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Comprehensive Advancements in Hydrogel, and Its Application in Microalgae Cultivation and Wastewater Treatment
1Institute of Biological Sciences, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
2Graduate School of Life Sciences and Health, Faculté des Sciences, Université Paris-Saclay, 91400, Orsay, France
3Centre for Research in Development, Social and Environment (SEEDS), Faculty of Social Sciences and Humanities, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia
4Department of Civil Engineering, Faculty of Engineering, Universiti Malaya, Kuala Lumpur 50603, Malaysia
J. Microbiol. Biotechnol. 2025. 35: e2407038
Published January 15, 2025 https://doi.org/10.4014/jmb.2407.07038
Copyright © The Korean Society for Microbiology and Biotechnology.
Abstract
Keywords
Graphical Abstract

Introduction
The rapid growth of the global population [1] has led to increased challenges [2, 3] such as food shortages [4], energy crises [5], and environmental degradation [6]. Traditional agricultural practices, including the excessive use of chemical fertilizers, have exacerbated soil degradation [1] and contributed to environmental pollution. For instance, in North China, heavy nitrogen input from chemical fertilizers [7] has resulted in soil acidity and increased salinity [8].
To address these challenges, the scientific community is increasingly turning to sustainable technologies, particularly the utilization of microalgae. Microalgae are considered as a promising resource for producing biofertilizers [9] and biofuels [10], offering environmental benefits such as carbon sequestration and pollution mitigation. Unlike conventional crops, microalgae do not require arable land and can be cultivated in various environments, making them an ideal candidate for sustainable agricultural [11] and energy solutions [10, 12-15].
Microalgae-based biofertilizers and biofuels not only meet the demand for these products but also contribute to achieving a circular economy and carbon neutrality . However, the large-scale cultivation of microalgae still faces several challenges, including high production costs, low productivity, and energy-intensive processes. Recently, hydrogel technology has emerged as a novel medium for microalgae cultivation to overcome these challenges, offering significant potential. Hydrogels provide a three-dimensional porous structure that supports efficient nutrient and light transport, thereby optimizing microalgal productivity [16].
This review focuses on the application of hydrogel technology in microalgae cultivation and its potential environmental benefits. The analysis highlights the critical role of hydrogels in advancing microalgae-based solutions for sustainable development. Moreover, the application of hydrogels in wastewater treatment has demonstrated significant environmental benefits. For instance, hydrogels can be used to recover nitrogen and phosphorus from wastewater, reducing the ecological problems caused by harmful algal blooms . This technology not only supports nutrient recovery from wastewater but also enhances microalgal biomass production, enabling effective wastewater treatment and resource recovery.
Hydrogel Applications in the Microalgae Cultivation System
Hydrogels are commonly utilized as biomaterial because of their high air permeability [17]. Calcium alginate-based hydrogels have been identified as having optimal permeability for essential elements such as nutrients, light, and gases (O2 and CO2), vital for facilitating the growth of microalgae [18-20]. Moreover, incorporating hydrogel into the microalgae cultivation process [21] can improve space utilization [22], protect microalgae against contamination [23, 24], minimize water consumption and reduce environmental impact [25], enhance photosynthesis [26] and growth rates, and increase the yield of secondary metabolites [27].
On the other hand, the conventional cultivation of microalgae, along with their symbiotic interactions with bacteria in wastewater treatment poses various challenges. Physically separating the microalgae from the bacteria is difficult [28], but this is crucial for the efficient extraction of microalgae biomass for subsequent utilization and the prevention of nutrient-rich microalgae being consumed by other heterotrophic bacteria [23]. Secondly, microalgae may receive insufficient light irradiation in a murky cell suspension with bacteria [29]. Lastly, due to the inherent slower growth rate of microalgae compared to bacteria, it is difficult to operate bioreactors with continuously reduced hydraulic retention durations. Therefore, entrapping microalgae in a porous and light-transmitting matrix such as alginate offers high-quality and acceptable permeability, biocompatibility, and transparency, which ensures unhampered gas exchange and allows for continuous operation, as well as easier harvesting of microalgae from wastewater.
Currently, the immobilization of microalgae, which involves encapsulating living cells in a polymeric matrix [30] allows the permeation of a solution containing nutrients and other components. This technique has emerged as a valuable method for various environmental applications?including the effective removal of undesired substances from water, efficient management of culture collections for CO2 capture, advancement of biosensor technologies [31], and the development of sustainable energy sources, among others [24]. Hydrogel-based immobilized cultivation not only promotes a high growth rate of microalgae in limited space but also eliminates physical-chemical concentration processes, such as flocculation and coagulation. Methods of immobilizing microalgae include using a nanoporous silica matrix [32] and utilizing naturally occurring polysaccharides including agars, carrageenans, and alginates [33].
Significantly, the continuous provision of nutrients through the channels enables microalgae residing within the hydrogel to proliferate and grow under specific ambient conditions over an extended period. This mode of cultivation offers advantages in terms of metabolic bioactivity compared to conventional flat sheet in large solution volume [34]. The quantification of microalgae growth inside the alginate beads exhibited an exponential pattern under symbiotic conditions, indicating that the efficiency of the bioreactor is constrained by the O2 production rates. In one previous study, Calcium alginate-based hydrogels demonstrated exceptional permeability to nutrients, CO2 and O2 [19]. Furthermore, alginate can protect microalgae from native microorganisms [24]. In the co-culture system of
González-Delgado,
Table 1 presents various types of hydrogels used alongside their advantages and disadvantages. It could be seen that different hydrogels exhibit diverse applications based on their distinctive properties. Nanoporous silica matrix hydrogels, characterized by mechanical stability, are particularly suited for environmental use due to their selective pollutant transport capabilities. Alginate-based hydrogels, with excellent biocompatibility and absorbent characteristics, are being used in wound dressing and drug delivery, albeit exhibiting potential adhesion to wounds. Silk fibroin hydrogels, while biocompatible, are limited in their utility for complex medical scenarios. Agar hydrogels have great gelling properties but are susceptible to variations in production conditions. Carrageenan hydrogels, commonly utilized in the food industries, exhibit a reduced swelling capacity. Silk protein-based hydrogels demonstrate biocompatibility and versatile application potential. Poly (2-hydroxyethyl methacrylate)(PHEMA) hydrogels are acknowledged for their durability and suitable for diverse industrial uses. The specific properties of each hydrogel type determine its applicability in various fields.
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Table 1 . Characteristics of hydrogels with their advantages and disadvantages.
