A Novel Application of Oceanic Biopolymers — Strategic Regulation of Polymer Characteristics for Membrane Technology in Separation Engineering

Abstract

Membranes prepared from oceanic biopolymers have a high potential in membrane separation processes and water purification. It is anticipated to result in more biocompatible and lower-cost materials compared with artificial polymers. This chapter describes the excellent performance of oceanic biopolymer membranes in separation engineering and the regulation factors controlling membrane properties. In particular, chitosan and alginate were picked up as intelligent membrane materials to provide the promised molecular size recognition and other membrane properties. Future prospective strategies for a simple methodology for preparing stable membranes from oceanic biopolymers and the development of selective separation processing were reviewed.

1. Introduction

1.1. Overview of oceanic biopolymers in membrane separation technology

Membrane separation technology has been applied in various fields, such as the chemical industry, food production, pharmaceutical products, environmental sciences, and water purification, because it can be operated without heating and residual toxicity. Its energy cost was lower than conventional thermal separation technologies [1–4]. It can be employed for broad components by selecting optimum membranes.

Development of oceanic biopolymer membranes such as chitosan and alginate has exponentially increased due to increases in the demand for biocompatibility, environment adaptability, renewability, and selective separation ability [5–6]. In the field of membrane science, most membranes have been made from petroleum-based polymers used as raw materials for the membrane body. Petroleum-based synthetic polymers such as polyethylene, polyamide, polyimide, and polysulfonate have been used as the main materials of the membrane body for a long time. The practical use of biopolymer membranes was less than synthetic polymer membranes, because controlling the physical and chemical properties of a biopolymer produced from natural bioresources was difficult in practical membrane applications. Cellulose-based membranes have been extensively used in practical applications since the development of the anisotropic cellulose acetate membrane in 1963 by Loeb and Sourirajan [7]. Applications of biopolymer materials have increased recently for membrane sciences [8]. Recent studies of biopolymer membranes are summarized in Table 1 referred to previous literatures [9-22]. In the past five years, oceanic biopolymers have attracted attention for their chemical functional ability. Investigations of oceanic biopolymer membranes, especially chitosan and alginate, for various applications have drastically increased.

Table 1.

Recent investigations of biopolymer membrane and applications.

Kashima, Nomoto and Imai 2015.

The common aim of membrane separation technology is to separate a target component from a mixture with the aid of permeation through the membrane, while rejecting other components. The key characteristics for practical use are as follows: mechanical strength, permeation flux, diffusivity of molecule, and micro-/nanostructure. This chapter describes the dominant factors in controlling important properties for practical membrane separation applications for oceanic polymer membrane, especially those of chitosan and alginate.

2. Chitosan membrane

Chitosan is a well-known sustainable and biocompatible oceanic biomaterial. It has been attracted in creating new polymer materials for broad application due to its nontoxicity, biocompatibility, and biodegradability [5]. Chitosan is refined by removing an acetyl group from chitin, which is mainly produced in oceanic bioresources such as the shells of crabs and shrimps. Chitin is the second most abundant natural polymer in nature, after cellulose [23]. Chitosan is generally discarded as industrial waste around the world [24]. It is strongly expected to be useful as a biocompatible and reactive material for making functional gel membranes. Recent studies of various chitosan membranes are summarized in Table 2 and Table 3 referred to previous literatures [5-6, 12, 15-19, 24-46].

2.1. Chemical composition and membrane formation

Chitosan is a heteropolymer obtained by alkaline deacetylation of chitin at the C-2 position as shown in Figure 1 (a) and (b). Chitosan is generally defined by a deacetylation degree (DD [%]) of 60–100%. The deacetylation degree is estimated as follows:

$$DD=\frac{n_{\text{GS}}}{n_{\text{GS}}+n_{\text{AGS}}}\times 100$$E1

Here, n_GS (n_AGS) is the molar number of glucosamine residues (acetylglucosamine residues) in the molecular chain. Chitosan has great potential for chemical modification because its molecular chain has a rich amino group and a hydroxyl group in the glucosamine residues [29].

Figure 1.

