Potential of Epipremnum aureum and Bacopa monnieri (L.) Wettst for Saline Phytoremediation in Artificial Wetlands

Abstract

The aim of this research was to evaluate the phytoremediative potential of Epipremnum aureum and Bacopa monnieri to improve the chemical properties of irrigation water exposed to the following two saline concentrations: highly saline (EC 2000 μS cm⁻¹) and severely saline (EC 4000 μS cm⁻¹). The artificial wetlands used in this experiment were of the free water surface type, considering a hydraulic retention time of 42 days. The evaluated treatments were configured as follows: T1 (B. monnieri [control, 300 μS cm⁻¹]), T2 (B. monnieri [2000 μS cm⁻¹]), T3 (B. monnieri [4000 μS cm⁻¹]), T4 (E. aureum [control, 300 μS cm⁻¹]), T5 (E. aureum [2000 μS cm⁻¹]), T6 (E. aureum [4000 μS cm⁻¹]), T7 (B. monnieri + E. aureum [control, 300 μS cm⁻¹]), T8 (B. monnieri + E. aureum [2000 μS cm⁻¹]), and T9 (B. monnieri + E. aureum [4000 μS cm⁻¹]). The results showed that the species B. monnieri and E. aureum (both separately and together) showed a good ability to reduce the salinity of the irrigation water. However, B. monnieri showed a greater ability of phytoremediation, to the point of improving its chemical properties and reducing potential damage to the soil to use this water. In the highly saline group, B. monnieri accumulated 7.992 g per experimental unit and achieved to reduce of the pH from 7.96 to 7.75, EC from 2000 μS cm⁻¹ to 670 μS cm⁻¹, SAR from 13.54 to 3.91 and ESP from 20.17 to 5.83, which allowed it to go from (C3-S3) to (C3-S1). In the severely saline group, B. monnieri accumulated 13.494 g per experimental unit and achieved to reduce the pH from 8.14 to 7.91, EC from 4000 μS cm⁻¹ to 1730 μS cm⁻¹, SAR from 27.35 to 8.73, ESP from 40.35 to 13.01, which allowed it to go from (C4-S4) to (C3-S2).

1. Introduction

Saline stress is a form of abiotic stress that inhibits germination and limits agricultural production worldwide [1]. It is estimated that approximately 800 million hectares in the world are affected by salinity [2] and that its influence has an effect on almost 50% of irrigated lands [3].

According to Zhao et al. [4], saline wastewater generated in agricultural drainage as a result of primary and secondary soil salinity problems negatively affects various agricultural areas, especially when this water resource tends to be used as an alternative for irrigation in the face of the growing scarcity of good quality water, which, in turn, causes an increase in salinity and toxicity in agricultural fields [5], as well as a reduction in the levels of survival, growth, and productivity of crops, since the ionic and osmotic stress induced by NaCl prevents them from having an adequate acquisition of nutrients, transpiration, and proper functioning of the photosynthetic apparatus [6].

In order to tackle the harmful effects of saline wastewater on ecosystems, the use of biological reactors, physicochemical technologies, or a combination of incorporated ecological engineering techniques has been proposed [7]. However, the problem with these technologies is that they use sophisticated equipment that requires large economic investments for their construction and operation, as well as high energy consumption [8].

For this reason, the treatment of saline agricultural wastewater through artificial or constructed wetlands (CWs) has received greater attention in recent years—especially in developing countries for being an environmentally friendly and aesthetic remediation technology with relatively low costs for its maintenance and operation [9].

Constructed wetlands (CWs) are structures made up of plant species, substrates, and microorganisms that function by imitating the natural processes of natural wetlands in a more controlled environment [10]. CWs are characterized by having a very slow water flow and shallow depth, which allows for the settlement of sediments, as well as a longer contact time between wastewater and wetland components in order to effectively reduce or eliminate salinity levels [11].

In recent years, the use of various halophyte species in CWs has been proposed as a natural way to reduce the salinity of agricultural wastewater, being that these plants have the capacity to accumulate, store or degrade large amounts of NaCl, while completing their life cycle [12]. The foregoing has led to the search for new halophyte plants with NaCl phytoremediation capacity for the treatment of agricultural wastewater, which could be used in CWs with the purpose of improving the edaphic quality of soils, increasing agricultural production, and promoting the development of a more sustainable agriculture [13].

Many studies have been conducted in this regard. For example, Farzi et al. [14] evaluated the feasibility of using three halophytic species of the Chenopodiaceae family (Salicornia europaea, Salsola crassa, and Bienertia cycloptera) for treating saline wastewater within CWs. Fountoulakis et al. [15] evaluated the ability of three halophytes (Atriplex halimus, Juncus acutus and Sarcocornia perennis) to phytodesalinate domestic wastewater through a vertical flow constructed wetland (VFCW). Jesus et al. [16] evaluated the ability of three halophytic species (Arundo donax, partina maritima, and Juncus maritimus) under hydroponic conditions, to reduce or remove of salt (EC, Na⁺, Cl⁻, SAR) and nutrients (NO₃⁻–N, NH₄⁺–N and PO₄³⁻–P) at saline wastewater. Guesdon et al. [17] evaluated the potential of two salt tolerant plants (Typha angustifolia and Eleocharis palustris) and one halophyte (Juncus maritimus) to remove deicing salts from runoff water in lab scale HSSF CW, while Buhmann et al. [18] evaluated the ability of the halophytes Tripolium pannonicum (Jacq.) Dobrocz. to be used as biofilters at nutrient rich saline water.

It is important to point out that, based on ecological aspect, halophytes can be classified as follows: (a) obligate, (b) facultative, and (c) habitat-indifferent halophytes [19]. Obligate halophytes grow only in salty habitats. They show sufficient growth and development under high saline condition. Facultative halophytes are capable of establishing themselves in saline soils, but their optimal condition lies in a salt-free condition, or at least low salt. However, they can tolerate salt. Halophytes indifferent to their habitat can cope with saline soils in nature. However, they usually grow in salt-free soils.

A facultative halophyte called Bacopa monnieri (L.) Wettst grows in the municipality of Villamar, Michoacán, Mexico. This is an herbaceous perennial, dicotyledonous, and C3 photosynthesis plant that belongs to the Scrophulariaceae family [20], which has a high growth rate at concentrations above 200 mM NaCl, as well as a great phytoremediative potential and phenotypic plasticity that make it ideal to be used in the agricultural sector [21].

As for the species Epipremnum aureum, it is a perennial plant salinity tolerant from the Araceae family [22], which has been widely used for ornamental and medicinal purposes, and for the removal of pollutants from air, soil, and water, either by metabolizing toxic pollutants, releasing harmless by products, or incorporating and sequestering toxic compounds in its tissues [23]. E. aureum is commonly marketed in Mexico [24] and it is known for growing very well—both hydroponically and in soil—and producing large amounts of biomass [25]. E. aureum is considered a plant with a high potential for the phytoremediation of wastewater due to its robustness, easy propagation, fibrous roots, and growth with minimal light [23].

