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Original Article
DOI: 10.1016/j.jmrt.2018.03.002
Development of amino-functionalized silica nanoparticles for efficient and rapid removal of COD from pre-treated palm oil effluent
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Syed Salman Shafqata, Amir Azam Khanb, Muhammad Nadeem Zafara,
Corresponding author
, Mohammed Haji Alhajic,d, Khairuddin Sanaullahd, Syed Rizwan Shafqate, Shahzad Murtazaa, Suh Cem Pange
a Department of Chemistry, University of Gujrat, Pakistan
b Department of Mechanical and Manufacturing Engineering, Universiti Malaysia Sarawak (UNIMAS), Malaysia
c Nigerian Institute of Leather and Science Technology, Zaria, Nigeria
d Department of Chemical Engineering and Energy Sustainability, Universiti Malaysia Sarawak (UNIMAS), Malaysia
e Department of Chemistry, Universiti Malaysia Sarawak (UNIMAS), Malaysia
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Received 22 December 2017, Accepted 20 March 2018
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Tables (2)
Table 1. Langmuir, Freundlich and Temkin isotherms parameters for COD adsorption.
Table 2. Comparison of COD removal by various adsorbents.
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Abstract

In this study, low cost synthesis of three-dimensional mesoporous amino-functionalized silica nanoparticles (AFS-NPs) were synthesized by sol–gel method with the addition of two different silica modifiers, 3-aminopropyltrimethoxysilane (APTMS) and 3-aminopropyltriethoxysilane (APTES) to two different silica sources, tetramethoxyorthosilicate (TMOS) and tetraethoxyorthosilicate (TEOS) separately. The presence of amino group on synthesized nanoparticles from the four different combinations was confirmed by Fourier transform infrared (FTIR) spectroscopy. The synthesized AFS-NPs were of narrow particle size within the range of 500–600nm and having high specific surface area (148m2g−1). The AFS-NPs were then utilized to investigate the adsorptive removal of COD from palm oil mil effluent (POME). The maximum removal efficiency (>90%) of AFS-NPs was found at pH 7, adsorbent dose 0.25g and temperature 30°C. At equilibrium the maximum adsorption capacity was found to be 271.11mgg−1. The Langmuir adsorption isotherm gave the best fit to the experimental data signifying the presence of adsorption monolayer on the surface of the adsorbent. The results have positively qualified grafted silica nanoparticles as one of the adsorptive media for removal of COD from POME.

Keywords:
Adsorption
Palm oil mil effluent
Silica nanoparticles
Chemical oxygen demand
Sol–gel synthesis
Full Text
1Introduction

Hybrid and composite materials have been extensively studied over a long period of time due to their diverse properties [1–7]. Their properties combine the advantages of the inorganic material (i.e., rigidity, thermal stability) and of the organic substances (i.e., flexibility, ductility, and process ability). In this respect, organically modified silica nanoparticles are a typical class of hybrid materials [8]. Modified silica nanoparticles have been studied in the past few decades due to their unique characteristics and widely studied synthesis techniques [9]. One of the main methods utilized to synthesize silica nanoparticles is the sol–gel method [10,11]. Silica nanoparticles with high specific surface area and silanol concentration compared to their bulk counterpart are expected to drastically alter the properties of composite material. However, the silica surface is hydrophilic in nature and thus has limited affinity for hydrophobic matrix [12]. The surface modification of silica with selected functional groups increases covalent bonding with the hydrophobic matrix, improves affinity between the two phases and thus avoids phase separation [13]. These organically modified silica nanoparticles find their applications in a number of fields [14–19].

Palm oil mill effluent (POME) has been treated with different methods such as coagulation–flocculation, biological anaerobic processes, evaporation, membrane technology and photocatalysis [20,21]. Among all these methods, biological and membrane technology are the most common while photocatalytic technology is still at developmental stage. However, all these methods though effective but have various setbacks in terms of their sustainability in the long run [21,22]. For example membrane technology is equally effective as it enhances quality treated water but is very expensive to maintain due to fouling of the membrane that need to be replaced overtime [23]. In contrast, adsorption provides an attractive, reliable and versatile alternative treatment [24,25] and adsorption of COD over activated carbon (AC), zeolite, kaolin has been previously studied to evaluate the overall adsorption behavior in wastewaters. The application of AC has been proved an effective absorbent in the removal of both organic as well as inorganic pollutants from wastewater. However, the high cost of commercially available AC limits their industrial applications and therefore great attention has been focused on the production of low-cost alternative adsorbents.