Chemical structure of chitosan showing β-1,4-D-glycoside linkage. (a) Chitin molecular chain consisting of poly-β-1,4-D-N-acetylglucosamine residues, (b) chitosan molecular chain consisting of poly-β-1,4-D-glucosamine residues.

The beneficial effects of chitosan are as a dietary fiber, such as its inhibition of fat digestibility [47] and its reduction of cholesterols [48]. The biodegradability and biocompatibility of chitosan are also suitable for biomedical applications [49].

Table 2.

Recent investigations of chitosan membrane (2012–2014).

Kashima, Nomoto and Imai 2015.

Chitosan is dissolved in an acid aqueous solution, such as acetic acid. To form a water-insoluble chitosan membrane, the acetic acid has to be neutralized by basic components. The authors have reported preparation of chitosan membrane using casting methods [16]. For example, 4 g of chitosan was dissolved in 198 mL of 1.7 mol･L^–1 acetic acid aqueous solution. The solution was mixed magnetically for 12 h at room temperature to obtain a mature solution. Some insoluble matter was removed by vacuum filtration using a filter paper (Grade No. 1, ADVANTEC, Japan). The concentration of the chitosan casting solution was 0.02 g_-chitosan･mL_{-chitosan casting solution}^–1. Ten grams of the casting solution was dispensed in a glass Petri dish (inner diameter 7.75 cm) and dried in a thermostatic chamber at 333 K for 24 h. A dried chitosan membrane formed on the glass Petri dish. A 20 mL of sodium hydroxide (NaOH) aqueous solution was directly introduced onto the dried chitosan membrane. The chitosan membrane was continuously immersed for 3 h in NaOH solution for neutralization. The concentration of the supplied NaOH ranged from 0.1 to 5 mol･L^–1. In this case, the stoichiometric equivalent concentration of NaOH was estimated to be 0.83 mol･L^–1. After neutralization, the swollen membrane was easily separated from the glass Petri dish and was washed fully with pure water to remove excess NaOH.

Table 3.

Most recent investigations of chitosan membrane (2015).

Kashima, Nomoto and Imai 2015.

2.2. Dominant role of the acid–base neutralization in chitosan membrane characteristics

The authors previously reported the dominant role of the acid–base neutralization process in forming chitosan membranes for controlling some membrane properties [16].

2.2.1. Mechanical strength

The mechanical strength, maximum tensile stress (Figure 2a), and maximum strain (Figure 2b) at membrane rupture presented bell-shaped curves with peaks when the NaOH concentration increased. For an 81% DD chitosan membrane, the maximum stress and the maximum strain reached maximum values of 8.6 MPa and 80.5%, respectively, in a membrane prepared with 1 mol･L^–1 NaOH. This concentration was almost the same as the stoichiometric equivalent concentration (0.83 mol･L^–1). In contrast, in a 98% DD chitosan membrane, the maximum stress and the maximum strain increase to 17.9 MPa and 134% when the membrane was prepared with 2 mol･L^–1 NaOH.

Figure 2.

Effect of NaOH concentration in neutralization on the maximum stress (a) and maximum strain (b) at membrane rupture in reference [16]. The dashed line represents the stoichiometric neutralization concentration of NaOH (0.83 mol･L^–1).

2.2.2. Water permeation flux

Figure 3 demonstrates that the water permeation flux and the volumetric void fraction of the swollen membrane were almost linearly correlated. The NaOH concentration in the neutralization process enhanced both the water permeation flux and the void fraction in the membrane. Neutralization using a high NaOH concentration weakens hydrogen bonding between chitosan polymer chains. The void fraction in the swollen membrane can assumed to be the volume of a water permeation channel occupying the membrane. This result suggests that the membrane structure involving water permeation was dominantly regulated by the NaOH concentration during neutralization.

Figure 3.

Correlation between the water permeation flux and the void fraction of 81% deacetylation degree chitosan membrane in reference [16]. The concentrations of NaOH used in the neutralization process are indicated at adjacent to keys, respectively.

