Versatile Strategy for the Preparation of Woody Biochar with Oxygen-Rich Groups and Enhanced Porosity for Highly Efficient Cr(VI) Removal.

Biochar is widely used to remove hexavalent chromium [Cr(VI)] from wastewater through adsorption, which is recognized as a facile, cost-efficient, and high-selectivity approach. In this study, a versatile strategy that combines delignification with subsequent carbonization and KOH activation is proposed to prepare a novel woody biochar from waste poplar sawdust. By virtue of the unique multilayered and honeycomb porous structure induced by delignification and activation processes, the resultant activated carbonized delignified wood (ACDW) exhibits a high specific surface area of 970.52 m2 g-1 with increasing meso- and micropores and abundant oxygen-containing functional groups. As a benign adsorbent for the uptake of Cr(VI) in wastewater, ACDW delivers a remarkable adsorption capacity of 294.86 mg g-1 in maximum, which is significantly superior to that of unmodified counterparts and other reported biochars. Besides, the adsorption behaviors fit better with the Langmuir isotherm, the pseudo-second-order kinetic model, and the adsorption diffusion model in batch experiments. Based on the results, we put forward the conceivable adsorption mechanism that the synergistic contributions of the capillary force, electrostatic attraction, chemical complexation, and reduction action facilitate the Cr(VI) capture by ACDW.

Introduction

Industrial wastewater containing chromium (Cr) has become one of the main sources of environmental pollution.[1−3] Hypotoxic trivalent chromium [Cr(III)] is an essential microelement in the human body, which participates in and maintains the metabolism of glucose, lipid, and protein. Meanwhile, hexavalent chromium [Cr(VI)] is a persistent pollutant with high toxicity and oxidizability.[4,5] Cr(VI) will inhibit cell metabolism, cause cancer, and induce gene mutations, provided that it is absorbed into the body.[6] Therefore, the processing of Cr(VI) wastewater has always been an important research topic in the field of water environmental protection.[7,8]

To alleviate the above-mentioned problems, several methods for collecting and removing Cr(VI) from polluted water have been developed. These include chemical precipitation,[6] membrane filtration,[9] ion exchange,[10] and adsorption. In particular, adsorption is considered a facile, cost-efficient, and high-selectivity method.[11] Various high-performance adsorbents for Cr(VI) removal, such as magnetic materials,[12] biomass-based porous materials,[13] and carbon materials (i.e., biochar), have been developed so far.[14] Among them, biochar, prepared via high-temperature (>600 °C) pyrolysis of agricultural or forestry waste under inert gases, is considered a promising candidate owing to its high specific surface area (SSA), outstanding adsorption capacities, inexpensiveness, and abundant resources.[15] However, it is difficult for pristine biochar to fulfill the requirement of higher adsorption capacities due to the finite nature of porosity and surface activity. The development of modification technologies is greatly indispensable for improving the SSA, pore structure, and surface chemistry.[16]

Chemical activation using acidic, alkaline, or metal salt solution as the modifier efficiently enhances the adsorption property of biochar on the Cr pollutant.[17,18] The biochar modified by KOH attains rich oxygen-containing functional groups such as −COOH, −OH, and O=CH–, which triggers strong electrostatic interactions with HCrO4– and Cr2O72– species, consequently promoting the Cr(VI) removal. Zhang et al.[19] increased the Cr(VI) adsorption capacity (45.88 mg g–1) of eucalyptus sawdust-derived hydrothermal biochar using a low-concentration KOH activation process. Notwithstanding their promising performance, due to the large amounts of additives and various interactions among cellulose fibers and amorphous lignin/hemicellulose, the internal cell structure of most biomass raw materials, especially wood that is characterized by a dense and tough block, is adverse to the biochar chemical activation at a high concentration of activated agents, which results in an adsorption that remains inadequate for practical applications.[20] To further optimize the raw wood structure, Hu’s group proposed a compelling approach that directly converts natural wood into compressible elastic carbon sponges through the removal of lignin and hemicellulose and subsequent carbonization, which partially destroyed the compact structure of the cell walls and led to a hierarchical architecture coupling with a large surface area, low density, and high porosity.[21−23] Subsequently, Guan et al.[24] prepared a lignin- and hemicellulose-free wood sponge with a lamellar structure of wave-like stacked layers using a similar method. The subsequent silylation reaction on the wood skeleton surface allowed the wood sponge to act as a superior oil adsorbent with excellent oil/water absorption selectivity and a high absorption capacity of 41 g g–1. What is more, Wan et al.[25] showed that carbon materials fabricated through facile delignification and carbonization have super-sensitive selectivity to the removal of organic pollutants from water. According to the above reports, we know that the natural structure of wood can be regulated by chemically removing lignin and hemicellulose from the cell walls of wood; meanwhile, the porosity, surface chemical structure, and micromorphology of plant-derived biochar materials were also highly dependent on their inherent cell wall structure. Inspired by these ideas, we tried to use a novel preparation method of biochar, that is, delignification combined with the carbonization–activation process. In addition, to the best of our knowledge, no reports have yet focused on the application of the delignified strategy for the preparation of woody carbon materials for the adsorption of heavy metals.

