Preparation, characterization, and efficient chromium (VI) adsorption of phosphoric acid activated carbon from furfural residue: an industrial waste

With the development of economic globalization, the problem of water pollution has gradually come into people's sight, among which industrial waste water is the most serious (Khamis et al. 2009). Chromium ion pollution is an important aspect of industrial waste water pollution. Some studies have found that chromium ion pollution is serious in many river basins in the world (Islam et al. 2015; Scheuhammer et al. 2015). The U.S. Environmental Protection Agency (USEPA) has classified chromium (VI) compounds as class A pollutants, which can cause skin problems, gastrointestinal diseases and high carcinogenic risk (Costa & Klein 2008; Desai et al. 2008; Wang et al. 2020). Therefore, it is meaningful to investigate the removal of chromium ions in waste water. Activated carbon (AC) adsorption is widely used in the removal of heavy metal ions in waste water. The specific surface area of AC is large, and the micropores are particularly developed, which helps to improve the adsorption capacity (Kumar & Mohan Jena 2015). However, commercial AC is usually expensive. In recent years, lots of researches have focused on the preparation of AC to pursue higher adsorption efficiency and lower production price. Agricultural by-products, with the advantages of low cost and renewable nature, have been used as the raw material for the production of AC, such as bamboo leaves, sugarcane bagasse, rice straw, peanut shells, herb residue, reedy grass leaves, jackfruit peel, and pinewood sawdust (Prahas et al. 2008; Xu et al. 2014; Marzbali et al. 2016; Ghosh et al. 2017; Yi et al. 2019).

Furfural residue (FR) (Zhu et al. 2016) is a waste by-product produced from commercial furfural production process. It is reported that approximately 13 tons of furfural residue will be produced per ton of furfural production. The annual emission of FR is huge, having reached 2.4 million to 3 million tons in China. This large amount of FR is a great threat to the environment (Xing et al. 2015; Wang et al. 2016). Some furfural factories burn FR as a fuel (Ma et al. 2014). However, FR contains 14% to 15% acid (Zhu et al. 2016), which will corrode the equipment. If this part of acid could be utilized properly, it will be beneficial for enhancing the economics of industrial furfural production.

For preparing AC, physical activation and chemical activation are common methods. Compared with physical activation, chemical activation has the advantages of low activation temperature, short activation time and low energy consumption. Among chemical activation methods, phosphoric acid activation is a hotspot in recent years. Phosphoric acid is a medium-strength acid with dehydrating properties, which enhances the de-polymerization, dehydration, and redistribution of constituent biopolymers (Danish & Ahmad 2018). In the presence of phosphoric acid, pyrolytic decomposition of the macromolecules composing the biomass is improved, including cellulose, lignin, and hemicellulose. In addition, phosphoric acid activation is beneficial for the formation of the cross-linked structure with both micropores and mesopores (Budinova et al. 2006; Heidari et al. 2014). FR mainly contains cellulose and lignin, which could be used as a precursor for AC production. Using phosphoric acid activation can make full use of the acidity of FR without washing or neutralization, which is conductive to reduce the cost of AC production.

Therefore, in this study, phosphoric acid activation was used to activate fresh FR. The effect of different conditions on AC preparation was explored, such as phosphoric acid dosages, activation temperatures, activation times, and heating rates. The AC prepared under the optimum conditions was characterized by elemental analysis, FT-IR, SEM, N₂ physisorption, and XPS. Cr(VI) removal from the waste water was used to evaluate the adsorption performance of AC. The effects of adsorption temperature, solution pH and Cr(VI) concentration on the adsorption efficiency of AC were studied. The adsorption mechanism of Cr(VI) by AC was proposed.

Fresh furfural residue (FR) was kindly provided by Shandong Kaiyuan Group Co. (Shandong, China). The moisture content of FR was 53%, and the initial pH of FR was around 3.0 (Shi et al. 2019). FR was used as received without any treatment. Sym-Diphenylcarbazide was purchased from Shanghai Zhanyun Chemical Co., LTD. HCl, H₃PO₄, H₂SO₄, K₂Cr₂O₇, and acetone came from KeLong Chemical Reagent Factory (Chengdu, China).