2.2.3. Mass transfer characteristics

The effective diffusion coefficient of the model components was evaluated based on the mass transfer flux. The membrane was sandwiched between twin mass transfer cells. The feed solution contained the model components (urea 60 Da, Methyl Orange 327 Da, Rose Bengal 1017 Da, and Sirius Red 1373 Da) at the desired concentrations, and the stripping solution was deionized water. The overall mass transfer coefficient K_OL was evaluated from the mass transfer flux according to Eq. (2).

$$\ln (1-\frac{2C_{\text{s}}}{C_{\text{fi}}})=-\frac{2A{K}_{\text{OL}}}{V}t$$E2

Here, C_s and C_fi are the concentrations of the stripping and initial feed solutions, A is the mass transfer area of the membrane, V is the volume of aqueous phase in each transfer cell, and t is the operation time.

The overall mass transfer coefficient K_OL included the film mass transfer resistances (k_L1^–1 and k_L2^–1) and the membrane mass transfer coefficient (k_m) as seen in Eq. (3).

$$K_{OL}{}^{-1}=k_{L1}{}^{-1}+k_m{}^{-1}+k_{L2}{}^{-1}$$E3

The aqueous phases in the mass transfer cells were sufficiently stirred to attain a fully developed turbulent condition (Re > 10⁴). The film mass transfer resistances (k_L1^–1 andk_L2^–1) within the overall mass transfer resistance (K_OL^–1) were ignored under fully turbulent conditions. This directly indicated the membrane mass transfer coefficient (k_m). The effective diffusion coefficient was evaluated from k_m using Eq. (4):

$$k_{\text{m}}=\frac{D_{\text{eff}}}{\ell }$$E4

The initial membrane thickness in the swollen state (ℓ) was measured with a micrometer (Mitutoyo Corporation, Kanagawa, Japan).

Figure 4 depicts the change in the effective diffusion coefficient in the chitosan membrane with the molecular weight of the tested components for 81% DD. The effective diffusion coefficient of a chitosan membrane prepared from NaOH of 1mol･L^–1 changed dramatically at a molecular size corresponding to Rose Bengal (1017 Da). This result suggested that the size distribution of the mass transfer channel in the membrane was monodisperse and similar to the molecular size of Rose Bengal.

Figure 4.

Change of effective diffusion coefficient (D_eff) versus molecular weight of tested components (81% deacetylation) [16].

The effective diffusion coefficient of a membrane prepared from 5 mol･L^–1 NaOH greatly increased relative to a membrane prepared from 1 mol･L^–1. The molecular diffusion channel formed by the polymer networks became enlarged due to the higher NaOH concentration employed.

The dominant role of the acid–base neutralization process in chitosan membranes was revealed to control membrane properties involving mechanical strength, the water permeation flux, and the effective diffusion coefficient.

2.2.4. Morphology of chitosan membrane

Figure 5 presents scanning probe microscope (SPM) photographs of chitosan membrane surfaces. The chitosan membrane made from 1 mol･L^–1 NaOH had a very smooth surface. In contrast, the membrane made from 5 mol･L^–1 NaOH had a somewhat rough surface.

Figure 5.

Scanning probe microscope photographs of chitosan membrane surface. (a) 1 mol L^–1 NaOH, (b) 5 mol L^–1 NaOH [16].

2.3. Other regulation factors

2.3.1. Deacetylation degree

The deacetylation degree (DD) of a chitosan membrane can be stoichiometrically controlled by an acetic anhydride additive. Effects of the deacetylation degree on membrane properties, such as water permeation flux, have been found [50]. The water permeation flux of a membrane prepared from 99.2% DD chitosan was remarkably enhanced about 100-fold compared with a 76.5% DD chitosan membrane. The water permeation flux increased exponentially with increasing deacetylation degree from 81.8% to 92.2% [51].

Increasing the deacetylation degree enhances the surface hydrophilicity of the membrane surface because the acetamido groups on the chitosan membrane are converted into amino groups. This advances the effective formation of hydrogen bonds between the amino and hydroxyl groups and between the amino groups in the chitosan molecules, thus resulting in a dense molecular chain in the membrane [18].

