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Heavy metal removal and crude bio-oil upgrade from Sedum alfredii Hance harvest using hydrothermal upgrading
Journal of Hazardous Materials 179 (2010) 1037–1041
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Heavy metal removal and crude bio-oil upgrade from Sedum alfredii Hance harvest using hydrothermal upgrading夽 Jian-guang Yang a,b,∗ , Chao-bo Tang a , Jing He a , Sheng-Hai Yang a , Mo-tang Tang a a b
Department of Metallurgical Science and Engineering, Central South University, Changsha 410083, China Institute of Powder Metallurgy Research, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 16 October 2009 Received in revised form 26 March 2010 Accepted 26 March 2010 Available online 2 April 2010 Keywords: Sedum alfredii Hance Biomass Hydrothermal upgrading process Heavy metals Hyperaccumulator
a b s t r a c t In this study, heavy metals were removed and crude bio-oil was yielded from a heavy metal hyperaccumulator harvest, Sedum alfredii Hance, through hydrothermal upgrading process. This paper reports on the optimization of process parameters for the removal of heavy metals (zinc, lead, and copper) and for the upgrading of crude bio-oil from this biomass in an autoclave. Parameters such as granularity, temperature, pressure, and duration were examined for their effect on the removal efficiency of heavy metals and upgrading efficacy of crude bio-oil. Maximum heavy metal removal efficiency of >99% and crude bio-oil upgrading efficiency of >60% were attained with an 18 mesh (1 mm) granularity, and 22.1 MPa at 370 ◦ C in the presence of 10 mg/L additives (K2 CO3 ) for 60 s. Under these optimized conditions, an oil phase (mostly composed of phenolic hydrocarbons and derivatives), a water phase raffinate (containing Zn2+ (0.39 g/L), Pb2+ (0.10 g/L), Cu2+ (0.15 g/L)), and a solid phase (the hydrothermal upgrading residue, which completely satisfies the limit set by China legislation related to biosolids disposal) were obtained. © 2010 Published by Elsevier B.V.
1. Introduction Phytoremediation is an environmental technology that uses plants to degrade, transform, immobilize, or stabilize various organic and inorganic pollutants present in soil, mud, or wastewater. Examples of these pollutants include atrazine, benzene, toluene, xylenes, trichloroethylene, arsenic, lead, antimony, nickel, copper, zinc, cadmium, etc. Phytoremediation is low-cost, simple, sustainable, compatible with the environment, and aesthetically more attractive compared with conventional technologies. It can be implemented in situ to treat vast expanses of contaminated ground or large volumes of diluted wastewater. Phytoremediation of soil or water using hyperaccumulator plants has been extensively explored in recent years [1–5]. One of the main drawbacks of this technology is related to the handling and disposal of a phytoremediation product, hyperaccumulator harvest. In literature [6], it has been reported that the harvest may be confined in landfills or used as compost. However, landfill disposal and composting are questionable options because toxic heavy metals could spread to the surrounding environment by leaching and other natural processes, polluting soil, surface water, and groundwater, and thereby threatening human and ani-
夽 Foundation item: Project (50804056) supported by the Nature Science Foundation of China; Project (20080431028) supported by China Postdoctoral Science Foundation. ∗ Corresponding author at: Department of Metallurgical Science and Engineering, Central South University, Changsha 410083, China. E-mail address: jianguang [email protected] (J.-g. Yang). 0304-3894/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.jhazmat.2010.03.109
mal health. Some authors have mentioned that the harvest can be dried, compacted, and incinerated to recover useful metals from the ash for recycling, in a manner similar to phytomining plants [7]. The harvest can also simply be confined [8]. Others have proposed that biomass could be used as an energy source [8]. However, detailed studies related to handling and using harvest are limited. SasNowosielska et al. reviewed phytoextraction crop disposal methods [9] and found little available information; however, they suggested that incineration could be the preferred disposal method because it is economically feasible and environment-friendly. Recently, Keller et al. conducted an experimental investigation on the thermal behavior of two different plants used in heavy metal phytoextraction [8]. In that study, pyrolysis was determined to be more efficient than incineration for recovering Cd and Zn from plant harvest, but its effectiveness depends on metal volatility, plant species growth form (i.e., herbs, shrubs, or trees), and incineration scheme (i.e., incineration alone or co-incineration with other solid waste). In our previous study [10,11], a process for detoxifying the hyperaccumulator harvest and converting this biomass to a suitable fertilizer or mulch was proposed. Heavy metals were separated from Sedum plumbizincicola harvest using ammonia–ammonium chloride solution as a leaching agent. After leaching, the heavy metal (zinc, copper, and lead) concentration in the harvest was sufficiently reduced to satisfy the limit set by China legislation related to biosolid disposal. Zinc, copper, and lead extraction efficiency reached 97.95, 89.48, and 95.52%, respectively. In the current study, another process, hydrothermal upgrading (HTU) process is proposed. HTU not only separates and removes heavy metals effectively but also yields crude bio-oil from Sedum alfredii Hance
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Table 1 Main chemical components of Sedum alfredii Hance harvest. Elements
Content/%
Zn
Pb
Cu
Fe
Ca
Mg
Na
S
P
0.53
0.14
0.20
0.70
2.80
0.55
0.48
0.76
0.61
Table 2 Optimum operating conditions for hydrothermal upgrading. Pressure Duration Quantity of added distilled water Additive concentration Agitation rate
22.1 MPa 60 s 100 mL 10 mg/L 60 rpm
