1 Introduction

Solar steam generation (SSG) has a great application prospect in producing clean drinkable water from polluted water at zero CO2 emission, since it uses the renewable and abundant solar energy as the only energy source [1, 2]. In typical SSG systems, solar energy is harvested and converted to thermal energy by photothermal materials and the thermal energy is used to heat up liquid water to generate steam [3]. Many efforts have been devoted to developing advanced photothermal materials, which have made the old concept of SSG become a rejuvenated green technology [4]. Carbonaceous materials are inherently suitable for broad light absorption owing to the π-band’s optical transitions. Their optically excited electrons can quickly relax via thermalization due to electron-electron and electron-phonon scattering [5]. Various carbonaceous materials, such as graphene, graphene oxide (GO), carbon nanotube (CNT), graphite, carbon black, etc. have been investigated as the photothermal materials [6]. Although CNT and graphene are two representative kinds of carbonaceous materials for SSG, both of them are still more expensive than gold. Moreover, the fabrication of graphene or CNT-based photothermal materials is complicated, making them unrealistic for widespread use [7, 8]. Therefore, it is highly desirable to develop inexpensive, easy-to-manufacture, mechanically robust carbonaceous materials for SSG [9].

Adsorption is considered as another effective technology for water purification due to its convenience for operation, effectiveness, and low consumption [10]. Carbonaceous materials with developed porous structure are commonly used as adsorbents by adsorbing pollutants from water. Porous carbon materials have been applied in the removal of nutrients [11], heavy metals [12], antibiotics [13], dyes [14], and natural organic matter [15]. Recently, the “treating contaminants with wastes” strategy has been considered as an inspiring approach to reducing the costs of contaminated water purification, and it has triggered the studies focused on the production of porous carbon materials from organic wastes, such as crab shell [16, 17], garlic skin [13], oil palm sell [18], rice husk [19], waste tire [20], and so on.

With the fast development of E-commerce in China, the express industry is undergoing unprecedented growth. The more people choose to shop online, the more delivery packages are need. In China, the number of corrugated cardboard box used for express packages increased from 9.92 billion in 2015 to 14.4 billion in 2016. Waste corrugated cardboard boxes (WCCB) have became one of the main constituents of municipal solid waste (MSW) in China, but less than 20% of them are collected and reused [21, 22]. Therefore, it is of great environmental and economic significance to develop a facile and feasible way for the recycling of WCCB. Since WCCB consists mainly of cellulose fibers [23, 24], it is an interesting subject to utilize WCCB as a carbonaceous precursor for porous carbon materials, which can simultaneously be used for SSG and adsorption.

Herein, we proposed a facile way of preparing a low-cost multi-functional porous carbon monolith (PCM) using WCCB as the raw material. The PCM can be used for SSG and adsorption because of its good photothermal conversion and absorption properties. The evolution of the physicochemical characteristics of the samples was analyzed by X-ray diffraction (XRD), N2-adsorption/desorption, Raman spectroscopy, ultraviolet visible near infrared spectroscopy (UV-Vis-NIR), and scanning electron microscope (SEM). Pyrolysis kinetic and preparation mechanism were revealed by a thermogravimetric analyzer coupled with Fourier transform infrared spectrometer (TG-FTIR).

2 Experimental

2.1 Materials

WCCB was collected from local offices and cut into small square-shaped pieces (about 1 cm × 1 cm in size) and then stored in polyethylene bags until needed. FeCl3·6H2O of analytical grade was purchased from Sinopharm, China. Methylene blue (MB) (chemical formula: C16H18ClN3S3·H2O) was purchased from Tianjin Fuchen chemical reagents factory.

2.2 Preparation of porous carbon monolith

FeCl3 solution (2 wt%) was prepared by dissolving FeCl3·6H2O in deionized water, and WCCB pieces were dipped into FeCl3 solution using a weight ratio: FeCl3/WCCB of 1/1 (on a dry basis). The mixture was vigorously stirred at room temperature for 24 h, then the uniform slurry was poured into tea filter bags for pre-dehydration. The pre-dehydrated material was placed in a manual tapping mold for further dehydration and molding into tablets, then the tablets were dried in an oven at 80 °C for 24 h. The dry tablets (labeled as Fe-WCCB) with diameter of 1.79 cm, and thickness of 0.61 cm were heated at a temperature increasing by 5 °C min−1 to final carbonization temperatures of 300, 400, 500, 600, 700, and 800 °C under pure N2 atmosphere with the flow rate of 200 mL min−1 in a tube furnace (CTF12/65/550, Carbolite, Great Britain), and the final temperature was maintained for 2 h. The carbonized products were cooled in pure N2 flow, and denoted PCMFe/300, PCMFe/400, PCMFe/500, PCMFe/600, PCMFe/700, and PCMFe/800, respectively, where PCM represents “porous carbon monolith.” For comparison, the tablets (labeled as H2O-WCCB) were obtained from WCCB according to the same process except using deionized water rather than FeCl3 for slurrying. The carbonized product (labeled as PCM800) was prepared from H2O-WCCB at 800 °C under the same heating and atmospheric conditions. The yield of product was calculated from the following equation:

$$ Yield\left(\%\right)=\frac{M_{\mathrm{PCM}}}{M_{\mathrm{Pre}}}\times 100 $$
(1)

where MPCM is the mass of the porous carbon monolith, and MPre is the mass of Fe-WCCB or H2O-WCCB used as carbon precursor. The fabrication process of the porous carbon monolith is illustrated in Fig. 1.