Herein, we present a novel strategy and have integrated the methods of delignification, carbonization, and KOH activation to prepare porous biochar from low-cost poplar wood sawdust. As illustrated in Figure , the facile delignification process endows the cell walls of delignified wood (DW) with a multilayer structure stacked by aligned cellulose fibers. The carbonized DW (CDW) retains the layered structure with a significantly increased porosity. Further KOH activation for the CDW optimizes the porous architecture and surface activity, leading to numerous meso- and micropores as well as rich oxygen-containing functional groups. The superiorities of a modulated nanostructure and surface modification result in activated CDW (ACDW) with a maximum Cr(VI) adsorption capacity of 294.86 mg g–1 at a pH of 2, which is not only higher than that of the carbonized wood (CW) without any treatments but also surpasses that of most of the biochar reported previously. Moreover, we study the adsorption behaviors of ACDW on Cr(VI) by using adsorption isotherms, adsorption kinetics, and thermodynamic models and propose the possible adsorption mechanism.

Figure 1

Schematic illustration of ACDW fabrication.

Results and Discussion

Characterizations

As is well known, wood is a kind of biopolymer composite material mainly comprising proportional cellulose, hemicellulose, and lignin and possessing a hierarchically anisotropic porous structure (Figure a). The structures and morphologies of pristine wood (PW) and DW were recorded through scanning electron microscopy (SEM). It can be observed that the delignification allowed the dense PW (Figure b) to evolve into loose DW (Figure c–e) with yellow turning to white in appearance (the inset of Figure b,c), which is a benefit of the SO32–-induced substitution reaction breaking the ether bonds of lignin and making it a soluble salt, and the HOO– destroying the chromophoric groups of lignin to further remove more lignin content. In particular, the preserved cellulose fibers stacked into a layer stacking structure due to strong intermolecular forces (Figure c), consequently resulting in the destruction of the cellular lumina without structure disassembly, which is expected to increase the porosity and the solution permeability.[25,26]

Figure 2

Characteristics of the structures and chemical components of PW and DW. (a) Representative molecular structures of cellulose, hemicellulose, and lignin in the wood cell wall. Low- and high-magnification SEM micrographs of (b) PW and (c–e) DW (the inset shows the corresponding digital photograph). (f) Composition evolutions, (g) FT-IR spectra, and (h) XRD patterns of PW and DW.

The chemical component contents of PW and DW were determined using a two-step sulfuric acid hydrolysis method (NREL). As shown in Figure f, cellulose, hemicellulose, and lignin decreased from 42.36 to 33.43, 14.56 to 7.65, and 28.75 to 6.39%, respectively, after delignification (eqs S1 and S2 and Table S1). What is more, Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD) were conducted to further reveal the chemical composition evolution of the wood induced by the delignification process. With regard to the FT-IR spectra (Figure g), the strong peaks at 1737 and 1243 cm–1 present a C=O stretching vibration of the ester group, which can be ascribed to the ester group, acetyl group, or uronic acid group of hemicellulose.[26] Furthermore, the peaks at 1596, 1910, and 1460 cm–1 are corresponding to the aromatic skeletal vibrations of lignin. Besides, the characteristic hemicellulose peaks almost completely wipe out and their lignin counterparts weaken on the FT-IR spectrum of DW, confirming that the large proportion of fillers (i.e., lignin and hemicellulose) in the wood cell wall were removed, whereas the cellulose fibers were maintained with an integrated structure after delignification. In the XRD patterns (Figure h), both samples display characteristic diffraction peaks at approximately 15.8° (101̅), 16.8° (101), 23.1° (200), and 34.7° (004), which can be well indexed to cellulose Iβ.[25] Furthermore, DW exhibits a higher crystallinity index (74%) than PW (44%) (calculated based on Segal’s method[28]), also reflecting the removal of amorphous lignin and exposure of a more cellulose phase.

