Elemental and isotopic behaviour of Zn in Deccan basalt weathering profiles: Chemical weathering from bedrock to laterite and links to Zn deficiency in tropical soils

Exploration of natural processes and anthropogenic inputs by Zn isotopes in suspended particulate matter: A case study from the Lancang River in Southwest China

River is an important pathway for the biogeochemical cycle of Zn. This study reports Zn concentration and δ66Zn composition for suspended particulate matter (SPM) from Lancang River basin in Southwest China, and explore the impact of natural processes and human activities on Zn cycle. The SPM samples have a much higher average Zn content (162 mg kg-1) than that of the upper crust (67.0 mg kg-1), but it is close to the value of the Pearl River (187 mg kg-1). The enrichment factor (EF) values of Zn in SPM range from 1.08 to 6.88, with an average of 2.15, which does not show significant pollution characteristics. The δ66Zn values in SPM range from -0.67‰ to +0.63‰, with an average of +0.13‰. The δ66Zn values showed positive correlation with Ca/Mg ratios while showed little correlation with Zn contents in SPM. It indicated that anthropogenic sources have limited influence on SPM, and the Zn isotope composition in SPM is more likely to be inherited from the weathered rocks materials and influenced by natural fractionation processes in river water. This result contributes to understanding of the geochemical cycling process of Zn and its environmental effects in water.

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.

Land-use conversion controls on the mobility of Zn in paddy soils revealed by stable Zn isotopes

Understanding Zn biogeochemical cycling is necessary for monitoring Zn supply for plants and life during land use conversion, which is critical for environmentally sustainable development. But little is known about how the conversion of paddy soil to abandoned land affects the Zn isotope signature. A comparative field observation was conducted in northeast Thailand to investigate the Zn isotope footprint of paddy soils and abandoned paddy soils (PL and NPL). Our results show that Zn (τ_Zn^int : 0.04) slightly retains on PL, but is lost from NPL (τ_Zn^int from - 0.81 to - 0.24) to the river during weathering. Compared to PL (Δ⁶⁶Zn_parent-soil: -0.29 ‰), more ⁶⁶Zn isotopes might enter the river when rice cultivation ceases in NPL (Δ⁶⁶Zn_parent-soil from -0.26 ‰ to -0.47 ‰). Rice harvest and then root decay might result in heavy ⁶⁶Zn isotopes accumulating at the topmost soil in PL (δ⁶⁶Zn: 0.14 ‰) and short-term abandonment (1-2 years) in paddy soils (NPL1 δ⁶⁶Zn: 0.18 ‰). The release of assimilated Zn, and then the high adsorption of Zn in the Fe-SOM-metal(loid)s ternary system positively contribute to the high [Zn] in PL, while this was not observed in NPL. Our findings provide a comprehensive insight into the Zn isotope signature in response to the conversion of land-use types, which is beneficial for understanding the terrestrial Zn geochemical cycle.

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.

Zn isotope fractionation in laterites from Yunnan province, southwest China: Implications for the Zn cycles and its environmental impacts in (sub-) tropics

The weathering and development of laterites can influence trace element cycling in (sub-) tropics. Zinc (Zn) is a ubiquitous trace metal that involves both abiotic and biotic processes in soils. To explore Zn behavior in laterites, Zn cycling in (sub-) tropics, and the environmental impacts, Zn isotope systematics were presented for two laterite profiles from Yunnan province, southwest China. The laterite samples exhibit the δ⁶⁶Zn of 0.02 ‰-0.56 ‰, indicating a light shift of Zn isotope ratios (Δ⁶⁶Zn_{laterite-parent rock} = -0.47 ‰-0.07 ‰) relative to bulk parent granite. This observation is attributed to the preferential preservation of light Zn isotopes on the surface of secondary Fe oxides. As a result, laterites are likely to control the instantaneous riverine δ⁶⁶Zn in (sub-) tropical regions heavier than unweathered rocks. The isotopic signature of different vegetation covered soils show that shrub-covered soils are stronger leached (average τ_Zn = -0.61) and have a smaller Δ⁶⁶Zn_{laterite-parent rock} (=-0.15 ‰), relative to forest-covered soils (=-0.20 ‰). Due to the strong loss of Zn (average τ_Zn = -0.61 to -0.12) and large amounts of low-bioavailable Zn preserved in oxides, the micronutrient supplies for plant growth are difficult to maintain and need more fertilization. This study is helpful for a better understanding of global Zn cycling and the management of micronutrients in (sub-) tropical soil-plant systems.

