Available online at http://ijcpe.uobaghdad.edu.iq and www.iasj.net Iraqi Journal of Chemical and Petroleum Engineering Vol.20 No.2 (June 2019) 23 – 32 EISSN: 2618-0707, PISSN: 1997-4884 Corresponding Authors: Name: Marah Waleed Khalid , Email: mw.alsharod@gmail.com, Name: Sami D. Salman, Email: sami.albayati@gmail.com IJCPE is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. Adsorption of Chromium Ions on Activated Carbon Produced from Cow Bones Marah Waleed Khalid and Sami D. Salman Alkhwarizmi College of Engineering, University of Baghdad, Baghdad, Iraq Abstract Due to the broad range uses of chromium for industrial purposes, besides its carcinogenic effect, an efficient, cost effective removal method should be obtained. In this study, cow bones as a cheap raw material were utilized to produce active carbon (CBAC) by physiochemical activation, which was characterized using: SEM to investigate surface morphology and BET to estimate the specific surface area. The best surface area of CBAC was 595.9 m 2 /gm which was prepared at 600 ᵒ C activation temperature and impregnation ratio of 1:1.5. CBAC was used in aqueous chromium ions adsorption. The investigated factors and their ranges are: initial concentration (10-50 mg/L), adsorption time (30-300 min), temperature (20-50 ᵒ C) and solution pH (2-11). Isotherm of adsorption and its kinetics were studied. The adsorption process was modeled statistically and was represented by an empirical model. Equilibrium data were fitted to the Langmuir and Freundlich isotherm models and the data best represented by Freundlich isotherm. Pseudo- first order and pseudo- second order kinetic equations were utilized to study adsorption kinetics, where chromium adsorption on CBAC fitted pseudo- second order fitted the data more adequately. The best removal efficiency was found to be 94.32%. Keywords: Adsorption, activated carbon, cow bones, chromium, physiochemical activation. Received on 06/02/2019, Accepted on 23/03/2019, published on 30/06/1029 https://doi.org/10.31699/IJCPE.2019.2.4 1- Introduction Due to the wide range and the exaggerated uses of organic solvents, oxidizing agents, phenols and heavy metals in industry, they were accumulated in the environment, which led some ecosystems to deteriorate. Heavy metals category is one of the most harmful pollutants of surface and ground waters. Industrial effluents are the primary source. Because almost all of the heavy metals could not be degraded to harmless components ‎[1]; therefore reduction of heavy metals concentrations before discharging them into rivers had become a necessity. Otherwise, they could be harmful to health and/or reduce drinking water quality ‎[2]. According to the World Health Organization (WHO, 1984) and International Program on Chemical Safety (IPCS, 1988), the most toxic metals are aluminum, zinc, mercury, arsenic, chromium, nickel, copper, cadmium and lead. The drinking water guideline value recommended by the World Health Organization (WHO) and Iraqi standard regulation is 0.01 and 0.015 mg Pb/l. Chromium is found in freshwaters due to its extensive use in petrochemical, electronics, tanneries, electroplating industries, mining operations, as well as in textile mill products ‎[3]. Chromium is carcinogenic ‎[4] and causes severe damage to human organs: e.g., to the kidneys, the brain, reproductive system, the liver, and the nervous system ‎[5]. As a result of the toxicity of this element and its compounds, removal has become an urgent priority. There are various available means to reduce heavy metals levels from water including ion exchange ‎[6], ‎[7], membrane filtration ‎[8], ‎[9], chemical precipitation ‎[10], ‎[11] and electrocoagulation ‎[12], ‎[13], but these methods generally require high cost and produce more lethal products ‎[14]. Adsorption on solid materials is one of the best existing methods for the refinement of water and the regulation of atmospheric and aquatic pollution, with active carbon being the most used adsorbent in the industry ‎[6]. In the last years, global consumption of active carbon has increased by an average annual increase