Distillation membrane constructed by TiO₂ nanofiber followed by fluorination for excellent water desalination performance

Highlights

• A superhydrophobic titania nanofibrous membrane for desalination was designed and prepared.

• The flux was much higher than that of ceramic membranes with particle aggregation structure.

• The prepared F-TNF membranes possess good thermal stability and chemical inertness.

Abstract

The hydrophobicity and module characteristic of membrane are key factors affecting the performance of the direct contact membrane distillation. In this paper, the superhydrophobic membrane, constrained by high porosity and large pore size, was prepared. Titania is regarded as a promising candidate material for environmental application, due to its high photocatalytic ability, relatively low cost, remarkable photostability, and toxicity. The membrane, consisted of titania nanofibers, was designed and fabricated by vacuum filtration and fluorination modification. Compared with ceramic particle aggregated membrane distillation (MD) membrane, an interconnected pore structure was constructed by entangled nanofibers to endow the prepared membrane with porosity higher than 80%. During direct contact membrane distillation process, the prepared membrane displayed an excellent desalination performance with flux of 12 LMH and salt rejection of 99.92%. Importantly, the flux was much higher than those of ceramic membranes with particle aggregation structure. Moreover, the prepared membrane possesses a good stability for long-term MD operation in pure water and even desalinating high saline water. The superhydrophobic titania nanofibrous ceramic membrane modified by fluorinated holds promise for practical applications due to its excellent performance for water desalination.

Introduction

Over the past decades, exploration of fresh water has been stimulated by growing population, increasing water demand and water pollution. Among various water treatment technologies, seawater desalination is considered as one of the potential solutions to obtain potable water resources [1], [2]. To date, three common technologies are widely used in water desalination, including reverse osmosis (RO), thermal evaporation (TE) and membrane distillation (MD) [3], [4]. In comparison with RO or TE, MD is expected to be a cost-effective technology due to its low demand on heat source and operating pressure [5], [6], [7]. Therefore, MD has drawn much attention in the past few decades [8], [9].

In the MD processes, a hydrophobic membrane required to separate saline feed and desalted condensate [10], [11], [12], [13]. Meanwhile, the low surface energy of the hydrophobic MD membrane would prevent pore wetted by water penetration. The water is evaporated across the MD membrane from the hot side to the cold side driven by the difference in vapor pressure [14], as shown in Scheme 1. To product fresh water, the process should ensure a theoretical 100% rejection of nonvolatile components, such as salts and proteins. Additionally, the membrane with good thermal stability and chemical inertness is also essential for water desalination. Compared to conventional polymeric MD membranes, such as polypropylene (PP), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), ceramic MD membranes catch increasing interest in recent years due to their physicochemical stability and anti-biocompatibility [10], [15], [16], [17], [23]. However, conventional ceramic membranes are inherently hydrophilic due to the presence of hydroxyl (OH-) groups on the surface of metal oxide, such as alumina, zirconia and silica. The membranes for MD should be hydrophobic. Therefore, they could be modified with hydrophobicity even superhydrophobicity by fluorination treatment [18], [19], [20], [21], [22]. The modified hydrophobic ceramic membrane can be successfully applied in MD processes. Additionally, the presented ceramic MD membranes were constructed with metal oxide particles, resulting in a porosity < 60% [23]. The low porosity results in a high resistance of vapor transport and low flux for MD processes. Hence, increasing the porosity of ceramic MD membrane is active to extend MD application in seawater desalination. Theoretically, ceramic membranes constructed with nanofibers are able to lead to interconnected pores, high porosity, low tortuosity and high surface area with them [24], [25], [26], [27]. Compared to the ceramic membranes consisting of particles, the nanofibrous membrane achieved higher permeation, as shown in Scheme 2. However, to our best knowledge, there is none reported about ceramic fibrous membrane with hydrophobicity that is used in MD processes.

In this work, as TiO₂ nanofiber can be easily fabricated through hydrothermal method and has been widely used in preparation of microfiltration and ultrafiltration membranes with low cost, ease of availability, high stability and non-toxicity, a superhydrophobic ceramic membrane was designed and prepared with TiO₂ nanofibers followed by fluorination modification. Its desalination performance was systematically investigated by desalinating artificial seawater with 3.5 wt.% NaCl concentration via direct contact membrane distillation. The prepared membrane properties for MD desalination, as well as gas and liquid permeation, were studied in the present work.