Prepared using a high-temperature pyrolysis in a nitrogen atmosphere, the CDW was characterized by a typical multilayer structure, whereas the cellulose microfibers arranged in the layers were retained (Figure b,c). Due to the inevitable generation of volatile carbonaceous derivatives during carbonization, the cellulose fiber layers of CDW exhibit a thinner dimension. Such a multilayer structure of the CDW is expected to contribute to an increasing SSA and a more developed porosity for excellent ion adsorption behaviors. In contrast, CW prepared from PW presents limited channels with a thick cell wall structure (Figure a). To further enrich the pore structure, the hierarchically porous ACDW with a multilayer structure was obtained through a simple KOH activation. As shown in Figure d, undergoing the KOH chemical activation does not destroy the structural integration of ACDW. Note that plentiful visible holes are carved inside the anisotropic direct channels (Figure e). Moreover, the transmission electron microscopy (TEM) images of ACDW were investigated. The TEM images of ACDW show porous granular aggregates with the amorphous structure (Figure S1a). The high-resolution TEM images (Figure S1b,c) exhibit alternately large and small aperture structures, indicating that many honeycomb-like pores in the nanoscale are uniformly distributed on the surface of ACDW, which corresponds with the observation in Figure f. Such a porous structure is derived from the drastic reactions between the activating agent and carbon species at a high temperature as follows[29]

Figure 3

Characteristics of the morphologies, porosities, and surface functional groups for the prepared biochar materials. Low- and high-magnification SEM images of (a) CW, (b,c) CDW, and (d–f) ACDW. (g) Nitrogen adsorption–desorption isotherms, (h) corresponding SSA values, and (i) pore size distribution plots based on the Barrett–Joyner–Halenda and density functional theory (DFT) methods (inset) of CW, CDW, and ACDW.

The excellent adsorption performance of biochar is governed by many characteristics of its own, such as a high SSA and a highly developed internal void structure.[27,30] N2 adsorption–desorption experiments were carried out for CW, activated CW (ACW), and ACDW samples to determine the SSA and the pore structures. As shown in Figure g, both the CW and CDW display a reversible type II isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classification, which is indicative of the typical mesoporous material. Note that CDW shows an increase of adsorption uptake in the initial stage, which suggests the existence of abounding micropores. Besides, the resultant ACDW isotherm clearly presents an integrated type of I and IV with a higher uptake at low relative pressures, which is indicative of to the existence of increasing micropores. Furthermore, the hysteretic loop between the adsorption and desorption isotherms becomes manifested, suggesting the development of mesoporosity due to the KOH activation. The respective SSA (based on the Brunauer–Emmett–Teller method) and the contribution of mesoporous/microporous volume are summed up in Figure i and Table S2. As shown in Figure h, the SSAs of CW, CDW, and ACDW were calculated as 57.3, 236.4, and 970.5 m2 g–1, respectively. In addition, the delignification and KOH activation endowed CDW and ACDW with improved meso- and microporous volumes compared with pristine CW, thereby reducing the average pore size and significantly increasing the SSA. Such results are also supported in the pore size distribution plots (Figure i).

The changes in the type of functional groups on the surface of samples were analyzed by FT-IR. As shown in Figure S2, the strong peaks at 3442 cm–1 that correspond to −OH groups were profound. The spectral bands between 2900 and 2800 cm–1 are assigned to the elongation of C–H aliphatic chains. The peaks at 1617 and 1574 cm–1 correspond to the aromatic systems in double bonds between carbons and the double bonds between carbon and oxygen in amides. Furthermore, the peaks at 873 and 809 cm–1 correspond to the aromatic CH bands. Lastly, the peaks between 1050 and 1093 cm–1 represent C–O–C groups, which are related to cellulose aliphatic ethers.[31−33]

