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Utilization of esterified sago bark fibre waste for removal of oil from palm oil mill effluent
Accepted Manuscript Title: Utilization of esterified sago bark fibre waste for removal of oil from palm oil mill effluent Author: Rafeah Wahi Luqman Chuah Abdullah Mohsen Nourouzi Mobarekeh Zainab Ngaini Thomas Choong Shean Yaw PII: DOI: Reference:
S2213-3437(16)30430-4 http://dx.doi.org/doi:10.1016/j.jece.2016.11.038 JECE 1351
To appear in: Received date: Revised date: Accepted date:
9-6-2016 24-11-2016 25-11-2016
Please cite this article as: Rafeah Wahi, Luqman Chuah Abdullah, Mohsen Nourouzi Mobarekeh, Zainab Ngaini, Thomas Choong Shean Yaw, Utilization of esterified sago bark fibre waste for removal of oil from palm oil mill effluent, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.11.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Utilization of esterified sago bark fibre waste for removal of oil from palm oil mill effluent Rafeah Wahia*, Luqman Chuah Abdullahb, Mohsen Nourouzi Mobarekehc, Zainab Ngainia and Thomas Choong Shean Yawb a
Department of Chemistry, Faculty of Resource Science and Technology, Universiti Malaysia
Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia b
Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti
Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia c
Department of Environment, Islamic Azad University of Isfahan (Khorasgan Branch), Isfahan,
Iran, 81595-158.
*Corresponding author: Rafeah Wahi, Department of Chemistry, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia; Email: [email protected]; Tel.: +6082-581000; Fax: +6082-583160 Highlights:
Esterified sago bark (ESB) was used for oil removal from palm oil mill effluent (POME).
ESB has superior oil removal properties than SB.
Esterification introduced long chains onto ESB surface for excellent oil uptake.
The maximum oil removal efficiency by ESB was 80.23% (at pH 4.18, 24 h, 30 °C).
Abstract With oil and grease content of 4000-8000 mg/l in palm oil mill effluent (POME), the commonly used ponding system often fails to produce treated effluent that meets the minimum standard of treated effluent. The present study investigates the efficiency of sago bark (SB) and esterified sago 1
bark (ESB) for removal of emulsified oil from POME. Oil removal experiments were conducted at different batch experimental conditions: namely adsorbent dosage, contact time, temperature and pH. In overall, the oil removal efficiency of both SB and ESB increased with the increasing of sorbent dosage and contact time. 24-h oil adsorption test afforded oil removal efficiency of 57.77% (SB) and 80.23% (ESB).On the other hand, the oil removal efficiency of both SB and ESB decreased with the increasing temperature. Acidic pH was favorable pH condition for high oil removal efficiency in POME. There was a good correlation (R2 > 9.5) between experimental data and the intra-particle diffusion model for both SB and ESB. The adsorption of oil in POME using SB was best described by Freundlich isotherm (R2=0.998), indicating heterolayer adsorption of oil on SB. The adsorption of oil in POME using ESB was better represented using Langmuir isotherm (R2=0.992), indicating a monolayer adsorption of oil onto the ESB surface. In conclusion, ESB showed better potential for use as sorbent for removing emulsified oil from wastewater, particularly POME.
Keywords: Sago bark, esterified sago bark, sago fibre, palm oil mill effluent, oil removal
1. Introduction One of the main sources of industrial oily wastewater in Malaysia is palm oil mill effluent (POME). POME is generated at significant level as a by-product during palm oil processing. In year 2008 alone, at least 44 million tonnes of POME was generated in Malaysia (Wu et al., 2010). The oil concentration in POME ranges between 4000-8000 mg/l (Ahmad et al., 2006; Ngarmkam et al., 2011). Most of the palm oil mills use conventional ponding system to treat POME. However, ponding system requires long treatment time and large area (Wu et al., 2010). Malaysian Department of Environment (DOE) has set a value of 50 mg/l as the maximum allowable limit for 2
oil and grease content in the effluent to be discharged in waterways. The ponding system quite often fails to produce treated effluent that meets the DOE standard (Chin et al., 1996). Thus, it is crucial for the palm oil industries to adapt an efficient treatment for oil removal.
Many technologies have been suggested for oil removal from water. Although ultrafiltration has a higher efficiency for the removal of oil, it is not suitable to be used in treating wastewater with high solid content such as POME due to the risk of premature membrane fouling (Wu et al., 2007; Yi et al., 2011). Other treatments like coagulation, flotation and biological treatment are either expensive, complex in operation or required highly skilled operators. Adsorption is the most preferred method due to its feasibility and effectiveness, provided an appropriate sorbent is used.
Natural fibres are potential sources of natural sorbent for removal of oil from POME. Raw natural sorbents generally has excellent adsorption capacity, comparable density with synthetic sorbent, chemical free and highly biodegradable (Annunciado et al., 2005; Rajaković-Ognjanović et al., 2008; Wang et al., 2012). Natural fibres comprise cellulose, hemicellulose, and lignin (Zhang et al., 2016), which is known responsible for oil adsorption (Tolba et al., 2011; Wahi et al., 2013; Yang et al., 2015). Examples of natural fibres used as oil sorbent are rice husk (Ali et al., 2012), kapok (Abdullah et al., 2010), barley straw (Ibrahim et al., 2010), sugarcane baggase (Said et al., 2009), sawdust (Cambiella et al., 2006) and grass (Suni et al., 2004).
Despite of their advantages, many natural fibres suffer low hydrophobicity and buoyancy, therefore are only suitable for oil removal in the absence of water (Ali et al., 2012). This is because most of the cellulose hydrophobic portions are covered by the hydroxyl groups, causing cellulose
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to behave more hydrophilic than hydrophobic (Said et al., 2009). To improve the oleophilicity and hydrophobicity of natural fibres, hydrophobization can be conducted by means of alkalization (Abdullah et al., 2010), chloroform treatment (Abdullah et al., 2010; Likon et al., 2013), acetylation (Adebajo and Frost, 2004; Ren et al., 2007), salt treatment (Wang et al., 2012), surfactant treatment (Ibrahim et al., 2010, 2009), combination of chemical-biotechnological treatment (Garcia-Ubasart et al., 2012), polymerization followed by ion-exchange process (Pan et al., 2016), and esterification (Banerjee et al., 2006; Said et al., 2009).
Sago, known scientifically as Metroxylon sagu Rottboll, comes from genus metroxylon and family palmae (Singhal et al., 2008). It is one of the potential natural fibers abundantly found in tropical lowland forest and freshwater swamps. In 2009, nearly 59,000 hectares of Sago is planted in Sarawak (Department of Agriculture Sarawak, 2009a). Sarawak is currently one of the world largest exporters of sago products with annual export of approximately 43,000 tonnes (Department of Agriculture Sarawak, 2009b, 2009c).
Every tonne of sago flour produced generates approximately 0.75 tonne of sago bark (SB) as a solid waste (Vikineswary et al., 1994). With this regards, SB is a potential source of natural fibre for oil removal. More than 85% of SB is left unutilized in sago processing mill. Common disposal of SB is via incineration, direct dumping into nearby rivers and natural degradation, which gives rise to environmental problems (Wahi et al., 2014).
