Adsorption of Cr6+ and Pb2+ on soy sauce residue biochar from aqueous solution

Abstract

Biochar produced by the pyrolysis of biomass can be used to counter water pollution from heavy metals. The purpose of this work was to develop a biosorbent based on soy sauce residue (SSR) for the removal of Cr6+ and Pb2+. The SSR biochar (SBC) from oxygen-limited pyrolysis under the temperatures of 300 to 700 °C were obtained, and their adsorption capability was evaluated. After determining the optimum pyrolysis temperature, the effects of initial pH values, contact times, and initial metal concentrations on the Cr6+ and Pb2+ adsorption by SBC prepared at 600 °C (SBC600) were investigated. With the increase of pyrolysis temperature, the physical and chemical properties of SBC developed in a direction favorable to heavy metal adsorption. The SBC600 reached the adsorption equilibrium at the time of 2 (Cr6+) and 24 h (Pb2+), and the maximum adsorption amounts of Cr6+ and Pb2+ were 25.80 and 135.3 mg/g, respectively. The adsorption kinetics followed the pseudo-second-order kinetic equation, and the adsorption isotherms was best described by the Langmuir isotherms. The SBC was an adsorbent with certain potential for heavy metals removal in wastewater.

Full Article

Adsorption of Cr⁶⁺ and Pb²⁺ on Soy Sauce Residue Biochar from Aqueous Solution

Xiaoxun Xu,^a,b,# Chenying Zhou,^a,# Shirong Zhang,^a,b,* Zhang Cheng,^a Zhanbiao Yang,^a Junren Xian,^a and Yuanxiang Yang ^a

Biochar produced by the pyrolysis of biomass can be used to counter water pollution from heavy metals. The purpose of this work was to develop a biosorbent based on soy sauce residue (SSR) for the removal of Cr⁶⁺ and Pb²⁺. The SSR biochar (SBC) from oxygen-limited pyrolysis under the temperatures of 300 to 700 °C were obtained, and their adsorption capability was evaluated. After determining the optimum pyrolysis temperature, the effects of initial pH values, contact times, and initial metal concentrations on the Cr⁶⁺ and Pb²⁺ adsorption by SBC prepared at 600 °C (SBC600) were investigated. With the increase of pyrolysis temperature, the physical and chemical properties of SBC developed in a direction favorable to heavy metal adsorption. The SBC600 reached the adsorption equilibrium at the time of 2 (Cr⁶⁺) and 24 h (Pb²⁺), and the maximum adsorption amounts of Cr⁶⁺ and Pb²⁺ were 25.80 and 135.3 mg/g, respectively. The adsorption kinetics followed the pseudo-second-order kinetic equation, and the adsorption isotherms was best described by the Langmuir isotherms. The SBC was an adsorbent with certain potential for heavy metals removal in wastewater.

Keywords: Soy sauce residue biochar; Adsorption; Heavy metal; Kinetics; Thermodynamics

Contact information: a: School of Environment Sciences, Sichuan Agricultural University, Chengdu, 611130, China; b: Key Laboratory of Soil Environment Protection of Sichuan Province, Sichuan Agricultural University, Chengdu, 611130, China; #: These authors contributed equally to this work and should be considered co-first authors; *Corresponding authors: srzhang01@aliyun.com

INTRODUCTION

Water pollution by heavy metals has become a major concern worldwide due to improper disposal of solid waste, smelting, mining, sewage irrigation, abuse of chemical fertilizers, and pesticides (Guan and Sun 2014). The toxic metals ions, such as Cr⁶⁺ and Pb²⁺, generally coexist in contaminated waters and some industrial effluents (Shaw and Dussan 2017; Diao et al. 2018). As the main sources of contamination toxicity in wastewater, they could threaten human health through direct drinking, skin contact, or through the food chain (Izah et al. 2016). Therefore, it is urgent to apply an eco-friendly and feasible method to remove Cr⁶⁺ and Pb²⁺ from wastewater.

