Adsorption of Pb(II) in Aqueous Solution by the Modified Biochar Derived from Corn Straw with Magnesium Chloride

Lead wastewater not only causes deterioration of water quality but also further enters the human body through the food chain and is harmful to human health. Therefore, there is an urgent need to find an economical, simple and efficient water treatment technology to treat lead-contaminated wastewater in waterbodies. In this paper, the modified biochar derived from corn straw by magnesium chloride is prepared. Adsorption experiments of Pb(II) in solution by the modified biochar are carried out. Experiment results show that the modified biochar mainly contains C and O elements, and a large number of functional groups. The adsorption amount of Pb(II) by modified biochar reaches 5.15 mg/g under 0.2 g of modified biochar, 25 mg/L initial concentration of Pb(II) ion, reaction time of 480 min, temperature 25°C and at a speed of 200 rpm. The adsorption process of Pb(II) ions in solution by the modified biochar fits on the Freundlich isotherm model. Pseudo-second order kinetic model can better describe the adsorption process of Pb(II) ion in solution by the modified biochar. The process of adsorbing Pb(II) ions in solution by modified biochar is dominated by multi-layer adsorption process and chemical adsorption process.


INTRODUCTION
With the widespread application of lead products and lead compounds, lead pollution in the environment has become increasingly serious (Fan et al. 2008, Zhu et al. 2014, Chen et al. 2018). Lead wastewater not only causes deterioration of water quality but also further enters the human body through the food chain and is harmful to human health (Shen et al. 2015, Ho et al. 2017. Therefore, there is an urgent need to find an economical, simple and efficient water treatment technology to treat lead-contaminated wastewater (Hang et al. 2017, Kwak et al. 2019. At present, the methods for removing Pb(II) ions in solution mainly include chemical precipitation, physical filtration separation, bio-concentration, physical adsorption, and chemical removal (Xiong et al. 2017, Zhang et al. 2019). Among these methods, the adsorption method has become one of the priority methods in practice because of its low cost, high efficiency, and easy operation. It is considered to be one of the most interesting treatment methods in current research (Mohan et al. 2014. Some adsorbents, such as activated carbon, activated zeolite, resin adsorbent, etc., have the disadvantages of high cost, difficulty in separation, and easy secondary pollution . Therefore, the development of new, cheap and effective adsorption material has attracted much attention by some researchers (Liu & Zhang 2009, Zhou et al. 2013. Biochar is mainly composed of carbon, hydrogen and oxygen. It is a light carbon material with high carbon content and high aromatization properties (Yahya et al. 2016, Hang et al. 2019. Therefore, it has a certain adsorption capacity for heavy metals, such as Pb(II) ions, Cu(II) ions, Cr(VI), etc., which affects the distribution, migration and bioavailability of these heavy metals in the environment (Devi & Saroha 2014). However, the adsorption capacity of single biochar for heavy metals is far less than that of a composite modified with other materials (Huang et al. 2015, Huang et al. 2018, Mostafa et al. 2018). The modified biochar by the composite material has the characteristics of low preparation cost, excellent physical and chemical properties, simple operation, good environmental compatibility, etc., and is widely used for the adsorption treatment of heavy metals wastewater . The biochar produced by the anoxic pyrolysis of agricultural and forestry waste is a material with developed pore structure, large specific surface area, rich oxygen-containing functional groups and excellent adsorption performance (Mallampati et al. 2012, Mandu et al. 2012. The yield of the corn stalk in China is huge, which is a common raw material for the preparation of biochar (Yuan et al. 2011).
In this study, the corn straw is chosen as the biomass raw material, which is pyrolyzed under anaerobic conditions to prepare the modified biochar derived from corn straw. Then biochar is modified by the magnesium chloride. The modified biochar by magnesium chloride is obtained. Adsorption experiments of Pb(II) in solution by the modified biochar are carried out in the laboratory. Adsorption kinetics and adsorption isotherms have been discussed in detail.

