Perspective for removing volatile organic compounds during solar‐driven water evaporation toward water production

Abstract

Solar evaporation has attracted considerable interest to alleviate the shortage of water resources in the past few years. Lots of works have been done focusing on evaporation materials, rate, and efficiency, however, little attempts have been made to remove the harmful volatile organic compounds (VOCs), a kind of contamination that inevitably evaporate and condense alongside with water, which seriously affect the distilled water quality. This perspective puts forward some technical ideas utilizing chemical or physicochemical methods, aiming at the development of multifunctional composite materials, and forming an integrated evaporation system, which manage to remove VOCs in situ at the interface of photothermal evaporation.

INTRODUCTION

Shortage of indispensable water and energy resources has become an urgent crisis. A practical strategy is to develop renewable energy source, such as solar power to compensate or substitute the diminishing fossil fuel.¹ Developing a low-cost solar-powered device, such as solar evaporation system, to produce fresh water from saline, could be an ideal solution for both water and energy supply. Great effort has been made in many aspects, such as developing new materials with higher light-to-heat conversion efficiency,² designing water channel,³ optimizing system salt crystallization resistance,^4,5 carrying out system thermal management to achieve higher light-to-vapor efficiency,^6,7 using solar photothermal water purification technology to decontaminate industrial wastewater,⁸ investigating the evaporation-driven electricity generation, and so forth.^9–11 Nowadays, photothermal evaporation system is developing toward higher water yield and higher efficiency in the continuous optimization process.

Water quality is a nonnegligible concern while pursuing high water yield. Using photothermal evaporation technology can remove most ionic impurities and microorganisms, and the effluent is able to meet the drinking water standard of seawater desalination.¹² Generally, the detection index of the water quality by photothermal evaporation technology is the concentration of four main ions (Na⁺, K⁺, Ca²⁺, Mg²⁺), which is relative incomplete when facing water with complex component.^12,13 Especially, the detection of volatile organic compounds (VOCs) or semi-VOCs in the wastewater has been ignored.¹⁴ The World Health Organization defines VOCs as organic compounds with saturated vapor pressure exceeding 133.322 Pa and boiling point ranging from 50 to 260°C under atmospheric pressure. A wide variety of VOCs with harmful properties possess saturated vapor pressures and boiling points that are close to those of water, which means that they could be vaporized together with the water, and be collected by condensation.¹⁵ It is reported that the concentration of the VOCs in the outlet even exceeds the original.¹⁶ As a consequence, VOCs severely affects the drinkability of the evaporated water, which is a major obstacle to overcome.¹⁷

However, few researches have focused on the removal of VOCs. Based on this concern, this perspective puts forward some chemical and physicochemical methods on VOCs removal during solar evaporation (Figure 1), which may inspire the future development. We have following prospects to solve the problem of VOCs volatilization in the process of photothermal evaporation according to existing VOCs control method. (1) Using chemical solution such as photocatalysis, advanced oxidation, and electrocatalysis to degrade the VOCs in situ during evaporation. (2) Combining photothermal materials with functional materials to prepare an integrated photothermal system, which could intercept VOCs at the evaporation interface through physicochemical effects such as adsorption and selective permeability. The key is to prepare integrated multifunctional materials that have both photothermal conversion and VOCs removal abilities, thereby ensure the outlet meet the qualification of drinking water. Besides, the object of this paper is the water contaminated by VOCs under emergency water supply conditions with fairly low VOCs concentration. Under rapid evaporating condition, the VOCs could potentially be collected together with water regardless of boiling point, which would bring a quiet large safety risk. Other than collecting water, solar distillation can also be used for the separation of VOCs in some industrial wastes, such as the recovery of ethanol or other chemicals.

FIGURE 1. Schematic illustration of the structure of this perspective. The methods for removing volatile organic compound (VOC) evaporation in solar evaporation process. Photocatalysis method is reproduced from Wang et al.,14 with permission. The method of selective interception is reproduced from Wang et al.,18 with permission

VOCS INTERCEPTION Chemical methods

In this section, we expound chemical methods applied in the photothermal evaporation system, such as photocatalysis, electrocatalytic, and advanced oxidation process (AOPs) with oxidant dosing, the detailed discussion is listed as below.