10 g of dried FR was suspended in 60 g of H₃PO₄ solution (the weight ratio of H₃PO₄/FR at 0.5:1 ∼ 2:1.) under stirring at room temperature for 6 h. After impregnation, the mixture was dried at 105 °C. Subsequently, the dried mixture was transferred in a muffle furnace, semi-carbonized at 170 °C for 1 h, and carbonized at different temperature (325 ∼ 525 °C) for different holding times (0.5 ∼ 3 h), and the heating rate of the muffle furnace was investigated at 3, 8, and 15 °C/min.

After calcination, the material was quenched to increase its hardness. After filtration, the solid was ground and then boiled with 150 mL 0.1 M HCl for half an hour. The final activated carbon (AC) was washed with distilled water, dried at 105 °C for 24 h, and ground into fine powder for further use.

The element contents of FR and AC were determined by an elemental analyzer (Elementar Vario EL III, Flash Smart, Germany). The surface area and pore structure of AC were determined by a Micromeritics ASAP 2460 apparatus at 77 K. The BET surface area (S_BET) was calculated with Brunauer-Emmett-Teller (BET) method. The total volume (V_tot) was obtained by single point adsorption of N₂ at a high relative pressure (∼0.99). The micropores area (S_mic) and volume (V_mic) were determined by the t-plot method. The external area (S_ext) and volume (V_ext) was calculated from the difference between S_BET and S_mic, V_tot and V_mic, respectively. The pore size distribution was analyzed by DFT method and average pore diameter (D_P) was calculated by 4 V/S_BET. FTIR was investigated by the KBr disk method using Fourier Transform Infrared Spectroscopy (Nicolet iS50 FTIR, Thermo Fisher Scientific) (Liang et al. 2020). The FTIR spectrum was taken in a wavelength range of 400–4,000 cm⁻¹. The surface characteristics of AC were measured by scanning electron microscopy (SEM) (Apreo, FEI, Hillsboro, USA), operated at the acceleration voltage of 2.0 kV. X-ray photoelectron spectroscopy (XPS) was measured on a VG ESCALAB 250XI spectrometer (Thermo Electron) with an Al Kα (1,486 eV) X-ray source.

A certain amount of potassium dichromate was dissolved in distilled water to prepare Cr(VI) solutions of different concentrations. The effect of pH (2–5), adsorption temperature (25–45 °C), contact time (1–120 min) was investigated. The pH value of the solution was adjusted with 0.1 M HCl solution. For each adsorption experiment, 50 mg of AC was added into a 250 mL conical flask containing 40 mL potassium dichromate solution. The mixture was continually stirred in a constant temperature shaker at a fixed shaking rate of 150 rpm. After a given period of time, a certain volume of solution was taken out for centrifugation at 10,000 rpm for 3 min. The supernatant was collected and diluted for Cr(VI) concentration determination at 542 nm by using a UV-visible spectrophotometer.

The removal rate of Cr(VI) by AC can be calculated by ⁠. The adsorbed amount of Cr(VI) on AC at equilibrium is calculated by ⁠. The adsorbed amount at t time is calculated by ⁠. Where C₀ (mg/L) and C_e (mg/L) are the initial concentration and equilibrium concentration of Cr(VI) solution, respectively; m (g) is the mass of AC, V (L) is the volume of Cr(VI) solution.

The flame atomic absorption spectrophotometer (TAS-986F, China) was used to characterize the variation of Cr(VI) and Cr(III) in solution. The concentration of Cr(III) was expressed by the difference between total chromium and Cr(VI).

