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 CHEMICAL ENGINEERING   TRANSACTIONS
 

VOL. 43, 2015 

A publication of 

The Italian Association 
of Chemical Engineering 
Online at www.aidic.it/cet 

Chief Editors: Sauro Pierucci, Jiří J. Klemeš 
Copyright © 2015, AIDIC Servizi S.r.l., 
ISBN 978-88-95608-34-1; ISSN 2283-9216                                                                               

 

Adsorption of FFA, Soap and Glycerine in Biodiesel 
 Using Magnesium Silicate 

Pornsawan Assawasaengrat*, Pornrin Jintanavasan and Prakob Kitchaiya  

Department of Chemical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 
10520, Thailand 
pornsawank@hotmail.com  

The effect of SiO2/MgO molar ratios of magnesium silicate on adsorption of free fatty acid (FFA), soap and 
glycerine in biodiesel was studied. The magnesium silicate adsorbent was prepared by the precipitation of 
magnesium sulfate and sodium metasilicate. The molar ratios of SiO2/MgO in synthetic magnesium 
silicate,i.e.,0.31:1, 0.76:1 and 2.04:1 were obtained by varying concentrations of Na2SiO3:MgSO4. The 
element analysis, crystal structures and surface area and pore size of the adsorbents were characterized by 
X-ray fluorescence method (XRF), X-ray diffraction method (XRD) and Brunauer-Emmett-Teller technique 
(BET), respectively. FTIR technique was used to confirm the chemical characteristics of the adsorbents. The 
adsorption efficiency of synthesized magnesium silicate in biodiesel containing different amount of SiO2  / MgO 
molar ratios was studied by varying dosage amount of adsorbents (1, 2 and 3 g) for 150 g of biodiesel at 60˚C 
for 5 hours. The results showed that the magnesium silicate at 2.04:1 SiO2/MgO molar ratio and 3 g of 
adsorbent showed the highest efficiency in adsorption of FFA, soap and glycerine at 77.39%, 77.11% and 
82.92%, respectively. While the adsorption capacity was 52.57 mg/g of FFA adsorption, 1.37 mg/g of soap 
adsorption and 155.06 mg/g of glycerine adsorption. The 2.04:1 molar ratio of SiO2/MgO in magnesium 
silicate had the biggest pore size, 105 A°, that was proportional to the contaminates adsorbed. Noticeably, the 
structure size of concerned contaminates were large, so the pore size of adsorbents could play a significant 
role on adsorptivity of adsorbate. 

1. Introduction 

Nowadays, the demand of energy and environmental concerns about air pollution from gas cars are steadily 
increasing. Biodiesel is an alternative and renewable fuel (Ceclan et al, 2012). It presents outstanding 
characteristics as non-toxicity, absence of sulphur, high energy content and biodegradability. 
Transesterification is a chemical reaction relating triglycerides and alcohol using homogeneous or 
heterogeneous substances as catalyst to yield alkyl esters (biodiesel) and glycerol as by-product. The methyl 
ester has contaminants frequently that have effects on the quality of biodiesel. Glycerol is a potential cause of 
biodiesel instability. During transesterification, reaction conditions could result in the formation of oxidizes 
byproducts that cause discoloration, especially when glycerol is exposed to either high temperature or alkaline 
pH. Glycerol can oxidize or thermally degrade to form diols or acids that can contribute to biodiesel instability. 
(Sangita and Pradeep, 2010).The water content in biodiesel causes corrosion of the automobile system (such 
as fuel tubes and injector) or might react with glycerides to form soap and glycerine as by-products 
(Predojevic, 2008).  In addition, soap and free fatty acids cause the deterioration of engine components. 
Therefore, the amount of water and free fatty acids affect the quality of biodiesel. Conventionally wet washing 
is the most employed technique to remove contaminants such as soap, catalyst, glycerine and residual 
alcohol from biodiesel. However, the major disadvantages in the use of water to purify biodiesel are emulsion 
formation (preventing the separation of the esters), formation of free fatty acids and soap, wastewater 
formation; which all increase in cost and production time (Ignat and Kiss, 2012). Adsorption technique was 
introduced to substitute wet washing to remove biodiesel contaminants. The use of adsorbents turns the 
process more rapid and there is not a formation of aqueous residues. The adsorbent such as magnesium 

