Alginate: Wastewater Treatment

Abstract

Human activities have led to the entry of various pollutants into rivers, seas, and oceans. Various methods are used to remove pollution, one of them is using biopolymers including alginate obtained from brown algae. Due to their special structure and physical properties, availability, biocompatibility, and biodegradability, they can be considered adsorption materials. Alginate hydrogel, composite, and nanocomposite could eliminate methyl violet dye and heavy metals, such as Pb2+, Cd2+, Sr2+, Cu2+, Zn2+, Ni2+, Mn2+, Li2+, and Pb2+. The immobilized microalgal cells in alginate exhibited higher removal efficiency of pollutants from the wastewater as compared to the suspended free cells of microalgal culture and alginate alone. The immobilization of the microalgal cells using alginate could eliminate palm oil, ammonium, phosphate, etc. More research needs to be done but according to researchers, alginate can be a safe substance to remove pollutants from the environment.

1. Introduction

The increase in population and human activities have caused various pollutants to enter the environment. Substances that exist in industrial, agricultural, and domestic wastewater can be toxic to humans and other organisms and have negative effects on them [1].

Hazardous pollutants can be classified into organic and inorganic materials. However, wastewater from chemical industries, the textile industry, industrial effluent, agricultural runoff, sewage treatment plants [2], urban runoff, boating activities [3], and mines include a high amount of chemicals, heavy metals, soaps, nutrients (nitrogen and phosphorous). There is no doubt that it poses a great threat to the ecological environment and human health. Therefore, the treatment of polluted water is very important.

Application of some methods to eliminate these pollutants like flocculation, membrane separation, and adsorption [4] for wastewater treatment has been used and studied for decades. In recent decades, the study and use of living organisms like microalgae and biodegradable components such as alginate as an absorbent of pollutants has increased. There are some modifications and changes also being made. Chemical modifications of biomaterials, including grafting, cross-linking, combining with other adsorbents, polymerization, and copolymerization. They can increase solubility, stability, and adsorbing capacity of natural polymers. Combining specific components with alginate creates composite materials containing functional groups that improve the elimination of a wide range of pollutants [5].

The objective of this chapter is introduction of new methods using alginate and adding algae and some materials in order to increase the capacity to absorb pollutants. It is hoped that the best method of pollutant absorption will be introduced in coming future.

2. Sources of alginate

Marine macroalgae or seaweeds are photosynthetic algae that are abundant in every ocean. There are three main classes, or phyla, of seaweed: Phaeophyta (brown algae), Rhodophyta (red algae), and Chlorophyta (green algae) [6].

It is estimated that 1800 brown, 6200 red, and 1800 green macroalgaehave existed in the marine environment [7]. Macroalgae have occurred across all coastlines, seas, and oceans. They are found from the intertidal zone to the greatest depths (>200 m) that has enough light for supporting their growth [8].

Macroalgae and macroalgal debris are important sources of food for benthic organisms [9]. Seaweeds have been used traditionally as food, drugs, fertilizers, and dyes [6]. Currently, seaweeds are used in many countries especially for food industry, human health (medicine, antioxidant, antibacterial properties), agriculture, biofuel production, bioactive compounds extraction, cosmetic industry, biofertilizing, and wastewater treatment [9, 10].

The cell wall of seaweeds produces large amounts of polysaccharides. Several of these compounds are of commercial value as phycocolloids [11].

Phycocolloids, such as alginate, carrageenan, and agar have gelling properties and are the most commonly used components because of their gelling properties in foods, pharmaceutical and biotechnological applications. Alginic acid, fucoidans, and laminarin are the main polysaccharides in brown algae [6]. Alginates and fucoidans are the major constituents of brown seaweed cell walls representing between 17 and 45%, respectively 5–20% of the algal dry weight [10].