Fig. 1
figure 1

Fabrication process of porous carbon monolith

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2.3 Characterization of porous carbon monolith

X-ray diffraction (XRD) analysis was carried out using an X-ray diffractometer (Bruker D8, Germany). N2-adsorption/desorption analysis was performed using a gas adsorption apparatus (Autosorb-iQ, Quantachrome Instruments, USA) at − 196 °C. BET surface area (SBET) was calculated using Brunauer-Emmett-Teller (BET) method. Total pore volume (VTotal) was estimated from liquid volume of N2 adsorbed at the saturation of relative pressure. Micropore volume (VMicro) and the mesopore volume (VMeso) were deduced using the Horvath-Kawazo (HK) and Barret-Joyner-Halenda (BJH) method, respectively. The average pore diameter (DAvg) was calculated as DAvg = 4VTotal/SBET. The carbonaceous structures of the samples were acquired by a Raman spectrometer (DXR, Thermo Scientific, USA). Optical absorption spectrum was measured using an ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer (Lambda 950, PerkinElmer, USA). The morphology was observed by using a field emission scanning electron microscope (FE-SEM) (Sigma 500, ZEISS, Germany). All thermal images were taken via an infrared (IR) camera (TiS10, Fluke, USA), and the change of temperature of the samples was also monitored by the IR camera.

2.4 Solar steam generation experiment

Figure 2 shows the experimental setup used to measure the mass loss of water. The optimum sample (PCMFe/600) was embedded in the central circular hole in a slice of expandable polyethylene (EPE) foam, and the assembly structure can float on the surface of water with only the bottom side of PCMFe/600 in direct contact with bulk water. A xenon lamp (HM-Xe330W, Hongming Instrument, China) was used as the solar light source, and the mass change of the water under 0.25 sun (250 W m−2) was recorded in real-time by an electronic balance (PL2002, METTLER TOLEDO, Switzerland) and transmitted to a computer.

Fig. 2
figure 2

Experimental setup of solar steam generation test

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2.5 Adsorption experiment

Adsorption isotherms of PCMFe/600 for MB were investigated referring a literature method [25]. PCMFe/600 was ground and passed through a 100 mesh sieve. Equilibrium adsorption were conducted in a set of Erlenmeyer flasks containing 0.015 g PCMFe/600 powder and 25 mL MB solutions with various initial concentrations (20 to 70 mg L−1). The flasks were agitated in a shaker at 200 rpm and 30 °C for 48 h to reach the equilibrium. After above treatments, the suspensions were filtered by 0.45 μm membrane filters, then the filtrates were diluted to suitable concentrations. The MB concentrations in the diluted supernatant solutions were determined using an UV-Vis spectrophotometer (DR5000, HACH, USA) at the maximum wavelengths (λ) of 664 nm. MB uptake per unit mass of adsorbent at equilibrium, qe (mg g−1), was calculated by

$$ {q}_{\mathrm{e}}=\frac{C_0-{C}_{\mathrm{e}}}{W}V $$
(2)

where C0 and Ce (mg L−1) are the initial and equilibrium liquid-phase concentrations of MB. V (L) is the volume of the solution and W (g) is the mass of porous carbon used. The equilibrium data were modeled using Langmuir, Freundlich, and Temkin isotherm models and the model equations are shown in Table 1.

Table 1 Linear and non-linear forms of Langmuir, Freundlich, and Temkin isotherm models
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To simulate the process of the purification of natural water body using a floating absorbent, PCMFe/600 embedded in EPE foam was added to a beaker containing 20 mL of MB aqueous solution with the concentration of 10 mg L−1. PCMFe/600 can float on the surface of MB solution with the help of EPE foam, and the beaker was sealed using parafilm and left undisturbed at room temperature. The MB solution was sampled using a pipette every 24 h and the concentration of the sample was determined using the UV-Vis spectrophotometer as mentioned above, then the small amount of solution after analysis was put back into the beaker for further adsorption.

