Abstract
The feedstock type and pyrolytic temperature used in producing biochar influence the characteristics of the obtained product and affect the adsorption behavior of naphthalene. In this study, different biochars were pyrolyzed from wheat straw, soybean straw, and corn straw at four temperatures (400 °C to 700 °C). Analyses of the elemental composition, pH, ash content, and specific surface area, scanning electron microscopy, and Fourier transform infrared spectroscopy were performed. The adsorption variations of the naphthalene for the different biochars were evaluated. The results showed that as the pyrolytic temperature increased, the carbon content, ash content, and pH of the biochars increased, the hydrophilicity, aromaticity, and polarity decreased, and the naphthalene adsorption was enhanced. The pseudo-second-order model fit the naphthalene adsorption kinetics better. Both the Langmuir and Freundlich isotherm models confirmed that naphthalene adsorption by all of the produced biochars were effective. Under moderate pyrolytic temperatures, the soybean straw-derived biochar showed a greater naphthalene adsorption capacity and faster adsorption rate than the wheat straw- and corn straw-derived biochars. This study provides a theoretical basis for selecting crop residues and optimizing the pyrolytic temperature with a high efficiency of naphthalene removal from water.
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Influence of the Pyrolytic Temperature and Feedstock on the Characteristics and Naphthalene Adsorption of Crop Straw-derived Biochars
Enzhu Hu,a,* Siyao Shang,a Nana Wang,a Xiangli Nan,a Shengjun Zhong,a and Zaijian Yuan b
The feedstock type and pyrolytic temperature used in producing biochar influence the characteristics of the obtained product and affect the adsorption behavior of naphthalene. In this study, different biochars were pyrolyzed from wheat straw, soybean straw, and corn straw at four temperatures (400 °C to 700 °C). Analyses of the elemental composition, pH, ash content, and specific surface area, scanning electron microscopy, and Fourier transform infrared spectroscopy were performed. The adsorption variations of the naphthalene for the different biochars were evaluated. The results showed that as the pyrolytic temperature increased, the carbon content, ash content, and pH of the biochars increased, the hydrophilicity, aromaticity, and polarity decreased, and the naphthalene adsorption was enhanced. The pseudo-second-order model fit the naphthalene adsorption kinetics better. Both the Langmuir and Freundlich isotherm models confirmed that naphthalene adsorption by all of the produced biochars were effective. Under moderate pyrolytic temperatures, the soybean straw-derived biochar showed a greater naphthalene adsorption capacity and faster adsorption rate than the wheat straw- and corn straw-derived biochars. This study provides a theoretical basis for selecting crop residues and optimizing the pyrolytic temperature with a high efficiency of naphthalene removal from water.
Keywords: Biochar; Crop residue; Pyrolytic temperature; Naphthalene; Adsorption kinetics; Adsorption isotherm
Contact information: a: Institute of Resources and Environmental Sciences, School of Metallurgy, Northeastern University, Shenyang, Liaoning 110819, China; b: Guangdong Key Laboratory of Agro-environmental Pollution Control and Management, Guangdong Institute of Eco-environmental Science & Technology, Guangzhou, Guangdong 510650, China; *Corresponding author: huez@smm.neu.edu.cn
INTRODUCTION
Biochar is a carbon-rich substance with a porous structure derived from pyrolyzed biomass under oxygen-limited conditions at relatively low temperatures (usually 200 to 900 °C) (Ahmad et al. 2014; Cha et al. 2016). It is presently receiving great attention for being a climate-friendly soil fertilizer and effective sorbent (Wang et al. 2018). Because of its particular physical and chemical properties, such as a high specific surface area (SBET), high density of negative surface charges, and characteristic pores and surface functional groups, biochar has shown a favorable adsorption capacity for heavy metals and organic compounds (Pan et al. 2013; Xi and Chen 2014).
Naphthalene is a nonpolar hydrophobic organic contaminant and common polycyclic aromatic hydrocarbon. It is the coking co-product of coal tar and petroleum distillation, and it is widely distributed in the environment (Yang et al. 2017). Naphthalene in natural water mainly comes from urban domestic sewage, industrial wastewater, atmospheric deposition, and soil leaching (Xi and Chen 2014). Its removal has been widely studied (Cooper et al. 2002; Goel et al. 2003; Ali and Tarek 2012). However, some typical treatment methods, such as chlorination, oxidation, and ultrasonic irradiation, are not effective because of the low solubility and resistance to mineralization of naphthalene (Changchaivong and Khaodhiar 2009). The adsorption method has been considered one of the most promising techniques because it has a high efficiency and economic performance (Bayazit et al. 2017). Among currently developed sorbents, biochars are distinctly advantageous because of the lower cost and abundant sources of biomass.
