Activated biochar from Co-hydrothermal carbonization of municipal sludge and enteromorpha for high-efficiency Pb(II) removal: Activation optimization and adsorption mechanism

The increasing accumulation of municipal sludge poses significant environmental challenges. Hydrothermal carbonization (HTC), as an advanced technology for the harmless treatment and resource recovery of sludge, enables the efficient conversion of organic matter into biochar. However, the resulting biochar is severely limited in its potential as an effective adsorbent due to its low carbon content and high ash content. This work addressed this limitation through co-hydrothermal carbonization of municipal sludge with Enteromorpha, a high-carbon, low-ash biomass waste. Activated biochar derived from the hydrothermal biochar was prepared by using KOH activation for Pb(II) adsorption in aqueous solutions. The effects of activation conditions on biochar yield and adsorption capacity were systematically investigated, along with the influence of adsorption parameters and the underlying mechanisms of Pb(II) removal. Optimal activation was achieved at 600 °C using 2 mol L⁻¹ KOH for 60 min, resulting in an adsorption capacity of 48.6 mg g⁻¹ and a biochar yield of 54.7 %. Adsorption kinetics analysis indicates that the process of active biochar adsorbing Pb(II) involves multiple mechanism interactions, including physical adsorption, complexation, electrostatic attraction, ion exchange, precipitation, and cation-π interactions.