Zn isotope signatures in soil FeMn nodules with karst high geochemical background.

Zn isotope has the potential to be used as an environmental tracer, due to its role in fingerprinting specific sources and processes. However, few studies have focused on Zn isotope system in terrestrial ferromanganese (FeMn) nodules, which is important on understanding the behaviors of Zn in soils. In this study, we analyse the isotopic composition in soil FeMn nodules and surrounding materials from a typical karst region in Guangxi Province, southwestern China and use advanced synchrotron-based methods to characterize Zn speciation. The Zn isotope compositions of the FeMn nodules range from 0.09 to 0.66 ‰, with an average value of 0.24 ‰. Pb isotope fingerprinting reveals that the major material sources contributing to the FeMn nodules are the surrounding soil (δ66Zn: ~0.36 ‰) and partly weathered carbonate bedrock (δ66Zn: ~0.58 ‰), which contain heavier Zn isotopes than the nodules. Synchrotron-based X-ray fluorescence (µ-SXRF) shows that Zn is well correlated with both Fe and Mn. X-ray absorption near edge spectroscopy (XANES) measurements reveal that Zn is associated with both goethite and birnessite phases, with goethite-sorbed Zn accounting for ~76 % of the total Zn and birnessite-sorbed Zn accounting for ~24 %. By combining these new results, the isotopically light Zn in the FeMn nodules compared to their sources can be explained by equilibrium sorption of Zn on goethite and birnessite, during which light Zn is preferentially sorbed. Our study provides important new data on Zn isotope compositions in terrestrial soil FeMn nodules and constrains associated mechanisms, and have implications for using Zn isotopes as environmental tracers.

A process-based model for describing redox kinetics of Cr(VI) in natural sediments containing variable reactive Fe(II) species.

Sediment-associated Fe(II) is a critical reductant for immobilizing groundwater contaminants, such as Cr(VI). The reduction reactivity of sediment-associated Fe(II) is dependent on its binding environment and influenced by the biogeochemical transformation of other elements (i.e., C, N and Mn), challenging the description and prediction of the reactivity of Fe(II) in natural sediments. Here, anaerobic batch experiments were conducted to study the variation in sediment-associated Fe(II) reactivity toward Cr(VI) in natural sediments collected from an intensive agricultural area located in Guangxi, China, where nitrate is a common surface water and groundwater contaminant. Then, a process-based model was developed to describe the coupled biogeochemical processes of C, N, Mn, Fe, and Cr. In the process-based model, Cr(VI) reduction by sediment-associated Fe(II) was described using a previously developed multirate model, which categorized the reactive Fe(II) into three fractions based on their extractabilities in sodium acetate and HCl solutions. The experimental results showed that Fe(II) generation was inhibited by NO3- and/or NO2-. After NO3- and NO2- were exhausted, the Fe(II) content and its reduction rate toward Cr(VI) increased rapidly. As the Fe(II) content increased, the three reactive Fe(II) fractions exhibited approximately linear correlations with aqueous Fe(II) concentrations ( [Formula: see text] ), which was probably driven by sorptive equilibrium and redox equilibrium between aqueous and solid phases. The model results indicated that the reaction rate constants of the three Fe(II) fractions (kn) significantly increased with incubation time, and log(kn) correlated well with [Formula: see text] [ [Formula: see text] , [Formula: see text] and [Formula: see text] ]. The numerical model developed in this study provides an applicable method to describe and predict Cr(VI) removal from groundwater under dynamic redox conditions.

Assessing the fractionation and bioavailability of heavy metals in soil-rice system and the associated health risk.

This study developed a method to build relationships between chemical fractionations of heavy metals in soils and their accumulations in rice and estimate the respective contribution of each geochemical speciation in the soils from the Yangtze River Delta, China. In contaminated areas, residue and humic acid-bound fractions in soils were the main phases for most heavy metals. The mobility of heavy metals was in this following order: Cd > Pb ≈ Zn > Ni > As ≈ Cr > Hg. Transfer factors calculated by the ratios of specific fractionations of heavy metals in the soil-rice system were used to assess the capability of different metal speciation transfer from soil to rice. The carbonate and Fe/Mn oxyhydroxides bound phase had significant positive correlations with total metal concentrations in rice. Hg uptake by rice might be related to the exchangeable and carbonate-bound fractions of soil Hg. Results of PCA analysis of transfer factors estimated that the labile fractions (i.e. water soluble, exchangeable and carbonate bound) contributed more than 40% of the heavy metal accumulations in rice. Effect of organic matter and residue fraction on metals transfer was estimated to be ~ 25 to ~ 30% while contribution of humic acid and Fe/Mn oxyhydroxides-bound fractions was estimated to be ~ 20 to ~ 30%. Modified risk assessment code (mRAC) and ecological contamination index (ECI) confirmed that the soil samples were polluted by heavy metals. Soil Cd contributed more than 80% of mRAC. Contrarily, the main contributors to ECI were identified as As, Hg, Pb and Zn. The average values of total target hazard quotient (TTHQ) and Risktotal were above 1 and 10-4 respectively, implying people living in the study area were exposed to both non-carcinogenic and carcinogenic risk. As and Pb were the main contributor to high TTHQ value while As, Cd and Cr in rice contributed mostly to Risktotal value. Spatial changes of ecological risk indexes and human health risk indexes showed that the samples with high TTHQ values distributed in the area with high values of mRAC. Likewise, the area with high ECI values and with high carcinogenic risk overlapped.

