Transcript Document

Plant Uptake Processes
in Phytoremediation of Organic Contamination
Guangyao Sheng (盛光遥)
University of Arkansas
Cary T. Chiou (邱成財)
National Cheng Kung University
Current Plant Uptake Models:
1. Kinetic Model (Trapp et al.)
Mass Balance
Differential Equations
2. Equilibrium Model (for roots only)
Briggs et al. (1982, 1983)
Trapp and Matthies (1995)
3. Quasi-equilibrium Model,
Mechanistic Model
dC
=
f
(C,
t)
dt
Objectives
1. Develop a partition-limited mechanistic model to
describe the passive uptake of organic
contaminants by plants from contaminated soils
or water.
2. Test the model with experimental data.
3. Establish the relationship between kinetic uptake
and equilibrium partition.
4. Offer plant selection guidelines for uptake-based
phytoremediation of organic-contaminated soils
and water.
References:
1. Chiou, C.T.; Sheng, G.; Manes, M. A partition-limited
model for the plant uptake of organic contaminants from
soil and water. Environ. Sci. Technol. 2001, 35, 1437-1444.
2. Li, H.; Sheng, G.; Chiou, C.T.; Xu, O. Relation of organic
contaminant equilibrium sorption and kinetic uptake in
plants. Environ. Sci. Technol. 2005, 39, 4864-4870.
Equilibrium Partitioning of Organic Chemicals
into SOM or Plants:
• Solubilization Processes
• Q = Kp  CW
Soil uptake: CS = Kp  CW = Ksom  fsom  CW
Plant uptake: Cpt = Kpl  CW = Kpom  fpom  CW
= fpw
+ 1Kpom  1fpom  CW
+ 2Kpom  2fpom  CW
+ ……
Model Development
System Parameters:
• Soil properties: effect of soil sorption
• Contaminant physicochemical properties
• Species of plants (or different plant tissues)
• Contaminant levels in soils or water
• Exposure time
Kinetic Uptake from Soil-Free Water Solution:
Qpt =  Cw Kpl
=  Cw ( fpw +  fpomiKpomi )
In which
fpw +  fpomi = 1
i = 1,2,3,…,n.
where:
fpomi = the organic-matter weight fraction for the ith component
Kpomi = the contaminant partition coefficient between ith
component plant organic matter and water
fpw = the plant-water weight fraction
 = quasi-equilibrium factor (1)
Kinetic Uptake from Contaminated Soils:
Qpt =  (Cs / fsom Ksom)( fpw +  fpomiKpomi )
with
Cw = Cs / fsom Ksom
Where:
Cs = the contaminant concentration in the whole soil,
fsom = the soil organic-matter (SOM) fraction,
Ksom = the contaminant partition coefficient between
SOM and water.
Important Plant Components and Their
Contaminant Partition Coefficients:
Plant Components:
Water; Nutrients; Proteins; lipids; Carbohydrates.
Relevant Partition Coefficients:
Kprt (protein-water);
Kch (carbohydrate-water);
Ksom (SOM-water).
Klip (lipid-water);
Kow (octanol-water);
Approximation:
Klip = Kow
Simplification of the Uptake Model:
Qpt =  Cw Kpl
=  Cw ( fpw +  fpomiKpomi )
=  Cw ( fpw + flip Klip + fch Kch )
Approximate Kch values for contaminants
log KOW
KOW
Kch
0
1
0.1
0.1-0.9
1-10
0.2
1.0-1.9
10-100
0.5
2.0-2.9
100-1000
1.0
3.0-3.9
1000-10000
2.0
 4.0
 10000
3.0
Experimental:
1. HCB, Lindane, PCE, TCE
2. Seedlings of wheat and ryegrass: roots and shoots
3. Composition: water, lipids, carbohydrates
4. Plant-water partition: batch equilibration
5. Plant uptake kinetics: constant solution-phase concentrations
Solution
reservoir
pump
sink
log KOW and Initial Concentrations of Chemicals
Chemical
HCB
LDN
PCE
TCE
log Kow
5.50
3.72
3.38
2.53
Concentration
(g/L)
4.96
503.7
1300
3300
Weight Compositions of Wheat and Ryegrass Parts
Plant
% water
% lipids
% carbohydrates
Ryegrass
roots
87.7
0.30
12.0
shoots
88.8
0.97
10.2
roots
84.4
0.51
15.3
shoots
85.2
1.10
13.7
Wheat
Contributions of Wheat Parts to Equilibrium Sorption
Qeq = Cw ( fpw + fch Kch + flip Klip )
Hexachlorobenzene:
shoots:
Qeq = Cw (0.852 + 0.137×3 + 0.0110×316228)
Lipids contribute 99.96%.
