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Green Treatment Strategies for Tannery Wastewater Employing

Natural Polymers

Thamaraiselvi. C*, J. Vijayalakshmi , C. V. Hemalakshmi

Department of Biotechnology, Mother Teresa Women’s University, Kodaikanal, TamilNadu, India-

624101

*Corresponding Author

DOI: https://doi.org/10.51583/IJLTEMAS.2026.150500071

Received: 02 April 2026; Accepted: 07 May 2026; Published: 25 May 2026

ABSTRACT

The leather tanning industry is one of the most water-intensive industrial sectors and generates highly polluted

wastewater containing organic matter, inorganic salts, sulfides, chromium, and recalcitrant compounds.

Conventional treatment methods rely heavily on chemical coagulants, which often produce non-biodegradable

sludge and pose potential environmental and health risks. The present study investigates an eco-friendly and

sustainable approach for tannery effluent treatment using a natural polysaccharide extracted from Strychnos

potatorum L. seeds. The extracted polysaccharide was instrumentally confirmed using Fourier Transform

Infrared (FTIR) spectroscopy, which revealed the presence of hydroxyl, carboxyl, and amine functional groups

responsible for coagulation activity and SEM analysis of raw polysaccharide and treated sludge for the

confirmation of coagulation process..Physicochemical characteristics of untreated tannery wastewater were

analyzed and compared with treated samples following coagulation–flocculation using varying doses of the

extracted polysaccharide. Process optimization studies identified an optimum coagulant dosage of 30 mg/L,

rapid mixing at 120 rpm for 2 min, slow mixing at 40 rpm for 20 min, and a settling time of 30 min for maximum

treatment efficiency. Significant reductions in turbidity, total suspended solids, chemical oxygen demand, and

biological oxygen demand were observed, with maximum removals of 78% color, 62.5% total dissolved solids,

70% COD, and substantial reduction in suspended solids and BOD at the optimized dosage, demonstrating

effective pollutant removal. The results highlight the potential of Strychnos potatorum seed polysaccharide as a

biodegradable, low-cost alternative to conventional chemical coagulants. This green treatment strategy offers a

promising pathway for sustainable tannery wastewater management and supports the transition toward

environmentally responsible industrial practices.

Keywords: Tannery wastewater, Natural coagulant, Strychnos potatorum, Coagulation–flocculation, FTIR

INTRODUCTION

Leather tanning is broadly classified into chrome tanning and vegetable tanning. Chrome tanning, which

accounts for nearly 80–85 % of global leather production, uses basic chromium sulfate and generates effluent

containing trivalent chromium, high salinity, and stable chromium–protein complexes. In contrast, vegetable

tanning employs plant-derived polyphenolic tannins and produces wastewater rich in organic matter but largely

free from heavy metals. The effluent investigated in this study originates predominantly from chrome-tanning

operations, which explains the elevated organic load, salinity, and potential chromium presence typical of

commercial tanneries in Tamil Nadu.

Industrial growth plays a decisive role in economic development; however, it is frequently accompanied by

severe environmental challenges (Manahan, 2010). The tannery and leather industry is particularly significant

in this context because it serves as a major contributor to foreign exchange earnings through international trade.

Leather and leather-based products such as finished leather, footwear, garments, gloves, and accessories are

highly demanded in global markets, especially in Europe, North America, and East Asia. Export-oriented

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production enables tanneries to earn foreign currency by supplying value-added leather products, thereby

strengthening national foreign exchange reserves and supporting economic stability (Bhardwaj et al., 2023;

Suman & Sangal, 2021). Among various industrial sectors, the leather tanning industry occupies a unique

position due to its dual role as a major foreign exchange earner and a significant source of environmental

pollution (Chowdhury et al., 2015; Sabur et al., 2013). In countries such as India, the tannery and leather goods

sector contributes substantially to national income through exports of finished leather, footwear, and leather-

based products, generating billions of dollars in foreign currency annually and providing employment to millions

of people (Bhardwaj et al., 2023; Suman & Sangal, 2021). Tamil Nadu alone accounts for a dominant share of

India’s leather exports, highlighting the economic indispensability of the tannery industry (Parveen et al., 2017).

Despite its economic importance, the tannery industry is recognized as one of the most water-intensive and

environmentally hazardous industries (Cassano et al., 2001; Alam et al., 2020). Large volumes of water are

consumed during soaking, liming, deliming, pickling, tanning, dyeing, and finishing operations. As a result,

tannery effluent is generated in enormous quantities and is characterized by high concentrations of organic

matter, total dissolved solids, suspended solids, sulfides, chlorides, chromium salts, dyes, and recalcitrant

compounds (Ali & Naher, 2015; Zhao et al., 2022 ). According to the Central Pollution Control Board (CPCB),

Government of India, the tannery industry is classified under the ‘Red Category’ of industries due to its highly

polluting nature and significant environmental risk. This classification is attributed to the generation of large

volumes of wastewater containing high organic load, excessive total dissolved solids, sulfides, chlorides, and

toxic heavy metals such as chromium, along with substantial sludge production. Industries falling under the Red

Category are subjected to stringent environmental regulations and are required to implement effective effluent

treatment systems to comply with CPCB discharge standards before disposal into the environment.

The discharge of untreated or inadequately treated tannery effluent into natural water bodies leads to serious

ecological degradation and public health concerns (Lejri & Younes, 2022).The environmental impacts of tannery

effluent are multifaceted and extend across aquatic, terrestrial, and atmospheric environments (Song & Williams,

2003; Wang et al., 2016). High biochemical and chemical oxygen demand levels deplete dissolved oxygen in

receiving waters, causing fish mortality, anaerobic conditions, and loss of aquatic biodiversity. Persistent organic

pollutants and tanning chemicals bioaccumulate in aquatic organisms, entering the food chain and posing long-

term ecological risks (Malik, 2014; Monira et al., 2018).