The effect of AC preparation conditions on Cr(VI) removal, including impregnation ratio, carbonization temperature, heating rate, and carbonization time, is shown in Figure 1. With increasing impregnation rate of H₃PO₄ and FR, the removal rate of Cr(VI) on prepared AC increased first and then decreased. The appropriate amount of H₃PO₄ was beneficial for the formation of pores, promoting the adsorption of Cr(VI) on AC. However, further increased dosage of H₃PO₄ might result in the collapse of the AC skeleton, which was not conducive to the adsorption of Cr(VI). In terms of carbonization temperature (Figure 1(b)), the AC prepared at 425 °C possessed the highest removal rate of Cr(VI). Although the removal rate of Cr(VI) by AC prepared at 325 °C was similar to that at 425 °C after 60 min adsorption, the initial 30 min removal rate of Cr(VI) by AC prepared at 425 °C was much higher. Therefore, the carbonization temperature of AC preparation was chosen as 425 °C. According to Figure 1(c), the optimum heating rate was 8 °C/min; the removal rate of Cr(VI) could be achieved at 58.9% without pH adjustment. On the premise of keeping the carbonization temperature of 425 °C and heating rate of 8 °C/min, the effect of carbonization time on Cr(VI) removal by AC was investigated and is shown in Figure 1(d). The holding time played an important role in the adsorption performance of prepared AC. With the extension of holding time, the adsorption performance of AC significantly decreased. According to the adsorption capacity of Cr(VI), the preparation condition of AC from untreated FR was as follows: impregnation rate (H₃PO₄ : FR) of 1.5, carbonization temperature of 425 °C, heating rate of 8 °C/min, and carbonization time of 30 min. At this condition, the corresponding AC was prepared and named FRAC.

A precursor with rich C content is the precondition for the preparation of AC. As shown in Table 1, the C content of FR was up to 49.79%, which was an excellent precursor for AC preparation. The S content of FR was 1.62%, which was attributed to the existence of sulfuric acid in FR material because the FR used for FRAC preparation was without any treatment. In terms of FRAC, the C content increased to 60.46%. The O content decreased from 42.25% to 33.81%. FRAC was rich in oxygen-containing groups, which was beneficial for the adsorption of Cr(VI). In addition, there was 1.15% of N in FRAC, which might originate from the N in the raw material corncob.

Figure 2(a) showed the N₂ adsorption-desorption curve of FRAC, which could be classified as an intermediate between Type I and IV according to the IUPAC. When P/P₀ was in the range of 0–0.2, the adsorbed volume increased rapidly with increasing P/P₀, which might be attributed to the filling of micropores (Zhang et al. 2004). It indicated that the prepared FRAC contained microporous structures. The hysteresis loop of type H4 at high P/P₀ (≥0.45) was observed, which was related to the capillary condensation in mesoporous structures (Brito et al. 2018). As observed in Figure 2(b), the pore size distribution of the prepared FRAC demonstrated the combination of microporous and mesoporous structures. The pore size distribution was wide with a range of pore widths from 0.4 to 50 nm. Table 2 lists the porous structural parameters of FRAC. The S_BET of FRAC was as high as 1,316.7 m²/g with 49.3% (648.8 m²/g) of micropores. The V_T of FRAC was 0.769 cm³/g with 36.4% (0.280 cm³/g) of micropores. The D_P of FRAC was 2.355 nm. Micropores could provide lots of adsorption sites, while mesopores are beneficial for the transfer of heavy metal ions (Ma et al. 2019). The S_BET of FRAC was much superior to that from raw material FR with other activation methods, such as hydrothermal microwave activation (Khushk et al. 2020), self-activation (Wang et al. 2001), water vapor activation (Yin et al. 2014), or pyrolysis auto-activation (Yin et al. 2018). In addition, BET surface area comparison of FRAC with other biomass AC using H₃PO₄ as the activator is listed in Table 3.

