Chemosphere 134 (2015) 257–262
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Physicochemical and sorptive properties of biochars derived from woody and herbaceous biomass Shengsen Wang a, Bin Gao b,⇑, Andrew R. Zimmerman c, Yuncong Li a,⇑, Lena Ma d, Willie G. Harris d, Kati W. Migliaccio e a
Tropical Research and Education Center, Soil and Water Science Department, University of Florida, Homestead, FL 33031, United States Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, United States Department of Geological Sciences, University of Florida, Gainesville, FL 32611, United States d Soil and Water Science Department, University of Florida, Gainesville, FL 32611, United States e Tropical Research and Education Center, Department of Agricultural and Biological Engineering, University of Florida, Homestead, FL 33031, United States b c
h i g h l i g h t s Prepare conditions affected biochar’s physicochemical properties. Prepare conditions affected the sorption of As and Pb onto biochars. All the tested biochars removed As and Pb form aqueous solutions. Electrostatic interaction played an important role in biochar sorption.
a r t i c l e
i n f o
Article history: Received 13 January 2015 Received in revised form 6 April 2015 Accepted 20 April 2015
Keywords: Biochar Temperature Feedstock Arsenate Lead
a b s t r a c t It is unclear how the properties of biochar control its ability to sorb metals. In this work, physicochemical properties of a variety of biochars, made from four types of feedstock at three pyrolysis temperatures (300, 450 and 600 °C) were compared to their ability to sorb arsenic (As) and lead (Pb) in aqueous solutions. Experimental results showed that both feedstock types and pyrolysis temperature affected biochar’s production rate, i.e., ratio of mass of biochar and biomass, thermal stability, elemental composition, non-combustible component (NCC) content, pH values, surface areas and thus their sorption ability to the two metals in aqueous solution. In general, the high temperature biochars had low O/C and H/C ratios, were more carbonized with larger surface area, and were more concentrated with alkaline cations. In addition, biochars made from woody feedstocks had larger surface area, but lower NCC contents than that made from grasses under the same conditions. Although all the tested biochars removed both As and Pb from aqueous solutions, they showed different sorption abilities because of the variations in properties. Statistical analyses suggested that feedstock type affected the sorption ability of the biochars to both As and Pb significantly (p < 0.001). Pyrolysis temperature, however, showed little influence on biochar sorption of Pb in aqueous solutions. Statistical analyses also showed that electrostatic interaction played an important role in controlling the sorption of both As(V) and Pb(II) onto the biochar. Other mechanisms, such as precipitation and surface complexation, could also control the sorption of Pb(II) onto the biochars. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Biochar is pyrogenic black carbon derived from the pyrolysis of biomass, such as wood and grass, under N2 or oxygen limited conditions (Lehmann et al., 2006; Rutigliano et al., 2014). Recent studies have suggested that biochar land application can mitigate global warming by fixing the carbon in soils (Lehmann et al., ⇑ Corresponding authors. E-mail addresses: bg55@ufl.edu (B. Gao), yunli@ufl.edu (Y. Li). http://dx.doi.org/10.1016/j.chemosphere.2015.04.062 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.
2006) and reducing N2O gas emission from soils (Spokas et al., 2009). In addition, biochar amendment can, in some instances, increase soil fertility and crop yield (Major et al., 2010), improve soil physical and chemical properties such as water holding and nutrient retention capacity (Bell and Worrall, 2011; Karhu et al., 2011). Biochar properties are strongly affected by their peak pyrolysis temperature (Yao et al., 2012; Park et al., 2013). As pyrolysis temperature increases, the degree of carbonization of the feedstock increases, as indicated by increased carbon (C) content as well as
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decreased hydrogen (H) and oxygen (O) contents in the resulting biochar (Harvey et al., 2011; Uchimiya et al., 2011). Biochar made at low temperature generally has lower pH, higher water holding capacity, lower specific surface area, more carboxylic and phenolic hydroxyl functional groups and higher cation exchange capacity (CEC) (Gaskin et al., 2008; Ippolito et al., 2012; Novak et al., 2012). These properties determine its potential environmental applications. For instance, biochar surface properties such as BET surface area and pore volume were correlated with its ability to sorb phenanthrene (Park et al., 2013). Feedstock types may also affect the physicochemical properties of the biochar. For instance, woody biomass often has higher cellulose, hemicellulose, and lignin contents than herbaceous or grass species (Lupoi and Smith, 2012). The higher lignin content in plant biomass was reported to promote carbonization and to increase biochar production rate (Demirbas, 2004). Furthermore, previous studies have shown that feedstock types may strongly influence biochar’s surface characteristics (surface area, pH, and functional groups), thereby affecting their potential environmental applications (Inyang et al., 2010; Kloss et al., 2012; Sun et al., 2014). The widespread degradation of ground and surface water, plant and animal communities, soils and air with metals and metalloids is a high environmental and human health risks (Ribeiro et al., 2010; Oliveira et al., 2012a,b; Quispe et al., 2012; Saikia et al., 2014). However, cost-efficient, safe and the least destructive recuperation techniques suitable for such impacted soils are wanted and their efficiency in the long-term needs to be evaluated (Silva et al., 2012; Oliveira et al., 2013; Ribeiro et al., 2013; Cutruneo et al., 2014). Because of its special surface properties, biochar has been suggested to be a low-cost option for soil remediation and water treatment adsorbents, or as a soil amendment to improve soil condition and increase carbon sequestration (Cao et al., 2009; Inyang et al., 2011; Yao et al., 2011; Abdel-Fattah et al., 2014; Elaigwu et al., 2014). Several previous studies have shown that the sorption ability of some biochars is comparable to or even higher than many commercial adsorbents including activated carbon (Cao et al., 2009; Inyang et al., 2011, 2012). Since most of the biochars are converted from waste materials, they are inexpensive and thus may not require regeneration after having been used as adsorbent (Inyang et al., 2010, 2011). In addition, some of the spent/ exhausted biochars can even be applied directly to soils as amendment or slow-release fertilizers (Yao et al., 2013a,b). Soil contamination by hazardous elements is detrimental to crop production and human health. Arsenic (As) and lead (Pb) are carcinogenic trace metals and are of great concern for human and animal health. Of the many well developed approaches for As and Pb removal from soil and groundwater systems, adsorption is the one of the most discussed techniques (Mohan and Pittman, 2007). Several studies have reported the strong sorption of hazardous elements, particularly lead, by biochars produced from various feedstocks and under different conditions (Uchimiya et al., 2010; Inyang et al., 2012; Xue et al., 2012). However, it is still unclear which types of biochar remove hazardous elements most efficiently and what biochar characteristics are most important in this process. To our knowledge, no previous works have systematically investigated the separate and combined effect of pyrolysis temperature and feedstock type on biochar sorption of hazardous elements in aqueous solutions. The main objective of this work was to understand how the physicochemical properties of biochar, arising from variations in pyrolysis temperature and feedstock type, affect biochar’s sorption of As and Pb. Two woody and two herbaceous plants were selected as the feedstocks to produce biochars at three pyrolysis temperatures. A range of laboratory tools were used to characterize and compare the properties of the biochars. In addition, batch sorption
experiments were conducted to measure their sorption of arsenate (As(V)) and cationic lead (Pb(II)) in aqueous solutions.