harvest. This paper aims to demonstrate that using HTU for heavy metal separation and removal, as well as crude bio-oil production from the hyperaccumulator harvest, is both feasible and efficient. 2. Materials and methods 2.1. Preparation of Sedum alfredii Hance Sedum alfredii Hance was cultivated in tap water as an outdoor mono-plant culture under environmental conditions prevailing from April to September in Hechi City, Guangxi, China (24◦ 42 31 N and 108◦ 03 35 W, altitude of 680 m above sea level). Culture temperature was held between 21 and 29 ◦ C, at pH 5.9. Harvested plants were first rinsed with deionized water then dried in an oven at 35 ◦ C to constant weight. Later, the dried plants were cut and crushed to definite granularity and analyzed at the Central South University Chemical Analysis Center. Table 1 shows the main chemical components of this harvest. 2.2. Method Hydrothermal upgrading experiments were conducted in a 400 mL stainless iron autoclave. In a typical hydrothermal upgrading experiment, the autoclave was loaded with 20 g of crushed Sedum alfredii Hance and a small amount of distilled water. The reactor was first filled with nitrogen to remove the air within the reactor. Reactants were agitated vertically at 100 r/min using a mechanism stirrer. Temperature was then raised to the setting temperature at a fixed heating rate of 5 ◦ C/min, with the setting duration maintained at the setting temperature. After the reaction duration, the reactor was cooled down to room temperature by circular tap water. The gaseous products were vented and collected within a sealed bag and were designated as Gas 1, then analyzed by a gas chromatograph (GC-TCD: GC-8A). The solid and liquid products were rinsed from the autoclave with distilled water. Solid
and liquid products were separated by filtration under vacuum for 15 min. During filtration, 200 mL of distilled water was used to wash the solid products three times. The filtrated liquid portion was extracted with aether. The aetheral solution thus obtained was dried over anhydrous sodium sulfate, filtered, and evaporated in an evaporator under reduced pressure. Upon aether removal, this proportion was weighed and designated as Oil 1. The water phase was then extracted with methanol. The obtained methanol solution was also dried over anhydrous sodium sulfate. Upon methanol removal under reduced pressure, this proportion was weighed and designated as Oil 2. After extraction, the remaining water phase contained the water-soluble hydrocarbons, heavy metal ions, inorganic anion, etc. Solid products were extracted with acetone in an extraction apparatus until the solvent became colorless. After acetone removal under reduced pressure in an evaporator, the proportion obtained was weighed and designated as Oil 3. The distilled-residue water fraction, that is, the raffinate, was designated as Water 1. Acetone insoluble fraction was dried at 100 ◦ C then weighed and labeled Solid Residue 1. The recovery of heavy metals, such as zinc, lead, and copper, was calculated by mass balance using the analysis of Water 1, solid residue, and original biomass. The upgrading efficiency is defined as the amount of solid harvest converted into other organic forms (i.e., oils, gas). Oils 1, 2, and 3, were analyzed by gas chromatograph equipped with a mass selective detector (GC–MS; HP 5973; column, HP-1; crosslinked methyl siloxane). Compounds were identified by means of the Wiley library-HP G1035A and NIST library of mass spectra and subsets-HP G1033A. In this research, the oil yield and heavy metal removal efficiency were calculated according to formulas (1) and (2), respectively: Oil yield(weight rate)% =
Total weight of oil products × 100%. 20 + 100 + 200
(1)
WMe × 100%. W
(2)
Heavy metal removal efficiency(%) =
In formula (1), 20 is the 20 g original Sedum alfredii Hance harvest, 100 represents the 100 mL added reagent distilled water, and 200 denotes 200 mL of distilled water used for washing the solid products. In formula (2), WMe represents the dissolved zinc, lead, and copper quantity in Water 1, while W denotes the total heavy metal weight in 20 g original Sedum alfredii Hance harvest. Oil yield was calculated according to formula (1), and not according to the commonly used formula (3). Formula (3) is expressed
Table 3 Effect of temperature on removal efficiency of heavy metals and upgrading efficiency of crude bio-oil from Sedum alfredii Hance. Temperature/◦ C
Oil yield/wt%
Oil 1
Solid phase/wt%
Water phase/wt%
Gas phase/wt%
Heavy metal concentration in Water 1 (g L−1 ) (removal efficiency (%))
Oil 2
Oil 3
Zn
Pb
Cu
270
0.33 0.58 Total: 1.78%
0.87
2.49
95.5
0.19
0.40 (99.2%)
0.09 (99.6%)
0.13 (99.5%)
320
0.82 1.03 Total: 2.81%
0.96
2.36
94.6
0.26
0.39 (99.3%)
0.10 (99.8%)
0.14 (99.4%)
370
1.04 1.88 Total: 3.82%
0.9
2.17
93.8
0.30
0.39 (99.7%)
0.10 (99.8%)
0.15 (99.6%)
420
1.22 1.78 Total: 3.88%
0.88
2.08
93.6
0.39
0.39 (99.8%)
0.10 (99.8%)
0.15 (99.6%)
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Table 4 Effect of pressure on removal efficiency of heavy metals and upgrading efficiency of crude bio-oil from Sedum alfredii Hance. Pressure/MPa
Oil yield/wt%
Oil 1
Solid phase/wt%
Water phase/wt%
Gas phase/wt%
Heavy metal concentration in Water 1 (g·L−1 ) (Removal efficiency (%))
Oil 2
Oil 3
Zn
Pb
Cu
18.2
0.21 0.49 Total: 1.46%
0.76
2.70
95.7
0.10
0.39 (99.2%)
0.10 (99.6%)
0.14 (99.4%)
20.5
0.75 1.11 Total: 2.86%
1.0
2.33
94.6
0.23
0.39 (99.3%)
0.10 (99.7%)
0.14 (99.4%)
22.1
1.21 1.75 Total: 4.37%
1.41
2.10
93.1
0.36
0.39 (99.8%)
0.10 (99.7%)
0.15 (99.6%)
25.4
1.34 1.68 Total: 4.29%
1.27
1.88
93.3
0.46
0.39 (99.8%)
0.10 (99.7%)
0.14 (99.8%)
Table 5 Effect of granularity on removal efficiency of heavy metals and upgrading efficiency of crude bio-oil from Sedum alfredii Hance. Granularity/mesh
Oil yield/wt%
Oil 1
Solid phase/wt%
Water phase/wt%
Gas phase/wt%
Heavy metal concentration in Water 1 (g L−1 ) (removal efficiency (%))
Oil 2
Oil 3
Zn
Pb
Cu
4 (4.75 mm)
0.67 0.81 Total: 2.41%
0.93
3.03
94.2
0.28
0.39 (99.2%)
0.09 (99.6%)
0.14 (99.5%)
8 (2.36 mm)
0.97 1.32 Total: 3.23%
0.94
2.51
94.0
0.25
0.39 (99.3%)
0.10 (99.7%)
0.14 (99.4%)
14 (1.40 mm)
1.08 1.42 Total: 3.32%
0.82
1.64
94.8
0.26
0.39 (99.7%)
0.10 (99.8%)
0.15 (99.6%)
18 (1.00 mm)
1.33 1.43 Total: 3.93%
1.17
1.40
94.3
0.33
0.39 (99.8%)
0.10 (99.8%)
0.16 (99.9%)
ent temperatures for 60 s at a system pressure of 22.1 MPa. Oil yield clearly increased with rising upgrading temperature, and the oil yield (weight rate) of bio-oil ranged from 1.78 to 3.88 wt%. Maximum upgrading efficiency was obtained at the temperature 420 ◦ C. However, under this temperature, the gaseous products reached 0.39 wt%, which is larger than that observed for 370 ◦ C, even though the total oil products were nearly the same. The removal efficiency of heavy metals did not significantly change. The heavy metals were almost dissolved and remained in the water phase under these temperatures. Table 3 also shows that by increasing the temperature, solid residue yield decreased because increased temperature enhanced the decomposition of Sedum alfredii Hance biomass. Comprehensively considering the oil and gas yields, as well as the heavy metal removal efficiency, 370 ◦ C was selected as the optimized upgrading temperature.
