Catchment Runoff in Industrial Areas Exports Legacy Pollutant Zinc from the Topsoil Rather than Geogenic Zn

Seasonal and multi-decadal zinc isotope variations in blue mussels from two sites with contrasting zinc contamination levels

Zinc (Zn) isotope compositions in soft mussel tissues help identify internal biological processes and track coastal Zn sources in coastal environments, thus aiding in managing marine metal pollution. This study investigated the seasonal and multi-decadal Zn isotope compositions of blue mussels (genus Mytilus) from two French coastal sites with contrasting Zn environmental contamination. Concurrently, we characterized the isotope ratios of sediments and plankton samples at each site to understand the associations between organisms and abiotic compartments. Our primary objective was to determine whether these isotope compositions trace long-term anthropogenic emission patterns or if they reflect short-term biological processes. The multi-decadal isotope profiles of mussels in the Loire Estuary and Toulon Bay showed no isotope variations, implying the enduring stability of the relative contributions of natural and anthropogenic Zn sources over time. At seasonal scales, Zn isotope ratios were also constant; hence, isotope effects related to spawning and body growth were not discernible. The multi-compartmental analysis between the sites revealed that Toulon Bay exhibits a remarkably lower Zn isotope ratio across all studied matrices, suggesting the upward transfer of anthropogenic Zn in the food web. In contrast, the Zn isotope variability observed for sediments and organisms from the Loire Estuary fell within the natural baseline of this element. In both sites, adsorptive geogenic material carrying significant amounts of Zn masks the biological isotope signature of plankton, making it difficult to determine whether the Zn isotope ratio in mussels solely reflects the planktonic diet or if it is further modified by biological homeostasis. In summary, Zn isotope ratios in mussels offer promising avenues for delineating source-specific isotope signatures, contingent upon a comprehensive understanding of the isotope fractionation processes associated with the trophic transfer of this element through the plankton.

Sizeable net export of base cations from a Carpathian flysch catchment indicates their geogenic origin while the 26Mg/24Mg, 44Ca/40Ca and 87Sr/86Sr isotope ratios in runoff are indistinguishable from atmospheric input

Nutrient imbalances may negatively affect the health status of forests exposed to multiple stress factors, including drought and bark beetle calamities. We studied the origin of base cations in runoff from a small Carpathian catchment underlain by base-poor flysch turbidites using magnesium (Mg), calcium (Ca) and strontium (Sr) isotope composition of 10 ecosystem compartments. Our objective was to constrain conclusions drawn from long-term hydrochemical monitoring of inputs and outputs. Annual export of Mg, Ca and Sr exceeds 5-to-15 times their atmospheric input. Mass budgets per se thus indicate sizeable net leaching of Mg, Ca and Sr from bedrock sandstones and claystones. Surprisingly, δ26Mg, δ44Ca and 87Sr/86Sr isotope ratios of runoff were practically identical to those of atmospheric deposition and soil water but significantly different from bedrock isotope ratios. We did not find any carbonates in the studied area as a hypothetical, easily dissolvable source of base cations whose isotope composition might corroborate the predominance of geogenic base cations in the runoff. Marine carbonates typically have lower δ26 Mg and 87Sr/86Sr ratios, and silicate sediments often have higher δ26Mg and 87Sr/86Sr ratios than runoff at the study site. Mixing of these two sources, if confirmed, could reconcile the flux and isotope data.

Anthropogenic processes drive heterogeneous distributions of toxic elements in shallow groundwater around a smelting site

Smelting activities have a far-reaching influence on the quality of soil and groundwater, while most studies have neglected the information on the pollution characteristics of groundwater. The hydrochemical parameters of shallow groundwater and the spatial distributions of toxic elements were investigated in this study. Correlations analysis and groundwater evolution revealed that the major ions were primarily determined by silicate weathering and calcite dissolution process, and anthropogenic processes had a significant effect on groundwater hydrochemistry. Almost 79%, 71%, 57%, 89%, 100%, and 78.6% of samples exceeded the standards of Cd, Zn, Pb, As, SO₄^2-, and NO₃^-, and their distribution is closely related to the production process. Analysis of soil geochemistry indicated that the relatively mobile forms of toxic elements strongly influence the origin and concentration in shallow groundwater. Besides, rainfall with high magnitude would lead to a decrease of toxic elements in shallow groundwater, whereas the area once stacked waste residue was the opposite. It is recommended to strengthen risk management of the limited mobility fraction while devising a plan for waste residue treatment in accordance with the local pollution conditions. The research on controlling the mechanism of toxic elements in shallow groundwater, along with sustainable development in the study area and other smelting zones may benefit from this study.