Types of hydrogels Advantages Disadvantages References Nanoporous silica Mechanically stable; resistant to degradation by microorganisms; porosity can be adjusted by manipulating the synthesis parameters enabling the unhindered diffusion of high molecular weight molecules; controlled porosity and the potential of silica surface derivatization allowing for selectively transport specific pollutants, imparting distinct selectivity to each module The controlled porosity and ability to modify the surface of the material with silica allow for the targeted transport of specific contaminants, thereby giving each module in the configuration its own unique selectivity. [32] Alginate-based hydrogels Good biocompatibility and liquid absorption capacity. be used in wound dressing, tissue engineering, and drug delivery applications. The risk of harm exists when removing the dried hydrogel or alginate because it adheres to the wound bed; alginate has a tendency to accumulate the exudate emanating from the wound and disperse it towards the intact skin adjacent to the lesion. [62] Silk fibroin hydrogelsmatrix Safety attributes, biocompatibility, regulate the process of degradation, and capacity to seamlessly integrate with other materials The current state of functional silk fibroin hydrogels is insufficient to meet the requirements of complex application scenarios. [63] Agars Excellent gelling properties; form relatively high thermal stability and strength gels. The implementation of rigorous conditions during the alkali pre-treatment led to the partial degradation of the agar fraction, leading to a reduction in molecular weight in comparison to the unpurified extracts; In regards to the agar extraction procedure, the implementation of sonication yielded extracts that exhibited reduced agar contents and molecular weights. [64] Carrageenans Adding κ-carrageenan to the bigel system will increase hardness and decrease cohesiveness. Lower swelling capacity [65] Silk protein-based hydrogel Biocompatibility and ambient conditions gelation; silk--- formidable mechanical characteristics, safe and compatible nature, green sourcing, and v versatility in terms of materials formation. long-term cell vitality and function [66] PHEMA hydrogels Highly durable; maintaining their structural integrity over an extended period of time when exposed to a liquid; PHEMA disks---altering porosities through the manipulation of the polymerization mixturés composition, modifiable surface properties by facilely attaching a diverse array of synthetic or natural molecules onto their surfaces [67]
Advantages of Hydrogel in Microalgae Cultivation
Improve Light Distribution within the Culture
During the cultivation process of microalgae, light is crucial for their growth. Fix microalgae in hydrogel, the adhesive surface has a stronger absorption of light. Therefore, light can be absorbed into the cultivation system. However, with the increase of the thickness of cultivation system, as light is emitted from top of the system, the reflectivity of light becomes stronger, and the permeability weakens. When the thickness of gel is 1 mm, the light transmittance of 2%, 4% and 6% (w/v) silk hydrogels measured at the maximum absorption wavelength (430 nm) of chlorophyll A in microalgae is higher than 60% [36]. The growth of microalgae under low layer of system were influenced, so Pierobon,
In wastewater treatment and CO2 emission reduction, biofilm-based microalgae culture makes it an effective technology for high photosynthetic efficiency and density. Given their large accessible adsorption area, porous materials are ideal substrates for supporting microalgae biofilms. In Fu,
Cultures encapsulated in gel result in the formation of cell aggregates due to the mechanical confinement of the cells. These aggregates significantly influence the light management within gel-encapsulated photobioreactors, thereby strongly affecting photosynthetic efficiency. The heterogeneous distribution of cell aggregates in a hydrogel matrix can enhance light penetration and more effectively mitigate photoinhibition compared to a flat biofilm. Additionally, incorporating scattering particles into the hydrogel matrix can further enhance light harvesting efficiency, resulting in a fourfold increase in biomass growth [21]. Chua,
Increasing Microalgae Productivity
Cultivating microalgae with hydrogels can greatly improve their productivity. Gorin,
Hydrogel culture increases the growth rate of microalgae. Krujatz,
The thickness of gels also influences the growth rate of microalgae. Yuhang Fu tested the effect of varying gel thickness (ranging from 1 to 3 mm) on microalgae activity, using a silk fibroin concentration of 6% and an initial microalgae density of 1 × 103 cells/ml. The microalgae growth rates in the three gel samples were comparable during the initial two days. However, from days three to seven, microalgae in the 1 mm thick gel grew more rapidly compared to those in the 2 mm and 3 mm gels. On day 7, the density of microalgae in sonicated gels with a thickness of 1 mm reached 5.28 × 105 cells/ml, whereas it was only 4.12 × 105 and 3.98 × 105 cells/ml for gels with a thickness of 2 and 3 mm, respectively [36]. Likewise, Pierobon,
Table 2 highlights the increased productivity of microalgae cultivated in hydrogel mediums. For instance, the cultivation of
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Table 2 . High growth of microalgae cultivated in hydrogel system.
Hydrogel Microalgae Cultivation Performance Reference Gelatin Marinichlorella kaistiae KAS603Co-cultivate with bacteria (Erythrobacter sp) 3-fold enhancement of growth compared with monoculture [22] Gelatin-modified poly 2-hydroxyethyl methacrylate (PHEMA) Nannochloropsis sp.,Dunaliella salina andBotryococcus braunii Immobilization and addition of crosslinking agnet 5-20 times higher photosynthetic activity than freshwater microalgae [67] Calcium alginate Synechococcus elongatus PCC 7942Immobilization Achieved 0.08-3.1 × 109 cells/ml production densities comparable to that of biofilms [37]
Immediate Release of Oxygen and Increased Hydrogen Production
The oxygen production efficiency in microalgae-embedded hydrogels is higher than those suspended in solution where microalgae are not immobilized. Fu,
For hydrogen (H) production, immobilized cell systems exhibit a distinctive advantage over conventional cell suspension methods. Das,
Encapsulating Microalgae in Hydrogel to Treat Wastewater
Recovery of Chemical Elements from Wastewaters
The excessive use of chemical materials has resulted in elevated concentrations of chemical elements in wastewater. To address this issue, microalgae can be cultivated in wastewater to utilize these elements, such as P and N, which are required for microalgae growth [44]. Further, in hydrogel system, microalgae are as well able to assimilate these elements effectively from wastewater. As shown in Table 3, Xiao,
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Table 3 . The applications of microalgae-based hydrogels in chemicals removal from wastewaters.
