INTRODUCTION:

Industrialization, urbanization and extensive agricultural production are mainly responsible for environmental pollution generating polluted effluents which need to be treated^1-6. The relevant technologies adopted for the wastewater treatment should be cost effective offering a less processing time and should leave a minimum impact on the environment^7-12. Coagulation-flocculation is a conventional method for water treatment to remove various dissolved contaminants, suspended solid (SS) particles and emulsified oil from wastewater¹³. The neutralization of repulsive force of colloids occurs during coagulation which causes formation of micro-particles.

In the next step, the micro-particles aggregate and flocs are formed which are removed effectively through sedimentation^14-17. During the process of coagulation, fragile flocs are produced leading to breakage and resuspension of flocs¹⁸. Multivalent metal like alum (Aluminum sulfate) is being used as traditional coagulant in large doses for the efficient removal of turbidity. But, the production of metal hydroxide sludge also causes disposal challenge. There is always concern about the residuals of the metals present in treated water or sludge, which may cause detrimental effects on human health and the environment¹⁹.

Flocculants are being used to solve these problems reducing the usage of traditional coagulants. Sedimentation of the flocs is possible applying proper doses and types of flocculant so that acceptable effluent can be obtained²⁰. Sometimes, Al-based cationic inorganic polymer flocculants viz. polyaluminumchloride (PAC), and polyaluminum sulfate and flocculants like polyacrylamide or acrylamide sodium acrylate are being used frequently for the water treatment which cause the adverse effects on human health due to the presence of trace acrylamide monomers which are potentially toxic and carcinogenic^21-23. Because of the drawbacks of conventional flocculants, it is therefore desirable to replace these chemical flocculants with bioflocculants or natural flocculants. Biomolecules based wastewater treatment technologies can provide suitable solutions^24-26. Thus, the development and application of efficient and economic natural flocculants or bioflocculants remains a great challenge for the control of environmental hazards.

Bioflocculants have gained tremendous attention owing to their unique flocculating characteristics, harmlessness, non-toxic and biodegradable nature which aids in creating an eco-friendly environment^27,28. Nucleic acids, proteins, glycoproteins and polysaccharides have been reported as the significant constituents of bioflocculants²⁹.Extracellular biopolymer flocculants are widely used in various fields including drinking water and wastewater treatment, removal of synthetics dyes and heavy metals, removal and recovery of cell biomass, emulsification, synthesis of nanoparticles, mining, cryoprotection, etc.³⁰. The bioflocculants can act as excellent alternatives to clarifying agents and will embark remarkable applications in dairy, food and other industries³¹. In recent years, several researchers have reported the applications of bioflocculants towards the treatment of wastewater, removal of heavy metals, decolorization of dye, harvesting of microalgae etc. for the remediation of environmental pollution.

The microorganisms viz bacteria, actinomycetes, fungi, and microalgae have been reported to produce variable constituents like polysaccharide, protein, cellulose etc. with good flocculation properties which are helpful for the removal of organic and inorganic pollutants from wastewaters³². A number of review articles covering different aspects of microbial bioflocculants have been published. Research updates on bioflocculants from isolated bacterial strains have been reported³¹. Significant amount of bioflocculants with high flocculating activity can be produced either from a single or mixed bacterial culture³³. Bacterial cultivation along with properties, characterization and application of bioflocculants was discussed by Abdullah et al.³⁴. The bacterial bioflocculants with extraordinary flocculation efficiency and their miscellaneous applications has also been published²⁸. Coagulation-flocculation technology, flocculation mechanism, testing of flocculation efficiency along with the applications of bioflocculants on wastewater treatments were discussed earlier³⁵.

Many studies have been published within the last decade covering the various aspects of bioflocculants. The present review is the compilation of research reports until March 2020 including more than a decade of research work on microbial bioflocculants. We hope that the research reports presented here will definitely inspire future researchers to implement the use of bioflocculants for environmental remediation.

Environmental Applications of Microbial Bioflocculants:

Several studies have been conducted for the beneficial outcomes of using microbial bioflocculants for the treatment of wastewater, removal of heavy metals and synthetic dyes, steroid, estrogen, Acanthamoeba cysts etc. which have been discussed below.

Wastewater treatment:

David et al.³⁶ demonstrated the potential use of two fungi species viz. Aspergillus flavus MCB 271 and Aspergillus niger MCBF 08 to produce effective bioflocculants which could be applied for the treatment of wastewater. The maximum flocculating activity of 97% and 86% was noted in case of A. flavus MCBF 271 and A. niger MCBF08 at pH 7 on the 3rd day of the study. Bacterial load of the wastewater samples was reduced significantly by 58.73% for A. flavus MCB 271 and 60.85% for A. niger MCBF respectively. Therefore, these bioflocculants can be used as potential replacement for the chemical flocculants which are traditionally being used for wastewater treatment. The treatment of coalmine wastewater was reported using the bioflocculant produced from Alcaligenes faecalis HCB2³⁷. The maximum flocculation activity was recorded as 92% at pH 9.0 and 30°C when urea and maltose were used as energy sources. The yield of bioflocculant obtained was 4g/L. The bioflocculant showed good removal efficiencies on BOD (59%), COD (72%), and sulfur (75%) respectively. K⁺ ion was reported as favourable cation and involvement of double layer compression by K+, chemical reactions and bridging were proposed as mechanisms for flocculation. The bioflocculant mediated synthesized Cu nanoparticles (CuNPs) was used for the treatment of wastewater³⁸. The synthesized CuNPs showed an excellent flocculation activity (96%) at very low concentration (0.2mg/mL) of copper nanoparticles which could remove pollutants present in domestic wastewater, river water and coal mine water. Removal of COD and BOD was found to be over 89% in case of river water and coal mine wastewater. The treatment of poultry slaughterhouse wastewater (PSW) was reported using 2 bacterial isolates viz. Bacillus sp. BF-2 and Comamonas aquatica (BF-3) which were used in a bioflocculant-supported dissolved air flotation (Bio-DAF) system. The reduction of the total suspended solids (TSS), proteins and lipids were noted by supplementing with mixed culture of C. aquatica BF-3 and Bacillus sp. BF-2 directly putting into the DAF system along with the bioflocculants produced³⁹. It was noted that the Bio-DAF system worked efficiently for the treatment of poultry slaughterhouse wastewater. Agunbiade et al.⁴⁰ reported the outstanding potential of the bioflocculant produced from Streptomyces platensis for river and wastewater treatment. The purified bioflocculant was found to flocculate the river water better than the wastewater with an excellent thermostability showing flocculation activity (90%) at a minimum dosage under optimized condition. In case of river water, the bioflocculant showed the maximum flocculation efficiency (91.4%), chemical oxygen demand (COD) removal (63.1%) and turbidity removal (84%) which were higher compared to wastewater. The COD removal efficiency of the bioflocculant was found to be 63.1% in river water and 46.6% in meat processing wastewater respectively. The turbidity removal was noted as 84.3% and 75.6% respectively. The dairy wastewater was treated efficiently using the bioflocculants produced from Rhizopus oligosporus, Aspergillus oryzae, and Bacillus subtilis⁴¹. The strain R. oligosporus produced more stable bioflocculants compared to others which were confirmed through thermostability analysis. The increase in pH caused an increase in the flocculating activity of bioflocculants produced by A. oryzae and B. subtilis. Better flocculation performance in acidic conditions was noted in bioflocculants from R. oligosporus. The addition of bioflocculants caused a significant decrease in COD, TSS, and dyes in case of dairy wastewater. The best performance was noted in case of bioflocculants from R. oligosporus. A complex bioflocculant MBF917 produced by the fungal strains Rhizopus sp. M9 and M17 was used for the treatment of potato starch wastewater (PSW)⁴². The higher flocculation efficiency was noted using lower dosage of bioflocculant (0.1 ml/L) without adjusting pH and 5mL/L of CaCl₂ (10%) was added as a coagulant. Turbidity and COD removal rates of PSW was found to be 92.11% and 54.09% respectively after flocculation.

H-acid is a polyaromatic hydrocarbon useful for the production of different types of reactive dyes, acids and medicine. A novel strain isolated from H-acid wastewater was identified as Klebsiella pneumoniae which could convert H-acid into bioflocculants (MBF-7) under optimized condition⁴³. The bioflocculant showed excellent flocculation activity in kaolin suspension without addition of any cation. The maximum bioflocculant production was noted after 60h of cultivation. H-acid wastewater treatment was suggested as viable treatment technology using the novel strain K. pneumoniae which could primarily reduce the costs of bioflocculants. Application of chemically modified zeolite and bioflocculant from activated sludge was reported for the treatment of swine wastewater⁴⁴. The polymeric substance was produced by Rhodococcus R3 using pre-treated sludge used as a raw material. The yields of bioflocculant was 4.2g/L for alkaline-thermal treated and 2.7g/L for sterilized sludge after the fermentation period of 60 h. The wastewater treatment process was optimized applying the composite of modified zeolite after calcination with MgO and bioflocculant using RSM. The use of bioflocculant producing two bacteria strains, xn11 + xn7, isolated from activated sludge was reported by Zhang et al.⁴⁵. It was also used for the treatment of swine wastewater with removal of turbidity (91%) and chemical oxygen demand (42%) respectively, which was found to be better than conventional flocculant polyaluminium chloride (PAM) alone. A bioflocculant-producing bacterium Serratia ficaria showed a flocculating activity of 95.4% under optimized condition and used for the treatment of river water, brewery wastewater, meat processing wastewater etc⁴⁶. Mechanism of flocculation was assessed through zeta potential analysis. The important role played by charge neutralization during flocculation was useful for the treatment of different types of real wastewaters. The bioflocculant showed better performance compared to traditional chemical flocculants.

Heavy metal removal:

The heavy metal pollution is a serious environmental concern and application of bioflocculant towards remediation of heavy metals remains a great challenge. Environmental hazards caused by heavy metals are becoming more severe with rapid industrial developments⁴⁷. Remediation of heavy metals is mandatory for environmental sustainability which needs the application of efficient, eco-friendly and cost- effective materials.