2. Materials and methods 2.1. Reagents All chemicals of used in this study were of analytical grade and were purchased from Fisher Scientific. Chemicals were dissolved in deionized (DI) water (18.2 MX) (Nanopure water, Barnstead) to produce stock solutions of sodium arsenate dibasic heptahydrate (Na2HAsO47H2O) and lead nitrate (Pb(NO3)2), which were stored in a refrigerator and later diluted with the same water to make experimental solutions. 2.2. Biochar production Loblolly pine (Pinus taeda) and Hamlin citrus (Citrus sinensis L. Osb.) trees, which are distributed widely in the southeastern U.S., were selected as the woody feedstock. The Hamlin citrus trees were infected with Huanglongbing (HLB) disease and are usually burned on site. Switchgrass (Panicum virgatum L.) and alfalfa (Medicago sativa), which are common bioenergy crops, were selected as the herbaceous feedstocks. The loblolly pine wood (PB) and HLB-affected Hamlin citrus wood (CT) were collected from the Citrus Research and Education Center of the University of Florida, Gainesville, FL. The switchgrass (SW) and alfalfa (AF) were obtained from USDA-ARS, Prosser, WA. The feedstock materials were rinsed with DI water several times except citrus wood. Citrus wood were washed with soap and cleaned with a brush to remove pesticide deposits and nutrient because the citrus wood was obtained from an intensively managed grove. The feedstocks were oven dried at 60 °C for 48 h and then crushed to a grain size of less than 2 mm with a mechanical mill. About 15–20 g of crushed dry samples were loaded into a quartz tubes and put inside a bench-top furnace (Barnstead 1500 M). The whole pyrolysis process was operated with constant N2 gas flow inside the tubes at peak temperatures of 300, 450, or 600 °C. The resulting biochars were designated by their biomass type and peak temperature such as PB300, CT300, SW300 and AF300. The biochars were passed through a sieve to isolate the 0.5–1 mm particle size intervals, and were then rinsed with tap water for one hour and DI water for 10 min and oven dried at 80 °C for 12 h. These were saved in sealed containers for subsequent analyses. 2.3. Biochar characterization Total C, N and H content in the biochar samples were analyzed with a CHN Elemental analyzer (Carlo-Erba NA-1500). The inorganic element contents (Ca, Mg, K, P, Al and Fe) of the biochar sampler were determined using the AOAC method by an inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin– Elmer Plasma 3200). Oxygen content was calculated as the weight difference between the raw biochar and sum of C, H, N and nonvolatile elements (Peterson et al., 2013). Total surface area was measured with NOVA 1200 analyzer using Brunauer–Emmett– Teller (BET) method using N2 adsorption isotherms. The pH value of biochar samples was determined using published method (Sun et al., 2014). Briefly, 1 g of sample was added to 20 mL DI water. The suspension was shaken with a mechanic shaker at 40 rpm for 1 h, and equilibrated for 5 min before measuring pH with a pH meter (Fisher Scientific Accumet basic AB15). Similar methods were previously reported by others authors (Cerqueira et al., 2011, 2012; Arenas-Lago et al., 2013).
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Thermal stability analysis of biochar samples was done under an air atmosphere using a Mettler Toledo thermogravimetric analyzer (TGA) between 25 and 700 °C. Non-combustible component (NCC) content was calculated as the weight left after thermal combustion at 700 °C relative to the starting biochar weight.
feedstock had higher C contents than the grass biochars. This is probably because the woody biomass has higher cellulose and lignin content but lower hemicellulose and NCC content than herbaceous biomass. The nitrogen (N) contents of the biochars were between 0.3% and 3%, which are also within the commonly observed value of previous studies (Sun et al., 2014). There was no obvious trend on the effect of pyrolysis temperature on N contents; however, the grass biochars contained more N than the wood biochars made at the same conditions. All of the tested biochars were low in non-volatile elements, except the CT and AF series had slightly high Ca contents (Table 1). Because high temperature can accelerate biomass decomposition (Silva et al., 2009; Kronbauer et al., 2013; Dias et al., 2014; Garcia et al., 2014; Perez et al., 2014; Sanchis et al., 2015), the higher temperature biochars contained more non-volatile elements than the low temperature ones.