Hydrogel Concentration Conditions Types of microalgae Targeted Chemicals Efficiency Advantages Disadvantages References Mineral-hydrogel composites (calcium phosphate and wollastonite particles) Dry weight: 36% alginate, 48% CaP mineral seed, and 16% wollastonite; 2.3% CaP, 1.7% alginate, and 0.8% wollastonite in water content. Municipal wastewater agricultural run-off Synechococcus elongatus 2973 P→Ca3(PO4)2 Removed 96%
Removed 91%A new approach for capturing P using heterogeneou s mineral nucleation and growth - [45] Na-alginate 2% Wastewater Chlorella vulgaris Total N
Total PFood grade (FG): 42 ± 3, 59 ± 4
Low viscosity (LV): 42 ± 5, 60 ± 1
Medium viscosity (MV): 43 ± 8, 58 ± 1
Wastewater control: −22 ± 37, 26 ± 3
FG: 20 ± 2, 44 ± 11
LV: 20 ± 9, 50 ± 13
MV: 21 ± 6, 55 ± 13
Wastewater control: 4.7 ± 12, −5.9 ± 14LV and MV alginate protect cells from physical damage in wastewater LV and MV alginate restrict nutrients diffusion; FG alginate has highest transmittance, leading greater light and inhibiting cells [68] Extracellula r polymeric substances, Fe-modified hydrogels Un-modified agarose (AG) hydrogel: 1%, 3%, and 5%
un-modified agarosehumic (AH) hydrogel: (SH): AG), 0.1:1, 0.5:1, 1:1, 2:1, and 5:1 (sodium humateSewage Microcystis P AG and AH hydrogels: 33.9 and 67.7 mgPg−1, respectively Adaptable to wide-range pH values (3−10) - [52] Alginate beads 2-4% (w/v) Sodium alginate Synthetic wastewater Chlorella vulgaris &Pseudomonas putida Glucose From 73% without aeration to 100% removal Gas exchange between suspended bacteria and immobilized algae - [33] Carbon black-added alginate 2.0 wt% Sodium alginate, 0.2 wt% carbon black particles - Chlorella sorokiniana NH4
NO3
NO21600 μmol Photons:
100% nitrificationLight-shielding properties enhanced nitrification and ammonia removal - [50] Sodium alginate 2% Acid mine drainage (AMD) Desmodesmus sp. MAS1 andHeterochlorella sp. MAS3 (acid-adapted strains)Fe 80% (in 24 h)
68% (in 24 h)>1.20–1.50- fold with alginate-beads Put dash in column without data or info [19] Polyvinyl alcohol (PVA) & sodium alginate (SA) 10 wt% PVA, 2 wt% SA Deionized water Initial ammonium concentration 77 ± 0.5 mg/l Chlorella vulgaris & nitrifying bacteriaAmmonium 24 h: 28.47 and 23.80 mg/l for 5%, 16% biomass (without activated carbon, AC) 48 h: 21.93 mg/l for 5% and 16.19 mg/l for 16% (without AC) [48]
When microalgae were encapsulated together with bacteria in hydrogel, the removal efficiency increased significantly.
The nitrification performance of the consortia was influenced by illumination conditions immobilizing consortia of
Improved hydrogel materials have higher ion adsorption capacity. Hydrogels made from extracellular polymeric substances (EPS) and modified with iron (Fe) were used to recover phosphorus. The hydrogels were composed of agarose (AG) and agarose-humic (AH). The AG and AH hydrogels exhibited adsorption capacities of 33.9 and 67.7 mg P g−1, respectively. Both hydrogels, in their original form, exhibited an adsorption capacity of above 75% at the pH circumstances studied (pH 3−10) [52]. And in these modified-hydrogels, the presence of coexisting anions (
Moreover, pH values and viscosity affect the chemicals removal. The phosphate removal was significantly hindered by pH values that ranged from neutral to acidic (28.9 ± 0.8% at pH = 6.00). However, a basic pH of around 8.50 was found to be the most advantageous, resulting in a phosphate removal rate of up to 90.8% at pH = 8.67 [53]. Therefore, by closely monitoring and regulating external parameters such as pH, temperature, and salinity, the removal process may be enhanced while working with existing technology [54]. Also, biopolymers promoted the cells for further reuse and treatment and protected cells from toxic environment [54]. Moreover, the pH, alkalinity, and salinity of the reverse osmosis concentrate (ROC) have a collective influence on the physical stability, chemical properties, biomass production, and nutrient removal efficiency. The alginate beads containing
Different types of microalgae have different ion absorption efficiency. Immobilized microalgae reached a removal efficiency of up to 60% (
Higher Removal Efficiency of Pollutants
Human activities have led to the indiscriminate discharge of a wide range of pollutants into extensive aquatic ecosystems, including rivers, seas, and oceans. Alginate, a biopolymer derived from brown algae, has emerged as a promising material for pollution mitigation. Its inherent structural composition, physical attributes, widespread accessibility, compatibility with biological systems, and biodegradability confer it with notable adsorptive properties. Consequently, these materials, including hydrogels, composites, and nanocomposites, have demonstrated potential in effectively removing methyl violet dye and heavy metals through various mechanisms. The immobilization of microalgal cells within an alginate matrix has shown superior capacity in removing pollutants from wastewater, surpassing the effectiveness of freely suspended microalgal cells and alginate alone. Based on the findings of Rafiee, it has been postulated that alginate possesses the potential to serve as a benign agent for eliminating contaminants from the environment [46].
For the removal of heavy metals from wastewater, Leon-Vaz,
Table 4 illustrates the effectiveness of hydrogels in pollutant removal. For instance, CMC-Fe3O4 hydrogels showed a maximum adsorption capacity of 600 mg/g for methylene blue and 100 mg/g for CdCl2, indicating their high efficiency in removing pollutants from water. These findings underlined the potential of hydrogels as effective materials for environmental remediation.
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Table 4 . Pollutants removal by treatment of microalgae-based hydrogel system.
Hydrogel Microalgae Pollutants Performance Assessment Reference CMC-Fe3O4 Methylene blue (MB) CdCl2 Maximum amount: 600 mg/g dried
hydrogel; qm = 620 ± 100 mg/g,
KL = 0.8 ± 0.1
qm =100 ± 10, KL = 0.06 ± 0.01Eco-safety
High adsorption of MB and Cd[69] Biochar and chitosan (BC) Ciprofloxacin (CIP)
enrofloxacin (ENR)pH = 7
1.0 g/l BC, 300min
1.5 g/l BC, 450 minA low-cost adsorbent [70] Nanocellulose from Chlorella vulgaris Methylene blue (MB) Add microalgae Nanocellulose into CMC/SA,
adsorption capacity: 28.57 mg/g
Dye removal: 24.4%; CMC/SA/
60AH: 109.03 mg/gShowed similar characteristics and adsorption capacity with commercial Nanocellulose [71]
Future Perspectives and Conclusion
The cultivation of microalgae within hydrogels and encapsulating microalgae in hydrogels for wastewater treatment represent burgeoning areas of microalgal research. In this review, a multitude of benefits of hydrogel-based microalgae cultivation have been summarized, exhibiting its considerable values as a viable alternative of conventional cultivation and advantages of this cultivating mode in wastewater treatment, compared with non-hydrogel. The integration of hydrogels in microalgae cultivation presents a host of opportunities to enhance productivity, optimize resource use, and expand applications. Here are key future perspectives to consider:
• Optimizing Light Distribution: Hydrogels can be engineered to improve light penetration and distribution within microalgae cultures, thereby increasing photosynthetic efficiency.