Excessive salinity, sulfides, and chlorides present in tannery effluents deteriorate soil structure, reduce soil

permeability, and inhibit microbial activity, ultimately leading to loss of soil fertility and reduced agricultural

productivity in irrigated lands (Sabur et al., 2013; Lejri & Younes, 2022). Groundwater contamination due to

percolation of untreated effluent further aggravates water scarcity and compromises drinking water quality in

tannery-dominated regions.

Chromium, particularly in its hexavalent form, represents one of the most critical environmental threats

associated with the tannery industry. Hexavalent chromium is highly toxic, mutagenic, and carcinogenic, and its

persistence in soil and water poses severe occupational and public health risks, including skin disorders,

respiratory diseases, and cancer (Can et al., 2019; Ashraf et al., 2020). Chronic exposure to chromium-

contaminated water has been reported to affect both human populations and livestock in tannery clusters.

In addition to water and soil pollution, tannery operations contribute to atmospheric pollution through the

emission of ammonia, hydrogen sulfide, volatile organic compounds, and particulate matter during various

processing stages. These emissions cause foul odors, respiratory irritation, and deterioration of ambient air

quality, adversely affecting the quality of life of nearby communities (Murugesan & Rajakumari, 2005).

Furthermore, the generation and improper disposal of tannery sludge enriched with heavy metals and organic

contaminants create long-term solid waste management challenges, leading to secondary pollution of land and

water resources (Environmental Protection Agency reports; Bernet & Béline, 2009).

Effective treatment of tannery effluent is therefore vital not only for environmental protection but also for the

sustainable continuity of the leather industry itself (Nazer, 2006; Bernet & Béline, 2009). Regulatory agencies

across the world have imposed stringent discharge standards, compelling tanneries to adopt efficient wastewater

treatment technologies (Freitas et al., 2015). Conventional treatment methods such as chemical precipitation,

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coagulation–flocculation using alum or ferric salts, adsorption, membrane filtration, and biological treatment

have been widely implemented (Yin, 2010; Ang & Mohammad, 2020). However, these methods often suffer

from limitations including high operational costs, excessive chemical consumption, generation of non-

biodegradable sludge, secondary pollution, and potential health hazards associated with residual metal ions

(Vishali et al., 2016; Kabir et al., 2023).

Hence, the search for alternative sustainable materials for tannery effluent treatment has gained significant

attention in recent years (Prabhakaran et al., 2020; Alazaiza et al., 2022). Sustainable treatment strategies aim to

minimize environmental impact while maintaining treatment efficiency and economic feasibility. Natural

coagulants derived from renewable biological sources have emerged as promising alternatives to conventional

chemical coagulants (Yin, 2010; Khan et al., 2021). These materials are biodegradable, non-toxic, cost-effective,

and capable of reducing sludge volume, making them particularly attractive for industrial wastewater treatment

applications (Fersi et al., 2018; Saranya & Shan, 2020).

Among plant-based natural coagulants, Strychnos potatorum L., commonly known as clearing nut or Nirmali,

has been traditionally used for water clarification in India (Karthikeyan et al., 2016). The seeds of S. potatorum

are rich in polysaccharides containing functional groups such as hydroxyl, carboxyl, and amine moieties, which

facilitate pollutant removal through charge neutralization, adsorption, and polymer bridging mechanisms

(Devipriya et al., 2020; Hwan et al., 2023). Previous studies have demonstrated the effectiveness of S. potatorum

seeds in turbidity removal, heavy metal adsorption, and dye removal, indicating their strong potential for

industrial wastewater treatment (Mageshkumar & Karthikeyan, 2016; Feng et al., 2007).

Considering the economic importance of the tannery industry, the severe environmental impacts of its effluents,

and the growing demand for sustainable treatment technologies, the present study focuses on the application of

polysaccharides extracted from Strychnos potatorum L. seeds for tannery effluent treatment. The work aims to

evaluate the physicochemical characteristics of untreated and treated effluent, assess the coagulation efficiency

of the natural polysaccharide, and elucidate the pollutant removal mechanisms using Fourier Transform Infrared

(FTIR) analysis. This study seeks to contribute toward environmentally responsible tannery effluent

management while supporting the long-term sustainability of an economically vital industry.Leather

manufacturing involves several sequential operations including soaking, liming, dehairing, deliming, bating,

pickling, tanning, dyeing, and finishing. Among these, tanning is the most critical step, where collagen fibers are

stabilized to prevent putrefaction. The two major tanning processes are chrome tanning and vegetable tanning.

Chrome tanning, which accounts for nearly 80–90% of global leather production, utilizes basic chromium sulfate

and generates effluent containing significant concentrations of trivalent chromium (Cr³⁺), sulfates, and high

organic load. In contrast, vegetable tanning employs plant-derived tannins and produces comparatively lower

chromium contamination but still contributes organic pollutants and suspended solids. The discharge from

chrome tanning units is particularly concerning due to chromium accumulation in wastewater streams.

MATERIALS AND METHODS

Collection and Preservation of Tannery Effluent

Raw tannery effluent was collected from the final discharge outlet of a leather processing industry located in

Tamil Nadu, India, which represents a typical composite wastewater generated from various tanning operations.

The samples were collected in clean, pre-washed high-density polyethylene containers to avoid contamination

and chemical interference. Prior to sampling, the containers were rinsed thoroughly with the effluent to ensure

representative collection.