2.3.2. Molecular weights of chitosan

Chitosan membranes prepared by Uragami and coworkers from different molecular weights (13–201 kDa) were tested during pervaporation dehydration of an ethanol aqueous solution. The permeation flux decreased with increasing molecular weight from 13 kDa to 90 kDa, then increased from 90 kDa to 201 kDa. In contrast, the water permeation selectivity from the ethanol aqueous solution increased remarkably with increasing molecular weight up to 90 kDa. Over 90 kDa, the selectivity decreased. These results were understood to indicate that the 90 kDa molecular weight effectively formed entanglements and thus produced the strongest hydrogen bonds between the molecular chains of chitosan [18], although the mechanical strength increased with increasing average molecular weight of chitosan [52].

2.3.3. Addition of a cross-linker

Basically, chitosan can be formed into an insoluble membrane by acid–base neutralization in the casting solution without using a cross-linker. However, the addition of a cross-linker to improve membrane characteristics was carefully investigated.

The most prominent effect of an additive cross-linker is improved mechanical strength. Figure 6 depicts the change of mechanical strength of chitosan membrane (maximum stress vs. maximum strain at membrane rupture) with increasing amount of cross-linker. In a common trend, increasing the amount of cross-linker forms a rigid membrane. The addition of genipin as a cross-linker increased the maximum stress and reduced the maximum strain at membrane rupture [39]. Increasing added amounts of glutaraldehyde, methanol with ethylene glycol diglycidyl ether, and gallic acid also increased the mechanical strength. However, excess addition of cross-linker formed rigid and fragile structures on the chitosan membrane [25–26, 53].

Figure 6.

Change of mechanical strength of chitosan membranes with increasing amount of cross-linker. ●: chitosan membrane cross-linked with genipin [54]. ■: chitosan membrane cross-linked with methanol and ethylene glycol diglycidyl ether [53]. ▲: chitosan membrane cross-linked with glutaraldehyde [25]. ◆: chitosan membrane cross-linked with gallic acid [26], ▼: chitosan cross-linked with tripolyphosphate [55]. Arrows indicate increasing additive amount of each cross-linker.

Other researchers have reported that cross-linking with sodium tripolyphosphate enhanced the mechanical strength of chitosan membrane [17]. From another viewpoint, Shenvi and coworkers determined the effect of the pH value of the cross-linking solution on the membrane properties. A chitosan membrane cross-linked by tripolyphosphate solution at pH 5 realized a higher rejection rate of NaCl and MgSO₄ than a membrane cross-linked at pH 9 [28].

The enhancement of pervaporative dehydration ability of water–alcohol mixtures by adding cross-linker has been frequently reported [56]. A chitosan membrane cross-linked with toluene-2,4-diisocyanate exhibited high pervaporative dehydration ability with an isopropanol aqueous solution in which the selectivity of water α was performed as 472 [57]. The selectivity of water α was calculated using the following equation:

$$\alpha =\frac{y(1-x)}{x(1-y)}$$E5

Here, y is the permeate weight fraction of water and x is the feed weight fraction of water.

2.3.4. Hybrid approaches with other materials

Some hybrid approaches using chitosan and other materials have been proposed to overcome problems in practical use [35]. To make a porous structure on a chitosan membrane, SiO₂, a porogen agent, was mixed with chitosan cast solution and removed from the membrane. The average pore size reached a maximum of 8.5 μm when the mixing ratio of chitosan and SiO₂ was 1:2 [19].

Chitosan membranes are frequently combined with nanomaterial to form a composite medium. A chitosan membrane composited with a multi-walled carbon nanotube was modified by adding perfluorooctanesulfonyl fluoride (PFOSF). The added PFOSF enhanced antibacterial activity due to its remarkable hydrophobicity. The mechanical strength increased with increasing multi-walled carbon nanotubes [44]. Chitosan composited with ZnO nanoparticles was also investigated for antibacterial activity [46]. Chitosan membrane has a good potential as a composited nanomaterial medium for biologically based nanotechnology.