In the work of Ngaini et al. (2014) and Noh et al. (2012), the SB was esterified using fatty acid derivatives to improve the oleophilicity and hydrophobicity of SB. It was found that the esterified
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SB (ESB) showed a good adsorption capacity on engine oil in water. Wahi et al. (2014) studied the optimization of the esterification process using stearic acid (SA). The optimum ESB synthesis conditions were at 1:1 SB: SA, 15 wt% CaO as catalyst and 8 h refluxing time. The application of ESB for removal of oil from POME, is however, a complex process. The high suspended solids and COD values, acidic pH, and moderate temperature (between 40-80 °C) of POME could affect the oil removal efficiency. The aforementioned factors made it crucial to carefully study the performance of ESB in removing oil from POME at various adsorption parameters before it could be applied at larger scale. Thus, this present study aims to investigate systematically the oil removal efficiency of ESB in POME via batch adsorption system at various adsorption parameters, namely the adsorbent dosage, contact time, POME temperature, and POME pH.
2. Material and Methods 2.1 Preparation of ESB and POME Shredded SB was collected from sago processing mill in Mukah, Sarawak. SB was ground and sieved into particle size range of 0.5 mm to 1.5 mm. Esterification of SB with fatty acid derivative was carried out using stearic acid (SA). SA was chosen due to the reason that it is a long-chain fatty acid with 18 carbon chains, with highly hydrophobic properties. Figure 1 shows the experimental set up for esterification of SB with SA. The ground SB (5 g) was placed in a round bottom flask containing ethyl acetate (50 mL). In this study, the SB to SA mass ratio was 1:1. Calcium oxide (0.75 g) was added to the mixture, to expedite the esterification process (Wahi et al., 2014). The mixture was heated under reflux for 8 hours. The resulting ESB was cooled down to room temperature, filtered, washed with ethyl acetate and dried in a desiccator prior to use. The hydrophobicity of ESB was measured based on Ribeiro, Rubio, & Smith (2003). The degree of
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esterification was estimated by calculating the ratio, R, between the intensity of C=O (ester peak) and C-O (cellulose backbone) in infrared spectra, as described in Adebajo & Frost (2004). Total pore volume was measured using BET Analyser (Quantachrome® ASiQwinTM). The hydrophobicity, average C=O:C-O intensity ratio, and total pore volume of ESB was 50.2 ± 9.7%, 1.251 ± 0.213, and 0.012 cm3/g, respectively. Full physicochemical characteristics of ESB including the surface functional groups and surface morphology has been described in detail in our previous work (Wahi et al., 2014).
POME was collected from FELCRA palm oil mill in Kota Samarahan, Sarawak. The POME sample was collected from the drain coming from the mill before entering the first treatment pond, whereby the temperature was around 40ºC (raw POME). The POME was filtered through a muslin cloth strainer to remove solid particles of millimeter size. The oil and grease content, total suspended solid content and pH of the POME sample was 4850 mg/L, 15,200 mg/L and 4.18 respectively. The POME sample was stored at 4°C before use.
2.2 Oil adsorption study The batch adsorption study was conducted using SB and ESB at different sorbent dosage, contact time, pH and temperature using rotary shaker (Luckham R100/TW). The effect of sorbent dosage was examined using 0.5, 1, 2, 3 and 4 g SB and ESB in 100 ml POME at 30ºC, POME, having pH of 4.18, 30 min of contact time and mixing speed of 200 rpm. The effect of contact time was studied at 10, 20, 30, 40, 50, 60, 90, 120 and 1440 min contact time (sorbent dosage: 2 g, temperature: 30 ºC, pH: 4.18, mixing speed: 200 rpm). Effect of pH was studied at pH 2, 4.18, 7, 8 and 10 (sorbent dosage: 2 g, contact time: 30 min, temperature: 30ºC, mixing speed: 200 rpm).
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Effect of temperature was studied at 15, 20, 30, 40 and 60ºC (sorbent dosage= 2 g, contact time= 30 min, pH= 4.18, mixing speed= 200 rpm). The oil and grease analysis method was adapted from Standard Methods for the Examination of Water and Wastewater, Section 5520B (Clesceri et al., 1999).
2.3 Reusability study Reusability study was conducted to evaluate the performance of ESB in removing oil from POME after certain number of cycle. The reusability experiment was conducted by placing 2 g of ESB in 100 mL of POME at room temperature. The mixture was shaken for 30 minutes at 200 rpm speed. The mixture was filtered to collect the filtrate for oil and grease analysis. The sorbent was drained for 30 minutes before commencing next cycle of batch oil removal. The steps were repeated for 15 cycles. The oil removal efficiency for each cycle was recorded.
2.4 Diffusion model for batch oil adsorption
In order to determine the adsorption mechanism during removal of oil from POME using ESB, the intra-particle diffusion model was integrated. The general representation of Weber and Morris intraparticle model is shown in Equation 1:
qt = Kpt1/2
(Equation 1)
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where qt is the concentration of oil adsorbed at time t (mg/g), Kp is the intra-particle diffusion rate constant (g/ g min-1/2). The equation describes the time evolution of the concentration in adsorbed state, where Kp is the slope of linear plot qt versus t1/2.
Assumptions are important prior to an employment of a model to experimental data. The following assumptions were made during the usage of Weber and Morris intraparticle model to describe oil adsorption from POME using SB and ESB in this study: 1) The Fick’s law is applicable in both liquid phase diffusion and solid phase diffusion. 2) The particles were considered in the isotropic medium. 3) Intraparticle diffusion controls the reaction or particle kinetics. 4) Pore and surface diffusivities were constant during the adsorption process. 5) Diffusion coefficients were not concentration dependent. 6) The bulk phase concentration of oil was not constant during the adsorption process.
3. Results and Discussion
There are many factors governing the oil adsorption process and the oil removal efficiency of a sorbent. In batch oil adsorption system, the common parameters studied were sorbent dosage, contact time, influent temperature and influent pH.
3.1 Effect of sorbent dosage on oil removal efficiency
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The influence of sorbent dosage on oil removal efficiency of SB and ESB in POME were investigated employing 0.5 g - 4 g SB and ESB in 100 ml of POME. Figure 2 shows that ESB afforded higher oil removal efficiency than SB. The high oil removal efficiency of ESB was due to several reasons. One of the reasons was due to extensive modification of the ESB surface via esterification which afforded rough surfaces and exposed internal structures thus increased the oil entrapment sites (Ibrahim et al., 2010). It is envisaged that the site for oil entrapment in SB and ESB was different. During oil removal from POME, oil was attached physically to the SB surface. In contrast, Attachment of oil to ESB surface is due to the hydrophobic tails available on the surface of ESB after esterification process, due to substitution of the –OH groups in ESB with hydrophobic groups in SA (Wahi et al., 2014). Another reason was due to the negative zeta potential of oil droplets in POME which was neutralized by calcium ions of calcium oxide in ESB. The neutralization of charge resulted in a reduction of the electrostatic repulsion and consequently enhanced oil droplets coalescence (Cambiella et al., 2006). However, the lower oil removal efficiency of SB was due to the tendency of SB to form hydrogen bonding with the aqueous phase in wastewater such as POME.
