Characterization of Moroccan cactus juice toward its use as a green flocculant in wastewater treatment

Chemical flocculants play an important role in different industrial sectors. In the mineral industry, the selective flocculation technique has been used for more than 30 years. It can be considered as one of the most widely used techniques in ore beneficiation (Brostow et al. 2007). This technique is also widely used in wastewater and drinking water treatment (Lee et al. 2014; Ma et al. 2019). In the last decades, the use of coagulation-flocculation processes has increased considerably. The success of these processes is due to the flocculants' properties, which contains monomers of acrylamide and acrylic acid (Benda et al. 1997) or ionic groups (Ma et al. 2013; Zheng et al. 2014) that contribute to the elimination of suspended particles in water (Nougbodé et al. 2013). The flocculation mechanism is governed by electrostatic interactions between the polymer functional groups (flocculants) and the surface of the particles (pollutants). Other factors such as the particle size and the intrinsic polymer flexibility play an important role in improving flocculation efficiency (Gregory & O'Melia 1989; Walker & Grant 1996; Thomas et al. 1999). Nevertheless, the use of synthetic chemical flocculants raises several questions about their impact on human health and environment safety (Harford et al. 2011; Okaiyeto et al. 2016). Alternatively, several research works have focused on the development of green and sustainable flocculants derived from plants, opening a new way for the use of green products without harmful effects on health and the environment. Pallavi et al. have used Moringa oleifera powder to treat dairy wastewater (Pallavi & Mahesh 2013). In the other work, the ability of the mucilaginous, an extract of Salvia hispanica seeds, to treat compost leachate wastewater was investigated (Tawakkoly et al. 2019). In addition, the suitability of mango kernels as a bioflocculant for the treatment of sewage wastewater has been studied (Das et al. 2021). On the other hand, Betatache et al. (2014), as well as other works (Zhang et al. 2006; Sellami et al. 2014; Ennawaoui et al. 2022), demonstrate that the juice of Opuntia Ficus Indica, a species of cactus, has the same flocculating effect as those of the industrial flocculants. Other studies have focused on the use of cactus juice for the removal of heavy metals and organic matter (Amari et al. 2019; El Bouaidi et al. 2020; El Mansouri et al. 2023). Pichler et al. showed that the treatment of drinking water by mucilage from a common cactus induces a decrease in turbidity. Such treatment provokes the removal of more than 83.3% of suspended solids and 59.1% of the chemical oxygen demand (Pichler et al. 2012).

In fact, due to the importance of water for human survival and environmental sustainability, water treatment continues to attract great interest. Recently, several investigations have been conducted to promote the use of natural materials for water treatment, such as cactus plants, which show promising performance as flocculant agents (Rachdi et al. 2017; Rebah & Siddeeg 2017). In order to understand the origin of flocculation induced by cactus, some studies have been dedicated to the identification of the chemical composition of cactus mucilage. Madera-Santana et al. (2018) reported that the mucilage of cactus is a heteropolysaccharide composed of galactose, arabinose, xylose, rhamnose and galacturonic acid. In addition to polysaccharides, cactus mucilage contains other components such as fibers, potassium and calcium (Sepúlveda et al. 2007).

All published works have focused on the use of cactus as a flocculant. Its characterization was in most cases limited to FTIR analysis. The aim of this work resumes in the in-depth analytical characterization of cactus juice, in order to understand its physical behavior, as well as the mechanisms responsible for its flocculating effect. To achieve this goal, many analytical techniques have been used. Optical inductively coupled plasma spectrometer (ICP-AES), Attenuated Total Reflection Fourier Transform Infrared spectrometry (ATR-FTIR), Nuclear magnetic resonance spectroscopy (NMR), and Gas Chromatography coupled with Mass spectrometry (GC-MS) were used to determine the chemical properties of cactus juice, while thermogravimetric analysis, Zeta potential and viscosity measurements were employed to study the physical properties. In addition, an application of cactus juice for the removal of chemical oxygen demand (COD) in wastewater by the coagulation/flocculation process was tested to confirm the cactus's efficiency as a natural flocculant in water treatment.