E.C.C. Stanford, a Scottish chemist, discovered alginates from British kelp in the 1880s. During World War II, production units of alginate were set up in Scotland and California using local seaweed resources of brown algae. After the war, other production units were launched close to natural seaweed beds in Norway, France, Germany, Japan, and, more recently, China. The main raw materials were obtained from Macrocystis pyrifera, the Giant Kelp, in California, from Ascophyllum nodosum, Laminaria hyperborea and L. digitata in north-eastern Atlantic, and from Durvillaea and other kelps in the southern hemisphere [12].

In China, the kelp Saccharina japonica (previously called Laminaria japonica), was considered the main source for the production of alginates and was introduced from Japan to China in the 1950s and grown in the Yellow Sea. In China, the mariculture of this alga yields over 2 million wet tons which makes it the single largest maricultural crop in the world by far [12] and it constitutes ~40% of the world’s seaweed production.

In Chile, brown seaweeds such as Lessonia spp., Macrocystis pyrifera and Durvillaea antarctica are used for alginate production and abalone feed and they harvest a total of 100,000 tones y⁻¹. Several other kelps are cultivated or harvested from nature for food and alginates. The cultivation of Saccharina latissima is occurring in eastern Canada and the eastern United States. Alaria esculenta is also cultivated in eastern Canada. Other brown seaweeds include Ascophyllum nodosum, harvested from wild populations for alginate production in eastern Canada and northern Europe. Chile is an important producer of brown seaweeds representing about 10% (250,000 wet tonnes y⁻¹) of the world supply. Lessonia spp. with a large natural population and Macrocystis with a much smaller wild population are used to feed abalone industry and for its alginate content [11].

3. Components and market of alginate

The cell walls of brown seaweeds have alginates as Ca, Mg and Na salts of alginic acid and are partially responsible for their flexibility.

The long chain of alginic acid consists of two uronic acids, mannuronic acid(M) and its C-5 epimer α-(1 → 4)-L-guluronic acid(G). Most of the chains consists of blocks as (−M-)_n, (−G-)_n, and (−MG-)_n. The strengths of alginates depend on Ca²⁺ binding, with guluronic acid having a much greater affinity for Ca²⁺ than mannuronic acid. This effect depends on the more zigzag conformation of polyguluronic acid, which allows Ca²⁺ to fit into the spaces like eggs in an egg carton. The hold fasts of kelp have stiff guluronate-rich alginate and elastic, mannuronate-rich alginates dominants in the blades [11].

The properties of alginates from various brown seaweeds vary considerably as the arrangement of these M and G units(blocks) in the chain and the overall ratio of M/G is species specific. For example, the low G/M ratio of the alginates obtained from Saccharina japonica yields weakly gelling alginates that are only useful in textile printing and paper coating [11].

Composition of the blocks depends on the species being used for extraction and the part of the thallus from which extraction is made. Extraction procedures probably also affect alginate quality. Alginates of one kind or another seem to be present in most species of brown algae but they occur in exploitable quantities (30–45% dry weight) only in the larger kelps and wracks (Laminariales and Fucales). Not all large brown algae have sufficiently large quantities of alginates to merit exploitation, for example, Sargassum muticum, an adventive species from Japan that has recently arrived in the Atlantic and Mediterranean, has, when dry, only 16–18% alginates [12].

The uses of alginates are based on several main properties: First, increasing the viscosity of solutions (i.e. thicken) when dissolved in water. Second, forming a gel when a Ca salt is added in order to displace the Na in the alginates. They do not melt by heating so many products can be produced. Third, having the ability to form films of Na or Ca alginates [11]. Forth, using Calcium in the manufacture of beads made by dropping a sodium alginate solution into a bath of calcium sulphate solution, forming spheres of water with a calcium alginate coating. These beads are used to immobilize cells or to deliver specific chemicals [12].

Alginates are generally of yellowish brown to white in color and available in powder, granular, and filamentous form. Areas with high seaweed reserves caused the creation of alginate plants with relatively low labor costs. Some companies produce high-quality alginate (e.g., Norway), some produce alginate for food, printing and textile (Scotland, France), and others have specialized in low-grade production (e.g., China, Chile), whilst yet other companies have specialized in buying low-grade alginates and then purify them. Over the years, this complex situation has led to establish of one factory in Scotland and harvesting of Macrocystis in California [12].

