Waste tires to fertilizer: Carbon black zinc fertilizer for maize in calcareous soils

Waste tires accumulate in massive quantities worldwide, posing significant environmental challenges. Pyrolysis under high vacuum offers a potential disposal solution, producing recovered carbon black (CB) enriched in zinc (Zn) from zinc oxide used in tire manufacture. Our objective was to evaluate recovered CB from pyrolysis of mining tires (CB4000) as a Zn fertilizer for maize (Zea mays L.) in calcareous soil with low plant-available Zn. This material contained 90 g Zn kg-1 and increased Zn bioavailability as measured by ion-exchange membranes (plant root simulator PRS probes). In 2 years of field and glasshouse trials, CB4000 enhanced Zn uptake and grain and silage yields. The effectiveness of CB4000 to increase stem width, yield and Zn uptake of maize was equal to traditional zinc sulfate (ZnSO4) fertilizer when applied at double the rate of Zn. Concentrations of toxic organic compounds were below detection or well below regulatory limits for use as a fertilizer and maize tissue concentrations of heavy metals of concern (lead Pb, chromium Cr, and nickel Ni) were unaffected. These results demonstrate that recovered CB from pyrolyzed mining tires can function as a safe and effective Zn fertilizer, while also offering a sustainable recycling pathway for end-of-life tires.