Overall, the oil removal efficiency increased as sorbent dosage was increased up to 4 g per 100 ml of POME whereby the oil removal efficiency was 54.7% (SB) and 66.8% (ESB). The phenomenon was associated with an increase in available binding sites for adsorption in higher sorbent dosage (Ahmad et al., 2005a). The oil removal efficiencies of SB and ESB in POME were lower than the previously reported sorbents. For example, the use of chitosan (Ahmad et al., 2005a), palm kernel shell magnetic composite (Ngarmkam et al., 2011) and commercial activated carbon (Ahmad et al., 2005b) afforded 90% - 99% oil removal efficiencies compared to SB (58.6%) and ESB
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(66.8%). However, the contact hours of the sorbents were longer than SB and ESB, which was 1 24 hr. Therefore, further investigation on the effect of longer contact time on oil removal efficiency of SB and ESB in POME were investigated.
3.2 Effect of contact time on oil removal efficiency
The effect of contact time on the oil removal efficiency of SB and ESB in POME were conducted to investigate the oil adsorption behavior in 10 - 120 min (Figure 3). The oil removal efficiency of SB and ESB was increased as the contact time increased.
Based on the slopes shown in Figure 3, the oil removal from POME by SB and ESB occurred in two phases. A very fast adsorption phase occurred during the first 40 min and a slow phase between 40 - 60 min. The amount of oil adsorbed in mg per gram of sorbent, qe was stabilized after 60 min and the constant oil removal efficiency reading indicates that the oil adsorption has reached equilibrium (Figure 3).
The minimum of 10 min contact time was sufficient to allow a small percentage of oil removal using SB and ESB. After 30 min, SB and ESB afforded 39.6% and 53.46% oil removal efficiencies respectively. The oil removal efficiency was increased to 67.62% (SB) and 83.81% (ESB) after 1 hr. However, there was a slight decrease in oil removal efficiency of both SB and ESB at 90 min contact time (SB: 57.45%, ESB: 80.98%).
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The oil removal efficiency of SB and ESB remained constant after 90 min contact time. Further study was also carried out by increasing the contact time up to 1440 min (not shown in Figure 3) which afforded oil removal efficiency of 57.77% (SB) and 80.23% (ESB). The results indicated that no significant changes were observed in oil removal efficiency of SB and ESB even though in longer contact time.
It was envisaged that during the first hour of the oil adsorption study, the increase in contact time has resulted in an increased oil-sorbent interaction. The interaction phenomena have increased the amount of oil attached to the sorbent and resulted in an increase in oil removal efficiency (Ahmad et al., 2005a). The constant oil removal efficiency of SB and ESB after 90 min contact time was attributed to the saturation effect (Rajaković-Ognjanović et al., 2008). It was due to the adsorptiondesorption process that occurred at the saturated sorbent surfaces, whereby the attached oil was released and re-attached interchangeably, causing oil removal efficiency reading to be reduced or constant (Figure 4). The contact time plays an important role only at the beginning of the oil adsorption activity and is less important near to equilibrium (Ahmad et al., 2005a; Ibrahim et al., 2009; Sokker et al., 2011). When near to equilibrium, only small increase in oil removal was observed due to limited availability of sorbent surfaces for oil entrapment (Sokker et al., 2011). The findings obtained in this study were in agreement with previously reported studies on oil removal from POME using chitosan, bentonite and activated carbon. The oil removal efficiency also remained constant after 30 - 40 min with maximum oil removal efficiency between 60% 99% (Ahmad et al., 2005a, 2005b).
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3.3 Effect of pH on oil removal efficiency
The study on the effect of pH during oil removal is of great importance due to the fact that the changes in pH could affect the surface properties and binding sites of the sorbent (Ibrahim et al., 2009) and emulsion breaking (Sokker et al., 2011). It is also interesting to note that different sorbents behave differently towards pH change during oil removal (Ahmad et al., 2005a; Aranđelović et al., 2009; Ibrahim et al., 2009; Rajaković-Ognjanović et al., 2008; Sokker et al., 2011).
The effect of POME pH on oil removal efficiency was studied at pH 2 - 10, and the results are illustrated in Figure 5. In general, ESB showed higher oil removal efficiency compared to SB. SB and ESB gave a similar trend, whereby higher oil removal efficiencies were found both at acidic and basic POME compared to neutral pH of POME (pH 7). The oil removal efficiency of SB and ESB was increased in the order of neutral condition < strongly basic condition < strongly acidic condition. A similar pattern was also demonstrated by chitosan (Ahmad et al., 2005a; Sokker et al., 2011) and recycled wool-based nonwoven (Rajaković-Ognjanović et al., 2008) during removal of various types of oil. The highest oil removal efficiency for both SB and ESB were recorded at pH 2 (SB: 48.17% and ESB: 68.49%). This was due to the fact that strong acidic condition promotes destabilization of oil droplets, thus resulting in demulsification and formation of larger oil droplets on sorbent surface (Ahmad et al., 2005a).
On the other hand, high oil removal efficiency recorded at pH 8 and pH 10 for both SB and ESB was found to be inaccurate. This is because the high oil removal efficiency at strong basic condition
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is usually associated with the saponification process (Figure 6) where the hydrolysis of oil in POME occurs (Ahmad et al., 2005a). Hydrolysis produced free fatty acid COOH, whereby in the high pH region, COOH become CO-OH. The hydrolysis resulted in solvent non-extractable hydrolyzed oil in treated POME, which contributed to low oil and grease in treated POME and consequently leads to inaccurate high removal efficiency. This phenomenon is prone to occur in light and medium oil such as palm oil compared to heavy oil which contains mainly alkanes, as described in the previous study (Aranđelović et al., 2009).
3.4 Effect of temperature on oil removal efficiency
The effect of temperature was studied at 15 ºC - 60 ºC. Figure 7 shows that oil removal efficiency decreased as temperature increased. The oil removal is caused by an adsorption process and temperature dependence (Sayed and Zayed, 2006). The process of oil removal from POME performed most efficiently at low temperature (15 ºC), where SB and ESB gave an oil removal efficiency of 50.21% and 79.03%, respectively.
At high temperature such as 80ºC, oil adsorption by plant fibers tends to decrease due to several reasons. Firstly, the viscosity of oil decreases at elevated temperature, resulting in only monolayer oil adsorption on sorbent surface (Sayed and Zayed, 2006). Secondly, there is the possibility of fiber degradation at high temperature (Rajaković-Ognjanović et al., 2008). Thirdly, oil solubility in water increases with high temperature, causing a decrease in oil separation from water (Likon et al., 2013). Fourthly, the randomness of oil adsorption-desorption increases at elevated temperature and causes less probability of oil attachment on sorbent surface (Cao et al., 2011).
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Finally, high temperature causes an increase in Brownian motion of oil molecules, which decreased the tendency of oil adsorption onto fiber surface (Likon et al., 2013). At a higher energy of adhesion, it is less likely that the desorption occurs, especially at elevated temperatures (Aranđelović et al., 2009).
