A contaminant is defined as a substance that is not natural to the soil, or a naturally occurring substance which has undesirable effects if its concentration exceeds a certain threshold. A number of non-metals have the potential to be contaminants in soil and water. Because of their high ionic potential, non-metals of Groups IIIA to VIA of the Periodic Table in their normal oxidation states form oxyanions (Sposito, 1984). Examples of non-metal oxyanions commonly found in the soil solution are borate (B(OH)−4), biocarbonate (HCO−3), nitrate (NO−3), silicate (H3SiO−4), sulphate (SO2−4), phosphate (H2PO−4), arsenate (AsO3−4) and selenate (SeO2−4). Other nonmetals such as Cl and F occur as the monovalent anions Cl− and F− The ubiquitous ion Cl forms soluble salts with the common cations in soil and is only a potential problem when the salt concentration of the soil solution or groundwater is high and specific Cl− toxicity effects in plants may occur. On the other hand, F− forms insoluble complexes with cations such as Al3+ and Ca2+ and is rarely an active contaminant except in some underground waters of the Great Artesian Basin. Of the other anions, HCO−3 derives naturally from the dissolution of CO2 in soil water and dissociation of the weak carbonic acid formed. Silicate is formed from mineral weathering and dissociation of the very weak acid Si(OH)4. Both are benign. Sulphate as SO2−4 and less commonly HSO−4 forms complexes with the hydroxyl surfaces of Fe and Al oxides, the edge faces of crystalline clay minerals, and Al3+ and hydroxy Al species adsorbed on charged surfaces.
Weitere Kapitel dieses Buchs durch Wischen aufrufen
- The fate of non-metal contaminants in the soil environment
R. E. White
A. N. Sharpley
- Springer Netherlands
Fallstudie Überschwemmungskarten/© Thaut Images | Fotolia