Water contamination due to heavy metals is a foremost concern and a low-cost & easy method of their removal is much required.Adsorption is an efficient technique for removing heavy metals (HMs) from water utilizing ZnO based nanoparticles (NPs)Large surface area, high sensitivity & non-toxicity of ZnO NPs make it an exceptional material for the potent removal of HMs.
The skyrocketing progress in industrialization is increasing the concentration of heavy metals in water resources. Most of the heavy metals are highly toxic and even a trace amount of these heavy metals in water resources can be ruthless for humans and aquatic biomes. Therefore, the alleviation of heavy metals from industrial effluents is a matter of serious concern. The adsorption is one of the widely used methods to remove such nonbiodegradable heavy metals due to its simple design as well as economical and eco-friendly nature. Over the last decade, nanoadsorbents has drawn a significant attention to remediate heavy metals contaminated water due to their high surface area to volume ratio, which provides a huge number of active sites for the adhesion of heavy metals. Among different nanoadsorbent materials, ZnO is quite promising for heavy metals remediation due to its excellent biocompatibility, low toxicity, negative zeta potential resulted due to the surface modifications during growth process and redox reactions resulted due to the generation of effeicient photoinduced electron hole pairs in ZnO NPs. Here in this article, we will systematically review the use of ZnO nanostructures for removing poisonous heavy metal ions from water. Various ZnO-based nanostorbents such as pristine ZnO NPs, doped ZnO nanostrutes, ZnO nanocomposites, and surface-modified ZnO NPs are reviewed thoroughly along with the comparisons of their maximum adsorption capacity for different heavy metal ions (Cd2+, Hg2+, As3+, Pb2+, Cr6+, Ni2+, Co2+, and Cu2+) in a tabular form.
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Waste water remediation of toxic metals and dyes could be complex to achieve owing to several factors. Various techniques have been developed for the removal of these pollutants. However, the success of these water treatment techniques mainly depends on the choice of appropriate media. Most of the commercial and chemically synthesized materials for water treatment create additional hazards due to the leaching of toxic chemicals into the process water. Consequently, studies on the development of better materials which could achieve high efficiency without posing any health risk led to the exploration of non-toxic composite materials such as those composed of cellulose and metal oxides. The focus is on the choice of materials and the best methods that allows their combination in order to address the pollution caused by heavy metals and dyes. This review critically analyses or assess the use of cellulose incorporated with metal oxides in the removal of toxic metals and dyes from industrial effluent. The nanocomposites have been accepted as a technique that circumvents the problems associated with the use of only metal oxides in water purification. They provide a platform for an improvement in the hydrophobicity of the composite with concomitant efficiency in adsorption and photocatalysis.
Nilanjana Das, Nupur Ojha, Sanjeeb Kumar Mandal; Wastewater treatment using plant-derived bioflocculants: green chemistry approach for safe environment. Water Sci Technol 15 April 2021; 83 (8): 17971812. doi: https://doi.org/10.2166/wst.2021.100
The rapid expansion of global trade and human activities has resulted in a massive increase in wastewater pollution into the atmosphere. Suspended solids, organic and inorganic particles, dissolved solids, heavy metals, dyes, and other impurities contained in wastewater from various sources are toxic to the atmosphere and pose serious health risks to humans and animals. Coagulationflocculation technology is commonly used in wastewater treatment to remove cell debris, colloids, and contaminants in a comfortable and effective manner. Flocculants, both organic and inorganic, have long been used in wastewater treatment. However, because of their low performance, non-biodegradability, and associated health risks, their use has been limited. The use of eco-friendly bioflocculants in wastewater treatment has become essential due to the health implications of chemical flocculants. Because of their availability, biodegradability, and protection, plant-derived coagulants/flocculants and plant-based grafted bioflocculants have recently made significant progress in wastewater treatment. This study will undoubtedly provide a clearer understanding of the current state, challenges, and solutions for bioflocculation in wastewater remediation using green materials for the sake of a cleaner climate.
Plant mucilage as a cost-effective and eco-friendly natural bioflocculant.It can serve as a potential bioflocculant for treatment of wastewaters released from different sources.Significant removal of pollutants from wastewaters revealed the novelty of plant mucilage as a bioflocculant.It throws light on their applications aiming to exploit the green materials for remediation of wastewater.
Water contamination has become more serious as a result of human economic growth all over the world. The persistent release of water polluted with various toxins into the atmosphere poses a risk to marine life as well as human health. As a result, there is a need to develop low-cost, high-efficiency methods for handling polluted water (Ahmed et al. 2020). Environmental pollution is largely caused by industrialization, urbanization, and intensive agricultural development, which produces vast volumes of wastewater that must be treated.