3.5 Diffusion model for batch oil adsorption
Figure 8 shows the intra-particle diffusion model of oil removal from POME using SB and ESB. The calculated values of Kp in SB and ESB were 22.51 and 26.88 respectively. Figure 8 showed that the adsorption of oil using ESB is relatively faster compared to SB. Further experiments at higher t1/2 were conducted to both SB and ESB, and the results are shown in Figure 9. It is found that at t1/2 higher than 8, the Kp was decreased.
The results are based on the adsorption process which occurred in two stages. The first stage (as shown in Figure 8), comprises the instantaneous adsorption stage. The second (normal) stage, (Figure 9), is the final equilibrium stage. In this stage, intraparticle diffusion started to slow down due to extremely low residue oil concentration in the POME sample (Ahmad et al., 2005). Overall, the results showed a good correlation (R2 > 9.5) between experimental data and the intra-particle diffusion model for both SB and ESB (Figure 8), justifying the adsorption mechanism. The value of effective diffusivities, Deff is determined from the slopes of the straight lines which are equal to 6 (Deff/ro2π)1/2 (Crank, 1975). For SB and ESB, the adsorption processes were 1.02 x 10-8 m2/s and 9.22 x 10-9 m2/s respectively.
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3.6 Reusability of ESB as oil sorbent The oil removal efficiency of ESB in POME is presented in Figure 10. The results shows that the oil removal efficiency of ESB was consistent up to the sixth cycle with 88-89% oil was removed from POME. Only after the sixth cycle the oil removal efficiency decreased to 55%. At the ninth and eleventh cycle the oil removal efficiency dropped to 44% and 33%, respectively. The results indicated that ESB has excellent reusability, which is an important characteristic for an oil sorbent.
4. Conclusion The present study showed that the ESB exhibits better oil removal efficiency compared to SB in all studied conditions, namely adsorbent dosage, contact time, temperature and pH. Overall, the oil removal efficiency of both SB and ESB increased by increasing the sorbent dosage and contact time. On the other hand, the oil removal efficiency of both SB and ESB decreased with the increasing temperature. Acidic pH was a favorable pH condition for high oil removal efficiency. Oil removal from POME by SB and ESB in the present study reached equilibrium at 60 min. 24 hr oil adsorption test afforded oil removal efficiency of 57.77% (SB) and 80.23% (ESB). On an overall basis, results showed a good correlation (R2 > 9.5) between experimental data and the intraparticle diffusion model for both SB and ESB. In conclusion, ESB showed better potential for use as a sorbent for removing residue oil from wastewater, particularly POME and further study on its application as a bed material in oil adsorption column system is worth investigated. On the other hand, POME contains various components and it is not known whether these components participate in the oil adsorption process or not. Therefore, further studies on the quantitative
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analysis of POME parameters such as COD, before and after adsorption, would be very helpful to understand the oil adsorption mechanism of sago bark in POME.
Acknowledgment: This
work
was
supported
by
the
Universiti
Malaysia
Sarawak
(Grant
No:
COESAR/PK07/07/2012(01)).
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hemicelluloses in ionic liquid using iodine as a catalyst. Carbohydr. Polym. 70, 406–414. doi:10.1016/j.carbpol.2007.04.022 Ribeiro, T.H., Rubio, J., Smith, R.W., 2003. A Dried Hydrophobic Aquaphyte as an Oil Filter for Oil/Water Emulsions. Spill Sci. Technol. Bull. 8, 483–489. doi:10.1016/S13532561(03)00130-0 Said, A.E.-A.A., Ludwick, A.G., Aglan, H.A., 2009. Usefulness of raw bagasse for oil absorption: a comparison of raw and acylated bagasse and their components. Bioresour. Technol. 100, 2219–22. doi:10.1016/j.biortech.2008.09.060 Sayed, S.A., Zayed, A.M., 2006. Investigation of the effectiveness of some adsorbent materials in oil spill clean-ups. Desalination 194, 90–100. doi:10.1016/j.desal.2005.10.027 Singhal, R.S., Kennedy, J.F., Gopalakrishnan, S.M., Kaczmarek, A., Knill, C.J., Akmar, P.F., 2008. Industrial production, processing, and utilization of sago palm-derived products. Carbohydr. Polym. 72, 1–20. doi:10.1016/j.carbpol.2007.07.043 Sokker, H.H., El-Sawy, N.M., Hassan, M. a, El-Anadouli, B.E., 2011. Adsorption of crude oil from aqueous solution by hydrogel of chitosan based polyacrylamide prepared by radiation induced graft polymerization. J. Hazard. Mater. 190, 359–65. doi:10.1016/j.jhazmat.2011.03.055 Suni, S., Kosunen, A., Hautala, M., Pasila, A., Romantschuk, M., 2004. Use of a by-product of peat excavation, cotton grass fibre, as a sorbent for oil-spills. Mar. Pollut. Bull. 49, 916–21. doi:10.1016/j.marpolbul.2004.06.015 Tolba, R., Wu, G., Chen, A., 2011. Adsorption of dietary oils onto lignin for promising pharmaceutical and nutritional applications. BioResources 6, 1322–1335. Vikineswary, S., Shim, Y.L., Thambirajah, J.J., Blakebrough, N., 1994. Possible microbial utilization of sago processing wastes. Resour. Conserv. Recycl. 11, 289–296. doi:10.1016/0921-3449(94)90096-5. Wahi, R., Abdullah, L.C., Ngaini, Z., Nourouzi, M.M., Choong, T.S.Y., 2014. Esterification of M. sagu bark as an adsorbent for removal of emulsified oil. J. Environ. Chem. Eng. 2, 324–331. Wahi, R., Chuah, L.A., Choong, T.S.Y., Ngaini, Z., Nourouzi, M.M., 2013. Oil removal from aqueous state by natural fibrous sorbent: An overview. Sep. Purif. Technol. 113, 51–63. doi:10.1016/j.seppur.2013.04.015 Wang, J., Zheng, Y., Wang, A., 2012. Effect of kapok fiber treated with various solvents on oil absorbency. Ind. Crops Prod. 40, 178–184. doi:10.1016/j.indcrop.2012.03.002 Wu, T.Y., Mohammad, A.W., Jahim, J.M., Anuar, N., 2010. Pollution control technologies for the treatment of palm oil mill effluent (POME) through end-of-pipe processes. J. Environ. Manage. 91, 1467–90. doi:10.1016/j.jenvman.2010.02.008 Wu, T.Y., Mohammad, a. W., Md. Jahim, J., Anuar, N., 2007. Palm oil mill effluent (POME) treatment and bioresources recovery using ultrafiltration membrane: Effect of pressure on membrane fouling. Biochem. Eng. J. 35, 309–317. doi:10.1016/j.bej.2007.01.029 Yang, Y., Yi, H., Wang, C., 2015. Supporting Information Oil Absorbents Based on Melamine / Lignin by a Dip Adsorbing. ACS Sustain. Chem. Eng. 3, 3012–3018. Yi, X.S., Yu, S.L., Shi, W.X., Sun, N., Jin, L.M., Wang, S., Zhang, B., Ma, C., Sun, L.P., 2011. 18