• Enhanced Nutrient Delivery: Hydrogels can serve as a medium for delivering nutrients directly to microalgae, ensuring optimal growth conditions and potentially reducing the need for external fertilization.
• Development of Smart Hydrogels: The advancement of responsive or "smart" hydrogels that can adapt to environmental changes could significantly improve the efficiency and adaptability of microalgae cultivation systems.
•Wastewater Treatment Applications: By encapsulating microalgae within hydrogels, the efficiency of nutrient and pollutant removal from wastewater can be significantly improved, creating a sustainable cycle of resource recovery.
• Agricultural Use: Hydrogel-based microalgae composites can be used as soil conditioners to enhance water and nutrient retention, providing a slow-release fertilizer effect and promoting plant growth.
• Bioenergy Production: Harnessing microalgae cultivated in hydrogels to produce biofuels, particularly hydrogen, presents a sustainable alternative to fossil fuels, given microalgaés rapid growth and CO2 sequestration capabilities.
• Environmental Remediation: Hydrogel-based microalgae systems have shown high efficiency in removing organic and inorganic pollutants from water, suggesting potential for broader environmental applications.
• Addressing Commercialization Challenges: While hydrogel-based cultivation offers many advantages, challenges such as light limitation in high-density cultures need to be addressed. Future research should focus on strategies to overcome these barriers, facilitating the large-scale commercial application of this technology.
By pursuing these directions, the future of microalgae cultivation using hydrogels may lead to more sustainable practices, higher yields, and a broader impact on various sectors, from agriculture to energy production.
Author Contributions
GT. Y, JL. Z: Conceptualization, Writing - Original Draft Preparation, Visualization; R. A: Literature Search, Supervision, Project Administration, Writing - Review & Editing; WY. C: Supervision, Project Administration, Writing - Review & Editing; DH. Z: Writing - Review & Editing, Visualization; TC. L: Supervision, Project Administration, Writing - Review & Editing.
Funding
This work was funded by the Universiti Malaya (Grant Number: MG017-2022) and the Universiti Kebangsaan Malaysia (Geran Gerakan Pensyarah Muda, Grant Number: GGPM-2022-048).
Conflict of Interest
The authors have no financial conflicts of interest to declare.
Highlights
•Microalgae are cultivated in using hydrogel system.
•Cultivation parameters to enhance pollutants removal and biomass productivity.
•Immobilized microalgae in using hydrogel system for environmental sustainability.
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Related articles in JMB

Article
Review
J. Microbiol. Biotechnol. 2025; 35():
Published online January 15, 2025 https://doi.org/10.4014/jmb.2407.07038
Copyright © The Korean Society for Microbiology and Biotechnology.
Comprehensive Advancements in Hydrogel, and Its Application in Microalgae Cultivation and Wastewater Treatment
Guangtao Yang1†, Jinglin Zhang2†, Rosazlin Abdullah1*, Wai Yan Cheah3*, Dehua Zhao4, and Tau Chuan Ling1
1Institute of Biological Sciences, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
2Graduate School of Life Sciences and Health, Faculté des Sciences, Université Paris-Saclay, 91400, Orsay, France
3Centre for Research in Development, Social and Environment (SEEDS), Faculty of Social Sciences and Humanities, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia
4Department of Civil Engineering, Faculty of Engineering, Universiti Malaya, Kuala Lumpur 50603, Malaysia
Correspondence to:Rosazlin Abdullah, rosazlin@um.edu.my
†These authors contributed equally to this work.
Abstract
Microalgae are recognized as a sustainable resource to produce biofertilizers, biofuels, and pigments, with the added benefits of environmental sustainability, such as carbon sequestration and pollutant removal. However, traditional cultivation methods face challenges like low biomass productivity and high operational costs. This review focuses on the innovative use of hydrogels as a medium for microalgae cultivation, which addresses these challenges by enhancing nutrient permeability, light distribution, and overall growth efficiency. Hydrogels provide a three-dimensional matrix that not only supports higher biomass yields but also facilitates the removal of pollutants from wastewater, contributing to circular economy goals. The review also explores the environmental benefits, challenges, and prospects of integrating hydrogel technology into microalgae cultivation systems. By highlighting influencing factors through which hydrogels improve microalgal productivity and environmental outcomes, this work aims to provide insights into the potential of hydrogel-based systems for sustainable development.
Keywords: Microalgae, cultivation, hydrogel, wastewater treatment
Introduction
The rapid growth of the global population [1] has led to increased challenges [2, 3] such as food shortages [4], energy crises [5], and environmental degradation [6]. Traditional agricultural practices, including the excessive use of chemical fertilizers, have exacerbated soil degradation [1] and contributed to environmental pollution. For instance, in North China, heavy nitrogen input from chemical fertilizers [7] has resulted in soil acidity and increased salinity [8].
To address these challenges, the scientific community is increasingly turning to sustainable technologies, particularly the utilization of microalgae. Microalgae are considered as a promising resource for producing biofertilizers [9] and biofuels [10], offering environmental benefits such as carbon sequestration and pollution mitigation. Unlike conventional crops, microalgae do not require arable land and can be cultivated in various environments, making them an ideal candidate for sustainable agricultural [11] and energy solutions [10, 12-15].
Microalgae-based biofertilizers and biofuels not only meet the demand for these products but also contribute to achieving a circular economy and carbon neutrality . However, the large-scale cultivation of microalgae still faces several challenges, including high production costs, low productivity, and energy-intensive processes. Recently, hydrogel technology has emerged as a novel medium for microalgae cultivation to overcome these challenges, offering significant potential. Hydrogels provide a three-dimensional porous structure that supports efficient nutrient and light transport, thereby optimizing microalgal productivity [16].
This review focuses on the application of hydrogel technology in microalgae cultivation and its potential environmental benefits. The analysis highlights the critical role of hydrogels in advancing microalgae-based solutions for sustainable development. Moreover, the application of hydrogels in wastewater treatment has demonstrated significant environmental benefits. For instance, hydrogels can be used to recover nitrogen and phosphorus from wastewater, reducing the ecological problems caused by harmful algal blooms . This technology not only supports nutrient recovery from wastewater but also enhances microalgal biomass production, enabling effective wastewater treatment and resource recovery.
