Formation of calcium sulfate through the aggregation of sub-3 nanometre primary species

The formation pathways of gypsum remain uncertain. Here, using truly in situ and fast time-resolved small-angle X-ray scattering, we quantify the four-stage solution-based nucleation and growth of gypsum (CaSO4 ·2H2O), an important mineral phase on Earth and Mars. The reaction starts through the fast formation of well-defined, primary species of <3 nm in length (stage I), followed in stage II by their arrangement into domains. The variations in volume fractions and electron densities suggest that these fast forming primary species contain Ca-SO4-cores that self-assemble in stage III into large aggregates. Within the aggregates these well-defined primary species start to grow (stage IV), and fully crystalize into gypsum through a structural rearrangement. Our results allow for a quantitative understanding of how natural calcium sulfate deposits may form on Earth and how a terrestrially unstable phase-like bassanite can persist at low-water activities currently dominating the surface of Mars

How Short-Lived Ikaite Affects Calcite Crystallization

The pathways of CaCO₃ crystallization are manifold, often involving one or several metastable amorphous or nanocrystalline intermediate phases. The presence of such intermediates is often overlooked, because they are short-lived and/or occur at small molar fractions. However, their occurrence does not just impact the mechanisms and pathways of formation of the final stable CaCO₃ phase, but also affects their crystal size, shape, and structure. Here we document the presence of a short-lived intermediate through in situ and time-resolved small and wide-angle X-ray scattering combined with high resolution electron microscope observations. When ikaite forms concomitant with the dissolution of amorphous calcium carbonate (ACC) but prior to calcite formation, fairly large glendonite-type calcite crystals grow despite the presence of citrate ligands that usually reduce crystal size. These were ideal seeding crystals for further crystallization from supersaturated ions in solution. In contrast, in the absence of ikaite the crystallization of calcite proceeds through transformation from ACC, resulting in fine-grained spherulitic calcite with sizes ∼8 times smaller than when ikaite was present. Noteworthy is that the formation of the intermediate ikaite, although it consumes less than 3 mol % of the total precipitated CaCO₃, still clearly affected the calcite formation mechanism

Understanding amorphous silica scaling under well-constrained conditions inside geothermal pipelines

Amorphous silica is a common precipitate in modern and ancient hot springs and in geothermal power plants, yet the corresponding precipitation rates and mechanisms are still highly debated, primarily due to the plethora of parameters that can affect the reactions in natural waters. Here, we report the results from a first ever industrial-scale time-resolved (1 day to 10 weeks) study of silica precipitation conducted at the Hellisheiði geothermal power plant (SW-Iceland). We show that such in-work pipelines of a geothermal power plant are ideal environments to investigate silica precipitation because the physicochemical conditions are well constrained and constantly monitored. Our results document that amorphous silica forms via two distinct precipitation modes: (1) the fast deposition of continuous botryoidal silica layers and (2) the growth of 3D fan- or ridge-shaped silica aggregates. The continuous layers grow by heterogeneous nucleation and subsequent surface controlled growth by monomer addition. In contrary, the 3D aggregates form through homogeneous nucleation of silica nano- and microparticles in solution, followed by deposition and cementation on the surface of the botryoidal layer. From the time-resolved data, silica precipitation rates of over 1 g m−2 day-1 are derived. Over time, this deposition of silica on pipelines and fluid handling equipment is detrimental to geothermal power production. Our data does not only help improve our understanding of silica precipitation from geothermal fluids, but the determined silica precipitation mechanisms and rates help improve mitigation strategies against silica scaling inside in-work geothermal power plants

Physicochemical and Additive Controls on the Multistep Precipitation Pathway of Gypsum

Synchrotron-based small- and wide-angle X-ray scattering (SAXS/WAXS) was used to examine in situ the precipitation of gypsum (CaSO4·2H2O) from solution. We determined the role of (I) supersaturation, (II) temperature and (III) additives (Mg2+ and citric acid) on the precipitation mechanism and rate of gypsum. Detailed analysis of the SAXS data showed that for all tested supersaturations and temperatures the same nucleation pathway was maintained, i.e., formation of primary particles that aggregate and transform/re-organize into gypsum. In the presence of Mg2+ more primary particle are formed compared to the pure experiment, but the onset of their transformation/reorganization was slowed down. Citrate reduces the formation of primary particles resulting in a longer induction time of gypsum formation. Based on the WAXS data we determined that the precipitation rate of gypsum increased 5-fold from 4 to 40 °C, which results in an effective activation energy of ~30 kJ·mol−1. Mg2+ reduces the precipitation rate of gypsum by more than half, most likely by blocking the attachment sites of the growth units, while citric acid only weakly hampers the growth of gypsum by lowering the effective supersaturation. In short, our results show that the nucleation mechanism is independent of the solution conditions and that Mg2+ and citric acid influence differently the nucleation pathway and growth kinetics of gypsum. These insights are key for further improving our ability to control the crystallization process of calcium sulphate

