Environ. Sci. Technol. 1001, 25, 1780-1787
Alkaline Precipitation and Aging of Copper from Dilute Cupric Nitrate Solution James W. Patterson,*,+Richard E. Boice,t: and Darlo Marani§ Illinois Institute of Technology, Environmental Engineering Department, 3200 South State, Chicago, Illinois 60616 ~~
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The aging characteristics of cupric hydrous oxides were monitored by use of concurrent measurements of pH, conductivity, free and filterable copper concentrations, turbidity, and precipitate composition. The precipitation was performed by mixing equal volumes of dilute solutions of NaOH and Cu(NO,),. The final copper content was 0.0005 M, while the base/copper ratio was varied from 1.05 to 3.4. Two methods of mixing were compared: slow addition of the base solution to the copper solution and vice versa. The trends obtained for pH, conductivity, and free copper concentration versus time suggest that there are three phases of aging. A middle period of relatively constant pH, conductivity, and free copper concentration suggests that the conversion of Cu(OH),(s) to CuO(s) is the predominant aging phenomenon during this phase. Reversing the order of addition significantly changed the trends, probably due to the very different nucleation conditions. The results for aged samples were compared with equilibrium model predictions.
Introduction As with many other metals, the standard treatment method for removal of copper from wastewaters is precipitation at alkaline pH. Despite its wide application, the precipitation process is still applied in industry on an empirical basis, normally using grossly oversaturated conditions. With copper being a relatively valuable metal and the costs for disposal of potentially hazardous wastes increasing, recovery of copper is becoming more desirable and has more potential to become cost effective. Experience has shown that, as with other metal hydrous oxides, copper hydrous oxide separation from solution is difficult due to the formation of colloidal precipitate [hydrous oxide designation taking here a general meaning, following the definition by Matijevie ( I ) ] . The precipitation process involves induction of supersaturation, nucleation, particle growth, and several aging processes such as Ostwald ripening, recrystallization, and agglomeration (2). Precipitate formation is very sensitive to constituent concentration, pH, temperature, time of aging, and the nature of the anions, as well as the method of mixing. The usual procedure for adding base leads to localized supersaturations and, consequently, to poorly defined stages of nucleation and particle growth. This explains the poor reproducibility in many studies of metal hydrous oxide formation and accounts for the finding that the resulting solids are mostly ill defined in shape and polydispersed ( I , 3). Typically, the characteristics of freshly precipitated metal hydrous oxides differ significantly from the expected thermodynamically stable solids at the given conditions. A large number of studies have been devoted to the mechanisms of aging processes, whereby fresh solids slowly convert to thermodynamically stable solids that possess Present address: Patterson Associates Inc., 39 South La Salle Street, Chicago, IL 60603. $Present address: US.EPA, 230 South Dearborn St., Chicago, IL 60604. 8 Present address: Water Research Institute, Via Reno, 1,00198 Rome, Italy. 1780
Environ. Sci. Technol., Vol. 25, No. 10, 1991
Table I. Reported Equilibrium Constants for ICso(Cu(OH), (s)), K , , (CuO(s)),B , (CuOH+), and B , (Cu(OH),O),at 25 “C and zero ionic strengtha ref 11 12
13 14 15 16 17 18 19 20
loe Kef, Cu(OH),(s) CuO(s) 8.68 8.66 8.68 8.64
log Bz
-8.06 -7.7