Preparation of biochar:
The corn straw from the farmland in the suburbs of Jinan city was washed several times with distilled water and dried at 105°C to constant weight. It was then pulverized and passed through a 40 mesh sieve. 20 g of 40-mesh corn straw was pyrolyzed in a muffle furnace at 250°C for 2 hours. The entire operational process was carried out under N 2 gas condition. It was cooled to room temperature, smashed and sieved into 100 mesh to obtain the biochar.
Preparation of the modified biochar by magnesium chloride: 2 g of biochar was taken in a 250 mL Erlenmeyer flask containing 100 mL of 0.01 mol/L magnesium chloride solution and stirred on a magnetic stirrer for 6 hours. It was washed 3 times with distilled water and dried at 105°C to constant weight to obtain the modified biochar by magnesium chloride. The characteristics of biochar were determined by SEM, EDS and FT-IR.
Adsorption experiment: 0.2 g of biochar or 0.2 g of modified biochar was taken in a 250 mL Erlenmeyer flask containing 100 mL of 25 mg/L Pb(NO 3 ) 2 solution. It was shaken for 480 minutes in a constant temperature shaking box at 25°C and a speed of 200 rpm. The sample was filtered through a 0.45 µm filter and determined by atomic absorption spectrometry. Each adsorption test was repeated three times.

Data analysis:
The amount of adsorption was calculated by the following formula: Where, q e (mg/g) is the amount of adsorption per unit mass of adsorbent at the end of adsorption. C 0 (mg/L) and C e (mg/L) are the initial concentration of the adsorbate in the solution and the concentration at the equilibrium of adsorption. V (mL) is the volume of solution and m (g) is the dosage of the modified biochar.

RESULTS AND DISCUSSION
The characteristics of modified biochar: The SEM image of modified biochar is shown in Fig.1. The surface of the modified biochar is very smooth and some fragments are present. The specific surface area of modified biochar is relatively large, which is favourable for adsorption.
200 rpm. The sample was filtered through a 0.45 µm filter and determined by atomic absorption spectrometry. Each adsorption test was repeated three times.

Data analysis:
The amount of adsorption was calculated by the following formula: Where, q e (mg/g) is the amount of adsorption per unit mass of adsorbent at the end of adsorption. C 0 (mg/L) and C e (mg/L) are the initial concentration of the adsorbate in the solution and the concentration at the equilibrium of adsorption. V (mL) is the volume of solution and m (g) is the dosage of the modified biochar.

RESULTS AND DISCUSSION
The characteristics of modified biochar: The SEM image of modified biochar is shown in Fig.1. The surface of the modified biochar is very smooth and some fragments are present. The specific surface area of modified biochar is relatively large, which is favourable for adsorption.   2 shows the EDS analysis of biochar. As shown in Fig.2, it implies that modified biochar mainly contains C and O elements, and their contents are 68.85% and 26.68% respectively. This can be thought that the modified biochar is hydrophilic and polar. A small amount of element, such as Mg, Si, S, Cl, K and Ca, is also present on the surface of the modified biochar.
The FT-IR spectra of modified biochar are shown in Fig.  3. It can be concluded that the surface of the modified biochar contains a large number of functional groups. The peak shape is wider between 3100 cm -1 and 3600 cm -1 , which is -OH stretching vibration . The peak at 1695 cm -1 is the stretching vibration peak of C=O in -COOH functional group. The absorption peak at 1440 cm -1 is a C=O stretching vibration (Zhang et al. 2018). The absorption peak at 1228 cm -1 is a C-O stretching vibration peak. The peak between 515 cm -1 and 735 cm -1 is C-H stretching vibration peak (Li et al. 2019). The presence of these functional groups will facilitate the adsorption of pollutant by the modified biochar (Günay et al. 2007).
Adsorption capacity experiment: It can be seen from Fig.  4 that the adsorption capacity of Pb(II) ions in aqueous solution by the modified biochar is significantly enhanced. The adsorption capacity of Pb(II) ions by the modified biochar is more than twice than the unmodified biochar. The adsorption amount of Pb(II) by modified biochar reaches 5.15 mg/g under 0.2 g of modified biochar, 25 mg/L initial concentration of Pb(II) ion, react time of 480 min, temperature 25°C and at a speed of 200 rpm.