as Total weight of oil products Oil yield(weight rate)% = × 100%. 20
(3)
3. Results and discussion 3.1. Effect of processing temperature Hydrothermal upgrading of Sedum alfredii Hance was first conducted by varying the upgrading temperature for 60 s at a system pressure of 22.1 MPa. The optimum operation conditions for the hydrothermal upgrading runs are summarized in Table 2, and the results are shown in Table 3. Table 3 shows the product distribution from hydrothermal upgrading treatment of Sedum alfredii Hance harvest under differ-
Table 6 Effect of duration on removal efficiency of heavy metals and upgrading efficiency of crude bio-oil from Sedum alfredii Hance. Duration/s
Oil yield/wt%
Oil 1
Solid phase/wt%
Water phase/wt%
Oil 2
Oil 3
10
0.10 0.17 Total: 0.43%
0.16
5.49
93.9
20
0.32 0.58 Total: 1.42%
0.52
4.17
40
0.95 1.27 Total: 2.99%
0.77
60
1.21 1.66 Total: 3.77%
80 120
Gas phase/wt%
Heavy metal concentration in Water 1 (g L−1 ) (removal efficiency (%)) Zn
Pb
Cu
0.08
0.31 (80.1%)
0.08 (85.1%)
0.12 (88.4%)
94.3
0.10
0.34 (87.2%)
0.09 (89.4%)
0.13 (91.4%)
3.32
93.5
0.21
0.38 (96.7%)
0.10 (96.5%)
0.14 (96.6%)
0.90
2.10
93.8
0.37
0.39 (99.8%)
0.10 (99.8%)
0.15 (99.6%)
1.10 1.69 Total: 3.72%
0.93
2.11
93.7
0.41
0.39 (99.8%)
0.10 (99.8%)
0.16 (99.8%)
1.12 1.67 Total: 3.71%
0.92
2.12
93.7
0.39
0.39 (99.8%)
0.10 (99.8%)
0.16 (99.7%)
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Table 7 Confirmation experiment conditions and results. Experiment conditions—temperature: 370 ◦ C; pressure: 22.1 MPa; duration: 60 s; granularity: 20 mesh; additive concentration: 10 mg/L; 100 mL of distilled water was used for reactant and 200 mL of distilled water was used for washing solid products. Experiment results
Oil yield/wt%
Solid phase/wt%
Water phase/wt%
Gas phase/%
Heavy metal concentration in raffinate (g L−1 ) Zn
Pb
Cu
1 2 3
3.66 3.79 4.15
1.99 1.92 1.96
93.7 94.1 93.5
0.33 0.32 0.30
0.39 0.39 0.39
0.10 0.10 0.10
0.15 0.15 0.16
Average
3.86
1.93
92.8
0.32
0.39
0.10
0.15
3.2. Effect of processing pressure
3.5. Confirmation experiments
The effect of processing pressure on the oil yield and heavy metal removal efficiency was conducted by varying the processing pressure from 18.2 to 25.4 MPa in the presence of 10 mg/L additive concentration at 370 ◦ C and upgrading for 60 s. The pressure inside the autoclave was controlled by altering the quantity of added distilled water. Table 4 shows that the oil yield increased with escalating upgrading pressure, and the oil yield (weight rate) of bio-oil ranged from 1.46 to 4.37 wt%. Gas phase increased from 0.1 to 0.46 wt%. However, heavy metal separation efficiency did not significantly change; heavy metals were nearly separated from the biomass and dissolved in the water phase. On considering yielding more oil products and avoiding yielding more gas products in this process, 22.1 MPa was selected as the optimized upgrading pressure.
Based on the experiments conducted, the optimum conditions for the removal efficiency of heavy metals and upgrading efficiency of crude bio-oil from Sedum alfredii Hance through HTU were as follows: temperature at 370 ◦ C, pressure at 22.1 MPa, duration of 60 s, 20 mesh granularity, and additive concentration of 10 mg/L. These conditions were then applied in a confirmation experiment to remove heavy metals and upgrade Sedum alfredii Hance biomass into crude bio-oil. The Sedum alfredii Hance harvest was first
3.3. Effect of granularity The effect of granularity on the removal efficiency of heavy metals and upgrading efficiency of crude bio-oil was conducted by varying the Sedum alfredii Hance harvest granularity from 4 to 20 mesh in the presence of 10 mg/L additive concentration at 370 ◦ C, and 22.1 MPa. Hydrothermal upgrading was performed for 60 s. Table 5 shows that >99.2% of the heavy metals Zn, Pb, and Cu were separated from Sedum alfredii Hance biomass and dissolved in water phase, while oil yield increased steadily with decreasing granularity. At the same time, the quantity of water phase and gas products changed only slightly. However, obtaining thinner granularity required more time and energy for cutting and grinding; when the granularity reached 90% >18 (1.00 mm) mesh, it became difficult to further diminish the granularity. Considering all these factors, 18 mesh was chosen as the optimum granularity in this study. 3.4. Effect of hydrothermal upgrading duration Table 6 shows the product distribution from hydrothermal upgrading treatment of Sedum alfredii Hance harvest under different upgrading durations at a system temperature of 370 ◦ C, and a 22.1 MPa pressure. We can see from Table 6 that the oil yield and heavy metal removal efficiency rose with increasing upgrading duration from 10 to 60 s. However, when the duration >60 s, no further increase was apparent in all product yields. At the same time, the removal efficiency of heavy metals did not change significantly when the upgrading duration reached 60 s. Table 6 also shows that, by increasing upgrading from 10 to 60 s, solid residue yield decreased. However, when the duration extended to more than 60 s, no considerable change occurred because the decomposable content of 20 g Sedum alfredii Hance biomass in this study requires 60 s to decompose. Certain undecomposed compositions and char-like solid products cannot be decomposed, even after upgrading time is prolonged. Therefore, 60 s was selected as the optimized upgrading duration in this study.
Table 8 Identification of compounds in a typical as-resultant crude oil by GC–MS analysis. No.
