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Microbial Effects on Nickel and Cadmium Sorption and Transport in Volcanic Tuff

New Mexico Environment Dep., State of New Mexico, Santa Fe, NM 87502;

Robert S. Bowman

Geoscience Dep. and Geophysical Res. Ctr., New Mexico Institute of Mining and Technology, Socorro, NM 87801;

Thomas L. Kieft^*

Biology Dep., New Mexico Institute of Mining and Technology, Socorro, NM 87801.

^* Corresponding author (tkieft{at}nmt.edu).

ABSTRACT

We performed sorption and transport experiments to determine how vadose-zone microbes affect sorption of Ni²⁺ and Cd²⁺ on crushed volcanic tuff and transport of Ni²⁺ in the same medium. Sorption of Ni²⁺ and Cd²⁺ to the tuff was less in samples inoculated with microbes than in sterile samples. Since Ni²⁺ sorption was the same both in the presence and absence of microbes when a buffer was used, microbes appear to decrease sorption by decreasing solution pH. Samples with Bacillus megaterium decreased Ni²⁺ sorption the most of the 14 subsurface microbial strains tested. Nickel isotherms were linear up to initial concentrations of 10 mg L^–1 with an average distribution coefficient (k_d) of 7.4 L kg^–1 for samples with freeze-dried B. megaterium and 50 L kg^–1 for sterile samples containing 1 g L^–1 NaN₃. Sterile samples without NaN₃ had a higher k_d (84–265 L kg^–1). We therefore predicted that the retardation of Ni²⁺ in inoculated columns would be lower than in sterile columns. A transport experiment with both inoculated (inoculum consisted of freeze-dried B. megaterium) and sterile (solutions contained NaN₃) columns indicated that retardation (R) of Ni²⁺ was less for columns with microbes (R = 205) than for sterile columns (R = 307), but retardation was higher than predicted by batch sorption experiments in both groups of columns. Dispersion was lower in inoculated columns, possibly due to microbial clogging of secondary pores. The results indicate that microbes can increase the mobility of cationic metals in subsurface environments.

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