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A numerical analysis of phytoextraction processes
Une analyse numérique des processus de phyto-extraction
Lugli F., Mahler C.F.
COPPE/UFRJ, Rio de Janeiro, RJ, Brasil
ABSTRACT: Phytoextraction is an in situ remediation technique involving the uptake of contaminants by plant roots and their
subsequent accumulation in plant tissues. Despite its many advantages, phytoextraction is not widely used because of difficulties in
estimating its efficiency and the required remediation time. The objective of our research was to numerically evaluate the
effectiveness of phytoremediation of Pb
2+
and Zn
2+
using a previously calibrated Hydrus-1D software package. The simulations
considered soil and climatological data representative of the coastal lowlands of the municipality of Rio de Janeiro in Brazil and were
organized in three steps: pre-contamination (analysis of the hydrological conditions), contamination (analysis of the contamination
plume before planting) and remediation. Several conditions were tested, including different root depth and irrigation schemes.
Although the results are specific for the assumed scenarios, it was possible to identify several trends in the simulations. While more
elaborate calibrations may be needed using long-term field data, the numerical analysis provided useful insight into the
phytoextraction process important for the design of future experiments.
RÉSUMÉ : La phyto-extraction est une technique de dépollution in situ basée sur l'absorption de contaminants par les racines des
plantes puis de leur accumulation dans les tissus végétaux. Malgré ses nombreux avantages, la phyto-extraction est peu utilisée en
raison de la difficulté à estimer son efficacité et la durée de traitement qu’elle implique. L'objectif de notre étude était d'évaluer
numériquement l'efficacité de la phyto-extraction du Pb
2+
et du Zn
2+
à l'aide d'un logiciel préalablement étalonné Hydrus-1D. Les
simulations ont pris en compte pour le sol et la climatologie des données représentatives de la plaine côtière de la municipalité de Rio
de Janeiro au Brésil et ont été organisées en trois étapes : la pré-contamination (analyse des conditions hydrologiques), la
contamination (analyse du panache de contamination avant le semis) et la dépollution. Plusieurs conditions ont été testées, en
particulier diverses profondeurs des racines et différents systèmes d'irrigation. Bien que les résultats soient spécifiques aux scénarios
pris en compte, il a été possible d'identifier plusieurs tendances dans les prédictions. Si des étalonnages plus élaborés peuvent être
nécessaires en utilisant des données de terrain à long terme, l'analyse numérique fourni des indications utiles sur le processus de
phyto-extraction qui sont importantes à prendre en compte pour la conception des expériences futures.
KEYWORDS: contaminant transport, plant solute uptake, irrigation, root density.
1 INTRODUCTION
Phyto-extraction is a remediation technique based on
contaminant uptake by plant roots. Pollutants are generally
accumulated in plant tissues. Plant based remediation processes
are currently used for many classes of contaminants, including
hydrocarbons, pesticides, explosives, metals, radionuclides,
chlorinated solvents, and waste landfill leachate. These
techniques can be complementary or alternative to chemical and
mechanical treatments. Their applicability depends basically on
the resistance of the plant to contaminants. In addition, the
efficiency of the process depends strongly on the site
characteristics such as soil, climate, hydrology and soil-
contaminant interactions. Due to this variability, it is difficult to
estimate the cost and the time necessary for a phytoremediation
project. Some studies (ITRC, 2009; Truong et al., 2010) show
that the costs due to installation, instruments and labor can be
significantly lower compared to other techniques.
The present research was focused on numerical modeling of
soil-plant-atmosphere continuum, proposing a method for the
calculation of:
1. time required for remediation (according to Brazilian
Law),
2. efficiency, defined as the ratio between contaminant mass
withdrawn from the soil and contaminant mass previously
leached into the soil.
The software Hydrus-1D (Šim
ů
nek et al., 2009) was applied
to scenarios of contamination by Pb
2+
and Zn
2+
chosen for their
different reactivity with the soil solid phase. The remediation
was carried by Vetiver grass (
Chrysopogon zizanioides
). Soil
and climate data were relative to an industrial area of the
municipality of Rio de Janeiro. The model parameters, relative
to the crop, were determined and calibrated in a previous study,
based on greenhouse experiment (Lugli et al., 2011).
2 MATERIALS AND METHODS
2.1
Numerical Model
The
Hydrus-1D
code uses linear finite element method of
Galerkin
type for spatial discretization and finite difference
method for time discretization of the Richards equation
(variably saturated flows). For solute transport,
Hydrus-1D
is
based on the resolution of the advection-dispersion equation by
finite elements (Šim
ů
nek et al., 2009).
The main hypotheses of this study were:
1. one-dimensional approach,
2. absence of preferential flow paths,
3. invariance of the potential root contaminant uptake,
4. remediation process based only on direct extraction of the
contaminant,
5. absence of growth, senescence, intoxication phenomena,
6. contaminants are considered separately.