The influence of important factors on ultrafiltration of oil/water emulsion using PVDF membrane modified by nano-sized TiO2/Al2O3. Desalination 281, 179–184. doi:10.1016/j.desal.2011.07.056 Zhang, Y., Shen, Y., Chen, Y., Yan, Y., Pan, J., Xiong, Q., Shi, W., Yu, L., 2016. Hierarchically carbonaceous catalyst with Brønsted-Lewis acid sites prepared through Pickering HIPEs templating for biomass energy conversation. Chem. Eng. J. 294, 222–235. doi:10.1016/j.cej.2016.02.092
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Figure Captions
Figure 1. Experimental set up for ESB preparation
Figure 2. Effect of sorbent dosage on oil removal efficiency of SB and ESB in POME (Experimental conditions: 100 ml POME at 30 ºC (room temperature), pH= 4.18 (as received), contact time=30 min and mixing speed= 200 rpm)
Figure 3. Effect of contact time on oil removal efficiency of SB and ESB in POME (Experimental conditions: sorbent dosage= 2 g, temperature= 30ºC, pH= 4.18, mixing speed= 200 rpm)
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Figure 4. Adsorption-desorption process caused by the saturation effect at prolonged adsorption time
Figure 5. Effect of POME pH on oil removal efficiency of SB and ESB in POME (Experimental conditions: sorbent dosage= 2 g, contact time= 30 min, temperature= 30ºC, mixing speed= 200 rpm)
Figure 6. Saponification in alkalinized POME
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Figure 7: Effect of POME temperature on oil removal efficiency of SB and ESB in POME (Experimental conditions: sorbent dosage= 2 g, contact time= 30 min, pH= 4.18, mixing speed= 200 rpm)
Figure 8. Intra-particle diffusion model of oil removal from POME using (a) SB and (b) ESB in the first stage of adsorption process (Experimental conditions: sorbent dosage= 2g, 100 ml POME at 30ºC (room temperature), pH= 4.18 (as received), contact time= 60 min and mixing speed= 200 rpm).
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Figure 9. Intra-particle diffusion model of oil removal from POME using (a) SB and (b) ESB in the second stage of adsorption process (Experimental conditions: sorbent dosage= 2g, 100 ml POME at 30 ºC (room temperature), pH= 4.18 (as received), contact time= 60 min and mixing speed= 200 rpm).
Figure 10. Reusability of ESB.
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S2213-3437(16)30430-4 http://dx.doi.org/doi:10.1016/j.jece.2016.11.038 JECE 1351
To appear in: Received date: Revised date: Accepted date:
9-6-2016 24-11-2016 25-11-2016
Please cite this article as: Rafeah Wahi, Luqman Chuah Abdullah, Mohsen Nourouzi Mobarekeh, Zainab Ngaini, Thomas Choong Shean Yaw, Utilization of esterified sago bark fibre waste for removal of oil from palm oil mill effluent, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.11.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Utilization of esterified sago bark fibre waste for removal of oil from palm oil mill effluent Rafeah Wahia*, Luqman Chuah Abdullahb, Mohsen Nourouzi Mobarekehc, Zainab Ngainia and Thomas Choong Shean Yawb a
Department of Chemistry, Faculty of Resource Science and Technology, Universiti Malaysia
Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia b
Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti
Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia c
Department of Environment, Islamic Azad University of Isfahan (Khorasgan Branch), Isfahan,
Iran, 81595-158.
*Corresponding author: Rafeah Wahi, Department of Chemistry, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia; Email: [email protected]; Tel.: +6082-581000; Fax: +6082-583160 Highlights:
Esterified sago bark (ESB) was used for oil removal from palm oil mill effluent (POME).
ESB has superior oil removal properties than SB.
Esterification introduced long chains onto ESB surface for excellent oil uptake.
The maximum oil removal efficiency by ESB was 80.23% (at pH 4.18, 24 h, 30 °C).
Abstract With oil and grease content of 4000-8000 mg/l in palm oil mill effluent (POME), the commonly used ponding system often fails to produce treated effluent that meets the minimum standard of treated effluent. The present study investigates the efficiency of sago bark (SB) and esterified sago 1
bark (ESB) for removal of emulsified oil from POME. Oil removal experiments were conducted at different batch experimental conditions: namely adsorbent dosage, contact time, temperature and pH. In overall, the oil removal efficiency of both SB and ESB increased with the increasing of sorbent dosage and contact time. 24-h oil adsorption test afforded oil removal efficiency of 57.77% (SB) and 80.23% (ESB).On the other hand, the oil removal efficiency of both SB and ESB decreased with the increasing temperature. Acidic pH was favorable pH condition for high oil removal efficiency in POME. There was a good correlation (R2 > 9.5) between experimental data and the intra-particle diffusion model for both SB and ESB. The adsorption of oil in POME using SB was best described by Freundlich isotherm (R2=0.998), indicating heterolayer adsorption of oil on SB. The adsorption of oil in POME using ESB was better represented using Langmuir isotherm (R2=0.992), indicating a monolayer adsorption of oil onto the ESB surface. In conclusion, ESB showed better potential for use as sorbent for removing emulsified oil from wastewater, particularly POME.
Keywords: Sago bark, esterified sago bark, sago fibre, palm oil mill effluent, oil removal
1. Introduction One of the main sources of industrial oily wastewater in Malaysia is palm oil mill effluent (POME). POME is generated at significant level as a by-product during palm oil processing. In year 2008 alone, at least 44 million tonnes of POME was generated in Malaysia (Wu et al., 2010). The oil concentration in POME ranges between 4000-8000 mg/l (Ahmad et al., 2006; Ngarmkam et al., 2011). Most of the palm oil mills use conventional ponding system to treat POME. However, ponding system requires long treatment time and large area (Wu et al., 2010). Malaysian Department of Environment (DOE) has set a value of 50 mg/l as the maximum allowable limit for 2
oil and grease content in the effluent to be discharged in waterways. The ponding system quite often fails to produce treated effluent that meets the DOE standard (Chin et al., 1996). Thus, it is crucial for the palm oil industries to adapt an efficient treatment for oil removal.
Many technologies have been suggested for oil removal from water. Although ultrafiltration has a higher efficiency for the removal of oil, it is not suitable to be used in treating wastewater with high solid content such as POME due to the risk of premature membrane fouling (Wu et al., 2007; Yi et al., 2011). Other treatments like coagulation, flotation and biological treatment are either expensive, complex in operation or required highly skilled operators. Adsorption is the most preferred method due to its feasibility and effectiveness, provided an appropriate sorbent is used.