A group of researchers reported the potential removal of Cu (II) and Zn (II) by a polymeric substance (EPS) produced by Bacillus licheniformis strain KX657843 which was isolated from the gut of earthworm (Metaphire posthuma)⁴⁸. The presence of protein and carbohydrates provided heavy metal binding capacity to the polymeric substance through electrostatic interactions. The maximum flocculation activity of polymeric substance was noted as (83%) at pH 11 with dosage 4mg/L. The metal absorption capacity by EPS was increased with the increase in pH for both the metals. It was concluded that EPS obtained from B. licheniformis strain KX657843 could serve as potential bioflocculant for heavy metal removal. The potential of Terrabacter sp. as producer of a biopolymeric flocculant was evaluated for remediating the heavy metals from wastewater⁴⁹. Significant removal of Fe (77.7%), Al (74.8%), Mn (61.9%) and Zn (57.6%) was confirmed using (ICP-OES) analyses. The optimum bioflocculant dosage was 0.5mg/mL for removal of suspended solids (SS), biological oxygen demand (BOD), chemical oxygen demand (COD), nitrate and turbidity in dairy wastewater. Higher flocculation activity was noted as compared to the traditional flocculants viz. polyaluminum chloride (PAC), polyethylenime (PEA) and alum. A novel bioflocculant QZ-7produced from Bacillus salmalaya 139S was reported for the removal of heavy metals from industrial wastewater⁶. The bioflocculant OZ-7 was tested for the adsorption of heavy metals viz As, Zn²⁺ Pb²⁺, Cu²⁺, and Cd²⁺ under optimal experimental conditions using synthetic wastewater samples. The maximum removal was achieved as 81.3%, 78.6%, 77.9%, 76.1%, 68.7% for Zn (II), AS (II), Pb (II), Cu (II) and Cd (II) respectively at pH 7 using optimal bioflocculant dosage 60mg/L. The experimental results confirmed the removal of As (89.8%), Zn (II) 77.4% and Cd (II) 58.4% respectively using real wastewater. It was concluded that QZ-7 can serve as potential bioflocculant for heavy metal removal from industrial wastewater. Fan et al.⁵⁰ reported the potential bioflocculant, A-GS408, produced by Klebsiella oxytoca GS-4-08 which was cultured in acetonitrile (ACN) used as a sole nitrogen source. It was composed of (46.3%) polysaccharides and (20.6%) proteins. The crude A-GS408 (4.6g) could be obtained in one liter of synthetic medium. The flocculant, A-GS408 showed an excellent potentiality for the remediation of Cu²⁺and Pd²⁺ through a process of chemisorption. ACN (1g/L) was completely degraded in 350h. This study provided a new technique for bioflocculant preparation which was applied for the dual application of degrading toxic compound like acetonitrile (ACN) along with heavy metal removal. The bioflocculants produced by Pseudomonas koreensis and Pantoea sp. isolated from gold mine soil were tested for heavy metal adsorption⁵¹. The maximum flocculating activity was noted as 51.7% and 71.3% with yield of 3.26g/L and 2.98g/L for Pantoea sp. and Pseudomonas koreensis respectively. The bioflocculants showed significant sorption capacity of the heavy metals viz. Cd, Pb and Cr from solution. The bioflocculant of Pantoea sp. showed removal of Cd (51.2%), Pb (80%) and Cr (52.5%) while P. koreensis removed Cd (48.5%), Pb (73.7%) and Cr (42.5%) respectively. The results of the study proved the potentiality of bioflocculants for remediation of heavy metals from industrial wastes. Removal of heavy metals from synthetic wastewater and lake water sample was reported using a potential bioflocculant produced by Bacillus subtilis⁵². The production of bioflocculant was found to be positively related with the growth of the bacterial strain. Heavy metals viz. Al, Zn, Fe, Cu present in the lake water samples were removed successfully showing removal of 92.9%, 94.3%, 86.2% and 68.1% respectively. Sajayan et al (2017)⁵³ reported the bioflocculant producing bacterial strain MSI021 isolated from marine sponge. The bioflocculant was polysaccharide in nature and showed high flocculation activity (94%) in kaolin clay suspension. The ability of the bioflocculant for the remediation of heavy metal was assessed through inhibition of bioluminescence expression in Vibrio harveyi. The bioflocculant could reduce the toxicity of heavy metals significantly and the expression of bioluminescence in Vibrio was improved. It was concluded that MSI021 could serve as potential bioflocculant in remediating wastewater containing heavy metals. The bioflocculant produced by a bacterium, Pseudomonas aeruginosa strain IASST201 was capable of degrading and utilizing n-hexadecane, which could serve as a sole energy source⁵⁴. It was carbohydrate in nature and flocculating activity was found to be 87.8%. The efficiency of the bioflocculant for the removal of Pb²⁺, Zn²⁺, Cu²⁺ Ni²⁺ and Cd²⁺ from the aqueous solutions was noted at the concentrations ranged from 1–50mg/L. The maximum bioflocculation efficiency was observed 79.29±0.12% in case of Ni²⁺. Arsenic removal was reported by Zhao et al.⁵⁵ using a bioflocculant (MBF-79) composed of polysaccharide (71.2%) and protein (27.9%) which was prepared from formaldehyde wastewater being used as carbon source. The maximum yield of MBF-79 was 8.97g/L observed under optimized conditions. The removal of arsenite (0.5mg/L) and arsenate (0.5mg/L) was noted as 84.6% and 98.9% respectively. The experimental results confirmed that MBF-79 could be used as effective bioflocculant for decreasing arsenic concentration during wastewater treatment. The adsorption of multi-metals using a bioflocculant produced by Achromobacter xylosoxidans TERI L1 isolated from oil refinery wastewater was reported⁵⁶. The bioflocculant was comprised of carbohydrate heteropolymer and showed good potentiality towards remediation of heavy metals from contaminated wastewater. Maximum bioflocculation activity was noted as 83.3% in absence of multi-metals which was found to be decreased to 73% in presence of Cd (II), Cu (II), Pb (II), Zn (II), Ni (II) respectively. The removal of toxic metal ion such as Cr (VI) was studied by Devi and Natarajan⁵⁷ using the bioflocculants produced by Bacillus firmus and Bacillus licheniformis. The bioflocculant yield was 10g/L from B. firmus (F) and 16.55g/L from B. licheniformis (L). The highest Cr (VI) removal was noted as 85% using the bioflocculant dosage 2g/L. The study demonstrated the potential applications of microbial bioflocculant in mineral processing for remediation of toxic metals in the solution.

