Acta Polytechnica doi:10.14311/AP.2016.56.0373 Acta Polytechnica 56(5):373–378, 2016 © Czech Technical University in Prague, 2016 available online at http://ojs.cvut.cz/ojs/index.php/ap EXPERIMENTAL INVESTIGATION ON CHROMIUM(VI) REMOVAL FROM AQUEOUS SOLUTION USING ACTIVATED CARBON RESORCINOL FORMALDEHYDE XEROGELS Eghe A. Oyedoha, b, ∗, Michael C. Ekwonuc a School of Chemistry and Chemical Engineering, Queen’s University Belfast, Northern Ireland, United Kingdom b Department of Chemical Engineering, University of Benin, Benin City, Edo State, Nigeria c Department of Chemical & Petroleum Engineering, Afe Babalola University, Ado-Ekiti, Nigeria ∗ corresponding author: egheoyedoh@uniben.edu Abstract. The adsorption of chromium(VI) metal ion in aqueous solutions by activated carbon resorcinol formaldehyde xerogels (ACRF) was investigated. The results showed that pore structure, surface area and the adsorbent surface chemistry are important factors in the control of the adsorption of chromium(VI) metal ions. The isotherm parameters were obtained from plots of the isotherms and from the application of Langmuir and Freundlich Isotherms. Based on regression analysis, the Langmuir isotherm model was the best fit. The maximum adsorption capacity of ACRF for chromium (VI) was 241.9 mg/g. The pseudo-second-order kinetic model was the best fit to the experimental data for the adsorption of chromium metal ions by activated carbon resorcinol formaldehyde xerogels. The thermodynamics of Cr(VI) ions adsorption onto ACRF was a spontaneous and endothermic process. Keywords: Adsorption, Chromium(VI) ion, Langmuir isotherm, Freundlich isotherm, Activated carbon resorcinol formaldehyde xerogels (ACRF). 1. Introduction Chromium is used in various industries such as the metallurgical industry (steel, ferro- and nonferrous alloys), refractories (chrome and chrome-magnesite), and in the chemical industry (pigments, electroplat- ing and tanning) [1]. As a result of these industrial processes, large amounts of chromium compounds are discharged into the environment. These compounds are toxic and have negative effects on humans and the environment. Persistent exposure to Cr(VI) causes cancer in the digestive tract and lungs, and may cause other health problems, for instance skin dermatitis, bronchitis, perforation of the nasal septum, severe diarrhoea, and haemorrhaging [2, 3]. The maximum level for chromium in drinking water permitted by the World Health Organization (WHO) is 0.05 mg/L [4]. Cr(III) and Cr(VI) species are the two stable forms of chromium present in the environment. They have different chemical, biological and environmental char- acteristics. The most toxic form of chromium is Cr(VI), which exists with oxygen as chromate CrO42– or dichromate Cr2O72– oxyanions. Cr(VI) compounds are highly soluble and mobile. Cr(III) is less mobile, less toxic and is mainly found bound to organic matter in soil and aquatic environments [5]. Typical methods for the removal of dissolved heavy metals from aqueous solution include chemical precip- itation, chemical oxidation or reduction, filtration, ion exchange, electrochemical treatment and application of membrane technology. However, these processes have some major drawbacks, which include incomplete metal removal, requirements for expensive equipment and monitoring system, high reagents and energy re- quirement and generation of toxic sludge with special disposal requirements, especially with the application of low cost adsorbents [6]. Adsorption can be an effective method for the re- moval of chromium from aqueous solution, especially in combination with suitable regeneration steps, which resolves the problems associated with sludge disposal and makes the process more economically viable [6]. Previous studies on the removal of Cr(VI) using ac- tivated carbons produced from coconut shells [7], clays [8], wheat bran [9], rice husk [10], tyres and sawdust [11], etc. have been reported in the litera- ture. This study investigates the adsorption of chromi- um(VI) onto activated carbon resorcinol formaldehyde xerogels using the Langmuir and Freundlich isotherms. The kinetics of adsorption was fitted with pseudo-first- order and pseudo-second-order and the controlling rate of adsorption described by intra-particle diffusion. 2. Material and methods 2.1. Material All chemical reagents and materials used were of an- alytical grade. Deionised water (18.0 Ω) was used as solvent in the preparation of stock solutions of chromium metal ions by dissolving 2.828 g of potas- sium dichromate in 1 dm3 of deionised water. 