Lauren Zink and Greg Pyle, University of Lethbridge, Canada
Sulphur has been the focus of modern polymer chemistry and materials science advances in recent years due to its abundance and unique characteristics. The oil and gas industry produces more than 60 million tons of sulphur as a refining and purification by-product each year, resulting in mountains of elemental sulphur. The demand for sulphur, in products such as fertilizers, is quickly fulfilled by a small portion of the supply, resulting in a major market imbalance and leaving companies with the challenge of storing the accumulating sulphur in accordance with environmental regulations. The benefits of finding uses for excess sulphur are obvious, and the unique chemical characteristics of sulphur have resulted in attempts to incorporate it into polymers.
Unlike many other elements, sulphur does not evaporate but rather forms a solid-like polymer at 200°C. Recent research has resulted in many polymer composites being created using sulphur. In addition to using sulphur in the vulcanization of many rubber products, enhancing asphalt with sulphur provides increased crosslinking, stability under high-heat conditions and mechanical resistance. Sulphur polymer cement is currently being produced and perfected as it provides low water absorbance, high strength and an increased resistance to acidic and basic environments. A new process called “inverse vulcanization” produces a stable plastic polymer using sulphur and nanoparticles, which can be used in a number of commercial and industrial applications. The utilization of sulphur to create polymers appears successful and diverse.
As these new polymers are created, studies investigating their degradation have begun, focusing on the structural integrity of the polymer when exposed to varying environmental conditions; however, what has yet to be considered is the biological implications of the inevitable introduction of these polymers to aquatic systems.
Sulphur compounds play important roles within aquatic systems. For example, compounds associated with the volatile organic sulphur compounds group, including dimethyl sulfide (DMS), contribute to climate change, acid precipitation and cloud formation through their role in the global sulphur cycle, in which DMS accounts for approximately 75% of the total sulphur flux. In aquatic environments, DMS is both produced and consumed by microorganisms. The consumption of DMS is suspected to be dependent on and proportional to the amount of DMS being produced, suggesting a potential mechanism to control the concentration of DMS within aquatic environments. Freshwater systems have received little attention in comparison with marine and estuarine ecosystems, though DMS and other sulphur compounds are detected in both water and sediment components of freshwater systems.
The function of DMS extends far beyond its role in the global sulphur cycle as many species interactions rely on DMS, such as allowing an animal to distinguish its prey, suggesting that DMS contributes to ecosystem health in addition to the global climate. The attraction to DMS by many species suggests that pollutants, such as plastics or other polymers, which have a similar sulphur signature, may be mistakenly ingested. While the surface of polymers may provide a substrate for DMS-producing microorganisms, the diverse make up of polymers, especially those that include sulphur, also presents the possibility for polymers to naturally mirror the sulphur signature of DMS.
As new sulphur-containing polymers are created, it is vital that we understand their degradation within aquatic environments and their effects on aquatic life. The inclusion of sulphur results in the inherent risk that, either initially or during degradation, new polymers may mirror sulphur signatures of important signalling chemicals, such as DMS, resulting in the potential for mistaken ingestion by aquatic species. The increase in the amount of sulphur within aquatic environments may also disrupt the balance of sulphur compounds being produced and consumed within the system. While sulphur polymers appear to provide a solution to the accumulation of sulphur waste, we should carefully consider the environmental fate of these polymers and ensure that any new products produced from them will not harm the ecosystems in which they will inevitably infiltrate.