Divergent repartitioning of antimony and arsenic during jarosite transformation: A comparative study under aerobic and anaerobic conditions

Occurrence of microplastics and distinct plastisphere in aquatic environments of metal mining areas in South China

Microplastic pollution is a growing global environmental concern, significantly impacting aquatic ecosystems. In metal mining regions, acid mine drainage (AMD) can exacerbate the issue as microplastics may carry heavy metals, heightening the risks to both human health and ecosystems. AMD can also form a distinct plastisphere further influencing the behavior and distribution of heavy metals. This study, beginning with tailing storage, examined microplastic contamination in river water and sediment in a mining region of South China. The concentration of microplastics in water ranged from 0.2 to 6.2 items/L, predominantly comprising PS, PE and PP. In contrast, the concentration of microplastics in the sediments ranged from 14.1 to 84.8 items/kg and were mainly composed of PP. Most microplastics in both water samples and sediments are smaller than 100 microns. Industrial wastewater from mining activities was identified as the predominant source of microplastics in the AMD-polluted upstream areas while agricultural and domestic waste contributed more significantly in the downstream areas. Plastisphere biodiversity was enhanced, while the colonization of pivotal species on plastic substrates was significantly facilitated in AMD-polluted aquatic environments. In these environments, the ecological niche afforded by plastics was of ecological significance. This study offers the direct evidence of microplastic pollution and highlights the critical ecological role of microplastics in aquatic ecosystems affected by AMD.

Effect mechanism of low-molecular-weight organic acids during sulfidation of As(V)-bearing ferrihydrite

Sulfide induces the reductive dissolution of iron (oxyhydr) oxides, the primary host phases for arsenic (As), thereby triggering As release. We investigates the physicochemical mechanisms of three types of low molecular weight organic acids (LMWOAs) on sulfide-mediated reductive dissolution of As(V)-ferrihydrite and As release using batch experiments combined with hydro-chemical, spectroscopic, and microscopic analyses. Arsenate dominated the aqueous (97.2-100 %) and solid phases throughout the experiment. LMWOAs accelerated S(-II) consumption and As release by inhibiting FeS formation, with rates ordered as citric acid (CA) > oxalic acid (OA) > malic acid (MA) > control (Kb). At S(-II): Fe = 0.5, maximum As release was 11.78 % (Kb) and 14.60 % (CA); at S(-II): Fe = 1, it was 27.58 % (Kb) and 30.71 % (OA). LMWOAs enhanced As release via non-reductive ligand dissolution of As(V)-ferrihydrite. Secondary mineral formation in later stages re-immobilized As, with mineral layers ≥50 nm thick. LMWOAs interacted differently with secondary minerals: CA primarily adsorbed on surfaces, while MA integrated into the matrix. LMWOAs influenced As redistribution in secondary minerals, increasing contamination risks. Thus, the complex effects of organic matter (OM) on Fe, S, and As biogeochemistry must be considered in risk assessments and remediation strategies for As-contaminated sites in sulfidic environments.

Birnessite enhanced Cr(III) oxidation during subsurface geochemical processes: Role of Mn(III)-induced nonphotochemical reactive oxygen species

Cr(III) oxidation by birnessite was the dominant geologic source of Cr(VI), which increases the environmental mobility and toxicity of Cr, threatening ecological safety. Photochemically hydroxyl radical (•OH) generated by birnessite was widely accepted to be the dominant reactive oxygen species (ROS) oxidating Cr(III). However, birnessite and Cr mainly co-exist in dark subsurface soils, with contribution of nonphotochemical ROS remaining unclear. In this work, free-radical quenching experiments, electrochemistry method and density functional theory (DFT) calculations were performed to elucidate ROS generation mechanisms during Cr(III) oxidation in simulated light-deprived environment. The results indicated that •OH was completely suppressed and nonphotochemical O2•- still accelerated Cr(III) oxidation in dark aerobic conditions with the contribution of 15.3%-19.1%. Moreover, DFT calculations proved that O2•- was produced by O2 molecules adsorbed on oxygen vacancies in the structure, thus being generated spontaneously in the dark. The oxidation contribution of O2•- was undetectable after extracting Mn(III), indicating that electron transfer occurred between Mn(III) and O2 to generate O2•-. Additionally, intervention of Cd2+ (for occupying oxygen vacancies) did not reduce participation of •OH, but resulted in suppression of electron transport which greatly reduced the production of O2•-, thereby affecting Cr(III) oxidation process. The above findings provide new insights on Cr(III) oxidation by manganese oxides and is able to have profound significance for predicting the fate of Cr in subsurface environments.

