Partitioning and Mobility of Chromium in Iron-Rich Laterites from an Optimized Sequential Extraction Procedure.

Chromium (Cr) leached from iron (Fe) (oxyhydr)oxide-rich tropical laterites can substantially impact downstream groundwater, ecosystems, and human health. However, its partitioning into mineral hosts, its binding, oxidation state, and potential release are poorly defined. This is in part due to the current lack of well-designed and validated Cr-specific sequential extraction procedures (SEPs) for laterites. To fill this gap, we have (i) first optimized a Cr SEP for Fe (oxyhydr)oxide-rich laterites using synthetic and natural Cr-bearing minerals and laterite references, (ii) used a complementary suite of techniques and critically evaluated existing non-laterite and non-Cr-optimized SEPs, compared to our optimized SEP, and (iii) confirmed the efficiency of our new SEP through analyses of laterites from the Philippines. Our results show that other SEPs inadequately leach Cr host phases and underestimate the Cr fractions. Our SEP recovered up to seven times higher Cr contents because it (a) more efficiently dissolves metal-substituted Fe phases, (b) quantitatively extracts adsorbed Cr, and (c) prevents overestimation of organic Cr in laterites. With this new SEP, we can estimate the mineral-specific Cr fractionation in Fe-rich tropical soils more quantitatively and thus improve our knowledge of the potential environmental impacts of Cr from lateritic areas.

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.

Temperature and organic carbon quality control the anaerobic carbon mineralization in peat profiles via modulating microbes: A case study of Changbai Mountain.

Peatlands account for a significant fraction of the global carbon stock. However, the complex interplay of abiotic and biotic factors governing anaerobic carbon mineralization in response to warming remains unclear. In this study, peat sediments were collected from a typical northern peatland-Changbai Mountain to investigate the behavior and mechanism of anaerobic carbon mineralization in response to depth (0-200 cm) and temperature (5 °C, 15 °C and 20 °C), by integrating geochemical and microbial analysis. Several indices including humification indexes (HI), aromaticity, and water extractable organic carbon (WEOC) components were applied to evaluate carbon quality, while 16S rRNA sequencing was used to measure microbial composition. Regardless of temperature, degradations of carbon quality and associated reduction in microbial abundance as well as diversity resulted in a decrease in anaerobic carbon mineralization (both CO2 and CH4) towards greater depth. Warming either from 5 °C to 15 °C or 20 °C significantly increased anaerobic carbon mineralization in all depth profiles by improving carbon availability. Enhanced carbon availabilities were mediated by the change in microbial composition (p < 0.01) and an increase in metabolic activities, which was particularly evident in the enhanced ß-glucosidase activity and microbial collaborations. A remarkable increase of over 10-fold in the relative abundance of the Geothrix genus was observed under warming. Overall, warming resulted in an enhanced contribution of CH4 emission and a higher ratio of hydrogenotrophic methanogenesis, as evidenced by carbon isotope fractionation factors. In addition, deep peat soils (>100 cm) with recalcitrant carbon demonstrated greater temperature sensitivity (Q10: ∼2.0) than shallow peat soils (Q10:∼1.2) when temperature increased from 15 °C to 20 °C. The findings of this study have significantly deepened our understanding for mechanisms of carbon quality and microbe-driven anaerobic carbon mineralization in peatlands under global warming.

Nucleation and Crystallization of Ferrous Phosphate Hydrate via an Amorphous Intermediate.

The fundamental processes of nucleation and crystallization are widely observed in systems relevant to material synthesis and biomineralization; yet most often, their mechanism remains unclear. In this study, we unravel the discrete stages of nucleation and crystallization of Fe3(PO4)2·8H2O (vivianite). We experimentally monitored the formation and transformation from ions to solid products by employing correlated, time-resolved in situ and ex situ approaches. We show that vivianite crystallization occurs in distinct stages via a transient amorphous precursor phase. The metastable amorphous ferrous phosphate (AFEP) intermediate could be isolated and stabilized. We resolved the differences in bonding environments, structure, and symmetric changes of the Fe site during the transformation of AFEP to crystalline vivianite through synchrotron X-ray absorption spectroscopy at the Fe K-edge. This intermediate AFEP phase has a lower water content and less distorted local symmetry, compared to the crystalline end product vivianite. Our combined results indicate that a nonclassical, hydration-induced nucleation and transformation driven by the incorporation and rearrangement of water molecules and ions (Fe2+ and PO43-) within the AFEP is the dominating mechanism of vivianite formation at moderately high to low vivianite supersaturations (saturation index ≤ 10.19). We offer fundamental insights into the aqueous, amorphous-to-crystalline transformations in the Fe2+-PO4 system and highlight the different attributes of the AFEP, compared to its crystalline counterpart.

