where, i is the extraction step number (1, least aggressive;
n, most aggressive) with the following values assigned to
i in Eq. (1) to represent various steps and components:
exchangeable fraction, 1; reducible fraction, 2; oxidizable
fraction, 3; and residual fraction, 4. So in our case, n equals
4. Fi is the percentage of the fraction bound to component i
in a metal-contaminated soil (each fraction divided by total
heavy metal concentration). The maximum of Ib is 1. This
parameter is introduced to describe the relative binding
intensity of metals in soils and enables comparison of the
binding intensity of a given metal among soils, and of
different metals in the same soil. A low Ib value represents
a distribution pattern where much of the metal resides in
the soluble and exchangeable fraction, while a high value
close to 1 indicates that a high proportion of the metal is
strongly bound in the residual fraction and the soil system
is stable.