2.2. Synthesis of activated carbon resorcinol formaldehyde xerogels Activated carbon obtained from the synthesis of re- sorcinol formaldehyde xerogels (ACRF) was used for the adsorption studies [12]. The RF xerogels 373 http://dx.doi.org/10.14311/AP.2016.56.0373 http://ojs.cvut.cz/ojs/index.php/ap Eghe A. Oyedoh, Michael C. Ekwonu Acta Polytechnica were synthesised from the polycondensation of re- sorcinol, C6H4(OH)2 (R), with formaldehyde HCHO (F) according to the method proposed by Pekala et al. [13, 14], RF solutions were prepared by mix- ing resorcinol (R), formaldehyde (F), sodium car- bonate Na2CO3 (C) and distilled water. The solu- tion was mixed vigorously for 45 min. The resorci- nol/formaldehyde ratio R/F was fixed at 0.5, while the molar ratio of R/C and the ratio of R/W (g/cm) were varied. The homogeneous clear solution was then poured into sealed glass vials to avoid water evaporat- ing during the gelation process. The sealed vials were then placed in an oven set at 25 °C for 24 h. Oven temperature was then increased to 60 °C for 48 h, and then finally it was increased to 80 °C for an additional 24 h to complete the curing process. The wet gels were then removed from the oven and allowed to cool to room temperature. In order, to remove water from the pores of the gels, the gels were immersed in acetone for solvent exchange at room temperature for three days. After the third day, the acetone was poured out and the gels were placed in a vacuum oven for drying. The gels were dried in a vacuum oven at 64 °C for 3 days. 2.3. Adsorption studies All adsorption experiments were carried out with batch reactors (glass bottles and beakers). Stock solu- tions (1000 ppm) of Cr(VI) metal ions were prepared. Different concentrations of standard solutions (25, 50, 100, 150, 200 and 250 ppm) were prepared by appro- priate dilutions of the stock solutions with deionised water. Chromium (VI) concentrations were analysed at 540 nm wavelength using HACH-DR-2800 UV vis- ible spectrophotometer with 1, 5-diphenylcarbazide reagent. The reagent was prepared by using 250 mg of 1, 5-diphenylcarbohydrazide which was dissolved in 50 ml of methanol (HPLC-grade). 250 ml of H2SO4 solution (contains 14 ml of 98 % H2SO4) was added into the above solution, which was then diluted with deionised water to 500 ml. 2.4. Adsorption isotherms Experimental data obtained from the batch tests were analyzed using the Langmuir and Freundlich isotherms to determine the isotherm model that described the experimental data more accurately. Langmuir Isotherm. The Langmuir isotherm as- sumes a monolayer, uniform, and finite adsorption site and therefore saturation is reached, beyond which no further adsorption takes place. It is also based on the assumption that there is no interaction between the molecules adsorbed on neighbouring sites [15]. The model developed by Langmuir (1916) is given by: qe = qmaxbCe 1 + bCe (1) The very important characteristic of the Langmuir isotherm can be expressed in terms of a dimensionless constant called the separation factor [6, 16]: RL = 1 1 + bCo (2) Freundlich Isotherm. The Freundlich isotherm is an empirical equation for multilayer, heterogeneous adsorption sites [17]. The Freundlich isotherm is given by: qe = KF C 1 n e (3) 3. Results and discussion 3.1. Effect of initial pH The effect of pH on the adsorption of the Cr(VI) metal ions was studied with the pH varied from 2.0–11.0. The studies were performed with constant initial metal ions of 100 ppm, adsorbent dose of 1 g/L solution and contact time of 72 h. The adsorption of chromium(VI) (Fig. 1) increases with the pH to a maximum at pH 3, and thereafter decreases with further increase in pH. This shows that adsorption of chromium ions is pH dependent. The maximum adsorption at pH 3 may be attributed to the existence of chromium ions as HCrO4 – which is the dominant form of Cr(VI) at pH 3. The high adsorption of Cr(VI) at pH 3 might be a result of electrostatic attraction between positively charged groups of the ACRF surface and HCrO4 – . This can also be attributed to fact that the surface charge on the ACRF. The pHzpc of ACRF is at 9.19 and below this pH, the surface charge of the ACRF is positive. Hence, adsorption of Cr(VI) might also be due to electrostatic attraction between positively charged ad- sorbent and negatively charged HCrO4 – [18]. As