Advancing resource sustainability with green photothermal materials: Insights from organic waste-derived and bioderived sources

2.1 Marine-derived materials

The marine environment, covering 72% of Earth’s surface and holding 97% of its water, extends beyond vast saline waters. It is a critical source of food, medicinal compounds, and diverse raw materials, anchoring substantial economic value through its rich biodiversity [48]. It includes over 80% of the planet’s species, many of which are pivotal for the wealth of marine-derived biomolecules (polysaccharides, proteins and amino acids, fatty acids, minerals, and vitamins) [49]. These biomolecules have garnered significant attention for their potential in medical and engineering applications, reflecting a growing interest in marine bioprospecting, which is the “exploration of biodiversity for commercially valuable genetic and biochemical resources” [50]. Additionally, the exotic biological materials found in marine environments are foundational for the development of manmade materials inspired by nature, called biomimetic materials, which offer innovative solutions to current challenges in the field of biomaterials science [51].

The escalating global demand for seafood underlines the intricate relationship between the marine ecosystem and human resource extraction activities. Global seafood production reached 184.6 million metric tons in 2022, with China leading at 44.9 million metric tons, as reported by the Food and Agriculture Organization (FAO) of the United Nations [52]. This marks a modest increase from the 178.1 million metric tons in 2021. The past few decades have seen a 3% annual increase in seafood consumption, surpassing population growth rates. With the expected surge in the global population to 9.8 billion by 2050, seafood demand is predicted to rise by 60%, leading to more and more marine biowastes [53].

In 2022, the FAO estimated that seafood waste comprised 35–50% of total production, amounting to 64–92 million metric tons [52]. This by-product, particularly prevalent at the processing stage, represents not only an environmental burden but also a loss of valuable nutrients. Since the large quantities of seafood waste remain underexploited, their utilization can bring ecological and economic benefits [53]. For example, marine-derived biowaste is an overlooked resource containing materials that can be utilized to create biomaterials with photothermal properties. Figure 2a showcases the step-by-step conversion of seafood waste into valuable materials. The process begins with the collection of seafood, followed by chemical and enzymatic treatments to extract and purify the target materials that also have photothermal properties or can be modified to have such properties.

Figure 2

Marine-derived photothermal materials. (a) Illustration of the valorization process of materials from the sea to obtain materials that have or can be modified to have photothermal properties. (b) Schematic presentation of the fabrication of a porous scaffold by mixing cuttlefish ink powder with a chitosan solution at high temperatures. The material is later frozen at −20°C and subjected to lyophilization to obtain a porous material with photothermal properties. Reproduced with permission from Liu et al. [35], Copyright 2021, The Royal Society of Chemistry. (c) Schematic illustration portraying melanin particles that were extracted from Lophius litulon fish skin in water and combined with a metal–organic framework – Fe(iii) benzene dicarboxylate (MIL-53 (Fe)). The homogeneous suspension is then poured into a suction filtration device equipped with a polyvinylidene fluoride (PVDF) membrane to obtain a photothermal evaporator. Reproduced with permission from Wang et al. [57], Copyright 2023, Elsevier B.V. (d) Photograph of the Enteromorpha algae bloom polluting the shores of the Yellow Sea, China, and illustration of the transformation of Enteromorpha algae bloom into a scaffold using the freeze-drying method to obtain a porous material and subsequently developing a partially carbonized surface for photothermal properties. The resulting porous scaffold exhibits self-floating characteristics, resembling jellyfish. Reproduced with permission from Wang et al. [58], Copyright 2022, Elsevier Inc. (e) Illustration of the utilization process of naturally air-dried dead fish by crushing to obtain a powder from carp meat and bonemeal. Later, meat and bonemeal biochars with photothermal properties are obtained through pyrolysis in controlled anoxic conditions. Reproduced with permission from Qiao et al. [59], Copyright 2021, the Authors.

2.2 Plant-derived materials

Plant-derived materials, as well as animal-derived materials (described in Section 2.3), are gaining attention due to their unique properties and sustainability. This section will explore materials derived from plants, such as SCGs, plant oils, and wood. Figure 3a highlights these materials, showcasing their structural properties and potential applications in various photothermal processes.

Figure 3

Plant-derived and animal-derived materials and (a) their potential to be used as photothermal platforms. (b) Visual representation of the fabrication process of pomelo peel functionalized with PPy for photothermal solar absorber application. The naturally existing interconnected porous framework of pomelo peel can act as a sponge-like template with photothermal properties attributed to the presence of PPy. Reproduced with permission from Zhang et al. [83], Copyright 2020, American Chemical Society. (c) Converting food wastes into carbonized materials with photothermal properties for use as solar evaporators. The carbonaceous food wastes have the capacity to trap solar heat, primarily owing to their broad optical absorption in the entire solar radiation spectrum. Reproduced with permission from Zhang et al. [84], Copyright 2019 Elsevier Ltd. (d) Schematic illustration of the preparation and characteristics of wood-derived ACCF. Natural wood owes its remarkable photothermal conversion ability and unique Joule-heating effect to delignification and carbonization. Reproduced with permission from Wang et al. [46], Copyright 2023, Elsevier Ltd. (e) Scheme showing the preparation of carbonized manure for solar evaporator application. Microchannels in carbonized manure trap the incoming light, low thermal conductivity permits heat localization and randomly arranged black carbon fibers create microchannels that pump the seawater, allowing salt dissipation. Reproduced under CC BY-NC-ND 4.0 license [101], Copyright 2021, The Authors.

2.3 Animal-derived materials

As we have explored the potential of plant-derived materials for photothermal applications, it is also important to consider the significant contributions of animal-derived materials in this field. This section covers the materials derived from animals, with noteworthy examples including eggshells and bones, manure, and silk fibroin (SF).

