Photocatalytic reduction of hexavalent chromium ions from aqueous solutions using polymeric microfibers surface modified with ZnO nanoparticles

MORALES, GRACIELA; CASTRO RUÍZ, ANDRÉS; RODRÍGUEZ TOBÍAS, HERIBERTO; ABRAHAM, GUSTAVO A.; RIVERO GUADALUPE; LOZANO-MORALES, ALEJANDRO

FIBERS AND POLYMERS

KOREAN FIBER SOC

Año: 2021 vol. 22 p. 3271 - 3280

1229-9197

The deterioration of the environment has become a priority issue for humanity, since it alters the balance of ecosystems and well-being in general[1]. In this context, water pollution is a strongly relevant issue that directly affects humans, flora and fauna of our planet. Particularly, the consumption of contaminated water can cause severe health problems, either through contact with the skin or through its direct intake[2]. Many organic and/or inorganic water pollutants are originated from various human activities[3]; although some are also naturally located in certain geographical areas.Specifically, heavy metals are among the main inorganic contaminants of water and they represent serious threats to human health due to their eventual accumulation in tissues, thus causing harmful physiological effects; even at very low concentrations[4]. Specifically, concentrations greater than 0.1 mg L-1 of chromium VI (Cr VI) have been closely related to diseases such as rhinitis, tuberculosis, diarrhea, bronchitis, hyperglycemia, dermatitis and also as a potentially carcinogenic agent[5?8]. In addition, the International Agency for Research on Cancer (USEPA) and the World Health Organization (WHO) have established that the allowable limit for Cr VI in drinking water is 50 μg L-1 [6?8].For these reasons, technological innovation in terms of processes for water treatment is crucial. Currently, outstanding research has been developed regarding the use of new methodologies for water treatment; among them, the technology of membranes made of micrometer polymer fibers, make up a new generation of devices that could offer significant advances in the techniques and methodologies involved in water treatment, since their preparation involves processes with lower energy consumption and cost compared to conventional procedures, including commercial membranes[9?11].In this sense, membranes based on electrospun polymeric fibers have been extensively investigated due to the simplicity of the obtaining process, its high efficiency and performance12. The main advantages of this type of membranes are their high porosity and high surface-to-volume ratio compared to their conventional counterparts. Electrospun membranes have been widely used for air filtration, but there are still many challenges for their applications in water treatment[13]. Therefore, in order to improve their performance, including permeability and selectivity, it is necessary a compositional and structural optimization. In this regard, some polymers have excellent thermal and mechanical stability that make them ideal materials for the preparation of composite membranes. Among them, poly(acrylonitrile-butadiene-styrene) (ABS) is a copolymer with great demand in the area of materials science due to its relevant properties that enhance its application as membranes for water treatment; however, to date it has been used to a limited extent in the preparation of fibers for use in membranes[14, 15]. Indeed, the application of hydrophobic polymers is frequently restricted due to low water penetration and high fouling[13]. For this reason, modification with hydrophilic polymers allowed the manufacture of composite membranes with greater separation capacity. However, in some cases thermal resistance, mechanical performance and/or chemical stability may be compromised. In general, dense membranes generally have low fluidity but high selectivity, while porous membranes have low selectivity but high permeability[16].On the other hand, some membranes based on polyacrylonitrile (PAN) have been recently implemented; though the resulting prototypes exhibited certain disadvantages such as dissolution in water and even degradation via hydrolysis[17, 18].Additionally, several reported investigations have taken another approach in the development of membranes for the reduction of heavy metals present in water. The incorporation of inorganic nanoparticles in polymeric matrices lead to the formation of composite membranes with improved mechanical and physicochemical properties. In this context, some investigations have been reported the addition of nanomaterials such as: carbon nanotubes[19], clay[20], silver[21], aluminum oxide (Al2O3), zirconium dioxide (ZrO2)[22], iron oxide (III) (Fe3O4)[23], titanium dioxide (TiO2)[24]and zinc oxide (ZnO)[25, 26], to different matrices.In this regard, PAN-based membranes combined with ZnO, obtained by electrospinning for photoreduction and metal removal were recently reported[25, 27]. The fabrication of polymer microfibers with ZnO nanoparticles (NpZnO) has been widely explored, mainly due to the fact that their surface chemical properties favor heavy metal photoreduction, and the synthesis and morphological control are relatively easy and implies low cost. Furthermore, ZnO has excellent antibacterial and photocatalytic activity and also favors the adsorption of different organic compounds, metals or ions; among other properties that are improved when the materials are arranged on a nanometric scale[28]. In this concern, the photocatalytic reduction of Cr VI to Cr III in aqueous solutions under UV irradiation and in the presence of ZnO as a photocatalyst has already been previously reported using potassium dichromate (K2Cr2O7) as the Cr VI substrate[29].Even though heterogeneous photocatalysis is considered an effective and low-cost technique; the recovery of the photocatalysts of the treated solution remains a challenge and limits the applications of this technology. To overcome this inconvenience, it has been proposed as an alternative, the fixation of the photocatalysts in appropriate supports, avoiding the material recovery stage. It is important to mention that in order for ZnO nanostructures to act as metal removers, they must preferably be placed on the surface of polymer fibers, since it allows a better use of the surface area prone to interact with metal ions[30].Several techniques such as dry spinning[31], template synthesis, phase separation, self-assembly and electrospinning, among others, have been used to obtain submicron fibers. However, the coaxial electrospinning process could be considered as the most viable method for the serial production of micrometer fibers of various polymers[12], where the possibility of locating nanostructures on the surface of the polymer fibers could improve the performance of these membranes.In this research work the coaxial electrospinning technique was used for the development of composite membranes based on ABS/PAN micrometer fibers loaded with ZnO nanoparticles (NpZnO) in order to evaluate their efficiency in the photoreduction of Cr VI ions in aqueous solutions.