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.

Deep weathering in the semi-arid Coastal Cordillera, Chile.

The weathering front is the boundary beneath Earth's surface where pristine rock is converted into weathered rock. It is the base of the "critical zone", in which the lithosphere, biosphere, and atmosphere interact. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones. Its depth and the degree of rock weathering are commonly linked to oxygen transport and fluid flow. By drilling into fractured igneous rock in the semi-arid climate zone of the Coastal Cordillera in Chile we found multiple weathering fronts of which the deepest is 76 m beneath the surface. Rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron oxidation and high porosity. Geophysical borehole measurements and chemical weathering indicators reveal more intense weathering where fracturing is extensive, and porosity is higher than in bedrock. Only the top 10 m feature a continuous weathering gradient towards the surface. We suggest that tectonic preconditioning by fracturing provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. Porosity was preserved throughout the weathering process, as secondary minerals were barely formed due to the low fluid flow.

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.

The interfacial reactivity of arsenic species with green rust sulfate (GRSO4).

Arsenic (As) contamination in groundwater is a significant health and environmental concern worldwide because of its wide distribution and toxicity. The fate and mobility of As is greatly influenced by its interaction with redox-active mineral phases, among which green rust (GR), an FeII-FeIII layered double hydroxide mineral, plays a crucial role. However, the controlling parameters of As uptake by GR are not yet fully understood. To fill this gap, we determined the interfacial reactions between GR sulfate (GRSO4) and aqueous inorganic As(III) and As(V) through batch adsorption experiments, under environmentally-relevant groundwater conditions. Our data showed that, under anoxic conditions, GRSO4 is a stable and effective mineral adsorbent for the removal of As(III) and As(V). At an initial concentration of 10 mg L-1, As(III) removal was higher at alkaline pH conditions (~95% removal at pH 9) while As(V) was more efficiently removed at near-neutral conditions (>99% at pH 7). The calculated maximum As adsorption capacities on GRSO4 were 160 mg g-1 (pH 8-9) for As(III) and 105 mg g-1 (pH 7) for As(V). The presence of other common groundwater ions such as Mg2+ and PO43- reduces the efficiency of As removal, especially at high ionic strengths. Long-term batch adsorption experiments (up to 90 days) revealed that As-interacted GRSO4 remained stable, with no mineral transformation or release of adsorbed As species. Overall, our work shows that GRSO4 is one of the most effective As adsorbents among iron (oxyhydr)oxide phases.

Removal of arsenic and mercury species from water by covalent triazine framework encapsulated γ-Fe2O3 nanoparticles.

The covalent triazine framework, CTF-1, served as host material for the in situ synthesis of Fe2O3 nanoparticles. The composite material consisted of 20 ±â€¯2 m% iron, mainly in γ-Fe2O3 phase. The resulting γ-Fe2O3@CTF-1 was examined for the adsorption of AsIII, AsV and HgII from synthetic solutions and real surface-, ground- and wastewater. The material shows excellent removal efficiencies, independent from the presence of Ca2+, Mg2+ or natural organic matter and only limited dependency on the presence of phosphate ions. Its adsorption capacity towards arsenite (198.0 mg g-1), arsenate (102.3 mg g-1) and divalent mercury (165.8 mg g-1) belongs amongst the best-known adsorbents, including many other iron-based materials. Regeneration of the adsorbent can be achieved for use over multiple cycles without a decrease in performance by elution at 70 °C with 0.1 M NaOH, followed by a stirring step in a 5 m% H2O2 solution for As or 0.1 M thiourea and 0.001 M HCl for Hg. In highly contaminated water (100 µg L-1), the adsorbent polishes the water quality to well below the current WHO limits.

UiO-66-(SH)2 as stable, selective and regenerable adsorbent for the removal of mercury from water under environmentally-relevant conditions.

The dithiol functionalized UiO-66-(SH)2 is developed as an efficient adsorbent for the removal of mercury in aqueous media. Important parameters for the application of MOFs in real-life circumstances include: stability and recyclability of the adsorbents, selectivity for the targeted Hg species in the presence of much higher concentrations of interfering species, and ability to purify wastewater below international environmental limits within a short time. We show that UiO-66-(SH)2 meets all these criteria.