Environmental Engineering Reference
In-Depth Information
reduction of soil, e.g. due to waterlogging, will cause reductive dissolution of Fe and
Mn oxides, and often destruction of the surfaces active in the retention of many met-
als and metalloid ions (Plekhanova 2007 ). Thus, water logging of soil will lead to the
release of elements strongly retained by these surfaces, e.g. arsenic concentrations
are much higher in pore waters of waterlogged soils than aerobic soils (Bowell 1994 ;
Marin et al. 1992 ).
8.3 Plant Acquisition of Metals and Metalloids from Soil
8.3.1 Root Uptake Pathway
In this section, we will elaborate on the processes by which metals and metalloids
are taken up from soil by plant roots and how these elements are subsequently
distributed with the plant tissues.
8.3.1.1 Speciation and Ion Uptake Rate
Plant roots absorb metals/metalloids via the root bathing pore water. The metals/
metalloids commonly enter the plant as ions (e.g. Cd 2+ ,H 2 AsO 4 ) via ion chan-
nels or carriers that have the capacity to concentrate the elements from solution.
Non-essential elements enter the plants using the uptake systems of nutrients that
resemble the contaminants in terms of charge and ionic radius. Passive uptake of
the elements through water uptake rarely explains the observed uptake of several
nutrients (Barber 1995 ) and the same is true for contaminants such as cadmium
and lead. The uptake rate generally increases with increasing concentration in pore
water. Short-term ion uptake studies with roots demonstrate that uptake of contam-
inants follows a concentration-dependent pattern that is similar to enzyme kinetics.
In a similar, but not identical, fashion it is observed that plant tissue concentrations
rise as soil concentrations rise . Such patterns are important in Risk Assessment
where the concept is to identify tolerable soil concentrations at which plant con-
centrations are below target values. Figure 8.2a summarises the general patterns for
non-essential and essential contaminants. The tissue concentrations of non-essential
elements such as arsenic, cadmium, mercury, and lead rise almost proportionally to
their concentrations in soil at low concentrations. This pattern is the basis of using
the BioConcentration Factor (BCF) concept in Risk Assessment, i.e. a constant
plant tissue:soil concentration relationship. As soil concentrations rise, however,
there are saturating processes and tissue concentration levels off, resulting in BCF
values that are lower than that at low concentrations. The saturating processes are
root uptake or translocation processes or potential feedback mechanisms under toxic
conditions. For essential elements, such as the micronutrients boron (B), copper
(Cu), manganese (Mn), molybdenum (Mo), and zinc (Zn), the pattern is distinctly
different. Tissue concentrations are maintained within narrow limits at widely differ-
ent external concentrations through homeostatic control mechanisms on ion uptake
and translocation. At elevated supply, however, the control mechanisms break down
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