Most dyes are poisonous, mutagenic, teratogenic, and carcinogenic to microorganisms, humans, and aquatic organisms, so wastewater containing dyes is considered a major environmental threat (Aljeboree et al. 2017). Textile, rubber, coating, and paper industries all use dyes in their various manufacturing processes. Even at extremely low concentrations, these compounds can cause substantial pollution and, as a result, kill ecosystem elements. Photo-catalysis, biodegradation, chemical coagulation, ion exchange, and adsorption are some of the currently available remediation techniques. Because of its high performance, ease of operation, low cost of operation, and ease of desorption, adsorption is one of the most useful methods in the treatment of aqueous solutions. However, the key operative point of adsorption is the selection or fabrication of the adsorbents, which must be highly stable, cost effective, environmentally friendly, and certainly successful. Activated carbon, zeolite, clay, chitosan, montmorillonite, and vermiculite are among the reactive materials that have been tested. However, these materials have many drawbacks, including a high cost, poor adsorption ability, and a low reuse rate, all of which restrict their use (Ahmed et al. 2020). Congo-red (CR) dye, a highly toxic and carcinogenic pollutant, has recently been degraded using gamma radiation (Shah et al. 2020). Furthermore, the removal of an organic dye by sonocatalysis using a TiO2/Montmorillonite nanocomposite was previously reported (Khataee et al. 2015).
Heavy metal contamination, in addition to dyes, is a significant environmental issue, and heavy metal remediation remains a major problem. With rapid industrialization, the environmental hazards posed by heavy metals are becoming more serious (Vimala et al. 2020). Heavy metals from wastewater must be remedied for environmental protection, which necessitates the use of efficient, environmentally sustainable, and cost-effective materials.
The wastewater treatment technologies used should be cost effective, have a short processing period, and have a low environmental impact (Abdollahi et al. 2018). For the removal of various contaminants from wastewater, various treatment approaches such as ion exchange, membrane filtration, activated carbon, and coagulation/flocculation processes have been used (Al-Wasify et al. 2017; Rachdi et al. 2017). Coagulation/flocculation systems are the most commonly used wastewater treatment methods for eliminating dissolved pollutants, suspended solid (SS) particles, and emulsified oil (Ortiz-Oliveros & Flores-Espinosa 2019). These techniques enhance the aesthetic appearance of turbid water and are used to extract suspended particles by destabilizing them and creating larger, heavier flocs that help them settle.
Inorganic and organic polymer flocculants are the most common flocculants used in wastewater treatment. Inorganic flocculants are mostly made up of aluminium and iron, and they have a strong treatment effect on suspended solids and colloidal particles, but they have some disadvantages. Sludge produced during wastewater treatment with an aluminium salt flocculant is often used as a fertilizer in agriculture. However, high levels of aluminium in the soil pollute the atmosphere and damage plants. Furthermore, free aluminium ions pollute water by infiltrating lakes, rivers, and groundwater by infiltration, diffusion, deposition, and migration, resulting in water contamination and health risks. According to Tietz et al. (2019), traces of aluminium in foods cause acute carcinogenic and genotoxic diseases. Alzheimer's disease may be related to residues of aluminium left in treated wastewater or marine environments in this context (Zhang et al. 2015). The use of iron flocculant is also discouraged due to its corrosive nature. It has the ability to speed up the aging of equipment and boost the cost of water treatment (Porwal et al. 2015). Polyacrylamide and its derivatives are major ingredients in organic polymer flocculants, which have advantages such as quick flocculation, low dosage, and easy separation. However, the most widely used water treatment products, polyacrylamide or acrylamide sodium acrylate, have harmful health effects due to the presence of trace acrylamide monomers, which are potentially toxic and carcinogenic (Im et al. 2019; Liu et al. 2019). Because of the disadvantages of traditional flocculants, bioflocculants or natural flocculants should be used instead.
Bioflocculants have gained much attention because of their unique flocculating properties, as well as their non-toxic, biodegradable nature, which helps to create an environmentally friendly climate (Kothari et al. 2017; Shahadat et al. 2017). Bioflocculants have been shown to contain large quantities of nucleic acids, proteins, glycoproteins, and polysaccharides (Dao et al. 2016). Drinking water and wastewater treatment, removal of synthetic dyes and heavy metals, removal and recovery of cell biomass, emulsification, nanoparticle synthesis, mining, cryoprotection, and other fields all use extracellular biopolymer flocculants (Mu et al. 2018). Bioflocculants can be used as excellent alternatives to clarifying agents and have a wide variety of uses in the dairy, poultry, and other industries (Lee & Chang 2018). Several researchers have documented the use of bioflocculants for wastewater treatment (David et al. 2019; Dlamini et al. 2019; Maliehe et al. 2019; Pham & Bui 2020), heavy metal removal (Tawila et al. 2018; Agunbiade et al. 2019; Fan et al. 2019; Biswas et al. 2020), and dye decolorization (Chouchane et al. 2018).
Chemical flocculants are now limited in many industrial applications by many authorities because they are expensive, pose health risks, and may not be available locally. As a result, other flocculants that provide a new sustainability approach must be considered. This environmentally friendly process relies on the use of bioflocculants extracted from plant sources in the coagulationflocculation treatment of wastewater to remove contaminants. Plant-based flocculants have recently gained more publicity because they are healthy, cost effective, biodegradable, non-toxic, do not cause secondary contamination, and can coagulate fine particles. This article reviews the production and current status of plant-derived flocculants and plant-based grafted bioflocculants, with the intention of shedding light on their potential applications in wastewater remediation. We hope that the research reports presented here will enable potential researchers to use plant-based bioflocculants as environmental remediation agents.
