Assessment of groundnut (Arachis hypogaea) as a natural coagulant for water treatment

Several treatment technologies are used to process raw water sources into drinking water. Most cannot be separated from the coagulation and flocculation stages. The coagulation–flocculation process is prevalent in water treatment due to its effectiveness in removing organic matter, suspended solids, turbidity, and colour (Abidin et al. 2013; Kakoi et al. 2016). The process requires the addition of positively charged, divalent chemical compounds. Negatively charged polymers are also used commonly in water treatment, notably as high molecular weight flocculants (Lapointe & Barbeau 2020).

The application of these compounds is not necessarily free from impacts (Barakwan et al. 2019b). The environmental impacts include increasing the corrosion rate of metallic utilities, changing the pH, limiting root elongation, and inhibiting seed germination. Water treatment involving conventional coagulants also generates excess chemical sludge in addition to the suspended solids to be removed; and this becomes another issue to resolve (Kurniawan et al. 2020). Aside from these impacts on the environment, concerns related to human health arise. Metal-based coagulants are non-biodegradable, and their residuals in drinking water can induce direct human health impacts when consumed and can accumulate in body cells (Kluczka et al. 2017). There have been reports of signs that chemical coagulants may have negative effects on human health, including central nervous system failure, dementia, Alzheimer's disease, and severe trembling (Ahmad et al. 2016; Exley 2017; Mirza et al. 2017; Barakwan et al. 2019a).

Natural coagulants may offer an alternative strategy to reduce the hazards to human health and the environment posed by chemical coagulants. They are entirely organic and biodegradable, and derived from living things; as a result, they are safe for the environment, with minimal potential negative effects on human health. However, a number of issues must be taken into account when using natural coagulants, mostly because of the complex extraction and purification processes, the limited availability of raw materials for continuous supply, and the varying characteristics of water and wastewater to be treated. The limitations of using natural coagulants open up new challenges for future study (Kurniawan et al. 2020). A lot of research related to natural coagulants is documented, but the findings are mainly focused on commonly known and established plants such as Moringa oleifera, Jatropha curcas, and Opuntia ficus indica. Although these sources have proven their ability to remove various pollutants, it is crucial to explore the possibility and feasibility of using other local raw material sources as well (Bahrodin et al. 2021).

This study's main objective was to evaluate the performance of groundnut extract in removing turbidity from water. Groundnuts or peanuts, taxonomically classified as Arachis hypogaea, are a legume crop of the Fabaceae family. It is known that peanut seeds contain arachin and coarachin, which are globulin proteins (Kristianto 2017). Proteins are proven active coagulant agents that work by charge neutralisation (Prihatinningtyas 2019). The focus of this study was on the development of a simple and reliable extraction method for the active coagulant agents contained in groundnuts, and to provide basic treatment process data, namely removal efficiency, change in pH, floc size, and flocculation and sedimentation durations. The effect of the water's initial turbidity was also determined.

Deshelled groundnuts were purchased from the local market, washed with water, and sun-dried. They were then ground and 5 g of the finely broken groundnuts were inserted in 500 ml of 1 M NaOH (which was prepared by dissolving 40 g of sodium hydroxide pearls in 1 l of distilled water). The mixture was stirred at 200 rpm for 15 min to extract the coagulation active components. It was then allowed to sediment, and the liquid portion was collected and used as a coagulant (Figure 1).

NaOH was chosen as the extraction solvent for the active coagulant agents (proteins) because it is efficient in protein extraction, and it is more readily available and affordable than other chemical solvents. Water and sodium chloride (NaCl) extracts were tested previously but showed poor results. This is in line with the work by Muda et al. (2020), who tested the same extraction solvents and concluded that NaOH was the most efficient.

The experiments aimed at evaluating not only the performance of the natural coagulant but also the effect of the water's initial turbidity. Synthetic turbid water was used for the experiments to enable control of the turbidity level while keeping other water characteristics constant.

Synthetic turbid water for treatment tests was prepared by adding kaolin to tap water. Kaolin was ground and sieved, and the finer fraction was used to prepare the turbid water. A 1% kaolin stock solution was made by adding 10 g of kaolin to 1 l of water. The solution was stirred for 1 h at 200 rpm, to achieve uniform kaolin dispersion, and then allowed to rest for 24 h for complete particle hydration (Figure 2). Immediately before the water treatment tests, synthetic turbid water samples with known initial turbidities were produced by diluting the stock solution with tap water. Three levels of turbidity were considered, as explained by Prihatinningtyas (2019):

• Low turbidity (T1): 50 NTU

• Medium turbidity (T2): 150 NTU

• High turbidity (T3): 300 NTU

The chemical coagulant was used for the jar tests in five dosages (Table 1), for ease of comparison with the performance of the natural coagulant. The groundnut extract dosages are given in Table 2.