Potential Ecological Risk Assessment of Heavy Metals in the Fe-Mn Nodules in the Karst Area of Guangxi, Southwest China.

We investigated the concentration of heavy metals in the Fe-Mn nodules in soils derived from the carbonate rocks of typical karst areas in Guangxi, Southwest China. Compared with the soil background values in Guangxi, heavy metals are substantially enriched in the following order: Cd (268.13) > Pb (39.46) > Cr (11.80) > Zn (8.43) > Ni (6.16) > Cu (3.65). CaCl2 extraction results indicate that heavy metals in the nodules are substantially stable, while the proportions of the metals released to the surrounding environment are extremely low (≤ 0.003%). Moreover, pH-static leaching experiments indicate that heavy metals can hardly be dissolved into the surrounding soil environment under natural conditions (pH 6-8). However, once the soil environment is acidified, heavy metals incorporated in the nodules will gradually release into the surrounding environment, thereby causing potential ecological risks.

Enrichment and source identification of Cd and other heavy metals in soils with high geochemical background in the karst region, Southwestern China.

The geographical distributions of Cd and several other metals (Cu, Ni, Pb, Zn, and Cr) were characterized in 308 terra rossa samples across the Guangxi karst region. We found significant enrichments of heavy metals in the saprolites and terra rossa developed in 30 profiles, which is mainly caused by the weathering of Cd-enriched carbonate rocks, while the subsequent pedogenic processes were the dominant factor of the enrichments for Cu, Ni, Pb, Zn, and Cr. Sequential extraction analysis indicated that geogenic Cd and Pb in terra rossa mostly distributed in the residual fractions and exhibited low mobility, whereas the amorphous Fe/Mn oxide fraction was the principal Cd-bearing phase in Cambisols. The good correlation of Fe, Al, and Ti in related bedrocks, saprolites, and terra rossa suggested that in-situ pedogenetic processes provided most of the parent materials for terra rossa. The residual accumulation during the special pedogenesis in the karst region, caused elevated Cd and Pb concentrations with increasing weathering intensity, which was indicated by chemical index of alternation (CIA). In addition, results of Pb isotopic fingerprinting confirmed that terra rossa mainly derived from insoluble residues of underlying carbonate rocks. The allochthonous input of Pb also occurred during pedogenesis, whereas the transport and deposition of non-carbonate materials (clasolite/granite derived soils) was only a minor source to Pb input in terra rossa and the anthropogenic impact seemed to be negligible.

Evaluation of various approaches to predict cadmium bioavailability to rice grown in soils with high geochemical background in the karst region, Southwestern China.

Evaluating the bioavailability of Cd to rice (Oryza sativa L.) was essential in the karst region, Southwestern China, where the soils have previously been shown to be anomalously enriched in Cd through geogenic processes. In this research, we examined the bioavailability of Cd to rice samples collected from 278 sites in Guangxi province, where rice is the most widely cultivated cereal crop that is responsible for the largest human dietary exposure to Cd. Both soil chemical extraction and soil-plant transfer modelling approaches were used to predict the bioavailability to rice. Some of the soil types were highly enriched in Cd, but their bioavailability was low, since the soil carbonates raised soil pH and remarkably reduced Cd bioavailability. In contrast, acidic soils (Ca was largely leached) with relatively low total Cd, the grown rice plants accumulated higher Cd in their grains. Results from CaCl2 extraction experiments provided good predictions for Cd in rice grain grown in soils of different types. Stepwise multiple regressions revealed soil pH and soil Ca content were the dominant factors that control the transfer of Cd from soil to rice. An extended Freundlich-type model and a polynomial surface model provided good prediction for Cd in rice grains. The diffusive gradients in thin films (DGT) technique gave the best estimation of soil Cd bioavailability, whereas water-extracted soil solution Cd provided relatively poor fits. Regional soil threshold that derived using the models, can avoid exceedance of Cd in rice and thereby enable local agricultural practitioners or authorities to develop appropriate management for croplands with high Cd background.