Amphiphilic engineering of MoS₂–g–C₃N₄ nanocomposites into a mangrove-inspired cascade system for sustainable drinking water production

Abstract

Drinking water contamination and water shortages are seriously exacerbated by industrial wastewater discharge. However, due to the high complexity of wastewater treatment systems, effective high-concentration pollutant removal and simplified wastewater recycling remain major challenges. Inspired by mangrove interconnected purification mechanisms, a novel cascade water treatment system has been developed using MoS₂–g–C₃N₄ (MoG), an amphiphilic material, as the main and single component to directly produce drinking water from wastewater with high efficiency. This cascade system integrates membrane filtration and solar-powered water evaporation processes to produce clean water, while also overcoming the requirement for less polluted source water that is typically required for standalone solar evaporation-based clean water production. The MoG membrane, featuring an amphiphilic platform, exhibits a high removal rate for organic and heavy metal contaminants and achieves a water flow of 966 L m⁻² h⁻¹ bar⁻¹ and an 80% efficiency in pollutant removal. The MoG-based aerogel enables nano- and micro-channels and exhibits a clean water production rate of 1.48 kg m⁻² h⁻¹ under 1 sun irradiation. The compact cascade system for practical use can produce drinking water that meets WHO standards from heavily polluted wastewater with an average hourly water production rate of 1.39 kg m⁻² h⁻¹. Life cycle assessment confirms that the cascade system displays significant environmental profile improvement with reduced CO₂ equivalent (CO_2e) levels with only 1/25 of that observed in conventional water treatment systems.

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Environmental significance

Water contamination and scarcity are exacerbated by the discharge of industrial wastewater. Heavy metal and organic pollution cause serious issues to public health and ecological environment. Nevertheless, the high cost and complexity of current wastewater treatment systems pose challenges to effective pollutant removal and simplified water resource recycling. Through emulating the ecosystem of mangroves, we innovatively integrated MoS₂–g–C₃N₄ (MoG) membrane filtration and MoG-based aerogel water evaporation, developing an efficient, energy-saving, and environmentally friendly cascaded wastewater treatment system. This innovative approach holds significant importance in tackling the scarcity of global potable water resources, providing a new solution to the global water crisis.

1. Introduction

Increased global industrialization and population have elevated the need for reliable drinking water resources.¹ Recycling wastewater into drinking water could be an efficient and long-term solution to this problem.² However, due to the high complexity of the conventional wastewater treatment system, there is a pressing need for a simplified and affordable new treatment method. Looking to nature for inspiration is usually helpful when tackling challenging issues.³ For instance, mangroves, which thrive in brackish coastal waters, use a cascade system that has evolved to be very adaptable and resilient to deal with high-salinity environments.^4,5 Mangrove roots and leaves cooperate to remove the salt from saline water to produce clean water: the mangrove roots, featuring a membrane architecture, act as a barrier to stop salt intrusion while the water evaporates from the leaves, generating freshwater droplets.^6–8 Compared with a variety of traditional and current water treatment processes, solar driven water evaporation interface technology shows significant low energy consumption and low carbon emission characteristics, this advantage not only meets the current global urgent demand for energy conservation and emission reduction, but also indicates its broad application prospects in the future field of water resources management. Most importantly, the technology does not rely on any fossil fuels or external power sources, and is powered entirely by clean, renewable solar energy, thus achieving an unprecedented level of autonomy and sustainability in energy supply.⁹

Inspired by the transpiration seen in mangrove leaves, solar evaporation offers a promising alternative for wastewater purification that could circumvent the need for complex and energy-intensive membranes.^10–12 However, solar evaporation techniques necessitate the use of a stock wastewater source with a limited pollutant concentration.¹³ For instance, high pollutant-containing wastewater (normally refers to heavy metal content exceeding 50 mg L⁻¹) can result in a failure of solar evaporation purification process, as the evaporated water may still contain harmful amounts of the contaminant due to the limited rejection rate of the photothermal materials.¹⁴ Thus, solar evaporation may be appropriate for saltwater desalination, but it is not ideal for sewage and industrial wastewater treatment. Previous studies on optimizing solar evaporation have typically focused on altering a single element, such as enhancing absorptivity or reducing heat loss. For instance, Li et al. demonstrated that optimizing the absorptive properties of the evaporative surface significantly increases desalination efficiency but does little to address the chemical and biological contaminants found in sewage.¹⁵ Similarly, Liu et al. highlighted how modifications to thermal insulation could improve performance in controlled conditions, yet the system remains inadequate for diverse industrial effluents.¹⁶ Leading to this poignant question: is it wise to only use a single step to achieve our water purification goals? Here, we merge membrane filtration and water evaporation technologies into one integrated system to manufacture drinking water cooperatively, mimicking the Mangrove roots and leaves cooperate to remove pollutants for freshwater production.