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.

The role of pedogenesis and natural fertiliser as vectors for essential metal content in agricultural topsoils, Central India

Essential trace metals are well known for their environmental toxicity and for being part of complex bio-chemical cycles. Their role as critical micronutrients, delivering vital health benefits, is less widely discussed and understood, holding back strategies for combatting malnutrition. Crops grown on many Indian soils suffer from deficiencies in essential metals, notably iron (Fe), zinc (Zn), and molybdenum (Mo). The list of deficient metals will likely grow due to increasing future crop demand. Geostatistical analysis of soils and farmyard manure (FYM), the predominant fertiliser, implies that residual oxide minerals carry high concentrations of the essential trace metals Fe, Zn, copper (Cu), chromium (Cr), nickel (Ni), cobalt (Co), manganese (Mn) not only in soil but also in FYM (especially Fe, Cr, Cu, Co and Ni). A geochemical survey across a road traverse of 600 km, encompassing an area of c. 15,000 km2, was conducted in Central India to evaluate reported essential metal deficiency in key agricultural topsoils. Importantly, our evaluation of the element cycling in this system reveals that despite high bulk concentrations, some key metals remain bio-unavailable. In effect, the existence of refractory (weathering-resistant) oxides is likely a significant factor for deficiency symptoms in the soil–plant-fertiliser cycle. Further, mass balance calculations of the bioavailable pool of metals imply that only Fe and Mn are present in sufficient quantities to combat deficiency problems. Notwithstanding this limitation of FYM, its high organic carbon content, as well as its importance for Zn, Cu and Fe, validates its traditional use to maintain the fertility and physical condition of Indian topsoils.

Using Zn isotopes to trace Zn sources and migration pathways in paddy soils around mining area

Paddy soils around mining areas suffer from the great threat of heavy metal pollution. The traditional source-tracing methods based on metal concentrations limit our ability to quantify the sources of heavy metals and trace their transport processes to paddy soils. In this study, Zn isotope compositions of paddy soils in Dabaoshan mine area, a typical sulfide deposit in southern China, have been systematically studied. According to a plot between 1/Zn (i.e. inverse concentration) and δ⁶⁶Zn value, all the polluted paddy soils fall on the mixing line between acid mine drainage precipitate (AMD-precipitate) and fertilizer while the unpolluted paddy soil falls on the mixing line between fertilizer and bedrock. This indicates the mixing of Zn sources at least three end-members: the mining end-member (i.e. AMD-precipitate), the agricultural end-member (i.e. fertilizer), and bedrock whose geochemical signature is often overprinted by the former two sources around the mining area. The quantitative calculations to apportion the end-member's contributions show that the mining activity contributes most Zn in the paddy soils with an average of ∼66.2%. The contribution of mining activities has significant spatial variations. Specifically, the mining activities have relatively low impacts on the lower reach and the deep soil. Additionally, the apparent Zn isotope fractionation between AMD and AMD-precipitate (Δ⁶⁶Zn_{AMD-precipitate} - _AMD of -0.35 to -0.08‰) in the tailings dam suggests that Zn cations in AMD coprecipitated with the secondary Fe-bearing minerals (e.g. jarosite and goethite). After being discharged from the tailings dam, Zn is mainly carried by the Fe-oxide minerals and migrated during surface runoff. Our study highlights the contribution of human activities to the Zn pollution in the paddy soils and the key role of Fe-bearing minerals in the migration of Zn. These findings provide a scientific base for the development of policy for pollution control in mining-affected region.

Association of Defects and Zinc in Hematite

Zn is an essential micronutrient that is often limited in tropical, lateritic soils in part because it is sequestered in nominally refractory iron oxide phases. Stable phases such as goethite and hematite, however, can undergo reductive recrystallization without a phase change under circumneutral pH conditions and release metal impurities such as Zn into aqueous solutions. Further, the process appears to be driven by Fe vacancies. In this contribution, we used ab initio molecular dynamics informed extended X-ray absorption fine structure spectra to show that Zn incorporated in the structure of hematite is associated with coupled O-Fe and protonated Fe vacancies, providing a potential link between crystal chemistry and the bioavailability of Zn.