2.6 TG-FTIR analysis

Weight loss behaviors and gaseous products obtained during the pyrolysis and carbonization of Fe-WCCB and H2O-WCCB were studied online using a TG-FTIR instrument that consists of a simultaneous thermal analyzer (STA 2500 Regulus, NETZSCH, Germany) and a Fourier transform infrared spectrometer (TENSOR II, Bruker, Germany). The samples (about 10 mg) were heated from 40 to 1000 °C at N2 flowing rate of 70 mL min−1 and the heating rate of 20 °C min−1. The volatile products were detected online by FTIR, in which IR spectra were recorded at 4000–650 cm−1 with a resolution of 4 cm−1. The transfer pipe and the gas cell in the FTIR were both heated at a constant temperature of 200 °C to minimize secondary reactions.

2.7 Pyrolysis kinetic theory

Kinetic study is important for the design of large-scale reactors to achieve practical production. Coats-Redfern method was used in this study to determine the kinetic parameters of Fe-WCCB and H2O-WCCB [26].

The rate of conversion, dx/dt, is a linear function of a temperature-dependent rate constant, k, and x temperature-independent function of conversion, f(α):

$$ \frac{d\upalpha}{d\mathrm{t}}= kf\left(\alpha \right) $$
(3)

where x is conversion degree, t is time. α is expressed as

$$ \alpha =\left({m}_{\mathrm{i}}-{m}_{\mathrm{t}}\right)/\left({m}_{\mathrm{i}}-{m}_{\infty}\right) $$
(4)

where mi is the initial mass of sample, mt is the mass of the sample at time t, m∞ is the final mass of the sample in the reaction.

The reaction rate constant, k, has been described by the Arrhenius expression

$$ k=A\exp \left(-{E}_{\mathrm{a}}/ RT\right) $$
(5)

where A is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the absolute temperature.

For a constant heating rate β during gasification, β = dT/dt, rearranging Eq. (5) and integrating by using Coats-Redfern method is given by

$$ \ln \left[g\left(\alpha \right)/{T}^2\right]=\ln \left[ AR/\beta {E}_{\mathrm{a}}\left(1-2 RT/{E}_{\mathrm{a}}\right)\right]-{E}_{\mathrm{a}}/ RT $$
(6)

As the term of 2RT/Ea can be neglected because it is much less than 1, Eq. (6) could be simplified as

$$ \ln \left[g\left(\alpha \right)/{T}^2\right]=\ln \left( AR/\beta {\mathrm{E}}_{\mathrm{a}}\right)-{E}_{\mathrm{a}}/ RT $$
(7)

Various reaction models define g(α) in different ways, and if the correct g(α) is used, plotting ln[g(α)/T2] versus 1/T should result in a straight line. Ea and A can be determined from the slop and the intercept of the line, respectively. The differential form f(α) and the integrated form g(α) for the basic model employed for the kinetic study of solid-state reactions are provided in Table 2 [27].

Table 2 Most frequently used solid-state reaction mechanism functions
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3 Results and discussion

3.1 TG-FTIR study

3.1.1 Weight loss behavior

TG and derivative thermogravimetry (DTG) curves of H2O-WCCB and Fe-WCCB can be seen from Fig. 3. For H2O-WCCB, the weight loss below 140 °C indicated the loss of moisture, and the major weight loss occurred within the temperature of 250–400 °C, with the maximum weight loss peak at 370 °C, indicating the pyrolysis of cellulose. The weight loss rate of H2O-WCCB became slow above 400 °C because the thermal decomposition of cellulose finished at this temperature. Instead, the carbonization of carbonaceous matters in the residue became the primary reaction, leading to the release of small molecules at a slow rate. The minor weight loss peak at 740 °C was caused by the decomposition of CaCO3 in the residue, which is one of the commonly used mineral fillers during paper-making [23, 27].

Fig. 3
figure 3

TG and DTG curves of H2O-WCCB and Fe-WCCB

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The weight loss of Fe-WCCB can be divided into three stages. The maximum weight loss rate of Fe-WCCB was achieved at 295 °C, and it was much lower than that of WCCB. However, Fe-WCCB had obviously higher weight loss rates between 140 and 320 °C, and its main pyrolysis reactions ended at 365 °C, indicating that the slurrying of WCCB using FeCl3 solution was helpful for the occurrence of pyrolysis reactions at low temperatures. The DTG peak belonged to CaCO3 disappeared due to the demineralization caused by H3O+ generated from the hydrolysis of FeCl3 during slurrying [28]. It is worth noting that the solid residue rate of Fe-WCCB was as higher as 12.19% than that of WCCB at 1000 °C, indicating that Fe(III) loaded on WCCB greatly benefited the formation of char.

