Sulfonated Polyethylene for Solar‐Driven Atmospheric Water Harvesting

ABSTRACT

Atmospheric water harvesting (AWH) is a promising approach to mitigate water scarcity, yet the practical deployment of many existing sorbents is hindered by cost, complex synthesis, and safety risks associated with additive leaching. Moreover, sorbent regeneration for water release often requires heating, which adds energy demand and operational expense. Here, we demonstrate sulfonated polyethylene (PE) as a low‐cost and scalable sorbent for AWH. Using a straightforward sulfonation process, hydrophilic sulfonic acid groups are introduced onto the PE backbone, transforming a hydrophobic commodity polyolefin into a hygroscopic material. Sulfonated PE shows strong water vapor sorption performance across a broad humidity range, reaching an uptake of up to 0.58 g g^{− 1} at 90% relative humidity (RH). Dynamic vapor sorption measurements further confirm excellent durability, with stable performance over 60 sorption cycles. In addition, without integrating solar absorbing fillers, the intrinsic photothermal feature of sulfonated PE enables an efficient solar‐driven water release without external energy input. This work presents a sustainable route to upcycle widely available polyolefin plastics effective sorbents for atmospheric water harvesting, supporting practical and energy‐efficient freshwater production.

Sulfonation of polyethylene (PE) is demonstrated as a simple and scalable strategy for atmospheric water harvesting, converting a hydrophobic polyolefin into a hygroscopic sorbent. Hydrophilic sulfonic acid groups enable high water vapor uptake across a wide humidity range, while intrinsic photothermal properties allow solar‐driven water release without external energy. Dynamic vapor sorption tests confirm excellent durability over multiple cycles, and field experiments verify fully reversible nighttime sorption and daytime desorption. This work provides a route to upcycle commodity plastics into effective, low‐cost sorbents for practical freshwater production.

1. Introduction

More than two billion people face severe water scarcity driven by uneven water distribution, overextraction of groundwater, inadequate infrastructure, and rising demand from agriculture and industry [1, 2]. This burden is expected to intensify as global populations grow, temperatures rise, environmental pollution further degrades water quality, and extreme weather events become more frequent [3]. While conventional approaches to secure freshwater, including desalination and groundwater pumping, can be effective, they largely depend on the availability of local water sources and often require substantial infrastructure and energy input [4, 5]. As these pressures strain traditional supplies, there is a strong demand for developing resilient, decentralized technologies that can produce water where and when it is needed [6]. Atmospheric water harvesting (AWH) offers an alternative solution to addressing water scarcity by extracting water directly from ambient air, enabling on‐demand water production [7, 8]. Conventional AWH approaches include fog collection and dew harvesting. Fog collection captures airborne microdroplets on meshes and requires persistently foggy, high‐humidity conditions [9, 10]. Dew harvesting condenses water vapor on surfaces cooled below the dew point, but it is typically energy‐intensive, particularly in hot and arid environments [11]. As a result, both methods are constrained by climate and geography, limiting their practicality for reliable, large‐scale water production in many regions.

In recent years, sorbent‐based AWH systems have attracted growing interest because they can spontaneously capture water vapor from air, enabling water collection without active cooling [12]. In these systems, water–sorbent interactions concentrate moisture and allow subsequent release and condensation under mild conditions. A range of hygroscopic materials, including metal–organic frameworks (MOFs), zeolites, and salt‐infused hydrogels, have been explored [13, 14, 15, 16, 17, 18]. For example, MOF‐based sorbents are promising due to fast sorption kinetics and strong performance in dry climates, with demonstrated operation at relative humidities as low as ∼14% [19]. Composite systems combining MOFs with hygroscopic salts could reach a water uptake of ∼0.77 g g^{− 1} at 30% RH [20]. Despite these advantages, several challenges still hinder the large‐scale deployment of current sorbents. Many high‐performing materials require complex, multi‐step syntheses and remain costly, while others can suffer from hydrolytic instability or gradual degradation under prolonged moisture exposure, leading to reduced uptake over time [21, 22, 23]. In addition, salt‐based and salt‐infused sorbents could raise safety concerns, where the deliquesced brines can leak from the sorbent matrix, and volatile carryover during desorption/condensation can introduce elevated salt concentrations in the collected water [24, 25].

Moreover, state‐of‐the‐art sorbent‐based AWH increasingly relies on solar‐driven regeneration, in which sunlight supplies the low‐grade heat required to desorb water without grid electricity [26, 27]. This approach is attractive because it reduces operation cost and carbon footprint while enabling portable water production [28, 29]. In particular, many studies show that integrating photothermal components into sorbent designs can accelerate desorption, minimize heat losses, and improve overall energy efficiency. For example, Schweng et al. developed brown/black sulfonated hypercrosslinked polymers (SHCPs) with strong photothermal conversion. Under simulated sunlight, the surface temperature of the SHCPs increased to approximately 90°C, enabling complete water desorption within one hour [30]. Liu et al. reported a poly(vinyl alcohol) (PVA) based sorbent system incorporating black ink as a photothermal absorber, which achieved efficient water desorption under natural sunlight [31]. Importantly. Recent studies have emphasized that AWH sorbents should be evaluated against several technical performance targets rather than equilibrium water uptake alone. These include working capacity under a defined humidity swing (reported in g g^{− 1}), sorption/desorption time required to reach practical uptake and release, regeneration temperature or energy input (°C or kJ L^{− 1}), cycling stability over repeated operation, and device‐level water productivity (e.g., L kg^{− 1} day^{− 1} or L m^{− 2} day^{− 1}). More broadly, recent perspectives also stress that the key bottlenecks for practical AWH are low water productivity and high energy consumption, meaning that an effective system must maximize water output while minimizing regeneration energy under realistic ambient conditions [32, 33].

