Multiple characterization for mechanistic insights of Pb(II) sorption onto biochars derived from herbaceous plant, biosolid, and livestock waste

Abstract

Biochars are considered as promising sorbents for the removal of aqueous metal ions. The aim of this study was to explore the adsorption mechanisms through the integrated characterization of the pristine and Pb(II)-loaded biochars derived from herbaceous plant, biosolid, and livestock waste with different physicochemical properties. The biochar derived from livestock waste exhibited higher Pb sorption capacity than the others. Experimental data of sorption kinetics and isotherms were well fitted by kinetic models and Langmuir isotherm model, respectively. Comparisons of Fourier-transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) between the pre- and post-adsorption biochars revealed the formation of Pb-carbonate, suggesting that the surface precipitation was the dominant adsorption mechanism. The combination of multiple characterizations and batch adsorption can make further exploration on the adsorption mechanism of Pb(II) adsorption onto the resultant biochars.

Full Article

Multiple Characterization for Mechanistic Insights of Pb(II) Sorption onto Biochars Derived from Herbaceous Plant, Biosolid, and Livestock Waste

Zhuhong Ding,^a,* Hailu Wu,^a and Xin Hu ^b

Biochars are considered as promising sorbents for the removal of aqueous metal ions. The aim of this study was to explore the adsorption mechanisms through the integrated characterization of the pristine and Pb(II)-loaded biochars derived from herbaceous plant, biosolid, and livestock waste with different physicochemical properties. The biochar derived from livestock waste exhibited higher Pb sorption capacity than the others. Experimental data of sorption kinetics and isotherms were well fitted by kinetic models and Langmuir isotherm model, respectively. Comparisons of Fourier-transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) between the pre- and post-adsorption biochars revealed the formation of Pb-carbonate, suggesting that the surface precipitation was the dominant adsorption mechanism. The combination of multiple characterizations and batch adsorption can make further exploration on the adsorption mechanism of Pb(II) adsorption onto the resultant biochars.

Keywords: Physicochemical property; Batch sorption; X-ray photoelectron spectroscopy; Sorption mechanism

Contact information: a: School of Environmental Science & Engineering, Nanjing Tech University, 30 Puzhu Southern Road, Nanjing 211816, P.R. China; b: State Key Laboratory of Analytical Chemistry for Life Science, Center of Material Analysis, 20 Hankou Road, Nanjing University, Nanjing 210093, P.R. China; *Corresponding author: dzhuhong@njtech.edu.cn

INTRODUCTION

Biochars, synthesized through slow pyrolysis or carbonization of feedstock biomass in an oxygen-limited environment, have received increasing attention for their potential environmental benefits such as carbon sequestration, soil fertility improvement, and pollution remediation (Ahmad et al. 2014; Mohan et al. 2014; Inyang et al. 2016). In particular, biochars have been used as low-cost, sustainable adsorbents to remove organic and inorganic contaminants from aqueous solutions (Mohan et al. 2014; Inyang et al. 2016). Current literature has covered a wide spectrum of biochars derived from various biomass feedstocks such as pine wood, oak wood, hard wood, corn straw, soybean straw, orange peel, anaerobically digested sugarcane bagasse, cow manure, sugar beet tailing, and coconut coir (Tan et al. 2015; Inyang et al. 2016). They can be simply classified as plant-based, sludge-based, and manure-based biochars. Traditionally, the basic physicochemical properties of these resultant biochars were analyzed or characterized firstly, which includes contents of C, H, N, and mineral elements, specific surface areas, surface functional groups, crystalline phases, and thermal stability, and morphology; and then batch sorption experiments (adsorption isotherm, kinetics and/or thermodynamics) and/or column sorption experiments were carried out to investigate the adsorption performance and the empirical or semi-empirical mathematical models such as the Langmuir isotherm equation, Freundlich isotherm equation, pseudo-first-order equation, and pseudo-second-order equation were used to fit the experimental data to discuss the adsorption mechanisms (Mohan et al. 2014; Inyang et al. 2016). For example, kinetic experiments of metal ions sorbed onto various biochars have shown that the pseudo-second-order model fits the data better than the first-order model (Mohan et al. 2014; Inyang et al. 2016). The equilibrium sorption isotherms are fitted with Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models, reflecting the sorbent surface properties and the affinity of contaminants to the biochars (Mohan et al. 2014; Inyang et al. 2016).

