Optimization Study on the Freshwater Production Ratio from the Freezing and Thawing Process of Saline Water with Varied Qualities

Abstract

Freezing saline water irrigation is an effective technique for leaching soil salts. However, the coupling effects between initial salinity, sodium adsorption ratio (SAR), and freezing temperature of saline water have not been systematically studied. Therefore, a three-factor and five-level quadratic general rotation combination design was adopted, employing indoor simulated freeze–thaw experiments to investigate the influence of these three factors on the freshwater production ratio. The results indicated the following: (1) Within the range of the set experimental factor levels, the freshwater production ratio after the freezing of saline water ranged from 23.34% to 81.11%. (2) The significant negative effect of the main factor on the freshwater production ratio was descending order: initial salinity > freezing temperature. Although the impact of saline water SAR on the freshwater production ratio is insignificant, the interaction between saline water SAR and the freezing temperature has a highly significant negative effect on the freshwater production ratio. (3) At least 83% of the salt content was leached out in the first 16 h when the ice was thawed at 6 °C. These findings are beneficial for selecting appropriate irrigation times based on irrigation water quality in the actual field application of saline water freezing irrigation, with the aim of more efficiently reducing soil salt content.

1. Introduction

Soil salinization is gaining attention as the expanse of saline agricultural land continues to rise globally [1]. Currently, over 1/6 of the global farmland is impacted by either primary or secondary salinization [2], with projections indicating that this figure may escalate to 1/2 by 2050 [3]. This anticipated increase is attributed to the effects of global warming, environmental degradation, and water scarcity, as reported by the Food and Agriculture Organization of the United Nations (FAO) [4]. Irrigation is recognized as an effective technological intervention for improving saline cropland [5]. However, the use of high-salinity water for irrigation exacerbates the problem rather than ameliorates it [6], particularly in arid and semi-arid regions where freshwater resources are limited, such as Xinjiang, China [7]. Therefore, the effective utilization of abundant underground saline water reserves to address freshwater scarcity presents a significant challenge that requires resolution. In terms of saline land area, China has the third largest area of saline land in the world, which is mainly distributed across 17 provinces located in the northeast, Northern China, and the northwest, as well as along the coastal regions [8]. Among them, the area of saline cultivated land in Xinjiang amounted to 2.33 million hectares, accounting for 37.70% of the total area of cultivated land (6.18 million hectares) in the irrigated areas of Xinjiang (including the Bingtuan farms) [9]. Based on the findings of the Third National Land and Resources Survey, as of the end of 2019, approximately 7.67 million hectares of saline–alkali land in China is categorized as unutilized. With stringent ecological management and control measures in place, this land presents a priority for development and utilization [10]. In the rehabilitation of saline–alkali soils, the availability of water resources is critically important. Numerous instances of reclaimed saline–alkali lands have been deserted due to intense soil salinization and insufficient supplies of freshwater [9]. Although Xinjiang experiences a shortage of freshwater resources, it possesses substantial reserves of subterranean brackish water. In China, the total volume of accessible brackish groundwater stands at 2599.51 × 10⁸ cubic meters, of which 131.42 × 10⁸ cubic meters are located within the borders of Xinjiang [11]. Consequently, advancing desalination technology for saline water and applying these resources judiciously to remediate saline–alkali soils is a critical agricultural strategy that must be pursued in the Xinjiang region.

Current desalination methods primarily utilize techniques such as reverse osmosis, electrodialysis, and thermal desalination [12,13]. Reverse osmosis effectively separates solutes from solvents in saltwater through the filtering action of semi-permeable membranes, consequently yielding desalinated water [14,15,16]. This technique is capable of eliminating nearly all dissolved salts, microorganisms, bacteria, and organic substances. In experimental applications conducted in Southern Xinjiang, this technology demonstrated its ability to decrease water salinity and remove up to 99% of ions. However, the general economic returns are not substantial [17]. Electrodialysis technology (ED) removes ions from a solution by their directional migration in an electric field, thereby reducing the concentration of salts [18,19]. The technology exhibits a high desalination rate, extended membrane lifespan, reusable electrodes, and the absence of secondary environmental contamination. Nonetheless, the energy consumption is directly proportional to the salt content of the inlet water, and the process necessitates water of high purity [20]. Thermal desalination technology operates by heating saltwater to vaporize volatile solvents through heat absorption, followed by condensing the vapor to produce freshwater [21]. Techniques such as multi-effect evaporation, multi-stage flash distillation, and membrane distillation are utilized in this process [22]. The desalinated water produced is of superior quality; however, the process is characterized by substantial energy consumption, considerable operational costs, vulnerability to corrosion and scaling, and challenges in scaling up the operation [22,23,24]. The financial implications of employing these technologies to extract and desalinate subterranean saline water in Xinjiang are notably high. Additionally, the distribution of subterranean saline water is extensive, with variations in salinity levels. Given the high costs and the challenge of balancing investment with returns, integrating these saline water desalination technologies is not deemed feasible.

