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Yusof, N. A., Mukhair, H., Malek, E. A., and Mohammad, F. (2015). "Esterified coconut coir by fatty acid chloride as biosorbent in oil spill removal," BioRes. 10(4), 8025-8038.

Abstract

Coconut coir, an agricultural waste, was chemically modified using esterification by fatty acid chloride (oleoyl chloride and octanoate chloride) for oil spill removal purposes. The modified coir (coir-oleate and coir-octanoate) were characterized by spectroscopy, thermal studies, contact angle, and morphological studies. The modified coir exhibited an enhancement towards the hydrophobic property but a decreased thermal stability. The oil adsorption performance was tested using a batch adsorption system. The effect of sorbent dosage, oil concentration, and effect of adsorption time on the adsorption capacity of the modified coir were also studied. From the analysis, the long chain oleoyl chloride (C18) was shown to be a better modifier compared to octanoate chloride (C8). The isotherm study indicated that the oil adsorption fitted well to a Langmuir model rather than Freundlich model. From the kinetic study, the result revealed a good fit in pseudo-second order model for all samples studied. The study therefore suggests that esterified coconut coir can serve as a potential biomaterial for the adsorption of spilled oil during operational failures.


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Esterified Coconut Coir by Fatty Acid Chloride as Biosorbent in Oil Spill Removal

Nor Azah Yusof,a,b,* Hayati Mukhair,b Emilia Abd. Malek,b and Faruq Mohammad a

Coconut coir, an agricultural waste, was chemically modified using esterification by fatty acid chloride (oleoyl chloride and octanoate chloride) for oil spill removal purposes. The modified coir (coir-oleate and coir-octanoate) were characterized by spectroscopy, thermal studies, contact angle, and morphological studies. The modified coir exhibited an enhancement towards the hydrophobic property but a decreased thermal stability. The oil adsorption performance was tested using a batch adsorption system. The effect of sorbent dosage, oil concentration, and effect of adsorption time on the adsorption capacity of the modified coir were also studied. From the analysis, the long chain oleoyl chloride (C18) was shown to be a better modifier compared to octanoate chloride (C8). The isotherm study indicated that the oil adsorption fitted well to a Langmuir model rather than Freundlich model. From the kinetic study, the result revealed a good fit in pseudo-second order model for all samples studied. The study therefore suggests that esterified coconut coir can serve as a potential biomaterial for the adsorption of spilled oil during operational failures.

Keywords: Modified coconut coir; Esterification; Natural fiber; Oil spill

Contact information: a: Institute of Advance Technology (ITMA), Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia; b: Department of Chemistry, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia;

* Corresponding author: azahy@upm.edu.my; Tel.: +6-03-8946-6782; Fax: +6-03-8943-5380.

INTRODUCTION

In recent years the contamination of sea water by oil spills has attracted a great deal of interest, and such interest was heightened due to the latest tragic incident that occurred in the Gulf of Mexico on the British Petroleum (BP)-operated Macondo Prospect. For such incidents in general, the water pollution caused by the oil spill leaves damage to the quality of human health and the environment, disturbs the marine life ecosystem, in addition to causing serious adverse economic impact on the life of people depended on fisheries, agriculture, and tourism (Lim and Huang 2007). The main reasons for these oil spills at sea are human mistakes and carelessness of daily activities during the oil drilling operations, oil storage tankers, runoff from offshore oil explorations and productions, and spills from tanker loading and unloading operations (Li et al. 2013). By keeping in view the serious impacts due to oil spill-resultant water contamination, it is therefore very important to remediate the spilled oil as quickly as possible to avoid further damage to the ecosystem.