of 5.5% and it is anticipated to continue to do so at a higher rate in next years (8.1% in 2018)‎[15]. Activated carbon has been established as effective adsorbents for the removal of a wide variety of pollutants dissolved in aqueous media, or from the gaseous environment due to its exceptionally high surface areas, well-developed internal micro porosity structure as well as the presence of a broad spectrum of surface functional groups ‎[16]. Thus, locally produced wastes such as cow bones, saw dust and others were examined for preparing active carbon. Because of the low cost of those wastes, a high percentage of carbonaceous contents and their abundance they were utilized to produce cost effective, efficient activated carbon and applicate it to remove heavy metals. https://doi.org/10.31699/IJCPE.2019.2.4 M. W. Khalid and S. D. Salman / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 23 - 32 46 2- Materials And Methods 2.1. Adsorbate Technical grade Potassium dichromate (K2Cr2O7) of 99.8% purity provided by Himedia, Germany was used to preparing the stock solution of chromium. All solutions were prepared using distilled water. The stock of 1000 mg/L was prepared by adding (2.8269) gm of K2Cr2O7 to a liter of DW; dilution law was used to prepare the required concentrations. 0.1 M HNO3 and 0.1 M NaOH were used to adjust the pH ‎[17] 2.2 Chemicals and Gases Name Formula Assay (%) Source or company Usage Nitrogen N2 99.9 local Inert gas to prevent raw material combustion Potassium Hydroxide pellet KOH 85 Himedia, India Chemical activating agent Carbon dioxide CO2 99.9 local Physical activating agent Hydrochloric acid HCl 2N England Washing and Neutralizing of AC Potassium dichromate K2Cr2O7 99.9 Thomas Baker, India Chromium ions source Nitric acid HNO3 70 J.T.Baker, Holland solution pH adjustment Sodium hydroxide NaOH 99.5 DIDACTIC, Spain solution pH adjustment 2.3. Preparation and Characterization of Activated Carbon Cow bones (CB) was collected from local butchers, Baghdad, Iraq as waste. The preparation methodology of CBAC is concise in Fig. 1. The conditions that were used in cow bone charring were obtained from Moreno-piraján ‎[18]. The surface area was analyzed using Brunaure-Emmett- Teller (BET: HORIBA, SA-900 series, USA) through nitrogen adsorption isotherm at 77 K. In order to determine the shape of the CBAC surface, the samples were scanned using a Scanning Electron Microscope (TESCAN, Vega III, Czech Republic). Fig. 1. Schematic diagram for the CBAC preparation steps 2.4. Design of Experiments The experimental design usually used to efficiently map the set of experiments to be conducted and to serve the following: - understand the effect of the factors and/or - model the relationship between response and factors with a minimum of experiments ‎[19]. Taguchi method was used in the optimization of the CBAC production and modeling of adsorption process due to its efficiency compared to other methodologies and its robustness. STATISTICA 10 (StatSoft, Inc. USA) was used to design the set of experiments. Table 1 shows the L9 orthogonal array that was chosen (2 factors, 3 levels) for CBAC preparation and Table 2 shows the L25 array (4 factors, 4 levels) for chromium adsorption process. Table 1. Taguchi DOE (L9 array) of CBAC optimization No. Activation temperature ( o C) IR 1 600 1:0.5 2 600 1:1 3 600 1:1.5 4 700 1:0.5 5 700 1:1 6 700 1:1.5 7 800 1:0.5 8 800 1:1 9 800 1:1.5 M. W. Khalid and S. D. Salman / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 23 - 32 47 Table 2. Taguchi DOE (L25 array) of Chromium adsorption experiments No. Initial concentration (mg/L) Contact time (minutes) Temperature (C o ) Solution pH 1 10 30 20 2 2 10 120 30 5 3 10 210 40 8 4 10 300 50 11 5 20 30 30 8 6 20 120 20 11 7 20 210 50 2 8 20 300 40 5 9 30 30 40 11 10 30 120 50 8 11 30 210 20 5 12 30 300 30 2 13 50 30 50 5 14 50 120 40 2 15 50 210 30 11 16 50 300 20 8 2.5. Batch Equilibrium Studies Bach mode adsorption experiments were conducted by adding a specific amount of adsorbent to a 100 ml chromium solution contained in a 100 ml capped plastic containers. The containers