001.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 83, 2021 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš Copyright © 2021, AIDIC Servizi S.r.l. ISBN 978-88-95608-81-5; ISSN 2283-9216 Fixed Bed Column Studies for the Adsorption of Cadmium onto Cockle Shell (Anadara Granosa) Powder Tuan-Anh Nguyena,*, Cam-Huy Nhana, Minh-Vien Lea, Phuong-Ha K. Huynha, Thanh Khoa Phungb, Anh Vy Tranc aFaculty of Chemical Engineering, Ho Chi Minh City University of Technology (HCMUT) – Vietnam National University Ho Chi Minh City (VNUHCM), 268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Vietnam bSchool of Biotechnology, International University, Vietnam National University Ho Chi Minh City (VNUHCM), Quarter 6, Linh Trung Ward, Thu Duc District, Ho Chi Minh City, Vietnam cDepartment of Chemical and Biological Engineering, Gachon University, 1342 Seongnamdaero, Seongnam-si, Korea anh.nguyen@hcmut.edu.vn Recently, the pollution of heavy metal ions is getting more attention due to economic development. A number of methods have been proposed for the removal of heavy metal ions in aqueous solutions, and the adsorption technique has been widely utilized due to the effectiveness and flexibility. Cockles are marine bivalve molluscs and their shells are discharged as waste by many restaurants and marine product manufacturers, which causes environmental problems. Cockle shells can be economically reused as an adsorbent for wastewater treatment. In this report, cockle shells were recycled as a material for the removal of cadmium (II) ion by adsorption in a fixed bed column. Experiments were designed according to the Box-Behnken scheme to investigate the effect of the column parameters such as inlet ion concentration, feed flow rate, and mass of adsorbent. The breakthrough curves from experiments were analyzed with Thomas, Bohart–Adams, and Yoon–Nelson models. The results also indicate that the powder derived from cockle shells can be used as a low cost and effective adsorbent for heavy metal removal such as cadmium in a fixed bed column. The observation suggests that the Thomas model and the Yoon-Nelson model are more suitable to predict the adsorption of cadmium (II) ions on onto cockle shell powder in fixed-bed operation mode. 1. Introduction Recently, the pollution of heavy metal ions is getting more attention in the developing world due to economic development. It was proved that heavy metal ion could cause severe problems to human health and the ecosystem (Khan et al., 2015). Numerous studies and several methods have been reported to treat heavy metal ions in water, such as precipitation, extraction, ion-exchange, coagulation-flocculation, flotation, membrane separation and electrochemical methods (Fu and Wang, 2011). Among them, the adsorption technique has been widely utilized due to its flexibility and cost-efficiency (Kumar et al., 2019). There are many kinds of adsorbents; however, the study of a low-cost and environmental friendly adsorbent is always desirable (Joseph et al., 2019). Food industry wastes such as desiccated coconut wastes (Rahim et al., 2019), oil palm wastes (Lee et al., 2017), coffee husk and spent coffee (Hernández Rodiguez et al., 2018), sugar cane bagasse (Busto et al., 2016), rice husk (Garba et al., 2017) have been utilized in adsorption study for the removal of heavy metals. Cockles are marine bivalve molluscs and their shells are discharged as waste by many restaurants and marine product manufacturers. Most of them are dumped into landfills, which becomes a potential environmental problem. According to recent research (Zuo et al., 2017), CaCO3 structure in seashells has a strong affinity with heavy metal ions, especially cadmium (II). Using seashells as adsorbent could be an economic and environmental friendly method as the process is highly efficient, low-cost materials for taking advantage of aquaculture’s waste product. For industrial-scale applications, the adsorbents are usually used in the fixed bed column because of the high capacity and continuous operation. The majority of cadmium adsorption studies have been carried out in batch DOI: 10.3303/CET2183044 Paper Received: 10/07/2020; Revised: 06/09/2020; Accepted: 27/09/2020 Please cite this article as: Nguyen T.