3.1.2 Pyrolysis kinetic

Pyrolysis kinetic parameters were determined by assuming single separate reaction for a particular stage of thermal decomposition. According to their DTG curves (Fig. 3), the main temperature ranges for pyrolysis of H2O-WCCB and Fe-WCCB were selected. Equation (7) was applied separately to each of them, and a series of straight lines can be obtained by plotting ln[g(α)/T2)] versus 1/T using OriginPro 8.6 software (Fig. 4). The highest correlation coefficient (R2) indicates that the corresponding reaction model reasonably fitted the experimental data. For the main pyrolysis stages of H2O-WCCB and Fe-WCCB, D1 and F2 models had the highest R2 values reached 0.9915 and 0.9943, respectively, and the change of reaction model suggested the change of pyrolysis reaction pathway. Moreover, the activation energies of the pyrolysis reactions of H2O-WCCB and Fe-WCCB were calculated to be 129.59 and 52.96 kJ mol−1, respectively. As known, cellulose was composed of numerous glucose units connected by glycosidic bonds, and the pyrolysis of cellulose is initiated from the cleavage of these glycosidic bonds [29].The break of glycosidic bonds can be facilitated through the attack of H3O+ generated from hydrolysis of FeCl3 [28]. Moreover, as a typical Lewis acidic ion, Fe3+ possibly interacted with the glucose units during slurrying, forming relatively stable complexes and changing electronic cloud distribution; thus, the break of glycosidic bonds can be promoted [30, 31]. As a result, much less energy was needed for the thermal cracking of the cellulose in Fe-WCCB.

Fig. 4
figure 4

Plots of ln[g(α)/T2] versus 1/T for pyrolysis of a H2O-WCCB and b Fe-WCCB using different models

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3.1.3 Analysis of gaseous products

The three-dimensional (3D) spectra (absorbance-wavenumber-temperature) of the volatiles produced from H2O-WCCB and Fe-WCCB can be seen from Fig. 5 a and b, respectively. Gram-Schmidt curve indicates the variation of the yields of the volatile compounds with temperature. As shown in Fig. 5c, Gram-Schmidt curves H2O-WCCB and Fe-WCCB had peaks at 368 and 322 °C, respectively, indicating the release of large amounts of gaseous products. When the temperature is fixed, absorbance information at different wavenumbers can be obtained to study the components released at this particular moment. Figure 5 d shows the FTIR spectra at the Gram-Schmidt peak temperatures for H2O-WCCB and Fe-WCCB. The gaseous products composed of a variety of molecules were evolved due to the pyrolysis of cellulose. By comparing the spectra of H2O-WCCB and Fe-WCCB, it was found that the types of the released products were not changed due to the presence of FeCl3, but the relative contents of these components were changed. The peaks centered at 3575 and 1506 cm−1 represented –OH bond stretching vibrations in H2O. The peak at 2819 cm−1 indicated the stretching vibration of C–H in acetaldehyde. The strongest peak at 2362 cm−1 was caused by the release of CO2. The release of CO led to the presence of two subtle peaks at 2184 and 2113 cm−1. The strong peak at 1748 cm−1 belonged to C=O stretching in furfurals. The peak at 1377 cm−1 can be ascribed to the bending vibration of phenolic –OH groups. The peaks at 1170 and 1106 cm−1 can be attributed to the stretching vibration of C–O–C in levoglucosan (LG) and –OH in hydroxyacetone (HA), respectively [32]. The subtle peak at 863 cm−1 suggested the presence of hydroxyacetaldehyde (HAA) [33].

Fig. 5
figure 5

3D spectra of volatiles produced from a H2O-WCCB and b Fe-WCCB, c Gram-Schmidt curves H2O-WCCB and Fe-WCCB, d FTIR spectra at the peak temperatures for H2O-WCCB (368 °C) and Fe-WCCB (322 °C)

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The evolution histories of gaseous products from H2O-WCCB and Fe-WCCB as a function of temperature are presented in Fig. 6. Basically, the release of gaseous products from Fe-WCCB started at lower temperatures, which was attributed to the destruction of cellulosic fibers by Fe3+ during slurrying, as mentioned in Sect. 3.1.2. More importantly, the yields of the volatiles significantly reduced due to the presence of Fe3+, resulting in a higher char yield of Fe-WCCB. It was assumed that Fe(III) species played an important role in the inhibition of the generation of the volatile organics. The organics gaseous products were mainly released from Fe-WCCB between 200 and 360 °C, and FeCl3 decomposed into amorphous Fe(III) species (e.g., FeOOH) within this temperature range [34, 35].

$$ {\mathrm{FeCl}}_3+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{FeOCl}\cdotp {\mathrm{H}}_2\mathrm{O}+2\mathrm{HCl}\to \mathrm{FeOOH}+3\mathrm{HCl} $$
(8)
Fig. 6
figure 6

Evolution histories of gaseous products from H2O-WCCB and Fe-WCCB as a function of temperature

