U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993,
Water-Resources Investigations Report 94-4015
  
Chemical Processes in Manganese Oxide and Carbonate Precipitation
in Pinal Creek, Arizona
by
John D. Hem (U.S. Geological Survey, WRD, Menlo Park, Calif.)
and Carol J. Lind (U.S. Geological Survey, WRD, Menlo Park, Calif.)
Abstract
Two water samples from well 503 near the head of
the perennial reach of Pinal Creek and one sample
of surface flow at the Setka Ranch crossing
about 2 kilometers downstream from the well were used
as the manganese-bearing solution in automated pH-stat
titration with a 0.10 molar NaOH solution while bubbling CO2-free
air into the mixture. The three samples had manganese
concentrations ranging from 63 to 94 mg/L and
dissolved solids concentrations ranging from of
3,500 to 3,570 mg/L, mostly consisting of Ca and
SO4. The ground-water samples initially were about
5 x 10-3 molar in undissociated
dissolved CO2 and had pH's near 6. The creek
water had a pH near 7 and a CO2 concentration
smaller than 10-3 molar.
Titration of one ground-water sample done without
removing the dissolved CO2 produced a mixture
of hausmannite (Mn3O4) and kutnahorite
(CaMn(CO3)2). The other two samples were
treated to decrease the dissolved CO2 species
before titration. One of these samples yielded mainly
hausmannite during titration that altered on aging
to manganite (MnOOH). The other sample yielded
a mixture of oxides that altered on aging to
a mixed Ca + Mn species similar to todorokite ((Mn2+, Ca) Mn4+5
O11 · 4H2O) in which most of the
manganese is in the 4+ oxidation state. X-ray-diffraction
studies of precipitates from Pinal Creek identified
kutnahorite in carbonate cemented crusts near the
head of perennial flow. The mixed Ca + Mn4+
oxides also were identified near the head of
perennial flow and in black precipitates farther downstream.
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