Integration of photothermal and AOPs with oxidant dosing

AOPs is a technology based on oxidants which can generate active species, for example, hydroxyl radicals (^•OH) and sulfate radicals (${\mathrm{SO}}_4^{·-}$), with strong oxidizability to oxidize organic compounds.¹⁹ This technology mainly includes Fenton catalytic oxidation, persulfate catalytic oxidation technology, ozone catalytic oxidation, and so on. Advanced oxidation technology has the advantages of wide application range, fast reaction rate, strong oxidation ability, making it a research hotspot for degradation of organic compounds in today's water treatment technology.²⁰

Fenton catalytic oxidation technology

The Fenton reagent named after chemist H. J. H. Fenton is a composite of ferrous ions/H₂O₂, which can degrade most organic compounds with non-selectively, even for those difficult to be biodegraded or treated by general chemical oxidation methods.²¹ Hence, a scenario that Fenton technology being applied in the photothermal evaporation process is expected to be an effective method to deal with the VOCs problem. It is assumed that Fenton reagent catalyzes the reaction to generate ^•OH under acidic conditions, and the ^•OH oxidizes and removes VOCs in water during the photothermal evaporation. However, there are also disadvantages of homogeneous Fenton method, like formation of ferric hydroxide flocs, low production rate and utilization rate of hydroxyl radicals, and so forth.

Fenton-like oxidation technology

In order to reduce the amount of H₂O₂ and improve the utilization rate of ^•OH, ultrasound, ultraviolet light, electricity and transition metals are introduced to oxidize organic compounds to make H₂O₂ generate ^•OH, which is Fenton-like method. The heterogeneous Fenton-like system prevents the formation of iron sludge compared to the homogeneous iron salts. Wang et al. reported a multi-functional honeycomb ceramic plate with a layer of CuFeMnO₄ on the surface.²² The CuFeMnO₄ coating act as both photothermal material and catalyst for VOCs removal by photon-Fenton reaction. Wang et al. provided forward-looking research ideas and work contents. It is suggested that combine other kinds of Fenton-like technology with photothermal materials to achieve efficient VOCs removal during fresh water producing.

However, integration of photothermal evaporation and Fenton/Fenton-like method have the following problems: the oxidant needs to be continuously supplemented, and the suitable pH range for Fenton method is relatively narrow (3–4). Furthermore, Fenton method would be greatly affected by the salinity of the water during the photothermal desalination, for the Cl⁻ in the saline will hinder the formation of ^•OH.

Persulfate catalytic oxidation technology

The traditional advanced oxidation technology uses hydroxyl radicals (^•OH) as oxides. In recent years, a new advanced oxidation technology, which takes the sulfate radicals radical (${\mathrm{SO}}_4^{·-}$) generated by persulfate activation as oxides to degrade organic compounds, has attract more attention. In general, Persulfate oxidation technology has a stronger oxidizability than Fenton system, and also be able to performed in much broader pH range (Figure 2A). However, due to the stable nature, it is difficult for persulfate to react with organic compounds without catalyst or at room temperature. Thermal activation,²³ UV activation,²⁴ alkali activation,²⁵ transition metal ion activation,²⁶ carbon material activation,²⁷ and other means are required to get a better activity.

FIGURE 2. (A) Schematic of the integration of photothermal and persulfate catalytic oxidation process during solar distillation. (B) Schematic of the integration of photothermal and photocatalysis process during solar distillation

The mechanism of thermal activation lies in that the activation energy in the reaction system is increasing by the temperature. When the activation energy is greater than 140.2 kJ mol⁻¹, the OO bond in persulfate will break to form ${\mathrm{SO}}_4^{·-}$. The photothermal evaporator absorbs light and converts it into heat, and the temperature of the evaporation interface increases. On the one hand, water can be heated at the water-air interface to form vapor, on the other hand, persulfate can be thermally activated to generate strong oxidizing ${\mathrm{SO}}_4^{·-}$ to decompose VOCs in situ. When permonosulphate (PMS) or peroxodisulfate (PDS) is added to the water, ions will be transported to the evaporation interface through water channel, and be activated thermally. Other than activating the persulfate, the heat can also increase the temperature, thereby increase the reaction rate and shorten the reaction time. This way, VOCs will be removed from distilled water by the as-generated ${\mathrm{SO}}_4^{·-}$ species.

