Introduction

Being one among the basic amenities of mankind, water apart becoming scarce is also becoming an expensive resource in the contemporary world attributed to overexploitation and unimpeded pollution of this resource. Given this scenario, there has been great emphasis on solutions to attain potable water. While several solutions have been put forth and adopted, each of these solutions come with their own disadvantages. Greener solutions are being looked up as promising alternatives attributed to their sustainable nature and no or negligible disadvantages. Further, the wide spectrum of biodiversity enables the provision of enhanced alternatives among greener solutions. Especially for drinking water treatment, extensive studies have been taken up, and numerous researchers have presented promising results. One such study is on using natural coagulants for water and wastewater treatment which comes with a plethora of published research results. It is of utmost importance that there is scientific dissemination of information on natural coagulants. In light of this, studies on properties and optimized conditions for coagulants have to be studied at levels much higher than existing studies, which is possible only through constant exploration of the coagulation process and chemistry of coagulants (Patchaiyappan and Devipriya 2022)

It is understood that extended polymer, which enhances the efficiency of natural coagulants, is possessed by natural resources having a higher molecular weight (Neerajasree et al. 2019; Jones 2017; Iqbal et al. 2019). Some of the reported efficiencies of natural coagulants include suitability and efficiency at a wide range of pH (Jayalakshmi et al. 2017; Deshmukh and Hedaoo 2019), also no alteration of pH in treated water unlike chemical coagulants, less sludge (Dwarapureddi et al. 2018), no or negligible effects on the environment. (Deshmukh and Hedaoo 2019; Yusoff et al. 2019).

Plant-based coagulants are gaining greater attention and acceptance as alternatives for conventional inorganic coagulants because these coagulants are sustainable, innoxious, biodegradable, and have the potential to reduce carbon emissions footprints associated with the treatment. However, the breakdown of the plant matter results in humic acids form of NOM. These are complex structures with molecular weights ranging from 500 to 5000 Daltons and highly aromatic (Fig. 1). The colour of NOM is brown to yellow as these substances have various functionalized groups, generally influenced by the moieties of the phenolic and carboxylic groups (Karnena and Saritha 2020).

Fig. 1
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Humic acid structure from the literature (Mirza et al. 2011; de Melo et al. 2016; Raj et al. 2022)

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Below is the figure illustrating published results of studies on the efficiency of natural plant-based coagulants towards producing potable water (Fig. 2).

Fig. 2
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Statistics of publications on coagulants derived from the plants in water treatment. (*Information attained from the database of Scopus and web of science, ** Publication till February 2022)

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Coagulation is reported to be the most widely adopted treatment method from individual households to community treatment levels owing to ease of the process, cost-effective, does not require skilled labour and can be adopted in all geographical conditions, specially using innate raw materials. The efficiency of natural coagulants is owed to their precise characteristics, which contribute to coagulation and flocculation processes. Active compounds in natural coagulants diverge and correspond to explicit working mechanisms, including charge neutralization, sweep coagulation, adsorption, bridging and patch flocculation (Kurniawan et al. 2022).

The primary objectives of the studies on natural coagulants are their properties, extraction methods and efficiency. Almost all studies have presented results on only a limited number of coagulants. Hence, there is a gap on cumulative information of suitable coagulants from plants studied. In these lines, the present research attempts to fill the gap and has presented 57 coagulants with reference to their selective properties and efficiency as reported by previous studies (Fig. 3).