RT (min)
Name of compounds
Area (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
1.01 1.42 2.04 3.11 3.51 4.21 5.22 5.41 5.48 5.79 5.88 6.62 6.69 6.86 7.07 7.15 7.35 7.51 7.68 7.85 8.11 8.13 8.32 8.42 8.54 9.17 9.25 9.42 9.70 10.09 10.15 10.36 10.43 11.47 11.79 12.20 12.53 12.77 13.40 13.81 14.75 15.54 16.84 17.52 21.41
Acetic acid Propanoic acid 2-Furancarboxaldehyde 2-methyl-2-cyclopenten-1-one 1-(2-Furanyl)-ethanone 3-Methyl-1-furancarboxaldehyde Etanamine, N-ethyl-N-nitroso2-Cyclopenten-1-one, 3,4-dimethyl 2-Cyclopenten-1-one, 2,3-dimethyl 2-Cyclopentan-1-one, 3,4,5-trimethyl Ethanol, 2,2 -oxybisPhenol, 2-methoxy1,1-Dimethyl-4-methylenecyclohexan 3-Cyclopenten-1-one, 2,2,5,5-tetramethyl Bicyclo[2.2.1]heptane, 2,2,3-trimethyl Lacthydrazide Benzenethiol 2-Hydmxy-3-methyl-2-cyelopenten-1-one 3-Cyclopenten-1-one, 2,2,5,5-tetramethyl1-Methoxy-5-hexene 2-Hydroxy-3-methylbenzaldehyde Hexaethylene glycol monododecyl ether 1,4-Bephenol, 4-ethyl-2-methoxyTriethylene glycol Phenol, 4-ethyl-2-methoxy 2-Propanol, 1,3-dimethoxyButane, 1,2,4-trimethoxy2-Methoxy-phenol Propne, 1,1-[tehylidenebis(oxy)]bis3-Methyl-phenol, 4-methyl-phenol Propne, 2,2-[ethylidenebis(oxy)]bisAlpha, D-xylofuranoside 3,6,9,12-Tetraoxahexadecan-1-ol 3-Methoxy-1-propene Propanoic acid, 3-methoxy, methylester Imidazo[4,5-e][1,4]diazepine-5,8-dione Hydrazine, 1,2-dipropyl4-Methyl-phenol 4-Hexen-3-ol, 2-methylOctaethylene glycol 1,2-Benzenediol 4-Ethyl-2-methoxy-phenol 4-Methyl-1, 2-benzenediol 2,6-Dimethyl-1,3-benzenediol Butylated hydroxytoluene
0.19 0.30 0.61 1.29 0.89 0.18 0.13 1.35 1.45 1.13 1.42 2.23 0.57 0.21 1.05 0.87 1.49 1.56 0.91 0.93 1.43 0.38 1.17 0.39 14.02 2.18 1.67 1.05 1.56 24.27 1.02 0.81 0.11 0.12 0.17 0.21 0.20 7.31 0.20 0.17 1.12 3.60 1.12 0.09 0.17
Total area
83.30
J.-g. Yang et al. / Journal of Hazardous Materials 179 (2010) 1037–1041 Table 9 Identification of main elements in the mixed raffinate by ICP-AES analysis. Element
Concentration (g/L)
Zn Cu Pb K Ca P S Na N Fe
0.39 0.15 0.10 0.14 0.49 0.02 0.11 0.29 0.01 0.31
prepared and then subjected to hydrothermal upgrading according to the optimized conditions. In each case, a quantity of crude biooil and raffinate loaded with heavy metals were obtained. Table 7 shows the confirmation experiment conditions and results. Calculated according to formula (1), the average oil yield was 3.86%; however, calculated according to formula (3), the oil yield was 61.76%. The heavy metals were nearly separated from the biomass and dissolved in the raffinate. The average concentration of the zinc, lead, and copper ions were 0.39, 0.10, and 0.16 g L−1 , respectively. Tables 8 and 9 provide the typical GS-MS analysis of as-resultant crude oil and ICP-AES analysis of relevant water phase raffinate, respectively. Table 8 shows that the major compounds in asresultant oil products were mainly phenolic hydrocarbons and derivatives. The identification of main elements in the raffinate by ICP-AES analysis in Table 9 shows that >99.8% of heavy metals Zn, Pb, and Cu were separated and dissolved in the raffinate. Therefore, the hydrothermal upgrading residue, after being washed with tap water, can be safely used as fertilizer because almost all the heavy metal content was separated from this biomass. The detoxified Sedum alfredii Hance harvest completely satisfies the limit set by China legislation related to biosolid disposal. 4. Conclusions An HTU process was developed for removal of heavy metals, including zinc, lead, and copper, and yielding crude bio-oil from a heavy metal hyperaccumulator harvest, Sedum alfredii Hance. Under optimized conditions, high oil yield (61.76%) and high heavy
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metal removal efficiency (>99.5%) were achieved. The as-resultant oil products were mainly phenolic hydrocarbons and derivatives. Through further purification and processing, more valuable organic products can be obtained from these compounds. Almost all the heavy metal content was separated from this biomass. The hydrothermal upgrading residue completely satisfies the limit set by China legislation related to biosolid disposal. Few research reports regarding the handling of hyperaccumulator biomass produced during heavy metal phytoremediation exist [12]. Considering that biomass harvested at the end of heavy metal phytoremediation can be quite abundant, and that its disposal in landfills represents a potential risk to living beings, the proposed method would allow for the detoxification of plant biomass for use as biofertilizer or mulch. Our proposed method may also be employed for the recovery of bio-energy and useful metals for confinement or recycling. References [1] S.L. Brown, R.L. Chancy, J.S. Angle, A.J.M. Baker, Zinc and cadmium uptake by hyperaccumulator Thlaspi caerulescens and metal tolerant Silene vulgaris grown on sludge-amended soils, Environ. Sci. Technol. 29 (1995) 1581–1585. [2] S.D. Cunningham, W.R. Berti, Remediation of contaminated soils with green plants: an overview, In Vitro Cell. Dev. Biol. Plant 29 (1993) 207–212. [3] B.H. Robinson, R.R. Brooks, P.E.H. Gregg, J.H. Kirkman, The nickel phytoextraction potential of some ultramafic soils as determined by sequential extraction, Geoderma 87 (1999) 293–304. [4] W.R. Peters, Chelant extraction of heavy metals from contaminated soils, J. Hazard. Mater. 66 (1999) 151–210. [5] S.L. Brown, R.L. Chancy, J.S. Angle, Phytoremediation potential of Thlaspi caerulescens and bladder campion for zinc- and cadmium-contaminated soil, J. Environ. Qual. 23 (1994) 1151–1157. [6] M. Ghosh, S.P. Singh, A review on phytoremediation of heavy metals and utilization of its byproducts, Appl. Ecol. Environ. Res. 3 (2005) 1–18. [7] R.R. Brooks, M.F. Chambers, L.J. Nicks, B.H. Robinson, Phytomining, Trends Plant Sci. 3 (1998) 359–362. [8] C. Keller, C. Ludwig, F. Davoli, J. Wochele, Thermal treatment of metalenriched biomass produced from heavy metal phytoextraction, Environ. Sci. Technol. 39 (2005) 3359–3367. [9] A. Sas-Nowosielska, R. Kucharski, E. Makowski, M. Pogrzeba, J.M. Kuperberg, K. Kryski, Phytoextraction crop disposal—an unsolved problem, Environ. Pollut. 128 (2004) 373–379. [10] Y. Jian-Guang, Y. Jian-Ying, P. Chang-Hong, T. Chao-Bo, Z. Ke-Cao, Recovery of zinc from hyperaccumulator plants: Sedum plumbizincicola, Environ Technol. 30 (2009) 693–700. [11] Y. Jian-Guang, P. Chang-Hong, T. Chao-Bo, Z. Ke-Cao, Study on recovery zinc from hyperaccumulator Sedum alfredii Hance biomass, Trans. Nonferrous Met. Soc. Chin. 19 (2009) 1353–1359. [12] M. le Clercq, T. Adschiri, K. Arai, Hydrothermal processing of nickel containing biomining or bioremediation biomass, Biomass Bioenergy 21 (2001) 73–80.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Heavy metal removal and crude bio-oil upgrade from Sedum alfredii Hance harvest using hydrothermal upgrading夽 Jian-guang Yang a,b,∗ , Chao-bo Tang a , Jing He a , Sheng-Hai Yang a , Mo-tang Tang a a b
Department of Metallurgical Science and Engineering, Central South University, Changsha 410083, China Institute of Powder Metallurgy Research, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 16 October 2009 Received in revised form 26 March 2010 Accepted 26 March 2010 Available online 2 April 2010 Keywords: Sedum alfredii Hance Biomass Hydrothermal upgrading process Heavy metals Hyperaccumulator
a b s t r a c t In this study, heavy metals were removed and crude bio-oil was yielded from a heavy metal hyperaccumulator harvest, Sedum alfredii Hance, through hydrothermal upgrading process. This paper reports on the optimization of process parameters for the removal of heavy metals (zinc, lead, and copper) and for the upgrading of crude bio-oil from this biomass in an autoclave. Parameters such as granularity, temperature, pressure, and duration were examined for their effect on the removal efficiency of heavy metals and upgrading efficacy of crude bio-oil. Maximum heavy metal removal efficiency of >99% and crude bio-oil upgrading efficiency of >60% were attained with an 18 mesh (1 mm) granularity, and 22.1 MPa at 370 ◦ C in the presence of 10 mg/L additives (K2 CO3 ) for 60 s. Under these optimized conditions, an oil phase (mostly composed of phenolic hydrocarbons and derivatives), a water phase raffinate (containing Zn2+ (0.39 g/L), Pb2+ (0.10 g/L), Cu2+ (0.15 g/L)), and a solid phase (the hydrothermal upgrading residue, which completely satisfies the limit set by China legislation related to biosolids disposal) were obtained. © 2010 Published by Elsevier B.V.