The common instrumental characterizations used in the various biochars include scanning electron microscope equipped with an energy-dispersive spectrometer (SEM-EDS), thermogravimetric analyzer (TGA), Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), CHN elemental analyzer, inductively coupled plasma optical emission spectroscopy (ICP-OES), and X-ray photoelectron spectroscopy (XPS) (Chen et al. 2015; Ding et al. 2016b; Fu et al. 2016; Xu et al. 2017). These characterizations can reveal the physicochemical properties of biochars, which have great influences on the adsorption capacities of metal ions onto biochars (Ahmad et al. 2014; Mohan et al. 2014; Inyang et al. 2016). Now they were used further to explore the adsorption mechanisms through the comparison of the pristine and the adsorbate-loaded biochars (Hu et al. 2015; Xu et al. 2017). Therefore, the combination of batch sorption and integrated characterization techniques may be more effective in identifying the governing adsorption mechanisms of contaminants onto the biochars and further study should be carried out on the biochars derived from different biomass feedstock.

In the present study, plant-based, sludge-based and manure-based biochars were derived from herbaceous plant (Spanish moss), biosolid, and livestock waste, respectively. Pb(II), one of the most investigated metal ions adsorbed by biochars (Ding et al. 2016a; Inyang et al. 2016; Chen et al. 2017), was selected as a represented heavy metal ion. Batch isotherm and kinetic sorption experiments were performed. The pristine and the Pb(II)-loaded biochar samples were analyzed using bulk and surface characterization techniques such as FT-IR, XRD, and XPS. The object of this study was to explore the adsorption mechanisms through the combination of batch sorption and integrated characterization.

EXPERIMENTAL

Chemical reagents

Analytical reagents including NaOH, HNO₃, Pb(NO₃)₂ were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Solutions were prepared using ultrapure water (18.2 MΩ*cm) (Milli-Q Advantage A10 System, Darmstadt, Germany).

Preparation of Sorbents

The herbaceous plant (Spanish moss) was collected from the campus of the University of Florida (UF) in Gainesville, FL, USA. The biosolid (sludge) was collected locally in Gainesville, FL, USA. The livestock waste was obtained from a local livestock breeding plant in Gainesville, FL, USA. Each biomass was ground in a knife mill (Model No. 4, Arthur H. Thomas Company, Philadelphia, PA, USA) into powder. The powders were placed inside tubular quartz reactors and then inserted into a benchtop muffle furnace (Barnstead 1500 M, Auburn, AL, USA) purged with N₂ gas.

The pyrolysis furnace temperature was increased at a rate of 20 °C/min and then held at 600 °C for 2 h with flowing N₂ gas. The obtained samples were rinsed with tap water and DI water and then oven dried at 80 °C. The pristine biochars were placed in zipper bags and denoted as SM (Spanish moss), BIOS (biosolid), and MAW (mixed animal waste). The details are given in a previous report (Wu et al. 2016)

Batch Aqueous Pb(II) Sorption

Batch sorption experiments consisted of an isotherm test and a kinetic test. All experiments were carried out at room temperature (22 ± 2 °C) by adding 0.05 g sorbent to 100 mL polyethylene centrifuge tubes containing 50 mL Pb(II) solution prepared from Pb(NO₃)₂ with pre-defined concentrations. The solution pH was adjusted to 5.5 ± 0.2 using 0.01 mol L⁻¹NaOH and 0.01 mol L⁻¹ HNO₃. After shaking for a pre-defined contact time in a rotary shaker, the mixture was centrifuged at 4000 rpm for 10 min. The concentrations of Pb(II) in the supernatants were determined using inductively coupled plasma optical spectrometry (ICP-OES, Optima 5300, Perkin-Elmer, Waltham, MA, USA). The amount of adsorbed Pb(II) (q_t or q_e) per unit adsorbent mass were calculated using the following equation,

q_t = [(C₀ ‒ C_t) V]/m (1)

where C₀ and C_t are the concentrations of Pb(II) at time 0 and t (mg L⁻¹), respectively; q_t is the amount adsorbed (mg g⁻¹) at contact time t (q_e is q_t at equilibrium time); V is the volume of the solution (L); and m is the mass of the adsorbent (g).

