Cryptomelane Modified Biomass Wastes for Solar Interfacial Evaporation and Stabilization of Cadmium

In this work, after carbonization of two biomass wastes, rice husk and coconut husk, they were modified by hydrothermally synthesized cryptomelane manganese and prepared into a solar interfacial evaporation system to explore their evaporation performance and stabilization performance of cadmium. The results demonstrate that the cryptomelane-modified biochar can obtain an evaporation rate of 1.247 kg m−2 h−1 and a photothermal conversion efficiency of 89.36% under one sun (1 kW m−2). In an outdoor experiment with an average solar power density of only 465.22W m−2, the evaporation capacity reached 4.7 kg m−2 and the evaporation rate reached 0.78 kg m−2 h−1 within 6 h. The cadmium adsorption capacity of biomass waste modified by manganese potassium ore increased by 50.94%, which could be well used for long-term stabilization and recycling of Cd. It could be verified that the cryptomelane-modified biochar material is a low-cost, scalable, and efficient material for interfacial evaporation and reduce wastewater pollution. The evaporation system designed in this work is a feasible way to obtain clean water and solve wastewater problems in the future.


Introduction
Fresh water and energy shortages are critical to the humankind's survival, as well as the economic development and social progress (Albert et al., 2021;Zhu et al., 2019a;Zhu et al., 2019c). According to The United Nations World Water Development Report, about 4 billion people, who make up nearly two-thirds of the world's population, experience severe water shortages for at least 1 month of the year. With the development of industry, the heavy metal pollution of water has also caused serious damage to clean water resources and posed a major threat to environmental organisms and human health (Carvalho et al., 1991;De Miguel et al., 2007;Haiyan and Stuanes., 2003). Heavy metals can accumulate in animals and humans for a long time, causing irreversible damage (Nateras-Ramírez et al., 2022;Pehlivan et al., 2008). Heavy metal particles are ingested and accumulated in water mainly through drinking and direct contact . When a certain concentration of heavy metals is present in the human body for a long time, it will have serious toxic effects on the organs and immune system. In severe cases, it will directly lead to cancer (Aslam et al., 2017;Vuković et al., 2010). In recent times, solar-driven evaporation has attracted considerable interest for replenishing the fresh water supply from seawater and polluted water (Wilson et al., 2020) as well as generating electricity .
Traditional solar evaporation devices generally deliver a low photothermal conversion efficiency of 30~45% because of poor solar absorption and large heat losses . Recently, many efforts have been done to enhance solar-to-heat energy conversion, thermally isolate the water-air interface from bulk water, tailor the morphology of the evaporating surface, and activate water to reduce the energy consumption of water evaporation . With the progress of the research Li et al., 2018;Qi et al., 2020;Zhang et al., 2018), many achievements have been made to improve the evaporation efficiency in the new structural design of the solar evaporator. Cooper et al., (2018) designed a laboratory-scale non-contact solar evaporation structure, which achieved 75.8% evaporation efficiency and collected 2.5 L m −2 fresh water in outdoor experiments. Qi et al., (2020) proposed a one-step method to load carboxylated multi-walled carbon nanotubes on cotton fabrics and assembled a jellyfish-like solar evaporator, which achieved an evaporation rate of 1.18 kg m −2 h −1 and an evaporation efficiency of 86.01% under one sun. Li et al., (2018) designed a 3D artificial transpiration device that can achieve evaporation efficiency of more than 85% in one sun and can achieve heavy metal treatment and recovery. Meanwhile, photothermal conversion materials for evaporation have also attracted extensive research attention (Yang et al., 2019b;Zhou et al., 2018). In terms of applications, natural materials, the main source of which is biomass, are used for solar evaporation by raw or carbonized processes with the advantages of low cost and low environmental impact (Fillet et al., 2021). Nowadays, more and more natural materials, such as wood (Xue et al. 2017;Yu et al., 2019;Zhu et al., 2017), plants (Fang et al., 2018;Sheng et al., 2020;Zhang et al., 2020), vegetables (Long et al., 2019), algae (Yang et al., 2019a), and other atypical biomass Zhou et al., 2019), have been proposed for solar thermal conversion materials with good practical results.
