Surface modification of activated carbon from rice husk for enhancing the nickel(ni2+) and cadmium (cd2+) adsorption capacity - Hoa Thai Ma

Journal of Science and Technology 54 (4B) (2016) 19-26 SURFACE MODIFICATION OF ACTIVATED CARBON FROM RICE HUSK FOR ENHANCING THE NICKEL(Ni2+) AND CADMIUM (Cd2+) ADSORPTION CAPACITY Hoa Thai Ma1, 2, Van Thi Thanh Ho3, Hung Cam Ly3, Nguyen Bao Pham2, Tuan Dinh Phan3* 1Vietnam National University, Hochiminh City University of Technology, 268 Ly Thuong Kiet street, Ward 14, District 10, HCMC, Vietnam 2Tra Vinh University, 126 Nguyen Thien Thanh street, Ward 5,Tra Vinh city, Vietnam 3Hochiminh City of University of Natural Resources and Environment Vietnam, 236 Le Van Sy street, District Tan Binh, HCMC, Vietnam *Email: pdtuan@hcmunre.edu.vn Received: 15th August 2016 / Accepted for publication: 10th November 2016 ABSTRACT Activated carbon (AC) has been proven to be an effective adsorbent for the removal of a variety of pollutants. AC is extensively used for adsorption because its high surface area is well- developed internal micro porosity. The objective of this study is to determine the optimal condition of the surface modification process of activated carbon from rice husk (ACRH) using HNO3. That increase the functional group, and improve affinity towards certain contaminants of ACRH for increasing the adsorption capacity of Ni2+ and Cd2+. Two factors were taken into account as: the concentration of HNO3 (1, 3, and 5 M), reaction time (1, 2, 3, 4 and 5 hours). The results showed that the optimal condition of the surface modification process was derived at the equilibrium concentration of HNO3 = 3 M and the equilibrium reaction time = 4 hour. Interestingly, in comparison with the control sample (ACRH not modified), the adsorption capacity of Ni2+ and Cd2+ increased from 10.0 to 17.2 mg/g and 10.4 to 29.6 mg/g, respectively. This adsorption capacity of modified ACRH shows the increase significantly. Keywords: surface modification, HNO3, activated carbon, rice husk. 1. INTRODUCTION Activated carbon (AC) has been proven to be effective for removal of a wide variety of inorganic and organic pollutants [1, 2, 3]. The demand of using ACs is increasing rapidly with enhanced awareness about environment protection. However, the price of ACs is very expensive due to the fact that most of the commercial AC products are derived generally from costly natural materials such as wood, or coal. Therefore, the exploration for a cheap and easily available precursor for AC production is of great importance. Hoa Thai Ma, Van Thi Thanh Ho, Hung Cam Ly, Nguyen Bao Pham, Tuan Dinh Phan 20 There is growing interest in the production of ACs using cheap agricultural and industrial wastes [4, 5, 6]. Several wastes including coffee husks, rice husks, cotton stalks, coconut husks, herb residues, and corn cobs have been investigated as precursors to produce activated carbon. The precursors are still receiving renewed attention because of their cheap prices and abundant resources. Many of the waste-based ACs also enabled comparable or even better performance than the commercial ACs in many applications. Consequently, the conversion of cheap biomass wastes into value-added ACs not only opens a low-cost path for AC production but also provides an efficient way to reduce the environmental pollution caused by disposing agricultural and industrial wastes [7]. Rice is an important staple food for 3.5 billion people on the world. According to Food and Agriculture Organization of the United Nations (FAO), the yield of paddy in 2015 reached 749.1 million tones and it will tend to increase in the following years. According to statistics rice husk (RH), a by-product from the milling process, made up about 20 % the weight of paddy [8]. Thus, The annual generated RH on the world is about 150 million tons. In Vietnam, it is about approximately 8.94 million tonnes, equivalent 5.96 % of the world's RH. Currently, the amount of husk is still underused in a reasonable manner especially in developing countries, including Vietnam. Most RH is