Nickel [Ni, 28]
227
or toxic Ni contents in the most plant species varies from 10 to 100 mg/kg. However,
native vegetation of serpentine soils (as well as Ni hyperaccumulators) may contain
very high Ni levels, up to above 7000 mg/kg in leaves, without any symptoms of
toxicity. The great ability of some plants to extract Ni from soils over Ni ore body
may be useful for biogeochemical prospecting.
The interaction between Ni and other trace metals, Fe in particular, is apparently
a common mechanism involved in its toxicity. In the presence of some cations,
Cu 2+ , Zn 2+ , and Fe 2+ , the absorption of Ni by plants may be inhibited. However, in
some conditions, synergistic interactions may also occur. Variable effects of Ni–Cd,
Ni–Pb, and Ni–Cr interactions were observed in soil bacteria.
The most common Ni accumulators are Alyssum spp., which are suggested for the
phytoextraction of Ni from contaminated soils (Chaney et al. 2005). There are calculations that about 25 kg Ni/ha may be extracted by these plant species. The mechanism
of the Ni hyperaccumulations is not well understood, but it is evidently associated
with the formation of organometallic complexes. The main ligands for Ni in these
plants are citrate and malate, which are involved in the metal transfer within plant
tissues (Someya et  al. vide Kabata-Pendias 2011). However, several other organic
ligands (e.g., histidine) are also involved in these processes.
The easy phytoavailability of Ni is a real environmental concern. Especially
elevated Ni contents in vegetable and fodder plants may be more of a health risk.
The background Ni mean contents in barley and wheat grains are established at 0.41
and 0.34 mg/kg, respectively. Oat grains contain a bit higher Ni levels, between 0.2
and 8.0  mg/kg (Eriksson 2001a). The highest Ni contents are reported for plants
from contaminated soils (in mg/kg): lettuce (leaves), 40–80; onion (bulbs), 47; celery
(stalks), 29; soybean (seeds), 7–26; clover (tops), 3–15; and alfalfa, 44.
The range of mean Ni amounts in grasses from various countries is 0.1–1.7 mg/kg,
whereas in clover it is 1.2–2.7 mg/kg. Much higher Ni contents have been reported
for meadow grass (13–75 mg/kg) and forest grass (10–100 mg/kg) from the taiga
zone of Western Siberia (Niechayeva vide Kabata-Pendias 2011). In ecosystems
where Ni is airborne pollutant, the tops of plants are likely to concentrate its great
amounts.
The mushrooms, common chanterelles (Cantharellus cibarius) grown in mountains of Poland, contain lower amounts of Ni, mean 1.4 mg/kg, than those grown in
the Baltic Sea coast, mean 2.0 mg/kg (Falandysz et al. 2012).
29.6 HUMANS
The estimated average body burden of Ni in adults (70 kg) is 15 mg (Emsley 2011).
Nickel concentrations are (in μg/kg FW) as follows: the lung, 7–137; the bone and
kidney, 9–14; the heart 6–8; the liver 8–10; the spleen 7; whole blood 0.34–2.3 μg/L,
and urine, 0.9–2.0 μg/L; and serum, 1.2 μg/L (IARC 1990).
The essentiality of Ni in humans has not been established, and Ni dietary recommendations have not been recommended for humans. Nickel deficiency has not been
observed in humans, although there may be benefits from small doses of Ni, whereas
exposure to its high levels may result in adverse health effects. These effects are
dependent on the route of exposure and, in the case of the inhalation, on Ni species.