The collected samples were transported immediately to the laboratory under cooled conditions and stored at 4

°C to minimize biological activity and physicochemical changes before analysis. All experimental analyses were

carried out within 24 hours of sample collection. Sampling, preservation, and handling procedures followed

standard protocols recommended for industrial wastewater characterization (APHA, 2017; Ali & Naher, 2015).

The collected effluent represents a composite wastewater generated predominantly from chrome-tanned leather

processing operations, as is typical of commercial tanneries in the study region. The collected effluent represents

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composite wastewater generated mainly from beamhouse, chrome-tanning, and post-tanning operations,

including soaking, liming, deliming, pickling, chrome tanning, dyeing, and finishing processes.

Extraction of Polysaccharide from Strychnos potatorum Seeds

Mature seeds of Strychnos potatorum L. were obtained from a local source and thoroughly washed with distilled

water to remove adhering dust and impurities. The cleaned seeds were shade-dried at room temperature and

ground into a fine powder using a mechanical grinder. The powdered material was subjected to aqueous

extraction to isolate the polysaccharide fraction.

Extraction was carried out by dispersing the seed powder in distilled water followed by continuous heating and

stirring to enhance polysaccharide solubilization. The extract was filtered to remove insoluble residues and

subsequently precipitated using ethanol to obtain the polysaccharide fraction. The precipitate was separated by

centrifugation, dried at controlled temperature, and stored in airtight containers for further use. Aqueous

extraction followed by solvent precipitation is widely reported as an efficient and environmentally benign

method for isolating plant-based polysaccharides used in wastewater treatment (Karthikeyan et al., 2016;

Devipriya et al., 2020; Hwan et al., 2023). The percentage yield of extracted polysaccharide was calculated

based on the initial dry weight of seed powder and is reported in the Results section.

Physicochemical Characterization of Effluent and Particle Size Distribution

The physicochemical characteristics of untreated and treated tannery effluent were analyzed to evaluate pollution

load and treatment efficiency. Parameters including color, odor, pH, temperature, total dissolved solids (TDS),

total suspended solids (TSS), alkalinity, hardness, chlorides, biochemical oxygen demand (BOD), and chemical

oxygen demand (COD) were determined following standard procedures recommended by the American Public

Health Association (APHA, 2017).

In addition to conventional parameters, particle size distribution of suspended solids was considered due to its

critical influence on coagulation–flocculation efficiency. Tannery effluent predominantly contains colloidal and

fine suspended particles originating from degraded hide proteins, lime residues, and organic macromolecules.

These particles generally fall within the size range of 0.1–10 µm, with a significant fraction below 1 µm, which

contributes to high turbidity and stability in suspension (Song & Williams, 2003; Wang et al., 2016).

The coagulation process in this study was designed to destabilize these fine particles and promote aggregation

into larger flocs suitable for sedimentation and removal.

Preparation of Natural Coagulant Solution

The polysaccharide extracted from Strychnos potatorum L. seeds was used as a natural coagulant. A stock

solution was prepared by dissolving 1 g of dried polysaccharide in 1 L of distilled water under continuous

magnetic stirring for 30 minutes to ensure complete dissolution. The solution was filtered to remove undissolved

residues and stored at 4 °C. Fresh working solutions were prepared prior to each experiment to maintain

coagulation efficiency (Yin, 2010; Prabhakaran et al., 2020)

Coagulation–Flocculation Experiment

Coagulation–flocculation experiments were conducted using a laboratory-scale flocculator (KEMI Make, India)

equipped with variable speed control. One liter of raw tannery effluent was transferred into square beakers, and

the required dose of natural coagulant was added. Rapid mixing was performed at 120 rpm for 2 minutes to

ensure uniform dispersion of the coagulant, followed by slow mixing at 40 rpm for 20 minutes to facilitate floc

formation. The flocculator provided controlled hydrodynamic conditions necessary for effective particle

collision and floc growth. After mixing, the samples were allowed to settle undisturbed (Vishali et al., 2016;

Fersi et al., 2018).

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All coagulation–flocculation experiments were performed in triplicate to ensure reproducibility, and the average

values with standard deviation were reported.

Optimization of Coagulant Dosage and Particle Growth

Optimization studies were carried out by varying the polysaccharide dosage in the range of 10, 20, 30, 40, and

50 mg/L while maintaining constant mixing conditions using the KEMI make flocculator. The influence of

dosage on particle aggregation and floc size was evaluated.Following coagulation–flocculation, the initial fine

particles (<1 µm) aggregated into visible flocs with effective particle sizes ranging between 100–500 µm,

facilitating rapid settling. Optimal coagulation performance was observed at 30 mg/L, beyond which floc

breakage and restabilization were noticed. Similar particle growth behavior has been reported for natural

polymer-based coagulants (Saranya & Shan, 2020; Khan et al., 2021).

Sludge Separation, Dosage, and Settling Time

After completion of the flocculation process, the treated effluent was allowed to settle for a fixed settling time

of 30 minutes. At the optimized coagulant dose of 30 mg/L, well-defined and compact sludge was formed at the

bottom of the beakers.The clarified supernatant was carefully decanted for further analysis, while the settled

sludge was collected. The sludge was filtered using Whatman No. 1 filter paper and dried in a hot air oven at 60

°C for 24 hours until constant weight was achieved. Controlled settling time and dosage ensured reproducible

sludge characteristics and effective pollutant removal (Bernet & Béline, 2009; Freitas et al., 2015).