In this study, we demonstrate sulfonated polyethylene (PE) as a scalable sorbent for solar‐driven AWH. PE is the most widely produced plastic globally, with annual production exceeding 100 million tons, making it inexpensive and broadly available while also underscoring the urgent need for value‐added upcycling routes [34, 35]. A simple sulfonation step introduces sulfonic acid groups and crosslinking, converting hydrophobic PE into a hydrophilic sorbent with strong AWH performance. We evaluate its water‐vapor sorption behaviors and uptake kinetics across a wide range of relative humidities and assess durability through repeated sorption cycling. Moreover, we conduct outdoor tests under natural conditions and demonstrate a custom‐built AWH device that collects water from ambient air with drinking‐water benchmarks, highlighting sulfonated PE as a practical upcycling strategy and a viable pathway toward safe, decentralized water harvesting.

2. Results and Discussion

We synthesized sulfonated PE using sulfonic acid as the sulfonation agent, as shown in Figure 1a, sulfonic acid reacts with PE at an elevated temperature (150°C in this work) to chemically attach sulfonic acid groups onto the polymer backbone [36, 37]. Under these conditions, sulfonation of PE in sulfuric acid proceeds via hydrogen abstraction from the polymer chain, followed by incorporation of sulfonic acid groups, and then the double bonds are introduced through an acid‐induced dehydrogenation/elimination pathway during sulfonation. (Figure S1). Subsequently, radical–radical coupling between neighboring chains may then occur, forming a thermally stable network as the outcome of sulfonation [38]. Notably, the PE sulfonation reaction is diffusion‐controlled; therefore, we used fine powder of PE (with an average particle size of 100–150 mm, Figure S2) to shorten diffusion pathways and promote more uniform functionalization throughout the material.

FIGURE 1.

(a) Sulfonation reaction scheme for PE. (b) FTIR spectra of sulfonated PE with different reaction times. (c) DSC result and crystallinity degree of sulfonated PE with different reaction times.

The FTIR spectra shown in Figure 1b demonstrate the changes in the chemical composition of PE as a result of the sulfonation reaction. Within the first 4 h of the sulfonation, the intensity of the alkyl stretching vibrations at 2850 and 2920 cm⁻¹ gradually decreases with reaction time and then reaches a plateau. The introduction of sulfonic acid groups onto the polymer backbone is confirmed by the emergence of a characteristic stretching vibration at 1125 cm⁻¹. In addition, the intensity of the band at 1650 cm⁻¹ increases with reaction time, corresponding to C═C stretching vibrations in sulfonated PE. This feature is attributed to the elimination of sulfonic acid groups and the formation of unsaturated double bonds, which provide the reactive sites for subsequent crosslinking reactions [39]. The FTIR spectra of sulfonated PE remain nearly constant after 12 h at 150°C in concentrated sulfuric acid, indicating the reaction reaches completion.

We further examined the sulfonation kinetics of PE by tracking changes in crystallinity as a function of reaction time. The progressive loss of crystallinity is attributed to disruption of PE chain packing caused by the introduction of bulky sulfonic acid groups, which restrict chain mobility and hinder crystallite formation. The pristine PE initially exhibits a crystallinity of ∼31%, which decreases rapidly with reaction time (Figure 1c), reaching 22% and 3% after 1 and 2 h, respectively. Notably, the melting temperature changes only slightly at early times, consistent with a diffusion‐controlled process in which a substantial fraction of unmodified, crystalline PE remains in the sample. With extended reaction (e.g., 4 h and above), the material becomes completely amorphous, as confirmed by the DSC results.

Mohr's titration was used to quantify the degree of sulfonation of the functionalized PE as a function of reaction time. As shown in Figure 2a, the sulfonic acid content rises rapidly at early times, reaching 1.4, 1.7, and 2.3 mmol g^{− 1} after 1, 2, and 4 h, respectively. With further reaction, the measured ─SO₃H content gradually decreases and then plateaus at ∼2.0 mmol g^{− 1}. This behavior indicates that sulfonation is initially fast, whereas prolonged heating promotes secondary processes that reduce the number of sulfonic acid groups retained on the polymer. In particular, a portion of the installed ─SO₃H groups can undergo elimination, generating unsaturated sites along the backbone. These unsaturated sites may participate in addition elimination steps that form radical intermediates, and subsequent radical coupling between chains can lead to intermolecular crosslinking. Additionally, XPS results can confirm the introduction of sulfonic acid groups on the PE surface upon functionalization. As shown in Figure 2b, the sulfur content (as evidenced by the S2p peak at ∼167 eV) increases rapidly during the early stage of reaction, reaching a maximum content of ∼0.9 at.% after 4 h, consistent with the results obtained from titration. With longer reaction times, the sulfur signal decreases slightly and then stabilizes at ∼0.8 at.%, suggesting that extended heating promotes secondary reactions (e.g., partial elimination/rearrangement) that could reduce the fraction of surface ─SO₃H groups. Notably, once the polymer network becomes fully developed, the surface sulfur content remains essentially constant. For O1s, a gradual increase in the signal is observed with increasing sulfonation time (from 3.2 at.% to 8.4 at.%), consistent with the introduction of both ─SO₃ and ─OH groups during the functionalization reaction. Moreover, high‐resolution XPS spectra of C 1s, O 1s, and S 2p for each sample are provided in Figure 2c and Figure S3. It was found that the C 1s spectra can be resolved into two sub‐peaks, which are centered at ∼284 and ∼285 eV, corresponding to C─C and C─S bonds, respectively. The C─C peak remains dominant with increasing sulfonation time, and the intensity ratio of the C─C to C─S component at ∼285 eV is approximately 10. The O 1s spectra can be deconvoluted into three subpeaks corresponding to S─O (∼531 eV), S─O (∼532.5 eV), and OH‐related species (∼533.5 eV), with the overall O 1s intensity increasing as the sulfonation time increases. Notably, the intensity of the OH–related subpeak increases significantly with sulfonation time (the relative area of the subpeak increases from ∼7.5 at% to ∼13.4 at%). The S 2p spectra can be resolved into a spin–orbit doublet with components centered at ∼167.5 eV (S 2p 3/2) and ∼168.5 eV (S 2p 1/2), which are both characteristic of sulfonic acid groups (Figure 2c).