the pH increased, the overall surface charge on the adsor- bents became negative and adsorption decreased [19]. The decrease in removal at higher pH may be due to the abundance of OH– ions which compete with the negatively charged Cr(VI) species for the active sites on the ACRF. Figure 1. Effect of pH on adsorption of Cr(VI) ions 374 vol. 56 no. 5/2016 Removal of Cr(VI) using ACRF 3.2. Effect of initial chromium(VI) concentration on ACRF The effect of initial chromium metal ion concentration on the adsorption was studied at optimum the pH of 3 which was observed from a previous study. The experimental data for the adsorption of Cr(VI) onto ac- tivated carbon resorcinol formaldehyde xerogels were fitted to the Langmuir and Freundlich isotherms using non-linear regression analysis. The isotherm parame- ters are given in Table 1. Langmuir isotherm (Fig. 2) was seen to have a better fit based on non-linear re- gression analysis. The value of the separation factor, RL, determines the type of isotherm either to be favourable (0 < RL < 1), linear (RL = 1), unfavourable (RL > 1) or irreversible (RL = 0) [20]. The low value of RL (0.000049) showed that the adsorption of chromium(VI) onto ACRF was favourable (Table 1). 3.3. Effect of temperature on chromium(VI) adsorption The effect of temperature on the adsorption of metal ions was carried out with the temperature varied from 20 °C (293 K) to 60 °C (333 K), with initial concentra- tion of 25–250 ppm, adsorbent dosage of 1 g/L and optimal pH. The adsorption of Cr(VI) ions was found to increase with an increase in temperature range 20–60 °C (Fig. 3). This increase in adsorption ca- pacity of ACRF is an indication of an endothermic process [21]. This might be a result of complexation and reduction reactions [22]. Also, diffusion is an Figure 2. Application of Langmuir and Freundlich Isotherms to adsorption of Cr(VI) ions Langmuir Isotherm Freundlich Isotherm qmax = 241.9 b = 0.3529 RL = 0.000049 R2 = 0.9740 KF = 79.06 1/n = 0.3426 R2 = 0.9441 Table 1. Isotherm parameters for Cr(VI) adsorption onto ACRF endothermic process and an increase in temperature increases the diffusion rate of the adsorbate molecules across the external boundary layers and into the pores of ACRF. Similar results were observed with adsorp- tion of Cr(VI) onto activated carbon [23]. 3.4. Effect of contact time The effect of contact time on the adsorption of Cr(VI) was studied by varying the contact time from 0– 420 min under pH of 3. In Fig. 4, it was seen that the uptake of the metal ions increased with increasing contact time until equilibrium was reached. The ad- sorption of Cr(VI) ions initially increased rapidly and then reached equilibrium. The optimum chromium removal was 74.86 % at 60 min for 25 ppm and 77.21 % at 240 min for 200 ppm; it was 100 % at 420 min for 25 ppm and 83 % at 420 min for 200 ppm. 3.5. Adsorption kinetics The pseudo-first-order, pseudo-second-order and intra- particle diffusion models were used to fit the exper- imental data for the different initial chromium ion concentrations. The results of pseudo-second-order kinetics observed in this study are supported by the findings of Bhattacharya [8]. The values of the second Figure 3. Effect of temperature on the adsorption of Cr(VI) Figure 4. Effect of contact time on the adsorption of Cr(VI). 375 Eghe A. Oyedoh, Michael C. Ekwonu Acta Polytechnica Co (mg/L) Pseudo-first-order Pseudo-second-order Intra-particle Diffusion k1 R 2 k2 h R 2 ki R 2 25 0.1705 0.9768 0.0136 3.2938 0.9743 1.5737 0.7411 50 0.0935 0.9742 0.0041 3.3706 0.9833 2.2374 0.7888 150 0.0706 0.9138 0.0015 8.3693 0.9647 3.0394 0.7584 200 0.0530 0.9499 0.0010 5.7566 0.9864 3.3937 0.8209 Table 2. Kinetic models and parameters of adsorption of Cr(VI) order rate constants (k2) were found to decrease from 0.0136–0.0010 g mg−1 min−1 as the initial concentra- tion increased from 25–200 mg/L. This indicated that the process is highly concentration dependent [24]. 