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.

Mesoporous phosphates are a group of nanostructured materials with promising applications, particularly in biomedicine and catalysis. However, their controlled synthesis via conventional template-based routes presents a number of challenges and limitations. Here, we show how to synthesize a mesoporous magnesium phosphate with a high surface area and a well-defined pore structure through thermal decomposition of a crystalline struvite (MgNH₄PO₄·6H₂O) precursor. In a first step, struvite crystals with various morphologies and sizes, ranging from a few micrometers to several millimeters, had been synthesized from supersaturated aqueous solutions (saturation index (SI) between 0.5 and 4) at ambient pressure and temperature conditions. Afterwards, the crystals were thermally treated at 70–250 °C leading to the release of structurally bound water (H₂O) and ammonia (NH₃). By combining thermogravimetric analyses (TGA), scanning and transmission electron microscopy (SEM, TEM), N₂ sorption analyses and small- and wide-angle X-ray scattering (SAXS/WAXS) we show that this decomposition process results in a pseudomorphic transformation of the original struvite into an amorphous Mg-phosphate. Of particular importance is the fact that the final material is characterized by a very uniform mesoporous structure with 2–5 nm wide pore channels, a large specific surface area of up to 300 m² g⁻¹ and a total pore volume of up to 0.28 cm³ g⁻¹. Our struvite decomposition method is well controllable and reproducible and can be easily extended to the synthesis of other mesoporous phosphates. In addition, the so produced mesoporous material is a prime candidate for use in biomedical applications considering that magnesium phosphate is a widely used, non-toxic substance that has already shown excellent biocompatibility and biodegradability.

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.