At present, the common treatment technologies of heavy metal in wastewater include coagulation flocculation, biological treatment, and adsorption methods (Lee et al. 2015; Brahmi et al. 2017; Kiran et al. 2017). Researchers have paid more attention to adsorption methods in the last years due to high adsorption efficiency, simplicity, less secondary pollution, and selectivity (Meng et al. 2019). Biochar is an adsorption material with high adsorption efficiency and low cost (Amin et al. 2017). Moreover, the biochar from waste residue used to remove heavy metal in wastewater can solve the problem of waste recycling to a large extent (Fan et al. 2018).

Soy sauce residue (SSR) from the sauce production process is often treated as fertilizer, feed, or garbage because it is difficult to transport and unsuitable for storage. It will not only cause a loss of resources, but also lead to environmental pollution (Li et al. 2013). The SSR is rich in coarse protein and fat, crude fiber of plant, and isoflavones. It can be used in the simultaneous extraction of oil and soy isoflavones and carbon source for sludge conditioning (Chen et al. 2014b; Duan et al. 2019). There has been a need for research on the preparation of biochar from SSR, while the problem of SSR loss and environmental pollution can be solved well by using it as the raw material of biochar.

The authors assumed that SSR was a potential material to prepare the biochar and that soy sauce residue biochar (SBC) could be effective for the adsorption of heavy metals in wastewater. However, little evidence is available for this hypothesis. Therefore, the specific purposes of this study were (1) to provide an adsorbent for the Cr⁶⁺ and Pb²⁺ in aqueous solution; (2) to investigate the effects of initial pH, contact time, and initial metal concentration on SBC for adsorption of Cr⁶⁺ and Pb²⁺; and (3) to discuss the possible mechanism of SBC for Cr⁶⁺ and Pb²⁺ removal. It is expected that this study can provide useful data to maximize the beneficial use of SBC as an effective biosorbent for Cr⁶⁺ and Pb²⁺ removal. Furthermore, the research would help the authors to develop a novel application strategy to suitably manage renewable solid waste by recycling them into the environment.

EXPERIMENTAL

Materials and Reagents

The SSR was supplied by a brewing enterprise (Qianhe Condiment and Food Co., Ltd.) of Chengdu, China. After the removal of the dust on the surface of SSR, it was air-dried at room temperature and then smashed to pass through a 20-mesh sieve (0.90 mm) and preserved as the raw material.

Lead nitrate (Pb(NO₃)₂), potassium dichromate (K₂Cr₂O₇), nitric acid (HNO₃), and sodium hydroxide (NaOH) were purchased commercially (analytical reagent grade, Sichuan YaHua Industrial Group Co., Ltd, Chengdu, China). A total of 1000 mg/L Cr⁶⁺ or Pb²⁺ stock solution was produced via dissolving K₂CrO₇ or Pb(NO₃)₂ in distilled water, respectively. The diverse concentrations of Cr⁶⁺ or Pb²⁺ solutions used in this study were acquired via diluting the stock solution.

Biochar preparation

The SBC was prepared through oxygen-limited pyrolysis. After it was filled in the closed ceramic crucible, the powder of SSR was placed in a furnace. The furnace was programmed to heat with a rate of 20 °C/min until it reached a specified temperature (300, 400, 500, 600, and 700 °C), and carbonization was completed at different pyrolysis temperatures for 2 h. After cooling to reach the room temperature, the SBC was removed and rinsed with ultra-pure water until the pH reached a stable state. The SBC was marked as SBC300, SBC400, SBC500, SBC600, and SBC700. After drying the SBC at 80 °C, then it was smashed to pass through a 100-mesh sieve (0.15 mm).

The productivity were determined by weighing the mass before and after pyrolysis, and the ash content were determined according to GB/T 12496.3 (1999), and the average aperture and specific surface area were determined via a multi-point Brunauer-Emmet-Teller (BET) method. The SBC’s basic physical and chemical properties are shown in Table 1.

