Synergy effect in blend Orange G/Rhodamine B ultrafiltration, using natural bentonite-based membrane

Organic dyes are now widely used across a number of industries like many other sectors that use dyes and pigments, such as paper, leather tanning, cosmetics, and plastics, and the textile industry is notable for its high water use as well as for being one of the main industrial producers of colored effluents (Wang et al. 2019; Seo et al. 2020). Additionally, these dyes are resistant to ultraviolet radiation and do not biodegrade, which results in long-term water pollution (Bouazizi et al. 2017a; Touahra et al. 2022). Reusing water is a major challenge for industrial companies as it allows them to conserve water while maintaining the sustainability of water supplies for future generations (Hamri et al. 2022). Multiple harmful compounds are emitted from industrial sites into water and soil, which is one of the negative repercussions of industrialization, For example, wastewater from the textile sector contains a complex mixture of cationic and anionic dyes, including some hazardous metal ions, and they discharge effluents that pose environmental problems (Atun et al. 2019; Liu et al. 2020). The aforementioned industries make extensive use of Rhodamine B (RB) and Orange G (OG), carcinogen compounds that not only harm the environment but also pose a risk to the health of both humans and animals (USEPA 2014; Mohamed et al. 2019; Tolkachev & Bespamyatnykh 2019). Therefore, it is very important to remove these contaminants from wastewater. Few studies, however, have focused on the simultaneous removal of both pollutants despite the fact that they frequently coexist in wastewater and may have a substantial impact on one another's destiny and travel. As reported in the literature, there are many different technologies to remove dyes from wastewater, such as coagulation (Liang et al. 2014), adsorption (Benhalima et al. 2017; Djamaa et al. 2019), photocatalytic degradation (Bouarroudj et al. 2021; Mekidiche et al. 2021), biological oxidation (Das et al. 2021), chemical oxidation (Oller et al. 2011), and membrane filtration (Bouzerara et al. 2006; Harabi et al. 2014). Unfortunately, these technologies are not always effective at removing soluble dyes because wastewater varies in terms of the type and quantity of dyes present, as well as in complexity and volume, and hence, new adequate and effective alternatives based on natural, abundant, and inexpensive materials have been developed for colored wastewater treatment. Given that sieving is mostly used in membrane separation, which makes it more suitable for the majority of industrial effluent, membrane separation might be seen in this context as an efficient and green technology to remediate dye wastewater (Cao et al. 2020). Membrane procedures are also acknowledged as being reasonably cost-effective methods, primarily because they require few chemical reagents and use little energy (Achiou et al. 2017). Reverse osmosis (RO), microfiltration, ultrafiltration (UF), and nanofiltration (NF) are four pressure-driven membrane technologies that are frequently employed to decontaminate textile wastewater (Dasgupta et al. 2015). It should be noted that additional membrane processes such as vacuum membrane distillation, membrane bioreactors, and electrodialysis (ED) may be applicable (Dasgupta et al. 2015; Parakala et al. 2019). Targeted species, wastewater volume, and operating conditions are just a few of the factors that should be taken into account when choosing an appropriate membrane process. Compared to NF and RO, UF requires lower pressure and might provide excellent separation efficiency in terms of flux and desirable rejection (saving energy) (Benkhaya et al. 2019). Due to these factors, UF is frequently chosen as the best procedure for removing soluble dyes (Bouazizi et al. 2017b; Tran et al. 2019). As described in the literature, many works that used UF membranes to remove dyes were published. Aloulou et al. developed an asymmetric microfiltration membrane from natural zeolite with a permeability of 176 L/m².h.bar and an average pore size of 0.55 μm. These membranes were characterized by good chemical oxygen demand removal potential (>85%) and applied in industrial wastewater treatment (Aloulou et al. 2020). Sawunyama et al. functionalized ceramic membranes for the removal of pharmaceuticals in wastewater (Sawunyama et al. 2023). On the other hand, a new alumina UF membrane with a permeability of 70 L/m².h.bar was developed by Zou et al., and this membrane was used for heat treatment of colored wastewater (Zou et al. 2017). Also, Ouaddari et al. elaborated membranes from natural clay for the UF of Direct Red 80 (Ouaddari et al. 2019). In another study, Jiang et al. desalinated several reactive dye species using a tight ceramic UF membrane made of TiO₂ with a molecular weight cutoff of 2,410 Da. A large percentage of the reactive dye was rejected by this membrane (>98.12%) (Jiang et al. 2018). Generally, industrial metal oxides like titania, zirconia, silica, and alumina were used to manufacture ceramic membranes that are more expensive (Achiou et al. 2018a), and hence, various materials were used for mineral membrane preparation, such as phosphate (Barrouk et al. 2014; Mouiya et al. 2018), ceramics (Fang et al. 2013; Bouazizi et al. 2016; Abdullayev et al. 2019; Mouiya et al. 2019), and zeolites (Wang & Peng 2010; Visa 2016). Due to their excellent mechanical properties such as thermal stability, good chemical resistance, and long lifetime, mineral membrane-based clays have real potential in the environmental protection field. Therefore, a great deal of inexpensive UF membranes have been effectively proposed based on low-cost materials (Al-Futaisi et al. 2007; Bhatt et al. 2012; Silva et al. 2012; Sharma et al. 2015; Foorginezhad & Zerafat 2017; Ouaddari et al. 2019; Saja et al. 2020; Ouachtak et al. 2021; de Almeida et al. 2022). As an example, for the elimination of bromothymol blue, methyl orange, and murexide from aqueous solutions, Karim et al. produced UF membrane from graphene oxide and natural pozzolan (Karim et al. 2018). In other works, the TiO₂-UF membrane was prepared by bentonite/phosphate support and was successfully used for the filtration of Direct Red 80 (Bouazizi et al. 2017a; Touahra et al. 2022). In a review on the development of nanocomposite membranes for water treatment, Ursino et al. recommend incorporating innovative nanoscale materials into polymeric membranes, such as carbon nanotubes, graphene oxide, zinc oxide, titanium dioxide, silver, and copper nanoparticles (Sikorski & Trzybiński 2014; Minitha et al. 2017; Ursino et al. 2018). So, one of the major problems, which can occur in all membrane classes, is membrane fouling, which refers to the accumulation of undesirable substances on the surface or within the pores of a membrane material, leading to such reduction of its efficiency. This membrane fouling can be essentially related to the feed water quality, the inadequacy of the selected filtration process, and/or the operating conditions. In addition, membrane fouling has a clear impact on the membrane performance by reducing the permeate flow, increasing transmembrane pressure and thus shortening membrane lifespan. Referring to these works and based on our previous study, the preparation of low-cost UF membrane-based bentonite clay with optimized conditions is a promising approach to prepare a cylindrical membrane for tangential filtration. In this work, bio-based material was elaborated by physical blending way of bentonite clay under a sintering temperature of 1,000 °C. The microstructure, pore size, chemical stability, point of zero charge, as well as permeability and other properties were investigated. The performance of the elaborated membrane was evidenced by tangential filtration of the cationic dye RB and the anionic dye OG in a single system and a simultaneous one (RB/OG), respectively. For this, the evolution of the dye's rejection was examined according to transmembrane pressure, pH, and concentration solution experimental parameters. The synergy on the simultaneous system (RB/OG) was also described to underline the important rejection results obtained in this system.