burned or dumped directly into canals causing environmental pollution. RH on burning gives 14–20 % ash which contains 80–95 % silica in the crystalline form and minor amounts of metallic elements [9]. RH is little commercial value and not useful to feed either humans or animals because it contains high content of silicon dioxide [10] which is dissolved in hot concentrated alkali [11]. The presence of heavy metals in the environment is a major problem due to their toxicity to many life forms. The treatment of metals using precipitation is not always able to meet the metal discharge standards. Technologies such as reverse osmosis, able to meet the standards, are expensive. The innovative technologies that are cost-effective and able to reduce heavy metal concentration to low levels is important [12, 13]. Surface modification of activated carbon, in order to modify their specific physical and chemical properties to facilitate metals removal from waste water, can be done through different methods. Based on the type and nature of the pollutant to be removed, surface modification in activated carbon has been carried out to increase its affinity towards the desired pollutant. The modification method has been proved effective for metal ions adsorption capability and the removal of them from water [14, 15, 16]. Acid treatment method have been studied to develop the surface modification of activated carbon which is considered as a promising and attractive way toward new applications of activated carbon in many fields. The aim of this study is to determine the interactive effect of the concentration of HNO3 and time on the adsorption capacity of Ni2+ and Cd2+ in the modification process of ACRH. That is highly significant and bring new information for the research. Because the actual adsorption process takes place simultaneously with the interaction of the elements without the individual effect. The efficiency of the modification showed improvement in the sorption of Ni2+ and Cd2+. We conducted experiments with each individual metal because each type of waste water contain a different metals content. In fact, depending on the demand for treatment of each metal in wastewater, we will choice the most appropriate condition for the metal. That promises to leverage agricultural by-product to produce modified ACRH using for metal ions treatment. Surface modification of activated carbon from rice husk for enhancing the nickel (Ni2+) 21 2. EXPERIMENTAL Preparation of ACRH RH from a rice mill at Tra Vinh province was used as raw material. It was washed thoroughly with distilled water to remove soil and dust, then it was dried at 105 oC in an oven for 24 hours. The dried rice husk was impregnated with 2 M NaOH solution with the ratio of rice husk per NaOH =1:8 and stirred at least 1 hour at 100 oC [7]. After the basis solution was drained, the RH was washed with distilled water until the filtrate was neutral, and then dried at 105 oC for 24 hours. 40 g of the dried rice husks were placed in a quartz reactor of the vertical electric heating furnace. The whole system was purged nitrogen gas to remove the oxygen. Then the reactor was heated at 500 oC in the presence of nitrogen flow with a heating rate of 10 oC/min. and was held at this temperature for 1 hour. Thereafter the temperature was raised to 800 oC. After the activation temperature was reached, the gas was switched to steam and sample was kept under this condition for 30 min. before it was finally cooled in nitrogen flow. Experimental Modification of ACRH by HNO3 0.5 g of each adsorbent was added with 25 ml of HNO3 concentrations (1 to 5 M) in erlen 50 ml. The erlen was stirred at 100 oC for 1 to 5 hours for each experimental concentration of HNO3 as shown in Table 1. The mixtures were filtered and washed with distilled water until the filtrate was neutral. Then the mixtures were dried at 100 oC for 6 hours. The sample was used to measure the adsorption capacity of Ni2+ and Cd2+ using atomic absorption spectrophotometry. Table 1. Experimental design with the interactive effect of the