Abstract

In this study, biochar was prepared from Alternanthera philoxeroides (AP) under O2-limited condition at 350 °C (LB) and 650 °C (HB) and treated with aging by HNO3/H2SO4 oxidation. Structural changes of the biochar after aging treatment and the treatment's effect on Pb(II) absorption were explored. The results showed that oxygen-containing functional groups, aromatic structure and surface area of the biochar increased after the aging treatment. However, the integrity of the tubular structure was broken into fragments. The adsorption process of Pb(II) was in accordance with the pseudo-second-order kinetic model and fitted by the Langmuir model. With the increase of pH, the adsorption capacities of Pb(II) increased gradually, and the adsorption effect was best at pH 5. The aged HB presented a decrease of the carboxyl group, which caused less adsorption capacity of Pb(II) than that of aged LB. The maximum adsorption capacities of Pb(II) on fresh biochar at 350 °C and 650 °C were 279.85 and 286.07 mg·g−1 and on aged biochar were 242.57 and 159.82 mg·g−1, respectively. The adsorption capacity of HB for Pb(II) was higher than that of LB, and the adsorption capacity of aged biochar for Pb(II) decreased obviously, which might be attributable to changes in physicochemical properties of biochar after the aging treatment.

INTRODUCTION

At present, lead is considered to be one of the most serious concerns for water contamination. It not only affects the quantity and quality of aquaculture products but also damages the human reproductive system and central nervous system mostly through the water and food chains (Jarup 2003). Lead pollution mainly comes from industrial and mining enterprises which smelt, manufacture and use lead products. Therefore, it is necessary to find an effective method for removal of Pb(II) from wastewater. Some treatment technologies for removing heavy metals from wastewater have been developed, such as adsorption, chemical precipitation, membrane filtration, ion exchange and electrochemical treatment (Hua et al. 2012; Zhao et al. 2018); in particular, adsorption has received increasing attention recently due to its low cost and high efficiency.

Biochar, a carbon-rich solid, was prepared using the pyrolysis of common organic compounds under relatively low temperature and oxygen-limited conditions (Kwapinski et al. 2010; Jones et al. 2011; Qian & Chen 2014). Numerous studies demonstrated that biochar had good ability in the adsorption of heavy metals (Park et al. 2011; Wang et al. 2015). Alternanthera philoxeroides (AP) is a kind of alien invasive malignant grass in China's waters and terrestrial ecosystems. Its invasion not only reduces the yield of farmland grain but also blocks waterways (Li & Liu 2002; Wu et al. 2017). Research on the biochar of AP is relatively rare in China and elsewhere. In this paper, the AP was selected as raw material to prepare biochar. The effect of AP biochar adsorbent on the adsorption/desorption of Pb(II) in soil was investigated.

Although biochar has excellent adsorption capacity, as reviewed elsewhere (Kookana et al. 2011; Zhao et al. 2017), once the biochar was applied to the soil, it may undergo a range of surface changes, which commonly was referred to as biochar aging (Kookana 2010), including oxidation (Cross & Sohi 2013) and biological consumption (Cross & Sohi 2011). Aging treatments may change the sorption characteristics of the adsorbent (Zhang et al. 2016). The biochar pore structure might be reduced because the micropores on the biochar surface were clogged with fine soil particles. In addition, aging processes might cause the introduction of functional groups (carbonyl, hydroxyl, and phenolic) on the biochar surface, thereby impacting its adsorption performance (Qian et al. 2015; Khorram et al. 2017).

In this study, two kinds of biochar were prepared from the AP under O2-limited condition at different temperatures (350 and 650 °C). The oxidative aging of biochar with different concentrations of HNO3/H2SO4 was simulated to investigate the effects of biochar chemical aging on the adsorption of Pb(II). The objectives of this study were as follows: (1) to prepare and characterize the fresh and aged biochar; (2) to compare the adsorption capabilities of the fresh and aged biochar for removing Pb(II) at different pyrolysis temperatures; and (3) to investigate the effects of various factors including contact time, initial Pb(II) concentration, and pH on Pb(II) removal efficiencies.