1. Introduction Phytoremediation is an environmental technology that uses plants to degrade, transform, immobilize, or stabilize various organic and inorganic pollutants present in soil, mud, or wastewater. Examples of these pollutants include atrazine, benzene, toluene, xylenes, trichloroethylene, arsenic, lead, antimony, nickel, copper, zinc, cadmium, etc. Phytoremediation is low-cost, simple, sustainable, compatible with the environment, and aesthetically more attractive compared with conventional technologies. It can be implemented in situ to treat vast expanses of contaminated ground or large volumes of diluted wastewater. Phytoremediation of soil or water using hyperaccumulator plants has been extensively explored in recent years [1–5]. One of the main drawbacks of this technology is related to the handling and disposal of a phytoremediation product, hyperaccumulator harvest. In literature [6], it has been reported that the harvest may be confined in landfills or used as compost. However, landfill disposal and composting are questionable options because toxic heavy metals could spread to the surrounding environment by leaching and other natural processes, polluting soil, surface water, and groundwater, and thereby threatening human and ani-
夽 Foundation item: Project (50804056) supported by the Nature Science Foundation of China; Project (20080431028) supported by China Postdoctoral Science Foundation. ∗ Corresponding author at: Department of Metallurgical Science and Engineering, Central South University, Changsha 410083, China. E-mail address: jianguang [email protected] (J.-g. Yang). 0304-3894/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.jhazmat.2010.03.109
mal health. Some authors have mentioned that the harvest can be dried, compacted, and incinerated to recover useful metals from the ash for recycling, in a manner similar to phytomining plants [7]. The harvest can also simply be confined [8]. Others have proposed that biomass could be used as an energy source [8]. However, detailed studies related to handling and using harvest are limited. SasNowosielska et al. reviewed phytoextraction crop disposal methods [9] and found little available information; however, they suggested that incineration could be the preferred disposal method because it is economically feasible and environment-friendly. Recently, Keller et al. conducted an experimental investigation on the thermal behavior of two different plants used in heavy metal phytoextraction [8]. In that study, pyrolysis was determined to be more efficient than incineration for recovering Cd and Zn from plant harvest, but its effectiveness depends on metal volatility, plant species growth form (i.e., herbs, shrubs, or trees), and incineration scheme (i.e., incineration alone or co-incineration with other solid waste). In our previous study [10,11], a process for detoxifying the hyperaccumulator harvest and converting this biomass to a suitable fertilizer or mulch was proposed. Heavy metals were separated from Sedum plumbizincicola harvest using ammonia–ammonium chloride solution as a leaching agent. After leaching, the heavy metal (zinc, copper, and lead) concentration in the harvest was sufficiently reduced to satisfy the limit set by China legislation related to biosolid disposal. Zinc, copper, and lead extraction efficiency reached 97.95, 89.48, and 95.52%, respectively. In the current study, another process, hydrothermal upgrading (HTU) process is proposed. HTU not only separates and removes heavy metals effectively but also yields crude bio-oil from Sedum alfredii Hance
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Table 1 Main chemical components of Sedum alfredii Hance harvest. Elements
Content/%
Zn
Pb
Cu
Fe
Ca
Mg
Na
S
P
0.53
0.14
0.20
0.70
2.80
0.55
0.48
0.76
0.61
Table 2 Optimum operating conditions for hydrothermal upgrading. Pressure Duration Quantity of added distilled water Additive concentration Agitation rate
22.1 MPa 60 s 100 mL 10 mg/L 60 rpm
harvest. This paper aims to demonstrate that using HTU for heavy metal separation and removal, as well as crude bio-oil production from the hyperaccumulator harvest, is both feasible and efficient. 2. Materials and methods 2.1. Preparation of Sedum alfredii Hance Sedum alfredii Hance was cultivated in tap water as an outdoor mono-plant culture under environmental conditions prevailing from April to September in Hechi City, Guangxi, China (24◦ 42 31 N and 108◦ 03 35 W, altitude of 680 m above sea level). Culture temperature was held between 21 and 29 ◦ C, at pH 5.9. Harvested plants were first rinsed with deionized water then dried in an oven at 35 ◦ C to constant weight. Later, the dried plants were cut and crushed to definite granularity and analyzed at the Central South University Chemical Analysis Center. Table 1 shows the main chemical components of this harvest. 2.2. Method Hydrothermal upgrading experiments were conducted in a 400 mL stainless iron autoclave. In a typical hydrothermal upgrading experiment, the autoclave was loaded with 20 g of crushed Sedum alfredii Hance and a small amount of distilled water. The reactor was first filled with nitrogen to remove the air within the reactor. Reactants were agitated vertically at 100 r/min using a mechanism stirrer. Temperature was then raised to the setting temperature at a fixed heating rate of 5 ◦ C/min, with the setting duration maintained at the setting temperature. After the reaction duration, the reactor was cooled down to room temperature by circular tap water. The gaseous products were vented and collected within a sealed bag and were designated as Gas 1, then analyzed by a gas chromatograph (GC-TCD: GC-8A). The solid and liquid products were rinsed from the autoclave with distilled water. Solid