The cactus leaves were collected from an agricultural field surrounding El Jadida town (Morocco). After washing, the cactus leaves were divided into two parts: the first part was cut and squeezed to extract the juice in a liquid state, and the second part was cut, dried, and crushed to a powder of less than 0.5 mm. Figure 1 shows the preparation steps.

Elemental analysis of the extracted juice was carried out by an optical inductively coupled plasma spectrometer ICP-AES (PerkinElmer AVIO 500). The maximum absorbance of cactus juice was determined using a Shimadzu UV-1900 series spectrophotometer in the range of 200 to 800 nm, and the spectrum obtained is shown in Figure SI.1 (Supplementary file). Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectrometry was performed to identify the functional groups in dried cactus and extracted juice using a Spectrometer BX-1000 FTIR (Perkin Elmer, Überlingen, Germany). The FTIR spectra were recorded in the range of 450–4,000 cm⁻¹. The ¹H, ¹³C, ¹³C DEPT, 2D ¹H-¹³C HSQC NMR spectra of cactus juice after extraction in ethanol solvent were performed on a Bruker Avance 400 MHz spectrometer (Bruker, Germany) at 25 °C and D₂O as solvent.

Gas Chromatography-Mass Spectrometry (GC-MS) analysis was performed on a GC TRACE 1300 TSQ 8000 evo, after treatment of the sample with ethanol, equipped with a TR 35MS (30 m × 0,25 mm × 0,25 μm) column. The injection volume was fixed at 0.5 μl in split mode with a flow of 1.5 ml/min and an injection temperature of 250 °C. Spectra were indexed using a NIST/EPA/NIH MASS SPECTRAL LIBRARY Version 2011.

Thermogravimetric analysis was conducted on raw cactus in the range of 20–900 °C with a heating rate of 5 °C/min under an inert atmosphere of N₂ at a flow rate of 60 mL/min using the Q50 TGA (TA instrument). The rheological behavior of the cactus juice was determined using an Anton Paar MCR 72 rheometer with an MS CC39 parallel plate measuring system at room temperature. The viscosity was measured as a function of shear rate ranging from 1,000 to 10 s⁻¹. On the other hand, the measurement of zeta potential (ζ) and particle size in cactus juice was carried out from a dilute solution of 10% and analyzed at 25 °C by using a Zetasizer Ultra zettameter (Malvern Panalytical, UK), equipped with a capillary cell (DTS 1070), and ZS XPLORER software (V 3.2.2.5).

The samples tested were taken from the municipal wastewater of the city of El Jadida (Morocco). The samples were collected, stored at 4 °C to minimize bacterial activity, and transported immediately to the laboratory. The coagulant was 10% (w/v) aluminum sulphate prepared by dissolving 10 g of aluminum sulphate ((Al₂(SO₄)₃,18H₂O), SOLVAPUR) in 100 mL of distilled water and stirring until complete dissolution. Cactus juice at 10% (v/v) (10 mL of the cactus juice solution in 100 mL of distilled water) was used as a flocculant. Turbidity was measured using a HACH LANGE 2100Q IS turbidimeter. The chemical oxygen demand (COD) was determined according to the ISO 15705:2002 standard (ISO 2002).

The extracted cactus juice, obtained by squeezing the leaves, has a green color and is very miscible in water. It contains about 95% humidity and less than 5% of dry residue. The liquid pH was 4.25, its conductivity was 6.36 mS/cm and its density was 0.95.

The elemental analysis of the cactus juice revealed the presence of six major elements as shown in Table 1. The content of these elements varies from 1 ppm (manganese) to 2,650 ppm (potassium). The presence of these elements explains the conductivity value mentioned above. In addition, the presence of aluminum ions is interesting because they are widely used as a flocculants in water treatment (Zouboulis et al. 2008; Bo et al. 2012; Betatache et al. 2014).

