Understanding the role of manganese oxides in retaining harmful metals: Insights into oxidation and adsorption mechanisms at microstructure level

The increasing intensity of human activities has led to a critical environmental challenge: widespread metal pollution. Manganese (Mn) oxides have emerged as potentially natural scavengers that perform crucial functions in the biogeochemical cycling of metal elements. Prior reviews have focused on the synthesis, characterization, and adsorption kinetics of Mn oxides, along with the transformation pathways of specific layered Mn oxides. This review conducts a meticulous investigation of the molecular-level adsorption and oxidation mechanisms of Mn oxides on hazardous metals, including adsorption patterns, coordination, adsorption sites, and redox processes. We also provide a comprehensive discussion of both internal factors (surface area, crystallinity, octahedral vacancy content in Mn oxides, and reactant concentration) and external factors (pH, presence of doped or pre-adsorbed metal ions) affecting the adsorption/oxidation of metals by Mn oxides. Additionally, we identify existing gaps in understanding these mechanisms and suggest avenues for future research. Our goal is to enhance knowledge of Mn oxides' regulatory roles in metal element translocation and transformation at the microstructure level, offering a framework for developing effective metal adsorbents and pollution control strategies.

Heavy metals concentration in zooplankton (copepods) in the western Bay of Bengal

Along the coastline, urbanisation and industrialization pose significant challenges to marine habitats and biodiversity. Most wastewater that reaches the marine environment contains toxic metals, which, as they are non-biodegradable, accumulate in the biota and enter the marine food chain. This study presents the concentration of nine heavy metals (Fe, Zn, Cu, Co, Cr, Mn, Ni, Pb, and Cd) in zooplankton (copepods) during the Spring Intermonsoon/Pre-Monsoon (April 2019) in the western Bay of Bengal (BoB). Higher Fe concentrations were found in copepods both nearshore (42,352 µg/g) and offshore (9835 µg/g). However, the relative amounts of heavy metals in copepods from 16 locations (eight nearshore and eight offshore) varied in the order of Fe > Zn > Cu > Co > Cr > Mn > Ni > Pb > Cd. The copepods in the nearshore locations had higher concentrations of heavy metals than those offshore. Compared to earlier data on heavy metals, particularly those that are physiologically non-essential and toxic, the current study demonstrates higher amounts of these metals in copepods in the inshore BoB [Ni (avg. 58.7 ± 5.5 µg/g), Pb (avg. 25.6 ± 4.7 µg/g), and Cd (avg. 16.4 ± 0.9 µg/g)]. Considering the strong East India Coastal Currents in the western BoB, trace metal-concentrated copepods could be transported too far along the Indian coastline from the current study area.

Micromorphology and environmental behavior of oxide deposit layers in sulfide-rich tailings in Tongling, Anhui Province, China

Sulfide-rich tailings produced by mineral processing are prone to oxidation and cause many pollution problems in the surrounding environment; therefore, this issue has become a focus of attention. The Tongling Shuimuchong tailings reservoir contains a large amount of sulfide minerals, especially pyrrhotite and pyrite. This reservoir features obvious oxidation in the surface layer, and the slab is very hard. Mineralogical and environmental geochemical analyses were performed on tailings with different degrees of oxidation in the Shuimuchong tailings reservoir to investigate the influence of the formation of the hard oxidized layer on environmental pollution in the tailings pond. The samples were first subjected to particle-size analysis. The shallow tailings were mainly composed of medium particle; the proportions of coarse particle and fine tailings particles were equal; and the proportions of clay and silt were less than those of the other size fractions. Mineralogical analysis showed that pyrrhotite and pyrite were replaced by residual structures in the oxide layer. The secondary minerals goethite, hematite and jarosite were attached to the edges and fractures of sulfide minerals. The samples were geochemically analyzed to determine the total concentrations of 5 elements, the pH and the major anions. The maximum SO₄^2- concentrations of 33,970 and 32,749 mg/kg were observed at a depth of 40 cm in profiles 1 and 2, respectively. Metal sulfide mineral oxidation in the tailings lowered the pH of the materials to values less than 4. The concentration of HCO₃^- (122-635 mg/kg) in the tailings samples was very low, and the concentration of CO₃^2- was zero. As (53.2-133.7 mg/kg), Pb (24.2-307.5 mg/kg) and Hg (0.03-0.06 mg/kg) were concentrated in the highly oxidized layer at the surface; the Cd content (0.23-10.5 mg/kg) increased with decreasing oxidation degree of the tailings; and the Cr content (38.0-54.9 mg/kg) fluctuated around a certain value.