Abstract
Phenol and p-nitrophenol (PNP) are priority pollutants widely present in wastewater. Developing superior or low-cost sorbents for their removal would be of great benefit. Here, corncob residues (CCR) were converted to hydrochars via hydrothermal carbonization (HTC) and further upgraded to carbon materials by thermal activation in an N2 atmosphere. The influence of HTC conditions including the temperature, residence time, and CCR/water weight ratio on the material properties and their performance for removing phenol and PNP from water were investigated and compared with those that were obtained from pyrochar (directly pyrolyzed CCR). Hydrochars showed lower adsorption capacities for phenols than pyrochar. The initial hydrothermal treatment at 220 °C and 2 h resulted in an improved porosity and 4- to 5-fold higher adsorption capacities for phenol and PNP compared with the pyrochar. However, hydrochars prepared at 250 °C or with a prolonged residence time (4 and 6 h) could not be upgraded to high performance carbon materials by thermal activation. The adsorption isotherms of both phenols on the best performance material were well correlated by the Sips model.
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Exceptional Adsorption of Phenol and p-Nitrophenol from Water on Carbon Materials Prepared via Hydrothermal Carbonization of Corncob Residues
Baojian Liu,a,b,* Yin Li,a,b Xikun Gai,a,b Ruiqin Yang,a,b Jianwei Mao,a,b and Shengdao Shan b,c,*
Phenol and p-nitrophenol (PNP) are priority pollutants widely present in wastewater. Developing superior or low-cost sorbents for their removal would be of great benefit. Here, corncob residues (CCR) were converted to hydrochars via hydrothermal carbonization (HTC) and further upgraded to carbon materials by thermal activation in an N2 atmosphere. The influence of HTC conditions including the temperature, residence time, and CCR/water weight ratio on the material properties and their performance for removing phenol and PNP from water were investigated and compared with those that were obtained from pyrochar (directly pyrolyzed CCR). Hydrochars showed lower adsorption capacities for phenols than pyrochar. The initial hydrothermal treatment at 220 °C and 2 h resulted in an improved porosity and 4- to 5-fold higher adsorption capacities for phenol and PNP compared with the pyrochar. However, hydrochars prepared at 250 °C or with a prolonged residence time (4 and 6 h) could not be upgraded to high performance carbon materials by thermal activation. The adsorption isotherms of both phenols on the best performance material were well correlated by the Sips model.
Keywords: Hydrothermal carbonization (HTC); Corncob residues (CCR); Phenol; p-Nitrophenol (PNP); Adsorption; Carbon material
Contact information: a: Zhejiang Provincial Key Lab for Biological and Chemical Processing Technologies of Farm Products, School of Biological and Chemical Engineering; b: Zhejiang Provincial Collaborative Innovation Center of Agricultural BioResources Biochemical Manufacturing; and c: Key Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province, School of Civil Engineering and Architecture, Zhejiang University of Science and Technology, 318 Liuhe Road, Hangzhou, Zhejiang 310023 China; *Corresponding authors: baojian_liu@126.com (B. Liu) and shanshd@vip.sina.com (S. Shan)
INTRODUCTION
Phenol and p-nitrophenol (PNP) are priority pollutants widely present in many industrial effluents including but not limited to the petrochemical, pharmaceutical, coal, pulp and paper, steel, and food industries (Zhang et al. 2014). The concentrations of phenol and its derivatives in wastewater should be less than 1 ppm (mg/kg) (Busca et al. 2008). Therefore, the removal of phenols from water has attracted considerable attention, but it is still challenging due to their high stability and water solubility. Adsorption is a promising method for the removal of phenols, especially at low concentrations. Activated carbons (Dabrowski et al. 2005; Kumar et al. 2007) and polymeric resins (Pan et al. 2007; Huang et al. 2009; Lin and Juang 2009) are the most widely used adsorbents. Superior adsorbents with higher selectivity or low-cost adsorbents with satisfactory performance are still needed.
Biomass resources are plentiful, renewable, and not competitive to human and animal consumption. Making value-added products from agricultural lignocellulosic biomass waste is attractive in terms of cost and pollution management. It is economical to use agricultural waste biomass as precursors for carbon materials that remove phenols from wastewater. To date, various techniques have been applied to biomass treatment and conversion, including combustion, pyrolysis, and gasification.
