Purification of Biotechnological Carboxylic Acids with an Adsorption Method Using Single-Walled Carbon Nanotubes O ¨ zge Nurhayat Arslanog ˘lu, † I ˙ smail I ˙ nci, † and S ¸ ahika Sena Bayazit* ,‡ Istanbul University, Engineering Faculty, Chemical Engineering Department, 34320, Istanbul, Turkey, and Beykent University, Engineering and Architecture Faculty, Chemical Engineering Department, 34396, Ayazag ˘a, Istanbul, Turkey Adsorption is a very important and valuable method for separation of carboxylic acids from aqueous solutions. In this study, single-walled carbon nanotubes (SWCNT) were used as an adsorbent for this purpose. Important biotechnological acids, tartaric acid and citric acid, were used. The equilibrium time and the adsorption capacity at three different temperatures (25 °C, 35 °C, and 45 °C) were investigated. Reaction kinetics and the diffusion between the acid molecules and SWCNT pores were investigated. The aim of this study was to determine the adsorption capability of SWCNT for carboxylic acids of biological importance from their fermentation media. Introduction Carbon nanotubes (CNTs) have porous-rich structures. That means they have very high adsorption capabilities, and CNTs can be used for removing many kinds of organic and inorganic pollutants. 1,2 Carbon nanotubes are hexagonal carbon networks that are capped by half fullerene molecules. There are two main types of carbon nanotubes: single-walled and multiwalled carbon nanotubes. Multiwalled carbon nanotubes (MWCNTs) were discovered by Iijima in 1991 and the related single-walled carbon nanotubes (SWCNTs) in 1993. 3,4 CNTs have excellent versatility and wide application fields. 5 Single-walled carbon nanotubes have become one of the most widely studied nanomaterials, primarily because of their unique physicochem- ical properties and wide-ranging applications in solid-state nanoelectronics, nanocomposites, nanolithography, sensing, and high-resolution imaging. 6 The rich-porous structure is the most important characteristic of an adsorbent that is used in carboxylic acid adsorption. SWCNTs were chosen in this study for their characteristic porosity. Carboxylic acids have a wide range of application fields, such as in the pharmaceutical, polymer, food industries, etc. Because of this, recovery of carboxylic acids is an important process in chemical engineering. In this study, a dicarboxylic acid (tartaric acid) and a tricarboxylic acid (citric acid) were adsorbed from their aqueous solutions by SWCNTs. 7-9 Citric acid is an important product in many industrial fields. The food industry is the largest consumer of citric acid. The amount of citric acid produced is more than one million tons per year. 10,11 Tartaric acid is used in the food and pharmaceutical industries. Wastewater from the winery industry contains tartaric acid. 12 It can be recovered from this wastewater. All these acids are produced by the fermentation method. Purification of the carboxylic acids from fermentation broth is a very challenging process. Adsorbents must have selectivity for these acids. In this study, the adsorption experiments were carried out from aqueous solutions of the carboxylic acids. The aim is to test the adsorption capability of SWCNTs. Numerous researchers have investigated the adsorption of carboxylic acids from aqueous solutions by using different adsorbents. Kawabata et al. separated carboxylic acid using a polymer sorbent with a pyridine skeletal structure and a cross- linked structure. The polymer sorbent showed good selectivity and high adsorption capacity for carboxylic acids even in the presence of inorganic salts. 8,13 SWCNTs are generally used for metal and protein adsorptions. For example, Lu et al. adsorbed Ni 2+ from aqueous solution using SWCNTs. 14 Karajanagi et al. adsorbed enzymes onto SWCNTs and investigated the structure and function of the adsorbed enzymes. 6 However, there is not enough information about carboxylic acid adsorption onto CNTs in the literature. The aim of this study is to define the sorption capability of SWCNTs, when used for the adsorption of carboxylic acids from aqueous solutions. Material and Methods Material. Tartaric acid and citric acid were obtained from Merck. SWCNTs were obtained from Sigma-Aldrich. SWCNTs are (40 to 60) % (by mass) carbon basis, and diameter × length is (2 to 10) nm × (1 to 5) µm and used without purification and activation. Acid solutions were prepared approximately (3, 6, 9, 12, and 15) %. Method. Adsorption experiments were carried out in a batch type. The aim of these experiments was to determine the effect of important variables on the adsorption, such as time, initial acid concentration, and temperature. For each experimental run, 3 mL of stock acid solution and a known amount of adsorbent were taken in a 50 mL flask. This mixture was shaken at a constant speed and temperature in a thermostatted shaker. A sample was taken out periodically, and the aqueous phase was titrated with 0.1 M NaOH solution and phenolphtalein used as an indicator. SWCNT particles remain suspended and do not settle down easily. Therefore, all the samples were separated with filter paper and then analyzed. The period of equilibration was determined as 70 min for citric acid and 80 min for tartaric acid. For determining the temperature effect on adsorption, five * Corresponding author. E-mail: sahikasena@gmail.com. † Istanbul University. ‡ Beykent University. J. Chem. Eng. Data 2010, 55, 5663–5668 5663 10.1021/je100596p  2010 American Chemical Society Published on Web 11/11/2010