Section Six
PLATING WASTES
55    IRON PROCESS FOR TREATMENT OF Cr(VI)
WASTEWATERS
Alan R. Bowers, Assistant Professor
Carlos A. Ortiz, Graduate Student
Roberto J. Cardozo, Graduate Student
Department of Civil and Environmental Engineering
Vanderbilt University
Nashville, Tennessee 37235
INTRODUCTION
The technology for treatment of Cr(VI)-bearing wastewaters has been well established. Conventional methods are based on the reduction of Cr(VI) to Cr(lII) followed by precipitation as Cr(OH)3
at high pH, ca. 8.0 to 10.0 [1-11]. Although other techniques such as ion exchange and activated
carbon adsorption have been successfully demonstrated in the laboratory, they have not been
employed to any significant degree in full-scale operations [12-15].
A variety of reducing agents have been suggested for Cr(VI) reduction,including sulfides, bisulfites,
metabisulfites, ferrous sulfate, and sulfur dioxide [1,7,8,11]. Although quite effective as reducing
agents, the cost of these chemicals may not be entirely justified due to the availability of equally
effective waste materials, such as scrap iron, for the reduction of Cr(VI). Furthermore, the cost of
reducing Cr(VI) with conventional chemicals is greatly increased by side reactions with dissolved
oxygen, which is ubiquitous in aqueous systems that are in contact with the atmosphere [16, 17].
Typically, the chemical dosage needed for Cr(VI) reduction is two times the stoichiometric requirement [11,16]. Based on this ratio, a comparison of chemical costs for Cr(VI) reduction is shown in
Table I.
The costs shown in Table I indicate that scrap iron has the potential to be significantly more
economical than conventional means of Cr(VI) reduction, e.g., > 10 times cheaper than the least
expensive alternative. In addition, the iron solubilized by oxidation may act as a conditioning agent
for the resulting Cr(OH)3 sludge and, thus, produce a greater overall benefit in cost that would not be
Table I. Cost Comparison of Chromium (VI) Reducing Agents
Chemical Costs"
Reducing Agent ($/Kg Cr)
S02 0.85"
NaHSO, 1.15c
FeS 3.33d
FeS04 1.25'
Scrap Iron 0.05f(0.08)
"Costs calculated assuming required dosage equal to 2 times the stoichiometric requirement!11].
°$210/ton for S02 [20].
c$0.174/lbfor NaHSOj [20].
d$0.45/Ib for FeS [20].
'$130/ton for FeS04 [20].
f$l.00/100 lb for scrap iron, current market price in Nashville, TN., based on telephone survey of
local dealers, August, 1985. Value in parenthesis represents 3.5 times the stoichiometric
requirement as found necessary in this work.
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