Impacts of mechanical and chemical factors on the water-holding capacity of polyacrylamide in sand: models and mechanisms

Abstract

Owing to its significant capacity to conserve water in soils, polyacrylamide (PAM) hydrogels have been applied extensively in croplands to combat droughts. However, the influences of different mechanical and chemical factors of sand on its swelling capacity is still unclear. The objective of this study is to reveal the relationships between swelling profile of PAM in sand and sand mechanical and chemical properties, using an originally designed laboratory-scaled column apparatus. Different water management strategies and sand properties, including water infiltration rates (0.6, 1.4, 1.9, and 2.5 m day–1), application depth (5.5, 11.0, and 16.5 cm), solution pH (5.0, 6.9 and 9.0), and ionic strengths (0.001, 0.01, and 0.1 M) were investigated in the present study. Chemical properties of sand medium (i.e. ionic strengths and pH conditions) reduced up to 70% of water holding capacity. Vertical stress limited water holding capacity to 50–60% of the maximum value, demonstrating that its effect has been underestimated in the past. Infiltration rate influenced the swelling profile but not the maximum water holding capacity. Results were fitted with the Korsmeyer-Peppas model, Gallagher-Corrigan model and First-Order model to derive parameters that highlight the impacts of the sand properties. Through model simulation, swelling behaviour was well explained with rapid swelling followed by further release in swelling. The discoveries in this study demonstrate the important roles of mechanical and chemical factors during the water absorption process of agricultural hydrogels.