Figure 3 shows the FTIR spectrum of FRAC. The broad peak at 3,800–3,000 cm⁻¹ originated from the stretching vibration of –OH from carboxyls, phenols or alcohols, as well as adsorbed water in FRAC (Xu et al. 2014). The peaks at 2,927 and 2,847 cm⁻¹ were assigned to C-H stretching in the methyl and methylene groups (Zhou et al. 2020). The sharp peak at 1,610 and 1,395 cm⁻¹ was attributed to the stretching vibration of C = O and C-O in the carboxyl groups, respectively (Maneerung et al. 2016). The band in the range of 1,300–1,000 cm⁻¹ is usually attributed to C-O stretching (Benadjemia et al. 2011). However, the characteristic of phosphorus and phosphocarbonaceous compounds in H₃PO₄ activated carbons is also in this range. The peak around 1,160 cm⁻¹ originated from the stretching mode of hydrogen-bonded P = OOH groups from phosphates or polyphosphates, and the O-C stretching vibration in the P-O-C (aromatic) linkage (Xu et al. 2014). The peak at 1,110 cm⁻¹ might be attributed to P⁺-O⁻ in acid phosphate esters as well as the symmetrical vibration in polyphosphate chain P-O-P (Puziy et al. 2007). Therefore, the prepared FRAC contained a large number of surface functional groups, such as hydroxyl groups, carboxylic groups.

The morphology of FRAC was investigated by SEM. The surface microstructures of FRAC were observed under 100,000 times magnification. As shown in Figure 4(a), a highly porous structure with various sizes and shapes was observed, which was consistent with the BET results of FRAC. The pores on the surface of FRAC were mainly caused by the evaporation of activator H₃PO₄ (Demiral et al. 2008). The porous structure of FRAC was beneficial for the adsorption of Cr(VI) on FRAC. After Cr adsorption, the surface became rough and some of the pores collapsed. In addition, the EDS and Cr mapping showed that a large number of Cr ions were detected on FRAC.

XPS analysis was used to investigate the chemical composition changes of FRAC before and after Cr(VI) (FRAC-Cr) adsorption. As observed in Figure 5(a), compared to FRAC, two new peaks assigned to Cr 2p at 587.1 and 577.3 eV are clearly observed in the spectrum of FRAC-Cr, indicating that Cr was on the surface of FRAC. In order to obtain more information about the adsorption process, the high resolution spectra of C 1s, O 1s, and Cr 2p were further analyzed. As shown in Figure 5(b), the characteristic bands of C 1 s presented at 284.7, 285.3, and 287.6 eV, assigned to graphitized carbon, carbon species in alcohol, phenols, ether groups, and/or C-O-P linkage, and carbon in carbonyl groups (Xu et al. 2014). After adsorption of Cr, the peak contribution at 287.6 eV increased, while the peak contribution at 285.3 eV decreased. This might be due to the oxidation of hydroxyl groups into carbonyl groups. Figure 5(c) exhibits the high resolution spectra of O 1s. The O 1s spectra could be fitted into three peaks, at 531.4, 532.6, and 533.4 eV, which are characteristic of oxygen double bond in carboxylic groups (C = O) and non-bridging oxygen in the phosphate group (P = O), singly bonded oxygen in C-O and C-O-P groups, and –O-H, respectively (Puziy et al. 2008; Liu et al. 2018). After the adsorption of Cr(VI), the same three peaks still existed; however, the intensity of C-O and –O-H decreased. On the contrary, the intensity of the oxygen double bond increased, which might be attributed to chemisorption during Cr(VI) adsorption on FRAC. As shown in Figure 5(d), the Cr 2p peaks could be fitted into two peaks. The binding energy peaks at 577.2 and 586.8 eV were indexed as Cr(III), which indicated that the Cr(VI) was reduced and adsorbed on FRAC in the form of Cr(III). The binding energy peaks at 578.9 and 587.1 eV corresponded to Cr(VI), indicating the existence of Cr(VI).