were placed in an isothermal shaker (JSSI-300CL, JSR, Korea) at an agitation speed of 180 rpm. The remaining concentration of chromium in each sample after adsorption at different times was determined by atomic-absorption spectroscopy (Shimadzu AA6200, Japan). All samples were filtered from the adsorbent with Whatmen filter paper to make it carbon free. The chromium concentration adsorbed on CBAC was predicted according to: ( ) (1) Where qe is the adsorption capacity at equilibrium (mg/g), Co and Ce are the concentrations at initial and equilibrium conditions (mg/L) for chromium solution, respectively; V is the volume (L); and W is the weight (g) of CBAC. 2.6. Adsorption Process Modelling After adsorption batch experiments were run, the equilibrium concentrations (Ce) were used to form a mathematical model that represents the adsorption process. This model relates Ce as a response with the investigated factors which are: initial concentration, contact time, temperature, and solution pH. STATISTICA 10 (StatSoft, Inc. USA) was utilized to form the model by nonlinear estimation method. This model was used to generate the equilibrium concentration at various conditions; these results were used in adsorption isotherm fitting, kinetics study, and adsorption thermodynamics. 2.7. Adsorption Isotherm Two isotherm models (Langmuir and Freundlich) were used to fit the equilibrium data. The linear form of the Langmuir ‎[20] model is: ( ) (2) Where Ce (mg/l) is the concentration of chromium at equilibrium; qe (mg/g) the equilibrium adsorption capacity; qm the adsorption capacity for a complete monolayer (mg/g); Ka (L/mg) is the constant of adsorption equilibrium. The linear form of Freundlich ‎[21] isotherm is: ( ) (3) The constants KF (mg/g) and n are the Freundlich constants. 2.8. Kinetic Studies The adsorption rate constants were predicted from the pseudo-first-order and pseudo-second-order equations. For the pseudo-first-order, the Lagergren ‎[22] the used expression is: ( ) (4) Where qe and qt (mg/g) are the adsorption capacities equilibrium and at time t (min), respectively and k1 (1/min) is the adsorption constant. The linear form of the pseudo-second-order ‎[23] reaction can give by: (5) Where the adsorption capacity of equilibrium (qe), and the constant of second order k2 (g/mg h) can be determined experimentally from the intercept and slope of t/qt versus t plot. 3- Results and Discussion 3.1. CBAC Production and Optimization The complete design array (L 9) for the surface area and yield as responses of CBAC preparation with two factors, temperature of activation and impregnation ratio (IR)(char: KOH wt: wt) from the experiments that were conducted are shown in Table 3. M. W. Khalid and S. D. Salman / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 23 - 32 48 Table 3. Preparation of CBAC experimental design array and the results for SSA and yield Runs CBAC preparation variables CBAC preparation responses IR Activation temperature (ᵒC) Specific surface area (m 2 /gm) Yield (%) 1 1:0.5 600 270.4 85.9 2 1:0.5 700 475.6 79.3 3 1:0.5 800 370.4 71.4 4 1:1 600 315.3 79.5 5 1:1 700 480.6 72.2 6 1:1 800 460.0 60.6 7 1:1.5 600 595.9 76.2 8 1:1.5 700 350.0 65.3 9 1:1.5 800 240.8 57.9 It was found that in CBAC preparation the SSA increases as IR rises from 1:0.5 to 1:1. These results clarify that the reduction of KOH occurs during the activation process, it was transformed to potassium oxide by a dehydration reaction. Potassium oxide reacts with carbon dioxide that is provided during physical activation to form K2CO3, which aids to form new pores and widen pores that were formed during chemical activation. So it was recognized that above 420 o C (melting point of KOH) the surface area of carbon activated by KOH is more than the area of carbon activated by K2CO3 ‎[24]. When the IR reached to 1:1.5 the SSA increases at activation temperature of 600 o C, this may be due to lower potassium deposition on pores compared to the experiments of activation temperatures of 700 and 800 o C where the SSA decreased, this was probably due to excessive potassium hydroxide molecules