Assessing the use of hydrogels to harvest atmospheric water for agriculture in arid and semi-arid areas : a thesis presented in partial fulfilment of the requirements for the degree of Master of Environmental Management at Massey University, Manawatū campus, New Zealand
Agricultural production in arid and semi-arid regions globally faces a growing challenge of water scarcity and initiatives to increase water-availability for crops are needed. The hygroscopicity of hydrogels underpins the real opportunity to desorb water that can be used to support agricultural production in water scarce areas. Research to date has predominantly focussed on direct contact absorption of water in a liquid phase. The opportunity for hydrogels to absorb water from the atmosphere is less studied. Specifically, the impact of relative humidity and temperature on hydrogel hygroscopicity and potential for desorption of this water under environmental pressures that might be expected in a plant root zone are poorly described in literature. Such information will underpin assessment of the extent to which atmospheric water absorption might serve as an alternative water source for plants use in the arid and semi-arid regions. This study was therefore undertaken to ascertain hydrogels hygroscopicity and desorption potential with specific consideration of agriculture in arid and semi-arid regions. The research aimed to provide information on the hygroscopicity potential of different hydrogels, and how different relative humidity percentages and temperature influence hydrogels hygroscopicity and different applied pressures impact water desorption from hydrogels. The effect of relative humidity, time and temperature on hygroscopicity was investigated using replicates of five hydrogels of different composition placed in five different relative humidity chambers (63 %, 76 %, 84 %, 95 % and 100 %) and under three different temperature levels (10 ºC, 20 ºC and 30 ºC ). The results showed that hydrogel type, relative humidity and time influences hygroscopicity significantly, and that the chemical composition of hydrogels can explain hygroscopicity. There was no influence of temperature on absorption. Hydrogels with no N content showed increased absorption of atmospheric water with time, and this is explained through the absence of an N-driven crosslinking effect on water absorption. Absorption of atmospheric water by the best performing hydrogel (Yates Waterwise Water Storage Crystals; at 3.139 g/g at 100 % relative humidity and 30 ºC) in this study was explained by first order model behaviour at 20 ºC for all relative humidity levels except at 63 %. Further research was conducted on the hydrogels defined as the best and worst absorbing in the initial experiments. These hydrogels were placed in contact with liquid water to yield the freely swollen state, and then desorption potential for plant access was investigated using different pressure levels on suction plates. The results clearly showed that increasing pressure increases water desorption between 0.1 and 1 bar pressure. However, between 1 bar and 15 bar no further water is lost. The best absorbing hydrogels identified in this study desorbed more water than the worst. However, this work finds that for both tested hydrogels, pressure beyond 15 bar would be required to desorb hygroscopic water for plant access and use. The study therefore infers that the hygroscopicity potential of hydrogels is optimum for hydrogels with no N content exposed to high relative humidity (above 84 %) over periods of daily cooling from late night and early morning where the dew point might be reached. Such conditions do overlap with some arid and semi-arid regions. However, even where these environmental conditions for optimal absorption are reached, plants are unlikely to be able to desorb the hydrogel water. Therefore, an engineering approach would be needed to physically or mechanical desorb water. In this scenario it is unlikely that hydrogels would be mixed into the soil. Instead, a system could be deployed where hydrogels are exposed to atmospheric water in ‘banks’ which can be closed periodically for desorption. Released water could then be channelled for irrigation. Solar power may be a viable energy source to drive this scenario, although further work is required to fully explore the opportunity.