Environmental Science and Pollution Research https://doi.org/10.1007/s11356-018-1356-0
RESEARCH ARTICLE
Kinetic and isothermal adsorption-desorption of PAEs on biochars: effect of biomass feedstock, pyrolysis temperature, and mechanism implication of desorption hysteresis Fanqi Jing 1,2 & Minjun Pan 1,2 & Jiawei Chen 1,2 Received: 3 July 2017 / Accepted: 22 January 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract Biochar has the potential to sequester biomass carbon efficiently into land, simultaneously while improving soil fertility and crop production. Biochar has also attracted attention as a potential sorbent for good performance on adsorption and immobilization of many organic pollutants such as phthalic acid esters (PAEs), a typical plasticizer in plastic and presenting a current environmental issue. Due to lack of investigation on the kinetic and thermodynamic adsorption-desorption of PAEs on biochar, we systematically assessed adsorption-desorption for two typical PAEs, dimethyl phthalate (DMP) and diethyl phthalate (DEP), using biochar derived from peanut hull and wheat straw at different pyrolysis temperatures (450, 550, and 650 °C). The aromaticity and specific surface area of biochars increased with the pyrolysis temperature, whereas the total amount of surface functional groups decreased. The quasi-second-order kinetic model could better describe the adsorption of DMP/DEP, and the adsorption capacity of wheat straw biochars was higher than that of peanut hull biochars, owing to the O-bearing functional groups of organic matter on exposed minerals within the biochars. The thermodynamic analysis showed that DMP/DEP adsorption on biochar is physically spontaneous and endothermic. The isothermal desorption and thermodynamic index of irreversibility indicated that DMP/ DEP is stably adsorbed. Sorption of PAEs on biochar and the mechanism of desorption hysteresis provide insights relevant not only to the mitigation of plasticizer mobility but also to inform on the effect of biochar amendment on geochemical behavior of organic pollutants in the water and soil. Keywords Biochar . PAEs . Adsorption . Desorption . Hysteresis
Highlights • Adsorption-desorption of DMP/DEP on biochars derived from different biomass and pyrolysis temperature was compared. • DMP/DEP adsorption on biochars is physically spontaneous and endothermic. • DMP/DEP could be immobilized after being adsorbed and biochars showed good adsorption stability. Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11356-018-1356-0) contains supplementary material, which is available to authorized users. * Jiawei Chen
[email protected] 1
State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, People’s Republic of China
2
School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, People’s Republic of China
Introduction Biochar is the carbon-rich solid product of biomass pyrolysis. The idea of converting agricultural residues into biochar has become attractive in recent years (Lehmann and Joesph, 2009; Kookana, 2010). As a soil amendment, biochar has great potential as an efficient use of biomass to improve soil fertility and sequester carbon in soil (Ahmad et al., 2012). In soil and standalone, much attention has focused on the absorptive properties of biochar. The notable capacity of biochar for organic pollutants has been attributed to a high specific surface area, well-developed pore structure, and rich diversity of surface functional groups (Cao et al., 2011; Chun et al., 2004). In pollutant management, a new high performance option has emerged: biochar as membrane composites such as biochar/ polyvinylidene fluoride (PVdF) and biochar/graphene (Abdul et al., 2017, 2018). The sorption behavior of heavy metals (Yang et al., 2016), polycyclic aromatic hydrocarbons
Environ Sci Pollut Res
(Chinedum et al., 2015), trichloroethylene (Ahmad et al., 2013), and 4-bromodiphengl ether (Du et al., 2016) in soil and water have been investigated extensively and mechanisms postulated (Zhang et al., 2013). This informs the potential for biochar to affect the geochemical behavior of pollutants in the environment. The widespread use of plastic in food packaging and agriculture products presents ecological and environmental risks from phthalic acid esters (PAEs). These organic compounds are typically used as plasticizer additives to plastic manufacture (Jin et al., 2014; Gao et al., 2013). These compounds are ubiquitous in the environment, especially near sites of PAE production and use. In