Guide for application of macroporous adsorption resins in polysaccharides purification

Abstract

As a highly selective, recoverable, and low‐cost process, macroporous resins (MPRs) are of interest for the purification of bioactive components from natural products. This guide takes super‐crosslinked MPR as an example, and focuses on application of macroporous adsorption resin in polysaccharide purification, including pretreatment and regeneration of macroporous adsorption resin, various factors affecting the adsorption effect of macroporous adsorption resin, and adsorption mechanism of resin.

INTRODUCTION

Macroporous resin (MPR) is a new kind of nonionic polymer adsorption material, which is prepared by polymerization of polymerization monomer and additives such as crosslinking agent, pore-forming agent and dispersant. The surface has a porous structure, particle size ranges from 0.2 to 1.25 mm, and has a large specific surface area. Adsorption is carried out by molecular sieve, hydrogen bond, and Van der Waals force, and adsorbate is adsorbed into the porous structure of resin by its hydrophobicity and interacts with adsorption sites, so as to achieve the purposes of separation, purification, impurity removal, and concentration (Jin et al., 2015). There are many types and models of resins, and their synthetic materials and structures make adsorption properties very different. Polarity is an important consideration when choosing the appropriate resin. According to its different polarity sizes, it is classified as non-polar MPR, weak-polar MPR, medium-polar MPR, and polar MPR (Table 1). Non-polar resins have strong hydrophobicity and no functional groups, and are suitable for adsorbing non-polar substances, including the most common styrenedivinylbenzene (SDVB) D-101, DM-21, DM-28, HPD-100, HP-20, ADS-5, and so on; the weak polar resin has hydrophilic groups attached to its skeleton structure, and only non-ionized functional groups in its structure, which has considerable specific surface area and suitable pore size, among which AB-8 is the most typical one; medium-polar resins with hydrophilic and hydrophobic surface properties contain ester groups and are suitable for adsorbing nonpolar and polar substances, such as HPD400 with styrene structure and XAD-6 composed of acrylate; polar resins with polar functional groups can adsorb polar substances by electrostatic interaction, such as NKA-9 and S-8 of polystyrene type (Wang et al., 2015; Yang et al., 2022).

Table 1 Physical and chemical properties of the macroporous resins.

Type	Polarity	Particle diameter (mm)	Surface area (m2/g)	Average pore diameter (nm)
D101	Nonpolar	0.3–1.25	600–700	10–12
DM21	Nonpolar	0.3–1.31	550–600	145–160
DM28	Nonpolar	0.3–1.31	600–650	145–160
HP-20	Nonpolar	0.3–1.25	600–650	24–26
HPD100	Nonpolar	0.3–1.25	550–650	90–100
X-5	Nonpolar	0.2–1.25	500–600	29–30
AB-8	Weak-polar	0.3–1.25	480–520	12–14
HPD400	Semi-polar	0.3–1.2	550	83
NKA-9	Polar	0.3–1.25	250–290	15–16.5
S-8	Polar	0.3–1.25	380–420	28–30
HPD600	Polar	0.3–1.2	550–600	85

The internal structure of polysaccharides is complex, and polysaccharides obtained by traditional extraction methods often contain more impurities, such as dyes, proteins, monosaccharides, and other substances, which reduce the purity of polysaccharides and have a greater impact on the subsequent application research of polysaccharides (Wang et al., 2022). Therefore, isolation and purification of crude polysaccharides are very important for obtaining homogeneous polysaccharide fractions, identifying structural characteristics, and determining biological activity. In addition to the efficiency of depigmentation and protein removal, the removal of impurities from plant polysaccharides should also reduce the loss of polysaccharides and avoid the influence of severe reactions on structure of polysaccharides. At present, commonly reported purification methods of polysaccharides include the hydrogen peroxide method, Sevag method for protein removal, activated carbon adsorption, and MPR adsorption (Hu et al., 2022). The decolorization by H₂O₂ oxidation depended on its strong oxidizing ability, which easily led to degradation of polysaccharides and reduction of molecular weight; although structural damage of polysaccharides was small by Sevag method, only a small amount of protein could be removed each time, resulting in low efficiency. In the activated carbon adsorption method, van der Waals force was used to decolorize and deproteinize polysaccharides, but selection coefficients of dyes and polysaccharides were slightly poor, and carbon powder was not easy to remove, resulting in high cost (Hu et al., 2019; Shao et al., 2020; Zhang et al., 2015). By contrast, macroporous adsorption resin was more suitable for polysaccharide impurity removal because of its good stability, strong adsorption, high selectivity, low cost, and reusable (Xi et al., 2015). In recent years, it has been widely used as an adsorbent in the fields of environmental protection, industrial decolorization, medicine, and separation and purification of effective active parts in natural products. Therefore, this paper attempted to introduce the application of several common macroporous adsorption resins in polysaccharide purification process.

