Journal of Colloid and Interface Science 316 (2007) 260–267 www.elsevier.com/locate/jcis Influence of anions on the adsorption kinetics of salicylate onto α -alumina in aqueous medium Jayanta M. Borah, Manash R. Das, Sekh Mahiuddin ∗ Material Science Division, North-East Institute of Science & Technology, Jorhat-785 006, Assam, India Received 18 May 2007; accepted 30 July 2007 Available online 29 September 2007 Abstract The adsorption kinetics of salicylate on α-alumina surfaces were studied at 25 ◦ C and pH 6 in the presence of 0.05 mM concentration of different anions (Cl − , Br − ,I − , SCN − , HCOO − , CH 3 COO − ,S 2 O 2− 3 , CO 2− 3 , and SO 2− 4 ) as a function of time. The experimental data were significantly better fitted to a pseudo-second-order kinetics equation of nonlinear form in the entire time duration and are in excellent agreement with corresponding estimated values. Considering adsorption data for salicylate in the presence of Cl − as the face value, all the monovalent anions (Br − ,I − , SCN − , HCOO − , CH 3 COO − ) promote the adsorption of salicylate onto α-alumina surfaces while the divalent anions (S 2 O 2− 3 , CO 2− 3 , and SO 2− 4 ) have the reverse effect under similar conditions. DRIFT spectra of α-alumina treated with salicylate reveal that the symmetric peak ν s (–COO − ) is shifted by ∼40 cm −1 to a lower wavelength region, which implies that salicylate forms an inner-sphere complex with α-alumina surface in the presence of both mono- and divalent anions.  2007 Elsevier Inc. All rights reserved. Keywords: Adsorption; Kinetics; Salicylate; Alumina; Anion effect; DRIFT 1. Introduction In the scope of adsorption of small organic (aliphatic and aromatic) anions on a metal oxide and oxy(hydroxide), gener- ally monovalent anions like Cl − , NO − 3 , and ClO − 4 (alkali salt) in the millimolar concentration range are used as the background electrolyte. Studies on the use of other monovalent and divalent anions as the background electrolyte and their influence on the adsorption of organic anions on metal oxide surfaces, despite marked depression of the zeta potential of an adsorbent [1,2], are seldom seen in the literature [3–7]. Adsorption of organic anions on metal oxides and oxy(hydr- oxides) depends on the pH vis-a-vis the surface charge of an adsorbent, notably the zeta potential. The monovalent anions at a particular concentration in the millimolar range that suppress the zeta potential of an adsorbent without altering the isoelec- tric point, except with iodide, are called indifferent electrolytes. * Corresponding author. Fax: +91 376 2370011. E-mail address: mahirrljt@yahoo.com (S. Mahiuddin). But in the case of divalent anions, the zeta potential of metal oxides and oxy(hydroxides) is drastically suppressed with low- ering of isoelectric points and reversal of zeta potential of an adsorbent [1,2]. Therefore, the adsorption behavior of an or- ganic anion onto an adsorbent in the presence of anions of 1:1 and 1:2 background electrolytes would be different. On the other hand, adsorption kinetics of an adsorbate (metal cations and organic anions) onto an adsorbent is generally ex- plained by using pseudo-first-order [8,9] and pseudo-second- order [9,10] kinetics equations. The pseudo-first-order kinetics equation fails to fit the experimental adsorption of an adsorbate at the metal oxide–water interface and the estimated equilib- rium concentration is many times lower than the experimental value [11,12]. Whereas for estimation of rate constant and the equilibrium concentration for the adsorption process, kinetics data are fitted to the pseudo-second-order kinetics equation of linear form. The estimated equilibrium concentration is based on the linear form of the pseudo-second-order kinetic equation, and even though roughly comparable with the experimental value, the results are not true [13,14]. 0021-9797/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.07.074