Mercury speciation, transformation, and transportation in soils, atmospheric flux, and implications for risk management : a critical review

Mercury (Hg) is a potentially harmful trace element in the environment and one of the World Health Organization's foremost chemicals of concern. The threat posed by Hg contaminated soils to humans is pervasive, with an estimated 86 Gg of anthropogenic Hg pollution accumulated in surface soils worldwide. This review critically examines both recent advances and remaining knowledge gaps with respect to cycling of mercury in the soil environment, to aid the assessment and management of risks caused by Hg contamination. Included in this review are factors affecting Hg release from soil to the atmosphere, including how rainfall events drive gaseous elemental mercury (GEM) flux from soils of low Hg content, and how ambient conditions such as atmospheric O3 concentration play a significant role. Mercury contaminated soils constitute complex systems where many interdependent factors, including the amount and composition of soil organic matter and clays, oxidized minerals (e.g. Fe oxides), reduced elements (e.g. S2−), as well as soil pH and redox conditions affect Hg forms and transformation. Speciation influences the extent and rate of Hg subsurface transportation, which has often been assumed insignificant. Nano-sized Hg particles as well as soluble Hg complexes play important roles in soil Hg mobility, availability, and methylation. Finally, implications for human health and suggested research directions are put forward, where there is significant potential to improve remedial actions by accounting for Hg speciation and transportation factors

Solidification/stabilization for soil remediation: an old technology with new vitality

No abstract available

Remediation of mercury contaminated soil, water, and air : a review of emerging materials and innovative technologies

Mercury contamination in soil, water and air is associated with potential toxicity to humans and ecosystems. Industrial activities such as coal combustion have led to increased mercury (Hg) concentrations in different environmental media. This review critically evaluates recent developments in technological approaches for the remediation of Hg contaminated soil, water and air, with a focus on emerging materials and innovative technologies. Extensive research on various nanomaterials, such as carbon nanotubes (CNTs), nanosheets and magnetic nanocomposites, for mercury removal are investigated. This paper also examines other emerging materials and their characteristics, including graphene, biochar, metal organic frameworks (MOFs), covalent organic frameworks (COFs), layered double hydroxides (LDHs) as well as other materials such as clay minerals and manganese oxides. Based on approaches including adsorption/desorption, oxidation/reduction and stabilization/containment, the performances of innovative technologies with the aid of these materials were examined. In addition, technologies involving organisms, such as phytoremediation, algae-based mercury removal, microbial reduction and constructed wetlands, were also reviewed, and the role of organisms, especially microorganisms, in these techniques are illustrated

New Trends in Biochar Pyrolysis and Modification Strategies: Feedstock, Pyrolysis Conditions, Sustainability Concerns and Implications for Soil Amendment

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Temporal effect of MgO reactivity on the stabilization of lead contaminated soil

Elevated soil lead (Pb) concentrations are a global concern owing to the toxic effects of this heavy metal. Solidification/stabilization (S/S) of soils using reagents like Portland cement (PC) is a common approach for the remediation of Pb contaminated sites. However, it has been reported that under long-term field conditions, the performance of PC treatments can diminish significantly. Therefore, novel reagents that provide longer-term stabilization performance are needed. In this study, four magnesium oxide (MgO) products of different reactivity values were applied (5 wt%) to a Pb contaminated clayey soil. The short-term (1–49 days) and long-term (25–100 years) temporal stabilization effects were investigated by laboratory incubation and accelerated ageing methods, respectively. The concentration of Pb in Toxicity Characterization Leaching Procure (TCLP) leachate was ~14 mg/L for the untreated soil; ~1.8 times higher than the TCLP regulatory level (5 mg/L). Only one day after treatment with MgO, the leachate concentration was reduced to below the regulatory level (a reduction of 69.4%–83.2%), regardless of the MgO type applied. However, in the long-term accelerated ageing experiments, only treatments using the most reactive MgO type could provide leachate concentrations that were consistently below the TCLP threshold throughout the 100 years of simulated ageing. The soil treated with the MgO of lowest reactivity was the first to exceed the regulatory level, at simulated year 75. It is thus demonstrated that MgO reactivity has a significant effect on its long-term effectiveness for contaminated soil stabilization. This is attributed to differences in their specific surface area and readiness to carbonate, which may facilitate the immobilization of Pb in the long term. It is also noteworthy that compared to PC, reactive MgO is more environmentally friendly owing to lower energy consumption and reduced CO2 emissions during its manufacture

Green remediation of Cd and Hg contaminated soil using humic acid modified montmorillonite: immobilization performance under accelerated ageing conditions