Advances in the application of metallic isotopes to the identification of contaminant sources in environmental geochemistry

The development of the economy and society makes heavy metals (HMs) pollution more and more serious. And, pollution source identification is the primary work of environmental pollution control and land planning. Notably, stable isotope technology has a high ability to distinguish pollution sources, and can better reflect the migration behavior and contribution of HMs from diverse sources, which has become a hot research tool for pollution source identification of HMs. Currently, the rapid development of isotope analysis technology provides a relatively reliable reference for pollution tracking. Based on this background, the fractionation mechanism of stable isotopes and the influence of environmental processes on isotope fractionation are reviewed. Furthermore, the processes and requirements for the measurement of metal stable isotope ratios are summarized, and the calibration methods and detection accuracy of sample measurement are evaluated. Besides, the current commonly used binary model and multi-mixed models in the identification of contaminant sources are also concluded. Moreover, the isotopic changes of different metallic elements under natural and anthropogenic conditions are discussed in detail, and the application prospects of multi-isotope coupling in the traceability of environmental geochemistry are evaluated. This work has some guidance for the application of stable isotopes in the source identification of environmental pollution.

High-precision zinc isotopic characterization of twenty soil reference materials from China determined by MC-ICP-MS

Zinc isotopic ratios serve as powerful tools for tracing biochemical cycling of metals at Earth's surface, including the distribution, transportation, and enrichment of zinc (Zn) in soil. To conduct such studies and enable inter-laboratory comparisons, high-precision Zn isotopic measurements require the use of soil reference materials (RMs). However, there have been limited reports on the high-precision Zn isotope ratios of soil RMs thus far. In this study, we have developed a two-step Zn chemical separation protocol utilizing Bio-Rad AG MP-1M resin columns. This method has demonstrated excellent reproducibility for measuring the external δ⁶⁶Zn values (relative to JMC-Lyon) of standard soil reference materials over an extended time period, with a better than 0.06‰ (2SD) precision. Remarkably, this study is the first to report the Zn isotopic compositions of 20 soil reference materials from various soil types in China. With the exception of one sample obtained from a mining area, the Zn isotopic compositions of all the analyzed soil reference materials exhibit remarkable similarity, with an average δ⁶⁶Zn value of 0.31 ± 0.12‰, which aligns closely with the values observed in igneous rocks. The exceptional sample, with a higher δ⁶⁶Zn value of 0.61 ± 0.02‰, indicates potential contamination during mining activities.

Modeling transport and fate of heavy metals at the watershed scale: State-of-the-art and future directions

A predictive understanding of the source-specific (e.g., point and diffuse sources) land-to-river heavy metal (HM) loads and HM dynamics in rivers is essential for mitigating river pollution and developing effective river basin management strategies. Developing such strategies requires adequate monitoring and comprehensive models based on a solid scientific understanding of the watershed system. However, a comprehensive review of existing studies on the watershed-scale HM fate and transport modeling is lacking. In this review, we synthesize the recent developments in the current generation of watershed-scale HM models, which cover a wide range of functionalities, capabilities, and spatial and temporal scales (resolutions). Existing models, constructed at various levels of complexity, have their strengths and weaknesses in supporting diverse intended uses. Additionally, current challenges in the application of watershed HM modeling are covered, including the representation of in-stream processes, organic matter/carbon dynamics and mitigation practices, the issues of model calibration and uncertainty analysis, and the balance between model complexity and available data. Finally, we outline future research requirements regarding modeling, strategic monitoring, and their combined use to enhance model capabilities. In particular, we envisage a flexible framework for future watershed-scale HM models with varying degrees of complexity to accommodate the available data and specific applications.

Integrated assessment of land-to-river Cd fluxes and riverine Cd loads using SWAT-HM to guide management strategies

