Role of Reduced Sulfur in the Transformation of Cd(II) Immobilized by δ-MnO₂

Abstract

Mn oxides are the major sinks for Cd(II) in the aquatic environment. At the redox interface, reduced sulfur might affect the fate of sorbed Cd(II) by either reducing Mn oxides or forming strong complexes with Cd(II). Here, we investigated the fate of Cd(II) immobilized on δ-MnO 2 affected by reduced sulfur (S 2− and cysteine). A low concentration of S 2− led to Cd(II) migration from vacant sites to edge sites, while a high concentration of S 2− largely converted Cd(II) adsorbed on the surface of δ-MnO 2 to CdS. At low pH, the cysteine addition led to the release of Cd(II) initially adsorbed at the δ-MnO 2 vacant sites into the solution and caused the migration of a small portion of Cd(II) to the δ-MnO 2 edge sites. At high pH, a high concentration of cysteine led to the detachment of Cd(II) from δ-MnO 2 , Cd(II) readsorption by Mn(III)-bearing minerals, and Cd−cysteine formation. Changes of Cd(II) speciation were caused by δ-MnO 2 dissolution induced by reduced sulfur, the competition of generated Mn(II/III) for the adsorption sites, and the precipitation of Cd(II) with reduced sulfur. This study indicates that reduced sulfur is a critical factor controlling the fate of Cd(II) immobilized on Mn oxides in the aquatic environment.

Facet-Dependent Photoinduced Transformation of Cadmium Sulfide (CdS) Nanoparticles

“…13 The presence of natural organic matter and soil minerals with the incorporation of Fe 2+ or Mn 2+ acts as stabilizers to prevent the aggregation and protect the dispersion of CdS-NPs. 6,7 Intercellular and extracellular biomolecules with rich amine groups and thiol groups, thus, various biogenic minerals provide preferential sites for nucleation and formation of mono-dispersed and solidsupported CdS-NPs. 8 The typical biogenic CdS-NPs showed a broad distribution of particle sizes ranging from 3 to 100 nm with poor crystalline to well crystalline spherical shapes.…”

“…Cadmium sulfide (CdS) is present in the environment originated from the wide applications of CdS in pigment materials, solar cells, and consumer products and more prevalently from biogeochemical transformations of anthropogenic Cd 2+ . Markedly, in paddy soil with characteristic periodical cycles of flooding and drainage, sulfate-reducing bacteria (SRB) utilize sulfate (SO 4 2– ) as an electron acceptor to produce sulfide (S 2– ) during the flooding period, − and dissolved Cd 2+ chemically precipitates in the presence of S 2– to form insoluble CdS under anaerobic conditions. , Recent findings suggested that the formation of Bio-CdS involved much broader microbial species and processes. For example, urease-producing bacteria induce calcite precipitation, which acts as an ideal supporter and stabilizer to promote the growth of CdS .…”

“…However, under environmental circumstances, both inorganically formed and biogenic CdS (Bio-CdS) were discovered to be prevailingly in the form of nanosized particles . The presence of natural organic matter and soil minerals with the incorporation of Fe 2+ or Mn 2+ acts as stabilizers to prevent the aggregation and protect the dispersion of CdS-NPs. , Intercellular and extracellular biomolecules with rich amine groups and thiol groups, thus, various biogenic minerals provide preferential sites for nucleation and formation of mono-dispersed and solid-supported CdS-NPs . The typical biogenic CdS-NPs showed a broad distribution of particle sizes ranging from 3 to 100 nm with poor crystalline to well crystalline spherical shapes. , The reactivities of nano-sized minerals significantly differ from bulk minerals due to the nano-specific effects including a high specific surface area (SSA), facet effects, and quantum effects; the reaction mechanisms derived from CdS-MPs cannot be properly extrapolated to CdS-NPs .…”

Experimental Investigation of Removal of SO3 from Flue Gas with Modified Fly Ash Adsorbents

“…There was an obvious characteristic peak in the S 2p spectrum after adsorption, which was located near the electron binding energy of 169 eV. The characteristic peak may come from Al 2 (SO 4 ) 3 , CaSO 4 , Na 2 SO 4 , and K 2 SO 4 . , It was indicated that there was metal sulfate in the products after adsorption, which was formed by the chemical reaction between SO 3 in flue gas and alkali metal oxides in the modified fly ash adsorbent.…”

Synthesis of Iron-Based Carbon Microspheres with Tobacco Waste Liquid and Waste Iron Residue for Cd(II) Removal from Water and Soil

“…Several techniques can be used to remove Cd­(II) from water, such as chemical ion exchange, precipitation, membrane technologies, and adsorption. ,− Among these methods, adsorption is widely used because of its practical and economic properties for Cd­(II) ion removal. − The commonly used adsorbents are metal and metal oxide nanoparticles, , clay minerals, zeolites, and carbon materials. , Carbon materials have attracted attention because of their outstanding adsorption performance, low cost, and ease of obtaining. Because of their surface charge and functional groups, the hydrochar has a high adsorption capacity for heavy metal ions, and the result has been reported in recent review papers. − …”