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Mechanistic Investigation of Mercury Sorption by Brazilian Pepper Biochars of Different Pyrolytic Temperatures Based on X‑ray Photoelectron Spectroscopy and Flow Calorimetry Xiaoling Dong,† Lena Q. Ma,*,‡,† Yingjia Zhu,† Yuncong Li,§ and Binhe Gu†,⊥ †

Soil and Water Science Department, University of Florida, Gainesville, Florida 32611, United States State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Jiangsu 210046, People’s Republic of China § Soil and Water Science Department, Tropical Research and Education Center, University of Florida, Homestead, Florida 33031-3314, United States ⊥ South Florida Water Management District, West Palm Beach, Florida 33411, United States ‡

S Supporting Information *

ABSTRACT: We investigated the mechanisms of Hg sorption onto biochars produced from Brazilian pepper (BP; Schinus terebinthifolius) at 300, 450, and 600 °C using different analytical techniques. The Hg sorption capacity of BP300, BP450, and BP600 was 24.2, 18.8, and 15.1 mg g−1 based on Langmuir isotherm. FTIR data suggested the participation of phenolic hydroxyl and carboxylic groups in Hg sorption by biochars. XPS analysis showed that 23−31% and 77−69% of sorbed Hg was associated with carboxylic and phenolic hydroxyl groups in biochars BP300−450, whereas 91% of sorbed Hg was associated with a graphite-like domain on an aromatic structure in BP600 biochar, which were consistent with flow calorimetry data. Based on flow calorimetry, sorption of K and Ca onto biochar was exchangeable with the molar heat of sorption of 3.1 kJ mol−1. By comparison, Hg sorption was via complexation with functional groups as it was not exchangeable by K or Ca with molar heat of sorption of −19.7, −18.3, and −25.4 kJ mol−1 for BP300, BP450, and BP600. Our research suggested that Hg was irreversibly sorbed via complexation with phenolic hydroxyl and carboxylic groups in low temperature biochars (BP300 and BP450) and graphite-like structure in high temperature biochar (BP600).

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INTRODUCTION Mercury (Hg) is one of the most toxic trace elements in the environment.1 Due to its toxicity to living organisms at low concentrations, Hg contamination has received great attention in recent years. Exposure to Hg can damage the nervous system in humans, especially for the developing nervous system of young children.2 Mercury is released to the environment by various industries, including coal combustion, chlorakali, paint, pulp and paper, and oil refining.3,4 Due to lack of knowledge and regulation, factory effluents containing Hg were commonly released into the environment in the past. To protect public health, the World Health Organization recommends a maximum human Hg uptake of 0.3 μg per week and a maximum acceptable concentration of 1 μg L−1 in drinking water.5 The USEPA sets an Hg limit of 10 μg L−1for wastewater discharge and 2 μg L−1for drinking water.6 It is therefore of great importance to reduce Hg contamination in the environment. Brazilian pepper (Schinus terebinthifolius) is the most aggressive evergreen shrub-like tree in Florida, which invades many habitats by forming large dense forests.7 It poses a serious © 2013 American Chemical Society

threat to species diversity in Florida’s native ecosystems, and several health and safety problems have been reported due to the poison ivy it releases.7 It now covers more than 700 000 acres in south and central Florida as well as many of the islands on the east and west coasts of the state.7 Recent developments of renewable bioenergy technologies make it possible to convert this invasive plant into value-added biochar and at the same time produce bioenergy. Biochars have an affinity for heavy metals, and their sorption capacity is comparable with other biosorbents. For example, the sorption capacity of rice husk biochar for Pb is 2.40 mg g−1,8 sugar beet tailing biochar for Cr is 123 mg g−1,9 and pine bark biochar for As is 12.2 mg g−1.10 Although little information is available on Hg sorption by biochar, its sorption by various biomaterials has been investigated. Das et al.11 proposed that complexation through functional groups such as hydroxyl and Received: Revised: Accepted: Published: 12156

