Investigating the mechanism of nanofiltration separation of glucosamine hydrochloride and N-acetyl glucosamine
© The Author(s) 2016
Received: 3 March 2016
Accepted: 20 June 2016
Published: 27 June 2016
Glucosamine hydrochloride (GAH) and N-acetyl glucosamine (NAG) are chitin derivatives. Owing to their excellent biological activity, they have long been used as pharmaceuticals and nutraceuticals. However, both of them exist simultaneously in chitin hydrolyzate or fermentation production. The aim of this study is to identify the feasibility of separating GAH and NAG by nanofiltration on the basis of appropriate adjustments of physical conditions.
One commercial spiral nanofiltration membrane (QY-5-NF-1812) was used. Experiments were carried out in full recycle mode and the membrane separation performance was investigated at various mass ratios (mass ratios of GAH to NAG were from 1:14 to 1:2), pressures (4–22 bar), temperatures (15–35 °C), and electrolytes (NaCl, MgSO4, and MgCl2). The influence of temperature on molecular characteristics that play an important role in the separation process was also studied.
Owing to the steric-hindrance effect, electrostatic effect, and different solute permeability, the GAH separation factor increased with increasing GAH concentration. Furthermore, upon temperature increasing, the permeability difference between GAH and NAG decreased, thus decreasing the GAH separation factor. Simultaneously, with increasing temperature, the polarities and calculated molecular diameters for both GAH and NAG increased evidently. The calculated reflection coefficients for both GAH and NAG can be well fitted by the steric-hindrance pore (SHP) model, suggesting that steric-hindrance effect played an important role on the separation process. Furthermore, owing to Donnan repulsion and solute diffusion effects, three electrolytes had noticeable effects on nanofiltration separation efficiency.
The nanofiltration separation efficiency of GAH and NAG was significantly affected by their physical properties in this system, and the mechanisms for GAH and NAG separation were elucidated. The current study could provide a certain basis for the nanofiltration separation of GAH and NAG on an industrial scale.
As chitin derivatives, glucosamine hydrochloride (GAH) and N-acetyl glucosamine (NAG) are widespread (Chen et al. 2012). Given that GAH and NAG have significant biological activity and can be used as ligands in coordination chemistry (Tao et al. 2014), both of them have long been used as pharmaceuticals and nutraceuticals to treat osteoarthritis and maintain cartilage and joint health (Zhu et al. 2015). GAH is usually produced by the HCl hydrolysis of chitin or fermentation (Zhu et al. 2005a; Chen et al. 2012) and is acidic in aqueous solutions. NAG can be prepared by GAH acetylization or glucose fermentation (Chen et al. 2012; Zhu et al. 2015) and is neutral in aqueous solutions. Both of them exist simultaneously in chitin hydrolyzate or in fermentation production (Deng et al. 2005; Chen et al. 2012). However, since both of them have similar molecular weights (GAH with molecular weight of 215.5 g/mol and NAG with molecular weight of 221.0 g/mol) and physical properties, it is difficult to separate them from their mixture solutions. Although some monosaccharides can be separated by chromatography (Brereton and Green 2012), the cost of this technique is relatively higher with poor selectivity of an appropriate stationary. Most columns face a number of problems such as column stability, lifetime, and separation reproducibility (Ghfar et al. 2015). Nanofiltration (NF) technology is a good approach for separation due to their advantages, including lower energy consumption, sustainable processing and relatively easy scale-up over other filtration procedures (Kolfschoten et al. 2011; Aroon et al. 2010).
Currently, NF membranes have been applied to many aspects, including the separation of multi-component solution in soybean molasses (Zhao et al. 2013), recycling of phosphoric acid from sewage sludge (Niewersch et al. 2010), recycling wastewater in the dairy industry (Chen et al. 2016), dye removal from aqueous and organic solutions (Kebria et al. 2015), and removal of fermentation inhibitors from wood extracts (Xie and Liu 2015). NF membranes have received increasing attention because of saccharides separation (Dong et al. 2014; Moreno-Vilet et al. 2014).
On the basis of previous research, NF performance can be affected by many factors during separation. Qin et al. (2014) reported that the increase of salt concentrations resulted in rejections for both salt and trisulfonic acid decrease. Sharma et al. (2003) indicated that with increasing temperature, pure water permeability increased because of the increase in polymeric membrane pore size and cutoff size (Desal-5 DL). Wang et al. (2002) demonstrated that the negatively charged membrane showed different rejections for different types of electrolytes with the order of R(MgSO4) > R(K2SO4) > R(MgCl2) > R(KCl) > R(NaCl) as a result of the Donnan and steric-hindrance effects. Sjoman et al. (2007) indicated that different mass ratios significantly influenced the rejections for both xylose and glucose through the steric-hindrance effect. Furthermore, saccharides, such as pectate oligosaccharides (which are acidic in aqueous solutions), carry an electric charge that affect their separation by NF membranes (Iwasaki and Matsubara 2000).
