Optimizing the interaction between poly(vinyl alcohol) and sandy soil for enhanced water retention performance

Abstract

Poly(vinyl alcohol) (PVA) was used as a low-cost and degradable water retention agent in combating drought and desertification. The effect of PVA with different degrees of hydrolysis on the enhancement of water retention capacity of sandy soil and the growth performance of Arabidopsis thaliana were investigated. The results showed that PVA could effectively enhance the water retention capacity of sandy soil and the growth of plants in it. After the addition of PVA, the survival rate, aerial biomass and chlorophyll content of Arabidopsis thaliana all increased substantially compared to those in untreated soil under the condition of water shortage. The relationship between PVA's degree of hydrolysis and its water retention performance in sandy soil was also studied. It was found that PVA with a middle degree of hydrolysis, 1795 and 1797, had the best performance, which even catches up with the traditional cross-linked hydrogel-PAM, suggesting that PVA could be an effective water retention agent for improving plant growth in sandy soil and combating desertification. Through this study, a few criteria were proposed for the selection of better water retention agents, considering their water absorbency, retaining ability in sandy soil and degradability.

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1. Introduction

Desertification is one of the most serious ecological problems worldwide.^1,2 According to a survey, the total area of desert, Gobi, and sandy desertification in China is about ∼1.6 × 10⁶ km², accounting for ∼16% of its territory.³ The area of land desertification has been found to increase in the past decades, making the situation even more serious. Desertification restrains plant growth, deteriorates the ecological environment and hinders economic development.⁴ Therefore, it is of immediate need to find an effective and economical method to prevent further desertification or even recover desert land.

Vegetation restoration is considered to be the primary choice of controlling desertification.^5–7 However, the water holding capacity of sandy soil is usually very poor; even with a large amount of rain, the rain water can easily infiltrate into the lower layer thus is unable to support the plant growth in surface layer.⁸ Moreover, in the desert, the temperature is usually high in the daytime, leading to rather fast moisture evaporation. Therefore, water content of the sandy soil is generally too low to support vegetation restoration. Thus enhancing water content of sandy soil is a prerequisite for controlling desertification. In recent years, chemical additives were used to improve water holding capacity of sandy soil, making it more suitable for plant growth, thus preventing desertification.⁹ Super-absorbent polymers (SAPs), which are macromolecules with segments of hydrophilic groups, are one type of the most maturely developed water retention agents. When irrigated, they can absorb large amount of water and hold water many times of their dry weight,^10–12 therefore effectively reducing water runoff from soil, improving water retention and increasing plant growth.^13,14 The most widely used SAPs in the past few decades are mainly cross-linked polymers such as polyacrylamide and polyacrylate hydrogels.^15–17 However, these hydrogels are difficult to degrade, thus may have potential risks to the environment and human health.^18,19 Therefore, developing economical and eco-friendly water retention agents is paramount.

Poly(vinyl alcohol) (PVA) is a synthetic hydrophilic polymer, which can degrade with the help of bacterial enzymes.^20,21 It can absorb large volume of water, thus is a potential water retention material. Compared to other polymeric water absorbents, such as PAM, PAA and potassium polyacrylate, PVA is inferior in water absorbency, thus was not the primary choice as water retention agent.²² However, plant growth trials under the normal irrigation condition suggested that PVA's performance was not worse than those super water absorbents, suggesting water absorbency may not be the paramount factor determining the improvement of plant growth in sandy soil.²³ Unfortunately, no comparison has been made under the water shortage condition between PVA and other super water absorbents. In addition, PVA also has different degree of hydrolysis, rendering various hydrophilicity, thus needs to be screened for the best candidate. In the present study, we will carry out a conceptual study to evaluate the performance of PVA as water retention agent for sandy soil, and to understand the relationship between PVA's degree of hydrolysis and its water retention performance. By doing such, we aim to identify the best PVA candidate with optimized water retention performance. Arabidopsis thaliana was planted in sandy soil as a model plant^24–26 to demonstrate the validity of our proposal. The commonly used cross-linked hydrogel, polyacrylamide (PAM), was used for comparison.