Fig. 3
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Flow diagram for the current review

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A brief timeline of plant-based coagulants

Saleem and Bachmann (2019) stated that “the utilization of coagulants derived from the plants towards water clarification can be observed from way back 2000 B.C”. The inscriptions of ancient Egypt’s middle kingdom revealed that the Prunus dulcis were used as water clarifying agents. The biomass of crushed P.dulcis was smeared to empty clay vessels followed by the subsequent raw water addition and stirring this mixture for some time and allowing it to stand for three hours in quiescent condition to produce clean supernatant. The clean water is transferred into smaller jars and consumed (Baker 1948). It was presumed that ancient Egyptians (primary settlers) utilized the “rolling stone pins as pestles and mortars flat surfaces for the crushing of material derived from plants” (Frazee et al. 1997), and vessels made up of coarse silts of different sizes for the storage and cooling od the drinking water for the shorter periods (Ogden et al. 2000). Pieces of evidence from the literary compositions (“Sanskrit writings”) (“Suśruta sũtra 45.13”) and different documents from India (Iyer 1884) furnish more examples of the plants and their materials used as coagulants in medieval and ancient India (Fig. 4a).

Fig. 4
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a Egyptian literary image shows water treatment (Smith 2017). b History of coagulation

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The utilization of plant materials in water treatment was also reported from different continents; According to Peruvians literature, in the sixteenth and seventeenth centuries, the sailors during journeys used to treat the water with Zea mays grains that are roasted and powdered (Jahn 1981). The Chilean folklore literature from the nineteenth century indicates that water clarification was done with the mucilage of Opuntia ficus indica (Beckman 2006). During a trip to Sudan in 1847, Alfred Edmund Brehm observed that people of the nomadic tribes used Faba fona and P. Dulcis to clarify water (Arndt 1975). Modern reports of the literature stated that Sudan and Tanzania rural communities (progeny of ancient Africa) and Mexico and Peru (progeny of old Columbia) utilized many materials obtained from plants to cause coagulation of water (turbid water) (Theodoro et al. 2013; Karnena and Saritha 2020; Saleem and Bachmann 2019).

The conventional methods of using materials derived from the plants for coagulation differ, and it also depends on the properties of raw materials. Plant material like leaves and roots consists of bodily fluids pounded to expose more biomass that helps release active coagulation compounds. The harvested seeds are deshelled, dried and powdered. The seed powder of Moringa oleifera is most commonly used to remove the turbidity of the water; upon adding seed powder to turbid water, the mixture is stirred with bare hands or wooden spoons for a little while, and the clarified water obtained is decanted and stored in the jars made of clay (Marobhe et al. 2013). Jahn and Dirar 1979, reported that Sudan’s tribal communities had placed the seed powders in a cloth bag with a thread attached to the string, like the tea bags wherein the crushed leaves are set.

Further, to use seed powder, a smaller quantity of water is added to a small bowl to extract the coagulant. After stirring for ten minutes, the extracts are added to the water’s having turbidity (Baxter 1981). Jung et al. (2018) and Saritha et al. (2019) reported and confirmed that the coagulation efficiencies were increased by using short extraction time, attributed to the release of active coagulating compounds. In conventional usage, leaves of Opuntia ficus indica are broken, and mucilage can drop into the water with turbidity (Theodoro et al. 2013; Saleem and Bachmann 2019). It should be noted that legumes, Manihot esculenta, M. oleifera and different species of the cacti utilized by our ancestors are furthermore explored in the twenty-first century through bench-scale coagulation experiments.

Research milestones in water clarification with the application of coagulants obtained from plants include separation and characterization of plant seed proteins (Ghebremichael et al. 2005; Gassenschmidt et al. 1995; Ndabigengesere et al. 1998), advances in simplifying procedures for the coagulant purification (Sánchez-Martín et al. 2010), the starch derivatization (Bratskaya et al. 2005) and the practical coagulant synthesis of tannin molecules (Reed and Finck 1997; Sánchez-Martín and Beltrán-Heredia 2012; Broin et al. 2002; Pavankumar et al. 2014), and recombinant coagulants protein synthesis on a large scale by using bioreactors (Pavankumar et al. 2014). Many researchers have adopted the conventional knowledge approach methods (Gunaratna et al. 2007; Mbogo 2008) and confirmed the coagulation activity of some known plants (Table 1) with the available modern technology; however, in most of the cases, the active coagulating compounds responsible for the coagulation activity are still unknown (Saleem and Bachmann 2019). Further much attention was gained, and more research was ongoing in Manihot esculenta as a coagulant for the treatment of water consisting of microbes and pollutants (Karnena et al. 2021a and Saritha et al. 2019, Vara 2012).

