Feasibility of sludge-based biochar for soil remediation: Characteristics and safety performance of heavy metals influenced by pyrolysis temperatures.

Sludge-based biochars (SBB) were prepared to evaluate their physiochemical properties and safety performance for the possible application in soil amendments in this study. SBB were produced at the temperatures ranging from 300 to 900 °C at 200 °C intervals. Both the solid fraction and the soluble organic fraction of SBB were analyzed. The pyrolysis temperature was found to affect the characteristics of solid fraction of the SBB greatly, in terms of the pH, surface area and functional groups. The content and composition of dissolved organic matter in SBB were influenced evidently by pyrolysis temperatures, which was mainly comprised of humic-like compounds with the molecular weight in a range of 0.13-2.4 × 10 kDa. The safety performance of heavy metals in SBB at different temperatures were analyzed: (i) The bioavailable fractions (F1+F2+F3) of heavy metals significantly decreased from 91.65% to 9.44% for Cu, from 98.82% to 63.34% for Zn, from 97.91% to 52.11% for As, from 55.91% to 4.87% for Pb, from 78.20% to 12.50% for Cd, and from 73.51% to 9.57% for Cr when sludge was pyrolyzed to biochars at 900 °C.; (ii) Acid and alkaline conditions promoted the leaching of heavy metals from SBB. The luminescence inhibition of Vibrio fischeri was significantly decreased from 81.41% to 6.01% with the increasing pyrolysis temperatures. Compared with the raw sludge addition, the shoot length, root length and activities of soil microbes in sandy soil and loamy soil with pyrolyzed sludge under different pyrolysis temperatures were increased by 37.5-53.32%, 66.81-96.45%, 92.31-157.69% and 154.74-195.76%, respectively. The biotoxicity tests indicated the relatively safe and reliable performance of SBB. The study provided significant perspectives on the application of SBB as the potential soil amendments.