2.4. As and Pb sorption For sorption experiment, solutions of As(V) (10 mg L 1) and Pb(II) (20 mg L 1) were made from the stock solutions (1000 mg L 1). About 0.05 g of each biochar was added to 20 mL solutions in 68 mL digestion vessels (Environmental Express) The vessels were placed onto a shaker and agitated at 40 rpm until sampling at room temperature (22 ± 0.5 °C). After 24 h equilibrium (predetermined), the suspensions were filtered through 0.22 lm pore size nylon membrane filters (GE cellulose nylon membrane) and filtrate was analyzed for As and Pb using the ICP-AES. Amount sorbed was calculated as the difference between initial and final solution metal concentrations. Each treatment was repeated three times and average values with error bars were reported.
3.2. Basic properties Biochar production rate (28–50%) was dependent on both pyrolysis temperature and feedstock type (Table 2), which agrees with the findings of other studies (Sun et al., 2014). All the biochars were alkaline (Table 2, pH 7.1–10), which is common for biochars prepared with dry-pyrolysis technology (Lehmann et al., 2006; Sun et al., 2014). In general, pH of the biochars increased with pyrolysis temperature (Table 2). This is because high pyrolysis temperature can increase the percent of alkaline cations (e.g., Ca, Mg, K). The surface area of the biochars is also strongly dependent on both pyrolysis temperature and feedstock type (Table 2). Wood biochars made at the two lower temperatures (i.e., 300 and 450 °C) and all the grass biochars had small surface area (0.1–15 m2/g), but PB600 and CT600 had surface area of 209 and 183 m2/g, respectively. NCC contents of the tested biochars increased with pyrolysis temperature with the exception of PB300 (Table 2). The NCC contents of the grass biochars were higher than that of the corresponding wood biochars of equivalent temperature, which can be attributed to the fact that herbaceous biomass has higher NCC than the woody biomass. The PB and SW series had the lowest and the highest NCC contents, respectively, compared to other biochars made at the same pyrolysis temperatures. All the biochars had good water holding capacity and maintained 72–86% of saturation under free-drainage conditions (Table S1, supporting information). Overall, the grass biochars showed slightly better water holding ability than the wood biochars; and SW was the most capable to retain water and CT was the least capable. The water holding capacity of the biochars was further evaluated with TGA analyses of the water-saturated
2.5. Statistical analyses Differences between As(V) and Pb(II) sorption in each treatment were tested for significance using a factorial analysis of variance (ANOVA; SAS Version 9.3; SAS Institute, Cary, NC) and Duncan’s multiple range tests. Pearson correlation analysis was used to determine the relationships between hazardous elements sorption and the measured biochar properties. 3. Results and discussion 3.1. Elemental composition All the tested biochars were rich in carbon (C, 64–86%), oxygen (O, 11–32%), and hydrogen (H, 2–5%) (Table 1), which is normal for biochars prepared from raw woody and grass feedstocks (Uchimiya et al., 2011). When the pyrolysis temperature increased, the C contents of the biochar also increased, but the O and H contents decreased. These trends are consistent with the findings of previous studies that pyrolysis can concentrate C of the biomass feedstocks (Kloss et al., 2012; Sun et al., 2014). The ratios of H/C and O/C were also used as carbonization indicators. Decreased H/C and O/C ratios of all the biochars made at higher temperatures indicated they lost more water and O-containing functional groups, and formed more aromatic and graphic structure to condense C (Uchimiya et al., 2011). Table 1 also shows that, under the same pyrolysis conditions, biochars derived from the wood
Table 1 Elemental composition of biochar made from loblolly pine (PB), citrus (CT), alfalfa (AF), and switchgrass (SW) pyrolyzed at 300, 450 and 600 °C under N2. Biochar
C
N
H
O
Al
Ca
Fe
K
P
Mg
O/C atomic ratio
H/C atomic ratio
0.34 0.31 0.33 1.53 1.23 1.28 3.10 2.42 2.22 2.34 2.23 1.90
4.95 3.31 2.13 4.73 3.61 2.08 5.26 3.01 1.91 4.64 2.87 2.21
25.17 15.76 11.40 27.07 25.02 14.90 24.24 22.13 19.43 31.68 28.27 24.99
0.028 0.036 0.041 0.012 0.011 0.013 0.023 0.024 0.025 0.039 0.039 0.042
0.14 0.20 0.19 1.51 1.71 2.28 1.19 1.33 1.53 0.43 0.56 0.52
0.019 0.022 0.025 0.015 0.017 0.019 0.039 0.03 0.038 0.066 0.072 0.067
0.02 0.02 0.05 0.34 0.41 0.66 0.51 0.70 0.76 0.84 1.05 1.21
0.03 0.04 0.04 0.10 0.11 0.15 0.15 0.17 0.19 0.21 0.30 0.32
0.078 0.12 0.12 0.21 0.25 0.35 0.56 0.53 0.65 0.44 0.59 0.61
0.27 0.15 0.10 0.31 0.28 0.14 0.28 0.24 0.20 0.40 0.33 0.28
0.86 0.50 0.30 0.88 0.64 0.32 0.98 0.52 0.31 0.94 0.54 0.39
%, mass PB300 PB450 PB600 CT300 CT450 CT600 AF300 AF450 AF600 SW300 SW450 SW600
69.24 80.18 85.68 64.48 67.63 78.28 64.72 69.66 73.25 59.32 64.02 68.15
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3.3. As(V) and Pb(II) sorption
Table 2 Basic physical and chemical properties of biochars. Biochar
PB300 PB450 PB600 CT300 CT450 CT600 AF300 AF450 AF600 SW300 SW450 SW600
pH
7.10 7.61 7.05 7.76 10.00 9.48 8.38 9.17 10.35 8.21 9.74 9.84
NCC
Production rate
BET surface area
%, mass based
m2 g
5.55 3.65 4.01 9.00 10.31 13.57 10.51 11.59 12.52 18.23 19.17 21.25
0.2 0.1 209 0.8 2.8 182 0.6 0.7 0.2 1.2 10 15
49.8 31.3 28.3 41.6 29.7 27.6 42.1 32.35 28.16 43.73 31.3 30.1
1
BJH pore vol (des. leg) cc g 1 0.002 0 0.003 0.005 0.009 0.013 0.006 0.005 0.006 0.01 0.02 0.024
biochars (Fig. S1, supporting information), which showed the similar trends. TGA analysis was carried out to study thermal stability of biochars in air (Fig. 1). When heating temperature was implemented between 25 and 100 °C, the mass of all the biochars only decreased slightly (due to water loss) and then became stable until at least 200 °C. No phase change was found on all the biochars at 100– 200 °C, indicating decomposition did not occur at this temperature range. Therefore, 100 °C was used as the starting points for plotting the TGA curves in Fig. 1. As pyrolysis temperature increased, the biochars became more resistant to decompose or more stable, which agrees well with the results of previous studies (Zhang et al., 2012; Sun et al., 2014; Yao et al., 2014).