Conditions for FTIR Sample Preparation (Dose and Time)

For FTIR analysis, sludge samples obtained at the optimal coagulant dose of 30 mg/L and 30 minutes settling

time were selected to represent maximum coagulation efficiency. The dried sludge was finely powdered and

mixed with spectroscopic-grade potassium bromide (KBr) in a ratio of 1:100 (sample:KBr). The mixture was

compressed into pellets under hydraulic pressure.The prepared pellets were scanned immediately to avoid

moisture interference. The selected dosage and contact time ensured that functional group interactions

responsible for coagulation were clearly detectable in the FTIR spectra (Karthikeyan et al., 2016; Devipriya et

al., 2020).

FTIR Instrumentation and Analysis

Fourier Transform Infrared (FTIR) analysis was carried out using a PerkinElmer Spectrum Two FTIR

Spectrometer. Spectra were recorded in the wavenumber range of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹.

Both raw Strychnos potatorum polysaccharide and treated sludge samples were analyzed.The FTIR spectra were

interpreted to identify characteristic functional groups such as hydroxyl (–OH), carboxyl (–COOH), and amine

(–NH₂), and to observe shifts in peak positions after treatment. These changes provided insights into the

mechanisms of pollutant binding through adsorption, charge neutralization, and polymer bridging. FTIR

spectroscopy is widely used to elucidate coagulation mechanisms in natural polymer-based wastewater treatment

systems (Devipriya et al., 2020; Karthikeyan et al., 2016).

Scanning Electron Microscopy (SEM) Analysis

Scanning Electron Microscopy (SEM) analysis was performed to investigate the surface morphology of the raw

and treated polysaccharide samples derived from Strychnos potatorum seeds. The analysis was carried out using

a scanning electron microscope under suitable operating conditions to obtain high-resolution surface

images.Prior to analysis, the samples were properly dried and mounted on specimen stubs, followed by coating

with a thin conductive layer to enhance image clarity. Both raw polysaccharide and treated sludge samples were

examined to evaluate morphological changes occurring during the coagulation process.SEM is a widely used

technique for analyzing surface characteristics such as texture, porosity, and structural variations in bio-

coagulants and wastewater treatment materials. It provides direct visual evidence of pollutant adsorption and

floc formation mechanisms (Bhatia et al., 2018; Vijayaraghavan et al., 2020; Ghernaout, 2020).

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RESULTS AND DISCUSSION

Characteristics of Untreated Tannery Effluent

The physico-chemical characterization of tannery effluent was carried out by evaluating both physical

parameters (color, odor, pH, total solids, TSS, and TDS) and chemical parameters (COD, BOD, chlorides,

sulphates, fluoride, phosphate, and dissolved oxygen). A comparative analysis of untreated and treated effluent

demonstrated substantial improvement in water quality after coagulation–flocculation using Strychnos

potatorum seed polysaccharide, confirming its effectiveness in reducing both organic and inorganic pollutant

loads.The physicochemical characteristics of the raw tannery effluent indicated a highly polluted nature typical

of leather processing industries. The effluent exhibited dark brown coloration with an unpleasant odor and

alkaline pH, reflecting the extensive use of lime, sulfides, and tanning chemicals during processing

operations.The intense colour of the untreated tannery effluent is mainly attributed to the presence of residual

dyes, vegetable tannins, chromium–organic complexes, and dissolved proteinaceous materials generated during

tanning and post-tanning operations (Chowdhury et al., 2015; Sabur et al., 2013). The objectionable odour arises

from the emission of reduced sulfur compounds and nitrogenous gases such as hydrogen sulfide, ammonia,

indole, and amines, which are produced during protein degradation in liming and unhairing processes

(Murugesan & Rajakumari, 2005; Malik, 2014). The alkaline pH of the effluent is a direct consequence of the

extensive use of lime (Ca(OH)₂) and sodium sulfide during beamhouse operations for hair removal and fiber

opening, resulting in elevated alkalinity in the wastewater (Song & Williams, 2003). Elevated chloride

concentration in tannery effluent originates from the large quantities of sodium chloride used during hide curing

and pickling stages, while additional dissolved solids are contributed by tanning salts such as basic chromium

sulfate in chrome tanning or polyphenolic compounds in vegetable tanning processes. These chemical additions

significantly increase the total dissolved solids and salinity of the effluent, as commonly reported for commercial

leather processing industries (Ali & Naher, 2015; Lejri & Younes, 2022). compare with CPCB standards.

Comparison with Central Pollution Control Board (CPCB) discharge standards indicates that the untreated

tannery effluent exceeds permissible limits for total suspended solids, total dissolved solids, chlorides, and

organic load. Discharge of such untreated effluent can lead to oxygen depletion in water bodies, soil salinization,

groundwater contamination, and bioaccumulation of toxic substances. Therefore, the adoption of effective and

eco-friendly treatment methods is essential for environmental protection and regulatory compliance.

Elevated concentrations of total suspended solids (TSS), total dissolved solids (TDS), chemical oxygen demand

(COD), and biochemical oxygen demand (BOD) confirmed the presence of high organic and inorganic loads.

The high COD and BOD values indicate a substantial amount of biodegradable and non-biodegradable organic

matter, which can severely deplete dissolved oxygen in receiving water bodies. High salinity and chloride

concentrations further highlight the potential risk of soil salinization and groundwater contamination if the

effluent is discharged without adequate treatment. These findings are consistent with earlier studies reporting

severe pollution potential of tannery wastewater (Ali & Naher, 2015; Chowdhury et al., 2015; Lejri & Younes,

2022).

Polysaccharide Extraction Yield

The aqueous extraction of Strychnos potatorum L. seeds yielded an appreciable amount of polysaccharide based

on the initial dry weight of the seed material. The obtained yield confirms the effectiveness of the extraction

method and demonstrates the suitability of this natural polymer as a sustainable coagulant for tannery wastewater

treatment. The yield obtained is comparable with previously reported values for plant-based polysaccharides

used in wastewater treatment.