Unlike a simple one-plus-one combination strategy, we utilize a single nanomaterial, MoS₂–g–C₃N₄ (MoG), as the main component to develop a cascade system and achieve high energy efficiency. MoG was synthesized by decorating MoS₂ nanospheres onto exfoliated g-C₃N₄ flakes using a hydrothermal method. Due to its 2-dimensional (2D) nature, the MoG can be used as the basic building block to form a well-orientated laminate structure. This laminate structure is applied as the first step in the cascade wastewater treatment system. Thanks to the amphiphilic character of g-C₃N₄, the MoG membrane can allow both aqueous and organic compounds to access its nano and micro-channels to achieve efficient purification.¹⁷ Additionally, because MoS₂ exhibits excellent light absorption capacity (absorbs ∼96% of sunlight), MoG-based aerogels are ideal for effective solar evaporation and clean water production.¹⁸

Herein, we present a novel cascade system inspired by the water purification mechanisms observed in mangroves. Specifically, the cascade system incorporates a MoG membrane to mimic the mangroves roots as the first step in treatment. The MoG membrane can achieve a high-water flux of 966 L m⁻² h⁻¹ bar⁻¹ and a 70% efficiency in pollutant removal. The MoG-based aerogel also serves for secondary treatment, facilitating water transport and achieving a water evaporation rate of 1.48 kg m⁻² h⁻¹ under one Sun irradiation. We confirmed that this cascade system is effective for high concentration contaminant removal and water sterilization, achieving removal efficiencies of 99.9% for heavy metals and 100% for organic dyes. The produced drinking water has contamination levels well below the standards set by the World Health Organization (WHO).¹⁹ Our system sacrifices some of the primary membrane's rejection rate to achieve a high flux rate and overcome the limitations of bulk water requirements in solar evaporation to directly recycle wastewater into drinking water with high efficiency. Moreover, the cascade treatment system significantly reduces energy consumption, providing distinct advantages over conventional treatment processes such as ion exchange, chemical oxidation, and magnetic separation.²⁰ When comparing this system with other evaporators and conventional wastewater treatment systems, the cascade system's CO₂ equivalence (CO_2e) levels are much lower than those of typical treatment systems, generating 1/25 that of conventional water treatment systems.

2. Experimental section

2.1 Materials and chemicals

Mercuric acetate was purchased from Jiang yan Global Reagent Factory. The melamine was purchased from TCI. The Sodium hydroxide (NaOH) was purchased from RON reagent. Chitosan (CS high viscosity >400 mPa s), potassium chloride (KCl), LB Broth Agar, chromium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O), lead nitrate (Pb(NO₃)₂), cupric nitrate (Cu(NO₃)₂) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Acetate acid (C₂H₄O₂, >99.8%) was purchased from Shanghai Macklin Biochemical Co., Ltd. Thioacetamide (C₂H₅NS), glutaraldehyde (50%), ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O), congo red (CR) and rhodamine B (RhB) were analytical-pure, nitric acid (HNO₃) were superior grade pure, and all purchased from Tianjin Kemio Chemical Reagent Co., Ltd. Nylon ultrafiltration membrane supports with 0.22 μm pores were obtained from Zhejiang Yibo Separator Company.

2.2 Materials characterizations

SEM images were acquired using an S-4800 field-emission scanning electron microscope (Hitachi, Japan) at magnifications ranging from 2500 to 100 000. The morphology and structure of the specimen were examined using a JEM-2100 transmission electron microscope (TEM) operating at an acceleration voltage of 200 kV. Analysis of surface elements and functional groups on the adsorbent was conducted using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) with resolutions of 0.48 eV (Ag 3d_5/2) and 0.68 eV (C 1s), respectively. UV-visible absorption spectra were recorded using the Beijing Puxi T6 New Century UV-visible spectrophotometer. AFS-933 atomic fluorescence photometer and ICP-OES were used to measure the Hg²⁺, Cu²⁺, Pb²⁺, Cd²⁺ concentration in the solution.

2.3 Preparation of MoG-based materials

2.3.1 Preparation of MoG composites. The graphitic carbon nitride (g-C₃N₄) was prepared by a thermal annealing of melamine at 550 °C for 2 hours at a heating rate of 5 °C min⁻¹.²¹ A solution containing 1.07 g of ammonium molybdate tetrahydrate, 0.77 g of thioacetamide, and 0.1 g of g-C₃N₄ was prepared by dissolving these components in 35 ml of deionized water within a beaker. The mixture was subjected to magnetic stirring for 0.5 hours and then transferred to a 100 ml autoclave lined with polytetrafluoroethylene. The reaction took place at 200 °C for a duration of 24 hours. Following centrifugation and drying, the resulting MoG composite material was obtained. Pure MoS₂ was synthesized as a separate preparation, without the inclusion of g-C₃N₄ suspension.

