Study on the production of modified biochar and activated carbon derived from corncob and their application in ammonium removal from domestic water

this dissertation □ Develop the optimal preparation procedure of modified biochar and activated carbon derived from an agricultural by- product, such as corncob wastes; □ Investigate the physical and chemical properties of modified biochar and activated carbon; □ Apply modified biochar and activated carbon in removal of ammonium from synthesised and real water under batch and column experiments; □ Propose adsorption mechanism. 4. The composition of the thesis 5 The thesis consists of 101 pages with 38 tables, 50 images, 123 references. The thesis was composed of 3 pages, 37 pages of literature review, 15 pages of research subjects and methods, 44 pages of research results and discussion, conclusion of 2 pages. THESIS CONTENT CHAPTER 1: LITERATURE REVIEW Ammonium contamination in ground water, methods of ammonium treatment, overview of methods of biochar production, modification methods in terms of biochar, activated carbon and application of biochar as organic adsorbent, heavy metals and ammonium treatment in water have been summarized. The research results show that: The researches focus on the application of biochar, modified activated carbon for ammonium treatment in water but there have not many researches focusing on the biochar surface modification for the adsorption of ammonium in water. The use of corncob to produce modified biochar for the adsorption of ammonium has not been investigated. Based on the review of the research materials, the thesis will focus on the following issues: - Providing optimum conditions for the production of modified biochar from corncob and modified activated carbon to enhance the ammonium adsorption capacity. - Determining the characteristics of dynamics and thermodynamics of ammonium adsorption in the water of the materials on the scale of batch adsorption and adsorption on the column. 6 CHAPTER 2: MATERIALS AND METHODS 2.1. Research subjects Adsorbent: corncob wastes were collected from Da Bac district, Hoa Binh province, Vietnam. Adsorbate: the ammonium solution was prepared in the laboratory by dissolving the suitable mass of NH4Cl in doubly distilled water to obtain a stock solution (1,000 mg/L). The synthesized water was used in the batch experiments. Meanwhile, the real water was collected from a well in a ammonium-polluted area (Mr. Nguyen Dinh Lam; Address: hamlet 3, Yen So commune, Hoai Duc district, Ha Noi city, Vietnam). The concentrations of NH4+, Fe3+, and Mn2+ ions in groundwater were 10.13 mg/L, 0.4 mg/L, and 0.02 mg/L, respectively. The real groundwater was used in the column experiments. groundwater 2.2. 1. Reagent All chemicals used in this study were of analytical reagent grade (purchased from Merck). 2.2.2. Device Equipment used in materials manufacturing and analysis at the Institute of Environmental Technology, Environmental Laboratory, Hanoi University of Natural Resources and Environment: - UV-VIS colorimeter (Hach, DR5000, USA) for ammonium content analysis - Atomic absorption spectrometer (AAS - Thermo Fisher, Solar- M6) for Mn, Fe analysis. 7 - Analytical balance, US, accuracy 10-5 and 10-2 mg - pH meter (Toledo, China). - Temperature controlled shaking apparatus (GFL 1083, Germany) for conducting static adsorption experiments. - Nabertherm kiln (L3/11/B170, Germany) used for making biochar, modified charcoal 2.3. Experimental 2.3.1. Adsorbent preparation Figure 2.1 represents the preparation procedure of modified biochar and activated carbon derived from corncob wastes. Briefly, biochar (Bio) was prepared at different pyrolysis conditions (i.e., pyrolysis temperatures and times) under an oxygen-limited environment. Subsequently, Bio was oxidized with HNO3 (BioN) to increase the concentration of oxygen-containing functionally groups (i.e., carboxylic group) on its surface. Lastly, BioN was treated with NaOH (BioN-Na) to enhance its capacity cation exchange. Meanwhile, corncob-derived activated carbon (BioP) was prepared through a one-stage chemical activation method using H3PO4. Similar to biochar, BioP was also treated with NaOH (BioP- Na) to enhance its capacity cation exchange. Notably, the pyrolysis process was done in the non-circulated air atmosphere (i.e., within lid-enclosed crucible) at different temperatures and heating times. 