Over the last century, rapid population growth and industrial activities have resulted in various environmental issues, particularly various forms of pollution, such as air and water pollution. The increased reliance on chemical-based products and discharge of various toxicants in wastewater effluents have created a numerous concern for sustaining a viable ecosystem [1, 2]. So far, various forms of contaminants have been identified with each having various degrees of physiological impacts not only on humans but animals alike. These contaminants originating mostly due to anthropogenic activities can be broadly classified as organic contaminants, pathogens, pesticides, as well as inorganic forms of pollutants such as ammonia, nitrates, and heavy metals. The scientific consensus is clear on the harmful impact of exposure to heavy metals such as lead (Pb), iron (Fe), and zinc (Zn), which usually results in acute or chronic poisoning [3]. Although heavy metals are mostly toxic, their relative physiological tolerance limit varies due to the role of some useful biological functions played by metals such as Zn. For instance, the established maximum tolerance limits for three main heavy metals, i.e., Pb, Zn, and Fe, are set at 0.1, 5, and 0.03 mg/L, respectively [4]. Nevertheless, the toxic physiological pathway of heavy metals has been established, i.e., generation of radical oxygen species, cellular disruption, and enzyme inactivation [5]. Considering the acute risks that heavy metals pose to not only humans but animals alike, research efforts have been devoted to the remediation of heavy metals on a priority basis [6].
So far, various techniques that have been explored are adsorptive separation processes, chemical processes, electric-based separation, photocatalytic separation, and membrane filtration [7, 8]. Adsorptive separation is based on using highly efficient adsorbents, such as carbon-based materials, chitosan-based materials, mineral adsorbents, bio adsorbents, and metal-organic frameworks [9, 10]. Chemical processes include conventional methods such as precipitation, coagulation, and flocculation, while electric methods employ redox reactions using suitable electrodes for the separation of heavy metals [7]. Among these technologies, membrane filtration based on pressure-driven separation processes has progressed significantly over the years and is considered a facile method for the remediation of various types of pollutants that otherwise are resistant to conventional wastewater treatment.
Membrane filtration processes are of three main types, i.e., microfiltration, nanofiltration, and ultrafiltration, which are used to separate different types of pollutants according to their pore sizes [11]. For the separation of heavy metals, ultrafiltration membranes are usually employed for treating heavy metals and synthetic organic pollutants. So far, various types of ultrafiltration membranes have been developed with mostly polymeric materials such as polysulfone (PSF), polyether sulfone (PES), cellulose acetate (CA), polyacrylonitrile (PAN), polyurethane (PU), and polyvinylidene fluoride (PVDF) [12–15]. These polymeric membranes can be fabricated using various techniques, with phase inversion and electrospinning the most common ones [16]. However, phase inversion offers great flexibility due to low-cost materials and processes that can be easily replicated in a lab without relying on costly electrospinning equipment. The morphology of membranes fabricated via phase inversion is reliant upon the physiochemical properties of casting solution and kinetics of solvent/non-solvent upon precipitation [17]. Among these, PVDF membranes are considered the preferred choice owing to their chemical resistance, structural stability, and mechanical resilience [18, 19]. However, pristine PVDF membranes exhibit poor performance for large-scale viability and are usually functionalized with various types of materials to impart fouling resistance, increase structural strength, and increase the rejection rate using metallic bonding agents [20]. In a recent study, Prasad et al. synthesized porous PVDF membranes for multifunction purposes. Their porous prepared PVDF membrane exhibited a higher removal capacity for Pb, Cd, Cu, and Zn ions than the reference non-porous sample [21]. In an interesting report by Wang et al., PVDF membranes were fabricated with the incorporation of two chelating agents, DTPA and EDTMPA, due to their high adsorption affinities for metal ions. Their prepared PDVF possesses a high Pb (II) uptake of 1.1 mmol g− 1, demonstrating excellent metal ions removal efficiency by PVDF [22]. Moreover, modified PVDF membranes have also been used for the remediation of other organic pollutants. For example, Alireza et al. synthesized PVDF membranes, which were blended with graphene oxide-polyvinyl alcohol-sodium alginate hydrogel. Their prepared modified hydrogel-based PVDF exhibited high membrane rejection rates of 92%, 95%, and 98% for methyl orange, Congo red, and bovine serum albumin, respectively [23]. The high efficiency of the PVDF membrane was ascribed to the incorporation of hydrogel, which altered the hydrophilicity, porosity, and surface roughness [23]. Nevertheless, currently investigated PVDF membranes are not viable for their commercial implementation on a large scale due to membrane fouling, low efficiency, and reduced regeneration [24]. It should be noted that the pore size of ultrafiltration membranes is usually larger than the metallic ions present in a targeted analyte. This results in a poor rejection rate of metallic ions which are being targeted [25]. To address the aforementioned issue, polymeric membranes, including PVDF, are usually incorporated with functional groups such as amine and carboxyl groups, which effectively bind with the heavy metals and thus improve the flux rate and reduce the rejection rate of metallic ions [26]. For instance, Zhao et al. blended commercially obtained PVDF membranes with 2-mercaptobenzothiazole, which has adsorption affinity for chromium (Cr) ions due to its C–H and H–C–H functional groups [27]. The prepared ultrafiltration membrane was able to exhibit a superior adsorption capacity of 241 µg/cm2 for Cr ions removal [27]. Carboxymethyl cellulose is another complexing agent that is known to significantly increase adsorptive capability for the removal of heavy metals owing to its abundant functional groups and tendency to form chemical complexes [28, 29]. Besides the fabrication and design of efficient ultrafiltration membranes for pollutant removal, commercial feasibility and operation viability are also considered important aspects. In this regard, practical suitability is important to consider as it should ideally be operated using the highest output while incurring the least costs. Response surface methodology (RSM) is an important tool in this regard by which the influence of various selected parameters can be obtained while the required output can be maximized by obtaining optimized conditions [30, 31]. RSM methods have been applied using various designs such as central composite design, Doehlert matrix, neural networks, and Box-Behnken design [13, 32–34]. However, it has been suggested that the BBD model is the most suitable for water treatment applications as it requires fewer experimental runs in comparison to other models, thus reducing time and costs [35].
Considering the preceding aspects, in this study, we report on the design and fabrication of carboxymethyl cellulose with PVDF ultrafiltration membranes for the removal of three heavy metals (Pb, Zn, and Fe), which are mainly present in wastewater bodies. Carboxymethyl cellulose was used as a complexation agent to bind the metallic ions and enhance the rejection rate for water treatment using an ultrafiltration system. The developed PVDF membrane was thoroughly characterized before and after filtration to investigate physicochemical characteristics. Considering sustainability aspects, RSM using the BBD model was applied to design, model, and optimize the influential filtration parameters such as solution pH, initial metal concentration, and CMC dosage. The reusability test was performed at optimized operational conditions to evaluate the economic potential of using PVDF integrated CMC ultrafiltration system for effective remediation of heavy metals from water.