Ultraviolet radiation with wavelength shorter than 270 nm can break the OO bond of ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ to form ${\mathrm{SO}}_4^{·-}$. When irradiated by ultraviolet light, the decomposition efficiency of persulfate will be significantly improved.²⁸ Transition metal ions can catalytically decompose persulfate to produce ${\mathrm{SO}}_4^{·-}$. The most common transition metal ion for activating persulfate is Fe²⁺, the activation energy required is about 50.2 kJ mol⁻¹, and transition metal ions like Ag⁺, Co⁺, Cu²⁺, Mn²⁺, Ce²⁺, Ti³⁺ as well have catalytic ability.¹² It is envisaged that transition metal ions can be combined with photothermal materials to prepare an integrated photothermal and transition metal activated persulfate catalyzed evaporator. This reaction can be carried out quickly at normal temperature and pressure without extra energy input. Carbon materials are widely used in the field of photothermal evaporation because of their high light absorption rate and easy modification. Many carbon-based materials can also activate persulfate. For example, Lee et al. reported that carbon nanotubes (CNTs) could activate persulfate through non-free radical mechanism, for which CNTs can effectively degrade organic compounds in water. This systems selectively oxidizes organic compounds, especially shows higher reactivity to phenolic compounds.²⁹ Carbon materials are widely used in the field of photothermal evaporation because of their high light absorption rate and easy modification. It is assumed to prepare simple integrated evaporation materials to solve the problem of VOCs in distilled water by photothermal evaporation.

Compared with Fenton oxidation technology, persulfate catalytic oxidation technology has higher salinity tolerance and wider pH range, but it unavoidably suffers from persulfate residue. As the evaporation continues, a certain amount of sulfate would generate in water, which may aggravate the problem of salt deposition during the evaporation process. Both of the technologies require the addition of chemical agents, which requires extra cost investment, let alone secondary pollution.

Integration of photothermal and photocatalysis

Photocatalysis can be introduced to degrade VOCs during solar distillation because it is also a light-driven interface process that can be inherently combined with photothermal processes.³⁰ Photocatalytic oxidation technology was first discovered by Fujishima and Honda.³¹ Practically, it involves adding semiconductor catalysts such as TiO₂, WO₃, CdS, GaN, or SnO₂ under the irradiation of light to generate strong oxidizing ^•OH.³² Another case is that valence band electrons of semiconductor materials transition to conduction band to form photogenerated electrons and holes under the irradiation of light, which have strong oxidizability and can oxidize and degrade almost all organic compounds. Additionally, photocatalyst excels itself in no need of extra reactive agents, which is meaningful for sustainable operation.

Song et al. proposed to integrate photothermal and photocatalytic materials, so that the materials could absorb light and convert part of the light energy into heat for evaporation, while use another part of the light energy for the generation of photogenerated carriers. In this way, during the water evaporation process, the VOCs contained are catalytically degraded into CO₂ and water at the evaporator interface, which not only improves the utilization rate of light energy, but also ensures the safety of the water quality. No other chemicals need to be added in the degradation process which might introduce extra contamination.

Nevertheless, the integrated system of photocatalytic and photothermal evaporation is facing the following problems: the photocatalytic reaction only occurs on the surface of the evaporator that irradiated by light, while the water evaporation rate rapidly (usually 1.5–4 kg m⁻² h⁻¹), which left nearly no time for VOCs to fully degraded. How to make VOCs fully contact with the active sites of the catalyst is a great challenge. In addition, whether the photothermal evaporation material and the photocatalyst will compete with each other and affect their respective light absorption, these above problems need to be further discussed. To solve these problems, researchers demonstrated a dual-scale porous photothermal/photocatalytic membrane based on the TiO_2−x nanofibers.¹⁴ This dual-scale porous structure can prolong the retention time of VOCs by providing more photocatalytic reaction sites and longer tortuous channels. Through the transition and relaxation of photogenerated electrons in TiO_2−x, photocatalysis and photothermal effects can be realized respectively. Hence VOCs were catalytically degraded (Figure 2B). Previous research provides a way for solving these existing problems, and the follow-up researchers should also consider these problems when carrying out their work.