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Many researchers suggested that the thermal conversion of cellulose follows free-radical mechanism [36,37,38]. It proposes that heat causes the vibration and cleavage of covalent bonds in cellulose, forming free radicals and radical terminals on the surface of the solid, and the reactions are not terminated till two radicals collide with each other to form stable compound [39, 40]. The stable gaseous products are mainly generated from the reactions between the hydrogen radical and other radicals in the gas phase, while the radical reactions occurred inside and on the surface of cellulose network result in the formation of char [41, 42]. It has been extensively reported that FeOOH or Fe2O3 can be used as a flame retardant and a smoke suppression agent, because Fe(III) has the ability to capture radicals generated via the thermal decomposition of polymers [43,44,45,46,47]. As illustrated in Fig. 7, instead of directly converting into stable gaseous products, the free radicals generated from the thermal cracking of WCCB were firstly trapped by the Fe(III) species deposited on the surface of the solid particles, enhancing the chances of these free radicals participating in char formation process via condensation and polymerization reactions. Thus, less gaseous products were released from Fe-WCCB and the char yield increased.

Fig. 7
figure 7

Schematic diagram of thermal decomposition of Fe-WCCB

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3.2 Characterization of porous carbon monolith

XRD patterns of PCMFe/300, PCMFe/400, PCMFe/500, PCMFe/600, PCMFe/700, and PCMFe/800 can be seen from Fig. 8. No Fe-related crystal structures (such as FeOOH, Fe2O3, Fe3O4) can be observed from XRD pattern of PCMFe/300, suggesting that Fe species in PCMFe/300 were in amorphous forms of short-range crystalline orders. The peak at 26.6 o indicated the presence of quartz (SiO2), which was used as a filler in WCCB [23]. For PCMFe/400, the peaks at 30.1°, 35.4°, 43.1°, 53.9°, 56.9°, and 62.8° corresponding to the (220), (311), (400), (422), (511), and (440) planes, respectively, confirmed the presence of magnetite (Fe3O4). With the carbonization temperature increasing to 500, 600, and 700 °C, the intensity of Fe3O4 peaks continued to increase, indicating that higher carbonization temperature promoted the transformation of amorphous Fe species into crystalline Fe3O4. When the carbonization temperature achieved 800 °C, Fe3O4 peaks dramatically weakened and a sharp peak emerged at 44.8°, which belonged to (110) plane of α-Fe and proved the formation of zero-valent iron (Fe0) due to the carbothermal reduction of Fe3O4. Unlike PCMFe/800, XRD pattern of PCM800 only had two peaks at 26.7° and 29.4° attributed to quartz (SiO2) and calcite (CaCO3), respectively.

Fig. 8
figure 8

XRD patterns of porous carbon monoliths

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In fact, the transformation of Fe species and their interaction with WCCB-derived char can also be reflected by the evolution histories of CO and CO2 from Fe-WCCB (see Fig. 6). The release of CO below 500 °C was caused by the thermal cracking of cellulose and the polymerization of the carbonaceous solid. The release of CO between 480 and 670 °C was possibly caused by the carbothermal reduction of the amorphous FeOOH or Fe2O3 by amorphous carbon [35, 48, 49].

$$ 6\mathrm{FeOOH}+4\mathrm{C}\to 2{\mathrm{Fe}}_3{\mathrm{O}}_4+4\mathrm{C}\mathrm{O} $$
(9)
$$ 2\mathrm{FeOOH}\to {\mathrm{Fe}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O} $$
(10)
$$ 3{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{C}\to 2{\mathrm{Fe}}_3{\mathrm{O}}_4+\mathrm{C}\mathrm{O} $$
(11)

A sharp CO peak emerged at 840 °C, and the yield of CO2 also ramped up above 820 °C. These phenomena can be explained by the reaction between Fe3O4 and amorphous carbon [34, 50].

$$ {\mathrm{Fe}}_3{\mathrm{O}}_4+3\mathrm{C}\to 3\mathrm{Fe}+2\mathrm{CO}+{\mathrm{CO}}_2 $$
(12)