Thus, the development of cheaper, eco-friendly and more efficient adsorbent capable of removing various pollutants including COD from wastewater is a subject of intensive research [26–28]. Silica molecules have high density of hydroxyl groups and can be modified with specific functional groups, such as carboxyl, amino and chloro for their various applications. Several studies have explored the utilization of organically modified silica in wastewater treatment [17,29,30]. Despite the uniqueness of modified silica nanoparticles as adsorbents the syntheses of reported modified silica involves expensive raw materials, multistep synthetic approaches and tangible workup processes.

In this study, syntheses of three-dimensional mesoporous silica nanoparticles modified with amino (NH2) group for removing the COD from POME was carried out. This synthesis involves the inexpensive raw materials and simple work up processes. The main objective of this work is to investigate the simultaneous removal of both organic and inorganic oxidizable moieties from POME. The different developed adsorption parameters such as pH, AFS-NPs dose, temperature, initial concentration of COD and contact time were optimized for COD.

2Materials and methods2.1Chemicals

3-Aminopropyltriethoxysilane (C2H5O)3Si(CH2)3NH2 (APTES) containing one amino group was purchased from Aldrich. 3-Aminopropyltrimethoxysilane (CH3O)3Si(CH2)3NH2 (APTMS), hexadecyltrimethylammonium chloride (CTAC) a surfactant, tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS) were purchased from Merck. All the chemicals were used as received. Distilled and deionized water was used to prepare the solutions, which was collected from the Bio Process Laboratory of Department of Chemical Engineering and Energy Sustainability, University Malaysia Sarawak.

2.2Synthesis of AFS-NPs

Amino-functionalized silica nanoparticles (AFS-NPs) were synthesized using sol–gel method [31]. Hexadecyltrimethylammonium chloride (CTAC) as template and 1M sodium hydroxide solution as catalyst were added in 800g of a methanol/water (50% w/w) solution. Further a mixture of 8.24mmol of tetraalkoxysilane (TAOS) and 0.43mmol of 3-aminopropyltrialkoxysilane (APTAS) were added with vigorous stirring at 298K. Herein, a ratio of TAOS 95% and APTAS 5% was selected to produce nanoparticles with higher degree of monodispersity. The solution was allowed to stir for 8h and afterwards the mixture was aged overnight. The white nanoparticles, which settled down during overnight were filtered, washed three times with distilled water and dried at 318K for 72h. To remove the templates, the AFS-NPs were treated with 1mL of 0.6M hydrochloric acid at 333K for 3h followed by filtration, washing with ethanol and dried at 318K. The AFS-NPs were then treated with 1mL of ammonia solution (28%) to remove residual Cl−1 ions and the solid was recovered by filtration, washed with ethanol, and finally dried in vacuum at 423K for 12h.

Using the method mention above, four combinations of gelator silica and modified silica were used to get the optimum yield of AFS-NPs. All the four combinations of silica source and silica modifier are represented in Scheme 1. In the first two combinations (i, ii), TMOS was treated with APTMS and APTES separately and in the next two combinations (iii, iv), TEOS was treated with APTMS and APTES separately to get the AFS-NPs. The product in all these cases has the same molecular structure as the methoxy (OCH3) and ethoxy (OC2H5) side chains present the silica source (TMOS/TEOS) and silica modifier (APTMS/APTES) are hydrolyzed. The presence of methoxy (OCH3) and ethoxy (OC2H5) side chains in the reactants has a remarkable effect on the reaction rates and morphology of the products. The effect of side chains on reaction kinetics and morphology of the product is explained in detail in Section 3.2

Scheme 1.
(0.17MB).

Schematic representation of the AFS-NPs synthesis from four combinations.

