Spatial patterns of Zn, Cd, and Pb isotopic compositions of ground and surface water in mine areas of South Korea reflecting isotopic fractionation during metal attenuation

Comprehensive multimetal isotopic model for regional-scale water management in a mining city: Tracing contamination sources, pathways, and geochemical processes using Cu, Mo, S-O, and Zn isotopes

Major mining cities worldwide have been suffering from diverse contamination sources such as tailings, mine drainages, geology enriched with toxic metals, and other industrial and domestic sources. This study established a multi-isotopic comprehensive model for elucidating water contamination sources and geochemical reactions in the city of Erdenet in Mongolia. The Erdenet city was contaminated with As, Cu, Mo, Zn, and SO42 -, and we used isotopes of Cu, Mo, Zn, and S-O in SO42-. Contamination sources for groundwater and surface water were differentiated as tailings dump, excavated ore, heap leaching, ash pond of a power plant, and argillic alteration zone. Groundwater in the residential area was influenced by the argillic alteration zone, as indicated by low δ34SSO4 and high δ65Cu values, while δ66Zn fingerprints may have been masked by adsorption and mixing. Additionally, δ98Mo fingerprints from two major Mo contamination sources (the ash pond and tailings dump) were evident in the stream. The tailings dump substantially impacted δ65Cu, δ98Mo, and δ66Zn values in the stream, with isotopic fractionation occurring through oxidative dissolution and adsorption. Furthermore, to assess oxidative dissolution of sulfides and adsorption, δ65Cu and δ98Mo+ δ66Zn were found to be particularly useful, respectively. This study highlights the effectiveness of multimetal isotopic ratios in tracing contamination sources and geochemical processes in regions with diverse contaminants, presenting a robust spatial model for isotopic fingerprinting.

Study on the migration pathway and isotopic composition of Zn in soil, plant and water in mining area

Isotopic tracing has been widely used to identify the sources and migration processes of Zn in diverse environments. However, Zn isotope fractionation during the migration process within the mining area poses challenges to the accuracy of isotopic tracing. To address this issue, a representative Pb-Zn mining area in the karst region of southwestern China was selected as the study area, given its long-term tailings' pollution history and the extensive spatial distribution of Zn migration. End-member samples and environmental media (soil, plants, river water and groundwater) were systematically collected, and heavy metal concentrations and isotopic signatures were analyzed. The migration pathways of Zn and associated isotopic fractionation from end members to surrounding environments were comprehensively investigated. Results indicated that tailings constitute the dominant source of Zn, with ZnS weathering being the primary driver of Zn isotopic variability in soils. Eluviation process, characterized by selective transport of soluble Zn2+ enriched in heavy isotopes, was identified as the key mechanism governing Zn migration across the soil-plant-river continuum. Retention processes (adsorption by organic matter, plant uptake, and mineral interactions) exhibited minimal influence on soil Zn isotopic composition. These findings advance the understanding of Zn and Zn isotope cycling in karst ecosystems and provide a scientific basis for formulating pollution control strategies in mining areas.

Assessing seepage sources of a tailings dump and fractionation of Mo and Zn isotopes

Tracing the sources of each contaminant and its geochemical reactions requires a variety of geochemical tools. In this study, chemical compositions and isotopic ratios of O-H, Mo, and Zn were utilized to identify the sources and geochemical reactions of water, As, Mo, and Zn in the seepage from a mine tailings dump. The distinct chemical compositions observed between the seepage and monitoring well, along with the O-H isotopic ratios, suggested that the seepage originated from creek water rather than nearby groundwater, which was supported by a large seasonal variation of δ98Mo in both the seepage and creek. Interpretation results indicated that Mo was predominantly supplied from the creek, while the majority of As and Zn originated from the tailings. During the transport of Mo and Zn, δ98Mo and δ66Zn increased and decreased, respectively, suggesting adsorptive removal, despite the δ66Zn increase during the leaching of the tailings. Notably, the combined use of Mo and Zn isotopic ratios proved to be a valuable tool for identifying geochemical reactions and determining sources and pathways in complex environmental systems. Additionally, although As does not have multiple isotopes, possible adsorption of As onto Fe (oxy)hydroxides could be inferred based on the isotopic behavior of Mo and Zn, as these two isotopes effectively reflected isotopic fractionation during adsorption.

