Upgrading of zirconia membrane performance in removal of hazardous VOCs from water by surface functionalization
Introduction
The continuous increase of the human population on the earth which is accompanied by their basic and industrial activities has put a strain on the present sources of freshwater [1]. In a technically and economically feasible processes, water can be purified by the implementation of a variety of membrane separation processes including membrane distillation (MD) [1], [2], [3].
MD is an example of a non-isothermal process, demonstrates potential applications in various niche areas of scientific and industrial interest, especially, for the production of high purity permeate, recovery of valuable minerals, and separation of contaminants from liquid solutions [4], [5]. Moreover, MD can be utilized to produce ultrapure water even from aqueous solutions with high salinity [6], [7]. Considering all MD processes, vacuum MD (VMD) can be used for the removal of volatile organic compounds (VOCs) from industrial wastewater [8]. Furthermore, in the case of utilization membranes with high affinity to VOCs, the MD is an efficient and relatively low-cost process to remove VOCs. The mentioned feature gives MD the ability to apply low-grade waste heat to drive the process and make the it a promising green technology towards integrated zero liquid discharge purification processes [9]. Since the vapour pressure is not highly reliant on the salt concentration, MD can be coupled with reverse osmosis (RO) for the treatment of highly saline water [10] and upgrade the efficiency of the entire process.
VOCs due to their negative impact on the health and environment need to be removed from water. Beside their direct effect on the respiratory system, eyes, and skin, the presence of VOCs in water, even at low concentration, can introduce cytotoxicity, carcinogenicity, and autoimmunity [11]. Methyl tert butyl ether (MTBE), ethyl acetate (EtAc), and butanol (BuOH), are among the VOCs utilized on an industrial scale. Hazardous classification according to REACH for the above mentioned VOCs follows: H225 – highly flammable liquid and vapour; H226 – flammable liquid and vapour; H302 – harmful if swallowed; H315 – causes skin irritation; H318 – causes serious eye damage; H319 – causes serious eye irritation; H335 – may cause respiratory irritation, and H336 may cause drowsiness or dizziness [11]. There are number of separation techniques available and suitable to remove VOCs e.g. photocatalytic [12], adsorption [13], and catalytic combustion [14]. However, the reason to implement VMD instead of other techniques, presented above is high effectives of VOCs removal by membrane distillation. It needs to be highlighted that VOCs are good wetting agents for hydrophobic materials, and an application of conventional hydrophobic membranes (e.g. PTFE – polytetrafluoroethylene, PP -polypropylene) for MD is challenging. For this reason, a stable and highly efficient membrane material needs to be implemented. The separation materials dedicated to membrane distillation have to meet the following requirements: hydrophobic character, porous structure and lack of wetting during the separation process. To meet these necessities very often the modification of the materials is additionally introduced. A number of separation materials have been used for membrane distillation operation including polymeric and ceramic ones [7]. The first type due to the possible natural hydrophobicity is more popular. The other one, ceramic membranes, are generally more robust owing to their intrinsic properties, long-life, and high mechanical strength. Furthermore, ceramic membranes are less subject to fouling in comparison to polymeric membranes [15]. Unfortunately, ceramic membranes cannot be used for the MD process directly on account of their natural hydrophilic character [8], [16]. Ceramic membranes must be hydrophobized prior to the application. This feature of the ceramic is actually beneficial due to the fact that properties of the material can be tune depend on the final application of the membranes. The membranes can be rendered hydrophobic by selective surface modification, such as the grafting of alkylsilanes and perfluoroalkylsilanes agents [17], [18]. Furthermore, functionalization can be done in a way that membrane will be not wetted by the solvents, including highly concentrated VOCs in water. This is a predominance of ceramic membranes implemented in VOCs removal by MD in comparison with polymeric materials.
The most common ceramic materials used to date are titania, alumina, and silica substrate [2], [6], [18]. Titania and alumina are broadly used in the preparation of membranes with different geometry (planar, tubular [8], [19], or hollow fibres [7]) for MD application [6], [9], [20], [21], [22], [23]. On the other hand, zirconia shows interesting properties since it can improve material strength [24], possesses enhanced biocompatibility, and can be utilized in clinical applications [25]. Recently, more attention has been paid to the utilization of ZrO2 in the MD owing to their lower thermal conductivity (2.1 Wm−1K−1) comparing to alumina (30 Wm−1K−1) and titania (11.8 Wm−1K−1) [26]. MD requires membrane materials with low thermal conductivity to prevent heat loss through the membrane. To date, such hydrophobized zirconia membranes have found very promising application in MD based desalination processes [27]. Liu et al. [28] presented hierarchically-structured ZrO2 ceramic membranes prepared on the yttria-stabilized zirconia support. Significant enhancement of transport properties (high flux −28.7 kg m−2h−1 and NaCl rejection coefficient >99%) in MD was observed, which was related to the hierarchically organized mesopore structure. The ZrO2 ceramic with the α-Al2O3 membrane was used by Wu at al. [29] to generate UiO-66-NH2 membranes with high performance for the deep desulfurization of model gasoline via the pervaporation process. Zhang et al. [30] compare two types of polymeric membrane modules in the removal of VOCs from wastewater implementing VMD. The synthetic feed contained 1 wt% sodium chloride and 2.2 wt% VOCs (dimethoxymethane, formaldehyde, and methanol at a mass ratio of 10:2:1) based on an industrial wastewater composition. Authors observed wetting of the membrane and suggested an additional modification of the membrane to avoid the tat problem.
