A slow pyrolysis biochar derived from Tetrapanax papyriferum petiole as an effective sorbent for removing copper ions from aqueous solution

Abstract

Biochar derived from Tetrapanax papyriferum petioles at different pyrolysis temperatures was used to remove copper from aqueous solution. Abundant porous structures were observed with scanning electron microscopy, and transmission electron microscope images revealed a unique layered nanopore structure. A high pyrolytic temperature resulted in a biochar with a higher surface area, ash content, and mineral element content. The maximum adsorption capacity of T. papyriferum petiole biochar (TBC) was 182 mg/g. The Langmuir adsorption isotherm model and pseudo-second-order kinetics model were most suitable for describing the adsorption process, indicating that adsorption takes place at specific homogeneous sites within the adsorbent. The calculated ΔH° values indicated that the adsorption process was endothermic. The adsorption mechanism for TBC was attributed to precipitation, ion exchange, C-π interactions, and complexation. Thus, the biochar used in this study is a promising environmentally friendly and effective adsorbent for removing Cu2+ ions from an aqueous solution.

Full Article

A Slow Pyrolysis Biochar Derived from Tetrapanax papyriferum Petiole as an Effective Sorbent for Removing Copper ions from Aqueous Solution

Wenqi Li,^a,# Liping Zhang,^a,# Ying Guan,^a Zhihan Tong,^b Xiang Chen,^a Guanqiao He,^a and Hui Gao ^a,*

Biochar derived from Tetrapanax papyriferum petioles at different pyrolysis temperatures was used to remove copper from aqueous solution. Abundant porous structures were observed with scanning electron microscopy, and transmission electron microscope images revealed a unique layered nanopore structure. A high pyrolytic temperature resulted in a biochar with a higher surface area, ash content, and mineral element content. The maximum adsorption capacity of T. papyriferum petiole biochar (TBC) was 182 mg/g. The Langmuir adsorption isotherm model and pseudo-second-order kinetics model were most suitable for describing the adsorption process, indicating that adsorption takes place at specific homogeneous sites within the adsorbent. The calculated ΔH° values indicated that the adsorption process was endothermic. The adsorption mechanism for TBC was attributed to precipitation, ion exchange, C-π interactions, and complexation. Thus, the biochar used in this study is a promising environmentally friendly and effective adsorbent for removing Cu²⁺ ions from an aqueous solution.

Keywords: Tetrapanax papyriferum; Biochar; Heavy metals; Adsorption; Isotherms

Contact information: a: School of Forestry and Landscape Architecture, Anhui Agricultural University, Hefei 230036, China; b: College of Materials Science and Engineering, Northeast Forestry University, Harbin 150040, China; ^*Corresponding author: huigaozh@163.com

#: These authors contributed equally to this study.

INTRODUCTION

Tetrapanax papyriferum (Family: Araliaceae) is native to northern Formosa and to the South China provinces of Hunan, Szechwan, Yunnan, Kweichow, Kwangsi, and Kwangtung. Under natural conditions, it is usually a shrub 3.0 to 6.0 feet tall but may attain a height of 30.0 feet (Ho et al. 2005). Tetrapanax papyriferum has large leaves. The diameter of the lamina is 55.0 ± 1.1 to 85.0 ± 1.2 cm, and the average length and diameter of its petiole is 70.0 ± 2.0 to 90.0 ± 3.5 cm and 0.8 ± 0.2 to 1.6 ± 0.4 cm, respectively. The petiole of T. papyriferum is hollow, and a white layer lies close to the inner wall. Furthermore, T. papyriferum has many special characteristics, such as low wood density, extremely fast growth, short felling period, wide adaptability, and strong self-reproduction ability. However, far too little attention has been paid to its potential applications. Tetrapanax papyriferum is inefficiently used and is not recycled. It has even been considered an invasive species in some places. In China, the stalk pith of T. papyriferum is a famous traditional Chinese medicine that is produced annually, but the leaves are discarded, resulting in serious waste. The comprehensive utilization of T. papyriferum, including its lamina and stalk, has become an increasingly challenging issue.

