Research paper Structural, textural and protein adsorption properties of kaolinite and surface modified kaolinite adsorbents R. Duarte-Silva a , M.A. Villa-García b, ⁎, M. Rendueles a , M. Díaz a a Department of Chemical Engineering and Environmental Technology, University of Oviedo, Julián Clavería 8, 33006 Oviedo, Spain b Department of Organic and Inorganic Chemistry, University of Oviedo, Julian Clavería 8, 33006 Oviedo, Spain abstract article info Article history: Received 27 June 2011 Received in revised form 12 December 2012 Accepted 22 December 2013 Available online xxxx Keywords: Kaolinite Metakaolinite Organo–kaolinite hybrid material Structural properties Textural characteristics Whey proteins adsorption The structural, textural and protein adsorption properties of kaolinite from clay sedimentary deposits, metakaolinite obtained by thermal dehydroxylation of kaolinite, and the organic derivative prepared by reacting kaolinite with the silane coupling agent tert-butyldimethylchlorosilane, were studied. The retention capacities for the proteins α-lactalbumin (A-LA), bovine serum albumin (BSA) and β-lactoglobulin (B-LG) and the nature of the interactions responsible for protein binding were studied by adsorption experiments, performed at room temperature and pH 5.0. The protein adsorption capacity and the selectivity show a clear dependence on the chemical nature of the adsorbents surface and on the textural properties. Kaolinite behaves as a strong adsor- bent for A-LA and BSA, and exhibits a very high affinity for B-LG. Metakaolinite shows good retention capacity for A-LA and B-LG, but does not retain significant amounts of BSA. The adsorption capacity of the organo–kaolinite hybrid considerably increases for BSA and A-LA. FTIR results indicate the absence of hydrogen bonding between the adsorbents surface and the polypeptides. The interactions responsible for protein binding are closely related to the hydrophilic or hydrophobic character of the adsorbent surface and the amino acid composition of the proteins, steric effects also should be considered for the adsorption patterns. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The interactions of peptides and proteins with clay surfaces have been studied extensively in the past (Bujdák and Rode, 1997; Bujdák et al., 1996; Causserand et al., 1997; Ding and Henrichs, 2002; Fusi et al., 1989; Gupta et al., 1983; Quiquampoix et al., 1993, 1995; Rigou et al., 2006; Violante et al., 1995; Yu et al., 2000). Since clays are very abundant, their use as inorganic hosts is important not only from an economic perspective, but also for their unique physical and chemical properties, and the ease with which these materials can be modified and adjusted to new uses. Kaolinite has a wide variety of applications in industry (Bergaya and Lagaly, 2001; Bergaya et al., 2000; Braggs et al., 2006). This mineral has two different basal cleavage faces. One face consists of a tetrahedral siloxane surface with inert –Si–O–Si– links; the other basal surface con- sists of an octahedral gibbsite (Al(OH) 3 ) sheet (Frost et al., 2002a, 2002b). Kaolinite has different surface structures between base planes (001) and edge planes (110) and (010). The charge on the edges is due to the protonation/deprotonation of hydroxyl groups and depends on the pH of the solution. The hydroxyl groups located at the edge planes are considered the major reactive sites of kaolinite surfaces (Rausell-Colom and Serratosa, 1987). Kaolinite surface is hydrophilic but it can be rendered hydrophobic by reaction with organo–functional molecules (Dai and Huang, 1999). Organic derivatives of clays are generally obtained by using silane coupling agents (Dai and Huang, 1999; Ishida and Miller, 1985; Waddell et al., 1981). After surface modification, the organic groups can be attached to the clay by chemical bonding, adsorption and coating. Kaolinite surface can also be modified by thermal treatment. At temper- atures higher than 450 °C dehydroxylation occurs to form metakaolinite (Al 2 O 3 · 2SiO 2 ), and at 650 °C dehydroxylation is by ca 90% complete (Grim, 1968). The dairy industry generates many by-products with high protein contents. These residues can cause severe environmental contamina- tion when they are not properly disposed, and it is necessary to find so- lutions to prevent this pollution problem. Furthermore, the recovered proteins can be used to obtain high quality protein rich food products. Protein recovery by adsorption on various types of supports is a com- monly used technique; however, the use of clay minerals as adsorbents of protein molecules has received considerably less attention. The adsorption/desorption of proteins on clay surfaces is a complex process controlled by different factors, such as the surface properties of the adsorbent, the structural stability of the proteins, the ionic strength and the pH of the adsorption/desorption experiments (Haynes and Norde, 1994). In a recent work we found that kaolinite showed a high adsorption capacity at the isoelectric point (IEP) of each protein (Barral et al., 2008). Moreover, there was a clear correlation between the adsorption patterns and the presence of hydrophobic or hydrophilic Applied Clay Science xxx (2014) xxx–xxx ⁎ Corresponding author. Tel.: +34 985102976; fax: +34 985103446. E-mail address: mavg@uniovi.es (M.A. Villa-García). CLAY-02888; No of Pages 8 0169-1317/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.12.027 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay Please cite this article as: Duarte-Silva, R., et al., Structural, textural and protein adsorption properties of kaolinite and surface modified kaolinite adsorbents, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2013.12.027