The properties and adsorption capacities of biochars are primarily ascribed to the nature of the feedstock (Conz et al. 2017; Zhang et al. 2017; Han et al. 2018; Mandal et al. 2018; Rodríguez-Vila et al. 2018). Moreover, the element composition, surface area, pore volume, and adsorptive characteristics of biochars are different when they are produced at different pyrolytic temperatures (Chen et al. 2014; Gai et al. 2014; Zhang et al. 2015; Tran et al. 2016; Li et al. 2017). Even though lessons about naphthalene adsorption can be learned from previous work (e.g., orange peels biochar and pine wood biochar) (Chen and Chen 2009; Chen et al. 2012), it is not always possible to extrapolate the specific adsorption characteristics from findings using other biochars, considering their differences in elemental compositions, pore structures, and surface properties.
Agricultural residues, such as wheat straw, soybean straw, and corn straw, are the most common low-cost biomass resources in Northeast China. Traditionally, they are combusted for kitchen fire and home-heating or burned in the field, which results in serious local air contamination. In recent years, the management of crop residues has become a notable issue because of the enormous quantities generated and associated environmental constraints (Zhang and Cao 2015; Chen et al. 2017; Mehmood et al. 2018). Also, these crop residues are generally considered to be favorable feedstock materials for making biochars because of their environmental and economic viability (Yuan et al. 2011; Yang et al. 2018). However, the surface characteristics and adsorption behaviors of different straw-derived biochars need to be further explored because of uncertainty.
In this study, biochars derived from three crop residues, i.e. wheat straw, soybean straw, and corn straw, at different temperatures (400 °C to 700 °C) were systematically characterized by elemental, pH, ash content, and Brunauer-Emmett-Teller (BET) surface area analyses, scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy to better understand their adsorption mechanisms for naphthalene. Batch adsorption experiments were conducted to establish the naphthalene adsorption kinetics and isotherms, which were used to fit the pseudo-first-order and pseudo-second-order kinetic models, and the Langmuir and Freundlich models. The main objective of this study was to examine the impact of the feedstock material and pyrolytic temperature on the biochar properties and naphthalene adsorption characteristics. And the correlation analyses between the adsorption capability, affinity and rates, as well as the characteristics of the biochars may fulfill the knowledge of adsorption mechanisms.
EXPERIMENTAL
Preparation of the Biochars
Biochar samples were produced with three common crop residues: wheat straw (W-BC), soybean straw (S-BC), and corn straw (C-BC). The raw straw materials were collected from the suburb of Shenyang, China, and they were washed several times with distilled water to remove any impurities. After being oven-dried (70 °C) for 3 d, the straw materials were crushed, ground, and sieved to pass through a 2-mm mesh. Then, they were converted to biochars using a muffle furnace equipped with a N2 purge to ensure oxygen-limited conditions. The pyrolytic temperature was increased to the desired values of 400 °C, 500 °C, 600 °C, and 700 °C, which was held for 3 h before cooling down to ambient temperature.
Characterization of the Biochars
The elemental contents (C, H, O, and N) of the biochars were determined using an elemental analyzer (Elementer Vario MACRO CUBE, Hanau, Germany). The ash content of the biochars was measured as the mass residual percentage after incinerating the samples in an open crucible in a muffle furnace at 750 °C for 4 h, which was modified from a American Society for Testing and Materials (ASTM) standard method (D 1762-84) (ASTM 2007). The weight of the samples was measured after cooling them to room temperature in a desiccator. The pH of the biochars was measured with a digital pH meter (PHS-3C, Leici, Shanghai, China) using a biochar/deionized water suspension mass ratio of 1:20, after stirring for 1.5 h and equilibrating for 1 h. The BET surface area of the biochars was calculated by fitting the N2 adsorption data to the BET equation. A 0.1000-g aliquot of the biochar sample with a particle size of 200 mesh was outgassed under vacuum (< 0.1 Pa) at 105 °C for 1 h and then heated to 350 °C for 3 h to remove volatiles and moisture. The N2 adsorption-desorption isotherm was measured under the condition of a liquid nitrogen temperature of -196 °C with a Surface Area and Pore Size Analyzer (NOVA 1200e, Quantachrome, Boynton Beach, USA). A scanning electron microscope (EV018, ZEISS, Oberkochen, Germany) was used to observe the micro-morphologies of the biochars. For the FTIR measurements, 1 mg of slightly ground biochar was gently mixed with 100 mg of oven-dried KBr, and then pressed into a pellet. The FTIR spectra were obtained using a NICOLET 380 FTIR spectrometer (Thermo Electron, Waltham, USA) over the wave number range of 4000 cm-1 to 500 cm-1.
