Synergistic inhibition of green rust crystallization by co-existing arsenic and silica.

Arsenic and silica are known inhibitors of the crystallization of iron minerals from poorly ordered precursor phases. However, little is known about the effects of co-existing As and Si on the crystallization and long-term stability of mixed-valence Fe minerals such as green rust (GR). GR usually forms in anoxic, Fe2+-rich, near-neutral pH environments, where they influence the speciation and mobility of trace elements, nutrients and contaminants. In this work, the Fe2+-induced transformation of As- and/or Si-bearing ferrihydrite (FHY) was monitored at pH 8 ([As]initial = 100 µM, Si/As = 10) over 720 h. Our results showed that in the presence of As(III) + Si or As(V) + Si, GR sulfate (GRSO4) formation from FHY was up to four times slower compared to single species system containing only As(III), As(V) or Si. Co-existing As(III) + Si and As(V) + Si also inhibited GRSO4 transformation to magnetite, contrary to systems with only Si or As(V). Overall, our findings demonstrate the synergistic inhibitory effect of co-existing Si on the crystallization and solid-phase stability of As-bearing GRSO4, establishing an inhibitory effect ladder: As(III) + Si > As(V) + Si > As(III) > Si > As(V). This further highlights the importance of GR in potentially controlling the fate and mobility of As in ferruginous, Si-rich groundwater and sediments such as those in South and Southeast Asia.

Nitrogen amended graphene catalyses fast reduction of vinyl chloride by nano zerovalent iron.

Vinyl chloride (VC) is a dominant carcinogenic residual in many aged chlorinated solvent plumes, and it remains a huge challenge to clean it up. Zerovalent iron (ZVI) is an effective reductant for many chlorinated compounds but shows low VC removal efficiency at field scale. Amendment of ZVI with a carbonaceous material may be used to both preconcentrate VC and facilitate redox reactions. In this study, nitrogen-doped graphene (NG) produced by a simple co-pyrolysis method using urea as nitrogen (N) source, was tested as a catalyst for VC reduction by nanoscale ZVI (nZVI). The extent of VC reduction to ethylene in the presence of 2 g/L of nZVI was less than 1% after 3 days, and barely improved with the addition of 4 g/L of graphene. In contrast, with amendment of nZVI with NG produced at pyrolysis temperature (PT) of 950 °C, the VC reduction extent increased more than 10-fold to 69%. The reactivity increased with NG PT increasing from 400 °C to an optimum at 950 °C, and it increased linearly with NG loadings. Interestingly, N dosage had little effect on reactivity if NG was produced at PT of 950 °C, while a positive correlation was observed for NG produced at PT of 600 °C. XPS and Raman analyses revealed that for NG produced at lower PT (<800 °C) mainly the content of pyridine-N-oxide (PNO) groups correlates with reactivity, while for NG produced at higher PT up to 950 °C, reactivity correlates mainly with N induced structural defects in graphene. The results of quenching and hydrogen yield experiments indicated that NG promote reduction of VC by storage of atomic hydrogen, thus increasing its availability for VC reduction, while likely also enabling electron transfer from nZVI to VC. Overall, these findings demonstrate effective chemical reduction of VC by a nZVI-NG composite, and they give insights into the effects of N doping on redox reactivity and hydrogen storage potential of carbonaceous materials.

Sulfidation extent of nanoscale zerovalent iron controls selectivity and reactivity with mixed chlorinated hydrocarbons in natural groundwater.

Sulfidated nanoscale zerovalent iron (S-nZVI) exhibits low anoxic oxidation and high reactivity towards many chlorinated hydrocarbons (CHCs). However, nothing is known about S-nZVI reactivity once exposed to complex CHC mixtures, a common feature of CHC plumes in the environment. Here, three S-nZVI materials with varying iron sulfide (mackinawite, FeSm) shell thickness and crystallinity were exposed to groundwater containing a complex mixture of chlorinated ethenes, ethanes, and methanes. CHC removal trends yielded pseudo-first order rate constants (kobs) that decreased in the order: trichloroethene > trans-dicloroethene > 1,1-dichlorethene > trichloromethane > tetrachloroethene > cis-dichloroethene > 1,1,2-trichloroethane, for all S-nZVI materials. These kobs trends showed no correlation with CHC reduction potentials based on their lowest unoccupied molecular orbital energies (ELUMO) but absolute values were affected by the FeSm shell thickness and crystallinity. In comparison, nZVI reacted with the same CHCs groundwater, yielded kobs that linearly correlated with CHC ELUMO values (R2 = 0.94) and that were lower than S-nZVI kobs. The CHC selectivity induced by sulfidation treatment is explained by FeSm surface sites having specific binding affinities towards some CHCs, while others require access to the metallic iron core. These new insights help advance S-nZVI synthesis strategies to fit specific CHC treatment scenarios.