Large quantities of heavy metal ions bioaccumulate throughout the food chain, where they threaten human health (Mishra et al. 2012; Lin et al. 2017b; Teodoro et al. 2017). Copper is an essential trace element that is not easily degraded. Many diseases, such as stomach and intestinal disorders, occur in humans when copper is ingested in excess (Lin et al. 2017a). Thus, efficient techniques are necessary to eliminate heavy metal ions from wastewater. There are many techniques for removing heavy metal ions (Lin et al. 2017a; Niu et al. 2017; Ren et al. 2017), including physical, chemical, and biological methods, such as chemical precipitation, ion exchange, adsorption, membrane filtration, and solvent extraction. Data from several studies suggests that adsorption is a highly efficient, cost-effective, and simple technique to remove heavy metal ions with minimal production of secondary pollution (Zhang et al. 2018a, b).

The most widely used class of adsorbents is carbon-based absorbents. Biochar (BC) is a carbon-rich solid derived from the thermal degradation of plant residue and agricultural waste in an oxygen-limited environment (Vyavahare et al. 2018). Biochar has been prepared from various feedstock materials, such as wood, sugarcane, rice husk, bamboo, dairy manure, and bioenergy residue (Li et al. 2017; Xu et al. 2017; Vyavahare et al. 2018). A great deal of research on biochar has focused on its adsorption of heavy metal ions (Han et al. 2016; Park et al. 2016; Qian et al. 2016). Idrees et al. (2018) used guinea fowl manure derived biochar and cattle manure derived biochar to adsorb Cu²⁺ ions, and the maximum sorption capacity values were 46.6 and 44.5 mg/g, respectively. Zhou et al. (2017) investigated the adsorption of Cu²⁺ ions using biochar derived from earthworm manure at different pyrolysis temperatures. The adsorption capacity was 24.27 mg/g. Kim et al. (2016) prepared KOH-activated Enteromorpha compressa biochars to investigate the removal efficiency of Cu²⁺ ions from an aqueous solution. The adsorption capacity of Cu²⁺ ions was 137 mg/g. Biochar is a low-cost adsorbent with high specific surface area and a high capacity to adsorb heavy metal ions. Thus, biochar is a promising alternative agent for removing heavy metal ions. The adsorption capacity of a biochar depends highly on its physicochemical properties, such as surface area, pore properties, and elemental constitution. The physicochemical properties of a biochar depend upon the raw material, pyrolysis temperature, environment (limited oxygen or N₂), and the heating rate. Hence, selection of the feedstock and pyrolysis conditions has an important impact on the final sorption properties of a biochar.

Tetrapanax papyriferum grows and reproduces at a very high rate and is considered an invasive plant species in some locations. No previous study has investigated adsorption of heavy metal ions using a biochar derived from T. papyriferum. Thus, this inexpensive and readily available waste biomass was used to remove Cu²⁺ ions from wastewater. In this study, a new low-cost biochar adsorbent derived from T. papyriferum petioles was prepared at different temperatures and used to remove Cu²⁺ ions. This study evaluated the absorption behavior of Cu²⁺ ions in T. papyriferum petiole biochar (TBC) under different pyrolysis conditions. The biochars were characterized via elemental analysis, the Brunauer-Emmett-Teller (BET) method, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectrometry (XPS), transmission electron microscopy (TEM), and atomic absorption spectroscopy (AAS). Batch experiments were conducted for different factors, such as initial pH, contact time, concentration of Cu²⁺ ions, and temperature.

EXPERIMENTAL

Materials

Tetrapanax papyriferum was planted at Anhui Agricultural University (Anhui, China), and its petioles were used as the raw material in this study. The mature petioles of T. papyriferumwere harvested, cleaned, air-dried, and cut into 5 cm lengths for the biochar samples. CuSO₄·5H₂O was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water (18 MΩ·cm^-1) was prepared using a Milli-Q purification system from Kertone (Changsha, China).

Methods

Preparation of the biochars

The air-dried samples were pyrolyzed in a tubular vacuum furnace (BTF-1200C-II, BEQ, Anhui, China) under a N₂ atmosphere. The samples were heated from room temperature to 500, 600, 700, or 800 °C at a rate of 5 °C·min^-1, and the temperature was held for 2 h. The temperature of the samples decreased to room temperature at a rate of 5 °C·min^-1. Finally, the biochars were sieved through 200 mesh. The resulting biochars were named TBC-500, TBC-600, TBC-700, and TBC-800.