Despite all the benefits that these plants can offer, so far there are no studies that have evaluated the NaCl phytoremediation potential of both E. aureum and B. monnieri in a controlled environment, such as CWs. Therefore, the aim of this study was to use CWs in order to evaluate the phytoremediation potential of B. monnieri and E. aureum to improve the chemical properties of irrigation water exposed to two saline concentrations over a period of 42 days.

2. Materials and Methods

2.1. Study Area

The establishment of the wetlands was carried out under greenhouse conditions, between the months of August and November 2021. The greenhouse was located at an altitude of 1560 m.a.s.l. and coordinates 19°59′57.6456″ N, −102°42′24.0336″ W. On average, the temperature and relative humidity conditions in the greenhouse were 36/10 °C (day/night) and 60% (±10%), respectively.

2.2. Vegetable Matter

In mid-August 2021, 81 plants of the species B. monnieri were extracted from a geothermal area known as Los Negritos in the municipality of Villamar, Michoacán, Mexico, at an altitude of 1540 m.a.s.l., with coordinates 20°03′28.4616″ N and −102°36′39.7008″ W. The plants were placed in black polystyrene bags to be taken to the greenhouse.

In the case of the species E. aureum, 81 plants were acquired from a commercial nursery located in the municipality of Sahuayo, Michoacán, Mexico, at an altitude of 1543 m.a.s.l. and coordinates 20°02′02.9364″ N, −102°43′01.7580″ W.

Once in the greenhouse, the plants of both species were washed with running water to remove the soil particles attached to their roots and leaves. Subsequently, the plants of both species were weighed using an Ohaus Compass™ CX scale (Naenikon, Switzerland). The plants of B. monnieri had an average fresh weight of 95.37 ± 3.13 g, while those of E. aureum had an average fresh weight of 118.14 ± 4.06 g.

2.3. Greenhouse Experiment

The artificial or constructed wetlands (CWs) used in this experiment were of the free-water surface type [16] (Figure 1). The different treatments were subjected to saline stress conditions, considering a hydraulic retention time of 42 days.

The CWs consisted of boxes with a capacity of 25 L, which were covered with light-colored polyethylene lids to reduce surface evaporation. On the bottom of each CW, a volume of 4134 cm³ of gravel was placed and distributed uniformly. The dimensions of the boxes were as follows: 39 cm wide × 53 cm long × 14 cm high. The roots of the plants of the different treatments were buried in the gravel to provide them with greater support and facilitate their growth. In addition, an air pump (Hagen^® ELITE 802, Hagen Deutschland GmbH & Co., Holm, GERMANY ) with 3 one-inch outlets was placed in each of the CWs to add movement and oxygen to the water, as well as to mitigate the possible formation of algae.

Based on the established treatments, six perforations—with a radius of 3 cm were made on the lid of each CW in order to place the emerging plants, considering a separation of 16–18 cm between each plant (a density of 29 plants/m²).

Moreover, this experiment aimed at having equal weights of total fresh biomass (FW) in each experimental unit (considering the six plants). This resulted in each CW being made up of 500 ± 7.52 g, which was equivalent to 58.14 ± 1.10 g of dry biomass (DW) of the species E. aureum (100%), 45.90 ± 0.87 g of DW of B. monnieri (100%), and 52.02 ± 0.98 g of DW of E. aureum (50%) and B. monnieri (50%).

Throughout the experiment, the daily evaporation was measured using a class A pan [26]. The potential evapotranspiration was determined by multiplying the pan coefficient by the amount of pan evaporation, and it was calculated according to the equation suggested by Allen [27]. Every week (seven days) the loss of water due to evapotranspiration in each of the treatments was compensated by adding distilled water to eliminate the effects produced by this phenomenon.

2.4. Treatments

The treatments consisted of the following two saline groups and their no saline controls, classified according to their electrical conductivity (EC): highly saline (EC 2000 μS cm⁻¹), severely saline (EC 4000 μS cm⁻¹) and no saline control (EC 300 μS cm⁻¹), based on the degree of restriction established for the use of irrigation water [28,29].

The evaluated treatments were configured as follows: T1 (B. monnieri [control, 300 μS cm⁻¹]), T2 (B. monnieri [2000 μS cm⁻¹]), T3 (B. monnieri [4000 μS cm⁻¹]), T4 (E. aureum [control, 300 μS cm⁻¹]), T5 (E. aureum [2000 μS cm⁻¹]), T6 (E. aureum [4000 μS cm⁻¹]), T7 (B. monnieri + E. aureum [control, 300 μS cm⁻¹]), T8 (B. monnieri + E. aureum [2000 μS cm⁻¹]) and T9 (B. monnieri + E. aureum [4000 μS cm⁻¹]).

A completely randomized block design was used, with three repetitions per treatment in a single experimental run; therefore, a total of 27 experimental units composed of six plants were considered (Figure 1).

In order to obtain an EC of 2000 μS cm⁻¹ in the experimental units, 28 g of analytical grade NaCl (1.12 g L⁻¹) were added, while 56 g of analytical grade NaCl (2.24 g L⁻¹) were added to obtain an EC of 4000 μS cm⁻¹.

2.5. Chemical Characteristics of the Irrigation Water Used for the Experiment

The nonsaline irrigation water presented the following chemical characteristics: electrical conductivity (EC) of 300 μS cm⁻¹, pH of 7.68, Ca²⁺ of 0.42 mmol L⁻¹, Mg²⁺ of 3.57 mmol L⁻¹, Na⁺ of 2.75 mmol L⁻¹, K⁺ of 0.34 mmol L⁻¹, Cl⁻ of 4.85 mmol L⁻¹, CO₃⁻² of 1.16 mmol L⁻¹, HCO₃⁻ of 3.58 mmol L⁻¹, SO₄⁻² of 2.47 mmol L⁻¹, residual sodium carbonates (RSC) of 0.75 mmol L⁻¹, sodium adsorption ratio (SAR) of 1.94, exchangeable sodium percentage (ESP) of 2.90, and effective salinity (ES) of 3.07. The classification of irrigation water according to USSL [28] and Shahid and Mahmoudi [29] was C2-S1.

The chemical characteristics of the highly saline irrigation water were as follows: electrical conductivity (EC) of 2000 μS cm⁻¹, pH of 7.96, Ca²⁺ of 0.42 mmol L⁻¹, Mg²⁺ of 3.57 mmol L⁻¹, Na⁺ of 19.18 mmol L⁻¹, K⁺ of 0.34 mmol L⁻¹, Cl⁻ of 33.89 mmol L⁻¹, CO₃⁻² of 1.16 mmol L⁻¹, HCO₃⁻ of 3.58 mmol L⁻¹, SO₄⁻² of 2.47 mmol L⁻¹, residual sodium carbonates (RSC) of 0.75 mmol L⁻¹, sodium adsorption ratio (SAR) of 13.54, exchangeable sodium percentage (ESP) of 20.17, and effective salinity (ES) of 19.52. The classification of irrigation water according to USSL [28] and Shahid and Mahmoudi [29] was C3-S3.