Plant-based coagulants/flocculants made from plant gum, vegetable extracts, tannins, seed kernels, and other sources are being studied in the field of bioflocculation. Amran et al. (2018) published a mini analysis on the coagulation/ flocculation potential of plant-based materials based on ten years of research reports. Saleem & Bachmann (2019) published a recent study on the efficacy of various plants as sources of cationic, anionic, and non-ionic coagulants, suggesting their potential for widespread application and commercialization. Mohd-Salleh et al. (2019) evaluated over 16 plants and reported on the potential of natural materials as aids and their value as sustainable composite coagulants/flocculants.
Coagulants/flocculants have been isolated from a number of plant sources, including Aloe vera (Irma et al. 2015; Jaouadi et al. 2020), Lepidium sativum (Allafchian et al. 2019), Moringa oleifera (Agarwal et al. 2019; Zaid et al. 2019), Salvia hispanica (Tawakkoly et al. 2019), guar gum (Dwari & Mishra 2019), Abelmoschus esculentus (Lee et al. 2018), Opuntia ficus-indica (Miller et al. 2008; Sellami et al. 2014; Nharingo et al. 2015; Bouaouine et al. 2019), Hibiscusesculentus and Trigonella foenum-graecum (Jones & Bridgeman 2019), Albizia gum (Afolabi & Adekanmi 2017), malva nut gum (Ho et al. 2014), guar, mesquite seed gum and Opuntia mucilage (Carpinteyro-Urban & Torres 2013), Plantago ovata (Al-Hamadani et al. 2011), date palm (Khiari et al. 2010) and Tamarindus indica (Mishra et al. 2006). Because of their natural abundance, safe and cost-effective existence, natural products from acorn leaves (Benalia et al. 2019), banana fruit peel (Zaidi et al. 2019), lime seeds (Seghosime et al. 2017), and tamarind (Buenao et al. 2019) have been identified as promising coagulants/flocculants (Muruganandam et al. 2017). The different sources of plant-based flocculants are depicted in Figure1.
Extraction of plant-based bioflocculants for wastewater treatment is a significant step toward creating a green and sustainable technology. Plant-derived flocculants have a mucilaginous texture, a neutral pH, and are polysaccharides in their natural state. The flocculating behaviour of mucilage is due to certain active ingredients in the mucilage. Mucilage is a heterogeneous mixture of monosaccharides present in plants, including arabinose, glucose, galactose, mannose, rhamnose, xylose, and uronic acid. In water, it exhibits viscous colloidal dispersion characteristics (Mirhosseini & Amid 2012). For the extraction of plant-derived coagulants/flocculants, two methods have been used so far: (i) solvent extraction and precipitation, and (ii) drying and grinding (Lee et al. 2014).
The mature seeds of Moringa oleifera were separated from their pods, shelled, ground, and sieved to extract the coagulant. The powder was then dried and used as a coagulant immediately (Sethupathy 2015). Active protein was extracted in another process by dissolving the powder in water, organic solvents such as alcohol or acetone, or salt solutions such as MgCl2, KCl, or NaCl. Since the proteins in M. oleifera seeds are active agents, salt extraction was favored. Salt increases protein solubility, which aids in increasing M. oleifera's coagulating ability against contaminants.
The preparation of flocculant from cactus (Opuntia ficus-indica) was reported by Sellami et al. (2014). Fresh cladodes were thoroughly rinsed, washed, cut into small parts, and crushed in a mixer to create a viscous mucilage that could be used as a coagulant or flocculant. To maintain the coagulating ability, the mucilage was held at a temperature below 4 C for up to two weeks. The powdered coagulant and/or flocculant from cactus was made by oven-drying the cladodes and then grinding them. The coagulation function of cactus mucilage was lowest when oven dried at 120 C or when only the skin of cladode was used (Miller et al. 2008). Furthermore, cactus coagulant/flocculant may be made from the mucilage or powder by combining it with water, salty water, or organic solvents. The use of water as a solvent should be promoted because it is environmentally friendly. In certain cases, however, the extract can be purified and then precipitated by alcohol, which can then be oven dried (Lassoued et al. 2018) or lyophilized (Bouaouine et al. 2019).
Ameena et al. (2010) documented the isolation of mucilage from Hibiscus rosasinensis. They gathered fresh leaves, washed them, and dried them. The dried powdered leaves were soaked in water for 56 minutes, then boiled for half an hour before being set aside for 1 hour to allow for full mucilage release into the water. The substance was squeezed out of a muslin cloth bag, and acetone was applied to the filtrate to precipitate the mucilage, which was then removed, dried, and stored at 50 C. The dried powder was gathered, sieved, and stored in desiccators until required.
