FöRFATTARE
Forsberg, Jerry

DATUM
2002-11-07

INSTITUTION
Samhällsbyggnadsteknik / Tillämpad geologi

SAMMANFATTNING
Natural attenuation processes involving metal sorption to colloids and suspended particles followed by colloidal flocculation and settling of particulate matter may be important as a final step in a sequence of remediation methods regarding mining waste.

The size distribution of metals in Vormbäcken, the recipient for mine drainage at the Kristineberg Zn-Cu mine in northern Sweden, was investigated in this study. In this stream, dissolved metals may be transferred into larger, settling particles downstream of the Kristineberg site. Further downstream, in the Vormbäcken system, metals may be immobilized in wetlands. The drainage water is heavily limed in a flooded impoundment before it is discharged into Vormbäcken.

To examine the size distribution of metal species, syringe filtration and cross flow ultrafiltration (CFF) were performed. Six different filter pore sizes were used in the syringe filtration. A 1 kDa regenerated cellulose Millipore® filter was used to isolate the colloidal and truly dissolved (< 1 kDa) fractions. 1 kDa is equivalent to approximately 0,5 - 1 nm.

In addition to ultrafiltration, diffusive gradient in thin film (DGT), was also used in this study to examine the distribution of truly dissolved metal species. The DGT-technique is a fairly new method designed to measure concentrations of truly dissolved and presumably bioavailable species. The DGT technique is based on a device which, in situ, accumulates dissolved ions on a binding agent after passage through a hydrogel which acts as a diffusion layer.

The major elements, Ca, Na, Mg, K and S occurred mainly in the truly dissolved phase (<1 kDa) and showed a conservative behaviour, i.e. no fractionation towards larger size fractions was evident downstream of the Kristineberg mine. Iron was mainly present in the colloidal- and particulate phase. A distribution towards larger sizes was seen for Fe in Vormbäcken, downstream of the Kristineberg mine. Any conclusions regarding flocculation could, however, not be drawn since a significant contribution of Fe from the surrounding catchment area was evident. The behaviour of Al was similar to that of Fe. A major part of the Mn occurred in the truly dissolved phase, and Mn is therefore less important as a potential carrier element for trace metals in Vormbäcken compared to Fe and Al.

Cadmium, Zn, Co and Ni mainly occured in the truly dissolved phase (<1 kDa). Flocculation should therefore be of little significance, and will probably not influence the transport of these metals in Vormbäcken. Arsenic, Pb and Cu, to a large extent, occurred in colloidal and particulate phases (> 1 kDa). These elements thus may be affected by flocculation, and sedimentation is likely to occur in the Vormbäcken system.

The major source for Cd, Zn and Cu is located upstream the point where drainage water from the Kristineberg mine discharges into Vormbäcken, i.e. the drainage water from the mine is not a major source for these metals in Vormbäcken. Significant contributions of As occur from the catchment area.

Almost all DGT concentrations were higher than the <1 kDa concentrations obtained with ultrafiltration. This may be due to an accumulation of metals bound to fulvic and humic substances by the DGT-units. Particularly large differences between ultrafiltrated concentrations and DGT concentrations, relative to the ultrafiltrated results, could be seen for metals mainly occurring in colloidal and particulate phases, i.e. for Fe, Al, Pb and Cu. It thus appears that the truly dissolved concentrations of Fe, Al, Pb and Cu are overestimated by the DGT-technique.

ISSN 1402-1617 / ISRN LTU-EX--02/310--SE / NR 2002:310