Table 1. Physical Characteristics of Untreated Tannery Wastewater

S. No

Parameter

Unit

Observed Value

CPCB Standard

Color

—

Greenish grey

Colorless

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—

8.5 ± 0.1

5.5-9.0

Odor

—

Objectionable

Odourless

Total Solids

mg/L

18,000

Not Specified

TSS

mg/L

2,000

100–600

TDS

mg/L

16,000

2,100

The observed values indicate that the tannery effluent is highly polluted, with elevated levels of total solids,

suspended solids, and dissolved solids exceeding CPCB permissible limits. The objectionable odor and alkaline

pH further confirm the presence of industrial contaminants. These results highlight the necessity for effective

treatment before discharge.

Table 2. Chemical Characteristics of Untreated Tannery Wastewater

S.NO

Parameter

Unit

Observed Value

CPCB Standard

Dissolved Oxygen

mg/L

0.0 ± 0.0

—

COD

mg/L

8,100 ± 0.1

—

Chloride

mg/L

1,554.6 ± 0.1

600–1,000

Fluoride

mg/L

0.195 ±0.1

2–15

Phosphate

mg/L

0.016 ± 0.1

Sulphate

mg/L

2,204 ± 0.1

1,000

The chemical characteristics of untreated tannery wastewater presented in Table 2 clearly indicate a high level

of pollution. The dissolved oxygen (DO) was observed to be nearly zero, which reflects the presence of a high

organic load and indicates unfavorable conditions for aquatic life. The chemical oxygen demand (COD) value

was extremely high, confirming the presence of significant amounts of oxidizable organic and inorganic matter

in the effluent.

The concentration of chlorides exceeded the permissible CPCB limits, which can contribute to increased salinity

and adversely affect soil and groundwater quality. Similarly, the sulphate concentration was also found to be

above the acceptable range, indicating the presence of inorganic contaminants originating from tanning

chemicals.

Although fluoride and phosphate levels were within permissible limits, the overall chemical profile of the

effluent confirms its highly polluted nature. The presence of high COD, chlorides, and sulphates suggests that

direct discharge of untreated effluent can lead to severe environmental consequences such as oxygen depletion,

toxicity to aquatic organisms, and long-term ecological imbalance.

Comparison with CPCB standards clearly indicates that the untreated effluent does not meet discharge

requirements. Therefore, effective treatment is essential before disposal. The results strongly justify the need for

adopting eco-friendly and sustainable treatment methods such as natural coagulant-based coagulation–

flocculation.

CPCB comparison & environmental impact

Comparison with Central Pollution Control Board (CPCB) discharge standards clearly indicates that the

untreated tannery effluent exceeds permissible limits for suspended solids, dissolved solids, chlorides, and

organic load. Discharge of such effluent without treatment can cause severe oxygen depletion in surface waters,

soil salinization, chromium accumulation, and groundwater contamination. These findings strongly emphasize

the necessity of effective and environmentally sustainable treatment technologies prior to discharge.

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The extracted Strychnos potatorum L seed polysaccharide was characterized using Fourier Transform Infrared

(FTIR) spectroscopy to confirm the presence of functional groups responsible for coagulation activity. The FTIR

spectrum exhibited characteristic bands corresponding to hydroxyl (–OH), carboxyl (–COOH), and amine (–

NH₂) groups, indicating the polymeric and bioactive nature of the extract. These functional groups are known to

facilitate pollutant removal through adsorption, charge neutralization, and polymer bridging mechanisms.

Effect of Strychnos potatorum L Polysaccharide on Turbidity and Suspended Solids Removal

Application of Strychnos potatorum seed polysaccharide as a natural coagulant resulted in a significant reduction

in turbidity and TSS of the tannery effluent. The coagulation–flocculation process effectively destabilized

colloidal particles in the size range of 0.1–10 µm, leading to their aggregation into larger, settleable flocs. Visual

observation during jar test experiments confirmed the formation of compact and well-defined flocs, particularly

at intermediate coagulant dosages.

The reduction in turbidity and suspended solids can be attributed to charge neutralization and polymer bridging

mechanisms facilitated by the polysaccharide chains. Hydroxyl, carboxyl, and amine functional groups present

in the natural polymer interact with negatively charged colloidal particles, reducing electrostatic repulsion and

promoting aggregation. Similar mechanisms have been reported for plant-based coagulants used in industrial

wastewater treatment (Yin, 2010; Vishali et al., 2016; Fersi et al., 2018). Suspended solids present in tannery

wastewater are predominantly composed of fine colloidal particles originating from degraded collagen fibers,

lime residues, fat emulsions, and organic macromolecules released during beamhouse and tanning operations.

These particles generally exist in the submicron to micrometer range and remain stable in suspension due to their

negative surface charge and high hydration, thereby contributing to persistent turbidity and poor settleability

(Song & Williams, 2003; Chowdhury et al., 2015). The polysaccharide extracted from Strychnos potatorum

possesses high molecular weight polymer chains enriched with hydroxyl, carboxyl, and amine functional groups,

which play a crucial role in destabilizing suspended particles through a combination of charge neutralization and

polymer bridging mechanisms. Upon addition to the effluent, the polymer chains adsorb onto the negatively

charged colloidal surfaces, reducing electrostatic repulsion and facilitating inter-particle bridging, resulting in

the formation of dense and compact flocs. Significant reduction in chemical oxygen demand (COD) was

observed, indicating effective removal of organic and inorganic pollutants from the tannery wastewater. At lower

coagulant dosages, incomplete surface coverage of colloidal particles leads to partial destabilization and lower

removal efficiency. In contrast, the optimized dosage enables effective bridging between multiple particles,

producing larger flocs with enhanced settling velocity. Beyond the optimum dosage, excess polymer may cause

steric stabilization and floc breakage, thereby reducing turbidity and suspended solids removal efficiency.

