Iron Sulfide Minerals as Potential Active Capping Materials for Mercury-Contaminated Sediment Remediation: A Minireview

Using recoverable sulfurized magnetic biochar for active capping to remediate multiple heavy metal contaminated sediment

Due to anthropogenic activities, heavy metals are discharged into the hydrosphere and deposit onto the sediment. Heavy metals remobilize through physical disturbance and change in environmental conditions, posing a risk to environments and human health. Among several remediation methods, active layer capping is considered to be more feasible due to its financial and technical advantages; however, its long-term effects remain unknown. To overcome this problem, this work applied a novel, recoverable amendment, sulfurized magnetic biochar (SMBC), to remediate multiple heavy metal (Cu, Ni, Zn, Cr, Hg, and MeHg) contaminated sediment. Physiochemical characterization shows magnetite (Fe₃O₄) crystalline in both magnetic biochar (MBC) and SMBC, with such characteristics resulting in a greater surface area (324.9 and 346.3 m²/g) than BC (39.6 m²/g) and SBC (65.0 m²/g). FeS crystalline was also observed in SMBC, which plays an important role in controlling heavy metal release from sediment. Microcosm experiments indicated the effectiveness of SMBC in lowering aquatic Cu, Ni, Zn, Hg, and MeHg releases was significantly greater than the other three biochar materials. Notably, the recovery of SMBC by magnetism was 87%, demonstrating the exceptional recoverability of SMBC from seawater and sediment. Based on its robust capability in lowering Cu, Ni, Zn, Hg, and MeHg release and excellent recoverability from seawater and sediment, this technique represents a practical alternative to conventional approaches for heavy metal immobilization from sediment.

Synthesis of Fe1-xS Nanoparticles with Various Superstructures by a Simple Thermal Decomposition Route and Their Magnetic Properties

Pyrrhotite nanoparticles with 5C and 3C superstructures were synthesized via a simple one-step thermal decomposition method in which hexadecylamine was used as a solvent at various reaction temperatures (TR). Structural analysis showed that at TR = 360 °C, almost uniform in size and shape Fe7S8 nanoparticles with 3C superstructure are formed, and an increase in the reaction temperature leads to the formation of Fe9S10 nanoparticles (5C superstructure), herewith a significant increase in the size of nanoparticles is observed. High-temperature magnetic measurements in 5 repeated heating-cooling cycles revealed that after the first heating branch in the Fe9S10 samples, the λ-Peak transition disappears, and the magnetization has a Weiss-type behavior characteristic of the Fe7S8 sample. The change in the behavior of magnetization can be explained by the redistribution of iron vacancies, which changes the initial phase composition of nanoparticles.

Preparation of a Novel Millet Straw Biochar-Bentonite Composite and Its Adsorption Property of Hg2+ in Aqueous Solution

The remediation of mercury (Hg) contaminated soil and water requires the continuous development of efficient pollutant removal technologies. To solve this problem, a biochar–bentonite composite (CB) was prepared from local millet straw and bentonite using the solution intercalation-composite heating method, and its physical and chemical properties and micromorphology were then studied. The prepared CB and MB (modified biochar) had a maximum adsorption capacity for Hg²⁺ of 11.722 and 9.152 mg·g⁻¹, respectively, far exceeding the corresponding adsorption value of biochar and bentonite (6.541 and 2.013 mg·g⁻¹, respectively).The adsorption of Hg²⁺ on the CB was characterized using a kinetic model and an isothermal adsorption line, which revealed that the pseudo-second-order kinetic model and Langmuir isothermal model well represented the adsorption of Hg²⁺ on the CB, indicating that the adsorption was mainly chemical adsorption of the monolayer. Thermodynamic experiments confirmed that the adsorption process of Hg²⁺ by the CB was spontaneous and endothermic. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and a thermogravimetric analysis (TGA) showed that after Hg²⁺ was adsorbed by CB, functional groups, such as the –OH group (or C=O, COO–, C=C) on the CB, induced complexation between Hg and –O–, and part of Hg (ii) was reduced Hg (i), resulting in the formation of single or double tooth complexes of Hg–O– (or Hg–O–Hg). Therefore, the prepared composite (CB) showed potential application as an excellent adsorbent for removing heavy metal Hg²⁺ from polluted water compared with using any one material alone.

The assessment and remediation of mercury contaminated sites: A review of current approaches

Remediation of mercury (Hg) contaminated sites has long relied on traditional approaches, such as removal and containment/capping. Here we review contemporary practices in the assessment and remediation of industrial-scale Hg contaminated sites and discuss recent advances. Significant improvements have been made in site assessment, including the use of XRF to rapidly identify the spatial extent of contamination, Hg stable isotope fractionation to identify sources and transformation processes, and solid-phase characterization (XAFS) to evaluate Hg forms. The understanding of Hg bioavailability for methylation has been improved by methods such as sequential chemical extractions and porewater measurements, including the use of diffuse gradient in thin-film (DGT) samplers. These approaches have shown varying success in identifying bioavailable Hg fractions and further study and field applications are needed. The downstream accumulation of methylmercury (MeHg) in biota is a concern at many contaminated sites. Identifying the variables limiting/controlling MeHg production-such as bioavailable inorganic Hg, organic carbon, and/or terminal electron acceptors (e.g. sulfate, iron) is critical. Mercury can be released from contaminated sites to the air and water, both of which are influenced by meteorological and hydrological conditions. Mercury mobilized from contaminated sites is predominantly bound to particles, highly correlated with total sediment solids (TSS), and elevated during stormflow. Remediation techniques to address Hg contamination can include the removal or containment of Hg contaminated materials, the application of amendments to reduce mobility and bioavailability, landscape/waterbody manipulations to reduce MeHg production, and food web manipulations through stocking or extirpation to reduce MeHg accumulated in desired species. These approaches often rely on knowledge of the Hg forms/speciation at the site, and utilize physical, chemical, thermal and biological methods to achieve remediation goals. Overall, the complexity of Hg cycling allows many different opportunities to reduce/mitigate impacts, which creates flexibility in determining suitable and logistically feasible remedies.