The UF tubular membrane was elaborated as follows: First, bentonite clay was sieved to a fine powder with a mean particle size of 100 μm. Then, 4 wt. % of both binding and plasticizer agents was dispersed in 50 mL of distilled water. The obtained material was purged for a few minutes before extrusion processing at a screw speed of 0.02 rpm. The material was dried at room temperature for 24 h and then annealed at 1,000 °C for 3 h at a heating rate of 1 °C.min⁻¹. The obtained tubular membrane dimensions are as follows: L: 150 mm, ED: 8 mm, and ID: 5 mm, with an effective filtration area of 33 cm². It worked inside our membrane module and was noticed as MB1000.

The point of zero charge of the MB1000 was determined according to the method described in the literature (Djebri et al. 2016; Saavedra-Labastida et al. 2019). Briefly, 0.1 g of materials was placed in aliquots of 0.01 N NaCl solution for 48 h, and then, the pH was varied using HCl or NaOH solutions (0.1N). The materials' pH_pzc was calculated by intersecting the bisector curve with the plot of pH final versus pH initial.

After each applied pressure, the UF membrane was cleaned by the clean water backwash method, using hot distilled water and then using acid and basic solutions. Finally, it is washed several times using hot distilled water.

Fourier transform infrared spectroscopy (FTIR) (Bruker Alpha) spectrometer was used to analyze the chemical composition of the membrane over the wavelength range 4,000–400 cm⁻¹, with a spectral resolution of 2 cm⁻¹ and 64 scans. The mineralogical composition of clay and modified sintered bentonite at 1,000 °C was carried out using X-ray diffraction (XRD) analysis and a diffractometer (BRUKER D8 ADVANCE) equipped with a Ni-filtered Cu-Kα (λ = 1.542A°) radiation. The apparatus operated at 40 KV and 25 mA in a step scan mode and a diffraction speed of 0.02°s⁻¹. Pore size distribution and surface area of the membrane were determined via N₂ adsorption/desorption at 77 K using an automated gas sorption apparatus by a well-known, Brunauer–Emmett–Teller method. The morphological and microstructure of the elaborated membrane were checked by using scanning electron microscopy (SEM) (QUANTA 250) operating at a working distance of 10.0 mm and under an acceleration voltage of 20.00 KV. A spectrophotometer device (Varian Cary 50 Scan) was used for the measurements of RB and OG concentration in the range 200–800 nm, at 2 nm resolution, at a wavelength of 554 and 478 nm for RB and OG, respectively.

As the pH of the solution changes, the charge of the material surface may be impacted. Therefore, the material exhibits a positive surface charge when the solution pH value is lower than the pH_pzc (Silva et al. 2012; Saavedra-Labastida et al. 2019; El Gaayda et al. 2021). The surface tends to have a negative charge when the pH solution exceeds pH_pzc, and pH_PZC was determined by using the batch equilibration method described in the previous work (Labied et al. 2022). In our case, the pH_pzc value is 7.25 (Figure 4(b)).

The prepared membrane MB1000 was applied for tangential filtration of RB and OG dyes, as organic pollutants. Experiments were conducted in single and simultaneous systems (RB/OG), respectively. For this, the effects of transmembrane pressure, pH solution, and dye feed concentration on permeate flux and rejection percent were studied.

Bentonite was used to prepare an UF membrane (MB1000), for removing RB and OG dyes from aqueous solutions in single and simultaneous (RB/OG) systems, respectively. The effect of transmembrane pressure, solution pH, and concentration on the dye tangential UF rejection percent was investigated, and the optimized parameters were described. The tangential UF rejection percent of RB and OG in the simultaneous systems (RB/OG) was remarkably important compared to similar single UF systems. So, their coexistence improves their UF in a synergistic manner. Finally, the results obtained in this study confirm that the used bentonite can be considered as a key material for membrane synthesis. Its low cost and soft elaboration make it a promoting bio-based membrane for industrial wastewater treatment.

The authors acknowledge that the financial support from the Ministry of Higher Education and Scientific Research (MESRS) and the General Agency of Scientific Research and Technological Development (DGRSDT), in the frame of the national projects is grateful.

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

The authors declare there is no conflict.