The removal of fulvic acids in natural water can be realised by several physico-chemical processes: activated carbon adsorption, coagulation-flocculation and oxidation. Activated carbon adsorption remains one of the most important processes used in water treatment. Many workers have demonstrated that the value of pH, the ionic composition, the temperature, the salt concentration and the type of carbon act upon the capacity of activated carbon to remove humic substances [Lee, M. C., Snoeyink, V. L. and Crittenden, J. C. (1981) Activated carbon adsorption of humic substances. JAWWA, 440-446; Randtke, S. J. and Jepsen, C. P. (1982) Effects of salts on actived carbon adsorption of fulvic acids. JAWWA 74, 84-93; Weber, W. J., Jr., Voice, T. C. and Jodellah, A. (1983) Adsorption of humic substances: The effect of heterogeneity and system characteristics. J. Am. Wat. Wks. Ass. 75, 612-619; Lafrance, P. and Mazet, M. (1985) Adsorption des acides humiques sur charbon active en poudre. Influence du triphosphate de sodium. Wat. Res. 19, 1059-1064]. Whereas the association of activated carbon and coagulant-flocculent for the removal of organic matter is usually used in the case of an accidental or chronical pollution, only few workers have studied the mechanisms of elimination. The purpose of the present study was to evaluate the percent of removal of natural fulvic acids onto powdered activated carbon and in the presence of aluminium salts. Initially, one method of dosage of the fulvic acids has been chosen. The KMnO4 oxidizability seemed to be the more efficient method because the pH and the presence of aluminium don't affect the results. Secondly, curves of the function of the order of introduction of the components, function of the pH values and function of the presence or not of aluminium salts have been drawn. Our results (Fig. 9) show that the adsorption depends upon the values of the acidity constants (pK(a)) of the organic molecule, the order of introduction of the components, the PAC electrokinetic potential and the aluminium species (Al3+, Al(OH)(2+), Al(OH)(2)(+), Al(OH)(3), Al(OH)(4)(-),...). The adsorption of the lone organic molecule was more important at low pH (80% of removal at pH 3) than at high pH (10% of removal at pH 8) and the removal decreased strongly between pH 4 and 6; this is certainly due to the formation of anionic substances (fulvates). The electrostatic interactions for a pH greater than 5.5 between PAC and aluminium salts are probably responsible for a modification in the adsorption of natural fulvic acids. If the pH is lower than 5.5 the adsorption of the organic molecule is increased because of a complexation between the fulvic acids and the aluminium ions. Many workers have demonstrated that aluminium strongly complexes with organic molecules [Backes, C. A. and Tipping, E. (1987) Aluminium complexation by an aquatic humic fraction under acidic conditions. Wat. Res. 21(2) 211-216; Tipping, E., Backes, C. A. and Hurley, M. A. (1988) The complexation of protons, aluminium and calcium by aquatic humic substances: A model incorporating binding-site heterogeneity and macroionic effects. Wat. Res. 22(5) 597-611; Rakotonarivo, E., Tondre, C., Bottero, J. Y. and Mallevialle, J. (1989) Complexation de l'aluminium (III) polymerise et hydrolyse par les ions salicylate. Etude cinetique et thermodynamique. Wat. Res. 23(9) 1137-1145; Sikora, F. J. and McBride, M. B. (1989) Aluminium complexation by catechol as determined by ultraviolet spectrophotometry. Environ. Sci. Technol. 23(3) 349-356; Thomas, F., Bottero, J. Y., Masion, A. and Genevrier, F. (1990) Mechanism of aluminium III hydrolysis with acetic acid and oxalic acid. Geochemistry of the earth's surface and of mineral formation. 2nd International Symposium, pp. 2-8. Aix en Provence; Cathalifaud, G., Ayele, J. and Mazet, M. (1997) Etude de la complexation des ions aluminium par des molecules organiques: Constantes et stoechiometrie des complexes. Application au traitement de potabilisation des eaux. Wat. Res. 31(4) 689-698]. The particular role of aluminium salts and pH in the adsorption of natural fulvic acids indicated that the carbon efficiency in removing humic substances can be significantly improved by the addition of aluminium salts in solution. (C) 1998 Elsevier Science Ltd. All rights reserved.
