Structural and magnetic characterization of the complete delafossite solid solution (CuAlO2){1-x}(CuCrO2){x}

We have prepared the complete delafossite solid solution series between diamagnetic CuAlO2 and the t2g^3 frustrated antiferromagnet CuCrO2. The evolution with composition x in CuAl(1-x)Cr(x)O2 of the crystal structure and magnetic properties has been studied and is reported here. The room-temperature unit cell parameters follow the Vegard law and increase with x as expected. The effective moment is equal to the Cr^3+ spin-only S = 3/2 value throughout the entire solid solution. Theta is negative, indicating that the dominant interactions are antiferromagnetic, and its magnitude increases with Cr substitution. For dilute Cr compositions, J_BB was estimated by mean-field theory to be 2.0 meV. Despite the sizable Theta, long-range antiferromagnetic order does not develop until very large x, and is preceeded by glassy behavior. Data presented here, and that on dilute Al-substitution from Okuda et al., suggest that the reduction in magnetic frustration due to the presence of non-magnetic Al does not have as dominant an effect on magnetism as chemical disorder and dilution of the magnetic exchange. For all samples, the 5 K isothermal magnetization does not saturate in fields up to 5 T and minimal hysteresis is observed. The presence of antiferromagnetic interactions is clearly evident in the sub-Brillouin behavior with a reduced magnetization per Cr atom. An inspection of the scaled Curie plot reveals that significant short-range antiferromagnetic interactions occur in CuCrO2 above its Neel temperature, consistent with its magnetic frustration. Uncompensated short-range interactions are present in the Al-substituted samples and are likely a result of chemical disorder

Sulfur isotope fractionation and sequential extraction to assess metal contamination on lake and river sediments

Purpose: The present study investigated lake and river sediments affected by metals from an acid mine drainage (AMD) from a former uranium mine. The role of bacterial sulphate reduction in the immobilization of contaminants was evaluated and analyses of acid volatile sulphide (AVS) and sequential extraction were performed. Consequently, the potential mobility and bioavailability of contaminants were established. Materials and methods: Sulphur isotopic fractionation (δ³⁴S), AVS and sequential extraction procedure were used to assess the sulphate bacterial reduction and the availability of contaminants in the environment at six sampling stations. Results and discussion: The δ³⁴S indicated that bacterial reduction is a key process in the natural attenuation of contamination in the Águas Claras reservoir, precipitating metal sulphides. According to the USEPA criteria, adverse biological effects are expected for sample S1 (inside the reservoir) which is likely to be toxic, whilst for sediment S4 (in the river) the toxicity is uncertain. The other samples were classified as non-toxic, likely because of the decreased solubility of zinc sulphide. A decrease in the concentration of the contaminants downstream of the reservoir was observed. The predominance of U (0.4 %) in the labile fraction and the elevated concentrations of Zn (0.5 %) and Mn (0.7 %) in the sediments inside the reservoir raise concerns regarding the availability of these contaminants in the environment. Conclusions: The main environmental impact appears to be concentrated in the Águas Claras reservoir whereas the Antas creek does not seems to be affected by the AMD process. Although the bacterial sulphate reduction is effective in its production of sulphides capable of immobilizing the contaminants, the presence of Zn and U in the labile and reducible fraction is a matter of concern due to its long term bioavailability. Thus, continuous monitoring of the redox potential of the waters and sediments, mainly in the reservoir, is recommended in order to assess and possibly prevent later dissolution of sequestered contaminants.The authors thank CAPES, FAPEMIG and CNPq - INCT-Acqua, PPM, CT Mineral- for the financial support and the staff of the Stable Isotope Laboratory of UFZ (Leipzig-Halle) for their isotopic analytical assistance. Also, the authors gratefully acknowledge the comments offered by the editor and two anonymous reviewers.This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s11368-016-1410-

Tracing freshwater nitrate sources in pre-alpine groundwater catchments using environmental tracers.