Figure 9 shows the N2-adsorption isotherm of PCMFe/400 within the relative pressure (P/P0) range of 0 to 0.3, and N2-adsorption/desorption isotherms of PCMFe/500, PCMFe/600, PCMFe/700, PCMFe/800, and PCM800 within the P/P0 range of 0 to 1. The adsorption isotherms of all the samples except PCMFe/400 showed steep N2 uptakes at P/P0 ≤ 0.01, suggesting the initial filling of micropores (pore diameter ≤ 2 nm). The adsorbed volumes increased gradually at P/P0 ≥ 0.01 with no plateau at high relative pressures, indicating the presence of mesopores (2 nm < pore diameter < 50 nm). Based on the N2-adsorption data, the calculated value of BET surface area (SBET) of PCMFe/400 was only 35 m2 g−1, suggesting the incomplete removal of volatile matters at 400 °C. Pore characteristic parameters of the other samples calculated according to the N2-adsorption/desorption data are shown in Table 3. SBET increased to 264 m2 g−1 when the carbonization temperature reached up to 500 °C, because the releasing of volatiles at high temperature greatly favored the formation of pores. With the carbonization temperature increasing to 600 °C, higher micropore volume (Vmicro) and SBET were achieved due to the further removal of volatiles and consumption of amorphous carbon via the reduction of Fe(III) species to Fe3O4. The mesopore volume (Vmeso) decreased because the sintering effect at high temperature resulting in the narrowing of pores and the shrinkage of carbon structure. The development of pore volume was affected slightly by the increase of carbonization temperature from 600 to 700 °C, because the releasing of volatiles finished within this temperature range, and only the consumption of amorphous carbon contributed to the development of porosity. SBET increased remarkably from 358 to 404 m2 g−1 because a lot of Fe3O4 formed below 800 °C was reduced to α-Fe, leading to the etching of char and formation of micropores. Compared with PCMFe/800, PCM800 had much lower SBET and Vmicro, but a higher Vmeso. These differences demonstrated that the charring process of WCCB was significantly affected by FeCl3, which was favorable to the formation of a more developed microporous structure. DFT pore size distribution of PCMFe/500, PCMFe/600, PCMFe/700, PCMFe/800, and PCM800 within different pore diameter ranges is shown in Fig. 10. PCMFe/500, PCMFe/600, PCMFe/700, and PCMFe/800 showed sharp peaks between 0.5 and 0.6 nm, indicating the presence of large amounts of micropores. Besides, more micropores with smaller diameters were formed with the carbonization temperature increasing from 500 to 800 °C. PCMFe/500, PCMFe/600, PCMFe/700, and PCMFe/800 also had mesopores with diameters between 2.25 and 4.30 nm. The distribution became weaker when the carbonization temperature increased from 500 to 600 °C due to sintering effect, while the consumption of amorphous carbon contributed to the generation of some new mesopores within the same diameter range. By comparing the DFT curves of PCMFe/800 and PCM800, it was found that the introduction of FeCl3 was obviously beneficial to the formation of micropores. However, the mesoporous structure became less developed due to the blockage of mesopores caused by the FeCl3-derived products.

Fig. 9
figure 9

N2-adsorption/desorption isotherms of porous carbon monoliths

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Table 3 Pore characteristics of porous carbon monoliths
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Fig. 10
figure 10

DFT pore size distribution curves of porous carbon monoliths within pore diameter ranges of a 0–2 nm and b 2–20 nm

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As mentioned, carbon materials can be used as solar absorbers due to optical transitions in the D band. Thus, Raman spectrometry analysis was necessary [51]. Raman spectra of PCMFe/500, PCMFe/600, PCMFe/700, and PCMFe/800 are shown in Fig. 11a. The spectra of all the samples exhibited D band and G band centered at 1334 and 1580 cm−1, respectively. D band is related to lattice defect and used to represent the disorder degree of graphite carbon, while G band is related to the vibration of sp2-hybridized carbon atoms in a two-dimensional hexagonal lattice [52]. The ratio of the relative intensity of D band and G band (ID/IG) is proportional to the crystalline degree of carbon materials [53]. The ID/IG values of the samples increased from 0.74 to 0.97 with the carbonization temperature increasing from 500 to 800 °C, suggesting the presence of more defect sites. For a quantitative characterization of the samples, the UV-Vis-NIR spectra were measured in the broadband solar spectrum. As shown in Fig. 11b), PCMFe/600 exhibited high light absorption above 80% within a broadband wavelength from 256 to 2140 nm, demonstrating its highest solar light absorption ability among these samples. Although the lowest ID/IG value of PCMFe/500 suggested that this sample contained the most sp2-hybridized carbon atoms and π electrons accounting for light absorption, PCMFe/600 and PCMFe/700 had higher light adsorption abilities than PCMFe/500. This was because the light adsorption ability of a porous carbon material was not only closely related with its π electrons, but also its pores. As shown in the inset, the incident light trapped in the pores of the carbon monolith can bounce back and forth between the walls, resulting in the light propagation and multiple light matter interactions and effective light harvest [4, 8, 54]. Thus, a higher BET surface area was no doubt conducive to improving the light adsorption.

Fig. 11
figure 11

a Raman and b UV-Vis-NIR spectra of porous carbon monoliths, the inset shows incident light trapped in the pores of porous carbon monolith bounces back and forth between the walls

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The surface morphology of PCMFe/600 observed by SEM under different magnification is shown in Fig. 12. PCMFe/600 had an irregularly twisted fibrous network structure, with many open pores and gaps between the flat ribbon shaped and crisscrossed fibers, which enabled the capillary-induced water transport. It is worth to note that a lot of particles presenting on the surface of PCMFe/600, indicating the presence of Fe3O4. Under high magnification, it can be observed that the Fe3O4 particles had polyhedron structures, with the particle size ranging from tens of nanometers to about 1 μm. Besides, the Fe3O4 particles were partly embedded in the dent parts and wrinkled areas or deposited deeply in the gaps and pores, suggesting good mechanical bonding between the carbon matrix and magnetic particles.