-A., Nhan C.-H., Le M.-V., Huynh P.-H.K., Phung T.K., Tran A.V., 2021, Fixed Bed Column Studies for the Adsorption of Cadmium onto Cockle Shell (Anadara Granosa) Powder, Chemical Engineering Transactions, 83, 259-264 DOI:10.3303/CET2183044 259 modes, which are only suitable for the treatment of small quantities of wastewater. There are not many studies that report the adsorption of cadmium ion by food industry waste in a fixed-bed column. In this study, cockle shells were recycled as a material for the removal of cadmium (II) ion by adsorption in a fixed bed column. The operation of the process depends on various parameters, and the effects of inlet cadmium ion concentration, flow rate, and mass of adsorbent on the behavior of the column are investigated. The column experimental data were fit to Thomas, Bohart–Adams, and Yoon–Nelson models to describe the breakthrough curves. 2. Materials and methodology 2.1 Adsorbent and absorbate Cockle shells (Anadara granosa) grew in the South-East of Vietnam were collected at Ho Chi Minh city’s retail store. The organic parts were taken out; after that, the shells were brushed to clean all remaining dirt and organic parts; then wash with deionized water. The shells were dried under the sunlight for three days. After the water had evaporated, the shells were crushed by pestle and sieved to 0.5-1 mm grain size by 0.5 and 1 mm laboratory sieves. Shell powder was preserved in airtight polyethylene bottle at room temperature and used for further column experiments. Cadmium solutions were prepared from cadmium sulfate octahydrate salt (3CdSO4·8H2O, China) dissolved in deionized water into the desired concentrations. 2.2 Fixed-bed column adsorption Continuous column adsorption experiments were conducted in a laboratory Pyrex glass tube with an inner diameter of 10 mm and a height of 300 mm. The column was initially packed with a predetermined mass of cockle shell powder. Then the prepared cadmium solution was pumped upward through the column at the desired flow rate by a peristaltic pump. At different time intervals, the samples were collected at the outlet of the column, and the concentration of the samples was determined. An ion selective electrode was used for the quantitative analysis of cadmium (II) ion in the liquid phase. The cadmium ion analytical device is from Hanna Instruments, consists of pH/ISE meter (HI98191), reference electrode (HI5315), and cadmium half-cell ISE electrode (HI4003). The flow to the column was continued until there was no adsorption on the adsorbent, or the cadmium (II) ion concentration of the effluent remained unchanged. The schematic diagram of the experimental set up is shown in Figure 1. Figure 1: Adsorption column system In this study, the second-order Box-Behnken design was employed to study the effect of several operating parameters of the column, such as inlet concentration, mass of adsorbent, flow rate on the adsorption performance of the fixed-bed. The detail of variables and levels is shown in Table 1. The full experimental design and results are shown in the next session. pH and temperature of all experiments were maintained in the range of 5.5-7.0 and 25 °C - 27 °C. Table 1: Levels and values of column