2.3.2 Preparation of MoG free-standing membranes. In the typical procedure, distinct high-quality as-synthesized ultrafine MoG nanocomposites were dispersed in deionized water and agitated for 30 minutes to generate a uniform solution with diverse content ratios. Subsequently, the solution was subjected to filtration on a commercial substrate utilizing a vacuum pump under a negative pressure of 1 bar. Regarding the permeation evaluation, the thickness (mass density) of the MoG membrane was adjusted within the range of 5.09 to 8.91 mg cm⁻². Meanwhile, the MoG membrane with a mass density of 7.64 mg cm⁻² was chosen for all subsequent measurements pertaining to the separation of contaminant molecules. The selection of a nylon substrate was based on its minimal impact on the separation performance.

2.3.3 Preparation of the 3D MoG-based aerogels. Typically, 200 mg of MoG composites were combined with 200 mg of chitosan (dissolved in 1% acetic acid solution) and thoroughly mixed for 12 hours. Following this, 1 ml of glutaraldehyde (25 wt%) was introduced, and mechanical stirring was conducted for 1 minute. Subsequently, the mixture was subjected to heating in a water bath at 60 °C for 2 hours, resulting in the formation of the MoG hydrogel with a distinctive black appearance. The hydrogel was subsequently subjected to freeze-drying under vacuum conditions at −50 °C for a minimum duration of 24 hours.²²

3. Result and discussion

3.1 Construction of a cascade water treatment system based on amphiphilic MoG

MoS₂ nanospheres are produced by a hydrothermal method and decorated on exfoliated g-C₃N₄ flakes to fabricate an amphiphilic material MoS₂–g–C₃N₄ (MoG) for enhanced pollutants removal in wastewater. Negatively charged g-C₃N₄ nanosheets facilitate the binding and reassembly of molybdenum precursors onto their surface, resulting in the formation of MoS₂ nanospheres.²³ Fig. S1† illustrates a general heterogeneous nucleation and diffusion-controlled growth process of MoG. The SEM image reveals that MoS₂ nanospheres, each with a diameter of approximately 0.6 μm, are uniformly dispersed on g-C₃N₄ nanosheets, which have a lateral size of about 15 μm (Fig. 1a). Simultaneously, energy-dispersive spectroscopy (EDS) mapping analysis shows that S and Mo are evenly distributed on the g-C₃N₄ surface (Fig. 1b). The specific surface area of MoG can reach ∼40.1 m² g⁻¹ (Fig. S2†), and the pore size is generally less than 5 nm, indicating a high porosity and nanochannel-containing structure. X-ray photoelectron spectroscopy (XPS) was used to determine the component elements and chemical valence states of MoG, and the resulting spectra are given in Fig. S3a–d.† The results confirm the successful integration of MoS₂ nano spheres and g-C₃N₄, as well as the coexistence of 1T-phase and 2H-phase MoS₂. In addition, because the superior adsorption capability of 1T-phase MoS₂ and the pronounced electronic and optical response of 2H-phase MoS₂, the MoG nanocomposite exhibits distinct advantages for both pollutant removal and photothermal absorption.^24,25Fig. 1c shows the amphiphilicity of MoG materials. The contact angle experiment confirm that MoG nanomaterials display amphiphilic behavior towards water and oil. This amphiphilicity of MoG arises from the hydrophilic properties of the edge groups in g-C₃N₄ and the hydrophobic characteristics of MoS₂. The designed cascade system utilizing amphiphilic MoG material exhibits impressive efficiency in removing pollutants and producing clean water.

Fig. 1 Cascade water treatment system based on amphiphilic MoG. a SEM images of MoG; b EDS elemental (Mo, S) mappings of prepared MoG; c schematic diagram of the mimetic hydrophobic–hydrophilic nanostructure of MoG; the water was repelled in the hydrophobic area and attracted in the hydrophilic area; d the schematic diagrams illustrate the water purification mechanism of the mangrove tree (left) and the cascade treatment system inspired by mangroves (right). The cascade treatment system as shown comprises a MoG-based aerogel and a MoG membrane, mimicking the leaf, stem, and root of natural mangrove trees, respectively. The negative pressure provided by the vacuum pump serves as the driving force for water passage through the MoG membrane, and the once-filtered, purified water is conveyed to the evaporator for water evaporation by a peristaltic pump. The SEM image displays the morphology of the MoG-based aerogel and the MoG membrane, and the digital image presents the materials' natural characteristics (being flexible, durable and light weight) (right).