FIGURE 2.

(a) Degree of sulfonation of functionalized PE as a function of reaction time. (b) XPS survey scans of sulfonated PE after different reaction times. (c) High‐resolution XPS scans of S2p for sulfonated PE with different sulfonation times.

Figure 3a shows the water uptake kinetics of the 24 h sulfonated PE measured at 25°C under controlled RH. At low humidity (13% and 35% RH), the mass gain from water sorption plateaus within ∼4 h. At higher humidity (≥57% RH), the uptake continues to increase for a much longer time (up to ∼20 h) before reaching equilibrium. This extended equilibration likely reflects the larger water loading at high RH, where continued sorption is accompanied by slower transport of water into the interior of the polymer network and the progressive formation of more strongly associated water domains (e.g., hydrogen‐bonded clusters) around the sulfonic acid groups. The corresponding equilibrium capacities increase systematically with humidity: 0.10 g g^{− 1} at 13% RH, 0.21 g g^{− 1} at 35% RH, 0.39 g g^{− 1} at 57% RH, 0.45 g g^{− 1} at 78% RH, and 0.68 g g^{− 1} at 100% RH. The sharp increase between 35% and 57% RH suggests a transition from primarily site‐limited sorption at low RH to more extensive water association and sulfonated PE hydration at intermediate‐to‐high RH.

FIGURE 3.

(a) Water‐uptake kinetics of the 24 h sulfonated PE measured at 25°C under different relative humidities. (b) Dynamic vapor sorption (DVS) sorption–desorption cycles of the 24 h sulfonated PE at 25°C. (c) Equilibrium water uptake of sulfonated PE samples prepared with different sulfonation times was measured at various relative humidities.

Figure 3b further supports this behavior by showing the sorption–desorption isotherms of the 24 h sulfonated PE at 25°C measured by dynamic vapor sorption (DVS), which capture the humidity‐dependent uptake and the reversibility of water release. At low to intermediate RH (≤50% RH), the water uptake increases gradually from 0 to 0.21 g g^{− 1}. A significant increase in water uptake is observed at higher humidities (>70% RH), rising from 0.21 to 0.58 g g^{− 1}, which is consistent with the results of the water uptake kinetics measurements. DVS measurements further confirm that water sorption–desorption in sulfonated PE is largely reversible at room temperature, and that the equilibrium water content is strongly governed by the ambient humidity. The water sorption isotherm shows a sigmoidal profile characteristic of IUPAC Type V behavior. The small sorption–desorption hysteresis likely originates from swelling of hydrophilic sulfonated PE and the delayed release of clustered or weakly confined water in hydrophilic domains, and its effect on practical water‐harvesting performance is rather limited. Interestingly, the sample with the highest sulfur content does not deliver the highest water uptake. As shown in Figure 3c, the 4 h sulfonated PE, despite exhibiting the maximum sulfur content, uptakes slightly less water than the 24 h sulfonated material, which achieves the greatest capacity (0.65 g g^{− 1} at 100% RH after 24 h of static sorption). Under 100% RH, the small additional mass gain at extended times is mainly attributed to the gradual accumulation of surface‐adsorbed water. As this contribution is minimal (<3% of the total uptake), it does not meaningfully affect the practical working capacity of the sorbent. This behavior indicates that water uptake is governed not only by the density of ─SO₃H groups, but also by the sorbent microstructure that develops with extended sulfonation reaction time. For these materials, two populations of sorbed water can exist, including strongly bounded water and weakly bounded or free water. Particularly, the first type of water molecules in the sorbent are strongly associated with sulfonic acid groups via hydrogen bonding and ion–dipole interactions, so its uptake is governed mainly by the concentration and accessibility of hydrophilic sites. Weakly bounded and free water, by contrast, can accumulate within capillaries and microvoids formed as the polymer network evolves, and its contribution depends on the available void volume and transport pathways [40, 41]. At low RH, sorption is dominated by bound water; therefore, samples with higher ─SO₃H content generally capture more moisture from air. At higher RH, free water becomes increasingly important as water penetrates and fills the network mesh space. The 24 h sulfonated PE appears to develop a more favorable internal architecture for this process, resulting in higher overall uptake despite having slightly lower ─SO₃H group content than the 4 h sample. DSC was performed on samples after completing moisture water uptake under different times and humidity conditions to further distinguish the different types of water present in the material. As shown in Figure S4a, the amount of water that can be frozen within the PE samples increased with their sulfonation time. The mass fractions of freezable water for the samples sulfonated for 2, 4, 12, and 24 h were 0.01, 0.09, 0.12, and 0.15 g g^{− 1}, respectively. Notably, the freezable water accounted for ∼20% of the total water content. Moreover, under lower RH conditions, a significant amount of water remains unfrozen in the samples even at −50°C, consistent with strong interactions between water and sulfonic acid groups that hinder complete water desorption. Similar behavior has been reported in various hydrogel systems, where nanoconfinement and hydrophilic groups promote strong ice inhibition [42, 43, 44]. As shown in Figure S4b, in the temperature range of −50°C to 30°C, no ice formation associated with sorbed water was detected for the 24 h sulfonated PE after equilibration at 78% RH or lower RH for 24 h, despite a total water uptake as high as 0.45 g g^{− 1}. Figure S4c shows the water uptake of sulfonated PE at 100% RH as a function of reaction time, along with the corresponding contributions of strongly bounded water and free/weakly bounded water. The amount of strongly bounded water increases markedly in the samples after the first 4 h of sulfonation treatment (from 0 to 0.43 g g^{− 1}). In contrast, the free or weakly bounded water content continues to increase steadily with prolonged reaction time.

