Ferrate and Alkaline Chlorination Treatment of Cyanide-Heavy Metal Maritime Wastewater

AbstractFerrate [Fe(VI)] and alkaline chlorination (AC) were investigated for the treatment of a maritime wastewater (CN, Cd, Cu, Ni, Pb, and Zn) in a seawater matrix. AC reduced CN concentrations but large dosages were required (0.018–0.056 mol CN consumed/mol HOCl). Metal removal occurred during the first stage (pH>10) but the amount of sludge produced was large due to Mg(OH)2(S) formation. Fe(VI) was effective in oxidizing CN and removing heavy metals to low levels without pH adjustment. Fe(VI) requirements [0.12 mol CN/mol Fe(VI)] were higher than those observed for other wastes but are considered more manageable compared with HOCl. Metal removal occurred by adsorption and coprecipitation with Fe(III) oxide, which results from the oxidation of Fe(VI). During staged treatment, metal removal and CN destruction increased [2× for Cd, 3× for Cu, 2.7× for Ni, 0.12 to 0.18 mol CN consumed/mol Fe(VI)]. Fe(III) oxide was relatively easy to separate using ultrafiltration tubular membranes and the amount of sludge produced is much less than what would occur with AC. The Fe(III) oxide was separated using ultrafiltration with the amount of sludge produced much less than AC.