2.3.5. Chemical modification of the chitosan molecular chain

Chitosan has great potential for chemical modification. Functionalized chitosan is often used as the main membrane body, coating agent, or additive agent for functionalization of surface modifications. Graphene oxide functionalizing a chitosan membrane as a surface modification agent on a commercial polyamide membrane has been investigated. The water flux and NaCl rejection of the chitosan membrane were increased by the modification, due to the formation of a dense, thin layer of graphene-oxide-functionalized chitosan, which is better than natural chitosan membrane [40]. Kumar and coworkers prepared various chitosan membranes: a natural chitosan blended with polysulfone membrane, an N-succinylchitosan blended with polysulfone membrane, and an N-propylphosphonyl chitosan blended with polysulfone membrane. These membranes were applied to a heavy metal purification process. N-succinylchitosan blended with polysulfone membrane sufficiently purified Cu, Ni, and Cd [32].

3. Alginate membrane

Alginate is abundantly and sustainably produced by marine biological resources, especially brown seaweed. It has been widely applied in the food industry [58] and as a thickener [59], a suspending agent [60], an emulsion stabilizer [61], a gelling agent [62], and a film-forming agent [63]. In addition, alginate was continuously developed as useful materials for biomedical applications, especially for controlled delivery of drugs and other biologically active compounds and for the encapsulation of cells [64]. In recent pioneering works, alginate has developed as membrane material with excellent molecular selectivity for water-soluble components [65]. Recent studies of various alginate membranes are listed in Table 4 and Table 5 referred to previous literatures [10-11, 15, 22, 66-95].

3.1. Chemical composition and membrane formation of alginate

The molecular chain of alginate is constructed of a block copolymer of β-D-mannuronate (Figure 7a) and α-L-guluronate (Figure 7b) [96]. These two uronates construct a polymeric block in an alginate polymer chain with the following three types of block: homopolymeric blocks of α-L-guluronate (GG blocks), blocks with an alternating sequence in varying proportions of guluronate and mannuronate (MG blocks), and homopolymeric blocks of β-D-mannuronate (MM blocks) [97]. GG blocks chelate alkaline earth-metal ions because of the spatial arrangement of the pyranose ring and the hydroxyl oxygen atoms, and thus create a much stronger interaction than MM blocks and MG blocks [98–99].

Figure 7.

Residues of alginate molecular chain. (a) β-D-mannuronate. (b) α-L-guluronate. (c) Chelation of homopolymeric blocks of α-L-guluronate junction with calcium ions. Alginate was cross-linked by divalent cations according to the “Egg-box” model.

Table 4.

Recent investigations of alginate membrane (2004–2013).

Kashima, Nomoto and Imai 2015.

Table 5.

Most recent investigations of alginate membrane (2014–2015).

Kashima, Nomoto and Imai 2015

Sodium alginate easily forms a cross-link with the presence of divalent cations such as Ca²⁺, resulting in a highly compacted and dense gel network. GG blocks are constructed mainly of a cross-linked zone with Ca²⁺. This is called an “Egg-box junction” (Figure 7c), where the ions were assimilated to “Eggs” [100].

Sodium alginate forms a cross-link in the presence of divalent cations such as Ca²⁺, resulting in a highly compacted gel network. Homopolymeric blocks of α-L-guluronate are constructed mainly of a cross-linked zone with divalent cations. This section describes the impact of this kind of divalent cations and the mass fraction of homopolymeric blocks of α-L-guluronate in an alginate polymer chain (F_GG).

The authors previously reported the preparation of alginate membrane described below [10–11]. A 20 mL sodium alginate aqueous solution (10 g･L^–1) was placed in a Petri dish. The solution was gradually dried at a mild temperature in a desiccator (298 K) or an electrical dryer (303 K) for 24 h to prevent heat degradation. A dried, thin film of sodium alginate was obtained on the Petri dish.