2. Experimental

2.1. Sandy soil

Sandy soil was collected at the Kubuqi Desert in Inner Mongolia, China. The soil was air-dried, ground, and passed through 2 mm sieves and fractions less than 2 mm were used in the experiments. The physical and chemical characteristics of the sandy soil are shown in Table 1.^27,28

Table 1 Physical and chemical characteristic of the soil used in the experiments

Characteristics of sandy soil
Total sand (%)	94.5
Texture of sand
Above 1.00 mm (%)	0.90
Between 1.00 and 0.5 mm (%)	11.1
Between 0.5 and 0.25 mm (%)	31.3
Between 0.25 and 0.05 mm (%)	51.2
Silt (%)	5.0
Clay (%)	0.50
pH	9.11
Organic matter (%)	0.62

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2.2. Materials and chemicals

Poly(vinyl alcohol) (PVA) with the same degree of polymerization (1700) but different degree of hydrolysis (99%, 97%, 95%, 92% and 88%, named 1799, 1797, 1795, 1792 and 1788 thereafter) were obtained from Yingjiashiye Corporation, Shanghai, China. All the PVA samples were dehydrated at 105 °C, ground, and passed through 60-mesh sieves.

Acrylamide (AM) was purchased from Sinopharm Chemical Reagent Corporation. Potassium persulfate (KPS) and N,N-methylene-bis(acrylamide) (MBA) were purchased from Alfa-Aesar. Tetramethylethylenediamine (TEMED) was purchased from Sigma-Aldrich. All these chemicals were used as received.

2.3. Preparation of PAM

PAM was prepared by free radical polymerization of AM at room temperature using KPS–TEMED as a redox initiating system and MBA as chemical cross-linker.²⁹ Briefly, a mixture of 7.1 g AM, 0.154 g MBA and 30 μL TEMED were added into 80 mL pure water in ice-water bath. Next, 0.027 g KPS was added while stirring. The solution was bubbled with N₂ for 30 min. Then, free radical polymerization was allowed to proceed under N₂ atmosphere at room temperature. After 20 h, the gel-like product was freeze-dried, ground, and passed through 60-mesh sieves.

2.4. Measurement of water absorbency of PVA and PAM

The procedure for determination of water absorbency is as following: 0.3 g of dried PVA with different degree of hydrolysis or PAM were immersed into excessive amount of distilled water and allowed to swell at room temperature for 24 h. The swollen samples were filtered through a 100-mesh sieve, blotted quickly with absorbent paper, and then weighed. The water absorbency (WA) was calculated using the equation:where M_s and M_d represent the weight of the swollen and dried polymers, respectively.

2.5. Measurement of instant water holding capacity of sandy soil

The procedure to determine the effect of PVA or PAM on the instant water holding capacity of sandy soil is as following: 150 g sandy soil with 3 g of PVA with different degree of hydrolysis or PAM were mixed thoroughly and then placed in a PE breaker. The bottom of the breaker was opened with a small hole, sealed with nonwoven fabrics and weighed (marked W₁). The sandy soil mixture was drenched slowly with distilled water from the top of the breaker until water seeped out from the bottom. When no water seeped out from the bottom any more, the breaker was weighed again (marked W₂). The instant water holding capacity (IWHC%) of sandy soil mixture was calculated from the following equation:³⁰

2.6. Measurement of flush away ratio of PVA in sandy soil

The initial experimental procedures were the same as that in Section 2.5. After the water seeped out from the bottom of the breaker, it was collected and its volume was recorded (marked as V_p). Next, the collected solution was filtered through 0.45 μm filter and a clear polymer solution was obtained. The polymer concentration was determined by the reaction of PVA with boric acid (H₃BO₃) and iodine solution (I₂).³¹ The absorbance of the resultant green complex was measured at 668, 667, 663, 650 nm for PVA 1797, 1795, 1792 and 1788 with a UV-vis spectrophotometer (TU-1901) 15 min after the start of the reaction. The concentration of PVA (marked as C_p) was obtained according to the predetermined calibration curve. The flush away ratio (FAR%) of PVA in sandy soil was evaluated as the following:where M_p is the weight of PVA added to the sandy soil.

