Efficient removal of anionic dye by protonated APTES modified hydrochar: The effect of cold-activation

The study focused on determining the effect of acidic and basic cold activation on hydrochar (HC) for the removal of methyl orange (MO). HC was prepared by hawthorn seeds (HS) under hydrothermal carbonization. HC was cold-activated with HCl and NaOH, respectively, and they were grafted with aminopropyltriethoxysilane (APTES) and protonated to obtain AHC-N⁺ (acid-activated and modified HC) and BHC-N⁺ (base-activated and modified HC) to determine the effect of acidic and basic activation. They were characterized by elemental analysis, IR, thermal analysis, zeta potential, N₂ adsorption–desorption measurements, and SEM–EDX analysis. The prepared adsorbents displayed MO adsorption due to abundant protonated amine groups. BHC-N⁺ showed higher MO adsorption than AHC-N⁺. The result showed that more protonated APTES groups grafted on the surface of HC via NaOH activation. The obtained data had a good fitting with the Langmuir isotherm and pseudo-second-order kinetic. The maximum adsorption capacity of BHC-N⁺ was 250.38 mg g⁻¹. The adsorption mechanism could be attributed to the electrostatic interactions between MO and protonated amine groups of APTES and hydrogen bonding.