Adsorption kinetic:
The process of describing the adsorption of solids to liquids is often described by Pseudo-first order kinetic equations and Pseudo-second order kinetic small amount of element, such as Mg, Si, S, Cl, K and Ca, is also present on the surface of the modified biochar. The FT-IR spectra of modified biochar are shown in Fig. 3. It can be concluded that the surface of the modified biochar contains a large number of functional groups. The peak shape is wider between 3100 cm -1 and 3600 cm -1 , which is -OH stretching vibration . The peak at 1695 cm -1 is the stretching vibration peak of C=O in -COOH functional group. The absorption peak at 1440 cm -1 is a C=O stretching vibration (Zhang et al. 2018). The absorption peak at 1228 cm -1 is a C-O stretching vibration peak. The peak between 515 cm -1 and 735 cm -1 is C-H stretching vibration peak (Li et al. 2019). The presence of these functional groups will facilitate the adsorption of pollutant by the modified biochar (Günay et al. 2007). small amount of element, such as Mg, Si, S, Cl, K and Ca, is also present on the surface of the modified biochar. The FT-IR spectra of modified biochar are shown in Fig. 3. It can be concluded that the surface of the modified biochar contains a large number of functional groups. The peak shape is wider between 3100 cm -1 and 3600 cm -1 , which is -OH stretching vibration . The peak at 1695 cm -1 is the stretching vibration peak of C=O in -COOH functional group. The absorption peak at 1440 cm -1 is a C=O stretching vibration (Zhang et al. 2018). The absorption peak at 1228 cm -1 is a C-O stretching vibration peak. The peak between 515 cm -1 and 735 cm -1 is C-H stretching vibration peak (Li et al. 2019). The presence of these functional groups will facilitate the adsorption of pollutant by the modified biochar (Günay et al. 2007).   Adsorption kinetic: The process of describing the adsorption of solids to liquids is often described by Pseudo-first order kinetic equations and Pseudo-second order kinetic equations. Its expression is Eq. 2 and Eq. 3 (Giles et al. 1960).
Where q t (mg/g) and q e (mg/g) are adsorption amount of the adsorbent to the adsorbate at the adsorption time t (min) and the adsorption equilibrium respectively. K 1 (min -1 ) and K 2 (min -1 ) are constants. Adsorption kinetic: The process of describing the adsorption of solids to liquids is often described by Pseudo-first order kinetic equations and Pseudo-second order kinetic equations. Its expression is Eq. 2 and Eq. 3 (Giles et al. 1960).
Where q t (mg/g) and q e (mg/g) are adsorption amount of the adsorbent to the adsorbate at the adsorption time t (min) and the adsorption equilibrium respectively. K 1 (min -1 ) and K 2 (min -1 ) are constants.
The adsorption experiments of the contact time influence were carried out according to the following experimental procedure. 0.2 g of modified biochar was taken in a 250 mL Erlenmeyer flask containing 100 mL of 25 mg/L Pb(NO 3 ) 2 solution. The contents Where q t (mg/g) and q e (mg/g) are adsorption amount of the adsorbent to the adsorbate at the adsorption time t (min) and the adsorption equilibrium respectively. K 1 (min -1 ) and K 2 (min -1 ) are constants.
The adsorption experiments of the contact time influence were carried out according to the following experimental procedure. 0.2 g of modified biochar was taken in a 250 mL Erlenmeyer flask containing 100 mL of 25 mg/L Pb(NO 3 ) 2 solution. The contents were shaken for a serial contact time (5,10,15,30,45,60,120,240,360 and 480 min) in a constant temperature at 25°C and a speed of 200 rpm. According to the test results, the fitting equation was performed according to Eq. 2 and Eq. 3. The adsorption kinetic parameters of modified biochar on Pb(II) ions in solution are given in Table  1. According to the value of R 2 , the pseudo-second order kinetic model can better describe the adsorption process of modified biochar on Pb(II) ion in solution. Therefore, it can be concluded that the process of adsorbing Pb(II) ions in solution by modified biochar is mainly chemical adsorption.