Barium isotopic fractionation in latosol developed from strongly weathered basalt

Weathering is a key process in the transfer of material from continents to the hydrosphere. A latosol profile in Zhanjiang, Guangdong Province, South China, formed through intense weathering of basalt, was studied to improve understanding of Ba isotopic fractionation during basalt weathering. Profile horizons were grouped into Ba-depleted and Ba-enriched layers (D- and E-layers, respectively) according to the mass fraction of Ba lost or gained from the weathered profile relative to bedrock. δ^137/134Ba values in the soil profile ranged from -0.22‰ to +0.02‰, lower than those of the parent basaltic rock (0.03‰ ± 0.03‰). In the D-layers, Ba isotopic fractionation can be explained by Rayleigh fractionation, implying that heavy Ba isotopes are preferentially leached. The Rayleigh fractionation model is not applicable to the E-layers because they preferentially acquired isotopically light Ba isotopes during weathering. Results indicate a net loss of heavy Ba isotopes during strong weathering of basalt due to the precipitation of FeMn (oxyhydr)oxides and adsorption on secondary minerals. A mass-balance model indicates that the average δ^137/134Ba value of materials leached from the weathered profile is ~0.08‰, slightly higher than that of the bedrock. This suggests a loss of heavy Ba isotopes into the hydrosphere during weathering of basalt, consistent with the enrichment of heavy Ba isotopes in river waters.

Environmental applications of metal stable isotopes: Silver, mercury and zinc

With developments in multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), applications of metal stable isotopes received increasing attentions in the studies of source and fate of heavy metals in the environment. In light of the rapid progresses in this emerging field, we attempted to review the recent findings comprehensively in a way that environmental scientists can easily read. This review started with an introduction of basic terminologies in isotope geochemistry, followed with detailed descriptions of instrumentation and analytical procedures, and finally focused on the cases of three typical metal stable isotopes (Ag, Hg and Zn) to illustrate how they were applied to address environmental issues. Additionally, future perspectives on the applicability, opportunities, and limitations of metal stable isotope techniques as novel approaches in advancing environmental chemistry were discussed.

Soil Salinity and pH Drive Soil Bacterial Community Composition and Diversity Along a Lateritic Slope in the Avon River Critical Zone Observatory, Western Australia

Soils are crucial in regulating ecosystem processes, such as nutrient cycling, and supporting plant growth. To a large extent, these functions are carried out by highly diverse and dynamic soil microbiomes that are in turn governed by numerous environmental factors including weathering profile and vegetation. In this study, we investigate geophysical and vegetation effects on the microbial communities of iron-rich lateritic soils in the highly weathered landscapes of Western Australia (WA). The study site was a lateritic hillslope in southwestern Australia, where gradual erosion of the duricrust has resulted in the exposure of the different weathering zones. High-throughput amplicon sequencing of the 16S rRNA gene was used to investigate soil bacterial community diversity, composition and functioning. We predicted that shifts in the microbial community would reflect variations in certain edaphic properties associated with the different layers of the lateritic profile and vegetation cover. Our results supported this hypothesis, with electrical conductivity, pH and clay content having the strongest correlation with beta diversity, and many of the differentially abundant taxa belonging to the phyla Actinobacteria and Proteobacteria. Soil water repellence, which is associated with Eucalyptus vegetation, also affected beta diversity. This enhanced understanding of the natural system could help to improve future crop management in WA since the physicochemical properties of the agricultural soils in this region are inherited from laterites via the weathering and pedogenesis processes.

Assessment of the metal contamination evolution in the Loire estuary using Cu and Zn stable isotopes and geochemical data in sediments

In this work, a multi-elemental approach combining Cu and Zn stable isotopes is used to assess the metal contamination evolution in the Loire estuary bulk sediments. Elemental geochemical data indicate an increase of metal concentrations from the beginning of the industrial period peaking in the 1990s, followed by an attenuation of metal contamination inputs to the estuary. Zinc isotope compositions suggest a binary mixing process between Zn derived from terrigenous material and multi-urban anthropogenic sources. Copper isotope systematics indicate a single natural dominant source represented by weathered silicate particles from soils and rocks. This work demonstrates the applicability of Zn isotopes to identify anthropogenic Zn sources in coastal systems, even under a low to moderate degree of contamination. Further studies are required to constrain Cu sources and to elucidate possible effects of grain-size and mineralogy in the Cu isotope composition of sediment in the Loire estuary.