The severely saline irrigation water had the following chemical characteristics: electrical conductivity (EC) of 4000 μS cm⁻¹, pH of 8.14, Ca²⁺ of 0.42 mmol L⁻¹, Mg²⁺ of 3.57 mmol L⁻¹, Na⁺ of 38.26 mmol L⁻¹, K⁺ of 0.34 mmol L⁻¹, Cl⁻ of 67.60 mmol L⁻¹, CO₃²⁻ of 1.16 mmol L⁻¹, HCO₃⁻ of 3.58 mmol L⁻¹, SO₄^2- of 2.47 mmol L⁻¹, residual sodium carbonates (RSC) of 0.75 mmol L⁻¹, sodium adsorption ratio (SAR) of 27.35, exchangeable sodium percentage (ESP) of 40.35, and effective salinity (ES) of 38.60. The classification of irrigation water according to USSL [28] and Shahid and Mahmoudi [29] was C4-S4.

2.6. Fertilization

Fertilization was performed manually on days 8, 20, and 32 after transplanting the plants to the CWs. In each application, each experimental unit was added with 100 mL (4 mL L⁻¹) of Bayfolan^® Forte (CDMX, Mexico) inorganic liquid fertilizer, with an NPK (nitrogen-phosphorus-potassium) ratio of 11-8-6.

2.7. Chemical Analysis of the Water

At the end of the phytoremediation process (42 days), the electrical conductivity and the pH were measured using a portable multiparameter meter (HANNA^® HI 83141, HANNA, Woonsocket, RI, USA)), and the alkalinity was measured using a HACH^® test kit (cat. 2270900, HACH, Loveland, CO, USA). The patterns established in the Standard Methods for the Examination of Water and Wastewater [30] were followed for the determination of CO₃²⁻, HCO₃⁻, Cl⁻, and SO₄²⁻.

An Anton Paar’s Multiwave GO (Anton Paar, Graz, Austria) was used for the acid digestion of the samples of water. The extracts were filtered through 42 Whatman^® paper and the Na⁺, K⁺, Ca²⁺, and Mg²⁺ concentrations were analyzed through atomic absorption spectroscopy [28], using a GBC, SensAA spectrometer (GBC Scientific Equipment Ltd., CDMX, Mexico).

The residual sodium carbonates (RSC) were evaluated according to Eaton [31].

$$\mathrm{RSC}=\left({\mathrm{CO}}_3^{2-}+{\mathrm{HCO}}_3^{-}\right)-\left({\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}\right)$$

where, all the concentrations are in milli moles per liter.

The exchangeable sodium percentage (ESP) and the sodium adsorption ratio (SAR) were evaluated according to USSL [29].

$$\begin{gathered} \mathrm{SAR}=\frac{{\mathrm{Na}}^{+}}{\sqrt{\frac{1}{2}\left({\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}\right)}} \\ \mathrm{ESP}=\frac{\left[100\left(-0.0126+0.01475\times \mathrm{SAR}\right)\right]}{\left[1+\left(-0.0126+0.01475\times \mathrm{SAR}\right)\right]} \end{gathered}$$

where, each of Na⁺, Ca²⁺ + Mg²⁺ concentrations are expressed in milli moles per liter and SAR is expressed as (milli moles per liter)^0.5 (mmoles L⁻¹)^0.5.

Additionally, the effective salinity (ES) was analyzed following the methods of Ayers and Westcot [32], Raju et al. [33], and Castellón-Gómez et al. [34], respectively.

$$\begin{array}{l}1.\mathrm{If}=\left[{\mathrm{Ca}}^{2+}\right]>\left[{\mathrm{CO}}_3^{2-}+{\mathrm{HCO}}_3^{-}+{\mathrm{SO}}_4^{2-}\right]\\ \hspace{1em}\hspace{1em}\hspace{1em}\mathrm{SE}=\left[{\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}+{\mathrm{Na}}^{+}+{\mathrm{K}}^{+}\right]-\left[{\mathrm{CO}}_3^{2-}+{\mathrm{HCO}}_3^{-}+{\mathrm{SO}}_4^{2-}\right]\\ 2.\mathrm{If}=\left[{\mathrm{Ca}}^{2+}\right]<\left[{\mathrm{CO}}_3^{2-}+{\mathrm{HCO}}_3^{-}+{\mathrm{SO}}_4^{2-}\right] \mathrm{and}\left[{\mathrm{Ca}}^{2+}\right]>\left[{\mathrm{CO}}_3^{2-}+{\mathrm{HCO}}_3^{-}\right]\\ \hspace{1em}\hspace{1em}\hspace{1em}\mathrm{SE}=\left[{\mathrm{Mg}}^{2+}+{\mathrm{Na}}^{+}+{\mathrm{K}}^{+}\right]\\ 3.\mathrm{If}=\left[{\mathrm{Ca}}^{2+}\right]<\left[{\mathrm{CO}}_3^{2-}+{\mathrm{HCO}}_3^{-}\right] \mathrm{and}\left[{\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}\right]>\left[{\mathrm{CO}}_3^{2-}+{\mathrm{HCO}}_3^{-}\right]\\ \hspace{1em}\hspace{1em}\hspace{1em}\mathrm{SE}=\left[{\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}+{\mathrm{Na}}^{+}+{\mathrm{K}}^{+}\right]-\left[{\mathrm{CO}}_3^{2-}+{\mathrm{HCO}}_3^{-}\right]\\ 4.\mathrm{If}=\left[{\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}\right]<\left[{\mathrm{CO}}_3^{2-}+{\mathrm{HCO}}_3^{-}\right]\\ \hspace{1em}\hspace{1em}\hspace{1em}\mathrm{SE}=\left[{\mathrm{Na}}^{+}+{\mathrm{K}}^{+}\right]\end{array}$$

2.8. Phytoremediation Capacity (PHC)

The phytoremediation capacity of the halophyte plants used in the different treatments was quantified from the amount of sodium accumulated in the aerial parts of their tissues and the dry weight obtained 42 days after transplantation to the experimental units (CWs). The phytodesalination capacity (PHC) was calculated using the following equation [35]:

$$\mathrm{PHC}={[\left({\mathrm{Na}}_{\mathrm{f}}-{\mathrm{Na}}_{\mathrm{i}}\right)]}_{\mathrm{plant}}\times {\left[\left({\mathrm{DW}}_{\mathrm{f}}-{\mathrm{DW}}_{\mathrm{i}}\right)\right]}_{\mathrm{plant}}\times \left(\mathrm{DP}\right)$$

where:

Na_f − Na_i = Difference between the amount of sodium accumulated at the beginning (Na_i) and at the end (Na_f) of the experiment (mg g⁻¹); DW_f − DW_i = Difference between the dry weight at the beginning (DW_i) and at the end (DW_f) of the experiment (g); DP = Number of plants per m².

