Unlocking the Fertilizer Potential of Waste-Derived Biochar

Mankind is facing a phosphorus (P) crisis. P recycling from anthropogenic waste is critical to close the P loop. Pyrolysis could be the ideal treatment for materials, such as sewage sludge (SS), producing a safe, nutrient-rich biochar product while sequestering the inherent carbon (C). However, pyrolyzed sewage sludge typically contains low levels of potassium (K) and plant available P, making the material rather unsuitable for use as fertilizer. Here, a novel treatment was investigated to produce an optimized P and K biochar fertilizer. We doped sewage sludge with a low-cost mineral (2 and 5% potassium acetate) and pyrolyzed it at 700 °C. The percentage water extractable of the total P content in biochar increased by 237 times with 5% K addition compared to the undoped biochar. After six water extractions, all of the K and 16% of P were obtained. Further optimization is feasible through adjustments of the biochar pH or doping the feedstock with other forms of K. Using X-ray absorption near-edge spectroscopy (XANES) and synchrotron X-ray fluorescence (XRF) mapping, we identified highly soluble potassium hydrogen phosphate up to 200–300 μm below the biochar surface. This simple and cost-effective modification enables the use of sewage sludge as safe biochar fertilizer with tailored P availability that also supplies K, improves soil properties, and sequesters C.