Study on the removal of ammonium in wastewater using adsorbent prepared from rice hull and magnesium chloride

Abstract. This study aimed to evaluate the ability of magnesium chloride modified carbonized rice hull (MCRH) material for ammonium removal from synthetic and domestic wastewater. The MCRH material was prepared by using waste rice hull from a household rice-processing factory and magnesium chloride salt via a simple mixing and annealing method. The material was then characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The effects of rice hull modification by magnesium chloride and experimental conditions such as initial ammonium concentration (20 - 100 mg/L), amount of adsorbent (0.8 - 2.0 g/L), and adsorption time (0 - 32 h) on the ammonium removal efficiency and adsorption capacity were investigated. Adsorption kinetic and isotherms were also studied for MCRH material. Results showed that magnesium was successfully added on carbonized rice hull with Mg/C molar ratio of 0.22. Ammonium adsorption isotherm fitted well to Langmuir model with maximum adsorption capacity of 65.36 mg/g. The adsorption was physical process and adsorption kinetic was best described by intra-particle diffusion model with the correlation coefficients ranged from 0.942 - 0.979. Ammonium adsorption feasibility of MCRH was proved through the treatment of domestic wastewater containing 80.7 ± 1.6 mg/L initial ammonium concentration with removal efficiency of 86.8 %. Ammonium concentration of the effluent met the allowable value (10 mg/L) as given by QCVN 14: 2008/BTNMT (column B) - National technical regulation on domestic wastewater. In conclusion, MCRH has a potential for being an economical and abundant material in Vietnam. The post adsorption material that accumulated ammonium would be potentially used for soil quality improvement.

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Vietnam Journal of Science and Technology 58 (3A) (2020) 113-123 doi:10.15625/2525-2518/58/3A/14322 STUDY ON THE REMOVAL OF AMMONIUM IN WASTEWATER USING ADSORBENT PREPARED FROM RICE HULL AND MAGNESIUM CHLORIDE Nguyen Thi Thuy 1 , Tran Dang Lan Van 2 , Nguyen Xuan Hoan 1 , Le Thi Bich Son 1 , Tran Thi Ngoc Mai 1 , Dang Van Thanh 3 , Nguyen Nhat Huy 2, * 1 Ho Chi Minh City University of Food Industry, 140 Le Trong Tan, Tan Phu District, Ho Chi Minh City, Viet Nam 2 Faculty of Environment and Natural Resources, Ho Chi Minh City University of Technology, VNU-HCM, 268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Viet Nam 3 Faculty of Basic Sciences, TNU-University of Medicine and Pharmacy, 284 Luong Ngoc Quyen, Quang Trung Ward, Thai Nguyen City, Thai Nguyen Province, Viet Nam * Email: nnhuy@hcmut.edu.vn Received: 23 August 2019; Accepted for publication: 23 October 2020 Abstract. This study aimed to evaluate the ability of magnesium chloride modified carbonized rice hull (MCRH) material for ammonium removal from synthetic and domestic wastewater. The MCRH material was prepared by using waste rice hull from a household rice-processing factory and magnesium chloride salt via a simple mixing and annealing method. The material was then characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The effects of rice hull modification by magnesium chloride and experimental conditions such as initial ammonium concentration (20 - 100 mg/L), amount of adsorbent (0.8 - 2.0 g/L), and adsorption time (0 - 32 h) on the ammonium removal efficiency and adsorption capacity were investigated. Adsorption kinetic and isotherms were also studied for MCRH material. Results showed that magnesium was successfully added on carbonized rice hull with Mg/C molar ratio of 0.22. Ammonium adsorption isotherm fitted well to Langmuir model with maximum adsorption capacity of 65.36 mg/g. The adsorption was physical process and adsorption kinetic was best described by intra-particle diffusion model with the correlation coefficients ranged from 0.942 - 0.979. Ammonium adsorption feasibility of MCRH was proved through the treatment of domestic wastewater containing 80.7 ± 1.6 mg/L initial ammonium concentration with removal efficiency of 86.8 %. Ammonium concentration of the effluent met the allowable value (10 mg/L) as given by QCVN 14: 2008/BTNMT (column B) - National technical regulation on domestic wastewater. In conclusion, MCRH has a potential for being an economical and abundant material in Vietnam. The post adsorption material that accumulated ammonium would be potentially used for soil quality improvement. Keywords: ammonium, rice hull, adsorption, wastewater treatment, magnesium chloride. Classification numbers: 2.4.2, 3.7.3, 3.3.2. Nguyen Thi Thuy, et al. 114 1. INTRODUCTION Domestic wastewater is a prominent source together with agricultural runoff contribution for eutrophication around the globe [1]. Together with organic materials and phosphorous, ammonium is one of the main contaminants presented in the domestic wastewater. Ammonium contributes to the surface water eutrophication, acidification of soil, vegetation fertilization, and changes in ecosystem. In addition, this component can be converted to nitrate in aerobic conditions which eventually leached to groundwater [2]. Hence, ammonium is a contaminant that needs to be removed from wastewater before discharging into the environment or water reuse. Several technologies have been developed to remove ammonium in wastewater, including biological treatment, ammonia stripping, chemical precipitation, and ion exchange, in which biological treatment is considered as the most common process used for ammonium removal from domestic wastewater. The advantages and disadvantages of these methods can be found elsewhere [3]. Bio-charcoal is a carbon-rich, fine-grained, and porous material produced from carbonization of biomass. This material can be prepared from various feedstocks (e.g., wood bark, dairy manure, sugar beet tailing, pinewood), which are considered as a promising adsorbent as cost-effective and environmentally sustainable products for integrated waste management [2]. Though many types of bio-charcoal have been synthesized using different feedstocks [2, 4, 5] and modified rice hull for ammonium removal has been reported [6-8], it is the first time that rice hull modified by magnesium chloride was applied for ammonium adsorption in domestic wastewater. In this study, magnesium chloride modified carbonized rice hull (MCRH) were synthesized using impregnation via simple mixing and annealing method. Since the adsorption treatment was developed to apply for municipal wastewater, synthetic wastewater was prepared with initial ammonium concentration ranged from 20 - 100 mg/L. Effect of magnesium chloride modification, initial ammonium concentration, adsorbent amount, and adsorption time on ammonium removal efficiency and adsorption capacity were investigated. Adsorption isotherms and kinetics studies were further conducted. In final step, domestic wastewater was collected and treated by MCRH to find the feasibility of the material in real application. 2. MATERIALS AND METHODS 2.1. Materials Rice hull was obtained from a household rice mill, dried at 55 o C overnight, and then used as the feedstock biomass. To prepare synthetic wastewater containing NH4 + at different concentrations, a desired amount of ammonium chloride salt (NH4Cl) was dissolved in distilled water. Domestic wastewater was collected from the campus of Ho Chi Minh City University of Food Industry (Ho Chi Minh City, Vietnam) during May and June 2019. MgCl2.7H2O was used to prepare 20 % w/w MgCl2 solution. All chemicals used in this study were of analytical grade. 2.2. Adsorbent preparation The porous MCRH adsorbent was produced from rice hull following the modified adsorption and pyrolysis method used by Li et al. [5]. At first, 200 mL of 20% MgCl2 solution was mixed with 10 g of rice hull in Erlenmeyer flask and the mixture was shaken by an oscillator (LabTech, LMS-2, Korea) at 120 rpm under room temperature (~ 26 o C in a Study on the removal of ammonium in wastewater using adsorbent prepared from rice 115 conditioned room) for 24 h. The mixture was subsequently heated at 80 o C until the water evaporated and the remained material was dried at 105 o C in an oven (Memmert, UN55, Germany) for 6 h. The dried mixture was finally transferred into a porcelain container and placed in a furnace (Lenton, EF11/8B, England) for pyrolysis at 550 o C for 1 h. This MCRH material was then characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) (JOEL, JSM-IT200, Japan). Sole carbonized rice hull (CRH) was also prepared with the similar procedure but without the addition of MgCl2. 