Mango seeds, like other plant sources, have a major coagulating effect due to their kernel content. The seed kernels were generally removed in the first step, then cleaned with distilled water and sliced. Then they were dried at various temperatures, such as 105 C for 24 hours (Dange & Lad 2015a) or 120 C for 1 hour (Qureshi et al. 2011), or sun-dried for 7 days and ground to make powder (Seghosime et al. 2017).
Although much research has been performed, there is no published report on the relationship between extraction process and flocculation quality. It is vital to understand whether the extraction process affects flocculation parameters such as pH, flocculant dose, and contact time. It may be important to optimize the extraction conditions for the development of a potential flocculant that can be compared to commercial flocculants in terms of efficiency and cost.
Because of their non-toxic and biodegradable nature, plant-based bioflocculants have become popular as an alternative to synthetic flocculants in wastewater treatment. They are abundantly available from renewable resources and have no negative environmental effects (Lee et al. 2014). Plant-based flocculation was used to treat wastewater obtained from a number of sources, including textile, compost leachate, landfill leachate, tannery, palm oil mill effluent, sewage, food industry, glue industry, laundry, and dairy wastewater. To maximize flocculant yields, flocculation operation, and pollutant removal, culture conditions such as pH, temperature, and flocculant dosage were optimized. The results obtained differed depending on the plant sources used for bioflocculant processing and the characteristics of the wastewater.
The use of plant-derived biopolymers in wastewater treatment has been noted. Several plant parts, such as stems, leaves, seeds, husks, piths, and kernels, have been evaluated for their ability to coagulate/flocculate turbidity (TN), chemical oxygen demand (COD), and total suspended solids (TSS) from various types of wastewaters (Adwuyi & Adwumi 2018). To remediate a range of wastewaters (Torres et al. 2014), researchers have examined the coagulatingflocculating behaviour of various plant parts and extracts extracted from stems (Alwi et al. 2013), leaves (Sellami et al. 2014), seeds (Ngbolua et al. 2016), fruits shells and kernels waste (Jayalakshmi et al. 2016).
Moringa oleifera seeds have recently been identified as a source of coagulant for textile wastewater treatment (Agarwal et al. 2019). The seeds contain cationic and dimeric proteins that could work as a coagulant to eliminate turbidity and color. The findings showed that 95% of textile wastewater was degraded at pH 10 with a dose of M. olifera seeds of 16 mg/L. The application of the coagulant was suggested as a cost-effective approach for large-scale water treatment because of the substantial reduction in COD. In another research study, mucilage from the cactus cladodes (Opuntia ficus-indica) was used as a bioflocculant in conjunction with alum to treat textile effluent (Bouatay & Mhenni 2014). COD (88.76%), TN (91.66%), and dye (99.84%) were found to have the highest removal rates. The use of cress seed mucilage magnetic nanocomposite to extract methylene blue dye from water has been confirmed (Allafchian et al. 2019). Tannins are polyphenol compounds found in the leaves, roots, bark, seeds, wood, and fruits of plants. Lopes et al. (2019) examined the decolorization of synthetic dye effluents using a tannin-derived coagulant in a coagulationflocculation process and compared it to a conventional inorganic coagulant, iron (III) sulfate. Two effluents were examined containing: (1) direct azo dye and salts and (2) dyeing auxiliary chemicals. The tannin coagulant was found to be efficient for color removal in both synthetic and natural textile effluents, eliminating all dyes completely over a pH range of 49. pH had a greater effect on the efficiency of iron (III) sulfate. At higher pH, the tannin coagulant performed better than the iron coagulant. Since it did not demonstrate substantial dye removal after a long period of contact time, the bioflocculant derived from Tamarindus indica (tamarind) could not be used as an effective flocculant for the removal of dyes such as direct (direct quick scarlet) and vat (golden yellow) from textile wastewater. For the highest flocculating efficiency of plant-based bioflocculants, the acceptable pH range was neutral in most of the recorded studies. Depending on the form and characteristics of treated wastewater, some were found to be stable in acidic or alkali environments.
Tawakkoly et al. (2019) used a natural coagulant, the mucilaginous extract of Salvia hispanica seeds, to treat compost leachate wastewater. COD and TN removal from compost leachate wastewater was tested. Response surface methodology was used to optimize the coagulationflocculation phase. Under optimal conditions, using 40 g/L of coagulant at pH 7, the TN removal and COD reduction percentages were found to be 62.4% and 39.7%, respectively.
The ability of psyllium husks as a coagulant aid and coagulant for the treatment of landfill leachate was investigated. When used as a primary coagulant, psyllium husk was found to be ineffective. When combined with poly-aluminium chloride (PACl), it was more successful as a coagulant aid, removing 64% of COD, 90% of color, and 96% of TSS (Al-Hamadani et al. 2011).
Moringa oleifera powder with particle sizes ranging from 150 to 425 m was used to treat dairy wastewater with a turbidity of 230 NTU and a COD of 2,240 mg/L (Pallavi & Mahesh 2013). In terms of COD (64.2%) and TN (86%) exclusion, the finest particle gave the best performance.