However, the possibilities of NF separations for monosaccharides with similar molecular weights have seldom been studied, and the transport mechanisms for monosaccharides are not fully understood. From the traditional point of view, membrane filtration would require a tenfold difference in molar mass or threefold difference in hydrodynamic radius for separation of components from each other (Sjoman et al. 2007). Simultaneously, many mathematical models have been proposed to describe and predict the process of NF. Generally, the filtration mechanisms involve mainly steric hindrance (Bowen et al. 1997), Donnan exclusion (Schaep et al. 2001), and dielectric exclusion effects (Yaroshchuk 2001). Nevertheless, the NF separation mechanisms for mixed monosaccharides with similar molecular weights were seldom studied and many of them focused on the study of neutral monosaccharides, such as xylose and glucose (Sjoman et al. 2007). Different from neutral monosaccharides, GAH is one kind of the cationic monosaccharides and the molecular weights of GAH and NAG are similar.
Thus, the aim of this study is to identify the feasibility of separating GAH and NAG using NF by regulating and controlling physical conditions, including different mass ratios, pressures, temperatures, and three types of electrolytes. Thereafter, the separation performance is evaluated by a series of models to lay the foundation for large-scale industrial utilization of monosaccharide purification with the same or similar molecular weights. Furthermore, varying molecular characteristics such as the changing molecular diameters and polarities caused by temperature are also investigated to obtain an insight into the processing of glucosamine fractions separation.
A series of models used for describing the membrane separation performance and molecular characteristics are explored as follows:
Concentration polarization model
Irreversible thermodynamic model
Calculation of the molecular diameter
The research in this study found that the pressure, feed concentration and ion strength hardly have influence on the calculated molecular diameter but the temperature has some influence on the calculated molecular diameter.
Simultaneously, molecular polarity can be obtained from HyperChem.
Steric-hindrance pore model
Max. Pressure (bar)
Pure water permeance (L m−2 h−1 bar−1)a
MgSO4 retention (%)b
Na2SO4 retention (%)b
NaCl retention (%)b
CaCl2 retention (%)b
Chemicals and reagents
GAH and NAG were of analytical grade and purchased from Shandong Aokang Biotechnology Co. Ltd. (Shandong, China) and Zhejiang Aoxing Biotechnology Co. Ltd. (Zhejiang, China), respectively. Several analytical grade salts, namely, MgSO4, Na2SO4, NaCl, MgCl2, CaCl2, NaOH, and EDTA (ethylene diamine tetraacetic acid), were supplied by Lingfeng Chemical Reagent Co. Ltd. (Shanghai, China). The deionized water (conductivity ≤10 μS cm−1) used for experiments and cleaning was supplied by Shanghai Huazhen Co. Ltd. (Shanghai, China).
The conductivity was measured by an electric conductivity meter (Shanghai Jingke, DDS-307, China), and the viscosity was measured by Ubbelohde Viscometer. The solution pH was measured by the FE20 pH meter (Mettler Toledo, Shanghai, China). The GAH and NAG contents of the samples were analyzed by the HPLC method (Agilent 1200, Agilent, USA) equipped with a high-performance sugar column (Sugarpak-I column, Waters, USA) and an RI detector.
Results and discussion
Influence of different mass ratios on separation performance
Influence of different mass ratios on feed volume flux
Estimated pore radius (r p ), osmotic pressure difference (Δπ) and effective thickness (Δx/A k )
Different mass ratios
R p (nm)
Δx/A k (μm)
Rejection difference between single solution and mixed solution
Figure 4b illustrates that compared with single solution, the NAG rejection in the mixed solution decreases; this result is consistent with that of Luo and Wan (2011). One reason is a partial dehydration of solute because the “salting-out” effect (also called the Hofmeister effect [Kunz et al. 2004)] causes a decrease of solute hydrodynamic radius, thus inducing a decrease in solute retention. Another reason is that, at a higher GAH concentration, the membrane becomes more compacted than that with a lower GAH concentration (Bargeman et al. 2014). Correspondingly, the double electric layer becomes compressed, leading to the thinner double electric layer and the increase of the channel in the membrane pore. Therefore, NAG rejection decreases in the mixed solution.
Influence of different mass ratios on GAH separation factor
The above results can be attributed to the following explanations: both the MWCO and molecular diameter [obtained from Eq. (9)] of GAH are smaller than NAG (at 25 °C, the molecular diameters for GAH and NAG are 0.61 and 0.67 nm, respectively). Therefore, the GAH rejection is smaller than NAG rejection because of the steric-hindrance effect (Sjoman et al. 2007). Furthermore, the positive charge of GAH can interact with the negative charge at the membrane surface because of electrostatic attraction, which reduces the electric quantity on membrane surface and diminishes the dielectric effect of the membrane (Qin et al. 2014). This result promotes that GAH penetrates the membrane. Different from GAH, neutral NAG cannot interact with the membrane, under the effect of steric hindrance, so GAH is less rejected than NAG. Furthermore, the permeability [obtained from Eq. (8)] of GAH and NAG is apparently different. As shown in Fig. 5c, GAH permeability is larger than NAG permeability at every mass ratio, and the permeability difference value is larger at higher GAH concentrations. Therefore, given the influence of permeability, MWCO, molecular diameter, and electrostatic effect, the rejection of GAH is less than NAG rejection, and the separation performance presents different results with different mass ratios. On the basis of the above results, the mass ratio of 1:2 (GAH:NAG) has been chosen in the next experiments.
