Colloids and Surfaces A: Physicochem. Eng. Aspects 254 (2005) 49–55 Kinetics and adsorption of benzoate and salicylate at the natural hematite–water interface Manash R. Das, Dipak Bordoloi, Prakash C. Borthakur, Sekh Mahiuddin ∗ MaterialScienceDivision,RegionalResearchLaboratory,Jorhat785006,Assam,India Received 2 July 2004; accepted 26 November 2004 Available online 30 December 2004 Abstract Kinetics of adsorption of benzoate and salicylate onto the natural hematite surfaces have been carried out at pH 5 and at a fixed ionic strength (I =1 × 10 -4 mol dm -3 ). The state of equilibrium was attained at 144 and 70h for benzoate and salicylate, respectively. Adsorption behaviour of benzoate and salicylate onto the natural hematite was studied over a wide range of pH at a fixed I =5 × 10 -4 mol dm -3 NaCl and at different temperatures. The adsorption isotherms for both the adsorbates were Langmuir type up to pH 8. The Γ max for benzoate is ≈2.7–13.7 times more than that of salicylate depending on the pH of the suspension. Unlike benzoate, salicylate is not only adsorbed at one surface site but also covers around three or more surface sites of hematite surface. The rate constant for adsorption, K 1 for salicylate onto the natural hematite is greater than that of benzoate and increases with the increase in temperature and ≈4 times greater at a fixed temperature. The activation energy for the adsorption of salicylate onto the natural hematite is 2.9 times more than that of benzoate on the same adsorbent resulting in lower Γ max . © 2004 Elsevier B.V. All rights reserved. Keywords: Adsorption; Benzoate; Kinetics; Natural hematite; Salicylate; Zeta potential 1. Introduction In the mineral processing industries the use of ionic or non-ionic polymers, surfactant and polyelectrolytes is an es- sential proposition to liberate and separate minerals from the ore [1–7]. The humic acid or its alkali salt finds applica- tion for beneficiation of iron ore fines and slimes [1,7–10]. The interaction between humate and mineral surfaces are not well understood due to its polydispersity, polyfunctional- ity, polyelectrolytic characteristics and the positioning of the different functional groups [11,12]. Humic substances are supramolecular species composed of humin, humic and ful- vic acids, which can be separated according to their solubil- ity in different pH values. The structural investigation using pyrolysis–gas chromatography/mass spectroscopy showed that there are no significant differences between the chem- ∗ Corresponding author. Tel.: +91 376 2370081; fax: +91 376 2370011. E-mailaddress: mahirrljt@yahoo.com (S. Mahiuddin). ical composition of fulvic and humic acids except fulvic acid has more functional groups [13]. The results also in- dicated that fulvic and humic acids are built up of aromatic rings with predominately carboxylic and phenolic groups. The simple and well-defined acids like benzoic, salicylic and phthalic acid are the constituting models of the functional groups occurring in humic acid. Therefore, depending on the structure, functionality and the conformational factors, the adsorption profile and the surface complexation are different [14,15]. It is reported that -Fe 2 O 3 , unlike -Al 2 O 3 , is relatively inert and no oxy (hydroxides) like goethite (-FeOOH) and lepidocrocite (-FeOOH) are formed when exposed to air, hu- mid air and water for 3 months [16]. On the other hand, scan- ning tunnelling microscopic studies of hematite [17] showed that the uppermost Fe is partially hydrated when exposed in water or humid air due to the chemisorption of water vapour [18]. Therefore, adsorption profile of small organic acids hav- ing carboxylic and phenolic groups onto hematite is different 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.11.025