To further evaluate the feasibility of sulfonated PE as a material for AWH, high‐density polyethylene (HDPE) and commercial PE foam were also investigated as precursors. As shown in Figure S5a, the degree of crystallinity of HDPE decreases progressively with reaction time, dropping from 83.6% to nearly 0% within the first 12 h. Compared to low molecular weight PE, which was used in this study as the model system, HDPE exhibits a higher initial crystallinity, which slows the diffusion of sulfonic acid and delays the initiation of the sulfonation reaction. In addition, Figure S5b shows that the degree of sulfonation increases rapidly during the first 12 h and reaches an equilibrium value of ∼2.3 mmol g^{− 1}. Although this value is comparable to that of fully sulfonated low‐molecular‐weight PE, a longer reaction time is required to reach equilibrium in HDPE. Figure S5c presents the water uptake kinetics of 24 h sulfonated HDPE at different relative humidities. Overall, both the equilibrium water uptake and sorption kinetics are comparable to those of low‐molecular‐weight PE, with equilibrium uptakes of 0.09, 0.36, 0.40, 0.45, and 0.66 g g^{− 1} at 13%, 35%, 57%, 78%, and 100% RH, respectively.

In addition, sulfonated PE foam demonstrates significantly enhanced water uptake performance due to the presence of macropores. As illustrated in Figure S6a, neat PE foam contains closely pores with diameters of ∼200 µm. After sulfonation, the foam retains its porous structure, while the pore size increases to ∼400 µm (reaction‐induced pore expansion) [45], as confirmed by optical microscopy. The presence of this porous architecture significantly improves both the water uptake rate and the equilibrium capacity, likely due to enhanced water diffusion pathways and capillary condensation within the interconnected pores. As shown in Figure S6b, water uptake reaches a plateau within ∼4 h at RH below 100%, whereas at 100% RH, the uptake continues to increase for up to 24 h before reaching equilibrium. Notably, the equilibrium water uptake of sulfonated PE foam is significantly higher than that of non‐porous low‐molecular‐weight PE and HDPE, reaching 0.14, 0.36, 0.66, 1.16, and 2.53 g g^{− 1} at 13%, 35%, 57%, 78%, and 100% RH, respectively. These results indicate the promise of pore engineering as an effective strategy to enhance sorption kinetics and overall water uptake performance. By introducing tailored porosity into the polymer precursors, mass transport can be significantly improved, enabling faster sorption and more efficient utilization of the sorbent in practical AWH.

To assess long‐term cycling stability, the 24 h sulfonated PE was subjected to 60 consecutive sorption–desorption cycles under controlled conditions. As shown in Figure 4, sorption was carried out at 25°C and 75% RH for 120 min to allow the material to approach, rather than fully reach, its equilibrium uptake at moderate humidity. We note that full equilibrium requires substantially longer times (>20 h, Figure 3); however, a 120 min sorption period was selected here as a more practically relevant timescale for cyclic AWH operation. Under these conditions, the sorbent already achieves substantial water uptake, enabling meaningful evaluation of cycling stability without requiring complete equilibration in each cycle. Desorption/regeneration was then performed at 90°C and 7.5% RH for 30 min. This temperature was selected as a fixed reference condition to ensure reproducible water release during repeated cycling and to enable consistent evaluation of sorbent stability, rather than to represent an optimized regeneration protocol. We note that lowering the regeneration temperature is an important goal for practical AWH applications, and optimization of desorption conditions will be the subject of future study [46]. Under these regeneration conditions, the release of water was rapid: more than 99.9% of the adsorbed water was removed within the first 20 min, indicating near‐complete drying and effective restoration of sorption sites for the subsequent cycle. The resulting working capacity (difference between sorption and desorption states) was ∼0.32 g g^{− 1} under this swing in humidity and temperature. Based on the results showing an approximately 2 h sorption–desorption cycle time and a cycling working capacity above 0.30 g g^{− 1}, the projected sorbent‐normalized water productivity is approximately 3.6 L kg^{− 1} day^{− 1}, assuming 12 cycles per day and complete water recovery. We note that this estimate is simplified, as device‐level water productivity is more challenging to determine due to additional factors such as heat and mass transfer limitations, incomplete water recovery, and system‐level thermal and energy losses. Therefore, this calculation is intended only to provide a material‐level estimate of the potential water productivity of sulfonated PE. After 60 cycles, the working capacity decreased by less than 2%, indicating that the material retains largely stable and reversible sorption–desorption behavior over repeated operation, albeit with a slight downward trend. These results support good short‐term cycling stability and suggest that the sorbent remains structurally robust under the tested regeneration conditions, while longer‐term durability for practical implementation over substantially more cycles warrants further investigation.