                                

 
 

 

 
   

                                                  
DOI: 10.3303/CET1543190 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Assawasaengrat P., Jintanavasan P., Kitchaiya P., 2015, Adsorption of ffa, soap and glycerin in biodiesel using 
magnesium silicate, Chemical Engineering Transactions, 43, 1135-1140  DOI: 10.3303/CET1543190

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silicate has the potential of selectively adsorbing hydrophilic materials such as glycerine, mono- and 
diglycerides. Magnesium silicate surface consists of partially hydrophobic and partially hydrophilic sites. The 
hydrophobic part includes siloxane (≡Si−O−Si≡) groups and the hydrophilic portion contains isolated hydroxy 
groups (−Mg−OH), individual silanol groups (≡Si−OH) and hydrogen bonds formed due to close vicinity of 
hydroxy groups linked to neighboring silicon atoms. In this paper, the adsorption of free fatty acid, soap, and 
total glycerine in biodiesel was studied by using magnesium silicate as an adsorbent  

2. Experimental 

2.1 Synthesis of magnesium silicate 
Three different concentration ratios of sodium silicate and magnesium sulphate (VI) were chosen for 
magnesium silicate synthesis. The concentration (Molar) ratios of Na2SiO4: MgSO4 were 0.5:0.5, 0.5:1.5 and 
3.0: 0.5. The sodium silicate was dropped into the magnesium sulphate (VI) solution with stirring speed of 200 
rpm at room temperature. Centrifugation at 2000 rpm was used to separate the precipitate and the liquid 
phase was removed by filtration. The precipitate was dried at 100°C for 24 h, then mashed and sieved. 

2.2 Characterization of magnesium silicate 
Chemical characterization of representative sample was undertaken by X-ray fluorescence spectrophotometer 
(XRFs) for major element detection. Analysis by X-ray diffractometer (XRD) revealed major crystalline phases 
in magnesium silicate samples. Structural characterization of the sample was investigated by Fourier 
transform infrared spectrometer (FT-IR). In order to determine magnesium silicate adsorptive potentials, their 
BET specific surface area and pore diameter were estimated using multipoint BET surface area, and meso 
pore volumes was determined by using BJH equation. 

2.3 Adsorption capacity of synthetic magnesium silicate 
1, 2 or 3 grams of adsorbent was added into 150 g of biodiesel (palm oil) to study the contaminates 
adsorption. The shaker machine (ZHWY-100B, ZHICHENG) was set at 60°C, 200 rpm and 2 h. After shaking, 
the adsorbent was removed and purified biodiesel was analysed for the contaminations. In order to determine 
the effectiveness of purification, the soap, free fatty acid, and total glycerine contents were measured before 
and after processing. The amount of remaining free fatty acid was determined by acid value titration according 
to AOCS Official Methods Cd 3a-63. Soap level was determined using AOCS Method Cc 17‐79. The amount 
of total glycerine was determined according to EN14105. Adsorption efficiency was defined as percentage of 
fraction between adsorbed and initial contaminate. 

3.  Results and discussion 

3.1 Adsorption synthesis and characterization 
The synthesized magnesium silicate was a fine white powder. The chemical composition of the synthesized 
magnesium silicates was analysed by X-ray fluorescence (XRF). Table 1 exhibited the chemical compositions 
of magnesium silicate which were synthesized by varying Na2SiO4: MgSO4 concentration ratios. The molar 
ratio of SiO2: MgO in synthetic magnesium silicate was 0.36:1, 0.76:1 and 2.04:1 according to the Na2SiO4: 
MgSO4 concentration ratio of 0.5:1.5, 0.5:0.5 and 3.0:0.5, respectively. Na2SiO4: MgSO4 concentration ratio of 
3.0:0.5 exhibited highest molar ratio of SiO2: MgO, while the concentration ratio of 0.5:1.5 showed lowest 
molar ratio of SiO2: MgO. Thus, the molar ratio of synthetic magnesium silicate was in agreement with the 
used concentration ratio. 