decomposing into metal. As a result, metal deposition on the already developed pores might have occurred and lead to a reduction of the surface area ‎[25]. The relation between SSA, activation temperature, and impregnation ratio is shown in Fig. 2a Regarding the activation temperatures, it was perceived that as the temperature rises from 600 C o to 700 o C with IR of the range 1:0.5- 1:1 the SSA increases with it. These results showed that as the activation temperature increases, the structure tends to become micro porous. That’s due to that porosity is formed by KOH evaporation, therefore; as temperature increases, more KOH evaporates which leads to micro porosity enhancement, except for the result of the experiments with IR of 1:1.5, this is may be explained by the excessive concentration of KOH that led to the formation of wider or exploded pores which caused the declined SSA ‎[26]. On the other hand, SSA decreased at 800 C o , this is probably because high activating temperatures caused pore explosion that led to lower values of the specific surface area ‎[27]. (a) (b) Fig. 2. a) Effect of activation temperature and impregnation ratio on SSA b) Effect of activation temperature and impregnation ratio on yield M. W. Khalid and S. D. Salman / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 23 - 32 49 In general, the CBAC yield was found to be inversely proportional to both temperatures of activation and IR. As temperature elevate more volatile components will be released due to intensified dehydration and elimination reaction that increases C-KOH and C-CO2 reaction rate, which causes lower ‎[28]. As the IR value rises, KOH amount increases, which lead to oxidation process promotion causing the carbon atoms gasification reaction to become more dominant, therefore; more weight of carbon would be lost ‎[24]. The relation between yield, activation temperature and impregnation ratio is shown in Fig. 2b. 3.2. SEM and BET Analysis Fig. 3 shows the SEM images of CB (a) and CBAC (b). It can be noticed that the CBAC surface has developed pores in which there is a good probability for chromium to be adsorbed. The BET surface area was 595.9 m 2 /g. Pore diameter in average was 3.46 nm, indicating that it was in the mesoporous region according to the International Union of Pure and Applied Chemistry (IUPAC). The pores are classified as micro pores (<2nm diameter), mesopores (2–50nm diameter) and macro pores (>50nmdiameter) ‎[29]. The CBAC has a high surface area, which makes it more efficient for the removal of chromium. The high SSA of the CBAC was a result of the used technique of activation. The activation process involved chemical and physical activating agents which are KOH and CO2 respectively. However, the developed pores during carbonization enhanced the surface area by diffusing more CO2 and KOH molecules inside the pores, therefore; the reaction between KOH-carbon and CO2-carbon promoted leading to more pores in the activated carbon. (a) (b) Fig. 3. SEM images, a) precursor b) CBAC (magnifications: 1000X) 3.3. Experimental Design and Empirical Model The set of experiments that were designed by Taguchi method and their results are shown in Table 4 Table 4. Batch adsorption experiments and their response initial concentratio n (mg/L) time (min) Temperature (C) pH equilibrium concentration (mg/L) 10 30 20 2 6.51 10 120 30 5 7.4 10 210 40 8 3.41 10 300 50 11 5.275 20 30 30 8 15.285 20 120 20 11 11.63 20 210 50 2 6.418 20 300 40 5 5.66 30 30 40 11 23.52 30 120 50 8 12.14 30 210 20 5 6.487 30 300 30 2 3.35 50 30 50 5 35.89 50 120 40 2 13.13 50 210 30 11 18.6 50 300 20 8 9.22 M. W. Khalid and S. D. Salman / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 23 - 32 4: In order to obtain the empirical model for the adsorption process the results from Taguchi experimental design was used. Y is the response variable, the obtained model with its four factors and their interaction is represented by: (6) Where b0, b1, b2, b3 and b4 are the linear coefficients, b12, b13, b14, b23, b24 and b34 are the second-order interaction terms, b11, b22, b33 and b44 are the quadratic