The XRD spectra of all the carbonized products are illustrated in Figure S3. Two broad peaks centered at 2θ = 23.6 and 43.5° are assigned to the (002) and (100) planes of the graphite (JCPDS no: 16-116), respectively. The crystal type of the ACDW is stable with a slight reduction in crystallinity due to the numerous micropores and mesopores generated on the samples with the activating reaction, which broke the order of the crystal structure of graphite. The Raman spectra of CW, CDW, and ACDW (Figure S4) show a D band at 1360 cm–1 and a G band at 1590 cm–1, where the D band is attributed to the disordered amorphous carbon and the G band can be allocated to the stretching motion of all the sp2 atoms in the carbon ring or long chain.[34,35] ACDW exhibits an increasing intensity (I) ratio of ID/IG, indicating more defects in the crystal structure of carbon after the KOH activation, which is consistent with the XRD results (Figure S3).

Evaluation of the Adsorption Performance of ACDW on Cr(VI)

Effect of Cr(VI) Adsorption at Different Temperatures in CW and CDW

We investigated the effect of different carbonization temperatures on the Cr adsorption capacity of CW and CDW [adsorbent dose: 0.01 g; initial Cr(VI) concentration: 60 mg L–1; volume: 30 mL; pH: 2.0; and solution temperature: 25 °C]. As shown in Figure S5a, the adsorption capacity of CW increases with the increase of carbonization temperature in the range of 400–700 °C but declines at 800 °C. CDW shows a similar trend with respect to the effect of carbonization temperature on Cr adsorption (Figure S5b). Such a phenomenon can be attributed to the plastic deformation of the biochar arising at higher carbonization temperatures, which inhibits the formation of micropores. Besides, the precipitated tar also blocks part of the pores, thus reducing the SSA of the biochar.[36−38]

Effects of Adsorption Materials

The adsorption process of Cr(VI) is affected by many factors, including adsorption time, pH values in the solution, and Cr(VI) concentration.[39,40] First, we performed a comparative adsorption experiment of the prepared CW, CDW, ACW, and ACDW, and the results are shown in Figure a. As is expected, CDW exhibits an adsorbing capacity of Cr (63.42 mg g–1), which is almost 3 times larger than that of CW (18.21 mg g–1) because the abundant nanolayered structure induced by the delignification process improves the porosity and increases the SSA. Moreover, the adsorbing capacity is further improved to 288.9 mg g–1 for ACDW. Considering that the active carbon material without delignification displays a moderate adsorbing capacity of 154.5 mg g–1, we conjecture that the cellulose fiber-dominated lamellar structure of CDW is more easily activated compared with the dense cellular architecture of DW. Therefore, we used ACDW to perform more profound adsorption experiments to investigate the adsorption mechanism.

Figure 4

(a) Comparison of the adsorbing capacity of the CW, DW, CDW, and ACDW [adsorbent dose, 0.01 g; initial Cr(VI) concentration, 100 mg L–1; volume, 30 mL; pH, 2.0; and temperature, 25 °C]. (b) Effect of various solution pHs on the Cr(VI) adsorbing capacity of ACDW (inset: zeta potential of ACDW at various pHs; adsorbent dose, 0.01 g; initial Cr(VI) concentration, 100 mg L–1; volume, 30 mL; and temperature, 25 °C). (c,d) adsorption behaviors of ACDW in the solutions with different Cr(VI) concentrations in 0–24 h (adsorbent dose, 0.01 g; volume, 30 mL; pH, 2.0; and temperature, 25 °C).

Effects of pH and Initial Concentration on Cr(VI) Adsorption

The adsorption performance of activated carbon is bound up with the pH values of the solution. Therefore, we tested the zeta potential and evaluated the chromium adsorption performance of ACDW in the pH range of 1.0 to 8.0. As shown in Figure b, the adsorption capacity exhibits the highest value when pH = 2. However, the adsorption capacity dramatically decreased with the increasing pH. The pH point of the zero charge (pHPZC) observed from the zeta potential for ACDW is 1.7 (inset, Figure b), revealing that the adsorbent is positively charged when the solution pH is below 1.7 and exhibits electronegativity at a pH above 1.7. Provided that pH < pHPZC, it will protonate the oxygen-containing functional groups and enhance the electrostatic potential of ACDW, thereby contributing to the capture of the negatively charged HCrO4– or Cr2O72– to the ACDW surface by virtue of the electrostatic attraction. However, the Cr(VI) uptake remarkably decreases with the gradual increase in the solution pH from 1.7 to 8.0 because of the electrostatic repulsive effect of the negatively charged ACDW to the anionic Cr(VI).

