Removal of As, Cr and Cd by Absorptive Filtration

The US Environmental Protection Agency (USEPA) recently introduced more stringent arsenic regulations
by lowering the maximum contaminant level (MCL) to 10 ìg l-1 (ppb) arsenic in drinking water.
During the present study, an iron solution injection sand filtration process was designed and tested to
selectively remove arsenic (As(V)), chromate (Cr(VI)) and cadmium (Cd(II)) from aqueous solutions
down to trace level. Bench scale sand columns with a chemical feeding system were used to conduct the
filtration study. The filtration results demonstrate that As(V) could be removed by ferric solution-treated
sand filters from 2,000 ìg l-1 (ppb) down to less than 5 ìg l-1 (ppb) using two sand filters connected
in series (two stage filtration). During the filtration, ferric concentrations in the first and second filters
were maintained at 5 and 2 ppm, respectively, through a continuous injection scheme. Bench scale filtration
results suggest that Cr(VI) could also be effectively removed by injection of ferrous solution
into the sand columns. Similar ferric treatment of the sand columns also significantly increased Cd(II)
removal. Batch adsorption experimental results suggest that when solution pH is lower than 8, arsenate
can be removed by iron-treated sand. Arsenate-saturated sand can be regenerated using a high pH
(pH > 12) solution. Scanning Electron Microscope (SEM) and energy dispersive x-ray (EDX) studies
suggest that very little amount of Fe on the sand surface was dissolved when the sand was regenerated
using a dilute NaOH solution (pH = 13).