A novel, direct-push approach for detecting sulfidated nanoparticulate zero valent iron (S-nZVI) in sediments using reactive and non-reactive fluorophores.

Injection of microparticulate and nanoparticulate zero valent iron has become a regularly used method for groundwater remediation. Because of subsurface inhomogeneities, however, it is complicated to predict the ZVI transport in the subsurface, meaning that tools capable of determining its distribution after injection are highly useful. Here, we have developed a new direct-push based technique, which combines fluorescent and visible imaging, for detection of sulfidized nanoparticulate zero valent iron (S-nZVI) in the subsurface. Laboratory experiments show that the redox sensitive fluorophore riboflavin is rapidly reduced by S-nZVI within 200 s. Because the reduced riboflavin losses its green fluorescence, it can be used as S-nZVI sensitive indicator. Secondly, S-nZVI is black and tints light coloured sediment to a degree that allows detection in images. For quartz sand, 70 mg/kg of S-nZVI can be detected by visible imaging. Based on these results, a new direct-push probe (Dye-OIP) was designed based on Geoprobe's Optical Image Profiler (OIP), which was equipped with a fluorophore injection port below the OIP-unit. The injectant consisted of the redox active riboflavin mixed with the redox inactive fluorophore rhodamine WT, which fluoresces red and was used to verify that the mixture was indeed injected and detectable. Small scale experiments show that the fluorescence of this mixture in S-nZVI amended sand changes within 150 s from green with a hue of ~50 to red with a hue of ~30 when imaged with Dye-OIP. Tests of the Dye-OIP after a S-nZVI injection in a 1 m3 sized tank show that the tool could detect S-nZVI via fluorescence and visible imaging, when S-nZVI concentration was >0.2 mg per g dry sediment. Thus, these novel methods should be able to detect S-nZVI in the subsurface, without relying on infrastructure such as wells. Based on our results, the Dye-OIP could be further improved to make it suitable for regular use in the field.

A density functional theory study of Fe(II)/Fe(III) distribution in single layer green rust: a cluster approach.

Green rust (GR) is a potentially important compound for the reduction of heavy metal and organic pollutants in subsurface environment because of its high Fe(II) content, but many details of the actual reaction mechanism are lacking. The reductive capacity distribution within GR is a key to understand how and where the redox reaction occurs and computational chemistry can provide more details about the electronic properties of green rust. We constructed three sizes of cluster models of single layer GR (i.e., without interlayer molecules or ions) and calculated the charge distribution of these structures using density functional theory. We found that the Fe(II) and Fe(III) are distributed unevenly in the single layer GR. Within a certain range of Fe(II)/Fe(III) ratios, the outer iron atoms behave more like Fe(III) and the inner iron atoms behave more like Fe(II). These findings indicate that the interior of GR is more reductive than the outer parts and will provide new information to understand the GR redox interactions.

Arsenic removal from natural groundwater using 'green rust': Solid phase stability and contaminant fate.

Arsenic (As) contamination in groundwater remains a pressing global challenge. In this study, we evaluated the potential of green rust (GR), a redox-active iron phase frequently occurring in anoxic environments, to treat As contamination at a former wood preservation site. We performed long-term batch experiments by exposing synthetic GR sulfate (GRSO4) to As-free and As-spiked (6 mg L-1) natural groundwater at both 25 and 4 °C. At 25 °C, GRSO4 was metastable in As-free groundwater and transformed to GRCO3, and then fully to magnetite within 120 days; however, GRSO4 stability increased 7-fold by lowering the temperature to 4 °C, and 8-fold by adding As to the groundwater at 25 °C. Highest GRSO4 stability was observed when As was added to the groundwater at 4 °C. This stabilizing effect is explained by GR solubility being lowered by adsorbed As and/or lower temperatures, inhibiting partial GR dissolution required for transformation to GRCO3, and ultimately to magnetite. Despite these mineral transformations, all added As was removed from As-spiked samples within 120 days at 25 °C, while uptake was 2 times slower at 4 °C. Overall, we have successfully documented that GR is an important mineral substrate for As immobilization in anoxic subsurface environments.