Sellami et al. (2014) used cactus juice as a bio-flocculant to treat a glue industry effluent (pH 6.7, suspended matter (SM): 270 mg/L, and COD: 99,200 mg/L). The removal rates for SM and COD were found to be 83.3% and 67.5%, respectively. The removal percentages for SM (79.6%) and COD (59.1%) were noted during polyacrylamide (PAM) testing.
A cactus juice-based coagulant was used to treat tannery effluent with a turbidity of 66960 NTU and COD of 28,000 mg/L. The removal of TN (78.5%) and COD (80.6%) was found to be successful. They also mentioned Moringa oleifera seeds being used to treat tannery wastewater. The removal of TN and COD was found to be 82.0 and 83.3%, respectively (Kazi & Virupakshi 2013).
The efficacy of an okra-based flocculant in treating laundry wastewater (pH: 56) was demonstrated (Freitas et al. 2015). The use of this natural resource as a flocculant improved the coagulationflocculation process. The authors indicated that color (93.6%), TN (97.2%), and COD (85.7%) were all significantly removed. De-Souza et al. (2014) used Opuntia ficus-indica mucilage to treat jeans laundry effluent and found that COD removal was approximately 64.8% and TN elimination was 91.3%. Al-Gheethi et al. (2017) found that Moringa oleifera seeds had a higher TN removal efficiency (84%) than FeSO4 in a study on laundry wastewater treatment (59%). However, COD removal was 47.2%, which appears to be equivalent to FeSO4 removal (54%). Moringa oleifera was also found to have better settling characteristics than FeSO4.
The seeds of M. olifera were used to treat a palm oil mill effluent with COD (40.2 mg/L) and an initial SM (17.9 mg/L) as a possible source of effective coagulant. It allowed for the removal of COD (92%) and SM (52%) respectively. Furthermore, they found that combining M. olifera with a commercial flocculant (NALCO 7751) greatly improved COD and SM elimination. Ullah & Rathnasiri (2015) also stated that using mango seed powder to treat a palm oil mill effluent allowed the effective removal of SM (96%), COD (89%), and color (97%) respectively.
The capacity of mango kernels to coagulate was investigated for the purpose of cleaning wastewater (Dange & Lad 2015a, 2015b). Mango kernels were used as the sole coagulant, resulting in removal efficiencies of 31.6% for SM and 33.4% for COD. When alum and mango were combined, however, the removal rate increased to 66.8% for SM and 69.2% for COD. Mango extract has gained importance as an important coagulant, according to research studies. Aloe gel (Alg), a natural extract from the Aloe sp. plant, was recently tested to see whether it could be used as a bioflocculant in the treatment of urban wastewater sewage sludge (Jaouadi et al. 2020). Under monitored laboratory conditions, the gel was used alone and in combination with water glass (WG). Alg was found to be successful in settling the flocculated sludge quickly and removing the sludge's unpleasant odours. It was discovered to be pH resistant, with no effect on the pH of the wastewater. To improve Alg's treatment efficiency, WG was tested as an alkali agent, which decreased ammonia (NH4-N) and COD levels in the wastewater.
Plant-based bioflocculants were found to be capable of removing heavy metals from both synthetic and real wastewater, in addition to organic pollutant removal. Adsorption and coagulation methods were used to remove cadmium (II) from wastewater using M. oleifera seeds (Swelam et al. 2019). pH, contact time, initial Cd(II) concentration, and temperature were used to compare the increase in Cd(II) removal percentage with the coagulation and adsorption technique. This research was critical in deciding on an environmentally safe water treatment strategy for removing Cd(II) from contaminated effluent. Nharingo et al. (2015) investigated the use of the Opuntia ficus-indica cactus as a low-cost, readily accessible, and environmentally friendly bioflocculant for the removal of heavy metals from wastewater. The pH, contact time, temperature, initial concentration of lead (II) ions, particle size, dosage, and ionic strength of the coagulationflocculation process were all optimized. After using Opuntia ficus-indica powder, heavy metals such as Cd (84.16%), Zn (85.74%), Cu (93.02%), and Pb (100%) were removed under optimal conditions. The bioflocculant removed lead from wastewater with impressive effectiveness, which could be tested on a commercial scale. Table1 displays a variety of wastewaters that have been treated with plant-derived flocculants.
