Elsevier

Chemical Engineering Journal

Volume 374, 15 October 2019, Pages 155-169
Chemical Engineering Journal

Upgrading of zirconia membrane performance in removal of hazardous VOCs from water by surface functionalization

https://doi.org/10.1016/j.cej.2019.05.160Get rights and content

Highlights

  • Highly efficient zirconia membranes for hazardous VOCs removal process were generated.

  • Improvement of transport properties after modification with fluorine-free molecules.

  • Extensive material and physiochemical studies of functionalizes zirconia membranes.

  • Correlation between physicochemistry and wetting properties of the modified ceramics.

Abstract

To upgrade the membrane separation process, there is a tremendous need to understand the process from the point of view of the membrane materials, e.g., their chemistry and material features. Functionalized by various alkyl- and fluoroalkylsilanes, zirconia membranes (pore size 3 and 200 nm) were systematically investigated from the material, physiochemical, and tribological points of view. The work shows the successful utilization of the modified ceramic membranes to niche application of water purification. Stable organic-inorganic membranes were applied in vacuum membrane distillation (VMD) for removal of hazardous volatile organic compounds (VOCs) butanol, methyl-tert-butyl ether, and ethyl acetate. Wettability, surface roughness, and adhesion were investigated, taking into account the type of modifiers (fluorine-free) and molecules with a different degree of fluorination. The impact of the type of reactive group in non-fluorinated modifiers on the material features and the separation effectiveness were discussed.

Furthermore, mechanical and chemical stabilities were studied. The most significant was that utilization of non-fluorinated modifier makes it possible to generate stable hydrophobic membranes, with high reactivity during grafting, suitable for VMD. These membranes possess better transport properties and high mechanical stability in comparison with fluorinated ones.

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.

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