Hydrothermal synthesis of needle-shaped manganese oxide nanoparticle for superior adsorption of thallium(I): characterization, performance, and mechanism study

Sequestration and oxidation of heavy metals mediated by Mn(II) oxidizing microorganisms in the aquatic environment

Microorganisms can oxidize Mn(II) to biogenic Mn oxides (BioMnOx), through enzyme-mediated processes and non-enzyme-mediated processes, which are generally considered as the source and sink of heavy metals due to highly reactive to sequestrate and oxidize heavy metals. Hence, the summary of interactions between Mn(II) oxidizing microorganisms (MnOM) and heavy metals is benefit for further work on microbial-mediated self-purification of water bodies. This review comprehensively summarizes the interactions between MnOM and heavy metals. The processes of BioMnOx production by MnOM has been firstly discussed. Moreover, the interactions between BioMnOx and various heavy metals are critically discussed. On the one hand, modes for heavy metals adsorbed on BioMnOx are summarized, such as electrostatic attraction, oxidative precipitation, ion exchange, surface complexation, and autocatalytic oxidation. On the other hand, adsorption and oxidation of representative heavy metals based on BioMnOx/Mn(II) are also discussed. Thirdly, the interactions between MnOM and heavy metals are also focused on. Finally, several perspectives which will contribute to future research are proposed. This review provides insight into the sequestration and oxidation of heavy metals mediated by Mn(II) oxidizing microorganisms. It might be helpful to understand the geochemical fate of heavy metals in the aquatic environment and the process of microbial-mediated water self-purification.

Influence mechanism of plant litter mediated reduction of iron and sulfur on migration of potentially toxic elements from mercury-thallium mine waste

The decomposition of plant litter in soil changes soil nutrient content and plays an important role in regulating soil pH and availability of potentially toxic elements (PTEs). However, there remains limited studies on the mechanism under which litter influences the transport of PTEs in the process of ecological restoration. This study examined the effect of plant litter decomposition mediated reduction of iron and sulfur components on migration of PTEs from mercury-thallium mine waste. The results showed that the four kinds of litter alleviated the acidity of the waste, especially the Bpa and Tre litter. The nitro and nitroso groups produced by the decomposition of the litter were adsorbed onto the waste, thereby providing an electron transfer medium for iron reducing microorganisms, such as Geobacter. This promoted the reduction and release of Fe³⁺ to Fe²⁺ and reduced the electronegativity (El) value of waste. The reduced El promoted the adsorption of metal cations such as Hg and Tl to maintain electrical neutrality. However, it was not conducive to the adsorption of oxygen containing anions of As and Sb. An increase in litter resulted in an increase in reductivity of mercury-thallium mine waste. This maintained the reduction of Fe³⁺ to Fe²⁺ and changed or destroyed the structure of silicate minerals. PTEs, such as Tl, Hg, As, and Sb, were released, resulting in reductions in their residual fraction. However, the strong reduction conditions, especially the decomposition of Bpa, caused part of the released Hg(II) combining with S^2- produced by the reduction of SO₄^2- to form insoluble HgS, thereby reducing its migration. The findings could provide a theoretical basis to guide the situ-control and ecological restoration of PTEs in waste slag site.

Efficient Adsorption of Tl(I) from Aqueous Solutions Using Al and Fe-Based Water Treatment Residuals

Iron and aluminum water treatment residuals from a water supply plant were used as adsorbents for Tl(I) to treat thallium-containing Tl(I) wastewater and realize the resource utilization of water treatment residuals. The feasibility study results showed that Fe-WTR and Al-WTR reached adsorption equilibria within 120 min. The Langmuir model showed maximum adsorption capacities of Tl(I) on Fe-WTR and Al-WTR as 3.751 and 0.690 mg g−1 separately at an initial concentration of 5 mg L−1. The adsorption capacities of Fe-WTR and Al-WTR positively correlated with pH. The removal of Tl(I) using Fe-WTR exceeded Al-WTR; the adsorbed Tl(I) in Fe-WTR occurred primarily in the reduced state, while the Tl(I) adsorbed in Al-WTR was mainly in acid-extractable and reduced states. FTIR and XPS data showed that Tl(I) and Fe/Al-OH-functional groups formed stable surface complexes (Fe/Al-O-Tl) during adsorption, and there was no redox reaction. This confirmed that WTR is a highly efficient adsorbent for the stable removal of Tl(I), which provides a practical foundation for industrial application in Tl(I)-containing wastewater treatment.