China, PAEs have been reported in the Haihe River, Yellow River (Taiyuan Section), and Yangtze River Delta amongst others, at concentrations up to 91.2 μg L−1 (Jie, 2009; Sha et al., 2007; Zhang et al., 2012). PAEs enter human body via multiple pathways (Blair et al., 2009). The accumulation of PAEs has suspected carcinogenic effects on humans (Wang et al., 2015; Mahmoud et al., 2012). Some researchers have considered potential application of carbon-based materials such as activated carbon (Gao et al., 2013), carbon nanotubes (Gao et al., 2013; Lu et al., 2018), and graphene (Abdul et al., 2017) regarding remediation of PAEs. Biochar offers a lower cost option that also draws only on residual biomass resources. Sun et al. (2012) used the Freundlich model to describe adsorption isotherms for PAEs on biochar, considering the effect of source biomass and pyrolysis temperature. They found that biochar pyrolyzed at low temperature to show maximum adsorption capacity. Abdul and Ghulam (2016) found that capacity decreases in the order of dimethyl phthalate (DMP) > diethyl phthalate (DEP). This was attributed to different hydrophobic properties of PAEs molecules. Beside from above limited information on the interact i o n o f PA E s w i t h b i o c h a r s ( S u n e t a l . , 2 0 1 2 ; Abdul and Ghulam 2016), a quantitative description for PAE adsorption on biochar based on a specific underlying mechanism is missing. Importantly, the desorption characteristics relevant to reactive migration of PAEs in the environment have not been established. Desorption hysteresis for graphene has previously been established, suggesting caution in their use as sorbents (Lu et al., 2018). Previous studies of biochar have focused on the thermodynamics of adsorption of PAEs, with limited exploration of the corresponding desorption. Our work evaluates the effect of biomass source and pyrolysis temperature on sorption and desorption processes; since, both the organic and mineral phases contributed to these phenomenon. A series of batch experiments on isothermal adsorption-desorption of DMP and DEP are undertaken, using a sequence of biochars made from peanut hull and wheat straw biomass.
Materials and methods Chemicals DMP (≥ 99.5%) and DEP (≥ 99.0%) were purchased from Dr. Ehrenstorfer GmbH (Augsburg, Germany). The physicochemical properties of DMP and DEP are shown in Table S1. Methanol (chromatographic purity, ANPEL Laboratory Technologies, Shanghai, China), CaCl2, and KBr (analytical grade, Sinopharm Group Chemical Reagent Co, Beijing, China) were used as received, without further purification. Deionized water (> 18.3 MΩ cm) was used for all the experiments.
Biochar preparation and characterization Biochar samples were made at laboratory scale using peanut hull and wheat straw as the biomass source material (feedstock). The biomass was washed, air-dried, crushed, and packed tightly in a ceramic pot, covered with a close-fitting lid, and placed in a muffle furnace (SX2-4-10, Tianjin Central Experimental Electric Furnace Co., Tianjin, China). The furnace was then heated to 450, 550, or 650 °C, at a rate of 3 °C/min. The set temperature was then maintained for 2 h. The biochar product was prepared for batch experiments using a pestle and mortar, with size grading to 0.3–0.5 mm through 35- and 60-mesh sieves. The peanut hull biochars and wheat straw biochars produced at temperatures of 450, 550, and 650 °C are hereafter denoted P450, P550, P650, W450, W550, and W650, respectively. The physicochemical properties of the biochar samples were determined using multiple techniques. Ash content was calculated as the mass difference before and after heating samples in the muffle furnace to 750 °C for 4 h. Bulk C, H, and N content was determined by dry combustion in an elemental analyzer (EA3000, Euro Vector, Italy). Bulk O content was calculated based on CHN and ash determinations. Surface functional groups were initially assessed in the 4000–400cm−1 region of the Fourier transform infra-red (FTIR) spectrum (Nicolet iS10 spectrometer, Thermo Fisher Scientific, USA). Further surface characterization was made using Xray photoelectron spectroscopy (XPS; ESCALAB 250Xi Thermo Fisher, USA). The surface morphology of samples was assessed using scanning electron microscopy (SEM; Zeiss Supra 55-VP, Germany). Specific surface area and pore size distribution of biochar were analyzed using the BET-N2 adsorption method (Quadrasorb Station 1 analyzer, Quantachrome Instruments, Boynton Beach, USA).
