6PPD Tire Additive Toxicity to Salmon and a Living Nickel MOF Discovery in Cable Bacteria

The latest Chemistry World Podcast examines a safety dilemma in tire chemistry, where the tire preservative 6PPD forms a toxic quinone that harms certain salmon species when washed into rivers. The episode also reports a striking discovery that cable bacteria on the sea floor biosynthesize and use nickel-based metal organic frameworks (MOFs) to transport electrons, offering insights for bio-inspired materials design. Alongside these stories are industry perspectives on substitution and environmental persistence, a nod to MOF chemistry's Nobel-winning history, and a brief look back at the chemist Ingold and his iconic curly arrows.

• Why 6PPD is hard to replace without new problems
• What makes the nickel MOFs in cable bacteria special and how they function as conductors
• Regulatory and toxicology considerations in pursuit of safer tire additives
• Connections between natural MOF construction and future materials design

Overview

The podcast covers two major stories in the chemistry world. The first centers on 6PPD, a tire preservative used for decades to prevent rubber degradation. While effective at surface ozone protection, 6PPD breaks down to form 6PPD quinone, a compound that is highly toxic to certain salmon species when it enters rivers via stormwater runoff. The episode explains how this byproduct is formed and why it is dangerous at very low concentrations, particularly in sediments where it can persist longer and be released back into water under anaerobic conditions. Listeners learn about the challenge of replacing 6PPD with alternatives that retain tire performance while avoiding new ecological hazards. The segment also touches on regrettable substitutions in industry, the broad toxicity questions across the 6PPD family, and regulatory moves in the US, Canada, and Europe to limit these compounds or their derivatives.

The second story highlights a surprising biological and materials discovery. Cable bacteria, known for moving electrons over centimeter-scale distances, are shown to biosynthesize nickel-based metal organic frameworks (MOFs). These naturally formed MOFs assemble into nanoribbons that conduct electricity as efficiently as some synthetic MOFs and conductive polymers. The podcast details how these organisms build MOFs using nickel and dithiolene-like ligands, and how the resulting nano-ribbon structures act as electrical conduits on the seafloor where energy is harvested by oxidizing sulfides in sediments. The discussion draws parallels to synthetic MOFs and reflects on how such natural motifs might inform the future design of sustainable, biologically grown electronics, and 3D MOF architectures that extend beyond conventional crystals.

A third portion of the episode offers a brief historical detour into the life and work of the chemist Christopher Ingold, known for formalizing reaction mechanisms and curly arrows, SN1 and SN2, and his broader influence on how chemists think about electron flow. The program closes with a note on how the chemistry community continues to explore safe, high-performance materials in a world of evolving environmental and regulatory expectations.

6PPD Tire Additive and Toxicology

The tire industry relies on 6PPD to stabilize rubber by quenching ozone at the rubber surface, preventing cracking that would compromise grip and safety. However, 6PPD is not stationary within the tyre. It diffuses through the solid rubber, driven by concentration gradients as surface reactions occur, replenishing the surface from deeper within the material. The byproduct, 6PPD quinone, has been identified as highly toxic to salmon species such as coho salmon and rainbow trout, even at minuscule concentrations. The toxicity profile appears species-specific, and not all 6PPD family members share the same toxicology, which complicates substitution strategies. The host of issues includes environmental persistence in sediments under low-oxygen conditions, where the compound can be re-oxidized to quinone, sustaining ecological risk. Europe has moved toward broader regulation of the 6PPD family due to this risk profile, raising questions about safe replacements that do not transfer toxicity to other organisms or ecosystems.

Industry teams have screened roughly 40 candidate compounds as potential replacements, including other PPD derivatives and non-PPD families. The goal is to achieve similar ozone protection within tyres while ensuring mobility within the rubber matrix and avoiding toxicology that could replicate or worsen the original problem. The conversation highlights the phenomenon of regrettable substitution, a common challenge in chemical safety where a known hazard is replaced with something that later proves problematic. The discussion also emphasizes the need for toxicology data across multiple aquatic species and ecosystems to guide safer decision-making and regulatory policy.

Environmental Fate and Regulation

6PPD degrades in air due to ozone and oxidation and is not considered highly persistent in the atmosphere. However, in river sediments, anaerobic environments slow degradation, enabling longer-term release back into the water column. The quinone byproduct is potent at very low concentrations, magnifying the risk to susceptible fish populations. The episode notes that while the problem has been prominently observed in North American waterways, tyres are used globally, and regulatory actions in Europe reflect concerns about rainbow trout and other local species. The path forward remains uncertain because the toxicity and environmental fate of many related PPDs are not fully understood. Replacements will need comprehensive toxicology and environmental fate data across global contexts to avoid simply shifting the risk elsewhere.

Living MOFs in Cable Bacteria

The second story introduces cable bacteria as a remarkable case of natural energy transport. These organisms extend long filaments that act as biological power lines, moving electrons over centimeter-scale distances to power metabolic processes. The researchers discovered nickel-based MOFs biosynthesized by these bacteria, forming nanoribbon MOFs that conduct electricity with performance comparable to that of synthetic MOFs and conductive polymers. In the MOF architecture, nickel centers coordinate with sulfur-containing, dithiolene-like ligands to create extended, ribbon-like networks. The natural MOFs serve as nanowires that shuttle electrons from sulfide oxidation in the sediment to bacterial cells, enabling chemosynthesis-based energy capture in the deep sea floor environment lacking light. The convergence between natural MOFs and synthetic MOFs suggests potential blueprints for sustainable electronics, including bio-compatible and bio-grown materials and new strategies for three-dimensional MOF structures that mimic biological nanoribbon morphologies.

Implications for Material Design

The discovery of biologically produced MOFs opens provocative questions for material science. Could we coax organisms to grow conductive MOFs with specific morphologies for applications in bioelectronics or energy storage? Could we replicate or adapt the nanoribbon MOF architecture in engineered systems to create flexible, networked nano-wires for low-dissipation electronics? The podcast notes that three-dimensional MOFs remain a focal point for researchers, with natural nanoribbon MOFs offering a potentially scalable blueprint for future materials design. The convergent evolution of MOF-like motifs, both in natural systems and synthetic chemistry, underscores the importance of exploring cross-disciplinary approaches that blend biology, chemistry, and materials science to design sustainable, high-performance devices.

Historical Context: Ingold and Curly Arrows

A short history segment introduces Christopher Ingold, whose work in the 20th century helped codify reaction mechanisms, including the use of curly arrows to depict electron movement. Ingold contributed to the SN1 and SN2 framework with Edward Hughes, shaping a language for discussing reaction mechanisms in solution. The segment also notes Ingold’s knighthood and his humanitarian actions during the 1930s. This historical reflection situates the episode within a broader tradition of chemical thinking that informs current practice in reaction mechanisms, catalysis, and materials design.

Conclusion

Overall, the podcast presents two threads that intersect in interesting ways: the environmental and safety challenges posed by established tire additives such as 6PPD, and the new biological MOFs discovered in cable bacteria that could guide future materials design. Together, they illustrate the ongoing tension between proven performance and ecological safety, while highlighting the potential for nature-inspired strategies to inform the next generation of materials and devices. The episode also anchors these stories in the scientific culture that includes mechanistic chemistry, toxicology, nanomaterials, and sustainability considerations as central to responsible innovation.