Dye removal:

An efficient bioflocculant BF-VB2 was explored using strain Bacillus sp. TERI VB2 and proposed for its application for the treatment of dye bearing wastewater⁵⁸. BF-VB2 showed remarkable flocculation activity without addition of cation at a wide range of pH for the treatment of synthetic wastewater. The flocculation efficiency of 1mg bioflocculant BF-VB2 was noted as 1980.0mg±5.0mg of kaolin particles in less time leading to the enhancement in flocculation activity (99.0%±0.5%). Reduction in turbidity, dye colour, COD and TSS was noted as 99.6%±1.0%, 82.78%±3.03%, 92.54%±0.24% and 73.59%±0.71% respectively. A novel bioflocculant, pKr produced by Kocuriarosea BU22S (KC152976) was reported for dye decolorization⁵⁹. Response surface methodology was adopted to optimize pKr production. Bioflocculant production was affected by the three significant factors viz. glucose concentration, peptone and incubation time. The maximum production of bioflocculant was 4.72± 0.02g/L using 15.61g/L of glucose and 6.45g/L of peptone after 3 days of incubation time. This bioflocculant showed an outstanding performance towards removal of soluble anionic dyes viz., Acid Yellow and Reactive Blue 4 and the decolourization efficiency was found to be 72.6% and 76.4% respectively. It was suggested that pKr could serve as a useful agent for the remediation of dye bearing wastewater. Devi and Natarajan⁵⁷ reported the bioflocculant producing two bacterial sp. viz. Bacillus licheniformis and Bacillus firmus and yield of bioflocculants were noted as 16.55g/L and 10g/L respectively. The bioflocculants extracted from the bacterial species were soluble in water and insoluble in organic solvents. The optimum pH was in the range 7-9 for the maximum bioflocculation. Bioflocculants exhibited efficiency in dye decolorization. Higher efficiency in decolorization of crystal violet, malachite green, and methylene blue were noted but relatively less decolorizing ability was observed for orange dye. The variation in decolorizing ability occurred due to the presence of different functional groups and composition of bioflocculants and dyes⁶⁰. Deng et al.⁶¹ noted the effective removal of anionic dyes viz. acid yellow 25 and reactive blue 4 compared to basic blue B using the bioflocculant produced by Aspergillus parasiticus. The highest flocculating efficiency was found to be 98.1%, obtained for Kaolin suspension after 72 h cultivation. The bioflocculant was mainly composed of sugar (76.3%) and protein (21.6%) and the decolorization efficiency was noted as 92.9% and 92.4% for Acid Yellow 25 and Reactive Blue 4 respectively. The dye decolorization depended on the solution pH and flocculant dosage. The presence of amine groups in the bioflocculant was confirmed through XPS analysis which was protonated at pH 5. Therefore, positively charged bioflocculant was attracted towards negatively charged dye molecules during decolourization.

Steroid estrogen removal:

The adverse effects of estrogen on wildlife and human have been reported which interfere with the endocrine system causing alteration in physiological functions including reproduction in aquatic organisms and human. The estrogens released into the effluents from wastewater treatment plants (WTPs) could not be eliminated to a safe level using the traditional techniques practiced in wastewater treatment plants. The removal of estrogen through application of bioflocculant was reported by Zhong et al.⁴³ using a potent strain SW-2 isolated from chromotropic acid wastewater. The bioflocculant consisting of polysaccharide and protein, showed flocculation efficiency 0.4% (w/w) kaolin suspensions at pH ranging from 3.0–9.0 and temperature 20–80°C. The application of bioflocculant SW-2 was believed to be an effective alternative for removal of estrogens from water. The highest estrogen removal efficiencies were found to be 87% for E1, 92% for E2, 88% for EE2 and 96% for E3.

Removal of acanthamoeba cysts:

The potentiality of a novel and non-toxic biopolymeric flocculant (MBF-5) was reported for the effective removal of Acanthamoeba cysts, a potential pathogen present in soil, air and water⁶². A strain of Klebsiella pneumoniae, isolated from a sputum sample was the producer of bioflocculant MBF-5 which was composed of polysaccharide (96.8 %) and protein (2.1%) respectively. MBF-5 was found to be nontoxic and could be applied for the removal of amoebae cysts from water. The response surface methodology (RSM) was used to optimize the flocculation process of Acanthamoeba cysts and kaolin suspension. This study ensured the application of non-toxic bioflocculant (MBF-5) as a substitute of chemical flocculants for the removal of amoebae cysts from aqueous environment. Table 1 summarizes different types of microbial bioflocculants with their applications.

Table 1: Applications of microbial bioflocculants for environmental remediation.