Turbidity is a critical parameter that must be tested for both wastewater treatment and healthy drinking water use. The presence of fine clay, silt, inorganic and organic elements, plankton, and microorganisms, among other things, causes it. According to a World Health Organization (WHO) survey, the maximum turbidity that can be used for disinfection is 0.1 Nephelometer Turbidity Units (NTU) (WHO 2008). The efficacy of disinfection methods can be greatly improved by reducing turbidity. M. oleifera seed powder was used as a biocoagulant in the water treatment process to remove turbidity (Zaid et al. 2019). Response surface methodology (RSM) was used to assess the effects of different parameters on turbidity removal. Lower turbidity (<5 NTU) obtained from the analysis demonstrated the possible use of M. oleifera seeds as an important coagulant for water treatment. The turbidity of the water was found to be less than the recommended value under optimal conditions, confirming the capacity of M. oleifera seed powder for the treatment of industrial and domestic river water. In the presence of KNO3, Dwari & Mishra (2019) recorded the use of guar gum as an effective flocculant that showed excellent turbidity removal performance. The bioresource Opuntia ficus-indica was discovered to be a strong source of bioflocculant for turbidity removal (Bouaouine et al. 2019). There were two molecules found in the bioflocculant that had synergistic flocculating power and operated in a bridging adsorption mode. The removal of turbidity was estimated to be 70%. Jones & Bridgeman (2019) used crude extracts of okra, kenaf, and sabdariffa as coagulants to investigate the characteristics of treated water using fluorescence excitationemission matrices (EEMs). Purified proteins from okra, sabdariffa, and kenaf were also tested for their effect. Fluorescence analysis verified that the purified proteins could extract dissolved organic carbon (DOC) from raw water. Srarfi et al. (2018) described the use of cactus Opuntia bioflocculant to treat industrial wastewater. TN was removed from industrial wastewater by the bioflocculant (19.59%). Rachdi et al. (2017) used a bioflocculant from the same plant to treat urban wastewater and found that TN (93.65%), SS (82.75%), and COD (64.30%) were all removed. Malva nut gum (MNG) was used as a safe and sustainable source for water treatment because it had excellent flocculation efficiency at low concentrations (Ho et al. 2014). It worked well as a powerful anionic flocculant, conformed well to the surface of particles with a branch-like surface structure, and performed well in water treatment. MNG (0.06 mg/L) and Fe (0.08 mg/L) were combined to make the composite Fe-MNG, which was used to treat water at pH 3.01. The Aloe vera leaf gel was evaluated as a flocculant and turbidity removal was assessed (Irma et al. 2015). The plant's clarification tests revealed a high level of SS, TN (72%), and color removal (15%). Strychnos potatorum seeds are said to act as a natural coagulant, allowing turbid waters to be clarified (Deshmukh et al. 2013). For the coagulation of SS, wastewater was treated with natural and synthetically prepared S. potatorum. The natural coagulant properties of S. potatorum were compared to those of alum, a conventional chemical coagulant. Lower dosages of S. potatorum, ranging from 0.25 to 3.5 mg/L, were found to be effective as coagulants, with higher turbidity of 10003000 NTU. Table2 compares the effectiveness of different plant-derived flocculants in removing TN.
There has been a surge in interest in the coagulationflocculation method for wastewater treatment. Polymer bridging, electrostatic patching, and charge neutralization are all common flocculation mechanisms. As shown in Figure2, polymer bridging is the most widely accepted mechanism for plant-based bio-flocculation. Depending on the particle present in wastewater, biopolymers function as a bridge, creating a particlepolymerparticle complex (Bolto & Gregory 2007). Polymer bridging has been proposed as a potential mechanism for flocculation during the treatment of textile effluent with Tamarindus indica mucilage (Mishra & Bajpai 2006) and Plantago psyllium mucilage (Mishra & Bajpai 2005). According to studies, the molecular weight and charge density of the flocculant played a critical role in determining its effectiveness (Morrissey et al. 2020). Experiments on municipal sludge dewatering revealed that hydrophobic association caused by copolymerization increased bridging and sweeping effects, promoting floc growth (Zhou et al. 2020). Furthermore, the flocs produced had a higher mechanical strength and were less compressible, resulting in improved permeability to mechanical dewatering.
Charge neutralization is a process that only works when the flocculants and colloid suspended particles have opposite charges. Adsorption and charge neutralization were the key mechanisms of coagulation/flocculation in Moringa oleifera. Proteins (34%) are contained in the seeds, as well as carbohydrates (15%) and lipids (15.5%). This high protein content may be the most effective coagulation agent. The protein's short chain and cationic surface charge allow it to adsorb and neutralize negatively charged colloids in water (Bolto & Gregory 2007).
Impurity particles may also be found to be negatively charged. Bioflocculants isolated from psyllium, okra, and Isabgol have been stated to have anionic charges, while fenugreek flocculants have neutral charges. The majority of confirmed bioflocculants have either anionic or neutral charges, according to reports (Lee et al. 2014). The interaction of solid waste with mucilage can result in flocculation. There was contact between the free hydroxyl groups of the polysaccharide and the contents of the wastewater when solid waste was treated with polysaccharides. During the flocculation phase, improvements in the composition of crystalline waste materials in wastewater were observed (Mishra et al. 2002, 2003; Mishra et al.2004a, 2004b).
