Journal of Nanomaterials

Journal of Nanomaterials / 2016 / Article

Research Article | Open Access

Volume 2016 |Article ID 5403976 | 11 pages | https://doi.org/10.1155/2016/5403976

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

Academic Editor: João Garcia
Received30 Jan 2016
Revised16 May 2016
Accepted05 Jun 2016
Published03 Jul 2016

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 diffusivity and 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 diffusivity and unsaturated hydraulic conductivity of soils under the effects of SAPs; in future research, these functions can be used to construct water movement models applicable to SAP-treated soil.

1. Introduction

The application of superabsorbent polymers (SAPs) for the purpose of enhancing soil water retention represents an important nonengineering water conservation technique for dryland farming. This technique is widely used in producing crops such as apples, grapes, wheat, and maize [14], and it has proven to be effective in saving water and increasing yields. In many farming areas with limited water supplies, crop growth relies completely on rainwater. However, the uneven spatiotemporal distribution of precipitation and the soil’s poor ability to conserve moisture keep rainwater use efficiency low in these areas, exerting a direct impact on crop growth [57]. Applying SAPs to the soil is effective in improving rainwater use efficiency in dryland farming areas [8, 9], because SAPs can repeatedly absorb and retain rainwater entering the soil to reduce deep seepage losses and then gradually release the water to the plants as the soil dries and the plants’ root pressure increases. This mechanism ensures a continuous water supply for plants during their growth periods [10, 11].

After treatment with SAPs, soil demonstrates remarkable changes in its hydraulic parameters and water holding properties. Han et al. [12] found that the periodic absorption and release of water by SAPs exert time-varying effects on the soil’s properties, causing the hydraulic parameters in SAP-treated soil to vary irregularly with time. Bai et al. [13] discovered that, during wet-dry soil cycles, the application of SAPs can reduce soil’s bulk density, with a higher SAP dosage producing a greater effect. Other studies [1417] have revealed that the repeated water absorption and release mechanism of SAPs not only ensures water supply for plants, but also alters the pattern of soil water movement by influencing the soil’s mechanical and chemical properties, local microbial communities, and root growth. This adds to the difficulty and complexity inherent in modeling water movement in soil to which SAPs have been applied.

Accurate characterization of soil’s hydraulic parameters and water retention properties is a key step in modeling water movement [1820]. However, only a few studies have provided quantitative descriptions of the dynamic characteristics of SAP-treated soil’s hydraulic parameters [12, 21, 22]. The dynamic effects of SAPs on soil’s hydraulic parameters and water retention properties are not yet clear, obstructing in-depth research into water movement models that are applicable to SAP-treated soils, as well as other related research.

In this context, a 120-day soil cultivation experiment was carried out, with horizontal soil column infiltration and high-speed centrifugation tests conducted to investigate the patterns of dynamic variation in the hydraulic parameters and water holding properties of SAP-treated soil. Time-varying functions were obtained to describe the unsaturated diffusivity and unsaturated hydraulic conductivity of soil, providing a basis for constructing relevant models, such as a model of soil water movement under the influence of SAPs.

2. Methods

2.1. Materials

This experiment tested a cross-linked soil SAP with particle sizes between 0.02 and 0.05 mm, manufactured from polyacrylamide and acrylic acid by Dongying Huaye New Material Co., Ltd., in Shandong. The soil tested was sand loam containing 52.4% sand, 36.1% silt, and 11.5% clay, taken from topsoil in the greenhouses and fields at the International Seed Industry Park in Yujiawu Town, Tongzhou District, Beijing. In terms of nutrients, the tested soil contained total nitrogen of  g/kg, available nitrogen of  mg/kg, total phosphorus of  g/kg, available phosphorus of  mg/kg, total potassium of  g/kg, rapidly available potassium of  mg/kg, and organic content of  g/kg. The soil was cultivated in round polyvinyl chloride pots with a height of 35 cm and a diameter of 25 cm.

