Comparative analysis of new natural coagulant extracts for turbidity removal in water systems Open Access

Growing population, increased economic activity, and industrialization have not only raised demand for clean water but have also resulted in the inefficient use of water resources. Water resources are depleted around the world due to complete exploitation and bad management, as well as natural depletion. Around 1.2 billion people still require clean drinking water, and more than 6 million children die from diarrhea each year (Sulaymon et al. 2023). The majority of water pollutants exist as colloidal solids that do not easily settle. Ordinary sedimentation cannot remove the exceedingly tiny suspended solids and colloidal particles that create turbidity (Pritchard et al. 2010a).

Electrochemical processes, such as electro-Fenton and electrocoagulation, are effective for treating complex wastewater, including saline and oily effluents. Recent studies highlight their potential for optimizing organic contaminant removal and improving treatment efficiency under diverse conditions (AlJaberi et al. 2020, 2022, 2023; Kilany et al. 2020; Saad et al. 2021; El Gheriany et al. 2022; AlJaberi 2023; Jasim & AlJaberi 2023; Alturki et al. 2024).

Recent advancements in wastewater treatment have focused on innovative materials such as metal-organic frameworks (MOFs) and bimetallic catalysts, demonstrating their potential for effective pollutant removal. MOFs, including ZIF-8 and ZIF-67, have been explored for their adsorption efficiency and stability, with promising results in removing organic dyes and heavy metals (Ahmad et al. 2024; Nazir et al. 2024b; Shah et al. 2024; Shahid et al. 2024). Additionally, hybrid MOF composites and membranes offer enhanced capabilities for selective pollutant removal and catalytic applications (Liu et al. 2008, 2018; Nazir et al. 2022, 2024a). Studies on phase transformation techniques and adsorption processes further emphasize the role of nanomaterials and adsorbents in addressing hazardous waste (Zhao et al. 2024).

Coagulation, flocculation, sedimentation, filtration, and disinfection are some of the most often used processes for treating raw water. These processes aimed to effectively extract strong particles from water. Coagulation and flocculation forms are linked. These processes attempt to reduce turbidity, organic compounds, bacteria, and algae, hence reducing the risk of waterborne infections and preventing filter clogging (Aboubaraka et al. 2017).

Chemical coagulants made of metal salts or polymers in the form of polyelectrolytes are used to improve the coagulation and flocculation processes of raw water (De Feo et al. 2008). Inorganic salts, particularly aluminum and iron salts, are extensively utilized as coagulants. Despite their high efficiency, chemical coagulants have environmental downsides and are expensive to use (Clasen 2008). For example, a high quantity of residual aluminum affects Alzheimer's disease and kills aquatic life (Teh et al. 2014). Furthermore, the presence of aluminum in generated sludge creates disposal issues and necessitates sludge treatment (Rondeau & Commenges 2001).

Thus, in water treatment, natural coagulants outperform chemical agents in a variety of ways, most notably their biodegradability and minimal residual sludge generation. The history of using natural coagulants to remove turbidity is long. For over 2000 years, natural polymers have been used in India, Africa, and China as viable coagulants and coagulants aid in high water turbidity (Phani Madhavi & Rajkumar 2013). They may be derived from plant seeds, leaves, or roots (Antov et al. 2012). In recent studies, a variety of plant materials have been reported as potential natural coagulants, including: Moringa oleifera seeds (Ndabigengesere & Subba Narasiah 1998), mesquite bean (Prosopis juliflora) (Diaz et al. 1999), cactus (latifaria) (Diaz et al. 1999), chestnut (Šćiban et al. 2009), acorn (Šćiban et al. 2009), quebracho (Schinopsis balansae) (Sánchez-Martín et al. 2010), nirmali seeds (Strychnos potatorum) (Vijayaraghavan & Sivakumar 2011), tannin (Vijayaraghavan & Sivakumar 2011), common bean seed (Antov et al. 2012), tamarind seed (Phani Madhavi & Rajkumar 2013), mallow (Sulaymon et al. 2023), arabic gum (Sulaymon et al. 2023), okra (Sulaymon et al. 2023), copra (Cocos nucifera) (Fatombi et al. 2013), Jatropha curcas seeds (Abidin et al. 2013), Phaseolus vulgaris (Muthuraman & Sasikala 2014), fava bean seeds (Vicia faba L.) (Kukić et al. 2015), basil seeds (Shamsnejati et al. 2015), banana pith (Kakoi et al. 2016), Margaritarea discoidea seeds (Oladoja et al. 2017), Plantago ovata seed (Dhivya et al. 2017), common oak, (Antov et al. 2018) and orange industry residues (Kebaili et al. 2018).