roots:
Qeq = Cw (0.844 + 0.153×3 + 0.0051×316228)
Lipids contribute 99.92%.
Lipid Contribution
Contributions of Wheat Lipids to Equilibrium Sorption
shoots (%)
roots (%)
Hexachlorobenzene
99.96
99.92
Lindane
98.09
95.88
PCE
95.91
91.41
TCE
79.03
63.41
Plant Uptake Model:
Sorption Model
Qeq =  Cw Kpl
Composition Model
(low log Kow)
Qeq =  Cw ( fpw + fch Kch + flip Klip )
Lipid Model
(high log Kow)
Qeq   Cw flip Klip
  Cw flip Kow
Concentration in Wheat, Qeq (g/kg)
Sorption of Hexachlorobenzene from Water by Wheat Seedlings
65000
52000
shoots:
measured
Kow
HCB
roots:
39000
measured
Kow
26000
13000
0
0.0
0.5
1.0
1.5
Concentration in Water, Cw (g/L)
2.0
Concentration in Wheat, Qeq (g/kg)
Sorption of Lindane from Water by Wheat Seedlings
25000
shoots:
measured
Kow
20000
Lindane
roots:
measured
Kow
15000
10000
5000
0
0
50
100
150
200
250
Concentration in Water, Cw (g/L)
300
Comparison of Determined log Klip to log Kow
shoots
roots
Hexachlorobenzene
log Kow
5.50
Kpl (L/kg)
37918
16900
log Klip
6.54
6.52
Lindane
3.72
Kpl (L/kg)
73.0
45.4
log Klip
3.82
3.95
Important Issues and Points:
 Are plant lipids more effective than octanol in uptake?
Triolein (C57H104O6) > Octanol (C8H18O)
O/C =
0.105
0.125
 Do current techniques underestimate plant lipid contents?
Selection of extracting solvents?
 Uptake limit (g/kg) can be defined by equilibrium
sorption.
