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.

Synergistic effect from combined use of scrap-recycling slag and hydrated lime to stabilize Pb and Zn in highly contaminated soil.

For the soil in an area which has been repeatedly chosen as one of the 10 most polluted places in the world, stabilization of Pb and Zn was assessed in batch, incubation, and column experiments. Single and combined amendment of scrap-recycling slag (Slag-R), charcoal, coal ash, hydrated lime, and basic oxygen furnace (BOF) slag were applied for the stabilization. Notably, the combined amendment of Slag-R and hydrated lime exhibited superior stabilization efficiencies than the individual use of all stabilizing agents and combined use of charcoal and hydrated lime. The combined amendment of Slag-R and hydrated lime decreased Pb levels by 92-99% and Zn levels by 63-88% in the incubation experiments and by 75% and 89-93%, respectively, in the column experiments. In particular, the combined amendment showed a synergistic effect for Pb stabilization because a higher pH enhanced sorption onto the slag and because sorption onto Fe (hydr)oxides of the sorbent possibly helped to remove Pb. Zinc had a relatively lower sorption tendency, so it was mainly controlled by the pH increase from hydrated lime. Although the addition of hydrated lime was very effective in stabilizing high concentrations of Pb and Zn, the dosage should be controlled carefully because excessively high pH redissolves Pb and Zn as anions.

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 δ34SSO4 and Δδ18OSO4-H2O were assessed in wells undergoing contamination by total petroleum hydrocarbons (TPH) and oxidation using H2O2 in 2021 and 2022. High δ34SSO4 and relevant parameters (e.g., dissolved sulfide and HCO3-) indicated the occurrence of sulfate reduction. The plot of δ65Cu versus δ34SSO4 effectively indicated precipitation of Cu sulfides and their reoxidation at oxidation wells. Although the plot of δ66Zn versus δ34SSO4 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.

Removal of Mn via coprecipitation and sorption by Fe(II), Fe(III), and Al in mine drainage.

In mine drainage, Fe and Mn are the two most abundant elements exceeding the discharge criteria. Although Mn removal generally requires a pH exceeding 9.5-10.0, its coprecipitation and sorption by Fe and/or Al can significantly reduce the required pH. In this study, Mn removal efficiencies, mechanisms, and required pH were investigated by experiments involving varying concentrations of Mn, Fe, and Al at different pH, and X-ray photoelectron spectroscopy (XPS) analyses. At pH > 7.9, precipitation as Mn (hydr)oxides was the principal Mn removal process, as indicated by the Mn removal plots, geochemical modeling, and XPS results. The precipitation was highly promoted by the heterogeneous oxidation of Fe and Al hydroxides. Coprecipitation-sorption experiments showed 65-80% lower Mn concentrations than those of sorption experiments at similar dosages and pH near 7.5. Fe(III) exhibited higher coprecipitation efficiency than Fe(II), possibly due to the prior oxidation of Fe(II). Fe(III) also displayed a coprecipitation-sorption efficiency five times more than Al. To decrease the Mn concentrations from 17-25 mg L-1 to <2 mg L-1 by coprecipitation-sorption, Fe(III)/Mn and Fe(II)/Mn ratios of ∼10 and ∼20, respectively, at pH 9.0 were required. Similarly, an Al/Mn ratio of ∼7 at pH 9.0 was required to reduce the Mn concentration to <5 mg L-1. Furthermore, the required Fe/Mn ratio decreased significantly when the initial Mn concentration decreased to 8-11 mg L-1. Utilizing the deduced relationships, required pH for Mn removal could be estimated and the design of Mn treatment facilities can be more efficient.