The freezing method is a technique for desalinating seawater that has a long history of research [25,26,27]. It is characterized by low energy consumption [26,28], minimal environmental pollution [27,29], and low investment costs [30], which are favorable factors for its popularization and application. The freeze desalination process effectively concentrates impurities, including salts, within the non-frozen portion of seawater as ice crystals precipitate during the freezing stage [27]. Under slow freezing conditions, theoretically, pure salt and pure water can be obtained. However, in practice, the majority of salts are excluded, with a small portion existing in the form of “salt cells” within the ice matrix and some adhering to the ice surface. Upon melting, the salts contained within the ice in the state of “salt cells” melt and flow out of the ice first, achieving desalination [31]. Both the freezing and melting processes yield a certain quantity of freshwater. Current studies have integrated the freezing method with the utilization of saline water/brackish water and saline–alkali soil improvement, enabling the desalination of salinized soils. This integrated approach has emerged as an innovative strategy for improving saline–alkali lands in recent years [32]. This innovative method harnesses the principle of freeze desalination. It involves irrigating underground saline water onto saline–alkali land when the ambient temperature persistently falls below the freezing point of saline water. By capitalizing on the natural cycle of freezing and thawing, the method effectively diminishes the salinity of the saline–alkali land during the early spring planting season. Furthermore, the presence of the ice layer in winter acts as an insulating barrier, preserving the land’s temperature [33,34]. This study is predicated on specific environmental criteria: it is primarily intended for saline–alkali soils to mitigate their salt content. Additionally, the targeted saline–alkali regions must possess a significant volume of utilizable brackish water. Moreover, the local winter temperatures should persistently remain below the freezing point for an extended duration. Given the high salinity of the irrigated brackish water, the agricultural fields require effective drainage systems. The research is particularly applicable to northern areas characterized by cold winter climates, soils with high salinity levels, substantial saline water resources, and well-developed drainage infrastructure. Numerous studies have investigated the improvement of saline soils through saline water freezing irrigation in coastal saline areas and the Hetao Irrigation District in Inner Mongolia, among other places. The findings indicate that employing freezing saline water irrigation significantly improves saline–alkali lands [32,35]. Specifically, irrigation with saline water at salinity levels of 8 g/L and 16 g/L resulted in a reduction of soil salinity within the 0 to 50 cm soil depth. Conversely, the use of water with salinity levels exceeding 24 g/L led to salt accumulation in the soil [36]. Furthermore, the SAR of the irrigation water significantly affects the efficiency of desalination in saline soils as well as the soil’s moisture content [37]. Irrigation using freezing saline water with a SAR of 5 and 10 has been shown to produce more effective leaching effects on coastal saline–alkali soils compared to water with a SAR of 30 [38]. The desalination of saltwater occurs through the processes of freezing and thawing, where thawing typically exhibits a superior desalination outcome compared to freezing [39,40]. It has been shown that the removal of salt from ice is significantly enhanced when the slow freezing method (i.e., higher freezing temperatures) is applied [41]. In addition, there is a positive correlation between the freezing temperature and the rate of desalination [31,42]. Notably, in practical scenarios, it is critical to acknowledge that the ambient temperature during the spring thawing of ice is subject to unpredictability and beyond direct control. Nonetheless, the selection of the freezing temperature presents a degree of controllability. By utilizing regional environmental temperature forecasts, we can strategically determine the optimal timing for irrigation.

Consequently, this study employed a quadratic general rotary combination design with three experimental factors: the initial salinity of saline water, SAR, and freezing temperature. Five levels were established for each factor, and an indoor freeze–thaw simulation experiment was conducted to mimic the natural winter conditions with a top–down freeze–thaw pattern. Following the experimental protocol, we measured the salinity degree and volume of the melt water during the melting process of the ice column. Through these measurements, we computed the volume of freshwater extracted and established a relationship equation with the three influential factors via regression analysis. The primary objectives of this study are threefold: (a) to delineate the correlation between the freshwater production ratio and the initial salinity of the brackish water, the sodium adsorption ratio (SAR), and the freezing temperature; (b) to pinpoint the critical factors influencing the freshwater extraction rate; and (c) to elucidate the patterns and proportions of salt release during the melting process of saline water with varying salinities. The findings of this research aim to inform regions conducive to brackish water freezing irrigation, enabling them to optimize irrigation schedules based on local environmental temperatures and groundwater quality, thereby maximizing the desalination efficacy on saline–alkali soils.

