Excellent performance of porous carbon from urea-assisted hydrochar of orange peel for toluene and iodine adsorption

In order to promote the physicochemical properties and the pollutants adsorption capacities of porous carbon, a novel urea-assisted hydrothermal method was developed to produce N-doped hydrochar as an ideal precursor from orange peel waste. Different from the modification technologies for nitrogen doping, the highly porous carbon prepared by KOH activation of the urea-assisted hydrochar contained extremely low N content. But such sample showed ultra-high specific surface area up to 3053 m2/g, with 63% and 39% higher than the porous carbons through conventional N-free method and hydrochar-urea mixing activation method respectively. In-depth study revealed that during 600–800 °C activation procedure, the N-containing groups including pyridinic-N, pyridonic-N and graphitic-N in the urea-assisted hydrochar reacted sufficiently with K2CO3, an intermediate of KOH activation, and generated NH3, creating richer porous structures in carbon. Particularly, there was a good linear relationship between the relative increment of specific surface area and the relative reduction of N content in porous carbon when the activation temperature increasing. This connection can be served as a reference to increase the porosity of porous carbon through the activation of N-doped carbonaceous precursor. As for pollutants adsorption, the highly porous carbon from urea-assisted hydrochar possessed excellent toluene adsorption capacity up to 724 mg/g and iodine adsorption capacity up to 2252 mg/g.