Comparison between activated carbon, carbon xerogel and carbon nanotubes for the adsorption of the antibiotic ciprofloxacin

Abstract

A comparison was made for the adsorption capacity of ciprofloxacin (CPX) on three types of carbon-based materials: activated carbon, carbon nanotubes and carbon xerogel. The obtained samples were characterised by adsorption of N₂ at −196 °C, determination of the point of zero charge and by temperature programmed desorption. The Langmuir and Freundlich models were used to describe the equilibrium isotherms obtained. Adsorption capacities ranging from approximately 112 to 231 mg_CPX g_C⁻¹ were obtained (for carbon xerogel and activated carbon, respectively). The influence of the textural and surface properties of the carbon materials on the CPX adsorption was discussed and it was shown that the carbon nanotubes sample presents the best performance per unit surface area.

Introduction

Ciprofloxacin (CPX) is a synthetic antibiotic widely used for the treatment of several bacterial infections that can end up into water courses due to incomplete metabolism in humans or coming from effluents of drug manufacturers [1], [2], [3], [4], [5]. It has an almost planar configuration (Fig. 1) and approximate dimensions of 13.5 Å × 3 Å × 7.4 Å. The pK_a values for CPX are 5.90 ± 0.15 (for the carboxylic acid group) and 8.89 ± 0.11 for the basic-N-moiety [6], [7], so it can exist as a cation, zwitterion and anion under typical soil and water pH conditions, as shown in Fig. 1.

CPX presence in waters, even at low concentrations, can lead to the development of antibiotic resistant bacteria [1], [2], [3], [4], [5], [8]. It has been measured in streams and waste water influents and effluents at concentrations typically <1 μg L⁻¹; however, orders of magnitude higher concentrations have been measured in effluents from hospitals (3–87 μg L⁻¹) and drug production facilities (31 mg L⁻¹) [1], [2], [3], [7], [8], [9]. Even though its removal is of extreme importance, there are not many studies performed on it, when compared with other antibiotics. A few studies in literature deal with CPX adsorption on activated charcoal and talc [10], sorption by dissolved organic carbon [7], photodegradation [11], photo-Fenton oxidation processes [12], oxidation by chlorine and chlorine dioxide [13], [14] and ozonation [15], [16].

Carbon materials (namely activated carbon) are well known “universal” adsorbents and present unique advantages due to their low cost, high adsorption capacity and easy disposal [17], [18]. The performance of these materials depends greatly on their texture and surface chemistry [18], [19]. The presence of oxygen atoms on their surface originates a variety of functional groups that can interact with the adsorbents [18], [19], [20], [21]. The effect of surface oxygen groups on the adsorption of organic compounds has been studied on oxidised activated carbons [17], [22], [23], carbon xerogels [24] and carbon nanotubes [25], [26], [27]. In general, it has been found that increasing the presence of oxygenated groups (i.e., more acidic samples), has detrimental effects on adsorption [17], [22], [24], [25], [26], [27].

Activated carbons (mostly microporous samples) have been widely used in studies of adsorption of pollutants from wastewater [17], [22], [23], [28], [29], [30], [31]. Similar works have been reported for carbon nanotubes [25], [26], [27] and carbon xerogels [24]. The latter are mesoporous materials and their texture can be easily tuned by modifying their synthesis conditions [32], [33].

In the present work, the adsorption of CPX was studied on activated carbon, carbon xerogel and carbon nanotubes.