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based on the mass titration procedure of Valdés et al.18 The elemental composition of the BP biochars was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Plasma 3200, Perkin-Elmer Crop, MA) after digesting with HNO3/H2O2 hot-block digestion procedure.19 Elemental C, N, and H concentrations were determined using a CHN Elemental Analyzer (Carlo-Erba NA-1500) via hightemperature catalyzed combustion followed by infrared detection of the resulting CO2, H2, and NO2 gases. Cation exchange capacity (CEC) was determined as described by Gaskin et al.20 The content of carboxylic and phenolic hydroxyl groups was determined by Bohem titration method.21 In short, a given amount of biochar was added to the alkali solutions (Na2CO3, NaHCO3, and NaOH) and the mixture was agitated at agitating bed for 24 h. The supernatant was then drawn and back-titrated with HCl.21 Hg Sorption Isotherm by Biochars. The stock solution of 1000 mg L−1 Hg was prepared by dissolving analytical grade Hg(NO3)2 in deionized water. All working solutions were freshly prepared before use by diluting the stock solution with deionized water, and the pH was adjusted to 6 with 1 M HNO3 or KOH solution. Batch experiments were conducted using 60 mL polyethylene bottles with sample volume of 50 mL, and Hg concentrations of 1, 5, 10, 20, and 50 mg L−1. The concentrations were within the range found in industry wastewater samples.22,23 The samples were agitated on a shaker at 200 rpm at room temperature (22 ± 0.5 °C). At the end of the reaction, a known volume of solution was removed and filtered with a 0.2 μm Whatman membrane filter. The effect of pH on Hg sorption by BP biochars was investigated at pHs ranging from 2.0 to 8.0 while keeping Hg at 20 mg L−1 and biochar at 2 g L−1. Biochar Characterization by FTIR and XPS. BP300, BP450, and BP600 biochar samples at 2 g L−1 biochar before and after reacting with 50 mg L−1 Hg for 24 h at pH 6.0 were analyzed by FTIR and XPS. FTIR spectroscopy (Bruker Optics Inc., Billerica, MA) was used to determine changes in functional groups on the biochar after Hg sorption. The spectra were obtained from 500 to 4000 cm−1 with a resolution of 8 cm−1. The resulting spectra were the average of 32 scans. The valence state of the Hg sorbed onto BP biochars was determined by XPS (Perkin-Elmer Model 5100, U.S.). The sample was analyzed at an Al X-ray source (Kα excitation at 1486.6 eV) at 100 W and pass energy of 0.1 eV, with 10 high-resolution scans. The system was operated at a base pressure of 2 × 10−8 mbar. The calibration of the spectra binding energy was performed with the C1s peak of the aliphatic carbons at 284.6 eV. Flow Calorimetry Experiment. Flow calorimetry was used to study Hg sorption characteristics onto biochars. The flow calorimeter was in-house designed and constructed at the University of Florida and its detailed operation was described by Rhue et al.24 All experiments were carried out at a solution flow rate of 0.30−0.35 mL min−1, and ionic strength of 30 mM at pH 6.0 with 25−40 mg of biochar. Prior to the sorption experiments, biochar samples were hydrated in 30m MKNO3, packed into the calorimeter, and flushed with 30 mM KNO3 until a stable baseline (indicating complete K saturation and equilibrium) was obtained. After a satisfactory baseline was obtained with 30 mM KNO3 equilibration, a solution of 15 mM Ca(NO3)2 was then introduced to the K-saturated BP biochar to determine the heat exchange of Ca for K. Several cycles of K/Ca exchange were recorded. The K and Ca peak areas were

