1. Introduction
Water is vital for the existence of any form of life on the earth. However, the current century is witnessing both the declining groundwater level and increased pollution of water resources [1]. Pollution is widely one of the most important environmental causes of disease and premature death in the world. According to a report published in 2017, almost 16% of total premature death throughout the world (9 million) in 2015 were due to the environmental pollution which is triple times higher than the cumulative death of malaria, tuberculosis, and AIDS [2–3]. Among various kinds of pollution, water, air, and soil pollution are the most common and major causes of toxicity towards environmental ecosystems [4–7]. Water pollution is a severe global problem that urgently requires concepts for monitoring and implementation of solution plans. Water pollution can be divided into different kinds of alteration in the physical, chemical and biological features of water that has a destructive consequence on human health and living creatures. Water not only, supports a range of human actions such as drinking, bathing, and household use, irrigation etc., but also, act as a key function in the biogeochemical reaction of underground water [1, 8].
Acid mine drainage (AMD) is well-known as an important source of pollution for surface and underground water all over the world. AMD, which can be produced by oxidation of metal sulfides in the presence of air oxygen and moisture in waste dumps and stockpiles can penetrate the underground water. Furthermore, water from rainfall and floods can also penetrate underground water in related to surface mines. Also, the acidic property of AMD increases the concentration of heavy metals and intensifies their toxic and destructive effects [9–11]. AMD from waste dumps, tailings, and mine structures contains pits and underground workings which can be the primer function of the mineralogy of the rock and the availability of water and oxygen. Obviously, the prediction of the potential for AMD sounds to be really complicated, costly, and of questionable reliability due to the mineralogy and other factors which can impact AMD formation with highly variable. Clearly, AMD from coal mining and mineral processing is a costly problem [9–14]. Moreover, acid can be generated at mine sites whenever metal sulfide minerals are oxidized. The metal sulfide presence in the host rock is associated with types of metal mining activities. Prior to mining, the oxidation of these minerals and the sulfuric acid formation can be considered as the function of natural weathering processes. The oxidation of undisturbed ore bodies is followed by the release of acid and slow mobilization of metals and presence of heavy metal contamination in the natural environment is recognized as a significant health human risk [9–16].
One of the most materials in AMD is sulfate, the sulfate or sulphate ion is a polyatomic anion with the empirical formula
. Sulfates as microscopic particles(aerosols) are resulting from fossil fuel and biomass combustion. They increase the acidity of the atmosphere and form acid rain [10]. Sulfate is a common non-toxic natural compound in water and wastewater, but its presence in high concentrations can have terrible detrimental effects [11–16]. High concentrations of
in aqueous environments can cause the mineralisation of water, corrosion of steel, scaling of equipment, damage to mammals and endangering human health [14, 17–20]. The
removal processes from wastewater include biological treatment with sulphate-reducing bacteria, membrane filtration, adsorption, ion exchange and chemical precipitation as gypsum, barium sulphate or ettringite, coagulation and flocculation, electrochemical treatment and solvent extraction. However, these methods suffer from some sorts of limitations such as high consumption of reagent and energy, low selectivity, high operational cost, disposal of chemicals, resins, and adsorbents to nature may themselves be harmful and difficult for reuses and production of toxic sludges [17, 18, 21]. Among these ways, adsorption may be preferred for
removal because it is inexpensive, ease of operation, effective and low cost [22–24].
An adsorbent is a material which can be used to adsorption from liquids or gases. It can be assumed as low cost if it requires little processing, abundant in nature, or a by-product or waste material from another industry. Some of the reported adsorbents include zeolite, iron-oxide-coated sand, carbon, fly ash, red muds, limestone, and minerals [25–29]. Considerably, the red mud was investigated between these wide range of adsorbents this research. Red mud is the solid waste residue of the digestion of bauxite ores with caustic soda in aluminum (Al2O3) production [34–35]. Also, it is a mixture of metallic oxides. The red color arises from iron oxides, which can comprise up to 60% of the mass. The mud is highly basic with a pH ranging from 10 to 13. In addition to iron, the other dominant components include silica, residual aluminum compounds, and titanium oxide [30–31].