8 Figure 2.1. Schematic illustration of the preparation procedure of modified biochar and activated carbon 2.3.2. Adsorption experiment The process of ammonium adsorption onto modified biochar and activated carbon was conducted in batch and column experiments. The batch experiments were run in the synthesized solutions at different operation conditions (i.e., varying solutions pH, initial ammonium concentrations, contact times, solution temperatures, NaCl concentrations). Meanwhile, the column experiments were conducted in the real groundwater to analyse the effects of different flow rates, influent concentrations, and bed heights on the adsorption capacity. Two fixed-bed systems comprised a downflow (using a glass laboratory mini-column) and an upflow (using a column in pilot 9 scale). Furthermore, the adsorption reversibility was determined through desorption experiments. 2.3.3. Adsorbent characterization The textural characteristics of adsorbent (i.e., specific surface area and total pore volume) were determined by the nitrogen adsorption/desorption isotherm at 77 K (ASAP-200, Micromeritics). Morphological property was obtained using an electron microscope S- 4800 (FE-SEM, Hitachi). The thermal stability of corncob was measured by a thermo-gravimetric analysis (TGA; DuPont TA Q50, USA). Qualitative information on functional groups present in the adsorbent surface was analysed by a Fourier Transform Infrared Spectrometer (FTIR, NEXUS 670, Nicolet, USA). The Boehm titration method was applied to determine the quantitative information on the acidic and basic groups on the adsorbent surfaces. The electrical state of adsorbent surfaces in solution was characterized by the point of zero charge (pHPZC) that was determined using the drift method. Proximate analysis was performed by following the international standard procedure (ASTM D2867-09, D2866, and D5832-98). CHAPTER 3: RESULTS AND DISCUSSION 3.2. The optimal preparation condition of modified biochar The results of ammonium adsorption (data not showed) indicated that the optimal preparation conditions of modified biochar were obtained. Briefly, BioN-Na was prepared at the optimal conditions as follows: 400 °C, 60 min, 6 M HNO3 (5/1, v/w) and 0.3 M NaOH (20/1, v/w). Therefore, the modified carbounous adsorbent 10 prepared at the optimal conditions (BioN-Na) were used for further experiments. 3.3. The optimal preparation condition of modified activated carbon. The results of ammonium adsorption (data not showed) indicated that the optimal preparation conditions of modified activated carbon (BioP-Na) were obtained. BioP-Na was prepared at 400 °C, 90 min, 50% H3PO4 (1.5/1, v/w), and 0.3 M NaOH (20/1, v/w). Therefore, the modified carbounous adsorbent prepared at the optimal conditions (BioP-Na) were used for further experiments. 3.4. Adsorbent characterization 3.4.1. Textural and morphology property As expected, the BET surface area (m2/g) and total pore volume (cm3/g) of adsorbent exhibited the following order: BioP-Na (1097 and 0.804) > BioN-Na (10.4 and 0.00664), respectively. The average pore width of BioP-Na (3.95 nm) and BioN-Na (3.71 nm) was greater 2 nm. The results of scanning electron micrographs (Figure 3.14) demonstrated that BioP-Na and BioN-Na had an irregular and heterogeneous surface morphology. The formation of well-developed pores of various sizes and shapes in BioP-Na was attributed to the chemical activation method used in the activated carbon preparation. 