Natural fibres are potential sources of natural sorbent for removal of oil from POME. Raw natural sorbents generally has excellent adsorption capacity, comparable density with synthetic sorbent, chemical free and highly biodegradable (Annunciado et al., 2005; Rajaković-Ognjanović et al., 2008; Wang et al., 2012). Natural fibres comprise cellulose, hemicellulose, and lignin (Zhang et al., 2016), which is known responsible for oil adsorption (Tolba et al., 2011; Wahi et al., 2013; Yang et al., 2015). Examples of natural fibres used as oil sorbent are rice husk (Ali et al., 2012), kapok (Abdullah et al., 2010), barley straw (Ibrahim et al., 2010), sugarcane baggase (Said et al., 2009), sawdust (Cambiella et al., 2006) and grass (Suni et al., 2004).
Despite of their advantages, many natural fibres suffer low hydrophobicity and buoyancy, therefore are only suitable for oil removal in the absence of water (Ali et al., 2012). This is because most of the cellulose hydrophobic portions are covered by the hydroxyl groups, causing cellulose
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to behave more hydrophilic than hydrophobic (Said et al., 2009). To improve the oleophilicity and hydrophobicity of natural fibres, hydrophobization can be conducted by means of alkalization (Abdullah et al., 2010), chloroform treatment (Abdullah et al., 2010; Likon et al., 2013), acetylation (Adebajo and Frost, 2004; Ren et al., 2007), salt treatment (Wang et al., 2012), surfactant treatment (Ibrahim et al., 2010, 2009), combination of chemical-biotechnological treatment (Garcia-Ubasart et al., 2012), polymerization followed by ion-exchange process (Pan et al., 2016), and esterification (Banerjee et al., 2006; Said et al., 2009).
Sago, known scientifically as Metroxylon sagu Rottboll, comes from genus metroxylon and family palmae (Singhal et al., 2008). It is one of the potential natural fibers abundantly found in tropical lowland forest and freshwater swamps. In 2009, nearly 59,000 hectares of Sago is planted in Sarawak (Department of Agriculture Sarawak, 2009a). Sarawak is currently one of the world largest exporters of sago products with annual export of approximately 43,000 tonnes (Department of Agriculture Sarawak, 2009b, 2009c).
Every tonne of sago flour produced generates approximately 0.75 tonne of sago bark (SB) as a solid waste (Vikineswary et al., 1994). With this regards, SB is a potential source of natural fibre for oil removal. More than 85% of SB is left unutilized in sago processing mill. Common disposal of SB is via incineration, direct dumping into nearby rivers and natural degradation, which gives rise to environmental problems (Wahi et al., 2014).
In the work of Ngaini et al. (2014) and Noh et al. (2012), the SB was esterified using fatty acid derivatives to improve the oleophilicity and hydrophobicity of SB. It was found that the esterified
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SB (ESB) showed a good adsorption capacity on engine oil in water. Wahi et al. (2014) studied the optimization of the esterification process using stearic acid (SA). The optimum ESB synthesis conditions were at 1:1 SB: SA, 15 wt% CaO as catalyst and 8 h refluxing time. The application of ESB for removal of oil from POME, is however, a complex process. The high suspended solids and COD values, acidic pH, and moderate temperature (between 40-80 °C) of POME could affect the oil removal efficiency. The aforementioned factors made it crucial to carefully study the performance of ESB in removing oil from POME at various adsorption parameters before it could be applied at larger scale. Thus, this present study aims to investigate systematically the oil removal efficiency of ESB in POME via batch adsorption system at various adsorption parameters, namely the adsorbent dosage, contact time, POME temperature, and POME pH.
2. Material and Methods 2.1 Preparation of ESB and POME Shredded SB was collected from sago processing mill in Mukah, Sarawak. SB was ground and sieved into particle size range of 0.5 mm to 1.5 mm. Esterification of SB with fatty acid derivative was carried out using stearic acid (SA). SA was chosen due to the reason that it is a long-chain fatty acid with 18 carbon chains, with highly hydrophobic properties. Figure 1 shows the experimental set up for esterification of SB with SA. The ground SB (5 g) was placed in a round bottom flask containing ethyl acetate (50 mL). In this study, the SB to SA mass ratio was 1:1. Calcium oxide (0.75 g) was added to the mixture, to expedite the esterification process (Wahi et al., 2014). The mixture was heated under reflux for 8 hours. The resulting ESB was cooled down to room temperature, filtered, washed with ethyl acetate and dried in a desiccator prior to use. The hydrophobicity of ESB was measured based on Ribeiro, Rubio, & Smith (2003). The degree of
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esterification was estimated by calculating the ratio, R, between the intensity of C=O (ester peak) and C-O (cellulose backbone) in infrared spectra, as described in Adebajo & Frost (2004). Total pore volume was measured using BET Analyser (Quantachrome® ASiQwinTM). The hydrophobicity, average C=O:C-O intensity ratio, and total pore volume of ESB was 50.2 ± 9.7%, 1.251 ± 0.213, and 0.012 cm3/g, respectively. Full physicochemical characteristics of ESB including the surface functional groups and surface morphology has been described in detail in our previous work (Wahi et al., 2014).
POME was collected from FELCRA palm oil mill in Kota Samarahan, Sarawak. The POME sample was collected from the drain coming from the mill before entering the first treatment pond, whereby the temperature was around 40ºC (raw POME). The POME was filtered through a muslin cloth strainer to remove solid particles of millimeter size. The oil and grease content, total suspended solid content and pH of the POME sample was 4850 mg/L, 15,200 mg/L and 4.18 respectively. The POME sample was stored at 4°C before use.
2.2 Oil adsorption study The batch adsorption study was conducted using SB and ESB at different sorbent dosage, contact time, pH and temperature using rotary shaker (Luckham R100/TW). The effect of sorbent dosage was examined using 0.5, 1, 2, 3 and 4 g SB and ESB in 100 ml POME at 30ºC, POME, having pH of 4.18, 30 min of contact time and mixing speed of 200 rpm. The effect of contact time was studied at 10, 20, 30, 40, 50, 60, 90, 120 and 1440 min contact time (sorbent dosage: 2 g, temperature: 30 ºC, pH: 4.18, mixing speed: 200 rpm). Effect of pH was studied at pH 2, 4.18, 7, 8 and 10 (sorbent dosage: 2 g, contact time: 30 min, temperature: 30ºC, mixing speed: 200 rpm).
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Effect of temperature was studied at 15, 20, 30, 40 and 60ºC (sorbent dosage= 2 g, contact time= 30 min, pH= 4.18, mixing speed= 200 rpm). The oil and grease analysis method was adapted from Standard Methods for the Examination of Water and Wastewater, Section 5520B (Clesceri et al., 1999).
2.3 Reusability study Reusability study was conducted to evaluate the performance of ESB in removing oil from POME after certain number of cycle. The reusability experiment was conducted by placing 2 g of ESB in 100 mL of POME at room temperature. The mixture was shaken for 30 minutes at 200 rpm speed. The mixture was filtered to collect the filtrate for oil and grease analysis. The sorbent was drained for 30 minutes before commencing next cycle of batch oil removal. The steps were repeated for 15 cycles. The oil removal efficiency for each cycle was recorded.
