Abstract Iron (oxyhydr)oxides mediate electron flux across coupled biogeochemical cycles in anoxic environments, yet whether their biogenic transformation feeds back to reshape interfacial electron transfer efficiency remains unresolved. Here, five structurally distinct iron (oxyhydr)oxide precursors (ferrihydrite, goethite, magnetite, wüstite, and hematite) were exposed to anaerobic sulfur-oxidizing bioreactors under Cr(VI) stress, and their transformation trajectories were tracked using synchrotron EXAFS spectroscopy, electrochemical impedance spectroscopy, DFT calculations, and metagenomic profiling. Mineral bioavailability decreased in the order ferrihydrite > goethite > magnetite > wüstite > hematite, a hierarchy governed by lattice iron-leaching energetics quantified via DFT (Efev from −8.96 eV for ferrihydrite to 5.76 eV for hematite). Despite this wide disparity in precursor lability, all biotransformation trajectories converged on magnetite as the thermodynamically stable end point. In the ferrihydrite system, biogenic magnetite reached 60% of the final solid phase and operated as an in situ “environmental battery” (storing and releasing electrons through reversible Fe(II)/Fe(III) lattice cycling), lowering interfacial charge transfer resistance and producing a 4.77-fold higher pseudo-first-order Cr(VI) reduction rate constant than the mineral-free biotic control (C-S). Metagenomic analysis revealed mineral-specific restructuring of the microbial community toward a tightly coupled sulfur–iron redox network. These results reframe precursor mineralogy from a determinant of intrinsic reactivity to a kinetic regulator of biogenic electron-buffer formation, providing a mechanistic basis for mineral-informed bioremediation design.
Current brine management strategies are based on the disposal of brine in nearby aquifers, representing a loss in potential water and mineral resources. Zero liquid discharge (ZLD) is a possible strategy to reduce brine rejection while increasing the resource recovery from desalination plants. However, ZLD substantially increases the energy consumption and carbon footprint of a desalination plant. The predominant strategy to reduce the energy consumption and carbon footprint of ZLD is through the use of a hybrid desalination technology that integrates renewable energy. Here, we built a computational thermodynamic model of the most mature electrified hybrid technology for ZLD powered by photovoltaic (PV). We examine the potential size and cost of ZLD plants in the US. This work explores the variables (geospatial and design) that most influence the levelized cost of water and the second law efficiency. There is a negative correlation between minimizing the LCOW and maximizing the second-law. And maximizing the second-law, the states that more brine produces, Texas is the location where the studied system achieves the lowest LCOW and high second-law efficiency, while California is the state where the studied system is less favorable. A multiobjective optimization study assesses the impact of considering a carbon tax in the cost of produced water and determines the best potential size for the studied plant.
Montmorillonite layer edge surfaces have pH-dependent properties, which arises from the acid–base reactivity of their surface functional groups. Edge surface acidity (with intrinsic reaction equilibrium constant, pKa) is a chemical property that is affected by crystal structure. While a cis-vacant structure predominates in natural montmorillonites, prior molecular-level studies assume a centrosymmetric trans-vacant configuration, which potentially leads to an incorrect prediction of montmorillonite acid–base surface properties. We computed intrinsic acidity constants of the surface sites of a montmorillonite layer with a cis-vacant structure using the first-principles molecular dynamics-based vertical energy gap method. We evaluated pKa values for both non-substituted and Mg-substituted layers on common edge surfaces (i.e., surfaces perpendicular to [010], [01̅0], [110], and [1̅1̅0] crystallographic directions). The functional groups ≡Si(OH), ≡Al(OH2)2/≡Al(OH)(OH2), and ≡SiO(OH)Al sites on surfaces perpendicular to [010] and [01̅0] and ≡Si(OH)U, ≡Si(OH)L, ≡Al(OH2), and ≡Al(OH2)2 on surfaces perpendicular to [110] and [1̅1̅0] determine the proton reactivity of non-substituted cis-vacant edge surfaces. Moreover, the structural OH sites on edge surfaces had extremely high pKa values, which do not show reactivity at a common pH. Meanwhile, Mg2+ substitution results in an increase in pKa values at local or adjacent sites, in which the effect is limited by the distance between the sites. A surface complexation model was built with predicted pKa values, which enabled us to predict surface properties as a function of pH and ionic strength. Edge surface charge of both trans- and cis-vacant models has little dependence on Mg2+ substitutions, but the dependence on the crystal plane orientation is strong. In particular, at pH below 7, edge surfaces are positively or negatively charged depending on their orientation. Implications of these findings on contaminant adsorption by smectites are discussed.