parameters Coded variables Factors Levels -1 0 +1 X1 Inlet concentration (mg/L) 190 200 210 X2 Mass of shell powder (g) 4.5 5.0 5.5 X3 Flow rate (mL/min) 7 8 9 2.3 Theory of models for fixed-bed studies The succeeding operation of a laboratory apparatus towards industrial-scale column can be well analyzed by simple mathematical models (Lakshmipathy and Sarada, 2015). The experimental data of the column from 260 different operating conditions were examined with well-known and straightforward mathematical models such as the Thomas, Bohart–Adams, and Yoon–Nelson models. Thomas model (Thomas, 1944) assumes the plug flow behavior and Langmuir adsorption-desorption equilibrium in the bed. The mathematical equation is expressed as Eq(1): 𝐶𝐶𝑡𝑡 𝐶𝐶0 = 1 1+𝑒𝑒𝑒𝑒𝑒𝑒� 𝐾𝐾𝑇𝑇𝑞𝑞0𝑚𝑚 𝑄𝑄 −𝐾𝐾𝑇𝑇𝐶𝐶0𝑡𝑡� (1) and the simplified linear form is as Eq(2): 𝑙𝑙𝑙𝑙�𝐶𝐶0 𝐶𝐶𝑡𝑡 − 1� = 𝐾𝐾𝑇𝑇𝑞𝑞0𝑚𝑚 𝑄𝑄 − 𝐾𝐾𝑇𝑇𝐶𝐶0𝑡𝑡 (2) where KT is the Thomas rate constant (mL min–1 mg–1); q0 is the adsorption capacity (mg g–1); m is the mass of adsorbent in the column (g); Q is the feed flow rate (mL min–1) The adsorption capacity q0 can be obtained from the experimental breakthrough curve by Eq(3): 𝑞𝑞0_𝑒𝑒𝑒𝑒𝑒𝑒 = 𝑄𝑄𝐶𝐶0 𝑚𝑚 ∫ �1 − 𝐶𝐶𝑡𝑡 𝐶𝐶0 �𝑑𝑑𝑡𝑡 ∞ 0 (3) The Bohart–Adams (Bohart and Adams, 1920) assumes that the equilibrium is not instantaneous, and the rate of sorption is proportional to the fraction of sorption capacity. The mathematical form and the linear form are expressed as the following Eq(4) and Eq(5): 𝐶𝐶𝑡𝑡 𝐶𝐶0 = 𝑒𝑒𝑒𝑒𝑒𝑒�𝐾𝐾𝐵𝐵𝐵𝐵𝐶𝐶0𝑡𝑡 − 𝐾𝐾𝐵𝐵𝐵𝐵𝑁𝑁0𝑍𝑍 𝑈𝑈0 � (4) 𝑙𝑙𝑙𝑙�𝐶𝐶𝑡𝑡 𝐶𝐶0 � = 𝐾𝐾𝐵𝐵𝐵𝐵𝐶𝐶0𝑡𝑡 − 𝐾𝐾𝐵𝐵𝐵𝐵𝑁𝑁0 � 𝑍𝑍 𝑈𝑈0 � (5) where KBA is the kinetic (mass transfer) constant (L mg–1 min–1); U0 is the superficial velocity (cm min–1); Z is the bed depth of column (cm) and N0 is the saturation concentration or adsorption capacity (mg L–1). Yoon–Nelson model (Yoon and Nelson, 1984) assumes that the rate of decrease in the probability of adsorption for each adsorbate molecule is proportional to the probability of sorbate sorption and the probability of sorbate breakthrough on the sorbent. The mathematical equation and the linear form of the model are expressed as Eq(6) and Eq(7): 𝐶𝐶0 𝐶𝐶𝑡𝑡 = 1 + 𝑒𝑒𝑒𝑒𝑒𝑒(𝐾𝐾𝑌𝑌𝑁𝑁𝑡𝑡 − 𝑡𝑡0.5𝐾𝐾𝑌𝑌𝑁𝑁) (6) 𝑙𝑙𝑙𝑙� 𝐶𝐶𝑡𝑡 𝐶𝐶0−𝐶𝐶𝑡𝑡 � = 𝐾𝐾𝑌𝑌𝑁𝑁𝑡𝑡 − 𝑡𝑡0.5𝐾𝐾𝑌𝑌𝑁𝑁 (7) where KYN is the Yoon–Nelson velocity rate constant (min–1); t0.5 is the time required for 50 % of adsorbate breakthrough (min). 3. Results and discussion The mechanism of the uptake of cadmium(II) ions by scallop shell powder are proposed by several studies such as in (Köhler et al., 2007). It can be explained by the surface precipitation of (Cd,Ca)CO3 solid solutions. The mechanism involves the exchange of calcium ion from the adsorbent and cadmium ions from the solution to form solid-solution nuclei as the following Eq(8). 𝑒𝑒𝐶𝐶𝑑𝑑2+ + 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶3(𝑠𝑠) = 𝑒𝑒𝐶𝐶𝐶𝐶2+ + 𝐶𝐶𝑑𝑑𝑒𝑒𝐶𝐶𝐶𝐶1−𝑒𝑒𝐶𝐶𝐶𝐶3 (8) The release of calcium ion also caused an increase in pH of the effluent solution (from 4.5 at the initial to 7). The dynamic adsorption behavior of the fixed bed column, the breakthrough curve, was simulated using Thomas, Bohart–Adams and Yoon–Nelson models. The coefficient of determination and the values of the models’ parameters were obtained using two methods: non-linear fitting (the original equation) and linear regression analysis (the linearized form). 