Quantification of the initial rates of environmental reactions at the mineral/water interface is a fundamental prerequisite to determining reaction mechanisms and contaminant transport modeling and predicting environmental risk. Until recently, experimental techniques with adequate time resolution and elemental sensitivity to measure initial rates of the wide variety of environmental reactions were quite limited. Techniques such as electron paramagnetic resonance and Fourier transform infrared spectroscopies suffer from limited elemental specificity and poor sensitivity to inorganic elements, respectively. Ex situ analysis of batch and stirred-flow systems provides high elemental sensitivity; however, their time resolution is inadequate to characterize rapid environmental reactions. Here we apply quick-scanning x-ray absorption spectroscopy (Q-XAS), at sub-second time-scales, to measure the initial oxidation rate of As(III) to As(V) by hydrous manganese(IV) oxide. Using Q-XAS, As(III) and As(V) concentrations were determined every 0.98 s in batch reactions. The initial apparent As(III) depletion rate constants (t < 30 s) measured with Q-XAS are nearly twice as large as rate constants measured with traditional analytical techniques. Our results demonstrate the importance of developing analytical techniques capable of analyzing environmental reactions on the same time scale as they occur. Given the high sensitivity, elemental specificity, and time resolution of Q-XAS, it has many potential applications. They could include measuring not only redox reactions but also dissolution/precipitation reactions, such as the formation and/or reductive dissolution of Fe(III) (hydr)oxides, solid-phase transformations (i.e., formation of layereddouble hydroxide minerals), or almost any other reaction occurring in aqueous media that can be measured using x-ray absorption spectroscopy.arsenic ͉ extended x-ray absorption fine structure ͉ manganese oxide ͉ x-ray absorption near-edge structure ͉ kinetics

BackgroundManganese-oxides are one of the most important minerals in soil due to their widespread distribution and high reactivity. Despite their invaluable role in cycling many redox sensitive elements, numerous unknowns remain about the reactivity of different manganese-oxide minerals under varying conditions in natural systems. By altering temperature, pH, and concentration of arsenite we were able to determine how manganese-oxide reactivity changes with simulated environmental conditions. The interaction between manganese-oxides and arsenic is particularly important because manganese can oxidize mobile and toxic arsenite into more easily sorbed and less toxic arsenate. This redox reaction is essential in understanding how to address the global issue of arsenic contamination in drinking water.ResultsThe reactivity of manganese-oxides in ascending order is random stacked birnessite, hexagonal birnessite, biogenic manganese-oxide, acid birnessite, and δ-MnO2. Increasing temperature raised the rate of oxidation. pH had a variable effect on the production of arsenate and mainly impacted the sorption of arsenate on δ-MnO2, which decreased with increasing pH. Acid birnessite oxidized the most arsenic at alkaline and acidic pHs, with decreased reactivity towards neutral pH. The δ-MnO2 showed a decline in reactivity with increasing arsenite concentration, while the acid birnessite had greater oxidation capacity under higher concentrations of arsenite. The batch reactions used in this study quantify the impact of environmental variances on different manganese-oxides’ reactivity and provide insight to their roles in governing chemical cycles in the Critical Zone.ConclusionsThe reactivity of manganese-oxides investigated was closely linked to each mineral’s crystallinity, surface area, and presence of vacancy sites. δ-MnO2 and acid birnessite are thought to be synthetic representatives of naturally occurring biogenic manganese-oxides; however, the biogenic manganese-oxide exhibited a lag time in oxidation compared to these two minerals. Reactivity was clearly linked to temperature, which provides important information on how these minerals react in the subsurface environment. The pH affected oxidation rate, which is essential in understanding how manganese-oxides react differently in the environment and their potential role in remediating contaminated areas. Moreover, the contrasting oxidative capacity of seemingly similar manganese-oxides under varying arsenite concentrations reinforces the importance of each manganese-oxide mineral’s unique properties.

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An in‐depth technical examination and conservation treatment of paintings by William Williams (Bristol 1727–1791 Bristol) has shed light on the artist's materials and technique. This investigation centered primarily on Williams's two 1766 portraits of William and David Hall. The paintings are considered the earliest life‐sized, full‐length portraits executed in the Philadelphia area. The analysis of the artist's palette indicated deliberate choices in the use of orpiment (As2S3). The mineral's tendency to oxidize to colorless and water‐soluble arsenic oxides likely caused color changes and degraded organic binder in the orpiment‐rich areas. μ‐XANES revealed orpiment photodegradation to arsenate species at the paint surface, with migration to the ground layers. Just below the paint surface, arsenic remains bound primarily as arsenite, with some associated with sulfur as orpiment. This As distribution suggests that the paint is liable to further degradation by photooxidation and use of moisture would be detrimental. Given this treatment‐critical degradation phenomenon, it was important to identify all arsenic‐containing areas of both portraits. Scanning XRF allowed rapid and accurate collection of maps from both portraits. Elemental maps of arsenic identified the orpiment‐rich areas of the painting, which would be susceptible to further degradation upon exposure to water during treatment. An aqueous adhesive was necessary to consolidate the cupped paint of the glue‐paste lined paintings. The arsenic maps guided the use of two different consolidants–BEVA 371 for the water‐sensitive orpiment‐rich paint and sturgeon glue for all other areas, striking a compromise between esthetic improvement and long term preservation.