3.6. Mechanism of adsorption As seen in Fig. 5, the ACRF spectra displayed a change of intensity and shift of the carbonyl stretching band around 1630 cm−1 after the contact with chromium solution. This is a result of the complexation of the carbonyl group with chromium. Another shift can be observed as a result of complexation of the oxygen from the carboxyl C–O bond at wave numbers 1166 and 1066 cm−1. The O–H (3434 cm−1) and C–O (2390 and 2361 cm−1) band absorption peaks are ob- served to shift when ACRF is loaded with chromium. Two new peaks were observed in FTIR spectra of Cr(VI)-loaded sorbents, which is attributed to Cr–O and Cr––O bonds of chromate anions, and which con- firms the sorption of Cr(VI) onto the activated carbon resorcinol formaldehyde xerogel (ACRF) at 719 and 910 cm−1 [25]. The mechanism of chromium(VI) adsorption from aqueous solution is attributed to physical adsorption by electrostatic attraction between positively charged adsorption sites in the adsorbent and the negatively charged Cr(VI) species. It can be seen that carboxyl groups are involved in the removal mechanism, as shown with the FTIR Figure 5. FTIR spectra of ACRF adsorbent (A) before and (B) after Cr(VI) adsorption. results [26]. Other functional groups may also be involved in metal ions adsorption. From the SEM (Figure 6a (Unloaded ACRF)), it can be seen that ACRF has a large surface area. The chromium metal ions were adsorbed onto the pores and surfaces of adsorbent as shown by the SEM image (Figure 6b (loaded ACRF)). The EDX analyses of ACRF before adsorption were: C: 97.48 %; O: 2.31 %; Na: 0.21 %. The EDX analyses for Cr(VI)-loaded ACRF were: C: 39.56 %; O: 3.0 %; Na: 0.05 %; Cr: 57.38 %. 3.7. Adsorption thermodynamics The thermodynamics parameters such as Gibbs free energy, enthalpy change and entropy change were obtained using the following equations [6]: Kc = qe Ce , (4) ∆Go = −RT ln Kc, (5) ln Kc = ∆So R − ∆H o RT . (6) The value of ∆H o and ∆So are obtained from the slope and intercept of the linear Van’t Hoff plot of ln Kc versus 1/T (6). Table 3 shows the calculated values of the thermodynamic parameters for the ad- sorption of Cr(VI) on ACRF. The negative values of ∆Go at various temperatures indicate the spontaneous nature of the adsorption pro- cess. The increase in ∆Go with temperature clearly indicates a more favourable adsorption at high tem- perature. The positive value of ∆H o indicates the adsorption process is endothermic. More so, the posi- tive value of ∆So indicates the degree of randomness of the system solid-solution interface during the ad- sorption process. Similar results were reported for Cr(VI) adsorption [6, 27]. As reported by Malkoc Figure 6. SEM images of ACRF (a) before Cr(VI) adsorption and (b) after Cr(VI) adsorption. 376 vol. 56 no. 5/2016 Removal of Cr(VI) using ACRF T Kc ∆Go ∆H o ∆So 293 3.0398 −2.71 7.51 0.0355 303 4.0450 −3.52 333 4.6185 −4.24 Table 3. Thermodynamics parameters for Cr(VI) adsorption onto ACRF and Nuhoglu [27], the positive value of ∆So reflects the affinity of the adsorbent for Cr(VI) ions and sug- gests some structural changes in chromium and the adsorbent. 4. Conclusions The effect of initial Chromium(VI) metal ion concen- tration on the adsorption on ACRF was studied at optimum pH observed from a previous study. The Langmuir isotherm was seen to have a better fit based on non-linear regression analysis. The maximum ad- sorption capacity of ACRF for Chromium(VI) was 241.9 mg/g. The pseudo-second-order kinetic model was the best fit to the experimental data for the ad- sorption of chromium (VI) metal ions by activated carbon resorcinol formaldehyde xerogels. The opti- mal removal was 74.86 % at 60 min for 25 ppm and 77.21 % at 240 min for 200 ppm. Different mecha- nisms were responsible for Chromium(VI) metal ion adsorption. The results showed that the adsorption of Cr(VI) is a result of electrostatic attraction, ion exchange/complexation and reduction reactions. The thermodynamic analysis showed that the Chromium adsorption process was endothermic and spontaneous in nature. 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Separation and Purification Technology 54(3):291–298, 2007. doi:10.1016/j.seppur.2006.09.017. 378 http://dx.doi.org/10.1021/ma00072a029 http://dx.doi.org/10.1021/ja02268a002 http://dx.doi.org/10.1021/i160018a011 http://dx.doi.org/10.1007/s13201-011-0014-1 http://dx.doi.org/10.1016/j.minpro.2010.08.003 http://dx.doi.org/10.1016/j.seppur.2006.09.017 Acta Polytechnica 56(5):373–378, 2016 1 Introduction 2 Material and methods 2.1 Material 2.2 Synthesis of activated carbon resorcinol formaldehyde xerogels 2.3 Adsorption studies 2.4 Adsorption isotherms 3 Results and discussion 3.1 Effect of initial pH 3.2 Effect of initial chromium(VI) concentration on ACRF 3.3 Effect of temperature on chromium(VI) adsorption 3.4 Effect of contact time 3.5 Adsorption kinetics 3.6 Mechanism of adsorption 3.7 Adsorption thermodynamics 4 Conclusions List of symbols References