9. Cadmium, Cd (atomic no. 48)

Transcription

1 9. Cadmium, Cd (atomic no. 48) - Cd is comparatively rare in the environment - Median concentrations of Cd in soils and sediments range from about 0.04 to 1.8 mg/kg. - Cadmium concentrations are elevated in association with sulfide ores of zinc and sometimes with ores of copper and lead. - Zn and Cd are chemically similar, but Cd is much less abundant. - Soil weathering can lead to release of the Cd 2+ ion, which is generally soluble and mobile in water at low to near-neutral ph.

2 Cadmium Occurence - The primary industrial uses of Cd are metal plating, Ni-Cd batteries (75%), as a stabilizer in plastics, and as a pigment. - The largest sources of Cd contamination to ground water and surface water are from sewage sludge, mine water, mine tailings leachate, metal smelter process waters, battery recycling plants, and wastes from electroplating facilities. - Cd and its compounds are very toxic to nearly all living organisms. - The EPA has set the maximum contaminant level (MCL) for Cd in drinking water at 5 ppb.

3 Geographic distribution of Cadmium concentrations in groundwater collected from wells as part of the National Water-Quality Assessment Program,

4 Concentration in NE Groundwater, (Ayotte, SIR )

5 Percent of Cd samples greater then 1 ppb in Groundwater sampled Across the U.S.; comparison to Cr USG-Unconsolidated Sand and Gravel GLA, Glacial Ayotte, SIR

6 Cd Solubility - Cadmium has some tendency to form aqueous complexes with both inorganic and organic ligands, although the noncomplexed Cd 2+ ion is fairly stable. - The distribution diagram for cadmium hydroxy complexes indicates that Cd 2+, Cd(OH) +, Cd(OH) 2 0 are the most significant species below ph 12 (fig. 1.1). - Figure 1.2 shows the cadmium species as a function of ph in a solution of chloride, sulfate, and carbonate species. - Complexation of Cd with chloride and sulfate is most important at near-neutral to low ph; carbonate complexation is most important at ph 9 to 11; and, hydroxyl species dominate Cd complexation at ph > 11 (Figure 1.2). - In general: Cd 2+ ion dominates below ph 8.5.

7 U.S. EPA, 2007b

8 U.S. EPA, 2007b Cadmium speciation and complex concentrations with ph; Cd = 1 ppm, Cl = 100 ppm, SO4 = 100 ppm, CO3 = 100 ppm.

9 Redox - In natural systems, Cd is only present as Cd Therefore, the geochemical transport of Cd isn't directly tied to changes in redox conditions. - Cd forms stable precipitates and aqueous complexes with redox-sensitive elements such as S and C; Cd mobility is indirectly tied to redox conditions. - In sulfate-reducing systems, Cd is expected to form insoluble CdS, or coprecipitate with FeS 2 (pyrite). - In moderately reducing, but non-sulfidic systems, however, reductive dissolution of hydrous ferric oxides with adsorbed Cd could result in Cd mobilization.

10 Eh-pH diagram for Cd; Cd = 10-8 M (~1 ppb), C = 10-3 M (12 ppm), S = 10-3 M (32 ppm) note: solid phase in yellow U.S. EPA, 2007b

11 Cd Adsorption - Adsorption/desorption behavior of Cd is a function of ph, (see below) and to a lesser extent, competing cations or complexing ligands. - At low concentrations of Cd, sharp adsorption edges indicate Cd forms strong bonds with mineral surfaces. - Ca and Mg will compete with Cd for adsorption by aquifer solids. - Zn (which tends to be more strongly adsorbed) can reduce Cd uptake by Fe and Al oxides. - Sorption study results indicate that hydrous ferric hydroxide (HFO) removes Cd at ph above 6.7.

12 Metal Cations % Note: Metal cation adsorption is greatest when H + concentrations are lower due to the increased negative charges on solid surfaces at higher ph

13 MOVEMENT OF WATERBORNE CADMIUM AND HEXAVALENT CHROMIUM WASTES IN SOUTH FARMINGDALE, NASSAU COUNTY, LONG ISLAND, NEW YORK (PERLMUTTER, et.al.; USGS PP 475-C) A plume of contaminated ground water, moving through glacial outwash and containing high concentrations of cadmium (up to 3.7 ppm) and hexavalent chrominum (up to 14 ppm), was discharged over a 20 year period ( ) by an industrial plant in South Farmingdale in east-central Nassau County. The plume(s) extended about 4,200 ft from the recharge basins at a plating plant to Massapequa Creek, where the waste discharged into the stream. Sampling indicated the ph of the contaminated water was about 5.

14 Map of Long Island, N.Y., showing location of South Farmingdale, Nassau County.

15 Aerial photograph of South Farmingdale area showing the extent of contamination in 1962 (ruled area), and location of section X-X' (N-S).

16 -Sections along X-X' show the vertical extent and concentrations of cadmium along the approximate direction of flow in the center of the plume in The maximum cadmium content was 3.7 ppm at well 67, a short distance south of the recharge basins.

17 Glacial Bedrock 0-1 ppm >1 ppm Sections showing geology and direction of movement of ground water (A), and lines of equal cadmium (C), in ppm, along line X-X' in Contaminated water body shaded.

18 Cadmium Attenuation and Mobilization USEPA, 2007b

19 Summary - Cd is comparatively rare in the environment, however, Cd contamination to ground water has occurred in NE, primarily from electroplating wastes (e.g. Farmingdale-LI plating waste plume). - Adsorption/desorption behavior of Cd is a function of ph, and to a lesser extent, Cd concentration and competing cations or complexing ligands. - Although only moderately sensitive to redox conditions, Cd is expected to form insoluble CdS precipitates in sulfate-reducing systems, or coprecipitate with FeS.

20 Next: Chromium, Cr