FIGURE 4.

Long‐term stability test of 24 h sample over 60 sorption–desorption cycles.

Building on the high‐water uptake content and excellent cycling stability, we next evaluated whether sulfonated PE could support solar‐driven regeneration via its intrinsic photothermal response. Figure 5a–e show infrared thermal images of the 24 h–sulfonated PE under different irradiation intensities, where the reported temperature corresponds to the sample surface during illumination. The thermal images indicate relatively uniform heating across the exposed surface upon sunlight exposure, suggesting efficient and homogeneous light absorption. As the simulated solar intensity increases from 200 to 320, and 630 W m^{− 2}, the equilibrium surface temperature rises from 51.0°C to 68.2°C, and 91.3°C, respectively, demonstrating effective light‐to‐heat conversion. Although solar intensity depends on geographical location, the global average solar input to the Earth is approximately 340 W m^{− 2} [47]. Consequently, solar intensities in most regions worldwide at noon typically are over 600 W m^{− 2}. It is important to note that the photothermal behavior of sulfonated PE arises from sulfonation‐induced chemical composition changes rather than carbonization, because the concentrated H₂SO₄ treatment at 150°C does not produce carbonized domains under these conditions. Compared with other photothermal polymer networks and moisture‐sorbing materials, sulfonated PE exhibits excellent intrinsic photothermal performance, which is mainly attributed to the formation of C─C─containing conjugated structures that broaden light absorption and give the material its black appearance. For example, in a core–shell system consisting of alginate/polyaniline networks coated with sulfonated poly(N‐isopropylacrylamide), solar irradiation at 600, 800, and 1200 W m^{− 2} produces equilibrium surface temperatures of approximately 40°C, 60°C, and 70°C, respectively [48]. Similarly, PVA loaded with black ink reached a peak temperature of 78.6°C under a solar flux of 471.5 W m^{− 2} [31]. Figure 5f summarizes the temperature evolution at each solar intensity (ambient temperature maintained at 25°C): upon illumination, the sample heats rapidly within the first few minutes and then gradually approaches a steady‐state plateau, with higher intensities yielding faster heating rates and higher equilibrium temperatures. In contrast, no appreciable temperature change is observed under dark conditions, confirming that the temperature rise originates from photothermal conversion rather than environmental heating.

FIGURE 5.

(a–e) The infrared images of surface temperature for the 24 h sulfonated PE sample under different light intensities. (f) Temperature changes as a function of time for 24 h sulfonated sample under different light intensities. (g) Normalized water uptake as a function of time for 24 h sulfonated sample under different light intensities.

We further linked the photothermal temperature rise to water release behaviors of sulfonated PE. Figure 5g shows the normalized water content as a function of illumination time under different irradiation intensities. Prior to irradiation, the 24 h sulfonated PE was equilibrated at 78% RH and 25°C in the dark for 24 h to ensure a completely saturated uptake. In the absence of light, the water content remains essentially unchanged over 30 min, indicating negligible desorption under the same ambient conditions. Upon illumination, however, water desorption accelerates dramatically, and the release rate increases monotonically with light intensity. At irradiation intensities ≥480 W m^{− 2}, more than 90% of the stored water is released within 10 min, whereas lower intensities (<480 W m^{− 2}) lead to slower kinetics and incomplete desorption over the same time window. This strong intensity dependence is consistent with two contributions to sorbed water. Weakly bounded and free water, residing in polymer meshes and capillaries, is released readily once the sample warms modestly. Strongly bounded water, associated more strongly with sulfonic acid sites through hydrogen bonding and ion–dipole interactions, may require higher temperatures (and/or longer illumination times) to overcome stronger interactions and diffuse out of the polymer matrix. These results demonstrate that the photothermally induced heating of sulfonated PE can efficiently drive rapid, solar‐powered regeneration without external energy input. Admittedly, while solar‐driven regeneration is attractive for low‐energy AWH operation, it often relies on a single daily cycle under sunlight, which can limit water production rates. Therefore, the dual operational pathways for AWH, including both solar‐driven cycling and temperature‐swing regeneration, are important for enabling more flexible and efficient water release, enhancing operational versatility for practical AWH systems.

