Desalination and Water Treatment 192 (2020) 61–77 www.deswater.com July doi: 10.5004/dwt.2020.25821 Applications of nanotechnology in membrane distillation: a review study Mamdouh El Haj Assada,*, Ehab Bani-Hanib, Israa Al-Sawaftaa, Salah Issaa, Abir Hmidac, Madhu Guptad, Rahman S.M. Atiqurea, Khaoula Hidouric aSustainable and Renewable Energy Engineering Department, University of Sharjah, P.O. Box: 27272, Sharjah, UAE, emails: [email protected] (M. El Haj Assad), [email protected] (I. Al-Sawafta), [email protected] (S. Issa), [email protected] (R.S.M. Atiqure) bSchool of Engineering, Australian College of Kuwait, Mishref, Kuwait, email: [email protected] (E. Bani-Hani) cNational Engineering School of Gabès, Applied Thermodynamics Research Laboratory, University of Gabès, Omar Ibn El Khattab Street, Gabès, 6029, Tunisia, emails: [email protected] (A. Hmida), [email protected] (K. Hidouri) dSchool of M.M.H. College, Ghaziabad, U.P. Resi-# 7/48 Sector-2, Rajendra Nagar, Sahibabad, Ghaziabad – 201005, India, email: [email protected] (M. Gupta) Received 27 June 2019; Accepted 10 March 2020 abstract Membrane distillation (MD) is an effective water treatment process with relatively low cost compared to conventional membrane processes. This study investigates several factors affecting the performance of the MD membrane such as fouling, porosity, pore size, mechanical stability, contact angle, salt rejection, and other physical and thermal properties. Membrane performance can be improved if membranes are manufactured using nanotechnology. This work presents a review of the application of recently discovered nanotechnology that improves the properties and enhances the performance of membranes used in water distillation processes. The use of carbon nanotech- nology-based membranes, nanoparticles, metal, and metal oxide nanocomposite is presented and discussed. The use of nanotechnology helps in making membranes less susceptible to fouling and compaction which results in more permeate flux. This study describes the use of scanning electron microscopy as a membrane characterization method and discusses the performance of MD under different operating conditions for the fabricated membranes by using nanotechnology applications. Due to the need for continuous improvement in membrane processes for water distillation and water treatment, the optimization of membrane performance and the parameters affecting this performance should be investigated. Keywords: Membrane distillation; Nanotechnology; Permeate flux 1. Introduction (FO/RO). Membrane parameters with ranges of separation and with Membrane distillation (MD) operation principles Most industrial membrane applications are water dis- are shown in Table 1. tillation, chemical treatment, and separation processes. As well as the operating conditions of the process, a In general, membrane separation processes can be divided membrane’s physical and mechanical properties are import- with respect to the pressure gradient across the membrane ant in determining the causes of membrane compaction and into four categories: microfiltration (MF), ultrafiltration (UF), fouling. MD is widely used for water treatment [1–10] and nanofiltration (NF), and reverse osmosis/forward osmosis separation processes [11]. Membrane separation processes * Corresponding author. Presented at the Sixth International Conference on Water, Energy and Environment 2019 (ICWEE-2019), 26–28 March 2019, Sharjah, United Arab Emirates 1944-3994/1944-3986 © 2020 Desalination Publications. All rights reserved. Table 1 Membrane features with ranges of separation [15] and MD operation principles [16] 62 Types Reverse osmosis Nanofiltration Ultrafiltration Microfiltration Membrane Asymmetrical Asymmetrical Asymmetrical Asymmetrical Thickness surface 150–1 µm 150–1 µm 150–250 µm 10–150 µm film Pore size <0.002 µm <0.002 µm 0.02–0.2 µm 0.2–5 µm Rejects HMWC, LMWC, sodium, chloride, glucose, amino acids, HMWC, mono, di and Macromolecules, proteins, Particulates, clay, proteins oligosaccharides, polyvalent polysaccharides, viruses bacteria anions 192(2020)61–77 Treatment M. ElHajAssadetal./DesalinationandWater Membrane mate- CA – thin film CA – thin film Ceramic, PSU, CA, PVDF, Ceramic, PP, PSU, rial(s) thin film PVDF Membrane Tubular, spiral – wound, Tubular, spiral, wound Tubular, hollow, fiber, spiral, Tubular, hollow, fiber, module wound, plate, and frame plate, and frame Pressure 15–150 bars 5–35 bars 1–10 bar 2 bars< MD operation principles [16] MD Operation MD Operation principles [16] principles [16] (a) Initial non-equilibrium osmotic state; (a)(a) Initial Initial non- non-equilibriumequilibrium osmotic state; osmotic state; (b)forward osmosis (FO) process (no pressure is applied) (b(b))fo rwardForward