Adsorption and desorption of DMP/DEP on biochar Batch experiments were conducted to obtain the adsorption thermodynamics and kinetics of adsorption-desorption of
Environ Sci Pollut Res
PAEs on the six as-prepared biochars (P450, P550, P650, W450, W550, W650). DMP and DEP were dissolved in methanol to create a stock solution. Working solutions of different concentrations were then prepared, together with a background solution of 0.01-M CaCl2. The methanol content of the working solutions did not exceed 0.1%, avoiding cosolvent effects. For adsorption kinetics, DMP/DEP at an initial concentration of 80 mg L−1 was mixed with biochar samples W450 and P450 in 40-mL vials, at a solid-to-liquid ratio of 1:50. All vials were sealed with Teflon screw caps and shaken at 160 r min−1 at 35 °C. About 0.3-mL supernatant was withdrawn from each vial after 0, 10, 30, 60, 120, 360, 720, 1440, 2160, 2880 and 4320 min. The supernatants were filtered through 0.22-μm polyether sulfone filters, and each added to a 2-mL vial with 0.2-mL microinsert for DMP and DEP analysis. For adsorption isotherms, DMP/DEP was added into the vials with desired concentrations of 5–160 mg L−1 at a solidto-liquid ration of 1:50. All vials were shaken at 160 r min−1 for 3 days, based on the equilibrium time established in the prior kinetics experiment. The vials were placed up to solidliquid separation, and about 2-mL supernatant withdrawn, filtered through a 0.22-μm polyether sulfone filter, and added to the 2-mL vial prior to DMP and DEP analysis. For desorption experiments, the same amount of fresh background solution of 0.01-mol L−1 CaCl2 was added after removal of liquid phase in the vials when the sorption reached equilibrium. Then the vials were shaken again for another 3 days and 2-mL supernatant was withdrawn for DMP and DEP analysis. All the batch experiments were run in duplicate and the DMP and DEP uptake by biochar was calculated by mass balance. The blank and control test indicated negligible loss of DMP and DEP during the experiments.
DMP/DEP analysis The DMP and DEP in solution were measured by HPLC (LC20A, Shimadzu, Japan) equipped with a reversed-phase C18 analytical column (5 μm, 4.6 mm × 250 mm). The mobile phase was methanol and deionized water with a ratio of 80:20 (v:v). The conditions were optimized as the flow rate 1 mL min−1, the detection wavelength 235 nm, the column temperature 35 °C, and the injection volume 10 μL. The retention time of DMP and DEP was 4.37 and 5.13 min, respectively.
Results and discussion Biochar characterization Source biomass and heating temperature had a significant effect on the physicochemical properties of biochars.
Decomposition of cellulose, hemicellulose, and fats is progressive during pyrolysis, so peak pyrolysis temperature can largely determine the porosity and specific surface area of chars. In the present study, there was a remarkable difference in the specific surface area of biochar samples; W650 (474.9 m2 g−1) and P650 (435.9 m2 g−1) were remarkably higher than that of W450 (19.9 m 2 g − 1 ) and P450 (2.7 m2 g−1), respectively. The N2 sorption-desorption isotherms (Fig. S1) were used to calculate pore size distribution (Table 1, Fig. S2). The pore volume of biochar increased with higher pyrolysis temperature. It showed biochar porosity comprising mainly 2–4-nm mecropores and 0.5–0.7-nm micropores. The isotherms (Fig. S1) of biochars made at 450 °C had overlapping, nearly horizontal sorption curves with no hysteresis loop. Biochars made at 650 °C did not undergo desorption under lower pressure, which could be attributed to the blocking of pores, particularly pores with a neck diameter markedly smaller than the pore interior (Oleszczuk et al., 2016). The elemental composition, ash content, atomic ratio, and surface area of all biochars (as prepared) are shown in Table 1. The organic matter was more decomposed and ash content of biochars increased with pyrolysis temperature. The ash also differed according to the source biomass (Sun et al., 2013). These findings reflect the general observation that increasing pyrolysis temperature is associated with biochar comprising more inorganic minerals (ash) and more graphitic carbon (Keiluweit et al., 2010). In the present study, the ash content of wheat straw biochar (13.48–15.69% ash) was higher than that of peanut hull biochar (3.53–4.12% ash). In other studies, wheat straw biochar has contained 12.12–18.66% ash (Wang and Liu, 2017), and 3.3–6.6% ash in biochar from in peanut hull (Abdul and Ghulam 2016). Examination by scanning electron microscope