Fig 1 :Schematic representation of coagulation–flocculation mechanism showing charge neutralization,

adsorption, polymer bridging, floc formation, and settling during tannery wastewater treatment using Strychnos

potatorum seed polysaccharide.

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Fig 2 : Kaolin turbidity removal test demonstrating coagulation efficiency of Strychnos potatorum L seed

polysaccharide at optimized dosage.

Removal of Organic Load (COD and BOD)

A substantial decrease in COD and BOD values was observed after treatment with Strychnos potatorum

polysaccharide, indicating effective removal of organic pollutants. The reduction in COD reflects the removal

of both particulate and dissolved organic matter, while the decrease in BOD suggests a significant reduction in

biodegradable organic load.

The removal of organic matter is primarily attributed to adsorption onto the polysaccharide matrix and

entrapment within flocs formed during coagulation–flocculation. Natural polysaccharides possess high

molecular weight and functional groups capable of binding organic molecules through hydrogen bonding and

van der Waals interactions. The observed COD and BOD reductions are comparable with those reported for

other natural coagulants and confirm the suitability of Strychnos potatorum polysaccharide for treating high-

strength industrial wastewater (Mageshkumar & Karthikeyan, 2016; Saranya & Shan, 2020). The high chemical

and biochemical oxygen demand of untreated tannery wastewater arises from the presence of dissolved and

particulate organic matter, including degraded proteins, fats, surfactants, residual dyes, and tanning auxiliaries

generated during beamhouse, tanning, and post-tanning operations Chowdhury et al., 2015; Yin, 2010 These

organic constituents contribute substantially to both biodegradable and non-biodegradable fractions of the

organic load.

The observed reduction in COD and BOD following treatment with Strychnos potatorum seed polysaccharide

can be attributed to the adsorption and enmeshment of organic pollutants within the polymer-induced flocs

formed during coagulation–flocculation. Bratby, 2016; Vijayaraghavan et al., 2011 High molecular weight

polysaccharide chains facilitate the aggregation of organic macromolecules through hydrogen bonding, van der

Waals interactions, and polymer bridging, thereby removing a significant fraction of oxygen-demanding

substances from the effluent. Although significant reduction in organic load was achieved, residual COD

observed after treatment suggests that natural coagulation alone may not ensure complete mineralization, and

subsequent biological or advanced treatment steps may be required to meet stringent discharge standards

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Fig: TWW treated with Strychnos potatorum seed polysaccharide ( 10 mg to 50 mg)

Table 3. Treatment Performance of Strychnos potatorum Seed Polysaccharide

S. No

Dosage (mg/L)

Color

removal

(%)

TDS

reduction (%)

COD reduction (%)

Sludge yield (g)

8.2 ± 0.1

72.0

37.5

55.0

0.169

8.4 ± 0.1

75.0

37.5

59.0

0.199

8.5 ± 0.1

78.0

62.5

70.0

0.174

8.5 ± 0.1

70.0

20.0

60.5

0.228

8.5 ± 0.1

71.0

11.3

59.6

0.177

The treated tannery effluent exhibited significant improvement in key water quality parameters. Reduction in

turbidity and TSS indicates efficient removal of colloidal and suspended matter, while decreased COD and BOD

reflect effective elimination of biodegradable and non-biodegradable organic pollutants. The observed reduction

in TDS and color further enhances the suitability of treated effluent for subsequent biological treatment or safe

discharge in compliance with regulatory standards.

Optimization of Coagulant Dosage and Floc Formation

The effect of coagulant dosage on treatment efficiency was evaluated over a range of 10–50 mg/L using a KEMI

make laboratory flocculator. Optimal pollutant removal was achieved at a dosage of 30 mg/L, beyond which no

significant improvement was observed. At lower dosages, incomplete destabilization of colloidal particles

resulted in reduced removal efficiency, whereas higher dosages led to floc restabilization due to excess polymer

coverage.

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At the optimal dosage, fine particles (<1 µm) aggregated into flocs with effective sizes ranging from 100–500

µm, facilitating rapid settling within 30 minutes. The formation of larger flocs under optimized hydrodynamic

conditions confirms the effectiveness of controlled mixing and appropriate dosage selection. Similar dosage-

dependent behavior has been reported in studies involving natural polymer-based coagulants (Khan et al., 2021;

Prabhakaran et al., 2020). The dosage-dependent coagulation behavior observed in the present study is

characteristic of polymer-based natural coagulants, where treatment efficiency increases with dosage up to an

optimum level due to enhanced particle collision and polymer bridging. Similar trends have been widely reported

for plant-derived polysaccharides and biopolymers used in industrial wastewater treatment (Yin, 2010; Bratby,

2016; Lee et al., 2014). At dosages beyond the optimum level, excess polysaccharide molecules may lead to

complete surface coverage of suspended particles, resulting in steric stabilization and re-dispersion of flocs,

thereby reducing removal efficiency. This phenomenon of overdosing-induced restabilization has been

extensively documented for natural and synthetic polymer coagulants (Roussy et al., 2005; Katal &

Pahlavanzadeh, 2011; Matilainen et al., 2010). The formation of larger flocs with effective particle sizes in the

range of hundreds of micrometers at the optimized dosage enhances settling velocity and sludge compactness,

which is a key requirement for efficient solid–liquid separation in tannery effluent treatment systems.