(i) TMOS+APTMS; (ii) TMOS+APTES; (iii) TEOS+APTMS; (iv) TEOS+APTES

2.3Characterization

AFS-NPs were isolated and characterized. Infrared spectra (IR) were recorded on Perkins Elmer FTIR (spectrum RXI) spectrometer. IR spectrographs helped to study the changes in the functional groups of reactant and product. Morphology and particle size of nanoparticles were studied using SEM Jeol (JSM-6390LA). The surfaces of the samples were coated with gold before the measurements. The average particle diameter was calculated from the diameters of 100 particles in an SEM picture. The nitrogen adsorption isotherm was measured using a Belsorp-mini II (BEL Japan) at 50°C. The sample was evacuated at 100°C under 10−3mm Hg before measurement. The pore diameter was calculated by using the Barrett–Joyner–Halenda (BJH) method and the specific surface area was calculated using adsorption data in the P/P0 considering the linearity of a Brunauer–Emmett–Teller (BET) plot. Thermal stability of the modified silica nanoparticles was studies using TGA Universal Analyzer 2000 with Universal V4.7A software.

2.4POME Collection

The pre-treated POME was collected from aerobic treatment pond of Salcra palm oil mill in Bau, State of Sarawak, Malaysia. The collected POME was stored in a refrigerator at 4°C to avoid its decomposition by air. The standard method for examination of water and waste water was followed for sample collection and storage [32]. The initial characteristics of the pre-treated POME were measured and presented in Table S1.

2.5Adsorption study of POME by AFS-NPs

For adsorption experiment, 0.1g of AFS-NPs was added into 100mL of POME at pH 5.0 and the beaker was then stirred on a magnetic stirrer for 30min. The amount of COD after adsorption was determined using COD digester (Rocker COD reactor CR25, USA) with subsequent chemical titration analysis as described in APHA standard methods for the examination of water and wastewater [32]. The % values of COD were obtained from Eq. (1) and at equilibrium, the amount of COD adsorbed by AFS-NPs (Qe) was calculated by Eq. (2).

where Ci (mgL−1) is the initial concentration of the COD before adsorption, while Ce (mgL−1) is final concentration of COD after adsorption, Qe (mgg−1) is the adsorption capacity, V (L) is the volume of the solution and m (g) is the mass of AFS-NPs.

To study the effect of initial pH on the adsorption of POME onto AFS-NPs, the experiment was performed at initial pH of 1.0, 3.0, 5.0, 7.0, 9.0, 11.0 and 13.0 using 0.1g adsorbent at temperature 25°C and stirred for 30min. To optimize the AFS-NPs dose, the experiment was performed for 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, and 0.45g of AFS-NPs at optimized pH, temperature 25°C and stirred for 30min. The effect of temperature was studied by varying the temperature at 10, 20, 30, 40, 50, 60 and 70°C.

3Results and discussion3.1Characterization of AFS-NPs

The morphology of the AFS-NPs was observed using SEM analysis and SEM images are shown in Fig. 1, which reveals that silica nanospheres were obtained in all the four combinations. From detailed images analysis, it was observed that the particle diameters were between 500 and 600nm for TMOS/APTMS combination (Fig. 1a), 150–650nm for TEOS/APTES combination (Fig. 1b), 200–650nm for TMOS/APTES combination (Fig. 1c) and 100–700 for TEOS+APTMS combination (Fig. 1d). Further the histograms of the particles size distribution of AFS-NPs from all four combinations were shown in Figure S1. General observation of the images revealed that the percentage of monodispersity was more close to monodispersity in case of product obtained from combination of TMOS/APTMS (Fig. 1a). This behavior of morphology of obtained AFS-NPs is in line with the similar results found in the literature. This is also related to the nature of chemical reaction used for these syntheses. Further explanation of these reactions with results is given in Supplementary Section S1.1.

Fig. 1.
(0.55MB).

SEM images of AFS-NPs from four combinations (a) TMOS+APTMS, (b) TEOS+APTES, (c) TMOS+APTES, (d) TEOS+APTMS.