Sr. No.	Microbial sources	Bioflocculant (BF): Flocculation Activity (FA)	Environmental Application	References
1.	Bacillus aryabhattai PSK1	BF: PSK1 FA: (94.56%) Composition: Glycoprotein in nature	Turbidity removal	33
2.	Aspergillus flavus MCB 271 Aspergillus nigerMCBF 08	FA (97 %) FA (6 %)	Wastewater treatment	36
3.	Alcaligenes faecalis HCB2	FA (92 %); yield 4 g/L; dosage: 200 µ/µL Composition: carbohydrate (88.6 %) & protein (9.5%)	Coalmine Wastewater treatment	37
4.	Streptomyces platensis	FA: (90 %)	River water and wastewater treatment	40
5..	A novel strain (designated as SW-2)	BF: MBF -2 Composition: Polysaccharide and protein. Dosage: 50 mg/L	Steroid estrogen removal	43
6.	Rhodococcus R3	Flocculation rate of fermented broth (92.1%) and supernatant 90.8%, respectively. Composition: total protein (84.6 %) and sugar content (15.2%); Dosage: 2.7 g/L	Treatment of swine wastewater	44
7.	Bacillus licheniformis strain KX657843	FA (83 %); dosage: 4mg/L; Composition: carbohydrates, some amounts of proteins, sugar acids & traces of nucleic acids	Heavy metal removal	48
8.	Terrabacter sp.	FA (85 %); dosage: 0.5 mg/mL; Composition: Total sugar (71.6 %) & total protein content (1.7%)	Heavy metal removal	49
9.	Klebsiella oxytoca GS-4-08	BF: A-GS408 FA: >90 %; Composition: polysaccharides (46.3%) and proteins (20.6%)	Heavy metal removal	50
10.	Pseudomonas koreensis and Pantoea sp.	FA (71.3 %), yield :2.98 g /L FA (51.7 %), yield: 3.26 g/L Composition: polysaccharides containing protein	Heavy metal sorption	51
11.	Bacterial strain MSI021	FA: 94 % Composition: Polysaccharide	Heavy metal removal	53
12.	Pseudomonas aeruginosa strain IASST201	FA: (87.8 %) Composition: Carbohydrate in nature	Heavy metal removal	54
13.	Aerobic bacteria, Strain ZCY-79	BF: MBF -79 FA (94.7%) Composition: Polysaccharide (71.2%) and protein (27.9%) Yield: 8.97 g/L	Arsenic removal	55
14.	Achromobacter xylosoxidans TERI L1	FA (83.3 %) Composition: Total sugar (75 %), neutral sugar (72 %) and protein (11.5 %). yield: 5 g/ L.	Heavy metal removal	56
15.	Bacillus licheniformis Bacillus firmus	BF dosage 2 g/L. Composition: polysaccharides in nature Yield: B. licheniformis (16.5 g/L) B. firmus (10 g/L)	Bioremoval of Cr (VI)	57
16.	Bacillus sp. TERI VB2	FA (99.0%± 0.5) %; Composition: Total sugar (97.13% ±1.3%) & protein (1.46% ± 0.005%)	Dye bearing wastewater treatment	58
17.	Kocuriarosea BU22S (KC152976)	BF: pKr Yield: 4.72 ± 0.02 g/L	Dye decolorization	59
18.	Turicibacter sanguinis	BF: MBF83 Composition: Polysaccharide (74.1%) and Protein (24.2%). Yield 4.61 g / L	Arsenite removal	63
19.	Bacillus licheniformis X14	BF: ZS-7 FA (98.5 %) Composition: Glycoprotein consisting of polysaccharide (91.5%) & total protein (8.4%)	Low temperature drinking water treatment	64
20.	Pseudomonas veronii L918	BF: MBF-L918 FA: (92.51%) Composition: Polysaccharides (77.14 %) and proteins (4.84%) Yield: 3.39 g/L	Treatment of coal ash-flushing wastewater	65
21.	Azotobacter chroococcum	The flocculation rate was increased by 22.86% when CTAB (20 mg/ L) was added to the coal waste slurry with the BF. Composition: Sugar (61.8 %) and protein (36.7%)	Coal waste slurry flocculation	66

CONCLUSION:

The present review addressed the significant amount of work that have been done in the past, with respect to microbial bioflocculants. Recently, number of bioflocculants were identified and explored for diversified applications but none has been practically applied in industry because of poor productivity and huge production costs. The challenge in bioflocculant production is its real-field application. Till now, many developing countries are depending on the synthetic flocculants which are toxic and non -degradable. Therefore, major research should be done to develop cost effective technology for the production of high quality bioflocculant in short period of time. Until then, industry has to rely on the chemical coagulants due to their efficiency and low cost, despite knowing the risks posed by the chemical coagulants. However, the researchers are attempting to develop safe, commercially viable, and eco-friendly bioflocculants with the help of new biotechnologies and advances in genetic engineering. It can be expected that bioflocculant production from renewable sources may open up new ways for their applications to control environmental pollution in the coming years. It is also needed to explore various green technologies for production of eco-friendly bioflocculants to meet the legislative requirements to create pollution free environment. We believe that this article will surely help future researchers for a better understanding about the nature of bioflocculants along with their wide applications. The development of new fermentation/recovery strategies and use of low-cost biomass resources can be recommended as key focus in future research for large scale production of bioflocculants towards the control of environmental pollution.

ACKNOWLEDGEMENTS:

The authors are grateful to Vellore Institute of Technology, Vellore - 632014, Tamil Nadu, India and Chaitanya Bharathi Institute of Technology, Hyderabad- 500075, Telangana, India for the support throughout the study.

