Uptake of Arsenic by Plants Grown Near CCA Preserved Wood
Arsenic Conc.In Media (mg/kg)
Arsenic Conc. In PlantTissue (mg/kg)
|Fresh Wt.*||Dry Wt.|
|Lettuce||8||Soil||6-20||CCA Boards||0.06-0||.1 0.8-1.7|
|Lettuce||7||ProMix**||Not Available||CCA Board||0.1||1.7± 0.3|
* Based on 93±2 % Moisture
(Lettuce) and 93±0.7 % Moisture (Mustard Greens).
** ProMix - 50% Peat Moss, 25% Perlite, 25% Vermiculite, lime to pH 5.6.
These preliminary data show
that the amount of arsenic in the mustard was about 8 times greater than
the lettuce under equivalent conditions. Also the arsenic levels tended
to increase in the plant tissue with increasing amounts of arsenic in
the soil, but in many instances reached a plateau or saturation region.
The relationship between soil arsenic and plant arsenic in the spiked
trials closely followed the observed plant uptake in the pots where CCA
boards were added, but not in those to which CCA powder was added. Somewhat
surprisingly, the amounts of arsenic in the lettuce were similar when
grown in the promix and the soil. Another unexpected result was the lack
of differences in plant uptake of arsenic as a function of compost or
added iron oxide, which we now attribute to the spike level (100 mg/kg
As) being in the saturation region (Onken and Hossner, 1995). We plan
to test this hypothesis by growing the plants under lower arsenic spike
concentrations. Finally, we noted that the use of CCA powder in the growth
studies was problematic due to the difficulty in growing plants at high
powder loading, and as such any further trials using CCA powder will be
Comparison of our preliminary
results to the earlier work (Speir et al., 1992; Thornton, 1994) shows
some discrepancies. The amounts of arsenic in the lettuce shown in the
table above is not only somewhat less than that reported by Speir, where
the lettuce was grown in CCA spiked soil, but is also much less than Speir
reported in the control lettuce tops (6-9 mg/kg, dry weight). The amounts
of arsenic in the lettuce grown in the spiked soils were much greater
than expected based on the English garden study reported by Thornton (1994).
For example, from the first row in the table it can be seen that we found
2.6-12.3 mg/kg arsenic (dry weight) in the lettuce leaves when grown in
soil containing 25-100 mg/kg arsenic, while, as given earlier, the arsenic
in the English garden soil averaged 322 mg/kg and the leaf arsenic in
the lettuce ranged 0.15-3.9 mg/kg dry weight. In all cases, however, the
arsenic in the lettuce did not exceed the English fresh weight limit of
1 mg/kg arsenic, but in some cases the more stringent limit of 0.5 mg/kg
in Chili (Queirolo et al., 2000) or 0.2 mg/kg in Germany (Arnold, 1988)
were exceeded in lettuce. All of the limits for arsenic were exceeded
in the mustard greens, even when grown at the lowest spike level of 25
mg/kg arsenic in the soil. The amounts found in the mustard greens are
similar to those given earlier for corn (1.8 mg/kg, fresh weight) and
potatoes (0.9 mg/kg fresh weight) when grown in arsenic contaminated soils
in Chile (Queirolo et al., 2000).
These discrepancies in uptake
may be partly explained by the use of differing soil conditions, which
may dramatically alter plant uptake of arsenic. In order for a plant to
take in inorganic constituents, including arsenic, the material needs
to be in solution form, not bound to the soil. On the other hand, through
a combination of material and soil properties, for continued uptake, the
constituent needs to be insoluble enough that it does not rapidly wash
away from the root zone. The form of arsenic that initially leaches from
the wood is the arsenate anion (AsO4-3). Some of the factors that tend
to decrease the solubility of arsenate in soil include sorption and precipitation
reactions with clay and iron oxides, while factors that tend to increase
the solubility are increasing amounts of sand and organic matter in soils
(Nriagu, 1994). Phosphates from fertilizing can release arsenate from
the soil by replacement reactions (Peryea, 1998; Woolson, 1973). Moreover,
plant uptake of arsenic can also be affected by plant-induced reactions.
An example is the formation of oxidizing environments near the roots (rhizosphere)
of wetland plants, leading to the precipitation of iron oxyhydroxides.
The precipitate, also termed ironplaque, binds with arsenic, which results
in a net accumulation of arsenic near the plant roots (Otte et al., 1995).
Further complicating the picture
is that arsenic can exist in many forms. After leaching from the wood,
and depending on the conditions, the arsenic, in the form of arsenate,
may undergo numerous transformations. Such transformations include reduction
to arsenite and even arsine, and through biological mechanisms transformations
to organo-arsenicals (Nriagu, 1994). Any arsenic species thus transformed
could be expected to differ in its sorption properties in the soil as
well as with in its uptake by plants (Burlo et al., 1999; Carbonell-Barrachina,
1999). Under the conditions of normal garden soils it is believed that
such transformations are increased by the presence of organic matter.
In summary, arsenic uptake by plants is affected not only by plant species and arsenic type and concentration in soils, but by soil properties such as the amounts of phosphorus, iron oxide and organic matter. The interaction of these factors on arsenic uptake by plants are not well understood and need to be investigated because of the potential for adverse health effects by arsenic in the food supply.
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