Characterization of the biochars

C, H, and N contents were measured with a CHN Elemental Analyzer (Vario EL Cube, Elementar, Langenselbold, Germany). The percentage of oxygen in each sample was estimated by mass difference (100% minus the percent of C, H, N, and ash). The biochar ash was obtained by heating the samples to 800 °C for 2 h in a muffle furnace (Wang et al. 2016). The elemental contents (e.g., Ca, Mg, Fe, K, Cu, Mn, and Zn) were analyzed using AAS (TAS-990, Pgeneral, Beijing, China) (Tong et al. 2011). The surface areas of the biochars were measured with a surface area analyzer (BET, ASAP20, Micromeritics Inc., Norcross, GA, USA) using the N₂ adsorption method. The FT-IR spectra of the samples were measured with a Nicolet 6670 spectrometer (Thermo Fisher, Waltham, MA, USA) in the range of 4,000 to 400 cm^-1 with a resolution of 2 cm^-1. The XRD measurements were performed with an XRD-3 diffractometer (PERSEE, Beijing, China) using Cu Kα radiation and an X-ray wavelength of 0.15406 nm at 36 KV and 20 mA. X-ray photoelectron spectrometry (XPS) data was obtained with an ESCALAB 250Xi electron spectrometer (Thermo Fisher). The morphology of each sample was observed via field emission SEM (S-4800, Hitachi, Tokyo, Japan) with the samples being sputter coated with gold. Several drops of the diluted suspension were deposited onto a freshly cleaved mica substrate and allowed to dry. Dispersibility of the biochar was investigated by TEM using the HT7700 (Hitachi, Japan) instrument. The pH of each sample was measured with a pH meter (Rex PHS-25, Shanghai, China) at a ratio 1:20 (m/V, g/mL) sample: water.

Batch adsorption experiments

A 1,000 mg/L solution of Cu²⁺ ions was prepared by accurately dissolving weighed CuSO₄·5H₂O in distilled water, and other experimental solutions were obtained by dilution. The concentration of Cu²⁺ ions was determined by AAS (TAS-990, Pgeneral, Beijing, China). Batch adsorption experiments were performed to remove the Cu²⁺ ions from a 15 mL solution of Cu²⁺ ions. The initial pH of the heavy metal solution was adjusted using 0.1 M HCl and 0.1 M NaOH solutions. A 15 mL aliquot of Cu²⁺ ion solution was mixed with 40 mg of adsorbent in a 50 mL centrifuge tube and shaken at 180 rpm and 30 °C. The effects of initial metal ion concentration (25 to 1,000 mg/L, adsorption time 24 h) and contact time (10 to 120 min) on adsorption performance were investigated in the batch mode of operation. The effects of various temperatures on the removal capacity of the adsorbent were also studied.

The amount of heavy metal adsorbed per unit mass of the adsorbent (Q_e) was calculated using the following mass balance equation,

Q_e = (C₀-C_e) × (V/m) (1)

where C₀ and C_e are the initial and equilibrium concentrations of Cu²⁺ ions (mg/L), respectively; V is the volume of solution (L); m is the weight of the dry biochars (g); and Q_e is the adsorption capacity of the adsorbent for Cu²⁺ ions (mg/g).

Reutilization of TBC biochar

Briefly, in the first sorption cycle, 40 mg of the adsorbent was mixed with 15 mL of 150 mg/L (pH = 5) Cu²⁺ ion solution and shaken at 30 °C for 12 h. The successfully adsorbed Cu²⁺ions from the first cycle were separated with a 0.45 μm filter and washed with distilled water. After drying, the adsorbent was reutilized for the next three cycles of Cu²⁺ ion sorption. This experiment investigated the recycling performance of the TBC.

RESULTS AND DISCUSSION

Characterizations of the Biochars

The SEM and TEM images of the biochars are presented in Fig. 1. An abundant number of pores was detected in the structures in Fig. 1, but the pore structures and pore sizes were irregular and heterogeneous. The TEM images of TBC provided further detailed structural information, revealing a unique stacked and multi-layered morphology with a layered nanopore structure and bulky aggregation. Undoubtedly, the pore structure provided more usable surface area causing higher adsorption of heavy metals.

