Water Purification and treatment using electrospun fibers

Water purification requires several sequential steps to make collected water drinkable. These steps include removal of particulate matters, pathogens and selected ions. With increasing pollution instances of pollution both above and underground, the need for technological advances to treat water becomes even more pressing. Researchers are investigating the use of electrospun membrane in the different stages of water purification and treatment. This starts from particulate filtration down to removal of selective contaminants such as arsenic and other heavy metals.

Electrospun fibers membrane typically has pore size of a few microns. This makes it suitable for use as a size exclusion filter for particles that are more than a few microns [Gopal et al 2006, Gopal et al 2007]. A comprehensive study on the microparticles rejection by Gopal et al (2007) using heat treated polysulfone (PSU) electrospun nonwoven membrane with bubble point of 4.6 µm showed that full flux recovery was possible for particle size 7, 8 and 10 µm with more than 99% separation factor. Full flux recovery was also reported for 5 µm size particles with separation factor greater than 90% using heat treated polyvinylidene fluoride (PVDF) membrane [Gopal et al 2006]. However, flux recovery was incomplete for particle sizes less than 3 µm. Examination of the cross section of the membrane showed that for larger particles (more than 5 µm), the membrane act as a size exclusion filter or screen filter with the particles trapped on the surface of the membrane [Gopal et al 2007]. Low adhesion force between the particles and the fiber surface meant that the flux can be recovered by stirring in the membrane module [Gopal et al 2006, Gopal et al 2007].

In many developing countries, rapid industrialization and mining operations have resulted in water sources polluted with heavy metals. Such heavy metal ions cannot be filtered out by microfiltration as the ions are too small for that. Instead of size exclusion, surface adsorption-based electrospun membrane has been developed. High surface area of electrospun nanofibers makes this technique suitable as more functional groups can be exposed on its surface. Open and interconnected pores formed by overlapping nanofibers allow the feed solution to pass through it.

Arsenic (As), a known cancer causing agent, are increasingly contaminating ground-water. Min et al (2017) fabricated iron functionalized chitosan elctrospun nanofiber (ICS-ENF) for the purpose of removing trace amount of arsenic from contaminated water. ICS-ENF was found to remove more than 90% of As under acidic to neutral condition (pH 4.3 to 7.3) which is better than purely chitosan (CS) electrospun membrane which shows little adsorption of As at neutral pH. When the pH is increased to 7.3, there is no adsorption of As even for ICS-ENF membrane. Adsorption by the nanofiber membrane is dependent on positive charge on its surface which interacts with negatively charged As. At high pH, the surface charge of the electrospun membrane may turn negative and this will repel As ions which lead to their drop in adsorption capacity.

In countries where there is limited available fresh water from the land, alternative source of water may come from the sea or from recycling waste water. These require technology that is able to separate salt and ions from the water source. While there are common industrial purification technology such as reverse osmosis, other emerging technologies such as membrane distillation and ultra/nano-filtration are been tested with the hope of bringing down costs.

For ultrafiltration (0.1 to 0.01 µm) and nanofiltration (0.01 to 0.001 µm), the pore size of the electrospun membrane is too large to use without any modifications. Instead, the electrospun membrane now functions as a supporting substrate to hold the separation layer as a thin film composite. Whether the composite membrane is used for ultrafiltration or nanofiltration is dependent on the coated separation layer. High porosity and small fiber diameter of electrospun membrane makes it an excellent supporting substrate as it gives a larger effective separation area (separation surface without underlying obstruction). Nano and ultrafiltration membrane using electrospun membrane will theoretically give it a higher flux compared to other conventional membranes. In a demonstration of electrospun thin film composite in salt water purification, Yalcinkaya et al (2016) used electrospun polyamide 6 nanofibers on polypropylene/polyethylene bicomponent spunbond nonwoven fabric as supporting substrate for piperazine or m-phenylenediamine (MPD) which function as the separation layer. An average rejection rate of 97.4% CaCl₂ and 96.3% NaCl was recorded for thin film nanofiber composite (TFNC) of MPD-Triethylamine(TEA)-Synferol AH(Sy-AH). Pure water flux of the TFNC was 22.5 L/m₂/h while permeate flux was 12.5 L/m₂/h. Using seawater, the TFNC membrane was able to retain more than 98% of the salt ions after 3 rounds of recirculation through the membrane.

Membrane distillation works by having a membrane to separate the salty water and pure water (permeate). There are a few parameters that affect the rate and efficiency of membrane distillation. First, there must be a temperature gradient between the feed side and permeate. Traditionally, the separation layer is relatively thick so that there will be less heat from the feed to permeate, thus maintaining an optimal temperature gradient. The porosity, pore size and tortuosity also affects the ease of water vapour passing through the membrane and to form permeate.

For an electrospun membrane to be used in membrane distillation, it must be able to maintain a separation between the feed and permeate. Thus, a superhydrophobic membrane is preferred as it is better able to maintain water separation. Comparative tests on electrospun nanofibrous membrane and commercially available membrane showed that the liquid entry pressure (LEP) of electrospun PVDF membrane is less than 0.64 bar compared to 9 bar from PTFE membrane [Jiricek et al 2016]. Electrospun PVDF membrane showed greater flux at higher crossflow velocity (80 mm/s) compared PTFE membrane and other commercial membranes. At lower water salinity up to 50 g/kg, thinner membrane has greater permeability and this is where electrospun membrane has an advantage. However, at higher salinity, flux for nanofibrous membrane drops quickly while PTFE membrane showed little changes [Jiricek et al 2016]. To increase the hydrophobicity of the membrane and maintain water separation for longer duration, Liao et al (2013) modified the surface of PVDF nanofibers with a coating of silver nanoparticles through electroless silver-plating followed by a coating of 1-dodecanethiol. The resultant membrane showed superhydrophobicity characteristic with contact angle of 153° and water sliding angle below 10°. The flux of the membrane is relatively unchanged from unmodified PVDF nanofibrous membrane at 32 L/m²/h for the test duration of 8 h. Instead of surface treating nanofibers with a superhydrophobic treatment, Zhou et al (2014) used a commonly known superhydrophobic material polytetrafluoroethylene (PTFE), for fabrication into nanofiber membrane. Since PTFE is resistance to most solvents, they used a suspension of polytetrafluoroethylene (PTFE) fine particles in water for blending with water soluble polyvinyl alcohol (PVA). The PVA with PTFE particles were electrospun and the PVA component removed through sintering up to 380°C for 30 minutes. At this temperature the PTFE particles melted and fused together to form an interconnected nanofibrous network of PTFE. The resultant membrane showed a water contact angle of 156.7°. When tested for vacuum membrane distillation, a pure water flux of 15.8kg/m²h and a stable salt rejection of more than 98% for ten hours were recorded.

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