Taking into account advantages of zirconia material (i.e. thermal conductivity, high mechanical stability and biocompatibility as well as the cost of the raw material) and their potential in the VOCs removal by MD it was possible to define the research gaps. Namely, there are lack of references focusing on the utilization of ZrO2 based membranes for VOCs removal. Bearing in mind the potential applicability of these membranes in water treatment, the environmentally friendly separation materials are desired. To meet the mentioned expectations, the authors proposed the utilization of zirconia ceramic membranes functionalized by non-fluorinated grafting agents.
The aim of the presented work is the base research focused on the grafting of zirconia ceramic membranes and assessment of their efficacy in a VMD process applied for the removal of VOCs. The membranes were furnished with alkyl-chains possessing varied degrees of fluorination and anchoring roots, i.e. chlorine, ethoxy and methoxy groups. An important part was to correlate and to discuss the wettability and roughness of the functionalized ceramic materials. The new materials were characterized, and the separation efficiency was correlated with the surface energy and wettability of the materials. This strategy opens up new avenues for fabrication ceramic, highly stable and resistant materials for water purification from hazardous volatile organic compounds address to niche and advanced application.
Section snippets
Materials
Tubular (single-channel) ceramic ZrO2 membranes with a molecular-weight-cut-off (MWCO) equal to 5kD (∼3 nm pore size) and 300kD (∼200 nm pore size) were purchased from TAMI (France). For material characterization, planar zirconia membranes with 3 and 200 nm pore size were utilized. 1H,1H,2H,2H-perfluorooctyltriethoxysilane (CAS 51851-37-7) marked hereafter as FC6; 1H,1H,2H,2H-perfluorodecyltriethoxysilane (CAS 101947-16-4) marked hereafter as FC8; 1H,1H,2H,2H-perfluorododecyltriethoxysilane
Pristine ceramic membranes
Pristine zirconia membranes have been systematically examined by the set of analytical methods. Goniometric measurements were done with planar samples. In the description of the materials designed for the membrane distillation, the crucial is a wettability of the material. Therefore, a detailed evaluation of water behaviour for pristine and hydrophobized samples was performed. Based on the Eq. (7) and dynamic contact angle measurements, water penetration profile into the ZrO2 ceramic structure
Conclusions
Highly efficient ceramic separation materials for removal of hazardous VOCs (ethyl acetate, methyl-tert-butyl ether, and butanol) from water were generated and comprehensively characterized. Zirconia ceramic membranes with various pore sizes (3 and 200 nm) were successfully modified employing a number of reagents including perfluoroalkylsilanes and non-fluorinated molecules. The modification turned the hydrophilic character of the material into a hydrophobic one making the membranes applicable
Future perspective
The presented results possess an important meaning in design, formation materials with tunable surface chemistry, as well as nano- or micro-architecture. Furthermore, the established data are valuable from the application point of view, e.g., lab on a chip, the fabrication of self-cleaning, and self-healing materials as well as microarrays and microsystems.
Declaration of Competing Interest
There are no conflicts of interest to declare
Acknowledgments
Special thanks are due to nLab Ltd. company (Warsaw. Poland) for the access to the goniometric equipment. Dr. Ludovic Dumée acknowledges the Australian Research Council for his ARC Discovery Early Career Research Award (DECRA) fellowship 2018 (DE180100130).
Funding: This work was supported by the 2017/26/D/ST4/00752 (Sonata 13) grant from the National Science Centre Poland. The research was supported partly by the statutory funds of Nicolaus Copernicus University in Toruń (Faculty of Chemistry.
References (63)
- et al.
Green silica-based ceramic hollow fiber membrane for seawater desalination via direct contact membrane distillation
Sep. Purif. Technol.
(2018) - et al.
Flat ceramic microfiltration membrane based on natural clay and Moroccan phosphate for desalination and industrial wastewater treatment
Desalination
(2018) - et al.
Air-gap membrane distillation as a one-step process for textile wastewater treatment
Chem. Eng. J.
(2019) - et al.
Membrane distillation pilot plant trials with pharmaceutical residues and energy demand analysis
Chem. Eng. J.
(2016) - et al.
Effect of hydrophobic surface modifying macromolecules on differently produced PVDF membranes for direct contact membrane distillation
Chem. Eng. J.
(2014) - et al.
Distillation membrane constructed by TiO2 nanofiber followed by fluorination for excellent water desalination performance
Desalination
(2017) - et al.
Morphological design of alumina hollow fiber membranes for desalination by air gap membrane distillation
Desalination
(2017) - et al.
Inorganic fouling mitigation by salinity cycling in batch reverse osmosis
Water Res.
(2018) - et al.
Efficient adsorption and sustainable degradation of gaseous acetaldehyde and o-xylene using rGO-TiO2photocatalyst
Chem. Eng. J.
(2018) - et al.
Adsorption of volatile organic compounds over MIL-125-NH2
Polyhedron
(2018)