The nitrogen adsorption-desorption isotherms and the pore size distribution of the biochars are shown in Fig. 2. The N₂ adsorption-desorption can provide preliminary qualitative information on the adsorption mechanism and on the porous structure of the carbons. Figure 2 shows a representative N₂ adsorption-desorption isotherm and the corresponding Barrett-Joyner-Halenda (BJH) pore size distribution curve. The adsorption-desorption isotherm plot of TBC-500 and TBC-600 represents a Type III (non-porous multi-layer adsorption) isotherm. The BET surface areas of the TBC-600 and TBC-500 were 3.82 and 1.81 m²/g, respectively. The isotherm plot of TBC-700 shows a hysteresis which implies the existence of the open channel in biochar structure. For the TBC-700, the N₂ isotherms were of type IV, with type-H₄ hysteresis loops, which implies the existence of micro- and mesoporous structures in the TBC-700, and narrow cracks and pores in the sorbent materials. Then the isotherm plot of TBC-800 resembles Type II isotherm (non-porous, mono to multi-layer adsorption). The Type II sorption curves are associated with micro and mesoporous solids. The fast-growing nitrogen adsorption curves indicated molecular monolayer adsorption or microporous multilayer adsorption. The surface area was dramatically increased from 1.81 m²/g to 236.4 m²/g when the pyrolysis temperature was increased to 800 °C, indicating an increase in the extent of raw material cracking and gradual development of the pore structure. The BET surface areas of the TBC (800, 700, 600, and 500) were 236.4, 207.3, 3.8, and 1.8 m²/g, respectively. The BET surface area of TBC increased sharply from 1.8 (500 °C) to 236.4 m²/g (800 °C), indicating that a well-developed porous structure formed at the higher pyrolysis temperatures. The pore volumes of the biochars were 0.177, 0.164, 0.038, and 0.023 cm³/g for TBC-800, TBC-700, TBC-600, and TBC-500, respectively. The mean pore sizes of the biochars were 1.50, 3.24, 40.0, and 51.2 nm for TBC-800, TBC-700, TBC-600, and TBC-500, respectively. The pore diameter indicated that microspores and mesospores were the main porous structures in the biochars. These results suggest that the BET surfaces depended on the pyrolysis temperature because surface area increased with rising pyrolysis temperature. In the current study, the adsorption of Cu²⁺ ions improved at higher pyrolysis temperatures of 500 to 800 °C, which may have been due to the increase in the BET surface area.

Fig. 1.SEM (a through d) and TEM (e through h) images of the biochars

Fig. 2. Nitrogen adsorption-desorption isotherms and pore size distribution of the biochars

The elemental compositions of the biochars are listed in Table 1. The carbon contents of TBC-500, TBC-600, TBC-700, and TBC-800 were 65.6, 67.4, 68.8, and 76.2%, respectively. Carbon content increased, whereas hydrogen, oxygen, and nitrogen contents decreased, resulting in a decrease in the H/C, O/C, N/C, and (N+O)/C molar ratios (due to high carbonization and removal of polar functional groups). The O/C ratio was lower, suggesting that the TBC surface became less hydrophilic at higher temperatures. The polarity index indicator ((N+O)/C) decreased, which was attributed to the formation of aromatic structures by higher carbonization of the TBC and removal of polar surface functional groups. The N content of TBC only depended on feedstock characteristics and not on pyrolysis temperature.

The physicochemical characteristics of the four biochars are shown in Table 1. The pH of the biochars increased slowly from 10.3 to 10.9 when the pyrolysis temperature was increased from 500 to 800 °C. Other studies have reported that a high biochar pH was obtained with a higher pyrolysis temperature (Chen et al. 2011). Cellulose and hemicellulose decompose at 200 to 300 °C to produce organic acids and phenolic compounds that decrease the pH of biochars (Rangabhashiyam and Balasubramanian 2019). Alkali salts are released from the pyrolytic structure when the pyrolysis temperature and pH of the biochar are higher (Goswami et al. 2016).

Table 1. Physicochemical Characteristics of the Four Biochars