The sheer volume of tires produced and discarded annually creates a substantial environmental challenge as they degrade in landfills, illegal dumps, or along roadsides. Their complex chemical makeup contains substances that can migrate into the surrounding environment. This raises the question of whether these chemicals leach into the soil, potentially impacting ecosystems and human health.
The Core Chemical Composition of Tires
A standard vehicle tire is a sophisticated blend of materials, far more complex than simple vulcanized rubber. The bulk of the tire matrix consists of a blend of natural rubber and synthetic polymers, such as styrene-butadiene rubber and polybutadiene rubber. These elastomers provide the necessary flexibility, traction, and durability.
Fillers and additives constitute over 40% of the tire’s mass. Carbon black is the most common filler, used to reinforce the rubber and increase its resistance to abrasion and tear. Silica is also used to improve a tire’s rolling resistance.
The remaining fraction consists of specialty chemicals, including vulcanizing agents and protective additives. Zinc oxide and sulfur facilitate the vulcanization process, chemically cross-linking the polymer chains to harden the rubber. Antioxidants and antiozonants protect the finished tire from degradation caused by heat and exposure to oxygen and ozone.
Mechanisms of Chemical Release and Leaching
Chemicals from tires enter the soil environment through two processes: continuous road abrasion and the slow degradation of disposed whole tires. The most widespread source of contamination comes from tire wear particles (TWP). This abrasion occurs every time a vehicle drives, shedding tiny fragments of rubber and embedded chemicals onto the roadway and surrounding landscape.
These microscopic particles are classified as microplastics and are easily mobilized by wind and rainwater runoff. They accumulate along road shoulders and are transported into nearby soils and aquatic systems, creating a diffuse source of pollution. Research suggests that nearly half of all microplastics found in soil and water originate from this tire abrasion.
The second mechanism involves bulk leaching from end-of-life tires stockpiled or buried in landfills and illegal dumps. When exposed to water, soluble components slowly diffuse out of the rubber matrix. This process is accelerated by the natural weathering and breakdown of the tire material over decades. The cumulative release of chemicals contaminates the surrounding soil and potentially the groundwater.
Key Contaminants and Their Interaction with Soil
Once chemicals are released from the tire matrix, their interaction is governed by their specific properties and the soil’s characteristics. The most abundant heavy metal contaminant released is zinc, introduced to the tire compound as zinc oxide. While zinc is a necessary micronutrient for plants, elevated concentrations from tire leaching can become phytotoxic, harming vegetation and suppressing soil microbial communities.
Polycyclic Aromatic Hydrocarbons (PAHs) are persistent organic pollutants formed during the manufacturing of certain tire components. These compounds are known for their potential carcinogenic and mutagenic properties. PAHs and other organic contaminants tend to bind strongly to the soil’s organic matter, which limits their mobility but prolongs their persistence in the environment.
The mobility of these contaminants in the soil solution is highly dependent on the soil’s acidity or alkalinity (pH). Heavy metals like zinc are more likely to leach and become mobile in acidic soil conditions. Conversely, some organic contaminants may show a higher propensity for leaching when the soil pH is higher.
The toxicity and environmental fate of these chemicals are also influenced by protective additives. For example, the antiozonant 6-PPD reacts in the environment to form the highly toxic compound 6PPD-quinone.
Managing End-of-Life Tires to Reduce Soil Impact
Effective management of discarded tires is a primary strategy for preventing large-scale soil contamination. The first step involves preventing the accumulation of whole tires in unmanaged stockpiles and illegal dumps, where they are guaranteed to leach chemicals. Many regions have implemented regulations banning the disposal of whole tires in landfills to address this issue.
After collection, end-of-life tires (ELTs) are increasingly processed into reusable materials through recycling programs. A major application for recycled tire material is in civil engineering, where shredded tires and tire chips are used as lightweight fill, drainage layers, or mixed into asphalt paving. This reuse diverts the material from unmanaged sites and puts it into controlled applications.
Emerging technologies offer more advanced ways to recover the tire’s raw materials, significantly reducing the environmental burden. Pyrolysis is one such process, which involves heating the tires in an oxygen-free environment to break them down. This thermal decomposition recovers valuable products like pyrolysis oil, steel, and a form of recovered carbon black, effectively closing the loop on the tire’s material components.