Adsorption isotherms: In this study, Langmuir isotherm model and Freundlich isotherm model were chosen to describe the adsorption isotherm. Their equations are as below , Foroughi-dahr et al. 2015 adsorption kinetic parameters of modified bio in Table 1. According to the value of R 2 , the better describe the adsorption process of mo Therefore, it can be concluded that the proces modified biochar is mainly chemical adsorptio   Table 1. According to the value of R 2 , the p better describe the adsorption process of modi Therefore, it can be concluded that the process modified biochar is mainly chemical adsorption  Where, C e (mg/L) is the equilibrium concentration in the solution, q e (mg/g) is the adsorbate adsorbed at equilibrium, q max (mg/g) is the maximum adsorption capacity, n is the Freundlich constant related to adsorption intensity, K L (L/mg) and K F ((mg/g) 1/n ) are the adsorption constants for Langmuir and Freundlich models respectively.
The adsorption experiments of the Pb(II) ions concentration influence were carried out. 0.2 g of modified biochar was taken in a 250 mL Erlenmeyer flask containing 100 mL of a serial of Pb(II) ions concentration (20, 40, 60, 80 and 100 mg/L). They were shaken for 480 minutes in a constant temperature shaking box at 25°C and a speed of 200 rpm. According to the results of the adsorption experiments and Eq. 4 and Eq. 5, parameters of Langmuir isotherm model and Freundlich isotherm model for the description of Pb(II) concentration of Pb(II) ion, react time of 480 min, temperature 25°C and at a speed of 200 rpm. Adsorption kinetic: The process of describing the adsorption of solids to liquids is often described by Pseudo-first order kinetic equations and Pseudo-second order kinetic equations. Its expression is Eq. 2 and Eq. 3 (Giles et al. 1960 Where q t (mg/g) and q e (mg/g) are adsorption amount of the adsorbent to the adsorbate at the adsorption time t (min) and the adsorption equilibrium respectively. K 1 (min -1 ) and K 2 (min -1 ) are constants.
The adsorption experiments of the contact time influence were carried out according to the following experimental procedure. 0.2 g of modified biochar was taken in a 250 mL Erlenmeyer flask containing 100 mL of 25 mg/L Pb(NO 3 ) 2 solution. The contents  ions in solution adsorption onto modified biochar are given in Table 2.
From Table 2, Freundlich isotherm model can better describe the adsorption isothermal process of modified biochar on Pb(II) ions in solution according to the value of R 2 . The adsorption process is monolayer adsorption process. Therefore, it can be concluded that the process of adsorbing Pb(II) ions in solution by modified biochar is dominated by multi-layer adsorption.

CONCLUSIONS
Based on the above experimental results, the following conclusions can be drawn.
(1) The adsorption ability of Pb(II) ions in solution by the modified biochar is better than that of the unmodified biochar. The surface of the modified biochar is very smooth structure and some fragments are present. It contains a large number of functional groups, such as C=O, C-H, -OH, etc.
(2) The adsorption amount of Pb(II) by modified biochar reaches 5.15 mg/g under 0.2 g of modified biochar, 25 mg/L initial concentration of Pb(II) ion, react time of 480 min, temperature 25°C and a speed of 200 rpm.
(3) Pseudo-second order kinetic model can better describe the adsorption process of modified biochar on Pb(II) ion in solution. The process of modified biochar on Pb(II) ions in solution fits the Freundlich isotherm model.
(4) The process of adsorbing Pb(II) ions in solution by modified biochar is dominated by multi-layer adsorption and chemical adsorption process.