The application of spent tyre activated carbons as low-cost environmental pollution adsorbents: A technical review

Following cleaner production principles, converting spent tyres to activated carbon (AC) as a low-cost adsorbent is an extremely environmentally responsible and economically attractive prospect. This review evaluates the application of spent tyre activated carbons (STAC) in dye adsorption, heavy metal removal from wastewater, and flue gas cleaning, and its general performance against commercial activated carbon (CAC). The relative performance of STAC against CAC was discussed in light of the general differences in their properties. Surface area, pore size distribution and surface groups all played major roles in adsorption but the importance of each property varied depending on application. STAC outperformed CAC in the liquid phase uptake of heavy metals, gas-phase uptake of Hg⁰, and the adsorption of dyes and polar organic molecules even when surface areas were lower (<800 m² g⁻¹). The improved performance in STAC for these applications is due to a combination of high mesoporosity (>50% total pore volume), a low surface pH (5–8) and an abundance of acidic surface functional groups (carbonyl and carboxyl). Conversely, the adsorption of non-polar organic molecules, gas impurities and CH₄ onto many STAC fail to achieve suitable performance against CAC, even when the STAC surface area is high (up to 1000 m² g⁻¹). Low uptake on STAC in these cases is largely due to the prevalence of carbonyl and carboxyl functional groups, high ash contents and a low surface pH, which are not favourable for the uptake of NO_x, CH₄ or phenols. Steam activated STAC had more comparable results to CAC over CO₂ and chemical activation methods, in general, as they produce the highest surface areas (>500 m² g⁻¹) and greater pore volumes (up to 1.6 cm³ g⁻¹). Nevertheless, the present analysis of the literature demonstrates that most spent tyre activated carbons can be cleaner, viable and sustainable alternatives to conventional coal and lignocellulosic commercial products.