Concentration-discharge relations and transient metal loads reveal spatiotemporal variability in solute-generation mechanisms in a mine-affected watershed

Concentration-discharge (CQ) relations are commonly used to understand geochemical and hydrologic controls on the generation of solutes in watersheds. Despite the widespread application of CQ relations, this technique has been infrequently applied to acid mine drainage (AMD) sites, but the CQ framework may allow mechanistic understanding of remedial outcomes such as impoundment of water within underground mines. Results of CQ analyses and changes in metal loads in an AMD affected watershed in Colorado, USA indicate that dissolved loads increased at many individual locations following water impoundment within mine workings. Although increased loads were observed at most individual locations, these increases were offset by a large decrease in loading from the largest mine. A loading analysis that included data from an instream monitoring location showed a statistically significant decrease in Fe and Zn after bulkhead emplacement, indicating a net positive effect of bulkheads. Streams generally displayed dilution CQ patterns whereas mines and springs showed either flushing or chemostatic patterns prior to bulkheading, which transitioned to chemostatic patterns following bulkheading, indicating a transition from dynamic to equilibrium geochemical processes. Saturation indices for sulfide and secondary minerals indicated that mines and springs were near equilibrium for phases including schwertmannite, fluorite, and gypsum. Saturation indices vary through time for mines suggesting progressive leaching of sulfide minerals as the mass of available minerals in the mine workings decreases. Together, these diverse analyses provide an integrated understanding of the variability in solute generating processes in this watershed and may inform remediation plans for similarly affected sites by indicating the nature of mineralogic controls on water quality.

Determination of contamination sources and geochemical reactions in groundwater of a mine area using Cu, Zn, and S-O isotopes

To determine contamination sources and pathways, the use of multiple isotopes, including metal isotopes, can increase the reliability of environmental forensic techniques. This study differentiated contamination sources in groundwater of a mine area and elucidated geochemical processes using Cu, Zn, S-O, and O-H isotopes. Sulfate reduction and sulfide precipitation were elucidated using concentrations of dissolved sulfides, δ34SSO4, δ18OSO4, and δ66Zn. The overlying contaminated soil was possibly responsible for the contamination of groundwater at <5 mbgl, which was suggested by low δ65Cu values (0.419-1.120‰) reflecting those of soil (0.279-1.115‰). The existence of dissolved Cu as Cu(I) may prevent the increase in δ65Cu during leaching of contaminated soil in the sulfate-reducing environment. In contrast, the groundwater at >5 mbgl seemed to be highly affected by the contamination plume from the adit water, which was suggested by high SO42- concentrations (407-447 mg L-1) and δ65Cu (0.252-2.275‰) and δ66Zn (-0.105‰-0.362‰) values at a multilevel sampler approaching those of the adit seepages. Additionally, the O-H isotopic ratios were distinguished between <5 mbgl and >5 mbgl. Using δ65Cu and δ66Zn to support the determination of groundwater contamination sources may be encouraged, particularly where the isotopic signatures are distinct for each source.

Ultrafast and highly efficient Cd(II) and Pb(II) removal by magnetic adsorbents derived from gypsum and corncob: Performances and mechanisms

The utilization of gypsum and biomass in environmental remediation has become a novel approach to promote waste recycling. Generally, raw waste materials exhibit limited adsorption capacity for heavy metal ions (HMIs) and often result in poor solid-liquid separation. In this study, through co-pyrolysis with corncob waste, titanium gypsum (TiG) was transformed into magnetic adsorbents (GCx, where x denotes the proportion of corncob in the gypsum-corncob mixture) for the removal of Cd(II) and Pb(II). GC10, the optimal adsorbent, which was composed primarily of anhydrite, calcium sulfide, and magnetic Fe3O4, exhibited significantly faster adsorption kinetics (rate constant k1 was 218 times and 9 times of raw TiG for Cd(II) and Pb(II)) and higher adsorption capacity (Qe exceeded 200 mg/g for Cd(II) and 400 mg/g for Pb(II)) than raw TiG and previous adsorbents. Cd(II) removal was more profoundly inhibited in a Cd(II) + Pb(II) binary system, suggesting that GC10 showed better selectivity for Pb(II). Moreover, GC10 could be easily separated from purified water for further recovery, due to its high saturation magnetization value (6.3 emu/g). The superior removal capabilities of GC10 were due to adsorption and surface precipitation of metal sulfides and metal sulfates on the adsorbent surface. Overall, these waste-derived magnetic adsorbents provide a novel and sustainable approach to waste recycling and the deep purification of multiple HMIs.

