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

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 Current Plant Uptake Models: dC dt = f (C, t)

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, 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,

Equilibrium Partitioning of Organic Chemicals into SOM or Plants: Solubilization Processes Q = K p  C W Soil uptake : C S = K p  C W = K som  f som  C W Plant uptake : C pt = K pl  C W = K pom  f pom  C W = f pw + 1 K pom  1 f pom  C W + 2 K pom  2 f pom  C W + ……

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 Model Development

Kinetic Uptake from Soil-Free Water Solution: Q pt =  C w K pl =  C w ( f pw +  f pom i K pom i ) In which f pw +  f pom i = 1 i = 1,2,3,…,n. where: f pom i = the organic-matter weight fraction for the i th component K pom i = the contaminant partition coefficient between i th component plant organic matter and water f pw = the plant-water weight fraction  = quasi-equilibrium factor (  1)

Kinetic Uptake from Contaminated Soils: Q pt =  (C s / f som K som )( f pw +  f pom i K pom i ) with C w = C s / f som K som Where: C s = the contaminant concentration in the whole soil, f som = the soil organic-matter (SOM) fraction, K som = 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: K prt (protein-water); K lip (lipid-water); K ch (carbohydrate-water); K ow (octanol-water); K som (SOM-water). Approximation: K lip = K ow

Simplification of the Uptake Model: Q pt =  C w K pl =  C w ( f pw +  f pom i K pom i ) =  C w ( f pw + f lip K lip + f ch K ch )

Approximate K ch values for contaminants log K OW K OW K ch  0   4.0 

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

Chemical HCBLDN PCE TCE log K ow Concentration (  g/L) log K OW and Initial Concentrations of Chemicals

Plant % water % lipids % carbohydrates Ryegrass roots shoots Wheat roots shoots Weight Compositions of Wheat and Ryegrass Parts

Q eq = C w ( f pw + f ch K ch + f lip K lip ) Contributions of Wheat Parts to Equilibrium Sorption Hexachlorobenzene: shoots: Q eq = C w ( × × ) Lipids contribute 99.96%. roots: Q eq = C w ( × × ) Lipids contribute 99.92%. Lipid Contribution

Contributions of Wheat Lipids to Equilibrium Sorption shoots (%)roots (%) Hexachlorobenzene Lindane PCE TCE

Plant Uptake Model: Sorption Model Q eq =  C w K pl Composition Model Q eq =  C w ( f pw + f ch K ch + f lip K lip ) (low log K ow ) Lipid Model Q eq   C w f lip K lip (high log K ow )   C w f lip K ow

Sorption of Hexachlorobenzene from Water by Wheat Seedlings

Sorption of Lindane from Water by Wheat Seedlings

Comparison of Determined log K lip to log K ow shoots roots log K ow Hexachlorobenzene 5.50 K pl (L/kg) log K lip Lindane 3.72 K pl (L/kg) log K lip

Important Issues and Points:  Are plant lipids more effective than octanol in uptake? Triolein ( C 57 H 104 O 6 ) > Octanol ( C 8 H 18 O ) O/C =  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) limit-to-C w ratio ( BCF ) Ryegrass shoots roots PCE TCE PCE TCE limit (  g/kg) limit-to-C w ratio ( BCF )

Uptake of Hexachlorobenzene from Water by Wheat Seedlings ( C w = 4.96  g/L)

Uptake of Lindane from Water by Wheat Seedlings ( C w =  g/L)

Uptake of Tetrachloroethylene from Water by Ryegrass Seedlings ( C w = 1300  g/L)

Uptake of Trichloroethylene from Water by Ryegrass Seedlings ( C w = 3300  g/L)

Uptake of Hexachlorobenzene from Water by Wheat Seedlings ( C w = 4.96  g/L) Uptake Time (Hours) Quasi-Equilibrium Factor,  Shoots Roots HCB

Uptake of Lindane from Water by Wheat Seedlings ( C w =  g/L) Quasi-Equilibrium Factor,  Uptake Time (Hours) Shoots Roots LDN

Uptake of Tetrachloroethylene from Water by Ryegrass Seedlings ( C w = 1300  g/L) Uptake Time (Hours) Shoots Roots Quasi-Equilibrium Factor,  PCE

Uptake of Trichloroethylene from Water by Ryegrass Seedlings ( C w = 3300  g/L) Uptake Time (Hours) Shoots Roots Quasi-Equilibrium Factor,  TCE

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) C w (  g/L) transpiration needed (L/kg/d)

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 K ow. 3.There is a need to develop a lipid extraction methodology suitable for plant uptake estimation and to verify the efficiency of K ow as a substitute for K lip. 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 time- dependent. 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.