An electrolyte aqueous solution (CaCl₂, SrCl₂, and BaCl₂) was directly introduced onto the dried, thin sodium alginate film as a source of divalent cation for cross-linking. The concentration range of the electrolyte aqueous solutions was 0.1–1.0 mol･L^–1. A stable alginate membrane was quickly formed in the Petri dish at room temperature. After 20 min, the prepared swollen membrane was spontaneously separated from the surface of the Petri dish. The membrane remained in the electrolyte aqueous solution for further 20 min. The membrane was totally immersed in the electrolyte aqueous solution for 40 min. The prepared membrane was repeatedly washed with pure water to remove excess electrolyte, and then stored in pure water.

3.2. Effect of homopolymeric block of α-L-guluronate (F_GG)

The authors previously reported that membrane properties are evidently controlled by the mass fraction (F_GG) [11, 65, 101]. At first, the mass of GG blocks in the actual alginate polymer chain was determined by acid hydrolysis combined with Bitter–Muir’s carbazole sulfuric acid method [102]. The actual alginate was separated into three fractions (GG block, MM block, and MG block) by the acid-hydrolysis method. Acid-hydrolysis protocols were employed according to a previously published method [11, 103]. Their obtained fractions were then determined by Bitter–Muir’s carbazole sulfuric acid method using an optical density of 530 nm (UV-1200, Shimadzu, Kyoto, Japan). The calibration curve for mannuronic acid lactone was obtained and used as the standard component of uronic acid. These methods produced good intensity and accuracy of coloration [104].

The mass of the GG block (W_GG) was obtained from a fractional solution of the GG block by acid hydrolysis. The masses of the MM block (W_MM) and MG block (W_MG) were determined in the same way. The mass fraction of the homopolymeric blocks of α-L-guluronate (F_GG) was then calculated using the following formula:

$$F_{\text{GG}}=\frac{W_{\text{GG}}}{W_{\text{GG}}+W_{\text{MM}}+W_{\text{MG}}}$$E6

F_GG was therefore shown to be a key factor in regulating membrane properties. In this study, two sodium alginates were examined, F_GG 0.18 and F_GG 0.56. Calcium alginate membrane prepared from five types of sodium alginate (F_GG 0.18, 026, 0.35, 0.45, and 0.56) were regulated by mixing with two types of sodium alginate (F_GG 0.18 and 0.56).

3.2.1. Mechanical strength

The effect of F_GG on mechanical strength is depicted in Figure 8. The mechanical strength was determined based on the membrane deformation. The maximum stress was defined at the rupture of the membrane. This value increased with increasing F_GG. In contrast, the maximum strain decreased with increasing F_GG. The mechanical properties of the calcium alginate membrane changed from elastic to plastic transfer, depending on F_GG.

Figure 8.

Effect of F_GG on mechanical strength of calcium alginate membrane [11].

3.2.2. Mass transfer characteristics

An alginate membrane performs superior molecular size recognition on low-molecular-weight components from 60 Da to 600Da [10]. Figure 9 demonstrates that the effective diffusion coefficient in the membrane (D_eff) of saccharides (glucose, G, 180Da; maltose, M, 324 Da; raffinose, R, 504 Da) can be efficiently changed by regulating F_GG. The effective diffusion coefficient in the membrane prepared from F_GG 0.56 alginate was proportional to the –4.6th power of the molecular volume. The diffusion coefficient in bulk aqueous phase (D₀) was inversely proportional to the 0.6th power of the molecular volume [105]. In contrast, the effective diffusion coefficient in the membrane prepared from F_GG 0.18 alginate was proportional to the –2.9th power of the molecular volume. The higher F_GG membrane exhibited more sensitive molecular size recognition. “Egg-box junction” zones constructed from calcium ions and homopolymeric blocks of α-L-guluronate dominantly regulated the mass transfer mechanism of the alginate membrane.

Figure 9.

The effective diffusion coefficient changed remarkably in our experimental range from 180 Da to 504 Da. The symbol ◆ indicates the diffusion coefficient in bulk aqueous solution. Temperature: 303 K. Agitation rate: 14.2 s^–1 [65].