Improving compliance to colorectal cancer screening using blood and stool based tests in patients refusing screening colonoscopy in Germany

Background Despite strong recommendations for colorectal cancer (CRC) screening, participation rates are low. Understanding factors that affect screening choices is essential to developing future screening strategies. Therefore, this study assessed patient willingness to use non-invasive stool or blood based screening tests after refusing colonoscopy. Methods Participants were recruited during regular consultations. Demographic, health, psychological and socioeconomic factors were recorded. All subjects were advised to undergo screening by colonoscopy. Subjects who refused colonoscopy were offered a choice of non-invasive tests. Subjects who selected stool testing received a collection kit and instructions; subjects who selected plasma testing had a blood draw during the office visit. Stool samples were tested with the Hb/Hp Complex Elisa test, and blood samples were tested with the Epi proColon® 2.0 test. Patients who were positive for either were advised to have a diagnostic colonoscopy. Results 63 of 172 subjects were compliant to screening colonoscopy (37%). 106 of the 109 subjects who refused colonoscopy accepted an alternative non-invasive method (97%). 90 selected the Septin9 blood test (83%), 16 selected a stool test (15%) and 3 refused any test (3%). Reasons for blood test preference included convenience of an office draw, overall convenience and less time consuming procedure. Conclusions 97% of subjects refusing colonoscopy accepted a non-invasive screening test of which 83% chose the Septin9 blood test. The observation that participation can be increased by offering non-invasive tests, and that a blood test is the preferred option should be validated in a prospective trial in the screening setting

Validity of the Modified Child Psychopathy Scale for Juvenile Justice Center Residents

Adult psychopathy has proven to be an important clinical and forensic construct, but much less is known about juvenile psychopathy. In the present study, we examined the construct validity of the self report modified Child Psychopathy Scale mCPS; Lynam (Psychological Bulletin 120:(2), 209–234, 1997) in a sample of 57 adolescents residing in a Dutch juvenile justice center, aged between 13 and 22 years. The mCPS total score was reliably related to high externalizing problems, low empathy, high anger and aggression, high impulsivity, high (violent) delinquency, and high alcohol/drug use. Unique relations were found for the antisocial-impulsive (mCPS Factor 2), but not the callous-unemotional facet of psychopathy (mCPS Factor 1). Our findings support the validity of the mCPS in that it encompasses the antisocial-impulsive facet of psychopathy, but it is less clear whether the mCPS sufficiently captures the affective-interpersonal facet of psychopathy

A highly reactive precursor in the iron sulfide system

Iron sulfur (Fe–S) phases have been implicated in the emergence of life on early Earth due to their catalytic role in the synthesis of prebiotic molecules. Similarly, Fe–S phases are currently of high interest in the development of green catalysts and energy storage. Here we report the synthesis and structure of a nanoparticulate phase (FeSnano) that is a necessary solid-phase precursor to the conventionally assumed initial precipitate in the iron sulfide system, mackinawite. The structure of FeSnano contains tetrahedral iron, which is compensated by monosulfide and polysulfide sulfur species. These together dramatically affect the stability and enhance the reactivity of FeSnano

Particle-Mediated Nucleation Pathways Are Imprinted in the Internal Structure of Calcium Sulfate Single Crystals

Calcium sulfate minerals are found in nature as three hydrates: gypsum (CaSO4·2H2O), bassanite (CaSO4·0.5H2O), and anhydrite (CaSO4). Due to their relevance in natural and industrial processes, the formation pathways of calcium sulfates from aqueous solution have been the subject of intensive research, and there is a growing body of literature, suggesting that calcium sulfates form through nonclassical nanoparticle-mediated crystallization processes. We showed earlier (Stawski et al. Nat. Commun.2016, 7, 11177) that at the early stages in the precipitation reaction, calcium sulfate nanocrystals nucleate through the reorganization and coalescence of aggregates rather than through classical unit addition. Here, we used low-dose dark field (DF) transmission electron microscopy (TEM) and electron diffraction and document that these restructuring processes do not continue until a final near-perfectly homogeneous single crystal is obtained. Instead, we show that the growth process yields a final imperfect mesocrystal with an overall morphology resembling that of a single crystal, yet composed of smaller nanodomains. Our data reveal that organic-free calcium sulfate mesocrystals grown by a particle mediated-pathway may preserve in the final crystal structure a “memory” or “imprint” of their nonclassical nucleation process, something that has been overlooked until now. Furthermore, the nanoscale misalignment of the structural subunits within these crystals might propagate through the length-scales, which is potentially expressed macroscopically as misaligned zones/domains in large single crystals. This is akin to observations in some of the giant crystals from the Naica Mine, Chihuahua, Mexico