This fact indicates that metals in soil 3 were in a poorly soluble form.
S3 was taken from the base of a tailing pile; receiving aqueous runoff and particles released from this waste by the weathering of mineralized rock.
Consequently, an important fraction of the hazardous elements would be held within a crystal lattice from the original mineral, which is not easily separated or removed.
Additionally, more soluble fractions can be quickly was hed away in runoff.
With regard to arsenic, the amount extracted by (NH4)2SO4 was lower than 0.1% of the
total concentration in all soils, concurring with values found by other
authors (Adriano, 2001).
This might be due to the anionic character of As, which favors adsorption at low pH.