Reusable composite membranes for highly efficient chromium removal from real water matrixes
Graphical abstract
Introduction
Achieving a zero toxic water environment is one of the main challenges of the XXI century. Nowadays, more than 1.5 million deaths are estimated to be directly or indirectly related to health problems arising from water pollution (WHO/UNICEF, 2019; Langford, 2005; Fisher, 2018). Indeed, more than 80% of the wastewater released into the environment lacks the appropriate treatment to mitigate pollution, as is the case of Cr(VI) derived from metal plating, leather tanning, or mining (Bakshi and Panigrahi, 2018; Li et al., 2021). The environmental and health risks arising from chromium are closely related to its oxidation state. For instance, the toxicity, bio-persistence, carcinogenicity, and environmental mobility of Cr(VI) are significantly higher than the ones of Cr(III) (Speer and Wise, 2018). As a result, Cr(VI) is considered by the Environmental Protection Agencies one of the top-priority hazardous contaminants (Murad et al., 2022a).
Cr(VI) removal from water is faced from different technological perspectives, such as co-precipitation (Gopalratnam et al., 1988), reverse osmosis (Slater et al., 1983), ion exchange (Kim and Benjamin, 2004), membrane filtration (Efome et al., 2018), coagulation (Bora and Dutta, 2019), and flocculation (Sun et al., 2020). Having each technical approach its pros and cons, all of them show similar Achilles' Neels: high operating costs, pH sensitivity, and in particular, the lack of efficiency to lower Cr(VI) concentrations below the legal thresholds under certain operation conditions (Hu et al., 2004). These technical limitations are closely linked to Cr(VI) speciation as highly soluble and mobile chromate (HxCrO4)−2+x and dichromate (Cr2O7)2- oxyanions.
In this context, specific adsorption is considered one of the greener alternative to tackle Cr(VI) water pollution. Sorbents should be highly selective and efficient, easy to process, maintain and apply, do not require, or induce the addition or generation of secondary chemicals, and have a low cost (Fiyadh et al., 2019; Pavithra et al., 2019; Zhu et al., 2021). Ideally, an efficient and fast adsorption is closely linked to the density and affinity of the adsorbent's specific sites for Cr(VI) anions (Wadhawan et al., 2020). A variety of carbon-based (Hashemi and Rezania, 2019), metal oxides (Valentín-Reyes et al., 2019), clays and zeolites (Ghani et al., 2020) and, more recently, metal-organic framework (MOF) materials (Far et al., 2020; Mahmoud et al., 2022; Fang et al., 2018; Daradmare et al., 2021) have been applied to efficiently capture Cr(VI) from polluted ideal or real water matrixes (Nasrollahpour and Moradi, 2017; Wu et al., 2018a; Gao et al., 2021; Zhang et al., 2021; Zhou et al., 2021). Nevertheless, the main drawback that hinders the powdered sorbents' real applicability is their time and energy-consuming recovery (i.e. most usually by centrifugation or magnetic- based) from the water media once saturated (Wadhawan et al., 2020).
The immobilization of the active materials into polymeric substrates stands out as one of the most appealing strategies to solve this handicap. Polymeric composites bring the opportunity to merge the functions of the sorbents with the easy manipulation of filtering or membrane technologies (Choi et al., 2014). The variety of possible polymers and sorbents combinations (Gupta et al., 2021; Zhang et al., 2021), and the control over the final macro to micro pore-structure of the composite membranes, open up the perspective to obtain easily recoverable, reactivable, and function/pollutant-tailored technologies for water remediation purposes (Ng et al., 2013; Salazar et al., 2016; Martins et al., 2019a; Grandcolas and Lind, 2022; Vinothkumar et al., 2022). In this scope, poly (vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, stands out compared to other polymeric materials because of its mechanical, thermal, and chemical stability, simplicity of processability, and precise control over its porous structure when manufactured by different means. Indeed, among the scarce research on PVDF-HFP for environmental purposes, PVDF-HFP composite membranes have proven their efficiency to capture and separate both inorganic and organic pollutants (Salazar et al., 2016, 2022; Zioui et al., 2020; Martins et al., 2022).
In this work, the potential of water filtering technologies based on sorbent/PVDF-HFP composite membranes has been studied for the specific case of hexavalent chromium. It is demonstrated that PVDF-HFP based composite membranes (CM) immobilize the active sorbents homogeneously, and in parallel, the sorbent particles themselves induce a templating effect on the porous structure of the PVDF-HFP matrix as well. The adsorption capacity, efficiency, and kinetics of the non-immobilized sorbents and of the PVDF-HFP/Sorbent membranes have been investigated.
Section snippets
Composite membranes preparation
Three different composite membranes (Al(OH)3/PVDF-HFP, UiO-66-NH2/PVDF-HFP, and MIL-88-B(Fe)/PVDF-HFP) were prepared following the general guidelines provided in (Salazar et al., 2016; Aoudjit et al., 2021). In short, a 10 wt % of sorbents were dispersed under ultrasonication in DMF for 3 h until a complete dispersion of particles was obtained (Salazar et al., 2020, 2021; Martins et al., 2022). This sorbent loading is selected to achieve the best compromise between the efficiency and the
Sorbents characterization
The sorbents were selected on the basis of the simplicity of their synthesis, the characteristics of their surface chemistry and charge, and surface or inner porosity. Metal-Organic Frameworks (MOFs), metal oxides and zeolite particles were pre-selected as potential sorbents to test their affinity to capture Cr(VI) oxyanions before and after their immobilization into polymeric matrices (Renu et al., 2016). All the selected materials can be acquired commercially. However, the MOFs have been
Conclusions
The adsorption capacity of varied sorbents over hexavalent chromium (i.e. Al(OH)3, Fe3O4, NanoNaY, MIL-125, MIL-88-B(Fe), and UiO-66-NH2) was evaluated before and after their immobilization into PVDF-HFP polymeric composite membranes. These hybrid systems were fully characterized to gain a complete understanding of their porous structure from the macro to the micrometer scale.
All the studied membranes show a well-defined and interconnected micrometric porous structure and a hydrophilic nature.
Author contributions statement
Joana M. Queirós: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision, Project administration, Funding acquisition. H. Salazar: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision, Project administration, Funding
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Projects UIDB/04650/2020 and UID/QUI/50006/2019 and project PTDC/FIS-MAC/28157/2017. H. Salazar and P.M. Martins thanks the FCT for grants SFRH/BD/122373/2016 and COVID/BD/151786/2021, and contract 2020.02802.CEECIND. Financial support from the Basque Government Industry and Education Departments under the ELKARTEK program is also acknowledged. Ainara Valverde acknowledges the
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These authors equally contributed to this work.