The presence of amino groups on silica surface was confirmed using FTIR and spectra of AFS-NPs obtained from all four combinations are shown in Fig. 2. A sharp and strong stretching peak at ∼1050cm−1 was observed for all samples that indicated SiOSi stretching frequency. Two peaks observed at ∼1630cm−1 and at ∼780cm−1 correspond to free water adsorbed on silica surface through hydrogen bonding. The shoulder observed at ∼1525cm−1 (NH bending) indicated the presence of NH2 groups. The peak corresponding to NH stretching is expected to overlap with the stretching vibrations of OH groups at ∼3500cm−1. A shoulder at ∼950cm−1 corresponds to the existence of silanol groups on the silica surface. This shoulder was available in the products of three combinations viz. TEOS/APTES, TEOS/APTMS and TMOS/APTES (Fig. 2B, C and D) representing the availability of OH group on silica surface. This shoulder was not observed for the product of TMOS/APTMS (Fig. 2A). The absence of shoulder at ∼950cm−1 in Fig. 2A is a confirmation toward the completion of condensation reactions between the silica surface (silanol groups-OH) and APTMS (NH2 groups). The product of TMOS/APTMS was denoted as product A. The FTIR spectra of all the samples showed absorption peak at ∼2950cm−1 (CH stretching), which indicated the presence of CH2 groups but the spectrum 2A showed two peaks in this region (2975cm−1 and 2922cm−1) corresponding to intense and clear existence of CH2 groups in product A as compared to other three combinations, possibly owing to the higher loading of propylamine groups onto Silica surface [12,14,33,34]. It could be concluded from the FTIR results that synthesized product from all the combinations is AFS-NPs, but the best product was obtained in the case of TMOS/APTMS. For further reactions and characterization, the product A was selected. Reaction efficiency between TMOS and APTMS is in accordance with the theoretical concept behind the reaction chemistry.

Fig. 2.
(0.14MB).

FTIR spectra of AFS-NPs from four combinations (A) TMOS/APTMS, (B) TEOS/APTES, (C) TMOS/APTES, (D) TEOS/APTMS.

In order to determine the thermal stability of amino (NH2) group onto silica surface, the AFS-NPs (product A) was calcined up to 450°C. Figure S2 shows the FTIR spectra for the aminofunctionalized silica before and after heat treatment. The non-calcined aminofunctionalized silica was used for adsorption purposes. The calcined silica was used only for assessment and to ascertain the presence of amino group in modified silica. Characteristic peak of silica in spectra (Figure S2) at 1035cm−1 corresponding to SiOSi bond of the siloxane backbone has been observed in all the spectra and this indicates that main backbone of silica remains the same before and after calcination [18]. Disappearance of peaks from 2975cm−1 and 2930cm−1 is evidence that propyl group along with amino group has been removed from silica surface. Appearance of a peak at 958cm−1 is due to retention of hydroxyl group (OH), which was retained because of decomposition of Schiff base group. FTIR results further revealed that calcination removes the aminopropyl group and the remaining product is mere silica (SiO2).

TGA has been used to determine thermal stability of AFS-NPs and both the TGA curve (black line) as well as the first derivatives curve (blue line) of the AFS-NPs is shown in Fig. 3A. There are three decomposition steps exhibited by AFS-NPs, the first step occurs within temperature range of 30–125°C (8wt%) representing the elimination of adsorbed water and gases, followed by decomposition of aminopropyl groups which occurred slowly between 125 and 380°C (4wt%) and this led to rapid degradation above 380°C (3wt%). In addition, this step also produced water, ammonia and ethylene groups. The final step produced ethylene, ammonia, methane and hydrazine.

Fig. 3.
(0.17MB).

(A) Thermal analysis of AFS-NPs, TGA (black line) and DTGA (blue line) curves, (B) BET isotherms of AFS-NPs.

3.2BET

The adsorption capacity and behavior of AFS-NPs can be predicted from the information about surface area and porosity. The adsorption isotherms of AFS-NPs have been presented in Fig. 3B, which indicates that all those samples having adsorption isotherm have shown much longer straight-line portion of the curve. The starting point of the straight line has been named as point “B” due to Brunauer and Emmett [35] and this shows that the adsorption at point “B” has been due to the capacity of the monolayer. However, formation of multi-layers of adsorbate occurs on the surface of the adsorbent by increasing pressure gradually. Moreover, no appreciable differences in the shape and the amount adsorbed on the adsorbent surface have been noticed in the isotherm plot before capillary condensation has commenced.