Molecular simulation methods were applied for microscopic analysis and investigation of physical phenomena at the molecular level. In these methods, the constituent species of a system such as: molecules and fine particles have been explored. Macroscopic and microscopic quantities of interest are derived from analyzing the behavior of these species. One of the most important software which was applied for modeling, visualization, and analysis of material systems is Materials Studio. This software was utilized in advanced research about various materials such as: polymers, carbon nanotubes, catalysts, metals. DMol3 module implements density functional theory (DFT) in the Materials Studio software package, broadly. It also provides a robust platform for accurate performing and reliable simulations of materials at the atomic scale [33–37]. Its capabilities extend beyond the traditional static DFT calculations, including molecular dynamics simulations, geometry optimization, and vibrational frequency calculations. Its accuracy and versatility saliently make it an essential tool for researchers seeking to gain insights into the properties and behavior of materials at the molecular level. To investigate the adsorption energy and behavior of sulfate ion on red mud, we tried to apply the Adsorption locator and Castep modules.
Here, we have investigated a new approach for sulfate adsorption by performing theoretical calculations on metal oxides which exist in red mud structure. Sulfate adsorption measurements were carried out for a series of metal oxides. Hematite (110), Al2O3, CaCO3, SiO2, TiO2, and Na2O surfaces were used for these calculations. Finally, the results of the adsorption energies for sulfate were reported, with an emphasis on hematite (110). For description of adsorption mechanism of sulfate on red mud surface, we investigated important thermodynamic properties of various compounds of red mud by means of the first principles density functional theory. In this paper, the interaction between sulfate ions and metal oxides in red mud structure were studied.
2. Materials and Methods
2.1. Materials
Red mud (RM) was provided from the Jajarm mine; located at 5 km of Jajarm in north Khorasan province of Iran and was applied as the base material adsorbent. The X-ray fluorescence (XRF) results and X-ray diffraction (XRD) studies of RM are provided in Table 1. The sulfate solution was prepared by sodium sulfate (Na2SO4), also the solution of ionic strength was adjusted via using NaCl (1 M). HCl and sodium hydroxide (NaOH) were employed for adjusting the pH of the solution. Noteworthy, all reagents were supplied from Merck.
2.2. Batch Adsorption Studies
The adsorption experiments were performed via using the batch method in a 250 ml Erlenmeyer flask with a constant stirring rate of 420 rpm, at a temperature of 298.15 K, and ionic strength of 0.01 M of NaCl. The sulfate solution was prepared by 1000 ppm sodium sulfate, followed by placing 2 gr/l adsorbent dosage (red mud, Al2O3, Fe2O3, SiO2, CaCO3, TiO2 and MgO) during 60 min contact time and stirring. The sulfate solution with a desired concentration (100–2000 ppm) and contact time (10–120 min) was prepared and placed in contact with raw and activated RM (0.5–5 g/L) and then underwent stirring. After equilibrium, the solution was filtered by Whatman filter paper No. 42 and a clear aliquot of the supernatant was gained. The sulfate percentage in the solution was determined by a UV-VIS spectrophotometer at 420 nm (model HITACHI U-2000), according to previous research. Ultimately, the sulfate removal (R) and adsorption capacity of the adsorbent (qe) were calculated using Eq. (1). and (2). [38].
In the above equation, R and C0 are the sulfate removal percentage (%) and the initial adsorbate concentration (mg/L), respectively. Furthermore, Ce and qe represent the final adsorbate concentration in the solution after equilibrium adsorption (mg/L) and the amount of the adsorbed sulfate per unit of adsorbents (mg/g), respectively. Finally, M and V denote the weight of the adsorbent (g) and the volume solution (L).
2.3. Molecular Modeling Simulation
To molecular simulation of sulfate ion adsorption on red mud surface, it was used Materials Studio software. Primarily, metal oxides exist in red mud structure; including clusters of Fe2O3, Al2O3, CaCO3, SiO2, TiO2, and Na2O were created by the built-in Cluster Builder tool in Materials Studio. The initial step was the 3D Atomistic documents creation and drawing atoms and bonds for each structure. Then it was cleaned and selected ball and stick option from display style dialog. It was sketched all molecules that exist in the chemical reaction. DMol3 module was applied as a tool for predicting thermodynamic properties such as enthalpy (H), entropy (S), free energy (G), total energy (E) and heat capacity at constant pressure (Cp) as functions of temperature. DMol3 module calculates all the above terms in the ground-state position. After completing geometry optimization was extracted the total energy (Etotal) from the last step and was recorded the Gtotal value at 298.15K. Following that, it was obtained ETcorr for compounds which exist in red mud via using Eq. (3). It can be converted Etotal from kcal/mol to Hartree (1 Hartree = 627.51 kcal/mol). Later, it was calculated the free energy of reaction from DMol3 output data (Eq. (4)) [39–41].