MATERIALS AND METHODS

Materials

The AP was collected from ponds near the Hongye pencil processing factory which is located at Rizhao, Shandong Province (119°30′N, 35°19′E), and was stored at the room temperature after being air-dried, milled and passed through a 2-mm sieve.

Preparation of biochar

The AP was washed with deionized water and air-dried for 3 days after being crushed. The biochar was prepared by pyrolyzing AP under O2-limited conditions at 350 and 650 °C for 2 h with a heating rate of 5 °C·min−1 (Qian & Chen 2014). The produced biochar was then screened, and only biochar particles with a diameter less than 0.830 mm were collected for the following analysis and batch sorption experiments. These biochar samples were labeled as LB (350 °C) and HB (650 °C) and the blank soil was set as the control.

Characterization of biochar

Determination of biochar ash content was calculated according to GB/T12496.3-1999 (Test methods of wooden activated carbon – Determination of ash content). The pH of the biochar was measured according to GB/T12496.7-1999 (Test methods of wooden activated carbon – Determination of pH). An elemental analyzer (MicroCube, Elementary, Germany) was used to determine the contents of C, H, O, N and S in biochar. The functional groups of biochar before and after aging treatment were analyzed by a Fourier transform infrared spectrometer (FTIR). The morphology and pore structure of biochar were observed by a scanning electron microscope (JSM-5600 LV, Japan).

Aging experiment

The AP biochar samples were oxidized with a 3:1 (v/v) H2SO4/HNO3 mixture for simulating the aging process in acidic environments, which could markedly increase the carboxyl content of biochar (Cho et al. 2010; Uchimiya et al. 2012). The biochar samples (5 g) were dipped into a beaker containing 400 mL of 20, 40, or 60% H2SO4/HNO3 dilution solution (Qian & Chen 2013). The remaining acid-free samples were treated with deionized water. The solution was maintained at 70 °C for 6 h in a water thermostat. These oxidized biochar samples were washed continuously with 1 L of deionized water for 50 min until the pH was stable before being dried at 50 °C and marked as LB-20%, LB-40%, LB-60%, HB-20%, HB-40% and HB-60% respectively according to different pyrolysis temperature and concentration of H2S O4/HNO3 dilution solution.

Batch sorption experiments

Adsorption kinetics experiments with the fresh and aged biochar for Pb(II) were conducted at room temperature (25 °C ± 1 °C). Blank soil (0.1 g) and the fresh and aged biochar samples (0.005 g) were placed separately into 40 mL of 0.01 mol·L−1 NaNO3 solution as the background electrolyte and a concentration of 30 mol·L−1 Pb(NO3)2 solution, and the initial pH of the solution was adjusted to 5.0. After that the samples were vibrated at a constant speed (200 rpm). Centrifuge tubes were withdrawn at different time intervals (5, 10, 30, 50, 90, 180, 300, 480, 660, 900 and 1,440 min), and the mixtures were filtered through a 0.45 pore size acetate fiber filter. Concentrations of Pb(II) in the filtrate after dilution were determined by an atomic absorption spectrophotometer (GFA-7000A, Japan). The adsorption capacity of biochar (qt) was calculated by Equation (1) 
formula
(1)
where qt represents the absorption amount of Pb(II) at time t (mg·g−1), C0 and Ct are the initial and at time t (min) concentration of Pb(II) (mg·g−1), respectively, V is the volume of solution (L) and m is the weight of adsorbents (g).
In order to analyze the adsorption kinetics of the fresh and aged biochar for Pb(II) at different temperatures, the pseudo-first-order (PFO) kinetic equation (Equation (2)) and pseudo-second-order (PSO) kinetic equation (Equation (3)) were used to describe the process of the adsorption (Inyang et al. 2012) 
formula
(2)
 