Immobilization of Cr(VI) by sulphate green rust and sulphidized nanoscale zerovalent iron in sand media: batch and column studies.

Chromate, Cr(VI), contamination in soil and groundwater poses serious threat to living organisms and environmental health worldwide. Sulphate green rust (GRSO4), a naturally occurring mixed-valent iron layered double hydroxide has shown to be highly effective in the reduction of Cr(VI) to poorly soluble Cr(III), giving promise for its use as reactant for in situ remedial applications. However, little is known about its immobilization efficiency inside porous geological media, such as soils and sediments, where this reactant would ultimately be applied. In this study, we tested the removal of Cr(VI) by GRSO4 in quartz sand fixed-bed column systems (diameter × length = 1.4 cm × 11 cm), under anoxic conditions. Cr(VI) removal efficiency (relative to the available reducing equivalents in the added GRSO4) was determined by evaluating breakthrough curves performed at different inlet Cr(VI) concentrations (0.125-1 mM) which are representative of Cr(VI) concentrations found at contaminated sites, different flow rates (0.25-3 ml/min) and solution pH (4.5, 7 and 9.5). Results showed that (i) increasing Cr(VI) inlet concentration substantially decreased Cr(VI) removal efficiency of GRSO4, (ii) flow rates had a lower impact on removal efficiencies, although values tended to be lower at higher flow rates, and (iii) Cr(VI) removal was enhanced at acidic pH conditions compared to neutral and alkaline conditions. For comparison, Cr(VI) removal by sulphidized nanoscale zerovalent iron (S-nZVI) in identical column experiments was substantially lower, indicating that S-nZVI reactivity with Cr(VI) is much slower compared to GRSO4. Overall, GRSO4 performed reasonably well, even at the highest tested flow rate, showing its versatility and suitability for Cr(VI) remediation applications in high flow environments.

Hydrotalcite stability during long-term exposure to natural environmental conditions.

Hydrotalcite-like compounds are a group of layered double hydroxides widely studied as sorbents to remove organic and inorganic contaminants under laboratory conditions. This study is a proof-of-concept of the long-term fate of hydrotalcite compounds under natural environmental conditions, to bridge the gap between laboratory studies and their field application as sorbents. Hydrotalcite (HT) with intercalated carbonate species (HT-CO3) and dodecyl sulphate (HT-DS) were synthesised and placed in two groundwater monitoring wells in Denmark, one contaminated with chlorinated hydrocarbons and another with uncontaminated groundwater. To assess the structural and surface compositional changes of hydrotalcite compounds upon prolonged exposure to groundwater, the material was analysed with powder X-ray diffraction (PXRD), Fourier-transformed infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The results showed that the stability and dissolution behaviour of hydrotalcite compounds under groundwater conditions depended on the intercalated anion (CO32- > DS) and groundwater dynamics (static flow > dynamic flow), while the hydrotalcite aggregate size only had a minor effect. Groundwater geochemistry influenced the precipitation of insoluble species (CaCO3, and adsorbed sulphate) on the hydrotalcite surface. The instability of hydrotalcite compounds, especially in the case of HT-DS, may constitute a significant limiting factor on their future application as sorbents under dynamic flow conditions.

Effects of metal cation substitution on hexavalent chromium reduction by green rust.