The threshold capillary condensation causes a rapid increase in the volume of the adsorbed amount, which has been noticed at relative pressure (P/P0) of 0.61. Similarly, there is saturation pressure (P/P0=1) very close to the horizontal branch where all the pores have been filled with adsorbed liquid. These assertions are in agreement with the results earlier obtained from the SEM analysis because it equally shows that small particles have higher adsorption capacity. Surface area analysis indicates that the specific surface area of the grafted silica (AFS-NPs) as determined by Brunauer–Emmett–Teller (BET) method is much higher (148m2g−1) and the pore volume and the diameter are 0.97cm3g−1 and 21nm respectively. Based upon the above-mentioned studies and characterization, the AFS-NPs (product A from TMOS/APTMS combination) were selected to study the adsorptive behavior of POME.

3.3Effect of pH on adsorptive behavior of COD onto AFS-NPs

The effect of pH is regarded as one of the important parameters affecting the adsorption technique. Adsorption removal of COD onto AFS-NPs was carried out in the pH range of 1.0–13.0 using 0.1g adsorbent for 30min at 25°C with shaking speed of 300rpm as shown in Fig. 4A. The results indicated that the % COD removal increased at low pH from 1.0 to 7.0 and with further increase in pH, the % COD removal was decreased. The higher values of removal efficiency of COD at pH<8 may be due to the positive charge on the surface of the adsorbent increases, resulted in significantly high electrostatic interactions between positively charged AFS-NPs and organic compounds (COD) [36]. This is not surprising as the point of zero charge (pHPZC) of AFS-NPs is around 8.5. Therefore, at a pH higher than pHPZC, the nanoparticle surface is negatively charged attracting cations, whereas at a lower pH, the surface is positively charged attracting anions. The results also showed that the adsorption is not favored in alkaline medium, this is indicated by a decline in % COD removal at pH>8 and this could be attributed to increasing hindrance to the diffusion of organic compounds due to abundance of OH ions or could be due to the increase of solubility of the pollutant at high pH values [37]. It is expected that adsorption of COD on AFS-NPs can take place through physical adsorption, chemical adsorption or ion exchange. However, it is rather difficult to predict the exact mechanism of adsorption and ion exchange could be ruled out as with increasing pH most functional groups become dissociated. Results for pH were similar to some previous reports where neutral pH was described for maximum COD removal [38,39]. The synthesis of AFS-NPs and application to remove COD from POME is illustrated in Scheme 2.

Fig. 4.
(0.19MB).

Effect of adsorption parameters on COD removal over AFS-NPs (A) pH, using 0.1g AFS-NPs for 30min at 25°C with shaking speed of 300rpm. (B) Adsorbent dose, at pH 7 for 30min at 25°C with shaking speed of 300rpm. (C) Temperature, at pH 7, 0.3g AFS-NPs for 30min with shaking speed of 300rpm. (D) Contact time, at pH 7, 0.3g AFS-NPs for 30min at 30°C with shaking speed of 300rpm.

Scheme 2.
(0.22MB).

Schematic representation of general synthesis of AFS-NPs and application in removal of COD.

3.4Effect of AFS-NPs dose on adsorptive behavior of COD

The effect of AFS-NPs dosage on percentage COD removal was studied at pH 7 for 30min at 25°C with shaking speed of 300rpm and the results were represented in Fig. 4B. The amount of AFS-NPs was increased from 0.1 to 0.4g and it was observed that the removal percentage of COD increased with increasing in the AFS-NPs dose and this may be attributed to the increase in the total surface area of the prepared AFS-NPs, and therefore more active sites were available for the adsorption of COD [40]. Equilibrium was reached at 0.3g of AFS-NPs dose and further increase in the adsorbent dose had a little effect on COD removal, which may be linked to saturation of adsorption sites due to particle interaction and this decreases the total surface area of the adsorbent available to COD [41]. The results of the present study also showed an agreement with a previously reported work where cow–dung ash was used to remove COD [38].

