Geochemical Transactions最新文献

Green rust (GR) is a potentially important compound for the reduction of heavy metal and organic pollutants in subsurface environment because of its high Fe(II) content, but many details of the actual reaction mechanism are lacking. The reductive capacity distribution within GR is a key to understand how and where the redox reaction occurs and computational chemistry can provide more details about the electronic properties of green rust. We constructed three sizes of cluster models of single layer GR (i.e., without interlayer molecules or ions) and calculated the charge distribution of these structures using density functional theory. We found that the Fe(II) and Fe(III) are distributed unevenly in the single layer GR. Within a certain range of Fe(II)/Fe(III) ratios, the outer iron atoms behave more like Fe(III) and the inner iron atoms behave more like Fe(II). These findings indicate that the interior of GR is more reductive than the outer parts and will provide new information to understand the GR redox interactions.

Detailed geochemical studies of both major and minor elements in Bangladesh surface waters are sparse, particularly in shrimp aquaculture pond environments. Therefore, water samples from shrimp aquaculture ponds and tidal channels were collected in high precipitation (July) and low precipitation (May) months from 2018–2019 in Southwest Bangladesh and analyzed for complete water chemistry. Selenium (Se) and arsenic (As) were elevated above WHO guidelines in 50% and?~?87% of samples, respectively, but do not show any recognizable spatial patterns. Shrimp pond and tidal channel water compositions in the dry season (May) are similar, illustrating their connectivity and minimal endogenous effects within shrimp ponds. Tidal channels are less saline in July than shrimp ponds still irrigated by tidal channels, suggesting that either farmers limit irrigation to continue farming saltwater shrimp, or the irrigation flux is low and leads to a lag in aquaculture-tidal channel compositional homogenization. δ¹⁸O and δ²H isotopic compositions from samples in May of 2019 reveal tidal channel samples are closer to the local meteoric water line (LMWL) than shrimp pond samples, because of less evaporation. However, evaporation in May shrimp ponds has a minimal effect on water composition, likely because of regular drainage/exchange of pond waters. Dissolved organic carbon (DOC) is positively correlated with both δ¹⁸O and δ²H in shrimp ponds, suggesting that as evaporation increases, DOC becomes enriched. Multiple linear regression reveals that As and Se can be moderately predicted (adjusted R² values between 0.4 and 0.7, p?<?0.01) in surface waters of our study with only 3–4 independent predictor variables (e.g., Ni, V and DOC for Se prediction; Cu, V, Ni and P for As prediction). Thus, this general approach should be followed in other regions throughout the world when measurements for certain hazardous trace elements such as Se and As may be lacking in several samples from a dataset.

A complete series of calcite-rhodochrosite solid solutions [(Ca_1-xMn_x)CO₃] are prepared, and their dissolution processes in various water samples are experimentally investigated. The crystal morphologies of the solid solutions vary from blocky spherical crystal aggregates to smaller spheres with an increasing incorporation of Mn in the solids. Regarding dissolution in N₂-degassed water, air-saturated water and CO₂-saturated water at 25?°C, the aqueous Ca and Mn concentrations reach their highest values after 1240–2400?h, 6–12?h and?<?1?h, respectively, and then decrease gradually to a steady state; additionally, the ion activity products (log_IAP) at the final steady state (≈ solubility products in log_K_sp) are estimated to be ??8.46?±?0.06, ??8.44?±?0.10 and ??8.59?±?0.10 for calcite [CaCO₃], respectively, and ??10.25?±?0.08, ??10.26?±?0.10 and ??10.28?±?0.03, for rhodochrosite [MnCO₃], respectively. As X_Mn increases, the log_IAP values decrease from ??8.44?~???8.59 for calcite to ??10.25?~???10.28 for rhodochrosite. The aqueous Mn concentrations increase with an increasing Mn/(Ca?+?Mn) molar ratio (X_Mn) of the (Ca_1-xMn_x)CO₃ solid solutions, while the aqueous Ca concentrations show the highest values at X_Mn?=?0.53–0.63. In the constructed Lippmann diagram of subregular (Ca_1-xMn_x)CO₃ solid solutions, the solids dissolve incongruently, and the data points of the aqueous solutions move progressively up to the Lippmann solutus curve and then along the solutus curve or saturation curve of pure MnCO₃ to the Mn-poor side. The microcrystalline cores of the spherical crystal aggregates are preferentially dissolved to form core hollows while simultaneously precipitating Mn-rich hexagonal prisms.