Comparable floc growth behavior has been reported for bio-coagulants applied to high-strength industrial

wastewaters (Vishali et al., 2016; Fersi et al., 2018; Khan et al., 2021). In tannery wastewater treatment,

optimization of coagulant dosage is particularly critical due to the complex mixture of colloidal proteins, fats,

and inorganic salts, and several studies have emphasized that improper dosage selection can significantly impair

treatment performance (Chowdhury et al., 2015; Sabur et al., 2013; Lejri & Younes, 2022).

The decline in color removal efficiency beyond the optimal dosage of 30 mg/L can be attributed to overdosing

of the polysaccharide coagulant. Excess polymer molecules may lead to complete surface coverage of colloidal

particles, resulting in steric stabilization and charge reversal. This phenomenon inhibits effective polymer

bridging and causes partial re-dispersion of previously formed flocs, thereby reducing color removal efficiency.

Table 4. FTIR Peak Assignments of Raw Strychnos potatorum Polysaccharide

S. No

Peak (cm⁻¹)

Vibration

Functional group

1025.9

C–O stretching

Alcohol / polysaccharide backbone

1622.8

COO⁻ / amide I

Carboxyl / protein residues

2915.8

C–H stretching

Aliphatic chains

3292.5

O–H / N–H stretching

Hydroxyl / amine

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Sludge Characteristics and Settling Behavior

The sludge generated during the coagulation–flocculation process was dense, compact, and exhibited good

settling characteristics at the optimized coagulant dosage. The reduced volume and improved dewaterability of

sludge produced using Strychnos potatorum polysaccharide offer significant advantages over conventional

chemical coagulants, which typically generate large quantities of metal-rich sludge.

The biodegradable nature of the natural coagulant further enhances the environmental compatibility of the

generated sludge and reduces challenges associated with disposal. These observations align with previous studies

highlighting the benefits of natural coagulants in minimizing sludge generation and improving sludge

management (Bernet & Béline, 2009; Freitas et al., 2015).

The results of the present study further substantiate this observation, as shown in Table 3, where an increase in

coagulant dosage up to the optimum level (30 mg/L) resulted in simultaneous enhancement of both total

dissolved solids (TDS) and chemical oxygen demand (COD) removal. The maximum TDS reduction (62.5%)

coincided with the highest COD removal (70%), demonstrating a direct proportional relationship between these

parameters. This trend indicates that a substantial fraction of dissolved solids in tannery wastewater consists of

organic constituents contributing to oxygen demand, and their removal through coagulation–flocculation leads

to concurrent reduction in both TDS and COD. Similar direct correlations between TDS and COD removal have

been reported for tannery effluents treated using coagulation-based processes, where dissolved organic matter

constitutes a major fraction of the total pollutant load (Ali & Naher, 2015; Wang et al., 2016; Lejri & Younes,

2022).

Table 5. FTIR Peak Assignments of Treated Sludge

S.NO

Peak (cm⁻¹)

Vibration Type

Interpretation

3213.7

N–H stretching

Amine / amide interaction

3008.4

O–H / N–H stretching

Hydrogen bonding

1398.1

C–O stretching

Carboxylate complexation

1050.0

C–N stretching

Aliphatic amines

609.3

C–X / metal interaction

Possible chromium association

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FTIR studies on Raw polysachharide and sludge (Treated polysachharide)

FTIR analysis of the raw Strychnos potatorum polysaccharide exhibited broad and intense absorption bands

characteristic of its biopolymeric nature. The broad peak observed around 3200–3400 cm⁻¹ corresponds to the

stretching vibrations of hydroxyl (–OH) groups, indicating the presence of abundant polysaccharide hydroxyl

functionalities capable of hydrogen bonding. Peaks appearing near 2920–2850 cm⁻¹ are attributed to C–H

stretching vibrations of aliphatic chains, confirming the polysaccharide backbone structure. The absorption band

around 1700–1730 cm⁻¹ is associated with the stretching vibration of carboxyl (–COOH) groups, while bands

observed near 1600–1650 cm⁻¹ may be assigned to amide I (C=O stretching) or –NH₂ bending vibrations.

Additional peaks in the region of 1000–1150 cm⁻¹ correspond to C–O–C and C–O stretching vibrations, typical

of glycosidic linkages in polysaccharides. Similar results have been reported by Devipriya et al. (2020) and

Karthikeyan et al. (2016).After coagulation treatment, the FTIR spectrum of the generated sludge showed

noticeable shifts in peak positions, peak broadening, and reduced band intensities, particularly in the –OH, –

COOH, and –NH₂ functional group regions. The shift and weakening of the hydroxyl band suggest hydrogen

bonding interactions between the polysaccharide and pollutant molecules. Changes in the carboxyl and amine-

related peaks indicate electrostatic interactions and complexation between negatively charged organic/inorganic

contaminants and protonated functional groups of the polysaccharide. (Yin, 2010; Chowdhury et al., 2015)

These spectral modifications provide strong evidence that these functional groups actively participated in

pollutant binding during the treatment process.