2.4 Diffusion model for batch oil adsorption
In order to determine the adsorption mechanism during removal of oil from POME using ESB, the intra-particle diffusion model was integrated. The general representation of Weber and Morris intraparticle model is shown in Equation 1:
qt = Kpt1/2
(Equation 1)
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where qt is the concentration of oil adsorbed at time t (mg/g), Kp is the intra-particle diffusion rate constant (g/ g min-1/2). The equation describes the time evolution of the concentration in adsorbed state, where Kp is the slope of linear plot qt versus t1/2.
Assumptions are important prior to an employment of a model to experimental data. The following assumptions were made during the usage of Weber and Morris intraparticle model to describe oil adsorption from POME using SB and ESB in this study: 1) The Fick’s law is applicable in both liquid phase diffusion and solid phase diffusion. 2) The particles were considered in the isotropic medium. 3) Intraparticle diffusion controls the reaction or particle kinetics. 4) Pore and surface diffusivities were constant during the adsorption process. 5) Diffusion coefficients were not concentration dependent. 6) The bulk phase concentration of oil was not constant during the adsorption process.
3. Results and Discussion
There are many factors governing the oil adsorption process and the oil removal efficiency of a sorbent. In batch oil adsorption system, the common parameters studied were sorbent dosage, contact time, influent temperature and influent pH.
3.1 Effect of sorbent dosage on oil removal efficiency
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The influence of sorbent dosage on oil removal efficiency of SB and ESB in POME were investigated employing 0.5 g - 4 g SB and ESB in 100 ml of POME. Figure 2 shows that ESB afforded higher oil removal efficiency than SB. The high oil removal efficiency of ESB was due to several reasons. One of the reasons was due to extensive modification of the ESB surface via esterification which afforded rough surfaces and exposed internal structures thus increased the oil entrapment sites (Ibrahim et al., 2010). It is envisaged that the site for oil entrapment in SB and ESB was different. During oil removal from POME, oil was attached physically to the SB surface. In contrast, Attachment of oil to ESB surface is due to the hydrophobic tails available on the surface of ESB after esterification process, due to substitution of the –OH groups in ESB with hydrophobic groups in SA (Wahi et al., 2014). Another reason was due to the negative zeta potential of oil droplets in POME which was neutralized by calcium ions of calcium oxide in ESB. The neutralization of charge resulted in a reduction of the electrostatic repulsion and consequently enhanced oil droplets coalescence (Cambiella et al., 2006). However, the lower oil removal efficiency of SB was due to the tendency of SB to form hydrogen bonding with the aqueous phase in wastewater such as POME.
Overall, the oil removal efficiency increased as sorbent dosage was increased up to 4 g per 100 ml of POME whereby the oil removal efficiency was 54.7% (SB) and 66.8% (ESB). The phenomenon was associated with an increase in available binding sites for adsorption in higher sorbent dosage (Ahmad et al., 2005a). The oil removal efficiencies of SB and ESB in POME were lower than the previously reported sorbents. For example, the use of chitosan (Ahmad et al., 2005a), palm kernel shell magnetic composite (Ngarmkam et al., 2011) and commercial activated carbon (Ahmad et al., 2005b) afforded 90% - 99% oil removal efficiencies compared to SB (58.6%) and ESB
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(66.8%). However, the contact hours of the sorbents were longer than SB and ESB, which was 1 24 hr. Therefore, further investigation on the effect of longer contact time on oil removal efficiency of SB and ESB in POME were investigated.
3.2 Effect of contact time on oil removal efficiency
The effect of contact time on the oil removal efficiency of SB and ESB in POME were conducted to investigate the oil adsorption behavior in 10 - 120 min (Figure 3). The oil removal efficiency of SB and ESB was increased as the contact time increased.
Based on the slopes shown in Figure 3, the oil removal from POME by SB and ESB occurred in two phases. A very fast adsorption phase occurred during the first 40 min and a slow phase between 40 - 60 min. The amount of oil adsorbed in mg per gram of sorbent, qe was stabilized after 60 min and the constant oil removal efficiency reading indicates that the oil adsorption has reached equilibrium (Figure 3).
The minimum of 10 min contact time was sufficient to allow a small percentage of oil removal using SB and ESB. After 30 min, SB and ESB afforded 39.6% and 53.46% oil removal efficiencies respectively. The oil removal efficiency was increased to 67.62% (SB) and 83.81% (ESB) after 1 hr. However, there was a slight decrease in oil removal efficiency of both SB and ESB at 90 min contact time (SB: 57.45%, ESB: 80.98%).
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The oil removal efficiency of SB and ESB remained constant after 90 min contact time. Further study was also carried out by increasing the contact time up to 1440 min (not shown in Figure 3) which afforded oil removal efficiency of 57.77% (SB) and 80.23% (ESB). The results indicated that no significant changes were observed in oil removal efficiency of SB and ESB even though in longer contact time.
It was envisaged that during the first hour of the oil adsorption study, the increase in contact time has resulted in an increased oil-sorbent interaction. The interaction phenomena have increased the amount of oil attached to the sorbent and resulted in an increase in oil removal efficiency (Ahmad et al., 2005a). The constant oil removal efficiency of SB and ESB after 90 min contact time was attributed to the saturation effect (Rajaković-Ognjanović et al., 2008). It was due to the adsorptiondesorption process that occurred at the saturated sorbent surfaces, whereby the attached oil was released and re-attached interchangeably, causing oil removal efficiency reading to be reduced or constant (Figure 4). The contact time plays an important role only at the beginning of the oil adsorption activity and is less important near to equilibrium (Ahmad et al., 2005a; Ibrahim et al., 2009; Sokker et al., 2011). When near to equilibrium, only small increase in oil removal was observed due to limited availability of sorbent surfaces for oil entrapment (Sokker et al., 2011). The findings obtained in this study were in agreement with previously reported studies on oil removal from POME using chitosan, bentonite and activated carbon. The oil removal efficiency also remained constant after 30 - 40 min with maximum oil removal efficiency between 60% 99% (Ahmad et al., 2005a, 2005b).
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3.3 Effect of pH on oil removal efficiency
The study on the effect of pH during oil removal is of great importance due to the fact that the changes in pH could affect the surface properties and binding sites of the sorbent (Ibrahim et al., 2009) and emulsion breaking (Sokker et al., 2011). It is also interesting to note that different sorbents behave differently towards pH change during oil removal (Ahmad et al., 2005a; Aranđelović et al., 2009; Ibrahim et al., 2009; Rajaković-Ognjanović et al., 2008; Sokker et al., 2011).
The effect of POME pH on oil removal efficiency was studied at pH 2 - 10, and the results are illustrated in Figure 5. In general, ESB showed higher oil removal efficiency compared to SB. SB and ESB gave a similar trend, whereby higher oil removal efficiencies were found both at acidic and basic POME compared to neutral pH of POME (pH 7). The oil removal efficiency of SB and ESB was increased in the order of neutral condition < strongly basic condition < strongly acidic condition. A similar pattern was also demonstrated by chitosan (Ahmad et al., 2005a; Sokker et al., 2011) and recycled wool-based nonwoven (Rajaković-Ognjanović et al., 2008) during removal of various types of oil. The highest oil removal efficiency for both SB and ESB were recorded at pH 2 (SB: 48.17% and ESB: 68.49%). This was due to the fact that strong acidic condition promotes destabilization of oil droplets, thus resulting in demulsification and formation of larger oil droplets on sorbent surface (Ahmad et al., 2005a).