The initial concentrations for the isotherm test were set at 0.5 mg·L⁻¹, 5 mg·L⁻¹, 10 mg·L⁻¹, 25 mg·L⁻¹, 50 mg·L⁻¹, and 100 mg·L⁻¹ with shaking times of 24 h. In the kinetic sorption test, the contact time was set as 5 min, 15 min, 30 min, 45 min, 60 min, 120 min, 240 min, 360 min, 480 min, 720 min, and 1440 min at pH of 5.5 ± 0.2 with Pb(II) concentrations of 10 mg·L⁻¹ and 50 mg·L⁻¹. All the adsorption experiments were conducted in duplicate and the mean values were reported. The sorption experiments without spiked Pb ([Pb(II) = 0 mg·L⁻¹]) were used as control groups.

Characterization of the Pre- and Post-Sorption Biochars

The basic physicochemical properties of the resultant biochars (e.g., contents of C, H, N, and mineral elements, pH value of the point zero charge (pH_PZC), specific surface areas, thermogravimetry analysis, and morphology) were presented in an earlier report (Wu et al. 2016).

The infrared spectra of the pristine and the post-sorption biochars were recorded using a Fourier-transform infrared spectroscopy (FT-IR, NEXUS870, NICOLET Co., Madison, WI, USA) in the range of 4000 cm⁻¹ to 500 cm⁻¹ with a resolution of 0.2 cm⁻¹, using a KBr disc containing 1% of finely ground sample produced via bench press. Crystalline minerals in the pristine biochar and the post-sorption biochars were detected using X-ray Powder Diffraction (XRD) (XRD-6000, Hitachi Co., Tokyo, Japan).

The composition of surface elements and Pb speciation in biochar samples were analyzed using X-ray photoelectron spectroscopy (PHI 5000 VersaProbe, UlVAC-PHI, Maoqi, Japan).

RESULTS AND DISCUSSION

Batch Sorption Kinetics and Isotherm

The sorption of Pb(II) onto the biochars increased considerably with increasing contact time in the course of the first hour and then slowed until reaching an equilibrium (Fig. 1a). Low initial concentrations of Pb(II) reached equilibrium adsorption more readily than high initial concentrations (Fig. 1a). However, the sorption curve of 10 mg L^-1 Pb(II) shifted unexpectedly, which might be due to the higher adsorption capacity of MAW for Pb(II). Also, most of the Pb(II) at lower initial concentrations was adsorbed at earlier contact time. The pseudo-first-order and pseudo-second-order model both were well fitted to the experimental data to characterize the sorption kinetics of Pb(II) ions onto biochars (Table 1). The pseudo-second-order kinetic model provided a somewhat better fit, except for that for 10 mg L^-1 Pb(II), which is in agreement with previous research on sorption characteristics ((Tan et al.2015).

Fig. 1. Sorption kinetics (a) and isotherm (b) of Pb(II) onto the resultant biochars

Table 1. Kinetic Parameters for Pseudo-First-order and Second-order Kinetic Models for the Adsorption of Pb(II) onto the Biochars

K₁ and K₂: the rate constant of the pseudo-first-order equation (min⁻¹) and the pseudo-second-order equation (g mg⁻¹ min⁻¹), respectively.

