Physical and Chemical Approaches for Water Micro-Droplet Capture

RÉSUMÉ: La pénurie d'eau est devenue un enjeu pressant dans le monde d'aujourd'hui. Sur la planète, 1,2 milliard de personnes n'ont pas accès à de l'eau propre et 2,7 milliards vivent en pénurie au moins un mois par an. Bien que cette ressource couvre 71 % de la surface de la Terre, seulement 2,5 % de celle-ci est accessible par des sources conventionnelles telles que les lacs et les eaux souterraines. Celles-ci se remplissent naturellement grâce au cycle de l'eau. Les sécheresses sont devenues plus fréquentes ces dernières années et ne font que s'aggraver avec les changements anthropiques apportés au climat. De plus, la population mondiale ne cesse de croître alors que la quantité d’eau renouvelable est en décroissance. Cette situation se traduit par une augmentation de la demande et une diminution de l’offre. Cela est dû en grande partie à l'industrialisation rapide depuis les années 1960, qui a entraîné une forte utilisation des ressources en eau. Pour répondre à cette demande, des processus de production d'eau douce, tel que l'osmose inverse sont des solutions plus en plus intéressantes. L’osmose inverse est devenue plus rentable au cours de la dernière décennie avec un coût d'exploitation de 0,50 $US par m3 d'eau. Toutefois, ce procédé nécessite la disponibilité de grandes quantités d'eau salée et d'énergie électrique pour fonctionner. Pour les régions intérieures ou non côtières, ce coût peut être prohibitif en raison de l'importance des investissements et des coûts d'opération. En revanche, l'atmosphère contient 13 000 km3 d'eau douce sous forme de vapeur, de gouttelettes liquides et sous forme solide (neige ou glace). Bien qu'elle ne soit pas facilement disponible, cette eau peut être extraite et purifiée en eau potable. Ainsi, la récolte du brouillard, un processus par lequel des microgouttelettes d’eau sont capturées par une surface et traitées pour une consommation ultérieure, est apparue comme une méthode alternative pour la production d'eau potable. Cette méthode consiste généralement à utiliser un matériau polymérique maillé permettant de capturer le brouillard à son contact. À l'heure actuelle, les principaux inconvénients dans ce domaine sont la dégradabilité des matériaux utilisés, due à l'exposition à des vents violents ou à des débris transportés dans l'air par exemple, et la faible efficacité de la collecte due à une mauvaise conception de la surface.----------ABSTRACT: Water scarcity has become a pressing issue in today’s world. On the planet, 1.2 billion people do not have access to clean freshwater and 2.7 billion experience this scarcity a minimum of one month out of the year. Although this resource covers 71% of the earth, a mere 2.5% of it is accessible through conventional sources such as lakes and groundwater that are naturally replenished through the water cycle. Droughts have become more common in the past years and is only getting worse with anthropogenic changes brought to the climate. Moreover, the world’s population continues to grow as the amount of renewable water decreases. This results in an increase in demand and a decrease in supply. This trend is largely due to the rapid industrialization as of the 1960s, forcing heavy usage of water resources. To fulfill this demand, processes for intensive production of freshwater such as reverse osmosis (RO) have become more and more interesting. However, this process requires the availability of large bodies of saline water as feedstock and high amounts of electrical power. For inland or noncoastal regions, this can be cost prohibitive due to high capital investment and operational costs. On the other hand, Earth’s atmosphere holds 13 000 km3 of freshwater in the atmosphere present in vapor, liquid droplets and solid (snow or ice) form. Although not readily available, this water can be extracted and purified into potable water. Thus, fog harvesting, a process by which water micro-droplets are captured by a surface and treated for further consumption, has emerged as an alternative method for freshwater generation. This is typically done by employing a mesh-like polymeric material through where fog is capture upon contact. At this time, major drawbacks in this field are the durability of widespread materials used for fog harvesting, stemming from exposure to high wind speeds or debris carried in the air for instance, and the low collection efficiency due to poor surface design

