Scavenging of As from acid mine drainage by schwertmannite and ferrihydrite: a comparison with synthetic analogues.
Ochreous precipitates containing 5.5-69.8 g/kg As were isolated from mine drainage in Finland and were composed of schwertmannite, ferrihydrite, and goethite. Schwertmannite formation was favored at pH 3-4, but its structure was degraded at high As levels. A series of coprecipitates were therefore prepared from mixed iron arsenate/sulfate solutions to define the limits of schwertmannite stability. Schwertmannite was replaced as the dominant phase by a poorly crystalline ironIII hydroxy arsenate (FeOHAs) when As/Fe mole ratios exceeded 0.15. The FeOHAs gave an X-ray diffraction pattern similar to that obtained from an "amorphous" ironIII arsenate (As/Fe = 1.0) with broad peaks at 0.30 and 0.16 nm. The FeOHAs possessed a magnetic hyperfinefield of 41.9T at 4.2 K that was intermediate to those of schwertmannite (46.1 T) and the ironIII arsenate (24.8 T). These data indicate a strong disruptive effect of arsenate on magnetic ordering and structure development in schwertmannite. Equilibration of 0.01 M arsenate solutions with freshly prepared schwertmannite and 2-line ferrihydrite at pH 3.0 for up to 60 d gave sorbed As contents of 175 and 210 g/kg, respectively. Arsenate sorption degraded the host schwertmannite and ferrihydrite, perhaps due to the formation of an FeOHAs surface phase.
Carlson
L
L
University of Helsinki, Finland.
Bigham
J M
JM
Schwertmann
U
U
Kyek
A
A
Wagner
F
F
D000327:Adsorption; D001151:Arsenic / Q000032:analysis / Q000737:chemistry*; D001682:Biological Availability; D011232:Chemical Precipitation; D004784:Environmental Monitoring; D005290:Ferric Compounds / Q000737:chemistry*; D005293:Ferritins / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D058085:Iron Compounds; D008906:Mining; D012989:Soil Pollutants / Q000032:analysis*; D014873:Water Pollutants / Q000032:analysis*; D014961:X-Ray Diffraction
D005290:Ferric Compounds; D058085:Iron Compounds; D012989:Soil Pollutants; D014873:Water Pollutants; C511969:schwertmannite; C092844:ferric oxyhydroxide; D005293:Ferritins; D001151:Arsenic