3.5Effect of temperature on adsorptive behavior of COD onto AFS-NPs

Adsorption of organic molecules on the surface of silica nanoparticles has been found to be relatively dependent on thermodynamic properties such as temperature rise as reported by Schreiber et al. [42]. The effect of temperature on the removal of COD was carried out in the temperature range of 10–40°C at pH 7, AFS-NPs dose 0.3g for 30min with shaking speed of 300rpm. The results showed in Fig. 4C revealed that the gradual rise in temperature increases the adsorptive removal of COD from POME. The highest percentage removal is attained at 30°C. However, as the temperature is increased there is gradual drop in the adsorption of COD, leading to a little decrease in the percentage of COD removed. This anomalous increase in adsorption as a result of temperature rise is due to increased penetration of POME inside micropores at higher temperatures which might have led to the creation of new active sites for adsorption of COD [43].

3.6Effect of contact time on adsorptive behavior of COD onto AFS-NPs

The effect of contact time is an important adsorption parameter and therefore the effect of time on COD removal by AFS-NPs was studied at pH 7, ASF-NPs dose 0.3g, temperature 30°C with shaking speed of 300rpm. The time interval was from 10 to 70min and the results were shown in Fig. 4D, which indicated that the removal of COD was very rapid during first 25–30min to reach about 94% due to a large number of available surface sites, representing the fast phase. The COD removal then continues to rise slowly until it reaches an equilibrium value at 50min where most of the adsorption sites were saturated to maximum removal efficiency, representing the slower phase [44]. After 50min (regarded as equilibrium time), the amount of COD removed remained constant, which might be attributed to saturation of the adsorption sites as contact time increased. The smooth and independent nature of curve, as in Fig. 4D, indicates the formation of monolayer of COD cover on the outer surface of AFS-NPs.

3.7Adsorption isotherms

The adsorption phenomenon was studied with respect to the effect of initial COD concentration. The % removal and adsorption capacities of the COD were investigated by changing the initial COD concentration (Ci) from 50 to 90mgL−1 as shown in Fig. 5A. With an increase in the initial COD concentration, the % removal of COD changed from 91.66% to 75.31% and this decrease may be due to the fact that a large number of free active sites of adsorbent were available for the COD molecules at the lower initial concentration and as the COD concentration increased, the saturated sites made it difficult to adsorb COD molecules. On the other hand as the COD concentration increased, the adsorption capacity increased until 271.12mgg−1. This sorption characteristic represented that surface saturation was dependent on the initial COD concentrations. The binding sites are limited keeping biomass loadings as constant.

Fig. 5.
(0.21MB).

Effect of initial COD concentration on the % removal and adsorption capacity of COD onto AFS-NPs (A), Langmuir isotherm model (B), Freundlich isotherm model (C) and Temkin isotherm model (D).

In the present work, the relationship between COD adsorption capacities of AFS-NPs and the initial concentrations of COD at equilibrium was analyzed with different isotherm models. The experimental data were fitted to the Langmuir, Freundlich and Temkin isotherm models using origin linear regression. Values for the determined parameters of each isotherm and respective R2 value for each regression are shown in Table 1 and the equations for the adsorption isotherm models are represented in Eqs. (3)–(5)[45–47].

where Qm and b are Langmuir constants indicating, respectively, the adsorption capacity (mgg−1) and energy of adsorption (Lmg−1). Values of Qm and b are determined from the linear regression plot of (Ce/Qe) versus Ce (Fig. 5B).
where KF is the Freundlich constant and 1/n is a measure of adsorption intensity. The values of KF and n can be measured from linear regression plot between logCe and logQe (Fig. 5C).
where B is a constant equal to RT/bT, AT (L/mg) and bT (J/mol) are the Temkin constants. bT is related to heat of adsorption and AT is the equilibrium binding constant corresponding to the maximum binding energy. R (8.314J/molK) is the universal gas constant and T (K) is the absolute temperature. The values of AT and bT are calculated from the linear regression plot of Qe versus lnCe (Fig. 5D).

Table 1.

Langmuir, Freundlich and Temkin isotherms parameters for COD adsorption.

