MINFIT: A Spreadsheet-Based Tool for Parameter Estimation in an Equilibrium Speciation Software Program

Determination of equilibrium constants describing chemical reactions in the aqueous phase and at solid–water interface relies on inverse modeling and parameter estimation. Although there are existing tools available, the steep learning curve prevents the wider community of environmental engineers and chemists to adopt those tools. Stemming from classical chemical equilibrium codes, MINEQL+ has been one of the most widely used chemical equilibrium software programs. We developed a spreadsheet-based tool, which we are calling MINFIT, that interacts with MINEQL+ to perform parameter estimations that optimize model fits to experimental data sets. MINFIT enables automatic and convenient screening of a large number of parameter sets toward the optimal solutions by calling MINEQL+ to perform iterative forward calculations following either exhaustive equidistant grid search or randomized search algorithms. The combined use of the two algorithms can securely guide the searches for the global optima. We developed interactive interfaces so that the optimization processes are transparent. Benchmark examples including both aqueous and surface complexation problems illustrate the parameter estimation and associated sensitivity analysis. MINFIT is accessible at http://minfit.strikingly.com

Effects of Mn(II) on UO₂ Dissolution under Anoxic and Oxic Conditions

Groundwater composition and coupled redox cycles can affect the long-term stability of U­(IV) products from bioremediation. The effects of Mn­(II), a redox active cation present at uranium-contaminated sites, on UO₂ dissolution in both oxic and anoxic systems were investigated using batch and continuous-flow reactors. Under anoxic conditions Mn­(II) inhibited UO₂ dissolution, which was probably due to adsorption of Mn­(II) and precipitation of MnCO₃ that decreased exposure of U­(IV) surface sites to oxidants. In contrast, Mn­(II) promoted UO₂ dissolution under oxic conditions through Mn redox cycling. Oxidation of Mn­(II) by O₂ produced reactive Mn species, possibly short-lived Mn­(III) in solution or at the surface, that oxidatively dissolved the UO₂ more rapidly than could the O₂ alone. At pH 8 the Mn cycling was such that there was no measurable accumulation of particulate Mn oxides. At pH 9 Mn oxides could be produced and accumulate, while they were continuously reduced by UO₂, with Mn­(II) returning to the aqueous phase. With the rapid turnover of Mn in the redox cycle, concentrations of Mn as low as 10 μM could maintain an enhanced UO₂ dissolution rate. The presence of the siderophore desferrioxamine B (a strong Mn­(III)-complexing ligand) effectively decoupled the redox interactions of uranium and manganese to suppress the promotional effect of Mn­(II)

X‑ray Absorption Spectroscopic Quantification and Speciation Modeling of Sulfate Adsorption on Ferrihydrite Surfaces

Sulfate adsorption on mineral surfaces is an important environmental chemical process, but the structures and respective contribution of different adsorption complexes under various environmental conditions are unclear. By combining sulfur K-edge XANES and EXAFS spectroscopy, quantum chemical calculations, and surface complexation modeling (SCM), we have shown that sulfate forms both outer-sphere complexes and bidentate–binuclear inner-sphere complexes on ferrihydrite surfaces. The relative fractions of the complexes vary with pH, ionic strength (<i>I</i>), and sample hydration degree (wet versus air-dried), but their structures remained the same. The inner-sphere complex adsorption loading decreases with increasing pH while remaining unchanged with <i>I</i>. At both <i>I</i> = 0.02 and 0.1 M, the outer-sphere complex loading reaches maximum at pH ∼5 and then decreases with pH, whereas it monotonically decreases with pH at <i>I</i> = 0.5 M. These observations result from a combination of the ionic-strength effect, the pH dependence of anion adsorption, and the competition between inner- and outer-sphere complexation. Air-drying drastically converts the outer-sphere complexes to the inner-sphere complexes. The respective contributions to the overall adsorption loading of the two complexes were directly modeled with the extended triple layer SCM by implementing the bidentate–binuclear inner-sphere complexation identified in the present study. These findings improve our understanding of sulfate adsorption and its effects on other environmental chemical processes and have important implications for generalizing the adsorption behavior of anions forming both inner- and outer-sphere complexes on mineral surfaces

Oxidative UO₂ Dissolution Induced by Soluble Mn(III)