While natural coagulants have been widely studied, agricultural residues and common legumes such as rice straw and lupin beans remain underexplored in water treatment applications. Rice straw, with its high cellulose content, and lupin beans, known for their protein-rich composition, suggest promising coagulation properties that have not been adequately investigated. This study aims to address this gap by systematically evaluating the coagulation performance of rice straw and lupin beans under various conditions (such as beginning turbidity, coagulant dose, water pH, and water temperature), analyzing their physicochemical properties, and exploring their synergistic effects with alum to enhance efficiency and reduce costs. The goal is to expand the range of sustainable, locally available coagulants for effective water treatment.

Lupin beans are yellow legume seeds from the genus Lupinus (Kurlovich 2002). Lupinus, sometimes known as lupin or lupine, is a commercially and agriculturally valuable plant that can grow in a variety of soils and climates (Sujak et al. 2006). Lupinus is normally composed of 36–52% protein, 5–20% oil, and 30–40% fiber (Petterson & Mackintosh 1994). These legume plants are primarily farmed in the Mediterranean and South America. Lupin production is becoming increasingly important due to its potential as a protein source for therapeutic uses and, due to its high alkaloid content, as a natural component of plant insecticides (Sujak et al. 2006).

Rice straw is one of the most common lignocellulosic waste products in the world. It is derived from the food sector and can be gathered after harvesting the main product, paddy rice/rough rice (Lin et al. 2018). Rice straw, a lignocellulosic substance, is mostly composed of cellulose (about 35%), hemicellulose (approximately 25%), and lignin (approximately 12%) (Hills & Roberts 1981; Swain et al. 2018). Rice is the world's third most important grain in terms of overall output, trailing only wheat and corn. According to Food and Agriculture Organization of the United Nations figures, global annual rice output in 2024 was approximately 780 million tons. Each kilogram of grain harvested yields 1–1.5 kg of straw (Swain et al. 2018). It estimates that between 780 and 1,170 million tons of rice straw are produced globally each year. The majority of rice straw is burned in open fields, resulting in significant air pollution and negative consequences for public health (Lin et al. 2018). Thus, there is a growing interest in finding new uses for agro-industrial leftovers (Lin et al. 2018).

A household mill pulverized the locally acquired lupin dry beans and rice straw to a fine powder. Ten grams of coagulant powder were suspended in 1 L of distilled water. The suspension was agitated for 10 min and then allowed to sit for 1 h before being filtered. The filtered solutions were utilized in the following jar tests. To prevent deterioration, fresh solutions were made daily. The concentrated alum solution employed in this investigation was solely for comparison reasons.

Raw turbid water was created right before the coagulation testing by adding kaolin suspension to tap water to achieve the desired beginning turbidity.

A jar test was performed to assess the effectiveness of natural coagulants. Six liter beakers were filled with manufactured synthetic turbid water samples. Each beaker received varying concentrations of natural coagulant extracts. For each sample in this study, the mixing regime was as follows: quick mix at 120 rpm for 2 min, slow mix at 40 rpm for 20 min, and sedimentation for 30 min.

The functional groups, surface architecture, and elemental composition of lupin beans and rice straw were determined using Fourier transform infrared spectroscopy (FTIR) (Bruker TENSOR 37), scanning electron microscope (SEM) (JSM 5300, JEOL), and energy dispersive X-ray spectroscopy (EDAX).

The following parameters were measured for crude extracts: zero charge point, zeta potential, and particle size distribution of aqueous extracts (Zetasizer, Malvern Instruments).

As depicted in Figure 6, the coagulation efficiency of the two extracts was found to be relatively consistent across the tested pH range (2–10), indicating no strict pH dependency for its effectiveness. However, at pH levels greater than 7, adsorption between negatively charged suspended particles and the positively charged coagulation agent is enhanced, while at pH levels less than 7, reduced negative charges on the suspended particles increase repulsion. These variations in interaction mechanisms do not significantly affect the overall efficiency of rice straw and lupin extracts as natural coagulants, thus supporting their broad applicability across a wide pH range. Sánchez-Martín et al. (2010) found that water extract from quebracho had similar effects. As a result, additional studies were conducted at a neutral pH.