In 2011, China invested US$9.8 billion to combat the severe heavy metal pollution in the Xiang River basin (XRB), aiming to reduce 50% of the 2008 industrial metal emissions by 2015. However, river pollution mitigation requires a holistic accounting of both point and diffuse sources, yet the detailed land-to-river metal fluxes in the XRB remain unclear. Here, by combining emissions inventories with the SWAT-HM model, we quantified the land-to-river cadmium (Cd) fluxes and riverine Cd loads across the XRB from 2000 to 2015. The model was validated against long-term historical observations of monthly streamflow and sediment load and Cd concentrations at 42, 11, and 10 gauges, respectively. The analysis of the simulation results showed that the soil erosion flux dominated the Cd exports (23.56-80.14 Mg yr^-1). The industrial point flux decreased by 85.5% from 20.84 Mg in 2000 to 3.02 Mg in 2015. Of all the Cd inputs, approximately 54.9% (37.40 Mg yr^-1) was finally drained into Dongting Lake; the remaining 45.1% (30.79 Mg yr^-1) was deposited within the XRB, increasing the Cd concentration in riverbed sediment. Furthermore, in XRB's 5-order river network, the Cd concentrations in small streams (1st order and 2nd order) showed larger variability due to their low dilution capacity and intense Cd inputs. Our findings highlight the need for multi-path transport modeling to guide future management strategies and better monitoring schemes to restore the small polluted streams.

Spatial and temporal trends in δ66Zn and 206Pb/207Pb isotope ratios along a rural transect downwind from the Upper Silesian industrial area: Role of legacy vs. present-day pollution

Transect sampling is an under-exploited tool in isotope studies of atmospheric pollution. Few studies have combined Zn and Pb isotope ratios to investigate whether atmospheric pollution at a receptor site is dominated by a different anthropogenic source of each of these toxic elements. It has been also unclear whether pollution abatement strategies in Central Europe have already resulted in regionally well-mixed background isotope signature of atmospheric Zn and Pb. Zinc and lead isotope ratios were determined in snow collected along a rural transect downwind from the Upper Silesian industrial area (southern Poland). Spatial and temporal gradients in δ⁶⁶Zn and ²⁰⁶Pb/²⁰⁷Pb ratios at four sites were compared with those of ore and coal collected in eight Czech and Polish mining districts situated at distances of up to 500 km. Snow pollution was extremely high 8 km from Olkusz in 2011 (1670 μg Zn L^-1; 240 μg Pb L^-1), sharply decreased between 2011 and 2018, and remained low in 2019-2021. Snow pollution was lower at sites situated 28-68 km from Olkusz. Across study sites, mean δ⁶⁶Zn and ²⁰⁶Pb/²⁰⁷Pb ratios of snow were -0.13‰ and 1.155, respectively. With an increasing distance from Olkusz, the δ⁶⁶Zn values first increased and then decreased, while the ²⁰⁶Pb/²⁰⁷Pb ratios first decreased and then increased. The δ⁶⁶Zn values in snow plotted closer to those of Upper Silesian ores (-0.20‰) than to the δ⁶⁶Zn values of Upper Silesian stone coal (0.52‰), showing predominance of smelter-derived over power-plant derived Zn pollution. The ²⁰⁶Pb/²⁰⁷Pb ratios of Upper Silesian coal (1.171) and Upper Silesian ores (1.180) were higher compared to those of snow. A²⁰⁶Pb/²⁰⁷Pb vs.²⁰⁸Pb/²⁰⁷Pb plot identified legacy pollution from leaded gasoline as the low-radiogenic mixing end-member. Across the transect sites, only the last sampling campaign exhibited a high degree of isotope homogenization for both Zn and Pb.

Environmental implications of land use change on the fate of Zn in agricultural soils: A case study of Puding karst soils, southwest China

Land use change threatens food security because it may cause the depletion and/or low bioavailability of micronutrients in agricultural soils. Therefore, it is significant to investigate the fate of micronutrients and predict the potential environmental problems. The zinc isotope technique is of particular interest in interpreting soil processes. In this study, Zn isotopic data of soil samples in five profiles based on different land uses were provided, and Zn behavior in different soils was discussed. The isotopic ratios of soil samples in the abandoned orchard, secondary forest, abandoned cropland, and cropland are similar, with the δ⁶⁶Zn varying from 0.15 to 0.29‰. However, the samples in shrub grassland show a lower δ⁶⁶Zn of 0-0.20‰, which may be affected by anthropogenic sources. For the vertical patterns, the non-cultivated long-rooted plants (i.e., abandoned orchard and secondary forest) show no significant difference in the distribution of δ⁶⁶Zn, but the patterns of cropland and abandoned cropland samples are reversed. The cropland samples show positive correlations between δ⁶⁶Zn and Fe₂O₃ (R² = 0.90) and MnO (R² = 0.75), indicating that Fe and Mn oxides preferentially adsorb heavy Zn isotopes on the mineral surfaces. The high affinity between Zn and oxides indicated that the concentration of bio-available Zn in cropland soils was getting lower. As a result, the supplies of micronutrients may be deficient and urged from fertilizer. This study provides a better understanding of Zn cycling in agricultural systems and gives improvements in soil management.

