This study elucidates the structural changes of sulfidized zerovalent iron (S-nZVI) in anoxic groundwater, presents a compelling evidence on the structural robustness of the material and explains how S-nZVI long-term reactivity is achieved.

GR SO4 is among the best performing Febearing minerals for As removal.• Both pH and the presence of competing aqueous ions affect As removal efficiency. • Long term experiments showed that GR SO4 with As adsorbed onto its surface is stable for up to 90 days. • High-resolution electron microscopy revealed that As is preferentially adsorbed at the GR particle edges.Editor: José Virgílio Cruz 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 Fe II -Fe III 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 (GR SO4 ) and aqueous inorganic As(III) and As(V) through batch adsorption experiments, under environmentally-relevant groundwater conditions. Our data showed that, under anoxic conditions, GR SO4 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 (N99% at pH 7). The calculated maximum As adsorption capacities on GR SO4 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 Mg 2+ and PO 4 3− reduces the efficiency of As removal, especially at high ionic strengths. Long-term batch adsorption experiments (up to 90 days) revealed that As-interacted GR SO4 remained stable, with no mineral transformation or release of adsorbed As species. Overall, our work shows that GR SO4 is one of the most effective As adsorbents among iron (oxyhydr)oxide phases.

No abstract

The dithiol functionalized UiO-66-(SH) 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) meets all these criteria.

Iron (oxyhydr)­oxides play an important role in controlling the mobility and toxicity of arsenic (As) in contaminated soils and groundwaters. Dynamic changes in subsurface geochemical conditions can impact As sequestration and remobilization since the fate of As is highly dependent on the dominant iron mineral phases present and, specifically, the pathways through which these form or transform. To assess the fate of arsenate [As­(V)] in subsurface settings, we have investigated the Fe2+-induced transformation of As­(V)-bearing ferrihydrite (As­(V)-FH) to more crystalline phases under environmentally relevant anoxic subsurface conditions. Specifically, we examined the influence of varying Fe2+ (aq)/Fe­(III)solid ratios (0.5, 1, 2) on the behavior and speciation of mineral-bound As species during the transformation of As­(V)-FH to crystalline iron-bearing phases at circumneutral pH conditions. At all Fe2+ (aq)/Fe­(III)solid ratios, goethite (GT), green rust sulfate (GRSO4), and lepidocrocite (LP) formed within the first 2 h of reaction. At low ratios (0.5 to 1), initially formed GRSO4 and/or LP dissolved as the reaction progressed, and only GT and some unreacted FH remained after 24 h. At Fe2+ (aq)/Fe­(III)solid ratio of 2, GRSO4 remained stable throughout the 24 h of reaction, alongside GT and unreacted As­(V)-FH. Despite the fact that majority of the starting As­(V)-FH transformed to other phases, the initially adsorbed As was not released into solution during the transformation reactions, and ∼99.9% of it remained mineral-bound. Nevertheless, the initial As­(V) became partially reduced to As­(III), most likely because of the surface-associated Fe2+-GT redox couple. The extent of As­(V) reduction increased from ∼34% to ∼40%, as the Fe2+ (aq)/Fe­(III)solid ratio increased from 0.5 to 2. Overall, our results provide important insights into transformation pathways of iron (oxyhydr)­oxide minerals in As contaminated, anoxic soils and sediments and demonstrate the impact that such transformations can have on As mobility and also importantly oxidation state and, hence, toxicity in these environments.

The covalent triazine framework, CTF-1, served as host material for the in situ synthesis of FeO nanoparticles. The composite material consisted of 20 ± 2 m% iron, mainly in γ-FeO phase. The resulting γ-FeO@CTF-1 was examined for the adsorption of As, As and Hg from synthetic solutions and real surface-, ground- and wastewater. The material shows excellent removal efficiencies, independent from the presence of Ca, Mg or natural organic matter and only limited dependency on the presence of phosphate ions. Its adsorption capacity towards arsenite (198.0 mg g), arsenate (102.3 mg g) and divalent mercury (165.8 mg g) 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% HO solution for As or 0.1 M thiourea and 0.001 M HCl for Hg. In highly contaminated water (100 μg L), the adsorbent polishes the water quality to well below the current WHO limits.

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.

No abstract