3.1 Underlying principles and mechanisms and factors contributing to photothermal behavior

Photothermal-based materials are stimuli-responsive materials that efficiently interact with light, leading to heat generation under a suitable electromagnetic excitation [113]. The capability to generate photothermal heating is crucial in different research fields, including physics, chemistry, biology, and medicine. Understanding the underlying principles, mechanisms, and factors contributing to the photothermal properties of tailored materials is essential for harnessing this phenomenon for diverse applications. The main aspects that regulate the capabilities of specific materials to convert light to heat are absorption of light, energy conversion, transfer and relaxation, and thermal diffusion. Photothermal conversion begins when the material absorbs external radiation. Indeed, when photons from the incident light interact with the material structure, they can be absorbed by the outer electrons displaced in the material atoms. Consequently, converting the light into a heating process starts because the absorbed energy is subject to various mechanisms, such as electronic transitions and vibrational excitations, that affect the light-to-heat conversion, depending on the specific material properties. Once the absorbed energy is stored in the material’s atomic structure, the energy is transferred by exploiting different modes, such as the redistribution of energy among the electrons and phonons. Consequently, the generated heat increases in the material structure with a subsequent thermal diffusion that spreads the heat from the local site to the surrounding regions. It is worth pointing out that several physical mechanisms can efficiently contribute to the photothermal properties of specific photo-converter materials. In some materials, light absorption creates electronic transitions that form electron–hole pairs. The consequent recombination of these pairs could release energy in the form of heat (this is called an electronic mechanism). In other materials, light absorption causes vibrational excitations (phonons) that produce heat as they dissipate (the vibrational mechanism). Other materials, such as plasmonic NPs (e.g., gold and silver), exhibit enhanced absorption due to their localized plasmonic resonance phenomenon, oscillating the bulk-free electrons at the metallic/dielectric interface. This physical phenomenon produces a very high light-to-heat conversion, possibly achieving nanomaterials with almost a 100% photothermal efficiency. Several factors influence the photothermal behavior of specific materials, such as (1) material properties: the absorption coefficient, the electronic structure, and the thermal conductivity can predominantly affect the photothermal behavior; (2) wavelength of the impinging electromagnetic radiation: materials possess different absorption properties at different wavelengths; (3) size and shape: there are nanostructured particles made of specific materials (gold, silver, copper) with particular dimensions (10–40 nm) and shapes (rod, prisms, triangles) that exhibit enhanced absorption due to resonance effects; and (4) dielectric constant: the optical and thermal properties of the medium surrounding the materials play a crucial role in the photothermal heating generation, confinement, and propagation. It is essential to point out that environmental conditions can dramatically affect the photothermal capabilities of several materials. Indeed, factors like temperature, pressure, humidity, pH and ionic strength, oxygen and reactive species, surface coating, and functionalization can influence the light absorption, heat dissipation, and the final efficiency of the photothermal conversion process.

3.2 Intrinsically photothermal organic waste-derived and bioderived materials

As mentioned earlier, some organic waste-derived and bioderived materials possess the photothermal property as an intrinsic characteristic. The presence of specific components like lignin, melanin, or melanoidins often coincides with the emergence of the photothermal capability of these materials. This suggests a mechanistic link between the unique molecular structures of those components and the ability to efficiently convert light energy into heat. The following subsections will focus on the photothermal properties of organic waste-derived and bioderived materials.

3.3 Process-enhanced photothermal organic waste-derived and bioderived materials

Pyrolysis and calcination are two of the most employed strategies to convert natural feedstock into new functional materials [30]. The starting material is decomposed by a thermochemical process performed in an inert atmosphere (pyrolysis) or in air (calcination) under high temperatures. The resulting biochar can be surface-modified by acid or base treatment, chloride functionalized, loaded with functional modifiers, or doped. The surface area, crystallinity, and aggregation of biochar are also strongly influenced by the processing conditions. Carbon-based materials, such as graphite, carbon nanotubes, graphene, fullerene, and carbon black, are efficient photothermal materials that are able to convert the light of the whole solar spectrum into thermal energy [126]. Porous nanostructured carbon-based materials can reduce the reflection of the light, in addition to being almost insensitive to the incidence angle, and have been successfully employed as valuable materials for steam generation. Further examples, based on natural materials, can be carbonized wood, mushrooms, and many vegetables, displaying high light absorption due to the multi-scattering reflection of light in their microcavities and channels [127]. Moreover, carbonaceous derivatives of plant-derived nanospheres, CDs, hollow particles, and fibrillar structures have been used for their high surface area and the possibility of being coated over different materials when suspended in organic solutions.

Wood-based materials, with their low density, open microchannels, capillary-induced hydrophilicity, and high thermal insulation, have recently gained attention for ISSG systems. ISSG systems use photothermal materials, which are materials that convert light into heat, to harness solar energy for water evaporation. These systems are designed to float on water and heat the surface, causing the water to evaporate more efficiently. The most employed strategy to enhance the photothermal activity of wood materials in ISSG systems is bulk or surface carbonization, which transforms the natural material into a black body that captures photons.

In this context, carbonized sugarcane was dried and pyrolyzed up to 700°C inside a tube furnace under nitrogen (N₂) and used as the top layer in an ISSG made by expanded polyethylene and fiber paper [128]. Similarly, the torrefaction (a mild form of pyrolysis performed from 200 to 320°C under an inert gas) of bamboo slices gave them an improved light absorption ability in the full solar spectrum (94.7% under one sun illumination) and a high water-transport capacity, thanks to the presence of vascular microchannels in their structure [129]. In another study, surface-carbonization was applied to the defoliated bamboo stems, and the product was used as an efficient biobased material for solar steam generation since the structure of bamboo already contains all the elements of a solar evaporator [130]. Indeed, its microstructure shows many vertical channels particularly efficient for transporting water, with a hollow core in its internals, which is an ideal air chamber for thermal insulation, while the external surface notably increases the absorption of the solar spectrum after carbonization (from 40 to 97%). A robust and natural-based sponge evaporator has been proposed using chitosan as an excellent and biocompatible hydrophilic material, carbonized pomelo peels particles as light-absorbing layer, and bamboo fibers acting as the tough matrix to support the device [131]. Owing to the bridged structure of bamboo fibers interspersed in chitosan lamellae and the high photothermal activity of carbonized pomelo peels, the composite solar sponge showed an evaporation rate up to 2.32 kg m⁻² h⁻¹ with a conversion efficiency of 89.23% under one sun illumination and the obtained results were almost insensitive to harsh water conditions. Other bioderived materials can also effectively acquire photothermal properties. Cattails from Typhaceae plants at different lengths and diameters were carbonized under an inert atmosphere using an increasing temperature profile (from 100 to 800°C) in a tube furnace [132]. Carbonized cattails have been fixed using expanded polystyrene foam and put into an evaporation chamber floating on the water surface. Shiitake mushrooms were also carbonized by a simple thermal treatment (500°C under argon for 12 h) and placed on a polystyrene foam [133].

Camphor nanospheres were recently obtained by calcination of Cinnamomum camphora extract pellets [134]. The obtained black soot was dissolved in ethanol, and the solution was deposited on polyvinyl alcohol sponge disks. Carbonized natural materials have also been widely applied in photothermal energy conversion for cancer therapy. For example, okara, a product of tofu preparation, has been calcined to form spherically shaped hollow particles with an average diameter of 200 nm, which have shown high biocompatibility and ability to act as photothermal converters for the NIR-triggered cancer treatment [135]. S-CDs, obtained by hydrothermal carbonization of Camellia japonica flowers, have killed cancer cells without damaging nearby healthy tissues after 5 min of NIR laser irradiation [96].