2.2. Process
2.2.1. Treatment

The soil was divided into a control group and three treatment groups. No SAP was applied to the soil in the control sample. In the first treatment group, labeled P-S1, SAP at a polymer concentration of 0.06% was applied to the soil. In the second treatment group, P-S2, 0.03% SAP was applied to the soil. In the third treatment group, P-S3, the soil was treated with 0.01% SAP. Each treatment process was repeated 12 times.

2.2.2. Methods

First, the SAP was evenly mixed with air-dried soil. The mixtures were then put into pots and compacted layer by layer, with each layer being 4 to 5 cm thick, to achieve a bulk density of 1.28 g/cm3. At the bottom of each pot, under the soil mixtures, a 2 cm thick filter bed of gravel was laid upon the drainage hole. After the soil thickness in the pots reached 28 cm, a 2 cm thick layer of quartz sand was applied on the soil surface to reduce evaporation, leaving a 3 cm hydraulic head above the quartz sand for irrigation. After these preparations, the field capacity (FC) of the soil was measured using the cutting ring method, yielding a result of 0.37 cm3/cm3. Every two to three days, four pots were randomly chosen from each sample group and weighed to measure average soil water content. During the experiment, the soil’s volumetric water content was maintained between 60% and 100% of FC. If the volumetric water content of the soil in a pot neared 60% of FC, the soil was irrigated until the value reached 100% of FC.

2.3. Sampling and Testing

Soil in the pots was sampled using cutting rings at 0, 5, 10, 15, 20, 25, 30, 40, 50, 60, 90, and 120 days. Part of each soil sample was packaged in filter paper and tested in the lab using a high-speed centrifugation method to analyze the water characteristic curves. After the remainders of the sampled soils were air-dried, a horizontal soil column infiltration test was conducted to measure their unsaturated diffusivity [12]. Soil’s unsaturated hydraulic conductivity can be calculated from the unsaturated diffusivity measurement according to the following relationships: and .

2.4. Data Analysis

The software RETC analyzed the soil water characteristic data to infer the values of unsaturated hydraulic conductivity. The parameters of time-varying functions for unsaturated diffusivity and unsaturated hydraulic conductivity were obtained by R programming and curve fitting.

3. Results and Analysis

3.1. Effect of SAP on Unsaturated Soil Water Diffusivity

Figure 1 displays the three-dimensional distribution of unsaturated diffusivity for each of the four groups at 0, 5, 10, 15, 20, 25, 30, 40, 50, 60, 90, and 120 days. The unsaturated diffusivity of the control group changed slightly throughout the experiment, and the population variance of the control group’s data at different times was 0.144. By contrast, all the SAP-treated groups demonstrated significant decreases in unsaturated diffusivity during the early period of the experiment, and the rate of the decreases increased with SAP concentration, such that P-S1 > P-S2 > P-S3. The treatment groups’ unsaturated diffusivities gradually increased with time and were nearly equal to the control group’s at the end of the experiment. The population variances of the unsaturated diffusivity data of the treatment groups were between 0.387 and 0.398.

Also, Figure 1 shows that soil water diffusion was significantly obstructed during the early period. At 0 days, the average decreases in unsaturated diffusivity were between 70.1% and 76.6%. These values fell to between 30.6% and 46.9% at 5 days and reached 49.5% to 68.1% at 10 days. After repeated absorption and release of water, the SAP incurred some structural damage, and SAP water absorbency decreased with time. As a result, the effect on water diffusion in the soil gradually weakened. The average decrease in unsaturated diffusivity dropped to between 9.5% and 25.5% at 60 days and between 9.4% and 26.1% at 90 days. At 120 days, the unsaturated diffusivities of the treatment groups were very close to the control group, and the average decreases in unsaturated diffusivity were 1.2% to 16.0%.

Table 1 presents the curves fitted to the unsaturated diffusivity data and their corresponding degrees of correlation (coefficient of determination, ) for the four groups at different times ( = 0, 5, 10, 15, 20, 25, 30, 40, 50, 60, 90, and 120 d). A comprehensive regression analysis of the data yielded time-varying functional equations for calculating the unsaturated diffusivity of soil under the effect of SAP. The regression equations show that the time-varying functions include the time variable , and the goodness of fit of each regression attains extremely high significance (), indicating that these functions can accurately describe the relationship between time and the unsaturated diffusivity of soil under the effect of SAPs.


