A Partition-Limited Model for the Plant Uptake of Organic Contaminants from Soil and Water

Abstract

In dealing with the passive transport of organic contaminants from soils to plants (including crops), a partition-limited model is proposed in which (i) the maximum (equilibrium) concentration of a contaminant in any location in the plant is determined by partition equilibrium with its concentration in the soil interstitial water, which in turn is determined essentially by the concentration in the soil organic matter (SOM) and (ii) the extent of approach to partition equilibrium, as measured by the ratio of the contaminant concentrations in plantwater and soil interstitial water, alphapt (< or = 1), depends on the transport rate of the contaminant in soil water into the plant and the volume of soil water solution that is required for the plant contaminant level to reach equilibrium with the external soil-water phase. Through reasonable estimates of plant organic-water compositions and of contaminant partition coefficients with various plant components, the model accounts for calculated values of alphapt in several published crop-contamination studies, including near-equilibrium values (i.e., alphapt approximately equals 1) for relatively water-soluble contaminants and lower values for much less soluble contaminants; the differences are attributed to the much higher partition coefficients of the less soluble compounds between plant lipids and plant water, which necessitates much larger volumes of the plant water transport for achieving the equilibrium capacities. The model analysis indicates that for plants with high water contents the plant-water phase acts as the major reservoir for highly water-soluble contaminants. By contrast, the lipid in a plant, even at small amounts, is usually the major reservoir for highly water-insoluble contaminants.

Interaction between cadmium and atrazine during uptake by rice seedlings (Oryza sativa L.)

“…The concentrations of ATR in roots are higher than those in shoots, due in part to their different compositions and possibly in part to the more active transformation processes (e.g., volatilization, metabolism, and complex formation) that took place with the shoots. It is well recognized that the plant uptake of nonionic compounds through roots occurs primarily by passive (partition) process (Briggs et al, 1982;Chiou et al, 2001) and the passive uptake is influenced by the properties of the compounds and plant growth and transpiration rates. The present data showed that the ATR concentrations in seedlings are much greater than those in external solution, as commonly found for chemicals having limited solubility in water (Briggs et al, 1982;Trapp and Matthies, 1995).…”

“…For the plant uptake of nonionic contaminants in general, the concentration of a contaminant in plants (or in a specific plant part) at a given time can be related to the source concentration in external solution (or soil water) and the quasi-equilibrium factor for the system as calculated by use of Eq. (1) (Chiou et al, 2001):…”

“…There has been a considerable interest in understanding the uptake of organic contaminants by plants during the last two decades (Briggs et al, 1982;Trapp and Matthies, 1995;Weiss, 2000;Chiou et al, 2001;Li et al, 2002;Trapp, 2004). Plants could be exposed to contaminants in different ways.…”

“…Plants could be exposed to contaminants in different ways. Organic contaminants may enter plant roots by passive and/or active processes and then translocate with plant transpiration stream to other plant components (Briggs et al, 1982;Chiou et al, 2001;Trapp, 2004). Passive transport, which proceeds in the direction of decreasing chemical potential, consists of a series of partitions between plant water and plant organic matter within various plant components (Chiou et al, 2001).…”

“…Passive transport, which proceeds in the direction of decreasing chemical potential, consists of a series of partitions between plant water and plant organic matter within various plant components (Chiou et al, 2001). It has been found to be largely responsible for the uptake of nonionic compounds (Briggs et al, 1982;Chiou et al, 2001). Active transport, which may proceed against the chemical potential gradient, occurs for certain nutrients and other (inorganic and organic) ions (Trapp, 2004).…”