Effects of methanol on CaSO4 ion associated species in mixed solutions: Solvation dynamics and hydrogen bond bridging structure

Seeds of imperfection rule the mesocrystalline disorder in natural anhydrite single crystals

In recent years, we have come to appreciate the astounding intricacies associated with the formation of minerals from ions in aqueous solutions. In this context, a number of studies have revealed that the nucleation of calcium sulfate systems occurs nonclassically, involving the aggregation and reorganization of nanosized prenucleation species. In recent work, we have shown that this particle-mediated nucleation pathway is actually imprinted in the resultant micrometer-sized CaSO₄ crystals. This property of CaSO₄ minerals provides us with the unique opportunity to search for evidence of nonclassical nucleation pathways in geological environments. In particular, we focused on large anhydrite crystals extracted from the Naica Mine in Mexico. We were able to shed light on this mineral's growth history by mapping defects at different length scales. Based on this, we argue that the nanoscale misalignment of the structural subunits, observed in the initial calcium sulfate crystal seeds, propagates through different length scales both in morphological, as well as in strictly crystallographic aspects, eventually causing the formation of large mesostructured single crystals of anhydrite. Hence, the nonclassical nucleation mechanism introduces a "seed of imperfection," which leads to a macroscopic "single" crystal whose fragments do not fit together at different length scales in a self-similar manner. Consequently, anisotropic voids of various sizes are formed with very well-defined walls/edges. However, at the same time, the material retains in part its single crystal nature.

Efficient removal of iron from red gypsum via synergistic regulation of gypsum phase transformation and iron speciation

Red gypsum is a type of iron-rich gypsum residue originated from industrial titanium dioxide process using Ilmenite. Currently, it has a low rate of comprehensive utilization about 20%, and deep removal of iron impurity is the crucial factor that directly limits its multipurpose utilization. In this study, the iron was efficiently removed from red gypsum residue by synergistic controlling the phase transformation of gypsum and the iron speciation under hydrothermal conditions. The iron removal efficiency was more than 99% under the optimized treatment condition (i.e. liquid-solid ratio of 10, with 1.5 M HCl as mineralizer, heating at 140 °C for 6 h). The X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) analyses showed that the phase transformation and crystal growth of gypsum accelerated by H⁺ was the essential reasons to fully remove iron. Moreover, H⁺ also provided acidic conditions (pH < 1) to change the iron speciation from amorphous oxide or hydroxide fine particles into soluble Fe³⁺ which release into the solution and easy to be removed by solid-liquid separation. In this work, based on the synergistic regulation of gypsum phase transformation and iron speciation, a feasible method for deep removal of iron from red gypsum was proposed, which is conducive to broadening the comprehensive utilization range of red gypsum. This work would inspire the treatment and resource utilization of industrial gypsum residues containing other contaminants or impurities, including heavy metals and organic matters.