Environmental Engineering Reference
In-Depth Information
Fig. 9.8 Principle of the ion
trap. AH is the neutral
contaminant (here an acid),
and A is the ion (here an
anion)
ionic inside. This means that weak acids (pKa 2-6) may accumulate in the alkaline
phloem (and are in consequence transported to fruits), while weak bases (pKa 6-10)
tend to accumulate in the acidic xylem (and are translocated to leaves). Examples of
such contaminants are herbicides like 2,4-D (Shone and Wood 1974 ) and sulfony-
lureas (Fahl et al. 1995 ), and drugs like the base trimethropin (Boxall et al. 2006 )
and the acid ibuprofen.
Briggs et al. ( 1987 ) determined the RCF and TSCF of weak organic acids with
log K OW between 0.06 and 4.51 and pKa-values near 3. At pH 7 in external solution,
RCF -values were low, between 0.5 and 4.5 L kg 1 .AtpH4,the RCF -values were
higher and ranged from 2.6 to 72 L kg 1 . Similar, the TSCF -values of weak acids at
pH7werelow,0.04to0.05LL 1 , while at pH 4 TSCF -values up to 4.2 L L 1 were
found. Briggs et al. used the ion trap process to interpret their results. Chamberlain
et al. ( 1998 ) studied the uptake into roots and translocation to shoots of two weak
bases, i.e. dodemorph (pKa 7.8) and tridemorph (pKa 7.4). At pH 5, RCF -values of
dodemorph were < 10 L kg 1 , and of tridemorph about 20 L kg 1 . With increasing
pH, RCF increased to 49 L kg 1 for dodemorph and 183 L kg 1 for tridemorph
at pH 8. In parallel, the TSCF increased from < 1 L L 1 for both bases at pH 5 to
24 L L 1 for dodemorph and slightly below 10 L L 1 for tridemorph at pH 8. To
our knowledge, the TSCF of dodemorph in this experiment was the highest TSCF
ever determined. A similar study was done by Inoue et al. ( 1998 ), who also found
a large increase of uptake and translocation of weak bases in plants with increasing
pH. The results from these studies underline the importance of pH in the soil pore
water for uptake and translocation of weak electrolytes. However, the results have
not yet been confirmed by field studies.
9.6 Influence of Plant-Specific Parameters
As pointed out in the introduction, there are a very high number of plant-
contaminant combinations, and the simulations done for a generic plant are not
necessarily valid for the whole plant empire and all contaminants. In this sec-
tion, some properties of plants are discussed, which vary and may lead to different
accumulation of contaminants.
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