Table 1:  Chemical compositions of magnesium silicate by various Na2SiO4: MgSO4 concentration ratios 

Concentration(Molar) ratios  
Na2SiO4: MgSO4 

SiO2 
(wt%) 

MgO 
(wt%) 

Other 
(wt%) 

Molar ratio of 
SiO2: MgO 

0.5:1.5  26.34  72.31  1.35 0.36:1  
0.5:0.5  
3.0:0.5 

43.18  
52.88 

56.81 
25.85 

0.01 
21.27 

0.76:1  
2.04:1 

The average surface area, pore volume and pore size of magnesium silicate were characterized by BET 
method. The result is showed in the Table 2. The magnesium silicate has surface area between 15  58 m2/g 
and average pore size is between 50-105 A° which could described that synthesized magnesium silicate was 
mesoporous. Higher SiO2: MgO molar ratio provided larger pore size. From the results, the 2.04:1 molar ratio 
of SiO2: MgO has the biggest pore size diameter, 105 A

°.  

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Table 2: Chemical compositions of magnesium silicate by various Na2SiO4: MgSO4 concentration ratios 

Molar ratio of 
SiO2: MgO  

Surface area  
(m2/g)  

Pore volume  
(cm3/g)  

 Pore size diameter 
(A°)  

 

0.36:1  15.1 0.0192  
 

 50.9  
0.76:1  
2.04:1  

67.2 
57.8 

0.1098  
0.1516  

 65.3  
105.0 

 

Crystalline structure of magnesium silicate was characterized by X-ray diffraction technique. XRD patterns of 
magnesium silicate were demonstrated in Figure 1.  The structure of the synthesized magnesium silicate was 
amorphous as there were no sharp peaks appearing at any 2θ. The XRD patterns of the samples were in an 
agreement with commercial magnesium silicate (Clowutimon et al., 2011). 

 

Figure 1: The XRD pattern of magnesium silicate with molar ratio of SiO2: MgO at (A) 0.36:1, (B) 0.76:1,      
(C) 2.04:1 

 

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The FT-IR spectra of magnesium silicate with different molar ratio of SiO2:MgO are shown in Figure 2. 
Normally, the strong band between 1,100 - 900 cm-1 represented the characteristics of siloxane (Si-O-Si) 
groups (Battisha et al., 2007) and the band around 600 cm-1 due to Si-O-Si bending vibration (Mostafa et al., 
2009).The synthesized adsorbent showed strongest absorption band at 1,025, 987 and 968 cm-1 according to 
Si-O-Si asymmetric stretching. At the right hand side of this band, there is a small shoulder, assigned to the 
Si-O stretching (Silva et al., 2011). The broad band between 3,800-3050 cm-1 assigned to the O-H stretching 
(Karbowiak et al., 2010) did not appeared in the adsorbent. The bending vibration band of molecular H2O 
appears at 1,632 cm-1 (Mostafa et al., 2009). The band at 1,400 cm-1 was related to the presence of Si-O-Na 
and Mg-O-Na (Narasimharao et al., 2014). The band at 400 - 450 cm-1 is attributed to Mg-O stretching 
vibration (Brew and Glasser, 2005). It was found that all magnesium silicate have a similar chemical nature.  
Nevertheless, only for magnesium silicate at 2.04:1 SiO2/MgO molar ratio was a peak detected at 1432 cm

-1 
that represent Si-O-Na and Mg-O-Na bonding. 