Researchers have recently subjected natural materials to surface (or bulk) chemical modifications in order to improve their intrinsic properties. The treatment of wood with tannic acid (TA) followed by a second treatment in the presence of a ferric sulfate solution gives it excellent photothermal properties, achieving 90% photothermal conversion under one sun illumination and improving water evaporation rate from 1.34 to 1.85 kg m⁻² h⁻¹ [30]. Similarly, coconut husks were first immersed into a solution of TA for 8 h at room temperature and then in a solution of ferric chloride for 2 h, washed, and freeze-dried. The obtained photothermal material showed fast water transportation in capillaries, large water-storage capability, and an evaporation rate 4.15 times higher than bulk seawater [136]. Hydrogen can be produced in a clean and green way by the dry reforming of methane (DRM). The SiO₂, extracted from bagasse and subjected to surface modification by KOH activation, has been recently employed as a support for a Ni catalyst for the DRM process. Thanks to the interaction of the catalyst with UV light to generate hydrogen, the temperature of the process could be lowered to 300°C instead of 700°C usually reached to obtain satisfactory yields [137]. Wind turbine blades have been treated with a super-hydrophobic layer with a photothermal effect based on biochar obtained from corncob carbonization. The biochar was loaded with Fe₃O₄, added to a solution of polydimethylsiloxane (PDMS) in n-hexane and spray-coated on the surface of the turbine blades, showing a high deicing performance under the sun illumination [85]. Similarly, Wang et al. developed an anti-icing surface treatment based on commercially available biochar and plasmonic TiN NPs, which were added into a fluorinated triethoxysilane solution [138]. The treated glass-fiber-reinforced plastic of the turbine blades showed a significant reduction in the icing mass.

Aluminophosphate-treated wood (W@AlP) can be easily obtained by means of facile, cost-effective, and scalable surface treatment [45]. AlP, prepared by reacting Al(OH)₃ powder with an aqueous solution of phosphoric acid (H₃PO₄), was brushed on the top surfaces of wood blocks, and the obtained samples were then heated at 130°C for 30 min. A black surface was obtained where the wood was treated due to the easy carbonization promoted by the Lewis acid catalyst, giving a hierarchical porous structure particularly suitable for broad solar spectrum absorption and water escape. Treated woods can float on seawater and exhibit a solar thermal efficiency of 90.8% with an evaporation rate of 1.423 kg m⁻² h⁻¹ under one sun illumination.

Loofahs, the fibrous skeletons of loofah fruit, are mainly composed of hydrophilic cellulose and show good mechanical properties and hydrophilicity. Zhang et al. recently proposed a carbon nanofiber decorated carbonized loofah (CL) steam generator supported on hydrophobic polyethylene-vinyl acetate foam as a thermal insulator [139]. CL sheets were first treated with a dopamine solution and heated up to 800°C in a nitrogen atmosphere for 2 h. PDA-coated CL was treated with an aqueous solution of nickel nitrate (Ni(NO₃)₂) for 24 h. Sheets were then calcined, obtaining the vapor deposition of carbon nanofibers over the whole surface. The proposed CL/carbon nanofiber layer gave a sewage water evaporation rate of 1.65 kg m⁻² h⁻¹ under illumination when used in an ISSG experimental device. Loofahs decorated with titanium carbide MXene nanocrystals have also been profitably employed in an evaporator for solar steam water desalination. The device showed high stability under sunlight, prevented salt deposition, and reached an evaporation rate of 1.57 kg m⁻² h⁻¹ under one sun illumination, with a thermal conversion efficiency of 95.5% [140].

3.4 Advantages of materials with “intrinsic” photothermal properties

The exploration of organic waste-derived and bioderived materials with intrinsic photothermal properties presents a promising avenue in the development of sustainable photothermal applications. These materials, by virtue of their natural composition, offer several benefits that reduce the need for extensive post-processing, which is often resource-intensive and environmentally taxing [31,33,35,40].

First, the intrinsic photothermal characteristics of these materials eliminate the necessity for additional chemical modifications or coatings that are typically required to enhance the photothermal response in synthetic materials [31,35]. This not only simplifies the production process but also minimizes the use of potentially hazardous chemicals, aligning with the principles of green chemistry. Second, materials with natural photothermal properties often exhibit a broad absorption spectrum, enabling efficient light-to-heat conversion across a wide range of wavelengths [33,35,41,141]. This broad-spectrum responsiveness is particularly advantageous in applications such as solar steam generation and PTT, where the utilization of the full spectrum of solar radiation or the flexibility in choosing an irradiation source can significantly improve performance. Finally, the integration of intrinsically photothermal bioderived materials with non-photothermal organic waste-derived counterparts yields a synergistic effect that significantly enhances the overall performance of photothermal applications [35,42,141]. This combination leverages the unique properties of each material, bringing together the best of both worlds to create a more efficient and sustainable solution.

For instance, natural melanin, found in abundance in squid ink and cuttlefish ink, exhibits intrinsic photothermal properties [142]. When used in conjunction with non-photothermal materials such as starch or chitosan in hydrogel form, they constitute a composite that not only retains the excellent photothermal properties of melanin but also benefits from the structural advantages of bioderived materials [35,141]. Starch and chitosan hydrogels provide a supportive matrix that facilitates water transport and retention, crucial for applications like solar steam generation. Therefore, cuttlefish ink-based solar absorbers use the natural pigments found in cuttlefish ink, such as melanin, to enhance light absorption. As mentioned, melanin is known for its broad-spectrum light absorption properties, which make it an effective material for converting solar energy into heat. Another compelling example of utilizing intrinsic photothermal properties is found in plants containing lignin [31,114,143]. Lignin, a complex organic polymer, forms a significant part of the cell walls in vascular plants, making it one of the most abundant biomasses on Earth. Its molecular structure is characterized by strong π−π stacking interactions, which not only facilitate self-assembly but also enhance photothermal conversion [144]. This unique feature of lignin allows it to act as a natural photothermal agent within the plant matrix. Separately, the integration of PDA and wood offers a unique approach inspired by natural processes [42]. PDA, known for its broad light absorption across the UV, Vis, and NIR regions, enhances the photothermal properties of the materials it coats [145,146]. By applying a PDA coating to wood, a natural superhydrophilic scaffold, the efficiency of solar evaporation systems can be significantly improved [42]. Mimicking the transpiration mechanism in plants, where water is translocated from roots to leaves, coated wood can float and create a thin water layer on its surface, making it an ideal candidate for such applications. SCGs present another case. They intrinsically contain melanoidins, which are responsible for their photothermal properties [123,147]. These high-molecular-weight polymers, formed during the roasting process of coffee beans, exhibit significant light absorption capabilities. Given their powdery nature, SCGs should ideally be embedded in a structurally stable platform for practical applications [40]. However, the development of such platforms from organic waste-derived or bioderived materials has not been widely explored, indicating a promising area for future research.

These examples highlight the potential of combining intrinsically photothermal materials with those that are not, where each component brings distinct advantages to the table. It enhances the photothermal performance and simplifies the manufacturing process. Moreover, the use of such composites in photothermal applications serves as a model for circular economy practices. By valorizing waste products and utilizing renewable bio-resources, these materials minimize environmental impact and offer a pathway to resource recovery and waste reduction.

4.1 Solar-driven water technologies

Solar-driven water technology provides a promising avenue in freshwater production, which depends on light-to-thermal conversion and distillation processes [148,149]. Additionally, the solar-driven water evaporation process is accompanied by mineral production [150], power generation [151], seawater desalination [152], and other applications [153,154]. Currently, the research on solar-driven water technologies depending on light-thermal conversion behavior mainly focuses on photothermal material development and optimal structural design [155]. Photothermal materials are regarded as ideal materials for water treatment due to their broad solar absorption and high conversion efficiency [156]. To date, a variety of unique topologies and structures have been developed to endow solar evaporators with outstanding light harvesting, water absorption, and photothermal conversion capabilities [157]. However, it still faces challenges in developing eco-friendly and cost-effective materials. In recent years, bioderived and organic waste-derived materials, as inexhaustible renewable energy sources, have attracted extensive attention because of their low cost and environmental friendliness [27]. This section reviews the development of green photothermal materials and their applications in seawater desalination, water purification, and energy production.

