58    CHROMIUM DETOXIFICATION BY
FIXED-FILM BIOREACTORS
Evans M. Nkhalambayausi Chirwa, Graduate Research Assistant
Y. T. Wang, Associate Professor
University of Kentucky
Lexington, Kentucky 40508
INTRODUCTION
The hexavalent form of chromium, chromate, is highly toxic and is known to be both carcinogenic and mutagenic to living organisms including humans.1-2 Higher than allowable concentrations of chromate are detected in groundwater and surface water receiving industrial wastewater.
Chromate is released into water bodies from a variety of processes such as photographic film or
negative making, electroplating, leather tanning, and wood preservation.3 Recent studies have
shown that certain species of bacteria are capable of catalytic transformation of chromate,
Cr(VI), into the less toxic trivalent form, Cr(III).4-10
One of the proposed mechanisms previously reported for Cr(VI) reduction in bacteria is
through aerobic activity."12 Aerobic reduction of Cr(VI) is generally associated with a soluble
protein fraction (enzyme) utilizing NADH as an electron donor during respiratory activity.1314 In
very simplified terms, Cr(VI) is visualized as being reduced to Cr(III) in the reaction:
Cr042" + 6.5H+ + 1.5 NADH -» Cr3+ + 1.5 NAD" + 4HzO; 1AG" = -121 U/electrons transferred. The energy needed to complete this reaction is presumed to come from the basic respiration reaction: C6H,206 + 602 -» 6C02 + 6H20; 1AG° = -<S5 U/electrons transferred.1516 However, the actual physiological functions of electron transfer to Cr( VI) are not yet known.
In this study, completely mixed, continuous flow bioreactors were utilized to detoxjfy
chromium. Glass beads were incorporated as a support medium for two strains of bacteria. Bacillus sp. and Pseudomonas fluorescens LB300 (LB300), growing aerobically in two separate reactors. Aerobic conditions were maintained in the reactors by continuously supplying fresh air to
the liquid through gas exchange chambers installed on the recycle line of the bioreactors. Results
obtained showed that near complete removal of chromate was possible for influent concentrations up to 200 mg/L for bacillus sp., and up to 100 mg/L for LB300 at 24 hours liquid detention
time. Similar results were obtained for corresponding loading rates at 12 hours and 6 hours liquid
detention time.
MATERIALS AND METHODS
Bioreactor System
The design of the reactor was based on the concept of a Completely Mixed Flow Reactor
(CMFR) with fixed media. The bench scale system arrangement is shown in Figure 1. The main
reactor was made out of a 12.5 cm long by 2.54 cm (1 inch) internal diameter glass column cut
from a Pyrex®* glass tube. The column was packed with 3 mm glass beads to provide inert support media of predictable surface area. The main reactor was operated in an up-flow mode. A gas
exchange chamber was made out of a 23 cm long by 0.8 cm internal diameter tubing closed on
one end. Air was continuously sparged in a counter-flow direction to the liquid flow in the chamber.  The gas exchange chamber was included to remove C02, and introduce 02 from and into
*Registered trademark of Corning Glass Works, Corning, New York.
50th Purdue Industrial Waste Conference Proceedings. 1995, Ann Arbor Press, Inc.. Chelsea. Michigan 48118.
Printed in U.S.A.
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