Comparative study of physico-chemical properties in by-products of the sugar industry

1. Introduction

The sugar sector is a crucial component of economic development as it boosts farming opportunities, job creation, and exports. [1]. Sugar is a widely used component in our local cuisine. In Pakistan, sugar cane and sugar beet are two prevalent crops cultivated for sugar production.[2]. However, sugar obtained from sugar cane is considered to be the best in the world [3]. The successful advancement in the sugar sector relies on its distinctive feature that nearly all of its products, including both primary and secondary products as well as waste, can generate profits [2]. There is a need to assess the stepwise elemental composition of the major and by-products of different sugar industries. The sugar industry primarily produces jaggery, brown sugar, and white sugar, along with by-products such as bagasse, filter (mud) cake, and molasses. Bagasse, the first by-product generated after crushing sugarcane and separating the raw juice, can be utilized as fuel and in the production of certain synthetic woods. The paper pulp and paper manufacturing industries are the main sectors that utilize bagasse. In several countries, it is also employed as fuel for generating self-electricity in sugar mills. The fly ash produced from these self-electricity units is often mixed with filter (mud) cake to create bio-compost, which is then provided to farmers as fertilizer. Additionally, molasses are marketed both nationally and internationally in large quantities, serving as a raw material for the production of amino acids, peptones, alcohols, phenols, acetone, and more.[2]. The primary potential sources of toxic metal contamination in sugarcane and its derivatives include the pollution of irrigation water (IW), agricultural soils, various fertilizers and pesticides, along with the chemicals employed in the bleaching process (such as lime, polyelectrolyte, and carbon dioxide) [4]. Toxic metal contamination can lead to various health issues. Nevertheless, the harmful effects of cadmium (Cd) and lead (Pb) are quite evident. Cadmium contamination in food can result in kidney disease, weakened bones, and damage to other organs [5]. Lead contamination can result in elevated blood pressure, anaemia, Alzheimer’s disease, and issues related to reproduction [2,5]. In this competition, various research studies have been presented to evaluate the elemental makeup and the contamination of toxic metals in sugar or sugar cane [6–10]. The sugar industry is experiencing rapid growth in India, making environmental contamination a pressing issue. Despite this, there is limited research on toxic metal levels in sugarcane products. In developing nations like India, vast and unregulated waste expansion can lead to the dumping of industrial effluents into land and water bodies, often neglecting the environmental consequences. Additionally, the by-products of the sugar industry have not been adequately monitored or managed, resulting in potential environmental and health risks. To address this, toxic risk assessments are essential for identifying contaminants in food products that could pose health threats to local populations. This study aims to evaluate the levels of selected heavy metals (Cadmium, Iron, Lead, and Zinc) in various sugar industry by-products. Furthermore, it includes a toxic risk assessment of Cadmium and Lead exposure from sugar consumption within families associated with the sugar industry, based on survey data.

2. Materials and methods

2.1. Sampling and pre-treatment strategy

Different products of the sugar industry were collected for analysis. A total of five specific products, including bagasse, mud-cake, molasses, bagasse fly ash, and effluent samples, were randomly gathered from a nearby sugar industry, with ten samples taken from each product. Solid samples were stored in polythene bags, while juice samples were collected in polyethylene bottles. The effluent samples from the same sugar industry were also included in the collection. Initially, all samples were dried for one hour at 105°C in an electric oven to aid in pre-concentration. Following this, a grinding machine was used to pulverize the pre-concentrated solid samples. The resulting powdered samples were then separated through a 140 µm plastic sieve and stored in desiccators for further analysis.

2.2. Sample preparation

In the analysis process, 100 mL samples of both molasses and industrial effluent were subjected to conventional pre-concentration at a temperature of 70 ± 5 °C using a hot plate, reducing the volume to 25.0 mL. Post-pre-concentration, these samples were filtered and stored at 4.0 °C for further analysis [11]. For the solid products harvested from the sugar industry, triplicate samples weighing 0.5 g each, along with 0.2 g of CRM (BCR-100) subsamples (n = 6), were placed in flasks. These were treated with a solution of 5.0 mL acid mixture composed of 2:1 HNO3 and H2O2 (v/v) and allowed to stand at room temperature for 30 minutes. Subsequently, the mixtures were heated at 80 °C on a hot plate for approximately 3 to 4 hours to achieve a semidried mass. After digestion, the solutions were cooled, diluted to a final volume of 10 mL using 0.2 N HNO3, and filtered through Whatman No. 42 filter papers to prepare them for subsequent analysis.

2.3. Physicochemical properties of By-products of Sugar Industry

The total metal contents in the By-products of the Sugar Industry were measured using atomic absorption spectrophotometry (AAS). The physico-chemical properties were analysed as per standard methods [12,13].