The adsorption mechanism of sulfate ions on red mud structure was studied by the molecular modeling simulation. DMol3 module was used for DFT calculation in Material Studio software, version 2020. The iron, aluminum, magnesium, calcium, silicon and titanium oxides were modeled via using the generalized gradient functional (GGA) and BLYP (Becke exchange plus Lee-Yang-Parr correlation). It was applied double numerical for all atoms plus a p-function polarization for hydrogen atoms (DNP) with the best accuracy. The most important convergence for geometry optimization is summarized in Table 2. In this study, all the results were based on “all electron relativistic” method without effective core potentials or pseudo-potentials.
In the next step, Fe2O3, Al2O3, CaCO3, SiO2, TiO2, and Na2O were built by crystal structures. The desired elements, the number of atoms, and appropriate lattice parameters for each structure were chosen including: the cubic structure for Fe2O3, CaCO3, and Na2O, and the hexagonal structure for TiO2. For Al2O3 and SiO2 which was chosen the trigonal and tetrahedral structures, respectively. Then the cleavage surface was created and 2×1 supercell was also expanded. Finally, it was built vacuum slab with appropriate thickness for all kinds of crystals. Hematite crystal with dimensions of 5.035×5.035×13.720 was shown in Fig. 1.
To investigate the adsorption of sulfate ions on red mud, it was used the optimized crystal (with Forcite module) as the basis of red mud models. Following that, sulfate ions were placed near the surface of the crystals and performed molecular simulations to study the adsorption behavior with Adsorption locator module. The details of the molecular simulations and the results of our study are presented in the results section. The adsorption energy is calculated by Eq. (5). [42]:
where Eads is the adsorption energy, Esurf + m ol is the total energy of the surface with the adsorbed molecule, Esurf is the total energy of the clean surface, and Em ol is the total energy of the isolated molecule.
3. Results and Discussion
3.1. Modeling Results
Single clusters simulation of red mud structure was conducted and can be observed in Fig. 2 Moreover, the single clusters; resulting from the sulfate ion reaction by various oxides in the red mud structure were simulated by using DFT (density functional theory) either. Fig. 2 shows the optimized structure of these clusters. Furthermore, various calculations were made to measure four main thermodynamic properties involving: enthalpy, entropy, heat capacity and Gibbs free energy of reactants and products. The diagrams in Fig. 3 clarify the values of these calculations. As can be seen, enthalpy (H) for all reactants and products are positive. Meanwhile, aluminum oxide is the most exothermic compound among others. Also, entropy (S) for all compounds is positive. The highest amount of entropy was iron oxide and then aluminum oxide. Ultimately, Gibbs energy can be available energy, and its maximum value has belonged to iron oxide. There is a significant difference in the G value of Fe2O3, Na2O and MgO with others.
The results obtained from thermodynamic calculations were shown the most amount of enthalpy, entropy, Gibbs free energy and heat capacity had belonged to Al2O3, Fe2O3, Fe2O3 and Al2O3, and equal to 13 kcal/mol, 77.4 Cal/mol K, −14.2 kcal/mol and 19.8 Cal/mol K, respectively. Among formed sulfate with these oxides the most amount of enthalpy, entropy and Gibbs free energy have belonged to SiO2 that were obtained 28.08 kcal/mol, 133.22 Cal/mol K and 41.1 Cal/mol K, respectively. In addition to calculation of thermodynamic parameters and free energy of structures through molecular simulation, laboratory methods were also applied to calculate these parameters. Fig. 4 compares the results of obtained free energy for these compounds by laboratory methods and molecular simulation. According to the obtained results, there is a good agreement between obtained data from two methods.
Materials Studio software computes thermodynamic property for different structures with various translational, rotational, and vibrational components. In this section, DMol3 module was applied for calculation the total energy of reactions between metal oxides in red mud structure and sulfate ion at T = 298.15 K by means of Gibbs free energy differences (Eq. (6)).