Table 1. Basic Physical and Chemical Properties of SBC

Methods

Characterizations of SBC

The appearance of SBC particles was observed via a scanning electron microscope (SEM; JSM-7500F; JEOL, Tokyo, Japan), and the functional groups of SBC were analyzed via FTIR (FTIR; Spectrum Two; PerkinElmer Inc., Waltham, MA, USA), and the elemental composition of SBC was determined using an elemental analyzer (Vario EL III; Elementar Analysen Systeme, Hanau, Germany).

Batch sorption experiment

The adsorption experiments were carried out in 50-mL centrifuge tubes, containing 80 mg of biochar sample and 40 mL of K₂Cr₂O₇ or Pb(NO₃)₂ solution. The ranges of experimental parameters were selected as follows: initial concentrations of K₂Cr₂O₇ (50-200 mg/L), initial concentrations of Pb(NO₃)₂ (50-200 mg/L), pH (1.0-6.0), and contact time (10-240 min for Cr⁶⁺ and 20-1440 min for Pb²⁺). The pH was adjusted with 1% HNO₃ and 1% NaOH. The tubes were capped and shook in a constant temperature oscillator (25 °C, 200 r/min; KT-86A, Changzhou Zhongbei Instrument Co. Ltd., Changzhou, China). All treatments were replicated three times.

The removal and adsorption capacity of Cr⁶⁺ and Pb²⁺ after adsorption were calculated according to the following formulas,

(1)

(2)

where p is the removal of Cr⁶⁺ and Pb²⁺ (%), q_e is the adsorption capacity of Cr⁶⁺ and Pb²⁺ (mg/g), C₀ is the initial concentration of Cr⁶⁺ and Pb²⁺ before adsorption (mg/L), C_t is the concentration of Cr⁶⁺ and Pb²⁺ after adsorption for t min (mg/L), m is the dosage of SBC, (g), and V is the volume of Cr⁶⁺ and Pb²⁺ solution (L).

Adsorption kinetics: The pseudo-first-order (Eq. 3) and pseudo-second-order (Eq. 4) kinetic models were used to describe the kinetics of adsorption of Cr⁶⁺ and Pb²⁺ by SBC,

(3)

(4)

where q_e is the equilibrium adsorption amount (mg/g), q_t is the adsorption amount of SBC for heavy metals (mg/g) at time t (min), k₁ is the constant of reaction rate of the pseudo-first-order kinetic model (/min), and k₂ is the constant of reaction rate of the pseudo-second-order kinetic model (g/(mg·min)).

Adsorption isotherm equation: Langmuir equation (Eq. 5) and Freundlich equation (Eq. 6) were used to fit the experimental results. The equations are as follows,

(5)

(6)

where Q_e is the equilibrium adsorption amount (mg/g), Q_m is the maximum adsorption amount (mg/g), C_e is the equilibrium concentration of Cr⁶⁺ in the solution (mg/L), K_L is the constant of Langmuir adsorption equilibrium (L/mg), K_f is the empirical Freundlich constants related to sorption capacity, and n is the empirical Freundlich constant related to sorption intensity.

The solution was centrifugally filtrated after adsorption, and the Cr⁶⁺ concentration was analyzed using 1,5-diphenylcarbazide colorimetric method by measuring the absorbance at 540 nm in the filtrate with a UV-vis spectrophotometer (UV-1780, Shimadzu, Co. Ltd., Shanghai, China). The concentration of Pb²⁺ in the filtrate was determined via flame atomic absorption spectrophotometry (Thermo Solaar M6; Thermo Fisher Scientific, Ltd., Waltham, MA, USA).

Data Processing

The data were analyzed by Statistical Product and Service Solutions 22.0 (SPSS Institute Inc., Chicago, IL, USA), including a single factor analysis of variance (ANOVA), relationship modeling of influencing factors, descriptive statistics, and comparisons of significance difference between treatments conducted by least significant difference (LSD). Data analysis was performed using Origin 9.1 (OriginLab, Origin 9.1, Northampton, MA, USA). The data are the average values of three repetitions.