In general, the concentration of Cr ions in wastewater is not greater than 300 mg L–1. Therefore, an experiment was designed to discuss the influence of different initial concentrations on the adsorption process. In this experiment, the concentration gradient, temperature, and adsorbent dosage were set to 40–250 mg L–1, 25 °C, and 0.01 g, respectively. As shown in Figure c, the adsorption capacity increases with the increase in the Cr(VI) concentration, whereas the removal rate decreases when the pollutant concentration exceeds 60 mg L–1. This result can be ascribed to the relatively small dosage of the adsorbent that cannot afford to take in and preserve the excess Cr ions.

Adsorption Kinetics

To further understand the mechanism of the adsorption and desorption processes for the ACDW adsorbent, the adsorption kinetics of 60 mg L–1 Cr(VI) was employed at 25 °C using 0.01 g of the adsorbent in a solution with 2.0 pH. The adsorption capacity was determined after 0.5, 1, 2, 3, 5, 8, 12, 18, and 24 h. The pseudo-first- and second-order kinetic models were used to evaluate the fitting data based on eqs S3 and S4. The adsorption kinetic model curves are illustrated in Figure a, and the parameters of pseudo-first-order and pseudo-second-order kinetic models for the Cr(VI) adsorption are summarized in Table S3. The experimental data fitting by the pseudo-second-order kinetic model (R2 = 0.96) is better than that of the pseudo-first-order kinetic model (R2 = 0.72). Compared with the experimental value of 179.9 mg g–1, the data of the pseudo-second-order kinetic model were more consistent, indicating that the adsorption process is mainly controlled by chemical adsorption.[41]

Figure 5

(a) Adsorption kinetics model fitting curves. (b) Adsorption isotherm fitting curves. (c) Intraparticle diffusion model curves. (d) Regeneration performance of ACDW [adsorbent dose, 0.01 g; initial Cr(VI) concentration, 100 mg L–1; volume, 30 mL; pH, 2.0; and temperature, 25 °C].

Adsorption Isotherms and Thermodynamics

We established the adsorption isotherm model to reveal the energy change during the adsorption process and the multilayer adsorption capacity of the adsorbent. The adsorption properties of the adsorbent were evaluated using the Langmuir and Freundlich models at 298, 308, and 318 K.

The Langmuir (eq S5) and Freundlich (eq S6) models were used to fit and analyze the data (Figure b, Table S4). The results show that the fitting degree of the Langmuir model is higher than that of the Freundlich model, indicating the occurrence of a monolayer adsorption process. With the driving force of the concentration gradient, the uptake capability of Cr(VI) at each temperature was gradually improved by the increasing initial concentration. Moreover, an uptrend on the Cr(VI) adsorption was found in the temperature range of 298 to 318 K, which is clearly indicative of its endothermic nature. At 298 K, the maximum adsorption capacity and adsorption per unit area of ACDW for Cr(VI) calculated using the Langmuir model are 294.86 mg g–1 and 0.30 mg m2, respectively, surpassing those of the previously reported biochar adsorbents (Table S5).

Table S6 shows the results of the adsorption thermodynamics. The values of the Gibbs free energy change (ΔG) are all negative, and the absolute value increased with the increasing temperature, suggesting that the adsorption process is spontaneous. Besides, a higher temperature is more conducive to the adsorption process. The ΔG value between −20 and 0 kJ mol–1 typically represents a physical adsorption, whereas the value between −400 and −80 kJ mol–1 indicates a chemical adsorption. The ΔG value in this experiment was estimated to be between −21.62 and −25.733 kJ mol–1, indicating that the adsorption process is the result of the combined effect of two adsorption processes. The positive ΔS value proved that the order of the liquid decreases and the degree of disorder increases after reaching the adsorption equilibrium, which may be related to the temperature increase after the adsorption process.[42]