 

Figure 2: FT-IR spectra of magnesium silicate with the molar ratio of SiO2: MgO at (A) 0.36:1, (B) 0.76:1,      
(C) 2.04:1 

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3.2 Adsorbent Efficiency 
Adsorption of free fatty acid, soap and total glycerine in biodiesel by synthesized magnesium silicate, which 
different molar ratio of SiO2/MgO were investigated and shown in Figure 3. 

 

Figure 3: Comparison of adsorption efficiency of contaminates from different molecular ratios of SiO2/MgO;  
(A) free fatty acid, (B) soap, and (C) total glycerine  

From the results, dosage amount of adsorbent had direct variation with efficiency of contaminates adsorption 
and trends of free fatty acid, soap and total glycerine adsorption. Magnesium silicate at 2.04:1 SiO2/MgO 
molar ratio and 3 g of adsorbent showed the highest efficiency in adsorption of FFA, soap and glycerine at 
77.39 %, 77.11 % and 82.92 %. In addition the adsorption capacity was 52.57 mg/g of FFA adsorption, 1.37 
mg/g of soap adsorption and 155.06 mg/g of glycerine adsorption.  Magnesol, a commercial adsorbent for 
biodiesel purification, was reported to adsorb FFA 90 mg/g of adsorbent and soap 0.75 mg/g of adsorbent 
(Jintanavasan and Assawasaengrat, 2012). On the other hand the trends of magnesium silicate, 0.36:1 
molecular ratio of SiO2/MgO, present low adsorbed efficiency of free fatty acid and soap (Figure 3A, 3B). This 
is because 2.04:1 molecular ratios of SiO2/MgO of magnesium silicate had high silica content than other 
concentration ratios of magnesium silicate which related to XRF result. It means that silica content in 
magnesium silicate effect on efficiency of free fatty acid and soap adsorption. However, the adsorption of 
glycerine was not affected by different molar ratios of SiO2/MgO as shown in Figure 3C. Physically, BET 
results presented the largest pore size, 105 A°, at 2.04:1 SiO2/MgO molecular ratio of magnesium silicate 
which influenced the adsorption efficiency. Free fatty acid and soap have larger molecular size than glycerine 
(Oleic acid and glycerine molecular size were estimated to be 28 A° (Eva et al., 2008) and 3.6 A° (Chen, 
2013), respectively. Therefore, larger pore size can enhance diffusion of large molecules into the adsorbents. 

 

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4. Conclusions 

The magnesium silicate adsorbent was prepared by the precipitation of magnesium sulphate and sodium 
metasilicate. The molar ratios of SiO2/MgO in synthetic magnesium silicate (0.31:1, 0.76:1 and 2.04:1) were 
obtained by varying concentrations of Na2SiO3:MgSO4. The adsorption efficiency of synthesized magnesium 
silicate in biodiesel containing different amount of SiO2/MgO molar ratios was studied by varying dosage 
amount of adsorbents (1, 2 and 3 g) for 150 g of biodiesel at 60˚C for 5 h. The magnesium silicate at 2.04:1 
SiO2/MgO molar ratio and 3 g of adsorbent showed the highest efficiency in adsorption of FFA, soap and 
glycerine at 77.39 %, 77.11 % and 82.92 %. The adsorption capacity was 52.57 mg/g of FFA adsorption, 1.37 
mg/g of soap adsorption and 155.06 mg/g of glycerine adsorption.  The adsorption efficiency of free fatty acid 
and soap was increased when silica content in magnesium silicate increased. However, the adsorption of 
glycerine was not affected by different molar ratios of SiO2/MgO. BET results presented the largest pore size 
at 2.04:1 SiO2/MgO molecular ratio of magnesium silicate which influenced the adsorption efficiency. Free 
fatty acid and soap have larger molecular size than glycerine. Therefore, larger pore size can enhance 
diffusion of large molecules into the adsorbents. 

Acknowledgements 

This research was financially supported by Faculty of Engineering, King Mongkut's Institute of Technology 
Ladkrabang and National Research Council of Thailand. Support from the Center of Advanced Instrument for 
Research in Engineering is also acknowledged. 

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