The flocculation mechanism of Opuntia ficus-indica mucilage was discovered. Galacturonic acid (812.7%), L-arabinose (24.642%), D-xylose (2222.2%), D-galactose (2140.1%), and L-rhamnose (713.1%) are all found in the mucilage of Opuntia ficus-indica (Nobel et al. 1992). Galacturonic acid, also known as polygalacturonic acid, is the most active compound that has a high coagulating activity (over 50%) when used alone to remove TN. Mucilage containing arabinose, galactose, rhamnose, and xylose, conversely, only reduces TN by 30% (Miller et al. 2008). Polygalacturonic acid interacts with anionic or cationic pollutants through its long anionic chain, which includes carboxyl (COOH), carbonyl (C = O), and hydroxyl (OH) classes. As a result, the mechanism of flocculation is deprotonation of the OH group, which increases anionic pollutant adsorption and causes agglomeration. Furthermore, according to Theodoro et al. (2013), the oxidation of the terminal carbon OH edge to a COOH group induces cationic pollutant adsorption and coagulationflocculation, and pollutant elimination from wastewater occurs by adsorption and bridging. Nharingo et al. (2015) also agreed with this mechanism.
According to Lee et al. (2014), bioflocculation is a direct flocculation process that is more effective than conventional coagulationflocculation due to the high charge density of amphoteric plant-derived coagulants/flocculants. Because of the dual presence of anionic and cationic active compounds, biopolymers are capable of bridging at any pH condition, so pH adjustment may not be necessary quite often (Mukhtar et al. 2015). In the case of a direct flocculation operation, bridging mechanisms and charge neutralization can occur simultaneously.
The fractionated components of Opuntia ficus-indica were used to create a bi-molecular principle in flocculation (Bouaouine et al. 2019). The flocculant, which was created using a three-step protocol of extraction, precipitation, and fractionation, showed 84% flocculation efficiency. Bridging and adsorption were found to be the underlying mechanisms on synthetic kaolin. Hassimi et al. (2020) demonstrated the importance of biomolecules in flocculation by studying novel anionic bioflocculants derived from fermented fractions of sago mill effluent and palm oil mill effluent. Due to charge neutralization and intra-particle bridging, fractions containing xylose and glucose were found to have a high flocculation quality.
It is safe to assume that research into the flocculation process for the possible use of plant-based flocculants in wastewater treatment is still minimal. As a result, researchers should keep looking for the mechanism behind the successful use of new coagulants/flocculants extracted from different plant sources that has yet to be discovered.
Plant-based grafted bioflocculants have received much attention in recent years as a way to boost flocculation properties and resolve disadvantages like unregulated biodegradability and varying efficiency due to different processing conditions (Ferasat et al. 2020). To extend shelf life and boost flocculating efficiency, natural polysaccharide biodegradability must be carefully monitored. Synthetic flocculants, conversely, are highly efficient and have a long shelf life, but they are non-biodegradable and environmentally harmful (Makhtar et al. 2020). The development of grafted copolymers can be used to enhance the properties of natural and synthetic polymers.
The aim of plant-based grafted copolymerization is to create a unique modified polymer that combines the best features of both groups. It has been demonstrated that grafting synthetic polymer branches onto the rigid backbone of natural polymers can result in efficient, relatively shear stable, and environmentally friendly flocculants. According to some research studies, acrylamide-grafted natural polymers like amylopectin, guar gum, and xanthan gum, as well as starch and sodium alginate, are widely used as flocculants. Polyacrylamide (PAM) was grafted onto the backbone of plant-based biopolymers (Plantago ovata, Plantago psyllium, and Tamarindus indica) and demonstrated significantly improved flocculating properties as compared to non-grafted natural polysaccharides (Ferasat et al. 2020). Table3 summarizes the flocculating efficiency of all plant-based grafted bioflocculants published in the literature.
Wastewater treatment is critical for ensuring a sustainable supply of clean water, especially for human consumption. In wastewater treatment, green or natural bioflocculants could resolve the drawbacks of chemical flocculants. For flocculant grafting, galactomannans, a branched natural biopolymer with good biodegradability, biocompatibility, and sustainability, was recently used. Sharma et al. (2020) used Taguchi's method of rigorous design of experiments with 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) under alkaline heterogeneous conditions to demonstrate the quaternization of galactomannan derived from Cassia tora gum (QCTG) under alkaline heterogeneous conditions, which could be used in a variety of industries for end-use applications (Sharma et al. 2020).
Lee et al. (2018) published a study on a techno-economic analysis of industrial scale bioflocculant processing using okra as a biomass feedstock. Using continuous mode microwave extraction at 90 C, a residence period of 10 minutes, a water loading of 3.5 w/w, and a production rate of 220 tonnes per year, the biosludge flocculant's dewatering capacity was optimized and showed significant results (Lee et al. 2018).
Furthermore, Guo et al. (2019) described a new modified rice straw biochar (RSB) that improved sludge dewaterability and served as a skeleton construct. Charge neutralization was the process used to improve the dewatering of the sludge. The sludge system was dosed with a changed RSB filled with positively charged aluminium, which destroyed the sludge colloidal system's stability and resulted in easier sludge particle congregation, which increased sludge dewatering. Another important mechanism was the development of skeleton structures in the sludge cake to facilitate water passage by lowering the sludge's extracellular polymeric substances (EPS) (Guo et al. 2019).