Table 1 List of prominent and identified plant-based coagulants widely explored for water treatment
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Plant-Based coagulants used for water treatment

The prime objective of the current section is to present the most explored and studied plant origin coagulants concerning their selective properties focussing on the compounds responsible for effective and active coagulation activity.

Arachis hypogea

These plants are, commonly called peanuts, belong to the family Fabaceae (Fig. 5). Han et al. (2014) identified the presence of alkyne and alkoxy groups in the seeds of these plants in their experiments. Saritha et al. 2019 stated that the presence of these groups enhances the coagulation activity and creates a stronger affinity with the impurities present in the aqueous medium. Birima et al. 2013 used the seed powders of these plants for a coagulation experiment with a dosage of 200 mg/L and achieved nearly 32% reduction. Further, previous studies stated that the proteins in the seeds are responsible for coagulation activity (Dwarapureddi et al. 2021).

Fig. 5
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Arachis hypogea plant seeds as a coagulant

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Brassica juncea

Mustard seeds are cruciferous vegetables generally used as spice and medicine worldwide. Mustard seeds are commonly called Chinese mustard (Fig. 6). These seeds have various phytochemicals which might be responsible for coagulation activity. Mustard seeds are cheap and consist of bioactive components and degradation products. (Tian and Deng 2020). Bodlund et al. 2014, in their studies, revealed that these plant seeds consist of positively charged proteins responsible for coagulation activity. The mustard coagulant protein reactivity involves arginine, histone, glutamines and threonine. Bodlund et al. 2014 used this coagulant and removed the turbidity of water with an efficiency of up to 85%.

Fig. 6
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Mustard seeds as a coagulant

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Ceratonia siliqua

Bazzo et al. 2021 stated that Ceratonia Siliqua is a legume that belongs to the family of Fabaceae consisting of edible pods and is widely cultivated in tropical regions (Fig. 7). The seed gums of the Ceratonia siliqua are commonly called locust beans or carob beans. These plants are native species of Mediterranean areas. The literature shows that these plants’ pods and seed powder are used for humans during a famine. The plants’ seed gums are considered heterogeneous gums (complex) as these consist of arabinose in significant amounts and form arabinogalactan (Kök 2007). However, mannose provides the majority of the reactive –OH sites. Bazzo et al. 2021 used these seeds to treat surface water with a turbidity of 83.7 NTU and achieved a removal efficiency of 85%.

Fig. 7
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Seeds gums of Ceratonia siliqua as a coagulant

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Coccinia indica

Coccinia Indica, commonly called ivy ground, generally grows in the world’s tropical zones and is mainly found in the Indian states (Fig. 8). These plants have edible fruits and shoots. These plants have a medicinal value for treating jaundice and asthma. These plants belong to the Fabaceae family and have anaphylactic and cell stabilizing properties (Karnena et al. 2020). Patale and Pandya, (2012) used the mucilage of these plants to remove the surface water’s turbidity and revealed that this mucilage shows higher efficiencies in removing the higher turbidity than the lower turbidity. The authors achieved 94% removal efficiencies in eliminating the pollutants from the surface water. Further, Sharma et al. (2021) analysed the mucilage of these plants using FTIR and revealed the presence of the functionalized group like hydroxyl and carboxyls. Saritha et al. (2019) showed that the presence of these groups enhances the efficiencies of the coagulation.