2.7. Measurement of dynamic water holding capacity of sandy soil

100 g sandy soil and 2 g PVA with different degree of hydrolysis or PAM were mixed thoroughly and placed in a PE breaker. The bottom of the breaker was opened with a small hole, sealed with nonwoven fabrics. Then distilled water was added slowly into the beaker to make the sandy soil saturated with water (reached the determined instant water holding capacity in previous step). The beakers were kept at room temperature and weighed every two days (marked as M₁) for 14 days. Then the breakers were dried at 80 °C for 48 hours to make the residual moisture evaporated and weighed (marked as M₀). The breakers were watered to saturation again and weighed once every two days (marked as M′₁) for the next 14 days. The breakers were dried again at 80 °C for 48 hours and weighed (marked as M′₀). The dynamic water holding capacity of sandy soil was evaluated by the water content (WC) remaining in the sandy soil as following:

WC = M₁ − M₀ (or WC = M′₁ − M′₀) (4)

2.8. Plant growth experiments

A certain amount of sandy soil were mixed thoroughly with 2% (wt% based on soil weight) of PVA with different degree of hydrolysis or PAM before plant growth. Arabidopsis thaliana ecotype ‘Columbia’ seeds, which had been sterilized using 3% (w/v) sodium hypochlorite, were kept in dark at 4 °C for 2 days and germinated on an MS medium.³² After 7 days, the seedlings were transferred to small pot each containing 250 g of sandy soil and water retention agents mixture that had been watered with MS nutrient solution and tap water until saturated. Five seedlings were planted in each pot and three replicas were prepared per treatment. Plants were grown in a greenhouse at constant temperature (25 °C). All the samples were only watered on the 15^th day. Survival rate was recorded by counting the living plants up to four weeks. After four weeks, the plants were harvested. The aerial part of the plants were removed from each pot, washed thoroughly with deionized water, weighed for weight and recorded as aerial biomass. Then the shoots were used for chlorophyll concentration determination.

2.9. Chlorophyll concentration determination

0.03 g fresh leaves of each treatment were ground in 1.5 mL centrifuge tube and extracted by 99% (w/w) ethanol. Pigments that were soluble in ethanol were separated from the colorless debris by centrifugation at 7000 rpm for 6 min. Absorbance was determined at wavelengths 649 and 665 nm using a UV-vis spectrophotometer (TU-1901). The concentration of chlorophyll was calculated using the following equations:³³

C_a = 13.95A₆₆₅ − 6.88A₆₄₉ (5)

C_b = 24.96A₆₄₉ − 7.32A₆₆₅ (6)

where C is the concentration of total chlorophyll (mg g⁻¹), C_a and C_b are the concentrations of chlorophyll a and chlorophyll b (mg L⁻¹), respectively, A is the absorbance at the respective wavelengths, V is the extraction volume (L) and M is the weight of the leaves (g).

3. Results and discussion

3.1. Water absorbency of PVA and PAM

The water absorbency of PVA and PAM is shown in Table 2. After swelling in water for 24 h at room temperature, PVA 1788 was completely dissolved and 1792 was partly dissolved. PVA 1799, 1797 and 1795 and PAM were swollen and formed transparent hydrogels; thus the comparison was only limited to these polymers. The water absorbency of PVA follows the order: 1795 > 1797 > 1799. This generally follows the order of PVA solubility in water, and can be interpreted based on the fact that residual acetyl group could reduce the hydrogen bonding formed between hydroxyl groups and release more free hydroxyl groups to interact with water.³⁴ PVA 1792 was partially dissolved, thus the effective water absorbency was lower than 1795 and 1797. PVA 1788 was completely dissolved, thus its water absorbency was not measurable. Meanwhile, due to its crosslinked structure and high abundance of hydrophilic groups, PAM had the highest water absorbency of all the samples measured, almost twice of that of the highest PVA (1795). Furthermore, considering the extremely low water absorbency of PVA 1799 (1.1 (g g⁻¹)), it was not used in the following experiments.