Hydrothermal carbonization (HTC) uses water as the reaction medium to reduce the oxygen and hydrogen content of biomass at low temperature (150 to 350 °C) and self-generated pressure. Compared with thermal pyrolysis, HTC has the advantages of high conversion efficiency, less energy required by avoiding drying wet feedstocks, and relatively low operating temperatures (Hoekman et al. 2011; Guo et al. 2015). HTC can be carried out very quickly and is suited for a broader range of feedstocks because the initial moisture level is not an issue. HTC was recently used to fabricate functional carbon-based materials with applications in adsorption and separation science through processing waste biomass. Various biomasses have been converted to carbon materials through the HTC method (Titirici et al. 2007; Sevilla and Fuertes 2009a, 2009b; Calucci et al. 2012; Gao et al. 2013; Budai et al. 2014). HTC hydrochars normally need to be further upgraded to functional carbon materials via physical activation using an activating agent such as air, CO2, or water steam (Román et al. 2013) or KOH chemical activation (Regmi et al. 2012; Falco et al. 2013a).
Corncob residue (CCR) is a waste lignocellulose. It has been exploited using HTC for various applications, such as high heating value biochar (Zhang et al. 2015), carbon precursors (Falco et al. 2013b), and soil fertility (Budai et al. 2014). In this work, HTC was used to convert CCR into hydrochars, and the hydrochars were further activated under N2 by thermal heating to prepare carbon materials. The hydrochars and carbon materials were characterized. Their efficiency in the adsorptive removal of phenol and PNP was compared with the CCR pyrochar prepared by direct thermal pyrolysis without hydrothermal treatment.
EXPERIMENTAL
Materials
All chemicals used in this work were purchased from commercial suppliers and used without further purification. Absolute ethanol (AR grade), phenol (99.5%), and PNP (99.5%) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). HPLC-grade methanol was supplied by Tedia Company Inc. (Fairfield, OH, USA). CCR was obtained from Zhejiang province, China. Prior to HTC, CCR was dried at ambient temperature, ground, and sieved. The particle size of 4 to 8 mesh (2.36 to 4.75 mm) was used to produce hydrochars.
Hydrothermal Carbonization Experiments
Either 5 or 10 g of CCR was weighed into a 100-mL PPL-lined stainless steel autoclave and then mixed with 40 mL of deionized water. The reactor was sealed airtight, heated from ambient temperature to the desired temperature, and maintained for varied times in an electric oven. The temperatures applied were 220 and 250 °C. After the preset residence time of 2, 4, and 6 h, the vessel was cooled to room temperature. Residence time was defined as the time the reactor was held at the desired reaction temperature, excluding preheating and cooling time. The hydrochar was recovered by centrifugation and washed twice with 25 mL of water, washed twice with 25 mL of ethanol, and then dried under vacuum at 100 °C overnight. The resulting hydrochar was named Hydrochar_X_Y_Z, where X, Y, and Z represented the HTC temperature (°C), residence time (h), and the solid/liquid weight ratio, respectively. The mass yield of hydrochars was calculated by the following equation:
(1)
Hydrochar Activation
In the thermal activation process, the dried hydrochars were placed in quartz boats within a tubular furnace and heated from ambient temperature to 750 °C under flowing nitrogen gas. An initial ramp was used to reach 400 °C in 90 min, and a second ramp was applied to reach 750 °C in 120 min. The sample was maintained at 750 °C for 2 h. Samples were then cooled to ambient conditions and removed from the oven. The as-synthesized carbon materials were denoted as Activated Hydrochar_X_Y, where X and Y represented the HTC temperature (°C) and HTC residence time (h), respectively. For comparison, raw CCR was also placed in a quartz boat within the tubular furnace and directly pyrolyzed using the same heating ramps, and the resulting carbon material was denoted as pyrochar.
Characterization Methods
The elemental analysis (C, H, N, and S) was performed using a Vario MICRO cube analyzer (Elementar, Germany), and the oxygen content was calculated by the balance. Fourier transform infrared (FTIR) spectra of the materials in powder form were recorded from 400 to 4000 cm–1 at a resolution of 4 cm–1 using a Bruker Vertex 70 FTIR spectrometer (Bruker, Germany) with the KBr disc method. The activated hydrochars and pyrochar were determined by the N2 adsorption-desorption isotherms at -196 °C using Autosorb-iQ (Quantachrome Instruments, Boynton Beach, FL, USA). Prior to adsorption analysis, the samples were outgassed at 300 °C for 6 h. The specific surface areas were evaluated using the Brunauer-Emmett-Teller (BET) method.