3.1 Thomas model The Thomas model parameters, the kinetic coefficient KT and the adsorption capacity of the column q0, were calculated by non-linear fitting from Eq(1) and linear regression from Eq(2). The values of model parameters obtained by two methods and the coefficients of determination are summarized in Table 2. From Table 2, it can be obtained that the adsorption capacity of the column increased, but the kinetic coefficient decreased with an increase in initial metal ion concentration. The reason was that the driving force for the adsorption is 261 the difference in concentration between the cadmium ion on the adsorbent and the cadmium ion in the solution. The higher driving force due to the higher cadmium ion concentration resulted in better column performance. The higher driving force caused an increase in mass transport resistance and thus reduced the kinetic coefficient kTH. The coefficients of determination (R2) obtained for all the breakthrough curves are high, which confirms the applicability of the model. The observations suggest that external and internal diffusion were not rate-limiting steps. The results also suggest that it is better to use non-linear regression to fit the experimental data. The value of q0 was also obtained from the experimental breakthrough curve for comparison and shown in Table 2. The values of q0 are comparable to the ones obtained from Thomas model by non-linear fitting method. The observation confirmed the advantage of non-linear regression. Table 2: Parameters of Thomas model obtained by non-linear fitting and linear regression C0 M Q Z Linear Non-Linear Experime -nts (mg/ L) (g) (mL/ min) (cm ) kTH (mL/ mg.min) q0 (mg/g) R2 kTH (mL/ mg.min) q0 (mg/g) R2 q0 (mg/g) 190 4.5 8 4.32 0.0505 163.51 0.9510 0.0488 152.48 0.9715 157.51 210 4.5 8 4.32 0.0576 243.95 0.9879 0.0549 156.53 0.9952 157.85 190 5.5 8 5.28 0.0743 161.10 0.9568 0.1041 86.58 0.9996 87.72 210 5.5 8 5.28 0.0679 219.59 0.9756 0.0851 124.64 0.9999 122.68 190 5.0 7 4.80 0.0598 197.59 0.8966 0.0767 125.33 0.9963 122.84 210 5.0 7 4.80 0.0547 220.11 0.9094 0.0670 135.75 0.9966 133.75 190 5.0 9 4.80 0.0509 157.75 0.9610 0.0503 133.64 0.9776 134.55 210 5.0 9 4.80 0.0460 167.24 0.9351 0.0462 133.68 0.9827 138.63 200 4.5 7 4.32 0.0508 169.70 0.9497 0.0544 101.55 0.9877 111.66 200 5.5 7 5.28 0.0588 167.02 0.9775 0.0772 95.53 0.9951 97.09 200 4.5 9 4.32 0.0561 173.24 0.9438 0.0484 123.33 0.9849 127.5 200 5.5 9 5.28 0.0645 180.72 0.9602 0.0803 111.23 0.9841 116.73 200 5.0 8 4.80 0.0505 167.36 0.8460 0.0696 129.32 0.9861 132.69 200 5.0 8 4.80 0.0571 182.52 0.8509 0.0828 128.71 0.9961 128.17 200 5.0 8 4.80 0.0621 226.90 0.9767 0.0587 141.06 0.9965 142.33 3.2 Bohart–Adams model The Bohart–Adam model parameters, the kinetic coefficient (KBA) and the adsorption capacity (N0), were calculated by non-linear fitting from Eq(4) and linear regression from Eq(5). The values of model parameters obtained by two methods and the coefficients of determination are summarized in Table 3. Table 3: Parameters of Bohart–Adams model obtained by non-linear fitting and linear regression C0 M Q Z Linear Non-Linear (mg/L) (g) (mL/min) (cm) kBA (mL/mg.min) N0 (mg/g) R2 kBA (mL/mg.min) N0 (mg/g) R2 190 4.5 8 4.32 0.0358 282.65 0.9149 0.0187 315.19 0.8783 210 4.5 8 4.32 0.0362 297.36 0.9227 0.0176 329.53 0.9160 190 5.5 8 5.28 0.0378 200.18 0.8500 0.0157 226.13 0.7846 210 5.5 8 5.28 0.0403 236.48 0.9304 0.0195 260.56 0.7060 190 5.0 7 4.80 0.0380 221.20 0.8475 0.0267 227.17 0.9589 210 5.0 7 4.80 0.0352 242.18 0.8571 0.0231 250.81 0.9504 190 5.0 9 4.80 0.0311 288.66 0.8476 0.0146 329.21 0.8727 210 5.0 9 4.80 0.0278 313.28 0.8027 0.0119 361.93 0.8432 200 4.5 7 4.32 0.0300 237.22 0.8334 0.0138 269.68 0.8404 200 5.5 7 5.28 0.0332 200.24 0.9241 0.0174 216.91 0.8698 200 4.5 9 4.32 0.0297 299.54 0.7656 0.0123 341.44 0.8578 200 5.5 9 5.28 0.0379 249.04 0.8457 0.0141 280.09 0.8068 200 5.0 8 4.80 0.0298 261.35 0.7592 0.0203 267.83 0.8952 200 5.0 8 4.80 0.0326 254.46 0.8265 0.0216 260.98 0.9149 200 5.0 8 4.80 0.0390 268.36 0.9004 0.0181 294.12 0.9211 262 From the results, many of the R2 values are less than 0.9, which reveals that the data do not fit sufficiently well to the model. The observation suggests a lack-of-fit to the experimental data because the Bohart–Adams model is used to describe the initial period of the adsorption column (Busto et al., 2016). As a result, it can be concluded that the Bohart–Adams model is not applicable to explain the overall adsorption behavior of the column. 