*Alginate: Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.110148*



**Table 1.**

*Removal of pollutants by algal immobilization in alginate beads.*

## **5. Alginate-based hydrogel, composite, and nanocomposite**

Agricultural, industrial, domestic, recreational, and environmental activities may cause water pollution. Therefore, the effluents from metallurgical processes, chemical industries, industrial plants, textile industries, agricultural runoff, and sewage treatment plants contain a high percentage of metal ions, radioactive substances, nutrients, and dyes.

In recent years, wastewater technologies have been developed and methods such as coagulation and oxidation or ozonation, electrocoagulation, flocculation, membrane

*Alginate: Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.110148*

separation, and absorption have been used to remove pollutants such as dyes, nutrients, and heavy metals from wastewater. According to the research, adsorption is economical, simple, has excellent recyclability, efficient process, and high absorption percentage [55].

Adsorption involves interactions like covalent, electrostatic, or in terms of physical and chemical bonds between the adsorbate and the adsorbent. Functional groups of polymers that may be involved in adsorption reactions are –COOH, –CO–, –NH2, –OH, and –SH. Several studies have shown that the best polymers for adsorption are chitosan and alginate.

Asadi and others [56], showed that the adsorption of Methyl Violet (MV)by calcium alginate hydrogel beads and magnetic hydrogel beads was at first fast, and most of the adsorption performs within 10 min (**Figure 2**). The rate of MV adsorption onto calcium alginate hydrogel beads was faster than that of magnetic hydrogel beads.

The use of polymer nanocomposites has been noted because they have a larger surface area, higher mechanical resistance, desirable porosity, and higher hydrodynamic radius [57].

Nanocomposite hydrogels in the presence of nanoparticles (NPs) are three-dimensional networks of hydrophilic polymers that can absorb and sustain a large amount of water.

Incorporation of NPs into the hydrogel matrix leads to improved mechanical strength, high adsorption capacity, and cost reduction [57]. In 2017, Thakur and Arobita [58]used hydrogel nanocomposites of acrylic acid in the presence of sodium alginate biopolymer for removing methyl violet. They concluded that the TiO2 nanoparticles incorporated sodium alginate-g-pacrylic acid hydrogel nanocomposite with a highly porous structure and high percentage grafting for MV dye elimination and successfully achieved high adsorption capacity.

Activated carbon has a very high absorption capacity, but its powder dispersion in treated environments is difficult to remove. This problem is solved by entrapping activated carbon in alginate polymer, which is economical and practical and helps in absorption, and allows it to absorb contaminated aqueous solutions.

Abdel Gawad and Abdel Aziz [59](used entrapped activated carbon in alginate polymer for the removal of ascorbic acid and lactose from aqueous solutions.

Maximum percent removal for ascorbic acid and lactose at pH 3 using a dose of 30 g for 60 min with a fixed stirring rate at 100 rpm was about 70% and 50%, respectively.

Various ways have been proposed to improve the adsorption performance of alginate, including increasing the percentage of calcium (II) in the matrix, which leads to the involvement of several adsorption mechanisms.

The cross-linking of alginate by calcium ions (Ca (II)) can be done by diffusion or internal regulation, which can easily elevate ion exchange as the primary adsorption process. The diffusion method produces gels with varying concentrations of Ca(II) ions, while the internal regulation produces gels with uniform ion concentrations throughout.

One of the advantages of cross-linking is that it offers additional possibilities regarding adsorption mechanisms.

According to the studies, alginate has the potential to be used as a very efficient adsorbent for purifying solutions containing metal ions and toxic pollutants. Alginate is an anionic polymer that has active groups, such as carboxylic, sulfate, phosphate, amine, and hydroxyl groups, which have the capacity to fix metal ions.

Composites, such as chitosan-alginate, play important role in increasing the adsorption capacity.

It seems that the purpose of modifications is to change some properties such as solubility, water absorption capacity, absorption capacity, and temperature resistance.

Derivatization by combining chemical groups in alginate, in general, includes –OH (non-specific reactions) and –COOH (specific reactions). The –OH and –COOH groups of alginate participate in several chemical reactions. The –OH can take part in oxidation, reductive-amination of oxidized alginate, sulfation, copolymerization, and cyclodextrin can be linked with alginate. The –COOH can interfere in esterification, ugi reactions, and amidation.

Modifications in adsorption make more functional groups available so that compounds can be selectively adsorbed by adsorption sites, which increases adsorption capacity and selectivity.

In addition, modifications can be performed by various methods, including mechanical and thermal procedures to make pores, and chemical processes to enhance the surface. There are some methods used for chemical modifications of biomaterials including grafting, cross-linking (ionic or covalent), combining with other adsorbents, polymerization, and copolymerization. There are also terms for physically modified materials, such as magnetic adsorbents (beads and powders), nano- and micro-particles, hydrogel, and aerogel adsorbents.

**Grafting or functionalization** is the addition of chemical groups in the structure of the polysaccharide by covalent bonds, to improve their adsorption capacity and selectivity toward a target metal. Grafting can increase the solubility, stability, and adsorbing capacity of natural polymers.

**Copolymers** such as alginate-bentonite have the benefit of physicochemical and mechanical properties, which are intermediate with the properties obtained by the corresponding homopolymers. Combining polysaccharides with other polymers creates composite materials containing functional groups that improve the elimination of a wide range of pollutants.

**Graft copolymerization** is a common method to increase the adsorption capacity and enhance the chelating or complexing properties by introducing functional groups into the primary structure of alginate.

**Crosslinking** develops a link between macromolecules and creates a threedimensional network via chemical or physical path. Crosslinking can be made by chemical methods (using crosslinking agents; crosslinking corresponds to the creation of covalent bonds between linear chains) or physical methods (resulting from non-covalent linkages between polymer chains by heat curing, electron-beam, or ultraviolet irradiation processes).

