In-situ immobilization of arsenic and antimony containing acid mine drainage through chemically forming layered double hydroxides

Primary Fe-(hydr)oxides and pyrite as carriers of arsenic and antimony: an overlooked problem in acid mine drainage areas

Unusual abundances of pyrite and goethite/hematite occur in quartzite quarries located in the Holy Cross Mountains (south-central Poland). Disseminated arsenical pyrite microcrystals and As/Sb-rich iron-(hydr)oxide accumulations, scarce scorodite (FeAsO4⋅H2O), and traces of löllingite (FeAs2) make up stratiform mineralization zones within an Upper Cambrian siliciclastic-volcanogenic formation. The abundances of arsenic and antimony vary over several orders of magnitude within alternating sandstone and clayey-silty shale beds, which is best evidenced in the Podwiśniówka bedrock (range of 40.4 to 5946 mg/kg As and < 0.01 to 125 mg/kg Sb). Fe-(hydr)oxides are typically more enriched in these two metalloids than microcrystalline pyrite, for example in the Podwiśniówka quarry mean contents of As in Fe-(hydr)oxides and pyrite microcrystals are 2.81 and 1.86 wt% whereas those of Sb are 0.40 and <0.015 wt%, respectively (based on EMP measurements). However, in another subordinate goethite type, mean contents of As and Sb are even higher amounting to 5.92 wt% and 11.40 wt%, respectively. In contrast to poorly soluble goethite/hematite, oxidation of microcrystalline pyrite easily releases arsenic into ponds, pools and seeps. A goethite/hematite crystal structure impedes liberation of As and Sb, which is well documented by trace concentrations of Sb (<1 μg/L) in most examined waters. As opposed to arsenical pyrite, the presence of high contents of As and partly Sb in goethite/hematite has so far been an overlooked problem. Although these two potentially toxic elements are not easily released from Fe-(hydr)oxides, their occurrence in rocks may pose a risk to miners and local residents that may inhale wind-borne mineral particles originating from erosion, mining and rock processing.

Long-term soil remediation using layered double hydroxides: Field evidence for simultaneous immobilization of both cations and oxyanions

Layered double hydroxides (LDHs) have great potential for immobilizing potentially toxic elements in soil. Nevertheless, their practical effectiveness under field conditions remains largely unknown. In this study, we conducted a 2.5-year field trial using pristine Mg-Al LDHs, Ca-Al LDHs, and iron (Fe)-modified LDHs to simultaneously immobilize both oxyanions (including As and Sb) and cations (including Cd and Pb) in historically contaminated soil affected by mining activities since the 1950s. The immobilization performance of LDHs was examined using various batch tests, including water and DTPA extraction, and by measuring metal(loid) concentrations in Coriandrum sativum (coriander). We found that both pristine and Fe-modified LDHs showed promising initial immobilization performance 7 days after application, achieving significant reductions in DTPA-extractable concentrations of As, Sb, Cd, and Pb by 45.6%-68.3%, 55.4%-94.2%, 11.2%-50.9%, and 62.9%-64.9%, respectively, compared to the control soil without amendment. Notably, pristine LDHs showed diminished immobilization performance in the long term, while Fe-modified LDHs exhibited long-term stability over 2.5 years. A conditional probability-based model was used to depict long-term metal(loid) leaching characteristics in LDH-amended soils. Temporal changes in metal(loid) concentrations in the aboveground edible parts (namely, stems and leaves) of coriander corroborated well with DTPA extraction results. Coriander grown in Fe-modified LDH-amended soils had much lower metal(loid) concentrations compared to those grown in pristine LDH-amended soils. As a result, reductions of 35.1%-42.2% for As, 54.4%-66.2% for Sb, 8.5%-22.8% for Cd, and 56.0%-62.7% for Pb concentrations in coriander were still observed 2.5 years after soil amendment with Fe-modified LDHs. To the best of our knowledge, this is the first field-based evidence using LDHs to simultaneously stabilize both cations and oxyanions in soil. The findings support the potential of LDHs for long-term immobilization of metal(loid)s in soil.

