Formation and properties of spindle-shaped aragonite mesocrystals from Mg-bearing solutions.

The formation of aragonite under ambient conditions is typically linked to Mg-rich aqueous environments. The grains that form in such environments show peculiar properties such as aggregate-like appearance and mesocrystalline character. We tested the effect of dissolved Mg2+ ions on the formation of aragonite mesocrystals by synthesizing aragonite with an automatic titrator at constant pH and at different dissolved Mg : Ca ratios, and by studying the properties of the precipitated material with various scanning transmission electron microscopy (STEM) techniques. At all studied Mg : Ca ratios the firstly condensed carbonate phase was Mg-bearing amorphous calcium carbonate (Mg-ACC) that transformed into aragonite during the synthesis experiments. The aragonite grains had typically aggregate-like appearance and spindle shapes, with the external morphologies of the spindles unaffected by variation in solution chemistry. The alignment of the nanocrystals within the aggregates was crystallographically highly coherent, the [001] directions of nanocrystals showing only a small misorientation with respect to one another; however, both parallel and twin assembly of neighbouring crystals occurred. An increase in the dissolved Mg concentration decreased the crystallographic coherence between the aragonite nanocrystals, suggesting an important role of Mg2+ ions in the assembly of aragonite spindles. Whereas the mesoscale-ordered arrangement of nanocrystals implies a particle-mediated assembly, the observed differences in particle size and composition between the amorphous precursor and the crystalline end-product suggest that the crystallization includes at least partial dissolution and re-precipitation. These findings provide insight into the formation of aragonite and could contribute to the understanding of important aspects of the formation of mesocrystals and hierarchically structured biogenic minerals.

Effect of Copper-Modification of g-C3N4 on the Visible-Light-Driven Photocatalytic Oxidation of Nitrophenols.

Graphitic carbon nitride (g-C3N4) has proved to be a promising heterogeneous photocatalyst in the visible range. It can be used, among others, for the oxidative conversion of environmentally harmful nitrophenols occurring in wastewater. However, its photocatalytic activity needs to be enhanced, which can be achieved by modification with various dopants. In our work, copper-modified g-C3N4 was prepared by ultrasonic impregnation of the pristine g-C3N4 synthesized from thiourea. The morphology, microstructure, and optical properties of the photocatalysts were characterized by XRD, FT-IR, DRS, SEM, XPS, and TEM. DRS analysis indicated a slight change in both the CB and the VB energies of Cu/g-C3N4 compared to those of g-C3N4. The efficiency of the photocatalysts prepared was tested by the degradation of nitrophenols. Copper modification caused a sevenfold increase in the rate of 4-nitrophenol degradation in the presence of H2O2 at pH = 3. This dramatic enhancement can be attributed to the synergistic effect of copper and H2O2 in this photocatalytic system. A minor Fenton reaction role was also detected. The reusability of the Cu/g-C3N4 catalyst was demonstrated through five cycles. Copper-modified g-C3N4 with H2O2 proved to be applicable for efficient visible-light-driven photocatalytic oxidative degradation of nitrophenols.

Nanoscale Pathway of Modern Dolomite Formation in a Shallow, Alkaline Lake.

Dolomite [CaMg(CO3)2] formation under Earth surface conditions is considered largely inhibited, yet protodolomite (with a composition similar to dolomite but lacking cation ordering), and in some cases also dolomite, was documented in modern shallow marine and lacustrine, evaporative environments. Authigenic carbonate mud from Lake Neusiedl, a shallow, episodically evaporative lake in Austria consists mainly of Mg-calcite with zoning of Mg-rich and Mg-poor regions in µm-sized crystals. Within the Mg-rich regions, high-resolution transmission electron microscopy revealed < 5-nm-sized domains with dolomitic ordering, i.e., alternating lattice planes of Ca and Mg, in coherent orientation with the surrounding protodolomite. The calcite with less abundant Mg does not show such domains but is characterized by pitted surfaces and voids as a sign of dissolution. These observations suggest that protodolomite may overgrow Mg-calcite as a result of the changing chemistry of the lake water. During this process, oscillating concentrations (in particular of Mg and Ca) at the recrystallization front may have induced dissolution of Mg-calcite and growth of nanoscale domains of dolomite, which subsequently became incorporated as ordered domains in coherent orientation within less ordered regions. It is suggested that this crystallization pathway is capable of overcoming, at least at the nanoscale, the kinetic barrier to dolomite formation.

Interplay between arsenic and selenium biomineralization in Shewanella sp. O23S.

Bacteria play crucial roles in the biogeochemical cycle of arsenic (As) and selenium (Se) as these elements are metabolized via detoxification, energy generation (anaerobic respiration) and biosynthesis (e.g. selenocysteine) strategies. To date, arsenic and selenium biomineralization in bacteria were studied separately. In this study, the anaerobic metabolism of As and Se in Shewanella sp. O23S was investigated separately and mixed, with an emphasis put on the biomineralization products of this process. Multiple analytical techniques including ICP-MS, TEM-EDS, XRD, Micro-Raman, spectrophotometry and surface charge (zeta potential) were employed. Shewanella sp. O23S is capable of reducing selenate (SeO42-) and selenite (SeO32-) to red Se(-S)0, and arsenate (AsO43-) to arsenite (AsO33-). The release of H2S from cysteine led to the precipitation of AsS minerals: nanorod AsS and granular As2S3. When As and Se oxyanions were mixed, both As-S and Se(-S)0 biominerals were synthesized. All biominerals were extracellular, amorphous and presented a negative surface charge (-24 to -38 mV). Kinetic analysis indicated the following reduction yields: SeO32- (90%), AsO43- (60%), and SeO42- (<10%). The mix of SeO32- with AsO43- led to a decrease in As removal to 30%, while Se reduction yield was unaffected (88%). Interestingly, SeO42- incubated with AsO43- boosted the Se removal (71%). The exclusive extracellular formation of As and Se biominerals might indicate an extracellular respiratory process characteristic of various Shewanella species and strains. This is the first study documenting a complex interplay between As and Se oxyanions: selenite decreased arsenate reduction, whereas arsenate stimulated selenate reduction. Further investigation needs to clarify whether Shewanella sp. O23S employs multi-substrate respiratory enzymes or separate, high affinity enzymes for As and Se oxyanion respiration.

Cave bacteria-induced amorphous calcium carbonate formation.

Amorphous calcium carbonate (ACC) is a precursor of crystalline calcium carbonates that plays a key role in biomineralization and polymorph evolution. Here, we show that several bacterial strains isolated from a Hungarian cave produce ACC and their extracellular polymeric substance (EPS) shields ACC from crystallization. The findings demonstrate that bacteria-produced ACC forms in water-rich environment at room temperature and is stable for at least half year, which is in contrast to laboratory-produced ACC that needs to be stored in a desiccator and kept below 10 °C for avoiding crystallization. The ACC-shielding EPS consists of lipids, proteins, carbohydrates and nucleic acids. In particular, we identified large amount of long-chain fatty acid components. We suggest that ACC could be enclosed in a micella-like formula within the EPS that inhibits water infiltration. As the bacterial cells lyse, the covering protective layer disintegrates, water penetrates and the unprotected ACC grains crystallize to calcite. Our study indicates that bacteria are capable of producing ACC, and we estimate its quantity in comparison to calcite presumably varies up to 20% depending on the age of the colony. Since diverse bacterial communities colonize the surface of cave sediments in temperate zone, we presume that ACC is common in these caves and its occurrence is directly linked to bacterial activity and influences the geochemical signals recorded in speleothems.