Some commercial methods for oil spill treatment such as booms, dispersants, skimmers, oil-water separators, and sorbent materials have been developed; however, a majority of these approaches are either costly or less environmental friendly or both. For example, the use of dispersants during oil spill treatment generates toxic reactions that may fail to balance the life in the ecosystem due to the usage of harmful chemicals linked to this process. In some methods, the treatment may cause the oil to sink by increasing its density, which makes the oil difficult to collect (Chapman et al. 2007). Besides these, the use of sorbents for oil spill removal has the advantages of making the oil separation process in an economical, efficient, and environmental friendly way; however, the use of this approach during the cases where thick layers of floating oils needs to be removed still remains a challenge (which is ideal for use of a skimmer and density-based separation) (Hubbe et al. 2013). In general, the sorbent materials synthesized from non-renewable petroleum based sources are widely used in floating booms for the containments assortment and for the collection of spilled oils. During this process, the addition of sorbent material concentrates and transforms the liquid oil to semisolid phase that can be removed easily from the surface of water (Adebajo and Frost 2004; Ali et al. 2012). They can be used to recover the oil through the mechanisms of absorption, adsorption or both. The other benefit of using a sorbent is its ease of handling, fast oil uptake, and the capability to capture and retain oil for retrieval at a later time (Parab et al. 2010).

Generally, the three major classes of oil sorbents include the inorganic mineral products, organic synthetic products, and organic natural products. Recently, the commercial sorbents that are widely used contains organic synthetic products such as polypropylene (PP) and polyurethane due to their oleophilic-hydrophobic properties (Lim and Huang 2007). However, the limitation for this sorbent material is that they are non-biodegradable and can be difficult to handle after their usage. The mineral products used in oil sorbents include perlite, vermiculite, and diatomite, as most of them have low buoyancy and oil sorption capacity (Warr et al. 2009). Thus, due to the limited availability of synthetic organic and mineral product sorbents, many researchers have embarked on a quest for alternative natural sorbent materials derived from agricultural waste products as a sorbent for oil spill treatment purposes.

Lignocellulose (mainly celluloses, hemicelluloses, and lignin) constitutes a renewable raw material that is widely available in the form of waste from agricultural residues such as sugarcane bagasse, cereal straw, peat moss, wool, rice hulls, coconut coir, and corn cob (Lundqvist et al. 2002). Because lignocellulose has high oil adsorption capacity, and is biodegradable, renewable, environmental friendly, inexpensive, and is easily available, it has been a subject of interest among researchers to further reveal its capability as oil sorbents. The organic sorbents can adsorb the oil up to 15 times their weight, but the limitation of these materials is their tendency to adsorb both water as well as oil, causing them to sink. This happens due to the presence of the hydroxyl group in the cellulose and hemicelluloses on the polymer backbone. This particular functional group is responsible mainly for the hygroscopic properties of lignocellulosic materials and these defects can however be reduced considerably by some chemical modification. One approach is by the replacement of the hydroxyl functional group from cellulose, hemicelluloses, and lignin present at the polymeric backbone with more hydrophobic groups by means of chemical reactions (Sun et al. 2004).

One form of lignocellulose material, coconut coir, is a fibrous layer outside the coconut shell and is one of the largest waste products of the copra industry. Coir is used in manufacturing of soil-treatment fibers, rope, and doormats around the world. Although coconut coir has been commercially available for many years, its capability as oil sorbent is still limited. Therefore, herein we chose coconut coir as a model biosorbent and studied its potential for removing oil from water. We carried out the esterification of coir by using fatty acid chloride in order to increase the oil adsorption capacity and to increase the hydrophobic properties of the material. Thus, chemically treated coir was thoroughly characterized by Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), thermal analysis, and static contact angle measurements. In addition, the oil adsorption studies were carried out to determine changes in sorbed amounts and adsorption rates. The system was further analyzed relative to kinetic studies and isotherm models.

EXPERIMENTAL

Biosorbent Preparation

The coconut coir raw material used in this study originated from Sungai Tengi Kiri Village, Selangor, Malaysia. The raw coir was ground into small and short shapes in a high speed grinder, and the visible dust/impurities were removed by washing and boiling with distilled water. Following this, the fiber was washed several times with acetone before drying in the oven at 60 °C overnight. To ensure the freshness of the raw materials, no further treatment was carried out.

