Spatial-temporal analysis and background value determination of molybdenum concentration in basins with high molybdenum geochemical background - A case study of the upper Yi River basin

Deriving ecological risk thresholds for soil molybdenum in China based on interspecies correlation estimation and quantitative ion character-activity relationship models

Soil molybdenum (Mo) levels can reach ecologically hazardous levels. China has not yet established the relevant thresholds, posing challenges for environmental management. Therefore, we present our data relevant to Mo toxicity for several important species. By normalizing soil properties, we obtained a correlation model of Mo toxicity to Hordeum vulgare, as well as 31 models for the toxicity of other elements including Cu and Ni to invertebrates and microbial processes. Using interspecies correlation estimation (ICE) extrapolation, the sensitivity coefficient (0.12-0.71) for five plants were found. For invertebrates and microbial processes lacking Mo data, we used regression analysis to establish Mo toxicity models based on the soil quantitative ion character-activity relationships (s-QICAR; R2 =0.70-0.95) and known toxicities of other metal elements to invertebrate and microbial processes. Furthermore, combining species sensitivity distribution calculations, the HC5 values for protecting 95% of soil species from Mo in three typical soil scenarios in China were calculated. After correction, the predicted no-effect concentrations were 6.8, 4.8, and 3.4 mg/kg, respectively. This study innovatively combined ICE and s - QICAR to derive soil Mo thresholds. Our results can provide a basis for decision-making in the assessment and management of soil Mo pollution.

Characterization and utilization potential of typical molybdenum tailings in Shaanxi Province, China

Shaanxi Province is located in the most important molybdenum ore district in the world, but a lot of molybdenum tailings have been released, polluting the environment and wasting resources seriously. Taking eleven tailing samples collected at the main molybdenum tailings ponds in Shaanxi Province as the research object, the physical, chemical, and mineralogical characteristics were studied through scanning electron microscope, X-ray fluorescence, X-ray diffraction, inductively coupled plasma mass spectrometer, and others. The ecological risk and utilization potential of molybdenum tailings were investigated through leaching test, geo-accumulation index, potential ecological risk assessment, and other methods. The results demonstrated that the main chemical and mineralogical composition of various molybdenum tailings in Shaanxi Province is similar, and the predominant mineral composition is muscovite, quartz, microcline, and calcite. The potential ecological risk of heavy metals in six molybdenum tailings is high, while Pb and Cd are the main pollution risk elements. Molybdenum tailings contain considerable amounts of critical minerals with huge potential economic value, and molybdenum tailings with high environmental hazards could be converted into a possible source for critical minerals by recovering the critical minerals and repurposing the secondary tailings as an additive or cement substitute. This study provides an innovative idea for the pollution treatment of molybdenum tailings and indicates the prospect of molybdenum tailings as a secondary source for critical minerals.

Diffusion kinetic processes and release risks of trace metals in plateau lacustrine sediments

The ecological risk posed by trace metals in the plateau lacustrine sediments of China has attracted worldwide attentions. A better understanding of the kinetic diffusion processes and bioavailability of these metals in plateau lakes is needed. Using the diffusive gradient in thin films (DGT) and Rhizon, concentrations of Mn, Mo, Ni, Cr, and Co in the sediments, labile fractions, and interstitial water of Lake Fuxian were comprehensively analyzed. According to the DGT-induced fluxes in sediments (DIFS) model, fully sustained and unsustained resupplies are possible ways in which metals are released from solids to the solution. Moreover, the resupply characteristics of metals varied at different depths in the sediments and at different sites in the lake. Based on the DIFS model, the effective concentrations (CE) of the trace metals were calculated and all except Cr showed good linear relationships with the DGT-labile concentrations, indicating that the CE values were valuable for predicting metal bioavailability. According to the CE values, the metal contamination released from the sediments was relatively low based on the Monte Carlo simulation. This study provides a comprehensive solution for studying the environmental behavior and potential ecological risks of toxic metals in sedimentary environment.

Identification of factors driving the spatial distribution of molybdenum (Mo) in topsoil in the Longitudinal Range-Gorge Region of Southwestern China using the Geodetector model

As a key component of plant nitrogen-fixing enzymes and a variety of human coenzyme factors, molybdenum (Mo) plays an essential role in supporting both plant growth and human health. Soil is a key medium for the cycling of Mo in the biosphere. However, the driving anthropogenic and natural factors governing the spatial distribution of Mo in soil and their interactions are not well understood. To determine the factors that affect the spatial patterns of Mo in topsoil, 6980 samples were collected from the Longitudinal Range-Gorge Region (Linshui County, Sichuan Province, China). In this area, tall mountains are adjacent to deep valleys. Topsoil with enriched Mo is mostly distributed in mountainous areas. The most important factors influencing Mo in topsoil are soil parent materials (q = 0.482), altitude (q = 0.256), and soil type (q = 0.259). There are synergistic effects among the various driving factors [q(X1 ∩ X2) > Max[q(X1), q(X2)]]. The Geodetector model was used to validate the magnitude of the interaction effects. The contribution to interacting factors is nonlinearly enhanced when the contribution of a single factor was low (any two factors of aspect, road distance, land use type, and S). The contribution to interacting factors is enhanced bidirectionally when the contribution of a single factor was high (any two factors of altitude, soil type, soil parent material, OM, and TFe2O3). When the contribution of one factor is high and the other is low, the contributing to interacting factors is mostly enhanced bidirectionally and a few are nonlinearly enhanced.