The pyrolysis process with little or no oxygen can enable cost-effective biochar (BC) such as rice husks and straws to have larger surface area and more porous structures (Baltrėnas et al., 2015;Leng et al., 2021). Furthermore, BC has been confirmed to remove and immobilize heavy metal pollutants compared with active carbon (Inyang et al., 2016). According to the research, rice straw biochar can better adsorb Cu (II) and Zn (II) in aqueous solution (Bhardwaj et al., 2022;Mei et al., 2020), and branch biochar can remove Pb (II), Cd (II), and Cr (II) well (Aslam et al., 2017). In addition, manganese oxide minerals most of which have dark black or grey color are commonly found in soil or as a thin coating on the surface of rocks (Lu et al., 2019). Most of manganese oxides have been noted for high oxidization potential and high adsorption capacity because of special tunnel or layered crystal structures and large specific surface area, so that they can be effectively applied in wastewater treatment (Islam et al., 2018). It has been reported that the manganese-modified biochar particles can greatly enhance the adsorption of heavy metal ions, such as Cu 2+ , Pb 2+ , and Cd 2+ , when dispersed in water (Liu et al., 2022;Sachan and Das, 2022;Tan et al., 2020).
Different from the existing research on cryptomelane-modified biochar dispersed in solution to explore the adsorption performance of heavy metals. This paper explores its use as a photothermal conversion material in a non-contact structural evaporation system and realizes evaporation to obtain clean water and Cd adsorption, which has never been reported. The water paths are constructed to link up the solar absorber to the water body with a filer-paper-PVA water pipe system. The objectives of the study are to determine a simple and cost-effective way for both a good photothermal evaporation efficiency and stabilize of heavy metals from the polluted water.
For the preparation of reaction solutions and purification of the synthesis products, deionized water was used as the solvent and scour. The rick husks were collected from the agricultural wastes in Anning City in Yunnan Province, southwest of China. The coconut shell biochar, filter paper, and PVA absorbent sponge form tube were purchased from JD. com. The brand, model, and country of the devices used are shown in Table 1.

Preparation
For the synthesis of BC, the rice husks were placed in a ceramic pot and dried at 80 °C overnight. Pyrolysis of the air-dried rice husks was conducted in a tube furnace under flowing N 2 by ramping the temperature at a rate of 10 °C per min and holding at a peak temperature of 900 °C for 2 h . Both of the rice husk biochar (BC-R) and coconut shell biochar (BC-C) were pulverized and ground into powder less than 80 mesh for further preparation.
Cryptomelane is prepared by hydrothermal method (McKenzie, 1971): 0.035 mol KMnO 4 was added into 80 mL deionized water, and 0.05 mol MnSO 4 ·H 2 O was added into 100 ml acetic acid solution with 2 mol L −1 concentration. The above two solutions were heated to 60 °C, and then the acetic acid solution dissolved with MnSO 4 was added to the KMnO 4 solution and transferred to a 500 mL conical flask. The conical flask was heated to 100 °C on a magnetic stirrer. After refluxing, the manganese oxide solid was filtrated, washed, and dried at 50 °C for 1 day. The solid particle size to less than 80 mesh and mixed mechanically with biochar powder 2.4 g unmodified biochar and manganese oxide-modified biochar powder was ultrasonically dispersed in 40 g water for 10 min and added into 40 g 2% sodium hydroxymethyl cellulose solution for continuing dispersing for 10 min, and finally get CBC-C and CBC-R.