Adsorption Experiments
The batch adsorption experiments were conducted at 30 °C using a temperature-controlled shaker. Prior to adsorption, the adsorbents were dried at 100 °C overnight, and 100 to 1000 mg/L phenol and PNP solutions were prepared using deionized water. Approximately 0.02 g adsorbents were mixed with 15 mL of solutions containing different concentrations of phenol or PNP in a set of 20-mL vials. To ensure sufficient mixing, the vials were placed horizontally in the shaker and shaken at 200 rpm for 24 h at 30 °C, which was sufficient to reach adsorption equilibrium. The solutions were then passed through a syringe filter (PTFE, hydrophilic, 0.25 μm).
The solute concentrations were analyzed by a Waters e2695 high-performance liquid chromatography (HPLC) apparatus (Milford, MA, USA) equipped with a Waters 2489 UV-Vis detector and an ODS-C18 column. The mobile phase was a methanol/water mixture (60:40, v/v), and the flow rate was 1.0 mL/min. The column was maintained at 50 °C, and phenol and PNP were detected at 270 nm. The amount adsorbed was calculated by the material balance:
(2)
where qe is the amount adsorbed (mg/g) at equilibrium, V is the volume of the liquid solution (L), C0 and Ce are the initial and equilibrium solute concentrations (mg/L), and m is the mass of adsorbent (g).
RESULTS AND DISCUSSION
Yield of Hydrochars
The CCR was submerged in water to ensure that the hydrothermal reaction occurred during the whole reaction process. A maximum of 10 g CCR was loaded when 40 mL of water was charged in the PPL cup; solid/liquid mass ratios of 1:4 and 1:8 were applied. HTC of CCR was carried out in hot compressed water at two different temperatures of 220 and 250 °C, and the residence times were set to 2, 4, and 6 h. The yields of hydrochars generated at different HTC conditions are compared with the yield of pyrochar in Fig. 1. As expected, product yields were higher in the hydrochars (26.7 to 45.2 wt%) than in the pyrochar (22 wt%). With increasing residence time from 2 to 6 h, when the solid/liquid ratio was 1:4, the hydrochar yield decreased from 45.2 to 35.9% at 220 °C, and from 34.8 to 31.8% at 250 °C. The influence of the residence time on yield at both temperatures and solid/liquid ratios was similar, and prolonged residence time led to a decreased yield. Meanwhile, the yields of hydrochars at 250 °C at the same residence time were lower than those of hydrochars at 220 °C. The higher solid/liquid ratio led to a higher yield of hydrochar; a small but noticeable decrease in mass recovery was observed when the solid/liquid ratio was decreased from 1:4 to 1:8. Based on these results, the CCR/water weight ratio of 1:4 was used for subsequent experiments.
Fig. 1. Solid mass yield of the pyrochar and hydrochars prepared in different HTC conditions
Adsorption Properties of Hydrochars
Preliminary adsorption tests were performed to evaluate the removal of phenol and PNP by the hydrochars and pyrochar; the adsorption isotherms are presented in Fig. 2. The adsorption capacities for both phenols followed the order of Pyrochar > Hydrochar_250_2_1/4 > Hydrochar_220_2_1/4. Hydrochar generated at 250 °C had higher adsorption than its counterpart at 220 °C, suggesting that the more carbonized hydrochar exhibits higher adsorption. However, both hydrochars exhibited lower adsorption capacities than the pyrochar, especially for phenol. Therefore, hydrochars prepared from CCR need further activation to increase their adsorption.
Fig. 2. The adsorption capacities of (a) phenol and (b) PNP on hydrochars and pyrochar from water at 30 °C
Adsorption Properties of Activated Hydrochars
KOH and phosphoric acid are known to be very effective activating agents for carbon materials, but it is environmentally harmful to use large amounts of caustic agents (Falco et al. 2013a; Qi et al. 2014). In this work, thermal activation under N2 atmosphere, an environment-benign method, was used to enhance the porosity and adsorption performance of HTC hydrochars. The temperature of physical activation is usually in the range of 750 to 900 °C (González et al. 2006). In order to avoid excessive degradation of the organic substrate and to retain some of the oxygen-containing functional groups, 750 °C was applied in this work. To understand the effect of the HTC residence time on the adsorption properties, hydrochars prepared at 2, 4, and 6 h were activated and used for the adsorption of phenol and PNP from water (Fig. 3). The adsorption capacities of phenol and PNP on Activated Hydrochar_220_2 were exceptional: superior to pyrochar, and greatly exceeding those of other activated hydrochars. Activated Hydrochar_220_6 and Activated Hydrochar_250_6 had almost identical adsorption capacities for phenol and PNP, which implied that for activated hydrochars, hydrothermal pretreatment temperature at 220 or 250 °C has little effect. Hydrochars obtained at longer residence times of 4 and 6 h adsorbed considerably less phenols than that obtained at 2 h. Prolonged HTC residence time showed a negative effect. It is unexpected that hydrochars with higher degrees of carbonization adsorbed less phenol and PNP after thermal activation than the pyrochar.
