wAbstract
@Clays and clay minerals are common components in soils, sediments,
and sedimentary rocks, and they play an important role in many
environmental processes. Iron is ubiquitous in clays and clay
minerals and its oxidation state, in part, controls the physical
and chemical properties of these fine-grained minerals. The structural
ferric iron in clay minerals can be reduced either chemically
or biologically. Biological reductants include mesophilic and
thermophilic microorganisms from diverse environments such as
soils, sediments, sedimentary rocks, and hydrothermal hot springs.
Multiple clay minerals have been used for microbial reduction
studies, including dioctahedral smectite-illite series, palygorskite,
chlorite, and their various mixtures in natural soils and sediments.
all of these clay minerals are reducible by microorganisms under
various conditions with smectite (nontronite) being the most reducible
and illite the least. The rate and extent of bioreduction depends
on many experimental factors, such as the type of microorganisms
and clay minerals, solution chemistry, and temperature. Despite
significant efforts, current understanding of the mechanisms of
microbial reduction of ferric iron in clay minerals is still limited.
Whereas some studies have presented evidence for a solid-state
reduction mechanism, other argue that the clay mineral structure
partially dissolves when the extent of reduction is high. This
inconsistency may be related to several experimental conditions,
and their specific effects are discussed in this paper. Whereas
past experiments have been largely conducted in well-controlled
laboratory systems, recent efforts have attempted to transfer
knowledge to the field to improve our understanding of more complex
soil systems for better agricultural practices. Biologically reduced
clay minerals are also important agents in remediating inorganic
and organic contaminants in soil and groundwater systems. This
paper reviews the most recent developments and suggests some directions
for future research.
Keywords: Bacteria; illite; mechanism; microbial Fe(III) reduction;
nontronite; smectitex
Introduction
Microorganisms used in bioreduction studies
Clay minerals used in bioreduction studies
Rate and extent of microbial reduction of Fe(III) in clay minerals
Relative reducibility of Fe(III) oxides and clay minerals
Mechanisms of microbial reduction of clay minerals
Mineral transformations as a result of microbial reduction of
clay minerals
Field demonstration of microbial reduction of Fe(III) in clay
minerals
Application of reduced clay minerals to remediation of heavy metals
and organic compounds
Future perspectives
Acknowledgments
References cited