Influence of different temperatures on separation performance
Influence of different temperatures on rejections for GAH and NAG and feed volume flux
Influence of different temperatures on molecular characteristics
The temperature influences not only the NF membrane but also the molecular characteristics (Jian et al. 2015; Paul and Paul 2015; Xing et al. 2013). Therefore, temperature plays an important part in the NF process. Figure 8a, b illustrates the effects of temperature on the molecular polarities and calculated diameters. Figure 8a shows that the GAH dipole moment increases from 3.54 to 3.74 Debye with increasing temperature from 15–35 °C. Compared with GAH, the dipole moment of NAG increases rapidly and the maximum dipole moment is up to 5.94 Debye when the temperature reaches 35 °C. The results provide a good explanation on why both GAH and NAG retentions decrease with increasing operating temperatures (Fig. 6a, b).
Influence of different temperatures on the GAH separation factor
Figure 10c shows that temperature influences both GAH and NAG solute permeability significantly. Compared with the solute reflection coefficient, solute permeability, which also represents the retention performance of membrane for GAH and NAG, shows rising tendency with increasing temperature. For GAH, the solute permeability increases rapidly from 15–20 °C and then increases slowly when temperature increases from 20–30 °C. Solute permeability of NAG increases slowly from 15 to 30 °C and then increases rapidly when temperature increases from 30 to 35 °C. Therefore, the permeability difference value between GAH and NAG decreases with increasing temperature. That is why the GAH separation factor decreases with increasing temperature. This result indicates that high temperatures have an adverse effect on the separation for the two mixed solutions.
Influence of different electrolytes on separation performance
Influence of different electrolytes on rejections for GAH and NAG
Diffusion coefficients of three electrolytes at 25 °C (Schaep et al. 1998) and permeability of NAG influenced by three electrolytes
Diffusion coefficient (10−9 m2 s−1)
Permeability of NAG (L m−2 h−1)
Influence of different electrolytes on GAH separation factor
NF membrane separation performance was significantly affected by physical conditions in this system. When the mass ratio of GAH:NAG was 1:2, the maximum membrane flux was up to 42.3 L m−2 h−1. Under this condition, the permeability difference obtained from the irreversible thermodynamic model was the largest. Therefore, the GAH separation factor under this condition was up to 1.22. When the temperature was 35 °C, the permeability of GAH and NAG was 9.6 and 8.7 L m−2 h−1, respectively, and the permeability difference was the minimum. Therefore, lower operation was not good for GAH and NAG separation. Simultaneously, the calculated molecular diameter and increasing temperature showed good exponential relationship, providing supplement for the separation process. After adding salts, by the analysis using the irreversible thermodynamic model, electrostatic repulsion was the essential influencing factor for GAH rejection, and solute diffusion was an important transport mechanism for NAG rejection. With NaCl addition, the GAH separation factor was up to 1.13. The explored mechanisms could be used to understand the process of NF separation for monosaccharides with similar molecular weights and provide a certain basis for large-scale separation of chitin derivatives in the future.
molecular weight cut-offs
- C p :
solute concentrations in the permeate (g L−1)
- C b :
solute concentrations in the bulk feed (g L−1)
- C m :
solute concentration on the membrane surface (g L−1)
- d c :
calculated molecular diameter (nm)
- J v :
permeat volume flux (L m−2 h−1)
- L p :
pure water permeability (L m−2 h−1 bar−1)
- k :
mass transfer coefficient (L m−2 h−1)
- ΔP :
transmembrane pressure (bar)
- P s :
solute permeability (L m−2 h−1)
- r p :
mean pore radius (nm)
- r s :
solute radius (nm)
- R :
- R o :
- Δπ :
osmotic pressure difference (bar)
- Δx/A k :
the ratio of the effective membrane thickness to membrane porosity (m)
- μ :
solute viscosity (mPa s)
- σ :
the reflection coefficient
This paper is the result of joint efforts. Prof. LZ designed the whole experimental plan and confirmed the main objective of this paper. Prof. JZ developed the statistical methods for experimental data. SZ was responsible for the optimization of the nanofiltration technology and the partial investigation of the mechanism in the process. YQ was responsible for the quantification of total sugars. Prof. LJ and Prof. LF helped us complete the paper writing and correcting some grammatical errors in details. All authors read and approved the final manuscript.
This work is financially supported by the National Natural Science Foundation of China (NO.31371725). It is also supported by the National High Technology Research & Development Program of China (863 Program) (No. SS2014AA021202).
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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