Sample  Exp. adsorption capacity  Langmuir
AFS-NPsQe (mgg−1)
271.1
Parameters  Qm (mgg−1b (Lg−1R2 
Values  307.7  0.274  0.9909 
Freundlich
Parameters  KF (mgg−1n  R2 
Values  128.4  4.25  0.9723 
Temkin
Parameters  AT (Lmg−1bT (Jmol−1R2 
Values  91.22  49.76  0.9580 

The results presented in Table 1 showed that the higher values of R2 (>0.9) obtained for isotherms models showed that the adsorption process of COD from treated POME by prepared AFS-NPs in this work could be well explained by Langmuir, Freundlich and Temkin models and R2 values for all three models are adsorbent specific. The values of Langmuir constant b in the range of 0–1, indicates that the AFS-NPs are favorable for adsorption of COD. The value of n from the Freundlich model for AFS-NPs falls within the range 1–10, which indicates favorable adsorption. The higher value of bT from the Temkin isotherm model demonstrates a strong interaction force between adsorbates and absorbents. But by comparing the three models with respect to R2 values, it can be concluded that the sorption equilibrium data of COD removal from POME onto the AFS-NPs predominantly can be described by Langmuir model than other two models. This is because of the R2 value of >0.99 and also the experimental Qe is close to theoretical Qm calculated from Langmuir model suggesting formation of monolayer coverage of COD molecules over outer surface of AFS-NPs. The similar behavior of COD removal was also reported in another study by Ligaray et al. recently [28].

Table 2 lists the comparison of the adsorption capacity and % COD removal by various adsorbents. The AFS-NPs prepared in this work showed comparable and relatively better efficiency to remove COD as compared to some previous works reported in the literature.

Table 2.

Comparison of COD removal by various adsorbents.

Adsorbents  Qe (mgg−1% COD removal  Reference 
Nano-CaO2@clinoptilolite    50.4  [26] 
Coffee-waste based AC    68.5  [48] 
Sugarcane bagasse derived AC    77.8  [49] 
Activated carbon fiber (ACF)    85.0  [50] 
Activated carbon cloth (ACC)  500  87.5  [51] 
Periwinkle shell-based granular activated carbon (PSC)    77.5  [52] 
Date-pit activated carbon (DP-AC)  241.4    [53] 
Commercially available BDH activated carbon (BDH-AC)  331.2    [53] 
γ-Fe2O3 nanoparticles  21.30    [54] 
Activated carbon prepared from date palm shell    95.5  [27] 
Charcoal ash    68.3  [55] 
Amino polyethylene glycol (PEG) functionalized CNTs    99.7  [41] 
Chiston-coated bentonite    73.34  [28] 
AFS-NPs  271.1  91.7  Present study 
4Conclusions

The AFS-NPs were designed and synthesized and the results showed that the modification could effectively increase its amino content, leading to an increase in adsorption capacity. The adsorbent showed a hybrid mesoporous structure and narrowly dispersed nanoparticles size distribution. The appearance of FTIR peak at 1050cm−1 corresponds to (Si–O–Si) of the siloxane backbone and (Si–O–Si) deformation confirmed that the modification process was successful. The experimental results of AFS-NPs fit well to Langmuir isotherm model and maximum theoretical adsorption capacity was 307.7mgg−1. Due to high surface area (148m2g−1) and hydrophilic and hydrophobic functionalities, AFS-NPs can adsorb both hydrophilic and hydrophobic species present in POME showing favorable adsorptive removal of COD from pre-treated POME. From the view of environmental impact, AFS-NPs could be regarded as cost-effective adsorbent in future water treatment.

Conflict of interest

The authors declare that there is no conflict of interest.

Acknowledgments

This work was supported by Faculty of Engineering, Universiti Malaysia Sarawak (UNIMAS), 94300 Kota Samarahan, Sarawak, Malaysia [DPI grant number 02(DPI08)/824/2011(08) and DPP grant number F02/(DPP52)/1281/2015(27)]. M.N. Zafar acknowledges the research funding from Higher Education Commission, Pakistan [grant number 6515/Punjab/NRPU/R&D/HEC/2016].

Appendix A
Supplementary data

The following are the supplementary data to this article:

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