Fig. 8
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Coccinia Indica Mucilage as coagulant

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Cocos nucifera

C. Nucifera belongs to the Arecaceae family, commonly known as coconut. C. Nucifera is widely abundant in tropical regions (Fig. 9). The fruits of these plants are believed to be obtained from East Africa to India (Lima et al. 2015). Casein is the protein mainly present in the seeds of C. Nucifera (Fatombi et al. 2013). The FTIR stretching of 1649, 1536 and 1237 cm−1 represents that the purified protein consists of primary, secondary and tertiary amine-reactive groups. Moreover, it was found that these proteins consist of hydroxyl and ammonia groups responsible for coagulation (Fatombi et al. 2013). Fatombi et al. (2011) used casein protein for the turbidity (Kaolinite) reduction of surface and achieved 99% removal efficiency by eliminating the collides present in the water. Fatombi and co-authors observed a strong bridging and adsorption between the casein protein and colloidal particles of the Kaolinite.

Fig. 9
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Seed protein of Cocos nucifera as coagulant

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Cyamopsis tetragonoloba

Cyamopsis tetragonoloba, commonly called cluster bean, are the sources of guar gum (Fig. 10). These plants are assumed to be developed from Cyamopsis senegalensis of Africa naturally grow in the semi-arid regions. The presence of gelling agents in these plant seeds has strong demand, and eight per cent of the production occurs in India (Pandey et al. 2020). These plants’ seed gums mainly consist of galactose and mannose (Mahungu and Meyland 2008). The mannose provides more –OH reactive sites compared to galactose. Arabinose and xylose are the additional residues in the plant seed gums (Rowe et al. 2009). Pandey et al. (2020) used these seeds to remove pollutants from the surface water by coagulation and attained good results. Further FTIR characterization of these seeds presented alkane, alkane and alkoxy groups responsible for bridging between impurities and coagulants.

Fig. 10
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Seeds of Cyamopsis tetragonoloba as a coagulant

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Manihot esculenta

Manihot esculenta belongs to Euphorbiaceae, commonly called cassava (Fig. 11). Starch, one of the most abundant biopolymers in nature, is a macromolecule consisting of linear amylose and branched amylopectin. Starch structure with reactive hydroxyl (–OH) functional groups enhances adsorption by creating hydrogen bonds (Maćczak et al. 2022). Studies on various starches obtained from peanut, rice and potato have been previously studied. Still, their adaptation has been hurdled due to these being stapled foods in many parts of the world.

Fig. 11
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Manihot esculenta as a coagulant

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On the other hand, sago starch is a staple food in a few places but is available in plenty in most regions. Hence, researchers have been inclined towards exploring and studying sago starch as a coagulant for water treatment. It has been reported that most of the starch is present in the trunk of the sago palm and that sago productivity is over four times in comparison to rice. Starches have been gaining interest as potential biopolymers owing to their properties such as their abundance, biodegradability, non-toxicity and being economical.

Turbidity removal of sago starch is owed to its coagulation mechanism, wherein the negative charge of the starch attracts the positively charged particles and destabilizes them, resulting in agglomeration. Further, particles of sago starch are discernible from their granular shape and particle size, with solid, smooth surfaces without pores (Copeland et al. 2009; Jane 2009). The property of sago starch in agglomeration is associated with the degree of crystallinity that might positively affect the removal of turbidity (Choy et al. 2017; Falconer 2019). Upon being subjected to FTIR analysis, the spectra of sago starch indicated numerous prominent peaks that correspond to functional groups such as –OCH3 –OH and –NH. In their study, Saritha et al. (2019) further reported good turbidity removal (Fig. 12).

Fig. 12
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Sago SEM images showing adsorption of impurities (Saritha et al. 2019)

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Dolichos lablab

These plants are commonly called Indianbeans and belong to Fabaceae (Fig. 13). These plants are found in tropical countries and are considered a widespread food crop (Unnisa et al. 2010a; b). The seed gums of Dolichos lablab consist of water-soluble non-galactomannans with a neutral charge, as reported by Salimath and Tharanathan (1982). The seed gums mainly constitute arabinose to galactan with the uronic acids; the –OH groups of the neutral sugars are in equal parts. Asopa and Korake, (2019) used the seed powder as a coagulant with a dosage of 8 mg/L reduced the stupidity by about 45% with an initial 160 NTU.