On the other hand, high oil removal efficiency recorded at pH 8 and pH 10 for both SB and ESB was found to be inaccurate. This is because the high oil removal efficiency at strong basic condition
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is usually associated with the saponification process (Figure 6) where the hydrolysis of oil in POME occurs (Ahmad et al., 2005a). Hydrolysis produced free fatty acid COOH, whereby in the high pH region, COOH become CO-OH. The hydrolysis resulted in solvent non-extractable hydrolyzed oil in treated POME, which contributed to low oil and grease in treated POME and consequently leads to inaccurate high removal efficiency. This phenomenon is prone to occur in light and medium oil such as palm oil compared to heavy oil which contains mainly alkanes, as described in the previous study (Aranđelović et al., 2009).
3.4 Effect of temperature on oil removal efficiency
The effect of temperature was studied at 15 ºC - 60 ºC. Figure 7 shows that oil removal efficiency decreased as temperature increased. The oil removal is caused by an adsorption process and temperature dependence (Sayed and Zayed, 2006). The process of oil removal from POME performed most efficiently at low temperature (15 ºC), where SB and ESB gave an oil removal efficiency of 50.21% and 79.03%, respectively.
At high temperature such as 80ºC, oil adsorption by plant fibers tends to decrease due to several reasons. Firstly, the viscosity of oil decreases at elevated temperature, resulting in only monolayer oil adsorption on sorbent surface (Sayed and Zayed, 2006). Secondly, there is the possibility of fiber degradation at high temperature (Rajaković-Ognjanović et al., 2008). Thirdly, oil solubility in water increases with high temperature, causing a decrease in oil separation from water (Likon et al., 2013). Fourthly, the randomness of oil adsorption-desorption increases at elevated temperature and causes less probability of oil attachment on sorbent surface (Cao et al., 2011).
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Finally, high temperature causes an increase in Brownian motion of oil molecules, which decreased the tendency of oil adsorption onto fiber surface (Likon et al., 2013). At a higher energy of adhesion, it is less likely that the desorption occurs, especially at elevated temperatures (Aranđelović et al., 2009).
3.5 Diffusion model for batch oil adsorption
Figure 8 shows the intra-particle diffusion model of oil removal from POME using SB and ESB. The calculated values of Kp in SB and ESB were 22.51 and 26.88 respectively. Figure 8 showed that the adsorption of oil using ESB is relatively faster compared to SB. Further experiments at higher t1/2 were conducted to both SB and ESB, and the results are shown in Figure 9. It is found that at t1/2 higher than 8, the Kp was decreased.
The results are based on the adsorption process which occurred in two stages. The first stage (as shown in Figure 8), comprises the instantaneous adsorption stage. The second (normal) stage, (Figure 9), is the final equilibrium stage. In this stage, intraparticle diffusion started to slow down due to extremely low residue oil concentration in the POME sample (Ahmad et al., 2005). Overall, the results showed a good correlation (R2 > 9.5) between experimental data and the intra-particle diffusion model for both SB and ESB (Figure 8), justifying the adsorption mechanism. The value of effective diffusivities, Deff is determined from the slopes of the straight lines which are equal to 6 (Deff/ro2π)1/2 (Crank, 1975). For SB and ESB, the adsorption processes were 1.02 x 10-8 m2/s and 9.22 x 10-9 m2/s respectively.
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3.6 Reusability of ESB as oil sorbent The oil removal efficiency of ESB in POME is presented in Figure 10. The results shows that the oil removal efficiency of ESB was consistent up to the sixth cycle with 88-89% oil was removed from POME. Only after the sixth cycle the oil removal efficiency decreased to 55%. At the ninth and eleventh cycle the oil removal efficiency dropped to 44% and 33%, respectively. The results indicated that ESB has excellent reusability, which is an important characteristic for an oil sorbent.
4. Conclusion The present study showed that the ESB exhibits better oil removal efficiency compared to SB in all studied conditions, namely adsorbent dosage, contact time, temperature and pH. Overall, the oil removal efficiency of both SB and ESB increased by increasing the sorbent dosage and contact time. On the other hand, the oil removal efficiency of both SB and ESB decreased with the increasing temperature. Acidic pH was a favorable pH condition for high oil removal efficiency. Oil removal from POME by SB and ESB in the present study reached equilibrium at 60 min. 24 hr oil adsorption test afforded oil removal efficiency of 57.77% (SB) and 80.23% (ESB). On an overall basis, results showed a good correlation (R2 > 9.5) between experimental data and the intraparticle diffusion model for both SB and ESB. In conclusion, ESB showed better potential for use as a sorbent for removing residue oil from wastewater, particularly POME and further study on its application as a bed material in oil adsorption column system is worth investigated. On the other hand, POME contains various components and it is not known whether these components participate in the oil adsorption process or not. Therefore, further studies on the quantitative
15
analysis of POME parameters such as COD, before and after adsorption, would be very helpful to understand the oil adsorption mechanism of sago bark in POME.
Acknowledgment: This
work
was
supported
by
the
Universiti
Malaysia
Sarawak
(Grant
No:
COESAR/PK07/07/2012(01)).
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Figure Captions
Figure 1. Experimental set up for ESB preparation
Figure 2. Effect of sorbent dosage on oil removal efficiency of SB and ESB in POME (Experimental conditions: 100 ml POME at 30 ºC (room temperature), pH= 4.18 (as received), contact time=30 min and mixing speed= 200 rpm)
Figure 3. Effect of contact time on oil removal efficiency of SB and ESB in POME (Experimental conditions: sorbent dosage= 2 g, temperature= 30ºC, pH= 4.18, mixing speed= 200 rpm)
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Figure 4. Adsorption-desorption process caused by the saturation effect at prolonged adsorption time
Figure 5. Effect of POME pH on oil removal efficiency of SB and ESB in POME (Experimental conditions: sorbent dosage= 2 g, contact time= 30 min, temperature= 30ºC, mixing speed= 200 rpm)
Figure 6. Saponification in alkalinized POME
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Figure 7: Effect of POME temperature on oil removal efficiency of SB and ESB in POME (Experimental conditions: sorbent dosage= 2 g, contact time= 30 min, pH= 4.18, mixing speed= 200 rpm)
Figure 8. Intra-particle diffusion model of oil removal from POME using (a) SB and (b) ESB in the first stage of adsorption process (Experimental conditions: sorbent dosage= 2g, 100 ml POME at 30ºC (room temperature), pH= 4.18 (as received), contact time= 60 min and mixing speed= 200 rpm).
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Figure 9. Intra-particle diffusion model of oil removal from POME using (a) SB and (b) ESB in the second stage of adsorption process (Experimental conditions: sorbent dosage= 2g, 100 ml POME at 30 ºC (room temperature), pH= 4.18 (as received), contact time= 60 min and mixing speed= 200 rpm).
Figure 10. Reusability of ESB.
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