The further activation of Hydrochar_220_2 via thermal pyrolysis under an inert N2 atmosphere increased the adsorption of phenol and PNP from water. As shown in Fig. 3, there was a 4 to 5-fold improvement in adsorption compared with the directly pyrolyzed CCR (calcination alone). Because Hydrochar_220_2 was preferable as a precursor for conversion to functional carbon material, this material is a good candidate for an adsorbent to remove phenols from water streams.
Fig. 3. The adsorption capacities of (a) phenol and (b) PNP on activated hydrochars and pyrochar from water at 30 °C
Materials Characterization
The adsorption properties of different hydrochars, activated hydrochars and pyrochar were quite different. To understand the experimental observations, adsorbents were characterized by the elemental analysis, FTIR, and N2 adsorption/desorption.
The elemental compositions of different adsorbents are present in Table 1. As expected, pyrochar has higher C content and lower O content than the hydrochars. Further thermal activation of the hydrochars increased the C content to a higher value than the pyrochar and decreased the O content to a lower value than the pyrochar. It is interesting to note that all the four activated hydrochars exhibit similar elemental compositions.
The primary differences in the FTIR spectra for the pyrochar, hydrochars, and activated hydrochars in the wavenumber range of 4000 to 400 cm–1 are shown in Fig. 4. The main identified peaks include –OH, aromatic C=O, aromatic C=C, and C–O. The types of functional groups are normally the same, but the number of functional groups are different when considering the different elemental compositions. The hydrochars have higher C=O, C–O, and –OH oxygen-containing functional groups than the pyrochar.
Table 1. Elemental Analysis of Pyrochar and Hydrochars Prepared from Corncob Residues
Fig. 4. FTIR spectra of (a) pyrochar and hydrochars (b) activated hydrochars and pyrochar
The N2 adsorption/desorption isotherms of pyrochar and several activated hydrochars at -196 °C are shown in Fig. 5. The specific surface areas, pore volumes, and average pore diameters of the samples are summarized in Table 2. When CCR was pyrolyzed directly at 750 °C without previous hydrothermal treatment, the pyrochar exhibited a moderate porosity with a BET surface area of 481 m2/g and a total pore volume of 0.273 cm3/g.
Fig. 5. Nitrogen adsorption (solid) and desorption (open) isotherms at –196 °C for activated hydrochars and pyrochar
Hydrochars produced via HTC usually have small pore volumes and low surface areas (< 50 m2/g) (Sevilla and Fuertes 2009a,b; Román et al. 2013). Lignocellulosic biomass-derived HTC hydrochars are good precursors for the preparation of porous carbonaceous materials via physical activation of calcination under inert atmosphere. For the hydrochar prepared at 220 °C and 2 h residence time, the post-synthesis carbonization at moderate temperature (750 °C) further increased the surface area and pore volume to 666.7 m2/g and 0.399 cm3/g, respectively. However, the N2 adsorption/desorption measurements revealed that both hydrochars prepared at 220 and 250 °C under 6 h residence time had very low porosities even after thermal activation, with BET surface areas of less than 25 m2/g. According to the IUPAC classification (Rouquerolet al. 1994), Activated Hydrochar_220_2 and pyrochar showed Type I isotherm behavior, which is characteristic of microporous materials. The other two activated hydrochars showed typical Type II isotherm behavior and were non-porous materials, which may explain their low adsorption performance for both phenols. Compared with pyrochar, Activated Hydrochar_220_2 exhibited improved porosity and dramatically higher adsorption capacities for phenol and PNP. Therefore, hydrochar prepared at 2 h has a higher tendency to develop micropores during thermal activation than hydrochars prepared at a longer residence time. Therefore, the exceptional adsorption properties of Activated Hydrochar_220_2 for phenols may be attributed to the combination of porosity and surface groups.