revealed differences in surface morphology (Fig. 1). The cleaner appearance of peanut hull biochar (Fig. 1a–c) contrasted with the rod shaped structure of wheat straw biochar, reflecting higher ash in the latter (Fig. 1d–f). The C content in biochars increased with pyrolysis temperatures, while H, N, and O contents decreased, indicating an increasing degree of carbonization and depolarizing functional groups. This was consistent with the properties of biochar prepared from pine needles and orange peels in others work (Chen et al., 2008; Chen and Chen, 2009). The measured status of H/C, O/C, and (N + O)/C ratios often used to as indicators of aromaticity, hydrophilicity, and polarity reflected the trend established in previous work (Wu et al., 2016; Novak et al., 2009). The ratios H/C, O/C, and (N + O)/C were gradually decreased as pyrolysis temperature increased, indicating higher aromaticity, lower hydrophilicity, and lower polarity (Table 1). The XPS spectra (Fig. 2) showed C─C/C─H and C─O to be dominant surface functional groups, with C═O groups comprising less than 10.94% (Table S2). Dominance
Environ Sci Pollut Res Table 1 straw
The elemental composition, ash content, atomic ratio value, surface area, and pore volume of biochars derived from peanut hull and wheat
Biochars
N (%)
C (%)
H (%)
O (%)
Ash (%)
H/C
O/C
(O + N)/C
BET-N2 surface area (m2 g−1)
Total pore volume (cm3 kg−1)
Average pore diameter (nm)
P450 P550 P650 W450 W550 W650
1.08 1.29 1.19 1.43 1.06 1.06
76.40 82.04 84.87 68.74 71.88 73.71
3.42 2.46 2.00 2.60 1.50 1.51
15.58 10.35 7.82 12.01 11.61 8.12
3.53 3.86 4.12 13.48 13.95 15.59
0.63 0.42 0.33 0.53 0.29 0.29
0.18 0.11 0.08 0.15 0.14 0.10
0.19 0.13 0.09 0.17 0.16 0.11
2.7 247.3 435.9 19.9 225.7 474.9
4.3 272.9 422.1 25.6 230.1 536.2
6.337 4.415 3.874 5.131 4.078 4.517
increased with biochar pyrolysis temperature owing to greater carbonization. According to FTIR spectra of biochars (Fig. 3), the bands of highest intensity were similar for all of the as-produced biochars. The total functional group density was slightly lower for 450-°C biochars, especially the oxygen-bearing functional groups of P550 such as ketones, amides, and carboxylic acid groups corresponding to the C═O at the peak of 1599 cm−1 and C─OH at peak 1113 cm−1 (Chen et al., 2008). The FTIR spectra recorded before and after adsorptiondesorption of DMP/DEP for W650 and P650 are shown in Fig. 3b, c, respectively. They show that the typical functional groups of P650 are affected little by interaction with DMP/ Fig. 1 SEM images of biochar samples P450 (a), P550 (b), P650 (c), W450 (d), W550 (e), and W650 (f)
DEP. By comparison, the O-containing functional groups in W650, such as ─OH at the peak at 3431 cm−1, aromatic C═C at 1690 cm−1, and ketones, amides, and carboxylic acids C═O at 1585 cm−1, were all markedly decreased after adsorption and increased after desorption. This indicates that these Obearing functional groups were central to the interaction of what straw biochars with DMP/DEP.
Adsorption kinetics of DMP/DEP on biochars The kinetic experiments were conducted on adsorption of initial 80-mg L−1 DMP/DEP on biochars of W450 and P450 as shown in Fig. 4. The sorption equilibrium was reached in
Environ Sci Pollut Res Fig. 2 Curve-fitting XPS (C1s) spectrum for biochar samples P450 (a), P550 (b), P650 (c), W450 (d), W550 (e), and W650 (f)
Fig. 3 FTIR spectra of the wheat straw and peanut hull derived biochars. a FTIR spectra of different biochars, b W650, and c P650 before and after DMP/DEP adsorption-desorption
Environ Sci Pollut Res Fig. 4 Adsorption kinetics of DMP (a) and DEP (b) on biochars P450 and W450
approximately 2–3 days. The quasi-first-order and secondorder kinetic models in the following Eqs. (1) and (2) could describe such sorption behavior. Quasi-first-order kinetic model: qt ¼ qe 1−e−kt ð1Þ 1 Quasi-second-order kinetic model: qt ¼ qe 2 k 2 t=ð1 þ qe k 2 t Þ
ð2Þ
where qe (mg kg−1) is the adsorption capacity at equilibrium, qt (mg kg−1) is the adsorption capacity for DMP/DEP on biochars at time t, k1 (min−1) is the equilibrium rate constant of quasi-first-order, and k2 (kg mg−1 min−1) is the quasi-secondorder rate constant. The kinetic models with fitting parameters of the adsorption of DMP/DEP on W450 and P450 are listed in Table 2. They show that the quasi-second-order kinetic model fitted quite well (R2 > 0.92). According to Fig. 4 and Table 2, the sorption capacity qe of DMP/DEP was relatively large, whereas the reaction rate constant k2 was small. The rate of sorption was low and the time taken for equilibrium longer.