CONFLICT OF INTERESTS:

The authors declare that there are no conflicts of interest regarding the publication of this manuscript

REFERENCES:

1. Sharmila S, Dinesh M, Kowsalya E, Kamalambigeswari R and Rebecca LJ. Biosorption of dye using Lawsonia sp. as adsorbent. Research Journal of Pharmacy and Technology. 2020; 13(4): 1651-1654.

2. Mandal SK and Das N. Biodegradation of benzo [a] pyrene by Rhodotorula sp. NS01 strain isolated from contaminated soil sample. Research Journal of Pharmacy and Technology. 2017; 10(6):1751-1757.

3. Gupta AK, Ganjewala D, Goel N, Khurana N, Ghosh S and Saxena A. Bioremediation of tannery chromium: A microbial approach. Research Journal of Pharmacy and Technology. 2014; 7(1):118-122.

4. Verma M and Ekka A. Decolorization and degradation of kraft lignin discharged from pulp and paper mill industry by axenic and co-culture of Bacillus sp. Research Journal of Pharmacy and Technology. 2018; 11(10):4386-4392.

5. Abbas BF, Al-Jubori WM, Abdullah AM, Shaaban H and Mohammed MT. Environmental pollution with the heavy metal compound. Research Journal of Pharmacy and Technology. 2018; 11(9):4035-4041.

6. Ragadevan V, Kanchanabhan TE, Dayakar P, Mani A and Chockalingam MP. Removal of heavy metal ions (Lead) using natural adsorbent. Research Journal of Pharmacy and Technology. 2019; 12(8):3693-3696.

7. Purushothaman V, Madhumathi R and Sakthiselvan P. Removal of Nickel (II) and Zinc (II) present in the electroplating industry wastewater by bioaccumulation method. Research Journal of Pharmacy and Technology. 2019; 12(4):1495-1503.

8. Sajen S, Jose JV, Bukke S, Subbarao SS and Mandla VR. Removal of basic dye from synthetic wastewater using sugarcane bagasse modified with propionic acid. Research Journal of Pharmacy and Technology. 2017; 10(6):1627-1634.

9. Meshram R and Jadhav SK. Treatment of oil refinery wastewater simultaneously with bioelectricity production in mediator-less microbial fuel cell using native gram-positive Bacillus sp. Research Journal of Pharmacy and Technology. 2019; 12(4):1953-1961.

10. Oda AM, Naji HK, Lafta AJ, Salih A, Ahmed L, Jawad H and Falah K. Congo red dye removal from simulated textile wastewaters over a neat and silver doped zinc oxide nanoparticles. A kinetics study. Research Journal of Pharmacy and Technology. 2019; 12(6):2669-2676.

11. Ramya M, Kalaivani RA and Raghu S. Microbial fuel cell: A renewable equipment for bio-power production and simultaneous treatment of industrial wastewaters. Research Journal of Pharmacy and Technology. 2019; 12(7):3551-3554.

12. Abdollahi K, Yazdani F, Panahi R and Mokhtarani B. Biotransformation of phenol in synthetic wastewater using the functionalized magnetic nano-biocatalyst particles carrying tyrosinase. 3 Biotech. 2018; 8(10):419.

13. Ortiz-Oliveros HB and Flores-Espinosa RM. Simultaneous removal of oil, total Co and 60 Co from radioactive liquid waste by dissolved air flotation. International Journal of Environmental Science and Technology. 2019; 16(7):3679-3686.

14. Liu H, Chen G and Wang G. Characteristics for production of hydrogen and bioflocculant by Bacillus sp. XF-56 from marine intertidal sludge. International Journal of Hydrogen Energy. 2015a; 40(3):1414-1419.

15. Liu Z, Huang M, Li A and Yang H. Flocculation and antimicrobial properties of a cationized starch. Water Research. 2017a; 119: 57-66.

16. Liu Z, Wei H, Li A and Yang H. Evaluation of structural effects on the flocculation performance of a co-graft starch-based flocculant. Water Research. 2017b; 118:160-166.

17. Abu Tawila ZM, Ismail S, Dadrasnia A and Usman MM. Production and characterization of a bioflocculant produced by Bacillus salmalaya 139SI-7 and its applications in wastewater treatment. Molecules. 2018; 23(10):2689.

18. Aguilar MI, Sáez J, Lloréns M, Soler A, Ortuño JF, Meseguer V and Fuentes A. Improvement of coagulation–flocculation process using anionic polyacrylamide as coagulant aid. Chemosphere. 2005; 58(1):47-56.

19. Yang R, Li H, Huang M, Yang H and Li A. A review on chitosan-based flocculants and their applications in water treatment. Water Research. 2016; 95:59-89.

20. Ferasat Z, Panahi R and Mokhtarani B. Natural polymer matrix as safe flocculant to remove turbidity from kaolin suspension: Performance and governing mechanism. Journal of Environmental Management. 2020; 255:109939.

21. Lapointe M and Barbeau B. Dual starch–polyacrylamide polymer system for improved flocculation. Water Research. 2017; 124:202-209.

22. Im D, Nakada N, Kato Y, Aoki M and Tanaka H. Pretreatment of ceramic membrane microfiltration in wastewater reuse: A comparison between ozonation and coagulation. Journal of Environmental Management. 2019; 251:109555.