Alginate market size was over USD 610 million in 2020 and is likely to grow at a CAGR of over 3.3% between 2021 and 2027 [13]. The global alginate market size is projected to reach USD 1.07 billion by 2028 registering a CAGR of 5.0% [14].

High M alginate market is projected to register over 3% CAGR through 2027. Alginates with a higher ratio of ß-D-mannuronic acid as compared to α-l-guluronic acid are referred to as high M alginates. They offer various benefits, such as high biocompatibility, ability to enhance cytokine production, and immunogenicity, stimulating their incorporation in biomedical science & engineering. Recent research indicate that natural biomaterials offer various benefits over synthetic alternatives, such as remodeling, biodegradability, and biocompatibility, triggering the requirement for high M products [13].

4. Application of alginate using microalgae

Pollutants can be removed via physical, chemical and biological methods. For a typical physical removal process, for example, Mechanical filtration and foam separation are difficult to achieve complete removal of pollutants.

Conventional chemical removal methods such as ultraviolet (UV) light, ozone, neutralization, and precipitation produce many byproducts during the process. Biological treatment is widely used due to its lower cost [15].

Application of microalgae for wastewater treatment and nutrients recovery has been studied for decades. Removal of pollutants from wastewater using conventional treatment involves high energy cost for mechanical aeration to provide oxygen for aerobic digestion system. During this process, the aerobic bacteria rapidly consume the organic matter and convert it into single cell proteins, water, and carbon dioxide. Alternatively, this biological treatment step can be accomplished by growing microalgae in the wastewater.

Microalgae have shown great potential to treat industrial wastes such as petroleum, heavy metals, dyes, and toxic gases [16].

Microalgae can remove color from different dyes by biosorption, bioagulation, and bioconversion mechanisms. The biosorption ability of these microorganisms can be related to the high surface area and high binding tendency during the treatment. Also, the functional groups of microalgal surface such as hydroxyl carboxylate and amino phosphate can accumulate ions or dyes on the surface of the microalgal cell biopolymer [17].

Algal cell walls contain a fibrillary skeleton (cellulose) and an amorphous embedding matrix. The algal cell wall is made up of polysaccharides (alginic acid, chitin, xylan), including functional groups as metal-binding sites. Sulfhydryl, hydroxyl, imidazole, amine, phosphate, sulfate, and carboxyl groups are metal ion binding sites of algal surfaces. The algae uptake metals by bonding metal ions with their surface followed by internalization. Two mechanisms in algal biosorption are: (1) ion exchange: ions present on algal surfaces, such as Ca, Mg, Na, and K are displaced by metal ions, (2) complex formation between metal ions and functional groups. Carboxyl and sulfate groups can link with metal ions by covalent bonding. Biosorption can occur using dead biomass (passive process) and bioaccumulation by live algae (active process) [18].

Microalgae can remove toxic, organic substances through cell uptake. Then, the organic contaminant can be biotransformed and mineraliszd within the cells and providing nutrients and energy for growth [19, 20].

Studies have also shown that removing of heavy metals from certain algae is much higher than from zeolite, activated carbon, and synthetic ion exchange resin. Microalgae are important in the secondary/tertiary treatment of municipal wastewater. They can increase removing of heavy metals, nutrients, and pathogens [18].

The advantages of using microalgae to treat wastewater include: the prevention of sludge handling problems; microalgae used in treating wastewater can also be used as fertilizer [21], the photosynthetic activity of microalgae releases oxygen into the water bodies for use by aquatic organisms and it does not require any carbon source for it to remove nutrients from wastewater [22].

Microalgae can grow in autotrophic and heterotrophic conditions, and biofuels like biodiesel and bioethanol can use as a result of the obtained biomass from the wastewater treatment process [17].

