International Proceedings of Chemical, Biological and Environmental Engineering, Vl. 11 (217) DOI: 1.7763/IPCBEE. 217. V11. 19 Use of Organo-Montmorillonite Nanoclay as an Environmentally Friendly Adsorbent for Removal of Hexavalent Chromium Havva Tutar Kahraman 1, Erol Pehlivan 2 1, 2 Department of Chemical Engineering, Selcuk University, Konya, TURKEY Abstract. In this work, an organo-montmorillonite nanoclay (n-ommt, commercial name; Cloisite 3B), modified with methyl, tallow, bis(2-hydroxyethyl) quaternary ammonium (MT2EtOH) ion, was used as an adsorbent to remove hexavalent chromium (Cr 6+ ) from aqueous solutions. Adsorption of hexavalent chromium (Cr 6+ ) ions was carried out in a batch arrangement. Experiments were performed as a function of solution ph, solute concentration, adsorbent concentration and contact time. Based on the difference in (Cr 6+ ) contents (measuring by a UV vis spectrophotometer) before and after adsorption, the removal percentage was calculated. The maximum adsorption capacity of organoclay was obtained at ph=2 and it tends to diminish with increasing ph from 1 to 5. The applicability of the Langmuir, Freundlich and Scatchard adsorption isotherms was tested. The adsorption isotherms of chromium are well fitted to Langmuir isotherm equations. The results from this study demonstrate that organo-montmorillonite could be used as an adsorbent for remediation of wastewater. Keywords: adsorption, hexavalent chromium, montmorillonite, organoclay. 1. Introduction In recent years, water treatment became a major issue worldwide. Different type of pollutants such as organic compounds, oxyanions, and heavy metal ions rise due to the industrial activities. Heavy metals are one of the main pollutant groups of water and wastewater. The protection of environment contaminated by heavy metals generates a great need to enhance the water treatment technologies and efficient adsorbents. Various physicochemical and biological techniques for removal of heavy metals have been studied [1]. Among treatment technologies adsorption process has been paid increasing attention due to low cost, flexibility and simplicity of design, ease of operation, insensitivity to toxic pollutants and avoiding the formation of secondary pollutants [2]-[4]. A great number of adsorbents have been reported in related literature including resins; biomaterials such as chitin, chitosan, starch; natural materials (clays, siliceous materials, and zeolites) and agricultural materials [5]-[1]. The selection of suitable adsorbent is the main significant point in adsorption processes. Because most adsorbents can adsorb only one type of contaminants such as organic contaminants or heavy metal ions based on their surface properties. However, waste water contains miscellaneous pollutants having a necessity of using different adsorbents for each pollutant. This type of treatment method increases the cost of process. Therefore, a need arise to develop new alternative adsorbents which can remove both organic contaminants and heavy metals from aqueous medium. Clays are naturally occurring attractive materials which have some important properties such as their charge, uptake abilities, high surface areas, colloidal or swelling capacities and high cation exchange capacities. Generally, organoclays can provide effective performance on removing non-polar organic pollutants such as oils, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, chlorinated solvents and gasoline hydrocarbons from waste water. However, there have been very few studies on the ability of this kind of adsorbents for removal of heavy metals. If organoclays show an adsorption behavior for heavy Corresponding author. Havva Tutar Kahraman Tel.: + 93322232242; fax: +9332241635. E-mail address: havvatutar@gmail.com 136