Zinc in soil reflecting the intensive coal mining activities: Evidence from stable zinc isotopes analysis

In the mining area affected by coal mining activities for a long time, heavy metal Zn pollution poses a serious threat to soil quality and human health, and direct evidence showing the relationship between Zn accumulation mechanism in soils and mining activities is lacking. In this study, the Zn content and isotopes composition (δ⁶⁶Zn) from soil and environmental samples around mining area were determined and analyzed to clarify the Zn characteristics in soil. Moreover, the distribution and source of Zn content in soil of mining area were analyzed by mathematical statistics, correlation analysis and isotope mass mixing model. The results showed that: (1) the Zn content in soil ranged from 95 to 327 mg·kg^-1 (mean: 233 mg·kg^-1), exceeding the control point and the soil background value of Anhui Province; (2) the results of Zn isotope analysis showed that Zn in soil mainly derived from the wind dispersion input of fine particles in gangue and fly ash, followed by the natural weathering of parent material; (3) isotopic mass mixing model can be used to distinguish the contribution of anthropogenic and natural Zn sources. Mining input was the main contribution source of Zn in soil (mean: 67%), followed by natural background (mean: 33%). The employment of Zn isotopes can effectively evaluate the impact of anthropogenic and natural long-term processes on Zn in the soil of the mining area, and provide important information for the formulation of soil metal pollution control measures.

Zinc isotopic signature in tropical soils: A review

The micronutrient cycling in tropical latitudes is an issue of great concern because tropical soils are not only suffering micronutrient deficiency, but also influencing the global cycling of trace metals. With the development of stable isotope techniques, Zn isotopic composition (δ⁶⁶Zn) has been an powerful tool to interpret the Zn behaviour, signature, and cycling in soils. This review compiles δ⁶⁶Zn ratios of ten types of soils from both tropical and non-tropical latitudes, to (i) discuss the Zn isotopic signature in tropical soils and at the interfaces of soil-plant-river-ocean, (ii) disclose the Zn mass balance in tropical latitudes, and (iii) provide an implication for the eco-environmental effects of Zn cycling in tropical latitudes. Zinc isotopic signature is constrained by soil constituents. Our review summarized that the precipitation of secondary Fe oxides and organic complexation in the aqueous phases are likely to result in the preferential preservation of light Zn isotopes in tropical soils. The extreme weathering and material leaching of tropical soils can remove large amounts of Zn and thus result in Zn deficiency in tropical latitudes and pose risks to plant growth. The removed Zn is likely to influence the instantaneous riverine δ⁶⁶Zn heavier than that of the crustal average. However, the modern oceanic δ⁶⁶Zn will ultimately approach those of the parent materials by mass balance, at large geological timescales. Future direction should be concerned with the isotopic studies on Zn speciation in tropical soils and the association of isotopic ratios with the flux of Zn to quantitatively estimate of the Zn mass balance in tropical regions. The prospect of this review is to help solve the issue of plant micronutrition, as well as riverine and marine bio-availablity.

Experimental and theoretical investigation for the cycloaddition of carbon dioxide to epoxides catalyzed by potassium and boron co-doped carbon nitride

Much endeavor has been devoted to efficient heterogeneous catalysts for carbon dioxide (CO₂) conversion to high-value chemicals. Meanwhile, the cycloaddition of CO₂ to epoxides is considered as a green and atom-economy reaction to produce cyclic carbonates. Herein, a series of K, B co-doped CN with various doping contents (K, B-CN-X) were developed by simple one-step calcination of melamine and KBH₄. B was confirmed to replace the C site and KN bond was formed, which was verified by XPS (X-ray photoelectron spectroscopy) and DFT (density functional theory) calculation. Particularly, K, B-CN-4 displayed the optimal catalytic performance in the presence of Bu₄NBr (tetrabutylammonium bromide) cocatalyst for the CO₂ cycloaddition with propylene oxide. Besides, K, B-CN-4/Bu₄NBr catalyst exhibited good substrate versatility to various epoxides and excellent recycling performance. According to the DFT calculation on CO₂ adsorption and experimental results, K, B-CN-4 presented satisfactory catalytic activity due to the enhanced CO₂ adsorption after K and B dopings then the possible reaction mechanism was proposed. The promising K, B-CN-X catalyst presented an attractive application due to the simple, eco-friendly synthesis route for the efficient fixation of CO₂.