The pollution and scarcity of freshwater resources are global problems that have a significant influence on human life. It is very important to remove harmful substances in the water to realize the recycling of water resources. Hydrogels have recently attracted attention due to their special three-dimensional network structure, large surface area, and pores, which show great potential for the removal of pollutants in water. In their preparation, natural polymers are one of the preferred materials because of their wide availability, low cost, and easy thermal degradation. However, when it is directly used for adsorption, its performance is unsatisfactory, so it usually needs to be modified in the preparation process.
1. Adsorption Mechanism and Kinetics of Polysaccharide-Based Hydrogels
The adsorption of the hydrogel can be divided into chemisorption and physisorption. Chemisorption is an irreversible process, mainly because its adsorbent and adsorbent are chemically bonded in the interaction; the destruction of the bond is permanent and, once destroyed, will not be able to bond again. In contrast, physisorption is an irreversible process, usually controlled by physical forces, such as hydrogen bonding, ionic bonding, π–π stacking, hydrophobic interactions, etc., and can be restored after being disrupted. In the preparation of hydrogels, physical action is usually combined with chemical action to strengthen their adsorption properties and mechanical properties, such as in self-healing hydrogels, which usually disperse energy consumption by physical action to ensure that the integrity of internal chemical bonds and mechanical properties is improved; such hydrogels are widely used in bioengineering
[1]. In the preparation of hydrogels in the field of adsorption, researchers have also chosen to combine the two modes of action. The specific adsorption mechanisms and their classification are as follows.
1.1. Adsorption Mechanism
In the adsorption process, different functional groups have different adsorption mechanisms and different modes of action, and functional groups determine the type and strength of intermolecular forces and the chemical reactivity of molecules. The main functional groups involved in the adsorption process of polysaccharide-based hydrogel materials can be divided into three main categories: oxygen-containing functional groups, nitrogen-containing functional groups, and sulfur-containing functional groups. N, O, and S class heteroatoms can contribute to one or more electrons and form coordination bonds with metal ions while also undergoing ion exchange or electrostatic attraction to achieve the adsorption of a wide range of contaminants. Among the various metal adsorption mechanisms reported for hydrogels, the three methods of electrostatic interactions, ion exchange, and surface complexation (including coordination and chelation) have been found to be closely related to surface functional groups
[2]. As shown in
Figure 1, the process of action when adsorption is carried out by these three modes of action is depicted, among which most adsorption mechanisms of the two electrostatic interactions and ion exchange are reversible, and the adsorbents can be reused and belong to physical adsorption.
Figure 1. Schematic diagram of the adsorption process in three modes.
1.2. Adsorption Kinetics
1.2.1. Pseudo-First-Order Kinetic
The pseudo-first-order kinetic model is based on the modal diffusion theory, which assumes that the arrival of the adsorbent from the solution to the adsorbent’s surface is controlled by the diffusion step and that the adsorbent’s surface has only one binding site
[3]. The form of the equation is as follows:
where q
t is the adsorption capacity at time t (mg/g), q
e is the adsorption capacity at the moment of adsorption equilibrium (mg/g), and K
1 is the rate constant of the first-order kinetic.
Although the first-order kinetic model has been widely used for various adsorption processes, it has limitations. It is often only suitable for the kinetic description of the initial stage of adsorption and cannot accurately describe the entire process of adsorption
[4].
1.2.2. Pseudo-Second-Order Kinetic
The pseudo-second-order kinetic model is based on the adsorption rate-limiting step and contains the adsorption mechanism, such as chemisorption, which involves electron sharing or electron transfers between the adsorbate and the adsorbent
[5]. The conformity to the pseudo-second-order kinetic model indicates that adsorption kinetics are mainly controlled by chemical interactions rather than by the material transport steps. The form of the equation is as follows:
where q
t is the adsorption capacity at time t (mg/g), q
e is the adsorption capacity at the moment of adsorption equilibrium (mg/g), and K
2 (g/mg·h) is the pseudo-second-order rate constant.
1.3. Adsorption Isotherms
1.3.1. Langmuir Isotherm Equation
The Langmuir adsorption isotherm model is the most widely used molecular adsorption model, which can predict the maximum adsorption capacity of adsorbents by considering the influence of the adsorbent’s surface and temperature
[6]. This theory is a single molecular layer adsorption theory, which requires a homogeneous solid surface with the same adsorption capacity and no interaction between the adsorbed molecules, but the assumptions of the model are far from the actual conditions, and the information obtained is sometimes highly inaccurate
[7]. The form of the equation is as follows:
where C
e is the equilibrium concentration of the solution, mg/L, Q
e is the adsorption capacity at the moment of adsorption equilibrium (mg/g), Q
m is the maximum adsorption capacity (saturation), mg/g, and K
L is the Langmuir constant related to the affinity and adsorption energy of the bonding site, L/g.