Hydrogel Applications in the Microalgae Cultivation System
Hydrogels are commonly utilized as biomaterial because of their high air permeability [17]. Calcium alginate-based hydrogels have been identified as having optimal permeability for essential elements such as nutrients, light, and gases (O2 and CO2), vital for facilitating the growth of microalgae [18-20]. Moreover, incorporating hydrogel into the microalgae cultivation process [21] can improve space utilization [22], protect microalgae against contamination [23, 24], minimize water consumption and reduce environmental impact [25], enhance photosynthesis [26] and growth rates, and increase the yield of secondary metabolites [27].
On the other hand, the conventional cultivation of microalgae, along with their symbiotic interactions with bacteria in wastewater treatment poses various challenges. Physically separating the microalgae from the bacteria is difficult [28], but this is crucial for the efficient extraction of microalgae biomass for subsequent utilization and the prevention of nutrient-rich microalgae being consumed by other heterotrophic bacteria [23]. Secondly, microalgae may receive insufficient light irradiation in a murky cell suspension with bacteria [29]. Lastly, due to the inherent slower growth rate of microalgae compared to bacteria, it is difficult to operate bioreactors with continuously reduced hydraulic retention durations. Therefore, entrapping microalgae in a porous and light-transmitting matrix such as alginate offers high-quality and acceptable permeability, biocompatibility, and transparency, which ensures unhampered gas exchange and allows for continuous operation, as well as easier harvesting of microalgae from wastewater.
Currently, the immobilization of microalgae, which involves encapsulating living cells in a polymeric matrix [30] allows the permeation of a solution containing nutrients and other components. This technique has emerged as a valuable method for various environmental applications?including the effective removal of undesired substances from water, efficient management of culture collections for CO2 capture, advancement of biosensor technologies [31], and the development of sustainable energy sources, among others [24]. Hydrogel-based immobilized cultivation not only promotes a high growth rate of microalgae in limited space but also eliminates physical-chemical concentration processes, such as flocculation and coagulation. Methods of immobilizing microalgae include using a nanoporous silica matrix [32] and utilizing naturally occurring polysaccharides including agars, carrageenans, and alginates [33].
Significantly, the continuous provision of nutrients through the channels enables microalgae residing within the hydrogel to proliferate and grow under specific ambient conditions over an extended period. This mode of cultivation offers advantages in terms of metabolic bioactivity compared to conventional flat sheet in large solution volume [34]. The quantification of microalgae growth inside the alginate beads exhibited an exponential pattern under symbiotic conditions, indicating that the efficiency of the bioreactor is constrained by the O2 production rates. In one previous study, Calcium alginate-based hydrogels demonstrated exceptional permeability to nutrients, CO2 and O2 [19]. Furthermore, alginate can protect microalgae from native microorganisms [24]. In the co-culture system of
González-Delgado,
Table 1 presents various types of hydrogels used alongside their advantages and disadvantages. It could be seen that different hydrogels exhibit diverse applications based on their distinctive properties. Nanoporous silica matrix hydrogels, characterized by mechanical stability, are particularly suited for environmental use due to their selective pollutant transport capabilities. Alginate-based hydrogels, with excellent biocompatibility and absorbent characteristics, are being used in wound dressing and drug delivery, albeit exhibiting potential adhesion to wounds. Silk fibroin hydrogels, while biocompatible, are limited in their utility for complex medical scenarios. Agar hydrogels have great gelling properties but are susceptible to variations in production conditions. Carrageenan hydrogels, commonly utilized in the food industries, exhibit a reduced swelling capacity. Silk protein-based hydrogels demonstrate biocompatibility and versatile application potential. Poly (2-hydroxyethyl methacrylate)(PHEMA) hydrogels are acknowledged for their durability and suitable for diverse industrial uses. The specific properties of each hydrogel type determine its applicability in various fields.
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Table 1 . Characteristics of hydrogels with their advantages and disadvantages..
Types of hydrogels Advantages Disadvantages References Nanoporous silica Mechanically stable; resistant to degradation by microorganisms; porosity can be adjusted by manipulating the synthesis parameters enabling the unhindered diffusion of high molecular weight molecules; controlled porosity and the potential of silica surface derivatization allowing for selectively transport specific pollutants, imparting distinct selectivity to each module The controlled porosity and ability to modify the surface of the material with silica allow for the targeted transport of specific contaminants, thereby giving each module in the configuration its own unique selectivity. [32] Alginate-based hydrogels Good biocompatibility and liquid absorption capacity. be used in wound dressing, tissue engineering, and drug delivery applications. The risk of harm exists when removing the dried hydrogel or alginate because it adheres to the wound bed; alginate has a tendency to accumulate the exudate emanating from the wound and disperse it towards the intact skin adjacent to the lesion. [62] Silk fibroin hydrogelsmatrix Safety attributes, biocompatibility, regulate the process of degradation, and capacity to seamlessly integrate with other materials The current state of functional silk fibroin hydrogels is insufficient to meet the requirements of complex application scenarios. [63] Agars Excellent gelling properties; form relatively high thermal stability and strength gels. The implementation of rigorous conditions during the alkali pre-treatment led to the partial degradation of the agar fraction, leading to a reduction in molecular weight in comparison to the unpurified extracts; In regards to the agar extraction procedure, the implementation of sonication yielded extracts that exhibited reduced agar contents and molecular weights. [64] Carrageenans Adding κ-carrageenan to the bigel system will increase hardness and decrease cohesiveness. Lower swelling capacity [65] Silk protein-based hydrogel Biocompatibility and ambient conditions gelation; silk--- formidable mechanical characteristics, safe and compatible nature, green sourcing, and v versatility in terms of materials formation. long-term cell vitality and function [66] PHEMA hydrogels Highly durable; maintaining their structural integrity over an extended period of time when exposed to a liquid; PHEMA disks---altering porosities through the manipulation of the polymerization mixturés composition, modifiable surface properties by facilely attaching a diverse array of synthetic or natural molecules onto their surfaces [67]
Advantages of Hydrogel in Microalgae Cultivation
Improve Light Distribution within the Culture
During the cultivation process of microalgae, light is crucial for their growth. Fix microalgae in hydrogel, the adhesive surface has a stronger absorption of light. Therefore, light can be absorbed into the cultivation system. However, with the increase of the thickness of cultivation system, as light is emitted from top of the system, the reflectivity of light becomes stronger, and the permeability weakens. When the thickness of gel is 1 mm, the light transmittance of 2%, 4% and 6% (w/v) silk hydrogels measured at the maximum absorption wavelength (430 nm) of chlorophyll A in microalgae is higher than 60% [36]. The growth of microalgae under low layer of system were influenced, so Pierobon,
In wastewater treatment and CO2 emission reduction, biofilm-based microalgae culture makes it an effective technology for high photosynthetic efficiency and density. Given their large accessible adsorption area, porous materials are ideal substrates for supporting microalgae biofilms. In Fu,
Cultures encapsulated in gel result in the formation of cell aggregates due to the mechanical confinement of the cells. These aggregates significantly influence the light management within gel-encapsulated photobioreactors, thereby strongly affecting photosynthetic efficiency. The heterogeneous distribution of cell aggregates in a hydrogel matrix can enhance light penetration and more effectively mitigate photoinhibition compared to a flat biofilm. Additionally, incorporating scattering particles into the hydrogel matrix can further enhance light harvesting efficiency, resulting in a fourfold increase in biomass growth [21]. Chua,
Increasing Microalgae Productivity
Cultivating microalgae with hydrogels can greatly improve their productivity. Gorin,
Hydrogel culture increases the growth rate of microalgae. Krujatz,
The thickness of gels also influences the growth rate of microalgae. Yuhang Fu tested the effect of varying gel thickness (ranging from 1 to 3 mm) on microalgae activity, using a silk fibroin concentration of 6% and an initial microalgae density of 1 × 103 cells/ml. The microalgae growth rates in the three gel samples were comparable during the initial two days. However, from days three to seven, microalgae in the 1 mm thick gel grew more rapidly compared to those in the 2 mm and 3 mm gels. On day 7, the density of microalgae in sonicated gels with a thickness of 1 mm reached 5.28 × 105 cells/ml, whereas it was only 4.12 × 105 and 3.98 × 105 cells/ml for gels with a thickness of 2 and 3 mm, respectively [36]. Likewise, Pierobon,
Table 2 highlights the increased productivity of microalgae cultivated in hydrogel mediums. For instance, the cultivation of
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Table 2 . High growth of microalgae cultivated in hydrogel system..