formula
(3)
where qt and qe represent adsorption capacities at time t (min) and at equilibrium (mg·g−1), k1 is the first-order adsorption rate constant (min−1), and k2 is the second-order adsorption rate constant (g·(mg·h)−1).
Adsorption experiments were performed in 50 mL centrifuge tubes containing the blank soil (0.1 g), and fresh and aged biochar (0.005 g) with Pb(II) solutions of varying initial concentrations (10–400 mg·L−1). The tubes were shaken at a constant speed (200 rpm) for 24 h after the pH of initial Pb(II) solutions was adjusted to 5. The pH-dependent adsorption experiments were respectively adjusted to the required pH range (2, 4, 5 and 6) by adding either 0.1 M HNO3 or 0.1 M NaOH. After shaking, the Pb(II) content in suspension liquid was measured by an atomic absorption spectrophotometer. The adsorption isotherms were expressed as the adsorption capacity of the adsorbents for Pb(II) at an equilibrium concentration calculated by Equation (4) and fitted according to Langmuir (Equation (5)) and Freundlich equations (Equation (6)): 
formula
(4)
 
formula
(5)
 
formula
(6)
where qe is the equilibrium adsorption capacity (mg·g−1), C0 and Ce are the initial and equilibrium concentrations respectively (mg·g−1), Kf is the adsorption capacity (mg·g−1), n is the Freundlich constant, qm is the maximum adsorption (mg·g−1), and K is the adsorption equilibrium constant (mg·L−1).

RESULTS AND DISCUSSION

Physicochemical properties of biochar

The basic properties of the fresh and aged biochar samples are shown in Table 1. The surface area (Brunauer–Emmett–Teller, BET) of the fresh and aged biochar at different pyrolysis temperature decreased in the order of HB-20% > LB-20% > HB > LB, which suggested that the BET of biochar was influenced by the pyrolysis conditions and aging process. The BET prepared at high temperature is larger, which indicated that higher pyrolysis temperature was conducive to the formation of a microporous structure in biochar. The BET of the aged biochar at the same pyrolysis temperature was higher than that of the fresh biochar, which suggested that oxygen-containing groups were introduced onto the biochar surfaces, which was also illustrated by the increase in O content and (O + N)/C ratios. In addition, the increase of O/C and (O + N)/C ratios explained that the biochar aging was a process of hydrophilicity and polarity enhancement. The H/C ratio of HB was lower than that of LB, which indicated that higher temperature biochar has a more complete aromatic structure, which was consistent with Ghaffar's findings using peanut shell (Ghaffar et al. 2015). Compared with the fresh biochar at the same pyrolysis temperature, the aromaticity (H/C) of the aged biochar did not change significantly, which was due to the release of soluble substances during aging treatment. The N content of biochar samples increased significantly from 2.68% to 5.94% in LB, and from 1.04% to 1.85% in HB; this was mainly due to the fact that N element was taken to the surface of biochar during the HNO3 oxidation treatment.

Table 1

Basic properties of the fresh and the aged biochar samples

sample pH BET
(m2·g−1
Element content (%)
 
Element ratio
 
Ash O/C H/C (O + N)/C 
LB 9.56 4.78 2.68 47.70 4.19 0.24 25.83 19.36 0.54 0.09 0.60 
LB-20% 2.44 6.43 5.94 56.21 4.92 0.14 32.00 0.79 0.57 0.08 0.62 
HB 10.39 4.93 1.04 45.55 1.47 0.45 18.73 31.76 0.41 0.03 0.43 
HB-20% 2.12 7.14 1.85 60.34 1.91 0.43 28.00 5.46 0.46 0.03 0.49 
sample pH BET
(m2·g−1
Element content (%)
 
Element ratio
 
Ash O/C H/C (O + N)/C 
LB 9.56 4.78 2.68 47.70 4.19 0.24 25.83 19.36 0.54 0.09 0.60 
LB-20% 2.44 6.43 5.94 56.21 4.92 0.14 32.00 0.79 0.57 0.08 0.62 
HB 10.39 4.93 1.04 45.55 1.47 0.45 18.73 31.76 0.41 0.03 0.43 
HB-20% 2.12 7.14 1.85 60.34 1.91 0.43 28.00 5.46 0.46 0.03 0.49 