Fig. 13
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Seed gum of Dolichos lablab as coagulant

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Hibiscus esculentus

These plants are called Okra and belong to the family of Malvaceae (Fig. 14). Zaharuddin et al. (2014) revealed that plant gums consist of galactose, rhamnose and galacturonic. (Zaharuddin et al. 2014). Mishra et al. (2017) used the gums of the Okra as a coagulant and achieved a turbidity removal of 72% in their studies. Further, Raji et al. (2016) conducted similar experiments and achieved removal efficiencies of 99% with an optimum neutral pH.

Fig. 14
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gums of Hibiscus esculentus as coagulant

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Luffa cylindrica

Luffa cylindrica is commonly called sponge guard (Fig. 15). These plants belong to the Cucurbitaceae family, originating from tropical Asia. Sponge guard has a medicinal value and proved in the tropical and subtropical regions. Alcalase and tryptic proteins are common in the seeds of these plants (Wu et al. 2020). Azeez et al. (2013) stated that these plants consist of light white coloured seeds having anti-inflammatory drugs like triterpenoids and a-tocopherol. Sowmeyan et al. (2011) used the extracts, performed coagulation and achieved a removal efficiency of up to 85%; however, these authors did not specify the extracts of these plants.

Fig. 15
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Extracts of the sponge guard used as a coagulant

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Moringa oleifera

These plants are native species of the northern part of India belonging to the family Moringacea; the coagulative properties of these species were identified in the eighteenth century (Fig. 16). The seed proteins are cationic and act as an excellent flocculant to remove pollutants from the water. These plants are available naturally in the regions like Africa, Madagascar, Arabia and India; further developing countries utilize these seed proteins for water treatment and are not new. The M.oleifera seeds are rich with functional cationic charged proteins with lower molar mass classified as MO2x proteins (Shebek et al. 2015) or also called lectins classified as cMOL (hemagglutinating) proteins (de Andrade Luz et al. 2013). Karnena et al. (2022) used the seed proteins for the water clarification and achieved 98% turbidity removal.

Fig. 16
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Moringa seed protein powder as a coagulant

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Opuntia ficus-indica

Opuntia belongs to the cacti species and consists of mucilage stores in the plant’s pulp. These species grow and are available in semi-arid regions. These plants gained much prominence in water treatment due to their vast availability and low cost (Karnena et al. 2020). Poly-galacturonic (Fig. 17) is also called pectic acid (Aspinall and Cañas-Rodriguez 1958), and these compounds are water soluble and derived from the mucilage of Opuntia (Miller et al. 2008). The reactivity of these substances is governed by the –OH groups representing the acid sites that offer more pollutant removal. The mucilage of the opuntia is used as a coagulant to remove the pollutants and clarify water.

Fig. 17
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Poly-galacturonic acid structure present in the mucilage of opuntia

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Parkinsonia aculeate

Parkinsonia aculeate spiny deciduous tree with a short trunk with drooping foliage remains green throughout the year and leafless after fall (Fig. 18). The leaves and seeds of these plants consist of glycoflavones in laevorotatory form (Divya et al. 2011). The Parkinsonia aculeate seed proteins consist of active coagulating compounds (proteins) responsible for the coagulation (Marobhe et al. 2007). These plants’ seeds showed high turbidity removal efficiencies from the water consisting of higher colloidal particles.

Fig. 18
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Parkinsonia aculeate seeds as coagulants

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Phaseolus vulgaris

These are commonly called beans and grow in the temperate zones within the subtropics (Fig. 19). These seeds are important crops in the central parts of Africa and southern parts of America (Caballero 2005). Antov et al. (2010) revealed that the P. Vulgaris seeds consist of anionic type protein coagulant; he used chromatography techniques and showed that seed proteins of these compounds have negatively charged amino acids. Although several researchers are still studying P. Vulgaris, these coagulants’ reactivity might be attributed to the polar amino acids (negatively charged). The seed proteins help remove the pollutants from the surface water (Karnena et al. 2020).