Naphthalene Adsorption Tests
Naphthalene (99%) (J & K Scientific Ltd., Beijing, China) was used to prepare the stock solution by dissolving 0.1 g of naphthalene in 100 mL of 99.5% ethanol. Then, it was diluted to the desired concentration with deionized water containing 10 mmol/L CaCl2 to simulate environmental water, and 200 mg/L NaN3 to inhibit biodegradation. The equilibrium concentrations of the naphthalene were determined by an ultraviolet-visible spectrophotometer (UV-2550, SHIMADZU, Kyoto, Japan) at 275 nm.
The effect of the adsorption time on naphthalene removal was studied by adding 2 g/L biochar to 30 mL of a 50 mg/L naphthalene solution in 50.0-mL screw-capped brown bottles. The bottles were placed on a rotating shaker and agitated end-over-end at 30 rpm and 25 °C in the dark for 1 h, 3 h, 6 h, 12 h, 24 h, 48 h, and 72 h to determine at which time equilibrium was reached. Then, the solution was immediately separated from the solids by centrifugation at 4000 rpm for 15 min, and the supernatants were taken to determine the naphthalene concentration. Because of the negligible adsorption on the glassware and unremarkable losses via biodegradation and evaporation, the amount adsorbed by biochar was calculated by the sorbate mass loss from the solution. To investigate the effect of the initial naphthalene concentration, 2 g/L biochar was added to solutions with various naphthalene concentrations (5 mg/L, 10 mg/L, 20 mg/L, 50 mg/L, and 100 mg/L). The samples were shaken at 30 rpm for 72 h before measurement. All of the experiments above were performed in triplicate.
Adsorption Kinetics
To understand the mechanisms of naphthalene adsorption by different biochars, pseudo-first-order (Eqs. 1 and 2) and pseudo-second-order kinetic models (Eqs. 3 and 4) were used to fit the data (Ho and McKay 1998; Ho and McKay 1999),
(1)
(2)
(3)
(4)
where Qe1 and Qe2 are the amounts of naphthalene adsorbed at equilibrium corresponding to pseudo-first-order and pseudo-second-order adsorption (mg/g), respectively, q denotes the amount of naphthalene adsorbed at time t (mg/g), and k1 and k2 are the rate constants for pseudo-first-order and pseudo-second-order adsorption (g/mg·h), respectively.
Adsorption Isotherm
Adsorption isotherms are used to describe how adsorbates interact with adsorbents at equilibrium. The Langmuir and Freundlich isotherm models were used to analyze the experimental data. The Langmuir isotherm model is widely implemented to determine the theoretical value of the maximum adsorption capacity, which assumes monolayer adsorption onto a surface (Langmuir 1918). It can be expressed as,
(5)
where Qe is the amount of naphthalene adsorbed at equilibrium (mg/g), Qm is the maximum adsorption capacity (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/L), and kLis the Langmuir constant (L/mg). A dimensionless separation factor (RL) for the Langmuir isotherm could be calculated with Eq. 6 (Weber and Chakravorti 1974),
(6)
where kL is the Langmuir constant and C0 is the initial adsorbate concentration (mg/L).
The separation factor indicates if the isotherm is unfavorable (R > 1), linear (R = 1), favorable (0 < R < 1), or irreversible (R = 0).
The Freundlich model is an empirical equation that deals with adsorption on heterogeneous surfaces or surface supporting sites with various affinities. It is assumed that stronger binding sites are occupied first and that the binding strength decreases with an increasing degree of occupation (Freundlich 1907). The Freundlich isotherm equation is,
(7)
where Qe is the amount of naphthalene adsorbed at equilibrium (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/L), kF is the Freundlich constant related to the adsorption capacity, and n is the degree of favorability of the adsorption process.