The FTIR spectrums of the fresh and aged biochar samples are shown in Figure 1. Some changes occurred in the surface functional groups of biochar after aging treatment. There were wide absorption peaks at 3,430 cm−1 and 3,244 cm−1 due to the stretching vibrations of O–H (Yu et al. 2018). The stretching vibrations near the band at 2,987 cm−1 and 2,839 cm−1 were aliphatic –CH2 (Zhang et al. 2011). After aging, the adsorption peaks were not as obvious as those of the fresh biochar, which indicated that a large number of hydroxyl and carboxyl groups were highly esterified and distorted at high pyrolysis temperatures, which made the aromatics more complete. The absorption peaks near the band at 2,355 cm−1 mainly corresponded to C ≡ C or C ≡ N or the cumulative stretching vibration of double bonds. The peak at 1,700 cm−1 represented the stretching vibrations of C = O and C = C, corresponding to aromatic characteristics. After the peak at 1,700 cm−1, the significant increase of amplitudes in all biochar samples suggested the development of new surface functional groups in biochar. The bands at 1,610 cm−1 and 1,394 cm−1 were characteristic of the C = C stretching vibration of the aromatic ring and the stretching vibrations of O–H (Zeng et al. 2013; Zhao et al. 2013). The band near 1,049 cm−1 represented the C–O, which generally existed in phenols or hydroxyl groups. The stretching vibration peak near band 779 cm−1 was considered to be produced by the inorganic mineral Si–O–Si (Qian & Chen 2013). After the acid oxidative aging of biochar, the disappearance of the Si–O–Si stretching vibration peak indicated that the aging effect changed the type and quantity of functional groups on the surface of biochar.

Figure 1

FTIR spectra of biochar samples and aged biochar samples.

Figure 1

FTIR spectra of biochar samples and aged biochar samples.

The different magnification (zoom 100, 500, 1,000 and 2,000 times, from left to right) scanning electron microscope (SEM) images of the fresh and aged biochar samples are shown in Figure 2. From the micrographs, the surface of all biochars exhibited a non-uniform distribution. There were significant differences in surface morphology of AP biochar of two pyrolysis temperatures. The number of micropores followed the order: HB > LB > LB-20% > HB-20%. In addition, low-temperature biochar presented a complete tubular structure. High-temperature biochar had a rougher surface and a more microporous and well-developed pore structure, which indicated that more active sites were provided for adsorption of Pb(II). With the increase of pyrolysis temperature, the disordered carbon of biochar formed a large number of graphene lamellae, which made the structure of biochar more similar to that of graphene. Some lamellae formed a microporous structure due to the destruction of structural defects. Therefore, the biochar pyrolysis at high temperature had stronger stability. The aged biochar had relatively thicker layering and a smooth external morphology compared with the fresh biochar, which was due to the dissolution of inorganic mineral on the surface of AP biochar during the acidic oxidative aging, which destroyed the micropores and pore structure of biochar and resulted in a decrease of adsorption sites (Spokas et al. 2014).

Figure 2

Scanning electron micrographs of the biochar.

Figure 2

Scanning electron micrographs of the biochar.

Kinetic studies

The adsorption experiments kinetic data was fitted by the PFO and PSO kinetic models. The fitting results are presented in Figure 3 and Table 2. It can be observed from Figure 3(a) and 3(b) that the adsorption capacity of Pb(II) by all biochar increased with time, reaching the adsorption equilibrium at about 400 min, being a rapid reaction stage within the first 90 min, reaching 86.02–95.21% of the equilibrium adsorption capacity, and then it tends to stabilize with time. This might be explained by the fact that there were more adsorption sites on the surface of the adsorbent at the beginning of the reaction, and Pb2+ was mainly adsorbed on the surface of the adsorbent. As the reaction proceeded, the adsorption sites gradually became saturated. Then Pb2+ diffused into the porous adsorbent material, and the adsorption rate was slow. Simultaneously, the remaining adsorption sites with lower affinity began to play a role, accompanied by interstitial diffusion, resulting in slowing down the adsorption rate and eventually tending to adsorption equilibrium.