ACTION PRINCIPLE OF MACROPOROUS ADSORPTION RESIN

Adsorption is an interface phenomenon, which is aggregation of adsorbed molecules at interface. Adsorption refers to the process in which one or more molecules of a substance adhere to surface of another substance (usually a solid). The process begins with diffusion of chemical substances to outer surface of adsorbent. As shown in Figure 1. In addition to solute moving toward outer surface of particles, solute molecules can penetrate fine spaces of resin (intragranular diffusion) and interact with available sites on the inner surface of polymeric material (Dos Santos et al., 2022). According to the difference of solid surface adsorption force, it can be divided into physical and chemical adsorption. Physisorption is generally a reversible and rapid process of adsorption in which adsorbate is bound to the surface only by van der Waals and electrostatic forces (dipole–dipole forces, polarization forces involving induced dipoles, dispersion force, and induced forces). Van der Waals forces are always present during adsorption, while electrostatic forces act only in the case of adsorbents with ionic structures (such as zeolites and resins). Under normal circumstances, adsorption heat is relatively small and is usually below 50 kJ/mol. Adsorption process generally does not need activation energy, no electron transfer, and no chemical bond formation and damage (Agboola & Benson, 2021; Wang & Guo, 2020). On the contrary, chemisorption generally has a large adsorption heat in the range of 60–450 kJ/mol. Through chemical interaction with electron transfer between adsorbent and adsorbed substance, adsorbed substance is bound by valence force, specific chemical force of association and complex forming free chemical bonds (such as hydrogen bonds), and often needs to be released at a high temperature to produce a new chemical substance which no longer has original properties and is mostly an irreversible process (Foo & Hameed, 2010).

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FACTORS AFFECTING ADSORPTION EFFECT OF MPR

Macroporous adsorption resin is a kind of adsorbent with large specific surface area and porous three-dimensional structure. Its adsorption capacity is affected by its polarity, specific surface area, pore volume, and pore size, as well as nature of loading liquid, nature of desorption solvent, experimental temperature, and other factors (Figure 2). For example, pore structure limits the size of molecules that can be adsorbed, while available surface area limits amount of material that can be adsorbed. The time for adsorbed molecules to adhere to resin surface depends directly on the energy that molecule needs to adsorb on the resin, that is, relationship between force on the surface of these molecules and field force of other adjacent molecules (Table 2).

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Table 2 The main measurement for the evaluation of polysaccharide decolorization.

Names	Resin	Temperature	Concentration	pH	Times/flow rate	Liquid–solid ratio/elution volume	Decolorization ratio (%)	Recovery ratio (%)	Deproteinization ratio (%)	References
Rhododendron dauricum	D101	50°C	8 mg/mL	-	70 min	6 mL/g	72.26	81.56	-	Hu et al. (2022)
Carex meyeriana Kunth	AB-8	-	2.10 mg/mL	-	1.88 BV/h	2.74 BV	66.43	49.63	82.06	Hu et al. (2019)
Meal	HP-20	25°C	30 mg/mL	-	3 BV/h	1.5 BV	-	62	91	Duan et al. (2020)
Toona sinensis (A. Juss.) Roem	D941	45°C	30 mg/mL	8.5	2 BV/h	5.5 BV	91.94	90.05	-	Shi et al. (2017)
Typha angustifolia	LKC100	30°C	1.5 mg/mL	7.0	1.5 BV/h	-	55.60	87.50	69.20	Shi et al. (2019)