Biochars were evaluated for their ability to sorb anionic (As(V)) and cationic (Pb(II)) hazardous elements. Under the tested experimental conditions, all the biochars showed limited sorption ability (110–280 mg kg 1, equivalent to 1.47–3.73 mmol kg 1) to As(V)) in aqueous solution (Fig. 2). Among all the biochars, the AF450 sorbed the greatest amount of As(V). For the rest of the biochars, pine derived biochars sorbed more As(V) than the others made under the same conditions. The mass sorption of Pb(II) on the biochars was higher than the As(V) (Fig. 2). However, pine derived biochars showed significantly lower Pb(II) sorption than other biochars derived from other feedstocks. The SW450 sorbed about 950 mg/kg (4.59 mmol kg 1) of Pb(II), which is much higher than all the other biochars. Statistical analyses indicated that feedstock type, pyrolysis temperature and their interaction had statistically significant effect on both As(V) and Pb(II) sorption (Table S2, supporting information). Duncan’s multiple range tests (a = 0.05) showed that PB and AF sorbed As(V) to the greatest extent (without significant difference between these), followed by SW, and then CT (Table S3, supporting information). For Pb(II) sorption, no statistically significant difference was found among CT, AF, and SW samples, but PB samples were lower. Duncan’s comparison indicated that biochars made at 450 °C has significantly greater Pb(II) sorption than at that made at 300 and 600 °C. All the biochars made at 300 and 450 °C showed no statistically significant difference in As sorption, but all of them showed statistically larger As sorption than the 600 °C biochars. Sorption of hazardous elements onto biochars has been ascribed to several potential mechanisms, including electrostatic attraction,
120
A
B
300 450 600
100
300 450 600
Weight (%)
80
60
40
20
0 120
C
D
300 450 600
100
300 450 600
Weight (%)
80
60
40
20
0 100
200
300
400
500 o
Temperature ( C)
600
700 100
200
300
400
500 o
Temperature ( C)
Fig. 1. Thermogravimetric curves of biochars derived from pinewood (A), citrus wood (B), alfalfa (C), and switchgrass (D).
600
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S. Wang et al. / Chemosphere 134 (2015) 257–262
8000
suggesting physisorption may not be the governing mechanism. Solution pH was strongly correlated with both As(V) and Pb(II) sorption with Pearson’s r of 0.808 and 0.661, respectively (Table 3). This indicates that electrostatic interactions may control the sorption of As(V) and Pb(II) onto the biochars because solution pH strongly affects the sorbate species and surface charge of the sorbent. Strong statistic correlations were also identified for the alkaline cations, particularly (Ca + Mg + K), and the sorption of the two hazardous elements (Table 3), further reflecting the importance of the electrostatic interaction to the sorption of As(V) and Pb(II) on the biochars. While no correlations between the sorption of As(V) and other elemental compositions was identified, the sorption of Pb(II) was correlated with the contents of P, N, and, C. This result is consistent with the findings in the literature that sorption of Pb(II) onto the biochar is controlled by multiple mechanisms, including precipitation (with phosphate ions) and surface complexation (with functional groups) (Cao et al., 2009; Ahmad et al., 2014). For example, strong correlation (r = 0.803) between Pb(II) sorption and the N content suggests N containing function groups such as amine may involve Pb sorption process (Yantasee et al., 2004).
6000
4. Conclusions
350
A -1
Sorbed As(V) (mg kg )
300
250
200
150
100
50
0 PB
0 0 0 0 0 0 0 0 0 0 0 0 30 B45 B60 T30 T45 T60 F30 F45 F60 W30 W45 W60 C C C A A A P P S S S
12000
B 10000 -1
Sorbed Pb(II) (mg kg )
261
4000
2000
0 PB
0 0 0 0 0 0 0 0 0 0 0 0 30 B45 B60 T30 T45 T60 F30 F45 F60 W30 W45 W60 C C C A A A P P S S S
Biochar Fig. 2. Sorption of As(V) (A) and Pb(II) (B) onto the biochars. Error bars represent standard deviations.
