Quantifying the combined effect of pH and salinity on the performance of water absorbing polymers used for drought management

Abstract

Water absorbing polymers (WAPs) are widely used in the agricultural field of arid and semiarid regions as soil amendments to increase the water retention capacity and minimize irrigation frequency. The agricultural soils in these regions generally have varying levels of pH and salinity, which would influence and inhibit the performance of WAP. The effect of pH and salinity on performance of WAP and its application rate has not been addressed adequately. This study quantifies the effect of different salt ions and pH on the performance of two different WAPs, including commercially available WAP (crosslinked potassium polyacrylate) and laboratory-grade WAP (crosslinked fly ash-polyacrylate superabsorbent composite). Experiments were performed to measure the water absorbing capacity (WAC) of the used WAPs in different solutions or swelling mediums with varying salt concentrations and pH. The combined effect of seven different commonly occurring inorganic salts with varying concentrations (0.01 M, 0.05 M, 0.1 M, 0.15 M, 0.2 M, and 0.3 M) and three different pH levels (5, 7, and 9) on the WAC of the WAPs was evaluated. It was found that the WAC was highly sensitive to the ionic strength and the type of cations/ anions present in the solution. The experimental results were used to develop a unique relationship between WAC and the electrical conductivity (EC) of the solvent, which needs to be incorporated for the basic characterization of WAP. This study demonstrates the usefulness of the proposed relationship for deciding appropriate WAP application rates in different agricultural soils based on their EC values.