To demonstrate the practical applicability of sulfonated PE for real‐world water collection, we constructed a prototype AWH device and evaluated it under outdoor ambient conditions. Because the collected water was produced by condensation of atmospheric moisture, its composition is expected to be similar to that of distilled water unless contaminants are introduced by the device materials or the surrounding environment. The outdoor test was therefore designed primarily as a proof‐of‐concept demonstration under realistic operating conditions, while also verifying that the integrated device could generate clean collected water. Prior to harvesting, the 24 h‐sulfonated PE powder was conditioned overnight to passively adsorb moisture from air. The hydrated sorbent was then placed in the device and irradiated with a full‐spectrum lamp to simulate sunlight (Figure 6a). Upon illumination, water vapor desorbed from the sorbent diffused through the enclosed headspace and condensed on the cooler inner wall of a glass collection jar, forming droplets that coalesced and drained to the bottom for collection (Figure 6b). A schematic of the device is shown in Figure 6c. The device consists of three stacked polymethyl methacrylate (PMMA) panels. The sorbent is held in a shallow vessel positioned between the top and middle panels, with a piece of filter paper at the bottom to retain the powder while allowing water vapor to pass. A glass jar mounted in the bottom panel functions as both the condenser and the collector. During operation, irradiation heats the sorbent via its photothermal response, driving water release; the resulting vapor is transported to the jar primarily by gas‐phase diffusion, where it condenses and accumulates as liquid water. In parallel, we conducted outdoor field tests to confirm that the sorption–desorption cycling observed under controlled irradiation also occurs under natural day–night conditions. Field experiments were performed in Hattiesburg, Mississippi, from 12th–14th October 2025. For this test, ∼100 mg of sulfonated PE powder was used and was pre‐dried under vacuum at 100°C for 3 h before outdoor exposure. Nighttime uptake experiments were performed outdoors to capture atmospheric moisture, while daytime conditions provided sufficient sunlight to regenerate the sorbent through photothermal heating. Solar irradiance was measured with a solar meter, sample surface temperatures were monitored using an infrared thermal camera, and ambient temperature and relative humidity were recorded with a Yitell temperature and humidity monitor. These experiments validate both the device‐level feasibility of water collection and the robust, passive day–night cycling behavior of sulfonated PE under realistic environmental conditions. As shown in Figure 6d, the water content and humidity increase steadily overnight (8 pm to 8 am), reaching a maximum uptake of ∼0.30–0.33 g g^{− 1} and 75%, respectively. In the absence of solar irradiation, the surface temperature remains the same as the ambient temperature (∼20°C). The corresponding changes in ambient temperature and humidity during the test are provided in Figure S7. After sunrise, increasing solar irradiance triggers rapid photothermal heating of the sulfonated PE, with the surface temperature rising from the ambient temperature (∼20°C) to ∼65°C at peak illumination (∼400 W m^{− 2}). Concurrently, the ambient RH drops to approximately 20%, while the water content drops sharply, indicating efficient daytime desorption driven by sunlight and the drop in ambient humidity, without external energy input. After noon, as the solar intensity decreases and the ambient humidity rises, the sample temperature gradually drops, and the water content slowly recovers. After sunset (∼7 pm), the sample cools to the ambient temperature and begins to re‐adsorb moisture with the increase of the humidity. The second nighttime cycle reaches nearly the same maximum uptake as the first. This repeatable day–night response demonstrates strong reversibility and stable cyclic performance, supporting sulfonated PE as a practical sorbent for passive atmospheric water harvesting.

FIGURE 6.

(a) Photograph of the prototype water‐harvesting device loaded with 24 h–sulfonated PE. (b) Representative images showing condensation, droplet growth, and coalescence on the inner wall of the collection jar during illumination. (c) Schematic of the device architecture, highlighting the sorbent holder, vapor‐diffusion pathway, and glass jar condenser/collector. (d) Outdoor field‐test results for 24 h‐sulfonated PE over 36 h (Hattiesburg, MS; 12th–14th October 2025), showing time‐dependent solar irradiance, infrared‐measured sample surface temperature, and gravimetrically determined water content (g g^{− 1}). (e) Water‐quality assessment of the collected condensate, including bacterial screening and heavy‐metal/ion, nitrite, and chlorine measurements using a commercial test kit.

The quality of the harvested water was evaluated using a commercial water‐analysis kit (Silver Lake Research Corporation). As shown in Figure 6e, no bacteria were detected, and the concentrations of common inorganic contaminants were low. Lead was below 15 ppb, while copper and iron were below 0.5 and 0.1 ppm, respectively. Total nitrate/nitrite was <0.5 ppm, total hardness was <50 ppm, and total chlorine was <2 ppm. Collectively, these results indicate that the condensed water using our sulfonated PE integrated device contains only trace levels of the measured inorganic species. We note that these water‐quality data are intended as a preliminary screening rather than a comprehensive compositional analysis. Because the collected water is condensed directly from atmospheric moisture, rather than produced from heavily contaminated feed streams or polluted water remediation, elevated concentrations of heavy metals or inorganic ions are not expected. Importantly, these results further confirmed that covalently tethered sulfonic acid groups on the PE backbone do not create the carryover risks commonly associated with salt‐infused or deliquescent sorbents. In salt‐based systems, water uptake can form mobile brines, which may leak from the sorbent matrix or be transported as droplets/aerosols during regeneration, leading to elevated solute levels in the condensate. The hydrophilic ionic functionalities in sulfonated PE are chemically bounded to the polymer chains rather than present as free salts, and therefore are not expected to leach or volatilize under typical sorption–desorption conditions.

