1 Introduction
1.1 Membrane distillation (MD)
1.1.1 Background
1.1.2 Membrane properties
1.1.3 Challenges
1.2 Carbon nanomaterials
1.2.1 Structure
1.2.2 Properties
2 Applications of carbon-based nanomaterials in MD
2.1 Modelling of graphene in MD
Tab.1 A summary of the uses of carbon-based nanomaterials in MD and the associated operating conditions a) |
Base polymer | Modification | Objective | MD operating conditions | Performance | Comments | Ref. |
---|---|---|---|---|---|---|
PP | Immobilization of nanodiamonds | Enhance flux via creation of specific sorption sites | Feed solution: 3.5 g∙L-1 NaCl Feed temp: 90 °C Feed flow rate: 24 L∙min-1 Gas flow rate: 60 L∙h-1 Configuration: SGMD | Flux: 13.8 LMH Rejection:>99.9% | Increase in flux by 118% using SGMD | [83] |
PTFE | Incorporation of carboxylated CNTs into PTFE membrane | Increase hydrophobicity and enhance permeability | Feed solution 3.4 g∙L-1 NaCl Feed temp: 70 °C Permeate temp: 20 °C Feed flow rate: 212 mL∙min-1 Permeate flow rate: 164 mL∙min-1 Configuration: DCMD | Flux: 69 LMH Rejection:>99.9% | Flux enhancement (54%) | [84] |
PTFE | GO with PVDF binder drop cast onto feed side of PTFE membrane | Increase permeability by improving water vapour-membrane interactions | Feed solution 34 g∙L-1 NaCl Feed temp: 80 °C Permeate temp: 20 °C Feed flow rate: 270 mL∙min-1 Permeate flow rate: 164 mL∙min-1 Configuration: DCMD | Flux: 83 LMH Rejection:>99.9% | Flux enhancement (35%) | [85] |
PTFE | GO with PVDF drop cast onto permeate side of PTFE membrane | Increase permeability by increasing vapour removal rate on permeate side | Feed solution 34 g∙L-1 NaCl Feed temp: 80 °C Permeate temp: 18 °C Feed flow rate: 150 mL∙min-1 Permeate flow rate: 200 mL∙min-1 Configuration: DCMD | Flux: 64.5 LMH Rejection: data not shown | Flux enhancement (15%) due to reduced permeate side boundary layer and faster vapour removal. | [86] |
PVDF | Addition of GNPs to PVDF by phase inversion | Increase hydrophobicity and permeability | Feed solution: brine from RO treated coal seam gas water Feed temp: 60 °C Coolant temp: 20 °C Feed flow rate: 24 L∙h-1 Coolant flow rate: 24 L∙h-1 Configuration: AGMD | Flux: 20.5 LMH Rejection: 99.99% | Flux enhancement (72%) and improved long term performance | [75] |
PVDF | Addition of GO and APTS-functionalised GO into phase inversion dope solution | Increase permeability by improving pore structure | Feed solution: 35 g∙L-1 NaCl Feed temp: 85 °C Coolant temp: 20 °C Feed flow rate: 380 mL∙min-1 Coolant flow rate: not specified Configuration: AGMD | Flux enhancements (52 and 86%) with GO and GO-APTS addition, respectively. Mostly attributed to higher surface and bulk porosity | [77] | |
CNT bucky paper | Thin sputtered PTFE coating followed by hot pressing at 80 °C | Improve in hydrophobic character and mechanical strength | Feed solution: 35 g∙L-1 NaCl Feed temp: 95 °C Permeate temp: 5 °C Feed flow rate: 300 mL∙min-1 Permeate flow rate: not specified Configuration: DCMD | Flux: 7.5 LMH Rejection: 99.9% | Higher contact angle and 30% higher porosity than commercial PTFE membrane (Pall) but a reduction in the flux by over 2-fold owing to increased active layer thickness | [87] |
PTFE | Few-layer graphene grown on a Ni substrate by ambient atmosphere CVD from soy bean oil and wet-transferred to PTFE commercial membrane | Reduce the fouling propensity of PTFE membranes when treating surfactant-containing feed water | Feed solution: (a) 70 g∙L-1 NaCl (A); (b) A+ 1 mmol∙L-1 SDS; (c) A+ 1 g∙L-1 mineral oil+ 1 mmol∙L-1 NaHCO3 Feed temp: 60 °C Permeate temp: 20 °C Feed flow rate/(L∙h-1): (a) and (c) 30; (b) 6 Permeate flow rate/(L∙h-1): (a) and (c) 30; (b) 6 Configuration: DCMD | Flux (initial)/LMH: (a) 50; (b) ca. 47; (c) ca. 56 | Enhancement in flux and antifouling properties when tested using feeds containing surfactant and oil emulsion over 72 h | [88] |
PVDF | PVDF/PDA/GO composite coating by evaporation-assisted deposition of GO cast with a casting knife | The relative content of different oxygenic groups of GO was tuned by varying oxidation temperature (50, 60 and 70 °C) | Feed solution: 1000 mg∙L-1 NaCl solution Feed temp: 60 °C Coolant temp: 20 °C Flow rate: not specified Configuration: DCMD | Flux/LMH: 15.4 initially and improved to 17.8 Rejection: 99.9% | Stable flux over 12 h operation with maintained conductivity at ca. µS∙cm-1 while plain PVDF dropped conductivity to 20 µS∙cm-1 | [89] |