Solidification/Stabilization (S/S) is an effective way to immobilize toxic metals in contaminated soil. However, utilization of ordinary Portland cement (PC) in this process has raised environmental concerns owing to the high carbon footprint from PC manufacturing and the risk of toxic element leaching in the long term. Hence there is an urgent need to seek for “green” immobilization approaches with long-term stability. In this study, a clay-based material, humic acid modified montmorillonite (HA-Mont) was applied to a Cd and Hg contaminated soil. Field emission scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (FESEM/EDS), N2 adsorption-desorption, Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analyses were performed to investigate the characteristics of this material. Compared to the soil without any treatment, dosage of 5% HA-Mont could effectively reduce Cd and Hg concentrations by 94.1% and 93.0%, respectively and to below the regulatory limits in the TCLP (Toxicity Characteristic Leaching Procedure) leachates. Compared to the soil treated with virgin montmorillonite, HA modification resulted in the reduction of leachate concentrations of Cd and Hg by 69.5% and 65.9%, respectively. Long-term immobilization performance of the HA-Mont treatment was examined using a quantitative accelerated ageing method. In order to examine the ageing features, a novel method based on conditional probability was developed, and the reliability of HA-Mont immobilization was found to fit the Weibull model well, as the ageing rate of immobilization effect increased with time. After 120 years of ageing, reliability of both metals could still remain above 0.95. Cd concentration in TCLP leachates at 120th year could still remain below the regulatory limit (294 μg/L vs 1000 μg/L), while Hg concentration reached the regulatory limit of 200 μg/L in 96th year. This is the first attempt developing a green S/S method of Cd and Hg contaminated soil using HA-Mont and examining the long-term ageing characteristics of the stabilized soil using a probability-based approach

Resilient remediation:Addressing extreme weather and climate change, creating community value

Recent devastating hurricanes demonstrated that extreme weather and climate change can jeopardize contaminated land remediation and harm public health and the environment. Since early 2016, the Sustainable Remediation Forum (SURF) has led research and organized knowledge exchanges to examine (1) the impacts of climate change and extreme weather events on hazardous waste sites, and (2) how we can mitigate these impacts and create value for communities. The SURF team found that climate change and extreme weather events can undermine the effectiveness of the approved site remediation, and can also affect contaminant toxicity, exposure, organism sensitivity, fate and transport, long-term operations, management, and stewardship of remediation sites. Further, failure to consider social vulnerability to climate change could compromise remediation and adaptation strategies. SURF's recommendations for resilient remediation build on resources and drivers from state, national, and international sources, and marry the practices of sustainable remediation and climate change adaptation. They outline both general principles and site-specific protocols and provide global examples of mitigation and adaptation strategies. Opportunities for synergy include vulnerability assessments that benefit and build on established hazardous waste management law, policy, and practices. SURF's recommendations can guide owners and project managers in developing a site resiliency strategy. Resilient remediation can help expedite cleanup and redevelopment, decrease public health risks, and create jobs, parks, wetlands, and resilient energy sources. Resilient remediation and redevelopment can also positively contribute to achieving international goals for sustainable land management, climate action, clean energy, and sustainable cities

Soil and geologic formations as antidotes for CO2 sequestration?

Rapid and far‐reaching transitions are required to combat climate change and its impacts. Carbon capture and storage within mineral deposits is a promising solution to remove CO2 from the atmosphere. In‐situ geological storage and ex‐situ mineral sequestration are practically sufficient for sequestering all the anthropogenic CO2. Recent research reports that more than 95% of injected CO2 was mineralized into carbonates in two years by using in‐situ geological approach, and mining wastes and secondary minerals were recycled as resources for ex‐situ CO2 sequestration. However, geological activity is the major risk of in‐situ storage, while high energy consumption and associated cost may limit the application of ex‐situ carbonation. Significant technical breakthroughs of mineral and geological CO2 sequestration are therefore of vital importance to realize a “net‐zero CO2 emissions” and even “carbon‐negative” society

Effective Dispersion of MgO Nanostructure on Biochar Support as a Basic Catalyst for Glucose Isomerization

Glucose isomerization to fructose is one of the most important reactions in the field of biomass valorization. We demonstrate wood waste valorization with MgCl2 salt to synthesize an environment-friendly catalyst (i.e., MgO-biochar), which exhibits effective glucose-to-fructose isomerization with over 30% fructose yield and 80% selectivity at only 100 °C for 30 min in water as a green medium. This study highlights that one-step synthesis can effectively disperse and tether MgO nanostructures to the biochar matrix, which displays a significant reduction of Mg leaching compared to MgO-biochars produced by two-step synthesis and pure MgO. The MgCl2 acts as a porogen that facilitates the formation of a porous biochar structure and dispersion of nanostructured MgO. We identify key parameters of impregnation media (ethylene glycol, ethanol, and water) and pyrolysis conditions (600/750 °C in N2/CO2 atmosphere) that are responsible for adjusting the reactivity and stability of MgO, which enable the design of more effective and recyclable biochar catalysts. Weak interactions between MgCl2 and biomass in the presence of aqueous miscible organic solvents as shape-directing agents are accountable for fast leaching of Mg from the MgO-biochar surface. The FTIR spectra confirm the existence of various coordinations on the hydroxylated surfaces of MgO-biochar surfaces. The mesoporous structures of the biochar support enhance the stability of MgO moieties as revealed by BET, XRD, and Raman analyses. Given the benefits of effective MgO dispersion on the biochar support, we can reduce the amount of MgO active species involved in each reaction run, which mitigates over-reaction compared to pure MgO catalysts and achieves high fructose yield and selectivity for three consecutive cycles