Fig. 19
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P. Vulgaris seed protein as coagulant

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Prosopis

Torres and Carpinteyro-Urban, (2012) stated that Prosopis trees grow to a maximum height of thirteen meters and are widely distributed around South America (Fig. 20). This plant was introduced to India in the semi-arid regions. These trees are used as firewood and pods for the fodder of cattle. The seed gums of mesquite (Prosopis spp) are branched heterogeneous galactomannans (Vernon-Carter et al. 2000). Torres and Carpinteyro-Urban’s (2012) studies revealed that turbidity removal efficiencies are achieved up to 70% with lower dosages of these seed gums.

Fig. 20
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mesquite seed gum as coagulant

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Scaphium scaphigerum

Scaphium scaphigerum is commonly called Malva nut (Fig. 21); these plants can yield up to 40 kg of fruit per year. These trees produce small brown-skinned fruits from March to April (Ho et al. 2015). Ho et al. (2015) conducted characterization of these fruit extracts and revealed the presence of esterified carboxyl groups, which help enhance coagulation. The gums of Malva nuts are classified as heterogeneous non-galactomannans (Somboonpanyakul et al. 2006). Ho et al. (2015) proved that using the seed gums as coagulants could enhance coagulation.

Fig. 21
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Gums of Malva nuts as a coagulant

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Strychnos potatorum

Strychnos potatorum is commonly called nirmali in India; the application of the nirmali seeds for turbidity removal is not new (Fig. 22), and it was mentioned in ancient literature like Sushruta Samhita. According to available literature, it is evident that over 4000 years, these seeds have been used for the treatment of water (Kumar et al. 2016). The gums of the Strychnos potatorum are the first coagulant solution derived from the plants and widely used to treat turbid water at a laboratory scale; these gums consist of mannose and galactose (Adinolfi et al. 1994). Kumar et al. (2016) used nirmali and attained turbidity removal with an efficiency of 65%.

Fig. 22
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Gums of the Strychnos potatorum as a coagulant

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Tamarindus indica

Tamarindus indica, commonly called tamarind, belongs to the Fabaceae family and these plants consist of edible fruits (Fig. 23). These plants are indigenous to Africa. There is evidence that these plant pulps have been used as medicines from ancient history. The tamarind plants are cultivated around the world in tropical regions. These plants’ gums consist of gummy polysaccharides (Saettone et al. 2000). The chemical components of seed consist of glucose, xylose and galactose (Saettone et al. 2000); the reactivity of these gums also depends on the −OH functionalized groups. Dwarapureddi et al. (2021) conducted preliminary studies to remove the turbidity by using the seed extract of the Tamarindus indica and removing the turbidity of the synthetic water with an efficiency of up to 90%

Fig. 23
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Seed gums of Tamarindus indica as a coagulant

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Trigonella foenum-graecum

These plants are commonly called fenugreek (Fig. 24) and have had medicinal properties for centuries. The seeds consist of proteins that have prominence in ancient ayurvedic medicine (Khan et al. 2018). Palanuvej et al. (2009) reported that fenugreek (T. foenum graecum) seeds consist of mannose and galactose molar ratio (Mathur 2011). The −OH groups of these seed gums are reactive and helpful for removing organic pollutants. The seeds are used for coagulation for eliminating the contaminants present in the water.

Fig. 24
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Seed gums of Trigonella foenum-graecum as coagulant

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Vigna unguiculate

Vigna unguiculata, commonly called cowpea (Fig. 25), are legumes cultivated in Africa and are available in tropical regions. These seeds are widely used as food for humans and animals. These seeds consist of phenolic substances responsible for coagulation activity (Avanza et al. 2021). The Vigna unguiculate seed proteins consist of active coagulating compounds (proteins) responsible for the coagulation (Marobhe et al. 2013). These seed proteins can be classified into cationic coagulants. High turbidity removal efficiencies were achieved during the coagulation experiment (Marobhe et al. 2013).