PVDF | Janus PVDF/f-MWCNTs membrane with spray-coated CNT and PVA layers | High water permeability and heat conduction of the CNT layer, as suggested by the mass-heat transfer studies. Moreover, antifouling properties of the modified membrane were noted for treating a hexadecane emulsion of 1000 mg∙L-1 | Feed solution: 5000 mg∙L-1 NaCl solution Feed temp: 55-75 °C Coolant temp: 15 °C Flow rate: 500 mL∙min-1 (feed), and 200 mL∙min-1 (permeate) Configuration: DCMD | Flux: 13.6 to 14.3 LMH Rejection: 99.9% | Improved antifouling features (when tested for 17.5 h) when treating a hexadecane emulsion | [90] |
Polysulfone | PVDF/MWCNT blended phase inversion membrane | Comparative performance evaluations of nanomaterials mixed into polysulfone. Best performance was obtained from MWCNT compared to SiO2, ZnO, and TiO2. Such behavior was attributed to higher hydrophobicity of MWCNT-based membrane | Feed solution: 2000-50000 g∙L-1 NaCl solution Feed temp: 40-60 °C Coolant temp: 20 °C Flow rate: 1-7 L∙min-1 (feed), and 5-35 L∙min-1 (permeate) Configuration: VEDCMD | Flux: 24.79 to 41.58 LMH Rejection: 99.9% | Low quantities of MWCNT improved the flux of polysulfone by 67% | [82] |
PTFE | Carbon nanotube coating immobilized on PTFE substrate | Ammonia removal by CNIMs was markedly superior to that of the original PTFE membrane, while functionalized CNIM showed the best performance in terms of flux, mass transfer coefficients and selectivity | Feed solution: 100-500 mg∙L-1 ammonia solution Feed temp: 30-50 °C Coolant temp: not specified Flow rate: 15 mL∙min-1 (feed) Configuration: DCMD | Flux: 5-30 LMH Rejection: 6%-9% | CNIMs-based membranes posed higher ammonia removal than that with the original PTFE membrane. The f-CNTs showed the highest flux, ammonia recovery and mass transfer coefficients under all operational conditions | [91] |
PVDF | Graphene-PVDF phase inversion membranes | Commercial PVDF polymer was functionalized with the aromatic rings of styrene to improve adhesion of graphene | Feed solution: 0.6 mol∙L-1 NaCl solution Feed temp: 50-70 °C Coolant temp: 20 °C Flow rate: 1 L∙min-1 (feed) Configuration: DCMD | Flux: 3-16 LMH Rejection: 99.9% | Functionalization of PVDF with styrene increased the porosity but reduced the mechanical properties compared to pristine PVDF which could be recovered after adhesion with graphene | [92] |
PAN | Vacuum-filtered GO coating with intercalated SiO2 nanoparticles | Increase the spacing between GO sheets and increase roughness for improved hydrophobicity | Feed solution: 35 g∙L-1 NaCl solution + SDS (0.4 mmol∙L-1) or humic acid (30 mg∙L-1) Feed temp: 40-60 °C Vacuum pressure: 300 Pa Flow rate: 120 L∙h-1 (feed) Configuration: VMD | Flux: 13.59 LMH (at 60 °C) Rejection: 99.99% | The intercalation of the nanoparticles increased the flux through the membrane compared to neat GO and the flux and rejection were stable with SDS and HA in the feed | [93] |
a) CVD: chemical vapour deposition; SDS: sodium dodecyle sulphate; PDA: polydopamine; PVA: poly(vinyl alcohol); VEDCMD: vacuum-enhanced direct contact membrane distillation. |
2.2 Mixed matrix membranes
2.2.1 Phase inversion
2.2.2 Electrospinning
Fig.4 (a) Surface and (b) cross-sectional scanning electron microscopy images of graphene/electrospun nanofiber membrane; (c) and (d) transmission electron microscopy images of the same membrane. The protrusions of graphene through the fibres provide nanoscale roughness which increases the membrane hydrophobicity. Reprinted with permission from ref. [102]. Copyright 2016, Elsevier. |
2.3 Coatings
2.3.1 Flux-enhancing coatings
2.3.2 Anti-fouling coatings
Fig.5 The effect of the polycrystalline graphene coating on the PTFE membrane is shown here to increase the measured temperature difference across the feed and permeate side due to the high thermal conductivity of the graphene layer. The feed used here was a 70 g∙L-1 NaCl solution at a temperature 90 °C and the permeate stream was deionised water at 20 °C. Reprinted with permission from ref. [88]. Copyright 2018, Nature. |