Adsorption isotherms of DMP/DEP on biochars Isothermal adsorption of DMP/DEP on biochars made from peanut hull and wheat straw biomass at an ambient temperature of 35 °C is illustrated in Fig. 5. Such nonlinear isotherms are normally fitted by Langmuir or Freundlich models in Eqs.
Table 2 The kinetic parameters of the adsorption of DMP/DEP on biochars P450 and W450
PAEs
DMP DEP
Biochars
P450 W450 P450 W450
(3) and (4). The associated fitting parameters are listed in Table 3. Based on this procedure, there is no single mode appropriate for all types of biochar, although the Langmuir or Freundlich models offered the best description for DMP/ DEP. This indicates that sorption behavior is complicated and depends on both biomass and pyrolysis temperature. Furthermore, the nonlinearity factor from Freundlich model (n < 0.5) suggests that the adsorption of DMP/DEP should be preferential. Langmuir model: qe ¼ KC e qmax =ð1 þ kC e Þ
ð3Þ
Freundlich model: qe ¼ K f C e n ;
ð4Þ
where qe (mg kg−1) is the solid-phase concentration, Ce (mg L−1) the aqueous-phase concentration, k (L mg−1) the adsorption affinity parameter in Langmuir model, qmax (mg kg−1) the Langmuir saturated adsorption capacity, kf (mg1–n kg−1 Ln) the Freundlich adsorption parameter, and n the nonlinearity factor, indicating the adsorption intensity. Adsorption isotherms for DMP/DEP showed that P650 had the highest capacity to adsorb DMP. The specific surface area and total pore volume were high for this sample (Table 1). N2 adsorption-desorption isotherms (Fig. S1) showed limited desorption at lower pressure owing to the blocking of pores (Oleszczuk et al., 2016), emphasizing
Quasi-first-order model
Quasi-second-order model
k1/min−1
qe/(mg kg−1)
R2
k2/(kg mg−1 min−1)
qe/(mg kg−1)
R2
1.56 × 10−2 1.27 × 10−2 2.63 × 10−2 1.74 × 10−2
1026.82 1933.60 1249.68 1967.59
0.807 0.888 0.883 0.841
2.31 × 10−5 9.55 × 10−6 3.49 × 10−5 1.38 × 10−5
1071.85 2027.41 1293.79 2047.15
0.925 0.950 0.965 0.962
Environ Sci Pollut Res Fig. 5 Adsorption isotherms of DMP (a) and DEP (b) on biochars at 35 °C
that pore filling effects are important in governing the sorption capacity of biochar (Chun et al., 2004). Made at equal pyrolysis temperature 650 , biochar made from wheat straw exhibited higher adsorption capacity for DMP than biochar made from peanut hull. The adsorption capacity of wheat straw biochars was slightly lower at the higher pyrolysis temperature, perhaps due to the slightly lower hydrophilicity and polarity. The capacity for DEP adsorption on wheat straw biochar increased with the pyrolysis temperature, and W650 had the highest adsorption capacity. This probably reflected the higher aromaticity and the potential dominant role of π-π EDA interaction (Abdul et al. 2017; Keiluweit et al., 2010). Differences in water solubility and hydrophobicity (Table S1) between DEP and DMP could also affect sorption capacity (Abdul et al. 2017). The effects of surface functional groups, pore filling, and π-π EDA interaction could explain observed patterns of DMP/ DEP sorption. The markedly lower functional group density of P550 measured inferred from FTIR (Fig. 3a) suggests lower hydrophobicity effects on DMP/DEP sorption. It was in accordance with the weaker adsorption behavior of P550 with respect to DMP/DEP. Table 3 The model parameters of isothermal adsorption of DMP/ DEP on biochars
PAEs
DMP
DEP
Biochars
P450 P550 P650 W450 W550 W650 P450 P550 P650 W450 W550 W650
Interestingly, the significant difference between the maximum saturated adsorption capacities (qmax) of DEP on these biochars (Table 3) was closely related to ash content, in that the wheat straw biochars showed higher values than biochar made from peanut hull (Table 1). In the wheat straw biochars, minerals could block a greater proportion of potentially available surface area. Furthermore, organic matter featuring O-bearing functional groups could be more easily involved, being directly exposed on the surface of minerals in when biochars of higher ash content (Jin et al., 2014). The FTIR spectra (Fig. 3) indicate that hydroxyl ─OH had an important role in the adsorption of DEP on wheat straw biochars. It indicated that such organic-mineral complexes greatly affected the adsorption of hydrophobic organic compounds, through the interactions between hydroxyl sites on mineral surface and carboxyl groups of the surfaces of biochar (Yang et al., 2011; Sun et al., 2013).