Esterification of Coir by Fatty Acid Chloride

The esterification reaction of coir was carried out in a reflux setup maintained under a fume hood. For that, approximately 5 g of coir was weighed and placed in a 250 mL three necked round bottom flask. To this, 100 mL of 1% (v/v) N-bromosuccinimide (NBS) catalyst prepared in a DMaC/LiCl solvent system was added and heated up to 100 °C with stirring. To this mixture, 10% (v/v) concentration of oleoyl chloride was added drop-wise, and the reaction was allowed to proceed continuously for 4 h under the same conditions of temperature and stirring. After the period during which the esterification process was completed, the acylated fiber was filtered and washed with a series of solvents including toluene, ethanol, water, and acetone to remove any unreacted oleoyl chloride and unwanted byproducts. Then the fiber was dried to a constant weight in an oven at 70 °C. The same synthetic process was repeated for the esterification of the coir by means of octanoyl chloride.

Oil Adsorption Study

The adsorption study was carried out in a batch system, and for that, the crude oil (engine oil) was mixed with 100 mL of water and stirred for 5 min at 120 rpm at room temperature. After the stirring, when oil was observed to be floating as a layer on the water surface, the modified coir (formed from 30% oleoyl chloride) was added to the oil/water mixture. The mixture was then shaken for 5 min at room temperature and the modified coir was then removed from the mixture using a mesh screen and drained for 1 min. The oil sorption capacity (Q, g-oil/g-sorbent) was calculated according to Wang et al. (2013), as shown in Eq. 1,

 (1)

where Ma (g) is the weight of wet modified coir after the adsorption, Mi (g) is the initial weight of modified coir, and Mw is the weight of water adsorbed by the modified coir. The amount of water adsorption was determined by the extraction-separation method using n-hexane as a solvent. All the tests were done in triplicates.

Instrumental Analysis

The functional group analysis in raw coir and esterified coir was performed by using FTIR spectroscopy (Perkin Elmer Spectrum 100 Series), and the spectra were recorded in the range of 400 to 4000 cm-1 wavenumbers. The thermal stability of the modified coir was determined by thermogravimetric (TGA) and derivative thermogravimetric (DTG) analyses, where the sample weight loss was monitored as a function of temperature by making use of Perkin-Elmer Thermogravimetry Analyzer TGA7. The samples were analyzed in a temperature range of 25 to 600 °C and at a heating rate 10 °C/min with nitrogen flow rate of 50 mL/min. For analyzing the surface morphology of the samples, a field emission scanning electron microscope model LEO 1455 VPFESEM was used. The sample surface was coated with gold by a Bio-rad coating system, and the scanning electron micrographs (SEM) were recorded at a magnification of 10000X. The static contact angle measurements were used to identify the solid-liquid phase interactions between sorbents and water. In this study, video contact angle ganiometer (vca 3000s) from Vistec Technology was used to analyze the samples.

RESULTS AND DISCUSSION

Physical Observation of Modified Coconut Coir

The hydrophobicity of the modified coir was initially evaluated by immersing it into a two immiscible solvent mixture of water (bottom) and hexane (top). The set-up is shown in Fig. 1. As one can see from the figure, the raw coir tended to be more homogenous in the water phase than in hexane, and this is attributable to the presence of hydroxyl groups at the polymer’s backbone, which by means of hydrogen bonding tends to be dispersible in water. The same coir on chemical modification, i.e. coir-oleate exhibited a more hydrophobic character as indicated by its migration and tendency to get remain in the hexane phase. The observation of such property may be due to the formation of hydrophobic ester bonds onto the coir by the substitution of hydroxyl groups of the polymer backbone. However, with an increase in the concentration of oleoyl chloride, the homogeneity of coir-oleate tended to be decreasing, i.e. the 10% oleoyl chloride product was more homogeneous in hexane phase than the corresponding 20% and 30% oleoyl chloride reaction products. The reason for this decreased homogeneity is likely due to the increased carbon content by means of esterification onto the surface of coir polymer, thereby promoting for the precipitation in the same hexane phase with a greater rate in the case of the 30% oleoyl chloride (Fig. 1d) than the 20% oleoyl chloride product (Fig. 1c). Similarly, the octanoate modified coir (Fig. 1e) also dispersed in hexane phase through the formation of hydrophobic octanoic groups and the presence of short chain C8 (compared to long chain oleoly of C18) surface groups is the reason for its dispersity in hexane solvent without any precipitation.