2.3. Adsorption performance tests General adsorption procedure: 1.0 g of MCRH were added into 1 L of ammonium synthesized sample. This sample was kept shaking at 120 rpm under room temperature and pressure. 10 mL of the treated sample was then filtered through membrane filter (0.22 µm, nylon cellulose, GE) and ammonium concentration was analyzed according to SMEWW 4500 – NH3.B&D, 2012 using an automatic analyzer (Kjeltec 2300). Effect of different factors on ammonium removal efficiency were investigated. (i) Effects of magnesium chloride modification (20 %) used in material synthesis on adsorption efficiency of 50 mg/L ammonia solution were firstly tested. The adsorption process was conducted for 24 h (followed a published literature, [5]) using MCRH and then followed similar procedure as mentioned above. For comparison purpose, the material without MgCl2 modification (carbonized rice hull, CRH) was also prepared and tested with ammonia adsorption under the same conditon with MCRH material. (ii) To test the effects of initial ammonia concentration and time of treatment, ammonia concentrations were prepared from 20 to 100 mg/L. (iii) To test the effect of time on the adsorption, experiments were conducted for ammonium concentration of 50 mg/L with a total adsorption time of 32 h. In this experiment, ammonium was measured every one hour. For the (i), (ii), and (iii) experiments, adsorbent concentration used were 1.0 g/L. (iv) Effect of adsorbent concentration (0.8 – 2.0 g/L) on the ammonia removal efficiency of 81.3 mg/L synthetic wastewater was tested for 27 h (obtained from experiment No. iii). All experiments were repeated at least 3 times and the average values were reported. Removal efficiency was calculated as followed equation (1): Removal efficiency (%) = (1) where Co is initial ammonia concentration (mg/L) and Ct is ammonia concentration at time t (mg/L). The data from experiment No. (iv) was then applied for Langmuir and Freundlich isotherm models, as given by Equations (2) and (3) [5]. Langmuir model: qe = KqmaxCe/(1+KCe) (2) Freundlich model: qe = KfCe 1/ n (3) where qe is the equilibrium adsorption capacity (mg/g); K is the Langmuir bonding term correlated to interaction energy (L/mg) and Kf is the Freundlich affinity coefficient (mg (1-n) L n g -1 ); qmax represents the Langmuir maximum capacity (mg/g); Ce is equilibrium solution concentration (mg/L) of ammonia; n is the Freundlich linearity constant. Nguyen Thi Thuy, et al. 116 A linearized form of the Dubinin–Radushkevich isotherm is given in Equation (4) [6]: ln(qe) = ln(qs)−(Kadε 2 ) (4) where qs is theoretical isotherm saturation capacity (mg/g), Kad is Dubinin–Radushkevich isotherm constant (mol 2 /kJ 2 ), and ε is Polanyi potential, which is expressed in Equation (5): ε = RTln(1+1/Ce) (5) where R is universal gas constant (8.314 J/mol K) and T is absolute temperature (K). Kad is the constant in Dubinin–Radushkevich isotherm indicating of the mean adsorption energy (E) (kJ/mol) of adsorption/mole of the adsorbate. It can be calculated by Equation (6): E=1/ (6) (v) To investigate the kinetic of the adsorption, initial ammonium concentration was prepared from 20 -100 mg/L. Adsorption was conducted for 27 h and ammonium concentration was measured for each four hours. Equations (7), (8) and (9) express pseudo-first-order, pseudo- second-order kinetic and intra particle diffusion models, respectively, which were applied to fit the adsorption kinetics experiment data [5]. Pseudo-first order kinetic model: ln (qe – qt) = ln qe – k1t (7) Pseudo-second order kinetic model: t/qt = 1/(k2qe 2 ) + t/qe (8) Intra-particle diffusion model: qt = kpt 1/2 + C (9) where k1 and k2 are the first-order and second-order apparent adsorption rate constants (1/h and g/mg/h), respectively; kp is the intra-particle diffusion rate constant (mg/g/h 1/2 ); C is the thickness of the boundary layer; qt and qe are the amount of ammonia adsorbed (mg/g) at time t and at equilibrium, respectively. To evaluate the potential of the adsorbent material for domestic wastewater treatment, adsorption experiment was conducted by mixing MCRH with domestic wastewater taken from the campus of Ho Chi Minh City University of Food Industry. The procedure was similar to the general adsorption procedure mentioned above with MCRH concentration was the value gained from the experiment No. (iv) as mentioned above. 