metals at a variety of environmental ph values, they could be effective and preferable adsorbents for mixedwaste systems containing relatively nonpolar organic pollutants and heavy metals. They can be characterized by their excellent cation exchange capacity and high surface area for efficient adsorption of heavy metals [11]. Hereby, we aimed to evaluate the adsorption performance of organo-montmorillonite nanoclay (n-ommt) on the removal of Cr 6+ ions. The effect of solution ph, contact time, adsorbent concentration and initial Cr 6+ ion concentration were optimized for the maximum adsorption capacity. These parameters would be useful in the understanding of specific adsorption of heavy metals by n-ommt from an aqueous environment. Also, obtained results can lead further studies based on the evaluation of simultaneous adsorption of organic pollutants and different heavy metal ions. 2µm 2. Materials and Method Fig. 1: Chemical and morphological structure (SEM image) of n-ommt. Organo-montmorillonite nanoclay (n-ommt, commercial name; Cloisite 3B), modified with methyl, tallow, bis(2-hydroxyethyl) quaternary ammonium (MT2EtOH) ion was provided by Southern Clay Products (Fig. 1). Aqueous solutions of Cr 6+ were prepared by dissolving K 2 Cr 2 O 7 powder with deionized water..1 M HCl and.1 M NaOH solutions were used to adjust the ph of solutions. 2.1. Adsorption Experiments Batch adsorption experiments were performed to evaluate the effects of several parameters such as solution ph, contact time, adsorbent concentration and solute concentration. Thus, for this purpose, varying amounts of n-ommt (.5-.4 g) were added to a certain volume of 52 ppm Cr 6+ solution and optimum adsorbent concentration was determined. The concentration of n-ommt was in the range of.5 4 g/l. Adsorption of Cr 6+ ions from solutions has been systematically investigated as a function of contact time from 1 min to 9 min at room temperature. Effect of initial concentration of Cr 6+ solution was changed between 1 to 15 ppm and the isotherm behavior of adsorption process was indicated. The effect of ph on Cr 6+ adsorption was assessed by adjusting the initial ph of the starting solutions within the range of 1 5. 2.2. Characterization of n-ommt 2.2.1. Fourier Transmission Infrared Spectroscopy (FTIR) Generally, pore size and chemical nature determine the adsorption behaviour of organoclays. In the case of liquid-phase adsorption, the chemical properties of surface groups of organoclays affect the extent of adsorption. Fourier Transform Infrared Spectra (FTIR) was used to investigate the effects of the presence of the surface functional groups on adsorption of Cr 6+. FTIR study for n-ommt is seen in Fig. 2. 1µm 137
462 522 147 363 2925 916 T, % 3396 285 1467 1637 236 As seen from the figure n-ommt includes bands at ~ 2925 cm 1 and ~ 285 cm 1, attributed to the CH 2 asymmetric and symmetric stretches respectively, as well as a band at ~ 1467 cm 1, attributed to the CH 2 bending vibration. The spectrum of n-ommt shows very clearly the stretching modes of Si OH and OH groups of interlayer water at ~ 363 cm 1 and ~ 3396 cm 1. Furthermore, a band at ~ 163 cm 1 can be attributed to the OH bending mode of water. Additionally, it comprises the band at ~ 15 cm 1 corresponding to the Si O bending and stretching modes; bands at ~ 522 cm 1 and ~ 462 cm 1 corresponding to the stretching modes of Al O and Mg O respectively [12]. 39 34 29 24 19 14 9 4 Wavenumber, 1/cm Fig. 2: FTIR spectrum of n-ommt. 3. Results and Discussion 3.1. Effect of Solution ph Due to the fact that ph affects the transition metal speciation and solubility and the charge of the functional groups of adsorbent, it is one of the most important parameters controlling the metal ions sorption process. At ph > 6 6.5, chromium metal precipitations occurred. Consequently, adsorption experiments were carried out at ph = 1 5 range in this study to determine the optimum ph for maximum removal of chromium ions. The plot of adsorption capacity vs. ph for n-ommt is shown in Fig. 3. It is observed that with an increase in ph from 1. to 2., the sorption capacity rate slightly increases. The maximum Cr 6+ adsorption (85%) by n-ommt was observed at ph 2. As seen from the figure, the percent of removal of metal ions decreases very sharply upon increasing the ph values from 2. to 5.. The great number of protons can easily coordinate with the functional groups present on the adsorbents at lower ph. This result meant that there is a strong attracting property between these oxyanions of Cr 6+ and the surface of n-ommt [13]. 3.2. Effect of Contact Time The cost of water treatment method is a very significant parameter. It is affected by the time required to reach equilibrium during metal sorption and energy consumption. Therefore, investigation of optimum contact time for this type of treatment is very crucial. For this purpose, calculated percentages of sorption by n-ommt are shown in Fig. 4 as a function of increasing contact time from 1 to 9 min keeping other parameters such as temperature 25 C; ph 2; adsorbent concentration 3 g/l, respectively constant. A very quick uptake of Cr 6+ ions in a short period is caused by the effect of a large number of vacant sites available on n-ommt. This meant that n-ommt had a high affinity for Cr 6+ ions. The removal of Cr 6+ increased with contact time until the equilibrium point for n- OMMt. Maximum removal of chromium ions was determined at optimal contact time of 6 min. 138
