Abstract

Polyacrylamide-based precipitating flocculants are commonly used to clarify juice and remove impurities in raw sugar production. However possible residual polyacrylamide in sugar can pose carcinogenic, neurotoxic, and environmental risks. To address this, lignocellulose nanofiber (LCNF)-based flocculants derived from sugarcane bagasse (a by-product of sugarcane processing) were introduced as a sustainable alternative. Polyacrylamide (PAM) was grafted onto LCNFs through microwave irradiation and a chemical-free radical initiator ((NH4)2S2O8), under optimized polymerization conditions. The flocculant was characterized through various analytical techniques confirming the successful grafting reaction. The flocculant achieved excellent turbidity removal rates of 99.3 %, 98.7 %, and 96.9 % for halloysite, kaolin, and sugarcane juice, respectively, at low dose (5 mg/L). It also demonstrated broad flocculation range and pH adaptability. A settling test conducted on sugarcane juice at 96 °C revealed that the 20 % commercial PAM and PAM@LCNF blend achieved a significantly enhanced initial settling rate of 111 mL/min, outperforming both the PAM@LCNF and commercial PAM. The PAM@LCNF flocculant represents a sustainable, highly effective alternative to conventional flocculants, for sugarcane juice clarification while utilizing sugarcane industry residual. Moreover, its flocculation performance is comparable to other reported biopolymer and other flocculants.

Clay minerals and other finely suspended particles of organic and/or inorganic nature are present in significant quantities within various industrial effluents, including those from papermaking, mineral processing, coal tailings, sludge dewatering, ceramic firing, and oil exploitation [1, 2], as well as raw product streams such as sugarcane juice. The fine particles remain suspended in water due to their small size, strong electronegativity, and the formation of a hydration film, which together result in significant electrostatic repulsion and steric hindrance, making natural settling difficult [3]. Disposal of these suspensions without proper treatment not only wastes valuable water and other industrial resources but also poses significant environmental risks, contradicting the principles of sustainable development. Therefore, the removal of fine particles from industrial wastewater and raw product streams remains a critical yet challenging task.

A wide range of technologies has been developed for treating finely suspended particles, encompassing physical, chemical, and biological approaches. Among these, flocculation technology stands out as one of the most effective and widely used technology. Flocculation works by destabilizing these dispersed particles, often through the addition of flocculants or coagulants that reduce electrostatic interaction energy, as described by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory [4]. This process helps aggregate these fine particles into larger flocs, facilitating their separation from water through sedimentation or filtration. The effectiveness of flocculation significantly depends on the choice of flocculants, which are categorized into two main types based on their chemical composition: inorganic and organic flocculants [5]. Traditional inorganic flocculants, such as iron salts, aluminum salts, and polyaluminum chloride (PAC), are moderately toxic and degrade slowly over time. Their use in wastewater treatment generates substantial amounts of precipitated sludge (mud) and residues and is highly sensitive to pH variations [6]. Organic flocculants, however, offer advantages such as low dosage requirements, cost-effectiveness, low toxicity, and compatibility with a wide pH range [7]. Nevertheless, to meet the growing demand for efficient particle separation, there is a need to enhance the efficiency and stability of these flocculants.

During sugar processing, the juice extracted from sugarcane contains high concentrations of suspended impurities, including fine clay particles, small plant fibers, proteins, colloidal particles, and other organic substances. Therefore, clarification is a critical step in the sugar production process, aiming to remove these impurities along with colorants to improve the purity of the juice and enhance processing efficiency [8]. Traditionally, this process relies on the addition of lime, which raises the pH of the juice and facilitates the coagulation and sedimentation of impurities [9]. However, despite its widespread industrial use, lime clarification alone is associated with significant challenges, including high chemical consumption, excessive sludge formation, and limited effectiveness in removing fine colloidal particles and organic substances. To overcome these limitations, various advanced clarification techniques have been investigated, particularly the use of synthetic polymeric flocculants such as polyacrylamide (PAM) derivatives [10]. These flocculants enhance flocculation efficiency through charge neutralization and bridging mechanisms, effectively aggregating fine particles and accelerating sedimentation. This not only improves the clarity and purity of the juice but also increases sugar recovery, reduces processing time, lowers energy consumption, and enhances overall production efficiency and profitability. Moreover, effective clarification reduces the burden on downstream processes such as filtration and evaporation, leading to improved. However, concerns regarding the environmental persistence and potential toxicity of synthetic flocculants have led to growing interest in natural and bio-based alternatives.