concentration of HNO3 and time for the modification of ACRH. Reaction time (hour) B1 (1 hour) B2 (2 hours) B3 (3 hours) B4 (4 hours) B5 (5 hours) Concetration of HNO3 (M) A1 (1 M) A1B1 A1B2 A1B3 A1B4 A1B5 A2 (3 M) A2B1 A2B2 A2B3 A2B4 A2B5 A3 (5 M) A3B1 A3B2 A3B3 A3B4 A3B5 Determination of Ni2+ adsorption capacity using 1-(2 Pyridylazo)-2-Napthol (PAN) method The method to determine the Cd2+ adsorption capacity was described by Lopez Garcia et al. [17]. The Ni2+ and Cd2+ adsorption capacity was calculated equation [18]: where wi is the initial mass of Ni2+ or Cd2+ before adsorption process and wt is the mass of Ni2+ or Cd2+ after adsorption process (mg), wM is the mass of sample (g). All of determinations were performed in triplicate. Hoa Thai Ma, Van Thi Thanh Ho, Hung Cam Ly, Nguyen Bao Pham, Tuan Dinh Phan 22 3. RESULTS AND DISCUSSION The effect of HNO3 concentration and reaction time on the Ni2+ adsorption capacity Table 2. The Ni2+ adsorption capacity at different conditions of the concentration of HNO3 and the reaction time. The concentration of HNO3 (M) Average ACRH is not modified Time (h) 1 3 5 10.04±0.2 1 11.2 11.9 13.0 12.0±0.1d 2 12.6 13.4 14.2 13.4±0.3c 3 13.5 15.5 15.7 14.9±0.3b 4 14.5 17.2 17.3 16.3±0.1a 5 14.6 17.0 17.4 16.3±0.2a Average 13.3±0.1b 15.0±0.3a 15.5±0.1a Note: a highest value denotes a statistically significant difference; b, c, d lower values Th e N i2 + ad so rp tio n ca pa ci ty (m g/ g) 7.69122 + 1.247*x + 2.18589*y - 0.151333*x*x + 0.0746667*x*y - 0.209444*y*y 0 1 2 3 4 5X 0 1 2 3 4 5 Y 7 9 11 13 15 17 19 Figure 1. The effect of the concentration of HNO3 and time on the Ni2+ adsorption capacity. (x is the concentration of HNO3 (1 to 5M), y is modification time (1 to 5 hours)). The Ni2+ adsorption capacity = 7.69122 + 1.247*x + 2.18589*y – 0.151333*x*x + 0.0746667*x*y – 0.209444*y*y (1) The equation (1) shows that the Ni2+ adsorption capacity of ACRH depends on the concentration of HNO3 and the modification time by quadratic of x and y. This relationship has high statistical significance due to P-Value <0.01 as shown in Table 3. Table 3. Analysis of Variance. Source Sum of Squares Df Mean Square F-Ratio P-Value Model 168.259 5 33.6518 192.31 0.0000 Residual 6.82447 39 0.174986 Total (Corr.) 175.083 44 Surface modification of activated carbon from rice husk for enhancing the nickel (Ni2+) 23 The concentration of HNO3 (1 to 5 M) and the reaction time (1 to 5 hour) using for the surface modification of ACRH have a significant impact on the Ni2+ adsorption capacity. The result in Table 2. shows that at the equilibrium the concentration of HNO3 from 3 to 5 M, the Ni2+ adsorption capacity is highest and there is no significant difference between them. At 3M of HNO3, the Ni2+ adsorption capacity reaches 15 mg/g and at 5 M, the Ni2+ adsorption capacity is 15.52 mg/g. While the Ni2+ adsorption capacity may be reached lower in the lower concentration condition. Besides, reaction time still plays a role in increasing the Ni2+ adsorption capacity. At equilibrium reaction time of 4 and 5 hours, the Ni2+ adsorption capacity is highest and reaches 16.33 and 16.31 mg/g, respectively. The results have statistically significant difference in comparison with the Ni2+ adsorption capacity at the shorter reaction time. The effect of variables to the Ni2+ adsorption capacity can be seen on the response surface as shown in Fig.1. This Ni2+ adsorption capacity of this study is higher than the results previously reported by Lakherwal, (2016) (7.6569 mg/g of granular AC) and Nabarlatz, (2012) (7.65 mg/g of activated carbon based on a native lignocellulosic precursor) [19,20]. But it is slightly lower than the result of Rahman, (2014) (19.61 mg/g of palm shell AC) [21]. The effect of the concentration of HNO3 and the reaction time on the Cd2+ adsorption capacity Table 4. The Cd2+ adsorption capacity at different conditions of the concentration of HNO3 and the reaction time. The concentration of HNO3 (M) Average AC is not modified Time (hour) 1 3 5 10.44±0.1 1 10.2 18.4 19.7 16.1±0.2d 2 13.7 21.6 24.8 20.0±0.1c 3 15.5 26.1 