As observed in Figure 6, the removal rate of Cr(VI) was greatly affected by the solution pH. With increasing pH in the range of 2–5, the removal rate of Cr(VI) by FRAC significantly decreased. The maximum removal rate was 99% at pH = 2. The adsorption of Cr(VI) was not only related to the surface functional groups, but also the chemistry of the Cr(VI) solution, which both varied with the solution pH. According to the solution pH, there might be different forms of hexavalent chromium ions, such as HCrO₄⁻, Cr₂O₇²⁻, CrO₄²⁻. In the pH range of 2–5, the main forms of Cr(VI) were HCrO₄⁻ and Cr₂O₇²⁻. At pH 2, HCrO₄⁻ is the dominant form of Cr(VI) (Shi et al. 2020). With increasing pH, the dominant form of chromium ions changed to Cr₂O₇²⁻. Due to the smaller ionic size, HCrO₄⁻ was more conducive to adsorption on FRAC than Cr₂O₇²⁻ (Liu et al. 2010). In addition, the highly protonated and positively charged surface at lower pH was beneficial for the adsorption of Cr(VI) anions by electrostatic attraction. With increasing pH, the protonation extent of surface groups decreased; meanwhile, there was competition between OH⁻ and Cr(VI) anions. Therefore, the removal rate significantly decreased.

FRAC was prepared from raw FR (pH around 3.0) without any treatment and H₃PO₄ was used as the activator. Therefore, the unwashed FRAC contained a large number of acids, which might be directly utilized for adjusting the initial pH of the Cr(VI) solution. The pH value of the prepared Cr(VI) solution was close to 5.2. After the addition of 50 mg unwashed FRAC, the pH value of the mixture was close to 2.7. As observed in Figure 6, the removal rate of Cr(VI) by unwashed FRAC was between pH 2.0 and pH 3.0. This indicated that unwashed FRAC could be used for Cr(VI) adsorption directly. It is a promising absorbent candidate for Cr(VI) uptake in industry since the washing procedure of AC and adjustment of Cr(VI) waste water pH can be omitted.

The effect of contact time on Cr(VI) adsorption at different initial Cr(VI) concentrations of 400, 500, and 600 mg/L is shown in Figure 7. Due to a large number of available adsorption sites in the beginning, a rapid increase was observed during the first 120 min. Subsequently, the adsorption rate became slow and finally achieved equilibrium. At initial Cr(VI) concentrations of 400, 500, and 600 mg/L, the adsorption capacity was 291.5, 334.6, and 355.3 mg/g, respectively. The increase of adsorption capacity might be due to the larger concentration gradient between Cr(VI) in solution and FRAC. Nevertheless, the removal rate of Cr(VI) decreased from 91.1% to 74.0% with Cr(VI) concentration increase from 400 to 600 mg/L.

In order to further explore the kinetics and the possible mechanism of Cr(VI) adsorption on FRAC, three commonly used kinetic models (PFO, PSO, and IPD) were employed to simulate the process. The plots of different kinetic models fitting for the adsorption of Cr(VI) on FRAC is shown in Figure S1 (Supplementary Information). The fitting parameters of different kinetic models are shown in Table 4. The regression coefficient R² and the difference between q_e and q_{e (exp)} were utilized to estimate the correlation of these models. Compared with the PSO model, the PFO model exhibited lower values of R². Moreover, the consistency between q_e and q_{e (exp)} was worse; for instance, the q_{e (exp)} was 291.5, 334.6, and 355.3 mg/g for initial Cr(VI) concentrations of 400, 500, and 600 mg/L, respectively, while it was only 142.6, 163.5, and 170.2 mg/g for PFO calculated q_e. In terms of PSO, excellent consistency existed between q_e and q_{e (exp)}, indicating that a chemical adsorption might be the rate-limiting step, which involved valence forces through sharing of electrons between Cr(VI) and FRAC (Mohan et al. 2011).

In order to explore the diffusion mechanism, the IPD model was used to fit the experimental data. The adsorption process is a multi-step process, usually containing outer diffusion (also called film diffusion), inner diffusion (also called intra-particle diffusion), and adsorption of adsorbate onto the active sites on the outer and/or inner surface of the adsorbent through strong adsorbate-adsorbent interactions equivalent to covalent bond formation or weak adsorption very similar to van der Waals forces (Singh et al. 2012). However, the last step is very quick, and is usually not the rate-limiting step. Therefore, the adsorption process is usually controlled by either the outer diffusion or inner diffusion or both. As shown in Figure S1(c), the plot of q_t versus t^0.5 exhibited multi-linearity; moreover, it did not pass through the origin. Therefore, the intra-particle diffusion was not the only rate-limiting step.