The equilibrium sorption capacities differed among different biochars (Fig. 1b). MAW shows the maximum sorption capacities while SM exhibits the least (Fig. 1b). The equilibrium sorption isotherms reflect the sorbent surface properties and Pb (II) affinity for the biochars. Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models were used to fit the isotherm data accordingly. The coefficients of determination (R²) listed in Table 2 indicated good fitting with these isotherm models (R² > 0.80), with the Langmuir adsorption isotherm being a generally better model. The Langmuir adsorption capacities for Pb (II) were 16.8 mg g^-1 for SM, 19.6 mg g^‒1 for BIOS, and 51.5 mg g^‒1 for MAW (Table 2). The observed values were larger than most of the reported values in the literature such as 4.13/3.00 mg g^‒1 for pine wood/bark char, 2.62/13.10 mg g^‒1 for oak wood/bark char (Mohan et al.2007), 3.89 mg g^‒1 to 4.25 mg g^‒1 for pine wood biochar, and 1.84 mg g^‒1 to 2.40 mg g^‒1 for rice husk biochar (Liu and Zhang 2009). Based on the Langmuir constant (K_L) and the initial Pb(II) concentration (Co), the dimensionless separation factor (R_L) can be calculated (R_L = 1 / ( 1 + K_L C₀)). It is believed that R_L value indicates the type of isotherm to be irreversible (R_L = 0), favorable (0 < R_L < 1), linear (R_L = 1), or unfavorable (R_L > 1). The R_L values for the adsorption of Pb(II) onto the biochars for all studied initial Pb(II) concentrations were below 1, indicating that the sorption of Pb(II) was a favorable process. Single point adsorption distribution coefficients (K_d) at low and high equilibrium concentrations (C_e = 5 and 50 mg L^‒1) are used to judge sorption ability. K_d values in Table 2 show that the sorption ability was in the order of MAW > BIOS > SM.

The slope of 1/n of the Freundlich model indicates a normal Freundlich isotherm when 1/n < 1 and cooperative adsorption when 1/n > 1. The range between 0.1 and 0.8 for 1/n in the present study (Table 2) is indicative of easy adsorption of Pb (II) onto biochars. The sorption mean free energy (E) from the D-R isotherm model (  ) can be used to evaluate whether physical sorption (E < 8 kJ/mol) or chemical sorption (8 kJ/mol < E < 16 kJ/mol) is the predominant sorption mechanism (Helfferich 1962). The E values were 12.1 KJ/mol, 8.34 KJ/mol, and 8.27 KJ/mol for the sorption of Pb(II) onto MAW, SM, and BIOS, respectively (Table 2), suggesting the chemical mechanisms.

Table 2. Isotherm Model Parameters for Pb Sorption onto Biochars

q_m: Maximum adsorption capacity (mg g⁻¹); K_L: Langmuir constant (L mg⁻¹); R²: coefficient of determination; K_F: affinity coefficient related to the bonding energy (mg^(1‒1/n) L^1/n g⁻¹); n: the heterogeneity factor that represents the bond distribution; β: the isotherm constant (mol/J)²; R: the gas constant (8.314 J/mol K); and T: absolute temperature (K).

FT-IR Spectra of the Biochars

FT-IR spectra (Fig. 2a) were used to obtain information about the kinds of the functional groups and their chemical environment located for the pre- and post-sorption biochars. In Fig. 2a, the broad band at 3200 cm⁻¹ to 3500 cm⁻¹ was assigned to OH groups. The broad bands at ~1710 cm⁻¹ and ~1600 cm⁻¹ represented carboxyl groups. The broad bands at ~1430 cm⁻¹ were assigned to asymmetric C‒O stretching in carbonates. Finally, the peaks at 890 cm⁻¹, 820 cm⁻¹, and 760 cm⁻¹ were assigned to the aromatic C−H bonding for all the tested biochars. The FT-IR spectra of the pre- and post-sorption biochars were very similar. The lack of obvious shift on the FT-IR spectra after adsorption may have been due to the strong light absorption of black biochars, which may have concealed the slight shift effect caused by the limited metal adsorption amount. For example, Table 2 showed that the adsorption amounts for Pb (II) were all below 6%. Also, it is known that FTIR gives information mainly about bulk properties, and changes associated with interactions at the surface of black biochars can be hard to detect. Similar results were reported previously (Lu et al. 2012; Yang et al. 2014).

