Adsorption of Cr(VI) from water sample onto the activated carbon cloth with an oxidized surface area

JOURNAL OF SCIENCE OF HNUE Chemical and Biological Sci., 2014, Vol. 59, No. 9, pp. 66-73 This paper is available online at ADSORPTION OF Cr(VI) FROMWATER SAMPLE ONTO THE ACTIVATED CARBON CLOTHWITH AN OXIDIZED SURFACE AREA Nguyen Thanh Binh1, Tran Van Chung1, Nguyen Hung Phong1, Bui Van Tai1 and Kieu Thanh Canh2 1Institute of Chemistry and Material, Academy of Military Science and Technology 2 Faculty of Natural Sciences, the Army College No. 1 Abstract.Cr(VI) is a substance which is toxic to humans and most living organisms but, due to improper handling of industrial wastes, it is being released into water resources. The removal of Cr(VI) from water can be carried out using one of many different methods. Activated carbon cloth with a surface area oxidized by the adsorbents H2O2 or HNO3 is commonly used to remove Cr(VI) from water by adsorption. The high adsorption capacity of these adsorbents is due to the formation of functional groups on them. In this study, the initial concentration of Cr(VI), the pH, the adsorption time and the temperature were all looked at as possible factors that influence the efficiency of Cr(VI) adsorption capacity and removal. The Freundlich isotherm provided the best correlation for adsorption of Cr(VI) onto the adsorbents. The kinetic equations corresponding to the adsorption process were established and fitted to the first- order reaction for these cases. Keywords: Chromium ion, adsorption of Cr(VI), activated carbon cloth, oxidized activated carbon. 1. Introduction Chromium compounds Cr(VI) are toxic to humans and the environment in general. They are placed in the ‘A’ group of human carcinogens due to propensity to cause mutations that result in cancer [6]. Some governments consider a level of 0.1 mg/L [10] in surface water and 0.05 mg/L [7] to be ‘safe’. Many methods are used to remove Cr(VI) ions from water [1, 3-5]. Adsorption isotherms and kinetics have been studied by many researchers [2, 8, 9]. This paper presents a method to adsorb Cr(VI) from water which makes use of activated carbon cloth with a surface area oxidized by chemicals. The preparation of activated carbon cloth with an oxidized surface area and its efficacy in adsorbing Cr(VI) ions is also presented in this paper. Received December 9, 2014. Accepted December 26, 2014. Contact Tran Van Chung, e-mail address: tranchunghhvl@gmail.com 66 Adsorption of Cr(VI) from water sample onto the activated carbon cloth... 2. Content 2.1. Experiments 2.1.1. Adsorbent preparation A commercially activated carbon cloth sample (20 g), 20  20 mm in size and denoted AC1, was oxidized with 60 mL of 30% H2O2 solution in a 500 mL bottle at the room temperature. The sample was continuously stirred for 3 h. It was then washed in distilled water and dried at 105 ◦C for 24 h. The obtained sample is denoted AC2. Another commercially activated carbon cloth (20 g) was oxidized with 60 mL of 67% HNO3 in a 500 mL bottle fitted with a condenser (cooling tube). The sample was heated at a temperature 90 - 100 ◦C for 15 min and then kept at room temperature for 2 h. The obtained oxidized activated carbon cloth was washed in distilled water to completely remove the HNO3 residue, then dried at 105 ◦C for 24 h and denoted AC3. The main physical parameters of the adsorbents were determined using the BET method listed in Table 1. Table 1. The physical parameters of the adsorbents (AC1, AC2, AC3) Nr Pore volume Units AC1 AC2 AC3 1 Vtotal pores cm3/g 1.210 1.190 1.220 2 Vmicropores cm3/g 0.462 0.446 0.439 3 Vmesopores cm3/g 0.089 0.099 0.105 4 Vmacropores cm3/g 0.659 0.645 0.676 5 SBET m2/g 710.678 669.851 647.895 The presence of functional groups on the oxidized adsorbent surface were determined using a Fourier transform Infra-Red Spectrophotometer (FTIR) and are presented in Table 2. Table 2. The functional groups on the adsorption surface Adsorbent O-H C=O (COOH) C=O (-C=C-COOH) C-O (C-OH) C=O (-COOR) OH -(COOH) AC1 + + + + - - AC2 + + + + + + AC3 + + + + + + Remarks: (+) : appearance of a functional group, (-) no appearance of a functional group 2.1.2. Batch adsorption studies A K2Cr2O7 sample was prepared by dissolving a calculated amount of potassium dichromate (0.5657 g) (PA) in distilled water (1000 mL). This was used as a stock solution having a concentration of 200 mg/L Cr(VI). The adsorption studies were carried out using 67 Nguyen Thanh Binh, Tran Van Chung, Nguyen Hung Phong, Bui Van Tai and Kieu Thanh Canh the batch technique to acquire rate and equilibrium data. The batch adsorption equilibrium tests were carried out on a rotary shaker for 2 h (stirring speed 250 rpm) and equal amounts of AC1, AC2 and AC3 (0.2 g) were thoroughly mixed with 100 mL of the Cr(VI) solution. The isotherm studies were performed by varying the initial concentrations of Cr(VI) from 5.0 to 50.0 mg/L, at pH = 6.0. The influence of pH on adsorption capacity was determined by mixing 0.2 g of each adsorbent (AC1, AC2 and AC3) with 100 mL of Cr(VI) solution of a concentration of 30 mg/L for 24 h. The samples were run varying the initial pH values from 2.0 to 10.0 at the stirring speed of 250 rpm. The influence of adsorption time was determined by mixing 0.2 g of the every adsorbent (AC1, AC2 and AC3) with 100 mL of Cr(VI) solution at a concentration of 30 mg/L, pH = 6.0. The samples were run varying the reaction time from 30 to 150 min at the stirring speed of 250 rpm. The influence of temperature on the adsorption was also determined by mixing each adsorbent (AC1, AC2 and AC3) with 100 mL of Cr(VI) solution at a concentration of 30 mg/L, pH = 6.0, for 2 h. The samples were run varying the reaction temperature from 0 - 60 ◦C stirring continuousely. 2.1.3. Analytical procedure After the completion of each experimental sample, the resulting mixture was filtered through a filter paper (blue mark) and the filtrate was analyzed. The Cr(VI) concentrations in the solutions were determined using atomic adsorption spectroscopy (AAS). The amount of adsorption at equilibrium (qe mg/g) and the removal efficiency E (%) were calculated using the following expressions: qe(mg=g) = (C0 Ce) V m E(%) = C0 Ce C0  100 here, C0, and Ce denote the concentration of Cr(VI) in solution initially and at the equilibrium step, respectively. m denotes the adsorbent mass. 2.2. Results and discussion 2.2.1. The influence of the initial concentration on the adsorption process The adsorption process with varying initial concentration of Cr(VI) with an adsorption time of 2 h is presented in Table 3 and Figure 1. From the results in Table 3, the maximum adsorption capacity of the each adsorbent may be calculated and given: qe(max) = 13.88 (for AC1), qe(max) = 15.28 (for AC2) and qe(max) = 15.54 (for AC3). This indicates that under the same adsorption conditions, the adsorption capacity of AC3 (activated carbon cloth oxidized by HNO3) is higher than that of AC1 and AC2 68 Adsorption of Cr(VI) from water sample onto the activated carbon cloth... Table 3. The influence of the initial concentration on adsorption Sample Factors Concentration of Cr(VI), C0(mg/L) 5 10 15 20 25 30 35 40 45 50 AC1 Ce(mg/L) 0.01 0.05 0.45 1.39 2.93 5.24 8.42 12.11 17.23 22.12 E(%) 99.80 99.51 97.00 93.02 88.28 82.53 75.91 69.72 61.86 55.76 qe (mg/g) 2.49 4.97 7.27 9.30 11.03 12.38 13.29 13.84 13.88 13.93 AC2 Ce(mg/L) 0.00 0.03 0.15 0.79 1.48 2.98 6.06 9.90 14.25 19.18 E(%) 100.00 99.70 99.00 96.01 94.08 90.06 82.68 75.25 68.33 61.64 qe (mg/g) 2.50 4.98 7.27 9.30 11.76 13.51 14.47 15.05 15.37 15.41 AC3 Ce(mg/L) 0.00 0.01 0.09 0.45 0.51 0.63 1.98 5.40 9.75 14.58 E(%) 100.00 99.80 99.75 99.31 99.24 97.90 94.34 86.47 78.33 71.40 qe(mg/g) 2.50 4.99 7.49 9.77 12.24 15.68 16.51 17.30 17.62 17.71 . Figure 1. The influence of Cr(VI) concentration on the adsorption capacity The high adsorption capacity of AC3 and AC2 may be explained by the high density of functional groups on their surfaces. 