The mechanism of water storage and evaporation in solar-driven water technologies involves several key processes [158,159]. First, water absorption and storage play a crucial role. Natural materials, particularly those derived from plants, often possess intrinsic capillary channels and vertically oriented porous structures designed for transpiration. These structures facilitate efficient water absorption and storage, as the water is absorbed through capillary action into the microchannels and pores. The hierarchical porous structure, from macro to micro scale, ensures a continuous supply of water to the evaporation surface. Solar absorption and photothermal conversion are the next critical steps. The photothermal material, often carbonized or modified to enhance solar absorption, captures sunlight across a broad spectrum, converting absorbed light into thermal energy, which significantly increases the temperature of the material. This heat is then localized on the surface of the photothermal material, where it heats the absorbed water. Due to the low thermal conductivity of these materials, heat is effectively confined to the evaporation zone, minimizing thermal losses. The localized heating increases the temperature of the water, causing it to evaporate. Finally, the produced water vapor rises and is subsequently collected and condensed into clean water. This process is facilitated by the porous structure of the material, allowing efficient vapor transport. In practical applications, the condensed vapor is collected using condensation systems designed to maximize the yield of purified water.

A wide variety of plant fruits have been used as starting materials for the production of solar evaporation devices. The carbonization approach provides a simple and robust route to improve the photothermal conversion capacity while maintaining the distinctive natural structures of plant fruits [160]. The natural materials with intricate architecture [161], possessing large specific surface areas and multilevel pores, are beneficial for solar absorbers. As an example, a solar evaporator with a 3D photothermal structure was fabricated via carbonization of durian skin at 700°C, which exhibited a macro-scaled 3D pyramid and micro-scaled porous petal-like architecture [162]. Benefiting from the multiple reflections and scattering on the unique hierarchical structure, carbonized durian skin exhibited better light trapping (99%). Beyond that, the outstanding capillary effect of the internal porous microchannels ensured adequate water transportation for evaporation. Subsequently, an evaporation rate of 2.22 kg m⁻² h⁻¹ with 93.9% energy efficiency was achieved upon one sunlight illumination. Furthermore, a carbonized durian-based outdoor device was built to demonstrate the practical application, and it was found that the daily collected purified water was able to meet the needs of more than 26 adults. Pomelo peel, a waste from pomelo, is lightweight with an interconnected foam-like structure and rich microscale pores. Inspired by the fractal structure in nature, Geng et al. developed fractal carbonized pomelo peel (FCPP) via direct graphitization followed by modification with macroscopic hole patterns by punching method and nanoscopic porosity by surface deposition of PPy nanowires [163]. The intricate fractal structure with macroscaled hole patterns and microscaled interconnected porous structure originated from the pomelo peel, and the nanoscaled porosity from the nanowire cluster benefited solar utilization significantly. As a result, the FCPP systems with 98% solar spectrum absorption were obtained. The evaporation rate and solar thermal conversion efficiency were 1.95 kg m⁻² h⁻¹ and 92.4%, respectively, showing a potential for seawater desalination. Furthermore, to meet the synchronous demands of fresh water and electricity, the discarded pomelo peel was converted into 3D porous carbon foams through freeze-drying and carbonization [164]. Owing to the superhydrophilic multichannel waterways, the carbonized pomelo peels showed a powerful water supply ability to avoid salt deposition. Then, the carbonized pomelo peel with solar-driven evaporation capability was integrated with a commercial thermoelectric generator. It was found that the as-prepared device could generate steam (1.39 kg m⁻² h⁻¹) as well as electricity (0.5 Wm⁻²) under one sun irradiation. Alternatively, chemical treatment provides another simple approach for developing photothermal materials. For example, the coconut husk-based photothermal materials was fabricated by immersion of coconut husk into a solution containing TA and Fe³⁺ [136]. The structural feature in combination with surface chemical composition decreased water enthalpy, leading to enhanced evaporation efficiency. The seawater evaporation rate of the resulting system reached 1.83 kg m⁻² h⁻¹. Another study reported a scale-producible SCGs-based photothermal material with the aid of in situ polymerization of PPy NPs on the delignified SCGs (PPy@D-SCG) [165]. The modification of PPy NPs on the porous D-SCG enhanced the light absorption sites and water evaporation area. A 3 kg of photothermal PPy@D-SCG was fabricated, demonstrating the scale-up production capacity. The PPy@D-SCG displayed a water evaporation performance under one sun illumination with a rate of 1.54 kg m⁻² h⁻¹. Moreover, a 3D solar evaporator was developed by 3D printing of polylactic acid and PPy@D-SCG composite, enabling the removal of organic dyes and heavy metal ions from wastewater to produce fresh water.

Vegetable-originated materials with exquisite structures have been widely applied in solar absorption systems. Since the porous structure is of importance for water transportation and thermal management, loofah, a sponge-like plant with hierarchical macroporous and microchannel structures, has been a favorable choice. Lu et al. proposed a Janus solar evaporator consisting of an alkalized and CL sponge, serving as a porous supporter and light absorber, respectively [166]. The Janus structure combining the solar evaporation and surface biosorption property imparted the photothermal evaporator with freshwater generation and heavy-metal ion recycling capacity. Accordingly, evaporation (1.36 kg m⁻² h⁻¹) and energy conversion performance (83.7%) were obtained at one sun illumination, along with self-desalting capability. In another work, a multi-layered evaporator composed of loofah and MXene nanosheets was prepared [140]. The upper hydrophobic loofah with fluorinated alkyl silane and MXene coating enhanced the light absorption performance. Meanwhile, the bottom superhydrophilic loofah with large holes and microchannels enabled the pumping of water to the top layer and prevented salt accumulation. The loofah/MXene-based evaporator exhibited not only a high evaporation rate and solar-thermal conversion efficiency but also superior self-desalination capability.