Based on the amphiphilicity of MoG nanomaterials, the primary aim of our cascade wastewater treatment strategy is to mimic the essential properties of the mangrove ecosystems. Fig. 1d illustrates the key elements of a mangrove, with desalination at the roots and water conduction and transpiration in the stem and leaf domains. For natural mangrove ecosystems, the wood vascular system connects the leaves and roots, facilitating the transport of desalinated water to the leaves while establishing negative pressure at the roots. To simulate the high-water flux and efficient salt exclusion observed in the roots of mangroves, our cascaded wastewater treatment system employs a MoG based membrane. Aqueous solutions permeate through the MoG membrane under a pressure of 0.8 bar; the subsequent water flux is determined using eqn (1):

where V is the volume of permeate (L), A is effective membrane area (for instance, 4 × 10⁻⁴ m² as the standard A in this research), t is the permeation time (h), and ΔP is the transmembrane pressure (0.8 bar).

Mangroves undergo transpiration through the stomata on their leaf surfaces, enabling the release of water vapor into the atmosphere through these microscopic pores. To achieve higher rates of water evaporation, a porous MoG-based aerogel was used as an evaporator to mimic the mangroves' stems and leaves. Due to the amphiphilic nature and porous structure of aerogels, water and organic pollutants can be continuously transported to the aerogel's surface through capillary action. Subsequently, it is heated on the surface and evaporates through small pores. The efficiency of water evaporation can be calculated using eqn (2):²⁶

where k is the evaporation rate, h_LV is the enthalpy of water evaporation (2444 J g⁻¹), C_opt is the optical concentration and q_i is the normal solar irradiation (100 mW cm⁻²).

3.2 Characterization and adsorption capacities of amphiphilic MoG

Research indicates that amphiphilic nanomaterials can disperse evenly throughout oil–water mixtures. To showcase the amphiphilic characteristics of MoG, we incorporated it into a cyclohexane/water mixture with equal volumes of each phase. The results revealed that, MoG flakes dispersed in the cyclohexane's nonpolar phase, enabling the creation of an emulsion in the vial through simple manual shaking (Fig. 2a).^27,28Fig. 2b presents the stabilized droplets as seen under an optical microscope. The microscope image clearly shows the droplets, whose sizes are influenced by the MoG concentration.

Fig. 2 Amphiphilic and adsorption performance of MoG. a Optical photographs of MoS₂–g–C₃N₄ stabilized pickering emulsions in a water/cyclohexane mixture at various concentrations; b optical microscopy images of MoG stabilized emulsion droplets; c molecular dynamic simulation of the interaction of rhodamine B (RhB) and MoG. Histograms and estimated densities of the distance between the atoms of RhB and the surface of MoG at 0 ns (blue), 10 ns (yellow), and 20 (green) ns respectively; d comparison of the effects of MoS₂ and MoG on the adsorption capacity of RhB at different contact times; e the change of fluorescent intensity of RhB upon addition of MoG; f adsorption isotherms of MO in water for MoG and MoS₂; g adsorption isotherms of Hg²⁺ in water for MoG and MoS₂.

In addition, molecular dynamics (MD) simulations were performed to explore the interaction between MoS₂ (2-layer)/g-C₃N₄ (2-layer) and RhB. As illustrated in Fig. 2c, S4 and S5,† RhB initially distanced from the surfaces of g-C₃N₄, MoS₂ and MoG, and gradually adsorbed to these surfaces. Notably, at approximately 10 ns into the simulation, RhB established contact with the surfaces of g-C₃N₄, MoS₂ and MoG. Subsequently, RhB remained attached to these surfaces until the conclusion of the simulation. Considering the conformational dynamics throughout this process, the transition state for the interactions was determined to be 10 ns. If we look at the histogram of distances, we can observe that the interaction between MoS₂ (2-layer)/g-C₃N₄ (2-layer) and RhB presents a narrower histogram centered at smaller distances at both 10 and 20 ns compared to MoS₂ and g-C₃N₄. This suggests that this composite presents a superior capacity for the adsorption of RhB, characterized by a more efficient process with a higher adsorption strength and specificity, with smaller interaction distances with a narrower interaction distribution. In light of these results, the compound exhibits pronounced amphiphilic characteristics, attributed not only to the active sites on g-C₃N₄ but also to the modulating role of MoS₂, which exerts its influence through additional active sites. To explore the disparity in RhB adsorption rates between MoG and MoS₂, their effects on the adsorption capacity of RhB over varied contact times were examined. Illustrated in Fig. 2d, MoG demonstrated a notably quicker adsorption rate, achieving equilibrium in merely 10 minutes, whereas MoS₂ required 60 minutes to reach the same state. This observation highlights MoG's superior efficiency in adsorption, attributed to its amphiphilic nature, suggesting its advantageous utility in adsorption processes involving organic cationic dyes.

