Adsorption of lead ions and methylene blue on acrylate-modified hydrochars

Hydrochars are promising sorbents for wastewater treatment. Herein, two acrylate-modified hydrochars (AMHC₁ and AMHC₂) were obtained by grafting acrylic acid on the surface of two hydrochars (MHC₁ and MHC₂ hydrothermally carbonized in water and acidic medium respectively) with free radical polymerization. Characterizations show that MHC₂ is more prone to free radical polymerization than MHC₁ does, and has higher carboxylate content after modification. The adsorption amounts of AMHC₂ over methylene blue (MB) and Pb(II) are much higher than those of AMHC₁. Pseudo-second-order kinetic and Langmuir isotherm equations well fit the Pb(II) and MB sorption data of AMHC₂. The Pb(II) adsorptive mechanism is mainly inner-surface complexation accompanied by ion exchange and cation-π interaction. MB adsorption involves ion exchange, electrostatic interaction, H-bonding and π-π interaction. Hence, the one-step modification method of free radical polymerization under alkaline condition has great potential for preparing carboxylate-modified hydrochars to adsorb cationic pollutants.