DIURNAL CYCLES IN DISSOLVED SELENIUM IN DRAINAGE FROM A PRECIOUS METAL ORE HEAP

Drainage water from a remediated ore heap at a southern California gold mine has been analyzed periodically through a 36-hr period to test for diurnal chemical variations. A number of parameters varied, including cyanide species, Fe, Mn, Cu, Ag, Au, NH₄, pH and temperature (ES&T 36, 840, 2002). Here, evidence is reported for diurnal variation in Se. Inductively coupled plasma-mass spectrometer analyses showed higher total Se during daytime (49±1 ppb) than at night (43±1 ppb). Drainage waters from other gold mines in the southwestern US were found to contain up to 660 ppb Se reflecting elevated Se in sulfides and(or) carbonaceous matter in some ores.

Se concentration was invariant at the point of discharge from the heap, but varied diurnally at a site downstream where the channel residence time was 34 min. The behavior cannot be explained by coprecipitation of Se with Fe-Mn oxides because dissolved Fe and Mn varied out-of-phase with Se. Incorporation of Se into organic matter is also unlikely because photosynthesis-respiration cycles would lower dissolved Se in daylight, the reverse of the observed behavior. Offgassing of methylated Se is possible, but Se was lowest at night when the water had cooled from 28 to 15°C, which is opposite the temperature dependence for methylation observed in incubations. The diurnal variation could reflect Se adsorption to Fe-Mn oxides because the nighttime pH decrease from 9.0 to 8.4 might have enhanced the uptake of any Se⁴⁺ that was present.

Another possible cause for the diurnal variation is release from channel-bottom sediment or particulates as selenocyanate (SeCN^-). Free cyanide is known to have been produced within the channel by photodissociation of an Fe cyanocomplex (see ES&T article). The free cyanide mobilized Cu, Au and Ag as cyanocomplexes during daylight, and it could have mobilized Se similarly. This would parallel the behavior of the chemically analogous element S, which reacts with cyanide to form SCN^-.

The observations reported here imply that daytime sampling can in some circumstances lead to overestimates of Se loads in drainage from gold mines. Further, they support recent discoveries by others that SeCN^- can be important for the mobility of Se, not only in cyanide-contaminated settings (power plants, oil refineries, gold mines), but also in natural aquatic environments.