The living cells move independently within the treatment bottles. To ensure uniform cell distribution the application of immobilized cells is a good approach [23].

Despite the low cost and shorter hydraulic retention time (HRT) compared to conventional wastewater treatment, this system has low cell loadings and biomass removal issues. The nature of the cells that are negatively charged and small in size (5–50 μm) prevents cell aggregation [24]. Therefore, the removal of the algal biomass from the treated water requires high energy intensive and costly operation.

Various methods have been used to address the biomass removals such as centrifugation, filtration, flotation, and flocculation. On the other hand, immobilization techniques can be used to solve the biomass harvesting issue and simultaneously retain the high-value biomass for further processing [25] lowers the greenhouse gas emission by sequestering carbon dioxide, requiring less space [17] and improving the quality of the treated wastewater [26]. Additionally, immobilized microalgae can grow in more stable and enhancing cell tolerance to unfavorable conditions including high salinity, extreme temperature, metal toxicity, and pH value [17, 27].

Microalgae immobilization using polymers like alginate, agar, and polyurethane was found to be effective in retaining high initial cell loading and recovering algal biomass without significant input of energy while protecting algal cells from toxic pollutants and predation [25].

In comparison to the other immobilization methods, such as adsorption on a surface, encapsulation, entrapment within a matrix,entrapment of microalgal cells in natural(agar, cellulose, alginate, pectin), and synthetic (polyacrylamide, polyurethane, polyvinyl, polypropylene) gel polymers carriers presents advantages of higher nutrients/products diffusion rates, non- toxic, hydrophilic, more environmentally friendly character, greater stability and produce less hazardous waste upon completion of the process [23] and retention time of cells in the reactor is prolonged, metabolic activity of the cells is maintained for long periods, protection of entrapped microorganism from toxic substances in the wastewater by the polymers, increased nutrient removal is achieved because the population of the microorganisms increase within the polymer, competition of the entrapped microorganism with other microorganism in the wastewater is avoided [22], inert aqueous matrix with high porosity that help preserve the physiological properties and functionality of the encapsulated cells [28], reduces the environmental impact. Besides, beads in nutrient-rich wastewater, can be recovered after cultivation, dry, and used as a soil fertilizer [27].

Since the immobilized beads have a larger size than suspended free cells, a simple method by sieving/netting can be employed to harvest the beads from water without requiring high energy input compared to suspended free cells. Solvents are used to dissolve the beads and then oven-dried (where this process is also done in suspended free cells). Also, the immobilized microalgal cells exhibited higher removal efficiency of pollutants from wastewater as compared to the suspended free cells of microalgal culture [18, 23].

Alginate has been regarded as an excellent polysaccharide for immobilization process due to its mild procedure during the process [29]. Studies have shown that in biosorption of pollutants such as nutrients algal beads with alginate are more absorbent than alginate alone [30].

Generally, sodium alginate is used as a matrix for entrapment of microalgae because it has the ability to organize gel beads in the presence of multivalent cations, like Ca⁺². The specific and strong interactions occur between calcium ions and guluronate blocks. During this process, known as the “egg-box” model, the solvent is confined within the spaces of a three-dimensional network, which is connected by cross-linking regions that involve the cooperative association of extended segments of polymer chains [31].

The “diffusion method” is the crosslinking of alginate to form gel beads usually resulting from external gelation with Ca⁺². According to this method, an alginate-cell solution is dripped into a solution including Ca⁺², like calcium chloride. The Ca⁺² ions diffuse from the continuous phase into the internal of the alginate drops and an alginate matrix gell is formed. Cations tend to cross-link the surface of the bead first, bringing the polymer chains closer together to create a less permeable surface.

This results in a highly cross-linked and internal matrix with fewer cross-links [27].