Figure 7 compares the water‐harvesting performance of sulfonated PE with representative classes of adsorbents, including metal–organic frameworks (MOFs), covalent organic frameworks (COFs), and state‐of‐the‐art polymer‐based materials, highlighting the competitive water uptake of sulfonated PE across a wide humidity range. All reported values were measured under static AWH conditions at room temperature (22°C–25°C) and varying relative humidity. Despite its simple and cost‐effective synthesis, sulfonated PE achieves water sorption capacities comparable to many state‐of‐the‐art adsorbents. For example, at 10% RH, the water sorption capacity of sulfonated PE reaches 0.06 g g^{− 1}, higher than that of COF‐480‐hydrazide (0.04 g g^{− 1}) and MOF‐841 (0.01 g g^{− 1}) [22, 49]. At a relatively low humidity of 30% RH, sulfonated PE (0.18 g g^{− 1}) also demonstrates water uptake comparable to other polymer‐based sorbents, such as poly(N‐vinyl caprolactam‐co‐acrylic acid)/polyacrylonitrile (P(VCL‐co‐AA)/PAN, 0.14 g g^{− 1}) and sulfonated hyper‐crosslinked polymer containing SO₂ (SHCP‐SO, 0.18 g g^{− 1}) [50, 51]. Under 90% RH, the water uptake of sulfonated PE increases to 0.58 g g^{− 1}, similar to that of high‐performing polymer‐derived sorbents, including sulfonated hyper‐crosslinked polymers (SHCPs, 0.9 g g^{− 1}) and poly(N‐isopropylacrylamide) and hydrophilic sodium alginate networks (PNIPAAm/Alg IPN gel, 0.89 g g^{− 1}) [30, 52]. Sulfonated PE exhibits comparable or exceeding water uptake performance over a wide range of relative humidity. Additionally, sulfonated PE offers several distinct advantages over other adsorbents. Specifically, recycled PE can be directly utilized as the precursor. This feature dramatically reduces the cost and energy input, while simultaneously providing a value‐added pathway for plastic waste upcycling, potentially promising for addressing mixed PE/PP wastes [53, 54]. In contrast to MOFs and COFs, which often rely on expensive ligands, metal salts, and multistep syntheses, sulfonated PE is derived from an abundant commodity polymer using scalable chemical processing. Coupled with its advantageous photothermal properties, sulfonated PE without extra additives enables an efficient and fully integrated sorption–desorption cycle driven solely by natural sunlight. Moreover, as shown in Table S1, the estimated lab‐scale material cost of sulfonated PE was compared with representative state‐of‐the‐art sorbents. Sulfonated PE shows one of the lowest estimated costs among the compared materials, at approximately $0.19 g^{− 1}. This value is much lower to P(VCL‐co‐AA)/PAN nanofibers ($0.57 g^{− 1}) and PNIPAAm/Alg IPN gel ($0.72 g^{− 1}), and lower than OHCP‐60 ($0.81 g^{− 1}), SHCPs ($1.02 g^{− 1}), SHCP‐SO₃H ($2.02 g^{− 1}), HCP‐SO₃H ($3.32 g^{− 1}), and G‐PDDA aerogel ($13.1 g^{− 1}). MOF‐841 and COF‐480‐hydrazide exhibit substantially higher estimated costs of approximately $200 g^{− 1} and $156 g^{− 1}, respectively [55, 56, 57], mainly due to the high cost of their organic linkers and/or metal precursors. Using recycled PE as a precursor could further reduce the material cost. We note that this cost analysis is a simplified lab‐scale estimate and may become more complex upon practical scale‐up, where factors such as reagent recovery, processing yield, solvent use, purification, waste treatment, energy consumption, and manufacturing logistics must also be considered. In our system, addressing acid waste and SO_x emission is critical and necessary for environmental protection. Nevertheless, these results highlight the potential cost advantage of sulfonated PE, which combines competitive water sorption performance with inexpensive polyolefin feedstocks and a simple sulfonation process. Finally, we emphasize that this comparison in Figure 7 is intended to position sulfonated PE within the broader landscape of reported polymer‐based AWH sorbents, rather than to claim superior overall performance over all material classes. The simplicity of the chemistry and the use of low‐cost commodity polymers suggest a potentially scalable platform, which we consider a key contribution and central motivation of this proof‐of‐concept study. At the same time, metrics such as kinetics and daily water yield are strongly influenced by material morphology, porosity, form factor, and device configuration, making direct comparison across fundamentally different sorbent systems difficult at the material level alone. As such, further system‐level optimization and performance validation are required to enable reliable cost analysis.

FIGURE 7.

Comparison of sulfonated PE to different classes of water‐harvesting adsorbents in the literature.

3. Conclusion

In conclusion, we demonstrate that sulfonated PE provides an economically feasible solution for atmospheric water harvesting that combines satisfactory performance with a straightforward, scalable material preparation route. Specifically, a simple sulfonation treatment introduces hydrophilic sulfonic acid groups onto a commodity PE backbone, yielding a hygroscopic material with relatively high moisture uptake across a broad humidity range and excellent durability over repeated sorption–desorption cycling. Unlike many state‐of‐the‐art sorbents that rely on complex syntheses or the introduction of salts, this approach leverages an abundant, low‐cost polyolefin and avoids concerns associated with salt leaching and solute carryover. In addition, the intrinsic photothermal response of sulfonated PE enables solar‐driven regeneration without active cooling or external energy input, providing an energy‐efficient pathway for water release. Outdoor field tests confirm reversible nighttime sorption and daytime desorption under natural conditions. While further optimization in material design and system integration is required, this work offers an accessible route to convert widely available polyolefin plastics into safe and effective sorbents and advances a scalable materials strategy for decentralized atmospheric water harvesting.

4. Experimental Section

4.1. Materials

High‐density polyethylene (HDPE) was obtained from NOVA Chemicals (SURPASSHPs667). Polyethylene foam (ZUSA‐XPE‐186) was obtained from USA Industrials. Sulfuric acid (98 wt.%) was purchased from Thermo Fisher. Polyethylene (PE) (weight‐averaged molecular weight (M_w): ∼4,000 g/mol and number‐averaged molecular weight (M_n): ∼1,700 g/mol) was obtained from Sigma–Aldrich. Lithium chloride (LiCl), magnesium chloride (MgCl₂), magnesium nitrate hexahydrate (Mg(NO₃)₂), sodium chloride (NaCl), and phenolphthalein were all purchased from Sigma–Aldrich. Millipore Sigma Milli‐Q IQ 7003 ultrapure lab water purification system was used to obtain deionized (DI) water. Poly(methyl methacrylate) (PMMA) panels for water harvesting device construction were sourced locally.