Uptake Limits (g/kg):
Wheat
shoots
roots
HCB
LDN
HCB
LDN
limit (g/kg)
188073
36770
83824
22868
limit-to-Cw ratio (BCF)
37918
73.0
16900
45.4
Ryegrass
shoots
roots
PCE
TCE
PCE
TCE
limit (g/kg)
31669
14113
10808
6645
limit-to-Cw ratio (BCF)
24.4
4.28
8.31
2.01
Plant Concentration, Qt (g/kg)
Uptake of Hexachlorobenzene from Water
by Wheat Seedlings (Cw = 4.96 g/L)
1200
HCB
Roots
1000
800
600
400
Shoots
200
0
0
50
100
150
200
250
Uptake Time (Hours)
300
350
Plant Concentration, Qt (g/kg)
Uptake of Lindane from Water by Wheat Seedlings
(Cw = 503.7 g/L)
16000
LDN
Roots
12000
8000
Shoots
4000
0
0
50
100
150
200
Uptake Time (Hours)
250
300
Plant Concentration, Qt (g/kg)
Uptake of Tetrachloroethylene from Water
by Ryegrass Seedlings (Cw = 1300 g/L)
5000
PCE
4000
Roots
3000
2000
Shoots
1000
0
0
40
80
120
Uptake Time (Hours)
160
Plant Concentration, Qt (g/kg)
Uptake of Trichloroethylene from Water
by Ryegrass Seedlings (Cw = 3300 g/L)
6000 TCE
Roots
4000
Shoots
2000
0
0
40
80
120
Uptake Time (Hours)
160
Quasi-Equilibrium Factor, 
Uptake of Hexachlorobenzene from Water
by Wheat Seedlings (Cw = 4.96 g/L)
0.10
HCB
0.08
Roots
0.06
0.04
0.02
Shoots
0.00
0
50
100
150
200
250
Uptake Time (Hours)
300
350
Uptake of Lindane from Water by Wheat Seedlings
(Cw = 503.7 g/L)
Quasi-Equilibrium Factor, 
0.8
LDN
Roots
0.6
0.4
0.2
Shoots
0.0
0
60
120
180
240
Uptake Time (Hours)
300
Quasi-Equilibrium Factor, 
Uptake of Tetrachloroethylene from Water
by Ryegrass Seedlings (Cw = 1300 g/L)
0.5
PCE
0.4
Roots
0.3
0.2
0.1
Shoots
0.0
0
40
80
120
Uptake Time (Hours)
160
Quasi-Equilibrium Factor, 
Uptake of Trichloroethylene from Water
by Ryegrass Seedlings (Cw = 3300 g/L)
1.0
TCE
0.8
Roots
0.6
0.4
Shoots
0.2
0.0
0
40
80
120
Uptake Time (Hours)
160
Shoot Uptake and Chemical Lipophilicity:
 PCE and TCE uptake reached steady state within 24 hours
 Lindane uptake reached steady state at 90 hours
 HCB uptake continued to rise at 300 hours
 An inverse correlation between uptake and lipophilicity or BCF
 Transpiration:
chemical
HCB
LDN
PCE
TCE
uptake at 24 h (g/kg)
70
1500
990
2380
Cw (g/L)
4.96
503.7 1300 3300
transpiration needed (L/kg/d)
14.1
2.98
0.76
0.72
Shoot Uptake versus Root Uptake:
 All the  values were <1 (even at steady state)
 Shoot uptake was consistently lower than root uptake, in
contrast to the measured lipid contents of plants
 Possible causes: various dissipation processes, i.e.,
foliar volatilization
plant metabolism
formation of bound residues
plant-growth-induced dilution
variation in plant composition / transpiration with growth
Concluding Remarks:
1. The model appears to give a satisfactory account of the
contaminant transport into plants in relation to contaminant levels
in water (and soil), the contaminant properties, the plant
composition, and the uptake time.
2. Uptake limit can be predicted from equilibrium sorption, which can
in turn be directly determined in laboratory or estimated from plant
composition and contaminant Kow.
3. There is a need to develop a lipid extraction methodology suitable
for plant uptake estimation and to verify the efficiency of Kow as a
substitute for Klip.
4. In-plant dissipation processes increase contaminant chemical
potential across the plant-water interface, thus maintaining the
driving force for continued uptake. A thorough understanding of
plant dissipation of contaminants is warranted for accurate
implementation of phytoremediation technology and assessment of
vegetable contamination.
Concluding Remarks (cont.):
5. Based on our results, the plant uptake capacity may be
categorized as:
 Low uptake for highly water-soluble compounds, e.g., MTBE,
much independent of plant species and not strongly timedependent. Use of high-transpiration plants.
 Moderate uptake for moderately lipophilic compounds, e.g.,
chlorinated solvents. Results should depend to a good
extent on plant composition and uptake time.
 High uptake for highly lipophilic compounds, e.g., PAHs and
PCBs. Results should depend very sensitively on plant
composition and uptake time. Use of high-lipids plants.