terms of each factor. X1, X2, X3 and X4 are the coded terms of initial chromium concentration, time, temperature and pH, respectively. The estimated values of the model coefficient, standard error of each model term and its p value are shown in Table 5 Table 5. Model coefficients, standard error and terms p- values Estimate Standard error p- value b0 7.506 1.869 0.00008 b1 0.443 0.047 0.00000 b2 -0.084 0.006 0.00000 b3 -0.016 0.815 0.00003 b4 -0.836 0.203 0.00005 b12 -0.001 0.000 0.00000 b13 -0.003 0.000 0.00000 b14 0.025 0.002 0.00000 b23 0.0001 0.000 0.265 b24 0.001 0.000 0.00000 b34 -0.005 0.002 0.044 b11 -0.001 0.000 0.035 b22 0.0002 0.000 0.00000 b33 0.0007 0.001 0.498 b44 0.063 0.11 0.00000 As it can be seen from table.4 that b23 and b33 have an insignificant effect on model accuracy, due to their p values, which are larger than 0.05. 3.4. Effects of Factors a. Effect Of Contact Time And Cr(VI) Initial Concentration The influence of adsorption time on chromium ions adsorbed by CBAC was investigated and presented in Fig. 4. It was observed that the chromium ions removal efficiency by CBAC increased with the initial chromium ion concentration. The adsorption was fast at the initial stage because of the high driving force which induced the metal ions to transfer rapidly from the bulk solution to the surface of CBAC ‎[17]. As time passed more active sited was occupied, this means less free active sites on the surface. Alongside with the declined driving force that made the adsorption to take more time to reach equilibrium, because metal ions slowly diffused to the intra-particle pores of the adsorbent ‎[30]. Thus, the adsorption rate is decreased. It is also clear from Fig. 4 that removal efficiency improved as the initial concentration of Cr (II). Because of the increased driving force of the concentration gradient ‎[31]. A similar trend of heavy metal adsorption, as a function of initial concentration, has also been reported previously ‎[32], ‎[33]. Fig. 4. Effect of contact time and initial concentration on removal efficiency b. Effect Of ph And Temperature The solution initial pH is the most significant factor to investigate the adsorption characteristic of an adsorbent because it affects not only surface charge of the adsorbent, but also the ionization degree and adsorbate speciation ‎[34]. The effect of initial solution pH on chromium ion removal by CBAC is presented in Fig. 5. As the negative charge density on CBAC surface increases due to COOH ionization, the adsorbed chromium will rise rapidly ‎[35]. At pH = 2, the efficiency on adsorption almost reached its maximum value. As temperature rises, the solution viscosity will slightly decrease, which enhances the diffusion rate of chromium into the pores of CBAC ‎[36]. In addition to that, the high temperatures will break internal bonds at the edges of the active sites ‎[37], which aids the enhancement of adsorption efficiency of the CBAC. Fig. 5 illustrates the direct proportion between temperature and efficiency of adsorption. As it can be seen from Fig. 5 that chromium adsorption on CBAC is an exothermic process. M. W. Khalid and S. D. Salman / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 23 - 32 4; Fig. 5. The effect of initial pH and temperature on adsorption efficiency 3.5. Adsorption Isotherm Studies The correlation between bulk solution concentration of sorbate and the amount of adsorbed heavy metal ions on the CBAC unit at equilibrium conditions is described functionally by the isotherms of adsorption. The adsorption isotherm was studied in order to understand the behavior of chromium ions in the solution – CBAC interphase,. Usually, adsorption isotherm analysis is conducted to find the fitter model to be used in equipment design purposes. Table.6 summarizes the capacities of adsorption for monolayer coverage as implied by the Langmuir model with the two isotherms constants and their correlation coefficients at 20, 30, 40 and 50 ◦ C. As it can be observed that Freundlich model fitted adsorption data of Cr(VI) more adequately due to higher R 2 values at all the mentioned temperature