Towards Improved Application of Super Absorbent Polymers in Agriculture and Hydrology: A Cross-Disciplinary Approach
Type of Degreethesis
Agronomy and Soils
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Fresh water is the essential and key component to support life on this planet. It constitutes about 0.01% of world’s total water. Though freshwater is renewable, it is finite. In recent years, the demand for fresh water increased due to increase in population and industrial growth and it is projected to increase further. To meet the food demand for the growing population more land is brought under agriculture with the aid of irrigation. Irrigated agriculture is the largest consumer of water for human consumption – 67% of current global water withdrawal and 87% of consumptive water use. It is beneficial to look for possible avenues to mitigate water consumption in this particular field. One such possible means is to alter the water holding capacity of the soils by changing the hydraulic conductivity. Hydraulic conductivity is the measure of ability of the soil to transmit water. The hydraulic conductivity of the soil could be altered by amending the soils with Super Absorbent Polymers (SAPs). SAPs are long chain, slightly cross-linked polymers capable of absorbing large quantities of water and retaining it within them. They were initially developed for agricultural usage to improve water holding capacity of soils and promote germination of seeds, but they have found extensive application in disposable pads, towels used in surgery and various other products. The water absorbing property of the SAPs can be altered by various means. Further, SAPs also undergo controllable volume changes when the ambient environment is altered. Non availability of SAPs with identical chemical and physical properties leads to preparation of poly (acrylic acid-co-acrylamide) with three different cross link ratios and particle sizes using acrylic acid (AA) and acrylamide (AAm) monomers. The SAPs thus prepared were characterized using spectroscopic methods. The water absorbing property of the SAPs was measured using de-ionized water and 9000 ppm sodium chloride solution. Mechanical strength increased and porosity decreased with increase in AAm content. The water absorbing capacity of the SAPs increased with decreasing AAm content and particle size. Also, the water absorbing capacity of SAPs decreases drastically in sodium chloride solution. The SAPs released the absorbed water when the ambient pressure is reduced. Further, the water absorbing capacity of the SAPs did not decrease drastically when reused. Earlier studies do not offer clarity on changes in hydraulic conductivity due to SAP amendment in soils. The effect of SAPs on hydraulic conductivity of soils is a function of the soils to expand. The effect of overburdened pressure in soils has not been accounted in the previous studies. Three different soils – Sandy Loam, Clay Loam and Sandy Clay Loam are amended with SAPs of three different water absorbing capacities and particle sizes. The application rates of SAPs in soils are 0.05%, 0.15% and 0.25%. Sandy Loam soil was further studied for effect of variable overlying pressure on the soil-SAP mixture with an application rate of 0.25%. The overlying pressures used were 0, 0.05, 0.11, 0.18 and 0.23 Pa. The free swelling conditions simulate amendment of SAPs near surface whereas restricted swelling conditions simulate that of amendment at subsurface of soil. In sandy loam soil, under free swelling condition, the hydraulic conductivity increases with increase in application rate. In clay loam and sandy clay loam soils, there is no significant change in hydraulic conductivity on amendment of SAPs. Further under free swelling conditions, the expansion of soil increases with increase in application rates of SAPs for all the three soils. The hydraulic conductivity of all SAP amended soils under restricted swelling conditions decreased drastically compared to free swelling conditions. In restricted swelling conditions, only the sandy loam showed increase in expansion with application rate, whereas clay loam and sandy clay loam soils showed mixed response. For sandy loam soil under varying overlying pressure the hydraulic conductivity of the soil-SAP mixture decreases with increase in overlying pressure. The soil amended with smallest SAPs particles has the lowest hydraulic conductivity and soil expansion compared to other SAP particle sizes. It can be concluded that, the hydraulic conductivity of the SAP amended soil increases under free swelling conditions and decreases under restricted swelling conditions.