A wood-based evaporator provides a feasible strategy owing to the aligned hierarchical porous structure, excellent mechanical stability, and low thermal conductivity [167]. In order to improve the evaporation rate and interfacial binding strength, Sheng et al. proposed an in situ synthesis method to assemble coordination polymers of Ni-dithiooxamidato (Ni-DTA) to the balsawood [168]. The Ni-DTA with both nanofiber and NP morphologies was grown on the wood walls, and 1D-nanofibers and 0D-NPs were formed by controlling methanol and dimethylformamide ratios (Figure 4a). The introduction of Ni-DTA with powerful hydration capacity to vertical channels of wood facilitated water supply and salt exchange. Notably, the presence of Ni-DTA was able to decrease evaporation enthalpy, leading to a higher water evaporation rate (2.75 kg m⁻² h⁻¹). Furthermore, the mechanism of reduced evaporation enthalpy was demonstrated by molecular dynamics simulations (Figure 4b). The molecular dynamics models of wood and wood-NiDTA evaporators were established to assess H-bonding density during the evaporation process. The escaped water molecules from the wood–NiDTA−water system are much larger than those from the wood–water system at 288 ps. The reason could be that the strong molecule interaction originating from Ni-DTA and H₂O reduces the H-bonding density between water molecules, thereby decreasing the vaporization enthalpy of the adjacent water molecules along with increased evaporation rate. As another example, copper-based MOF (Cu-CAT), which is an electronically conductive material with broadband light absorption capability, was integrated with the wood sheets for the preparation of the solar evaporator [169]. Unlike the conventional bi-layered structure, the Cu-CAT was in situ grown on the top and bottom sides of wood sheets without any binder. The upper and bottom Cu-CAT layers provided light adsorption and anti-oil-fouling performances, respectively. Benefiting from the Cu-CAT coating, the evaporator could produce drinkable water from a variety of contaminated waters. Importantly, electricity could be generated along with the solar vaporization process.

Figure 4

Wood-based evaporator. (a) Schematic illustration of morphological structures of Ni-DTA on the wood channels, as well as improved salt exchange and accelerated water evaporation attributed to the presence of Ni-DTA. (b) Molecular dynamics simulation of wood−water and wood–NiDTA−water evaporators at different times. Reproduced with permission from Sheng et al. [168], Copyright 2023, American Chemical Society.

The plant stems, which transport water from the ground to the leaves, possess a large amount of cellulose and a connected porous structure [170]. The luffa vine-based evaporator is an innovative example of using natural materials for efficient solar water evaporation. Figure 5a displays the internal porous structure of the natural luffa vine, highlighting its xylem vessels and piths, which are crucial for water transport. Figure 5b presents a schematic that illustrates the water and salt exchange process during solar water evaporation. Mentioned solar evaporator have been fabricated using surface carbonization followed by KOH-freeze-drying [171]. The lucerne vine possesses vascular bundles with multiple large vessels inside, and abundant pits exist on the wall of vertical hollow vessels. The small-diameter vessels, high capillary pressure difference, and abundant pits on the cell walls contributed to rapid and excellent water absorption and transfer performances (Figure 5a). The water vapor generation rate reached 3.26 kg m⁻² h⁻¹, and the evaporation efficiency was 118%. Furthermore, a spontaneous salt backflowing system was formed owing to the hierarchical architecture consisting of vessels and pits of different sizes (Figure 5b). During solar evaporation, a salt concentration gradient was formed in the lucerne vine evaporators since a larger salt concentration was reached in cell channels than in vessel channels. Furthermore, an enhanced salt concentration gradient was achieved in the upper layer of the vessels with quick evaporation of the surface water. Next benefitting from capillary pressure, the seawater continues to flow upward in the cell channels, accelerating the salt backflow process from the vessels into the water body. The salt backflowing behavior could reduce the surface salt crystallization and increase the lifespan. This setup demonstrates how the natural architecture of the luffa vine can be leveraged to enhance water evaporation rates while simultaneously managing salt accumulation.

Figure 5

Luffa vine-based evaporator. (a) The internal porous structure of natural luffa vine with xylem vessels and piths. (b) Schematic showing water and salt exchange during solar water evaporation. Reproduced with permission from Lv et al. [171], Copyright 2023, Elsevier Ltd.

Sunflower stalk (SS) consists of pith, vascular, and dermal tissue, which guarantee storage, transport, and protection of internal tissues, respectively. The effect of storage issue stem pith on evaporation behavior was investigated by Feng et al. [172] In their work, carbonized sunflower stalk (CSS), centerless carbonized sunflower stalk (C-CSS) with partial removal of piths inside the stalk, and hollow carbonized unflower stalk (H-CSS) with full removal of piths were prepared. Attributed to the water storage function of carbonized stem pith, the CSS showed a better water evaporation rate under one solar irradiation, which was 1.82-fold and 1.34-fold higher than those of C-CSS and H-CSS, respectively. In addition, the influence of carbonization temperature and height on the evaporation capability was studied. With a high energy conversion efficiency of 344.69%, the optimized CSS exhibited a water evaporation rate of 11.62 kg m⁻² h⁻¹ at one solar irradiation. The corn straw was able to quickly supply and diffuse water owing to the special structure of vascular bundles and abundant closed tracheid. Therefore, the corn straw has been another choice for evaporator fabrication. For instance, Zhang et al. prepared a double-layered solar vapor generation device via carbonization of the top surface of corn straw [173]. It was found that the resultant bilayered corn straw-based structure was capable of diffusing water, evaporating water, and converting solar energy into thermal energy, holding great promise for solar vapor generation.

Besides plant-derived materials, the utilization of animal-derived materials and their derivatives has attracted substantial interest in the solar-driven water evaporation systems. Natural melanin, as extensively mentioned in Section 2.1, with excellent broadband light absorption ability has been extensively used to improve the photothermal conversion efficiency for the development of solar-steam generators [26]. Notably, natural melanin exists in many animal tissues, especially the ink sacs of some marine animals. Instead of using biomass in its raw state, the carbonization of animal-derived biomass into biochar offers another type of photothermal materials.

As a kind of black liquid, squid ink has the advantage of absorbing broadband light; however, its application is limited due to the lack of freestanding structure. A biomass hydrogel evaporator was prepared by using squid ink as photothermal material and starch as hydrogel skeleton material [141]. Under the light irradiation, a rapid increase in temperature was observed on the hybrid hydrogel compared with the pure hydrogel. Benefitting from the microporous structure of the starch hydrogel and the light absorption ability of the squid ink, the water evaporation rate and energy efficiency of squid ink-starch hydrogel were 2.07 kg m⁻² h⁻¹ and 93.7%, respectively, under one sun irradiation. In addition, its seawater desalination capability was proved, in which a declined salinity of <1 mg L⁻¹ was achieved. Excessive water transport is prone to the loss of unnecessary heat, which reduces conversion efficiency. Li et al. developed a cuttlefish juice-based solar absorber in which hydrophobic SiO₂ NPs were introduced to regulate water transportation [174]. The optimized absorber was able to block excessive water penetration, leading to reduced heat loss. The solar absorber yielded an evaporation efficiency of 85.8% under one sun irradiation, which is higher than the theoretical value (81%).

Animal bones are another type of easily accessible and low-cost biomass material, and their special hierarchical porous structure plays a valuable role in solar-driven evaporation. Carbonized cattle bone, which is cattle bone that has been heated to high temperatures in the absence of oxygen to convert it into a carbon-rich material, has shown great potential as a solar absorber. This process creates a unique porous structure with a high surface area, which enhances its ability to absorb solar energy. As an example, carbonized cattle bone was fabricated after treatment at 600°C, which showed interlinked microchannels and fibrous mesoporous structure [107]. Owing to efficient water transportation, reduced vaporization enthalpy, broadband light absorption, and excellent photothermal performance, the outstanding solar steam generation capability was proved. Furthermore, the ions removal efficiency of the collected freshwater reached 99.99%. The salt fouling, which causes evaporation rate degradation, remains a huge challenge for interfacial solar-vapor generation. Li et al. developed a salt-resistant cuttlebone-based solar evaporator, modified with PDA, reducing graphene oxide and PPy [175]. The superhydrophilic 3D directional channels are conducive to rapid water transport, which allows an adequate water supply, rapid automatic salt exchange, and a decrease in the salt concentration gradient. As a result, after 10 h of exposure to natural sunlight, the modified cuttlebone collected 8.32 kg m⁻² of fresh water, which could meet the needs of several people.