carboxyl groups is the main mechanism for Hg sorption by Aspergillus versicolor biomass. Cation exchange is involved in Hg sorption by activated sludge biomass,12 which is supported by higher release of alkali metal ions (Ca2+, Na+, and K+) during Hg sorption. Flow calorimetry is a powerful tool to study mechanisms of metal sorption on different solids.13,14 It provides a direct measure of the heat of reaction on a liquid/solid interface.13 It has several advantages over conventional batch experiments, including separating reactions occurring simultaneously but at different rates, applying multiple sorption/desorption cycles to the same sample, and distinguishing between reversible and irreversible processes.15 For example, Cao et al.16 identified that Cu and Zn sorption onto phosphate rock was through cation exchange since the heat release of K/Ca exchange before and after Cu and Zn sorption were unchanged. However, by comparison, the heat release for K/Ca exchange after Pb sorption was half of that before Pb sorption, consistent with an irreversible precipitation of fluoropyromorphite.16 To our knowledge, flow calorimetry has not been applied to study Hg sorption on biochar. In addition to flow calorimetry, FTIR (Fourier transform infrared spectroscopy), and XPS (X-ray photoelectron spectroscopy) are also powerful tools in identifying functional groups and surface speciation involved in metal sorption. FTIR is useful in identifying types of chemical bonds in a molecule based on an infrared absorption spectrum, whereas XPS is a surface sensitive technique that measures elemental composition and chemical state in a material.9,17 For example, FTIR analysis suggest that both hydroxyl and carboxyl groups are involved in Cr(VI) sorption by sugar beet tailing biochar and XPS analysis confirms that both Cr(VI) and Cr(III) are present on the biochar.9 In this study, biochars were produced from Brazilian pepper (BP) via slow pyrolysis using a laboratory-scale reactor at 300 to 600 °C. Our overall objective was to investigate the characteristics and mechanisms of Hg sorption by BP biochars using various analytical techniques. The specific objectives were to (1) investigate the effectiveness of BP biochars in Hg sorption under different conditions, and (2) determine the mechanisms governing Hg sorption by BP biochars using flow calorimetry, SEM-EDS (scanning electron microscopy-energy dispersive X-ray spectroscopy), FITR and XPS.

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MATERIALS AND METHODS Biochar Preparation and Characterization. Fresh Brazilian pepper plant collected from Florida, U.S. was ovendried at 105 °C and then fed into a lab-scale tubular reactor within a muffle furnace (1500 M, Barnstead Intl. Corp, IA). Nitrogen gas at 10 psi was used to maintain an oxygen-free environment. The furnace temperature was programmed to increase at a rate of 10 °C min−1 until it reached a specified temperature (300, 450, and 600 °C). The entire procedure took ∼2 h. The biochars were allowed to cool at room temperature under a flow of N2 gas. They were then washed with deionized water, dried in an oven at 60 °C for 24 h, and sieved to a size ≤106 μm. The dried biochars were stored in airtight plastic containers prior to use and will be referred as BP300, BP450 and BP600. The surface morphology of BP biochars were examined using JSM-6400 SEM (JEOL, Peabody, MA, U.S.) at 15 keV equipped with EDS (Oxford Instruments, Concord, MA, U.S.). Evaluation of its zero point of charge (pHZPC) was 12157

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Figure 1. Fourier transform infrared spectra of BP biochars produced at different temperatures before and after reaction of 2 g biochar L−1 of 50 mg L−1 Hg for 24 h at pH 6.0.

dried, and digested in HNO3/H2O2with USEPA Method 3050. The amount of Hg remained on the biochar was believed to have reacted with the biochars via a chemisorption process as they were not exchangeable with 30 mM KNO3. Mercury and Statistical Analysis. The concentrations of Hg were analyzed by hydride generation atomic fluorescence spectrometry (HG-AFS, AI3300, Aurora Instruments Ltd., Vancouver, BC). The detection limit was 0.3 ng L−1. All experiments were conducted in triplicate, and the results were calculated as the mean values of the samples. Data were analyzed using JMP, version 9.

of equal and opposite magnitude, indicating a reversible process. Following the last 30 mM KNO3 wash, the solution was changed to 29.5 mM KNO3 solution containing 50 mg L−1 Hg as Hg(NO3)2 to obtain reaction heat of Hg sorption. After the signal being returned to baseline, the system was then flushed with 30 mM KNO3 to remove the sorbed Hg on the biochar. All three BP biochar samples underwent several cycles of K/ Ca solution before and after reaction with Hg. This was done to compare the energetics of K/Ca exchange before and after Hg sorption to ascertain the changes in the energetics was due to Hg introduction. The biochar residues were then removed from the flow calorimeter column, washed with deionized H2O, air12158