1.3.2. Freundlich Isotherm Equation
The Freundlich isothermal adsorption equation is an empirical equation with no assumptions. The form of the equation is as follows:
where Q
e is the adsorption amount, mg/g, when adsorption reaches equilibrium, C
e is the concentration of adsorbate in solution at adsorption equilibrium, mg/L, K
F is the constant related to adsorption capacity and adsorption strength under the Freundlich model, and 1/n is the Freundlich constant. A large value of K
F is a sign of a better adsorption performance of the adsorbent. Freundlich adsorption isotherms can be obtained by plotting lnq
e against lnC
e at different temperatures
[8].
2. Adsorption Applications of Polysaccharide-Based Hydrogels
2.1. Heavy Metal Ion Adsorption
Heavy metal ions are highly toxic, non-degradable, and bioaccumulative in the environment and circulate through the food chain in water and biological systems, seriously affecting the organisms at the top of the food chain
[9][10]. Heavy metals are metals with a density greater than 4.5 g/cm
3, mainly including Au, Ag, Cu, Pb, Zn, Ni, Co, Cd, Hg, Cd, and more than 40 other kinds of metals
[11]. The five most toxic to humans are lead, mercury, chromium, arsenic, and cadmium. These heavy metals cannot be decomposed in water, and their toxicity is amplified when they enter the human body; therefore, efficient and special methods are needed to remove heavy metal contaminants from water systems
[12]. The adsorption process of heavy metal ions is directly related to the functional groups of the adsorbent materials themselves, and most current studies mainly revolve around the adsorption of divalent heavy metal ions, of which Cu
2+, Pd
2+, and Cd
2+ are predominant.
Table 1 summarizes the polysaccharide-based adsorbent materials in the last two years for this application of heavy metal ion adsorption.
Table 1. Adsorption of heavy metal ions by different polysaccharide-based composite hydrogels.
2.2. Dye Adsorption
There are many methods for removing dyes from wastewater: biological dye removal, acoustic chemical degradation, electrocatalytic degradation, cation exchange membrane technology, etc. However, these processes produce toxic residues that cause secondary pollution and are costly to implement. In contrast, the gel’s adsorption method is simple, efficient, and inexpensive to operate. Hydrogels have a strong dye removal capability, and dyes are more easily diffused in the dissolved hydrogel, which enhances the adsorption capacity via electrostatic interactions with oppositely charged dyes
[39].
There is a wide range of adsorbed dyes, among which methylene blue (MB), malachite green (MG), and methyl orange (MO) fuels are more widely adsorbed. MB is a phenothiazine cationic dye, an alkali, that is used to treat methemoglobinemia in histology and microscopy to identify and detect bacteria, to treat fungal infections by staining tissues
[40], and to stain cotton and wood. The waste dye discharged into the environment after use can be harmful, causing dizziness, headaches, tremors, and mental confusion, among other symptoms
[41]. Malachite green (MG) is a toxic trityl methane chemical, both as a dye and as a bactericidal and parasiticidal chemical, is an alkali, and is prohibited for use in aquaculture. In industries, it is used to color leather, paper, cotton, and silk. However, it is potentially carcinogenic, teratogenic, and mutagenic and is difficult to remove from water
[42].
Table 2 summarizes polysaccharide-based adsorbent materials used in the past two years for this application of dye adsorption.
2.3. Drug Antibiotics Adsorption
In the treatment of contaminants in water, attention has been focused mainly on the adsorption of textile dyes and heavy metals. However, the harmful effects of these emerging contaminants, such as pesticides, herbicides, fungicides, pharmaceutical compounds, and personal care products, on the water environment cannot be ignored. Once the toxic substances of pharmaceutical and medical waste enter the soil, they will be adsorbed by the soil, pollute the soil, kill microorganisms and protozoa in the soil, and destroy the microecology in the soil, which in turn will reduce the soil’s ability to degrade pollutants. Furthermore, the acid, alkali, and salts in the substances will change the nature and structure of the soil, leading to the acidification, alkalization, and hardening of the soil, affecting the development and growth of plant roots, and damaging the ecological environment; meanwhile, many harmful drug pollutants can cause serious damage to the liver and nervous systems. Table 3 summarizes the polysaccharide-based adsorption materials used for antibiotic adsorption in the past two years.