3. RESULTS AND DISCUSSION 3.1. Magnesium chloride modified carbonized rice hull characterization In this experiment, MCRH prepared using 20 % MgCl2 solution was characterized by EDS analysis. Figure 1 showed the presence of C, O, Mg, and Cl as major components of MCRH material. In the inset of Figure 1, mapping result of MCRH proved the successful adding and distribution of Mg element on rice hull support. The data from Table 1 identified that Mg contributed 13.75 % (w/w) and the Mg/C molar ratio of MCRH material was calculated to be 0.22. Study on the removal of ammonium in wastewater using adsorbent prepared from rice 117 Figure 1. EDS result of MCRH (inset is mapping result). Table 1. Element composition of MCRH. 3.2. Comparison of ammonium adsorption between MCRH and CRH In this experiment, we compared ammonium removal ability between MCRH and the sole carbonized rice hull (CRH) to find whether modification of rice hull with MgCl2 positively changed the treatment efficiency. As can be seen in Figure 2, MCRH provided significantly higher ammonium removal efficiency and capacity (90.3 % and 45.9 mg/g, respectively) than CRH material (21.6 % and 8.7 mg/g, respectively). According to Li et al. [5], material (sugarcane residue) modified by MgCl2 provided MgO at the surface of the modified material which helped to increase the removal efficiency of pollutants such as ammonium. Hence, we also expected the addition of MgO on MCRH which enhanced the adsorption ability of this adsorbent. CRH MCRH R e m o v a l e ff ic ie n c y ( % ), a d s o rp tio n c a p a c tit y q e ( m g /g ) 0 20 40 60 80 100 Removal ef f iciency qe Figure 2. Removal efficiency and adsorption capacity of CRH and MCRH (initial concentration of 50 mg/L). 3.3. Effect of initial ammonium concentration Element Mass % Atom % C 30.22 ± 0.51 44.36 ± 0.75 O 29.44 ± 0.58 32.44 ± 0.64 Mg 13.75 ± 0.38 9.97 ± 0.27 Cl 26.60 ± 0.71 13.23 ± 0.35 Total 100.00 100.00 Nguyen Thi Thuy, et al. 118 Ammonia concentration in domestic wastewater can vary from different studies, e.g. 17 - 48 mg/L [1], 20 - 75 mg/L [9], and 33.4 mg/L [10]. In this experiment, initial ammonia concentration was prepared from 20 to 100 mg/L and the adsorption was conducted for 24 h with MCRH concentration of 1.0 g/L. As can be seen from Figure 3, increasing ammonia concentration from 20 to 100 mg/L resulted in the significant increase of adsorption capacity, which reached 64.4 mg/g at initial ammonium concentration of 100 mg/L. This trend was similar to the one found by Sarkhot et al. [4] when NH4 + concentrations were in the low concentration range in pure solution (0-100 mg/L). Accordingly, further increase of ammonium concentration (100-1000 mg/L) also increased adsorption capacity but at a lower rate. As observed in Figure 3, the change of ammonium concentration in the effluent suggested that the wastewater with initial ammonium concentration less than or equal to 60 mg/L treated by 1.0 g/L of adsorbent can produce the effluent ammonium concentration lower than the allowable value for domestic wastewater (10 mg/L, QCVN 14 : 2008/BTNMT, column B). For initial ammonium concentrations higher than 60 mg/L, the applied adsorbent concentration of 1.0 g/L may not be sufficient for the treatment. Initial concentration (mg/L) 20 40 60 80 100 C e ( m g /L ) 0 10 20 30 40 50 q e ( m g /g ), r e m o va l e ffi c ie n c y ( % ) 0 20 40 60 80 100 Ce qe Removal efficiency Figure 3. Ammonium effluent concentration, removal efficiency and adsorption capacity of MCRH at various initial ammonium concentrations of 20 - 100 mg/L. 3.4. Effect of adsorption time In this experiment, the effect of adsorption time was evaluated by conducting the treatment for 32 h and ammonium concentration was measured