Effect of preparation conditions of biochar from rice straw by hydrothermal carbonization - Ngoc Bich Nguyen

Journal of Science and Technology 55 (1B) (2017) 223–229 EFFECT OF PREPARATION CONDITIONS OF BIOCHAR FROM RICE STRAW BY HYDROTHERMAL CARBONIZATION Ngoc Bich Nguyen1, 2, *, Cao Thanh Tung Pham1, 3, Dinh Thanh Nguyen1, 4 1 Graduate University of Science and Technology, Vietnam Academy of Science and Technology 18 Hoang Quoc Viet Street, Nghia Do Ward, Cau Giay District, Ha Noi, Vietnam 2Centre of Chemical Analysis, Dong Thap University 783 Pham Huu Lau Street, Ward 6, Cao Lanh City, Dong Thap Province, Viet Nam 3 Institute of Chemical Engineering, Viet Nam Academy of Science and Technology 1 Mac Dinh Chi Street, Ben Nghe Ward, District 1, Ho Chi Minh City, Vietnam 4 Institute of Applied Material Science, Viet Nam Academy of Science and Technology 1 Mac Dinh Chi Street, Ward 14, District 1, Ho Chi Minh City, Vietnam *Email: nnbich2906@gmail.com Received: 30 December 2016; Accepted for publication: 6 March 2017 ABSTRACT Biochar was prepared from rice straw using hydrothermal carbonization method which consisted of zinc chloride treatment. The effects of hydrothermal temperature, time and biomass/water rate on methylene blue removal from aqueous solution were investigated. Besides, response surface modeling (RSM) and the central composite design – face centered (CCF) method will be used in designing experiments. It was found that the optimal conditions for high efficiency of biochar were such that: temperature of 194.07 °C, 9.00 hours and biomass/water ratio of 10 wt%, which resulted methylene blue removal of 92.56 %. Keywords: hydrothermal carbonization, biochar, rice straw, methylene blue. 1. INTRODUCTION Agricultural waste would be an attractive starting precursor for carbonaceous materials because of its abundance, low cost, excellent properties and special structures [1, 2]. Accordingly, numerous carbonaceous materials derived from renewable biomass have been reported, and some of them were applied in water purification [3]. In addition, most of the reported adsorbents were synthesized in hash conditions with high temperature and toxic reagents, which was uneconomic, energy–intensive and non–environmentally friendly. Therefore, it is of great importance to develop a novel approach to synthesize biomass–derived adsorbent under mild conditions. Hydrothermal carbonization (HTC) is a novel thermal conversion process, which provides an eco–friendly approach to obtain various carbonaceous materials under mild conditions (up to Effect of preparation conditions of biochar from rice straw by hydrothermal carbonization 224 180 °C). It shows distinct advantages over pyrolysis in that it can process wet biomass, thus avoiding a substantial drying cost for typically wet biomass feedstock [4]. In our efforts towards biomass waste disposal and resource recovery, we have developed a facile and low–cost approach for synthesizing a carbonaceous adsorbent from rice straw under low temperature hydrothermal condition. 2. MATERIALS AND METHODS 2.1. Materials Rice straw (RS) as precursor was obtained from Binh Chanh farmer in Ho Chi Minh City, Vietnam. Zinc chloride (ZnCl2) and methylene blue (MB) were provided by Xilong Chemical Reagent Co., China. Deionized water was used to prepare all solutions. 2.2. Preparation of biochar RS was washed with water and subsequently dried at 105 °C for 24 h to remove moisture content. The dried RS was milled into powder and sieved through a 250 μm sieve before loading into a teflon–lined autoclave (100 mL) with ZnCl2 (7.5 g) and deionized water (50 mL). Hydrothermal carbonization step was carried out at temperature from 160 to 180 °C, under time from 3 to 9 h and with RS/water ratio from 5 to 15 wt%. After cooling to room temperature, the solid product was collected by vacuum filtration, washed with deionized water and stirred in 1 L of deionized water overnight to remove the residual ZnCl2. Then, the biochar (BC) was got by vacuum filtration, and washed with deionized water and dried in a vacuum. 