Superabsorbent polymer is a multi-functional polymer material . It has excellent water absorption and water retention performance because of its slightly cross-linked three-dimensional network structure, and a large number of hydrophilic groups, such as hydroxyl and carboxyl groups [2, 3]. It can absorb hundreds times or even thousands times of water higher than its own quality and swell into hydrogel, the absorbed water cannot removable under a certain range of pressure [4, 5] and has repeated water absorbency capacity, safe and non-toxic, good processing and use performance. Therefore, superabsorbent polymers have been widely used in agriculture and forestry [6, 7], medical and health [4, 8], adsorption [9–12], construction [13, 14] and petrochemical industry , etc. In particular, the use of superabsorbent polymer in soil can not only increase water-holding capacity of soil, but also firmly lock rainwater and irrigation water, slowly release stored water in soil to supply water to crops, thereby enhancing soil water retention . It can also promote the formation of soil aggregate structure to improve soil permeability, improve soil structure, reduce the invalid evaporation and deep leakage of soil moisture and nutrients, and improve the utilization rate of water and fertilizer . At present, there are still few reports on the evaporation performance of superabsorbent polymer in soil.
As an important performance factor, the water absorbency of superabsorbent polymer is restricted by many factors, including structure composition, cross-linking density, surface morphology, and solution properties. Therefore, improving the comprehensive properties of the polymer can be considered from the following aspects. I. Introducing diverse hydrophilic groups. According to the literature, although ionic superabsorbent polymer has excellent water absorbency capacity, its gel strength is relatively low after water absorbed. The polymer of non-ionic hydrophilic monomer has low water absorbency, but its water absorption rate is fast, water absorbency is not affected by the external electrolyte, and gel strength is high after water absorbed. If superabsorbent polymer has both ionic hydrophilic groups (carboxyl, sulfonic acid group, tertiary amine group, etc.) and non-ionic hydrophilic groups (hydroxyl, amide group, ester group, etc.), its salt tolerance, water absorption rate, and gel strength will be significantly improved . II. Compound with inorganic components. The strength of inorganic materials is higher than that of polymers, and the price is low. However, inorganic hydrogels have some disadvantages, such as structural instability and irreversibility, but the salt resistance is generally better. Therefore, the composite of polymers and inorganic materials can not only improve the gel strength, but also reduce its cost [19, 20]. Ⅲ. Increase the specific surface area. Increasing the specific surface area of superabsorbent polymers can increase the contact area between water and polymer particles , but excessively reducing the particle size of polymer can have the opposite effect. If the superabsorbent polymers have multi-porous structure, it can increase the contact area with water to improve the water absorption rate .
Cellulose is a kind of water absorbent material containing multiple hydroxyl groups and has a certain water absorbency capacity. In addition, it has large specific surface area, biocompatible, biodegradable, non-toxicity, low cost and renewable [8, 10]. AMPS is a strong anion water-soluble monomer with strong hydrophilic groups (amide group and sulfonic acid group), it has good salt resistance, acid and alkali resistance, and hydrolytic stability . In addition, laterite from Lanzhou was used to produce organic-inorganic superabsorbent polymer, which could not only improve the water absorption capacity of the polymer, but also reduce the production cost, and promote the regional economic development and resource utilization. Herein, HEC, AMPS and laterite were used as raw materials to fabricate a kind of superabsorbent polymer with excellent water absorption, water retention, re-swelling ability, and swelling rate. The swelling mechanism was also studied through the kinetic model, and the swelling process was more in line with the pseudo-second-order swelling kinetics model, indicating that the swelling process was controlled by chemical absorption. Based on the previous laboratory study of the influence of the ratio of laterite to loess on soil evaporation performance, the mixed soil of laterite and loess with mass ratio of 2:4 was selected, and hydrogels after swelling of superabsorbent polymer were used to prepare soil-based anti-evaporation composite materials to study the performance of hydrogels to inhibit soil water evaporation.