Time (d)Sample group
CGP-S1P-S2P-S3













Overall

indicated significant correlation at the 0.01 level.
3.2. Effect of SAP on Soil Water Characteristic Curves

Figure 2 shows the soil water characteristic curves of the four sample groups at 0, 15, 30, 50, 90, and 120 days. The trends of the curves suggest that the SAP improved the water retention properties of the treated soil during the early period. A comparison of the soil water content at different suction pressure ranges demonstrated that the improvement in soil water retention mainly occurred in the suction pressure range of 0 to 3,000 cm, and it was especially marked in the 100 to 800 cm range. At 0 days, the soil in the P-S1, P-S2, and P-S3 groups showed water content increases of 10.6% to 26.4%, 14.2% to 17.0%, and 7.7% to 10.6%, respectively, compared to the control group; the P-S1 group experienced the increase of 26.4% at a suction pressure of 300 cm. By 15 days, the soil water content in the P-S1, P-S2, and P-S3 groups increased by 15.4% to 26.5%, 14.8% to 21.6%, and 9.7% to 14.7%, respectively; the largest increase of 26.5% occurred in the P-S1 group at a suction pressure of 500 cm. After a duration of 30 days, the soil water content in the three treatment groups increased by 11.8% to 20.7%, 10.2% to 14.4%, and 5.1% to 9.2%, respectively, with the largest increase of 20.7% occurring in the P-S1 group at a suction pressure of 500 cm. By 50 days, the soil water content increases in each of the three treatment groups were 8.3% to 23.2%, 12.4% to 18.3%, and 8.8% to 14.1%, respectively; the maximum of 23.2% occurred in the P-S1 group at a suction pressure of 100 cm. By 90 days, these values were 10.3% to 13.0%, 7.3% to 11.6%, and 7.1% to 10.4%, with the maximum of 13.0% occurring in the P-S1 group at a suction pressure of 500 cm. At 120 days, the soil water content in the treatment groups was only 5.6% to 7.7%, 3.4% to 4.0%, and 2.7% to 4.2% higher than in the control group, and the P-S1 group saw the highest soil water content increase of 7.7% at a suction pressure of 100 cm. Those soil water characteristic curves were used for predicting specific moisture capacity to calculate unsaturated hydraulic conductivities of the soil. Finally, time-varying functions for the unsaturated hydraulic conductivity under the effect of SAP were derived from a series of calculated unsaturated hydraulic conductivities.

3.3. Effect of SAP on Unsaturated Hydraulic Conductivity

Figures 38 show the unsaturated hydraulic conductivities of the control group and the treatment groups at 0, 15, 30, 50, 90, and 120 days, respectively. Compared to the control group, we can see the unsaturated hydraulic conductivities of treated soil sample were greatly decreased after SAP application during the early period, similar to the effect of SAP on unsaturated diffusivity. The average decreases in unsaturated hydraulic conductivity were 85.5% to 94.1% at 0 days, 75.1% to 82.9% at 30 days, and 65.6% to 76.2% at 50 days. Notably, abnormalities were observed in regions with high water content in all treatment groups at 30, 90, and 120 days. It was therefore impossible to calculate values for these regions.

Table 2 displays the curves fitted to unsaturated hydraulic conductivity data at different times ( = 0, 15, 30, 50, 90, and 120 d), as well as corresponding degrees of correlation. Time-varying functional equations describing soil’s unsaturated hydraulic conductivity were constructed through a comprehensive regression analysis. As the regression equations show, these functions include the time variable , and each regression’s goodness of fit attained extremely high significance (), demonstrating that these functions can precisely reflect the relationships of unsaturated hydraulic conductivity varying with the test time under the influence of SAP.


Time (d)Sample group
CGP-S1P-S2P-S3