Effect of polarity of macroporous adsorption resin

The adsorption effect of MPR is directly related to polarity of separated substances. Adsorption force of resin on compound follows the principle of “similar compatibility,” so resin with similar polarity to compound should be selected. However, in actual separation and purification process, if the polarity of resin and compound is too similar, adsorption is too strong, and adsorbed substances will be difficult to elute from resin, thus affecting recovery and utilization of resin; if the polarity difference is too large, adsorption force is too weak to achieve the purpose of separation. Therefore, compounds with relatively high polarity were generally suitable for separation on moderate-polarity resins, while compounds with relatively low polarity were suitable for separation on nonpolar resins. For medium-polarity macroporous adsorption resins, the more groups capable of forming hydrogen bonds in the molecules of compounds to be separated, the stronger the adsorption capacity (Jia & Lu, 2008; Ma et al., 2011). For example, Tea seed saponin (TSS) was more efficiently adsorbed on weakly polar (AB-8, XDA-6) or non-polar (D4020) resins than tea seed polysaccharide (TSP), and TSP adsorption on polar resin (NKA-9) is more effective than TSS. The polarity of TSP was stronger than that of TSS, and based on the principle of polarity matching, adsorption rate of TSS on AB-8 resin was higher than that on XDA-6 resin (Figure 3) (Yang et al., 2015).

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Effect of pore structure of macroporous adsorption resin

The physical properties of MPRs are usually very important in applications. For example, specific surface area, pore size, and pore distribution have a great impact on adsorption process. The compound molecules to be separated diffuse to the inner surface of resin through pore structure of resin and are adsorbed, and pore size of resin and molecular weight of compound can influence free access of compound molecules, and pore size also influence specific surface area. Specific surface area can play a role only when the aperture ratio separated substance is large enough, and pore distribution is more important at the moment. When pore size was appropriate, adsorption capacity of resin for surfactant increased with the increase of specific surface area. 50 # and 38 # acrylic acid vinegar adsorption resins were the optimal resins screened out by adsorption of two surfactants. Their unique pore distribution characteristics were the important factors affecting their adsorption capacity.

Effect of adsorption temperature on crude polysaccharide solution

The nature of adsorption process depends on the physical or chemical properties of adsorption system and also on environmental conditions such as temperature. For example, at low temperature (20°C), AB-8 resin and DM-130 resin can remove 62.43% and 66.38% colored impurities in soybean oligosaccharide extract respectively within 6 h; however, at higher temperatures (60°C), resin tends to saturate, which generally results in lower removal efficiency, and desorption rates of resins AB-8 and DM-130 decrease to 23.39% and 26.09% respectively, mainly due to the higher temperature which makes molecules more mobile and thus desorbs more rapidly (Wang et al., 2012).

Effect of concentration of crude polysaccharide solution

Loading concentration was also an important factor affecting the adsorption capacity of resin. The relationship between adsorption amount and solution concentration was in accordance with Freundlich and Langmuir's classical adsorption formulas. That is to say, with the increase of loading mass concentration, adsorption rate of resin increased, and then showed a downward trend. This was attributed to the fact that under the condition of low mass concentration, there were more adsorption sites on resin. With the increase of mass concentration of adsorption liquid, total amount of polysaccharides was increased, and adsorption amount of resin was also increased, while desorption amount was very little. And as that mass concentration of adsorption liquid continues to improve, adsorption amount of resin increases, but impurity mass competing for adsorption with polysaccharide also increase, so that resin is easily blocked, and active site can be competitively adsorbed with a target compound at the same time, so that diffusion capacity of polysaccharide in resin is reduced; when adsorption balance is reached between resin and adsorption liquid, that is, mass concentration of solution is continuously increased after saturation state is reached, adsorption amount of resin is reduced (Duan et al., 2020; Nie et al., 2018).