2. Materials and Methods

2.1. Test Materials

Soil surface salt crusts were collected in Jiashi County, China (77.0340 E, 39.5780 N), and after removing impurities such as stones in the laboratory, the salt crusts were soaked and washed with distilled water to prepare the initial salt solution. The initial salt concentration and composition of the salt ions in the prepared saline solution are as follows: Ca²⁺: 802.55 mg/L; Mg²⁺: 872.7 mg/L; K⁺: 216.7 mg/L; Na⁺: 30,868.0 mg/L; salt concentration: 96.1 g/L; and SAR: 179.55 (mmol/L)^1/2.

2.2. Experimental Design

In this study, a quadratic general rotary combination design was employed, encompassing three experimental variables: the initial salinity of saline water, the SAR of the saline water, and the freezing temperature. Each variable was evaluated at five distinct levels, resulting in a total of 20 treatment combinations, with each treatment replicated three times. The coded values and actual levels of these experimental variables are delineated in

Table 1. Brackish water/saltwater desalination rule processing code value and actual value.

Experimental Factor Encoding Value	Actual Value of Test Factor
	Salinity (g/L)	SAR	Freezing Temperatures (°C)
1.682	23	52	−6
1	19	42	−9
0	13	27	−14
−1	7	12	−19
−1.682	3	2	−22

, while the details of the experimental treatments are provided in

Table 2. Brackish water/saltwater desalination process number.

Processing Number	Coded Value			Actual Value
	Salinity	SAR	Freezing Temperatures	Salinity (g/L)	SAR	Freezing Temperatures (°C)
1	1	1	1	19	42	−9
2	1	1	−1	19	42	−19
3	1	−1	1	19	12	−9
4	1	−1	−1	19	12	−19
5	−1	1	1	7	42	−9
6	−1	1	−1	7	42	−19
7	−1	−1	1	7	12	−9
8	−1	−1	−1	7	12	−19
9	1.682	0	0	23	27	−14
10	−1.682	0	0	3	27	−14
11	0	1.682	0	13	52	−14
12	0	−1.682	0	13	2	−14
13	0	0	1.682	13	27	−6
14	0	0	−1.682	13	27	−22
15	0	0	0	13	27	−14
16	0	0	0	13	27	−14
17	0	0	0	13	27	−14
18	0	0	0	13	27	−14
19	0	0	0	13	27	−14
20	0	0	0	13	27	−14

.

Based on the original salt solution ion composition, different amounts of NaCl, CaCl₂, and MgCl₂ (analytical grade) were added to the original salt solution. According to Formula (1), five types of brackish water/saline solutions with different SAR and salinity were prepared corresponding to the actual values in (Table 2). The formula of SAR is shown in Formula (1).

$$\mathrm{SAR}=\left[{\mathrm{Na}}^{+}\right]/{\left(\left[{\mathrm{Ca}}^{2+}\right]+\left[{\mathrm{Mg}}^{2+}\right]\right)}^{{\displaystyle \frac{1}{2}}}$$

(1)

In the formula, SAR represents the sodium adsorption ratio of the aqueous solution; [Na⁺] is the concentration of sodium ions in the aqueous solution, measured in millimoles per liter (mmol/L); [Ca²⁺] is the concentration of calcium ions in the aqueous solution, also measured in millimoles per liter (mmol/L); and [Mg²⁺] is the concentration of magnesium ions in the aqueous solution, measured in millimoles per liter (mmol/L).

Fill the pre-configured saltwater into a numbered 600 mL plastic container, each filled with 500 mL of solution, with three replicates for each treatment. To mimic the natural freezing pattern from top to bottom in situ, the sides and bottom of the container were insulated with a 1 cm thick aluminum foil-backed foam insulation, allowing for the conduction of low temperatures from the top to the bottom. Following the experimental protocol, the containers were positioned within a cold storage facility, where the temperature was precisely controlled, as specified in Table 2. The freezing sequence initiated from the highest designated temperature, beginning with the −6 °C treatment. Upon the achievement of complete solidification, the temperature was systematically lowered to −9 °C for the subsequent treatment, and this procedure was repeated until all treatments were executed. The samples remained stored in the cold storage until the water samples for the −22 °C treatment were entirely frozen. Subsequently, all containers were uniformly transferred to a cold storage environment maintained at 6 °C. Below each container, a beaker was placed to collect the meltwater, with collections scheduled at 4 h intervals. This process yielded a total of 16 sets of meltwater samples, which were subsequently analyzed for salinity and volume. The detailed experimental setup is illustrated in Figure 1.