Table 3 Pearson correlation coefficients (r) and significance of linear relationship between As(V) and Pb(II) sorption and physiochemical properties of biochars. Factors
Pb(II) r
As(V) Significance level
r
Significance level
pH Ca Mg K
0.808 0.613 0.801 0.826
⁄⁄
Ca + Mg + K P Al O N C
0.931 0.790 0.318 0.500 0.803 0.609
⁄⁄⁄
Surface area
0.321
NS
0.218
NS
Pore volume
0.655
⁄
0.600
⁄
NCC
0.793
⁄⁄
0.452
NS
⁄ ⁄⁄ ⁄⁄⁄
⁄⁄
NS NS ⁄⁄ ⁄
0.661 0.615 0.522 0.653
⁄⁄
0.790 0.601 0.292 0.157 0.484 0.233
⁄⁄⁄
⁄
NS ⁄
NS NS NS NS NS
Note: ⁄, ⁄⁄, and ⁄⁄⁄ are significance level of
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Physicochemical and sorptive properties of biochars derived from woody and herbaceous biomass Shengsen Wang a, Bin Gao b,⇑, Andrew R. Zimmerman c, Yuncong Li a,⇑, Lena Ma d, Willie G. Harris d, Kati W. Migliaccio e a
Tropical Research and Education Center, Soil and Water Science Department, University of Florida, Homestead, FL 33031, United States Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, United States Department of Geological Sciences, University of Florida, Gainesville, FL 32611, United States d Soil and Water Science Department, University of Florida, Gainesville, FL 32611, United States e Tropical Research and Education Center, Department of Agricultural and Biological Engineering, University of Florida, Homestead, FL 33031, United States b c
h i g h l i g h t s Prepare conditions affected biochar’s physicochemical properties. Prepare conditions affected the sorption of As and Pb onto biochars. All the tested biochars removed As and Pb form aqueous solutions. Electrostatic interaction played an important role in biochar sorption.
a r t i c l e
i n f o
Article history: Received 13 January 2015 Received in revised form 6 April 2015 Accepted 20 April 2015
Keywords: Biochar Temperature Feedstock Arsenate Lead
a b s t r a c t It is unclear how the properties of biochar control its ability to sorb metals. In this work, physicochemical properties of a variety of biochars, made from four types of feedstock at three pyrolysis temperatures (300, 450 and 600 °C) were compared to their ability to sorb arsenic (As) and lead (Pb) in aqueous solutions. Experimental results showed that both feedstock types and pyrolysis temperature affected biochar’s production rate, i.e., ratio of mass of biochar and biomass, thermal stability, elemental composition, non-combustible component (NCC) content, pH values, surface areas and thus their sorption ability to the two metals in aqueous solution. In general, the high temperature biochars had low O/C and H/C ratios, were more carbonized with larger surface area, and were more concentrated with alkaline cations. In addition, biochars made from woody feedstocks had larger surface area, but lower NCC contents than that made from grasses under the same conditions. Although all the tested biochars removed both As and Pb from aqueous solutions, they showed different sorption abilities because of the variations in properties. Statistical analyses suggested that feedstock type affected the sorption ability of the biochars to both As and Pb significantly (p < 0.001). Pyrolysis temperature, however, showed little influence on biochar sorption of Pb in aqueous solutions. Statistical analyses also showed that electrostatic interaction played an important role in controlling the sorption of both As(V) and Pb(II) onto the biochar. Other mechanisms, such as precipitation and surface complexation, could also control the sorption of Pb(II) onto the biochars. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Biochar is pyrogenic black carbon derived from the pyrolysis of biomass, such as wood and grass, under N2 or oxygen limited conditions (Lehmann et al., 2006; Rutigliano et al., 2014). Recent studies have suggested that biochar land application can mitigate global warming by fixing the carbon in soils (Lehmann et al., ⇑ Corresponding authors. E-mail addresses: bg55@ufl.edu (B. Gao), yunli@ufl.edu (Y. Li). http://dx.doi.org/10.1016/j.chemosphere.2015.04.062 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.
2006) and reducing N2O gas emission from soils (Spokas et al., 2009). In addition, biochar amendment can, in some instances, increase soil fertility and crop yield (Major et al., 2010), improve soil physical and chemical properties such as water holding and nutrient retention capacity (Bell and Worrall, 2011; Karhu et al., 2011). Biochar properties are strongly affected by their peak pyrolysis temperature (Yao et al., 2012; Park et al., 2013). As pyrolysis temperature increases, the degree of carbonization of the feedstock increases, as indicated by increased carbon (C) content as well as
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decreased hydrogen (H) and oxygen (O) contents in the resulting biochar (Harvey et al., 2011; Uchimiya et al., 2011). Biochar made at low temperature generally has lower pH, higher water holding capacity, lower specific surface area, more carboxylic and phenolic hydroxyl functional groups and higher cation exchange capacity (CEC) (Gaskin et al., 2008; Ippolito et al., 2012; Novak et al., 2012). These properties determine its potential environmental applications. For instance, biochar surface properties such as BET surface area and pore volume were correlated with its ability to sorb phenanthrene (Park et al., 2013). Feedstock types may also affect the physicochemical properties of the biochar. For instance, woody biomass often has higher cellulose, hemicellulose, and lignin contents than herbaceous or grass species (Lupoi and Smith, 2012). The higher lignin content in plant biomass was reported to promote carbonization and to increase biochar production rate (Demirbas, 2004). Furthermore, previous studies have shown that feedstock types may strongly influence biochar’s surface characteristics (surface area, pH, and functional groups), thereby affecting their potential environmental applications (Inyang et al., 2010; Kloss et al., 2012; Sun et al., 2014). The widespread degradation of ground and surface water, plant and animal communities, soils and air with metals and metalloids is a high environmental and human health risks (Ribeiro et al., 2010; Oliveira et al., 2012a,b; Quispe et al., 2012; Saikia et al., 2014). However, cost-efficient, safe and the least destructive recuperation techniques suitable for such impacted soils are wanted and their efficiency in the long-term needs to be evaluated (Silva et al., 2012; Oliveira et al., 2013; Ribeiro et al., 2013; Cutruneo et al., 2014). Because of its special surface properties, biochar has been suggested to be a low-cost option for soil remediation and water treatment adsorbents, or as a soil amendment to improve soil condition and increase carbon sequestration (Cao et al., 2009; Inyang et al., 2011; Yao et al., 2011; Abdel-Fattah et al., 2014; Elaigwu et al., 2014). Several previous studies have shown that the sorption ability of some biochars is comparable to or even higher than many commercial adsorbents including activated carbon (Cao et al., 2009; Inyang et al., 2011, 2012). Since most of the biochars are converted from waste materials, they are inexpensive and thus may not require regeneration after having been used as adsorbent (Inyang et al., 2010, 2011). In addition, some of the spent/ exhausted biochars can even be applied directly to soils as amendment or slow-release fertilizers (Yao et al., 2013a,b). Soil contamination by hazardous elements is detrimental to crop production and human health. Arsenic (As) and lead (Pb) are carcinogenic trace metals and are of great concern for human and animal health. Of the many well developed approaches for As and Pb removal from soil and groundwater systems, adsorption is the one of the most discussed techniques (Mohan and Pittman, 2007). Several studies have reported the strong sorption of hazardous elements, particularly lead, by biochars produced from various feedstocks and under different conditions (Uchimiya et al., 2010; Inyang et al., 2012; Xue et al., 2012). However, it is still unclear which types of biochar remove hazardous elements most efficiently and what biochar characteristics are most important in this process. To our knowledge, no previous works have systematically investigated the separate and combined effect of pyrolysis temperature and feedstock type on biochar sorption of hazardous elements in aqueous solutions. The main objective of this work was to understand how the physicochemical properties of biochar, arising from variations in pyrolysis temperature and feedstock type, affect biochar’s sorption of As and Pb. Two woody and two herbaceous plants were selected as the feedstocks to produce biochars at three pyrolysis temperatures. A range of laboratory tools were used to characterize and compare the properties of the biochars. In addition, batch sorption
experiments were conducted to measure their sorption of arsenate (As(V)) and cationic lead (Pb(II)) in aqueous solutions.