In Table 3, all possible reactions between red mud compounds and Na2SO4 (was used for preparing sulfate synthetic solution for adsorption experimental test) and ETcorr, ΔH, ΔS and ΔG; related to each reaction are investigated profoundly. Results have illustrated the biggest difference between ETcorr of reactants and products in reactions which occurs for MgO and Fe2O3. As can be seen, the obtained total energy values for the reaction of all oxides in the red mud structure are regarded to be positive. The highest total energies were 756.2, 324.7, 273.2 and 242.3 kcal/mol for MgO, Fe2O3, Al2O3 and SiO2, respectively. These values indicate that reaction between sulfate ion and these oxides are non-spontaneous. Hence, the adsorption can be as a main mechanism for sulfate removal by red mud. In other words, main oxides which existed in red mud structure have not high tendency to the reaction with sulfate ions in water pollutants without spending energy and it can also be considered adsorption as an effective mechanism in sulfate removal sulfate removal.
It was applied Forcite module to predict of sulfate adsorption mechanism on the red mud surface and the adsorption energies calculation of system. After creation of appropriate cleavage for each crystal, various number and thickness layers were created on their surface and structures and they were optimized in different cutoff distances. Fig. S1 shows energy optimization results for oxides in red mud structure. These results demonstrate that the lowest total energies were −582.37, −657.78, −445.5, −673.2 and −698.4 kcal/mol for Fe2O3, Al2O3, CaCO3, TiO2 and SiO2 crystal structures, respectively. These energies were obtained in 6.25, 12.99, 25.13, 15.57 and 20.6 Å thickness.
In the next step, the adsorption energies of sulfate ion for all the oxides in the red mud structure was calculated via using adsorption locator module. Table 4 shows the results of adsorption energy per sulfate ion unit and for different oxides. These energies are −819.09, −561.7, −268.8, −105.4, and −314.7 kcal/mol for Fe2O3, Al2O3, CaCO3, TiO2 and SiO2, respectively. The adsorption energy values exhibits that the most tendency to adsorb sulfate ions among metal oxides belongs to Fe2O3, Al2O3, SiO2, CaCO3, and TiO2, respectively. As expected, iron and aluminum oxides with almost similar structures have a higher adsorption tendency for sulfate adsorption than other oxides in the red mud structure. It can be said that the adsorption mechanism between iron and aluminum oxides and sulfate ions is a spontaneous process without the need to add additional energy to the system. Therefore, the most active compounds in the red mud structure to remove sulfate ions were iron and aluminum oxides. These oxides account for 22.5% and 13.3% of the red mud structure, respectively.
Moreover, modeling steps of sulfate ion adsorption on hematite were done by the means of Castep module. At first, two types of optimal crystal lattice, including rhombohedral and hexagonal in which hematite crystallizes were modeled (Fig. S2) and the obtained lattice energy from these structures that were equal to −2239.66 kcal/mol. In the next step, the crystal structure of sulfate ion was designed, and the resulting lattice energy was also calculated as −170.71 kcal/mol. Then, to determine the most optimal part of the hematite surface for sulfate ion adsorption, the hematite cleavage surface was evaluated. According to the modeling results, the best desired cleavage surface was a surface with a miller index of (0 0 1) [43] and a lattice energy of −584.56 kcal/mol. After determining the adsorption site on the hematite structure, sulfate ions were added to the surface of 1×1 and 2×1 hematite, which can be seen in Fig. S3. The lattice energy values after sulfate adsorption on hematite in the two cases were equal to −1119.83 and −2239.66 kcal/mol, respectively. As can be observed, the lattice energy values after the sulfate ions adsorption by hematite were significantly more negative than the lattice energy values of the raw materials. This explores that the hematite crystal which releases more lattice energy after the sulfate ions adsorption has potential to reach to a more stable state than its initial state. In other words, sulfate ions adsorption on hematite produces a more stable product than hematite and it can be done spontaneously without the need to spend energy [44].
Also, the energy values of chemisorption and repulsion were calculated using the following equations and their values are shown.