Table 2 Water absorbency of PVA with different degree of hydrolysis and PAM

	Water retention agent
	1799	1797	1795	1792	1788	PAM
Water absorbency (g g^−1)	1.10	4.36	6.65	2.38	—	13.98

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3.2. Instant water holding capacity of sandy soil with PVA and PAM

Fig. 1 presented the instant water holding capacity of sandy soil added with PVA and PAM. It can be seen that the instant water holding capacity follows the order: PAM > 1795 > 1797 > 1792 > 1788 > control (the untreated sandy soil), which was in agreement with the results of the water absorbency. The undissolved water retention agents-PAM, PVA 1795 and 1797 could swell to absorb and maintain large amounts of water in soil, thus improving its instant water holding capacity. However, it was noticed that the improvement of instant water holding capacity of sandy soil with PVA 1792 and 1788 were not pronounced but only a little higher than the control. This might be caused by the flushing away of PVA 1788 and 1792 due to their higher solubility in water, which will be studied later.

Fig. 1 Instant water holding capacity (IWHC) of sandy soil samples added with PVA with different degree of hydrolysis and PAM.

3.3. Flush away ratio of PVA in sandy soil

The results of flush away ratio of PVA with different degree of hydrolysis in sandy soil were shown in Fig. 2. It is clear that the flush away ratio of PVA 1792 and 1788 were rather high, especially 1788 (∼22%), that means over 1/5 of the added PVA were flushed away with excess water. This might because that PVA 1792 and 1788 with lower degree of hydrolysis were highly soluble in water, thus were hard to be retained in sandy soil. In contrast, the values of flush away ratio of PVA 1797 and 1795 were ∼1.1% and 0.8%, indicating these two PVAs had higher retaining ability in sandy soil and would not be easily flushed away by water. This is one of the important factors that determine whether the water retention agent can effectively improve the water retention performance of sandy soil. The results explain rather well the instant water holding capacity of sandy soil in the previous step.

Fig. 2 The flush away ratio of PVA with different degree of hydrolysis in sandy soil.

3.4. Dynamic water holding capacity of sandy soil with PVA and PAM

Fig. 3 is the dynamic water holding capacity profiles of sandy soil added with different PVAs or PAM. We mimicked the plant growing cycle (28 days) and watered the samples again on the 15^th day just like the plant grown experiment in the next step. It can be seen that in the first periods (1–14^th days), the water content of all the soil samples decreased over the time of exposure. The water content of the control sample (untreated soil) decreased fastest and it was completely dried on the 11^th day. However, the addition of water retention agent could slow down the reduction of water content significantly. The sandy soil samples added with the water retention agents still remained some water even at the end of the first period (14^th day), and the water contents were greater than that of the control during the whole observation period. The effects on the improvement of the water content of sandy soil by different water retention agents were different. The water content at the end of the first 14 days period follows the order: 1795 ≈ PAM > 1792 > 1788 > 1797 > control. The trend of change in water content in the second periods (15–28^th days) is similar to that in the first period. The only difference is that at the end of the second 14 days period, the water content follows the order: 1795 ≈ PAM > 1797 > 1792 > 1788 > control.

Fig. 3 Dynamic water holding capacity of sandy soil samples added with PVA with different degree of hydrolysis and PAM (the samples were watered again at the 15^th day).