To further obtain insight into the amphipathic superiority of MoG towards the adsorption of pollutant. The adsorption properties of MoG and MoS₂ for RhB were evaluated using fluorescence titration experiments (Fig. S6a and b†). The emission of RhB at 580 nm decreases significantly upon the addition of MoG, while the fluorescence intensity of RhB did not decrease much even when additional MoS₂ was used. As shown in Fig. 2e, when MoG and MoS₂ were added into the RhB solution, RhB was adsorbed onto the MoS₂–g–C₃N₄ and MoS₂, and the fluorescence intensity decreased continuously continued to decrease until reaching a minimum level. At the intersection, adsorption equilibrium was reached, indicating RhB has been completely adsorbed by MoG and MoS₂. Furthermore, as shown in Fig. 2f compared to pure MoS₂, MoG exhibits superior performance in the adsorption other dyes (the maximum adsorption capacity is about 20% higher). Additionally, MoG exhibits good adsorption capabilities for a wide range of dyes with more than 85% removal efficiency under both low and high concentration conditions (Fig. S7†).

Industrial wastewater always contains not only organic dyes but also a complex mixture of heavy metal ions coexisting with the dyes. Therefore, it is necessary for MoG to exhibit effective adsorption of both dyes and heavy metals.²⁹Fig. 2g presents the initial investigation into the adsorption capacities of MoG and MoS₂ for Hg²⁺. The findings revealed that MoG possesses a superior adsorption capacity for Hg²⁺, demonstrating its effectiveness in capturing this specific ion.

3.3 Removal performance of MoG membranes for pollutants

The effective filtering performance of the MoG membrane depends on the adsorption performance of MoG. Moreover, amphiphilicity can improve membrane purification by increasing the hydrophobic attraction between the membrane and aromatic dye molecules, which boosts the effectiveness of removal. The channel's hydrophilic areas make it easier for water to access; this encourages increased water flow.^17,30 The MoG adsorption capability for RhB and Hg²⁺ was investigated using isotherm adsorption models and kinetic models (Fig. S8–S13†). The adsorption behavior of MoG for RhB and Hg²⁺ adheres to the Langmuir model, underscoring the attainment of monolayer coverage. The adsorption capacity of MoG for Hg²⁺ decreases with increasing temperature. This could be ascribed to the attenuation of interactions between the active sites of the adsorbents and adsorbate species, as well as among neighboring molecules within the adsorbed phases.³¹ Furthermore, a series of adsorption experiments have substantiated the concurrent adsorption capabilities of MoG nanomaterials for various pollutants (Fig. S14 and S15†).

Due to the exceptional removal ability, MoG was used to fabricate MoG membranes to act as the primary step in the cascade system. The nanochannel-containing MoG materials are deposited onto a porous nylon substrate through vacuum filtration (Fig. S16†). Through fine tuning of the MoG dispersion volume, the loading mass of MoG can be precisely controlled within the range of 5.09 to 8.91 g cm⁻². Fig. 3a, illustrates a schematic representation of a nanocomposite membrane used for the removal of pollutants from an aqueous solution, which provides insights into the interaction mechanisms at play between the membrane and the pollutants, including electrostatic attraction, π–π interactions, and hydrogen bonds, which are instrumental in capturing the pollutants. This underscores the potential application of the material in various pollution scenarios, particularly in water treatment technologies that require efficient filtration of multiple types of contaminants.

Fig. 3 Pollutant removal performance of MoG based membrane. a Schematic diagram of MoG based membrane for removing pollutants from wastewater; b removal efficiency of MoG membrane loaded with 7.64 mg cm⁻² of various dyes, insert: color changes before and after dye removal; c UV absorption spectra of MoG membrane removing RhB-CR mixed dyes; d removal efficiency of MoG membrane for multicomponent system at different loading levels; e comparison of membrane separation efficiency and static adsorption efficiency for multicomponent systems.

RhB was used as an organic pollutant model to test the filtration performance of the membrane. Due to variations in transit channel distance, the loading mass of MoG has a significant impact on both water permeability and rejection rate.³² As illustrated in Fig. S17,† the rejection rate increased from 86% to 99% with increased MoG loading mass, whereas water flux decreased from 1190 to 837 L m⁻² h⁻¹ bar⁻¹. An optimized loading mass of approximately 7.64 mg cm⁻² was chosen for further performance assessment which exhibited a water permeability at 940 L m⁻² h⁻¹ bar⁻¹ with a RhB rejection of 97.52%. Additionally, a range of dye molecules with different weights, (ranging from 327.33 to 799.8 g mol⁻¹), were selected for comparison, including methyl orange (MO), malachite green (MG), methyl violet (MV), rhodamine B (RhB), bromocresol purple (BP), and methyl blue (MB). The chemical compositions and surface charge attributes for each molecule are shown in Table S5.† As shown in Fig. 3b, the MoG membranes consistently exhibit rejection rates surpassing 95% for MO, MG, MV, and RhB, irrespective of the solute charge characteristics. From Table S5,† we can see that BP, and MB have larger molecular sizes and accumulated charge centers compared to MO. The larger molecular size may create a spatial barrier effect, which is likely the reason for a different removal rate.^33–35