The stability of UO₂ is critical to the success of reductive bioremediation of uranium. When reducing conditions are no longer maintained, Mn redox cycling may catalytically mediate the oxidation of UO₂ and remobilization of uranium. Ligand-stabilized soluble Mn­(III) was recently recognized as an important redox-active intermediate in Mn biogeochemical cycling. This study evaluated the kinetics of oxidative UO₂ dissolution by soluble Mn­(III) stabilized by pyrophosphate (PP) and desferrioxamine B (DFOB). The Mn­(III)–PP complex was a potent oxidant that induced rapid UO₂ dissolution at a rate higher than that by a comparable concentration of dissolved O₂. However, the Mn­(III)–DFOB complex was not able to induce oxidative dissolution of UO₂. The ability of Mn­(III) complexes to oxidize UO₂ was probably determined by whether the coordination of Mn­(III) with ligands allowed the attachment of the complexes to the UO₂ surface to facilitate electron transfer. Systematic investigation into the kinetics of UO₂ oxidative dissolution by the Mn­(III)–PP complex suggested that Mn­(III) could directly oxidize UO₂ without involving particulate Mn species (e.g., MnO₂). The expected 2:1 reaction stoichiometry between Mn­(III) and UO₂ was observed. The reactivity of soluble Mn­(III) in oxidizing UO₂ was higher at lower ratios of pyrophosphate to Mn­(III) and lower pH, which is probably related to differences in the ligand-to-metal ratio and/or protonation states of the Mn­(III)–pyrophosphate complexes. Disproportionation of Mn­(III)–PP occurred at pH 9.0, and the oxidation of UO₂ was then driven by both MnO₂ and soluble Mn­(III). Kinetic models were derived that provided excellent fits of the experimental results

Measurement and Modeling of U(IV) Adsorption to Metal Oxide Minerals

Chemical or biological reduction of U­(VI) produces a variety of poorly soluble U­(IV) species. In addition to uraninite (UO₂) and biomass-associated noncrystalline U­(IV), recent research has found adsorbed U­(IV) species on mineral surfaces. To build on these observations, we evaluated equilibrium adsorption of U­(IV) to magnetite and rutile as a function of pH and total U­(IV) loading. Surface complexation models that could successfully simulate the uptake of U­(IV) by accounting for UO₂ precipitation and adsorption of U­(IV) to both the minerals and the reactor surfaces were developed. Application of the models could determine the conditions under which adsorption as opposed to precipitation would dominate U­(IV) uptake with solids. The model-predicted U­(IV) surface coverages of the minerals were consistent with a recent spectroscopic study. Such models advance our ability to predict the equilibrium speciation of U­(IV) in the subsurface

Synergistic Effects between Biogenic Ligands and a Reductant in Fe Acquisition from Calcareous Soil

Organisms have developed different strategies to cope with environmental conditions of low Fe availability based on the exudation of reducing, ligating, and acidifying compounds. In the context of Fe acquisition from soil, the effects of these reactive compounds have generally been considered independent and additive. However, highly efficient Fe acquisition strategies may rely on synergistic effects between reactive exudates. In the present study, we demonstrate that synergistic effects between biogenic ligands and a reductant (ascorbate) can occur in Fe mobilization from soil. Synergistic Fe mobilization was found for all ligands examined (desferrioxamine B (DFOB), 2′-deoxymugineic acid (DMA), esculetin, and citrate). The size and duration of the synergistic effect on Fe mobilization varied with ligand: larger effects were observed for the sideorphores compared to esculetin and citrate. For DFOB, the synergistic effect lasted for the 168 h duration of the experiment; for DMA, an initial synergistic effect turned into an antagonistic effect after 4 h because of enhanced mobilization of competing metals; and for esculetin and citrate, the synergistic effect was temporary (less than 24 h). Our results demonstrate that synergistic effects greatly enhance the reactivity of mixtures of compounds known to be exuded in response to Fe limitation. These synergistic effects could be decisive for the survival of plants and microorganisms under conditions of low Fe availability

Sulfolane Crystal Templating: A One-Step and Tunable Polarity Approach for Self-Assembled Super-Macroporous Hydrophobic Monoliths

Freeze-casting (ice templating) is generally used to prepare super-macroporous materials. However, water solubility limits the application of freeze-casting in hydrophobic material fabrication. In the present work, inexpensive and low-toxic sulfolane was used as a novel crystallization-induced porogen (sulfolane crystal templating) to prepare super-macroporous hydrophobic monoliths (cryogels) with tunable polarity. The phase transition of sulfolane consisted of reversible processes in the liquid, semi-crystalline, and crystalline states. Because of the density change during phase transition, liquid sulfolane experienced a 16.4% volume shrinkage per unit mass. Thus, the cryogels obtained using the conventional freezing method contained obvious hollow-shaped defects. Furthermore, a novel route of pre-cooling, pre-crystallization, crystal growth, freezing, and thawing (PPCFT) was employed to prepare cryogels with defect-free macroscopic morphology and uniform pore structure. The as-obtained cryogels were composed of a super-macroporous structures and interconnected channels, and their porosity ranged between 85 and 97%. Moreover, the cryogels manifested good hydrophobicity (contact angle = 120–130°) and had absorption capacities greater than 10 g g–1 for oils and organic liquids. The maximum absorption capacities of the resultant cryogels in dichloromethane, ethyl acetate, and liquid paraffin were 60.3, 35.8, and 15.2 g g–1, respectively. Moreover, sulfolane could conveniently dissolve hydrophobic and hydrophilic monomers to generate amphiphilic cryogels (contact angle = 130–0°). Therefore, sulfolane crystal templating is a potential fabrication method for super-macroporous hydrophobic materials with tunable polarity