and liquid products were separated by filtration under vacuum for 15 min. During filtration, 200 mL of distilled water was used to wash the solid products three times. The filtrated liquid portion was extracted with aether. The aetheral solution thus obtained was dried over anhydrous sodium sulfate, filtered, and evaporated in an evaporator under reduced pressure. Upon aether removal, this proportion was weighed and designated as Oil 1. The water phase was then extracted with methanol. The obtained methanol solution was also dried over anhydrous sodium sulfate. Upon methanol removal under reduced pressure, this proportion was weighed and designated as Oil 2. After extraction, the remaining water phase contained the water-soluble hydrocarbons, heavy metal ions, inorganic anion, etc. Solid products were extracted with acetone in an extraction apparatus until the solvent became colorless. After acetone removal under reduced pressure in an evaporator, the proportion obtained was weighed and designated as Oil 3. The distilled-residue water fraction, that is, the raffinate, was designated as Water 1. Acetone insoluble fraction was dried at 100 ◦ C then weighed and labeled Solid Residue 1. The recovery of heavy metals, such as zinc, lead, and copper, was calculated by mass balance using the analysis of Water 1, solid residue, and original biomass. The upgrading efficiency is defined as the amount of solid harvest converted into other organic forms (i.e., oils, gas). Oils 1, 2, and 3, were analyzed by gas chromatograph equipped with a mass selective detector (GC–MS; HP 5973; column, HP-1; crosslinked methyl siloxane). Compounds were identified by means of the Wiley library-HP G1035A and NIST library of mass spectra and subsets-HP G1033A. In this research, the oil yield and heavy metal removal efficiency were calculated according to formulas (1) and (2), respectively: Oil yield(weight rate)% =
Total weight of oil products × 100%. 20 + 100 + 200
(1)
WMe × 100%. W
(2)
Heavy metal removal efficiency(%) =
In formula (1), 20 is the 20 g original Sedum alfredii Hance harvest, 100 represents the 100 mL added reagent distilled water, and 200 denotes 200 mL of distilled water used for washing the solid products. In formula (2), WMe represents the dissolved zinc, lead, and copper quantity in Water 1, while W denotes the total heavy metal weight in 20 g original Sedum alfredii Hance harvest. Oil yield was calculated according to formula (1), and not according to the commonly used formula (3). Formula (3) is expressed
Table 3 Effect of temperature on removal efficiency of heavy metals and upgrading efficiency of crude bio-oil from Sedum alfredii Hance. Temperature/◦ C
Oil yield/wt%
Oil 1
Solid phase/wt%
Water phase/wt%
Gas phase/wt%
Heavy metal concentration in Water 1 (g L−1 ) (removal efficiency (%))
Oil 2
Oil 3
Zn
Pb
Cu
270
0.33 0.58 Total: 1.78%
0.87
2.49
95.5
0.19
0.40 (99.2%)
0.09 (99.6%)
0.13 (99.5%)
320
0.82 1.03 Total: 2.81%
0.96
2.36
94.6
0.26
0.39 (99.3%)
0.10 (99.8%)
0.14 (99.4%)
370
1.04 1.88 Total: 3.82%
0.9
2.17
93.8
0.30
0.39 (99.7%)
0.10 (99.8%)
0.15 (99.6%)
420
1.22 1.78 Total: 3.88%
0.88
2.08
93.6
0.39
0.39 (99.8%)
0.10 (99.8%)
0.15 (99.6%)
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Table 4 Effect of pressure on removal efficiency of heavy metals and upgrading efficiency of crude bio-oil from Sedum alfredii Hance. Pressure/MPa
Oil yield/wt%
Oil 1
Solid phase/wt%
Water phase/wt%
Gas phase/wt%
Heavy metal concentration in Water 1 (g·L−1 ) (Removal efficiency (%))
Oil 2
Oil 3
Zn
Pb
Cu
18.2
0.21 0.49 Total: 1.46%
0.76
2.70
95.7
0.10
0.39 (99.2%)
0.10 (99.6%)
0.14 (99.4%)
20.5
0.75 1.11 Total: 2.86%
1.0
2.33
94.6
0.23
0.39 (99.3%)
0.10 (99.7%)
0.14 (99.4%)
22.1
1.21 1.75 Total: 4.37%
1.41
2.10
93.1
0.36
0.39 (99.8%)
0.10 (99.7%)
0.15 (99.6%)
25.4
1.34 1.68 Total: 4.29%
1.27
1.88
93.3
0.46
0.39 (99.8%)
0.10 (99.7%)
0.14 (99.8%)
Table 5 Effect of granularity on removal efficiency of heavy metals and upgrading efficiency of crude bio-oil from Sedum alfredii Hance. Granularity/mesh
Oil yield/wt%
Oil 1
Solid phase/wt%
Water phase/wt%
Gas phase/wt%
Heavy metal concentration in Water 1 (g L−1 ) (removal efficiency (%))
Oil 2
Oil 3
Zn
Pb
Cu
4 (4.75 mm)
0.67 0.81 Total: 2.41%
0.93
3.03
94.2
0.28
0.39 (99.2%)
0.09 (99.6%)
0.14 (99.5%)
8 (2.36 mm)
0.97 1.32 Total: 3.23%
0.94
2.51
94.0
0.25
0.39 (99.3%)
0.10 (99.7%)
0.14 (99.4%)
14 (1.40 mm)
1.08 1.42 Total: 3.32%
0.82
1.64
94.8
0.26
0.39 (99.7%)
0.10 (99.8%)
0.15 (99.6%)
18 (1.00 mm)
1.33 1.43 Total: 3.93%
1.17
1.40
94.3
0.33
0.39 (99.8%)
0.10 (99.8%)
0.16 (99.9%)
ent temperatures for 60 s at a system pressure of 22.1 MPa. Oil yield clearly increased with rising upgrading temperature, and the oil yield (weight rate) of bio-oil ranged from 1.78 to 3.88 wt%. Maximum upgrading efficiency was obtained at the temperature 420 ◦ C. However, under this temperature, the gaseous products reached 0.39 wt%, which is larger than that observed for 370 ◦ C, even though the total oil products were nearly the same. The removal efficiency of heavy metals did not significantly change. The heavy metals were almost dissolved and remained in the water phase under these temperatures. Table 3 also shows that by increasing the temperature, solid residue yield decreased because increased temperature enhanced the decomposition of Sedum alfredii Hance biomass. Comprehensively considering the oil and gas yields, as well as the heavy metal removal efficiency, 370 ◦ C was selected as the optimized upgrading temperature.