The adsorption isotherms study was studied at three different temperatures (25, 35, and 45 °C) with the initial Cr(VI) concentration of 350–1,000 mg/L at pH 2.0. Three isotherm models were used to simulate the experimental data. The model parameters of Langmuir, Freundlich, and Temkin (plot figures shown in Figure S2) are listed in Table 5. The higher R² value of the Langmuir model indicated that the adsorption of Cr(VI) on FRAC was better fitted by this model. This suggested that the adsorption occurred on a homogeneous adsorbent surface (Sun et al. 2016; Duan et al. 2017). The maximum monolayer capacity (q_m) increased from 400.0 to 454.6 mg/g. The adsorption capacity of Cr(VI) exhibited an upward trend with increasing temperature. It indicated that the adsorption process of Cr(VI) on FRAC was endothermic and higher temperature was conducive to improve the process. For evaluating the adsorption capacity of Cr(VI) on FRAC, the comparison of q_m calculated from Langmuir with other AC prepared using different raw materials or different methods is listed in Table 6. From the comparison, it can be seen that the FRAC produced in this study exhibited higher q_m, indicating an excellent adsorption efficiency for Cr(VI).

The linear fitting of lnKd and 1/T is shown in Figure S3, and the calculated thermodynamic parameters are tabulated in Table 7. The value of ΔG⁰ was negative, indicating the spontaneous nature of Cr(VI) adsorption on FRAC. Moreover, at higher temperature, the ΔG⁰ value was more negative, indicating that the adsorption process was more favorable at higher temperature. The value of ΔH⁰ was positive, further proving the endothermic nature of the adsorption process. In addition, the value of ΔS⁰ was positive, suggested that there was an increase in the randomness at the solid-liquid interface during the adsorption process (Demiral et al. 2008). Similar findings have been reported for Cr(VI) adsorption by other groups (Demiral et al. 2008; Ma et al. 2019).

According to the XPS results, there were Cr(III) ions immobilized on the surface of FRAC. In order to explore whether all Cr(III) was adsorbed by FRAC, the variation tendency of total Cr, Cr(VI) and Cr(III) in solution at initial Cr(VI) concentrations of 600 and1,000 mg/L was investigated. As shown in Figure 8, with increasing contact time, the total Cr concentration gradually declined, therefore the removal efficiency of Cr(VI) increased. The Cr(III) was detected in the solution, especially in the initial stage of adsorption. However, with the extension of adsorption time, the content of Cr(III) significantly decreased. After 48 h adsorption, the content of Cr(III) in solution was close to 0. This indicated that a proportion of Cr(VI) was reduced to Cr(III). Moreover, FRAC was also capable of adsorbing Cr(III).

The H₃PO₄-activated porous carbon with high surface area (1,316.7 m²/g) and excellent Cr(VI) adsorption capacity has been successfully prepared from the furfural residue waste. The kinetics and isotherm studies illustrated that the adsorption of Cr(VI) by FRAC was better described by the PSO and Langmuir models. According to the Langmuir model, the maximum adsorption capacity of Cr(VI) on FRAC was 454.6 mg/g. The adsorption mechanism could be summarized as electrostatic attraction, complexation, reduction, and physical adsorption. Therefore, furfural residue could be considered as a promising precursor for the production of AC. Moreover, H₃PO₄ activation could utilize the acidity of furfural residue itself. FRAC has a considerable potential for Cr(VI) removal from aqueous solution.

The authors would like to acknowledge the financial support of Qingchuang Science and Technology Program of Shandong Province University (2019KJD004), and SDUST Research Fund (2018YQJH102, S202010424057).

All relevant data are included in the paper or its Supplementary Information.