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eV. 18A Ca2p peak was observed on BP 600 biochar, which was consistent with its relative high Ca content. It was interesting to note the absence of N signal, which indicated its presence in the inner part of biochar since XPS is sensitive to surface 5−10 nm.32 Hg Speciation Impacted Its Sorption by Biochars at Different pHs. Hg sorption capacity onto biochars was determined by Langmuir isotherm (Table S3 of the SI). It decreased from 24 to 15 mg g−1 as pyrolysis temperature increased, implying that low temperature was beneficial to form biochars with higher Hg sorption capacity, which was related to more functional groups on the biochars (Figure 1). The sorption capacity of BP biochars was comparable with those of biosorbents reported in the literature, which range from 10.1 mg g−1 by organic sewage sludge derived activated carbon to 25 mg g−1 by sugi wood carbonized at 1000 °C.33,34 Hg sorption by biochars depended on the interactions between Hg ions and the functional groups on biochar, which was impacted by solution pH. The starting Hg concentration was 20 mg L−1, which was high enough to provide sufficient Hg to saturate biochars and low enough to maintain Hg soluble in pH range of 2−8.35 Hg sorption by biochars increased with increasing pH, reaching a plateau at pH of 4.0−8.0 (Figure S3 of the SI). At pH 2.0, the amount of Hg sorbed was 3.2−7.0, 2.6−4.6, 2.1−3.0 mg g−1 and at pH 4−8, it was 8.7−9.0, 6.1− 7.8 and 6.3−7.0 mg g−1 for BP300, BP450 and BP 600, respectively. On the basis of analysis using the speciation model MINEQL+, at pH of 2−3, Hg(OH)+ and Hg2+ were the dominant species in the solution, accounting for >70% of Hg in solution (data not shown). At this pH, biochars were positively charged as their pHPZC was higher than solution pH (6.65, 8.35, and 9.91 for BP 300, 450, and 600 biochar respectively; Table S2 of the SI). The low Hg sorption at this pH was attributed to the positive charge of the sorbent surface ( 7. Hence, deprotonated carboxylic group was likely the primary source of CEC on biochar. Furthermore, the carboxyl group content in BP biochars was close to the CEC value (Table S2 of the SI), indicating that carboxylic group was probably the major cation exchange sites for K/Ca exchange on biochars. A similar conclusion was made by Harvey et al.14 on plant-derived biochars. The heat of exchange for K/Ca after Hg sorption decreased by 41%, 70%, and 100% for BP300, BP450, and BP600 biochar, consistent with a nonexchangeable Hg sorption onto biochars (Figure 2d−f). The data also indicated that a similar amount of charge at 0.12, 0.11, and 0.16 mmolc kg−1 for BP300, BP450, and BP600 biochar was consumed during Hg sorption (ΔC: calculated by the CEC after Hg sorption). The smaller heat signals suggested that Hg sorption occurred on the same cation exchange sites, but this process was not exchangeable with K and Ca. Stability constants of Ca, K, and Hg with carboxylic group on biochar are not available, but the stability constant of

Table 1. Surface Composition for Hg Loaded BP Biochars Based on XPS Analysis after Reaction of 2 g Biochar L−1 of 50 mg L−1 Hg for 24 h at pH 6.0 atomic content (%) element

functional groups

C1s

total graphitic or aromatic C in ethers or hydroxyl C in carboxyl total carbonyl or quinone ethers or hydroxyl anhydride, lactone or carboxylic acids

O1s

binding energy (eV)

