Indeed,
the concentration of free metal ions in soil solution
has been shown to be closely linked to toxicity (Ma et
al., 1999; McBride, 2001). This is because the free ion
species of cationic metals is the dominant form
absorbed by organisms (Kochian, 1991). Lokeshwari
and Chandrappa 2006 reported highest bioavailability
of Fe for plants and animals in sewage-fed lake water.