At the end of the phytoremediation process, the plants were washed with distilled water and quantified by their total fresh weight (FW). Subsequently, the samples were dried in a Terlab^® TE H45DM (Terlab, Guadalajara, Jal. Mexico) oven at 70 °C for 48 h; at the end of this period the total dry weight (DW) in grams was determined using a precision balance (Brainweigh^® B5000, Brainweigh, Florham Park, NJ, USA).

2.9. Statistical Analysis

The data obtained from the evaluated variables were averaged and subjected to the Shapiro–Wilk normality test (p ≤ 0.05) and Levene’s test for homogeneity of variance.

The variables that met the criteria of both tests were subjected to the analysis of variance (ANOVA) and Tukey’s mean comparison test (p ≤ 0.05). The variables that did not meet the criteria of normality or homogeneity of variance were transformed to the natural logarithm (ln) until normality and homoscedasticity were observed; subsequently, the ANOVA and Tukey’s mean comparison test were performed (p ≤ 0.05).

The data that did not comply with any of the tests were subjected to the non-parametric analysis of Kruskal-Wallis and the Wilcoxon rank-sum test. For all cases, the program Statistical Analysis System (SAS, Cary, NC, USA) version 9.1 was used [36].

3. Results

3.1. Cations

At the end of the experiment (42 days), the accumulated cations in E. aureum and B. monnieri in the different treatments were quantified (

Table 1. Concentration of cations of Na⁺, Ca⁺, Mg²⁺, K⁺ and Cl⁻ in Epipremnum aureum and Bacopa monnieri after having been exposed to different saline concentrations for a period of 42 days.

					mg g−1 DW
Treatments	Na+		Ca2+		Mg2+		K+		Cl−
	I	F	I	F	I	F	I	F	I	F
T1	18.52 a ± 0.38	29.15 g ± 0.61	8.32 c ± 0.18	82.49 a ± 1.69	3.93 a ± 0.1	51.84 a ± 1.08	6.04 c ± 0.12	98.19 a ± 2.06	32.72 a ± 0.68	51.50 g ± 1.08
T2	18.52 a ± 0.38	102.14 d ± 2.14	8.32 c ± 0.18	66.67 c ± 1.35	3.93 a ± 0.1	38.25 c ± 0.80	6.04 c ± 0.12	83.45 c ± 1.75	32.72 a ± 0.68	180.48 d ± 3.79
T3	18.52 a ± 0.38	195.68 a ± 4.10	8.32 c ± 0.18	47.54 e ± 0.91	3.93 a ± 0.1	23.61 f ± 0.45	6.04 c ± 0.12	64.92 d ± 1.36	32.72 a ± 0.68	345.76 a ± 7.26
T4	3.78 c ± 0.12	12.26 i ± 0.25	16.51 a ± 0.34	62.17 d ± 1.30	3.15 c ± 0.06	34.78 d ± 0.73	11.07 a ± 0.23	79.44 c ± 1.79	6.67 c ± 0.14	21.66 i ± 0.45
T5	3.78 c ± 0.12	53.39 f ± 1.12	16.51 a ± 0.34	23.81 g ± 0.92	3.15 c ± 0.06	17.27 h ± 0.36	11.07 a ± 0.23	37.35 g ± 1.41	6.67 c ± 0.14	94.34 f ± 1.98
T6	3.78 c ± 0.12	117.32 c ± 2.46	16.51 a ± 0.34	19.26 h ± 0.61	3.15 c ± 0.06	12.19 i ± 0.25	11.07 a ± 0.23	25.58 h ± 0.95	6.67 c ± 0.14	207.30 c ± 4.35
T7	11.15 b ± 0.23	20.61 h ± 0.43	12.41 b ± 0.27	74.32 b ± 1.56	3.54 b ± 0.08	45.69 b ± 0.95	8.55 b ± 0.17	87.83 b ± 1.82	19.70 b ± 0.41	36.41 h ± 0.76
T8	11.15 b ± 0.23	76.27 e ± 1.60	12.41 b ± 0.27	46.77 e ± 1.19	3.54 b ± 0.08	29.76 e ± 0.62	8.55 b ± 0.17	60.66 e ± 1.48	19.70 b ± 0.41	134.77 e ± 2.83
T9	11.15 b ± 0.23	143.68 b ± 3.01	12.41 b ± 0.27	28.93 f ± 0.81	3.54 b ± 0.08	20.05 g ± 0.43	8.55 b ± 0.17	43.48 f ± 1.12	19.70 b ± 0.41	253.88 b ± 5.33

Notes: Different letters in each column indicate significant differences according to Tukey’s test (p ≤ 0.05). Values are the mean ± standard deviation (n = 80). I: initial stage of the plants before being placed in the CWs; F: final stage of the plants after the phytoremediation process. T1 (B. monnieri [control, 300 μS cm⁻¹]), T2 (B. monnieri [2000 μS cm⁻¹]), T3 (B. monnieri [4000 μS cm⁻¹]), T4 (E. aureum [control, 300 μS cm⁻¹]), T5 (E. aureum [2000 μS cm⁻¹]), T6 (E. aureum [4000 μS cm⁻¹]), T7 (B. monnieri + E. aureum [control, 300 μS cm⁻¹]), T8 (B. monnieri + E. aureum [2000 μS cm⁻¹]), and T9 (B. monnieri + E. aureum [4000 μS cm⁻¹]).

). The treatments that contained the species B. monnieri (T2 and T3), E. aureum (T5 and T6), and both species (T8 and T9) showed the following concentration order: Na⁺ > K⁺ > Ca²⁺ > Mg²⁺. As for the treatments that contained the species B. monnieri (T1), E. aureum (T4), and both species (T7) showed the following concentration order: K⁺ > Ca²⁺ > Mg²⁺ > Na⁺.

It should be noted that the different treatments showed a greater accumulation of Na⁺ and Cl⁻ in plant tissues and a decrease in K⁺, Ca²⁺, and Mg²⁺ as the levels of saline concentration in the CWs increased.

In the highly saline group (2000 μS cm⁻¹), the treatment T2 showed a significantly increased accumulation (p ≤ 0.05) of Na⁺ and Cl⁻ compared to T5 and T8 (Table 1). The accumulation of Na⁺ and Cl⁻ in T2 was 1.91 times greater than in T5 and 1.33 times greater than in T8. On the other hand, in the severely saline group (4000 μS cm⁻¹), T3 was the treatment that achieved the most significant accumulation (p ≤ 0.05) of Na⁺ and Cl⁻, with respect to T6 and T9, being that its accumulation was 1.66 times greater than T6 and 1.36 times greater than T9.