23. Liu Z, Wei H, Li A and Yang H. Enhanced coagulation of low-turbidity micro-polluted surface water: properties and optimization. Journal of Environmental Management. 2019; 233:739-747.

24. Abdollahi K, Yazdani F and Panahi R. Covalent immobilization of tyrosinase onto cyanuric chloride crosslinked amine-functionalized superparamagnetic nanoparticles: synthesis and characterization of the recyclable nanobiocatalyst. International Journal of Biological Macromolecules. 2017; 94:396-405.

25. Abdollahi K, Yazdani F and Panahi R. Fabrication of the robust and recyclable tyrosinase-harboring biocatalyst using ethylenediamine functionalized superparamagnetic nanoparticles: nanocarrier characterization and immobilized enzyme properties. Journal of Biological Inorganic Chemistry. 2019; 24(7): 943-959.

26. Firooz NS, Panahi R, Mokhtarani B and Yazdani F. Direct introduction of amine groups into cellulosic paper for covalent immobilization of tyrosinase: support characterization and enzyme properties. Cellulose. 2017; 24(3): 1407-1416.

27. Kothari R, Pathak VV, Pandey A, Ahmad S, Srivastava C and Tyagi VV. A novel method to harvest Chlorella sp. via low cost bioflocculant: Influence of temperature with kinetic and thermodynamic functions. Bioresource Technology. 2017; 225: 84-89.

28. Shahadat M, Teng TT, Rafatullah M, Shaikh ZA, Sreekrishnan TR and Ali SW. Bacterial bioflocculants: a review of recent advances and perspectives. Chemical Engineering Journal. 2017; 328: 1139-1152.

29. Dao VH, Cameron NR and Saito K. Synthesis, properties and performance of organic polymers employed in flocculation applications. Polymer Chemistry. 2016; 7(1):11-25.

30. Mu J, Zhou H, Chen Y, Yang G and Cui X. Revealing a novel natural bioflocculant resource from Ruditapes philippinarum: Effective polysaccharides and synergistic flocculation. Carbohydrate Polymers. 2018; 186:17-24.

31. Lee DJ and Chang YR. Bioflocculants from isolated stains: A research update. Journal of the Taiwan Institute of Chemical Engineers. 2018; 87:211-215.

32. Siddeeg SM, Tahoon MA and Rebah FB. Agro-industrial waste materials and wastewater as growth media for microbial bioflocculants production: a review. Materials Research Express. 2020; 7(1):012001.

33. Abd El-Salam AE, Abd-El-Haleem D, Youssef AS, Zaki S, Abu-Elreesh G and El-Assar SA. Isolation, characterization, optimization, immobilization and batch fermentation of bioflocculant produced by Bacillus aryabhattai strain PSK1. Journal of Genetic Engineering and Biotechnology. 2017; 15(2):335-344.

34. Abdullah AM, Hamidah H and Alam MZ. Research progress in bioflocculants from bacteria. International Food Research Journal. 2017; 24:402-409.

35. Agunbiade MO, Pohl CH and Ashafa AO. A Review of the application of biofloccualnts in wastewater treatment. Polish Journal of Environmental Studies. 2016; 25(4):1381-1389.

36. David OM, Oluwole OA, Ayodele OE and Lasisi T. Characterisation of fungal bioflocculants and its application in water treatment. Current Journal of Applied Science and Technology. 2019; 34(6):1-9.

37. Maliehe TS, Basson AK and Dlamini NG. Removal of pollutants in mine wastewater by a non-cytotoxic polymeric bioflocculant from Alcaligenes faecalis HCB2. International Journal of Environmental Research and Public Health. 2019; 16(20):4001.

38. Dlamini NG, Basson AK and Pullabhotla VS. Optimization and application of bioflocculant passivated copper nanoparticles in the wastewater treatment. International Journal of Environmental Research and Public Health. 2019; 16(12):2185.

39. Dlangamandla C, Ntwampe SK and Basitere M. A bioflocculant-supported dissolved air flotation system for the removal of suspended solids, lipids and protein matter from poultry slaughterhouse wastewater. Water Science and Technology. 2018; 78(2):452-458.

40. Agunbiade M, Pohl C and Ashafa O. Bioflocculant production from Streptomyces platensis and its potential for river and waste water treatment. Brazilian Journal of Microbiology. 2018; 49(4):731-741.

41. Didar Z and Ferdosi-Makan A. Bioflocculant production by different microbial species and their potential application in dairy wastewater treatment. Journal of Advances in Environmental Health Research. 2016; 4(1):18-24.

42. Pu SY, Qin LL, Che JP, Zhang BR and Xu M. Preparation and application of a novel bioflocculant by two strains of Rhizopus sp. using potato starch wastewater as nutrilite. Bioresource Technology. 2014; 162:184-191.

43. Zhong C, Xu A, Chen L, Yang X, Yang B, Hong W, Mao K, Wang B and Zhou J. Production of a bioflocculant from chromotropic acid waste water and its application in steroid estrogen removal. Colloids and Surfaces B: Biointerfaces. 2014; 122:729-737.

44. Guo J, Yang C and Zeng G. Treatment of swine wastewater using chemically modified zeolite and bioflocculant from activated sludge. Bioresource Technology. 2013; 143:289-297.

45. Zhang CL, Cui YN, Wang Y. Bioflocculant produced from bacteria for decolorization, Cr removal and swine wastewater application. Sustainable Environment Research. 2012; 22(2):129-134.