Additionally, 1/n ranges between 0 and 1 and is a measure of the adsorption intensity or surface heterogeneity. A lower 1/n value indicates a greater degree of heterogeneity on the biochar surface.
Data Analyses
Pearson correlation analyses were conducted to evaluate the relationships between two pyrolytic temperatures, elemental compositions, ash contents, pH values, and BET surface areas, as well as the model parameters of the naphthalene adsorption kinetics and isotherms. The performance of the adsorption kinetic and isotherm models was assessed using the coefficient of determination (R2), as well as the Akaike information criteria (AIC). When the averaged R2 is higher and the AIC is lower, the model performs better, and vice versa. All of the calculations and plots were performed and generated using MATLAB software (version R2017a, MathWorks, Inc., Natick, USA).
RESULTS AND DISCUSSION
FTIR Spectra and SEM Images of the Biochars
The FTIR spectra of the three types of biochars (W-BC, S-BC, and C-BC) produced at different temperatures are presented in Fig. 1. The peak at 3436 cm−1 to 3433 cm−1 was assigned to the stretching vibration of the phenolic or alcoholic hydroxyl groups (-OH) (Leng et al. 2015); the broad band at 2927 cm−1 to 2919 cm−1 was assigned to symmetrical and asymmetrical stretching vibrations of aliphatic hydrocarbons or cycloalkanes (-CH3 and -CH2) (Leng et al. 2015); and the peak at 1650 cm−1 to 1580 cm−1 was ascribed to C=C and C=O stretching vibration on aromatic rings (Xiao et al. 2014). Among these peaks, the sub-strong peak near 1600 cm−1 indicated the presence of aromatic C=C or C=O groups in the biochars (Gai et al.2014). The peak near 1630 cm−1 is generally considered to be the C=O stretching vibration of alcohol, phenol, and ether (Li et al. 2014). Above all, these peaks demonstrated that the surface of the biochar samples may have contained oxygen-containing functional groups, such as carboxyl, carbonyl, and ester groups. The bands at 1440 cm−1 and 1380 cm−1corresponded to C=C stretching vibration of lignin and methyl C-H stretching vibration of alkanes, respectively (Wu et al. 2012). The peak at 1220 cm−1 was designated stretching of aromatic CO- and phenolic -OH. The peak that shifted from 1098 cm−1 to 1056 cm−1 was assigned to C-O stretching vibration in a carbohydrate or polysaccharide structure, while the peak at 900 cm−1 to 750 cm−1 was ascribed to C-H bending aromatic CH out-of-plane deformation (Wu et al. 2012). In the FTIR spectra of the three different biochars, the same functional groups mentioned above were found around corresponding wavenumbers. Some of the peaks disappeared or shifted. Also, some of them became weak as the pyrolytic temperature increased.
Fig. 1. FTIR spectra of the W-BC (a), S-BC (b), and C-BC (c) prepared from 400 °C to 700 °C
Bands from O-H stretching (3436 cm−1 to 3433 cm−1) and aliphatic C-H stretching (2927 cm−1 to 2919 cm−1) lost their intensities as the pyrolytic temperature increased from 400 °C to 700 °C, which implied accelerated dehydration, greater demethylation (loss of CH3), and increased decarboxylation (loss of CO2) in this temperature range (Leng et al. 2015). However, a higher pyrolytic temperature resulted in the disappearance of the peak for aromatic CO- and phenolic -OH (approximately 1220 cm-1) and a weaker peak for aromatic C=C (approximately 1440 cm−1) and C-H (900 cm−1 to 750 cm−1). This was because of the condensation of the aromatic structure at higher temperatures (Keiluweit et al. 2010). Additionally, with a progressively increasing temperature, the straw-derived biochars tended to develop a more stable graphene-like structure, but still maintained a pronounced O-contained functionality. Peaks for aliphatic C-O at 1098 cm−1 to 1056 cm−1 were detected even at 700 °C, which suggested incomplete aromatization and condensation (McBeath and Smernik 2009). The spectra of all three biochars (W-BC, S-BC, and C-BC) were relatively similar, except with slight differences for the peak wavenumbers.