Influence of pyrolysis temperature on physical and chemical properties of biochar made from sewage sludge

• The rich-nutrients were intensified with the temperature rising except nitrogen.
• Pyrolysis abated the bioavailability of Mn, Fe, Zn and Cu.
• Pyrolysis reduced the leaching toxicity of heavy metals.
• The biochar was safer than sludge as a soil amendment.

This work investigated the influence of pyrolysis temperature on biochar production and evaluated its properties for agricultural applications. Biochar yield decreased and its pore structure developed with the increase of temperature from 300 to 700 °C. Biochar produced at 300 °C was acidic whereas at higher temperatures it was alkaline. Fewer dissolved salts were contained in the biochars produced at higher temperatures. The rich nutrients, other than nitrogen, were intensified with the temperature rise. The results of (diethylenetriaminepentaacetic acid) DTPA-extraction showed the pyrolysis process abated the bioavailability of trace nutrient elements (Mn, Fe, Zn and Cu). The leaching toxicity of Pb, Zn, Ni, Cd, As, Cu and Cr in the biochars was lower than that in the sewage sludge although the pyrolysis process enriched the heavy metals in the biochars. Hence, the biochar was safer than sewage sludge as a soil amendment.