Equilibrium studies for trimethoprim adsorption on montmorillonite KSF

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Abstract

In this study, the adsorption of trimethoprim (TMP) on montmorillonite KSF was studied under different conditions (pH, ionic strength, temperature). The results indicate that a pH value of 5.04 is optimum value for the adsorption of TMP on KSF. The adsorption kinetics was interpreted using pseudo-first-order kinetic model, pseudo-second-order kinetic model and intraparticle diffusion model. The pseudo-second-order model provides the best correlation with the experimental data of KSF adsorption. The adsorption data could be fitted with Freundlich, Langmuir and Dubinin-Radushkevich equation to find the characteristic parameters of each model. It was found that linear form of Langmuir isotherm seems to produce a better model than linear form of Freundlich equation. From the Langmuir and Freundlich equation, the adsorption capacity values raised as the solution temperature decreased. From DR isotherm, it was also determined that the type of adsorption can be considered as ion-exchange mechanism. Determination of the thermodynamic parameters ΔH0, ΔS0 and ΔG0 showed that adsorption was spontaneous and exothermic in nature. It was also added that adsorption of TMP by KSF may involve physical adsorption.

Introduction

Human and veterinary drugs are continually being released in the environment mainly as a result of manufacturing processes, disposal of unused or expired products, and excreta [1]. Antibiotics have wide range of uses in both human and veterinary medicine. In the livestock industry, the use of antibiotics as growth promoters as well as therapeutic agents is very common [2]. As a result, it is possible to determine antibiotics in the environment. Antibiotics have been determined in diverse environmental samples, e.g. wastewater [3], ground and river water [4], [5], hospital wastewater [6], sludge [7], soil and manure [8], [9]. For that reason, antibiotics released into the environment are of concern for the following reasons; (i) contamination of raw, treated and recycled water used for drinking, irrigation and recreation; (ii) potential to accelerate wide spread bacterial resistance to antibiotic; (iii) negative effect on important ecosystem bacteria (through death or inhibition) [10]. Their continual input into the environment may lead to a high, long-term concentration and promote unnoticed adverse effects on aquatic and terrestrial organisms. Effect can accumulate so slowly that changes remain undetected until they become irreversible [1].

Trimethoprim (TMP) is among the most important antibacterial agents (synthetic antibiotics) used in human and veterinary medicine worldwide acting as an inhibitor in the chemotherapy treatment due to its antifolate effect by interaction with dihydroflate coenzymes [11], [12]. In respect of many articles, TMP was detected at 0.6–7.6 μg L−1 level in hospital sewage water in Sweden [13], at 0.12 and 0.16 μg L−1 levels in wastewater effluents from East Aurora and Holland, respectively [2], and 12.4 mg/kg level in manure and soil in a German farming area [14], and at 40–705 ng L−1 in two municipal wastewater treatment plants in USA [15], and 0.013–0.15 μg L−1 in US streams [5]. Additionally, a study made in Australia showed that TMP was detected in effluent entering receiving waters and detectable at 50 m from the site of discharge, and also in one bacterium (E. coli), resistance was observed to TMP [10]. Trimethoprim, 2,4-diamino-5-(3,4,5-trimethoxybenzyl)-pyrimidine is shown in Fig. 1.

It is impossible and impractical to halt usage of antimicrobial agents within our environments, though management of their overuse and disposal within catchments is likely to be an environmental management agenda in the near future [10].

To overcome destructive impacts of antibiotics released into the environment, it is very significant to design and develop a system to remove pharmaceuticals present in wastewater treatment plants. For this purpose, adsorption is appropriate process that is widely used in the removal of contaminants (e.g. pesticides) from wastewater. In view of the above fact that clays have high specific surface areas, high adsorption capacity, mechanical stability and variety of surface and structural properties, they are used as adsorbents for removing pollutants. This work reports the adsorption of trimethoprim onto montmorillonite KSF from aqueous solution as function of pH, temperature and ionic strength. Adsorption capacity of KSF for trimethoprim has been determined by using some common adsorption equilibrium models. In order to understand the adsorption process, kinetic and thermodynamic parameters obtained have also been evaluated at different temperatures.

Section snippets

Materials and methods

KSF supplied from Fluka Company was utilized for adsorption experiments as adsorbent. The properties of KSF were summarized in Table 1.

TMP was purchased from Merck and used without further purification. In the preparation of 10−3 M TMP stock solution, 0.02903 g TMP was weighed and dissolved in the mixture of 60 mL H2O and 40 mL C2H5OH. Further solutions were freshly prepared from stock solution for each experimental study. For batch adsorption experiments, 0.1 g of KSF was added into 25 mL of TMP

Adsorption isotherms

The adsorption isotherm of TMP onto montmorillonite KSF was determined by plotting the amount of TMP adsorbed by KSF (Cs, mmol g−1) versus the equilibrium concentration of TMP (Ce, mmol L−1). The adsorption isotherms of TMP from aqueous solution onto KSF at different temperatures with initial pH 3.0 and without any salt addition are shown in Fig. 2. In terms of the slope of initial portion of the curves, the shapes of these isotherms correspond to L-type according to the Giles classification [16]

Thermodynamic parameters

In order to obtain activation energy of sorption process, Arrhenius equation was used in the following form:lnk2=lnAEaRTwhere Ea is the activation energy of sorption, k2, the pseudo-second-order rate constant, A, the Arrhenius constant, R, the gas constant (8.314 J mol−1 K−1) and T is the solution temperature (K). The Ea value is obtained from the slope of Arrhenius plot of ln k2 against 1/T. The importance of activation energy is that it is used to determine the type of adsorption (physical or

Conclusion

To get under control the release of drugs to environment, montmorillonite KSF has been suggested for removing drugs from wastewater and hospital sewage water. If releasing of drugs to environment goes on in this way, antibiotics and resistant bacteria will increase in the local waterways and this will affect the biotic process because of transferring resistance genes from harmless bacteria to pathogenic bacteria and on to humans interacting with the aquatic environment. Additionally, it is

Acknowledgement

The Authors also thank to Murat Kuşoğlu (Dokuz Eylül University, Faculty of Engineering, Department of Metallurgical and Materials Engineering).

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