Some studies, though with different protocols, have also demonstrated that groundnut extract can remove turbidity successfully from water. Raed et al. (2023) opted for 1 M NaCl extraction of de-oiled peanut seeds and achieved 75.4–85.0% turbidity removal from water samples with 35.2 NTU mean turbidity, while alum exceeded 90.0%. Prasad et al. (2017) achieved 84% turbidity removal from a 100 NTU water sample using a water-based groundnut seed extract dosed at 20 mg/L. Abood et al. (2017) treated a 340 NTU water sample using a 4 mol/l KCl groundnut extract and obtained 88.3% turbidity removal efficiency. Birima et al. (2013) reported that peanut (after oil removal) extracted with 6 mol/l NaCl solution could effectively remove 92% of the 200 NTU turbidity using 20 mg/L, while peanut seeds extracted with distilled water could remove only 31.5% of the same turbidity with the same dosage and those extracted with 1 M NaCl removed about 40%. In our experiments, however, water-based groundnut extract was not able to induce coagulation in water and 1 M NaCl did not give significant results, hence the choice of 1 M NaOH as extraction solvent. It is possible that increasing the dosage of NaOH increases the turbidity removal efficiency of groundnut extract. Removing the oil from groundnuts before extraction is also likely to increase our extract's performance. Half of the dry weight of peanut seeds consists of lipid fractions that do not contribute to their removal efficiency. Therefore, these lipidic fractions should be removed to enhance the coagulation power of peanut seed cake (Choy et al. 2015). This explains the high coagulant dosages that were required for our treatment.

This water pH conservation is consistent with that reported by Prasad et al. (2017), who observed a pH change from 8.3 to 8.4 at the optimum dosage. Abood et al. (2017) also reported only a small pH change from 7.5 to 7.3 at optimum dosage.

Table 3 details the time taken for flocculation with Al₂(SO₄)₃ to appear complete at each dosage and turbidity level. The flocculation time is constant (7 min) for every dosage at T1, except 2.5 ml where it is 20 min, the duration of the mixing phase. At T2, more water samples flocculated extremely slowly, except for dosages 0.5 and 1 ml, where the flocculation time was 9 min. At T3, 7 min was enough for dosages between 0.5 and 1.5 ml, and 10 min for higher dosages. The water samples which flocculated slowly are those in which turbidity removal was low.

Table 4 details the time taken for flocculation with groundnut extract to appear complete at each dosage and turbidity level. At dosages between 10 and 25 ml, the flocs formed quickly whereas the 5 ml dosage always had the longest flocculation time. At 5 ml, half the mixing duration had passed before a significant number of flocs could be observed in the water, hence the low turbidity removal. Overall, flocculation is quicker with groundnut extract than with Al₂(SO₄)₃ at optimum dosages and it can be concluded that it is time-saving to use groundnut extract rather than Al₂(SO₄)₃ for water treatment. The total duration of the mixing phase can be reduced to 10 min or less for low turbidity water and 5 min for medium and high turbidity waters.

Table 5 details the sizes of the flocs formed at each dosage and turbidity level using Al₂(SO₄)₃. The flocs formed were small at both T2 and T3 for smaller dosages, and very small for higher dosages. At T1, from 1 to 2 ml, the flocs were of medium size. The higher dosage resulted in the formation of very small flocs while the lower dosage resulted in the formation of small flocs.

Table 6 details the sizes of the flocs formed at each dosage and turbidity level, using groundnut extract. Big flocs were formed at T1 and T2 at optimum dosages of 10 and 25 ml, respectively, while the 5 ml dosage had the smallest floc sizes. At T1, medium sizes were observed at higher dosages. At T2 and T3, however, floc size increased with dosage. This is in accordance with the flocculation times and coagulation activities that were reported. Overall, groundnut extract produced bigger flocs than Al₂(SO₄)₃ and does not require an additional product to aid with flocculation.

Table 7 details the time taken for more than 90% of Al₂(SO₄)₃ flocs to sink to the bottom of the beaker at each dosage and turbidity level. At T1, sedimentation was quick (less than 5 min), except for the 2.5 ml dosage where the sedimentation time allowed (15 min) was not enough. The 15-min duration was also insufficient for the three last dosages at T2. The sedimentation time was medium at 0.5 and 1 ml at this turbidity level. 8 and 10 min were obtained, respectively. At T3, there were variations ranging between 4 and 8 min for dosages 0.5–1.5 ml. A correlation can be seen between sedimentation time and removal efficiency.

Table 8 details the time taken for more than 90% of groundnut extract flocs to sink to the bottom of the beakers at each dosage and turbidity level. Sedimentation time increased as turbidity increased. It was between 3 and 5 min for T1 and T2, but got higher at T3, to the point where most flocs were still in suspension when the allowed sedimentation time was over for lower dosages. This obviously resulted in lower coagulation activities. A better performance can be expected if the sedimentation time is prolonged. Sedimentation is quicker with groundnut extract than with Al₂(SO₄)₃ due to the bigger floc sizes. The sedimentation duration in the water treatment protocol can be reduced to 5 min for low and medium turbidity waters, and 10 min for high turbidity waters.

Groundnut extract is a promising natural coagulant for turbidity removal from water. The best performance was obtained at medium turbidity level (150 NTU), where the extract removed about 89% of the turbidity using a dosage of 500 mg/L. Sodium hydroxide is efficient in extracting the active coagulant agents from groundnuts. However, the extract was not as efficient as Al₂(SO₄)₃, whose coagulation was better at every turbidity level. The use of groundnut extract does not modify the pH of water significantly. The floc size decreases as turbidity increases but they are bigger than those produced by Al₂(SO₄)₃, and flocculation and sedimentation are quick (each less than 10 min), quicker than those with Al₂(SO₄)₃. Groundnut extract can be used efficiently to treat water with any turbidity level, the required dosage depending on the characteristics of the water to be treated. Groundnuts are readily available and environmentally friendly, both of which tend to facilitate remote area application as a source of natural coagulants. Hence, groundnut extract is a viable alternative to chemical coagulants.

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

The authors declare there is no conflict.