2.3. Index Measurement

The salinity of melted water was determined by the gravimetric method (HJ/T 51-1999; Water quality-Determination of total salt-Gravimetric method. Ministry of Ecology and Environment of the People’s Republic of China: China, 1999). The operational procedure for the gravimetric method involves filtering the collected meltwater, followed by evaporation using a steam bath. Subsequently, a hydrogen peroxide solution is employed to remove organic matter. The sample is then dried to constant weight at 105 °C ± 2 °C, and the weight of the residue is measured. The calculation formula is presented in Formula (2).

$$\mathrm{\rho }={\displaystyle \frac{\mathrm{\omega }-{\mathrm{\omega }}_0}{\mathrm{V}}}\times {10}^3$$

(2)

In the formula, $\mathrm{\rho }$ is the total salt content of the sample, g/L; $\mathrm{\omega }$ is the weight of the evaporating dish with the sample, g; ${\mathrm{\omega }}_0$ is the weight of the evaporating dish, g; and $\mathrm{V}$ is the volume of the water sample, mL.

The measurement of the volume of the melted water was carried out using a graduated cylinder with an accuracy of 0.5 mL.

According to the ‘Agricultural Irrigation Water Quality Standards’ (GB5084-2021; Standard for Irrigation Water Quality. Ministry of Ecology and Environment of the People’s Republic of China: China, 2021.), which stipulate a salinity of less than 2 g/L for irrigation water in saline–alkali soil regions, this research classified water with salinity below 2 g/L, resulting from the thawing of saline ice, as freshwater. The formula of the freshwater production ratio is shown in Formula (3).

$$\mathrm{Freshwater} \mathrm{production} \mathrm{ratio}={\displaystyle \frac{\mathrm{The} \mathrm{volume} \mathrm{of} \mathrm{freshwater}(<2 \mathrm{g}/\mathrm{L}) \mathrm{produced} \left(\mathrm{mL}\right) }{\mathrm{Total} \mathrm{water} \mathrm{volume} \left(\mathrm{mL}\right)}}$$

(3)

2.4. Data Processing

Data were collated using Microsoft Excel 2019 (Microsoft Corporation, Redmond, WA, USA). Data processing was conducted using the Central Composite Design feature within the Design-Expert 13 (Stat-Ease, Inc., Minneapolis, MN, USA), and graph plotting was carried out using Origin 2021 (OriginLab Corporation, Northampton, MA, USA).

3. Results

3.1. The Primary Factors Affecting the Freshwater Production Ratio After Saline Water Freezing and Thawing

Regression analysis was performed on the data of freshwater production ratio after the thawing of saltwater ice with a concentration less than 2 g/L, yielding the following regression equation:

$$\mathrm{Y}=44.07-12.22{\mathrm{X}}_1-0.7186{\mathrm{X}}_2+3.29{\mathrm{X}}_3+0.4515{\mathrm{X}}_1{\mathrm{X}}_2+2.16{\mathrm{X}}_1{\mathrm{X}}_3-3.27{\mathrm{X}}_2{\mathrm{X}}_3+3.05{\mathrm{X}}_1^2+1.45{\mathrm{X}}_2^2-2.28{\mathrm{X}}_3^2$$

(4)

$$\mathrm{Y}=86.12062-3.42828{\mathrm{X}}_1-1.08078{\mathrm{X}}_2-1.93261{\mathrm{X}}_3+0.005017{\mathrm{X}}_1{\mathrm{X}}_2+0.07195{\mathrm{X}}_1{\mathrm{X}}_3-0.043553{\mathrm{X}}_2{\mathrm{X}}_3+0.086791{\mathrm{X}}_1^2+0.006623{\mathrm{X}}_2^2-0.101663{\mathrm{X}}_3^2$$

(5)

In Equation (4), $\mathrm{Y}$ is the freshwater production ratio (%), ${\mathrm{X}}_1$ is the initial salinity of saline water, ${\mathrm{X}}_2$ is the initial SAR of saline water, and ${\mathrm{X}}_3$ is the freezing temperature. All values are coded value levels.

In Equation (5), $\mathrm{Y}$ is the freshwater production ratio (%), ${\mathrm{X}}_1$ is the initial salinity of saline water (g/L), ${\mathrm{X}}_2$ is the initial SAR of saline water, and ${\mathrm{X}}_3$ is the freezing temperature (°C). All values are actual values.

The analysis of variance (ANOVA) was conducted on the regression equation, and the findings are presented in

Table 3. Results of variance analysis of the regression equation.

Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Model	2578.32	9	286.48	33.75	<0.0001 **
${\mathrm{X}}_1$	2040.27	1	2040.27	240.35	<0.0001 **
${\mathrm{X}}_2$	7.05	1	7.05	0.8309	0.3834
${\mathrm{X}}_3$	147.81	1	147.81	17.41	0.0019 **
${\mathrm{X}}_1{\mathrm{X}}_2$	1.63	1	1.63	0.1921	0.6705
${\mathrm{X}}_1{\mathrm{X}}_3$	37.27	1	37.27	4.39	0.0626
${\mathrm{X}}_2{\mathrm{X}}_3$	85.36	1	85.36	10.06	0.0100 **
${\mathrm{X}}_1^2$	134.10	1	134.10	15.80	0.0026 **
${\mathrm{X}}_2^2$	30.11	1	30.11	3.55	0.0890
${\mathrm{X}}_3^2$	74.85	1	74.85	8.82	0.0141 *
Residual	84.89	10	8.49
Lack of Fit	62.21	5	12.44	2.74	0.1462
Pure Error	22.68	5	4.54
Cor Total	2663.21	19
R2	0.9681

Note: * indicates a significant level (p ≤ 0.05); ** indicates a highly significant level (p ≤ 0.01).

.

The analysis of the table above reveals that the regression equation, with an R² value of 0.9681 and a p-value less than 0.01, is highly significant. The lack-of-fit test yields an F-value of 2.74 and a p-value of 0.1462, indicating no significant difference. This suggests that the control factors have a minimal impact on the experimental outcomes, making them suitable for fitting the regression equation. Consequently, this regression equation has been validated and is capable of explaining the freshwater production ratio for various water qualities at different freezing temperatures, as well as the correlation between salinity degree, SAR, freezing temperature, and freshwater production ratio.

The absolute values of the partial regression coefficients (or partial correlations) serve as indicators of the significance of each determinant factor, with the signs of the coefficients representing the positive or negative impact of these factors. Analysis of regression in Equation (5) and the data in Table 3 reveal that the initial salinity of the saline water and the freezing temperature are highly significant factors influencing the freshwater production ratio. In contrast, SAR exhibits no significant effect on the freshwater production ratio. In the scope of the experimental values, the factors affecting the freshwater production ratio are ranked as follows: initial salinity has the greatest impact, followed by freezing temperature and then SAR, with all factors showing negative effects. The interaction effect of saline water SAR and freezing temperature on the freshwater release rate is significant and negative. The interaction between the initial salinity of saline water and SAR, as well as the initial salinity and freezing temperature, has no significant effect on the freshwater production ratio from saline ice. The regression model predicts that, within the tested range of factors, the production ratio of freshwater from saline water with concentrations ranging from 3 to 23 g/L after the freezing and melting process will fall between 23.34% and 81.11%, with a 95% confidence interval.

3.2. The Impact of Individual Factors on the Freshwater Production Ratio of Saline Ice

By fixing two factors at the 0 level and conducting a dimensionality reduction analysis on the actual values of the regression equation ((Equation (5)), a single-factor partial regression equation can be obtained.

$${\mathrm{Y}}_1=86.12062-3.42828{\mathrm{X}}_1+0.086791{\mathrm{X}}_1^2$$

(6)

$${\mathrm{Y}}_2=86.12062-1.93261{\mathrm{X}}_3-0.101663{\mathrm{X}}_3^2$$

(7)

The quadratic term in the equation relating initial salinity to freshwater production ratio has a positive coefficient, suggesting a characteristic curve that is a parabola opening upwards. This implies that as the initial salinity rises, the freshwater production ratio tends to decrease. In the equation between freezing temperature and freshwater production ratio, the coefficient of the quadratic term is negative, indicating that the curve it represents is a downward-opening parabola. As the freezing temperature increases, the freshwater production ratio shows a trend of first increasing and then decreasing, suggesting that both salinity and freezing temperature have optimal values, and their levels will affect the effectiveness of the freshwater production ratio.

The influence of single factors, including initial salinity, SAR, and freezing temperature, on the freshwater production ratio from saline ice is depicted in (Figure 2).

Table 3 and Figure 2 reveal that the initial salinity and freezing temperature of saline ice significantly influence the freshwater production ratio (p ≤ 0.01), whereas the SAR of saline water does not. Within the tested range of factors, a decrease in the initial salinity of saline ice corresponds to a higher freshwater production ratio. Under optimal conditions, with a 95% confidence interval, the freshwater production ratio from saline water with a salinity of 3 g/L can reach a maximum of 81.11% following the freezing and melting process. Conversely, for saline water with a salinity of 23 g/L, the freshwater production ratio is at least 23.34% after undergoing the same process.

3.3. The Influence of Factor Interactions on the Freshwater Production Ratio from Saline Ice

The interaction effects of initial salinity, SAR, and freezing temperature on the freshwater production ratio of saline ice are shown in Figure 3.