Crosslinking agents bind to pollutants such as metal ions by several methods. Biosorbents like alginate have functional groups as an active site, such as carboxyl, hydroxyl, and amine groups, to bind heavy metals, by ion exchange (where –COOH groups are involved) or by a complexation mechanism (where –COOH groups, –OH, –SH and –NH2 may be involved).

A large amount of nitrogen can activate chelation mechanisms and thus increase the absorption percentage. Thus, a high percentage of –S and –O and the increase of –N offer advantages in terms of the mechanisms of sorption.

Benettayeb et al. [2] showed that the addition of amine functions in the alginate structure with a simple bond of urea and biuret increases the adsorption percentage of various metal ions, such as Pb(II), Cu(II), and Cd(II), Ni(II), Zn(II) and Hg(II). Such grafting can improve selectivity and create new reaction possibilities in the adsorption process.

Ca (II)-alginate (CA) does not contain active binding sites for the sorption of Cr (VI). The study of Navarro et al. [60] used Polyethyleneimine (PEI) -Calcium alginate for absorbing Cr (VI) ions from aqueous solutions. PEI has metal chelating properties because of having a large number of amine groups and is used to modify the sorbent surface area to increase sorption capacity.

Isawi [61] synthesized Polyvinyl alcohol/sodium alginate (PVA/SA) beads via blending Polyvinyl alcohol (PVA) with sodium alginate (SA) and the glutaraldehyde used as a cross-linking agent. The zeolite nanoparticles (Zeo NPs) incorporated PVA/ SA resulting Zeo/PVA/SA nanocomposite (NC) beads were made for the elimination of some heavy metals from wastewater. The results indicated that the removal percentage using Zeo/PVA/SA NC beads reached a maximum for Pb2 **+**, Cd2 **+**, Sr2 **+**, Cu2+, Zn2+, Ni2+, Mn2+, and Li2+ with 99.5, 99.2, 98.8, 97.2, 95.6, 93.1, 92.4 and 74.5%, respectively, and the highest removal was achieved for Fe3+ and Al3+ with 96.5 and 94.9%, respectively.

Crosslinking is used to provide Polyvinyl alcohol (PVA)/graphene oxide (GO) sodium alginate (SA) nanocomposite hydrogel beads for removing Pb2+.

It was shown that PVA and SA molecules are embedded in the GO layers through hydrogen bonding interactions. It leads to the destruction of the regular structure of GO, while GO is uniformly distributed in the PVA matrix.

As the concentration of the PVA solution increased, the hydrogel beads became more regular, and a large number of polygonal pores with thin walls and open pores were formed, the average pore size was reduced and a dense network was formed.

At the same time, with the reduction of composite hydrogel permeability, the Pb2+ absorption capacity of hydrogel decreased. As the GO content increased, the ability to become a ball of the hydrogel beads was weakened, the pore size increased, and a relatively loose network structure was formed, which led to an increase in the permeability and Pb2+ adsorption capacity of the hydrogel [5].

Graphene oxide (GO) has the ability to remove contaminants from water. Graphene oxide adsorbents have a higher specific surface area (2630 m2 /g) and several chemical functional groups than the other adsorbents.

Chelation of divalent cations with G block of alginate generates hydrogels.

However, due to the ion exchange between divalent ions in alginate and monovalent ions in solutions, this material lacks mechanical stability and adjustment ability, which often reduces its wider applications.

In the study of Zhuang et al. [62] graphene was introduced into alginate hydrogel that led an increase in mechanical strength from 0.29 MPa (pure alginate hydrogel) to 2.14 MPa (GAD-network), and enhanced adsorption capacity of Cu2+ and Cr2O7 2− up to 169.5 mg g−1 and 72.46 mg g−1 individually.

Graphene/alginate hydrogel can also remove small organic compounds such as ciprofloxacin [63]. Zhuang et al. [62] used CaCO3 as a pore formation agent and a hydrogel with a porous skeleton formed that increased the adsorption of ciprofloxacin from aqueous solutions.

However, reusability is another obstacle to the use of alginate hydrogel. A typical hydrogel swells in solution. If this hydration and volume expansion is unavoidable and irreversible, the mechanical properties will deteriorate drastically. One of the solutions is to make a double network (DN) hydrogel because DN has been proven to show a higher specific surface area, better thermal stability than single network and more resistant to ionic strength:

Conventional double network gels are composed of two interwoven polymer components with complementary structural and mechanical properties. One component is stiff and acts as a skeleton while the other remains elastic and loosely cross-links the hydrogel.

Nanocomposite and DN hydrogels have excellent mechanical performance.

Due to GO's 3D nature and chemical modifiability, it can be effectively integrated with alginate to form a DN nanohydrogel. A new DN hydrogel bead was made composed of GO and sodium alginate (SA) (GO/SA) bead. This new hydrogel can consider as an adsorbent because of the following benefits: (1) the hydrogel bead integrates the characteristics of high specific surface area and thermal stability of graphene oxide and the biocompatibility of sodium alginate; (2) the hydrogel bead retains the functionality of GO and allows reaction in aqueous systems with improved biocompatibility; (3) the hydrogel bead can be quickly separated from wastewater, recycled and regenerated for sustained application.

The adsorption capacity of GAD beads for removing Mn (II) increases 27 folds compared to commonly used granular activated carbon, four times increase than (PVA/CS) hydrogel and double the Al-zeolite composites' capacity.

Finally, it can be mentioned that the direct comparison of these absorbers may be incorrect due to the different characteristics and experimental conditions of the absorbers [64, 65].