Table 2

Kinetics parameters of the PFO kinetic model and the PSO kinetic model

Sample qe,exp
(mg·g−1
PFO kinetic model
 
PSO kinetic model
 
qe1
(mg·g−1
K1
(min−1
R2 qe2
(mg·g−1
K2
(g·(mg·h)−1
R2 
LB 10.33 9.94 0.196 0.971 10.19 0.033 0.974 
LB-20% 9.46 8.95 0.067 0.951 9.45 0.011 0.981 
LB-40% 9.68 9.23 0.124 0.974 9.60 0.016 0.975 
LB-60% 10.17 9.69 0.133 0.973 10.06 0.021 0.977 
soil 8.07 7.90 0.162 0.981 8.14 0.034 0.998 
HB 10.92 10.23 0.202 0.978 10.48 0.036 0.973 
HB-20% 8.89 8.45 0.078 0.953 8.72 0.011 0.985 
HB-40% 9.49 9.04 0.142 0.975 9.66 0.026 0.983 
HB-60% 10.08 9.73 0.181 0.986 9.98 0.033 0.997 
Sample qe,exp
(mg·g−1
PFO kinetic model
 
PSO kinetic model
 
qe1
(mg·g−1
K1
(min−1
R2 qe2
(mg·g−1
K2
(g·(mg·h)−1
R2 
LB 10.33 9.94 0.196 0.971 10.19 0.033 0.974 
LB-20% 9.46 8.95 0.067 0.951 9.45 0.011 0.981 
LB-40% 9.68 9.23 0.124 0.974 9.60 0.016 0.975 
LB-60% 10.17 9.69 0.133 0.973 10.06 0.021 0.977 
soil 8.07 7.90 0.162 0.981 8.14 0.034 0.998 
HB 10.92 10.23 0.202 0.978 10.48 0.036 0.973 
HB-20% 8.89 8.45 0.078 0.953 8.72 0.011 0.985 
HB-40% 9.49 9.04 0.142 0.975 9.66 0.026 0.983 
HB-60% 10.08 9.73 0.181 0.986 9.98 0.033 0.997 
Figure 3

Adsorption kinetic fitting of the fresh and aged biochar: (a) the low pyrolysis temperature biochar, (b) the high pyrolysis temperature biochar.

Figure 3

Adsorption kinetic fitting of the fresh and aged biochar: (a) the low pyrolysis temperature biochar, (b) the high pyrolysis temperature biochar.

It can be seen from Table 2 that both of the kinetic models could elucidate the mechanism of Pb(II) adsorption, which indicated that Pb(II) adsorption might be controlled by physical and chemical adsorption processes and it occurred mainly on the surface of biochar. However, the correlation coefficients (R2) of the PSO kinetic model (0.973–0.998) were higher than those of the PFO kinetic model (0.951–0.986) for all biochar samples. Simultaneously, the values of parameter qe2 (10.19 (LB), 9.45 (LB-20%), 9.60 (LB-40%), 10.06 (LB-60%), 8.14 (soil), 10.48 (HB), 8.72 (HB-20%), 9.66 (HB-40%), 9.98 (HB-60%) mg·g−1) were closer to the actual adsorption capacity (qe,exp) (10.33 (LB), 9.46 (LB-20%), 9.68 (LB-40%), 10.17 (LB-60%), 8.07 (soil), 10.92 (HB), 8.89 (HB-20%), 9.49 (HB-40%), 10.08 (HB-60%) mg·g−1) than were the values of parameter (qe1), suggesting that Pb(II) adsorption changes to a chemisorption process (Hao et al. 2017; Zhang et al. 2018). In addition, the qe2 inferred from the PSO kinetic model decreased in the order HB > LB > LB-60% > LB-40% > LB-20% > HB-60% > HB-40% > HB-20%. The qe,exp and qe2 of aged biochar at low temperature was higher than that of aged biochar at high temperature, which might be due to the damage degree of the pore structure of the biochar at the high pyrolysis temperature being greater than that of the biochar at the low pyrolysis temperature. In addition, it can be seen from Table 1 that the aged biochars at low temperature have more oxygen-containing functional groups compared with the aged biochars at high temperature.

The adsorption rate constant (K1 and K2) could reflect the adsorption rate of Pb(II) by biochar. The larger value of K1 and K2 means the shorter time required to achieve equilibrium. It can be seen from the K1 and K2 values of the PSO and PFO kinetic model shown in Table 2 that fresh and aged biochar at the high temperature have higher K values than that at the low temperature, and the K1 and K2 values gradually increased and finally approached the unaged K value as the degree of aging increased, which might be because the physicochemical characteristics of biochar were changed during the aging process.