We present experimentally determined trace element partition coefficients (D) between pyrochlore-group minerals (Ca₂(Nb,Ta)₂O₆(O,F)), Ca fersmite (CaNb₂O₆), and silicate melts. Our data indicate that pyrochlores and fersmite are able to strongly fractionate trace elements during the evolution of SiO₂-undersaturated magmas. Pyrochlore efficiently fractionates Zr and Hf from Nb and Ta, with D_Zr and D_Hf below or equal to unity, and D_Nb and D_Ta significantly above unity. We find that D_Ta pyrochlore-group mineral/silicate melt is always higher than D_Nb, which agrees with the HFSE partitioning of?all other Ti–rich minerals such as perovskite, rutile, ilmenite or Fe-Ti spinel. Our experimental partition coefficients also show that, under oxidizing conditions, D_Th is higher than corresponding D_U and this implies that pyrochlore-group minerals may fractionate U and Th in silicate magmas. The rare earth element (REE) partition coefficients are around unity, only the light REE are compatible in pyrochlore-group minerals, which explains the high?rare earth element concentrations in naturally occurring magmatic pyrochlores.

Chromate, Cr(VI), contamination in soil and groundwater poses serious threat to living organisms and environmental health worldwide. Sulphate green rust (GR_SO4), a naturally occurring mixed-valent iron layered double hydroxide has shown to be highly effective in the reduction of Cr(VI) to poorly soluble Cr(III), giving promise for its use as reactant for in situ remedial applications. However, little is known about its immobilization efficiency inside porous geological media, such as soils and sediments, where this reactant would ultimately be applied. In this study, we tested the removal of Cr(VI) by GR_SO4 in quartz sand fixed-bed column systems (diameter?×?length?=?1.4?cm?×?11?cm), under anoxic conditions. Cr(VI) removal efficiency (relative to the available reducing equivalents in the added GR_SO4) was determined by evaluating breakthrough curves performed at different inlet Cr(VI) concentrations (0.125–1?mM) which are representative of Cr(VI) concentrations found at contaminated sites, different flow rates (0.25–3?ml/min) and solution pH (4.5, 7 and 9.5). Results showed that (i) increasing Cr(VI) inlet concentration substantially decreased Cr(VI) removal efficiency of GR_SO4, (ii) flow rates had a lower impact on removal efficiencies, although values tended to be lower at higher flow rates, and (iii) Cr(VI) removal was enhanced at acidic pH conditions compared to neutral and alkaline conditions. For comparison, Cr(VI) removal by sulphidized nanoscale zerovalent iron (S-nZVI) in identical column experiments was substantially lower, indicating that S-nZVI reactivity with Cr(VI) is much slower compared to GR_SO4. Overall, GR_SO4 performed reasonably well, even at the highest tested flow rate, showing its versatility and suitability for Cr(VI) remediation applications in high flow environments.

Inorganic carbon exists in various dissolved, gaseous and solid phase forms in natural waters and soils. It is important to accurately measure and model these forms to understand system responses to global climate change. The carbonate system can, in theory, be fully constrained and modelled by measuring at least two out of the following four parameters: partial pressure (pCO₂), total alkalinity (TA), pH and dissolved inorganic carbon (DIC) but this has not been demonstrated in soils. In this study, this “internal consistency” of the soil carbonate system was examined by predicting pH of soil extracts from laboratory measurement of TA through alkalinity titration for solutions in which pCO₂ was fixed through equilibrating the soil solution with air with a known pCO₂. This predicted pH (pH_CO2) was compared with pH measured on the same soil extracts using spectrophotometric and glass electrode methods (pH_{spec and} pH_elec). Discrepancy between measured and calculated pH was within 0.00–0.1 pH unit for most samples. However, more deviation was observed for those sample with low alkalinity (≤?0.5?meq L^?1). This is likely attributable to an effect of dissolved organic matter, which can contribute alkalinity not considered in the thermodynamic carbonate model calculations; further research is required to resolve this problem. The effects of increasing soil pCO₂ was modelled to illustrate how internally consistent models can be used to predict risks of pH declines and carbonate mineral dissolution in some soils.

Solution ³¹P nuclear magnetic resonance (NMR) spectroscopy has been widely applied to analyze the speciation of soil organic P; however, this time-consuming technique suffers from a low analytical efficiency, because of the lack of fundamental information such as the spin–lattice relaxation (T₁) of ³¹P nucleus for model P compounds. In this study, we for the first time determined the T₁ values of twelve typical soil organic P compounds using the inversion recovery method. Furthermore, we examined the effect of co-existing paramagnetic ions (e.g., Fe³⁺ and Mn²⁺) on the reduction of the T₁ values of these compounds. Without the addition of paramagnetic ions, the T₁ values of twelve model P compounds ranged from 0.61?s for phytic acid to 9.65?s for orthophosphate. In contrast, the presence of paramagnetic ion significantly shortened the T₁ values of orthophosphate, pyrophosphate, and phytic acid to 1.29, 1.26, and 0.07?s, respectively, except that of deoxyribonucleic acid (DNA) remaining unchanged. Additionally, we evaluated the feasibility of improving the efficiency of quantitative ³¹P NMR analysis via addition of paramagnetic ion. Results show that, after an addition of 50?mg L^?1 paramagnetic ions, ³¹P NMR measurement can be 3 times more efficient, attributed to the reduced T₁ and the corresponding recycle delay.