Adsorption Diffusion Model

The factors controlling adsorption and diffusion were discussed herein using an intraparticle diffusion model, and the corresponding results of the Cr(VI) adsorption data at different initial Cr(VI) concentrations are shown in Figure c and Table S7.[43] The fitting data with the intraparticle diffusion model imply that this adsorption process comprised more than two steps. It is clear that the fitting curves reflect the two-stage external diffusion, followed by the intraparticle diffusion of Cr(VI) onto ACDW.[44] The multilinearity of the plots indicates that two or more steps (multistep adsorption) could have occurred in the Cr(VI) adsorption on ACDW. Table S7 presents the intraparticle rate constants and correlation coefficients. The first stage (0–180 min), corresponding to the interface diffusion, shows the largest slope, which is suggestive of the highest adsorption rate caused by the high solution concentration and the large mass-transfer driving force. In addition, the increase in the initial pollutant concentration leads to an increase in the adsorption rate and K value. The second stage (180–720 min) is controlled by the membrane and intraparticle diffusion, which shows an adsorption saturation process caused by the increasing internal diffusion resistance. Finally, the absorption equilibrium is achieved at the third stage (720–1440 min), manifesting that the adsorption rate is equal to the desorption.

Adsorption Mechanism

The element mapping images of ACDW after Cr(VI) adsorption experiments display a uniform distribution of the C, O, and Cr elements (Figure a). Besides, the energy-dispersive X-ray spectroscopy (EDS) pattern (Figure b) also implies the existence of Cr with a weight ratio of 45.14% and an atomic ratio of 17.81% in the state of various valences, further demonstrating the excellent adsorption property of ACDW and successful reduction of Cr(VI) to Cr(III). To better understand the possible adsorption mechanism, X-ray photoelectron spectroscopy (XPS) was conducted to investigate the characteristic differences existing in the chemical ingredients of ACDW before and after the adsorption experiment. The XPS survey spectra of ACDW (Figure S6) clearly depict that the additional intense signal associated with Cr 2p was detected after adsorbing Cr in the aqueous solution, which is consistent with the EDS results (Figure b). The Cr 2p core-level spectrum (Figure c) illustrates that the characteristic spin–orbit peaks located at 576.38, 580.93, and 586.36 eV represent Cr(III), whereas those located at 588.86 and 577.79 eV correspond to Cr(VI), suggesting that partial of Cr(VI) ions were reduced into low-valence Cr(III) due to the electronegative oxygen-containing groups that are capable of donating electrons for reduction reaction.[17,44] In addition, the XPS analysis shows that about 69.73% of the Cr species bound to the ACDW surface existing in the form of Cr(III), whereas the rest of the ingredients are Cr(VI). The O 1s spectra (Figure d,e) imply that abundant oxygen-containing functional groups were introduced on the ACDW surface. The split peak of OI located at 531.07–531.16 eV represents carbonyl oxygen (O=C), such as lactone and the quinone structure. The peak labeled as OII located at 532.29–532.64 eV can be ascribed to the oxygen atoms of the carboxyl (COOH) and hydroxyl groups (−OH), and the peak labeled as OIII centered at 533.5–534.8 eV is associated with the inner ether (C–O), which is consistent with the result of FT-IR.[45−48]Figure e displays an obvious decrease in the OII peak and a binding energy shift after the Cr uptake, which is indicative of the consumption of the carboxyl and hydroxyl groups for the complexation between ACDW and Cr ions and the formation of complex compounds resulting from the interaction between the lone pair electrons of the these groups and Cr ions.[49]

Figure 6

Characterizations of ACDW before and after Cr(VI) adsorption. (a) Elemental mapping images of C, O, and Cr and (b) EDS spectra for ACDW after Cr(VI) adsorption. (c) Cr 2p XPS spectrum of ACDW after Cr(VI) adsorption. O 1s spectra of ACDW (d) before and (e) after Cr(VI) uptake respectively. (f–k) Mapping diagrams of the electrostatic potential on the electron isopycnic surface.