Effect of initial Pb(II) concentration on Pb(II) adsorption

In this study, at different initial Pb(II) concentrations (10, 30, 60, 120, 300, 400 mg·L−1), the sorption capacities of the fresh and aged biochar were measured and fitted to the analysis using the Langmuir and Freundlich models. As illustrated in Figure 4, the adsorption capacity of biochar increased with the increase of Pb(II) concentration, grew rapidly in the range of 0–250 mg·L−1, and tended to be smoothly saturated in the range of 250–400 mg·L−1.

Figure 4

Adsorption isotherm fitting of the fresh and aged biochar: (a) the low pyrolysis temperature biochar, (b) the high pyrolysis temperature biochar.

Figure 4

Adsorption isotherm fitting of the fresh and aged biochar: (a) the low pyrolysis temperature biochar, (b) the high pyrolysis temperature biochar.

The parameters of the models are listed in Table 3. Both the Langmuir and Freundlich adsorption model could fit the adsorption of Pb(II) before and after oxidative aging. However, the R2 (0.990–0.999) of the Langmuir model was higher than that of the Freundlich model (0.942–0.994). The Langmuir model assumes that the adsorption occurs in a monolayer; that is, adsorbates were uniformly distributed on the adsorbent surface in a monomolecular layer, and finally reached the maximum adsorption capacity. (Yang et al. 2014). The values of the maximum adsorption capacities (qm) are also shown in Table 3. By comparing the Pb(II) adsorption capacity (286.07 mg·g−1) of HB with that (279.85 mg·g−1) of LB, it was found that the qm increased by 2.222%. According to the parameters of the Langmuir model, the qm decreased in varying degrees after the aging treatment. For example, compared with LB (279.85 mg·g−1), the qm of LB-20%, LB-40% and LB-60% of Pb(II) were 134.55, 222.75, and 242.57 mg·g−1, respectively, which decreased by 51.921%, 20.404%, and 13.321%. Compared with HB (286.07 mg·g−1), the qm of HB-20%, HB-40% and HB-60% of Pb(II) were 93.01, 131.05, and 159.82 mg·g−1, respectively, which decreased by 67.487%, 54.181%, and 44.133%. This might be related to the leaching loss of alkaline substances in biochar during acid oxidation aging. For the aged biochar at the same pyrolysis temperature, the higher the oxidation degree was, the higher qm was, which might be due to more adsorption sites produced by acid oxidation on the surface of the aged biochar.

Table 3

Freundlich and Langmuir isotherm fitting results of Pb(II) adsorption

Sample Langmuir
 
Freundlich
 
K (L∙mg−1qm (mg·g1R2 Kf (mg∙g−1n R2 
LB 0.002 279.85 0.996 1.271 0.767 0.983 
LB-20% 0.004 134.55 0.995 1.595 0.656 0.978 
LB-40% 0.002 222.75 0.997 1.156 0.749 0.994 
LB-60% 0.002 242.57 0.999 1.132 0.760 0.993 
soil 0.005 79.79 0.994 1.763 0.578 0.983 
HB 0.002 286.07 0.994 1.287 0.767 0.985 
HB-20% 0.005 93.01 0.964 1.572 0.614 0.942 
HB-40% 0.002 131.05 0.990 1.516 0.776 0.983 
HB-60% 0.003 159.82 0.995 1.527 0.682 0.978 
Sample Langmuir
 
Freundlich
 
K (L∙mg−1qm (mg·g1R2 Kf (mg∙g−1n R2 
LB 0.002 279.85 0.996 1.271 0.767 0.983 
LB-20% 0.004 134.55 0.995 1.595 0.656 0.978 
LB-40% 0.002 222.75 0.997 1.156 0.749 0.994 
LB-60% 0.002 242.57 0.999 1.132 0.760 0.993 
soil 0.005 79.79 0.994 1.763 0.578 0.983 
HB 0.002 286.07 0.994 1.287 0.767 0.985 
HB-20% 0.005 93.01 0.964 1.572 0.614 0.942 
HB-40% 0.002 131.05 0.990 1.516 0.776 0.983 
HB-60% 0.003 159.82 0.995 1.527 0.682 0.978 

Effect of the initial pH on Pb(II) adsorption

The initial pH had a marked effect on the removal of Pb(II) because different pH values affect the form, ionization degree and surface charge of lead in solution, which is one of the important factors affecting the adsorption of heavy metal ions (Atkinson et al. 2010).