Continuous reactive-roll-to-roll growth of carbon nanotubes for fog water harvesting applications

ABSTRACT: A simple method is presented for the continuous generation of carbon nanotube forests stably anchored on stainless-steel surfaces using a reactive-roll-to-roll (RR2R) configuration. No addition of catalyst nanoparticles is required for the CNT-forest generation; the stainless-steel substrate itself is tuned to generate the catalytic growth sites. The process enables very large surfaces covered with CNT forests to have individual CNT roots anchored to the metallic ground through primary bonds. Fog water harvesting is demonstrated and tested as one potential application using long CNT-covered wires. The RR2R is performed in the gas phase; no solution processing of CNT suspensions is used, contrary to usual R2R CNT-based technologies. Full or partial CNT-forest coverage provides tuning of the ratio and shape of hydrophobic and hydrophilic zones on the surface. This enables the optimization of fog water harvesters for droplet capture through the hydrophobic CNT forest and water removal from the hydrophilic SS surface. Water recovery tests using small harp-type harvesters with CNT-forest generate water capture of up to 2.2 g/cm2·h under ultrasound-generated fog flow. The strong CNT root anchoring on the stainless-steel surfaces provides opportunities for (i) robustness and easy transport of the composite structure and (ii) chemical functionalization and/or nanoparticle decoration of the structures, and it opens the road for a series of applications on large-scale surfaces, including fog harvesting

Nanoporous sponges as carbon-based sorbents for atmospheric water generation

Water scarcity threatens more and more people in the world. Moisture adsorption from the atmosphere represents a promising avenue to provide fresh water. Nanoporous sponges (“NPSs” ), new carbon-based sorbents synthesized from the pyrolysis of resorcinol-formaldehyde resin, can achieve comparable performance to metal organic framework-based systems, but at a significantly lower cost. Oxygen and nitrogen functionalities can be added to the NPS surface, through oxidation and addition of phenanthroline to the initial reagent mixture, respectively. The resulting NPS sorbents have high specific surface areas of 347 to 527 m2·g–1 and an average capillary-condensation-compatible pore size of 1.5 nm. When oxidized, the NPS can capture up to 0.28 g of water per gram of adsorbent at a relative pressure of 0.90 (0.14 g·g–1 at P/Psat = 0.40) and maintain this adsorption capacity over multiple adsorption/desorption cycles. Scaled-up synthesis of the NPS was performed and tested in an experimental water capture setup, showing good agreement between small- and larger-scale adsorption properties. Water adsorption isotherms fitted with the theoretical model proposed by Do and Do demonstrate that hydroxyl functionalities are of key importance to NPS behavior

Suppression of Hydrophobic Recovery in Photo-Initiated Chemical Vapor Deposition

Photo-initiated chemical vapor deposition (PICVD) functionalizes carbon nanotube (CNT)-enhanced porous substrates with a highly polar polymeric nanometric film, rendering them super-hydrophilic. Despite its ability to generate fully wettable surfaces at low temperatures and atmospheric pressure, PICVD coatings normally undergo hydrophobic recovery. This is a process by which a percentage of oxygenated functional group diffuse/re-arrange from the top layer of the deposited film towards the bulk of the substrate, taking the induced hydrophilic property of the material with them. Thus, hydrophilicity decreases over time. To address this, a vertical chemical gradient (VCG) can be deposited onto the CNT-substrate. The VCG consists of a first, thicker highly cross-linked layer followed by a second, thinner highly functionalized layer. In this article, we show, through water contact angle and XPS measurements, that the increased cross-linking density of the first layer can reduce the mobility of polar functional groups, forcing them to remain at the topmost layer of the PICVD coating and to suppress hydrophobic recovery. We show that employing a bi-layer VCG suppresses hydrophobic recovery for five days and reduces its effect afterwards (contact angle stabilizes to 42 ± 1° instead of 125 ± 3°)