Overall, the biomass and waste-derived materials provide a promising platform for solar-driven water evaporation. Significant achievement has been made in the development of solar evaporators with favorable thermal conversion efficiency and water evaporation rate (Figure 6). Although a variety of eco-friendly and cost-effective materials have been developed, the complicated manufacturing process and poor durability are critical issues for their real-world application. Most of the photothermal conversion materials for solar-driven water systems require complex processing or introduce fictitious materials. Further research is still needed to simplify the treatment process without reduction in the required properties. Furthermore, the practical application of solar-driven water devices is largely overshadowed due to instability. Few bioderived photothermal materials reported the previously mentioned service lifespan. Future studies should pay more attention to the improvement of material stability in the actual harsh environments. The development of sustainable organic waste-derived or bioderived solar-evaporators with low cost, simple preparation process, and long service life will boost their practical applications.

Figure 6

Comparative analysis of evaporation rates and efficiencies for photothermal materials derived from organic waste and bioderived sources. The materials included are as follows: Carbonized durian skin [162], FCPP [163], carbonized pomelo peel [164], coconut husk [136], delignified SCG [165], loofah sponge [166], balsawood [168], luffa vine [171], corn straw [173], squid ink-starch [141], and carbonized cattle bone [107].

4.2 Biomedical applications

Photothermal waste- and bioderived materials hold immense promise in revolutionizing healthcare through their unique ability to convert light energy into heat. Their biocompatibility, sustainability, and photothermal properties make them versatile candidates for a wide range of applications in the biomedical field. These materials offer environmentally friendly alternatives to conventional resources, contributing to the development of greener technologies in healthcare.

A prime example of developing eco-friendly, budget-friendly, and easily synthesized photothermal materials entails utilizing leftover coffee grounds. The photothermal characteristics of this material have demonstrated notable efficacy in eradicating planktonic bacteria and eliminating biofilms [176]. An excellent example of an antimicrobial bioproduct is rice husk biochar-mediated red phosphorus. The designed product generates ˙OH and ˙O₂ free radicals that cause damage to cellular structures, including membranes, proteins, and enzymes. Additionally, bioproduct photothermal properties enhance cell membrane permeability, allowing more free radicals to enter the cell. The combined action of these two factors accelerates cell death, ultimately achieving highly efficient bacteria elimination [177].

The interest in utilizing materials derived from marine organisms or resources in biomedical applications is steadily increasing. Cuttlefish ink natural nanoparticles (CINPs), a subcategory of MNPs, have garnered researchers’ interest due to their ease of extraction by simply washing the ink, nearly zero cost, and avoidance of complex processing as opposed to artificial nanomaterials. CINPs possess remarkable affinity and coupling efficiency for proteins without additional chemical modification and have photothermal conversion capabilities, attributed to the high concentration of eumelanin pigments [33]. Drawing inspiration from the adhesive properties found in mussel proteins, PDA has become a prevalent technique for enhancing the photothermal functionality of material surfaces. Implementation of PDA opens up opportunities for improving medical devices and implants [178,179,180,181]. Leveraging the outstanding ability of PDANP-PEI-rPEG to convert light into heat within living organisms, the combined approach of gene and PTT presents a compelling strategy for treating cancer [178].

In recent decades, a significant focus has been on precisely detecting biological molecules, drugs, and pesticides. Harmful molecules are commonly found in hospital waste, sewage water, and groundwater, posing severe risks to aquatic and human life. A cutting-edge approach to detecting such molecules involves utilizing the highly fluorescent and water-soluble nature of C-dots. Vijeata et al. have used leaves from the traditionally medicinal Ocimum sanctum plant to create stable C-dots capable of swiftly detecting ciprofloxacin [182]. Thakur et al. harnessed recycled carbon from Citrus limetta organic waste for photo-electrocatalytic, sensing, and biomedical applications [183]. Another sensing application was producing adsorptive materials to eliminate hazardous substances commonly found in the environment, such as mercury. Mercury, when absorbed by the body, harms the digestive tract, kidneys, capillaries, and the nervous system. The group led by Liu et al. has developed an effective and rapid removal of Hg(ii) from aqueous solutions by synthesizing a highly efficient adsorbent using activated carbon from corn cob by KOH activation [184].

One of the critical issues in biomedical applications lies in developing an efficient delivery platform capable of facilitating the passage of drugs through the highly selective blood–brain barrier (BBB). This barrier, composed of specialized endothelial cells, restricts the entry of most substances into the brain, presenting a significant hurdle for effectively treating neurological disorders and diseases. PTT, due to its notable advantages, including high efficacy, easy application, and minimal invasiveness, has become an effective strategy to improve BBB penetration via photothermal agents. An interesting solution presented by Zhang et al. involves utilizing NPs extracted from the beard hair of young Asian males, which are primarily composed of keratin and melanin that possess remarkable photothermal conversion capacity (Figure 7a). The results show that the negatively charged surface of extracted NPs attracts positively charged fluoxetine, which enhances BBB permeability under NIR irradiation and exhibits a strong antioxidant effect [185]. This innovative approach shows promise for improving the treatment of depression. Another waste- and bioderived material application is transdermal drug delivery systems (TDDS), which present an appealing alternative for administering drugs and bioactive compounds through the skin. Compared to oral drug delivery, TDDS provide the advantages of bypassing first-pass metabolism, enhancing drug bioavailability, and reducing the likelihood of gastrointestinal disorders. Due to the protective barrier of the stratum corneum, the percutaneous penetration of low molecular weight (<500 Da) and lipophilic drugs through passive diffusion is limited. However, photothermally controlled drug release of naproxen incorporated mainly into mungbean starch-containing MNPs overcomes these challenges [186].

Figure 7

Biomedical, deicing, anti-icing, and environmental applications of photothermal organic waste-derived and bioderived materials. (a) A human beard-derived photothermal drug delivery platform for depression therapy. Reproduced with permission from Zhang et al. [185], Copyright 2020, Elsevier Ltd. (b) Hollow carbon nanospheres derived from biomass by-product okara for imaging-guided photothermal treatment of cancers. Reproduced with permission from Weng et al. [135], Copyright 2019, The Royal Society of Chemistry. (c) Anti-icing/deicing performance of modified biochar coating, representing the freezing and defrosting process of water droplets. Reproduced with permission from Wang et al. [138], Copyright 2022, Elsevier Ltd. (d) Heat transfer diagram of leaves-derived leaf biochar during the solar-driven interfacial evaporation process tested for seawater desalination and sewage treatment. Reproduced with permission from Wang et al. [189], Copyright 2022, Elsevier B.V.