The observed FTIR spectral changes validate that pollutant removal occurred through multiple synergistic

mechanisms. Initially, charge neutralization takes place when oppositely charged functional groups of the

polysaccharide interact with suspended particles and dissolved contaminants, reducing repulsive forces and

promoting aggregation. Simultaneously, adsorption mechanisms dominate through hydrogen bonding, van der

Waals forces, and electrostatic attraction between polysaccharide functional groups and pollutants. Furthermore,

due to the long-chain polymeric structure of the polysaccharide, polymer bridging plays a crucial role, where a

single polymer chain binds multiple particles, leading to the formation of larger and denser flocs that readily

settle(Yin, 2010; Chowdhury et al., 2015)

The FTIR spectrum of the treated sludge showed peak shifts in regions corresponding to carboxylate and amine

functional groups, indicating interaction between chromium ions and the polysaccharide matrix. These

interactions likely occur through coordination bonding and electrostatic attraction, facilitating chromium

entrapment within the floc structure. The presence of chromium-associated complexes in sludge confirms its

removal from the aqueous phase during treatment (Yin, 2010; Chowdhury et al., 2015).

The combined action of adsorption, charge neutralization, and polymer bridging enhances floc formation

efficiency and results in effective removal of organic matter, suspended solids, and metal ions from wastewater.

Similar FTIR-based confirmation of functional group involvement and coagulation mechanisms has been widely

reported for natural polysaccharide-based coagulants and bioadsorbents, reinforcing the reliability of the

proposed mechanism proposed by Karthikeyan et al., (2016); Devipriya et al., (2020). Thus, the FTIR results

conclusively demonstrate that Strychnos potatorum polysaccharide functions as an efficient, eco-friendly

coagulant through chemically active surface interactions rather than mere physical entrapment.

The coagulation–flocculation of tannery effluent using Strychnos potatorum polysaccharide occurs through a

synergistic mechanism involving charge neutralization, adsorption, and polymer bridging. Initially, protonated

functional groups neutralize negatively charged colloids. Subsequently, long polymer chains adsorb onto

multiple particles simultaneously, forming inter-particle bridges that result in dense, settleable flocs. This multi-

mechanistic action explains the effective removal of suspended solids, organic matter, and chromium-associated

complexes. Bratby, 2016; Vijayaraghavan et al., 2011

Environmental and Practical Implications

The results demonstrate that Strychnos potatorum seed polysaccharide is an efficient and sustainable alternative

to conventional chemical coagulants for tannery effluent treatment. The significant reduction in turbidity,

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suspended solids, COD, and BOD, combined with reduced sludge generation and biodegradability, highlights

its potential for large-scale application. The shift and broadening of –OH and –NH stretching bands in the treated

sludge indicate strong hydrogen bonding interactions between polysaccharide functional groups and organic

pollutants. Yin, 2010; Bratby, 2016)The appearance and intensification of carboxylate-related peaks suggest

electrostatic interactions and complexation with metal–organic species, including chromium complexes. These

spectral changes confirm that pollutant removal occurred through a synergistic mechanism involving adsorption,

charge neutralization, and polymer bridging. Chowdhury et al., 2015; Vijayaraghavan et al., 2011The economic

importance of the tannery industry as a major foreign exchange earner, adoption of eco-friendly treatment

technologies is essential to ensure regulatory compliance and long-term sustainability. The use of plant-based

natural coagulants can support cleaner production practices while minimizing environmental impacts associated

with tannery operations.

Scanning Electron Microscopy (SEM) Analysis of Polysaccharide sludge:

The SEM micrograph of the raw polysaccharide exhibited a smooth, compact, and relatively uniform surface

morphology, indicating limited availability of active binding sites. In contrast, the treated polysaccharide showed

a rough, irregular, and highly porous structure with the presence of cracks, cavities, and flake-like formations.

These morphological changes clearly indicate the adsorption and accumulation of pollutants onto the surface of

the polysaccharide during the coagulation process. The development of a porous and aggregated structure

suggests enhanced interaction between the functional groups of the polysaccharide and the wastewater

contaminants. (Yin, 2010; Chowdhury et al., 2015)The formation of a net-like structure further confirms the

occurrence of flocculation, where particles are bound together through polymer bridging and charge

neutralization mechanisms. This structural transformation demonstrates the effectiveness of the natural

polysaccharide as a bio-coagulant for wastewater treatment. Similar results have been reported by

Vijayaraghavan et al. (2011) and Bratby (2016).

Figure 2 : . SEM images of

(a) Raw polysaccharide

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(b) Treated polysaccharide

CONCLUSION

The present study demonstrates the effective application of Strychnos potatorum L. seed polysaccharide as a

sustainable and eco-friendly coagulant for the treatment of tannery effluent predominantly generated from

chrome-tanning operations. Physico-chemical characterization of untreated effluent revealed extremely high

pollution load, with elevated TSS, TDS, COD, chlorides, and sulphates, exceeding CPCB discharge standards

and confirming the hazardous nature of tannery wastewater. Coagulation–flocculation treatment using the

extracted natural polysaccharide resulted in significant reductions in turbidity, suspended solids, colour, COD,

and TDS, with optimal performance achieved at a dosage of 30 mg/L. Beyond this dosage, a decline in treatment

efficiency was observed due to overdosing-induced steric stabilization and partial restabilization of colloidal

particles. The reduction in organic load indicates effective removal of both particulate and dissolved organic

matter through adsorption and polymer bridging mechanisms.

Compared to conventional chemical coagulants, the use of Strychnos potatorum polysaccharide offers clear

advantages such as biodegradability, reduced sludge volume, absence of secondary metal contamination, and

cost-effectiveness. The findings highlight the strong potential of this natural coagulant as a green alternative for

preliminary treatment of high-strength tannery effluent. Integration of this eco-friendly approach with biological

or advanced treatment systems could further enhance compliance with stringent discharge standards, supporting

environmentally responsible and sustainable tannery operations.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the Tamil Nadu State Council for Higher Education (TANSCHE),

Government of Tamil Nadu [File No. RGP/2019-20/MTWU/HECP-0075], for providing financial support to

carry out this research.

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