Fig. 25
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Seeds of Vigna unguiculate as coagulants

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Coagulation–flocculation process

In general, coagulation mechanisms are used to remove turbidity, organic matter, etc., from the water and wastewater. The coagulation process involves adding plant-based coagulants, which helps aggregate smaller particles to form larger flocs that can be sedimented. The coagulation and flocculation process is interlinked with each other. Coagulation involves clustering phenomena with rapid mixing. In contrast, the flocculation process consists of the settling process with gentle mixing. After the chemical phenomenon of neutralizing pollutants, flocculation occurs, a physical process that leads to the formation of flake-like substances by balancing the particles during the coagulation process; this helps in aggregating the particles and settling down at a faster level. Coagulation mechanisms rely on operating conditions like mixing rate, temperature, settling time, dosage and coagulant type. The coagulation process mainly has four mechanisms (neutralization of charge, bridging of polymers, sweep coagulation and compression by double layer) for destabilizing the pollutants with the coagulants. Figure 26 shows the general coagulation process, removing the contaminants from the aqueous medium (Karnena et al. 2021b).

Fig. 26
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Illustrative representation of the coagulation process

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Even though the innovation and research on the plants’ coagulants are advanced, moving these treatment technologies beyond the bench-scale studies is still in the initial phases. Laboratory scale testing on coagulants derived from the plants have produced potable water having standards laid down by the World Health Organization (WHO). The plant-based coagulants soluble in water exhibited excellent performance in removing the colloidal particles and microbes from the turbid water; moreover, reduction in some of the elevated physicochemical parameters from the water was also observed and is presented in Table 2. The plants’ extracts also produced lower sludge volumes during the coagulation, along with the benefits, like lower dosages and insignificant effects on the pH. The full-scale coagulation process for seed extracts of Moringa oleifera is abundant as far as is known (Table 3).

Table 2 Laboratory scale analysis by the coagulants derived from the plants in water treatment (Saleem and Bachmann 2019)
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Table 3 Coagulation studies by Moringa oleifera in water treatment (Saleem and Bachmann 2019)
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Plant-Based coagulants sludge management

Łukasiewicz, (2016), in her review, showed post-coagulation sludge management after water treatment, and many researchers have adopted these methods. The traditional inorganic coagulants produce more sludge than natural coagulants (Dwarapureddi et al. 2021). However, much research on sludge management after the water treatment by coagulation using natural coagulants was not researched. The sludge obtained by the traditional inorganic coagulation process requires further advanced treatment like bio-nano remediation, electro-remediation, etc., for treating the sludge as these substances consist of more toxic substances (Karnena et al. 2020). However, the plant-based coagulation sludges do not have more impurities/secondary products as inorganic coagulation and can be used as a fertilizer for agriculture (Table 4). Ezemagu et al. 2021 digested the sludge obtained by the plant-based coagulation process, was made as compost, and on subject to characterization, revealed that the sludge consists of both macro and microelements essential for the growth of the plants and improves the soil quality. Mtshali et al. (2014) showed that the sludge obtained after the coagulation process consists of trace elements, nitrogen and phosphorous that enhanced the plant growth. Hamid et al. (2014) stated that sludge obtained by treating aquaculture wastewater with Moringa oleifera consists of recovered chlorella and is further used for biofloc cultivation in the closed pond ecosystem. Turunen et al. 2019 used the post-coagulation sludges of the Acacia mearnsii and Solanum tuberosum as a phosphorous fertilizer for agriculture.