Effect of solution pH

pH determines the ionization degree of adsorbed molecules and thus affects their adsorption affinity. Hydrogen bond plays an important role in the adsorption process of MPR. Under the condition of lower pH, dyes and protein are more easily absorbed. Under the condition of higher pH, hydrogen bond interaction is reduced, resulting in reduction of adsorption capacity of resin. Most polysaccharides are acidic, and decolorization rate and deproteinization rate of S-8 resin in acidic solution are higher than those in neutral and basic solutions. In addition, polysaccharide yield of S-8 resin at pH 6.0 is significantly higher than resin adsorption conditions at pH 4.0 and pH 5.0 (Liu et al., 2010).

Effect of adsorption flow rate

Adsorption capacity of resin is achieved through surface adsorption, size screening, surface electrical properties, hydrogen bond interaction, and so on. When adsorption process reaches critical point, adsorption capacity of resin will reduce or even disappear, and solute will leak from resin. In addition, adsorption of polysaccharides by resin was a dynamic process. The excessive adsorption flow rate shortened the contact time of polysaccharide solution and resin, resulting in dyes or proteins not being fully adsorbed and flowing out with the sample solution, and adsorption efficiency was decreased. Adsorption flow rate is slow, although it reduces leakage of polysaccharides, improves the adsorption rate, but the operation cycle is too long, which also can cause resin regeneration ability to be poor (Wei et al., 2012).

PURIFICATION OF CRUDE POLYSACCHARIDE BY CROSS-LINKED POLYSTYRENE MPR

The widely used polymeric adsorbents polystyrene-divinylbenzene (PS-DVB) copolymers belong to class of super crosslinked MPRs that have a very hard network and a large adsorption capacity compared to other known organic and inorganic adsorbents. In addition, PS-DVB resins are being increasingly used due to their excellent properties, such as high analyte retention, low solvent consumption, short processing time, ease of regeneration, and environmental friendliness. Taking separation and purification of crude polysaccharide from pumpkin residue fermentation broth by MPR as an example, purification effects of different polarity, diameter, and surface area resins (AB-8, S-8, HPH480, HPD100, X-5, and D101) on crude polysaccharide were studied through static adsorption–desorption and adsorption kinetics experiments (Yang et al., 2012).

Preparation of crude polysaccharide

Hot water (Wang et al., 2020), ultrasound (Shi et al., 2022), enzymolysis (Jiang et al., 2020), microwave (Mirzadeh et al., 2021), and other methods are often used for preparation of polysaccharides. Different extraction methods are selected according to test materials and properties of polysaccharides. The main difference lies in different extraction reagents and auxiliary instruments, and yield is different. However, the same point lies in that crude polysaccharides extracted by various methods contain more impurities, and purity is slightly lower. For example, under the optimal conditions (temperature 53°C, ultrasonic time 39 min, and power 672 W), yield of polysaccharides extracted from notoginseng flower was 2.72%, and contents of polysaccharide and protein were 68.21% and 5.33% respectively (Wu et al., 2022). The onion polysaccharide was successively extracted with four different solvents (thermal buffer, chelating agent, dilute alkali, and concentrated alkali) to obtain four components, HBSS, CHSS, DASS, and CASS, with the yield of 8.392%, 3.45%, 5.265%, and 12.982% respectively, and polysaccharide purity was 72.12%, 70.47%, 71.02%, and 76.82% (Zhu et al., 2017).

Pretreatment and regeneration of macroporous adsorption resin

The newly prepared macroporous adsorption resin generally contains fat-soluble substances such as unpolymerized monomers, pore-forming agents, and dispersing agents, and impurities need to be removed through pretreatment before use to obtain better adsorption effect. First, resin was immersed in deionized water for 24 h to fully swell, followed by sequential treatment with 5% HCl and 3% NaOH solutions to remove monomers and porogen trapped in the pores during synthesis, and resin was washed to neutrality with deionized water, respectively. Then soaking resin in 95% alcohol for 24 h, finally washing resin with deionized water until resin has no alcohol smell, collecting resin, and storing resin in an environment with a humidity of about 40°C (Shi et al., 2017).