3.2. Experimental Analyses
The study also focuses on the effect of various factors on sulfate removal, including pH, adsorbent dose, adsorbate concentration, and contact time. To determine optimum adsorption conditions, batch experimental tests were conducted in different pH (4–10), sulfate concentration (100–2000ppm), adsorbent dosage (0.5–5 g/L) and contact time (10–120 min). The results were brought in Tables S1–S4 and optimum adsorption conditions were 5, 1000 ppm, 2 g/L and 60 min for pH, sulfate concentration, adsorbent dosage and contact time. Among these factors, pH is the most important and effective factor in the adsorption of sulfate ions and in strongly acid medium, sulfate adsorption increases due to the high concentration of hydrogen ions and more positive charge on the adsorbent surface.
The performance of red mud was evaluated in different pH levels in Fig. S4. The adsorption tests performed on red mud and all the main oxides in its structure (Al2O3, SiO2, Fe2O3, CaCO3, TiO2) as adsorbents in different pHs, the sulfate removal and its adsorption capacity are higher in the case of iron oxide than the other types of adsorbents and metal oxides which have been used. The adsorption capacity and sulfate removal for iron oxide at pH=5 was 24.5 mg/g and 42.1%, at pH =6 was 18.5 mg/g and 25.3%, and at pH =7 was 25.0 mg/g and 20.6%. The highest percentage of sulfate removal and adsorption capacity was obtained at pH 5 for all metal oxides and red mud. This issue can demonstrate that the tendency of sulfate adsorption on all desired metal oxides and red mud composition is higher at acidic pHs. According to the results of adsorption tests at different pHs, the sulfate removal and adsorption capacity for iron oxide is higher than other metal oxides in the red mud structure. Also, in the case of molecular simulation values, adsorption energy of metal oxides and sulfate ions belongs to Fe2O3 and is equal to −819.09 kcal/mol. However, the adsorption capacity and sulfate removal for red mud were lower than all the metal oxides in its structure. These values for red mud at pH =5 was 12.7 mg/g and 25.5%, at pH =6 was 6.3 mg/g and 12.8%, and at pH=7 was 4.2 mg/g and 8.5%. This controversial issue illustrates that combination and interaction of these metal oxides in red mud structure cannot help sulfate ions absorption. But the red mud can still be considered as the potential adsorbent of sulfate.
3.3. Discussion
Sulfate is a common anion; found in various industrial processes as a water contaminant. Excessive sulfate in wastewater can cause environmental pollution and pH reduction in aqueous solutions. Thus, the removal of sulfate from wastewater is essential to maintain water quality and protect the environment. RM as a byproduct of bauxite processing in aluminum production industries has been utilized as an adsorbent due to the high content of various metal oxides and active compounds. Therefore, RM as an industrial waste has recently gained attention as an influential adsorbent for various inion contaminant including sulfate, because it can aid to neutralize the acidity of these wastes with its alkaline pH.
Also, investigation of various factors effect on sulfate removal (including pH, adsorbent dose, adsorbate concentration, and contact time) indicate that among these factors, pH is the most important and effective factor and in strongly acid medium, sulfate adsorption increases due to the high concentration of hydrogen ions and more positive charge on the adsorbent surface. In a strongly acid medium, sulfate adsorption increases due to the high concentration of hydrogen ions and more positive charge on the adsorbent surface. Specific adsorption can occur through the ligand exchange mechanism, the displacement of OH− groups by sulfates. Therefore, the adsorption of sulfate ions is more intensive under acid conditions [45–47]. Also, the adsorption experimental tests for red mud and all the main oxides in its structure show the percentage of sulfate removal and its adsorption capacity; higher for iron oxide comparing to other types of applied metal oxides in all levels of pH.
To clarify the red mud, various simulations were performed by appropriate modeling software (DMol3, Adsorption locator and Castep modules), and laboratory analyses which were also carried out to investigate the adsorption mechanism. RM contains high concentrations of oxides such as: iron, aluminum, and titanium oxide, which have a high affinity for sulfate ions. Adsorption bond length measurements for crystal oxide existed in RM with the DFT method, and the DMol3 module illustrated the shortest bond lengths, which belong to Fe2O3, Al2O3, TiO2, CaCO3 and SiO2 equal to 2.134, 2.745, 2.949, 3.194 and 18.229 Å, respectively. The bond length for α-Fe2O3 as adsorbent in the same research was obtained at 2.7–2.82 Å [44]. Thermodynamic properties of these oxides as reactants and their products which have been obtained during reaction with sulfate ion indicate that whole total energy of different reactions has been positive, and this issue shows that all these oxides need the external energies for their reactions and these reactions are non-spontaneous. The highest total energies were gained: 756.2, 324.7, 273.2 and 242.3 kcal/mol for MgO, Fe2O3, Al2O3 and SiO2, respectively. Therefore, these values indicate that the main oxides that existed in the RM structure do not have a high tendency to react with sulfate pollutants in water and can also be considered adsorption as an effective mechanism in sulfate removal.