osmosis osmosis (FO) (cprocess) PR(FO)O (noprocess process pressure (application (nois applied) pressure of hydrostatic is applied); pressure lower (c(c)) PR PROO process process (application (application of hydrostatic of hydrostatic pressure lowe pressurer lower than than the transmembrane osmotic pressure on the thanthe transmembranethe transmembranedraw osmoticossolutionmotic );pressure pressure onon thethe draw solution); drawsolution); (d) RO process used(d) forRO desalination process used foofr adesalinati saline onfeed of solutiona saline feed (d) RO process used fosolutionr desalinati (applicationon of a ofsaline a hy drostaticfeed pressure greater than solution(application (application of aof hydrostatic athe hy drostatictransmem pressure branepressure os mogreate ticgreater rpressure) than than the transmem- thebrane transm osmoticembrane os pressure)motic pressure) CA – cellulose acetate; PSU – polysulfone; PVDF – polyvinylidene fluoride; PP – polypropylene; HMWC – high molecular weight compounds: 100,000–1 million mol g–1; LMWC – low molecular-weight compounds: 1,000–100,000 mol g–1; macromolecules: 1 million mol g–1. M. El Haj Assad et al. / Desalination and Water Treatment 192 (2020) 61–77 63 are preferred among other technologies as they are inex- general performance in desalination. One particular pensive, easy to scale-up, and have low energy demand graphene-based nanomaterial has a unique structure and [12]. They can also be used to separate proteins, viruses, and has tunable physicochemical, biological, electrical, and gasses. Membranes play a part in the separation of organic mechanical properties that improve the performance of chemicals. Such separation is known as organic solvent desalination processes that include desalination of water nanofiltration (OSN) membrane technology [13,14]. with high salinity (Na+ and Cl–). Carbon nanotubes (CNTs) In the MD process, in order to reduce flux, the mem- have the same advantages in terms of excellent separation brane should be hydrophobic [17] to avoid plugging its while graphene-based membranes are much easier to scale pores due to wetting (pore wetting). Thus, a hydrophobic up with low-cost. membrane with high wetting resistance is required for bet- With regard to cost, producing distilled water by tra- ter MD performance. Fouling is one of the most common ditional desalination is currently more expensive. For problems which happens in MD after periodic use and example, the unit cost of RO seawater desalination is on affects membrane efficiency [18,19]. average about US$2.0 m–3 compared to US$0.83 m–3 for Current improvements to MD processes include the desalination by nano-filtration. The high cost increases creation of new construction materials with better char- again when the price of energy goes up. Additionally, the acteristics. The use of nanomaterials, either in membrane high salt concentration in seawater imposes a thermo- manufacturing or the direct use of nanoparticles in the fluid, dynamic limit of 1.1 kWh m–3, and the theoretical mini- results in better mass transfer processes in the membrane mum energy consumption at 50% recovery, significantly [20]. This review focuses on the use of different types of contributing to the overall cost of seawater desalination. nanoporous membranes for water distillation with the aim Therefore, the development of high-performance desali- of enhancing the performance of the membranes. nation membranes using nanomaterials plays a key role in improving and developing membrane-based desalina- tion technology. In fact, future membranes coupled with 2. Review of nanoparticle use in the membrane fabrication nanotechnology should have the following properties in order to be efficient for drinking water production: foul- 2.1. Importance of nanomaterial in membrane desalination ing resistance, low cost, high salt rejection, high water flux, Traditional membranes suffer from many problems and good mechanical stability [21]. Table 2 shows differ- in desalination processes such as permeability, selectiv- ent applications of nanoparticles in membrane separation ity, chemical stability, and fouling. Such problems affect processes along with their costs. Table 2 Cost of different application of nanoparticles in membrane water treatment Membrane Membrane Wastewater Used Cost process material type nanoparticles Ultrafiltration Phosphatidic acid Salt removal Zinc oxide (ZnO) Cost for this type of project can typically [22,23] run about 10%–15% of the cost of the entire Ultrafiltration Polysulfone Bacterial removal Zinc oxide (ZnO) project [25] from aqueous solutions [24] Microfiltration Polyvinylidene Heavy metal ions Zinc oxide (ZnO) Typical installation costs for microfiltration fluoride removal from with