and recorded for each hour. As can be seen from Figure 4, ammonium concentration decreased sharply for the first 4 h, then continuously decreased but at slower speeds. After 13 h of treatment, ammonium concentration in the effluent was approximate 10 mg/L. Further treatment of the solution leaded to the decrease of ammonium concentration to 1.6 mg/L after 27 hours, and then remained unchanged for 3 h later. Hence, we considered the adsorption process reached its equilibrium after 27 h of treatment. After 27 h of adsorption, 97.0 % of ammonium was removed, corresponding to the adsorption capacity of 50.8 mg/g. This adsorption capacity is significantly higher than the values gained from Sarkhot et al. [4] (i.e. about 0.75 mg/g), who used the material made by hardwood shavings to treat 50 mg/L NH4 + solution for 24 h, and the capacity (i.e. 20 mg/g) found by Li et al. [5] who synthesized MgO impregnated material of sugarcane crop harvest residue using MgCl2 solutions with mass concentration from 2 to 20 % to treat swine wastewater. Study on the removal of ammonium in wastewater using adsorbent prepared from rice 119 t (h) 0 5 10 15 20 25 30 C t (m g /L ) 0 10 20 30 40 50 q t (m g /g ), r e m o va l e ff ic ie n c y (% ) 0 20 40 60 80 100 Ct qt Removal efficiency QCVN 14 : 2008/BTNMT, column B Figure 4. Changes of ammonium concentration (Ct), adsorption capacity (qt) and removal efficiency by time (initial concentration of 50 mg/L). 3.5. Effect of adsorbent concentration As mentioned above, using 1.0 g/L of adsorbent to treat the solution with initial ammonium concentration less than or equal to 60 mg/L would result in the effluent meeting well the allowable value of ammonium. For the wastewater containing higher initial ammonium concentrations, effect of adsorbent concentration on the treatment efficiency need to be investigated to find the suitable adsorbent amount. Since initial ammonium concentration in the domestic wastewater taken from the dormitory were 80.7 ± 1.6 mg/L (n = 3), we tested the effect of MCRH concentration on the treatment of synthetic wastewater prepared at initial concentration of 81.3 mg/L. As can be seen from Figure 5, increase of adsorbent led to the increase of ammonium removal, significantly from 0.8 to 1.5 g/L, slightly from 1.5 - 1.8 g/L, and almost unchanged after that. After 27 h of treatment, using 1.8 g/L or higher concentration of adsorbent resulted in the ammonium concentration in the effluent below 10 mg/L. The result from this experiment hence suggests the ratio of adsorbent and initial ammonium concentration was 22.5 g/g. Biochar concentration (g/L) 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 C e ( m g /L ), q e ( m g /g ), r e m o va l e ff ic ie n c y (% ) 0 20 40 60 80 100 qe Ce Removal efficiency QCVN 14 : 2008/BTNMT, column B 3.6. Adsorption isotherms and kinetics Ammonium adsorption data were fitted to Langmuir, Freundlich, and Dubinin– Radushkevich isotherm models as given in Figure 6 and values of parameters for these three models are provided in Table 2. Figure 5. Effect of adsorbent dose on effluent ammonium concentration, removal efficiency and adsorption capacity of MCRH (initial concentration of 81.3 mg/L). Nguyen Thi Thuy, et al. 120 Obviously, Langmuir and Dubinin–Radushkevich isotherm models described ammonium adsorption better than Freundlich isotherm model, which is consistent to the results from the literature [2, 5]. However, several previous studies found that the fitness of Freundlich isotherm model was similar (for rice husk biochar) [6] or slightly better than Langmuir isotherm model [7]. Langmuir isotherms Ce(mg/L) 0 10 20 30 40 C e /q e ( g /L ) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 y = 0.0138x+0.0753 R 2 = 0.976 Freundlich isotherm log (Ce) 0.6 0.8 1.0 1.2 1.4 1.6 1.8 lo g ( q e ) 1.0 1.2 1.4 1.6 1.8 2.0 y = 0.209x+1.473 R 2 = 0.695 Dubinin–Radushkevich isotherm ε2 1000 2000 3000 4000 5000 6000 L n ( q e ) 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 y = -0.0002x + 4.376 R 2 = 0.967 Figure 6. Langmuir, Freundlich, and Dubinin–Radushkevich isotherms. Table 2. Isotherm model parameters from adsorpti
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