In addition, significant differences were observed in the treatments of the different saline groups (p ≤ 0.05) with regard to the accumulation of Na⁺ and Cl⁻ compared to the controls T1, T4, and T7. Regarding the highly saline group (2000 μS cm⁻¹), the accumulation of Na⁺ and Cl⁻ in T2 was 3.5 times greater than in T1, in T5 it was 4.35 times greater than in T4, and in T8 it was 3.7 times greater than in T7. As for the severely saline group (4000 μS cm⁻¹), the accumulation of Na⁺ and Cl⁻ in T3 was 1.01 times greater than in T1, in T6 it was 1.09 times greater than in T4, and in T9 it was 1.03 times greater than in T7.

3.2. Phytoremediation Capacity (PHC)

At the end of the experiment, the treatments of the different saline groups showed significant differences (p ≤ 0.05) regarding their Na⁺ phytoremediation capacity (per experimental unit) and their dry biomass yield, with respect to the controls T1, T4, and T7 (

Table 2. Phytoremediation capacity (PHC) of the species Epipremnum aureum and Bacopa monnieri at the end of the experiment (42 days).

				Parameters
Treatments	Final FW per Plant (g)	Final DW per Plant (g)	Initial FW per Plant (g)	Initial DW per Plant (g)	PHC (g Plant−1)	PHC CW (g)	PHC (g m−2)
T1	284.13 a ± 5.68	26.08 d ± 0.49	83.42 a ± 1.73	7.66 c ± 0.14	0.195 f ± 0.006	1.170 f ± 0.041	5.655 f ± 0.171
T2	257.09 c ± 5.14	23.60 e ± 0.43	83.61 a ± 1.68	7.66 c ± 0.12	1.332 c ± 0.027	7.992 c ± 0.167	38.628 c ± 0.812
T3	221.83 e ± 4.43	20.36 f ± 0.38	83.55 a ± 1.62	7.66 c ± 0.15	2.249 a ± 0.047	13.494 a ± 0.283	65.221 a ± 1.369
T4	270.94 b± 5.41	31.48 a ± 0.57	83.35 a ± 1.72	9.69 a± 0.17	0.184 f ± 0.006	1.104 f ± 0.034	5.336 f ± 0.155
T5	228.68 e ± 4.57	26.57 c ± 0.50	83.47 a ± 1.59	9.69 a ± 0.20	0.837 e ± 0.016	5.022 e ± 0.105	24.273 e ± 0.509
T6	182.56 g ± 3.65	21.21 f ± 0.42	83.43 a± 1.64	9.69 a± 0.19	1.307 c ± 0.025	7.842 c ± 0.164	37.903 c ± 0.795
T7	277.19 b ± 5.54	28.82 b ± 0.54	83.46 a ± 1.73	8.67 b ± 0.16	0.190 f ± 0.005	1.140 f ± 0.029	5.510 f ± 0.147
T8	241.93 d ± 4.83	25.16 d ± 0.47	83.39 a ± 1.65	8.67 b ± 0.15	1.073 d ± 0.022	6.438 d ± 0.135	31.117 d ± 0.653
T9	203.37 f ± 4.06	21.15 f ± 0.40	83.34 a ± 1.56	8.67 b ± 0.15	1.655 b ± 0.034	9.930 b ± 0.208	47.995 b ± 1.007

Notes: Different letters in each column represent the minimum significant difference according to Tukey’s test (p ≤ 0.05). Values are the mean ± standard deviation (n = 80). FW: fresh weight of plants; DW: dry weight of plants. A density of 6 plants was considered for each CW, and a density of 29 plants per m². T1 (B. monnieri [control, 300 μS cm⁻¹]), T2 (B. monnieri [2000 μS cm⁻¹]), T3 (B. monnieri [4000 μS cm⁻¹]), T4 (E. aureum [control, 300 μS cm⁻¹]), T5 (E. aureum [2000 μS cm⁻¹]), T6 (E. aureum [4000 μS cm⁻¹]), T7 (B. monnieri + E. aureum [control, 300 μS cm⁻¹]), T8 (B. monnieri + E. aureum [2000 μS cm⁻¹]), and T9 (B. monnieri + E. aureum [4000 μS cm⁻¹]).

).

Regarding the highly saline group (2000 μS cm⁻¹), the Na⁺ phytoremediation capacity in T2 was 6.83 times greater than in T1, in T5 it was 4.54 times greater than in T4, and in T8 it was 5.64 times greater than in T7. As for the severely saline group (4000 μS cm⁻¹), the Na⁺ phytoremediation capacity in T3 was 11.53 times greater than in T1, in T6 it was 7.1 times greater than in T4, and in T9 it was 8.71 times greater than in T7.

On the other hand, in the highly saline group (2000 μS cm⁻¹) the dry biomass yield achieved by T2 was 9.5% lower than T1, in T5 it was 15.59% lower than in T4, and in T8 it was 12.7% lower than in T7. Regarding the severely saline group (4000 μS cm⁻¹), the dry biomass yield of T3 was 21.93% lower than that of T1, in T6 it was 32.62% lower than in T4, and in T9 it was 26.61% lower than in T7 (Table 2).

It is important to note that, in the highly saline group (2000 μS cm⁻¹), the treatment T2 achieved a significantly higher phytoremediation capacity (p ≤ 0.05) compared to T5 and T8 (Table 2). The Na⁺ phytoremediation capacity (per experimental unit) in T2 was 1.59 times greater than in T5 and 1.24 times greater than in T8. As for the severely saline group (4000 μS cm⁻¹), the treatment T3 achieved a significantly greater phytoremediation capacity (p ≤ 0.05) with respect to T6 and T9, since the accumulation of Na⁺ (per experimental unit) in T3 was 1.72 times greater than in T6 and 1.35 times greater than in T9.

The association of the species E. aureum and B. monnieri (T8 and T9) did not show significant increases (p ≤ 0.05) with regard to Na⁺ phytoremediation capacity, as did the treatments T2 and T3, where only plants of B. monnieri were used.

3.3. Chemical Characteristics of the Irrigation Water

At the end of the experiment, it was observed that the pH, EC, SAR, ESP, and ES values were significantly reduced (p ≤ 0.05) in the different saline groups (

Table 3. Chemical properties of the irrigation water at the end of the experiment (42 days).