Chromium contamination is a serious environmental issue in areas affected by leather tanning and metal plating, and green rust sulfate has been tested extensively as a potential material for in situ chemical reduction of hexavalent chromium in groundwater. Reported products and mechanisms for the reaction have varied, most likely because of green rust's layered structure, as reduction at outer and interlayer surfaces might produce different reaction products with variable stabilities. Based on studies of Cr(III) oxidation by biogenic Mn (IV) oxides, Cr mobility in oxic soils is controlled by the solubility of the Cr(III)-bearing phase. Therefore, careful engineering of green rust properties, i.e., crystal/particle size, morphology, structure, and electron availability, is essential for its optimization as a remediation reagent. In the present study, pure green rust sulfate and green rust sulfate with Al, Mg and Zn substitutions were synthesized and reacted with identical chromate (CrO42-) solutions. The reaction products were characterized by X-ray diffraction, pair distribution function analysis, X-ray absorption spectroscopy and transmission electron microscopy and treated with synthetic δ-MnO2 to assess how easily Cr(III) in the products could be oxidized. It was found that Mg substitution had the most beneficial effect on Cr lability in the product. Less than 2.5% of the Cr(III) present in the reacted Mg-GR was reoxidized by δ-MnO2 within 14 days, and the particle structure and Cr speciation observed during X-ray scattering and absorption analyses of this product suggested that Cr(VI) was reduced in its interlayer. Reduction in the interlayer lead to the linkage of newly-formed Cr(III) to hydroxyl groups in the adjacent octahedral layers, which resulted in increased structural coherency between these layers, distinctive rim domains, sequestration of Cr(III) in insoluble Fe oxide bonding environments resistant to reoxidation and partial transformation to Cr(III)-substituted feroxyhyte. Based on the results of this study of hexavalent chromium reduction by green rust sulfate and other studies, further improvements can also be made to this remediation technique by reacting chromate with a large excess of green rust sulfate, which provides excess Fe(II) that can catalyze transformation to more crystalline iron oxides, and synthesis of the reactant under alkaline conditions, which has been shown to favor chromium reduction in the interlayer of Fe(II)-bearing phyllosilicates.

Bone Char Mediated Dechlorination of Trichloroethylene by Green Rust.

Biochars function as electron transfer mediators and thus catalyze redox transformations of environmental pollutants. A previous study has shown that bone char (BC) has high catalytic activity for reduction of chlorinated ethylenes using layered Fe(II)-Fe(III) hydroxide (green rust) as reductant. In the present study, we studied the rate of trichloroethylene (TCE) reduction by green rust in the presence of BCs obtained at pyrolysis temperatures (PTs) from 450 to 1050 °C. The reactivity increased with PT, yielding a maximum pseudo-first-order rate constant (k) of 2.0 h-1 in the presence of BC pyrolyzed at 950 °C, while no reaction was seen for BC pyrolyzed at 450 °C. TCE sorption, specific surface area, extent of graphitization, carbon content, and aromaticity of the BCs also increased with PT. The electron-accepting capacity (EAC) of BC peaked at PT of 850 °C, and EAC was linearly correlated with the sum of concentrations of quinoid, quaternary N, and pyridine-N-oxide groups measured by XPS. Moreover, no TCE reduction was seen with graphene nanoparticles and graphitized carbon black, which have high degrees of graphitization but low EAC values. Further analyses showed that TCE reduction rates are well correlated with the EAC and the C/H ratio (proxy of electrical conductivity) of the BCs, strongly indicating that both electron-accepting functional groups and electron-conducting domains are crucial for the BC catalytic reactivity. The present study delineates conditions for designing redox-reactive biochars to be used for remediation of sites contaminated with chlorinated solvents.

Direct Visualization of Arsenic Binding on Green Rust Sulfate.

"Green rust" (GR), a redox-active Fe(II)-Fe(III) layered double hydroxide, is a potential environmentally relevant mineral substrate for arsenic (As) sequestration in reduced, subsurface environments. GR phases have high As uptake capacities at circum-neutral pH conditions, but the exact interaction mechanism between the GR phases and As species is still poorly understood. Here, we documented the bonding and interaction mechanisms between GR sulfate and As species [As(III) and As(V)] under anoxic and circum-neutral pH conditions through scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray (EDX) spectroscopy and combined it with synchrotron-based X-ray total scattering, pair distribution function (PDF) analysis, and As K-edge X-ray absorption spectroscopy (XAS). Our highly spatially resolved STEM-EDX data revealed that the preferred adsorption sites of both As(III) and As(V) are at GR crystal edges. Combining this data with differential PDF and XAS allowed us to conclude that As adsorption occurs primarily as bidentate binuclear (2C) inner-sphere surface complexes. In the As(III)-reacted GR sulfate, no secondary Fe-As phases were observed. However, authigenic parasymplesite (ferrous arsenate nanophase), exhibiting a threadlike morphology, formed in the As(V)-reacted GR sulfate and acts as an additional immobilization pathway for As(V) (∼87% of immobilized As). We demonstrate that only by combining high-resolution STEM imaging and EDX mapping with the bulk (differential) PDF and extended X-ray absorption fine structure (EXAFS) data can one truly determine the de facto As binding nature on GR surfaces. More importantly, these new insights into As-GR interaction mechanisms highlight the impact of GR phases on As sequestration in anoxic subsurface environments.