Resin after many times of use, its adsorption capacity is weakened, will be on the surface and its internal residual part of impurities, after regeneration treatment to continue to use. That is, the surface or internal adsorption resin with a variety of pollutants through special treatment, makes it active again, and restores adsorption capacity, to reuse the adsorption process. During regeneration, resin was firstly washed with 95% alcohol to be colorless, and then washed with a large amount of deionized water to remove ethanol, so that resin can be used again. If resin adsorption of impurities is greater, color is deeper, adsorption capacity is reduced, and it should strengthen regeneration treatment. The method is to add 2%–3% hydrochloric acid solution, which is two to three times the volume of resin, soak it for 6 h, and then wash the resin to be neutral with deionized water. Then soak and rinse with 5% sodium hydroxide solution in that same way, and reuse. Generally, the resin of same variety should not be used again when its adsorption capacity drops by 30%.

Determination and analysis of pigment, protein, and polysaccharide content

Dye content was determined by high-performance liquid chromatography (HPLC). Column [Diamonsil C18 (250 mm × 4.6 mm i.d., 5 μm particle size)] was obtained by Bonna-Agela Technologies Inc (Venusil). The mobile phase is acetonitrile-methanol-dichloromethane (6:2:2, v/v/v) with a flow rate of 1.0 mL/min and an injection volume of 20 μL.

Protein content in polysaccharide solution was determined by Coomassie brilliant blue method (Shi et al., 2019). Using bovine serum albumin as the standard, coomassie brilliant blue G250 (100 mg) was dissolved in 50 mL of 95% ethanol, followed by 100 mL of 85% phosphoric acid, and mixture was diluted to 1000 mL with deionized water to obtain Coomassie brilliant blue reagent. The polysaccharide solution (1 mL) and 5 mL of Coomassie brilliant blue were then mixed in 10 mL tubes and UV-visible values were recorded at 595 nm.

Polysaccharide content of test solution was analyzed by phenol-sulfuric acid method (Zeng et al., 2022). Using glucose as the standard, mix 2 mL of polysaccharide solution with 1 mL of 5% phenol (w/v) and 5 mL of concentrated sulfuric acid, and react in a boiling water bath for 20 min before cooling the tube to room temperature in an ice bath and measuring absorbance at 490 nm.

Static adsorption and desorption tests of resins

Static adsorption and desorption experiments were conducted to evaluate adsorption and desorption ratios and the most effective resin was initially selected. In the adsorption test, 1 g of resin (on a dry weight basis) was added to a closed conical flask containing 50 mL of crude polysaccharide solution and shaken in a 20°C water-bath shaker for 10 h until adsorption was balanced. Liquid phase was then separated from sample solution by centrifugation at 4000g for 10 min.

In the desorption test, 60 mL of NaCl solution (50 mM, pH 7.0) and 60 mL of water in ethanol (70: 30, v/v) were rinsed in a gradient fashion in a water-bath shaker (170 rpm, 20°C) until desorption equilibrated.

In static adsorption and desorption experiments, concentrations of dye, protein and polysaccharide in liquid phase were determined every 20 min. The quantization equation of its parameters is expressed as follows:

Adsorption rate: 1$$A( \% )=\frac{\,{C}_{0}-{C}_{{e}}}{{C}_{0}}\times 100 \% .$$

Desorption rate: 2$$D( \% )={C}_{d}\times \frac{\,{V}_{d}}{\,({C}_{0}-{C}_{{e}}){V}_{{i}}}\times 100 \% .$$

Adsorption capacity: 3$${q}_{{t}}=\frac{\,({C}_{0}-{C}_{{t}}){V}_{{i}}}{W}\times 100 \% .$$

In formula, A and D are adsorption rate (%) and desorption rate (%), respectively. Q_t is adsorption capacity at time t (min) (protein, polysaccharide mg/g dry resin, dye μg/g dry resin). C₀, C_e, C_t, and C_d are concentrations of dye (μg/mL), protein (mg/mL), and polysaccharide (mg/mL) in solution during the initial adsorption phase, adsorption equilibrium, time t (min), and desorption phase, respectively. V_i and V_d are the volume (mL) of crude polysaccharide solution and the volume (mL) of desorption solution, respectively, initially used in the study. W is the weight (g) of dry resin used.