range, where R 2 was more than 0.95. Fitting the Freundlich isotherm refers to the heterogeneous surface energies and high values of KF indicate the high adsorption capacity of CBAC for Cr(VI) ‎[38]. All 1/n values were larger than 1 which is an indication of the chemical nature of the process ‎[39]. Fig. 6 shows the plot of ln Ce versus ln qe with temperature range of (20-50 o C). other studies had also confirmed the same results ‎[40], ‎[41]. Table 6. Values of Freundlich and Langmuir constants for Cr(VI) adsorption on CBAC Langmuir Temperature (Cᵒ) KL (L/mg) qm (mg/g) R 2 KF (mg/g ) 1/n R 2 20 0.32 14.401 0.93 16.232 1.59 0.952 30 0.241 13.346 0.914 9.804 1.593 0.962 40 0.161 13.333 0.906 4.907 1.663 0.966 50 0.099 12.774 0.939 1.287 1.853 0.962 Fig. 6. Adsorption data fitted into Freundlich isotherm at temperature range of (20-50 C o ) 3.6. Kinetics Studies To figure out the mechanism that controls the adsorption of chromium on CBAC, such as physical interactions and chemical reaction, pseudo-first-order and pseudo-second-order equations were utilized to model the kinetics of adsorption. The comparison between the experimental and calculated concentration of equilibrium and correlation coefficients were used to evaluate kinetics equations fitting. As the difference between experimental equilibrium concentration (qe,exp) and calculated equilibrium concentration (qe,cal) get smaller and R2 goes to unity, the kinetic equation represents the adsorption more accurately. The kinetics were studied at different initial concentration of chromium. Chromium adsorption obeyed pseudo-second order more clearly compared to pseudo- first order. Pseudo-first order and pseudo-second order adsorption rate constants, calculated and experimental qe values for different initial concentration chromium are summarized in table 7. Other researches had confirmed the same results ‎[37], ‎[42] and ‎[33]. Table 7. Comparison of the pseudo-first and pseudo- second order rate constants, and calculated and experimental qe values chromium adsorption on CBAC for various initial concentrations Initial concentration (mg/L) Pseudo-first order Pseudo-second order qe,exp (mg/g) K1 (1/min) qe,cal (mg/g) R 2 K2 (g/mg min) qe,cal (mg/g) R 2 10 3.45 0.014 3.836 0.972 0.02 3.339 0.991 20 7.388 0.01 5.419 0.929 0.013 7.865 0.993 30 11.232 0.011 7.496 0.945 0.019 10.68 0.997 40 14.96 0.006 7.888 0.931 0.025 13.979 0.998 50 18.864 0.005 9.185 0.975 0.026 18.511 0.999 M. W. Khalid and S. D. Salman / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 23 - 32 53 4- Conclusion In this study, cow bones active carbon showed a promising prospect in chromium adsorption from aqueous solution over a wide range of conditions, the optimum removal efficiency was 94.32%. Highest SSA of CBAC was 595.9 m 2 /g at the activation temperature of 600 o C and IR of 1:1.5. 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Salman / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 23 - 32 54 الكروم بواسطة كاربون منشط منتج من عظام البقرامتزاز أيونات الخالصة نتيجة لالستعماالت الواسعة لمكروم في الصناعة، باإلضافة الى أن الكروم عنصر مسرطن، وجب البحث عن طريقو فعالو واقتصاديو إلزالتو من المياه العادمة. في ىذه الدراسة، تم استخدام عظام البقر، كماده اوليو رخيصة، إلنتاج الكاربون المنشط. تم تشخيص الكاربون المنتج بالمجير االلكتروني لمتحقق من شكمو السطحي متر مربع/ 555.5( لمعرفة مساحتو السطحية. أفضل مساحو سطحيو تم الحصول عمييا BETوتم استخدام ال) .1.1.5درجو سيمزيو كجرارة تفعيل و نسبة تصبيغ 600غم ب الكاربون المنتج من عظام البقر المتزاز الكروم من المحاليل المائية. تم دراسة تأثير العوامل تم استخدام دقيقو(، درجة الحرارة 300-30ممغم/لتر(، وقت االمتزاز ) 50-10األتية مع مدياتيا. التركيز االولي لمكروم ) زاز رياضيا بواسطة معادلو قياسيو (. تم تمثيل عممية االمت11-2درجو سيمزيو( واالس الييدروجيني ) 20-50) تجريبيو. تم استخدام معادلة النغماير وفريندليتش لقولبة عممية االمتزاز، حيث كانت العممية منطبقة أكثر عمى نموذج فريندليتش. استخدمت معادلة الدرجة األولى والدرجة الثانية لدراسة حركيو االمتزاز، حيث كان االمتزاز .%54.32لة الدرجة الثانية. أفضل كفاءه امتزاز كانت متوافق أكثر مع معاد . امتزاز، كاربون منشط، عظام بقر، الكروم، تفعيل فيزيائي كيميائي. الكممات الدالة