To better understand the adsorption behavior of ACDW, we conducted a theory calculation based on the DFT (the details of the DFT calculation are shown in S3, Supporting Information) and illustrated the mapping diagrams of the electrostatic potential on the electronic isodensity surface for the simplified molecular models (Figure f–k). The surface electrostatic potential of biochar can directly describe the ability of donating electrons, which greatly affects its adsorption behavior on the chromium ions in the aqueous solution.[50] The introduction of electronegative phenolic hydroxyl, carboxyl, lactone, ether, and semiquinone enhances the electrostatic potential of the graphene structure, thereby contributing to the capture of HCrO4– or Cr2O72– on the ACDW surface by the electrostatic attraction. Moreover, these functional groups allow the contribution of more electrons for the reduction reaction of Cr(VI) to Cr(III).

Given the above systemic analysis, we put forward the conceivable mechanism of the Cr(VI) adsorption on ACDW. Figure manifests the intuitionistic schematic of the Cr adsorption mechanism. In the initial stage, the Cr(VI) ions reach the interface between graphite carbon and the aqueous solution through a pore filling effect that is induced from a fast passive capillary absorption with a low flow resistance. Compared with CW, CDW with a multilayered structure and increasing micropores can reserve more Cr-containing aqueous mediums and provide more active sites available for adsorption. The subsequent KOH activation further upgrades such capability and endows ACDW with abundant oxygen-containing groups. Therefore, the ACDW is equipped with numerous positive charges and then absorbs Cr2O72–/CrO42– anions to the reaction interface as much as possible via the electrostatic interaction. Simultaneously, the oxygen groups of −OH and −COOH donate sufficient electron pairs for the complexation reaction and coordinate with Cr ions to form complex compounds.[17,51] On the other hand, a large number of electrons provided from −OH and −COOH of ACDW facilitate the reduction of Cr(VI) to Cr(III), and the reduced products of Cr(III) are immobilized on ACDW via intermolecular force.

Figure 7

Schematic of the conceivable mechanism of Cr(VI) adsorption on ACDW.

Regeneration and Cyclic Performance

It is vital to ensure desorption for realizing the cyclic utilization of ACDW. Thus, the ACDW after the uptake of Cr(VI) was desorbed with 2.0 mol L–1 NaOH. As shown in Figure d, the removal efficiency of the adsorbent was 95.43% after the first adsorption–desorption cycle. It was still maintained at 84.82% after three cycles. After the fifth adsorption treatment, the removal efficiency decreased sharply to 59.82%, which could be put down to two reasons: (1) the pore of the ACDW was blocked in the process of adsorption and desorption and (2) the complex formed between the functional groups and Cr(VI) cannot be desorbed during the adsorption process, resulting in the decrease of surface functional groups. Overall, the results suggested ACDW to be a high-performance adsorbent with reliable reusability for practical applications.

Experimental Section

Materials and Reagents

PW is derived from the remains of forestry processing, that is, poplar wood sawdust. K2Cr2O7, HCl, KOH, and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Na2SO3, H2O2, and H2SO4 were purchased from Tianjin Guangfu Fine Chemical Industry Research Institute, China. All chemicals were of analytical purity and used without any further purification. The Cr(VI) model solution with different concentrations was prepared by dissolving K2Cr2O7 in deionized water.

Preparation of DW

DW fabrication was conducted according to the following procedures:[52] PW saw dust was immersed in a mixed aqueous solution of 1.25 M sodium hydroxide and 0.2 M sodium sulfite and cooked at 100 °C for 5 h; the samples were rinsed with deionized water several times to remove the chemical residues; the obtained samples were then treated by 3% H2O2 solution at 80 °C for 2 h for removing residual lignin, and finally, the obtained white DW samples were dried by vacuum freezing for 24 h.

Preparation of CDW

The CDW was prepared by pyrolyzing DW in a tubular furnace in a N2 atmosphere with a yield of 29.4%. Specifically, the pyrolysis temperature was increased from room temperature to 700 °C and maintained for 2 h with a 5 °C min–1 heating rate and a 100 sccm N2 flow rate. In contrast, CW was prepared under the same pyrolysis conditions with a yield of 27.3%.

Preparation of ACDW

The typical high-temperature activation process for fabricating ACDW was performed as follows: at the beginning, CDW and KOH with a 1:4 mass ratio were ground uniformly for 5 min using an agate mortar. Moderate deionized water was then added to the hybrid to dissolve the KOH and stirred overnight. Subsequently, the CDW/KOH mixture was dried and heated up to 700 °C for 2 h with a 5 °C min–1 heating rate and a 100 sccm N2 flow rate. Finally, the obtained ACDW was washed numerous times with HCl (6%) and deionized water, and its yield is 40.3%. As a control, ACW was prepared under the same pyrolysis conditions, and the yield is 39.5%.