In order to construct a non-contact structural evaporation system, the filter papers were cut into discs of 6 cm diameter. The filter paper was coated evenly with the above solutions using a brush and dried in the oven at 60 °C. As shown in Fig. 1, a PVA sponge rod with a diameter of 1 cm was used as a water delivery channel to transport water from the beaker to the surface of the filter paper and support the filter paper to keep it stable.

Experiments
The solar steam generators were placed on top of a 100 mL plastic beaker containing deionized water and placed on the analytical balance with automatic counting. The vessel was illuminated by a solar simulator with 1 sun intensity in the laboratory. The steam generation performances were measured for at least 3 h for each generator. In order to obtain the real outdoor evaporation effect, the evaporator also conducted all-day evaporation experiments at an altitude of about 2200 m in Anning City, Yunnan Province. All the surface temperatures of steam generators were recorded using an infrared camera, and the sunlight intensity was enhanced by a mobile weather monitoring station on the mountain.
In order to evaluate the Cd treatment performance of the system, 200 mg L −1 Cd (II) solution was loaded in the 100 mL breaker. The vessel was also put on the weighing balance and illuminated with 1 sun intensity by a solar simulator for the adsorption experiments. Adsorption experiments were performed for each sample for 6 h, and the Cd solution was sampled and diluted 100-fold every hour, and the concentration was determined by inductively coupled plasma mass spectrometry (ICP-MS). After 4 h of sufficient adsorption, the evaporation material was completely dried in an oven at 60 °C. Each sample was randomly selected three times using energy dispersive spectroscopy (EDS) to obtain elemental semi-quantitative analysis results. In order to study the salt crystallization phenomenon in the long-term solar evaporation adsorption process of the steam generator, the steam generator was put into 10g L −1 of high-concentration Cd (II) solution for 24 h and 7 days in a solar experiment.  Figure 2 shows the SEM images of CBC-C (a) and CBC-R (b) under different magnifications. From the characterization results, it can be seen that compared with the smooth block coconut shell BC, the rice husk BC has certain ravines and holes. Meanwhile, both CBC-C and CBC-R adhered well to the surface of the filter paper. The XRD patterns of CBC-C and CBC-R are shown in Fig. 2c, and the cryptomelane was well generated by the hydrothermal method. The rod-like crystals uniformly loaded on the sample are the synthetic cryptomelane (Meng et al., 2020b). This indicates that more potential active adsorption sites can be generated after the modification of cryptomelane (Tao et al., 2019). Figure 3a and b show the N2 adsorption/desorption isotherms of CBC-R and CBC-C. According to the International Union of Pure and Applied Chemistry (IUPAC) classification corresponding to micromacropores materials, the adsorption isotherms for both materials are a combination of type V. Nitrogen uptake at lower relative pressures (P/P0 ≤ 0.01) indicates the filling of micropores (diameter ≤ 2 nm) (Long et al., 2009). Nitrogen uptake at higher relative pressures and explicit hysteresis loops indicate the presence of mesopores (2 nm < diameter < 50 nm) (Tao et al., 2009). The dramatic increase in the adsorption volume in the high relative pressure region (P/P0 = 0.95 to 1) indicates the presence of larger mesopores and macropores (> 50 nm in diameter) (Robertson and Mokaya, 2013;Wang et al., 2012). According to the IUPAC classification, the adsorption-desorption hysteresis loops of CBC-C and CBC-R are of H1 type. Due to the pore network effect is the smallest, the most obvious sign is the steep narrowing of the hysteresis loop, which is the result of delayed aggregation of the adsorption branch. It shows that the two samples are cylindrical uniform mesoporous materials with narrow pore size distribution, and most of the pore type in the materials are pores and cavities. Comparing with the pore distribution curve shown in Fig. 3c, it can be seen that CBC-R and CBC-C have a sharp peak at 1.5 to 2 nm, indicating the existence of a certain microporous structure. And there are a large number of mesopores and a small amount of mesopores and macropores. Meanwhile, pore characteristic parameters including BET surface area (S BET ), total pore volume (V total ), and average pore diameter (D avg ) of each sample were calculated from the N2 adsorption/desorption data, and the results are shown in Table 2. It can be clearly seen from the results that the specific surface area and pore volume of BC-C and BC-R before modification are small, and there is only a small amount of micropores and mesopores. After cryptomelane modification, the specific surface area and pore volume were increased. The above results demonstrate that the modified CBC-C and CBC-R are a typical hierarchical porous structure with better adsorption performance and better ability to stabilize Cd.