Arsenic redistribution associated with Fe(II)-induced jarosite transformation in the presence of polygalacturonic acid

Jarosite exists widely in acid-sulfate soil and acid mine drainage polluted areas and acts as an important host mineral for As(V). As a metastable Fe(III)-oxyhydoxysulfate mineral, its dissolution and transformation have a significant impact on the biogeochemical cycle of As. Under reducing conditions, the trajectory and degree of abiotic Fe(II)-induced jarosite transformation may be greatly influenced by coexisting dissolved organic matter (DOM), and in turn influencing the fate of As. Here, we explored the impact of polygalacturonic acid (PGA) (0-200 mg·L-1) on As(V)-coprecipitated jarosite transformation in the presence of Fe(II) (1 mM) at pH 5.5, and investigated the repartitioning of As between aqueous and solid phase. The results demonstrated that in the system without both PGA and Fe(II), jarosite gradually dissolved, and lepidocrocite was the main transformation product by 30 d; in Fe(II)-only system, lepidocrocite appeared by 1 d and also was the mainly final product; in PGA-only systems, PGA retarded jarosite dissolution and transformation, jarosite might be directly converted into goethite; in Fe(II)-PGA systems, the presence of PGA retarded Fe(II)-induced jarosite dissolution and transformation but did not alter the pathway of mineral transformation, the final product mainly still was lepidocrocite. The retarding effect on jarosite dissolution enhanced with the increase of PGA content. The impact of PGA on Fe(II)-induced jarosite transformation mainly was related to the complexation of carboxyl groups of PGA with Fe(II). The dissolution and transformation of jarosite drove pre-incorporated As transferred into the phosphate-extractable phase, the presence of PGA retarded jarosite dissolution and maintained pre-incorporated As stable in jarosite. The released As promoted by PGA was retarded again and almost no As was released into the solution by the end of reactions in all systems. In systems with Fe(II), no As(III) was detected and As(V) was still the dominant redox species.

Divergent redistribution behavior of divalent metal cations associated with Fe(II)-mediated jarosite phase transformation

The Fe(II)/Fe(III) cycle is an important driving force for dissolution and transformation of jarosite. Divalent heavy metals usually coexist with jarosite; however, their effects on Fe(II)-induced jarosite transformation and different repartitioning behavior during mineral dissolution-recrystallization are still unclear. Here, we investigated Fe(II)-induced (1 mM Fe(II)) jarosite conversion in the presence of Cd(II), Mn(II), Co(II), Ni(II) and Pb(II) (denoted as Me(II), 1 mM), respectively, under anaerobic condition at neutral pH. The results showed that all co-existing Me(II) retarded Fe(II)-induced jarosite dissolution. In the Fe(II)-only system, jarosite first rapidly transformed to lepidocrocite (an intermediate product) and then slowly to goethite; lepidocrocite was the main product. In Fe(II)-Cd(II), -Mn(II), and -Pb(II) systems, coexisting Cd(II), Mn(II) and Pb(II) retarded the above process and lepidocrocite was still the dominant conversion product. In Fe(II)-Co(II) system, coexisting Co(II) promoted lepidocrocite transformation into goethite. In Fe(II)-Ni(II) system, jarosite appeared to be directly converted into goethite, although small amounts of lepidocrocite were detected in the final product. In all treatments, the appearance or accumulation of lepidocrocite may be also related to the re-adsorption of released sulfate. By the end of reaction, 6.0 %, 4.0 %, 76.0 % 11.3 % and 19.2 % of total Cd(II), Mn(II), Pb(II) Co(II) and Ni(II) were adsorbed on the surface of solid products. Up to 49.6 %, 44.3 %, and 21.6 % of Co(II), Ni(II), and Pb(II) incorporated into solid product, with the reaction indicating that the dynamic process of Fe(II) interaction with goethite may promote the continuous incorporation of Co(II), Ni(II), and Pb(II).