DOI: 10.3303/CET2291099 Paper Received: 11 February 2022; Revised: 8 April 2022; Accepted: 11 May 2022 Please cite this article as: Roslan F.A.S., Yub Harun N., Saeed A.A.H., Ramli R.M., Mannikam S.D., Hussein Al-Qadami E.H., 2022, Methylene Blue Removal Using Palm Oil Mill Effluent Sludge-derived Adsorbent, Chemical Engineering Transactions, 91, 589-594 DOI:10.3303/CET2291099 CHEMICAL ENGINEERING TRANSACTIONS VOL. 91, 2022 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Valerio Cozzani, Bruno Fabiano, Genserik Reniers Copyright © 2022, AIDIC Servizi S.r.l. ISBN 978-88-95608-89-1; ISSN 2283-9216 Methylene Blue Removal Using Palm Oil Mill Effluent Sludge- Derived Adsorbent Farah Amelia Shahirah Roslan, Noorfidza Yub Harun*, Anwar Ameen Hezam Saeed, Raihan Mahirah Ramli, Sharvinaa Devi Mannikam, Ebrahim Hamid Hussein Al-Qadami Department of Chemical Engineering, Universiti Teknologi PETRONAS (UTP), 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia noorfidza.yub@utp.edu.my Textile industries release a large amount of dye effluent into the wastewater that can cause health issues and endanger aquatic animals. In this study, sludge derived biochar (SB) adsorbent was produced from sewage sludge to remove methylene blue (MB) dye from wastewater. The surface area and porosity of SB are analysed by using Field Emission Scanning Electron Microscope (FESEM) and Surface Area and Pore analyser (SAP). The factors that affect the adsorption of MB in aqueous solution with SB adsorbent in batch mode are adsorbent dosage (0.5g, 1g), initial MB concentration (50 – 250 mg/L), and contact time (0 – 120 minutes). The Langmuir isotherm model fits the adsorption equilibrium results, and the pseudo-second-order kinetic model best describes the sorption kinetics of MB. The maximum adsorption capacity of MB decreases from 15.37 mg/g to 8.61 mg/g as the adsorbent dosage increases from 0.5 g to 1.0 g and contact time increases from 45 minutes to 60 minutes. Hence, sludge derived biochar is an effective adsorbent for the removal of MB dye from wastewater. 1. Introduction The synthetic origin of industrial dyes has a high concentration of suspended solids, aromatic pollutants, alkaline properties, chemical oxygen demand, and biochemical oxygen demand in wastewater. The defiant nature of dyes possesses a threat to the environment and also to human health such as hypertension, fever, and mental disorder (Chaukura et al., 2017; Fan et al., 2017). The methylene blue (MB) dye referred to in this study is discharged in the lake and river (Saeed et al., 2019). Moreover, MB cannot be removed by conventional treatment systems such as advanced chemical oxidation technology, membrane treatment, and photo-degradation due to its high solubility and non-degradable nature (Chaukura et al., 2017; Fan et al., 2017). The adsorption method favours MB removal due to its low cost, high efficiency, and easy operational system. Activated carbon is an effective adsorbent for the adsorption process, but it comes with a high capital cost for small-scale applications (Sajab et al., 2010). Therefore, low-cost, green and efficient adsorbent from biomass such as sludge has demonstrated outstanding MB removal capability from wastewater compared to activated carbon (Otero et al., 2003). Sludge is the by-products of wastewater treatment plants (Otero et al., 2003). The process of upcycling the sludge waste into adsorbent can reduce the pollutants in the environment while removing MB from wastewater with a lower adsorbent material cost. SB is prepared at a lower temperature than commercial activated carbon, which shows that SB is cheaper than activated carbon (Chaukura et al., 2017; Fan et al., 2017). In general, the adsorption capacity is determined by the chemical structure and the porous structure of the adsorbent, which leads to alternative models for adsorption capacity based on the adsorption from cationic and anionic solution adsorbate (Saeed et al., 2020). Biochar as an adsorbent has recently gained attention, especially the ones derived from agricultural residues like rice husk (Saeed et al. 2021a), Kenaf (Saeed et al. 2021b), coconut coir ( Paranavithana et al. 2016), sawdust (Mashkoor and Nasar, 2020), corn straws (Lian et al., 2016), pineapple bark (Guo et al., 2018), durian husk (Ahmad et al., 2015), hickory wood (Ding et al. 2016), and tea waste (Vithanage et al., 2016). Biochar is 589 effective, affordable and environmentally friendly adsorbent material for dye removal from wastewater (Saeed et al., 2018, Saeed et al., 2020). Therefore, the study aims to study the adsorption performance of biochar using sludge from wastewater treatment plants by evaluating the initial concentration, adsorbent dosage and the contact time that affects the removal of MB through adsorption. 2. Methodology 2.1 Sample pre-treatment Sludge was acquired from the Nasiruddin Palm Oil Mill in Bota. The sodium hydroxide (NaOH), hydrochloric acid (HCl) and methylene blue (MB) dye were acquired from Sigma-Aldrich. MB trihydrate was dissolved with deionised water to prepare a 1000 milligram per litre (mg/L) stock solution of MB, and it was then diluted into desired initial concentrations. The MB concentration was measured with UV-Vis Spectrophotometer with the wavelength set at 665 nm (Fan et al., 2017). A standard solution of MB between 20 mg/L and 220 mg/L was prepared for the calibration curve. Preliminary washing of sludge with distilled water to remove contaminations on the surfaces. The sample is then weighted before drying in the oven for 24 hours at 105 degrees Celsius (°C) to calculate the moisture content of the sludge, which is around 80 %. The dried sludge was ground using a blender and sieved into 250 µm to 500 µm and kept in air-tight containers for subsequent work. 2.2 Preparation of sludge derived biochar The dried sludge was impregnated with sodium hydroxide (NaOH) solution with a 3:1 weight ratio of sludge to NaOH with a magnetic stirrer before placing in the oven for 24 hours at 105 ℃ for the drying process. Then, the dried impregnated sludge underwent pyrolysis process in a tube furnace under a nitrogen atmosphere with a heating rate of 10 degrees Celsius per minute (0C/min) for 2 hours at 750 ℃ to produce sludge derived biochar (SB). The mass of SB was measured after the pyrolysis process. SB was washed with deionised water and diluted sulphuric acid (H2SO4) solution until the effluent reaches the neutral pH between 6 and 8. The sample was then filtered and dried in the oven for 24 hours at 105 ℃. Finally, the sample is stored in an air-tight container before further work. The physical and chemical features of raw sludge and SB were analysed using Surface Area and Porosity (SAP) Analyser, Field Emission Scanning Electron Microscopy (FESEM). The textural property of SB was analysed by SAP analyser. The BET multipoint was used to calculate the surface area of the SB. The surface morphology and element distribution were determined by using FESEM. 2.3 Adsorption studies Adsorbent dosages of 0.5 g and 1 g with a contact time of 45 minutes and 60 minutes for the adsorption process were investigated in this study. The adsorbent at different doses was added to the 250 mL conical flask with 50 millimetres (mL) of 200 mg/L MB solution and was then shaken at different contact times on the orbital shaker at 120 rpm. The solution was filtered, and the filtered adsorbents were dried at 105 ℃ for 24 hours and characterised using FESEM. Absorbance test was done on the filtrate. The nomenclature used for the adsorption studies was SB(D)-T, where D is the adsorbent dosage, and T is the contact time. The removal efficiency (R) and adsorption capacity of MB were calculated using the following formulae, respectively: R (%) = (C0 − Ct) C0⁄ × 100% (1) qt = (C0 − Ct) × (V/m) (2) where, qt is the amount of MB adsorbed at time, t (mg/g), C0 and Ct is the concentration of MB solution at initial and at time, t (mg/L), V is the volume of MB solution (L) and m is the mass of adsorbent (g). 