Table 3 and Figure 3 reveal that in the pairwise interactions among initial salinity, SAR, and freezing temperature, the interaction between SAR and freezing temperature is the most significant, followed by the interaction between initial salinity and freezing temperature, and, finally, the interaction between initial salinity and SAR. Among them, the interaction between SAR and freezing temperature has a highly significant impact on the freshwater production ratio, while other interactions have no significant effect. According to the coefficients of the regression (Equation (5)), the interactive effect between SAR and freezing temperature is negative. The interaction between SAR and freezing temperature shows a dual-peak pattern. In the range of values, the freshwater production ratio increases with freezing temperature when SAR is −1.682 (2). Conversely, when SAR is 1.682 (52), the freshwater production ratio decreases as the freezing temperature rises. The maximum freshwater production ratio is achieved when SAR is −1.682 (2) and the freezing temperature is 1.682 (−6 °C). When the salinity is 1.682 (23 g/L) and the freezing temperature is −1.682 (−22 °C), the freshwater production ratio reaches the minimum value of 14.05%.

3.4. Characteristics of Water Quantity and Quality Changes During the Melting Process

During the melting process, a constant temperature of 6 °C was maintained, with the collected meltwater being taken every 4 h. Based on the actual values of regression (Equation (5)) and the variance analysis results presented in Table 3, salinity was found to have the most significant impact on the freshwater production ratio.

Therefore, only different levels of salinity were considered to investigate the patterns of water quantity and salinity of the melt water every 4 h (as depicted in Figure 4). The initial quantity of meltwater from saline ice with varying initial salinities decreases as the salinity decreases, with the volumes being 23 g/L > 19 g/L > 13 g/L > 7 g/L > 3 g/L. The salinity of the melt water follows the sequence 19 g/L > 23 g/L > 13 g/L > 7 g/L > 3 g/L. The higher salinity of 19 g/L compared to 23 g/L might be attributed to the melting temperature of 6 °C, where the saltier components melt along with the freshwater ice. The volume of meltwater influences the alteration in salinity, and the freezing temperature during the freezing process also affects the salinity of the meltwater. The meltwater diagram also reveals that the first batch of meltwater from an initial salinity of 23 g/L contains more salt than that from an initial salinity of 19 g/L. The salinity of meltwater from saline ice with varying initial salinities exhibits a pattern of rapid decline followed by a trend toward 0 g/L. Specifically, the salinity of the fourth batch (12–16 h) of meltwater from saline ice with an initial salinity of 3 g/L is 1.80 g/L, which is less than 2 g/L. Similarly, the salinity of the seventh batch (24–28 h) of melt water from saline ice with initial salinities of 7 g/L, 13 g/L, 19 g/L, and 23 g/L also falls below 2 g/L. Starting from the sixth batch (24 h), the amount of meltwater from saline ice with different initial salinities every 4 h exhibits a decreasing trend. The change in water volume every 4 h before the sixth batch (24 h) is due to the fact that the ice body needs to absorb heat during the melting process. When it reaches the freezing point temperature of the saline water, the saltier parts will flow out first, causing an increase in water volume; when the saltier parts are discharged, the freshwater ice body continues to absorb heat until it is completely melted. At a constant melting temperature of 6 °C, it takes 48 h for saline ice with an initial salinity of 23 g/L to fully melt, 52 h for 19 g/L, 60 h for both 7 g/L and 13 g/L, and 64 h for 3 g/L. Consequently, at the same ice melting temperature, higher initial salinities result in shorter melting durations.

During the melting process of saline ice, a phenomenon of salty and freshwater separation occurs. The higher the initial salinity of the saline ice, the greater the amount of water released during the early stage of melting (before the fourth batch) at the same time. As the melting time increases, the amount of melt water continues to decrease, and the salinity of the melt water approaches 0 g/L. For saline ice with an initial salinity of 3 g/L, 51% of the salt is released in the first batch, and the proportion of salt released in the first batch increases with the increase in initial salinity. Regardless of the initial salinity, at least 83% of the salt in the saline ice is released in the first four batches (16 h), and the subsequent freshwater can leach soil salinity.

4. Discussion

4.1. The Influence of Saline Water’s Salinity, SAR, and Freezing Temperature on the Freshwater Production Ratio

In the application of saline water freezing irrigation to ameliorate saline–alkali soils, the factors of irrigation water salinity, SAR, and freezing temperature are crucial considerations for the desalination efficiency of the soil [36,38,42]. In this study, a three-factor, five-level quadratic universal rotatable combination design was used for the experiment, aiming to minimize the number of trials while seeking the best outcomes. The predictions of the model were used to investigate the interrelationships among the three factors and to optimize their combinations more effectively, resulting in a higher rate of freshwater extraction. According to the analysis of variance, the model is highly significant and represents the actual situation well. This model can be effectively applied to saline field reclamation by controlling the freezing temperature according to the saline water quality to maximize the freshwater production ratio and improve the soil salt leaching rate.