The effect of F_GG on the mass transfer characteristics was evaluated using a permeation test with urea aqueous solution as a typical molecule model. The effective diffusion coefficient (D_eff) of urea in the membrane decreased with increasing F_GG. Figure 10 illustrates the correlation of the volumetric void fraction of the swollen membrane with the ratio between the effective diffusion coefficient of urea in the membrane and the diffusion coefficient in bulk aqueous phase (D₀). Both the effective diffusion coefficient and the volumetric water fraction were restrained by increasing F_GG. The strong dependency of the effective diffusion coefficient of urea on the void fraction contributes to good understanding of the formation of a mass transfer channels in the alginate membrane. These channels are speculated as being monodisperse in a similar size with urea molecule. The mass transfer channel was governed by the mass fraction of homopolymeric blocks of α-L-guluronate (F_GG).

Figure 10.

Correlation of the volumetric void fraction of the swollen membrane with the effective diffusion coefficient of urea [11].

3.2.3. Morphology of the alginate membrane

Figure 11 presents SPM photographs of the calcium alginate membrane surface. The distribution of membrane asperity clearly decays with increasing F_GG. Especially in higher F_GG conditions, many of the GG blocks formed a higher population density of Egg-box junction chelating with Ca²⁺, and the atomic force of the polymer networks was uniform.

Figure 11.

Scanning probe microscope photographs of calcium alginate membrane surface. (a) F_GG = 0.18, (b) F_GG = 0.56.

3.3. Effect of cross-linking divalent cations

The authors prepared a stable alginate membrane cross-linked with CaCl₂, SrCl₂, and BaCl₂. These membranes are sufficiently stable in aqueous phase to apply for practical use.

3.3.1. Water permeability

The water permeation coefficient was evaluated based on the pure water permeation flux as presented in Figure 12. It was decreased logarithmically with increasing concentration of cross-linker. This suggests that the number of permeation channels and/or the size of the permeation channels decreased with increasing concentration of cross-linking ions. The effect of cross-linker concentration on water permeability appeared strongly in the BaCl₂ used, suggesting that Ba²⁺ structures have a much stronger interaction in α-L-guluronate block. Binding of divalent cation to the three kind of copolymer blocks have been investigated [106–107]. The following orders of binding strength indicated by sign of equality were reported:

$$\begin{array}{l}\text{GG block: Ba > Sr> Ca >> Mg}\hfill \\ \begin{array}{l}\text{MM block: Ba > Sr}\approx \text{Ca}\approx \text{Mg}\\ \text{MG block: Ba}\approx \text{Sr}\approx \text{Ca}\approx \text{Mg}\end{array}\hfill \end{array}$$BB1

Hence, the alginate membrane cross-linked with BaCl₂ constructed a dense polymer network due to strong bonding between alginate molecular chain and Ba²⁺.

Figure 12.

Effect of concentration of cross-linking electrolyte solution on water permeation coefficient.

3.3.2. Volumetric water fraction

The volumetric water content of a swollen membrane can be assumed to an indicator of the void fraction of the membrane structure [108]. Figure 13 illustrates the effect of the concentration of electrolyte solution on the void fraction of the membrane. The void fraction decreased with increasing concentration of the cross-linking electrolyte solution. An alginate membrane cross-linked with BaCl₂ was highly densified by the provided Ba²⁺.

Figure 13.

Effect of concentration of cross-linking electrolyte solution on void fraction of swollen alginate membrane.

3.4. Other regulation factors for controlling membrane properties

3.4.1. Addition of cross-linker other than metal ions

Conventional cross-linkers forming a polymer membrane were also examined to realize the alginate membrane. Glutaraldehyde is most commonly employed as a cross-linker to form sodium alginate membranes [83–84]. These membranes cross-linked with glutaraldehyde were developed for use in organic dehydration by pervaporation [66, 79, 82]. A sodium alginate membrane cross-linked with phosphoric acid was prepared for pervaporative dehydration of ethanol aqueous solution [67]. This resulted in 3.5 × 10^–2 kg･m^–2･h^–1 of permeation flux and 2182 of selectivity as defined by Eq. (4). A sodium alginate membrane cross-linked by sodium tartrate was characterized by CO₂ capture from CO₂/N₂ [78].