Moreover, to assess the viability of mixed molecule separation, binary mixture separation trials were executed utilizing the MoG membrane (Fig. 3c and S18†). The experiment was conducted at 0.8 bar with a combined concentration of 100 ppm for the RhB-CR binary feed solution. In the UV-vis spectra, the mixed feed solution showcases two absorbance peaks corresponding to RhB and CR, the resultant permeates solutions exhibit a solitary peak attributed to the smaller CR molecules post-filtration. This result illustrates the effective separation of the majority of smaller RhB molecules from the larger CR molecules using the MoG membrane. As shown in Fig. S19,† the filtration efficiency of the MoG membrane for Hg²⁺ was also evaluated under different loading conditions. As with dye molecules, the loading mass of MoG has an impact on the removal efficiency for Hg²⁺. As the loading mass increases, the water flux of the membrane experiences a gradual reduction, accompanied by a progressive enhancement in removal efficiency.³⁶

A synthetic wastewater was prepared to mimic the real application scenario to evaluate further the membrane's purification ability (methods S6†).³⁷ This synthetic wastewater was used as a solvent to prepare model dye effluent containing 50 mg L⁻¹ of Cu²⁺, Cd²⁺, Pb²⁺, Hg²⁺, RhB and CR. The removal efficiency of the prepared membranes was assessed through membrane filtration experiments using MoG membranes with simulated wastewater (Fig. 3d). It was found that, as the loading capacity increases, the removal efficiency of dyes and Pb²⁺ remains relatively stable, while the removal efficiency for Cu²⁺ and Cd²⁺ gradually increases. The stability of dye and Pb²⁺ removal efficiency can be primarily attributed to the minimal hydration ion radius of Pb²⁺ (40.1 pm) and the various adsorption mechanisms of dyes. These factors result in the rapid migration of both dyes and Pb²⁺ to adsorption sites, thereby maintaining their stability. The improved removal rates for Cu²⁺ and Cd²⁺ can be linked to the heightened availability of MoG adsorption sites within the membrane.^38,39 However, there is a slight reduction in the removal efficiency of Hg²⁺. This phenomenon can likely be attributed to heightened competition for adsorption sites between additional heavy metal ions and mercury ions. In addition, due to the confined space and the capillary force, the removal efficiency of the MoG membrane is superior to that of using only MoG powder for adsorption (Fig. 3e).

3.4 Solar evaporation performance of MoG-based aerogel

We then evaluated the evaporation capacity of the MoG-based aerogel, as the secondary step in the cascade system. As shown in Fig. 4a, under the irradiation of one sun, the water evaporation rate of MoS₂-based aerogel is 0.78 kg m⁻² h⁻¹, Mog-based aerogel is 1.48 kg m⁻² h⁻¹, 1 sun which exceeds the evaporation rate of pure water (0.29 kg m⁻² h⁻¹, 1 sun). Compared with MoS₂, MoG shows better photohot water evaporation performance. To comprehensively assess the efficacy of the MoG evaporation system in solar steam generation under 1 sun, a detailed analysis was performed to compare the system's heat loss and energy gain (calculated using eqn (S4) and (S5), ESI†).⁴⁰ The results indicated that, the top and side energy losses of the MoG evaporative system were calculated to be 0.0839 W, while the environmental energy gain, attributed to the lower temperature of the MoG-based aerogel compared to the ambient temperature, was 0.1108 W. This net effect of the disparity confirms a significant energy gain of 0.0269 W from the environment, effectively enhancing the overall water evaporation rate of the system. In addition, we use infrared cameras to capture real-time temperature changes. Fig. 4b illustrates that the surface temperature of MoG enhanced from 12.3 to 36.3 °C within 20 minutes and sustained equilibrium. In contrast, the temperature variation for pure water was minimal. This solar evaporation performance can be attributed to the extensive spectral absorption and hydrophilic channel surface of MoG. To date, one of the critical challenges in the practical application of solar water evaporation technology is its ability to address diverse and complex water quality environments.⁴¹Fig. 4c illustrates the water evaporation rates of MoG-based aerogel for different solutions (wastewater, RhB, pure water, acid, and alkali solutions: under 1 sun irradiation). The evaporation rates for wastewater, RhB, acid, and alkali solutions were 1.47, 1.30, 1.48, and 1.29 kg m⁻² h⁻¹, respectively, which are close to the water evaporation rate in deionized water (1.41 kg m⁻² h⁻¹). This indicates that the aerogels exhibit adaptability to various challenging water quality environments. Furthermore, simulated wastewater was formulated through controlled indoor experiments to assess the evaporation efficiency of MoG for wastewater treatment. During the evaporation process, although the concentrations of the four major ions notably decreased (rejection rate >95%), they still fell short of the required drinking water standards (Fig. S20†).