Thermodynamic analysis The sorption isotherms of DMP/DEP on biochars P550 and W550 were conducted at different ambient temperatures (15,
Langmuir model
Freundlich model
k/(L mg−1)
qmax/(mg kg−1)
R2
kf/(mg1 – n kg−1 Ln)
n
R2
0.03 0.11 0.99 0.08 0.13 1.30 0.03 0.06 0.13 0.05 0.04 0.10
3174.93 923.74 4120.25 2800.06 1980.96 1359.01 2977.77 950.17 1287.16 2661.54 3588.91 5595.05
0.932 0.895 0.967 0.923 0.969 0.766 0.987 0.879 0.885 0.992 0.993 0.986
290.05 261.23 1929.46 580.49 546.78 646.69 298.11 183.48 370.88 309.15 403.36 997.40
0.45 0.26 0.20 0.33 0.28 0.19 0.45 0.33 0.26 0.45 0.44 0.40
0.994 0.966 0.888 0.994 0.951 0.980 0.928 0.997 0.982 0.910 0.961 0.978
Environ Sci Pollut Res Fig. 6 Adsorption isotherms of DMP (a) and DEP (b) on biochars P550 and W550 at different temperatures
25, and 35 °C). The isotherms were well fitted using the Freundlich model (Fig. 6). The fitted parameters are listed in Table 4. The parameters of standard Gibbs free energy (ΔrGm0), standard enthalpy (ΔrHm0), and standard entropy (ΔrSm0) associated with the adsorption process were calculated using the following thermodynamic equations. Gibbs free energy ΔrGm0 ¼ −RTlnk f
ð5Þ
ΔrGm0 ¼ ΔrHm0 −T ΔrSm0 :
ð6Þ
lnk f ¼ ΔrHm =RT þ ΔrSm =R
ð7Þ
0
0
where ΔrSm0 (J mol−1 K−1) and ΔrHm0 (kJ mol−1) are obtained from the slope and intercept of the plot lnkf versus 1/T, respectively; R is the universal gas constant (8.314 J mol−1 K−1); T is the absolute temperature (K), and kf is the thermodynamic equilibrium constant (mg1 − n kg−1 Ln) of the Freundlich model.