3.4.2. Hybrids with other polymers or materials

Many efforts have been made to enhance the performance of the alginate membrane by blending it with different hydrophilic polymers. An alginate-blended DNA membrane cross-linked with Mg²⁺ or Ca²⁺ was investigated with regard to the permeation flux of ethanol aqueous solution for pervaporation [95]. The permeation flux and selectivity of a calcium alginate membrane with DNA in pervaporative dehydration for an ethanol aqueous solution were measured as 1.2×10^–2 kg･m^–2･h^–1 and 5500, respectively. In contrast, a magnesium alginate membrane with DNA exhibited a permeation flux and selectivity of 1.2×10^–2 kg･m^–2･h^–1 and 6500.

Hybrid membranes of sodium alginate and dextrin cross-linked with glutaraldehyde had a permeation flux of 9.65 × 10^–2 kg･m^–2･h^–1 permeation flux and a selectivity of 8991 in pervaporative dehydration for an isopropanol aqueous solution [68].

Alginate membranes are frequently combined with nanomaterials to form a composite medium. A calcium alginate membrane containing multi-walled carbon nanotubes was prepared as a new nanofiltration membrane. It had high mechanical strength, antifouling ability, and high rejection of small organic molecules (Congo Red, 697Da) [92].

4. Conclusion

Strategic regulation of an oceanic biopolymer membrane to control its characteristics in membrane separation technology was demonstrated. An oceanic biopolymer chitosan membrane can be easily prepared by casting chitosan in dilute aqueous organic acids and neutralizing it with an alkaline aqueous solution. The dominant role of neutralization for chitosan membrane involves the mechanical strength, the permeation flux, and the mass transfer characteristics. Other regulating factors, such as the deacetylation degree, the average molecular weight, and the addition of a cross-linker were presented.

A calcium alginate membrane performs superior molecular size recognition on low-molecular-weight components from 60 Da to 600 Da due to a dense polymer network consisting of homopolymeric blocks of α-L-guluronate and calcium ions. The kind of cross-linking ion used is also able to control the membrane properties. Barium ion performed a stronger cross-linker in the alginate membrane rather than Ca²⁺ and Sr²⁺, and a highly complex structure was formed. The mass fraction of homopolymeric blocks of α-L-guluronate and cross-linking metal ions were impact factors in regulating the mass transfer characteristics, water permeability, and mechanical strength.

These oceanic polymers are biodegradable, biocompatible, environmentally friendly, stable, and easily available from renewable agricultural resources. In the future, oceanic biopolymer membranes should be developed as an alternative to artificial polymer membranes.

Nomenclature

A Area of membrane surface [m²]

C_fi Initial concentration in the feed solution [mol･L^–1]

C_s Concentration in the stripping solution [mol･L^–1]

D₀ Diffusion coefficient in a bulk aqueous phase estimated from Wilke–Chang’s empirical formula [m²･s^–1]

DD Deacetylation degree [%]

D_eff Effective diffusion coefficient [m²･s^–1]

F_GG Mass fraction of GG block inalginate determined by Eq. (5) [–]

K_OL Overall mass transfer coefficient [m･s^–1]

K_OL^–1 Overall mass transfer resistance [(m･s^–1)^–1]

k_m Membrane mass transfer coefficient [m･s^–1]

k_L1 Film mass transfer coefficient in the feed phase [m･s^–1]

k_L2 Film mass transfer coefficient in the stripping phase [m･s^–1]

ℓ Membrane thickness in the initial state [m]

n_AGS Molar number of acetylglucosamine residues in polymer chain of chitin or chitosan [mol]

n_GS Molar number of glucosamine residues in polymer chain of chitin or chitosan [mol]

Re Reynolds number [–]

t Time [s]

V Volume of aqueous solution in the transfer cell [m³]

W_GG Mass of GG block in sodium alginate [kg]

W_MG Mass of MG block in sodium alginate [kg]

W_MM Mass of MM block in sodium alginate [kg]

x Feed weight fraction of water in pervaporation [–]

y Permeate weight fraction of water in pervaporation [–]

α separation factor on pervaporative dehydration defined by Eq. (4) [–]