2.2.2. Influence of pH on adsorption pH is one of most important factors in assessing the adsorption capacity of an adsorbent of metallic ions. The results of the influence of pH on the adsorption capacity of Cr(VI) are presented in Table 4 and Figure 2. The experimental results show that maximum adsorption of Cr(VI) onto the adsorbents took place in an acidic medium. This is consistent with the work [3]. Here, the adsorption is said to be due to the adsorption of Cr(VI) through HCrO−4 anions as follows: CrO2+4 +H + (surface of adsorbent) ! HCrO−4-adsorption This can be accepted because in a pH of 2 to 4, HCrO−4 ions are the dominant anionic form of Cr(VI). Other authors [4] have claimed that the adsorption of Cr(VI) on 69 Nguyen Thanh Binh, Tran Van Chung, Nguyen Hung Phong, Bui Van Tai and Kieu Thanh Canh activated carbon is due to the formation of Cr3+ by the following reductive reaction with carbon: Cr2O 2− 7 + 14H + + 6e− ! 2Cr3+ + 7H2O The being small size of Cr3+can be easily replaced by the positive charge species (H+) on the adsorbent surface. This is consistent with our work. Table 4. Influence of pH on the adsorption process of Cr(VI) Sample Factors pH 2 4 6 8 10 AC1 Ce (mg/L) 2.59 3.15 5.31 11.04 17.29 qe(mg/g) 13.70 13.42 12.34 9.48 6.35 E(%) 91.36 89.50 82.30 62.30 42.36 AC2 Ce (mg/L) 1.42 1.65 2.89 9.86 14.61 qe(mg/g) 14.29 14.17 13.55 10.06 7.69 E(%) 95.26 94.48 90.36 67.33 51.50 AC3 Ce (mg/L) 0.19 0.22 1.06 7.73 11.92 qe(mg/g) 14.90 14.89 14.47 11.13 9.04 E(%) 99.36 99.26 96.46 74.23 60.09 Figure 2. The influence of pH on the adsorption process 2.2.3. The influence of adsorption time The influence of time on the adsorption process is presented in Table 5 and Figure 3. The experimental data shows that after 2 h the adsorption process reached equilibrium. 70 Adsorption of Cr(VI) from water sample onto the activated carbon cloth... Table 5. The influence of time on the adsorption of Cr(VI) Sample Factors Adsorption time (min) 30 60 90 120 150 AC0 Ce (mg/L) 14.72 9.03 6.12 4.51 4.52 qe(mg/g) 7.64 10.05 11.94 12.74 12.75 E(%) 50.93 70.03 79.60 84.96 85.02 AC1 Ce (mg/L) 13.13 7.20 4.47 2.95 2.70 qe(mg/g) 8.43 11.40 12.76 13.52 13.65 E(%) 56.23 76.06 85.10 90.16 91.05 AC2 Ce (mg/L) 10.31 4.37 1.44 0.264 0.14 qe(mg/g) 9.84 12.81 14.27 14.86 14.92 E(%) 65.63 85.43 95.19 99.12 9953 Figure 3. The influence of time on the adsorption process 2.2.4. The influence of temperature on the adsorption process Figure 4. The influence of temperature on the adsorption process of Cr(VI) 71 Nguyen Thanh Binh, Tran Van Chung, Nguyen Hung Phong, Bui Van Tai and Kieu Thanh Canh The experimental results of the influence of temperature on the adsorption process of Cr(VI) onto the adsorbent surface are presented in Figure 4. The experimental data shows that as the temperature increases from 10 to 60 ◦C, the adsorptive efficiency also increased. The increased adsorption efficiency may be explained by the chemical adsorption of Cr(VI) onto the adsorbents. 2.2.5. Adsorption isotherm and kinetic studies * Adsorption isotherm Langmuir and Freundlich equations were used to describe the adsorption equilibrium for Cr(VI). Here the adsorption isotherm was fitted only to Freundlich: log qe = logKF + (1=n) log Ce and presented in Figure 5. KF denoted the Freundlich constant, qe is the adsorption capacity and Ce is the concentration of Cr(VI) in a state of equilibrium. yAC1 = 0.257logCe + 0.924 with R2 = 0.994. yAC2 = 0.223logCe + 1.015 with R2 = 0.996. yAC3 = 0.225logCe + 1.139 with R2 = 0.997. Figure 5. Freundlich plots of Cr(VI) loaded onto AC1, AC2 and AC3 From the plots, the values of n and KF , were determined to be: n(AC1) = 3.89, n(AC2) = 4.48, n(AC3) = 4.44; KF (AC1) = 8.39, KF (AC2) = 10.35, KF (AC3) = 13.77. * Kinetic studies The experimental data were better fitted to the adsorption kinetics of the first order equation (Lagergren equation); log(qe qt) = log qe (k1=2:303)t which are: yAC1 = - 0.0101t + 1.021 with R2 = 0.994; yAC2 = - 0.0137t + 1.129 with R2 = 0.995; yAC3 = - 0.0143t + 1.166 with R2 = 0.994. From here the reaction constants corresponding to every adsorbent are as follows: k1(AC1) = 0.0233, k1(AC2) = 0.0316, k1(AC3) = 0.0329. 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