Solar Interfacial Evaporation Experiment
To evaluate the evaporation performance of solar absorbers, an indoor simulated evaporation experiment was carried out under the conditions of an ambient temperature of 10 °C and 25% relative humidity. During the evaporation experiment, an infrared camera was used to record the temperature of the material surface, as shown in Fig. 4. Under the irradiation of 1 sun, the top surface of each device can absorb light and convert it into heat, resulting in a rapid increase in surface temperature. As shown in Fig. 4b, compared with the blank group, each device has a good photothermal conversion performance. The surface temperature of the device rises rapidly within 10 min, and then the rising trend rapidly slows down, and then the surface of each device reaches the highest temperature at 60 min. Among them, the surface temperatures of the modified CBC-C and CBC-R were higher than those before modification, respectively, reaching 43.0 °C and 42.3 °C. It can be verified from the results that after cryptomelane modification, the photothermal conversion capability of the device has been improved to a certain extent, and more light energy can be converted into heat, so that the surface temperature is higher, which can be better used for the interface water evaporates (Lenert and Wang, 2012).
As shown in the mass change curve shown in Fig. 5a, the evaporation rate did not decay during the whole experimental process, showing good durability. To further evaluate the performance of CBC, in addition to the evaporation rate, thermal efficiency (η) is also introduced to account for the utilization of sunlight by the device. The formula for calculating η is as follows (Ghasemi et al., 2014): where η is the efficiency (dimensionless, but more often expressed in %), m is the net evaporation rate (that is, the rate of evaporation due to light alone) in kg m −2 h −1 , λ is the latent heat of phase transition in (kJ kg −1 ), C is the specific heat capacity of water (4.2 J g −1 K −1 ), ΔT is the interfacial water temperature difference, and Q s is the power density of the solar flux in kW m −2 . The calculation results are shown in Fig. 5b and c; the evaporation rate of CBC-C with the highest surface temperature reaches 1.247kg m −2 h −1 , which is the highest among all samples, and is about 2.1 times of the evaporation rate of the blank device. The evaporation rates of BC-C, CBC-R, and (1) = m( + C ΔT) Qs  BC-R were 1.061, 1.246, and 1.167 kg m −2 h −1 , respectively. In the case of high radiation energy loss caused by low ambient temperature in winter , the four steam generators also showed good photothermal conversion efficiency. Among them, CBC-C had the highest evaporation efficiency, reaching 89.36%, and CBC-R, BC-C, and BC-R reached 89.19%, 75.78%, and 83.22%, respectively. Compared with the recent similar research work in Table 3, the contactless evaporation system designed in this work has good evaporation performance. The modification of cryptomelane improves the photothermal conversion ability of the material, so that the evaporation interface temperature increases, and the system has better evaporation performance (Lenert and Wang., 2012). Also, the good linear relationship of the evaporation curve also shows that the system has good evaporation stability . There are two main aspects of heat loss that affect photothermal conversion: heat radiation to the environment and heat conduction to bulk water (Fillet et al., 2021). The specific calculation methods are as follows (Ni et al., 2016): where P Rad is the radiative heat loss to the environment, A is the surface area of the interface material facing the sun, ε is the reflectivity of the interface material, σ is the Stefan-Boltzmann constant (5.67 × 10 −8 W m −2 K −4 ), T is the ambient temperature, T M is the interface material temperature, P Con is the heat conduction loss to the bulk water, and h is the convective heat transfer coefficient. The calculation results are shown in Table 4 According to Formulas (2) and (3), the ambient temperature will affect the heat loss of the whole system. The greater the difference between the ambient temperature and the evaporation interface temperature, the