In turbid water with the lowest initial turbidity (50 NTU), the best coagulation efficiency was achieved with a coagulant dose of 50 mg L⁻¹. In water with an initial turbidity of 200 NTU, dosages of 40 and 50 mg L⁻¹ demonstrated the highest coagulation efficiency. The highest coagulation efficiency in water with 500 NTU was achieved at a coagulant dose of 50 mg L⁻¹.

The differences in coagulation efficiency between lupin beans and rice straw arise from their distinct chemical compositions. Lupin beans, which are rich in proteins and fatty acids, provide functional groups such as hydroxyl (–OH), carbonyl (C = O), and amine (–NH₂), which actively participate in charge neutralization and bridging mechanisms. Conversely, rice straw, with its lignocellulosic structure and silicon content, relies primarily on adsorption mechanisms for turbidity removal.

The stability of coagulation performance at elevated temperatures highlights the suitability of these natural coagulants for application in regions with tropical or arid climates, where water temperatures often exceed 25 °C. For instance, these materials could be effectively implemented in decentralized water treatment systems or small-scale facilities in warm regions, offering a sustainable alternative to chemical coagulants. This reinforces the practical relevance of the study, as it demonstrates the potential for consistent performance in real-world conditions.

Table 1 illustrates the coagulation efficiency of different mixtures of alum and natural coagulants (lupin beans and rice straw) for reducing 500 NTU turbidity. The alum concentration in the mixtures ranged from 0 to 100%.

For lupin beans, the coagulation efficiency initially decreased as the percentage of alum was reduced, reaching 96.56% at 50% alum concentration. Further reductions in the alum percentage led to a more pronounced decline in efficiency. A comparable pattern was observed for rice straw, although the highest coagulation efficiency (98.7%) occurred at 75% alum concentration.

The percentages (50% for lupin beans and 25% for rice straw) highlight the optimal points where the natural coagulants effectively complement alum in achieving high coagulation performance, minimizing the required amount of alum for turbidity reduction. Consequently, the use of these natural coagulants can reduce overall treatment costs while maintaining high coagulation performance.

Table 2 provides a comparison of the present study's coagulants, lupin beans and rice straw extracts, with previously reported natural coagulants. It highlights the competitive performance of the current coagulants, with lupin beans achieving a turbidity reduction of 86.4% and rice straw extract achieving a 63.2% turbidity reduction, comparable to other coagulants, while introducing an innovative use of agricultural waste.

Notably, the use of rice straw provides a sustainable alternative by repurposing agricultural by-products, which are readily available in many regions, particularly in rice-growing countries. This accessibility ensures a cost-effective supply chain, especially compared to synthetic coagulants. Similarly, lupin beans, widely cultivated for food and fodder, offer a low-cost source for coagulant extraction, making them feasible for large-scale applications in regions with limited resources.

Additionally, the methodology employed in the present study is notably simple and cost-effective compared to previous works. The preparation of coagulants from lupin beans and rice straw requires no complex steps or the consumption of additional chemicals, unlike some conventional and reported natural coagulants that involve elaborate processing or chemical treatments. This simplicity not only reduces production costs but also minimizes environmental impact, making these coagulants highly suitable for deployment in low-resource settings.

This table highlights the significant contributions of this study by presenting effective, sustainable, and economically viable alternatives to conventional coagulants. By leveraging abundant natural materials, these coagulants align with environmental and economic goals, particularly in communities striving for low-cost and eco-friendly water treatment solutions.

This study demonstrates the effectiveness of lupin beans and rice straw as natural coagulants for turbidity removal in water treatment. Both materials exhibited significant potential as standalone coagulants, with lupin beans achieving up to 86.4% turbidity removal and rice straw achieving 73.2% under optimal conditions. The performance of these natural coagulants was consistent across a broad pH range and improved with increasing temperature, highlighting their suitability for diverse environmental conditions.

The findings demonstrate the unique potential of lupin beans and rice straw not only as standalone coagulants but also in combination with alum to achieve significant reductions in chemical usage. Their combination with alum significantly reduced alum dosage requirements by 25–50%, thereby lowering treatment costs and mitigating the environmental impacts associated with chemical coagulants providing a novel perspective on their applicability in sustainable water treatment processes.

While turbidity removal is critical, future research should investigate the efficacy of lupin beans and rice straw extracts in addressing other water quality parameters, such as chemical oxygen demand (COD), biochemical oxygen demand (BOD), and pathogen removal, to comprehensively evaluate their potential as sustainable coagulants in water treatment processes.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.