Quantification of rapid environmental redox processes with quick-scanning x-ray absorption spectroscopy (Q-XAS)

Abstract

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

The effects of iron(II) on the kinetics of arsenic oxidation and sorption on manganese oxides

“…Many important mineral surface processes (e.g., adsorption, oxidation-reduction, precipitation) are characterized by a rapid initial reaction on time scales of milliseconds to minutes in which a significant portion of the reaction process may occur [3,25]. In this study, we are also interested in knowing how Fe(II) could affect the initial As(III) oxidation rate by d-MnO 2 .…”

“…In environmental settings, manganese (Mn) oxide minerals commonly occur as coatings and fine-grained aggregates with large surface areas and exert chemical influences far out of proportion to their prevalence [2,3]. Manganese oxides minerals act as potent sorbents of heavy metals and nutrients, serving as natural sinks for environmental contaminants.…”

Real-time QEXAFS spectroscopy measures rapid precipitate formation at the mineral–water interface

“…Several researchers have used quick scanning X-ray absorption spectroscopy (Q-XAS) batch techniques to investigate redox reactions or iron oxide transformations [26][27][28][29][30] at the mineral-water interface, but there are no examples of Q-XAS to illustrate realtime adsorption/precipitation under dynamic/flow conditions. We combine a custom-built flow cell and Q-XAS 31 to study these sorption processes at the clay mineral-water interface in real-time, in situ and at the molecular scale.…”

Applications of synchrotron-based X-ray techniques in environmental science

“…Similarly, Mn-oxides are extremely significant in affecting oxidation of both inorganic and organic species in soils, as evidenced by Landrot et al [16]. They firstly quantified the initial kinetics of Cr III oxidation and As III on Mn-oxides, separately, using a quick scanning XAS method and calculated the chemical rate constants for the oxidation reaction during the first seconds [16,17].…”

“…Among the researchers, Brown [9,[12][13][14], Sparks [15][16][17] and Manceau [18][19][20] acquired much attention for their excellent works in the fields. Here we discuss some of the most mainstream applications of synchrotron-based X-ray spectroscopy and spectromicroscopy in environmental science.…”