Crystalline Phase Analyses

The XRD patterns of the pre- and post-sorption biochars are shown in Fig. 2b. Carbon in the biochars existed mainly in amorphous form while quartz and calcite were found in the pre- and post-sorption biochars (Fig. 2b). Quartz and calcite were reported in biochars produced at relatively high temperatures from various feedstocks (Cao et al. 2009; Xu et al. 2013). Some new peaks occurred in the biochars after Pb sorption (Fig. 2b). The d values and the corresponding 2θ degree of the new peaks were assigned to cerussite (PbCO₃). The possible peak of Pb₃(PO₃)₂ was noted in the post-sorption MAW, supporting the previous reports of Pb adsorption onto dairy manure biochars (Cao et al. 2009; Xu et al. 2013). Surface precipitation of PbCO₃ phase was also reported when Pb(II) was exposed to calcite in DI water (Rouff et al. 2005). Separating Pb(II) removal by biochars into organic and inorganic fractions indicated that the sorption capacities were only around 0.4% to 0.6% for organic fractions and more than 99% for the inorganic fractions (Xu et al. 2014). Therefore, surface precipitation may be a significant mechanism responsible for adsorption of Pb onto biochars.

Fig. 2. FT-IR spectra (a) and XRD patterns (b) of the pre- and post-sorption biochars (pre-sorption biochar data are from Wu et al. (2016))

Surface Speciation of C, O, and Pb for Adsorption Mechanism Insight

The survey scans of XPS analysis for the pre- and post Pb-sorption biochars (Fig. 3) showed that carbon and oxygen were the dominant elements in the biochar surfaces, while Pb appeared on biochar surfaces after Pb(II) sorption. No mineral elements were found in the pre- and post-sorption biochars in the detection range of XPS (Fig. 3). The presence of Pb_4dprovides evidence of Pb(II) fixation onto the surface of biochars. The surface Pb contents were in the order of MAW > BIOS > SM (Fig. 3), consistent with the results of batch sorption.

High-resolution XPS spectra of the pre- and post Pb-sorption biochars for C_1s, O_1s and Pb_4d were also analyzed (Fig. 4). Only one dominant peak appeared at the binding energy of around 284.8 eV for C_1s and around 533 eV for O_1s in the pre- and post Pb-sorption biochars, which can be ascribed to graphite C for C_1s and C‒OH or O‒C for O_1s (Datsyuk et al.2008) (Fig. 4a, b). The lack of significant changes on chemical states of surface carbon and oxygen in biochars observed during the sorption process (Fig. 4a, b) may explain the extremely low sorption capacities of the organic fractions (around 0.4% to 0.6%) for Pb(II) removal as indicated by Xu et al. (2014). Two prominent Pb peaks corresponding to Pb_4d5/2and Pb_4d7/2 were found on the surface of biochars after Pb sorption (Fig. 4c). Peaks of Pb_4d7/2 in BIOS, SM and MAW were found at the binding energy of 138.1eV to 138.4eV (Fig. 4c), which can be associated with PbCO₃ (Taylor and Perry 1984). These further confirmed the above-mentioned adsorption mechanism from XRD analyses that surface precipitation was the important mechanism for Pb(II) adsorption onto biochars.

Fig. 3. Survey scan of XPS analysis for the pre- and post-sorption biochars

Fig. 4. High-resolution XPS spectra of (a) C_1s, (b) O_1s and (c) Pb_4d in the pre- and post-sorption biochars

CONCLUSIONS

1. Both isotherm and kinetic adsorption showed the highest adsorption capacities of Pb(II) for manure-based biochars, and then sludge- and plant-based biochars. Experimental data of adsorption kinetics and isotherms were well fitted by either the pseudo-first-order or pseudo-second-order kinetic model and by the Langmuir isotherm model, respectively.
2. Multiple characterization of FT-IR, XRD, and XPS for the pre- and post-adsorption biochars revealed the dominant sorption mechanism of the surface precipitation of Pb-carbonate during Pb(II) adsorption onto biochars.

ACKNOWLEDGMENTS

The authors are thankful for the support of the Natural Science Fund of China (21677075).

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Article submitted: April 5, 2017; Peer review completed: July 7, 2017; Revised version received and accepted: July 21, 2017; Published: July 31, 2017.

DOI: 10.15376/biores.12.3.6763-6772