SEA WATER ELECTROLYSIS FOR CHROMIUM REMOVAL
Calvin P. C. Poon, Professor
Cheng-Fa Lu, Graduate Student
Department of Civil and Environmental Engineering
University of Rhode Island
Kingston, Rhode Island 02881
The federal government through the Environmental Protection Agency has promulgated regulations for industrial waste disposal, requiring the plating industry to treat wastewater with the best practical technology. In 1983, the best available technology will be
required for zero discharge requirements except small shops with less than 33 sq m/hr of
production or less than 2,000 A of installed dc. However, they must remove chromium and
cyanide and adjust the pH of the discharge. The interim effluent guideline for the metal
finishing industry set by EPA is 0.05 mg/l for Cr(VI) and 0.5 mg/l for total Cr, both for
filtered effluent and on a daily average basis.
The prevalent method of chromate removal is to reduce Cr(VI) to Cr(III) followed by
Cr(III) precipitation. Cr(VI) reduction can be brought about by using ferrous sulfate, sodium
bisulfide, and sulfur dioxide [ 1-3]. Alkaline precipitation of CrfHI) is effective and widely
used. Permutit [4] has developed a Sulfex process in which ferrous sulfide is used to remove
Cr(VI) in one step, converting to Cr(OH)3 for precipitation. Other methods of chromate
removal include ion exchange, activated carbon adsorption, reverse osmosis, ion flotation
(foam separation) using surfactants, liquid-liquid extraction and evaporation [ 1,3-5].
Electrochemical processes have always interested researchers as a means of introducing an
oxidant without adding extraneous chemicals or ions. It has long been accepted as a cost-
effective means of destroying concentrated cyanide solutions [6]. Only recently the practical adaptation of the electrolytically induced oxidation techniques has been achieved in the
treatment of dilute solutions using conductive carbon particles [7] or closely spaced electrodes [8] for cyanide removal or for the oxidation of organic pollutants. Electrolysis of
seawater is a special application of the electrochemical process useful for sewage disinfection, nitrogen and phosphorus removal [11-16] ; laundry waste treatment with aluminum
as the sacrificial anode [17] ; and copper, nickel, cadmium, zinc and cyanide removal [18,
19].
Electrochemical reduction has been applied for chromate removal using sacrificial iron
electrodes [20,21,23]. The reactions involved in the process have been described [22] as
follows:
Fe ** Fe 2 + 2e~ Anode
2H20 + 2e" - H2 t + 20H" Cathode
SFe*2 + Cr04"2 + 4H20 *• 3 Fe+3 + Cr+3 + 80H" Solution
Flocculants, e.g., anionic polymers, are required to agglomerate the insoluble iron and
chrome into larger size particles for precipitation. Either centrifugation or filtration is
normally required after precipitation to polish the effluent. The electrochemical reduction
process has an advantage over the chemical reduction process in that it can tolerate a wide
PH influent of 6.0 to 9.0. This research investigates the electrochemical reduction process
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