Abstract

Despite the demand for clean water, it is universally deficient. Although methods to harvest drinkable water exist, they are often expensive, inefficient, or highly dependent on the availability of water sources in the area, limiting their effective uses. Recently, there has been renewed interest in the development of a versatile clean water generation processes from atmospheric moisture. The key to achieving high efficiency in such moisture harvesting systems is the absorbing agent that absorbs moisture. Several materials are currently under research, including but not limited to metal-organic frameworks (MOF) and hygroscopic salts. However, most of these approaches would likely be challenging to be scaled up from technical and economic perspectives.

This project aims to develop a simple, cost-effective, environmentally friendly, and highly effective moisture absorber. Calcium chloride salt was chosen as the main salt of interest due to its deliquescent properties; however, it is known to suffer from agglomeration upon repeated absorption-desorption cycles which decreases its efficiency. To overcome this problem, we have discovered that a simple infusion of the salt into a cellulose sponge significantly reduces the agglomeration problem of the salt while also improving its maximum water uptake by ~30 % at 25°C and 80% relative humidity (RH) compared to a sample without the cellulose sponge. The absorbed moisture is released at 60°C when dried for 12 hours, and even after three absorption-desorption trials, it did not show signs of degradation in its absorption capabilities, validating its reusability. To elucidate the science behind this synergistic interaction, time-dependent water uptake measurements at controlled conditions were carried out using a microbalance in an environmental chamber. Then the data was analyzed using a double exponential equation to extract the fitting constants and propose a physical model of the moisture absorption mechanism in the salt/sponge system.

Data Analysis

First the sponge sample exhibits higher water uptake compared to that of the salt control sample. Secondly, sponge sample absorbs moisture more rapidly. It was found that the curves do not fit well with a single exponential equation but rather it fits well with a double exponential equation ( Equation 1 ) [ 1 ] . The equation is a simplified version to the Langmuir - type diffusion model which is also described as a double exponential equation . To interpret the meaning behind the fitting parameters A₁ , A2 , T₁ , and T2 , the equation gives at time t = 0 can be considered . When , t = 0 : Wsatuarated = A₁ + A₂ . Hence , A₁ and A₂ can be thought of as the capacity of two separate reservoirs for absorbing the moisture . T₁ and ₂ are time constants that relate to A , and A₂ respectively . It is interesting to note that r , and r₂ have different values . This suggests that there are two separate moisture absorption mechanisms that are taking place . Furthermore , T₂ tends to show lower value than T₁ , indicating that the mechanism that is associated with A₂ and T₂ is that of a faster process than the mechanism associated with A₁ and T₁ The data obtained and the analysis mentioned above make it possible to describe the two mechanisms by a simple model illustrated in figure 11. Here there are two separate mechanisms . First ( figure 11a ) , water absorption at the surface of the salt particle , which is in direct contact with the ambient moisture and is the primary process taking place first . Then a second mechanism ( figure 11b ) takes place where water at the surface diffuses into the inner core of the salt due to a moisture concentration gradient . In this model , the moisture absorption at the salt particle's surface would be a much faster process ( like T₂ ) than that of the water diffusion into the core of the salt ( like T₁ ) . Thus , here A , and r , can be referred to the first stage of the process , the moisture absorption at the surface of the salt particle . Whereas A , and T , refers to the second stage of the process , the water diffusion from the outer surface into the inner core of the salt particle . Figure 11 - First mechanism ( a ) and second mechanism ( b ) of moisture A , can be defined as the capacity to hold water in the first stage of the absorption process , which would be proportional to the total effective surface area of the salt particle . Whereas A , would correlate to the total absorption capacity of the inner core of the salt particle . Likewise , r , can be defined as the rate of completing the A , process and r₂ can be defined as the rate of completing the A , process . absorption .

Interpretation

This phenomenon is best explained using the Kelvin equation equation 2 ) . P is the vapor pressure over a curved surface , P , is the vapor pressure over a flat surface , y is the liquid - vapor surface tension , Vis the molar volume of the liquid , r is the radius of the droplet , R is the universal gas constant , and 7 is the temperature . When " liquid saturation in a solid porous media is low , it creates a concave interface between the water and the trapped gas within the pores " [ 2 ] . The equation suggests that this concavity in the liquid surface lowers the vapor pressure of the porous media as r becomes negative , as radius is not being measured from the water droplet , making < P figure 12 ) . The liquid - vapor interface in the cellulose sponge also may be concave given that the round pores force the water droplets to form a concave surface as shown figure 13. Liquid - gas interface as the sponge pores allow air to pass through . Deliquescence occurs when the vapor pressure of the liquefied solution is lower than the partial pressure of water vapor in the air . Upon further lowering of vapor pressure , the salt will be forced to absorb more moisture . It becomes more susceptible to higher water vapor pressure in the atmosphere and tend to equilibrate . Furthermore , a concave surface means there are more surrounding molecules due to larger surface area ; this leads to stronger intermolecular attractions which attract water molecules from the air . Subsequently , it will theoretically absorb more water compared to the flat surface obtained in the salt control . Figure 13 can be referred to explain the quicker rate of the cellulose sponge . Its microscopic pores minimized agglomeration , exposing more surface area and ultimately quickening the rate of reaction .