2.4. Statistical Analysis

All the observations were conducted in triplicate (n = 3), and data were presented as mean ± SD.

3. Results and discussion

3.1. Physicochemical properties of By-products of Sugar Industry

3.1.1. Bagasse

Bagasse is a waste product of the sugar manufacturing unit that is created during the juice extraction process and has various physical and chemical characteristics (Table 1). Bagasse fibers exhibited an average length of 2.96 ± 0.22 mm with a high moisture content (52.66 ± 2.73%). The lignocellulosic composition was dominated by cellulose (52 ± 1.73%), followed by hemicellulose (25 ± 1.73%), making it a promising feedstock for bioethanol production and biocomposite materials, and lignin (25.33 ± 0.88%) shows its suitability for thermochemical conversion (e.g., pyrolysis, gasification) to produce biofuels. Minor constituents included pectin (0.60 ± 0.24%) and ash (4 ± 0.58). However, the high moisture content may necessitate pre-treatment (e.g., drying or torrefaction) before processing. The low pectin and ash levels are favorable for enzymatic hydrolysis in biorefineries, as excessive ash can hinder fermentation efficiency. 

3.1.2. Bagasse fly ash

BFA showed a moisture content of 14.4 ± 1.16% and a high ash content (54.95 ± 1.04%), indicating significant inorganic residues. The material had a large surface area (470.33 ± 25.52 m²/g) and alkaline pH (8.37 ± 0.11). Its bulk density (1.77 ± 0.10 g/cc) and dry density (1.10 ± 0.05 g/cc) suggest moderate compaction properties. BFA’s high ash content (54.95%) and large surface area (470 m²/g) indicate its potential as an adsorbent for wastewater treatment, particularly for heavy metal and dye removal. Its alkaline pH (8.37) further supports its use in neutralizing acidic effluents. Additionally, its moderate bulk density (1.77 g/cc) suggests possible applications in lightweight construction materials or as a filler in composites. However, the high ash content may limit its direct combustion efficiency, necessitating blending with other fuels for energy applications. 

3.1.3. Press mud

Press mud was highly alkaline (pH 8.54 ± 0.31) with a very high total ash content (80.84 ± 0.70%), indicating minimal organic matter. The organic carbon (0.32 ±0.11%) and total nitrogen (0.17 ± 0.03%) levels were low, while residual sucrose (0.51 ± 0.08%) was detected. The electrical conductivity (EC) was 0.84 ± 0.01 µS/cm, suggesting low salinity. The extremely high ash content (80.84%) and alkaline pH (8.54) of press mud make it a candidate for soil conditioning in acidic soils, improving pH and providing micronutrients. However, its low organic carbon (0.32%) and nitrogen (0.17%) suggest limited value as a standalone organic fertilizer. Instead, it could be composted with nitrogen-rich waste to enhance its agronomic utility. The residual sucrose (0.51%) indicates incomplete sugar extraction, which could be further valorized through microbial fermentation. 

3.1.4. Molasses

Molasses is the leftover syrup from the production of sugar from sugarcane (Table 4). Molasses contained 33.66 ± 3.53% sucrose and 28.66 ± 2.03% glucose, making it rich in fermentable sugars. The moisture content was 25.13 ± 1.02%, with 16.83 ± 0.60% ash, indicating substantial mineral content. The pH was slightly acidic (5.16 ± 0.00), and the EC was high (336.13 ± 10.81 µS/cm), reflecting its ionic strength. Molasses’ high sucrose (33.66%) and glucose (28.66%) content highlight its suitability for bioethanol, citric acid, and yeast production. The slightly acidic pH (5.16) is conducive to microbial fermentation, while the high EC (336 µS/cm) reflects its mineral richness, which may require dilution in fermentation processes to avoid microbial inhibition. The substantial ash content (16.83%) suggests the need for purification steps if used in high-value biochemical synthesis. 