Fig. 12
figure 12

Surface morphology of PCMFe/600 under different magnification

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3.3 Photothermal conversion and solar steam generation tests

Since PCMFe/600 had the highest light adsorption ability among all the samples, and it was expected to exhibit excellent photothermal conversion performance. Thus, the study of photothermal activity was focused on PCMFe/600. Surface temperature measurement under a certain intensity of solar irradiation is widely used to evaluate the light-to-heat conversion performance of photothermal materials [55]. The variation of the recorded temperature of PCMFe/600 surface with irradiation time can be seen from Fig. 13, and the inset shows the optical image of PCMFe/600 being irradiated under 0.25 sun and the IR images of PCMFe/600 surface under 0.25 sun irradiation for 1830 s in the dry state. The surface temperature of PCMFe/600 increased rapidly from 25.2 to 51.6 °C within the first 60 s. The increase of temperature became slowly after 150 s, and finally attained a high equilibrium value (61.9 °C) within 1830 s. The temperature variation proved that PCMFe/600 had a good photothermal conversion performance, implying it can be used as a promising candidate of SSG.

Fig. 13
figure 13

Variation of temperature of PCMFe/600 surface with irradiation time, the inset shows the optical image of PCMFe/600 being irradiated under 0.25 sun and the IR images of PCMFe/600 surface under 0.25 sun irradiation for 1830 s in the dry state

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Mechanical stability is important during the use of the monolith for SSG, because the easy destruction of the samples must be avoided. As shown in Fig. 14, PCMFe/600 was pressed under a beaker filled with water weighing 1267.75 g, and the monolith character of PCMFe/600 can be perfectly retained. This proved that PCMFe/600 was able to withstand the pressure at least 4875 times of its own weight (2.55 N), thus we can say that PCMFe/600 had a good mechanical rigidity. Moreover, PCMFe/600 can float well just above the water surface with the help of EPE foam. A hot region is expected to appear on the top of PCMFe/600 under solar irradiation via photothermal conversion. Benefitting from the porous framework providing water transport channels, water can be continuously pumped to the hot upper surface of PCMFe/600. In this configuration, the solar energy is localized at the water-air interface to heat thin layer where evaporation occurs, instead of wastefully heating the entire body of water, which conforms to the definition of interfacial SSG (ISSG) [3, 56].

Fig. 14
figure 14

Optical images of PCMFe/600 being pressed under a beaker filled with water and floating on water surface and schematic diagram of PCMFe/600-based interfacial solar steam generation platform

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Figure 15 a shows the photographs of the two different steam generation scenes (the temperature measuring points were marked by “+”) and the IR images of the floating PCMFe/600, water surrounding PCMFe/600 and pure water under 0.25 sun irradiation for 3600 s. Figure 15 b shows the variation of temperature of the floating PCMFe/600 surface, neighboring water of PCMFe/600 and pure water with irradiation time. The temperature of PCMFe/600 surface rapidly increased from 22.7 to 31.7 °C within 300 s, and slowly increased to 36.5 °C over the remaining time. Meanwhile, the surface temperature of the neighboring water of PCMFe/600 reached 32.9 °C within 3600 s, while that of pure water increased to 30.2 °C. The results proved that the top of the floating PCMFe/600 was able to become a hot layer for generating steam. Besides, the neighboring water surface can be heated due to heat transfer from PCMFe/600. Figure 16 shows that the evaporation of pure water within 3600 s was 0.37 g, while the evaporation of water in the presence of the floating PCMFe/600 increased to 0.54 g under the same condition. The increase in water evaporation can be attributed to ISSG occurred on the hot surface of PCMFe/600. Moreover, as shown in the inset, PCMFe/600 acted as the heat source distributing heat to the neighboring water, thus this heat island effect no doubt contributed to the improvement of steam generation.

Fig. 15
figure 15

a Photographs of two different steam generation scenes and the IR images of the floating PCMFe/600, water surrounding PCMFe/600 and pure water under 0.25 sun irradiation for 3600 s. b Variation of surface temperature of floating PCMFe/600, neighboring water of PCMFe/600 and pure water with irradiation time

