. 2009. Effect of humic acid coating on arsenic adsorption on ferrihydrite-kaolinite mixed systems. Can. J. Soil Sci. 89: 421Á434. The reactivity of the colloids formed in soil environments results from complex interactions between clay minerals, (oxy)hydroxides and organic matter of soils. The adsorption/desorption equilibria and kinetics of anionic and cationic species on/from natural colloids could substantially differ compared with their interaction with pure minerals. The objective of this study was to investigate the arsenite and arsenate equilibria and the kinetics of arsenite adsorption on ferrihydrite-coated kaolinite as influenced by the presence of humic acid coverage. The Langmuir adsorption maximum and affinity of arsenite and arsenate adsorption on the kaolinite-ferrihydrite system was reduced by the presence of the humic acid. The enthalpy of adsorption on the kaolinite-ferrihydrite system, measured with isothermal titration calorimetry, was (24.4 kJ mol(1 for arsenite and (39.9 kJ mol(1 for arsenate, and was only slightly decreased by the humic acid coating. The kinetics of arsenite adsorption was substantially influenced by the presence of the humic coverage, resulting in the decrease in the rate constant, the Arrhenius heat of activation, and the pre-exponential factor (indicative of the collision frequency of the sorbate and the sorbent). The hampering effect of the humic acid on arsenic adsorption was attributable to the blocking by the organic ligand of the adsorbing sites, coupled with the steric hindrance of the large organic molecules, which restricted the arsenic to approach the surfaces.Key words: Arsenic, adsorption, kinetics, kaolinite, ferrihydrite, humic acid Martin, M., Celi, L., Barberis, E., Violante, A., Kozak, L. M. et Huang, P. M. 2009. Incidence de l'acide humique sur l'adsorption de l'arsenic dans les syste`mes mixtes ferrihydrite-kaolinite. Can. J. Soil Sci. 89: 421Á434. La re´activite´des colloı¨des forme´s dans le sol de´pend des interactions complexes entre les mine´raux pre´sents dans l'argile, les (oxy)hydroxydes et la matie`re organique du sol. L'e´quilibre entre adsorption et de´sorption, et la cine´tique des anions et des cations sur les colloı¨des naturels ou qui en e´manent pourraient sensiblement diffe´rer face a`ce qu'on observe dans les interactions avec les mine´raux purs. La pre´sente e´tude devait pre´ciser l'e´quilibre entre les arse´nites et les arse´niates ainsi que la cine´tique de l'adsorption des arse´nites sur de la kaolinite enduite de ferrihydrite en pre´sence d'une couche d'acide humique. L'acide humique re´duit le maximum d'adsorption de Langmuir et l'affinite´des arse´nites ainsi que l'adsorption des arse´niates dans le syste`me kaolinite-ferrihydrite. Mesure´e par calorime´trie a`titrage isotherme, l'enthalpie de l'adsorption dans le syste`me kaolinite-ferrihydrite s'e´tablit a`(24.4 kJ par mole pour les arse´nites et a`(39,9 kJ par mole pour les arse´niates; la couche d'acide humique ne la re´duit que le´ge`rement. La cine´tique de l'adsorption...