There are several palm-oil mills around the world. Most of them have generated around 800 tones/day of palm oil effluent (POME) resulting from palm oil processing. POME has a high concentration of organic matter identified as chemical oxygen demand (COD) and biochemical oxygen demand (BOD) which are up to 51,000 and 25,000 mg/L, respectively could result in eutrophication of water bodies. Conventional technologies for POME treatment are sometimes still not capable of reducing the level of pollutants. Pollutants present in the wastewater can be assimilated by microalgae that their cell wall are porous which allow free passage of molecules and ions in aqueous solution. These microalgae can be used to produce high-level products such as biodiesel.

In a study, immobilization of the microalgal cells of Nannochloropsis sp. using sodium alginate was carried out to enhance the harvesting and efficiency of POME treatment [23]. The microalgal cell culture was mixed with the sodium alginate solution at a ratio of 1:1 (v/v) and extruded using a syringe to form a spherical shape of the beads in CaCl₂ solution. The microalgal beads were then left overnight for the hardening stage. The procedure was repeated without the addition of microalgal cells culture with blank sodium alginate bead samples. According to the color of the beads microalgal cells were successfully grown inside the beads (Figure 1). The immobilized cells used in the study were more robust against high concentrations of POME, as indicated by a higher microalgal growth cell than the aforementioned suspended free cells of the alga.

Figure 1.

Image of blank sodium alginate beads (A), immobilized sodium alginate microalgal cell beads before treatment (B), and immobilized sodium alginate microalgal cell beads after 10% POME treatment [23].

A noticeable reduction of COD in 9 days was observed, which was 55% for 10% POME (p < 0.05) and 50% for 25% POME (p > 0.05), respectively. The Nannochloropsis sp. cells were in the heterotrophic growth metabolism, which consumed the organic matter as a carbon source for growth resulting in the reduction of COD over the treatment period. Organic carbon is one of the important nutrients for building up microalgal cells. Most of the composition of microalgal cells was made up of carbon [32]. These findings indicated that the immobilized cells were very effective in removing COD at 10% POME with initial COD of 252.5 mg/L [23]. Suspended free cells of Chlamydomonas sp. UKM 6 reduced COD within the range of 8–29% from various concentrations of POME [33], which was lower than immobilized Nannochloropsis sp. cells reported in the study of Emparan et al.

According to Maria and Anggraini [34], the cellular respiration of microalgal cells results in organic carbon assimilation process in the growth media. In this process, the organic compound and oxygen were used as electron donor and final electron acceptors, respectively. Cellular respiration was a source of energy for treatment and biosynthesis in dark conditions. The microalgal cells will consume inorganic carbon sources such as CO₂ and organic carbon derived from the media for their growth during the mixotrophic culture mode [35].

According to [36], microalga Nannochloropsis sp. was immobilized in alginate gel beads and cultivated under optimal conditions that their growth. The immobilized cells were used in phytoremediation of secondary effluent from a palm oil mill and easily recovered by simple sieving method. The immobilized cells contributed to the removal of nitrogen and phosphorus >90% and CO₂ mitigation >99%.

In the study by [25], the combination of Scenedesmus obliquus, Chlorella vulgaris, and Chlorella sorokiniana microalgae were co-immobilized with sludge bacteria in alginate beads for treatment of real meat processing wastewater and an annular photobioreactor was designed to facilitate efficient light transmission at high beads-to-wastewater ratios. Results showed that increasing alginate mixture from 1.25% to 2.00% (w/v) extended beads stability in real wastewater from three to seven days. When beads were suspended in wastewater the overshading effect at beads-to-wastewater ratio of 20% (v/v) was significant and average algal growth (0.17 mg/bead) was lower than 2% (0.46 mg/bead) and 5% (0.36 mg/bead). Nevertheless, the ratio of 20% (v/v) was preferred as the higher beads count may compensate for the disadvantage, however, the light transmission efficiency needed to be improved. In comparison with the suspended photobioreactor, in improved annular photobioreactor algal growth increased from 1412.6 to 3191.0 mg/L, and removal of chemical oxygen demand (COD) and total nitrogen (TN) was improved from 78.5 to 82.9% and from 68.5 to 84.4% respectively with 89.4% total phosphate (PO4³⁻) being removed. By the end of treatment, 135.9 mg/L TN and 99.2 mg/L total PO4³⁻ were removed and it was speculated that most of the nitrogen and phosphorus were removed by algal assimilation rather than the physical adsorption of alginate gel.