Fig. 4 Clean water production based on a two-step cascade system. a Time-dependent mass change of the water sample for MoS₂ based aerogel, MoG based aerogel and pure water under 1-sun illumination; b surface temperatures of the MoG-based aerogel and bulk water as a function of time; c water evaporation amount and efficiency of MoG based aerogel using different water sources; d photograph of outdoor cascade system to produce clean water during the day; e measured concentrations of four heavy ions in the simulated wastewater before and after solar evaporation. The dotted lines refer to the WHO standards for drinkable water; f the bacterial quantity in water after evaporation and antibacterial activity for different conditions; g solar endurance test results of evaporator continuously exposed under one sun illumination in a closed system batch purification prototype for 30 days with 8 h every day.

It is noteworthy that although the MoG membrane can effectively remove most pollutants from simulated wastewater, the purified water obtained after just one purification process, still does not meet the WHO drinking water standards. In addition, in the presence of excessively high pollutants and/or salt content in wastewater, the performance of standalone solar water evaporation systems is also significantly affected. This is because the excessive impurities in water increase the enthalpy of evaporation, leading to a reduction in the evaporation rate, and certain impurities can be carried into the distilled water along with water vapor during the evaporation process.^42,43 Therefore, to solve this problem, drawing inspiration from mangroves, with this research we explored the integration of a membrane filtration system and a water evaporation system to form a two-stage cascade system, with the goal of achieving energy and cost efficient, high-quality and the rapid production of drinking water. Fig. S21† presents the schematic diagram of the cascade system, the simulant wastewater is filtered once using vacuum suction, and then the primary purified water is pumped into the evaporation system using a peristaltic pump. To verify the wastewater treatment capability of the cascade system, simulated wastewater was prepared,³⁷ containing heavy metal ions Cu²⁺, Hg²⁺, Pb²⁺, Cd²⁺ at a concentration of 50 mg L⁻¹, along with 50 mg L⁻¹ of RhB and CR dyes. In June 2023, using an outdoor cascade system running experiments were conducted from 9 : 00 to 17 : 00 in Baoding, China. Fig. 4d illustrates the operational photographs of the cascade system under real sunlight. It was observed that by 17 : 00, condensed water droplets appeared on the inner walls of the system, and the collection area had accumulated a significant amount of distilled water. After treating with the cascade system, no peak corresponding to RhB and CR was detected in the collected water, confirming complete removal of organic pollutants (Fig. S22†). In addition the concentration of Hg²⁺, Cu²⁺, Pb²⁺ and Cd²⁺ in the produced purified water was reduced to 0.92 μg L⁻¹, 16.7 μg L⁻¹, 9.8 μg L⁻¹ and 4.6 μg L⁻¹, which was below the WHO standards (Table S7†) for drinking water (Fig. 4e).⁴⁴ In addition, following the WHO drinking water standards Escherichia coli (E. coli) should not be detected in the water. We further evaluated the antibacterial activity of the cascade system by inoculating with E. coli. After the bacteria solution was treated by the cascade system under 1 sun illumination, no colonies were found in the collected condensed water on the solid medium, indicating excellent removal of bacteria by the evaporation system. At the same time, the effect of light on the antibacterial effect of MoG-based aerogel was also evaluated. As shown in Fig. 4f, for MoG-based aerogel samples without light treatment, a large number of colonies can be seen on the solid media. However, after light treatment, significant inhibition zones were observed for the MoG-based aerogel indicating excellent bacterial purification ability. The inhibitory effect of MoG-based aerogel on bacteria may be due to the high temperature generated. As shown in Fig. S21,† the infrared images of MoG-based aerogel were also obtained, and the surface temperature of MoG-based aerogel increased rapidly from 27.5 to 70.2 °C within 40 min and then remained stable. This temperature could effectively kill bacteria in conventional wastewater.^45,46 The excellent anti-bacterial behavior is also attributed to the visible light induced antibacterial action of MoS₂. When MoS₂ was irradiated with visible light, it can catalyze the production of reactive oxygen species (ROS) containing, ·OH and ·O₂⁻ to accelerate the inactivation of bacteria, therefore exhibiting excellent antibacterial effects.^47,48 To validate the stability of the cascade system, it was operated continuously for 30 days under real sunlight, for eight hours each day. The results indicated that the solar energy system exhibited excellent stability (Fig. 4g).