According to the results from modeling of sulfate adsorption on hematite using Castep module, the best cleavage surface was (0 0 1) with lattice energy of −584.56 kcal/mol. The energy optimization of cluster sulfate ion was calculated as −170.71 kcal/mol. After sulfate adsorption on the 1×1 and 2×1 hematite surface, the lattice energy values in these two cases were equal to −1119.83 and −2239.66 kcal/mol, respectively. Chemisorption and repulsion energy for hematite structure were obtained: −364.5 and −1119.6 kcal/mol. As it seems, adding sulfate ions to hematite surface produces a more stable structure than hematite and can be done spontaneously without the need to spend energy.
For simulation of sulfate adsorption on red mud surface and adsorption energy prediction, adsorption locator module was applied. The negative adsorption energy results demonstrated that all main crystal components in the RM structure tend to adsorb sulfate ions. The highest adsorption energy: −819.09, −561.7, −314.7, −268.8, −105.4 kcal/mol for Fe2O3, Al2O3, SiO2, TiO2 and CaCO3 in order. The highest calculated adsorption energy for α-Fe2O3 as adsorbent obtained −83.7 kJ/mol in previous literature [43]. The values of adsorption energy demonstrated that the most tendency to adsorb sulfate ions among metal oxides belong to Fe2O3 and Al2O3. These oxides amounts are 22.5% and 13.3% in red mud structure. This issue shows that main metal oxides in red mud structure have high tendency for sulfate adsorption. All obtained results illustrate that red mud can help reduce the effects of AMD pollution. Although a column composed of RM as adsorbent and AMD passing through it and with sulfate adsorption on the RM surface can reduce the alkaline pH of RM, AMD pH approaches neutral values.
4. Conclusions
RM can be introduced as an unexpansive adsorbent for sulfate removal from AMD due to its high metal oxide content and high pH level, increasing this drainage’s low pH. Also, the adsorption of sulfate pollutants from aqueous solution using hematite, alumina and other red mud’s clusters as potential adsorbents separately were investigated. Key adsorption parameters, involving adsorption energy and Gibbs free energy were calculated by Dmol3, Adsorption locator and Castep modules which allowed us to model the adsorption process at the molecular level. Results illustrate the total energy values which have been obtained for the reactions between all oxides in the red mud structure and sulfate ions that can be determined to be positive. Therefore, reactions between sulfate ions and these oxides are non-spontaneous and adsorption can be viewed as the main mechanism for sulfate removal by red mud as an adsorbent. This issue was confirmed with the results of Adsorption locator module that showed adsorption energies for all the oxides in RM composition which can be negative. The results of our study highlighted that sulfate ions can be effectively adsorbed onto the hematite surface with favorable adsorption energies (−819.09 kcal/mol). This indicates that the adsorption process is energetically favorable and can also occur spontaneously. As expected, iron and aluminum oxides with almost similar structures have a higher adsorption tendency for sulfate ions than other oxides. Lattice energy values after sulfate adsorption on hematite by Castep module shows that the lattice energy after the adsorption is more negative than the lattice energy values of the raw materials and hematite crystal reaches, a more stable state than its initial state.
The experiments further validated red mud’s efficacy as an adsorbent for sulfate removal, demonstrating its potential as a low-cost alternative to conventional adsorbents. However, the adsorption capacity and sulfate removal percentage for red mud was lower than all the metal oxides in its structure whereas red mud can still be considered as a potential adsorbent of sulfate. Although RM do not have high adsorption capacity and sulfate removal percentages, applying new activation methods, including acid treatment, sea water washing, heating and combination with surfactant and bio absorbent, can increase the adsorption amount and reduce destructive environmental effects of both RM and AMD. The regeneration study revealed the high reusability (> 90%) and acceptable stability of RM even after five successive cycles. These findings indicate that RM use for pollutant removal from wastewater offers a green and environmentally sustainable approach.