2. Materials and methods 2.1. Reagents All chemicals of used in this study were of analytical grade and were purchased from Fisher Scientific. Chemicals were dissolved in deionized (DI) water (18.2 MX) (Nanopure water, Barnstead) to produce stock solutions of sodium arsenate dibasic heptahydrate (Na2HAsO47H2O) and lead nitrate (Pb(NO3)2), which were stored in a refrigerator and later diluted with the same water to make experimental solutions. 2.2. Biochar production Loblolly pine (Pinus taeda) and Hamlin citrus (Citrus sinensis L. Osb.) trees, which are distributed widely in the southeastern U.S., were selected as the woody feedstock. The Hamlin citrus trees were infected with Huanglongbing (HLB) disease and are usually burned on site. Switchgrass (Panicum virgatum L.) and alfalfa (Medicago sativa), which are common bioenergy crops, were selected as the herbaceous feedstocks. The loblolly pine wood (PB) and HLB-affected Hamlin citrus wood (CT) were collected from the Citrus Research and Education Center of the University of Florida, Gainesville, FL. The switchgrass (SW) and alfalfa (AF) were obtained from USDA-ARS, Prosser, WA. The feedstock materials were rinsed with DI water several times except citrus wood. Citrus wood were washed with soap and cleaned with a brush to remove pesticide deposits and nutrient because the citrus wood was obtained from an intensively managed grove. The feedstocks were oven dried at 60 °C for 48 h and then crushed to a grain size of less than 2 mm with a mechanical mill. About 15–20 g of crushed dry samples were loaded into a quartz tubes and put inside a bench-top furnace (Barnstead 1500 M). The whole pyrolysis process was operated with constant N2 gas flow inside the tubes at peak temperatures of 300, 450, or 600 °C. The resulting biochars were designated by their biomass type and peak temperature such as PB300, CT300, SW300 and AF300. The biochars were passed through a sieve to isolate the 0.5–1 mm particle size intervals, and were then rinsed with tap water for one hour and DI water for 10 min and oven dried at 80 °C for 12 h. These were saved in sealed containers for subsequent analyses. 2.3. Biochar characterization Total C, N and H content in the biochar samples were analyzed with a CHN Elemental analyzer (Carlo-Erba NA-1500). The inorganic element contents (Ca, Mg, K, P, Al and Fe) of the biochar sampler were determined using the AOAC method by an inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin– Elmer Plasma 3200). Oxygen content was calculated as the weight difference between the raw biochar and sum of C, H, N and nonvolatile elements (Peterson et al., 2013). Total surface area was measured with NOVA 1200 analyzer using Brunauer–Emmett– Teller (BET) method using N2 adsorption isotherms. The pH value of biochar samples was determined using published method (Sun et al., 2014). Briefly, 1 g of sample was added to 20 mL DI water. The suspension was shaken with a mechanic shaker at 40 rpm for 1 h, and equilibrated for 5 min before measuring pH with a pH meter (Fisher Scientific Accumet basic AB15). Similar methods were previously reported by others authors (Cerqueira et al., 2011, 2012; Arenas-Lago et al., 2013).
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Thermal stability analysis of biochar samples was done under an air atmosphere using a Mettler Toledo thermogravimetric analyzer (TGA) between 25 and 700 °C. Non-combustible component (NCC) content was calculated as the weight left after thermal combustion at 700 °C relative to the starting biochar weight.
feedstock had higher C contents than the grass biochars. This is probably because the woody biomass has higher cellulose and lignin content but lower hemicellulose and NCC content than herbaceous biomass. The nitrogen (N) contents of the biochars were between 0.3% and 3%, which are also within the commonly observed value of previous studies (Sun et al., 2014). There was no obvious trend on the effect of pyrolysis temperature on N contents; however, the grass biochars contained more N than the wood biochars made at the same conditions. All of the tested biochars were low in non-volatile elements, except the CT and AF series had slightly high Ca contents (Table 1). Because high temperature can accelerate biomass decomposition (Silva et al., 2009; Kronbauer et al., 2013; Dias et al., 2014; Garcia et al., 2014; Perez et al., 2014; Sanchis et al., 2015), the higher temperature biochars contained more non-volatile elements than the low temperature ones.