Table 4 PAEs
DMP
The model parameters of isotherms for adsorption of DMP/DEP on biochars P550 and W550 at different temperatures Biochars
P550
W550
DEP
Based on the thermodynamic analysis in Table 5, the values of ΔrGm0 were negative at the experimental temperatures, indicating that adsorption of DMP/DEP on P550 and W550 was spontaneous and thermodynamically favorable. Generally, the values of ΔrGm0 were in the range from 0 to − 20 kJ mol−1 and/or from – 80 to − 400 kJ mol−1, indicating physical and/or chemical sorption; ΔrHm0 for physical and chemical sorption was 8–20 and 40–800 kJ mol−1, respectively (Zhang et al., 2014; Carter and Kilduff, 1995). Based on the thermodynamics established here (Table 5), the sorption processes were controlled physically by nature, while wheat straw biochars displayed weak chemical adsorption. The latter is supported by the observation of O-bearing functional groups in FTIR, namely ─OH, aromatic C═C, ketones, amides, and carboxylic acids C═C were related to the adsorption of DMP/DEP on wheat straw biochars (Fig. 3). The positive values of ΔrHm0 were indicative of the fact that the sorption was endothermic, consistent with the sorption behavior of DMP/DEP on biochars. Abdul et al. (2015) previously found
P550
W550
T (K)
Langmuir model
Freundlich model
K/(L mg−1)
qmax/(mg kg−1)
R2
kf/(mg1 – n kg−1 Ln)
n
R2
288 298 308 288
0.07 0.23 0.11 0.08
877.36 803.68 923.74 1523.28
0.959 0.877 0.895 0.974
181.59 229.05 261.24 312.77
0.32 0.29 0.26 0.33
0.926 0.972 0.966 0.920
298 308 288 298 308 288 298 308
0.15 0.13 0.02 0.05 0.06 0.03 0.03 0.04
1643.72 1980.96 1663.21 952.00 950.17 2007.78 2781.69 3588.91
0.916 0.969 0.959 0.987 0.879 0.976 0.978 0.993
479.98 546.78 94.17 147.87 183.48 191.51 313.73 403.36
0.28 0.28 0.52 0.36 0.33 0.45 0.42 0.44
0.988 0.951 0.993 0.995 0.997 0.979 0.988 0.961
Environ Sci Pollut Res Table 5
Thermodynamic analysis for adsorption of DMP/DEP on biochars P550 and W550
PAEs
Biochars
T/(K)
kf/(mg1 – n kg−1 Ln)
ΔrGm0/(kJ mol−1)
ΔrHm0/(kJ mol−1)
ΔrSm0/(J mol−1 K−1)
DMP
P550
288 298 308 288 298 308 288 298 308 288 298 308
181.74 229.05 261.23 312.77 479.98 546.78 94.17 121.04 147.87 191.51 313.73 403.36
− 12.49 − 13.39 − 14.29 − 13.87 − 15.07 − 16.27 − 10.96 − 12.20 − 13.44 − 12.67 − 14.06 − 15.46
13.45
90.06
20.71
120.06
24.68
123.78
27.56
139.67
W550
DEP
P550
W550
that the adsorption process of carbon nanotubes is spontaneous and exothermic, indicating different physicochemical properties of carbon nanotubes and biochars. In summary, the adsorption of DMP/DEP on both peanut hull and wheat straw biochars was physically spontaneous, while being endothermic.
Desorption isotherms and hysteresis Despite the finding above, the stability of PAEs on biochars has to be considered, i.e., whether the adsorption of DMP/DEP is a reversible process, with potential in the environment for future release of adsorbed compounds. To assess this potential risk, we compared desorption of DMP/DEP on biochars made from wheat straw and peanut hull at an ambient temperature of 35 °C (Fig. 7). The model-fitting parameters for the adsorption are shown in Table 6. Isothermal desorption was fitted well using the Freundlich model. More importantly, the different shapes of sorption behavior between the nonlinear adsorption (Fig. 5) and desorption isotherms (Fig. 7) revealed that
Fig. 7 Desorption isotherms of DMP (a) and DEP (b) on biochars derived from peanut hull and wheat straw
such sorption of DMP/DEP on biochars was not a reversible process, displaying obvious desorption hysteresis. The thermodynamic index of irreversibility (TII) is often used to characterize desorption hysteresis, where TII = 0 indicates a completely reversible system and TII = 1.0 a process that is completely irreversible (Sander et al., 2005). The value for TII can be obtained from the adsorption-desorption isotherm coefficient n using the following equation. TII ¼ 1−n⋅nd−1
ð8Þ