32.4 24.7±0.2b 4 16.6 29.6 34.1 26.8±0.3a 5 24.8 33.6 33.6 30.7±0.2a Average 16.1±0.2b 25.9±0.2a 28.9±0.1a Note: a highest value denotes a statistically significant difference; b, c, d lower values The conditons of the modification affect not only the Ni2+ absorption capacity but also the Cd2+ adsorption capacity as shown in Table 4. The experimental factors achieving the highest adsorption capacity of Cd2+ (25.86 mg/g) can be determined as the equilibrium concentration of HNO3 of 3 M and this value has not significant difference as HNO3 concentration increases to 5M (28.91 mg/g). The results have statistically significant difference in comparison with the Cd2+ adsorption capacity at the concentration of HNO3 and that values are sill higher the control sample. This can be ascribed to the enhancement of the ions migration to the absorbent surface by the increased concentration [14]. Besides, reaction time still plays a role in increasing the Cd2+ adsorption capacity as of Ni2+. At the equilibrium reaction time of 4 and 5 hours, the Cd2+ adsorption capacity is highest, that reaches 16.33 and 16.31 mg/g respectively. While the Cd2+ adsorption capacity may be reached lower values in the shoter reaction time. The results have statistically significant difference. The effect of variables to the Cd2+ adsorption capacity can be seen on the response surface as shown in Fig. 2. This Cd2+ adsorption capacity of this study is Hoa Thai Ma, Van Thi Thanh Ho, Hung Cam Ly, Nguyen Bao Pham, Tuan Dinh Phan 24 higher than the results previously reported by Alslaibi [22] (11.72mg/g of microwaved olive stone AC), Tounsadi [23] (0.85 mg/g of AC from Glebionis coronaria L.) and Wang [24] (8.08 mg/g of granular AC supported magnesium hydroxide) [22 - 24]. But it is lower than the result of Tounsadi [25] (31.6 mg/g of AC from Diplotaxis Harra biomass). Th e C d2 + ad so rp tio n ca pa ci ty (m g/ g) -2.20486 + 7.8235*x + 4.3428*y - 0.834083*x*x + 0.125167*x*y - 0.186587*y*y 0 1 2 3 4 5X 0 1 2 3 4 5 Y -3 7 17 27 37 Figure 2. The effect of the concentration of HNO3 and time on the Cd2+ adsorption capacity. (x is the concentration of HNO3 (1 to 5 M), y is modification time (1 to 5 hours)). Table 5. Analysis of Variance. Source Sum of Squares Df Mean Square F-Ratio P-Value Model 2509.65 5 501.929 162.92 0.0000 Residual 120.153 39 3.08084 Total (Corr.) 2629.8 44 The Cd2+ adsorption capacity = -2.20486 + 7.8235*x + 4.3428*y - 0.834083*x*x + 0.125167*x*y - 0.186587*y*y (2) The equation (2) shows that the Cd2+ adsorption capacity of ACRH depends on the concentration of HNO3 and the modification time by quadratic of x and y. This relationship has high statistical significance due to P-Value <0.01 as shown in Table 5. The Ni2+ and adsorption capacity increase when the concentration of HNO3 increases from 3 to 5 M and the reaction time increases from 4 to 5 hours. That can be explained so that the oxidation treatment by HNO3 gives rise to a large increase in the amount of total acidity resulting from the increase of surface oxide groups such as, carboxyl, lactone and phenol. The increase of the total acidity changed the point of zero charge of ACRH and led to an increase of adsorption capacity for metal ions [16]. Besides, the increase of the Ni2+ and Cd2+ adsorption capacity mainly attributes to the abundant vacant sites on activated carbon surface. The high adsorption capacity of Ni2+ and Cd2+ by the modified RHAC confirms that the adsorption process is related to the concentration of HNO3 and reaction time. The concentration of HNO3 of 3 M for 4 hours was selected for the modification process of ACRH sample regarding the cost of the process. 4. CONCLUSION The effect of the fixed conditions of ACRH modification on the Ni2+ and Cd2+ adsorption capacity investigated. The results showed that concentration of HNO3 and reaction time of the Surface modification of activated carbon from rice husk for enhancing the nickel (Ni2+) 25 modification process effect significantly to the Ni2+ and Cd2+ adsorption capacity. 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