11 Figure 3.14. Scanning electron microscope (SEM) image of the (a) NaOH-treated biochar and (b) NaOH-treated activated carbon 3.4.2. Surface chemistry Figure 3.15 represents qualitative information about the functional groups on the adsorbent surfaces. The presence of several important function groups on the surfaces of six target adsorbents was identified at peaks at approximately 3430 cm-1 (the hydroxyl groups, –OH, in the carboxylic groups, phenol groups, or adsorbed water), 1700 cm-1 (C=O in the carboxylic and lactonic groups), 1380 cm-1 (stretching C–O groups), and 1620 cm‒1 (the C=C double bonds in the aromatic rings). The decrease in intensity was attributed to the change of corresponding surface chemistry of the adsorbents, which is consistent with the change of (1) the concentration of oxygen- containing functional groups on the adsorbent’s surfaces, and (2) the point of zero charge (pHPZC) (Table 3.8). The results demonstrated that the treatment process (pyrolysis, chemical activation, oxidation, and NaOH impregnation) significantly affected the surface chemistry of the adsorbents. 12 Figure 3.15. Fourier transform infrared spectroscopy (FTIR) spectra of prepared adsorbents Table 3.8. Concentration of oxygen-containing functional groups on the surface of adsorbent pHPZC Oxygen-containing groups (mmol/g) Total acid groups (mmol/g) Carboxylic Lactonic Phenolic Biosorbent CC 7.0 0.131 0.490 0.873 1.494 Biochar Bio 5.3 0.619 1.479 0.486 2.584 BioN 4.6 1.382 2.745 0.171 4.298 Activated carbon BioP 4.3 0.988 1.601 0.980 3.569 3.4.3. Physical property The results of proximate analysis demonstrated that the modified biochar and activated carbon exhibited a low percentage of moisture and ash content, suggesting a high quality of BioN-Na and BioP-Na. In addition, a low volatile content reflects a high potential 13 for industrial applications or real water treatment in household scales. Notably, a high fixed carbon content demonstrated that modified biochar and activated carbon consist mainly of carbon. Table 3.9. Proximate analysis of modified biochar and activated carbon BioN-Na BioP-Na Yield (%)a 34.9 81.5 Moisture (%) 4.36 5.01 Volatile (%) 18.1 13.0 Total ash (%) 18.0 13.1 Fixed carbon (%) 71.9 79.3 Note: athe yield was calculated from the different mass between before and after pyrolysis for the samples of biochar and activated carbon. 3.5. Adsorption result in batch experiment 3.5.1. Effect of pH The effects of solution pH on the NH4-N adsorption process are provide in Figure 3.16 and 3.1. The result showed that the adsorption process was strongly dependent on the solution pH (pHsolution). At strong acidic condition (pH = 4), the amount of ammonium uptake onto M-CCAC and M-CCB seems negligible. This is because (1) the excess H+ ions in the system strongly competed with the NH4+ ions for the active adsorption sites, and (2) repulsion occurred between the positively charged surface of adsorbent (M-CCAC of M-CCB) and the NH4+ ions. Furthermore, the adsorption efficiency decreased when pHsolution >9.0. The decrease in adsorption capacity resulted from the transformation of ammonium (NH4+) ion into gaseous ammonia 14 (NH3), which makes the electrostatic attraction mechanism no longer effective. In general, optimal pHsolution was obtained at 7.0–8.0. Figure 3.16. Effects of initial solution pH on the capacity of ammonium adsorption onto BioN-Na Figure 3.17. Effects of initial solution pH on the capacity of ammonium adsorption onto (a) BioP-Na 3.5.3. Adsorption isotherms The adsorption isotherms of corncob-derived adsorbents (Figure 3.22) were classified according to their shapes as L-type (Langmuir) isotherms, which are characterized by an initial concave region relative to the concentration axis (concave downward curve). Typically, the Langmuir model better fits the experimental data on the adsorption of ammonium onto BioP-Na, BioN-Na, BioN, Bio, and CC than dose the Freundlich model. The maximum Langmuir adsorption capacity (qm; mg/g) at 30 °C decreased the following order: BioN-Na (qm = 22.6 mg/g) > BioP-Na (15.4 mg/g) > BioN (8.60 mg/g) > Bio (3.93 mg/g) > CC (2.05 mg/g), suggesting that the treatment processes efficiently enhanced the NH4+ adsorption capacity onto biochar and activated carbon. 