where n is the adsorption isotherm Freundlich coefficient; nd is the desorption isotherm Freundlich coefficient. Except for DEP on biochars P550 and W550, PAE sorption on other biochars showed that the values of TII (Table 7) increased with biochar pyrolysis temperature, indicating that the desorption hysteresis was gradually strengthened and DMP/DEP could be immobilized after being adsorbed. Such resistance to desorption indicates high stability for PAEs adsorbed on freshly produced biochars. Explanation for this hysteresis draws on the irreversible pore deformation mechanism proposed by Bmida
Environ Sci Pollut Res Table 6 The model parameters of isotherms for desorption of DMP/ DEP on biochars
PAEs
DMP
DEP
Biochars
Langmuir model
P450 P550 P650 W450 W550 W650 P450 P550 P650 W450 W550 W650
kd/(L mg−1)
qmax/(mg kg−1)
R2
kfd/(mg1 – n kg−1 Ln)
nd
R2
2.57 × 10−5
2.65 × 107
0.917
258.21
1.56
0.998
−5
3.24 × 107 1.82 × 107 1.18 × 107 1.47 × 107 2.31 × 107 4.67 × 107 2.28 × 107 1.05 × 107 2.62 × 106 1.75 × 107 1.41 × 107
0.911 0.955 0.986 0.971 0.928 0.796 0.971 0.969 0.990 0.972 0.937
364.82 824.79 287.44 420.99 473.49 218.59 429.45 599.04 426.43 321.85 451.12
1.53 1.21 1.18 1.20 1.54 2.05 1.29 1.13 1.04 1.27 1.45
0.989 0.975 0.997 0.985 0.999 0.998 0.999 0.977 0.991 0.998 0.995
2.73 × 10 6.20 × 10−5 3.65 × 10−5 4.25 × 10−5 4.29 × 10−5 2.11 × 10−5 3.31 × 10−5 7.29 × 10−5 1.78 × 10−4 3.10 × 10−5 5.17 × 10−5
Conclusion
et al. (2003). Elementary charcoal is considered to contain open sectors and closed sectors with respect to an adsorbed solute. During the process of adsorption, solute molecules may enter open sectors that display a high degree of connectivity. The expansion of some pores occur due to the pressure of solute molecules on the pore walls then open up new pathways for solute molecules to penetrate previously closed sectors, until further sector rearrangement. During desorption, the corresponding closed sectors cannot leak solute molecules on short time scales. Biochar is a kind of porous charcoal where the porosity is dramatically increased with pyrolysis temperature. This increases TII for biochars made at higher pyrolysis temperature (Table 7), resulting in greater hysteresis in desorption. Table 7 PAEs
DMP
DEP
Freundlich model
Biomass and pyrolysis temperatures can affect the adsorption characteristic of biochars. The aromaticity and specific surface area of wheat straw and peanut hull biochars increased with the pyrolysis temperature, whereas the total density of functional groups was decreased. Higher capacity for adsorption of DEP was measured for wheat straw biochar compared to those from peanut hull, possibly resulting from O-bearing functional groups of organic matter on exposed minerals within the biochars. The sorption of DMP/DEP on biochars is both physically spontaneous and endothermic. Furthermore, desorption hysteresis reflected the porosity and specific surface area of biochars. Based on measured desorption hysteresis, the TII suggests that DMP/DEP is stably adsorbed.
Freundlich model and hysteresis index (TII) for adsorption-desorption of DMP/DEP on biochars Biochars
Adsorption Freundlich model
Desorption Freundlich model
TII
kf/(mg1 – n kg−1 Ln)
n
R2
kfd/(mg1 – n kg−1 Ln)
nd
R2
P450 P550 P650 W450
290.05 261.24 1929.46 580.49
0.45 0.26 0.20 0.33
0.994 0.966 0.888 0.994
258.21 364.82 824.79 287.44
1.56 1.53 1.21 1.18
0.998 0.989 0.975 0.997
0.710 0.827 0.835 0.722
W550 W650 P450 P550 P650 W450 W550 W650
565.57 646.69 241.28 183.48 370.89 309.15 403.36 997.40
0.28 0.19 0.52 0.33 0.26 0.45 0.44 0.40
0.916 0.980 0.943 0.997 0.982 0.910 0.961 0.978
420.99 473.48 218.59 429.45 598.56 426.40 321.85 451.12
1.20 1.54 2.05 1.29 1.13 1.04 1.27 1.12
0.985 0.999 0.998 0.999 0.977 0.991 0.998 0.995
0.782 0.877 0.748 0.744 0.767 0.570 0.656 0.644
Environ Sci Pollut Res Acknowledgements We thank for CUGB Famous Teacher Auditorium Program 2017 for Dr. Saran P. Sohi from University of Edinburgh. We also appreciate Dr. Saran P. Sohi to comment and polish our revised paper thoroughly. We are grateful to the editor and three anonymous reviewers whose comments improved the quality of the manuscript. Funding information This study was supported by the National Nature Science Foundation of China (41472232, 41272061, 41731282), Fundamental Research Funds for the Central Universities (2652015113, 2652016062), and National Innovation Experiment Program for University Students (201711415002).
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