greater the heat radiation loss and heat conduction loss. For this experiment, the lower the ambient temperature, the more loss dissipation. And because of the advantages of the interface evaporation system, that is, the evaporation interface does not contact with the bulk water, the loss caused by conduction is very small, and the heat loss mainly occurs in the form of thermal radiation. The radiative heat loss fluxes (Q rad ) of CBC-C and CBC-R are about 202.09W m −2 and 197.09W m −2 , and their radiative heat losses are 0.5 W and 0.49 W, respectively. According to the results of Table 4, it can be seen that the modified material will produce higher thermal radiation loss due to higher surface temperature. Compared with other experimental results , the radiative heat loss increases more. This shows that the lower ambient temperature in the experiment also increases the thermal radiation loss. It is not difficult to predict that the excellent evaporation system designed in this work can achieve better evaporation performance under higher ambient temperature (> 10 °C). Meanwhile, according to research, as the surface area of the interface evaporated material gradually increases within a certain  range, the loss caused by heat conduction is smaller (Yan et al., 2022). The evaporation interface designed in this work has good scalability, and the evaporation capacity and evaporation efficiency can be further improved by increasing the size. Yunnan Province (20°8′-29°16′ N, 97°31′-106°12′ E), located in southwestern China, borders the Tibetan Plateau and the Himalayas in the west, Laos and Vietnam in the south, covering an area of about 3.83 × 105 square kilometers (Duan et al., 2016). The elevation of this area increases from 76 m in the southeast to 6740 m in the northwest, with an average elevation of 2000 m and high solar radiation intensity . Due to its cold zone, temperate zone, and tropical climate type, Yunnan Province has relatively small annual temperature difference and long sunshine hours. The hottest month (July) averages between 19 and 22 °C, and the coldest month (January) averages over 6 °C; however, the daily temperature difference in Yunnan is larger (Yunling, 2019). In order to obtain a certain evaporation effect in winter, the outdoor simulated evaporation experiment was selected on a mountain about 2000 m above sea level in Anning City, Yunnan Province. The experimental results are shown in Fig. 6.
The CBC-C and CBC-R with the best evaporation effect were selected for the test. The surface temperature of the device before and after the test is shown in Fig. 6a-c. The ambient temperature during the experiment was 10-15 °C, and the highest temperature reached on the surfaces of the two devices was about 33.7 °C. The evaporation per day was about 4.7kg m −2 , and the evaporation rate was 0.78kg m −2 h −1 . The surface temperature of the blank group was only 12.6 °C, which was lower than the ambient temperature. Assuming 12 h of sunlight per day (6:00-18:00), the production of purified water is estimated to be 9.36 kg m −2 per day. Therefore, a solar steam generator with an area of 0.28-0.39 m 2 can produce enough clean water to meet an adult's daily water intake (about 2.7 l per day for women and about 3.7 l per day for men) (Sawka et al., 2005). Figure 6d shows the sunlight intensity curve on the day of the experiment. It can be seen that the highest solar power density on the day of the experiment is 1070W m −2 , and the average power density is 465.22W m −2 . Compared with the water quality change curve in Fig. 6e, it can be seen that during the period of 13:00-14:00 when the solar power density is high, the evaporation rate increases significantly, and in the afternoon, with the decrease of the solar power density, the evaporation rate decreases obviously, which is in line with the theoretical law. It can be seen from the experimental results that the modified CBC-C and CBC-R still have excellent evaporation performance under the condition of low solar power density. It is foreseeable that the evaporation system designed in this work can be an efficient way to obtaining clean water in areas with low ambient temperature or weak power intensity.