Effects of common groundwater ions on the transformation and reactivity of sulfidized nanoscale zerovalent iron.

Sulfidized nanoscale zerovalent iron (S-nZVI) is an Fe-based reactant widely studied for its potential use for groundwater remediation. S-nZVI reactivity has been widely investigated testing various contaminants in various water matrices, but studies on S-nZVI corrosion behaviour and reactivity upon exposure to complex groundwater chemistries are limited. Here, we show that anoxic aging of S-nZVI for 7 days in the absence and presence of key groundwater solutes (i.e., Cl-, SO42-, Mg2+, Ca2+, HCO3-, CO32-, NO3-, or HPO42-) impacts Fe0 corrosion extent, corrosion product and reduction rates with trichloroethene (TCE). White rust was the dominant corrosion product in ultrapure water and in SO42-, Cl-, Mg2+ or Ca2+ solutions; green rust and/or chukanovite formed in HCO3- and CO32- solutions; magnetite, formed in NO3- solutions and vivianite in HPO42- solutions. The aged S-nZVI materials expectedly showed lower reactivities with TCE compared to unaged S-nZVI, with reaction rates mainly controlled by ion concentration, Fe0 corrosion extent, type(s) of corrosion product, and solution pH. Comparison of these results to observations in two types of groundwaters, one from a carbonate-rich aquifer and one from a marine intruded aquifer, showed that S-nZVI corrosion products are likely controlled by the dominant GW solutes, while reactivity with TCE is generally lower than expected, due to the multitude of ion effects. Overall, these results highlight that S-nZVI corrosion behaviour in GW can be manifold, with varied impact on its reactivity. Thus, testing of S-nZVI stability and reactivity under expected field conditions is key to understand its longevity in remediation applications.

Sorption of chlorinated hydrocarbons from synthetic and natural groundwater by organo-hydrotalcites: Towards their applications as remediation nanoparticles.

Chlorinated hydrocarbons (CHCs) are recalcitrant compounds frequently found as contaminants in groundwater. Hydrotalcites (HT) have emerged as promising sorbents due to their tunable properties and anion exchange capacity. Here, two types of organo-HT were synthesized, via coprecipitation, by intercalation of two different anionic surfactants, sodium dodecyl sulfate and sodium 1-dodecane sulfonate. These compounds were first characterized by a suite of techniques to quantify surfactant intercalation and to evaluate their physico-chemical properties. Next, the sorption affinity of these organo-HT towards a suite of CHCs was tested under various conditions, including interlayer surfactant type, single and multiple CHCs systems, and different water chemistry (pH, ionic composition). Sorption coefficients (Kd) and organic-matter-normalized partition coefficient (Kom) derived from linear sorption isotherms for individual CHC were inversely correlated to their hydrophobicity in the order of: tetrachloroethylene > tetrachloromethane > trichloroethylene> 1,1,2-trichloroethane > trichloromethane. Kom values were further affected by the organo-HT drying process. In contrast, varying water chemistry and pH, and the co-existence of multiple CHCs had little effect on Kom values, indicating that competition between CHCs and ionic strength have a marginal effect on the sorption affinity. The inverse linear relationship between CHC hydrophobicity and Kom is shown to be a suitable tool to predict organo-HT's sorption efficiency in complex CHCs contaminated groundwaters. Overall, organo-HT's might be used as potential sorbents for ex situ treatment of CHCs in groundwater.

The Structure of Sulfidized Zero-Valent Iron by One-Pot Synthesis: Impact on Contaminant Selectivity and Long-Term Performance.