2.4. As and Pb sorption For sorption experiment, solutions of As(V) (10 mg L 1) and Pb(II) (20 mg L 1) were made from the stock solutions (1000 mg L 1). About 0.05 g of each biochar was added to 20 mL solutions in 68 mL digestion vessels (Environmental Express) The vessels were placed onto a shaker and agitated at 40 rpm until sampling at room temperature (22 ± 0.5 °C). After 24 h equilibrium (predetermined), the suspensions were filtered through 0.22 lm pore size nylon membrane filters (GE cellulose nylon membrane) and filtrate was analyzed for As and Pb using the ICP-AES. Amount sorbed was calculated as the difference between initial and final solution metal concentrations. Each treatment was repeated three times and average values with error bars were reported.
3.2. Basic properties Biochar production rate (28–50%) was dependent on both pyrolysis temperature and feedstock type (Table 2), which agrees with the findings of other studies (Sun et al., 2014). All the biochars were alkaline (Table 2, pH 7.1–10), which is common for biochars prepared with dry-pyrolysis technology (Lehmann et al., 2006; Sun et al., 2014). In general, pH of the biochars increased with pyrolysis temperature (Table 2). This is because high pyrolysis temperature can increase the percent of alkaline cations (e.g., Ca, Mg, K). The surface area of the biochars is also strongly dependent on both pyrolysis temperature and feedstock type (Table 2). Wood biochars made at the two lower temperatures (i.e., 300 and 450 °C) and all the grass biochars had small surface area (0.1–15 m2/g), but PB600 and CT600 had surface area of 209 and 183 m2/g, respectively. NCC contents of the tested biochars increased with pyrolysis temperature with the exception of PB300 (Table 2). The NCC contents of the grass biochars were higher than that of the corresponding wood biochars of equivalent temperature, which can be attributed to the fact that herbaceous biomass has higher NCC than the woody biomass. The PB and SW series had the lowest and the highest NCC contents, respectively, compared to other biochars made at the same pyrolysis temperatures. All the biochars had good water holding capacity and maintained 72–86% of saturation under free-drainage conditions (Table S1, supporting information). Overall, the grass biochars showed slightly better water holding ability than the wood biochars; and SW was the most capable to retain water and CT was the least capable. The water holding capacity of the biochars was further evaluated with TGA analyses of the water-saturated
2.5. Statistical analyses Differences between As(V) and Pb(II) sorption in each treatment were tested for significance using a factorial analysis of variance (ANOVA; SAS Version 9.3; SAS Institute, Cary, NC) and Duncan’s multiple range tests. Pearson correlation analysis was used to determine the relationships between hazardous elements sorption and the measured biochar properties. 3. Results and discussion 3.1. Elemental composition All the tested biochars were rich in carbon (C, 64–86%), oxygen (O, 11–32%), and hydrogen (H, 2–5%) (Table 1), which is normal for biochars prepared from raw woody and grass feedstocks (Uchimiya et al., 2011). When the pyrolysis temperature increased, the C contents of the biochar also increased, but the O and H contents decreased. These trends are consistent with the findings of previous studies that pyrolysis can concentrate C of the biomass feedstocks (Kloss et al., 2012; Sun et al., 2014). The ratios of H/C and O/C were also used as carbonization indicators. Decreased H/C and O/C ratios of all the biochars made at higher temperatures indicated they lost more water and O-containing functional groups, and formed more aromatic and graphic structure to condense C (Uchimiya et al., 2011). Table 1 also shows that, under the same pyrolysis conditions, biochars derived from the wood
Table 1 Elemental composition of biochar made from loblolly pine (PB), citrus (CT), alfalfa (AF), and switchgrass (SW) pyrolyzed at 300, 450 and 600 °C under N2. Biochar
C
N
H
O
Al
Ca
Fe
K
P
Mg
O/C atomic ratio
H/C atomic ratio
0.34 0.31 0.33 1.53 1.23 1.28 3.10 2.42 2.22 2.34 2.23 1.90
4.95 3.31 2.13 4.73 3.61 2.08 5.26 3.01 1.91 4.64 2.87 2.21
25.17 15.76 11.40 27.07 25.02 14.90 24.24 22.13 19.43 31.68 28.27 24.99
0.028 0.036 0.041 0.012 0.011 0.013 0.023 0.024 0.025 0.039 0.039 0.042
0.14 0.20 0.19 1.51 1.71 2.28 1.19 1.33 1.53 0.43 0.56 0.52
0.019 0.022 0.025 0.015 0.017 0.019 0.039 0.03 0.038 0.066 0.072 0.067
0.02 0.02 0.05 0.34 0.41 0.66 0.51 0.70 0.76 0.84 1.05 1.21
0.03 0.04 0.04 0.10 0.11 0.15 0.15 0.17 0.19 0.21 0.30 0.32
0.078 0.12 0.12 0.21 0.25 0.35 0.56 0.53 0.65 0.44 0.59 0.61
0.27 0.15 0.10 0.31 0.28 0.14 0.28 0.24 0.20 0.40 0.33 0.28
0.86 0.50 0.30 0.88 0.64 0.32 0.98 0.52 0.31 0.94 0.54 0.39
%, mass PB300 PB450 PB600 CT300 CT450 CT600 AF300 AF450 AF600 SW300 SW450 SW600
69.24 80.18 85.68 64.48 67.63 78.28 64.72 69.66 73.25 59.32 64.02 68.15
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3.3. As(V) and Pb(II) sorption
Table 2 Basic physical and chemical properties of biochars. Biochar
PB300 PB450 PB600 CT300 CT450 CT600 AF300 AF450 AF600 SW300 SW450 SW600
pH
7.10 7.61 7.05 7.76 10.00 9.48 8.38 9.17 10.35 8.21 9.74 9.84
NCC
Production rate
BET surface area
%, mass based
m2 g
5.55 3.65 4.01 9.00 10.31 13.57 10.51 11.59 12.52 18.23 19.17 21.25
0.2 0.1 209 0.8 2.8 182 0.6 0.7 0.2 1.2 10 15
49.8 31.3 28.3 41.6 29.7 27.6 42.1 32.35 28.16 43.73 31.3 30.1
1
BJH pore vol (des. leg) cc g 1 0.002 0 0.003 0.005 0.009 0.013 0.006 0.005 0.006 0.01 0.02 0.024
biochars (Fig. S1, supporting information), which showed the similar trends. TGA analysis was carried out to study thermal stability of biochars in air (Fig. 1). When heating temperature was implemented between 25 and 100 °C, the mass of all the biochars only decreased slightly (due to water loss) and then became stable until at least 200 °C. No phase change was found on all the biochars at 100– 200 °C, indicating decomposition did not occur at this temperature range. Therefore, 100 °C was used as the starting points for plotting the TGA curves in Fig. 1. As pyrolysis temperature increased, the biochars became more resistant to decompose or more stable, which agrees well with the results of previous studies (Zhang et al., 2012; Sun et al., 2014; Yao et al., 2014).
