Nickel is essential for plants, bacteria, humans and other mammals, but toxic at higher concentrations to most plants and fungi, moderately toxic to mammals and may induce allergic effects in humans. Certain Ni compounds and maybe also metallic Ni are human carcinogens. Like Cu, also Ni shows elevated surface water levels found in the surrounding agricultural areas. Scandinavian field surveys indicate that both acidity and humic substances influence its distribution in these types of waters. Most often aqueous Ni seems positively correlated with the amount of humus (TOC) substances but negatively correlated with water pH. The positive corrrelation observed between Ni and pH in a Swedish lake survey may be due to the high pH and Ni concentrations found in agricultural areas. Data collected between 1980 and 1995 in Finish, Norwegian and Swedish surface waters show a median Ni concentration of < 0.5 |g/l (see Lydersen et al., 2002). The median concentration found in Swedish and Norwegian low TOC lakes was 0.21 |g/l, whereas in medium and high TOC lakes, median values were found to be 0.41 and 0.58 |g/l, respectively. Although the obtained field data provided no significant correlation between pH and Ni in these waters, the median concentration of Ni in low pH lakes (0.22 |g/l) was somewhat different from medium to high pH waters (0.20 - 0.33 |g/l).

The global mean soil concentration of Ni is estimated to be 35-40 mg/kg dw. In the soil profile, Ni tends to follow iron and humus and can accumulate in the B horizon and in the mor layer. Also clay minerals are marked accumulators of Ni. The solubility of Ni increases rapidly if pH drops down to 4.0-4.5 in the B horizon. In uncontaminated river sediments, Ni concentrations vary in general between 1 and 150 mg/kg dw, but can reach up to 1000 mg/kg dw in the vicinity of nickeliferous deposits. Concentration levels in surface lake sediments from an unpolluted area in central and northern Sweden were in the range of 3.6 to 21 mg/kg dw. As in soils, Ni is mainly associated with hydrous oxides of Fe and Mn, with clay minerals and organic matter in aquatic sediments.

Concerning its chemical speciation, Ni occurs predominantly in the divalent form Ni(II), which forms strong complexes with organic ligands, like carboxylates, fulvates and humates. Ni forms soluble salts of chloride, sulfate and nitrate, whereas Ni oxide is only soluble under acid conditions, in contrast to Ni hydroxides, sulphides, arsenides, arsenates, and silicates, which are almost insoluble. The stability of metal complexes with humic acids decreases in the order: Pb2+ > Cu2+ > Cd2+ > Zn2+ > Ni2+ > Co2+. Due to the chemical similarity between Ni2+ and Fe2+ and Co2+, Ni can substitute these metals in a number of compounds.

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