It is well known that a shear-thinning fluid is characterized by a decrease in apparent viscosity as the velocity gradient increase (Perrin et al. 2006). Therfore, the observed behavior may be due to the fact that the time required for the rearrangement of the chains exceeds the time scale of deformation. Hence the elastic deformation of the entangled network becomes progressively larger and the system behaves like an elastic solid, exhibiting a nonlinear model that characterizes non-Newtonian fluids (Denn 2004). This behavior can be attributed to the disorder of the macromolecules, which are mainly composed of D-xylose, D-galactose, L-arabinose, L-rhamnose and D-galacturonic acid (Cárdenas et al. 1997; Pichler et al. 2012).

According to the obtained results, the rheological behavior of cactus juice is in perfect agreement with Casson's model (Nazeer et al. 2021; Verma & Mondal 2021). Several industrial flocculants used to remove suspended matter from water or to condition sludge show identical behavior (Nasser & James 2007; Feng et al. 2020).

Zeta potential is a technique that measures the total surface charge of particles present in a liquid sample. Its principle is based on measuring the velocity of particles when they are subjected to an electric field and become charged, migrating towards electrodes of opposite polarity in proportion to the intensity of the field and the zeta potential (Bean et al. 1964; Pestana et al. 2015). This technique can be used to distinguish between cationic and anionic flocculants.

The objective of this section is to provide an application of cactus juice in order to confirm its flocculant activity as suggested by the characterization discussed above. As previously reported elsewhere (Bouaouine et al. 2018; Othmani et al. 2020), the presence of neutral sugars and uronic acids are assumed to be the active flocculating agents in cactus juice. Polygalacturonic acid, consisting of a long polymer chain containing functional groups, mainly carboxyl (-COO)-/-COOH) and hydroxyl (-OH), provides both positively and negatively charged adsorption sites that interact electrostatically to create charge neutrality between the flocculant and the cationic pollutant. For anionic particles that carry the same charge as the flocculant, flocculation mechanisms do not operate in the same way as for cationic pollutants, but rather through an adsorption and bridging mechanism between the particles and the flocculant (Bouaouine et al. 2018; Choudhary et al. 2019).

In the experimental part, a series of experiments were carried out for the removal of chemical oxygen demand (COD) from wastewater by coagulation/flocculation process using aluminum sulphate as a coagulant and cactus juice as a bioflocculant. The effect of aluminum sulfate and catus juice doses, as well as pH on the coagulation/flocculation process were investigated. The efficiency of the treatment was based on the COD removal. Table 3 shows the characteristics of the wastewater used and the coagulated wastewater after treatment.

In summary, the removal of organic matter in wastewater is effective with a COD removal of more than 97%. These results are in agreement with several studies carried out on the effectiveness of cactus juice as a green flocculant in the treatment of wastewater such as petrochemical effluent, food industry effluent, and controlled discharge leachate, with 72, 88 and 82% of COD removal, respectively (Rebah & Siddeeg 2017). In addition, the obtained COD removal yield (97%) proves the high flocculating activity of cactus juice, compared to other plant derivatives, such as Moringa oleifera, Salvia hispanica seeds and mango kernels (COD removal yield of 88.76; 39,7 and 33,4%, respectively; Das et al. 2021).

The structural and physicochemical properties of Moroccan cactus juice were investigated in this study. Several powerful techniques such as ICP-AES, FTIR, NMR, GC-MS, TGA, zeta potential, and rheology were used to achieve this objective. The physicochemical characterization of the cactus juice indicated the presence of numerous organic compounds such as polysaccharides and mineral elements (potassium, sodium, calcium, magnesium, aluminum, and manganese), which are considered to be the origin of the significant reduction of the chemical oxygen demand in the wastewater, which decreases drastically with the increase in the added quantity of cactus juice. These results demonstrate the flocculating behavior of cactus juice as a green and sustainable biopolymer.

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

The authors declare there is no conflict.