Geospatial patterns and geochemical compositional characteristics of molybdenum in different mediums of an urban environment

Urban areas are characterized by the presence of certain potentially toxic elements including molybdenum (Mo). Therefore, compositional data analysis combined with geospatial mapping was applied in this study to reveal the spatial distribution characteristics of Mo in courtyard surface dust (dust), soils, and river sediments (sediments), to identify potential sources of Mo, and to reveal Mo geochemical associations in different urban environmental mediums. The mean contents of Mo decreased in the following order: dust (11.9 mg/kg) - soil (5.84 mg/kg) - sediment (4.87 mg/kg). The highest maximum Mo content among the studied mediums (61.8 mg/kg) was detected in dust. It was the only investigated medium where a very high level of Mo enrichment was observed (4.4% of samples). Moreover, a significant level of enrichment predominated in dust (47% of samples) whereas in soil, moderate enrichment prevails (68.5%). A significant correlation of Mo contents was observed with Zn contents in all the studied mediums, and with Ca contents in soil and sediments. A significant negative correlation was observed only between Mo contents in dust and sampling site altitudes suggesting that high-rise buildings might play the role of geochemical barriers. Principal component analysis, k-means and hierarchical clustering showed that in the geogenic elements soil group Mo showed an affinity to be bound by Fe/Mn oxide/hydroxides whereas the Mo coprecipitation, complexation and absorption by carbonates predominated in the Mo-related soil group (geochemical compositional association of Mo, Zn, Cu, Pb, and Ca) under anthropogenic influence. For dust, the geochemical compositional association was the same, but in the geogenic-related group, Cu was the most closely associated element instead of Zn. The spatial location of the Mo-related group of samples identified by k-means clustering indicates that Mo concentrate processing plant may be a potential source of Mo introduction into the urban environment.

Enrichment mechanisms of Mo in soil in the karst region Guangxi, China

Soils developed in karst regions have naturally high background values of molybdenum (Mo) due to geological factors. However, the enrichment mechanism of Mo in these soils are not fully understood, making it challenging to assess their ecological risk and utilize Mo-rich land resources. To shed light on this issue, this study collected and analyzed data from the 1:50,000 geochemical survey in Guangxi, including 536,503 sets of soil data and 3043 sets of rock data, as well as 40 sets of carbonate rock-soil from typical karst regions. The results showed that soil Mo enrichment is highly correlated with the distribution of carbonate rocks in karst regions. The carbonate rocks in these regions contain Mo ranging from 0.03 to 1.06 mg·kg^-1 (with a mean of 0.22 mg·kg^-1). In comparison, the soil Mo derived from carbonate rocks can reach up to 6.00 mg·kg^-1 (with a mean of 2.75 mg·kg^-1), representing an average enrichment of soil Mo that is 24 times higher compared to the carbonate parent rock. The enrichment of soil Mo in karst regions is primarily controlled by secondary enrichment during the weathering process of carbonate. During the insoluble residue accumulation process, the dissolution of carbonate leads to a dramatic reduction in bedrock volume, and the adsorption of clay minerals and Fe minerals in insoluble residues plays an essential role in Mo enrichment during these stages. During the soil-forming stage of the insoluble residue, most Mo leaches into the water body due to the mineral transformation of insoluble residue. Consequently, as Fe-Mn nodules in soils become more enriched with increasing weathering intensity, some Mo is absorbed and passivated by iron and manganese oxides (hydroxides). Accordingly, the contribution of Fe-Mn nodules and the degree of leaching were closely related to the enrichment of soil Mo in karst regions. This study provides insights into the enrichment mechanisms of Mo in soils developed in karst regions, which will help to evaluate their ecological risk in these environments.

Isotope evidence for temporal and spatial variations of anthropogenic sulfate input in the Yihe River during the last decade

Pyrite oxidation and sedimentary sulfate dissolution are the primary components of riverine sulfate (SO₄^2-) and are predominant in global SO₄^2- flux into the ocean. However, the proportions of anthropogenic SO₄^2- inputs have been unclear, and their tempo-spatial variations due to human activities have been unknown. Thus, field work was conducted in a spatially heterogeneous human-affected area of the Yihe River Basin (YRB) during a wet year (2010) and drought years (2017/2018). Dual sulfate isotopes (δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2-) and Bayesian isotope mixing models were used to calculate the variable anthropogenic SO₄^2- inputs and elucidate their temporal impacts on riverine SO₄^2- flux. The results of the mixing models indicated acid mine drainage (AMD) contributions increased from 56.1% to 83.1% of upstream sulfate and slightly decreased from 46.3% to 44.0% of midstream sulfate in 2010 and 2017/2018, respectively, in the Yihe River Basin. The higher upstream contribution was due to extensive metal-sulfide-bearing mine drainage. Sewage-derived SO₄^2- and fertilizer-derived SO₄^2- inputs in the lower reaches had dramatically altered SO₄^2- concentrations and δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2- values. Due to climate change, the water flow discharge decreased by about 70% between 2010 and 2017/2018, but the riverine sulfate flux was reduced by only about 58%. The non-proportional increases in anthropogenic sulfate inputs led to decreases in the flow-weighted average values of δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2- from 10.3‰ to 9.9‰ and from 6.1‰ to 4.4‰, respectively. These outcomes confirm that anthropogenic SO₄^2- inputs from acid mine drainage (AMD) have increased, but sewage effluents SO₄^2- inputs have decreased.