46. Gong WX, Wang SG, Sun XF, Liu XW, Yue QY and Gao BY. Bioflocculant production by culture of Serratia ficaria and its application in wastewater treatment. Bioresource technology. 2008; 99(11):4668-4674.

47. Vimala RT, Escaline JL and Sivaramakrishnan S. Characterization of self-assembled bioflocculant from the microbial consortium and its applications. Journal of Environmental Management. 2020; 258:110000.

48. Biswas JK, Banerjee A, Sarkar B, Sarkar D, Sarkar SK, Rai M and Vithanage M. Exploration of an extracellular polymeric substance from earthworm gut bacterium (Bacillus licheniformis) for bioflocculation and heavy metal removal potential. Applied Sciences. 2020; 10(1):349.

49. Agunbiade MO, Pohl C, Heerden EV, Oyekola O and Ashafa A. Evaluation of fresh water actinomycete bioflocculant and its biotechnological applications in wastewaters treatment and removal of heavy metals. International Journal of Environmental Research and Public Health. 2019; 16(18):3337.

50. Fan HC, Yu J, Chen RP and Yu L. Preparation of a bioflocculant by using acetonitrile as sole nitrogen source and its application in heavy metals removal. Journal of Hazardous Materials. 2019; 363:242-247.

51. Ayangbenro AS, Babalola OO and Aremu OS. Bioflocculant production and heavy metal sorption by metal resistant bacterial isolates from gold mining soil. Chemosphere. 2019; 231: 113-120.

52. Dih CC, Jamaluddin NA and Zulkeflee Z. Removal of heavy metals in lake water using bioflocculant produced by Bacillus subtilis. Pertanika Journal of Tropical Agricultural Science. 2019; 42(1):89-101.

53. Sajayan A, Kiran GS, Priyadharshini S, Poulose N and Selvin J. Revealing the ability of a novel polysaccharide bioflocculant in bioremediation of heavy metals sensed in a Vibrio bioluminescence reporter assay. Environmental Pollution. 2017; 228:118-127.

54. Pathak M, Sarma HK, Bhattacharyya KG, Subudhi S, Bisht V, Lal B and Devi A. Characterization of a novel polymeric bioflocculant produced from bacterial utilization of n-hexadecane and its application in removal of heavy metals. Frontiers in Microbiology. 2017; 8:170.

55. Zhao H, Zhong C, Chen H, Yao J, Tan L, Zhang Y and Zhou J. Production of bioflocculants prepared from formaldehyde wastewater for the potential removal of arsenic. Journal of Environmental Management. 2016; 172:71-76.

56. Subudhi S, Bisht V, Batta N, Pathak M, Devi A and Lal B. Purification and characterization of exopolysaccharide bioflocculant produced by heavy metal resistant Achromobacter xylosoxidans. Carbohydrate Polymers. 2016; 137:441-451.

57. Devi KK and Natarajan KA. Production and characterization of bioflocculants for mineral processing applications. International Journal of Mineral Processing. 2015; 137:15-25.

58. Bisht V and Lal B. Exploration of performance kinetics and mechanism of action of a potential novel bioflocculant BF-VB2 on clay and dye wastewater flocculation. Frontiers in Microbiology. 2019; 10:1288.

59. Chouchane H, Mahjoubi M, Ettoumi B, Neifar M and Cherif A. A novel thermally stable heteropolysaccharide-based bioflocculant from hydrocarbonoclastic strain Kocuria rosea BU22S and its application in dye removal. Environmental Technology. 2018; 39(7):859-872.

60. Buthelezi SP, Olaniran AO and Pillay B. Textile dye removal from wastewater effluents using bioflocculants produced by indigenous bacterial isolates. Molecules. 2012; 17(12):14260-14274.

61. Deng S, Yu G and Ting YP. Production of a bioflocculant by Aspergillus parasiticus and its application in dye removal. Colloids and Surfaces B: Biointerfaces. 2005; 44(4):179-186.

62. Zhao H, Liu H and Zhou J. Characterization of a bioflocculant MBF-5 by Klebsiella pneumoniae and its application in Acanthamoeba cysts removal. Bioresource Technology. 2013; 137:226-232.

63. Cao G, Zhang Y, Chen L, Liu J, Mao K, Li K and Zhou J. Production of a bioflocculant from methanol wastewater and its application in arsenite removal. Chemosphere. 2015; 141:274–281.

64. Li Z, Zhong S, Lei HY, Chen RW, Yu Q and Li HL. Production of a novel bioflocculant by Bacillus licheniformis X14 and its application to low temperature drinking water treatment. Bioresource Technology. 2009; 100:3650–3656.

65. Liu W, Hao Y, Jiang J, Zhu A, Zhu J and Dong Z. Production of a bioflocculant from Pseudomonas veronii L918 using the hydrolyzate of peanut hull and its application in the treatment of ash-flushing wastewater generated from coal fired power plant. Bioresource Technology. 2016; 218:318–325.

66. Yang Z, Liu S, Zhang W, Wen Q and Guo Y. Enhancement of coal waste slurry flocculation by CTAB combined with bioflocculant produced by Azotobacter chroococcum. Separation and Purification Technology. 2019; 211:587–593.

Received on 03.01.2021 Modified on 17.04.2021

Research J. Pharm.and Tech 2022; 15(4):1883-1890.

DOI: 10.52711/0974-360X.2022.00315