Sulfidized zerovalent iron (sZVI) is widely studied because of its remarkable reactivity with a number of groundwater contaminants. Nonetheless, its nanoscale structure is not well understood. As such, there is an uncertainty on how sZVI structure controls its reactivity and fate in the subsurface environment. Using pair distribution function analyses, we show that sZVI made from one-pot synthesis using dithionite as sulfur precursor consists of an Fe0 core with a shell composed dominantly of short-range ordered Fe(OH)2 and FeS having coherent scattering domains of less than 8 Å. Reactivity experiments show that increasing shell material significantly decreases rate for cis-dichloroethene (cis-DCE) reduction, whereas the opposite is observed for trichloroethene (TCE). The results are consistent with a conceptual model wherein cis-DCE reduction requires active Fe0 sites, which become largely inaccessible when shell material is abundant. Conversely, an increase in FeS shell volume led to faster TCE reduction via direct electron transfer. Aging experiments showed that sZVI retained >50% of its TCE removal efficiency after 30-day exposure to artificial groundwaters. The decline in sZVI reactivity due to long-term exposure to groundwater, is attributed to Fe0 oxidation from water reduction coupled by reorganization and recrystallization of the poorly ordered shell material, which in turn reduced access to reactive FeS sites.

Mechanism of silica-lysozyme composite formation unravelled by in situ fast SAXS.

A quantitative understanding of aggregation mechanisms leading to the formation of composites of inorganic nanoparticles (NPs) and proteins in aqueous media is of paramount interest for colloid chemistry. In particular, the interactions between silica (SiO2) NPs and lysozyme (LZM) have attracted attention, because LZM is well-known to adsorb strongly to silica NPs, while at the same time preserving its enzymatic activity. The inherent nature of the aggregation processes leading to NP-LZM composites involves structural changes at length scales from few to at least hundreds of nanometres but also time scales much smaller than one second. To unravel these we used in situ synchrotron-based small-angle X-ray scattering (SAXS) and followed the subtle interparticle interactions in solution at a time resolution of 50 ms/frame (20 fps). We show that if the size of silica NPs (ca. 5 nm diameter) is matched by the dimensions of LZM, the evolving scattering patterns contain a unique structure-factor contribution originating from the presence of LZM. We developed a scattering model and applied it to analyse this structure function, which allowed us to extract structural information on the deformation of lysozyme molecules during aggregation, as well as to derive the mechanisms of composite formation.

A Silicate/Glycine Switch To Control the Reactivity of Layered Iron(II)-Iron(III) Hydroxides for Dechlorination of Carbon Tetrachloride.

Layered FeII-FeIII hydroxide chloride (chloride green rust, GRCl) has high reactivity toward reducible pollutants such as chlorinated solvents. However, this reactive solid is prone to dissolution, and hence loss of reactivity, during storage and handling. In this study, adsorption of silicate (Si) to GRCl was tested for its ability to minimize GRCl dissolution and to inhibit reduction of carbon tetrachloride (CT). Silicate adsorbed with high affinity to GRCl yielding a sorption maximum of 0.026 g of Si/g of GRCl. In the absence of Si, the pseudo-first-order rate constant for CT dehalogenation by GRCl was 2.1 h-1, demonstrating very high reactivity of GRCl but with substantial FeII dissolution up to 2.5 mM. When Si was adsorbed to GRCl, CT dehalogenation was blocked and FeII dissolution extent was reduced by a factor of 28. The addition of glycine (Gly) was tested for reactivation of the Si-blocked GRCl for CT dehalogenation. At 30 mM Gly, partial reactivation of the GRCl was observed with pseudo-first-order rate constant for CT reduction of 0.075 h-1. This blockage and reactivation of GRCl reactivity demonstrates that it is possible to design a switch for GRCl to control its stability and reactivity under anoxic conditions.

A field and modeling study of fractured rock permeability reduction using microbially induced calcite precipitation.