2.4 Adsorption Isotherm and Kinetic The adsorption isotherm and adsorption kinetic studies were conducted in a conical flask with 200 mL of MB solution with a varying initial concentration of 50 – 250 mg/L. The experiment was carried out with 2 g of SB adsorbent. The conical flasks were placed on the orbital shaker for 120 minutes at 150 rpm. MB samples of 10 mL were collected every 10 minutes. Langmuir and Freundlich's isotherm adsorption models were evaluated to depict the adsorption mechanism of MB. The adsorption equilibrium and kinetics are defined in the linear form shown in the Table1 (Fan et al., 2017; Ferrentino et al., 2020), where, qm is the maximum monolayer adsorption capacity (mg/g), qe and qt is the amount of MB solution adsorbed at equilibrium and at time, t, respectively (mg/g), Ce and C0 is the equilibrium and initial concentration of MB in the liquid phase, respectively (mg/L), KL is the Langmuir adsorption constant (L/mg), KF is the relative sorption capacity of the sorbent ((mg/g)(mg/L)-1/n), n is the degree of dependence of sorption on the equilibrium concentration of MB, t 590 is the MB adsorbed at time, t (min), and k1 (min-1) and k2 (g/mg.min) are the first-order and second-order adsorption rate constant, respectively. Table 1: Equilibrium and kinetic equations Models Name Equilibrium 1 qe = 1 KLqm 1 Ce + 1 qm (3) Langmuir ln qe = ln KF + (1/n)ln Ce (4) Freundlich RL = 1/(1 + KLC0) (5) Separation factor Kinetic log (qe − qt) = log qe − ((k1t)/2.303) (6) Pseudo-first-order t qt = 1 k2qe 2 + 1 qe t (7) Pseudo-second-order 3. Result and Discussions 3.1 FESEM Analysis FESEM micrographs of raw sludge, SB, and SB after adsorption are shown in Figure 1 below. In Figure 1(a), the surface of raw sludge had a smooth surface with small pores. An adsorbent with bigger pore size has a good adsorption capacity (Kutty et al., 2019). As a result of the channels from the pores and voids to the active sites in the internal pores of the sludge, an improvement in the MB molecules' adsorption was observed in this study (Ahmed et al., 2019). The surface of SB in Figure 1(b) has a more homogeneous structure, with semi-spherical forms and no apparent pores. However, there might be a possibility of a large porous structure. The surface of SB became a bit rougher because of the adsorption of MB molecules onto the adsorbent surface, as shown in Figure 1(c). Figure 1: FESEM Micrographs of (a) raw sludge; (b) SB; (c) SB after adsorption 3.2 SAP Analysis The pore structure of raw sludge and SB was analysed with an SAP analyser. Figures 2(a) and 2(b) exhibit type IV isotherms according to the IUPAC classification with hydrogen hysteresis loops, indicating favourable adsorption on mesoporous solids. Moreover, the high volume of nitrogen adsorbed shows high proportions of mesoporous (Ahmed et al., 2019), which suggests that the textural characteristics of the adsorbent have improved. The BET and Langmuir surface area of SB is higher than raw sludge at 25.28 m2/g and 36.84 m2/g, respectively. The presence of alkaline salts and other mineral elements is shown to increase the surface area, which contributes to simultaneous thermal and chemical activation during pyrolysis (Ahmed et al., 2019). The Barrett-Joyner-Halenda (BJH) method was used to determine pore size and volume. The pore volume of SB increases to 0.034 cm3/g, but the pore size has decreased from 42.91 nm to 29.29 nm. The pores indicate that SB is mesoporous, which leads to high MB adsorption with minimal resistance to diffusion (Ahmed et al., 2019). Organics of carbon-based materials stimulated higher adsorption from high porosity and large pore volume with a pore-filling effect (Ahmed et al., 2019). 3.3 Effect of contact time and adsorbent dose on adsorption of MB The performance of MB adsorption was studied using adsorbent dosage of 0.5 g and 1 g with a contact time of 45 minutes and 60 minutes. Equation (1) and Equation (2) are used to evaluate the removal efficiency and adsorption capacity, respectively. According to Figure 3, as the adsorbent dose and contact time increases, the removal efficiency also increases. The increase in the binding sites for adsorption enhances the adsorption rate, which increases the removal efficiency (Fan et al., 2017). However, the adsorption capacity decreases as the adsorbent dose increases. The decrease in adsorption capacity might be due to adsorbate saturation from the decrease