Characterization of hydrogen peroxide (H2O2) modified hydrochars from walnut shell for enhanced adsorption performance of methylene blue from aqueous solution

Abstract

Hydrothermal carbonization of waste biomass materials is becoming more prevalent in the production of carbonaceous adsorbents. In this work, mesoporous hydrochars were prepared by the hydrothermal carbonization of walnut shell (HTC-WS) as novel adsorbents of methylene blue (MB) from aqueous solution. In order to enhance the adsorption ability, HTC-WS was chemically modified using H2O2. Both materials were characterized by elemental analysis, Fourier transform infrared spectroscopy, scanning electron microscope, zeta potential analysis and nitrogen adsorption–desorption measurements. Additionally, the adsorption performances of these two hydrochars toward MB were evaluated. The results showed that H2O2 modification of hydrochars might have increased the oxygen-containing functional groups, such as carboxyl groups, resulting in more than three times increase of MB adsorption capacity (173.92 mg/g) compared with unmodified hydrochars (50.56 mg/g). MB adsorption kinetics of HTC-WS and hydrothermal carbonization of H2O2-activated walnut shell could be interpreted with pseudo-second-order models and adsorption equilibrium data were best fitted by the Langmuir isotherm model. Oxygenated functional groups of these two hydrochars were significant adsorption sites and appeared to be associated with molecular electrostatic interactions between MB and hydrochars. The results suggested that H2O2 modified hydrochars were environmental friendly, effective and low-cost adsorbents for the removal of MB from wastewater.