From the results we can conclude that the water content of soil added with PVA 1795 and PAM were almost the same and were the highest two among all the samples for most of the time in the plant growth cycle. The water content of soil added with PVA 1797 was lower than PVA 1792 and 1788 in the first 14 days, but was higher than them in the following 14 days, highlighting the importance of flushing away ratio, as shown earlier. In general, the water retention agents could effectively improve dynamic water holding capacity of the sandy soil, which may prolong the irrigation cycles for plants growing in them. PVA 1795 and PAM had the best effect in the whole experimental period, in this aspect.

3.5. Plant growth

Fig. 4 showed the optical photos of Arabidopsis thaliana planted in sandy soil treated with PVAs and PAM before harvest. It was observed that under the condition of water shortage, the Arabidopsis thaliana planted in control samples (untreated soil) were all dead, but the vast majorities of the plants in sandy soil with the addition of PVA or PAM survived and even grew rather well. The survival rate of the Arabidopsis thaliana (Table 3) was significantly increased with water retention agents, especially for PVA 1795 and 1797, which were 100% survived, the same level as PAM had achieved. The survival rate of PVA 1792 and 1788 were 87%, which were lower than that of other water retention agents. Thus, it seems to suggest that PVA 1795 and 1797 can retain sufficient water for plant growth, while PVA 1792 and 1788 can only partially satisfy the water supply.

Fig. 4 Optical images of Arabidopsis thaliana grown in sandy soil samples added with PVA with different degree of hydrolysis and PAM (a) control (b) 1797 (c) 1795 (d) 1792 (e) 1788 (f) PAM.

Table 3 The survival rate of Arabidopsis thaliana grown in sandy soil samples added with PVA with different degree of hydrolysis and PAM

	Water retention agent
	Control	1797	1795	1792	1788	PAM
Survival rate (%)	0	100	100	87	87	100

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To quantify the growth condition of Arabidopsis thaliana as well as the efficiency of different water retention agents, the average biomass of the aerial part and the chlorophyll content of the plants were measured (Fig. 5 and 6). It is clear that the water retention agents could effectively improve the average biomass and chlorophyll concentration of Arabidopsis thaliana. The average biomass follows the order: 1795 > 1797 > PAM > 1792 > 1788. The chlorophyll concentration also had a similar trend: PVA 1795 and PAM were almost the same and were the highest among all the samples; PVA 1797 was in the third place and PVA 1792 and 1788 were at a lower level. Considering survival rate, biomass and chlorophyll concentration, PVA 1795 was the best choice, although its instant water holding capacity is much lower than PAM.

Fig. 5 The average biomass of the aerial part of Arabidopsis thaliana grown in sandy soil samples added with PVA with different degree of hydrolysis and PAM.

Fig. 6 Chlorophyll content in Arabidopsis thaliana grown in sandy soil samples added with PVA with different degree of hydrolysis and PAM.

3.6. Criteria for a good water retention agent in sandy soil

Generally, water content in soil is one of the most important factors affecting plant growth, and it could be evaluated by the instant or dynamic water holding capacity. As seen from the plant growth result, PVA 1795 showed the best performance, even better than PAM. PVA 1797 also showed higher biomass, similar chlorophyll concentration and survival rate, if not better than PAM samples, although PAM has a much higher water absorbency and instant water holding capacity, and higher water content in most of the time in the plant growth cycle. This seems to suggest that to improve the growth condition of plant in sandy soil, as far as a threshold water content is reached, further increasing in water holding capacity is not necessary. Higher water holding capacity may sometimes bring about adverse effect, as suggested by the case of PAM in this study, probably due to the competition of water between plants and PAM. This is rather different from the previous study;²³ however, it is worth noting that the previous study was carried out under the normal irrigation condition, i.e. there was enough water for both plants and PAM, thus no intensive competition would happen. PAM has been widely used as water retention agent, in merit of its high water holding capacity; however, it also brings about serious environmental issues as it is non-degradable. The accumulation of PAM with time may hamper further growth of plant in sandy soil, as suggested in this study. PVA, a degradable polymer, although with medium water holding capacity, can perform as well as PAM, or even better in some aspects. Therefore, PVA may be a potential candidate of water retention agent, to improve plant growth in sandy soil, circumventing the adverse effect of PAM while maintaining sufficient water level for plant growth.