as Total weight of oil products Oil yield(weight rate)% = × 100%. 20
(3)
3. Results and discussion 3.1. Effect of processing temperature Hydrothermal upgrading of Sedum alfredii Hance was first conducted by varying the upgrading temperature for 60 s at a system pressure of 22.1 MPa. The optimum operation conditions for the hydrothermal upgrading runs are summarized in Table 2, and the results are shown in Table 3. Table 3 shows the product distribution from hydrothermal upgrading treatment of Sedum alfredii Hance harvest under differ-
Table 6 Effect of duration on removal efficiency of heavy metals and upgrading efficiency of crude bio-oil from Sedum alfredii Hance. Duration/s
Oil yield/wt%
Oil 1
Solid phase/wt%
Water phase/wt%
Oil 2
Oil 3
10
0.10 0.17 Total: 0.43%
0.16
5.49
93.9
20
0.32 0.58 Total: 1.42%
0.52
4.17
40
0.95 1.27 Total: 2.99%
0.77
60
1.21 1.66 Total: 3.77%
80 120
Gas phase/wt%
Heavy metal concentration in Water 1 (g L−1 ) (removal efficiency (%)) Zn
Pb
Cu
0.08
0.31 (80.1%)
0.08 (85.1%)
0.12 (88.4%)
94.3
0.10
0.34 (87.2%)
0.09 (89.4%)
0.13 (91.4%)
3.32
93.5
0.21
0.38 (96.7%)
0.10 (96.5%)
0.14 (96.6%)
0.90
2.10
93.8
0.37
0.39 (99.8%)
0.10 (99.8%)
0.15 (99.6%)
1.10 1.69 Total: 3.72%
0.93
2.11
93.7
0.41
0.39 (99.8%)
0.10 (99.8%)
0.16 (99.8%)
1.12 1.67 Total: 3.71%
0.92
2.12
93.7
0.39
0.39 (99.8%)
0.10 (99.8%)
0.16 (99.7%)
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Table 7 Confirmation experiment conditions and results. Experiment conditions—temperature: 370 ◦ C; pressure: 22.1 MPa; duration: 60 s; granularity: 20 mesh; additive concentration: 10 mg/L; 100 mL of distilled water was used for reactant and 200 mL of distilled water was used for washing solid products. Experiment results
Oil yield/wt%
Solid phase/wt%
Water phase/wt%
Gas phase/%
Heavy metal concentration in raffinate (g L−1 ) Zn
Pb
Cu
1 2 3
3.66 3.79 4.15
1.99 1.92 1.96
93.7 94.1 93.5
0.33 0.32 0.30
0.39 0.39 0.39
0.10 0.10 0.10
0.15 0.15 0.16
Average
3.86
1.93
92.8
0.32
0.39
0.10
0.15
3.2. Effect of processing pressure
3.5. Confirmation experiments
The effect of processing pressure on the oil yield and heavy metal removal efficiency was conducted by varying the processing pressure from 18.2 to 25.4 MPa in the presence of 10 mg/L additive concentration at 370 ◦ C and upgrading for 60 s. The pressure inside the autoclave was controlled by altering the quantity of added distilled water. Table 4 shows that the oil yield increased with escalating upgrading pressure, and the oil yield (weight rate) of bio-oil ranged from 1.46 to 4.37 wt%. Gas phase increased from 0.1 to 0.46 wt%. However, heavy metal separation efficiency did not significantly change; heavy metals were nearly separated from the biomass and dissolved in the water phase. On considering yielding more oil products and avoiding yielding more gas products in this process, 22.1 MPa was selected as the optimized upgrading pressure.
Based on the experiments conducted, the optimum conditions for the removal efficiency of heavy metals and upgrading efficiency of crude bio-oil from Sedum alfredii Hance through HTU were as follows: temperature at 370 ◦ C, pressure at 22.1 MPa, duration of 60 s, 20 mesh granularity, and additive concentration of 10 mg/L. These conditions were then applied in a confirmation experiment to remove heavy metals and upgrade Sedum alfredii Hance biomass into crude bio-oil. The Sedum alfredii Hance harvest was first
3.3. Effect of granularity The effect of granularity on the removal efficiency of heavy metals and upgrading efficiency of crude bio-oil was conducted by varying the Sedum alfredii Hance harvest granularity from 4 to 20 mesh in the presence of 10 mg/L additive concentration at 370 ◦ C, and 22.1 MPa. Hydrothermal upgrading was performed for 60 s. Table 5 shows that >99.2% of the heavy metals Zn, Pb, and Cu were separated from Sedum alfredii Hance biomass and dissolved in water phase, while oil yield increased steadily with decreasing granularity. At the same time, the quantity of water phase and gas products changed only slightly. However, obtaining thinner granularity required more time and energy for cutting and grinding; when the granularity reached 90% >18 (1.00 mm) mesh, it became difficult to further diminish the granularity. Considering all these factors, 18 mesh was chosen as the optimum granularity in this study. 3.4. Effect of hydrothermal upgrading duration Table 6 shows the product distribution from hydrothermal upgrading treatment of Sedum alfredii Hance harvest under different upgrading durations at a system temperature of 370 ◦ C, and a 22.1 MPa pressure. We can see from Table 6 that the oil yield and heavy metal removal efficiency rose with increasing upgrading duration from 10 to 60 s. However, when the duration >60 s, no further increase was apparent in all product yields. At the same time, the removal efficiency of heavy metals did not change significantly when the upgrading duration reached 60 s. Table 6 also shows that, by increasing upgrading from 10 to 60 s, solid residue yield decreased. However, when the duration extended to more than 60 s, no considerable change occurred because the decomposable content of 20 g Sedum alfredii Hance biomass in this study requires 60 s to decompose. Certain undecomposed compositions and char-like solid products cannot be decomposed, even after upgrading time is prolonged. Therefore, 60 s was selected as the optimized upgrading duration in this study.
Table 8 Identification of compounds in a typical as-resultant crude oil by GC–MS analysis. No.