ZnFe-LDH synthesized by seed-induced method to simultaneous enhance arsenic removal, stabilization and sludge reduction

To simultaneously enhance arsenic removal, stabilization and sludge reduction, a seed-induced method was applied to in-stiu synthesize ZnFe-LDH containing arsenic (ZnFe-As-LDH). The optimal seed was determined to be ZnFe-LDH by analyzing the effects of the unseeded system and seed-induced system (FeⅡFeⅢ-LDH, ferrihydrite and ZnFe-LDH). In the ZnFe-LDH seed system, the arsenic removal efficiency increased and the arsenic leaching concentration were drastically reduced by 91.33 % under the optimal conditions as the seeds dosage of 2.5 g/L, pH 10 and Zn/Fe ratio of 1.5. The addition of seed crystals promoted crystal growth and aggregation and finally regulated the formation of dense bulk LDH with fewer pores and smaller pore sizes. The arsenic removal and stabilization were achieved by converting active arsenic into more stable crystalline bound arsenic through coprecipitation, ion exchange and complexation. The SV30 was reduced by 73.68 % and it was due to reduction of unstable surface water and interspace and crystallinity enhancement. The formation pathway of ZnFe-As-LDH by seed-induced was elucidated. The seed-induced method promoted the formation of arsenic-containing ferrihydrite and Zn(OH)2 coprecipitations on the surface of ZnFe-LDH seed, and led to a structural rearrangement to generate ZnFe-As-LDH.

Unveiling interfacial interaction between antimony oxyanions and boehmite nanorods: Spectroscopic evidence and density functional theory analysis

In natural environments, the fate and migratory behavior of metalloid contaminants such as antimony (Sb) significantly depend on the interfacial reactivity of mineral surfaces. Although boehmite (γ-AlOOH) is widely observed in (sub)surface environments, its underlying interaction mechanism with Sb oxyanions at the molecular scale remains unclear. Considering Sb-contaminated environmental conditions in this study, we prepared boehmite under weakly acidic conditions for use in the systematic investigation of interfacial interactions with Sb(III) and Sb(V). The as-synthesized boehmite showed a nanorod morphology and comprised four crystal facets in the following order: 48.4% (010), 27.1% (101), 15.0% (001), and 9.5% (100). The combined results of spectroscopic analyses and theoretical calculations revealed that Sb(III) formed hydrogen bonding outer-sphere complexation on the (100), (010), and (001) facets and that Sb(V) preferred to form bidentate inner-sphere complexation via mononuclear edge-sharing configuration on the (100), (001), and (101) facets and binuclear corner-sharing configuration on the (010) facet. These findings indicate that the facet-mediated thermodynamic stability of the surface complexation determines the interaction affinity toward the Sb species. This work is the first to document the contribution of boehmite to (sub)surface media, improving the ability to forecast the fate and behavior of Sb oxyanions at mineral-water interfaces.

Investigation of the migration of natural organic matter-iron-antimony nano-colloids in acid mine drainage

Colloids can potentially affect the efficacy of traditional acid mine drainage (AMD) treatment methods such as precipitation and filtration. However, it is unclear how colloids affect antimony (Sb) migration in AMD, especially when natural organic matter (NOM) is present. To conduct an in-depth investigation on the formation and migration behavior of NOM, iron (Fe), Sb and NOM-Fe-Sb colloids in AMD, experiments were performed under simulated AMD conditions. The results demonstrate significant variations in the formation of NOM-Fe-Sb colloids (1-3-450 nm) as the molar ratio of carbon to iron (C/Fe) increases within acidic conditions (pH = 3). Increasing the C/Fe molar ratio from 0.1 to 1.2 resulted in a decrease in colloid formation but an increase in particulate fraction. The distribution of colloidal Sb, Sb(III), and Fe(III) within the NOM-Fe-Sb colloids decreased from 68 % to 55 %, 72 % to 57 %, and 68 % to 55 %, respectively. Their distribution in the particulate fraction increased from 28 % to 42 %, 21 % to 34 %, and 8 % to 27 %. XRD, FTIR, and SEM-EDS analyses demonstrated that NOM facilitates the formation and crystallization of Fe3O4 and FeSbO4 crystalline phases. The formation of the colloids depended on pH. Our results indicate that NOM-Fe-Sb colloids can form when the pH ≤ 4, and the proportion of colloidal Sb fraction within the NOM-Fe-Sb colloids increased from 9 % to a maximum of 73 %. Column experiments show that the concentration of NOM-Fe-Sb colloids reaches its peak and remains stable at approximately 3.5 pore volumes (PVs), facilitating the migration of Sb in the porous media. At pH ≥ 5, stable NOM-Fe-Sb colloids do not form, and the proportion of colloidal Sb fraction decreases from 7 % to 0 %. This implies that as pH increases, the electrostatic repulsion between colloidal particles weakens, resulting in a reduction in the colloidal fraction and an increase in the particulate fraction. At higher pH values (pH ≥ 5), the repulsive forces between colloidal particles nearly disappear, promoting particle aggregation. The findings of this study provide important scientific evidence for understanding the migration behavior of NOM-Fe-Sb colloids in AMD. As the pH gradually shifts from acidic to near-neutral pH during the remediation process of AMD, these results could be applied to develop new strategies for this purpose.