BP300

BP450

BP600

530.7 532.1 533.3

79.9 71.4 23.0 5.60 18.9 13.9 52.7 33.4

82.5 69.7 25.8 4.50 16.4 6.50 51.2 42.3

88.6 62.5 33.7 3.80 11.0 3.00 53.5 43.5

101.1 101.3 102.0

1.1 76.9 23.1 0

0.7 69.5 30.5 0

0.4 0 9.00 91

284.6 286.0 288.6

Hg4f phenolic Hg carboxylic Hg graphite Hg

functional groups in Hg sorption. After Hg sorption, total oxygen content on biochar surface decreased by 2.2%, 1.7%, and 1.1% on BP300, BP450, and BP600, respectively (Table 1). The high resolution spectra of Hg4f on BP biochars could be fitted with two doublet-peaks, both separated by 4.1 ± 0.1 eV (the distance between Hg4f7/2 and 4f5/2 peaks) (Figure S4 of the SI). The absence of Hg4f7/2 peaks at bending energies 5.5. If these were the case, then the mole of charge consumed per mol of Hg sorbed should be 2 for bidentate complexation. However, the ratio of ΔC/QHg was 0.90, 0.75, and 0.21 for BP300, BP450, and BP600, pointing the sorption also occurred on undeprotonated functional groups, possibly phenol group and graphite-like structure. These results were consistent with XPS analysis, where Hg101.3 (phenolic hydroxyl group) and Hg101.9 (graphite-like structure) peaks were identified (Figure S4 of the SI). On this consideration, Hg complexation with phenolic hydroxyl group and graphite-like structure were also involved. During Hg sorption, solution pH increased by 0.32 to 0.52 unit, which could be explained by the complexation between deprotonated carboxylic group and Hg(OH)2°, the dominant Hg species at pH 6.0. Moreover, on the basis of the Hg speciation of 50 mg L−1 Hg at pH 6, Hg precipitation was unlikely. Thus, Hg sorption on BP biochars might be explained by the following equations: carboxylic and phenolic hydroxyl were the predominantly binding sites for BP300 and BP450 biochars; while carboxylic and graphite-like structure were the major binding sites for BP600 biochar.

Figure 3. Heat signals for Hg sorption at pH 6 for BP biochars produced at 300 (a), 450 (b), and 600 °C (c).

Table 2. Heats of Sorption and Quantity of Hg Sorbed after Reaction with 50 mg L−1 Hg Solution at pH 6 Based on Flow Calorimetry and Batch Sorption Experiment

Hg(OH )°2 + 2RCOO− = (RCOO)2 Hg + 2OH −

Hg(OH )°2 + 2RCOH = (RCO)2 Hg + 2H2O

sorption capacity QHg, mg g−1 sample

flowa

batchb

heats of reactiona (ΔHHg, kJ mol−1)

BP300 BP450 BP600

2.61 2.81 14.8

21.3 16.4 13.2

−19.7 −18.3 −25.4

Hg(OH )°2 + −Cπ = −Cπ Hg(OH )°2

Environmental Implication. Results of this study suggested that Brazilian pepper can be converted into biochar as an effective sorbent for Hg removal. Brazilian pepper is an invasive plant in Florida, therefore, converting it into biochar improving the invasive plant management and protecting the environment. Two distinct mechanisms were responsible for Hg sorption on BP biochars. The first mechanism applied to low temperature biochars and involved complexation with carboxylic and phenolic hydroxyl groups. The second mechanism was evident for high temperature biochar and was involved in Hg binding with graphite-like structure. Further studies are needed to verify removal efficacy of biochars for treating Hg-contaminated wastewater.

a Flow rate = 0.30−0.35 mL min−1 until reaching baseline. b2 g biochar L−1 of 50 mg L−1 Hg shaken for 24 h.

L−1 at pH 6 (Table 2). The data implied that Hg did not react with all of the sorption sites after one Hg cycle for BP300 and BP450 biochar. Rhue et al.24 observed that phosphate saturation in a Ultisol soil was completed in five 40-min cycles. However, for BP600 biochar, all sorption sites were occupied after one cycle, coupled with the longer time required for the heat signal to return to baseline (∼50 min, twice that of BP300 or BP450), it indicated that Hg sorption onto these sites was 12162

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ASSOCIATED CONTENT

S Supporting Information *

(1) Chemical composition and surface properties of the BP biochars, (2) Langmuir isotherm parameters, (3) influence of pH on Hg removal by BP biochars, and (4) XPS spectra of Hg4f for Hg loaded biochars. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (352)-294-3135; fax: (352) 392-3902; e-mail: lqma@ ufl.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported in part by the National Natural Science Foundation of China (No. 21277070), Jiangsu Provincial Innovation Project, USDA-TSTAR, and a matching assistantship provided by UF-IFAS. The authors would like to thank the following professors at University of Florida: (1) Dr. Bala Rathinasabapathi in the Horticultural Sciences Department for collecting Brazilian pepper wood samples, (2) Dr. Bin Gao in the Department of Agriculture and Biological Engineering for providing Brazilian pepper biochar samples, (3) Dr. Dean Rhue in the Soil and Water Sciences Department for his help with the flow calorimetry experiment, and (4) Dr. J. C. Bonzongo in the Environmental Engineer Sciences Department for Hg analysis.

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REFERENCES

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dx.doi.org/10.1021/es4017816 | Environ. Sci. Technol. 2013, 47, 12156−12164