Batch Adsorption Experiments

The Cr(VI) adsorption behaviors of the samples were evaluated by performing batch adsorption experiments in a 50 mL Erlenmeyer flask, in which a specific amount of the adsorbent was added into a 30 mL Cr(VI)-containing solution. Various Cr(VI) concentrations (40–200 mg L–1) in the solutions were calibrated using K2Cr2O7, and 0.1 M HCl or 0.1 M NaOH solution was used to adjust the pH of the solution. The adsorption equilibrium process was performed through continuous flask shaking at 150 rpm on a rotating shaker for 1440 min. The adsorption time (0–24 h), initial concentration (40–200 mg L–1), pH (1–8), and amount of the adsorbent as the variables were discussed. As a cyclic adsorption experiment, a 30 mL solution with a Cr(VI) concentration of 100 mg L–1 was treated using ACDW (0.01 g) at 25 °C and pH 2 for 24 h. Then, it was filtered and desorbed using NaOH (2 mol L–1) at 25 °C for 12 h and cleaned thoroughly with deionized water to remove any residual solution. Finally, the ACDW were dried at 105 °C for 3 h.

The adsorption capacity and Cr(VI) removal rate are calculated followswhere qe refers to the adsorption capacity (mg g–1), E is the removal rate (%), and C0 and C represent the initial concentration (mg L–1) and adsorption solution concentration (mg L–1), respectively, determined using a UV spectrophotometer (UV-2450); V (L) and m (g) are the solution volume and the adsorbent mass, respectively. The adsorption kinetics data were fitted with a pseudo-first-order kinetics model, a pseudo-second-order kinetic model, an intraparticle diffusion model, and a thermodynamics model. The adsorption isotherms were fitted by the Langmuir and Freundlich models. The Supporting Information provides a detailed description of these models.

The morphology of the samples was carried out by TEM (FEI-Talos F200S, USA) and SEM (Hitachi, S4800) equipped with an EDX detector for elemental analysis. The chemical components of PW and DW were measured through FT-IR (PerkinElmer, USA) in the wavenumber range of 600–4000 cm–1. XRD was implemented on a Bruker D8 Advance TXS XRD instrument with S5 Cu Kα (target) radiation (λ = 1.5418 Å) at a scan rate (2θ) of 4° min–1 and a scan range from 5 to 85°. XPS measurements were performed using a Thermo Scientific ESCALAB250 spectrometer (Thermo VG, USA) with a dual Al Kα X-ray source. A Raman spectrometer (LabRAM HR Evolution, Horiba, JPN) test was carried out in the spectral range of 0–2000 cm–1. The pore properties of the carbon materials were studied by N2 adsorption–desorption experiment measurements at 77 K (3H-2000PS2 unit, Beishide Instrument S&T Co., Ltd.).

Conclusions

In summary, we have successfully fabricated a novel porosity-enhanced biochar with oxygen-rich functional groups from low-cost wood sawdust using a versatile and efficiently integrated strategy of delignification and subsequent carbonization and KOH activation. The resultant CDW shows a multilayered structure with increasing micropores induced by the extractions of lignin and hemicellulose from PW. The further KOH activation renders ACDW a more developed microporous/mesoporous architecture, a high SSA of 970.52 m2 g–1, and abundant oxygen-containing functional groups, resulting in a remarkable Cr(VI) adsorption capacity of 294.86 mg g–1 in solution. These results are better fitted with the Langmuir isotherm, the pseudo-second-order kinetic model, and the adsorption diffusion model in batch experiments. Moreover, according to the studies on the zeta potential, EDS, XPS, and adsorption experiments for ACDW, it was concluded that the synergistic contributions of the capillary force, electrostatic attraction, chemical complexation, and reduction action facilitate the Cr(VI) capture by ACDW. The Cr(VI) removal efficiency was constantly higher than 70% during four recycle tests, indicating the effective reuse performance of ACDW. These results highlight the significant potential of ACDW as an excellent adsorbent for Cr(VI) removal from aqueous solutions.