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= Tungsten Pollution and Remediation =

Tungsten, also known as Wolfram, is a dense metal utilized in a wide range of industrial and military applications. Despite its common use, the full extent of its environmental impact remains unclear. Previously, tungsten was thought to be stable in soil environments and non-soluble in water. However, recent studies have challenged this assumption, indicating that tungsten may behave differently under certain conditions.

Sources of Tungsten Pollution
While naturally occurring in soil and rocks, human activities are the primary drivers of tungsten pollution, posing potential risks to ecosystems and human health. Mining and industrial processes are the major culprits.

Mining: Tungsten extraction releases trace elements into the environment, contaminating soil, air, and water. Despite efforts to reuse mining waste, it often contains harmful elements like arsenic, posing potential risks. Dissolved tungsten in wastewater further exacerbates the problem, making mining a leading contributor to environmental contamination.

Metal Production and Manufacturing: Semiconductor manufacturing, particularly wafer production, utilizes tungsten, leading to its release through treated wastewater effluents. This, along with other heavy metals, can be suspended and deposited in sediments, impacting riverine environments.

Military Ammunition: Military training bullets often contain tungsten-nylon composites. Upon impact, they fragment into tiny particles that coat soil, increasing the surface area exposed to environmental conditions. This enhances dissolution and corrosion, dispersing tungsten more widely and potentially leaching into water sources, further contributing to pollution.

W in soil
Tungsten (W) in soil typically ranges from 0.1 to 5 mg/kg, but in contaminated areas like military and mining sites, levels can soar up to 1000 times higher.

In soil, W can exist in various chemical forms, affecting its mobility and bioavailability. Metallic W, generally considered environmentally benign due to its low solubility, can become more mobile under certain conditions, such as oxidation to soluble forms. The solubility and behavior of W in soil are influenced by factors like soil pH, with increased solubility and potential for entering the food chain in alkaline conditions The bioavailability of tungsten in soil is influenced by its organic matter content; soils with higher organic matter typically retain more tungsten than less organic-rich soils.

W's interactions with soil components are complex, involving adsorption to minerals and organic matter, and can form various tungstate species, including polymers and polyoxometalates (POMs), which influence its environmental fate The presence of natural dissolved organic matter (DOM) and humic substances can further decrease W mobility through adsorption and complexation processes.

W in water
In aquatic environments, tungsten behavior varies with pH, W concentration, and ionic conditions. Under acidic conditions (pH < 5) and high W levels, polymerized W species dominate, while in neutral to alkaline conditions with low W levels, monomeric tungstate oxyanions are more common. Thiotungstate forms can also occur in anoxic conditions.

In water affected by industrial effluents, tungsten dissolved concentrations can reach 400 μg/L and particulate levels up to 300 μg/g in sediments, far surpassing typical river averages and underscoring the pronounced impact of industrial activities on its distribution in aquatic environments.

Tungsten in marine ferromanganese oxides forms stable inner-sphere complexes with manganese oxides and exhibits distinct adsorption patterns compared to molybdenum, driven by complex stability and adsorption mechanisms.

W's behavior in aquatic and terrestrial systems is complex, involving adsorption, precipitation, redox reactions, and polymerization, affected by concentration, pH, and water conditions. Groundwater studies in the USA show that areas with high arsenic levels might also exhibit high W levels, particularly near W-rich minerals or geothermal activity. This underscores the intricate interactions and transformations of W in water, shaped by both natural and anthropogenic factors.

W in the Atmosphere
In the atmosphere, tungsten is typically found in low volatility and concentrations, often below 1 ng m−3, yet it can moderately increase in areas impacted by urban and industrial activities. Despite tungsten's tendency to remain in solid form, recent studies have shown that it can form gaseous compounds like tungsten hexacarbonyl in landfills, with observed concentrations ranging from 0.005 to 0.01 micrograms per cubic meter, prompting closer examination of its atmospheric impact. Additionally, other tungsten compounds, such as tungsten carbide and oxide fibers, are known to contribute to atmospheric pollution in industrial environments and are linked to health concerns, including interstitial pulmonary fibrosis.

Environmental and Health Risks
Effective remediation of tungsten (W), a potentially toxic element, is dependent on understanding its bioavailability and effects on different organisms. W impedes plant and soil microorganism growth by disrupting essential enzyme activities and cellular structures, induces plant cell death,  and creates harmful acidic environments. Some bacteria, however, can lessen tungsten nanoparticles’ (WNPs) harmful effects.

Elevated W levels can alter soil microbial populations, harm earthworm reproduction, and suppress beneficial bacteria growth. Microorganisms have developed W resistance mechanisms, such as efflux and siderophore entrapment, with recent studies uncovering specific genes and pathways for resistance.

W compounds poses health risks to animals and humans, including lung and cardiovascular diseases, genotoxicity, inflammation, and cancer risks, particularly near mining sites. W primarily accumulates in bones, with kidney excretion, and may lead to chronic kidney disease upon long-term exposure.

Remediation Strategies
Despite limited research on W remediation, the need to mitigate its ecological risks is growing due to W's increasing industrial use. Electrokinetic remediation (EKRT) and phytoremediation offer potential solutions but require further development for efficient W recovery.

Water remediation techniques, including adsorption, membrane filtration, and chemical precipitation, have shown effectiveness in removing W from contaminated waters. However, their applicability varies based on water characteristics and W chemical forms, which are influenced by pH, co-occurring inorganic species, and other geochemical and environmental factors.

While promising, remediation and recycling technologies require further refinement and field-scale validation to become commercially viable for W recovery, aligning with the goals of a circular economy and addressing the ecological risks posed by W contamination.