2.3. Adsorption studies For batch adsorption studies, 0.1 g of adsorbent were mixed with 100 mL aqueous dye solutions of 50 mg/L initial concentration in 250 mL glass flasks. The mixture was agitated at 400 rpm for 120 minutes to reach the adsorption equilibrium. The MB concentration of the solution was analyzed at wavelength of maximum absorbance (664 nm) by UV–vis spectrometry (Labomed Spectro UV – 2650), and the MB concentration after adsorption was calculate according to the standard curve. All samples were filtered prior to analysis in order to minimize interference of the solid with the analysis. All the experiments were conducted at 30 °C. The percentage removal at equilibrium was calculated as follows: ۻ۰ ܚ܍ܕܗܞ܉ܔ ሺ%ሻ ൌ ۱ܗି ۱܍۱ܗ ൈ ૚૙૙ (1) where Co and Ce are the liquid–phase MB concentrations at initial state and at equilibrium (mg/L), respectively. 2.4. Design of experiments In this work, response surface methodology (RSM) is a collection of mathematical and statistical techniques that are useful for modeling and analysis of problems in which a response of interest is influenced by several variables [5]. A standard RSM design called the central composite design – face centered (CCF) was applied to study the variables for preparing BC from RS. This method can reduce the number of experimental trials needed to evaluate multiple parameters and their interaction. Generally, the CCF consists of three kinds of runs which are the Ngoc Bich Nguyen, Cao Thanh Tung Pham, Dinh Thanh Nguyen 225 2n factorial runs, 2n axial runs and six center runs, where n is the number of factors. The BC was prepared using hydrothermal carbonization method. The variables studied are hydrothermal temperature (x1), hydrothermal time (x2) and RS/water ratio (x3). For each categorical variable, a 23 full factorial CCF for the three variables, consisting of 8 factorial points, 6 axial points and 6 replicates at the center points were employed, indicating that altogether 20 experiments were required, as calculated from equation (2) [3]: N = 2n + 2n + nc = 23 + 2 * 3 + 6 = 20 (2) where N is the total number of experiments required and n is the number of factors. The complete design matrix of the experiments carried out, are shown in Table 1. Table 1. Design matrix of the experiments. Variables (factors) Code Coded variable levels –1 0 +1 HTC temperature, °C x1 160 180 200 HTC time, h x2 3 6 9 RS/water ratio, wt% x3 5 10 15 The experimental sequence was randomized in order to minimize the effects of the uncontrolled factors. The response was MB removal (Y). It was used to develop an empirical model which correlated the response to the three BC preparation variables using a second degree polynomial equation as given by equation (3): (3) where Y is the predicted response, bo is the constant coefficient, bi is the linear coefficients, bij is the interaction coefficients, bii is the quadratic coefficients and xi, xj are the coded values of the activated carbon preparation variables. 2.5. Model fitting and statistical analysis The experimental data were analyzed using a MODDE statistical software version 5.0 (Umetrics Inc., Switzerland) for regression analysis to fit the second–degree polynomial equation and also for the evaluation of the statistical significance of the equations developed. 2.6. Characterization of optimized biochar Scanning electron microscopy (SEM) by using Hitachi – S4800 analysis in Nano Technology Laboratory, The Research Laboratories of Saigon Hi–Tech Park was carried out on the biochar prepared under optimum conditions, to study its surface morphology. Eff 22 3.1 exp 92 Ru 1 2 3 4 5 6 7 8 9 1 tem we fac res ect of prepa 6 . Model fitt The com erimental w .35%. Coeff n HTC temperat x1 (°C 160 200 160 200 160 200 160 200 160 0 200 Based on perature wa ll as the qu tor do not ponse surfac ration condit ing and stat plete design ork are giv icient List of Tab ure, ) HTC time, x2 (h) 3 3 9 9 3 3 9 9 6 6 Figu the values s found to h adratic effec significantly es which w ions of bioch 3. RESU istical analy matrixes en in Table regression m le 2. Experim RS/water ratio, x3 (wt%) 5 5 5 5 15 15 15 15 10 10 re 1. Coeffici of regress as significan ts of HTC affect on t as constructe ar from rice LTS AND sis together wi 2. The MB odel equat ental design MB removal, Y (%) Ru 69.54 1 85.62 1 72.20 1 92.35 1 67.24 1 86.52 1 70.49 1 91.58 1 70.04 1 87.00 2 ent List of re ion model t effects on temperature he MB rem d to show t straw by hy DISCUSSI th