3.5 Life-cycle assessment

To date, there has been limited research on the global warming potential (GWP) associated with solar evaporation technologies, especially within the context of wastewater purification systems. Nonetheless, for a comprehensive validation of these systems, it is essential to assess their environmental impact—specifically their GWP—and conduct a comparative analysis with traditional wastewater treatment methods. Fig. 5a illustrates a comparative analysis between the cascade wastewater treatment system and conventional wastewater treatment methodologies. The process delineated on the left is markedly streamlined, presenting an advantageous approach for environments where energy efficiency and water scarcity are critical concerns. Its ease of operation, reduced energy requirements, and sustainable nature make it an optimal solution for resource-constrained settings. As shown in Fig. 5b, the cascade system achieves the same level of treatment effect (more than 99%) as that for conventional six-step treatment processes using only two steps, indicating significant simplification of processing when compared to the traditional industrial wastewater treatment process. A preliminary life cycle assessment (LCA) was conducted based on ISO Standard 14 040,⁴⁹ utilizing lab- and pilot-scale data. Evaluating the CO_2e produced by the raw materials needed for the system with that produced by other traditional processes, the environmental advantages of the cascade system were assessed. Given that the systems evaluated in this study are not present in the ecoinvent v3.5 database, we constructed and compared each system side by side using industry and literature data.^50–55 As shown in Fig. 5c and methods S7,† when the water yield is 5000 L h⁻¹, the CO_2e of the raw material required for the cascade system is 3.95 × 10³ kg, while the CO_2e of the raw material in the production of conventional wastewater treatment process under the same conditions is 9.96 × 10⁴ kg, which is 25 times that of the cascade system. Furthermore, the first pie chart within Fig. 5c illustrates the percentage CO_2e contribution of raw materials used in the cascade system, with thioacetamide constituting more than half of the emissions at 50.57%. The second pie chart details the CO_2e distribution for the traditional process, highlighting activated carbon as the predominant contributor at 95.95%. To further mitigate carbon dioxide emissions, strategic optimization of raw materials with the most significant emissions contribution is essential. For instance, substituting thioacetamide with raw materials that have a reduced carbon footprint could render the cascade system more eco-efficient. Fig. 5d shows the results of the IPCC 2013 20a method for each system. This method assesses the CO_2e of the raw materials required to generate 1 L pure water from each evaporator. The CO_2e produced by the cascade system is lower than other evaporators, which means that our cascade system is more environmentally friendly. In addition, the cascade system offers a relatively low-cost for materials in production and operation, estimated as $5.22. (see Table S8†). Over a stable operational period of 30 days, the average daily water production rate stands at 11.2 kg m⁻², with daily production rates consistently ranging between 9.92–12.16 kg m⁻². Therefore, this low-cost approach readily enables the production of small-scale systems suitable for household generation of drinking water, while also offering a promising concept for the development of more environmentally sustainable and cost-effective large-scale water purification facilities. As such, the cascade system proposed in this study holds significant potential for addressing water-related challenges, representing a sustainable and energy-efficient approach for real world applications.

Fig. 5 Environmental benefits of cascading systems compared with other systems. a Cascade system process and traditional water treatment process; b comparison of removal efficiency of heavy metal ions by cascade system and conventional wastewater treatment process; c the relative CO_2e of the raw materials involved in the production of the cascade system and the conventional water treatment process at the effluent rate of 5000 L h⁻¹; d comparison of the amount of carbon dioxide generated during the production of the cascade system and other evaporators.

4. Conclusions

Unlike the single-step process, herein, we investigated a system-level, nature inspired cascade system. Consisting of two systems: membrane filtration and water evaporation. The MoG membrane, featuring an amphiphilic platform, exhibits a high removal rate of organic and heavy metal contaminants and achieves a water flow of 966 L m⁻² h⁻¹ bar⁻¹. The MoG-based aerogel exhibits nano and micro-channels and has a production rate of 1.48 kg m⁻² h⁻¹ under 1 sun irradiation. Using two steps, the cascade system can produce drinking water that meets WHO standards from heavily polluted water with an average hourly water production rate of 1.39 kg m⁻² h⁻¹. The ability to directly purify highly contaminated water into drinking water cannot be achieved by individual membrane filtration techniques or solely solar vapor purification approaches. From the LCA, our cascade system has significantly reduced CO₂ equivalent (CO_2e) levels in manufacturing, only 1/25 of that for conventional water treatment systems. Due to its facile design, low cost, consistent performance, and improved environmental profile, our cascade systems exhibit the potential to be a practical solution for wastewater treatment.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Sichen Liu: methodology, investigation, validation, data curation, formal analysis, writing – original draft. Haotian Wang: resources, investigation, validation, funding acquisition. Yumeng Xiao: software. David G. Calatayud: supervision, Boyang Mao: validation, Gaoqi Zhang: supervision, Chenhui Yang: data curation, Lidong Wang: validation, Meng Li: supervision, resources, writing – review & editing.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The present work is supported by the National Natural Science Foundation of China (Grant #: 52370110). This work was also supported by the Fundamental Research Funds for the Central Universities (Grant #: 2023MS146).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4en00633j

‡ Indicates the co-first author.

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