Environmental Engineering Reference
In-Depth Information
metal speciation in the rhizosphere are the gradients in pH, soluble organic mat-
ter, depletions or accumulation of ions and redox gradients. We refer the interested
reader to reviews dedicated to this topic (McLaughlin et al. 1998 ) and only explore
the causes of concentration gradients, since this has practical consequences as
shown below. Concentration gradients in the rhizosphere are the result of the bal-
ance between the ion uptake rate and resupply by the uptake of water. A simple
calculation allows estimating if the contaminant is either accumulated or depleted
in the rhizosphere: a plant typically transpires about 200 L of water per kg dry
weight produced (Barber 1995 ). The product of the transpiration and the concen-
tration of the contaminant in the pore water is termed the mass flow of elements to
the plant root. If mass flow matches uptake perfectly, then the ratio of the contam-
inant concentration in the plant (mg/kg dw ) to that in pore water (mg/L) should be
200. Larger ratios mean that mass flow is not sufficient to match the rate of element
uptake by the root, and concentrations of that element will have been depleted in
the rhizosphere. The concentration depletion is followed by a diffusion flux towards
the roots and this flux can be several fold larger than mass flow. Conversely, if con-
centration ratios are lower than 200, then mass flow (induced by transpiration) has
exceeded root uptake and, consequently, the contaminant may have accumulated
around roots. Field-based data for several plants and soils show that cadmium and
lead concentration ratios exceed this threshold by 1-2 orders of magnitude, while
arsenic concentrations ratios are typically 1 order of magnitude below that (Chen
et al. 2009 ). This means that cadmium and lead are, on average, depleted in the rhi-
zosphere, while arsenic generally accumulates around the roots. The differences are
related to differences in the so-called root absorption power (uptake rate per unit
concentration), for example a low value for arsenic uptake per unit time from the
pore water due to the strong competition with phosphate ions. Similar data for other
metals and metalloids allows ranking as given in Fig. 8.4 . The practical consequence
of these gradients is that the causal relationship between pore water concentrations
and tissue concentrations, observed in stirred solution, may not be detectable in
soils anymore because we fail to measure the pore water concentrations in the rhi-
zosphere. The rhizosphere conditions can be estimated by modelling the diffusion
and mass flow (Barber 1995 ). If the element is depleted in the rhizosphere, then it is
replenished by either solid or liquid complexed forms. Practically, this means that
a fraction of the complexed ions is also part of the directly available forms in soil,
provided that the dissociation rate is sufficiently rapid. For that reason, bioavailabil-
ity is always a complex function of both the activity in the pore water and a fraction
of the labile bound forms. It is also logical that assessments of diffusive fluxes (i.e.
fluxes under conditions of zero-sink) correlates well with the uptake, provided that
the metal or metalloid is indeed depleted in the rhizosphere (Nolan et al. 2005 ).
8.3.1.3 Ion Competition Effects for Metal and Metalloid Uptake
Ion uptake is furthermore affected by interionic effects , i.e. the uptake rate of the ion
decreases or increases as the concentration of an ion competing with the same uptake
site is increased or decreased, respectively. One of the most striking examples of
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