As shown in Figure 4A, adsorption rates of dyes and protein by S-8 and AB-8 resins were higher than those of other four resins, and adsorption ratios of polysaccharides on the six resins were lower than those of dyes and protein. Except for low adsorption rate on HPD100 resin, there was no significant difference in other resins. Figure 4B shows that S-8, AB-8, and HPH480 exhibited higher desorption rates for dyes and protein than other three resins, while six resins exhibited almost identical desorption rates for polysaccharides. In conclusion, S-8 and AB-8 resins had good purification effects on crude polysaccharides.

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Adsorption kinetics

Adsorption kinetics describe the evolution of adsorption process over time before equilibrium is reached. 1 g of resin (based on dry weight) was added into a closed conical flask containing 50 mL of crude polysaccharide solution, and shaken in a water bath shaker at 20°C for 3 h until adsorption balance. The liquid phase was then separated from sample solution by centrifugation at 4000g for 10 min. The Langmuir adsorption rate equation was used to evaluate time course of adsorption: 4$$\mathrm{ln}\frac{{q}_{e}}{{q}_{e}-{q}_{t}}={k}_{t}+m,$$q_t and q_e are time t (min) and adsorption capacity in equilibrium stage (protein, polysaccharide mg/g dry resin, pigment μg/g dry resin), respectively. k is equilibrium adsorption rate, and m is the constant.

Adsorption kinetics curves and adsorption kinetics equilibrium rate constant K of S-8 and AB-8 resins are shown in Figures 5 and 6. Adsorption capacity and adsorption rate of S-8 resin for dyes, proteins, and polysaccharides were higher than those of AB-8 resin. Considering the performances of six resins, S-8 resin was selected as the most suitable resin for the purification of crude polysaccharide due to its high adsorption and desorption ratio as well as high adsorption capacity for dyes and protein.

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Adsorption isotherm

Adsorption isotherm equation was used to describe the relationship between adsorption amount on the solid phase and concentration of adsorbate in the solution when adsorption system reached equilibrium. Langmuir and Freundlich models were the two most commonly used adsorption models, and they had a good fitting effect on solid-solution adsorption system (Table 3). Langmuir isothermal model describes single-layer adsorption on a uniform surface, and there is no interaction between the adjacent adsorbed molecules. Freundlich isotherm model reflects multi-layer adsorption process on a heterogeneous surface. Sample solutions were diluted to different concentrations, and pretreated resin (1 g) was poured into 150 mL conical flasks and 50 mL of crude polysaccharide from pumpkin residue fermentation broth was added, respectively. All conical flasks were sealed and oscillated using a water-bath oscillator at 20°C for 3 h until adsorption equilibrated. Liquid phase was then separated from sample solution by centrifugation at 4000g for 10 min. Two standard theoretical models, Langmuir and Freundlich, are used to describe adsorption behavior: 5$$\frac{{C}_{{e}}}{{q}_{{e}}}=\frac{{C}_{{e}}}{{q}_{{m}}}+\frac{1}{K{L}_{{q}{m}}},$$ 6$${q}_{e}={K}_{F}{{C}_{e}}^{1/n}.$$

Table 3 Langmuir and Freundlich parameters of pigment, protein and polysaccharide on a column packed with S-8 resin.

