Transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), have gained significant attention due to their unique electronic, optical, and mechanical properties. These materials are widely explored for applications in electronics, energy storage, and catalysis. However, as their production and usage increase, concerns regarding their biocompatibility and environmental toxicity have emerged. Understanding the potential risks associated with TMDCs is critical for safe handling, disposal, and large-scale deployment.

Cytotoxicity studies of TMDCs have been conducted on various biological systems, including mammalian cells, bacteria, and aquatic organisms. In vitro experiments with MoS2 nanosheets have shown dose-dependent cytotoxicity in human lung epithelial cells, with reactive oxygen species (ROS) generation being a primary mechanism of cellular damage. Concentrations exceeding 50 μg/mL have been reported to reduce cell viability by over 30% within 24 hours. Similarly, WSe2 nanosheets exhibit cytotoxic effects at high concentrations, though their impact varies with surface functionalization and oxidation state.

Bacterial studies reveal that MoS2 nanosheets can inhibit the growth of Escherichia coli and Staphylococcus aureus, with minimal inhibitory concentrations ranging between 10-100 μg/mL depending on lateral size and layer thickness. The sharp edges of TMDC nanosheets are thought to physically disrupt bacterial membranes, while oxidative stress contributes to antimicrobial activity. However, prolonged exposure may also lead to microbial resistance, raising concerns about ecological imbalances.

Aquatic toxicity assessments demonstrate that MoS2 and WSe2 can affect freshwater and marine organisms. Daphnia magna, a model aquatic organism, experiences reduced survival rates when exposed to TMDC concentrations above 1 mg/L over 48 hours. Fish embryos, such as those of zebrafish, show developmental abnormalities, including delayed hatching and spinal deformities, at similar exposure levels. The accumulation of TMDCs in aquatic systems may disrupt food chains, necessitating further research on long-term ecological effects.

Degradation byproducts of TMDCs are another critical consideration. Under environmental conditions, MoS2 and WSe2 undergo oxidation, releasing soluble molybdate (MoO4^2-) and tungstate (WO4^2-) ions, along with selenium or sulfur compounds. These byproducts may exhibit different toxicity profiles compared to the parent materials. For instance, selenite (SeO3^2-), a degradation product of WSe2, is toxic to aquatic life at concentrations as low as 0.1 mg/L. Similarly, molybdate ions, while essential in trace amounts, can become harmful at elevated levels, affecting plant growth and microbial activity in soil.

Photocatalytic degradation of TMDCs under sunlight or UV irradiation accelerates the release of metal ions and chalcogen species. This process is influenced by pH, oxygen availability, and the presence of organic matter. In acidic environments, MoS2 degrades more rapidly, increasing molybdenum bioavailability. Conversely, alkaline conditions stabilize the material but may promote aggregation, altering its environmental mobility.

Safe handling protocols for TMDCs are essential to minimize occupational and environmental exposure. In laboratory and industrial settings, engineering controls such as fume hoods and glove boxes should be used to prevent inhalation or dermal contact. Personal protective equipment, including N95 respirators, nitrile gloves, and lab coats, is recommended when handling powdered or dispersed forms of TMDCs.

For liquid dispersions, containment measures should prevent spills and accidental release into water systems. Waste disposal must comply with local regulations, with particular attention to heavy metal content. Incineration of TMDC-containing waste should be avoided due to the potential release of toxic oxides. Instead, chemical stabilization or recycling methods are preferred to reduce environmental impact.

In manufacturing facilities, air filtration systems with high-efficiency particulate air (HEPA) filters can capture airborne TMDC particles. Regular monitoring of workplace air quality ensures that exposure remains below permissible limits. For large-scale production, closed-loop systems minimize waste generation and prevent environmental contamination.

The environmental persistence of TMDCs depends on their structural stability and interaction with natural components. Studies indicate that MoS2 nanosheets can persist in soil for several months, with slow oxidation rates. In contrast, WSe2 may degrade more rapidly due to selenium’s higher reactivity. Sedimentation and biofouling can influence the long-term fate of TMDCs in aquatic environments, potentially leading to benthic accumulation.

Efforts to mitigate TMDC toxicity include surface passivation, biocompatible coatings, and the development of less toxic alternatives. Polyethylene glycol (PEG) functionalization, for example, reduces the cytotoxicity of MoS2 by minimizing ROS generation. Similarly, embedding TMDCs in polymer matrices can limit ion leaching while maintaining functional properties.

Further research is needed to establish standardized toxicity assessment protocols for TMDCs, accounting for variations in size, shape, and surface chemistry. Regulatory frameworks must evolve to address the unique challenges posed by these emerging nanomaterials, ensuring their sustainable integration into technology and industry.

In summary, while TMDCs offer remarkable technological potential, their biocompatibility and environmental toxicity require careful evaluation. Cytotoxicity studies highlight risks to cells and organisms, while degradation byproducts introduce additional hazards. Implementing rigorous safety measures and advancing eco-friendly modifications will be crucial in harnessing the benefits of TMDCs without compromising human health or ecosystems.