15 Figure 3.22. Adsorption isotherms of ammonium onto corncob derived-biosorbent (CC), biochar (Bio), oxidized biochar (BioN), modified biochar (BioN-Na), pristine activated carbon (BioP), and modified activated carbon (BioP-Na) 3.5.4. Adsorption kinetics The effects of contact time on the adsorption process were examined at different initial ammonium concentrations (10 mg/L, 20 mg/L, and 40 mg/L) and operation temperatures (20 °C, 30 °C, and 40 °C). As expected, the adsorption process reached a fast equilibrium at approximately 60 min (Figure 3.17 and 3.18). The experimental data of adsorption kinetics were adequately described by the pseudo- second-order equation. The adsorption rate (k2; g/mg × min) was calculated from this model. The results demonstrated that the adsorption rate of ammonium onto BioP-Na and BioN-Na at an initial NH4+ concentration of 10 mg/L increased when the temperature increased. The k2 values exhibited the following order: 20 °C (k2 = 0.04 16 g/mg × min) < 30 °C (0.09) < 40 °C (0.14) for BioP-Na, and 20 °C (0.06) < 30 °C (0.15) < 40 °C (0.21) for BioN-Na. Moreover, in the same operation conditions, BioN-Na exhibited higher k2 values than BioP-Na, suggesting that the ammonium adsorption process onto BioN-Na occurred faster than that onto BioP- Na. Notably, the activated energy (calculated from the Arrhenius equation) of the process of ammonium adsorption onto BioP-Na (Ea = 47.89 kJ/mol) and BioN-Na (52.46 kJ/mol) demonstrated that ion exchange played an important role in the adsorption mechanism. Figure 3.18. Effects of contact time on the capacity of ammonium adsorption onto BioN-Na Figure 3.19. Effects of contact time on the capacity of ammonium adsorption onto BioP-Na 3.5.5. Adsorption thermodynamics As showed in Figure 3.25, the adsorption process was strongly dependent on the operation temperature. The amount of ammonium adsorption onto modified biochar and activated carbon decreased when the temperature increased, which implies that the ammonium adsorption was an exothermic process. The qm values at 20 °C, 35 °C, and 50 °C were as follows: 24.52 mg/g > 22.58 mg/g > 10.40 mg/g for 17 BioN-Na, and 17.03 mg/g > 15.40 mg/g > 11.99 mg/g for BioP-Na, respectively. Essentially, when the adsorption process reached a true equilibrium, the equilibrium constant (KC; dimensionless) can be obtained (Figure 3.25). In this case, the adsorption thermodynamic parameters (∆G°, ∆H°, and ∆S°) can be directly calculated from the well-known van’t Hoff equation. Table 3.20 shows that the negative value of Gibbs energy change (∆G°) all investigated temperatures indicate that the NH4+-N adsorption process onto modified biochar and activated carbon occurred spontaneously. Meanwhile, the positive values of the change in entropy (∆S°) suggest that the organization of NH4+ ions at the solid/liquid interface becomes more random during the adsorption process. Furthermore, the negative values of the change in enthalpy (∆H°) reflect the exothermic nature of the adsorption process, which was demonstrated by a decrease in the adsorption capacity (qe; Figure 3.25) and equilibrium constant (KC; Table 3.20) at a higher temperature. Figure 3.25. Effects of temperature on the adsorption process of (a) BioN-Na and (b) BioP-Na 18 Table 3.20. Thermodynamic parameters of ammonium adsorption onto modified biochar and activated carbon T (K) Van’t Hoff equation KC ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (kJ/mol × K) Modified biochar (BioN-Na) 293 y = 140x + 3.02 R² = 0.9185 32.92 –8.512 –1.164 0.0251 308 32.53 –8.917 323 31.48 –9.263 Modified activated carbon (BioP-Na) 293 y = 39x + 3.18 R² = 0.982 27.35 –8.060 –0.320 0.0264 308 27.13 –8.452 323 27.02 –8.852 3.5.6. Co-existent effects of other cations The Fe3+, Ca2+, and Mn2+ cations are commonly present in groundwater in