Groundwater is one of the main resources for drinking water. Its quality is still threatened by the widespread contaminant nitrate (NO3−). In order to manage groundwater resources in a sustainable manner, we need to find options of lowering nitrate input. Particularly, a comprehensive knowledge of nitrate sources is required in areas which are important current and future drinking water reservoirs such as pre-alpine aquifers covered with permanent grassland. The objective of the present study was to identify major sources of nitrate in groundwater with low mean nitrate concentrations (8 ± 2 mg/L). To achieve the objective, we used environmental tracer approaches in four pre-alpine groundwater catchments. The stable isotope composition and tritium content of water were used to study the hydrogeology and transit times. Furthermore, nitrate stable isotope methods were applied to trace nitrogen from its sources to groundwater. The results of the nitrate isotope analysis showed that groundwater nitrate was derived from nitrification of a variety of ammonium sources such as atmospheric deposition, mineral and organic fertilizers and soil organic matter. A direct influence of mineral fertilizer, atmospheric deposition and sewage was excluded. Since temporal variation in stable isotopes of nitrate were detected only in surface water and locally at one groundwater monitoring well, aquifers appeared to be well mixed and influenced by a continuous nitrate input mainly from soil derived nitrogen. Hydrogeological analysis supported that the investigated aquifers were less vulnerable to rapid impacts due to long average transit times, ranging from 5 to 21 years. Our study revealed the importance of combining environmental tracer approaches and a comprehensive sampling campaign (local sources of nitrate, soil water, river water, and groundwater) to identify the nitrate sources in groundwater and its vulnerability. In future, the achieved results will help develop targeted strategies for a sustainable groundwater management focusing more on soil nitrogen storage

Anoxic nitrogen cycling in a hydrocarbon and ammonium contaminated aquifer

Nitrogen fate and transport through contaminated groundwater systems, where N is both ubiquitous and commonly limits pollutant attenuation, must be re-evaluated given evidence for new potential microbial N pathways. We addressed this by measuring the isotopic composition of dissolved inorganic N (DIN = NH₄⁺, NO₂¯, and NO₃¯) and N functional gene abundances (amoA, nirK, nirS, hszA) from 20 to 38 wells across an NH₄⁺, hydrocarbon, and SO²₄− contaminated aquifer. In-situ N attenuation was confirmed on three sampling dates (0, +6, +12 months) by the decreased [DIN] (4300 - 40 μM) and increased δ¹⁵N-DIN (5‰–33‰) over the flow path. However, the assumption of negligible N attenuation within the plume was complicated by the presence of alternative electron acceptors (SO²₄−, Fe³⁺), both oxidizing and reducing functional genes, and N oxides within this anoxic zone. Active plume N cycling was corroborated using an NO₂¯dual isotope based model, which found the fastest (∼10 day) NO₂¯ turnover within the N and electron donor rich central plume. Findings suggest that N cycling is not always O₂ limited within chemically complex contaminated aquifers, though this cycling may recycle the N species rather than attenuate N

The isotopic (δ18O, δ2H, δ13C, δ15N, δ34S, 87Sr/86Sr, δ11B) composition of Adige river water records natural and anthropogenic processes

The water composition of the river Adige displays a Ca–HCO3 hydrochemical facies, mainly due to rock weathering. Nitrate is the only component that has increased in relation to growing anthropogenic inputs. The aim of this paper was to identify the origin of the dissolved components in this river and to establish the relationship between these components and critical zone processes within an evolving framework where climatic and human impacts are influencing the riverine system. In particular, emphasis is given to a wide spectrum of isotope data (δ18O, δ2H, δ13C, δ15N, δ34S, 87Sr/86Sr, δ11B), which is considered useful for determining water origin as well as natural and anthropogenic impacts on riverine geochemistry. Together with oxygen and hydrogen isotopes, which are strictly related to the climatic conditions (precipitation, temperature, humidity), the carbon, sulphur, strontium and boron signatures can describe the magnitude of rock weathering, which is in turn linked to the climatic parameters. δ13CDIC varies regularly along the riverine profile between −4.5‰ and −9.5‰, and δ34SSO4 varies regularly between +4.4‰ and +11.4‰. On the other hand, δ15NNO3 shows a more scattered distribution between +3.9‰ and +10.5‰, with sharp variations along the riverine profile. 87Sr/86Sr varies between 0.72797 in the upper part of the catchment and 0.71068 in the lower part. δ11B also shows a rough trend, with values approaching 7.6‰ in the upper part and 8.5‰ in the lower part. In our view, the comparatively low δ34S, δ11B, and high 87Sr/86Sr values, could be a proxy for increasing silicate weathering, which is a process that is sensitive to increases in temperature