Table 2. Physical Properties of Activated Hydrochars and Pyrochar Prepared from Corncob Residues
Adsorption Isotherms
The most commonly used Langmuir, Freundlich, and Sips isotherm models were tested to correlate the experimental equilibrium data of phenol and PNP on the best performance material, Activated Hydrochar_220_2. The Freundlich model is given by Eq. 3. The Sips and Langmuir (n= 1) models take the same form as Eq. 4,
(3)
(4)
where Kf is an indication of the adsorbent capacity, 1/nf is a measure of the surface heterogeneity, ranging between 0 and 1; qm is the maximum adsorption capacity (mg/g), b is the adsorption equilibrium constant (L/mg), and n is the Sips parameter.
The best-fit curves are shown in Fig. 6, and the resulting parameter values are presented in Table 3. It is evident that both the Langmuir and Freundlich models failed to correlate the adsorption isotherms of both phenols, resulting in considerable deviation. The adsorption isotherms of both phenols were correlated by the three-parameter Sips equation with excellent fits (R2 > 0.999). The adsorption capacities were tremendous, with qm values for phenol and PNP up to 216.3 and 326.8 mg/g, respectively.
Fig. 6. Adsorption isotherms of phenol and PNP on Activated Hydrochar_220_2 showing the experimental data and fitting by different isotherm models. The dotted, dashed, and solid lines are the Freundlich, Langmuir, and Sips models, respectively.
Table 3. Fitting of Various Adsorption Isotherms for the Adsorption of Phenol and PNP on Activated Hydrochar_220_2
The comparison of the adsorption capacities of phenol and PNP on the best performance material of this work with some alternative adsorbents reported in the literature are summarized in Tables 4 and 5, respectively. It is not easy to compare the adsorption performance of different adsorbents because the experimental conditions (especially the concentration range) were usually different. Here the amounts adsorbed at equilibrium concentration of 100 mg/L (or 100 mg/g adsorbed for phenol) and the maximum adsorption capacities of various adsorbents were selected for comparison. Activated Hydrochar_220_2 exhibited exceptional adsorption capacities for phenol and PNP, far exceeding most other adsorbents.
The maximum adsorption capacities of HPR, CC-1.6, RH-1.6, and NDA-701 were higher than those of Activated Hydrochar_220_2, which is attributed to the higher concentration ranges applied in the liquid phase. Activated Hydrochar_220_2 was superior or equal to the best-performing activated carbons (Filtrasorb 400 and AQ40) and only slightly lower than the activated carbon fiber. Therefore, the conjunction HTC and thermal activation processes are a promising approach to upgrade CCR to a high-quality carbon material under mild conditions.
Table 4. Comparison of the Adsorption Capacities of Phenol from Water on Various Adsorbents
CONCLUSIONS
- Corncob residue (CCR) hydrochar prepared by hydrothermal carbonization (HTC) at 220 °C and 2 h was further activated at 750 °C using thermal pyrolysis under N2 atmosphere to fabricate a porous carbon material. This material exhibited very promising performance towards the adsorption of phenol and p-nitrophenol (PNP) from water, with 4- to 5-fold improvement of the adsorption capacity compared with the directly pyrolyzed CCR (calcination alone).
- The effect of the HTC conditions on hydrochar yields was examined at two different temperatures (220 and 250 °C), three residence times (2, 4, and 6 h), and two CCR/water weight ratios (1:4 and 1:8). With increasing temperature and time, the HTC hydrochar yield decreased. A higher solid/liquid ratio resulted in higher HTC hydrochar yield. The hydrochars exhibited lower adsorption performance than the directly pyrolyzed CCR.
- The HTC residence time was extremely important in the porosity development and adsorption properties. HTC hydrochars with higher degree of carbonization (4 and 6 h residence time) could not be further upgraded to functional carbon materials via thermal activation.
- This paper illustrates that HTC hydrochars of lignocellulosic biomass are good precursors to produce carbon materials with particular application for water purification. The conjunction of HTC and thermal activation processes is a promising way to convert CCR to high-quality carbon materials under mild conditions.
Table 5. Comparison of the Adsorption Capacities of PNP from Water on Various Adsorbents
ACKNOWLEDGMENTS
The authors are grateful for the support of the Ministry of Science and Technology of China (2014DFE90040), the Science Technology Department of Zhejiang Province, China (2015C33006), and the National Natural Science Foundation of China (21576242).
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Article submitted: February 24, 2016; Peer review completed: April 23, 2016; Revised version received: July 14, 2016; Accepted: July 15, 2016; Published: July 21, 2016.
DOI: 10.15376/biores.11.3.7566-7579