the resp removal w ion are show matrix for MB n HTC temperat x1 (°C 1 180 2 180 3 180 4 180 5 180 6 180 7 180 8 180 9 180 0 180 gression mod equation as the MB rem were consi oval. Figure he interactio drothermal c ON onse values as found to s in Figure 1 removal. ure, ) HTC time, x2 (h) 3 9 6 6 6 6 6 6 6 6 el equation. shown in oval. The ef dered moder 2 shows th n effects of arbonization obtained f range from . RS/water ratio, x3 (wt%) 10 10 5 15 10 10 10 10 10 10 Figure 1, t fects of HTC ate. RS/Wa e three–dim the HTC tem rom the 67.24 to MB removal, Y (%) 82.44 91.56 87.51 83.22 84.58 85.16 84.90 84.32 83.68 83.94 he HTC time as ter radio ensional perature an Th du HT res dev mo d time on th e MB remo e to the incre C temperat ult was in ag Figure 2. Th Summary eloped was dels develo e MB remov val generally ase in the d e with ZnCl reement wit ree–dimensio F plot of the evaluated b ped seems to Ngoc B al. For this increased ecompositio 2 treatment. h the work d nal response HTC igure 3. Sum regression ased on the be the best ich Nguyen, plot, the RS with increas n of organic The obtaine one by Hua surface plot o time, RS/W r mary Plot of model is gi correlation at low stan Cao Thanh /water ratio e in the two compounds d material h n Ma et al. [ f MB remova atio = 10 %). the regression ven in Figu coefficient dard deviatio Tung Pham, was fixed at variables s in the raw st as high abso 4]. l (effect of HT model. re 3. The q s, R2 and Q n and high Dinh Thanh zero level ( tudied. This raw when in rption capa C temperatu uality of th 2 values. In R2 statistics Nguyen 227 10 wt%). result is creasing city. The re and e models fact, the which is Eff 22 clo va sm 0.8 tha thi 3.2 pro res stu Th in ob wt rem va rel H 3.3 pre po tre Zn ect of prepa 8 ser to unity lue reaches all predictio 74, respecti t there was s model. . Process op One of th duced shou ponses of M died. The ex e predicted a Table 3. In the op tained by us %, respectiv oval obtain lues obtaine atively smal TC temperatu 194.0 . Character Figure 4a paration. T res were pr atment and Cl2 was effe ration condit as it will g 0.7 or larger n errors [6] vely. It was a good agre timization e main aims ld have a h B removal perimental nd experim timization a ing HTC tem ely. At op ed are 93.41 d were in l errors betw re, x1 (°C) 7 ization of o , b, respect he precursor esence as ob the surface ctive in crea Figure ions of bioch ive predicte , indicates t . In this exp considered r ement betwe of this study igh MB rem while the v conditions w ental results nalysis in perature, H timum cond % and 92.5 good agreem een the pred Tab Variabl HTC time, x 9.00 ptimized BC ively show ’s surface t served in o of this was ting surface. 4. SEM mic ar from rice d value clo hat the mod eriment, the elatively hi en the expe was to find oval. The alues of the ith the high of MB remo order to ma TC time and itions, the 6 %, respec ent with t icted and th le 3. Model v es 2 (h) RS/ the SEM im extures wer ther bioma larger. Thi rographs; (a) straw by hy ser to the ac el has good R2 and Q2 v gh as the va rimental and the optimum target was three variab est desirabi val obtained ximum MB RS/water ra predicted a tively. It wa he values p e actual valu alidation. water ratio, x 5.00 ages of th e rough, un ss [7–9]. Th s result reve raw RH and ( drothermal c tual value f predictive alues (Figu lue was clos the predicte process pa set as maxi les were se lity were se at optimum removal, th tio of 194.0 nd experime s observed redicted fro es. 3 (wt%) P e raw RS a even, undul e size of B aled that th b) BC (500x) arbonization or the respo ability and w re 3) were 0 e to unity, i d MB remo rameters wh mum value t in the rang lected to be conditions e optimum 7 °C, 9.00 h ntal results that the expe m the mod MB remova rediction Ex 93.41 nd BC at o ating and v C was sma e HTC proc . nses. Q2 ill have .983 and ndicating val from ich HTC s for the es being verified. are listed BC was and 5.00 of MB rimental els, with l (%) periment 92.56 ptimized ery little ller after ess with Ngoc Bich Nguyen, Cao Thanh Tung Pham, Dinh Thanh Nguyen 229 4. CONCLUSIONS RSM was successfully used to investigate the effects of preparation variables on the MB removal. 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