Natural polymeric flocculants have become increasingly popular in water and wastewater treatment in recent years because of their non-toxicity and biodegradability. Flocculants derived from natural biopolymers are favored over synthetic ones to prevent biomass contamination as well as being more sustainable or eco-friendly. Biomass flocculants have become a significant category within the developed and applied organic flocculants [11]. These flocculants are typically semi-synthetic, produced by molecular recombination and other modification techniques using waste biomass substrates as raw materials. They leverage renewable resources and are highly effective in treating industrial wastewater, aligning with the concept of “Pollution Control with Waste.” This approach holds substantial practical significance [12]. However, advancing biomass flocculants remains a key challenge in particle separation. Efforts are needed to explore and develop new raw materials, such as cellulose [13], lignin [14], chitosan [15], and their derivatives, which are sustainable and environmentally friendly biological resources.

Cellulosic nanomaterials (CNMs) have emerged as an excellent sorbent for water contaminants. Unlike conventional materials, they offer advantages such as a high specific surface area, versatile surface chemistry, environmental stability, and renewability [16]. Traditional adsorbents often exhibit low adsorption efficiency and capacity due to their limited surface area or active sites for adsorption [17]. By reducing the adsorbent size to the nanoscale, the specific surface area increases significantly [18], and shorter intraparticle diffusion distances further enhance adsorption performance. Additionally, the strong potential for surface chemical modification [19] allows for the introduction of numerous active sites. As renewable, abundant, and environmentally inert biomaterials, CNMs present a sustainable solution with minimal environmental impact. CNMs can help lower costs and enhance performance by optimizing raw materials and pretreatment methods. Typically, these nanomaterials are derived from purified cellulose sources, such as cellulose fibers, where non-cellulosic components (primarily lignin) are removed [20]. This process often involves complex and hazardous bleaching steps. A more sustainable approach involves producing nanomaterials directly from lignocellulosic raw materials using little or no chemical treatment, ensuring the full utilization of lignocellulose. As a result, lignocellulose nanofibers (LCNFs) have emerged as a sustainable alternative to bleached CNMs.

Several studies have utilized modified cellulose with both anionic and cationic charges as flocculants in particle separation from water [[21], [22], [23], [24]]. Graft copolymerization has been demonstrated as an effective and straightforward method for modifying cellulose [25]. Under mild conditions, cellulose can react with olefin-containing monomers, allowing for targeted modification to introduce desired functional groups and prepare flocculants, thus enhancing adsorption and bridging effects [26]. Currently, widely used polymerization initiation methods include thermal, radiation, microwave, and photo-initiation polymerization [27]. In conventional grafting, chemical initiators are used to generate free radical sites on the polymer in an inert atmosphere, where monomers can attach and form graft chains. However, this method suffers from low reproducibility, making it unsuitable for large-scale commercial production [28]. In contrast, microwave-assisted grafting uses electromagnetic radiation to generate radicals via redox initiators, significantly reducing the activation energy while increasing the reaction rate and energy efficiency [29]. Unlike conventional heating, microwave radiation selectively excites polar bonds without breaking the nonpolar polymer backbone, thereby preserving structural integrity [30]. Moreover, the combination of microwave radiation with chemical initiators enhances grafting efficiency compared to conventional methods. Microwave-initiated grafting is also faster, more economical, and environmentally friendly, with higher monomer conversion rates, making it a superior alternative for modifying cellulose [28]. In this context, Mishra et al. [31] synthesized polyacrylamide-grafted starch as a flocculant for kaolin suspension using a microwave-assisted method. Ghosh et al. [32] grafted acrylamide to tamarind kernel polysaccharide using a microwave-assisted approach for the removal of kaolin from water. They stated that microwave synthesis enhances flocculant efficiency by preserving the rigidity of the polysaccharide backbone, achieving higher grafting through a synergistic combination of ring-opening via free radical initiation and grafting without ring-opening, which extends the polymer chains for improved contaminant capture. Wu et al. [33] prepared a chitosan-based flocculant using microwave-assisted polymerization for sludge conditioning and dewatering. Zeng et al. [28] used microwave-assisted synthesis for the novel bio-based flocculant from dextran and chitosan for kaolin removal. Sen et al. [34] grafted poly(2-hydroxyethylmethacrylate) to agar by microwave-assisted method for wastewater treatment. However, to the best of our knowledge, there are few reports on the microwave-assisted synthesis of bio-flocculants based on LCNFs.

This study aimed to develop an eco-friendly flocculant derived from the LCNFs under various synthesis conditions using microwave-assisted heating. The new cationic LCNF-based flocculant, named as PAM@LCNF (P@L), was developed by grafting cationic PAM, synthesized from acrylamide and [2-(Methacryloyloxy)ethyl] trimethylammonium chloride, onto LCNFs. The thermal stability, crystal structure, functional groups, charge density, morphology, and electronic properties of P@L systems were analysed. The flocculation performance of the synthesized flocculant, including its effectiveness and mechanism in treating halloysite, kaolin suspensions, and for clarification of sugarcane juice.