Hydrogel Microalgae Cultivation Performance Reference Gelatin Marinichlorella kaistiae KAS603Co-cultivate with bacteria (Erythrobacter sp) 3-fold enhancement of growth compared with monoculture [22] Gelatin-modified poly 2-hydroxyethyl methacrylate (PHEMA) Nannochloropsis sp.,Dunaliella salina andBotryococcus braunii Immobilization and addition of crosslinking agnet 5-20 times higher photosynthetic activity than freshwater microalgae [67] Calcium alginate Synechococcus elongatus PCC 7942Immobilization Achieved 0.08-3.1 × 109 cells/ml production densities comparable to that of biofilms [37]
Immediate Release of Oxygen and Increased Hydrogen Production
The oxygen production efficiency in microalgae-embedded hydrogels is higher than those suspended in solution where microalgae are not immobilized. Fu,
For hydrogen (H) production, immobilized cell systems exhibit a distinctive advantage over conventional cell suspension methods. Das,
Encapsulating Microalgae in Hydrogel to Treat Wastewater
Recovery of Chemical Elements from Wastewaters
The excessive use of chemical materials has resulted in elevated concentrations of chemical elements in wastewater. To address this issue, microalgae can be cultivated in wastewater to utilize these elements, such as P and N, which are required for microalgae growth [44]. Further, in hydrogel system, microalgae are as well able to assimilate these elements effectively from wastewater. As shown in Table 3, Xiao,
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Table 3 . The applications of microalgae-based hydrogels in chemicals removal from wastewaters..
Hydrogel Concentration Conditions Types of microalgae Targeted Chemicals Efficiency Advantages Disadvantages References Mineral-hydrogel composites (calcium phosphate and wollastonite particles) Dry weight: 36% alginate, 48% CaP mineral seed, and 16% wollastonite; 2.3% CaP, 1.7% alginate, and 0.8% wollastonite in water content. Municipal wastewater agricultural run-off Synechococcus elongatus 2973 P→Ca3(PO4)2 Removed 96%
Removed 91%A new approach for capturing P using heterogeneou s mineral nucleation and growth - [45] Na-alginate 2% Wastewater Chlorella vulgaris Total N
Total PFood grade (FG): 42 ± 3, 59 ± 4
Low viscosity (LV): 42 ± 5, 60 ± 1
Medium viscosity (MV): 43 ± 8, 58 ± 1
Wastewater control: −22 ± 37, 26 ± 3
FG: 20 ± 2, 44 ± 11
LV: 20 ± 9, 50 ± 13
MV: 21 ± 6, 55 ± 13
Wastewater control: 4.7 ± 12, −5.9 ± 14LV and MV alginate protect cells from physical damage in wastewater LV and MV alginate restrict nutrients diffusion; FG alginate has highest transmittance, leading greater light and inhibiting cells [68] Extracellula r polymeric substances, Fe-modified hydrogels Un-modified agarose (AG) hydrogel: 1%, 3%, and 5%
un-modified agarosehumic (AH) hydrogel: (SH): AG), 0.1:1, 0.5:1, 1:1, 2:1, and 5:1 (sodium humateSewage Microcystis P AG and AH hydrogels: 33.9 and 67.7 mgPg−1, respectively Adaptable to wide-range pH values (3−10) - [52] Alginate beads 2-4% (w/v) Sodium alginate Synthetic wastewater Chlorella vulgaris &Pseudomonas putida Glucose From 73% without aeration to 100% removal Gas exchange between suspended bacteria and immobilized algae - [33] Carbon black-added alginate 2.0 wt% Sodium alginate, 0.2 wt% carbon black particles - Chlorella sorokiniana NH4
NO3
NO21600 μmol Photons:
100% nitrificationLight-shielding properties enhanced nitrification and ammonia removal - [50] Sodium alginate 2% Acid mine drainage (AMD) Desmodesmus sp. MAS1 andHeterochlorella sp. MAS3 (acid-adapted strains)Fe 80% (in 24 h)
68% (in 24 h)>1.20–1.50- fold with alginate-beads Put dash in column without data or info [19] Polyvinyl alcohol (PVA) & sodium alginate (SA) 10 wt% PVA, 2 wt% SA Deionized water Initial ammonium concentration 77 ± 0.5 mg/l Chlorella vulgaris & nitrifying bacteriaAmmonium 24 h: 28.47 and 23.80 mg/l for 5%, 16% biomass (without activated carbon, AC) 48 h: 21.93 mg/l for 5% and 16.19 mg/l for 16% (without AC) [48]
When microalgae were encapsulated together with bacteria in hydrogel, the removal efficiency increased significantly.
