Phosphogypsum impacts on soil chemical properties and vegetation tissue following reclamation.

Phosphogypsum (PG) is a by-product of phosphorus fertilizer that is typically stacked near production sites. Phosphogypsum contains trace elements and naturally occurring radioactive materials which may be hazardous to the surrounding environment. Phosphogypsum stack reclamation typically involves placing a soil cap and seeding grass to create a barrier for reducing environmental impacts; using woody species is uncommon. This study used three soil treatments with grass and woody species to determine whether mixing PG with soil affects soil chemical properties, and metal and radionuclide concentrations in tissue. None of the elements in soil was above Canadian guidelines for industrial land use. Aluminum, beryllium, chromium, copper, iron, magnesium, manganese, nickel, and vanadium were significantly higher in both study and reference sites than in pure PG; cadmium, calcium, fluoride, and strontium were significantly higher in pure PG. There was a poor correlation between soil and plant concentrations for most elements indicating trace elements were not in a bioavailable form. Trace elemental concentrations in plant tissue generally differed significantly with vegetation type but not within similar species. Trace elements and isotopes in PG were not high enough to affect plant growth. Among the isotopes, 222Ra emissions differed significantly with vegetation covers; activity of 226Ra in pure PG was above Canadian guidelines, but lower in vegetation tissue. This study suggests 15 cm soil mixed with PG can be used for PG stack revegetation when fast-growing Salix and Populus species are used in reclamation.

A Voltammetric Perspective of Multi-Electron and Proton Transfer in Protein Redox Chemistry: Insights From Computational Analysis of Escherichia coli HypD Fourier Transformed Alternating Current Voltammetry.

This paper explores the impact of pH on the mechanism of reversible disulfide bond (CysS-SCys) reductive breaking and oxidative formation in Escherichia coli hydrogenase maturation factor HypD, a protein which forms a highly stable adsorbed film on a graphite electrode. To achieve this, low frequency (8.96 Hz) Fourier transformed alternating current voltammetric (FTACV) experimental data was used in combination with modelling approaches based on Butler-Volmer theory with a dual polynomial capacitance model, utilizing an automated two-step fitting process conducted within a Bayesian framework. We previously showed that at pH 6.0 the protein data is best modelled by a redox reaction of two separate, stepwise one-electron, one-proton transfers with slightly "crossed" apparent reduction potentials that incorporate electron and proton transfer terms ( E app 2 0 > E app 1 0 ). Remarkably, rather than collapsing to a concerted two-electron redox reaction at more extreme pH, the same two-stepwise one-electron transfer model with E app 2 0 > E app 1 0 continues to provide the best fit to FTACV data measured across a proton concentration range from pH 4.0 to pH 9.0. A similar, small level of crossover in reversible potentials is also displayed in overall two-electron transitions in other proteins and enzymes, and this provides access to a small but finite amount of the one electron reduced intermediate state.

Using Purely Sinusoidal Voltammetry for Rapid Inference of Surface-Confined Electrochemical Reaction Parameters.

Alternating current (AC) voltammetric techniques are experimentally powerful as they enable Faradaic current to be isolated from non-Faradaic contributions. Finding the best global fit between experimental voltammetric data and simulations based on reaction models requires searching a substantial parameter space at high resolution. In this paper, we estimate parameters from purely sinusoidal voltammetry (PSV) experiments, investigating the redox reactions of a surface-confined ferrocene derivative. The advantage of PSV is that a complete experiment can be simulated relatively rapidly, compared to other AC voltammetric techniques. In one example involving thermodynamic dispersion, a PSV parameter inference effort requiring 7,500,000 simulations was completed in 7 h, whereas the same process for our previously used technique, ramped Fourier transform AC voltammetry (ramped FTACV), would have taken 4 days. Using both synthetic and experimental data with a surface confined diazonium substituted ferrocene derivative, it is shown that the PSV technique can be used to recover the key chemical and physical parameters. By applying techniques from Bayesian inference and Markov chain Monte Carlo methods, the confidence, distribution, and degree of correlation of the recovered parameters was visualized and quantified.