Furthermore, Chen et al. (2020) demonstrated an octopus-like lignin-grafted cationic polyacrylamide (L-CPA) with excellent flocculation performance, was inexpensive, environmentally friendly, and technically feasible, indicating a promising future in wastewater treatment. Using acrylamide (AM), methylacryloyloxyethyltrimethyl ammonium chloride (DMC), and enzymatic hydrolysis lignin (EHL) as raw materials, a high-efficiency L-CPA bioflocculant was synthesized. In this case, a linear pre-polymer of cationic polyacrylamide (CPA) terminated with chlorine was first prepared, and then grafted onto EHL using chlorine reactions with phenolic hydroxyl groups in lignin molecules.
The main mechanism behind this formulation was that the prepared L-CPA could self-assemble into octopus-like nanospheres with CPA segments suspended in water and hydrophobic lignin skeletons condensed in the center, giving the L-CPA excellent flocculation efficiency for kaolin suspension under acidic, neutral, or alkaline conditions (pH = 59). For flocculation of the kaolin suspension, only a small amount of L-CPA (4.04.5 mg/L) was needed (Figure3).
Furthermore, a recent study demonstrated the formulation of polyacrylamide-grafted fenugreek gum (FG-g-PAM) as a bioflocculant. Fenugreek gum was modified in this study using two different graft copolymerization methods: microwave-assisted and thermal synthesis. The effective-grafted product was created using an optimized concentration of acrylamide monomer and ceric ammonium nitrate (CAN) initiator. In the refloccculation method, the flocculation efficacy was significantly increased from 92.05 to 95.98% in this analysis. In kaolin, coal fine, and iron ore suspensions, the flocculation efficacy was found to be 95.98%, 40.52%, and 68.94%, respectively. In comparison to chemical flocculants, the novel grafted flocculant was predicted to be a highly effective flocculant in the future (Mishra & Kundu 2019).
Furthermore, Makhtar et al. (2020) investigated a plant-based Tacca leontopetaloides biopolymer flocculant (TBPF) for leachate treatment, which showed excellent removal of TN, TSS, and color. The tuber of the Tacca leontopetaloides plant was used to extract a new natural-based bioflocculant, which was then processed via gelatinization to create TBPF for leachate treatment. The presence of COOH and OH groups in TBPF matched bioflocculant properties for leachate therapy. In this analysis, 240 mg/L of TBPF at pH 3 was found to be very effective in reducing turbidity, TSS, and color from 218 NTU, 214 mg/L, and 14201 PtCo to 45.854.5 NTU, 19.319.9 mg/L, and 852994 PtCo, respectively, indicating that this new plant-based TBPF has much potential for leachate and other waste-water treatment applications.
The development of superior flocculation technology has so far focused on combining the properties of chemical and biological flocculants, with the primary goal of lowering flocculant dosage and improving flocculation efficiency. PAM has been considered as a suitable co-polymer graft for this reason. PAM's neurotoxicity, conversely, may be the big discrepancy. As a result, further research into the development of natural and healthy organic co-grafts that could replace acrylamide is needed. In this regard, Ferasat et al. (2020) have recently identified yeast cell wall (YCW) as a so-called safe polymer. This polymer has recently been shown to be an efficient kaolin coagulation matrix in an aqueous environment.
The possible use of plant-derived flocculants and plant-based grafted bioflocculants as a replacement for traditional chemical coagulants/flocculants in wastewater treatment was examined in this study. Bioflocculants derived from various plant sources have been studied extensively, and the potential use of these green flocculants for wastewater treatment has been identified. They also shown important results in the removal of SS, TN, and COD, total nitrogen, dye, and heavy metals from wastewater. These products eco-friendliness has been well known, and they have paved the way for the implementation of green chemistry, clean, healthy, and cost-effective technology. Plant-based flocculants can, in reality, be used to replace chemical flocculants, which have been shown to have negative effects on the environment and human health. The data on green flocculant production conditions are needed to develop a comprehensive strategy for scientific research and commercial implementation in wastewater treatment. Many plant-based flocculants showed substantial flocculating behaviour in the laboratory, with pollutant removal efficiency exceeding 90% depending on the type of plant material used to make the flocculant and the characteristics of the wastewater. Plant-based grafted copolymerization has been shown to be an efficient green flocculant for wastewater treatment, containing the best properties of both the plant and polymer groups. Green flocculants for wastewater treatment have been found to be efficient, shear stable, and environmentally friendly by researchers. However, extensive research is needed to optimize bioflocculation procedures for each form of wastewater treatment based on the plants and polymers that should be statistically optimized to improve flocculation operation. Statistical optimization, which should be the primary focus and potential research prerequisite for realistic applications, has received little attention from researchers so far. Furthermore, several studies are required to understand the process of bioflocculation, taking into account the changes in various treatment systems. Finally, the effectiveness of green flocculants should be tested on a broad scale, in real-world environments, and for a wide range of wastewater systems, followed by a techno-economic evaluation.
The authors are grateful to Vellore Institute of Technology, Vellore 632014, Tamil Nadu and Sri Shakthi Institute of Engineering and Technology, Coimbatore, 641062, Tamil Nadu, India and Thiruvalluvar University, Serkadu, Vellore, 632115, Tamil Nadu, India for the support throughout the study.