Potential of superabsorbent hydrogels to improve agriculture under abiotic stresses

Highlights

• Hydrogels have the potential to contribute to water use efficiency under stresses.

• •Residual acrylamide contents were well below permissible levels.

• •Up to 95% of the residual acrylamide was degraded within 30 days.

• •There was no depolymerization of acrylamide across the temperatures and salinities tested.

Abstract

Hydrogels are a promising management option to increase the efficiency of water use in agriculture in arid and semi-arid regions. However, abiotic factors may affect hydrogel efficacy. The effect of abiotic stress on the swelling and residual acrylamide in hydrogels was evaluated. The treatments were three hydrogels (A, B and C), four temperatures (20, 30, 40 and 65 °C) and two salinities (0.003 and 3.0 dS m⁻¹). The swelling degree and the residual acrylamide concentration of the hydrogels were measured. Data were analyzed with univariate and multivariate statistics. The swelling order of hydrogels under low salinity was A>C>B; at high salinity was B>A>C. All hydrogels presented swelling reduction at high salinity, with swelling reductions in A and C of 97% and 85%, respectively, while B presented a swelling reduction of 37% at 20 °C and greater than 89% at higher temperatures. Hydrogel B achieved good swelling under saline conditions, but only at 20 °C. Thus, the temperature tolerance of this hydrogel should be improved so it can be used to improve water use efficiency in regions with high temperatures and salinity levels. Residual acrylamide levels of hydrogels decreased by 95% over 30 days, being safe for agricultural applications.

Introduction

Semiarid and arid areas comprise 41% of the global land area and provide food to more than 38% of the global population (Reynolds et al., 2007). Careful management of natural resources in these regions is essential in order to feed growing populations (Thombare et al., 2018).

Arid and semi-arid regions around the world face increasing problems of water scarcity for both domestic and agricultural water use. Rainfed agriculture is the predominant cropping system in these areas, but aridity and climate uncertainty are the main challenges facing farmers (Ammar et al., 2016). Water scarcity has compromised agricultural productivity in many developing countries, leading to a growing interest in developing practical and affordable solutions (Zain et al., 2018).

Due to their high-water absorption capacity, superabsorbent hydrogels are being extensively studied as soil conditioners to increase water use efficiency in agriculture (Chen et al., 2017; Coello et al., 2018; Paradelo et al., 2019; Thombare et al., 2018; Yang et al., 2018; Zain et al., 2018).

Hydrogels are crosslinked hydrophilic polymers that can absorb large amounts of water (Guilherme et al., 2015).The characteristics of the external solution, such as charge valency and mineral salts concentration (e.g. Na⁺, Mg²⁺), influence the expansion of superabsorbent polymers (Raju et al., 2003). Water absorption efficiency of hydrophilic polymers decreases with increasing solution electrical conductivity (Andry et al., 2009). Salinity impairs water absorption because hydrophilic groups of hydrogels bind to salts, blocking water ingress. Salts also suppress the electrostatic interactions among the polymer and water molecules (Xiong et al., 2018; Zhao et al., 2019). Andry et al. (2009) also observed that hydrophilic polymers can exhibit thermal sensitivity. When hydrogels are applied to the soil, increasing temperature increases the adsorption of anionic polymers to clays due to the reduction of hydrogen bonds among the polymer and water molecules which increases polymer penetration in the internal clay structure (Xiong et al., 2018). Thus, the efficiency of superabsorbent polymers is affected by the salinity of the soil and/or irrigation water applied as well as temperature. Both these abiotic stresses characterise arid and semiarid regions.

Anionic polyacrylamide (PAM) is the most commonly used hydrogel as a soil conditioner and, although it is an inert polymer, may contain residual acrylamide monomer as a result of incomplete polymerization (Xiong et al., 2018). Residual acrylamide concentrations in commercial PAM products are typically <500 mg/kg as regulated by the Food and Drug Administration (FDA), the U.S. Environmental Protection Agency (EPA), and the National Resources Conservation Services (NRCS) (Xiong et al., 2018).

Residual acrylamide although soluble and mobile in the soil is susceptible to biodegradation in soil and surface waters, which significantly reduces the risks of accumulation in the environment (Neely et al., 1974; Tepe and Çebi, 2017). Residual acrylamide associated with PAMs is subject to rapid biological degradation in soils with a half-life of 18–45 h (Barvenik, 1994).

In arid and semiarid regions superabsorbent hydrogels are increasingly seen as a viable option to significantly increase water retention in soils and increase plant available water. However, as salinity and high temperatures compromise water absorption by hydrogels, it is necessary to develop polymers tolerant of these conditions. Further, there is a paucity of data on the effect of salinity and temperatures commonly associated with arid and semiarid regions on superabsorbent polymers.

The objective of this study was to evaluate the water absorption potential and residual acrylamide degradation rate of three superabsorbent polymers subjected to thermal and saline stress. The hypotheses tested were: i) anionic superabsorbent polymers can contribute to water absorption efficiency under abiotic conditions of arid and semiarid regions; ii) despite the adverse effects of temperature and salinity on the efficiency of superabsorbent polymers, a polymer that provides improvements in the soil of arid and semi-arid regions can be indicated; iii) the rate of residual acrylamide degradation is influenced by the abiotic stresses of high temperatures and/or salinity and does not pose an environmental risk.