Halim and Haron [16] used C. vulgaris immobilized in calcium alginate to study the removal efficiency of main nutrients in wastewater such as ammonium and phosphate.

They used synthetic wastewater and concluded that the immobilized cells showed a higher percentage of ammonium and phosphate removal of 83% and 79% respectively, compared to free-suspended cells (76% and 56%). COD removal recorded was 89% and 83% for immobilized cells and free-suspended cells, respectively. Immobilized cells showed better growth than free suspended cells as reported by other studies reported similar observations on cell growth where the cell number increased rapidly once the beads were added into the medium.

The enhanced removal performance can be described based on the chemical interactions between the alginate matrix and the nutrients. The presence of carboxyl group (COO-) attracts cations and the free binding site on the surface of the alginate matrix provides the medium for physical adsorption for both NH⁴⁺ and PO₄³⁻ ions before assimilation process occur by the encapsulated algae [37]. The calcium ions associated with alginate gelling can precipitate the PO₄³⁻-P ions [38]. In this study, the removal of nutrients by the alginate bead alone was reported to be minimal compared to the alginate-immobilized microalgae cells [37].

Even though the contribution of alginate in removing nutrients is evident, the primary mechanism of nutrient removal through microalgae uptake is more significant compared to the chemical and physical processes. Moreover, the utilization of immobilized cells using alginate matrix provides a protective environment against mechanical shear stress and external microorganisms contamination which enhanced the overall nutrients removal performance [16].

Oluwole and colleagues, in 2019, used C. vulgaris entrapped in calcium alginate beads for removing ammonium nitrogen from synthetic wastewater for 43 hours. They found that there was a rapid reduction in the concentration of NH₄⁺ in the first 19 hours of treatment using the three beads (20, 40, and 80 mg/L concentrations. There was 86%, 97%, and 93% reduction in ammonium nitrogen at the end of the 43 hours experiment using 20 g/l, 40 g/l, and 80 g/l bead concentrations respectively. Too high and too low concentrations of beads are not very effective. High bead concentration (80 g/l) prevents proper light penetration into the bioreactor due to selfshading of dense beads. The metabolic activity and growth of the cells are thus affected. However, too low bead concentration (20 g/l) implies a low concentration of cells available for the treatment of wastewater.

In the research of Tam and Wong [39], C. vulgaris entrapped in calcium alginate as algal beads were employed to remove nutrients (N and P) from simulated settled domestic wastewater. A significantly higher nutrient reduction was found in bioreactors containing algal beads (at concentrations ranging from 4 to 20 beads ml⁻¹ wastewater) than the blank alginate beads (without algae). Complete removal of NH₄⁺-N and around 95% reduction of PO₄³⁻-P was achieved within 24 h of treatment in bioreactors having the optimal algal bead concentration (12 beads ml⁻¹, equivalent to 1:3 algal beads: wastewater, v/v).

Chen-Lin et al. [40] also showed that the specific growth rate and maximumNH₄⁺ uptake of the immobilized Nannochloropsis sp. cells in calcium alginate bead were markedly higher than free cells.

Some research showed that algal-bacterial symbiotic system (ABSS) can remove pollutants from wastewaters higher than single algal or bacterial cultures. The algae consume CO₂, nitrogen and phosphate, and other nutrients in wastewater and release O₂ required for aerobic bacteria to grow and reproduce [15].

There are some examples of the removal of pollutants by algal immobilization in alginate beads in Table 1.

