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Clay–Microbe Interactions and Implications for Environmental Mitigation

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Clay–Microbe Interactions and Implications for Environmental Mitigation Clay–Microbe Interactions and Implications for Environmental Mitigation Hailiang Dong1,2 1811-5209/12/0008-0113$2.50 DOI: 10.2113/gselements.8.2.113 lay minerals are ubiquitous in soils, sediments, and sedimentary rocks, Some of these interactions generate and they play important roles in environmental processes. Microbes distinct mineral assemblages in the ancient rock record, producing Care also abundant in these geological media, and they interact with biosignatures. clays via a variety of mechanisms, such as reduction and oxidation of struc- Although clay mineralogy has tural iron and mineral dissolution and precipitation through the production been studied for more than half a of siderophores and organic acids. These interactions greatly accelerate clay century and various applications mineral reaction rates. While it is certain that microbes play important roles have been developed, the impor- tant role of microbes in clay in clay mineral transformations, quantitative assessment of these roles is mineral transformations has been limited. This paper reviews some active areas of research on clay–microbe recognized for only about 20 years. interactions and provides perspectives for future work. In the last decade, interest in this fi eld has risen dramatically, as it KEYWORDS: clay minerals, microorganisms, oxidation, reduction, transformation has become clear that reduced clays can sequester toxic metals INTRODUCTION and radionuclides and that clays can degrade organic compounds (Stucki and Kostka 2006; Clay minerals are likely the minerals that we encounter Dong et al. 2009). In this article, I will start with a short most commonly in our daily lives. They form the soils in review of clay mineralogy and then follow with a compre- which plants grow, and they are the primary materials in hensive overview of clay mineral–microbe interactions. I a range of products and applications, including cat litter, will end with a consideration of the environmental impli- animal feed, pottery, china, oil absorbants, pharmaceuti- cations of clay mineral–microbe interactions and the cals, cosmetics, wastewater treatment, and even antibacte- outlook for future developments. rial agents. Clay minerals such as kaolinite and smectite have long been used to treat ailments of the digestive tract in certain countries (Ferrell 2008). CLAY MINERALOGY Clays are built of tetrahedral and octahedral sheets, typi- The diversity of clay mineral applications can be attributed cally in a 1:1 or 2:1 ratio. A 1:1 clay structure consists of to the chemistry and structure of these minerals. Clay one tetrahedral sheet and one octahedral sheet, and minerals are hydrous aluminum layer silicates with struc- tures similar to those of micas. Unlike the micas, however, interlayers of clay minerals contain a low cationic charge to “glue” adjacent silicate sheets, and water molecules can enter or exit the interlayer regions easily. Thus, these mate- rials swell or shrink under wet or dry conditions. Clays are common products of the weathering and hydrothermal alteration of various rocks, but they can also precipitate directly from aqueous fl uids. Because of their low-temper- ature genesis, clay minerals are very common in soils, sedi- ments, and sedimentary rocks. Depending on their specifi c mode of formation, clay minerals can form extensive deposits of economic value (FIG. 1). Because clay minerals are common in low-temperature environments where microorganisms thrive, the interac- tions between clays and microbes are important to a number of surfi cial processes. Microorganisms can dissolve, precipitate, and transform clay minerals and thus change their physical and chemical properties. Some of these changes are benefi cial, whereas others are not desirable. 1 State Key Laboratory of Biogeology and Environmental Geology This vermiculite deposit in Weili, Xinjiang, China, FIGURE 1 China University of Geosciences, Beijing, 100083, China was formed from the weathering of phlogopite by meteoric water. PHOTO COURTESY OF TONGJIANG PENG, SOUTHWEST 2 Department of Geology and Environmental Earth Science UNIVERSITY OF SCIENCE AND TECHNOLOGY, CHINA Miami University, Oxford, OH 45056, USA E-mail: [email protected] or [email protected] ELEMENTS, VOL. 8, PP. 113–118 113 APRIL 2012 examples include kaolinite and serpentine. A 2:1 clay struc- trapping of hydrocarbons (Pevear 1999). Thus, the S-I reac- ture consists of an octahedral sheet sandwiched between tion is often used as an index for the generation of petro- two tetrahedral sheets (FIG. 2), and examples include smec- leum and natural gas. In terrigenous sediments, the S-I tite, illite, and vermiculite. reaction