RESULTS AND DISCUSSION

Structure Characterization and Physicochemical Properties of Biochar Prepared at Different Temperatures

FITR spectra

The preparation temperature has a great influence on the surface functional groups of biochar (Zhang et al. 2013). The Fourier infrared spectrum of five kinds of biochar prepared at different temperatures is shown in Fig. 1. They had strong absorption peaks at 3432, 2874, 1607, 1102, and 832 cm^-1. The appearance of an absorption peak located at 3432 cm^-1 was due to –OH, which might be derived from carbohydrates in organic matter (Wu et al. 2017). The characteristic peak at 2874 cm^-1 was attributed to the symmetric and asymmetric stretching vibration of -CH₂ of aliphatic hydrocarbon and the characteristic absorption peak of benzene ring was 1607 cm^-1. The absorption peak appearing at approximately 1102 cm^-1 was produced by the stretching vibration of C-O, which indicated that SBC might contain surface groups such as ether bonds, alcohols, and lipids. In addition, the absorption peak at 832 cm^-1 could be attributed to the stretching deformation vibration of the C-H in aromatic compounds (Lammers et al. 2009).

It could be seen from the FTIR spectrum that the SBC was mainly composed of an aromatic skeleton and contained functional groups, such as the hydroxyl group and the aromatic ether. As the pyrolysis temperature increased, the groups that SBC contained were slightly different, but only SBC prepared at 300 °C contained -CH₂, and SBC600 and SBC700 showed a C-O (Fig. 1). This phenomenon indicated that as the pyrolysis temperature increased, the alkyl group was deleted, and the degree of aromatization of biochar gradually increased. The main reason for this phenomenon was that the increase of temperature caused the break of C-C, C-O, and C-H bonds in the cellulose molecules, and various free radicals formed. After a series of cyclization reactions and collisions, these radicals formed –OH and other functional groups. These groups provided sites for the binding of heavy metal cations. Thereby, they provided a basis for the adsorption of heavy metal ions by SBC (Zhang et al. 2016). In addition, the presence of C-O ether bond groups increased the adsorption and ion exchange capacity of biochar for heavy metals (Radwan et al. 2010). Therefore, the presence of an ether bond in SBC600 and SBC700 indicated that they had good adsorption potential for heavy metal ions.

Fig. 1. Infrared spectrum of biochar at different pyrolysis temperatures

SEM analyses

The scanning electron micrograph group visually reflected the pore variation of the biochar surface prepared at different pyrolysis temperatures (Hamza et al. 2016). In this study, the pores exhibited by the SBC were mostly transverse tubular structures (Fig. 2). Under low temperature conditions (< 400 °C), SBC was at the degree of low carbonization and had few pore structure and small pores. When the pyrolysis temperature exceeded 400 °C, the pore structure of the SBC surface gradually formed, and the tubular structure began to deform remarkably. From the results shown in Fig. 2, the fiber structure of SBC600 and SBC700 were broken, exhibiting an irregular pore-like structure that became looser, and the surface roughness was noticeably increased. However, the surface porosity of the biochar was reduced when the pyrolysis temperature was as high as 700 °C.

As the preparation temperature increased from 300 °C to 400 °C, the formation of large pores in SBC might have resulted from the honeycomb structure of soybean fiber in the SSR. As the preparation temperature continued to increase, more volatile mass escaped from SBC to form a distinctly porous structure (Wang et al. 2018a). On the one hand, the presence of these micropores provided sufficient contact between the biochar and the water sample. In contrast, it also provided a large number of heavy metal adsorption sites, which provided an ideal environment of physicochemical reaction between the biochar surface groups and the heavy metals in the water sample. The SBC600 had higher adsorption performance (Que et al. 2018). The SBC tended to be fragmented at 700 °C. This situation was caused by excessive temperature that damaged the porous structure to some extent, and might have reduced the adsorption performance of SBC700.