Effects of Superabsorbent Polymers on the Hydraulic Parameters and Water Retention Properties of Soil

Abstract

Superabsorbent polymers (SAPs) are widely applied in dryland agriculture. However, their functional property of repeated absorption and release of soil water exerts periodic effects on the hydraulic parameters and water-retention properties of soil, and as this property gradually diminishes with time, its effects tend to be unstable. During the 120-day continuous soil cultivation experiment described in this paper, horizontal soil column infiltration and high-speed centrifugation tests were conducted on SAP-treated soil to measure unsaturated diffusivityDand soil water characteristic curves. The experimental results suggest that the SAP increased the water retaining capacity of soil sections where the suction pressure was between 0 and 3,000 cm. The SAP significantly obstructed water diffusion in the soil in the early days of the experiment, but the effect gradually decreased in the later period. The average decrease in water diffusivity in the treatment groups fell from 76.6% at 0 days to 1.2% at 120 days. This research also provided parameters of time-varying functions that describe the unsaturated diffusivityDand unsaturated hydraulic conductivityKof soils under the effects of SAPs; in future research, these functions can be used to construct water movement models applicable to SAP-treated soil.

Establishing and validating a root water uptake model under the effects of superabsorbent polymers

“…In this study, if a unique time‐dependent effect was not considered in the calculation method, the numerical experimental results indicated a pronounced error when the soil hydraulic parameters were decreased by 50–90%. As reported in previous studies, compared with the treatment without SAP application, significant decreases in the soil hydraulic parameters were observed, that is, up to 71.6% in the experiment carried by Han, Yu, et al () and 70–76% in the test performed by Liao et al (). Therefore, the time‐dependent effect of SAPs on soil hydraulic parameters should be fully considered when using the proposed calculation method to estimate RWU in response to SAPs.…”

“…The parameters used in the calculations were as follows: time step 1000; distance step 0.125 (5 cm/40 cm); instrument precision w = 0.01; and the error threshold used in the iterative process was 1 × 10 −5 . The functional forms of D ( θ ) and K ( θ ) for the control group and of D ( θ , t ) and K ( θ , t ) for the SAP‐treated groups presented in a previous study (Liao et al, ) were used to calculate the RWU. The rates of soil surface evaporation measured in the controlled soil column experiment were as follows: CK: 0.026 cm·d −1 ; SFL group: 0.030 cm·d −1 ; SL group: 0.027 cm·d −1 ; and SH group: 0.029 cm·d −1 .…”

“…In this equation, D ( θ , t ) and K ( θ , t ) are time‐dependent soil hydraulic parameters, cm 2 ·min −1 ; S ( z,t ) is the RWU source/sink term, cm 3 ·cm −3 ·d −1 ; θ is the soil moisture content, cm 3 ·cm −3 ; z is the depth of soil, cm; and t is the experimental duration, d. The forms of D ( θ , t ) and K ( θ , t ) can be determined through continuous soil experiments described in a previous study (Liao, Wu, Ren, & Yang, ), and the parameters for equation θ = θ re + ( θ sa − θ re ) ∕ [1 + ( αh ) d ] f fitted by the soil water retention curve are listed in Table . We ignored the changes in soil volume affected by the SAP; therefore, the SAP‐treated soil was regarded as a rigid soil with time‐dependent soil hydraulic parameters.…”

“… Note. θ re is the residual moisture content, cm 3 ·cm‐ 3 ; θ sa is the saturated moisture content, cm 3 ·cm‐ 3 ; α, d , and f are the fitted parameters (Liao et al, ). SAP = superabsorbent polymer.…”

Valorization of Superabsorbent Polymers from Used Disposable Diapers as Soil Moisture Conditioner

“…When soil is treated with SAP, its water retention properties are related to both porosity and the SAP's water absorbency. These results suggested that the adding of hydrogels into soil could obviously improve the water-retention capacity of soil and lessen the amount of water evaporation [35,38,39]. The trends of the curves suggest that the SAP improved the water retention properties of the treated soil during the early period.…”