1 8 6 Percent of Sorption, % 4 2 1 2 3 4 5 6 ph. Fig. 3: Effect of ph on the sorption of Cr 6+ by n-ommt. Percent of Sorption, % 1 8 6 4 2 5 1 Contact time, min Fig. 4: Effect of contact time on the sorption of Cr 6+ by n-ommt. 3.3. Effect of Adsorbent Concentration Due to the fact that, adsorbent concentration specifies the capacity of adsorbent to uptake a certain initial concentration of Cr 6+, it has a great effect on this process. While keeping other parameters, varying concentrations of n-ommt (range:.5 4 g/l) were used for batch adsorption experiments. Obtained profile is seen in Fig. 5. The maximum removal was about 9.81% at the adsorbent concentration of 4. g/l. For the concentration of 3. g/l, obtained percentage was 85.16% and it is used as an optimum adsorbent concentration for further experiments. As it can be seen from the figure, the increase in the adsorbent concentration causes an increase in the percentage of metal sorption. It can definitely be explained that a greater number of n-ommt creates a greater surface area. By this way, the number adsorption sites in the matrix of n-ommt were increased. This useful experimental section of the study guided to determine the economical adsorbent amount for water remediation technologies. 1 8 Percent of Sorption, % 6 4 2. Concentration 2. of 4. n-ommt, 6. g/l Fig. 5: Effect of adsorbent concentration on the sorption of Cr 6+ by n-ommt. 139
3.4. Adsorption Isotherms Since providing detailed information for making a comparison between various adsorbents, determination of the equilibrium data and the maximum adsorption capacity of the adsorbent are very important for large scale applications. Because well-defined and modeled of obtained data lead to optimize and design the process conditions very well. Characterization of this type of process is generally performed with a series of isotherm models [14]. In this study, in order to describe the equilibrium of Cr 6+ ions on n-ommt, Langmuir, Freundlich and Scatchard isotherms were used. For this investigation, a series Cr 6+ solution was used at various Cr 6+ concentrations ranging from 1 to 15 mg/l. As seen from the Fig. 6, an increase in the initial Cr 6+ concentration resulted in increased Cr 6+ adsorption. The profile of the sorption isotherm curve in figure deviates from a straight line, and the curvature increases, as the equilibrium concentration is higher. It can be claimed that with increasing the concentrations, more Cr 6+ ions cannot adsorb onto the surface of n-ommt and stay in the medium due to the saturation of binding groups of n-ommt. It can be seen from the sorption isotherm, Cr 6+ adsorption rate was rapid in the initial stage, then gradually reached a plateau toward equilibrium. Correlation coefficients (R 2 ) definitely showed that the adsorption data for Cr 6+ ions on n-ommt correlated well with the Langmuir isotherm model (Table 1) and Cr 6+ uptake was calculated from the equation.43 mmol/g for n-ommt. As a result, it can be concluded that n-ommt can be used in order to remove hexavalent chromium from wastewater. Table 1: Adsorption capacities and correlation coefficients for Cr 6+ sorption on n-ommt. Adsorbent K f a Freundlich Isotherm Langmuir Isotherm Scatchard Isotherm n R 2 a K b A s R 2 Q s K s R 2 n-ommt 1.233 7.189.983 5813.95.43.9994.438 1572.4.968 a mmol/g dry adsorbent. Scatchard plot analysis was implemented to the result of the adsorption isotherm data [15]. In the case of the plot of q e /C e versus qe results a straight line, the adsorbent comprises of only one type of binding site. Thus, it can be interpreted as a formation of a homogenous surface. However, if the plot deviates from linearity, then the adsorbent has more than one type of binding site. In this study, the value of the obtained regression R 2 (.968) means that n-ommt consists of only one type of binding site..5 qe, (mmol/g adsorbent).4.3.2.1.5.1.15.2 Ce, mmol Fig. 6: Sorption isotherm of Cr 6+ on n-ommt. 4. Conclusions 14
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