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 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 the mineral-bound As species during the transformation of As(V)-FH to crystalline iron-bearing phases at circum-neutral pH conditions. At all Fe²⁺_(aq)/Fe(III)_solid ratios, goethite (GT), green rust sulfate (GR_SO4) and lepidocrocite (LP) formed within the first 2 h of the reaction. At low ratios (0.5 to 1), initially formed GR_SO4 and/or LP dissolved as the reaction progressed, and only goethite and some unreacted FH remained after 24 h. At Fe²⁺_(aq)/Fe(III)_solid ratio of 2, GR_SO4 remained stable throughout the 24 h reaction, alongside GT and unreacted FH. Despite the fact that majority of the starting As(V)-FH transformed to other phases, the initially adsorbed As was not released into the 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⁺ and goethite 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 dithiol functionalized UiO-66-(SH)₂is 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)₂meets all these criteria.

The valorization of coffee wastes through modification to activated carbon has been considered as a low-cost adsorbent with prospective to compete with commercial carbons. So far, very few studies have referred to the valorization of coffee parchment into activated carbon. Moreover, low-cost and efficient activation methods need to be more investigated. The aim of this work was to prepare activated carbon from spent coffee grounds and parchment, and to assess their adsorption performance. The co-calcination processing with calcium carbonate was used to prepare the activated carbons, and their adsorption capacity for organic acids, phenolic compounds and proteins was evaluated. Both spent coffee grounds and parchment showed yields after the calcination and washing treatments of around 9.0%. The adsorption of lactic acid was found to be optimal at pH 2. The maximum adsorption capacity of lactic acid with standard commercial granular activated carbon was 73.78 mg/g, while the values of 32.33 and 14.73 mg/g were registered for the parchment and spent coffee grounds activated carbons, respectively. The Langmuir isotherm showed that lactic acid was adsorbed as a monolayer and distributed homogeneously on the surface. Around 50% of total phenols and protein content from coffee wastewater were adsorbed after treatment with the prepared activated carbons, while 44, 43, and up to 84% of hydrophobic compounds were removed using parchment, spent coffee grounds and commercial activated carbon, respectively; the adsorption efficiencies of hydrophilic compounds ranged between 13 and 48%. Finally, these results illustrate the potential valorization of coffee by-products parchment and spent coffee grounds into activated carbon and their use as low-cost adsorbent for the removal of organic compounds from aqueous solutions.

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.