C:\Users\Admin ITMA\Desktop\Figure-1.tif

Fig. 1. Effect of fatty acid chloride concentration towards the hydrophobic character of coir where (a) raw coir, (b) coir-oleate with 10% oleoyl chloride, (c) coir-oleate with 20% oleoyl chloride, (d) coir-oleate with 30% oleoyl chloride, and (e) coir-octanoate

FTIR Analysis of the Modified Coir

The FTIR spectroscopic analysis was performed so as to determine the nature of chemical bonding in raw coir and the modified coirs (coir-oleate). The FTIR spectra of raw coir and coir-oleate when treated with different oleoyl chloride concentrations, different NBS concentration, and different reaction temperatures are illustrated in Figs. 2, 3, and 4, respectively.

Effect of oleoyl chloride concentration

The comparison of FTIR spectra of raw coir with those of coir-oleate (various concentrations) and coir-octanoate is shown in Fig. 2(a-e). From the figure, the presence of absorption peaks at 3372 cm-1, 2914 cm-1, 1638 cm-1, 1385 cm-1, 1241 cm-1, and 1068 cm-1 in the raw coir (Fig. 2a) spectrum was associated with raw coconut coir. The broad peak at 3372 cm-1 originated from O-H stretching of the hydroxyl group. The narrowing of hydroxyl band around 3374 to 3329 cm-1 for the treated coir samples (Fig. 2b-2d) suggests the occurrence of partial esterification. In general, the broadness of hydroxyl band around 3300 to 3500 cm-1 is related to the inter- and intra-molecular hydrogen bonding in polysaccharides; it can be concluded from the present case that the narrowing of this band is also attributable to a lower degree of hydrogen bonding (Calado et al.2000). Similarly, the peak around 2914 cm-1 indicates the asymmetric and symmetric stretching vibration of CH2 and CH3, and the strong band at 1068 cm-1 is attributed to C-O stretching in cellulose, hemicelluloses, and lignin (Rana et al. 1996). The peak at 1457 cm-1in the raw coir represents the aromatic C=C stretch of aromatic ring of lignin, and the bands at 1241 cm-1 and 1385 cm-1 are assigned to the C-O and C-H bending vibrations respectively (Wang et al. 2013). The peak at 1638 cm-1 can be assigned to C=C alkene stretching of lignocelluloses material.

After the modification reaction, the existence of (C=O) carbonyl ester peak in coir-oleate FTIR spectra at 1729 cm-1 indicates that the esterification had occurred. Besides that, the decrease in (O-H) hydroxyl peak intensity also showed that some of the hydroxyl groups were substituted by long chain acyl groups. As the concentration of oleoyl chloride increased, the vibration signal of C=O ester showed higher intensity and is an indication of a formation of more ester bonds. The high intensity of CH2 and CH3 peaks at 2915 cm-1 also indicates an increase of alkyl groups in coir-oleate. The coir-octanoate (Fig. 2e) showed a low vibration signal of C=O compared to coir-oleate, an indication of a low concentration of coir ester product being formed.

Figure-2

Fig. 2. FTIR spectra of (a) raw coir, (b) coir-oleate in 10% (v/v) oleoyl chloride, (c) coir-oleate in 20% (v/v) oleoyl chloride, (d) coir-oleate in 30% (v/v) oleoyl chloride, and (e) coir-octanoate in 10% (v/v) octanoate chloride

Effect of N-bromosuccinimide (NBS) catalyst concentration

The comparison of the FTIR spectra of coir-oleate towards NBS concentration is shown in Fig. 3(a-d). From the figure, the NBS’s concentration was observed to have some effect on the esterification of coir, i.e. as the concentration of NBS reached 3% (w/v), the C=O carbonyl ester peak at 1739 cm-1 became intense when compared to reaction carried without the NBS catalyst (Fig. 3a). Besides that, the intense peak of C-H alkyl group around 2915 cm-1 in Fig. 3d also indicates the increase of alkyl groups after the coir’s modification reaction.