Identification of anthropogenic source of Pb and Cd within two tropical seagrass species in South China: Insight from Pb and Cd isotopes

Seagrass beds are susceptible to deterioration and heavy metals represent a crucial impact factor. The accumulation of heavy metal in two tropical seagrass species were studied in South China in this study and multiple methods were used to identify the heavy metal sources. E. acoroides (Enhalus acoroides) and T. hemperichii (Thalassia hemperichii) belong to the genus of Enhalus and Thalassia in the Hydrocharitaceae family, respectively. Heavy metal concentrations in the two seagrasses followed the order of Cr > Zn > Cu > Ni > As > Pb > Co > Cd based on the whole plant, and their bioconcentration factors were 31.8 ± 29.3 (Cr), 5.7 ± 1.3 (Zn), 7.0 ± 3.8 (Cu), 3.0 ± 1.9 (Ni), 1.2 ± 0.3 (As), 1.7 ± 0.9 (Pb), 9.1 ± 11.1 (Co) and 2.8 ± 0.6 (Cd), indicating the intense enrichment in Co and Cr within the two seagrasses. The two seagrasses were prone to accumulate all the listed heavy metals (except for As in E. acoroides), especially Co (BCFs of 1124) and Cr (BCFs of 2689) in the aboveground parts, and the belowground parts of both seagrasses also accumulated most metals (BCFs of 27) excluding Co and Pb. The Pb isotopic ratios (mean 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb values of 38.2054, 15.5000 and 18.3240, respectively) and Cd isotopic compositions (δ114/110Cd values ranging from -0.09‰ to 0.58‰) within seagrasses indicated the anthropogenic sources of Pb and Cd including coal combustion, traffic emissions and agricultural activities. This study described the absorption characteristics of E. acoroides and T. hemperichii to some heavy metals, and further demonstrated the successful utilization of Pb and Cd isotopes as discerning markers to trace anthropogenic origins of heavy metals (mainly Pb and Cd) in seagrasses. Pb and Cd isotopes can mutually verify and be helpful to understand more information in pollution sources and improve the reliability of conclusion deduced from concentrations or a single isotope.

Fractionation behaviors of Cu, Zn, and S-O isotopes in groundwater contaminated with petroleum and treated by oxidation

Fractionation behaviors of Cu and Zn isotopes have been increasingly studied at the field scale, but those in various redox conditions of groundwater contaminated with petroleum and treated by oxidation have not been assessed. In this study, δ⁶⁵Cu and δ⁶⁶Zn as well as δ³⁴S_SO4 and Δδ¹⁸O_SO4-H2O were assessed in wells undergoing contamination by total petroleum hydrocarbons (TPH) and oxidation using H₂O₂ in 2021 and 2022. High δ³⁴S_SO4 and relevant parameters (e.g., dissolved sulfide and HCO₃^-) indicated the occurrence of sulfate reduction. The plot of δ⁶⁵Cu versus δ³⁴S_SO4 effectively indicated precipitation of Cu sulfides and their reoxidation at oxidation wells. Although the plot of δ⁶⁶Zn versus δ³⁴S_SO4 could also indicate reoxidation of Zn sulfides, the Zn isotopic fingerprint of sulfide precipitation may have been masked by fractionation by sorption. The advantage of using δ⁶⁵Cu in the redox reactions resulted from the wider range of δ⁶⁵Cu owing to the redox behavior of Cu. The plot combining isotopic fractionations of Cu and S can assist in assessing sulfide precipitation and oxidative treatment in TPH-contaminated groundwater.