Absorbate	Langmuir equation	Freundlich equation
qmax	KL	R2	a	1/n	R2
Pigment	24.60	11.30	0.9324	22.64	0.36	0.9278
Protein	2.01	13.36	0.9218	1.86	0.44	0.9356
Polysaccharide	5.31	47.27	0.9289	5.15	0.62	0.9434

In the formula, C_e and q_e represent same parameters as those in formulae (1) and (4); q_m is theoretically calculated maximum adsorption capacity; K_L is Langmuir constant, and 0 < K_L < 1 indicates that isothermal model is available. K_F characterizes the adsorption capacity of the adsorbent. The n value reflects the heterogeneity of adsorbent or adsorption strength coefficient, which is related to the size of adsorption position, adsorption driving force, and energy distribution. The larger the value of n, the better adsorption performance. When the value of 1/n is between 0.1 and 0.5, adsorption easily occurs, but when the value of 1/n is between 0.5 and 1, adsorption hardly occurs.

As shown in Figure 7, with the increase of initial adsorption concentration, adsorption capacity of resin S-8 for dye, protein, and polysaccharide was gradually increased, and when initial concentration of crude polysaccharide was 1.49 mg/mL, adsorption capacity of resin reached saturation. Table 3 summarizes Langmuir and Freundlich parameters used to describe the relationship between solute and liquid phases at equilibrium. When Langmuir model was used to fit resin adsorption process, K_L values were all greater than 1, so Langmuir model was not suitable for explaining adsorption of dyes, protein and polysaccharides by S-8. During resin adsorption process fitted by Freundlich model, 1/n values of dye, protein, and polysaccharide were 0.36, 0.44, and 0.62, respectively, and all showed high correlation coefficients, indicating that S-8 resin was suitable for separation of dye and protein.

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Dynamic adsorption and desorption test

The pretreated S-8 resin was loaded into a glass column (16 mm × 400 mm) with a resin bed volume (BV) of 20 mL for dynamic decolorization. In dynamic adsorption test, crude polysaccharide solutions were successively loaded onto glass columns and tested with S-8 resin at a flow rate of 1 mL/min.

In dynamic desorption test, sodium chloride solution (50 mm, pH 7.0) and water in ethanol (70: 30, v/v) were sequentially eluted at a constant flow rate (1 mL/min) until desorption was balanced. The concentrations of dyes, proteins, and polysaccharides in decolorizing solution were determined.

According to dynamic adsorption curve 8A, critical point capacity can be obtained, that is, the maximum sample volume that can be pre-concentrated without losing analyte during the sample loading process. Critical point is concentration at which outlet solute concentration reaches 15% of inlet concentration. As shown in Figure 8A, before 2 BV (critical point), the dye, protein, and polysaccharide in solution were almost completely absorbed by S-8 resin, and then concentrations of dye, protein, and polysaccharide in effluent rapidly increased until a steady state was reached at 3 BV. Therefore, critical point for S-8 resins is 2 BV. Dynamic desorption curve is shown in Figure 8B, protein and polysaccharide can be desorbed with 8 BV NaCl solution, and then dye can be completely desorbed with 8 BV ethanol-water solution.

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CONCLUSION

The wide application of macroporous adsorption resin technology has brought considerable economic benefits to food, pharmaceutical, and cosmetic industries. When using macroporous adsorption resin as adsorbent, adsorption mechanism, and main influencing factors need to be explored to improve separation and purification efficiency. In the past ten years, with continuous development of macroporous adsorption resin technology, not only adsorption and desorption process has been further optimized, but new resins have been continuously emerging. With the deepening of basic theory and application research, this technology will be more mature in the separation and purification of plant polysaccharides.

ACKNOWLEDGMENTS

This study was financially supported by the Research Fund for the National Natural Science Foundation of China (No. 32172151), the Foreign Cooperation Projects of Fujian Province of China (No. 2021I0007), and the Projects for Scientific and Technological Development of Fujian Agriculture and Forestry University (Nos. CXZX2018069, CXZX2019095G, and CXZX2020120A).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT

Data sharing is not applicable—no new data generated, or the article describes entirely theoretical research.

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