Integration of photothermal and electrocatalytic oxidation technology

Electrocatalytic oxidation technology is an environment-friendly technology, which has many advantages. There is no need to add other oxidizing substances in the process of electrolysis, which is a clean technology; electrons are transferred to each other in the solution to avoid secondary pollution³³; electrocatalysis can be divided into two types: one type is direct electrocatalysis, which mainly converts organic compounds into small molecule organic compounds even CO₂ and H₂O through electrochemical oxidation in the anode. The other type is indirect oxidation, which means that an additional medium generates oxidizing substances under the electrocatalytic reaction to oxidize and remove organic compounds.^34,35 The electrocatalytic oxidation reaction process is controlled easily. It is envisaged to combine photothermal technology and electrocatalytic technology to form an integrated system for solar desalination. The photothermal material is loaded on the electrode sheet and served as the photothermal layer at the same time, which will decompose the organic compounds in the interface water through electrocatalytic oxidation while photothermal evaporation, therefore, avoid its evaporation into the distilled water. In addition, electrocatalytic technology has a potential trend in the treatment of high salinity water, which is suitable for solar distillation to desalinate.

In the above discussion, several possible and effective chemical strategies were proposed for VOCs controlling. What needs to be considered is that the generation of by-products. Besides of the mineralization of VOCs, side reactions may also occur in the complex actual water and undesirable by-products may generate. By far, little attention has been paid to the problem of residual products of chemical oxidant and the incompletely mineralized organic compounds at present. They may remain in water, enter the air, or appear in distilled water. These problems are expected to be addressed in future research. In addition, it should be noted that in chemical methods, the strong oxidizing substances produced in photocatalysis and AOPs systems have no evidence to prove that they can selectively degrade VOCs, and whether the oxidant will degrade other components of the evaporation system needs further study. Therefore, in the follow-up research, more attention should be paid to the selection of materials. Macromolecules and easily degradable materials may be prone to photoaging, which may reduce the stability of evaporators. It tends to choose inorganic materials, such as carbon and ceramics.

Physicochemical methods

In addition to the combination of chemical methods and photothermal technology, we also propose specific ideas for physicochemical methods with photothermal technology. The VOCs in the system are retained in raw water to ensure that they do not appear in distilled water.

Integration of photothermal and adsorption

Adsorption is an effective technique for organic compounds removal in water treatment, which is widely used because of its low operational cost, high removal efficiency, highly selective, highly specific, and easy operation.³⁶ Porous materials with large specific surface area, such as carbon nanomaterials, zeolites, and MOFs, are often used as the adsorbents. During solar-driven evaporation, the adsorbent with large specific surface area and loose porous structure can be used to temporarily adsorb VOCs and prevent their evaporation. Physical adsorption could separate VOCs using the difference of physical properties of components, and then remove VOCs by desorption for recycling. For example, carbon-based nanomaterials (graphene oxide, CNTs, etc.) have a good prospect as adsorbents for the removal of organic compounds in water owing to their low-dimensional structure and abundant surface functional groups, excellent adsorption performance and stability. Meanwhile, carbon-based nanomaterials has the advantages of optimal light absorption rate, wide absorption spectrum thus excellent evaporation rate, it also has relatively low cost and outstanding stability, which makes it widely used materials in photothermal evaporation.³⁷ It is envisaged to prepare a photothermal and adsorption integrated evaporator with a porous structure, and utilize van der Waals force to adsorb VOCs in water through a single physical adsorption or combination with other technologies. Another solution is to combine the water transport channel with the adsorption technology, so that the organic compounds would be absorbed during the water transport process thus could not reach the evaporation interface (Figure 3). Adsorption and desorption are a dynamic equilibrium process. In the first solution, the adsorbent is combined with the photothermal absorber, and the heat generated during irradiation may cause its adsorption of VOCs to decrease. The use of cold solar evaporation systems with low surface temperatures may be a development direction.^38,39 In addition, adjusting the surface temperature change the Henry's coefficient of VOCs vaporization, which can be further explored in the follow-up research. Obviously, main parameters such as the evaporation efficiency of materials in practical applications and the concentration of VOCs in the distillated water are of great importance, as well as the regeneration of photothermal integrated adsorbent. The surface charge properties of organic compounds should be considered in the selection of adsorbent materials. The integrated system of adsorption and photothermal evaporation is simple without producing residual products. However, there is a problem of adsorption saturation. Only after solving the in-situ regeneration of adsorbents in future can this method get further development.