The nitrification performance of the consortia was influenced by illumination conditions immobilizing consortia of
Improved hydrogel materials have higher ion adsorption capacity. Hydrogels made from extracellular polymeric substances (EPS) and modified with iron (Fe) were used to recover phosphorus. The hydrogels were composed of agarose (AG) and agarose-humic (AH). The AG and AH hydrogels exhibited adsorption capacities of 33.9 and 67.7 mg P g−1, respectively. Both hydrogels, in their original form, exhibited an adsorption capacity of above 75% at the pH circumstances studied (pH 3−10) [52]. And in these modified-hydrogels, the presence of coexisting anions (
Moreover, pH values and viscosity affect the chemicals removal. The phosphate removal was significantly hindered by pH values that ranged from neutral to acidic (28.9 ± 0.8% at pH = 6.00). However, a basic pH of around 8.50 was found to be the most advantageous, resulting in a phosphate removal rate of up to 90.8% at pH = 8.67 [53]. Therefore, by closely monitoring and regulating external parameters such as pH, temperature, and salinity, the removal process may be enhanced while working with existing technology [54]. Also, biopolymers promoted the cells for further reuse and treatment and protected cells from toxic environment [54]. Moreover, the pH, alkalinity, and salinity of the reverse osmosis concentrate (ROC) have a collective influence on the physical stability, chemical properties, biomass production, and nutrient removal efficiency. The alginate beads containing
Different types of microalgae have different ion absorption efficiency. Immobilized microalgae reached a removal efficiency of up to 60% (
Higher Removal Efficiency of Pollutants
Human activities have led to the indiscriminate discharge of a wide range of pollutants into extensive aquatic ecosystems, including rivers, seas, and oceans. Alginate, a biopolymer derived from brown algae, has emerged as a promising material for pollution mitigation. Its inherent structural composition, physical attributes, widespread accessibility, compatibility with biological systems, and biodegradability confer it with notable adsorptive properties. Consequently, these materials, including hydrogels, composites, and nanocomposites, have demonstrated potential in effectively removing methyl violet dye and heavy metals through various mechanisms. The immobilization of microalgal cells within an alginate matrix has shown superior capacity in removing pollutants from wastewater, surpassing the effectiveness of freely suspended microalgal cells and alginate alone. Based on the findings of Rafiee, it has been postulated that alginate possesses the potential to serve as a benign agent for eliminating contaminants from the environment [46].
For the removal of heavy metals from wastewater, Leon-Vaz,
Table 4 illustrates the effectiveness of hydrogels in pollutant removal. For instance, CMC-Fe3O4 hydrogels showed a maximum adsorption capacity of 600 mg/g for methylene blue and 100 mg/g for CdCl2, indicating their high efficiency in removing pollutants from water. These findings underlined the potential of hydrogels as effective materials for environmental remediation.
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Table 4 . Pollutants removal by treatment of microalgae-based hydrogel system..
Hydrogel Microalgae Pollutants Performance Assessment Reference CMC-Fe3O4 Methylene blue (MB) CdCl2 Maximum amount: 600 mg/g dried
hydrogel; qm = 620 ± 100 mg/g,
KL = 0.8 ± 0.1
qm =100 ± 10, KL = 0.06 ± 0.01Eco-safety
High adsorption of MB and Cd[69] Biochar and chitosan (BC) Ciprofloxacin (CIP)
enrofloxacin (ENR)pH = 7
1.0 g/l BC, 300min
1.5 g/l BC, 450 minA low-cost adsorbent [70] Nanocellulose from Chlorella vulgaris Methylene blue (MB) Add microalgae Nanocellulose into CMC/SA,
adsorption capacity: 28.57 mg/g
Dye removal: 24.4%; CMC/SA/
60AH: 109.03 mg/gShowed similar characteristics and adsorption capacity with commercial Nanocellulose [71]
Future Perspectives and Conclusion
The cultivation of microalgae within hydrogels and encapsulating microalgae in hydrogels for wastewater treatment represent burgeoning areas of microalgal research. In this review, a multitude of benefits of hydrogel-based microalgae cultivation have been summarized, exhibiting its considerable values as a viable alternative of conventional cultivation and advantages of this cultivating mode in wastewater treatment, compared with non-hydrogel. The integration of hydrogels in microalgae cultivation presents a host of opportunities to enhance productivity, optimize resource use, and expand applications. Here are key future perspectives to consider:
• Optimizing Light Distribution: Hydrogels can be engineered to improve light penetration and distribution within microalgae cultures, thereby increasing photosynthetic efficiency.
• Enhanced Nutrient Delivery: Hydrogels can serve as a medium for delivering nutrients directly to microalgae, ensuring optimal growth conditions and potentially reducing the need for external fertilization.
• Development of Smart Hydrogels: The advancement of responsive or "smart" hydrogels that can adapt to environmental changes could significantly improve the efficiency and adaptability of microalgae cultivation systems.
•Wastewater Treatment Applications: By encapsulating microalgae within hydrogels, the efficiency of nutrient and pollutant removal from wastewater can be significantly improved, creating a sustainable cycle of resource recovery.
• Agricultural Use: Hydrogel-based microalgae composites can be used as soil conditioners to enhance water and nutrient retention, providing a slow-release fertilizer effect and promoting plant growth.
• Bioenergy Production: Harnessing microalgae cultivated in hydrogels to produce biofuels, particularly hydrogen, presents a sustainable alternative to fossil fuels, given microalgaés rapid growth and CO2 sequestration capabilities.
• Environmental Remediation: Hydrogel-based microalgae systems have shown high efficiency in removing organic and inorganic pollutants from water, suggesting potential for broader environmental applications.
• Addressing Commercialization Challenges: While hydrogel-based cultivation offers many advantages, challenges such as light limitation in high-density cultures need to be addressed. Future research should focus on strategies to overcome these barriers, facilitating the large-scale commercial application of this technology.
By pursuing these directions, the future of microalgae cultivation using hydrogels may lead to more sustainable practices, higher yields, and a broader impact on various sectors, from agriculture to energy production.
Author Contributions
GT. Y, JL. Z: Conceptualization, Writing - Original Draft Preparation, Visualization; R. A: Literature Search, Supervision, Project Administration, Writing - Review & Editing; WY. C: Supervision, Project Administration, Writing - Review & Editing; DH. Z: Writing - Review & Editing, Visualization; TC. L: Supervision, Project Administration, Writing - Review & Editing.
Funding
This work was funded by the Universiti Malaya (Grant Number: MG017-2022) and the Universiti Kebangsaan Malaysia (Geran Gerakan Pensyarah Muda, Grant Number: GGPM-2022-048).
Conflict of Interest
The authors have no financial conflicts of interest to declare.
Highlights
•Microalgae are cultivated in using hydrogel system.
•Cultivation parameters to enhance pollutants removal and biomass productivity.
•Immobilized microalgae in using hydrogel system for environmental sustainability.
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Table 1 . Characteristics of hydrogels with their advantages and disadvantages..