3.3 Yoon–Nelson model The Yoon–Nelson model parameters, the rate constant (KYN) and 50 % breakthrough time (t1/2), were calculated by non-linear fitting from Eq(6) and linear regression from Eq(7). The values of model parameters obtained by two methods and the coefficients of determination are summarized in Table 4. From the results, the rate constant, KYN, increased with an increase in inlet cadmium ion concentration. The reason was that the higher concentration offers the higher driving force for the adsorption, which results in a greater uptake rate. The values of t1/2 decreased with an increase in flow rate. This happened because the higher flow rate results in faster saturation of cadmium ion in the column. The coefficients of determination obtained for all the breakthrough curves are as high as compared to the Thomas model and closer to 1 than that of the Bohart– Adam model. The observation suggests the goodness of fit of the experimental data to the Thomas model and the Yoon–Nelson model. Table 4: Parameters of Yoon-Nelson model obtained by non-linear fitting and linear regression C0 M Q Z Linear Non-Linear (mg/L) (g) (mL/min) (cm) kYN (min-1) t1/2 (min) R2 kYN (min-1) t1/2 (min) R2 190 4.5 8 4.32 0.0096 484.09 0.9510 0.0093 451.44 0.9715 210 4.5 8 4.32 0.0121 432.39 0.9879 0.0115 419.28 0.9952 190 5.5 8 5.28 0.0141 330.28 0.9568 0.0198 313.29 0.9996 210 5.5 8 5.28 0.0143 403.31 0.9756 0.0179 408.04 0.9999 190 5.0 7 4.80 0.0114 457.73 0.8966 0.0146 471.16 0.9963 210 5.0 7 4.80 0.0115 456.51 0.9094 0.0141 461.74 0.9966 190 5.0 9 4.80 0.0097 429.31 0.9610 0.0096 390.75 0.9776 210 5.0 9 4.80 0.0097 411.79 0.9351 0.0097 353.66 0.9827 200 4.5 7 4.32 0.0102 376.19 0.9497 0.0109 326.42 0.9877 200 5.5 7 5.28 0.0118 390.56 0.9775 0.0154 375.30 0.9951 200 4.5 9 4.32 0.0112 347.71 0.9438 0.0097 308.31 0.9849 200 5.5 9 5.28 0.0129 385.56 0.9602 0.0161 339.88 0.9841 200 5.0 8 4.80 0.0101 414.67 0.8460 0.0139 404.14 0.9861 200 5.0 8 4.80 0.0114 399.56 0.8509 0.0166 402.23 0.9961 200 5.0 8 4.80 0.0124 456.73 0.9767 0.0117 440.80 0.9965 Figure 2: Experimental and predicted breakthrough curves for cadmium adsorption in the fixed-bed column at center point of experimental design (C0=200 mg/L, M=5.0 g, Q=8 mL/min) The predicted breakthrough curves were obtained by fitting experimental data to the three models by two methods for cadmium adsorption in the fixed-bed column at the center point of experimental design (C0=200 263 mg/L, M=5.0 g, Q=8 mL/min) are presented in Figure 2. From the comparison between the predicted value and experimental data in the figure and the coefficients of determination, it can be concluded that the Thomas model and the Yoon-Nelson model are suitable for predicting the adsorption behavior of cadmium ion onto cockle shell powder in a fixed-bed column. The observation also suggests that the non-linear fitting method provides better prediction compare to the linear regression method. 4. Conclusions This study explores the efficiency of the adsorbent derived from cockle shells in removing the heavy metal ion cadmium (II) from aqueous solution in a fixed-bed column. The effect of various operating parameters such as inlet concentration, mass of adsorbent and flow rate was investigated. The experimental breakthrough curves were analyzed by several models and two fitting methods. 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