1. Introduction
Heavy metals have been found in potentially
harmful concentrations in numerous
drinking water systems due to natural or industrial
pollution sources. Two of the most toxic
heavy metals are arsenic (As) and mercury
(Hg), and thus each compound must be
removed to very low levels in order to prevent
health problems. The maximum concentrations
of arsenic and mercury recommended by
the World Health Organization are 10 ppb and
1 ppb, respectively.
The main limitations of existing treatment
technologies are secondary contamination
by microbial cells, nutrients, and metabolic
by-products in biological processes; difficulties
in disposing the brine solution from pressure
driven membrane processes; high energy
demands in distillation processes; competition
for adsorption sites in ion-exchange systems by
other ions that are usually present in much
higher concentrations; and the undesirable
addition of chemicals, as well as difficulties in