Hanoi. Therefore, they were selected as the foreign cations in this study. The results showed that the amount of NH4+-N adsorbed onto the adsorbents (BioN-Na and BioP-Na) remarkably decreased with an increase in the concentrations of Fe3+, Ca2+, and Mn2+ ions (Figure 3.26). This is presumably because: (1) a screening effect (known as the electrostatic screening) occurred between the positively charged adsorbent surfaces and the NH4+ ions, and (2) there is a competition between the NH4+ ions and the Fe3+, Ca2+, and Mn2+ ions for the adsorbing or exchanging sites on the adsorbent’s surfaces (i.e., —COO– or —COONa+). 19 Figure 3.26. Effects of the presence of other cations on the adsorption capacity of modified biochar and activated carbon 3.5.7. Desorption study and adsorption mechanism The adsorption efficiency of ammonium ions using various desorbing agents is provided in Figure 3.27. The order of ammonium desorption from BioN-Na and BioP-Na was as follows: (43% and 41%) > NaCl (34% and 29%) > NaCl + NaOH (28% and 23%) > NaOH (22% and 17%) respectively. The percentage of ammonium desorbed by HCl was assumed to correspond to both electrostatic attraction and ion exchange mechanisms, and thus it can be concluded that approximately 41% of ammonium ions was removed from the solution (adsorbed onto BioN-Na and BioP-Na) through electrostatic attraction and ion exchange mechanisms. 20 Figure 3.27. Percentage of NH4 +-N desorbed using various desorbing agents 3.6. Adsorption result in column experiment 3.6.1. Adsorption performance in laboratory mini-column (downflow) 3.6.1.1. Effect of solution flow rate Figure 3.28. Breakthrough curves for different flow rates BioN-Na Figure 3.29. Breakthrough curves for different flow rates BioP-Na 0 10 20 30 40 50 HCl 0,1M HCl 1M NaCl 0,5M NaOH 0,5M + NaCl 0,5M NaOH 0,5M % D e so rp ti o n BioP-Na BIoN-Na 21 When the adsorption system was controlled at a low flow rate, the NH4+ ions would have more time to contact with modified biochar, resulting in greater removal of NH4+ ions from the solution. As a result, the breakthrough and exhaustion time (Table 6) dramatically decreased from 3700 min to 1020 min and from 4980 min to 1620 min as the flow rate increased from 1 mL/min to 3 mL/min, respectively (BioN-Na). For BioP-Na, the breakthrough and exhaustion time (Table 6) dramatically declined from 3000 min to 700 min and from 4500 min to 1300 min as the flow rate rose from 1 mL/min to 3 mL/min, respectively. A higher flow rate causing a higher adsorption rate. 3.6.1.2. Effect of influent ammonium concentration Figure 3.30. Breakthrough curves for different feed concentrations, BioN-Na Figure 3.31. Breakthrough curves for different feed concentrations BioP-Na The effects of initial ammonium concentration (Co; mg/L) on the process of ammonium adsorption are described in the breakthrough curves Figure 3.30 (BioN-Na) and Figure 3.31 (BioP-Na). Generally, The change in concentration gradient demonstrated a strong effect on the breakthrough and exhaustion times. The breakthrough time of the ammonium adsorption process onto BioP-Na and BioN-Na was as follows: 1100 min and 1500 min at Co = 10 mg/L > 450 min and 650 min at 20 mg/L > 120 min and 500 mg/L at 40 mg/L, respectively. 22 The breakthrough and exhaustion times remarkably decreased with an increase in inlet NH4+ concentrations because the adsorbing sites on the modified adsorbent’s surface became quickly saturated. Generally, a higher influent concentration resulted in the higher amount of ammonium adsorbed because of the greater driving force to the modified adsorbent’ surface (Chowdhury, 2013). 