	Treatments
	CMS	T2	T5	T8	CFS	T3	T6	T9
Parameters	I	F	F	F	I	F	F	F
pH	7.96 c ± 0.02	7.75 f ± 0.02	7.84 e ± 0.02	7.81 e ± 0.03	8.14 a ± 0.02	7.91 d ± 0.03	8.06 b ± 0.02	7.98 c ± 0.02
EC (μS cm−1)	2000 d ± 50	670 h ± 50	1170 f ± 50	920 g ± 50	4000 a ± 50	1730 e ± 50	2690 b ± 50	2340 c ± 50
Na+ (mmol L−1)	19.18 d ±0.12	6.82 h ± 0.12	10.35 f ± 0.18	9.34 g ±0.17	38.26 a ±0.12	16.66 e ±0.29	25.63 b ± 0.46	23.34 c ± 0.42
Ca2+ (mmol L−1)	0.42 e ± 0.03	1.58 d ± 0.03	1.72 b ± 0.03	1.67 c ± 0.03	0.42 e ± 0.03	1.67 c ± 0.32	1.83 ª ± 0.03	1.76 b ± 0.03
Mg2+ (mmol L−1)	3.57 f ± 0.08	4.49 e ± 0.08	4.79 d ± 0.08	4.61 c ± 0.08	3.57 f ± 0.08	5.61 b ± 0.10	5.97 ª ± 0.11	5.83 a ± 0.10
K+ (mmol L−1)	0.34 g ± 0.09	5.08 f ± 0.09	6.16 d ± 0.11	5.58 e ± 0.10	0.34 g ± 0.09	6.97 c ± 0.12	8.53 ª ± 0.15	7.94 b ± 0.14
SO42− (mmol L−1)	2.47 a ± 0.04	0.91 f ± 0.01	1.32 e ± 0.02	1.18 e ± 0.02	2.47 a ± 0.04	1.55 d ± 0.02	2.15 b ± 0.03	1.87 c ± 0.03
CO32− (mmol L−1)	1.16 a ± 0.02	0.55 g ± 0.01	0.81 e ± 0.02	0.67 f ± 0.02	1.16 a ± 0.02	0.93 d ± 0.02	1.10 b ± 0.02	1.04 c ± 0.02
HCO3− (mmol L−1)	3.58 a ± 0.07	1.57 g± 0.03	1.92 e ± 0.03	1.75 f ± 0.03	3.58 a ± 0.07	2.45 d ± 0.04	3.29 b ± 0.06	2.91 c ± 0.05
Cl− (mmol L−1)	19.15 c ± 0.10	6.04 g ± 0.10	9.18 e ± 0.17	8.28 f ± 0.15	33.89 c ±0.10	14.76 d ±0.26	22.70 a ± 0.41	20.67 b ± 0.37
RSC (mmol L−1)	0.75 a ± 4.51	ND	ND	ND	0.75 a ± 4.51	ND	ND	ND
SAR (mmol L−1)0.5	13.54 b ± 0.07	3.91 h ± 0.07	5.73 f ± 0.12	5.27 g ± 0.09	27.35 a ±0.07	8.73 e ± 0.16	12.98 c ± 0.24	12.02 d ± 0.21
ESP	20.17 b ± 0.10	5.83 h ± 0.10	8.54 f ± 0.15	7.85 g ± 0.14	40.35 a ±0.10	13.01 e ±0.23	19.34 c ± 0.35	17.91 d ± 0.33
ES (mmol L−1)	19.52 f ± 0.23	15.85 h ± 0.19	19.29 e ± 0.27	18.78 g ±0.2	38.60 a ±0.42	27.53 d ±0.28	37.57 b ± 0.36	34.92 c ± 0.33

Notes: In each row, the different letters represent the minimum significant difference according to Tukey’s test (p ≤ 0.05), ND: not detected = 0. Values are the mean ± standard deviation (n = 27). I: initial stage; F: final stage; CMS: initial chemical characteristics of irrigation water at 2000 μS cm⁻¹; CFS: initial chemical characteristics of irrigation water at 4000 μS cm⁻¹. RSC: residual sodium carbonate; SAR: sodium absorption ration; ESP: exchangeable sodium percentage; ES: effective salinity. T2 (B. monnieri [2000 μS cm⁻¹]), T3 (B. monnieri [4000 μS cm⁻¹]), T5 (E. aureum [2000 μS cm⁻¹]), T6 (E. aureum [4000 μS cm⁻¹]), T8 (B. monnieri + E. aureum [2000 μS cm⁻¹]), and T9 (B. monnieri + E. aureum [4000 μS cm⁻¹]).

).

In the highly saline group, T2 showed the most significant capacity (p ≤ 0.05) to reduce the pH by 2.63%, the EC by 66.5%, the SAR by 71.12%, the ESP by 71.09%, and the ES by 18.80%, compared to the initial chemical characteristics of irrigation water at 2000 μS cm⁻¹.

Moreover, significant differences were observed (p ≤ 0.05) among the treatments of the different saline groups regarding pH, EC, SAR, ESP, and ES. In the highly saline group, T2 showed a significantly greater capacity (p ≤ 0.05) to reduce the values of these indicators, compared to T5 and T8. In T2, the pH was 1.14% and 0.76% lower, the EC was 42.73% and 27.17% lower, the SAR was 31.76% and 25.80% lower, the ESP was 31.73% and 25.73% lower, and the ES was 21.88 and 15.60% lower than T5 and T8, respectively.

The treatment T2 also achieved a significant reduction (p ≤ 0.05) of Na⁺ and Cl⁻ regarding the initial chemical characteristics of irrigation water at 2000 μS cm⁻¹, presenting sodium (Na⁺) and chlorine (Cl⁻) reductions of 64.44% and 68.45%, respectively. Likewise, the carbonates (CO₃²⁻) were reduced by 52.58%, bicarbonates (HCO₃⁻) by 56.14%, and sulfates (SO₄⁻²) by 63.15%.

Reductions were also observed in T5 and T8, where sodium was reduced by 34.10% and 26.98%, chlorine by 34.20% and 27.05%, carbonates by 32.09% and 17.91%, bicarbonates by 31.06% and 22.88%, and sulfates by 31.06% and 22.88%, respectively.

In the severely saline group, T3 was the treatment that showed significant reductions (p ≤ 0.05), where the pH was reduced by 2.82%, EC by 56.75%, SAR by 68.08%, ESP by 67.75% and ES by 28.67%, with respect to the initial chemical characteristics of irrigation water at 4000 μS cm⁻¹.

Compared to T6 and T9, the reduction capacity of T3 was significantly greater (p ≤ 0.05), being 1.86% and 0.87% greater for reducing the pH, 35.68% and 26.06% for the EC, 32.74% and 27.37% for the SAR, 32.73% and 27.35% for the ESP, and 26.97% and 21.16% for the ES, respectively.

In addition, T3 achieved a significant reduction (p ≤ 0.05) in Na⁺ and Cl⁻ regarding the Initial chemical characteristics of irrigation water at 4000 μS cm⁻¹, considering that sodium was reduced by 56.45% and chlorine by 56.44%, which meant significant differences (p ≤ 0.05) between this treatment and T6 and T9, being that T3 showed differences of 35% and 28.62% when reducing sodium, and 34.97% and 28.59% when reducing chlorine, respectively (Table 3).

Likewise, in the treatment T3, carbonates were reduced by 19.82%, bicarbonates by 31.56%, and sulfates by 37.24%, which indicated a significant reduction (p ≤ 0.05) with respect to treatments T6 and T9 (Table 3). Compared to T6 and T9, the capacity of T3 to reduce carbonates showed a difference of 15.45% and 10.57%, 25.53% and 15.80% for bicarbonates, and 27.9% and 17.11% for sulfates, respectively (Table 3).