Stabilization Experiment of Cadmium from Aqueous Solution
After 6 h of adsorption experiment, the experimental results are shown in Fig. 7. Considering that the volume of the solution decreases with the evaporation of water during the experiment, the volume change is taken as an important factor for the concentration change. The variation curve of solution volume with experimental time is shown in Fig. 7a. Corresponding to the evaporation rate, the solution volume of each device gradually decreases with the evaporation of water. The concentration obtained by ICP-MS was corrected by introducing the volume change to obtain a curve of mass versus time, as shown in Fig. 7b. The volume change, like the adsorption of Cd by the device, leads to the change of the solution concentration. Under the combined influence of these two factors, the concentration curve is unstable and fluctuates. After 6 h of evaporation, BC-C and BC-R adsorbed and stabilized 3.20 mg and 3.00 mg of Cd, with removal rates of 16.00% and 14.99%, respectively. CBC-C and CBC-R adsorbed and stabilized 4.83 mg and 4.44 mg, with removal rates reached 24.16% and 22.18%, respectively. It can be seen from the curve that BC-C and BC-R with good porous structure can adsorb Cd in the solution. After modification, the adsorption capacity of Cd by CBC-C and CBC-R increased by 50.94% and 47.96%, respectively. The modified CBC-C and CBC-R can form amphiphilic sites on the carbonaceous lattice, which has high immobilization potential for divalent metals and enhance the adsorption of Cd(II) (Shaheen et al., 2021). At the same time, the surface of biochar modified by cryptomelane will enrich functional groups such as C-O, C=O, -OH, which can complex with Cd (II), thereby improving the adsorption effect (Zhu et al., 2019b;Choppala et al., 2012). So, complexation may be an important mechanism for Cd removals from metal solutions. The EDS semi-quantitative analysis results in Table 5  show that the more Mn element content, the more Cd element adsorbed, which just confirms this hypothesis. This proves that cryptomelane can improve the device's ability to capture Cd and can better adsorb and stabilize Cd from the solution. In addition, the higher specific surface area and pore volume brought by modification are also important reasons for enhancing Cd adsorption capacity (Baltrėnas et al., 2015).
The surface crystallization of the obtained material is shown in Fig. 8a, and only a small amount of crystalline salt appears on the surface of the outer ring. To further illustrate the crystallization of Cd (II), the whole experimental device was placed in the natural environment for 7 days, and the obtained crystallization is shown in Fig. 8b and c. It can be seen that compared with the large amount of white crystalline salt produced by the internal measurement of the filter paper, only a small amount of crystalline salt appeared on the outer ring of the material surface. This shows that the water supply at the evaporation interface close to the water delivery channel is stable for a long time in the experiment and does not lead to supersaturation of the salt solution (Liu et al., 2020). Meanwhile, the crystal salt is well fixed on the outer ring of the filter paper and the surface of the material, which does not affect the evaporation performance and is beneficial to the recovery of the crystal salt. It is not difficult to see that greater removal of Cd by increasing the biochar treatment duration may be due to high exposure and contact of Cd with biochar (Sun et al., 2014;Inyang et al., 2012). In addition, the electrical conductivity and pH value of the solution also affect the Cd adsorption capacity (Aslam et al., 2017). It can be expected that the system designed in this work will have better Cd adsorption and stabilization ability under some conditions.

Conclusion
In this paper, cryptomelane-modified biomass waste was successfully synthesized, and its evaporation performance and adsorption stability of heavy metal Cd in a contactless evaporation system were evaluated.
The evaporation system achieved excellent evaporation performance of 89.36% evaporation efficiency and 24.16% Cd adsorption capacity. This indicates that the modification of cryptomelane is an effective means to improve the photothermal conversion performance and Cd adsorption capacity of biomass carbon materials. At the same time, the good scalability and optimization conditions make the evaporation system designed in this paper still have great application and research potential.