Microplastic Emission from Soil-Air Interface

To bridge the gap in understanding soil–air microplastic emissions, here we studied the soil–air transfer mechanism of microplastics with laboratory simulations using microplastic particles varying in size from 1 to 5000 μm. Our findings indicate that the size and shape of the microplastics together with the soil type and moisture levels are major determinants of the microplastic enrichment ratio (ER) in dust, overshadowing the role of polymer types. Notably, microplastic pellets larger than 20 μm do not enrich in dust, while microplastic fibers, even those as large as 500–5000 μm in length, do enrich in dust. Based on our ER measurement, the global microplastic size distribution in soil and dust aerosol production flux, we made a preliminary bottom-up assessment of global soil microplastic emissions of 48[0.4–1217] kilotons/year. Our pioneering findings on the global atmospheric soil–air microplastic flux provide insights that could guide the future development of a more precise microplastic emission inventory

Synergistic Effect of Reductive and Ligand-Promoted Dissolution of Goethite

Ligand-promoted dissolution and reductive dissolution of iron (hydr)­oxide minerals control the bioavailability of iron in many environmental systems and have been recognized as biological iron acquisition strategies. This study investigated the potential synergism between ligands (desferrioxamine B (DFOB) or <i>N,N</i>′-Di­(2-hydroxybenzyl)­ethylenediamine-<i>N,N</i>′-diacetic acid (HBED)) and a reductant (ascorbate) in goethite dissolution. Batch experiments were performed at pH 6 with ligand or reductant alone and in combination, and under both oxic and anoxic conditions. Goethite dissolution in the presence of reductant or ligand alone followed classic surface-controlled dissolution kinetics. Ascorbate alone does not promote goethite dissolution under oxic conditions due to rapid reoxidation of Fe­(II). The rate coefficients for goethite dissolution by ligands are closely correlated with the stability constants of the aqueous Fe­(III)–ligand complexes. A synergistic effect of DFOB and ascorbate on the rate of goethite dissolution was observed (total rates greater than the sum of the individual rates), and this effect was most pronounced under oxic conditions. For HBED, macroscopically the synergistic effect was hidden due to the inhibitory effect of ascorbate on HBED adsorption. After accounting for the concentrations of adsorbed ascorbate and HBED, a synergistic effect could still be identified. The potential synergism between ligand and reductant for iron (hydr)­oxide dissolution may have important implications for iron bioavailability in soil environments

Quantification of Coexisting Inner- and Outer-Sphere Complexation of Sulfate on Hematite Surfaces

Sulfate adsorption on hematite surfaces controls sulfate mobility and environmental behavior but whether sulfate forms both inner- and outer-sphere complexes and the type of the inner-sphere complexes remain contentious. With ionic strength tests and S K-edge X-ray absorption near-edge structure spectroscopy, we show that sulfate forms both outer- and inner-sphere complexes on hematite surfaces. Both S K-edge extended X-ray absorption fine structure spectroscopy and the differential pair distribution function analyses determine the S–Fe interatomic distance (∼3.24 Å) of the inner-sphere complex, suggesting bidentate-binuclear complexation. A multivariate curve resolution (MCR) analysis of the attenuated total reflection–Fourier-transform infrared spectra of adsorption envelope samples shows that increasing ionic strength does not affect the inner-sphere but decreases the outer-sphere complex adsorption loading, consistent with the ionic strength effect. The extended triple layer model directly and successfully models the MCR-derived inner- and outer-sphere surface loadings at various ionic strengths, indicating weaker sulfate inner-sphere complexation on hematite than on ferrihydrite surfaces. Results also show that sample drying, lower pH, and higher ionic strength all favor sulfate inner-sphere complexation, but the hematite particle size does not affect the relative proportions of the two types of complexes. Sulfate adsorption kinetics show increasing ratio of exchanged OH^– to adsorbed sulfate with time, attributed to inner- and outer-sphere complexation dominating at different adsorption stages and to the changes of the relative abundance of surface OH^– and H₂O groups with time. This work clarifies sulfate adsorption mechanisms on hematite and has implications for understanding sulfate availability, behavior and fate in the environment. Our work suggests that the simple macroscopic ionic strength test correlates well with directly measured outer-sphere complexes