It should be noted that the treatments of both saline groups showed a significant reduction (p ≤ 0.05) in residual sodium carbonates (RSC) with respect to the initial chemical characteristics of irrigation water at 2000 and 4000 μS cm⁻¹ (Table 3), to the point of RSC not being detected at the end of the phytoremediation process.

3.4. Degree of Restriction of Irrigation Water for Agricultural Use

At the end of the experiment (42 days), it was observed that the treatments exposed to 2000 μS cm⁻¹ went from a high salinity and high sodium hazard level (C3-S3) to a medium salinity and low sodium hazard level (C2-S1) in the case of T2, and to a high salinity and low sodium hazard level (C3-S1) in the case of treatments T5 and T8 (Figure 2).

As for the treatments exposed to 4000 μS cm⁻¹, they went from a severe or very high salinity and sodium hazard level (C4-S4) to a medium salinity and low sodium hazard level (C3-S2) in the case of T3, and to a high salinity and low sodium hazard level (C4-S3) in the case of T6 and T9 (Figure 2).

4. Discussion

When the different treatments were exposed to higher levels of NaCl, increases in extracellular Na⁺ concentrations were observed, which facilitated the passive transport of sodium from the outside to the inside of the cells, causing a certain degree of competition with Ca²⁺, K⁺, and Mg²⁺ cations on the membrane of cell binding sites [37].

In the case of Ca²⁺, it has been proven that it is one of the cations with the greatest positive influence on the regulation of cellular metabolism in the face of ionic and osmotic stress caused by salinity [38], while K⁺ is one of the cations that influence energy metabolism, especially with regard to protein and carbohydrate synthesis during plant growth [39]. As for Mg²⁺, it is related to photosynthesis, protein biosynthesis, and chlorophyll—processes that are necessary to maintain a high growth rate in plants [40].

In the treatments where the species E. aureum was subjected to high (2000 μS cm⁻¹) and severe (4000 μS cm⁻¹) salinity levels, greater difficulties were observed in maintaining high growth rates, both in the roots and in the parts of the young shoots, which, in turn, limited the yield of fresh and dry biomass, as well as the phytoremediation capacity (Table 2), indicating that high concentrations of NaCl imposed both ionic and osmotic stress on this species [41], in addition to altering the absorption of plant nutrients at different levels of tissue organization [42].

According to Alaoui et al. [43], the abundance of Na⁺ and Cl⁻ ions reduces the plant’s capacity to access and absorb some elements, such as N, P, K, and Mg, also affecting the synthesis of phytohormones and the maturation of cell walls.

Unlike the negative effects that were observed in the species E. aureum, the treatments that involved the species B. monnieri showed a greater capacity to regulate their cellular metabolism and maintain a high growth rate, which resulted in a higher biomass content, as well as a greater capacity to bioaccumulate Na⁺ in their tissues (Table 2), as has been reported in other studies [44].

According to Aslam et al. [45], the increase in biomass in the species B. monnieri can be attributed to a channel called aquaporin, which is involved in the intracellular compartmentalization of water and plays an important role in maintaining osmotic homeostasis and turgor in plant cells under salt stress conditions. While aquaporins are primarily responsible for the absorption of Na⁺ by plant roots, the movement of Na⁺ could also be favored by the low-affinity cation transporter (LCT1), and the Arabidopsis K⁺ transporter 1 (AKT1) [46].

Biomass growth can also be explained by the fact that the species B. monnieri has the ability to synthesize and accumulate organic osmolytes (osmoprotectants), such as proline and glycine-betaine, which allow it to maintain osmotic balance under conditions of high salt concentration [47]. Usually, proline accumulates in the cytosol and acts on the osmotic adjustment of the cytoplasm [48]. In addition, it has been found that proline can tolerate the increase in redox potential due to the formation of nicotinamide adenine dinucleotide phosphate (NADP) and that it has a vital role in the defense mechanisms of stressed cells [49].

According to Ghosh et al. [50], proline serves as a carbon and nitrogen storage sink and as a free radical scavenger, in addition to stabilizing subcellular structures (membranes and proteins), and buffering the cellular redox potential under stress, while glycine betaine acts as an osmoprotectant that helps stabilize the quaternary structures of proteins and highly organized membrane states [51]. Hence, the synthesis of osmolytes is part of a process that depends on the energy consumed by a large number of ATP molecules and that directly impacts plant growth [52].

The greater amount of biomass in B. monnieri could also be attributed to the fact that the species has a series of efficient defense mechanisms—such as the antioxidant enzymes SOD (superoxide dismutase), CAT (catalase), APX (ascorbate peroxidase), GPX (guaiacol peroxidase), and GR (glutathione reductase)—that allow it to protect itself from oxidative stress, which is induced by the over-accumulation of ROS (reactive oxygen species) in high salinity conditions [47]. In fact, the salinity tolerance in various halophyte species has been associated with their ability to maintain ROS homeostasis by means of an adequate activation of their antioxidant system when exposed to high salinity conditions [53].

Furthermore, the growth in the biomass of B. monnieri could be due to the fact that—like any other halophyte—this species has endogenous phytohormones (auxins, cytokinins, gibberellins, and zeatins) that allow it to improve its growth, development, and physiological processes, in addition to allowing it to regulate several cellular defense mechanisms, including antioxidant defense [54]. According to Devireddy et al. [55], the formation of ROS and the subsequent redox processing is an integral part of hormonal regulation and plays an important role in controlling plant development and stress tolerance.

The biomass obtained in both E. aureum and B. monnieri in this study could also be regarded as a result of the longer growth period to which the plants were exposed during the experiment (42 days), the characteristics of the utilized substrate (gravel) and the nourishment of the plants through fertilization. In this regard, Jesus et al. [16] and Zhou et al. [56] have indicated that the growth period, the type of substrate, and the nutrients in the medium are essential elements in the phytoremediation process of plants and their associated microorganisms when growing under conditions of toxic stress.

In this sense, numerous studies have suggested that the microbial community of the rhizosphere promotes plant health, productivity, and resistance to saline stress through a series of direct and indirect mechanisms—such as the use of plant growth-promoting bacteria (PGPB) [57]—and, in turn, plant growth can benefit microbes through the release of carbon dioxide and the secretion of photosynthate [58], all of which is a virtuous cycle that could explain the growth of biomass and phytoremediation capacity of both species in this research.

Moreover, the species E. aureum and B. monnieri were able to reduce the concentrations of Na⁺ and Cl⁻ in the water, as well as the levels of pH, EC, SAR, ESP, and ES, including some anions, such as carbonates (CO₃²⁻), bicarbonates (HCO₃⁻), and sulfates (SO₄²⁻). According to de la Fuente et al. [59] and Merino-Martín et al. [60], reductions in pH, EC, SAR, ESP, and ES could be due to the activity of the roots of both species to absorb Na⁺ cations from the irrigation water, which caused the Ca²⁺/Na⁺ exchange reaction to become a slow process and that the low flow rate increased the desodification efficiency through sodium translocation to the harvestable parts of the plants.