Figure 5 shows the adsorption capacity of Pb(II) by the fresh and aged AP biochar at different pH conditions. It was found that the qe increased continuously with the change of pH value from 2 to 5. This trend was similar to previous research (Yang et al. 2014) because of the free Pb(II) and a small amount of Pb(OH)+ in aqueous solution at low pH. Competition of H+ caused a decrease in the binding sites of Pb(II) on biochar. The effect of pH on the adsorption process was significant. With the increase of solution pH, the competition for adsorption sites decreased due to the decrease of H+ concentration, and the adsorption of Pb(II) by hydrolysis, precipitation, and electrostatic attraction was gradually strengthened, which improved the adsorption performance of biochar and the adsorption capacity of Pb(II). At pH 5, the maximum qe of all biochar samples was obtained. Nevertheless, the further increase of solution pH resulted in a decrease of the qe, which might be due to the difficulty of dissolving mineral components in the biochar. The precipitation or formation of Pb(II) hydroxide complexes also resulted in the decrease of qe (Kolodynska et al. 2012). Moreover, compared with the fresh biochar samples, the qe of the aged biochar samples decreased, indicating that the decrease of the pH during the aging process caused reduction of the qe of the biochar, which was attributed to the reduction of the number of carboxyl groups and changes of functional groups on the surface of the biochar.

Figure 5

Effect of pH on the adsorption of Pb(II) by the fresh and aged biochar: (a) the low pyrolysis temperature biochar, (b) the high pyrolysis temperature biochar.

Figure 5

Effect of pH on the adsorption of Pb(II) by the fresh and aged biochar: (a) the low pyrolysis temperature biochar, (b) the high pyrolysis temperature biochar.

CONCLUSIONS

Based on the experimental results, the following conclusions can be drawn. After the aging treatment with HNO3/H2SO4, there were significant changes in surface morphology and adsorption capacity of biochar. The content of O and (O + N)/C increased at two preparation temperatures, which resulted in the increase of polar oxygen-containing functional groups. In addition, the pH and ash content of the aged biochar were lower than that of the fresh biochar.

The adsorption equilibrium of Pb(II) by the fresh and aged biochar was reached at about 400 min, with a rapid reaction stage within the first 90 min, and reaching 86.02–95.21% of the equilibrium adsorption capacity. The adsorption kinetics of the fresh and aged biochar fitted the PSO kinetic model, which was greatly affected by the chemical adsorption mechanism. The Pb(II) adsorption data of the fresh and aged biochar samples was better fitted by the Langmuir model, which meant that the adsorption might occur in a monolayer. At the same reaction temperature, the maximum equilibrium adsorption increased significantly with the increase of the initial concentration of Pb(II). Furthermore, the destruction of pore structure and the decrease of carboxyl functional groups during the aging process weakened the adsorption effect for Pb(II). The effect of pH on the adsorption process was significant. With the increase of pH, the adsorption capacities of Pb(II) on the fresh and aged biochar increased gradually, and the adsorption effect was best at pH 5. The adsorption capacity of HB for Pb(II) was higher than that of LB. In addition, the adsorption capacity of aged biochar for Pb(II) decreased obviously and the adsorption capacities of the aged biochar did not increase with the increase of aging degree.

In the experiment of simulating natural aging, the aging mode of acidic oxidation was simulated under laboratory conditions. Although the aging behavior of biochar for many years can be simulated by this aging mode, its aging factors are variable once biochar is applied into the environment, such as humus in soil, temperature, and microbial action. Therefore, changes of biochar properties under different aging conditions and the biochar's adsorption/desorption behavior for Pb(II) need to be further studied.

ACKNOWLEDGEMENTS

This research is financially supported by the Natural Science Foundation of Shandong Province (No. RZ2013DM005) and Laboratory Open Fund Project of Qufu Normal University (No. SK201714).

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