RT (min)
Name of compounds
Area (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
1.01 1.42 2.04 3.11 3.51 4.21 5.22 5.41 5.48 5.79 5.88 6.62 6.69 6.86 7.07 7.15 7.35 7.51 7.68 7.85 8.11 8.13 8.32 8.42 8.54 9.17 9.25 9.42 9.70 10.09 10.15 10.36 10.43 11.47 11.79 12.20 12.53 12.77 13.40 13.81 14.75 15.54 16.84 17.52 21.41
Acetic acid Propanoic acid 2-Furancarboxaldehyde 2-methyl-2-cyclopenten-1-one 1-(2-Furanyl)-ethanone 3-Methyl-1-furancarboxaldehyde Etanamine, N-ethyl-N-nitroso2-Cyclopenten-1-one, 3,4-dimethyl 2-Cyclopenten-1-one, 2,3-dimethyl 2-Cyclopentan-1-one, 3,4,5-trimethyl Ethanol, 2,2 -oxybisPhenol, 2-methoxy1,1-Dimethyl-4-methylenecyclohexan 3-Cyclopenten-1-one, 2,2,5,5-tetramethyl Bicyclo[2.2.1]heptane, 2,2,3-trimethyl Lacthydrazide Benzenethiol 2-Hydmxy-3-methyl-2-cyelopenten-1-one 3-Cyclopenten-1-one, 2,2,5,5-tetramethyl1-Methoxy-5-hexene 2-Hydroxy-3-methylbenzaldehyde Hexaethylene glycol monododecyl ether 1,4-Bephenol, 4-ethyl-2-methoxyTriethylene glycol Phenol, 4-ethyl-2-methoxy 2-Propanol, 1,3-dimethoxyButane, 1,2,4-trimethoxy2-Methoxy-phenol Propne, 1,1-[tehylidenebis(oxy)]bis3-Methyl-phenol, 4-methyl-phenol Propne, 2,2-[ethylidenebis(oxy)]bisAlpha, D-xylofuranoside 3,6,9,12-Tetraoxahexadecan-1-ol 3-Methoxy-1-propene Propanoic acid, 3-methoxy, methylester Imidazo[4,5-e][1,4]diazepine-5,8-dione Hydrazine, 1,2-dipropyl4-Methyl-phenol 4-Hexen-3-ol, 2-methylOctaethylene glycol 1,2-Benzenediol 4-Ethyl-2-methoxy-phenol 4-Methyl-1, 2-benzenediol 2,6-Dimethyl-1,3-benzenediol Butylated hydroxytoluene
0.19 0.30 0.61 1.29 0.89 0.18 0.13 1.35 1.45 1.13 1.42 2.23 0.57 0.21 1.05 0.87 1.49 1.56 0.91 0.93 1.43 0.38 1.17 0.39 14.02 2.18 1.67 1.05 1.56 24.27 1.02 0.81 0.11 0.12 0.17 0.21 0.20 7.31 0.20 0.17 1.12 3.60 1.12 0.09 0.17
Total area
83.30
J.-g. Yang et al. / Journal of Hazardous Materials 179 (2010) 1037–1041 Table 9 Identification of main elements in the mixed raffinate by ICP-AES analysis. Element
Concentration (g/L)
Zn Cu Pb K Ca P S Na N Fe
0.39 0.15 0.10 0.14 0.49 0.02 0.11 0.29 0.01 0.31
prepared and then subjected to hydrothermal upgrading according to the optimized conditions. In each case, a quantity of crude biooil and raffinate loaded with heavy metals were obtained. Table 7 shows the confirmation experiment conditions and results. Calculated according to formula (1), the average oil yield was 3.86%; however, calculated according to formula (3), the oil yield was 61.76%. The heavy metals were nearly separated from the biomass and dissolved in the raffinate. The average concentration of the zinc, lead, and copper ions were 0.39, 0.10, and 0.16 g L−1 , respectively. Tables 8 and 9 provide the typical GS-MS analysis of as-resultant crude oil and ICP-AES analysis of relevant water phase raffinate, respectively. Table 8 shows that the major compounds in asresultant oil products were mainly phenolic hydrocarbons and derivatives. The identification of main elements in the raffinate by ICP-AES analysis in Table 9 shows that >99.8% of heavy metals Zn, Pb, and Cu were separated and dissolved in the raffinate. Therefore, the hydrothermal upgrading residue, after being washed with tap water, can be safely used as fertilizer because almost all the heavy metal content was separated from this biomass. The detoxified Sedum alfredii Hance harvest completely satisfies the limit set by China legislation related to biosolid disposal. 4. Conclusions An HTU process was developed for removal of heavy metals, including zinc, lead, and copper, and yielding crude bio-oil from a heavy metal hyperaccumulator harvest, Sedum alfredii Hance. Under optimized conditions, high oil yield (61.76%) and high heavy
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metal removal efficiency (>99.5%) were achieved. The as-resultant oil products were mainly phenolic hydrocarbons and derivatives. Through further purification and processing, more valuable organic products can be obtained from these compounds. Almost all the heavy metal content was separated from this biomass. The hydrothermal upgrading residue completely satisfies the limit set by China legislation related to biosolid disposal. Few research reports regarding the handling of hyperaccumulator biomass produced during heavy metal phytoremediation exist [12]. Considering that biomass harvested at the end of heavy metal phytoremediation can be quite abundant, and that its disposal in landfills represents a potential risk to living beings, the proposed method would allow for the detoxification of plant biomass for use as biofertilizer or mulch. Our proposed method may also be employed for the recovery of bio-energy and useful metals for confinement or recycling. References [1] S.L. Brown, R.L. Chancy, J.S. Angle, A.J.M. Baker, Zinc and cadmium uptake by hyperaccumulator Thlaspi caerulescens and metal tolerant Silene vulgaris grown on sludge-amended soils, Environ. Sci. Technol. 29 (1995) 1581–1585. [2] S.D. Cunningham, W.R. Berti, Remediation of contaminated soils with green plants: an overview, In Vitro Cell. Dev. Biol. Plant 29 (1993) 207–212. [3] B.H. Robinson, R.R. Brooks, P.E.H. Gregg, J.H. Kirkman, The nickel phytoextraction potential of some ultramafic soils as determined by sequential extraction, Geoderma 87 (1999) 293–304. [4] W.R. Peters, Chelant extraction of heavy metals from contaminated soils, J. Hazard. Mater. 66 (1999) 151–210. [5] S.L. Brown, R.L. Chancy, J.S. Angle, Phytoremediation potential of Thlaspi caerulescens and bladder campion for zinc- and cadmium-contaminated soil, J. Environ. Qual. 23 (1994) 1151–1157. [6] M. Ghosh, S.P. Singh, A review on phytoremediation of heavy metals and utilization of its byproducts, Appl. Ecol. Environ. Res. 3 (2005) 1–18. [7] R.R. Brooks, M.F. Chambers, L.J. Nicks, B.H. Robinson, Phytomining, Trends Plant Sci. 3 (1998) 359–362. [8] C. Keller, C. Ludwig, F. Davoli, J. Wochele, Thermal treatment of metalenriched biomass produced from heavy metal phytoextraction, Environ. Sci. Technol. 39 (2005) 3359–3367. [9] A. Sas-Nowosielska, R. Kucharski, E. Makowski, M. Pogrzeba, J.M. Kuperberg, K. Kryski, Phytoextraction crop disposal—an unsolved problem, Environ. Pollut. 128 (2004) 373–379. [10] Y. Jian-Guang, Y. Jian-Ying, P. Chang-Hong, T. Chao-Bo, Z. Ke-Cao, Recovery of zinc from hyperaccumulator plants: Sedum plumbizincicola, Environ Technol. 30 (2009) 693–700. [11] Y. Jian-Guang, P. Chang-Hong, T. Chao-Bo, Z. Ke-Cao, Study on recovery zinc from hyperaccumulator Sedum alfredii Hance biomass, Trans. Nonferrous Met. Soc. Chin. 19 (2009) 1353–1359. [12] M. le Clercq, T. Adschiri, K. Arai, Hydrothermal processing of nickel containing biomining or bioremediation biomass, Biomass Bioenergy 21 (2001) 73–80.