The ongoing challenge that stands prominently before humanity is finding a reliable method to eradicate solid tumors while simultaneously preventing cancer from recurring. Over the years, a diverse range of agents has been developed for PTT, encompassing noble metal nanomaterials, organic reagents, and materials like black phosphorus and carbon-based compounds. Considering biomass’s sustainability, affordability, and abundant availability, there has been a recent surge in innovative approaches to transform diverse biomass sources into valuable functional materials based on carbon. A noteworthy study led by Weng et al. employed dried okara, a residual product of soybean food production (Figure 7b). The results demonstrate that the carbonized okara forms sphere-shaped hollow particles with an average diameter of 200 nm, exhibiting an excellent photothermal conversion efficiency. The distinctive structure of the carbon particles imparts them with superior efficiency in converting light into thermal energy, resulting in a robust photoacoustic response and serving as an imaging-guided agent for PTT in cancer treatment [135]. Another work was presented by Kim et al., who engineered bioinspired Camellia japonica CDs with NIR solid absorbance for effective photothermal cancer therapy [96]. Lung cancer was the challenge of Wang et al., who employed photothermal treatment by utilizing graphene sheets functionalized with plant extract polyphenols [187]. In discussions about PTT, it is crucial to underscore a critical point. This technique can still adversely affect neighboring tissues, highlighting the importance of limiting exposure to healthy areas. One effective strategy to address this concern is using materials with the capacity to attenuate both radiation and generated heat. Aerogels hold promise for this application due to their insulating properties. The study conducted by Ferreira-Gonçalves et al. successfully engineered silica (SiO₂) and pectin-based aerogels to serve as optical and thermal insulators, acting as constraints in PTT applications. In detail, the fabrication of organic and inorganic aerogels involved combining pectin and SiO₂, with cotton fibers added for reinforcement. The aerogels were formed through ethanol- and thermal-induced gelation, followed by supercritical drying for the organic aerogels and a two-step catalyzed sol–gel process with subsequent oven drying for the inorganic ones. The safety of both types of aerogels confirms their suitability as controllers of light and heat and as insulators in the context of PTT systems [188].

Photothermal materials from waste and biological sources show promise in healthcare but face some challenges. Ensuring material consistency and purity is vital for reliable photothermal properties. Scaling production to meet demand must be cost-effective and efficient. Biocompatibility and toxicity testing are essential due to potential new toxic or immunogenic compounds. Targeting diseased tissue without harming healthy tissue is a key challenge, especially in cancer therapy. Controlling heat distribution to prevent damage is difficult. Regulatory approval necessitates rigorous testing, which is time-consuming and expensive. Material stability over time, particularly in biological environments, is a concern. Integration with current medical technologies may require further development. The cost-effectiveness of the overall production process is crucial for competitiveness. Environmental considerations are important in the extraction and processing of these materials. Interdisciplinary research is needed to overcome these challenges and safely apply these materials in healthcare.

4.3 Deicing and anti-icing solutions

Icing is a commonly observed natural phenomenon that can lead to significant challenges and risks in various sectors, including powerlines, buildings, highways, railways, aircraft, and wind turbines [190]. The accumulation of ice over large areas, whether in the form of sizeable masses or minute crystals like freezing rain, damaging frosts, and heavy snow, presents significant challenges to infrastructure, transportation, and crops, leading to substantial economic losses and posing threats to human life [191,192]. Conventional methods for ice removal, encompassing mechanical, electric heating, or chemicals to eliminate ice accretion persist as widely used practical solutions [193,194]. Unfortunately, these approaches often come with drawbacks such as high energy consumption and difficulties in designing environmentally and economically friendly equipment and systems or contributing to environmental pollution [195]. Hence, there is a pressing need for cost-effective alternative materials and simpler fabrication methods. In response to these challenges, a variety of innovative techniques have emerged to mitigate ice growth and accumulation [196]. In recent times, there has been growing interest in passive anti-icing and deicing approaches driven by superhydrophobic coatings with a photothermal effect. Unlike traditional methods, this approach efficiently converts absorbed light into heat, rapidly accelerating the melting of ice. The superhydrophobic nature of the coating facilitates the quick-rolling off of melted droplets, leaving the surface dry [197]. These coatings encompass superhydrophobic coatings/surfaces, ice phobic materials with low-interfacial toughness, slippery liquid-infused porous surfaces, phase transformation materials, and photothermal trap structures [198]. Furthermore, photothermal materials can keep the surface temperature above freezing, avoiding the production and deposition of ice. Continuous surface warming is especially advantageous in minimizing the frequency and intensity of deicing treatments [199,200]. Among them, the use of organic waste-derived and bioderived materials has the intrinsic advantage of being environmentally friendly. For instance, researchers recently introduced an innovative solution – a scalable flexible bilayer bamboo (BLB) for deicing purposes [194,201]. Crafted through a process involving homogeneous carbonization and hydrogel coating of bamboo veneer, this biomaterial possesses necessary attributes essential for efficient anti-icing surfaces. These qualities include strong photothermal trapping capability, an ion reservoir effect, and exceptionally low ice adhesion strength. By leveraging established manufacturing techniques alongside the abundant concave parenchymal cell lumina microstructure intrinsic in bamboo veneer, this method integrates a dual-action mechanism and can combat ice formation through both photothermal heat generation and nucleation-inhibition processes [202,203]. In other studies, the BLB has been highlighted for not only demonstrating its effectiveness in removing ice blocks, frozen condensate drops, and large amounts of snow but also for establishing itself as a versatile and long-lasting anti-icing solution suitable for various surfaces, including challenging curved ones commonly found on buildings and communal facilities. Additionally, the use of bamboo veneer shows a commitment to sustainability, as bamboo is a rapidly renewable resource that aligns with modern eco-conscious sensibilities. With its benefits and eco-friendly profile, the BLB represents an advancement in anti-icing technology, a development supported by the findings of previous studies in the field [204].

