Applewood Biochar at Different Smoldering Conditions Passivates Pyrite by Promoting the Formation of Jarosite

To control acid mine drainage (AMD) from the source, a new environmentally and green passivator (biochar) has been introduced to passivate pyrite. To reduce the difficulty of biochar preparation and cost, and improve its production scale, in-situ pyrolysis of applewood by smoldering to produce biochar. Here, particle size, moisture content and gas flow rate were selected to prepare biochar by smoldering through orthogonal combination, and the pyrite was passivated with different conditions and biochar concentrations (2 g/L, 3 g/L, 4 g/L). The results revealed that when the particle size is 200 mm×200 mm×20 mm, the water content is 20–30%, and the gas flow rate is 0.4 L/m³, the biochar yield is the highest. Biochar promotes the formation of passivating layer (jarosite), inhibits the release of metal ions. Increasing biochar concentration can promote the formation of jarosite and enhance the passivation effect on pyrite.