Contaminant Transport in Soil

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Contaminant Transport in Soil

• Sanjay K Mohanty
• University of Hawaii at Manoa
• 3rd June, 2005

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Overview

• Objective
• Background
• Theory of sorption
• Mechanism
• Experimental Techniques
• Batch sorption
• Miscible Displacement Study
• Modeling

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Objective

• To determine the transport parameters (Kd,
  diffusion coefficient, Retardation factor etc.)
  of selected organic contaminants in a tropical
  soil by understanding the fundamental mechanism
  of sorption at soil water interface.

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Questions need to be answered!

• What are the factors that can control solute
  transport in soil?
• What are the mechanisms that govern this process?
• What are the experimental techniques that can
  quantify this process with fair degree of
  accuracy?

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A. Controlling factors
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1. Contaminant Properties

• Organic contaminants
• Water solubility (polar interaction)
• Kow (Hydrophobicity)
• Functional Groups (mechanism)
• Carboxyl-COOH, Phenolic -OH (H bonding,
  Deprotonation)
• Amide -NH2 (Protonation)
• Aromatic and aliphatic chain (hydrophobic)
• Esther group CO (H-bond)
• Half life period (Volatility/degradation)

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Contaminants of concern
(NDMA)
(Atrazine)
(Lindane)
(Octylphenol)
(Nonylphenol)
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2. Soil Constituents

• Soil Mineral (inorganic)
• Layer silicate clays
• Oxides, hydroxides and oxyhydroxides
• Carbonates and sulfates
• Natural Organic Matter
• Humic substances (Humic, Fulvic acids non
  extractable Humin)
• Biomacromolecule

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Soil Constituent cont.
Source Chorover 2005
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Soil Properties

• Surface charge (Ionic interaction)
• Soil pH (activity of solute)
• Structure/texture (surface area)
• Soil Hydraulic properties (residence time)
• Organic carbon content (hydrophobic)
• Mineralogy Clay (oxide) content (Inner/outer
  surface complexation)
• Cation exchange capacity (CEC) (amide group)

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Soil in use1 Poamoho site

• Type Oxysol/Rhodic Eutrustox (Wahiawa series)
• Surface charge Net Negative (variable) charge
• Soil pH 5.7-6.7
• Mineralogy Kaolinite, Iron Oxide/oxyhydrate,
  clay content (75)
• OC 1.34 (A) 0.85 (B)
• Structure/texture low surface area (highly
  aggregated), coarse structure.

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Soil Mineralogy

• Low surface reactivity (CEC)
• Charge developed at the mineral edge

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3. Ambient Conditions

• Disolved Organic Carbon (DO)
• Colloid
• Ionic Strength (Contaminant activity)
• pH (Surface charge)
• Temperature
• Suction (water content)

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B. Mechanism of Sorption

• Proton Complexation Reaction (less probable)
• R-COOH (s) ? R-COO- (s) H (aq)
• R-NH4 (s) ? R-NH3 (s) H (aq)
• Ion Complexation (excluding H/OH-)
• R-COOH (s) M (ligand) ? R-COOM (s) H (aq)
• Hydrophobic Interaction (favorable)
• Ion Exchange (less probable)

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Surface Complexation
Chemisorption
Irreversible, Pore diffusion, Slow reaction rate
Reversible, Film diffusion, Fast reaction rate
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C. Experimental Methodology
Column Experiment (BTC)
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Characterization of soil

• OC (Dry combustion method/acidic soil)
• Humic substance extraction (acid wash2) and
  characterization (CPMAS 13C NMR3)
• Determination of CEC (if applicable)
• Determination of pHpzc (Batch study)
• Ionic strength of soil solution (Batch study)
• Clay mineral characterization (x-ray
  diffraction4)
• Surface functional group (FTIR)

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Batch Experiments

• Sorption Isotherm (Non Linear /hydrophobic)
• Competitive sorption
• Kinetic study (expected for the contaminant of
  interest)
• Non equilibrium study
• Time variability of Kd
• Effect of pH and Ionic Strength
• Sorption Hysterics (asymmetry)

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Batch exp. (precaution)

• Bottle losses
• Phase distribution artifacts (volatility)
• Artifacts caused by agitation
• Method of Analyte introduction
• Sampling technique (filtration/centrifugation)
• Recovery/extraction techniques
• Cross contamination

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Column Experiments

• Break Through Curve (BTC) analysis
• Non equilibrium (high foc) study (flow
  interruption method, extended tailing)
• Kinetic study (Flow variation)
• Sorption singularity (asymmetry)

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Model Characterization

• HYDRUS 1D
• Used convention diffusion equation might not be
  the best representative of sorption of organic
  compounds !!
• Calibration
• Inverse Modeling (Parameter Estimation)

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Questions or Comments !
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References

• Gavenda et al. (1996) Research series 075. CTAHR,
  UH manoa.
• Swift, R. S. (1996) Soil Sci. Soc. Am. Book
  Series No. 5. pp 1011-1069
• Mahieu et al (1999) Soil Sci. Soc. Am. J. 63. pp
  307-319
• Essington, M. E. (2004) Soil and Water chemistry.
  CRC Press.
• Sposito, G. (1989) The Chemistry of Soils. Oxford
  University Press.
• Pignatello, J. J. (2000) Advances in Agronomy.

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