3.1.5. Wastewater

Physico–chemical analyses of sugar mill wastewater are summarized in Table 1. The evaluation confirmed that the wastewater is acidic and contains a significant pollution load, which can adversely affect the flora and fauna of aquatic ecosystems. In this investigation, the pH value of the sugar factory effluent was found to be 3.34, indicating its acidic nature. Elevated temperatures can accelerate the rates of chemical reactions and alterations in aquatic conditions, further contributing to the negative impact on water quality and aquatic life. These findings underscore the need for effective treatment methods to mitigate the contamination associated with sugar mill effluents before they are released into the environment [14]. For agricultural irrigation, the acceptable temperature limit is 40ºC. Therefore, the temperature of this sugar mill wastewater is suitable for irrigation purposes. The presence of suspended particles in the water body affects light intensity, contributing to increased turbidity and decreased transparency. The total dissolved solids (TDS), total suspended solids (TSS), and total solids (TS) in the wastewater were measured at 1239.48 mg/l, 348.77 mg/l, and 1605 mg/l, respectively. These findings align with those reported by Vinod and Chopra [15]. The investigation revealed that the wastewater has high biochemical oxygen demand (BOD) at 468.03 mg/l and chemical oxygen demand (COD) at 774.12 mg/l. Additionally, high concentrations of toxic heavy metals were detected in the collected wastewater: nickel (0.21 ppm), cadmium (0.05 ppm), zinc (8.05 ppm), iron (43.99 ppm), copper (1.76 ppm), lead (0.31 ppm), chromium (2.78 ppm), cobalt (0.25 ppm), and manganese (11.85 ppm). These findings are consistent with those of Singh et al. (2024). Spent wash has a high concentration of heavy metals. Iron (Fe), zinc (Zn), cadmium (Cd), copper (Cu), lead (Pb), and chromium (Cr) accumulate in the soil, and their concentration increases in plants with the increasing application of spent wash [15].

3.2. Heavy metals in sugar industry products

The concentrations of selected heavy metals, viz. Ni, Cd, Cu, Zn, and Fe were quantified in five samples obtained from the sugar industry: bagasse, BFA, press mud, molasses, and wastewater (Table 3). Ni concentrations ranged from 0.10 ± 0.02 mg/kg in BFA to 0.66 ± 0.18 mg/kg in press mud. Press mud exhibited the highest Ni accumulation, while bagasse and molasses showed moderate levels (0.13 ± 0.00 and 0.14 ± 0.14 mg/kg, respectively). Wastewater contained 0.20 ± 0.18 mg/L of Ni. While Cd concentrations were relatively low in all samples, ranging between 0.04 ± 0.00 mg/kg in wastewater to 0.07 ± 0.01 mg/kg in press mud. This indicates a minimal presence of cadmium contamination in the sugar industry by-products. The highest concentration of Cu was detected in wastewater (1.65 ± 0.05 mg/L), followed by molasses (1.16 ± 0.06 mg/kg) and BFA (1.14 ± 0.01 mg/kg). Bagasse showed moderate Cu content (1.02 ± 0.00 mg/kg), whereas press mud recorded the lowest Cu level (0.04 ± 0.00 mg/kg). On the other hand, Zn was found in significant amounts in molasses (1.67 ± 0.11 mg/kg) and BFA (1.31 ± 0.06 mg/kg). Bagasse and wastewater also exhibited notable Zn levels (0.70 ± 0.07 and 1.14 ± 0.00 mg/L, respectively), whereas press mud had the lowest Zn concentration (0.24 ± 0.11 mg/kg). Fe recorded at maximum levels in wastewater (44.41 ± 7.42 mg/L), followed by BFA (38.13 ± 2.10 mg/kg) and molasses (20.20 ± 3.29 mg/kg). In contrast, press mud contained only 0.37 ± 0.29 mg/kg of Fe, indicating its relatively lower iron load. The values of heavy metals are aligned with Fareed [16]. Fe may accumulate in sugarcane due to excessive levels of Fe in the growth medium (soil), IW, and fertilisers. Another possible reason for the elevated levels of Fe is the leaching of Fe from the processing units during sugar production. Overall, wastewater and BFA were found to contain the highest concentrations of most heavy metals analyzed, raising concerns about their potential environmental impact if disposed of or used untreated in agriculture. These findings underline the need for appropriate treatment or management strategies before the application or disposal of these by-products.

4. Conclusion 

The heavy metal content of five important by-products of the sugar industry is reported in this study. Furthermore, the high content of Fe, Zn, Cu, Ni, and Cd found in all samples suggests a possible risk for the environment and also for human health (in case these materials might be reused in agriculture, as fuel, or for industrial needs). Although the by-products have significant economic and agronomic values, particularly in power and composting, ethanol production also causes pollution by toxic elements such as Cd and Pb. Consistent toxicological evaluations, appropriate waste management, and rigorous regulatory supervision are crucial to guarantee the safe recycling of these by-products. Future research should also investigate bioremediation strategies and more streamlined processing techniques to minimize heavy metal build-up in the waste stream of the sugar industry.

Acknowledgement

The author is grateful to the Head of the Department, Department of Botany, University of Lucknow, for granting access to the central laboratory facility of the botany department.

Author contribution

Prof. Alka Kumari supervised this research and provided suggestions. Priyanshi Singh conducted the experimental work, analyzed the data, and wrote the manuscript. Aanchal Verma and Pratibha were also involved in data analysis.

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