Species	Immobilization technique	Type of effluent	Type of pollutant	Removal efficiency	Source
Scenedesmus rubescens; Chlamydomonas sp.; Chroococcus dispersus	Calcium alginate	Municipal	Orthophosphate	68.8%; 71.9%; 72.3%	[26]
Chlorella vulgaris	Spherical beads	Domestic	N; P; Free cells	95%; 99%; 50%	[38]
Chlorella sorokiniana	Calcium alginate	Synthetic wastewater	Ammonium; Free	43.59%; 38.57%	[41]
Chlorella sp.		Batik effluent	Decolorization; N	80%	[17]
Desmodesmus	Calcium alginate	Batik effluent	methylene blue and malachite green	98%	[42]
Scenedesmus spp. and Chlorella spp.	sodium alginate	Aquaculture system	ammonia-N; total P	42.85%; 44.90%	[30]
Chlorella vulgaris and Azospirillum brasilense	Ca-alginate	municipal wastewater	Ammonium; Phosphate	>50%; >90%	[43]
Tetraselmis chui	Calcium alginate	CuSO4; CdCl2	Cu; Cd	100%; 20%	[44]
Chlorell vulgaris	Alginate-entrapped (microalgae-bacterial consortium & PAC	Municipal	N; P	98.47%; 98.49%	[45]
Desmodesmus sp.	Alginate	Raw domestic	N; P	40–86%; 26–72%	[46]
Chlorella vulgaris; Chorella sp.		Textile wastewater	NH4-N; COD; Color	90%; 75%; 60%	[47]
Scendesmus obliquus,; Chlorella Vulgaris; C. sorokiniana; activated sludge bacteria	Alginate; co-immobilized microalgae-bacteria	Meat processing wastewater	COD; N	72–82%; 75–80%	[48]
Selenastrum capricornutum; Scenedesmus acutus	Alginate	Benzo(a)anthracene (BaA); benzo(a)pyrene (BaP)	P; BaA; BaP	86–92%; 67–85%; 56–83%	[49]
Microalgae bacteria	Microalgae-bacterial flocs (MaB-flocs) using polyvinyl alcohol (PVA) crosslinked with sodium alginate	Synthetic wastewater solutions	NH4-N; PO4^3	61%; 82%	[50]
Nannochloropsis sp	Alginate	Marine water	NH4-N	82.2%	[51]
Scenedesmus bijugatus	Alginate	Pinang River estuary, Malaysia	NH4-N; PO4^3	81–94%; 62–88%	[36]
Chlorella sp.; Oocystis sp.	Alginate	Artificial wastewater	NH4-N; PO4^3; NH4-N; PO4^3	62.27%; 100%; 65.2%; 69%	[52]
Chlorella vulgaris	Ca-alginate immobilized live algae; Immobilized dead algae; Blank beads	Municipal wastewater	Pb(II)	89%; 72%; 68%	[18]
Chlorella sp.and Nitzschia acicularis	Co-immobilized microalgae in alginate; Microalgae; Alginate beads		NH4–N; TP; NH4-N; TP; NH4–N; TP	89%; 96%; 64%; 90%; 40%; 70%	[53]
Chlorella vulgaris	Alginate	Artificial wastewater	Nonylphenol	>98%	[54]
Chlorella vulgaris	Alginate	Artificial wastewater	Tributyltin (TBT)	>90%	[19]
Chlorella vulgaris and activated sludge	Immobilized algal-bacterial beads (IABB) using alginate	Synthetized mariculture tail water	NH3-N; TP; COD	66.7 %; 81.0 %; 90.7%	[15]

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 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.

Figure 2.

Adsorption of methyl violet by alginate-based hydrogel beads [56].

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 three-dimensional 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 Cr₂O₇²⁻ 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].

6. Conclusion

The result of several studies showed that the immobilized algae in alginate beads can be a potential alternative system in the removal of nutrients, heavy metals and organic compounds from primary settled effluent to reduce the contaminant load from wastewater treatment plants to the environment. Meanwhile, application of alginate hydrogel, composite and nanocomposite can be safe methods to remove organic and inorganic substances from water.