The impact of individual factors on the freshwater production ratio was as follows: initial salinity > freezing temperature > SAR (with insignificant impact). This was supported by the findings of Rich et al. [41] that the initial concentration of the solution and the rate of freezing significantly impact the purity of the ice layer. Additionally, Wang et al. [43] have provided further evidence, using a combined model and microscopic experiments, to show that both the initial concentration of the solution and the freezing temperature plays a crucial role in the crystallization process of seawater, leading to changes in the purity of sea ice and the salt exclusion channels in the ice body. Chu et al. [44] employed a perspective view of salt solution droplet freezing and time-lapse snapshots of brine ice via molecular dynamics simulation to demonstrate that ice crystal nucleation is accompanied by the exclusion and concentration of brine, with concentrated saltwater residing between the ice dendrites. Within the range of experimental factor values, the higher the initial salinity, the more salt is in the solution and the lower the freshwater production ratio. This finding is consistent with existing research results, where salt is nearly insoluble in ice, and freezing ice tends to exclude salt from the unfrozen water [43,45]. The freshwater that first freezes forms a solid phase in the ice, while the brine at the bottom of the ice is in a liquid phase. After warming and melting, the liquid and solid phases separate, resulting in the formation of less desalinated water from a higher initial salinity solution [46,47,48]. Conversely, solutions with lower initial salinity have less salt solution to remove, resulting in a higher purity of ice and a higher rate of freshwater melting out after freezing.

Across the spectrum of experimental factor values, the relationship between freezing temperature and freshwater production ratio exhibits a trend of initially increasing followed by a decrease. It is crucial to maintain an appropriate freezing temperature during the freezing process, as excessively low temperatures can hinder the effectiveness of salt and freshwater separation. This observation aligns with the findings of Barma’s investigation, which demonstrated that supercooled solutions undergo rapid freezing at the commencement of the freezing process, while the elevated freezing rate impedes the prompt expulsion of salts [49]. Salts with low solubility may solidify with the ice or be trapped in liquid form at the base of the ice block, forming “salt cells”. These salts tend to form a dendritic branching structure within the ice [31,50]. As the ice body thickens and the freezing rate decreases, the conditions become more favorable for the expulsion of the salt solution, thereby enhancing the desalination rate [51,52].

The present study found that the influence of individual factors in SAR on the freshwater production ratio is not significant. This is consistent with the findings of Guo et al. [53], which state that the SAR exhibits a minimal influence on the melt-out rate of freshwater. During salt ice melting with identical initial salinity but varying SAR, no significant differences were observed in the volume and salinity of the resultant meltwater. Furthermore, both SAR and salinity demonstrated a similar trend, gradually decreasing throughout the melting process. Wang et al. [43] discovered in their research that the peaks of cations in the meltwater of this study arrived in sequence as Na⁺ > K⁺ > Mg²⁺ > Ca²⁺. During the melting process of saline ice with varying SAR values, Na⁺ exhibits a higher migration rate compared to Ca²⁺ and Mg²⁺. The SAR of the meltwater progressively diminishes and tends to converge toward a consistent value [53]. In the soil environment, the early displacement of sodium ions during the initial stages of ice melting causes their migration to deeper soil layers, resulting in the dispersion of soil particles. Subsequently, the release of calcium and magnesium ions promotes the aggregation of soil particles. The increased soil porosity resulting from this process aids in the deeper penetration of freshwater produced during the later stages of ice melting, effectively leaching salts from the soil and thereby achieving the desalinization of the topsoil [38]. Although the influence of saline water SAR on the freshwater melting rate is not significant, our study revealed a highly significant interaction between saline water SAR and freezing temperature on the freshwater melting rate. During the melting process, the migration of ions affects soil particles, resulting in varied impacts and altering the physicochemical properties of the soil. This necessitates further in-depth research on the patterns of ion migration in soil following the freeze–thaw cycles of saline water with different SAR values.

The impact of the two-factor interactions on the freshwater production ratio is ranked as follows: the interaction between SAR and freezing temperature > the interaction between initial salinity and freezing temperature > the interaction between initial salinity and SAR. Among these, only the interaction between SAR and freezing temperature demonstrates a highly significant effect. Within the tested parameter range, the freshwater production ratio showed a rise in the freezing temperature of saline water and a decrease in SAR. Similarly, a reduced freezing temperature coupled with an elevated SAR leads to an increased ratio of freshwater production. The interaction between freezing temperature and SAR reveals a dual-peaked pattern. This may be related to the migratory behavior of ions during freezing and melting. Yang et al. [54] investigation has elucidated that the freezing of saline water is characterized by pronounced variations in the migration velocities of diverse ions. Ions with elevated concentrations in aqueous solutions and larger monatomic ions are more susceptible to being “trapped” by ice crystals. Furthermore, at lower temperatures of freezing, these ions are more likely to be “trapped”. The melting of saline ice is also marked by disparities in the dissolution concentrations of ions. Lv et al. [55] observed that Na⁺ and Cl⁻ tend to migrate and accumulate in the lower layers of the lake ice. Similarly, Zhang et al. [56] noted that Mg²⁺ migrates rapidly toward the water layer beneath the ice when studying the migration pattern of magnesium ions during freeze–thaw cycles and that the SAR is correlated with the cations Ca²⁺, Mg²⁺, and Na⁺. The pronounced influence of both freezing temperature and SAR on the freshwater production ratio indicates that the freezing temperature modifies the ion migration rates during freezing and melting, consequently affecting the freshwater production ratio. Consequently, in practical applications, selecting an optimal freezing temperature according to the salinity and SAR of underground saline water can improve the freshwater production ratio in saline water freezing irrigation, thereby enhancing soil salt leaching efficiency.