FIGURE 3. Schematic of the integration of photothermal and adsorption process during solar distillation

Integration of photothermal and selectively permeable membrane

VOCs pollutants exist in some water bodies, which are easily evaporated together with water to form azeotropes, thereby increases the difficulty of removal. Due to the different dissolution and diffusion abilities of water and VOCs in the membrane, water molecules are more likely to pass through the membrane, while VOCs are intercepted. We envisage to make use of this property to prepare a membrane with both photothermal and organic compounds interception functions, or to combine photothermal membrane and selective permeable membrane into an evaporation system. The permeable layer can selectively permeate water, and after the water reaches the photothermal layer, it will be heated and evaporated under sunlight. The different vapor pressures between water and VOCs are the driving force in the evaporation interface. The membrane materials include polyvinyl alcohol, chitosan, sodium alginate, and so forth, which enhance the solubility selectivity of the membrane to water through the formation of hydrogen bonds. Hildebrand and Hanson solubility parameters are used to explain the interaction between membrane and separated substances, or the affinity of water and VOCs molecules to membrane materials. Generally, the closer the solubility of the membrane and components are, the easier it is for components to penetrate the membrane.

However, this method tends to sacrifice membrane porosity for selective separation through molecular networks, so the vapor generation flux will be reduced to a certain extent. In this case, it is necessary to optimize the thickness of the membrane or increase the roughness to create an evaporation surface with a high specific surface area, which is expected to balance the trade-off between selectivity and permeability. Researchers reported a PPy membrane with micro-pyramid structure, has selective permeability to water and VOCs (Figure 4).¹⁸ During solar evaporation, 90% interception of the evaporation of VOCs was achieved. Meanwhile, the membrane thickness is only 8 μm. The micropyramid structure reduces the light reflection and increases the area of the air/ membrane interface, resulting in an evaporation rate of 1.12 kg m⁻² h⁻¹.

FIGURE 4. Schematic of the selective transmission evaporator to intercept volatile organic compounds (VOCs) during solar distillation

This method provides a new sight for selective VOCs intercepting, and show the promising application of selectively permeable membrane in the photothermal evaporation. However, challenge still exists in maintaining both evaporation flux and separation efficiency. Through new material development or structure designing could enlarge the flux with a high separation efficiency, which could further promote the competitiveness.

CONCLUSION AND OUTLOOK

To conclude, the purpose of this review is to put forward some strategies to deal with the VOCs issues in solar evaporation water generation. Solar evaporation technology is facing the following problems: VOCs in water can be evaporated together with water, which would seriously affect the safety of distilled water. However, few studies have paid attention to this problem at present. In this paper, several solutions are proposed and commented forward-looking. Chemical methods including photocatalysis, electrocatalytic, and AOPs with oxidant dosing need to consider the production of undesirable by-products, residual chemical agents, and incompletely mineralized organic compounds. These difficulties are expected to be taken seriously in future research. Physicochemical methods include adsorption and selectively permeable interception. The choice of adsorption materials has to consider the surface charge properties of VOCs. Desorption and regeneration of adsorbents are now the bottleneck of research. Using permeable membranes to selectively intercept VOCs in photothermal evaporation are still challenging in maintaining the evaporation flux and separation efficiency. In the future, it is necessary to develop new materials or design structures to achieve the goal of expanding flux under high separation efficiency. We summarized and imaged the integration of the photothermal evaporation technology with the above methods to achieve the in-situ removal of VOCs at the evaporation interface. The principle, feasibility, advantages and disadvantages of this method are expounded. Hopefully, the ideas and examples in this perspective can provide more inspirations. We also hope that more disciplines will be integrated to provide more technical support for our lack of fresh water resources.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the National Natural Science Foundation of China (52070052, 51873047, and 52000161), the 68th batch of China Postdoctoral Science Foundation (2020M682356), the Natural Science Foundation of Heilongjiang Province (YQ2020B003), the State Key Laboratory of Urban Water Resource and Environment in HIT of China (No. 2019DX10), the Singapore National Research Foundation (NRF2017NRFNSFC001-048), and the TOUYAN Project of Heilongjiang Province (AUEA5640201520-01).

CONFLICT OF INTEREST

The authors declare no conflict of interest.