Within the series of PVA, it was found that sandy soil with PVA 1795 had the highest level of water content (Section 3.4), which, as expected, yielded the healthiest plants, indexed by the survival rate, biomass and chlorophyll content. Sandy soil with PVA 1797 showed very little disparity compared with 1795. It is interesting to see that PVA with low degree of hydrolysis – 1792 and 1788, had higher level of water content in the last part of the first 14 days period than PVA 1797, but lower level of water content after watered again on the 15^th day, eventually, plants grown in sandy soils with these two PVAs were in less healthy conditions, lower survival rate, biomass and chlorophyll content. This suggests that not only the instant and dynamic water holding capacity are important criteria to be considered when evaluating water retention capacity, but also the immobilization of water retention agent in sandy soil needs to be taken into account. As shown in Section 3.3, PVA 1792 and 1788 were easy to migrate in sandy soil when flushed with water, therefore, their concentrations will be reduced by each irrigation and finally fail to maintain a sufficient level of water content for plant growth. In particular for PVAs with different degree of hydrolysis, too high the degree of hydrolysis will render the sample hard to absorb water, for example PVA 1799, thus unable to improve water retention performance of sandy soil; while too low the degree of hydrolysis will make the sample highly soluble in water, thus although the polymer chains can associate with many water molecules, they could not be retained in sandy soil and still cannot help the sandy soil to improve water retention performance, like the cases of PVA 1792 and 1788. With appropriate degree of hydrolysis, PVA chains could associate with enough water molecules while still be efficiently retained in sandy soil, thus maintaining sufficient water level for plant growth, like the cases with PVA 1795 and 1797.

Therefore, according to this study, we conclude that these issues should be considered when choosing water retention agents: (i) water absorbency; (ii) retaining ability in sandy soil; (iii) degradability. The first one should be able to help maintain sufficient level of water in sandy soil but not too strong to compete with plant for water and the second one should be sufficiently strong so that the added agents will not be flushed away by water. The degradability is an issue needing more attention as the added water retention agent may have environment concerns in the long term. PVA (for example PVA 1795 and 1797 shown in this study) seems to satisfy these criteria thus may be worth of further investigation for the application as water retention agent in sandy soil treatment.

4. Conclusion

The feasibility of using a degradable polymer—poly(vinyl alcohol) (PVA) as water retention agent for conserving water in sandy soil was investigated. The results showed that PVA could effectively improve water retention capacity of sandy soil and the growth of plants in it. After the addition of PVA, the survival rate, aerial biomass and chlorophyll content of Arabidopsis thaliana all increased substantially compared with those in untreated soil under the condition of water shortage. The relationship between PVA's degree of hydrolysis and its water retention performance in sandy soil was also studied. It was found that with the decrease in the degree of hydrolysis, their water holding capacity improved accordingly, however, their retention ratio in sandy soil decreased, which eventually led to poorer water holding capacity for the part of sandy soil of interest. Only with appropriate degree of hydrolysis, the best performance could be achieved, such as the cases of PVA 1795 and 1797, which could guarantee a sufficient level of water content in the sandy soil and stay locally in the sandy soil, consequently give a better growth condition of the plants in it. It is also interesting to see that although with much higher water holding capacity in most of the plant growth cycle, PAM does not show better improvement of plant growth than PVAs, or sometimes even worse (as compared to PVA 1795), probably due to its competition of water with plants, which suggests that extremely high water absorbency is not the primary pursuit when developing high efficiency water retention agents. This study suggests that PVA could be effective in enhancing the water retention capacity of sandy soil to improve the growth of plants, and together with its degradability, may be a good alterative of PAM in this field.

Acknowledgements

This work was supported by the Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, CAS.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22309a

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