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Fig. 16
figure 16

Mass loss of water with and without PCMFe/600 at various time points

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3.4 Adsorption study

Based on the above experiments, PCMFe/600 was proved to be an excellent photothermal material and able to promote SSG by interface evaporation and heat island effects. To determine whether PCMFe/600 was simultaneously suitable for SSG and adsorption, the adsorption behavior of PCMFe/600 for MB was studied. The adsorption isotherm of MB on PCMFe/600 is shown in Fig. 17a, as can be seen, the increase in initial concentration of MB led to the increase of adsorption equilibrium, this was because the initial concentration provided a driving force for MB transferred from the aqueous phase to the surface of PCMFe/600. The inset of Fig. 17a shows the linear fitting of adsorption equilibrium data by Langmuir, Freundlich and Temkin isotherm models, and the best fitting model can be determined by the highest R2 value. R2 values and the isotherm parameters calculated from the slopes and intercepts can be seen in Table 4. Langmuir model had the best fitting, suggesting the formation of MB monolayer on the PCMFe/600 surface and no further adsorption occurred after the formation of the monolayer, and there was no MB molecules transmigrated in the plane of the neighboring surface. Moreover, each MB molecule had similar enthalpy and activation energy. Based on the Langmuir constant, the maximum monolayer adsorption capacity of PCMFe/600 for MB was calculated to be 70.9 mg g−1. The comparison of MB adsorption capacities of PCMFe/600 and some other waste-derived porous carbon materials are shown in Table 5. As can be seen, PCMFe/600 showed a competitive MB adsorption capacity, outperforming some other porous carbons in the literatures.

Fig. 17
figure 17

a Adsorption isotherm of MB on powdered PCMFe/600, the inset shows linear fitting of equilibrium isotherm data by Langmuir, Freundlich and Temkin isotherm models. b Variation of MB concentration in the presence of floating PCMFe/600 with contact time, the inset shows the optical images of PCMFe/600 floating on surface of MB solution and PCMFe/600 being attracted by a magnet, and the schematic diagrams of potential applications of PCMFe/600

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Table 4 Fitting parameters for Langmuir, Freundlich, and Temkin isotherm model
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Table 5 Comparison of MB adsorption capacities of various waste-derived porous carbons
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To simulate the process of the purification of natural water body using a floating absorbent, PCMFe/600 embedded in EPE foam was put into a beaker containing 20 mL of MB aqueous solution with the initial concentration of 10 mg L−1. PCMFe/600 kept floating on water surface with the help of EPE foam, then the beaker was sealed using parafilm and left undisturbed. As shown in Fig. 17b, the removal efficiency of MB by PCMFe/600 reached 77% within 5 days, and it continued to increase smoothly to nearly 100% on day 20. The adsorption rate was not as fast as that of a floatable carbon monoliths reported in a previous study [62, 67], and this was possibly because of the following reasons. First, PCMFe/600 needed more macro- and mesopores that provided channels for the fast diffusion of the MB dye to the active adsorption sites. Second, a stronger interaction between the functional groups on PCMFe/600 surface and MB molecules was required. Third, in the present study, the low initial concentration of MB solution provided limited driving force for mass transfer between the aqueous phase and solid medium. Besides, the MB solution was left undisturbed without stirring that can increasing contact between MB and PCMFe/600. The deficiencies of PCMFe/600 are pointed out, as well as the future research directions. Anyway, the floating PCMFe/600 was proved to be useful for absorbing the dissolved pollutants from water. Moreover, As can be seen from the inset, the MB-absorbed PCMFe/600 can be recovered from water via magnetic separation, and this phenomenon also reflected the potential of the floating PCMFe/600 being manipulated by magnetic force during its use. Based on the adsorption ability and magnetic response exhibited by the floating PCMFe/600, this material may find applications in pond water purification and environmental clean-up of oil spills.

4 Conclusion

Porous carbon monoliths were prepared from waste corrugated cardboard box (WCCB) via slurrying in FeCl3 solution, molding, and carbonization. FeCl3 was helpful for the pyrolysis of WCCB at lower temperatures, and the activation energy was reduced by 76.63 kJ mol−1 because Lewis acidic Fe3+ promoted the breaking of glycosidic bonds of cellulose contained in WCCB. FeCl3 was also beneficial to char formation because the radicals released from WCCB during pyrolysis can be captured by Fe(III) species derived from FeCl3. Amorphous Fe(III) species in PCMs continuously converted into crystalline Fe3O4 when the carbonization temperature increased from 400 to 700 °C, and the carbothermal reduction of Fe3O4 at 800 °C led to the formation of α-Fe. Both the release of volatiles and etching of carbon contributed to the development of the porous structure of PCMs, and FeCl3 was favorable to the formation of micropores. The optimal sample (PCMFe/600) exhibited high optical absorption above 80% within a broadband wavelength, because it was rich in both sp2-hybridized carbon atoms and pores. The surface temperature of PCMFe/600 in dry state achieved 61.9 °C under 0.25 sun irradiation due to its good photothermal conversion ability. PCMFe/600-assisted water evaporation was 1.46 times that of the pure water because of the occurrence of interfacial solar-derived evaporation and heat island effect. Methylene blue can be effectively absorbed from water by PCMFe/600 floating on water surface, and the adsorption equilibrium data of PCMFe/600 for MB followed Langmuir model with a monolayer adsorption capacity of 70.9 mg g−1.