Determination of contamination sources and geochemical behaviors of metals in soil of a mine area using Cu, Pb, Zn, and S isotopes and positive matrix factorization

The use of multiple isotopic ratios and statistical methods can substantially increase the reliability and precision of determining contamination sources and pathways. In this study, contamination sources were differentiated in three subareas in one mine area and geochemical processes were investigated using Cu, Pb, Zn, and S isotopes and positive matrix factorization (PMF). Soil samples downstream of the adit seepages exhibited distinctly higher δ⁶⁵Cu values than those from other areas. δ⁶⁵Cu in adit seepages increased substantially from ore sulfides owing to large isotopic fractionation during oxidative dissolution. Although δ⁶⁵Cu decreased during sulfide precipitation in seepage-contaminated soil, the discrimination of δ⁶⁵Cu was still valid. Therefore, δ⁶⁵Cu is particularly useful for differentiating between contamination by sulfides (tailings) and water (adit seepages). Moreover, sulfide precipitation following sulfate reduction was verified by the decreased δ⁶⁶Zn and δ³⁴S in the soil. In addition, the plot of ²⁰⁸Pb/²⁰⁶Pb versus Pb^-1 distinguished contamination sources. Furthermore, PMF analysis confirmed the determination of sources and differentiated between contamination by As- and Cu-enriched tailings. The effect of Cu-enriched tailings further downstream suggested that the lower specific gravity of chalcopyrite compared to that of arsenopyrite affected the distribution of soil contamination.

Mechanisms of Ni removal from contaminated groundwater by calcite using X-ray absorption spectroscopy and Ni isotope measurements

A flow-through cell (FTC) experiment was conducted to identify mechanisms of Ni removal by calcite through study of changes in Ni speciation and Ni isotope signature during the treatment of simulated Ni-contaminated groundwater. Synthetic Ni-contaminated groundwater was pumped through a FTC packed with crushed natural calcite. Effluent samples were collected to determine concentrations of anions, cations, and for Ni isotope-ratio measurement. X-ray absorption spectroscopy (XAS) was performed on chosen spots of the solid phase along the FTC length. Isotope data indicated multiple mechanisms affected Ni removal in the FTC system. Ni adsorption to and coprecipitation with calcite dominated the early part of the experiment yielding a fractionation factor of ε = -0.5 ‰. Subsequently, Ni precipitation as a Ni-hydroxide phase became the major process controlling Ni removal, resulting in a fractionation factor ε = -0.4 ‰. XAS analysis confirmed the presence of both Ni(OH)₂ and (Ni, Ca)CO₃ types of Ni local structural environments. Results from this study highlight the potential of Ni isotopes as auxiliary tools to determine the processes involved in Ni attenuation from the environment. The characterization of mechanisms involved in Ni removal from solution is necessary to evaluate potential impacts to the environment and to develop effective remediation strategies.

Bottom Ash Modification via Sintering Process for Its Use as a Potential Heavy Metal Adsorbent: Sorption Kinetics and Mechanism

Heavy metal pollution in the environment is a critical issue, engendering ecosystem deterioration and adverse effects on human health. The main objective of this study was to evaluate heavy metal adsorbents by modifying industrial byproducts. The bottom ash was sintered and evaluated for Cd and Pb sorption. Three adsorbents (bottom ash, sintered bottom ash (SBA), and SBA mixed with microorganisms (SBMA)) were tested to evaluate the sorption kinetics and mechanism using a lab-scale batch experiment. The results showed that the highest sorption efficiency was observed for Cd (98.16%) and Pb (98.41%) with 10% SBA. The pseudo-second-order kinetic model (R² > 0.99) represented the sorption kinetics better than the pseudo-first-order kinetic model for the SBA and SBMA, indicating that chemical precipitation could be the dominant sorption mechanism. This result is supported by X-ray photoelectron spectroscopy analysis, demonstrating that -OH, -CO₃, -O, and -S complexation was formed at the surface of the sintered materials as Cd(OH)₂ and CdCO₃ for Cd and PbO, and PbS for Pb. Overall, SBA could be utilized for heavy metal sorption. Further research is necessary to enhance the sorption capacity and longevity of modified industrial byproducts.