4.2. Sample Preparation

Approximately 1 g of PE powder was weighed and transferred to a reaction glass flask, where 30 g of sulfuric acid was introduced. Subsequently, the flask was placed on a thermal plate and heated to 150°C with stirring for varying amounts of time. After the sulfonation reaction, the sample was carefully collected from the flask and washed with DI water three times and immersed in DI water overnight to remove residual acid. The sulfonated PE was removed from the water by filtration, and the pH of the water was measured using indicator paper. This process was repeated until the water was at pH 7. We note that water use is an important consideration in developing a sustainable process. In practice, the washing step can be readily scaled using counter‐current washing or recycled rinse water, which would substantially reduce overall water consumption while preserving process sustainability. Subsequently, sulfonated PE was completely dried at elevated temperatures (100°C) under vacuum for at least 12 h. The sulfonation and washing processes for HDPE and PE foam are the same as those used for PE powder. Because the sulfonated PE foam exhibited limited mechanical stability after sulfonation and washing, the sorption measurements were conducted using powdered fragments that retained macroporous features.

4.3. Characterization

Optical microscopy images of the pore structure were obtained using a ZEISS Smartzoom 5 digital microscope. Fourier‐transform infrared (FTIR) spectra were collected on a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific) equipped with a GladiATR accessory. Spectra were recorded from 4000 to 600 cm^{− 1} using 32 scans at 8 cm^{− 1} resolution. Differential scanning calorimetry (DSC) was performed on a Discover DSC 250 (TA Instruments) to quantify changes in crystallinity and freezable water content as a function of sulfonation time. A heat–cool–heat protocol was used for crystallinity calculation: samples were first heated from 40°C to 200°C at 10°C min^{− 1} to erase thermal history, cooled to 0°C at 5°C min^{− 1}, and then reheated to 200°C at 10°C min^{− 1}. The cool–heat process was used for freezable water content calculation: samples were first cooled from 40 to −90°C at 5°C min^{− 1}, and then heated to 40°C at 5°C min^{− 1}. The freezable water uptake can be calculated using the following equation:

$${\mathrm{w}}_{\mathrm{fw}}=\frac{\mathrm{\Delta }{H}_{\mathrm{sample}}}{\mathrm{\Delta }{H}_{\mathrm{water}}}$$

In this calculation, w_fw represents the mass fraction of freezable water within the sample, and ΔH _water is the theoretical melting enthalpy of bulk ice (333.5 J g⁻¹) [58]. ΔH _sample corresponds to the melting enthalpy of water in the sample, normalized with respect to the total sample mass.

X‐ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific ESCALAB Xi+ spectrometer with a monochromatic Al Kα source (1486.6 eV) and a MAGCIS Ar⁺/Ar_n ⁺ gas cluster ion sputter gun. Binding energies were referenced to the C 1s peak at 284.8 eV. High‐resolution spectra were processed in Thermo Avantage to determine heteroatom identities and quantify their content in sulfonated PE powders. Scanning electron microscopy images (SEM) were employed on a Zeiss Ultra 60 field‐emission SEM with an accelerating voltage of 5 kV to explore the morphology of samples.

To determine the degree of sulfonation through Mohr's titrations, ∼100 mg of the sulfonated PE was soaked in a 0.1 m NaCl solution for 48 h. This allowed ion exchange between the Na⁺ ions from the solution and the H⁺ ions from the sulfonic acid groups in the polymer. After the exchange, the solution was titrated with 0.026 m NaOH until the pH reached 7. This titration measured the concentration of acid in the solution and suggested the amount of sulfonic acid present in the sulfonated fibers. The degree of sulfonation was then calculated using the following equation:

$$\text{Degree of sulfonation}=\frac{V_{\mathrm{NaOH}}{M}_{\mathrm{NaOH}}}{m_{\mathrm{PE}}}$$

In this calculation, V _NaOH represents the volume of NaOH solution required to neutralize the solution, M _NaOH is the molarity of the NaOH solution, and m _PE is the mass of the sulfonated polymer added to the NaCl solution. This process determines the number of moles of acid groups per gram of polymer, reflecting the extent of sulfonation after the sulfonation reaction.

Static water‐vapor sorption was evaluated using a sealed humidity‐chamber method. Briefly, ∼100 mg of the 24 h‐sulfonated PE powder was placed in an open container and transferred to an airtight chamber maintained at 25°C. Relative humidity (RH) was controlled using saturated salt solutions and verified with a humidity meter. These salts establish well‐defined equilibrium RH values of ∼13% (LiCl), 35% (MgCl₂), 57% (Mg(NO₃)₂), 78% (NaCl), and ∼100% (DI water). The sample was equilibrated in each chamber for 24 h to ensure the RH stabilized and the sorbent reached steady mass. Water uptake was then determined gravimetrically by recording the sample mass before and after exposure, and sorption capacity was calculated from the mass increase relative to the dry sample mass. Alternatively, dynamic vapor sorption (DVS Carbon Advanced, Surface Measurement Systems Ltd.) measurements were performed on sulfonated PE powder after 24 h of sulfonation. Water vapor sorption‐desorption isotherms were performed at 25°C. The mass stability criterion (dm/dt) was set to 0.01% min⁻¹. The RH increased stepwise from 0% to 90% RH and subsequently decreased back to 0% RH with a ramp rate of 10% RH per step. The measurements were conducted under a total gas flow rate of 200 sccm. Long‐term cycling stability was assessed by alternating between sorption at 75% RH and 25°C and desorption at 7.5% RH and 90°C. Each cycle consisted of 120 min of sorption followed by 30 min of desorption, and a total of 60 cycles were collected. The measurements were conducted under a total gas flow rate of 200 sccm.

Funding

This work was supported by the National Science Foundation (No. CMMI‐2239408).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: marc70356‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.