typically takes place over the temperature range of about 90–120 oC (Moore and Reynolds 1997). To better Of special importance are two 2:1 clay mineral families: understand the kinetics and the controlling conditions for smectite and illite (FIG. 3). Smectite minerals are 2:1 layer this reaction, laboratory simulation experiments are typi- silicates with a total negative charge between 0.2 and 0.6 cally performed (Huang et al. 1993). These efforts reveal per half unit cell. The low charge accounts for the capacity that, in the absence of any microbial activity, conditions of smectites to swell. A member of the smectite family of 250–350 °C and 50–100 MPa are often needed to achieve called nontronite (iron-rich smectite) has the following realistic reaction rates (a few months) (Huang et al. 1993; ideal formula: Kim et al. 2004). However, in the presence of microbial 3+ 2+ activity, this reaction occurs via various mechanisms, at (K0.01 Na0.30 Ca0.15)(Al0.55 Fe 3.27 Fe 0.06Mg0.12) 3+ much lower temperatures and pressures over shorter time (Si7.57Al0.15Fe 0.28)O20(OH)4 . durations. The large cations (Na+, Ca2+, and K+) enter the interlayer space to balance the net negative charge created by the MICROBE–CLAY MINERAL INTERACTIONS substitution of trivalent Al and Fe for tetravalent Si in the tetrahedral site and divalent Mg for trivalent Fe in the Microbial Dissolution of Clay Minerals octahedral site. In soils and sediments, microorganisms play important Illite minerals have the same basic structure as smectite, roles in the dissolution of clay minerals, contributing to but with a higher net negative layer charge of 0.6 to 0.9 elemental cycling, soil fertility, and water quality. Microbes per half unit cell, a result of the greater degree of tetrahe- accelerate clay mineral dissolution either through redox dral and/or octahedral substitution. A typical illite from reactions of structural iron (see below) or through the Silver Hill, Montana, USA, has the following formula: release of metabolic by-products. In contrast to abiotic dissolution where pH is a primary driving force, biotic 3+ 2+ (Mg0.09Ca0.06K1.37)(Al2.69Fe 0.76Fe 0.06Mg0.43Ti0.06) dissolution requires many organic compounds to destabi- (Si6.77Al1.23)O20(OH)4 . lize the mineral structure. These compounds include sider- ophores (a group of element-scavenging compounds that Because of the higher charge of the interlayer cations, the contribute to the weathering of Fe oxides and silicate electrostatic attraction between the basal surface and the minerals), organic acids, iron chelators (small organic interlayer cations is stronger in illite than in smectite. molecules that bind to Fe and help its transport), and extra- Consequently, illite does not expand when hydrated. cellular polymeric substances (high-molecular-weight Smectite is stable at low temperature and pressure, which compounds composed of polysaccharides that are secreted are the typical conditions in soils and surfi cial sediments. by microorganisms into the environment). In microbially As soils and sediments are buried, smectite becomes mediated clay mineral dissolution, the mineral surface unstable and transforms into illite according to the reactivity seems to play a secondary role (Grybos et al. 2011). following reaction: Microbially mediated mineral dissolution is usually incon- smectite + Al3+ + K+ → illite + silica . gruent, meaning that certain elements are removed pref- erentially relative to others, producing nonstoichiometric Three important variables drive the smectite-to-illite (S-I) minerals. Silica is a common dissolution product and has reaction: time, temperature, and potassium concentration. been observed in both laboratory experiments and natural The S-I reaction is of special signifi cance because the extent mineral assemblages. This material is largely amorphous, of the reaction, termed “smectite illitization,” is associated with particle sizes in the nanometer range. Other biogenic with a specifi c combination of temperature and time. These minerals include pyrite, siderite, and vivianite. A mineral same conditions can trigger maturation, migration, and assemblage of quartz, pyrite, and calcite has been used to recognize the role of microbes in mediating mineral reac- tions in ancient marine sediments and in regulating global Si and Fe fl uxes (Vorhies and Gaines 2009). Microbial Formation of Clay Minerals Clay minerals are common weathering products at low temperatures and neutral to acidic pH. Therefore, it is common to fi nd that microbes are involved in the process of rock weathering and clay formation. In fact, microbially catalyzed rock and mineral weathering rates can be orders of magnitude higher than the chemical equivalent, espe- cially when rocks and minerals contain nutrients such as phosphorus, a limiting nutrient in microbial metabolism (Rogers and Bennett 2004).
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