Elsevier

Environment International

Volume 31, Issue 2, February 2005, Pages 255-262
Environment International

A whole-plant mathematical model for the phytoextraction of lead (Pb) by maize

https://doi.org/10.1016/j.envint.2004.10.002Get rights and content

Abstract

Phytoextraction is a technology that uses plants to remove heavy metals from contaminated soils. Although it is economically attractive compared to other methods, little attention has been paid to the mathematical modeling of the mechanisms involved. In this work, we simulate the phytoextraction of Pb using a mechanistic system dynamics modelling approach and the physiology model of maize (Zea mays) as a model system as it is a good Pb accumulator and translocator. Simulation results showed that precipitation is the most important mechanism related to the uptake of Pb from the ground. The most important model parameters have been identified through sensitivity analysis.

Introduction

Lead is a hazardous heavy metal pollutant that originates from various sources which include paints, gasoline additives, Pb smelting and refining, pesticide production, Pb acid battery breaking, and battery disposal (Eick et al., 1999, Paff and Bosilovich, 1995).

Many studies have been conducted on the use of green plants for the removal of lead from the environment in an ecological and cost-effective way. This includes techniques, such as phytoextraction, which is a type of phytoremediation that involves the removal of heavy metals from contaminated soils/groundwater with plants that accumulate large amounts of heavy metals (hyperaccumulators). It involves growing plants that hyperaccumulate heavy metals at a contaminated site, harvesting the plants and processing the cuttings with the heavy metals for further use (recycling), or disposing them as hazardous waste (Cunningham et al., 1995).

Lead is usually accumulated in the roots, and only a very small amount is accumulated in the shoots. For a successful application of phytoextraction, lead should not only be removed form the soil, but it should also be translocated to the harvestable parts of plant. The use of heavy metal chelators, such as ethylene-diamine-tetraacetic acid (EDTA), can enhance Pb desorption from soil to soil solution, can facilitate Pb transport into the xylem, and can increase Pb translocation from roots to shoots (Huang et al., 1997). Chelator-assisted shoot lead accumulation has been found to be two or more times higher than shoot accumulation of nonchelated lead within the same species (Jarvis and Leung, 2001). Unfortunately, Pb-EDTA is highly water-soluble, and hence, the application of chelators poses potential risks to groundwater (Madrid et al., 2003, Wu et al., 1999, Kos and Lestan, 2003).

Some plants translocate Pb effectively to shoots without chelators. One of them is maize (Zea mays), which translocates Pb from roots to shoots, and thus a smaller amount of lead is accumulated into the roots (Brennan and Shelley, 1999). Despite this, maize is an excellent model system for mathematical modeling of plant uptake and translocation of metals.

From a site remediation design point of view, it is important to have a mathematical model that can yield quantitative information about the rate of Pb removal from the soil/groundwater so that a successful site decontamination strategy can be developed.

Section snippets

Model development

The purpose of this work is to simulate the key mechanisms that control plant uptake of the metal from the soil/groundwater and its translocation from the roots to the shoots, as well as the interaction of these mechanisms to control heavy metal accumulation. A mechanistic system dynamics modelling approach is used, employing the physiology model of maize (Z. mays), as it is a good model system because maize is a significant Pb accumulator and translocator.

Simulation results and discussion

The complete model was programmed in MATLAB (version 6.1), where the system of 54 ODEs was solved using MATLAB's “ode15s” differential equation solver which can handle stiff ODE systems satisfactorily. The time period over which the differential equations were solved is from the 12th to the 125th day and coincides with the growing period of a maize plant. Two test cases have been examined. In the first one, the phytoextraction of lead from contaminated groundwater with a constant Pb

Concluding remarks

The presented model appears to be a valid tool for simulating a maize plant taking up and translocating Pb. The model could also be used as a baseline for studying other plant–heavy metal systems. In particular, the model structure and its basic flow pattern could be used to develop models that simulate uptake of different heavy metals, such as Cd or Zn. The model could also be adjusted to different soil conditions and coupled to mass transfer models for the movement of Pb rhizosphere area of

Acknowledgement

This work has been supported in part by IHP-Marie Curie Development Host Fellowships Program (EC Contract Ref: HPDM-CT-2001-00061).

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