Insights into the synergistic removal mechanisms of thallium(I) by biogenic manganese oxides in a wide pH range

The behavior and mechanism of thallium (Tl) adsorption by biogenic manganese oxides (BMnOx) are poorly understood. In this study, BMnOx was applied for Tl(I) removal from aqueous solution, and the adsorption interactions were systematically revealed for the first time. BMnOx was successfully prepared with high productivity by effectively oxidizing Mn(II) with a manganese oxide bacterium in an optimal Mn(II) concentration range of 4.0-28 mg/L. Compared with other adsorbents, the prepared BMnOx achieved high Tl(I) adsorption capacity over a wide pH range from 3.0 to 9.0 and high humic acid (HA) concentration (40 mg/L) interference. The experimental results were well depicted by pseudo-second-order kinetics and the Langmuir isotherm model, indicating that chemisorption played the dominant role during the adsorption process. The adsorption mechanisms were verified as synergetic interactions of oxidation-precipitation, electrostatic attraction, ion exchange and surface complexation. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) results suggested that 19.46% of the highly toxic Tl(I) was transformed into the much less toxic product Tl₂O₃ after adsorption onto BMnOx. This study provides theoretical guidance for high-concentration Tl(I) decontamination from groundwater by biogenic manganese oxides.

Which is Better for Nanomedicines: Nanocatalysts or Single-Atom Catalysts?

With the rapid advancements in nanotechnology and materials science, numerous nanomaterials have been used as catalysts for nanomedical applications. Their design and modification according to the microenvironment of diseases have been shown to achieve effective treatment. Chemists are in pursuit of nanocatalysts that are more efficient, controllable, and less toxic by developing innovative synthetic technologies and improving existing ones. Recently, single-atom catalysts (SACs) with excellent catalytic activity and high selectivity have attracted increasing attention because of their accurate design as nanomaterials at the atomic level, thereby highlighting their potential for nanomedical applications. In this review, the recent advances in nanocatalysts and SACs are briefly summarized according to their synthesis, characterizations, catalytic mechanisms, and nanomedical applications. The opportunities and future scope for their development and the issues and challenges for their application as nanomedicine are also discussed. As far as it is known, the review is the systematic comparison of nanocatalysts and SACs, especially in the field of nanomedicine, which has promoted the development of nanocatalytic medicine.

Migration characteristics of heavy metals in the weathering process of exposed argillaceous sandstone in a mercury-thallium mining area

Lanmuchang mercury-thallium mine, a typical polymetallic mine is located in southwestern Guizhou, China, is the most serious and typical area resulted from multi-metal contamination (Tl, Hg, As, and Sb). After the mercury-thallium mining, a large area of surrounding rocks such as argillaceous sandstone with high contents of Tl, Hg, As, and Sb is exposed to air. Weathering caused the argillaceous sandstone to form different weathering layers, including the grey-black external layer, the brown-yellow middle layer and the gray-white inner layer, and the external layer was enriched with higher heavy metals. However, the reason of heavy metal migration and transformation in argillaceous sandstone caused by weathering is unclear. The objective of this paper was to investigate the migration, transformation and release characteristics of Tl, Hg, As, and Sb in argillaceous sandstone during the weathering. The results indicated that weathering not only promoted an acidic oxidation environment in argillaceous sandstone, but also increased its specific surface area, pore volume and hydrophilicity, which are beneficial to the permeability of oxygen and etching liquids during the process of weathering and leaching. Meanwhile, weathering led to the transformation or decomposition of hydrophilic groups, such as -OH and -C˭O in the grey-black external layer of argillaceous sandstone, resulting in the further release of heavy metals bound to these groups. The concentration of Tl, Hg, As, and Sb in the leaching solution of argillaceous sandstone represented a positive correlation with that of Fe, Ca, Mg at different levels, indicating that Tl, Hg, As, and Sb were released with the dissolution of Fe, Ca and Mg during weathering and leaching. In summary, these results indicated that weathering caused the dissolution and migration of heavy metals in the argillaceous sandstone. Tl, Hg, As, and Sb migrated from the grey-white inner layer to the grey-black external layer and partially adsorbed by free alumina (Al_d), jarosite and Ca-bearing minerals, showing enrichment phenomena, partially released into the environment, causing environmental pollution.