3.6.1.3. effect of bed height The breakthrough curves (Figures 3.32–3.33) describe the effects of ammonium adsorption onto BioN-Na and BioP-Na at different column heights at an influent rate of 1 mL/min and an influent ammonium concentration of 10 mg/L. Figure 3.32. Breakthrough curves for different bed height, BioN-Na Figure 3.33. Breakthrough curves for different bed height, BioP-Na The increasing breakthrough and exhaustion times at a higher bed height were attributed to the enlargement of mass transfer zones (increases the contact time between the ammonium solution and the modified adsorbents in the column). In addition, as the increase in the column height of modified biochar and activated carbon also increased, which means increasing the 23 amount of adsorbing sites on, so that more NH4+ ion will be adsorbed on modified biochar and activated carbon. The highest column adsorption efficiency was obtained at a column height of 16.2 cm (BioN-Na) and 15.8 cm (BioP-Na) under fixed other operation parameters, such as a flow rate (1 mL/min) and an initial ammonium concentration (10 mg/L). 3.6.2. Adsorption performance in pilot scale (upflow) On the basis of the adsorption results conduced in the laboratory-scale column (Section 3.6.1), the pilot scale adsorption study of BioN-Na was performed to evaluate the change of adsorption capacity of BioN-Na toward ammonium. The appropriate operating conditions at the column mode in the laboratory scale were hydraulic speed of 0.6 m/h (1 ml/min), the contact time of 15 minutes and over, therefore, on the pilot column, hydraulic speed should be selected in the range of 0.4 to 0.8 m/h to investigate the appropriate hydraulic speed. In order to utilize the existing adsorption column (column height of 60cm, column diameter of 14cm), the volume of biochar and incoming water flow was selected to achieve the contact time of water with biochar greater than 15 minutes. Table 3.26. The length of mass transfer layer L of BioN-Na biochar on the pilot system Nồng độ amoni đầu vào, Co, mg/l Lưu lượng nước, ml/phút Chiều cao cột, H (cm) Thời gian thoát, tb (phút) Thời gian bão hòa, ts (phút) Độ dài tầng chuyển khối, L (cm) Hiệu suất hấp phụ cột, η (%) 10 115 30 8420 11220 7.49 75.04 10 154 30 3620 7620 15.75 47.51 10 205 30 2420 6020 17.94 40.20 24 Table 3.26 shows that the adsorption capacity of BioN-Na was insignificant dependent on the flow rate (i.e., 115, 154, and 205 min/L). In contrast, the breakthrough and saturation times was strongly dependent on the flow rate: 8420 min and 11220 min at 115 mL/min > 3620 min and 7620 min at 154 mL/min > 2420 min and 6020 min at 205 mL/min, respectively. The adsorption capacity of BioN-Na in the pilot-scale column ranged from 6.83 mg/g to 7.05 mg/g, which is significantly lower than that in the laboratory-scale column (10.8 mg/g). Table 3.27. Ammonium adsorption capacity of BioN-Na on pilot scale. Nồng độ amoni đầu vào, Co (mg/l) Tốc độ dòng chảy Q (ml/phut) Chiều cao cột, h (cm) Dung lượng hấp phụ cột, q (mg/g) 10 115 30,0 6,83 10 154 30,0 7,05 10 205 30,0 7,05 As expected, the ammonium adsorption capacity of BioN-Na and BioP-Na in the laboratory-scale column (ranging from 8.08 to 10.8 mg/g) was generally higher than that of some other adsorbents reported in the literature. Chapter 4: CONCLUSIONS The dissertation developed a simple method to prepare modified biochar and activated carbon derived from corncob wastes. They were characterized and apply to remove ammonium ions from environmental water under various batch and column experiments. It can be concluded that:  The optimal preparation conditions of target adsorbents were obtained at (1) a pyrolysis temperature of 400°C and time of 60 min, 25 6 M HNO3 (an impregnation ratio of 5/1; volume of HNO3 per mass of precursor; w/v), and 0.3 M NaOH (20/1; w/v) for modified biochar (BioN-Na), and (2) 400 °C, 90 min, 50% H3PO4 (1.5/1, v/w), and 0.3 M NaOH (20/1, v/w) for modified activated carbon (BioP-Na).  Thermogravimetric analysis indicated that the endset temperature of corncob was approximately 400°C. The results of FTIR, pHPZC, and Boehm titration suggested that BioP-Na and BioN- Na possessed abundant oxygen-containing functional groups. BioP- Na and BioP-Na were considered as a carbonaceous mesoporous material. BioP-Na and BioN-Na possessed a low moisture, volatile, and total ash