Constantes et stoechiométrie des complexes. Application au traitement de potabilisation des eaux

Abstract

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 Al 3+ 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 μm 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 Al 3+ ions indicating that they probably did not complex (at pH 4.6) with Al 3+ 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 Al 3+ ions, even though the ionization of these molecules depends on their pKa. The complex stoichiometry was evaluated by Job's method (Job, 1928). The excess function variation, the molar extinction coefficients for Al 3+ and acids and the value of the excess function for several wavelengths were given for salicylic acid (Fig. 3, Tables 1 and 2) and for benzoic acid (Fig. 4, Tables 3 and 4). For salicylic acid only one stoichiometry has been determined, for benzoic acid, three stoichiometries were found and they were to be (1-1) for Al 3+ and salicylic acid (Al-AS (1-1) and (3-1)(2–3) and (1–4) for Al 3+ and benzoic acid (Al-AB (3-1); (Al-AB (2–3)); (Al-AB (1–4)). The spectrophotometric absorbance of phthalic acid was determined at 235 nm. In the presence of Al 3+ ions the variations of the intensity of the specific peak of the spectrum were significant, certainly due to the formation of complexes between the aluminium and the phthalic acid. There was however no significant shift of the peak. Tannic acid (277 nm) was harder to analyze because of the non reproducible results and because, in the presence of Al 3+ ions, the modification of the spectrum was very great. Complexation was observed for these two molecules but Job's method did not give useful results, as shown in Fig. 5 for tannic acid, and complexation stoichiometry was not determined. For salicylic and benzoic acids, whose stoichiometries of complexes were evaluated, the Benesi-Hildebrand method was applied and the complexation constants were calculated. The different values of the parameters were given in Tables 7 and 8 for different wavelengths, and the final results showed that the (Al-AS (1-1)) complex was stable ( K = 10 4.00), so that (Al-AB (1–4)) with K = 10 3.00. The other constants for (Al-AB (2–3)) ( K = 10 0.70) and (Al-AB (3-1)) ( K = 10 0.04), on the other hand, show a certain weakness for these complexes. Only 70% of benzoic acid is ionized at pH 4.6 and it is easy to imagine the combination between the metal and the ligand for the third complex (log K = 3.00). For this percentage of ionization, the stoichiometries of the complexes would be: 4Al for 1AB; 1Al for 1AB and 1Al for 3AB. At pH 4.6, and only if the monomeric forms are taken into consideration, Al 3+ ions represent 82.7% of the total aluminium concentration, while Al(OH) 2 +, Al(OH) 2+ and Al(OH) 3 represent 13.8%, 1.7% and 1.7%, respectively. The polymeric forms were not taken into consideration in this work, but the maximum aluminium concentration is 10 −4 mol 1 −1 and, under these conditions, the hydrolysis products of aluminium ions are mainly monomers with rapid formation kinetics. Only Al 3+ ions were studied here and it could be thought that Al(OH) 2 + ions could be involved in the complexation with salicylic and benzoic acids. All our organic complexes with Al 3+ are positively charged. With the monomeric form Al(OH) 2 +, or the polymeric forms such as Al 2(OH) 4+ 2, Al 2(OH) 5+ 4 etc. other complexes could appear, especially in water treatment plants where aluminium concentrations can range from 20 to 120 mg l −1.