EFFECTS OF PH AND PROPORTIONING OF FERROUS AND
SULFIDE REDUCTION CHEMICALS ON ELECTROPLATING
WASTE TREATMENT SLUDGE PRODUCTION
Lt. James R. Aldrich, Environmental Research Engineer
Air Force Engineering and Services Laboratory
Tyndall AFB, Florida 32403
INTRODUCTION
In general, electroplating waste waters bearing heavy metals such as copper, cadmium, nickel, and
chromium are relatively easy to treat in chemical precipitation systems. The effluent quality obtained
is limited only by the solubility of the metal salts formed in the reaction. This is because these heavy
metals all readily form hydroxides or sulfides with the notable exception of chromium which will not
form a sulfide at standard temperature and pressure. Additionally, chromium generally requires an
additional treatment step to reduce the ion from the hexavalent to the trivalent state. Many treatment
chemicals can be used for this reduction. Among these are ferrous sulfate, sodium bisulfite, sulfur
dioxide, and sodium sulfide. While all these chemicals work well from the standpoint of effluent quality, the quantity of sludge produced by the different processes can vary dramatically.
With the advent of the Resource Conservation and Recovery Act (RCRA), producing clean water
is no longer sufficient. Hauling and disposal charges for hazardous sludge are over $100/ton in many
areas, hence the volume of sludge for disposal is nearly as important as the effluent quality. Some of
the more exotic treatment chemicals, such as sodium borohydride, are extremely efficient from a
sludge production standpoint (8 moles of electrons are available per mole of reactant), but they are
quite expensive and have not been fully tested on mixed metal wastewaters. To address this problem,
an Air Force research program was initiated to investigate the sludge volumes produced by the more
common reduction chemicals. Even though Air Force treatment plants must treat mixed-metal wastes,
chromium was singled out for this study because it alone requires both reduction and hydroxide precipitation.
BACKGROUND
Present treatment technology for chromium reduction is predicated on work by Espensen [1] who
determined that the rate of chromium reduction depends upon the pH of the waste solution. Taking
the "standard" practice as ferrous reduction, the reaction is:
3FC24  + HCKV  + 7H+ - 3 Fe3+  + Cr3+  + 4H2°
Espensen quantified the rate of this reduction reaction as:
d[HCrQ4] m   _ [Fe2 + ]2[H + ]3(k,[HCrQ4   ] + k2[HCrQ4]2)
dt [Fe3*]
Since this rate equation is third order with respect to the hydrogen ion concentration ([H + ]), it has
been the basis for claims that chromium reduction is very slow at all but acidic pH levels [2]. Each unit
increase in pH (i.e., decrease in [H + ]) would decrease the rate of the reaction by three orders of magnitude. For example, Thomas [3] calculated that reducing 100 mg/l of hexavalent chromium at pH 3
would take over 1000 times longer than at pH 2 (90 minutes versus 5 seconds).
Completely contrary to the Espensen rate equation, work performed by TerMaath [4,5,6 and 7]
and Higgins [5,6,7,8 and 9] indicated that hexavalent chromium could be rapidly reduced at pH 8.0.
In follow-on work, Higgins clarified this apparent dichotomy [10]. Espensen's work was done with
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