in the adsorption active sites or the increase in the adsorbent surface area, 591 which decreases the number of MB molecules per unit of adsorbent. The highest adsorption capacity of 15.37 mg/g is achieved with SB (0.5)-60. However, some studies indicate that when the adsorbent dosage further increases, the removal rate and adsorption capacity become stagnant due to MB molecule saturation in the adsorption sites (Fan et al., 2017). As the contact time increases, the adsorption capacity and removal efficiency of MB from the aqueous solution increases. However, as the adsorbent dosage for adsorption of MB increases, the adsorption capacity decreases while the removal percentage increases. Figure 2: Adsorption-desorption isotherms of: (a) raw sludge; (b) SB Table 2: Porous textural parameters analysis of raw sludge and SB Material Raw sludge SB BET surface area (m2/g) 1.10 25.28 Pore volume (cm3/g) 0.0108 0.0340 Pore size (nm) 42.91 29.29 Figure 3: Adsorbent dosage and contact time effect of MB: (a) adsorption capacity, (b) removal efficiency 3.4 Adsorption Isotherms A different range of MB concentration (50 mg/L – 250mg/L) was used to carry out the isotherm study of SB on MB solution. The Langmuir and Freundlich isotherm models were used to fit the adsorption equilibrium data for SB using Equation (3) and Equation (4), respectively, and the respective parameters and correlation are reported in Table 3. The Langmuir model fit the data better than the Freundlich model with a correlation coefficient (R2) of 0.9895, implying monolayer adsorption and a homogeneous surface of MB onto SB (Ferrentino et al., 2020). The value of RL obtained is greater than zero and less than one, which suggests that it is favourable adsorption (Fan et al., 2017). The R2 shown on these models showed a strong affinity for the adsorption of MB onto SB. Table 3: The parameters of Langmuir and Freundlich adsorption isotherm Langmuir Freundlich qm (mg/g) KL (L/mg) R2 RL n KF R2 2.023 0.629 0.9895 0.0063 7.446 1.199 0.9288 592 3.5 Adsorption Kinetics Equation (2) was used to calculate the adsorption capacity of MB at equilibrium. Figure 4 demonstrates that as the MB concentration increases, the adsorption capacity also increases. The increase in MB molecules for adsorption or increased interaction between adsorbent and MB molecules due to higher mass transfer driving force causes the increase in the adsorption capacity (Nasrullah et al., 2018). The adsorption capacity exhibits a drastic increase, and then it plateaus as it reaches equilibrium around 30 – 70 minutes into the adsorption process. Based on Table 4, the R2 for pseudo-first-order kinetics was between 0.0281 to 0.6053 for all concentrations ranges, and the calculated qe does not match the experimental qe values. Conversely, the experimental data were well fitted with pseudo-second-order kinetics with an R2 higher than 0.9994 and closer to unity. Based on the assumption that chemisorption involving valency forces may be the rate-limiting step through electron exchanging and sharing, pseudo-second-order kinetics is likely to describe the system in this study (Chaukura et al., 2017). Figure 4: Kinetic curves for MB adsorption onto SB Table 4: Parameters of pseudo-first-order and pseudo-second-order kinetic model C0 (mg/L) qe, exp (mg/g) Pseudo-first-order Pseudo-second-order qe, cal (mg/g) K1 (min-1) R2 qe, cal (mg/g) K2 (min-1) R2 50 1.35 0.1466 0.0051 0.0281 1.3245 -0.6377 0.9995 100 1.76 0.2729 0.0207 0.3002 1.7759 0.4642 0.9994 150 1.88 0.3797 0.0281 0.6053 1.8871 1.3631 0.9999 200 1.94 0.3167 0.0251 0.2231 1.9451 1.8838 0.9999 250 2.01 0.1244 0.0170 0.1557 2.0145 1.0058 0.9998 4. Conclusions This study has achieved the main objective which is to study the effect of activating agents NaOH on SB biochar where SB had mesoporous pores with a surface area of 25.28 m2/g and pore volume of 0.0340 cm3/g. The batch adsorption experiment shows a promising result in which the highest adsorption capacity of MB obtained was 2.0235 mg/g mg/g at 60 min contact time,250 mg/L initial MB concentration, and 1 g adsorbent dosage. 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