Microbially induced calcite precipitation (MICP) offers an attractive alternative to traditional grouting technologies for creating barriers to groundwater flow and containing subsurface contamination, but has only thus far been successfully demonstrated at the laboratory scale and predominantly in porous media. We present results of the first field experiments applying MICP to reduce fractured rock permeability in the subsurface. Initially, the ureolytic bacterium, Sporosarcina pasteurii, was fixed in the fractured rock. Subsequent injection of cementing fluid comprising calcium chloride and urea resulted in precipitation of large quantities (approximately 750 g) of calcite; significant reduction in the transmissivity of a single fracture over an area of several m(2) was achieved in around 17 h of treatment. A novel numerical model is also presented which simulates the field data well by coupling flow and bacterial and solute reactive transport processes including feedback due to aperture reduction via calcite precipitation. The results show that MICP can be successfully manipulated under field conditions to reduce the permeability of fractured rock and suggest that an MICP-based technique, informed by numerical models, may form the basis of viable solutions to aid pollution mitigation.

Bacterial diversity in five Icelandic geothermal waters: temperature and sinter growth rate effects.

The microbial ecology associated with siliceous sinters was studied in five geochemically diverse Icelandic geothermal systems. Bacterial 16S rRNA clone libraries were constructed from water-saturated precipitates from each site resulting in a total of 342 bacterial clone sequences and 43 species level phylotypes. In near-neutral, saline (2.6-4.7% salinity) geothermal waters where sinter growth varied between 10 and ~300 kg year(-1) m(-2), 16S rRNA gene analyses revealed very low (no OTUs could be detected) to medium (9 OTUs) microbial activity. The most dominant phylotypes found in these waters belong to marine genera of the Proteobacteria. In contrast, in alkaline (pH = 9-10), meteoric geothermal waters with temperature = 66-96°C and <1-20 kg year(-1)m(-2) sinter growth, extensive biofilms (a total of 34 OTUs) were observed within the waters and these were dominated by members of the class Aquificae (mostly related to Thermocrinis), Deinococci (Thermus species) as well as Proteobacteria. The observed phylogenetic diversity (i.e., number and composition of detected OTUs) is argued to be related to the physico-chemical regime prevalent in the studied geothermal waters; alkaliphilic thermophilic microbial communities with phylotypes related to heterotrophic and autotrophic microorganisms developed in alkaline high temperature waters, whereas halophilic mesophilic communities dominated coastal geothermal waters.

In-situ grown silica sinters in Icelandic geothermal areas.

Field in-situ sinter growth studies have been carried out in five geochemically very different Icelandic geothermal areas with the aim to quantify the effects of water chemistry, (e.g. silica content (250 to 695 p.p.m. SiO(2)), salinity (meteoric to seawater), pH (7.5 to 10)), temperature (42-96 degrees C) and microbial abundance (prevalence, density) on the growth rates, textures and structures of sinters forming within and around geothermal waters. At each location, sinter growth was monitored over time periods between 30 min and 25 months using glass slides that acted as precipitation substrates from which sinter growth rates were derived. In geothermal areas like Svartsengi and Reykjanes, subaqueous sinters developed rapidly with growth rates of 10 and 304 kg year(-1 )m(-2), respectively, and this was attributed primarily to the near neutral pH, high salinity and medium to high silica content within these geothermal waters. The porous and homogeneous precipitates that formed at these sites were dominated by aggregates of amorphous silica and they contained few if any microorganisms. At Hveragerdi and Geysir, the geothermal waters were characterized by slightly alkaline pH, low salinity and moderate silica contents, resulting in substantially lower rates of sinter growth (0.2-1.4 kg year(-1 )m(-2)). At these sites sinter formation was restricted to the vicinity of the air-water interface (AWI) where evaporation and condensation processes predominated, with sinter textures being governed by the formation of dense and heterogeneous crusts with well-defined spicules and silica terraces. In contrast, the subaqueous sinters at these sites were characterized by extensive biofilms, which, with time, became fully silicified and thus well preserved within the sinter edifices. Finally, at Krafla, the geothermal waters exhibited high sinter growth rates (19.5 kg year(-1 )m(-2)) despite being considerably undersaturated with respect to amorphous silica. However, the bulk of the sinter textures and structure were made up of thick silicified biofilms and this indicated that silica precipitation, i.e. sinter growth, was aided by the surfaces provided by the thick biofilms. These results further suggest that the interplay between purely abiotic processes and the ubiquitous presence of mesophilic and thermophilic microorganisms in modern silica rich terrestrial hydrothermal settings provides an excellent analogue for processes in Earth's and possibly Mars's ancient past.