Another example is biochar, which is derived from organic waste and bioderived materials and has emerged as a versatile player [85,205]. Derived from the pyrolysis of straw, rice husk, sawdust, and wood, its eco-friendly traits and remarkable photothermal characteristics have positioned it as a candidate for applications in water purification and soil enhancement [138]. Despite its intrinsic potential, the practical application of biochar has been hampered by challenges associated with its intricate composition, encompassing diverse functional groups (−OH, –NH₂, –COOH) and minerals (N, P, S, Si, Ca, Mg, and K). This complexity renders biochar intrinsically hydrophilic or weakly hydrophobic. In a quest to unlock the full potential of biochar, various methods, including acid modification, metal oxide modification, and metal salt modification, have been explored to enhance its hydrophobicity. Additionally, many reported hydrophobic biochars are self-synthesized through intricate and time-consuming processes, showing the need for commercially low-cost biochar to facilitate its widespread industrial application [201]. As an example, researchers presented a straightforward method to craft a superhydrophobic photothermal anti-icing/deicing coating utilizing readily available biochar and economically viable titanium nitride (TiN) NPs [138]. The synergistic integration of TiN NPs and biochar yields a coating that remarkably enhances water-repellency and photothermal conversion performance. Notably, when the TiN content surpasses 20 wt%, the coating achieves a superhydrophobic state, boasting an impressive contact angle of up to 156° and a roll-off angle of less than 5°. This achievement highlights the pivotal role played by the micro-nano structure combined with low surface energy in conferring superhydrophobicity upon the coating. Further elevating the performance of the coating, a thermal insulating layer is inserted between the superhydrophobic surface and substrate. In practical terms, with a temperature increase of up to 63.3°C under one sun’s illumination in subzero conditions, the coating effectively melts, covering frost and ice without leaving water droplets on the surface (Figure 7c). Mechanical durability is substantiated through water droplet impact, sand abrasion, and bending tests. Notably, the scalability of the coating is demonstrated through the fabrication of a 900 cm² superhydrophobic photothermal coating. Therefore, photothermal coatings hold great promise for multifunctional applications, leveraging solar energy efficiently. However, a current challenge lies in identifying a readily prepared material with high photothermal performance. Drawing inspiration from mussel adhesion and lotus leaf surfaces, researchers have developed superhydrophobic photothermal coatings featuring a hierarchical structure. This is achieved through the sequential deposition of melanin-like PDA and dip-coating PDMS/hydrophobic fumed SiO₂. Additionally, the coatings offer UV protection, ensuring prolonged functionality under extended outdoor sunlight exposure [206].

4.4 Environmental remediation and beyond

Environmental remediation, the process of mitigating the impact of pollutants on ecosystems, has become a critical area of scientific research and innovation. The exploration of sustainable materials for this purpose has led to a growing field focusing on the applications of photothermal organic waste-derived and bioderived materials. This section explores the diverse applications of these materials in environmental remediation, highlighting their potential to address pollution challenges and contribute to a more sustainable future. In general, photothermal materials serve an important role in environmental remediation because of their ability to transform solar energy into heat, accelerating the destruction of contaminants in water, soil, and air via photodegradation processes [207,208]. Moreover, photothermal materials enhance photocatalytic reactions, by raising reaction rates. In disinfection, the heat produced by these materials can inactivate pathogenic germs in water by breaking cellular structures [209]. They aid in oil–water separation by lowering oil viscosity during heating and drive membrane distillation by providing vapor pressure gradients for water purification [210]. Heat enhances adsorption capacity by increasing the contaminant diffusion rates and widening adsorbent pores. Thermo-catalytic processes use localized heat to transform pollutants, such as turning CO₂ into fuels or breaking down complex organic molecules. This combines increased temperatures and catalytic activity for effective remediation [211].

Therefore, those organic waste-derived or bioderived materials that are intrinsically photothermal or acquired this property are promising candidates for environmental applications [212]. These materials, produced from renewable sources such as cellulose, lignin, chitosan, and agricultural by-products such as corn starch, excel in a variety of pollutant removal processes. Bioderived materials play an important part in sustainable pollutant removal solutions by utilizing physical adsorption, chemical binding, and biological processes, providing both efficiency and environmental compatibility [213]. Cellulose-based materials have a hierarchical porous structure rich in hydroxyl groups (–OH), allowing contaminants to bind to large surface areas through physical interactions. Modified cellulose nanofibers containing functional groups such as carboxyl (–COOH) improve their ability to selectively adsorb heavy metals and pollutants in water, greatly increasing removal efficiency [214,215]. Lignin, a complex aromatic polymer found in plant tissues, has strong adsorption properties for organic contaminants and colors due to its polyphenolic composition. Lignin NPs functionalized with amine groups (–NH₂) show greater effectiveness in removing dyes such as methylene blue, boosting environmental remediation efforts. They create strong π–π interactions and hydrogen bonds with aromatic contaminants [216,217]. Chitosan amino groups (–NH₂) allow the efficient chelation and binding of heavy metal ions via coordination chemistry. Chitosan beads and membranes are particularly successful for pollutant removal from aqueous solutions due to their positively charged surface, which allows for electrostatic interactions with negatively charged contaminants including anionic dyes and phosphates [218]. Modified corn starch compounds have amphiphilic characteristics, allowing them to produce biodegradable films that can capture oil and grease from water surfaces. These materials adsorb non-polar contaminants through hydrophobic interactions and provide stability and adhesion through hydrophilic groups, making them suited for use in oil spill cleaning and water remediation [219]. Moreover, algae and microorganisms provide biological channels for pollutant removal in aquatic environments and polluted soil. Algae bioaccumulate toxins by active transport systems, concentrating them inside their biomass. Meanwhile, microorganisms such as bacteria and fungi breakdown complex organic contaminants through enzymatic processes, transforming harmful substances into simpler, less toxic forms and so contributing to long-term environmental cleanup efforts [220]. Another example is carbon-based bioderived materials that exhibit remarkable light absorption across a broad spectrum, possess a highly adaptable structure, and are cost-effective with abundant resources. Recently, they have become a focal point in solar energy research. Enhancing the photothermal conversion efficiency of carbon-based materials involves compounding carbon-based materials with other substances to form heterojunctions and altering surface functional groups. Adjusting the microstructure of carbon-based materials holds promise for advancing solar steam generation efficiency, though further research is needed. Wang et al. proposed the adjustment of sp² hybrid carbon content in leaf biochar derived from discarded leaves. Biochar demonstrates potential applications in seawater desalination and sewage treatment, effectively removing ions and pollutants from industrial wastewater (Figure 7d). The practical utility of biochar is evaluated by observing its evaporation rate under natural light conditions [189].

The use of organic wastes like compost, animal manure, and agricultural residues for soil remediation has also gained traction owing to their capacity to enhance soil structure, nutrient levels, and microbial activity. These materials play a pivotal role in promoting soil health, reducing erosion, and sequestering carbon dioxide, thereby aiding in climate change mitigation. Moreover, organic wastes serve as a rich source of essential nutrients for plant growth and microbial activity. Composts, for instance, are teeming with nitrogen, phosphorus, potassium, and various micronutrients crucial for plant health. When applied to the soil, these nutrients become readily available to plants, promoting robust growth and development [221]. Additionally, organic amendments stimulate microbial populations in the soil, fostering a dynamic ecosystem of beneficial microorganisms. These microbes play vital roles in nutrient cycling, decomposition of organic matter, and suppression of soil-borne pathogens, thereby contributing to overall soil health and productivity [222]. In a study conducted by Medyńska-Juraszek and Ćwieląg-Piasecka, the potential of wheat straw biochar for cobalt sorption in contaminated soil was investigated [223]. Their study revealed that the investigated material exhibited high sorption efficiency, effectively reducing the mobility and availability of CO 2 + ions in soil. This mitigation of cobalt mobility is crucial for minimizing health risks associated with human exposure to contaminated soil environments. The primary sorption mechanisms were attributed to interactions between cobalt ions and functional groups such as carboxylic and hydroxyl groups present on the biochar surface. However, it was noted that the immobilization of cobalt is a multifaceted process, influenced by factors such as biochar oxidation and interactions with soil constituents [224,225].