Table 4 Sludge management after coagulation reported from the literature
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Sustainability with plant-based coagulants

Even though many researchers highlighted the efficiencies of plant-based coagulants in treating water and wastewater, their reliability in the applications might be evaluated and given importance, as stated by the United Nations, to achieve sustainable development. The word “sustainable development” means “the development that meets the needs of the present without comprising the ability of future generations to meet their own needs”, as stated United Nations Brundtland Commission. The combination of the parameters illustrated in Fig. 27 should be considered for sustainability. The main moto of the current review is to present the plant-based coagulants’ efficiencies in treating the water; thus, the main principles of sustainability are present in Fig. 27 to justify the current review. Further, in their review, Karnena et al. (2021) evaluated the coagulants’ interpretation of sustainability and showed how these plant-based coagulants enhance water treatment efficiencies, improving water treatment and public health.

Fig. 27
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Parameters considered for the sustainability of plant-based coagulants in water treatment

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Challenges in adopting plant-based coagulants

Plant-based coagulants face several challenges towards their adaptation. As presented by Karnena et al. (2021), the applicability of natural coagulants at a large scale has not evolved to the fullest as most of the studies that have been reported are restricted to bench-scale studies. Hence, it is challenging to transfer technology to industrial and real-time application levels. Further, it is also understood that several parameters govern this technology, including consistency in water quality, availability of coagulants having active coagulating compounds, their handling and storage, etc. Moreover, from an ecological point of view, there are also challenges in extracting the active coagulating compounds that should not be affected in the extraction process. Further, there are concerns about the amount of waste generated in preliminary, secondary treatments and remnants in treated water that can threaten the population and environment.

Potential of plant-based coagulants in future

The challenges mentioned above can be taken up as opportunities for subsequent research; addressing these challenges would resolve ambiguities and provide more information for real-time applications. Some of the aspects that can help in boosting the acceptance of natural coagulants include: upscaling coagulants that have shown extreme performance from laboratory scale to bench scale and pilot scale; constant availability of natural coagulants without any effect on the environment and humans in the process of their extraction and purification; in the future blended coagulants capable of multifunctioning are to be identified and studied for enhanced coagulation properties. The existing methods for extracting the coagulant protein from the plants are complex. Thus, new and reliable extraction methods must be developed for using natural coagulants towards sustainable development. Few studies reported higher removal efficiencies of pollutants by natural coagulants than conventional inorganic coagulants.

Nevertheless, optimizing the plant-based coagulants for extraction and purification might enhance the efficiencies of the plant-based coagulants in water treatment. Thus, much focus on the current research is needed to strengthen the coagulant's efficiencies. The sources of the plant-based coagulants towards utilization and production is becoming challenging as the water treatment systems require a significant quantity of coagulants. Meticulous research on the plants must be conducted to search for inedible plant parts as coagulants. In addition, further studies are required to optimize the parameters needed for the coagulation process for the treatment of water from different sources/types. In-depth studies are required to test the efficiency of these coagulants in removing the microbes and micropollutants from the other aqueous medium.

Conclusion

The world has been resorting to natural or green solutions as an initiative towards sustainable development. In water treatment, coagulation is understood to be an important step that can remove colloidal particles usually associated with organic and microbial contaminants. Though several researchers have presented the efficiency of various coagulants, the initial database on explored coagulants cumulatively has not been presented. Hence, the current review offers a cumulative database of 57 coagulants that have been studied globally in terms of their specific chemical characteristics required for coagulation and their efficacy. The review also focussed on the sludge management from plant-based coagulants over inorganic coagulants and challenges faced by these coagulants towards commercialization.

In conclusion, it is presented that more research is required in exploring natural coagulants for water treatment towards developing innate technologies. Also, the optimization of the coagulation process and understanding the properties of coagulants (physical, chemical, phytochemical, etc.) would provide baseline data for future research where efforts in the exploration of coagulants can be reduced, and research on related precise aspects of coagulants can be enhanced. These efforts would result in strengthening the commercialization of coagulants.