4.2. Characteristics of Water Quantity and Quality Changes in the Melting Process

According to our research, within the range of the experimental factors, freshwater with salinity ranging from 23.34% to 81.11% can be produced. The amount of freshwater that can be produced from the process of freezing and thawing of saline ice will significantly impact the desalination effectiveness of saline–alkali soil. Guo et al. [53] indicated in their study that the melting of 10 g/L saline ice at room temperature yields approximately 49% water with a salinity of less than 3 g/L. However, the volume of brackish and freshwater produced by brines of different salinities after freeze–thaw cycles remains unclear. Our study provides insights into the freshwater production ratio of less than 2 g/L from saline water with different qualities at different freezing temperatures. In this study, the salinity of the first four batches of melt water decreased sharply and gradually approached 0 g/L as the melting progressed. This also verifies the formation of “salt cells” within the saline ice, which absorb heat and migrate downward under the influence of gravity [50]. As the melt water flows through these channels, the concentration of salt between the ice crystals is higher in the early stages, leading to an increase in melt water salinity. Subsequently, the salinity of melt water decreases due to the deposited salts being washed away [54,57]. These findings can further inform the practice of saline ice irrigation for saline–alkali land reclamation, tailored to local water quality conditions and irrigation timing (freezing temperature). In nature, temperatures fluctuate throughout the day, with colder nights and warmer days. The optimal freezing temperature determined in this study can be applied by maintaining the highest local air temperature at the chosen optimal temperature. It is crucial to acknowledge that discrepancies may exist between the freshwater production ratio obtained in this controlled experiment and those occurring under dynamic environmental conditions due to factors such as temperature variations and the sublimation of ice. Under ideal conditions, the freshwater production ratio within the range of experimental factor values is between 23.34% and 81.11%, with a salinity of less than 2 g/L. In this study, the melting temperature was constant at 6 °C, resulting in both saline and freshwater reaching their melting points and subsequently flowing out together. In the natural environment, the melting process occurs slowly and continuously. Due to the discrepancy between soil and air temperatures [58,59,60], the more saline regions of the ice mass may begin to melt and infiltrate into the soil at −3 °C. This process facilitates a better separation of the more saline fraction from the freshwater ice mass, potentially increasing the ratio of freshwater melting. Therefore, the freshwater produced by slow melting under variable temperature conditions may be more than that obtained at a constant temperature, and the melting temperature also affects the freshwater production ratio.

5. Conclusions

This study investigates the influence of the initial salinity of saline water, SAR, and freezing temperature on the ratio of freshwater produced during thawing and provides an algorithm for calculating the proportion of freshwater that can be melted out from saline water with different water qualities at various freezing temperatures. Within the range of experimental factors, the impact of each factor on the freshwater production ratio is as follows: initial salinity > freezing temperature, with a highly significant and negative effect. The impact of interaction effects on the freshwater production ratio is as follows: SAR and freezing temperature > initial salinity of brine and freezing temperature > initial salinity of brine and SAR. Different initial salinity of brine ice melts over time, and the salinity of the melted water shows a trend of rapid decline and then approaches 0 g/L. The higher the initial salinity of the brine ice, the shorter the melting time required. Moreover, when the brine ice with different salinity melts at 6 °C, at least 83% of the salt is melted out in the first 16 h. This study provides theoretical foundations and practical guidance for the improvement of saline–alkali soil through the irrigation of saline ice, considering irrigation water quality and timing. However, this research only takes into account the freshwater production ratio of saline ice with different water qualities at a constant temperature. In field conditions, the temperature at which the ice melts varies, potentially affecting the amount of freshwater produced. The results indicate that the impact of SAR on the freshwater production ratio is not significant. Nonetheless, in practical applications, the ionic composition of the melted water can undergo ion exchange with the soil, which may affect soil structure and, consequently, the desalination effectiveness. Therefore, when acting on the soil, special attention should be given to the influence of SAR on desalination outcomes. Future research should involve soil column experiments for further validation, aiming to provide ample theoretical support for the utilization of saline water in saline–alkali soil improvement.