Other factors that could explain these reductions derive from the addition of distilled water to compensate for the losses generated by evapotranspiration [8], as well as the ion exchange adsorption—mainly of Na⁺—which was generated with the use of the gravel-based substrate throughout the phytoremediation process [14]. In this regard, Zhou et al. [56] indicate that external substrates—such as gravel—can affect to a certain extent the purification function of wetlands by altering the sodium concentration of the water through deposition, resuspension, and adsorption exchange of ions in the colloids of the substrate.

On the other hand, the movement of Cl⁻ towards the interior of the cells of the roots of E. aureum and B. monnieri could have been mediated by transporters of H⁺/Cl⁻ symporters, cotransporters of Cl⁻/H⁺ and transporters of nitrate (NRT) [61], as reported in numerous species [62].

It should be noted that both E. aureum and B. monnieri were able to reduce the concentration of carbonates (CO₃²⁻), bicarbonates (HCO₃⁻) and sulfates (SO₄²⁻) in the irrigation water of the different treatments, which can be beneficial for agricultural soils, since, according to López-García et al. [63], the reduction of these anions positively influences the sodium adsorption levels of the soil, to the point of significantly improving its physicochemical and microbiological properties—structure, composition and bacterial activity [64].

The total reduction of residual sodium carbonates (RSC) in the irrigation water of the different treatments is another factor that could positively influence the safeness of agricultural soils by preventing hazards induced by alkalinity in the soil—once calcium and magnesium cations have reacted with carbonate and bicarbonate anions—as well as preventing reductions in plant growth and development [65].

Regarding the degree of restriction of irrigation water for agricultural purposes, it was observed that the treatments of the highly saline group (2000 μS cm⁻¹) went from being in a category C3-S3 to achieving categories C2-S1 and C3-S1 at the end of the experiment (42 days). As for the treatments of the severely saline group (4000 μS cm⁻¹), they went from being in a category C4-S4 to achieving categories C3-S2 and C4-S3 at the end of this research, being B. monnieri the species that achieved the best conditions for agricultural use in both saline groups (Figure 2).

According to USSL [28], irrigation water in the category C4-S4 can occasionally be used on permeable soils with low salinity, but it should be applied to crops, which are highly tolerant to sodium. Likewise, irrigation water in the category C4-S3 can occasionally be used on permeable soils in conditions of low to moderate salinity, specifically where the soil water solution is rich in calcium. As for the irrigation water in the category C3-S3, it can cause high levels of salinity in agricultural soils, therefore, it cannot be used in areas that have restricted drainage and/or poor leaching capacity; even with adequate drainage, special management may be required to control salinity. Similarly, irrigation water in the category C3-S2 can represent a considerable risk of sodium in fine-textured soils with high cation exchange capacity, especially in low leaching conditions. On the other hand, while irrigation water in the category C3-S1 cannot be used in soils that have restricted drainage, it can be used for irrigation in almost every soil that presents a low risk of developing toxic levels of exchangeable sodium, being that sensitive crops can accumulate harmful concentrations of sodium. Lastly, the irrigation water in the category C2-S1 can be used in agricultural soils that have moderate leaching conditions; in addition, it can be used in soils where plants with moderate tolerance to salinity are being grown.

It is important to note that the main problem with a high concentration of sodium in irrigation water is the increased osmotic pressure in the soil solution, which can result in a physiological drought condition and cause a negative effect on the physical properties of the soil, such as degradation of the soil structure [16]. Therefore, it is recommended to avoid an electrical conductivity in irrigation water greater than 750 μS cm⁻¹ and a SAR greater than 10 (mmol L⁻¹)^0.5, especially when that water is the only source of irrigation for prolonged periods of time [66].

5. Conclusions

At the end of the experiment (42 days), B. monnieri and E. aureum species (both separately and together) showed a good tolerance to the two saline concentrations used in this study (high [EC 2000 μS cm⁻¹] and severe [EC 4000 μS cm⁻¹]) and the ability to reduce the salinity of the irrigation water. However, B. monnieri showed a greater ability to phytoremediation, to the point of improving its chemical properties and reducing potential damage to the soil.

Regarding the species B. monnieri, increases in dry biomass of 208% (highly saline) and 165.8% (severely saline) were observed with respect to its initial biomass, that is, at the time of beginning the experiment; while, for the species E. aureum, the increases in dry biomass were 174.2% (highly saline) and 118.8% (severely saline) respect to their initial biomass.

As for the highly saline group (2000 μS cm⁻¹), the B. monnieri species showed a Na⁺ phytoremediation capacity (per experimental unit) of 7.992 g, and the potential to reduce the pH from 7.96 to 7.75, EC from 2000 μS cm⁻¹ to 670 μS cm⁻¹, SAR from 13.54 to 3.91, ESP from 20.17 to 5.83, and ES from 19.52 to 15.85, which allowed it to go from a high salinity and high sodium hazard level (C3-S3) to a medium salinity and low sodium hazard level (C2-S1); while, the species E. aureum, showed a Na⁺ phytoremediation capacity (per experimental unit) of 5.022 g and the potential to reduce the pH from 7.96 to 7.84, EC from 2000 μS cm⁻¹ to 1170 μS cm⁻¹, SAR from 13.54 to 5.73, ESP from 20.17 to 8.54, and ES from 19.52 to 19.29, which allowed it to go from a high salinity and high sodium hazard level (C3-S3) to a high salinity and low sodium hazard level (C3-S1).

In addition, it was observed that in the severely saline group (4000 μS cm⁻¹), the species B. monnieri showed a Na⁺ phytoremediation capacity (per experimental unit) of 13.494 g and the potential to reduce the pH from 8.14 to 7.91, EC from 4000 μS cm⁻¹ to 1730 μS cm⁻¹, SAR from 27.35 to 8.73, ESP from 40.35 to 13.01, and ES from 38.60 to 27.53, which allowed it to go from a severe or very high salinity and sodium hazard level (C4-S4) to a medium salinity and low sodium hazard level (C3-S2), while, the species E. aureum, showed a Na⁺ phytoremediation capacity (per experimental unit) of 7.842 g and the potential to reduce the pH from 8.14 to 8.06, EC from 4000 μS cm⁻¹ to 2690 μS cm⁻¹, SAR from 27.35 to 12.98, ESP from 40.35 to 19.34, and ES from 38.60 to 37.57, which allowed it to go from a severe or very high salinity and sodium hazard level (C4-S4) to a high salinity and low sodium hazard level (C4-S3).

Therefore, the use of B. monnieri in CWs is presented as a green technology that would permit the development of more sustainable agriculture in a short period of time (42 days), by reducing the salinity levels of the wastewater that is generated in the agricultural drains and for its wide spectrum of adaptation to various saline concentrations in irrigation water. In addition to their use for phytoremediation, these plants can also serve as ornamental and medicinal plants, which would also be a short-term solution for the growing problem of disposing of the seemingly useless biomass of this type of weeds.