Fluorine [F, 9]
113
HF is the most abundant gaseous F released into the atmosphere. Its natural
sources of F are from volcanoes, emitting 0.6–6 Mt/yr. About 10% emission is from
large eruptions. Its concentration is higher in air of industrial regions. The largest anthropogenic emissions of HF are the electrical utilities, and the Al industries.
Also, the production of P fertilizers release some amounts of F. In gaseous phase,
F occurs mainly as F 2 and HF. However, it may be emitted from industries in various
other compounds, but the most abundant gaseous F released into the atmosphere is
HF. Due to reactions with various materials, both in vapor and in aerosols, several
compounds, including S-hexafluoride and fluorosilicic acid, are formed in the air.
The predominant mode of degradation of inorganic fluorides in the air is hydrolysis.
However, some fluorides emitted by industries in particulate matter are stable compounds that do not readily hydrolyze.
The common F concentrations in air of inhabited regions in various countries
range from 1 to 7 μg/m 3 (Table 15.1). Longer influence of increased F levels in the
atmosphere has detrimental effects on plants, humans, and animals. Especially sensitive to elevated F concentration in air are coniferous trees. Acceptable F levels for
the forest regions in Poland have been established for 0.02 μg/m 3 , whereas for the
whole country it is 2 μg/m 3 (Gramowska and Siepak 2002).
15.5 PLANTS
Phytoavailability of fluorine is relatively low. It is taken up by plants passively by
roots, and is apparently easily transported in plants. There are some observations
that F is bound in plants to mobile organic complexes. Its concentrations in some
plants show a relationship with hot-water soluble F in soil. Often, its availability
increases with decreasing soil pH. In soils with higher pH, and especially with soluble Ca compounds, F bioavailability decreases due to the precipitation of CaF 2 on
the root surface, which resulted in lower F contents in shoots and higher in roots
(Maćkowiak et al. 2003).
Usually, F concentrations in shoot tissues of plants do not exceed 30 mg/kg, and
are higher than in roots. Although plants can uptake F easily from soils, especially
from F-contaminated soils, its increased levels in plants are very often from airborne
F compounds. Plants grown in F-contaminated soils and/or exposed to industrial
emissions may significantly contain elevated amounts of F (Table 15.3). Especially
some plants growing in acidic soil enriched in F may accumulate elevated amounts
of F. Pine needles are common and good indicators for the F pollution. Pine needles
of trees from vicinities of the Al smelter contain F above 1000 mg/kg, whereas its
background contents is calculated at below 20 mg/kg. Needles of trees surrounding
phosphate factories contain following amounts of F (in mg/kg): Scots pine, >200;
Norway spruce, >100; Douglas fir, >50; and control trees, around 20 (Karolewski
et al. 2000). Lichens used for the biomonitoring of pollution accumulate F up to
243 mg/kg, near the F point source (Geebelen at al. 2005).
Usually leaves of most plants absorb F deposited on leaf surfaces easily. Great
accumulation of F in old tea leaves resulted from both root uptake and leaf absorption (Siemiński vide Kabata-Pendias and Mukherjee 2007). In most cases, F taken
up by plants from air is much higher than those from soils. Several factors affect