Biochars were evaluated for their ability to sorb anionic (As(V)) and cationic (Pb(II)) hazardous elements. Under the tested experimental conditions, all the biochars showed limited sorption ability (110–280 mg kg 1, equivalent to 1.47–3.73 mmol kg 1) to As(V)) in aqueous solution (Fig. 2). Among all the biochars, the AF450 sorbed the greatest amount of As(V). For the rest of the biochars, pine derived biochars sorbed more As(V) than the others made under the same conditions. The mass sorption of Pb(II) on the biochars was higher than the As(V) (Fig. 2). However, pine derived biochars showed significantly lower Pb(II) sorption than other biochars derived from other feedstocks. The SW450 sorbed about 950 mg/kg (4.59 mmol kg 1) of Pb(II), which is much higher than all the other biochars. Statistical analyses indicated that feedstock type, pyrolysis temperature and their interaction had statistically significant effect on both As(V) and Pb(II) sorption (Table S2, supporting information). Duncan’s multiple range tests (a = 0.05) showed that PB and AF sorbed As(V) to the greatest extent (without significant difference between these), followed by SW, and then CT (Table S3, supporting information). For Pb(II) sorption, no statistically significant difference was found among CT, AF, and SW samples, but PB samples were lower. Duncan’s comparison indicated that biochars made at 450 °C has significantly greater Pb(II) sorption than at that made at 300 and 600 °C. All the biochars made at 300 and 450 °C showed no statistically significant difference in As sorption, but all of them showed statistically larger As sorption than the 600 °C biochars. Sorption of hazardous elements onto biochars has been ascribed to several potential mechanisms, including electrostatic attraction,
120
A
B
300 450 600
100
300 450 600
Weight (%)
80
60
40
20
0 120
C
D
300 450 600
100
300 450 600
Weight (%)
80
60
40
20
0 100
200
300
400
500 o
Temperature ( C)
600
700 100
200
300
400
500 o
Temperature ( C)
Fig. 1. Thermogravimetric curves of biochars derived from pinewood (A), citrus wood (B), alfalfa (C), and switchgrass (D).
600
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8000
suggesting physisorption may not be the governing mechanism. Solution pH was strongly correlated with both As(V) and Pb(II) sorption with Pearson’s r of 0.808 and 0.661, respectively (Table 3). This indicates that electrostatic interactions may control the sorption of As(V) and Pb(II) onto the biochars because solution pH strongly affects the sorbate species and surface charge of the sorbent. Strong statistic correlations were also identified for the alkaline cations, particularly (Ca + Mg + K), and the sorption of the two hazardous elements (Table 3), further reflecting the importance of the electrostatic interaction to the sorption of As(V) and Pb(II) on the biochars. While no correlations between the sorption of As(V) and other elemental compositions was identified, the sorption of Pb(II) was correlated with the contents of P, N, and, C. This result is consistent with the findings in the literature that sorption of Pb(II) onto the biochar is controlled by multiple mechanisms, including precipitation (with phosphate ions) and surface complexation (with functional groups) (Cao et al., 2009; Ahmad et al., 2014). For example, strong correlation (r = 0.803) between Pb(II) sorption and the N content suggests N containing function groups such as amine may involve Pb sorption process (Yantasee et al., 2004).
6000
4. Conclusions
350
A -1
Sorbed As(V) (mg kg )
300
250
200
150
100
50
0 PB
0 0 0 0 0 0 0 0 0 0 0 0 30 B45 B60 T30 T45 T60 F30 F45 F60 W30 W45 W60 C C C A A A P P S S S
12000
B 10000 -1
Sorbed Pb(II) (mg kg )
261
4000
2000
0 PB
0 0 0 0 0 0 0 0 0 0 0 0 30 B45 B60 T30 T45 T60 F30 F45 F60 W30 W45 W60 C C C A A A P P S S S
Biochar Fig. 2. Sorption of As(V) (A) and Pb(II) (B) onto the biochars. Error bars represent standard deviations.
Table 3 Pearson correlation coefficients (r) and significance of linear relationship between As(V) and Pb(II) sorption and physiochemical properties of biochars. Factors
Pb(II) r
As(V) Significance level
r
Significance level
pH Ca Mg K
0.808 0.613 0.801 0.826
⁄⁄
Ca + Mg + K P Al O N C
0.931 0.790 0.318 0.500 0.803 0.609
⁄⁄⁄
Surface area
0.321
NS
0.218
NS
Pore volume
0.655
⁄
0.600
⁄
NCC
0.793
⁄⁄
0.452
NS
⁄ ⁄⁄ ⁄⁄⁄
⁄⁄
NS NS ⁄⁄ ⁄
0.661 0.615 0.522 0.653
⁄⁄
0.790 0.601 0.292 0.157 0.484 0.233
⁄⁄⁄
⁄
NS ⁄
NS NS NS NS NS
Note: ⁄, ⁄⁄, and ⁄⁄⁄ are significance level of