Types of hydrogels Advantages Disadvantages References Nanoporous silica Mechanically stable; resistant to degradation by microorganisms; porosity can be adjusted by manipulating the synthesis parameters enabling the unhindered diffusion of high molecular weight molecules; controlled porosity and the potential of silica surface derivatization allowing for selectively transport specific pollutants, imparting distinct selectivity to each module The controlled porosity and ability to modify the surface of the material with silica allow for the targeted transport of specific contaminants, thereby giving each module in the configuration its own unique selectivity. [32] Alginate-based hydrogels Good biocompatibility and liquid absorption capacity. be used in wound dressing, tissue engineering, and drug delivery applications. The risk of harm exists when removing the dried hydrogel or alginate because it adheres to the wound bed; alginate has a tendency to accumulate the exudate emanating from the wound and disperse it towards the intact skin adjacent to the lesion. [62] Silk fibroin hydrogelsmatrix Safety attributes, biocompatibility, regulate the process of degradation, and capacity to seamlessly integrate with other materials The current state of functional silk fibroin hydrogels is insufficient to meet the requirements of complex application scenarios. [63] Agars Excellent gelling properties; form relatively high thermal stability and strength gels. The implementation of rigorous conditions during the alkali pre-treatment led to the partial degradation of the agar fraction, leading to a reduction in molecular weight in comparison to the unpurified extracts; In regards to the agar extraction procedure, the implementation of sonication yielded extracts that exhibited reduced agar contents and molecular weights. [64] Carrageenans Adding κ-carrageenan to the bigel system will increase hardness and decrease cohesiveness. Lower swelling capacity [65] Silk protein-based hydrogel Biocompatibility and ambient conditions gelation; silk--- formidable mechanical characteristics, safe and compatible nature, green sourcing, and v versatility in terms of materials formation. long-term cell vitality and function [66] PHEMA hydrogels Highly durable; maintaining their structural integrity over an extended period of time when exposed to a liquid; PHEMA disks---altering porosities through the manipulation of the polymerization mixturés composition, modifiable surface properties by facilely attaching a diverse array of synthetic or natural molecules onto their surfaces [67]
-
Table 2 . High growth of microalgae cultivated in hydrogel system..
Hydrogel Microalgae Cultivation Performance Reference Gelatin Marinichlorella kaistiae KAS603Co-cultivate with bacteria (Erythrobacter sp) 3-fold enhancement of growth compared with monoculture [22] Gelatin-modified poly 2-hydroxyethyl methacrylate (PHEMA) Nannochloropsis sp.,Dunaliella salina andBotryococcus braunii Immobilization and addition of crosslinking agnet 5-20 times higher photosynthetic activity than freshwater microalgae [67] Calcium alginate Synechococcus elongatus PCC 7942Immobilization Achieved 0.08-3.1 × 109 cells/ml production densities comparable to that of biofilms [37]
-
Table 3 . The applications of microalgae-based hydrogels in chemicals removal from wastewaters..
Hydrogel Concentration Conditions Types of microalgae Targeted Chemicals Efficiency Advantages Disadvantages References Mineral-hydrogel composites (calcium phosphate and wollastonite particles) Dry weight: 36% alginate, 48% CaP mineral seed, and 16% wollastonite; 2.3% CaP, 1.7% alginate, and 0.8% wollastonite in water content. Municipal wastewater agricultural run-off Synechococcus elongatus 2973 P→Ca3(PO4)2 Removed 96%
Removed 91%A new approach for capturing P using heterogeneou s mineral nucleation and growth - [45] Na-alginate 2% Wastewater Chlorella vulgaris Total N
Total PFood grade (FG): 42 ± 3, 59 ± 4
Low viscosity (LV): 42 ± 5, 60 ± 1
Medium viscosity (MV): 43 ± 8, 58 ± 1
Wastewater control: −22 ± 37, 26 ± 3
FG: 20 ± 2, 44 ± 11
LV: 20 ± 9, 50 ± 13
MV: 21 ± 6, 55 ± 13
Wastewater control: 4.7 ± 12, −5.9 ± 14LV and MV alginate protect cells from physical damage in wastewater LV and MV alginate restrict nutrients diffusion; FG alginate has highest transmittance, leading greater light and inhibiting cells [68] Extracellula r polymeric substances, Fe-modified hydrogels Un-modified agarose (AG) hydrogel: 1%, 3%, and 5%
un-modified agarosehumic (AH) hydrogel: (SH): AG), 0.1:1, 0.5:1, 1:1, 2:1, and 5:1 (sodium humateSewage Microcystis P AG and AH hydrogels: 33.9 and 67.7 mgPg−1, respectively Adaptable to wide-range pH values (3−10) - [52] Alginate beads 2-4% (w/v) Sodium alginate Synthetic wastewater Chlorella vulgaris &Pseudomonas putida Glucose From 73% without aeration to 100% removal Gas exchange between suspended bacteria and immobilized algae - [33] Carbon black-added alginate 2.0 wt% Sodium alginate, 0.2 wt% carbon black particles - Chlorella sorokiniana NH4
NO3
NO21600 μmol Photons:
100% nitrificationLight-shielding properties enhanced nitrification and ammonia removal - [50] Sodium alginate 2% Acid mine drainage (AMD) Desmodesmus sp. MAS1 andHeterochlorella sp. MAS3 (acid-adapted strains)Fe 80% (in 24 h)
68% (in 24 h)>1.20–1.50- fold with alginate-beads Put dash in column without data or info [19] Polyvinyl alcohol (PVA) & sodium alginate (SA) 10 wt% PVA, 2 wt% SA Deionized water Initial ammonium concentration 77 ± 0.5 mg/l Chlorella vulgaris & nitrifying bacteriaAmmonium 24 h: 28.47 and 23.80 mg/l for 5%, 16% biomass (without activated carbon, AC) 48 h: 21.93 mg/l for 5% and 16.19 mg/l for 16% (without AC) [48]
-
Table 4 . Pollutants removal by treatment of microalgae-based hydrogel system..
Hydrogel Microalgae Pollutants Performance Assessment Reference CMC-Fe3O4 Methylene blue (MB) CdCl2 Maximum amount: 600 mg/g dried
hydrogel; qm = 620 ± 100 mg/g,
KL = 0.8 ± 0.1
qm =100 ± 10, KL = 0.06 ± 0.01Eco-safety
High adsorption of MB and Cd[69] Biochar and chitosan (BC) Ciprofloxacin (CIP)
enrofloxacin (ENR)pH = 7
1.0 g/l BC, 300min
1.5 g/l BC, 450 minA low-cost adsorbent [70] Nanocellulose from Chlorella vulgaris Methylene blue (MB) Add microalgae Nanocellulose into CMC/SA,
adsorption capacity: 28.57 mg/g
Dye removal: 24.4%; CMC/SA/
60AH: 109.03 mg/gShowed similar characteristics and adsorption capacity with commercial Nanocellulose [71]
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