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The present study discusses a novel biochar activation technique consisting of physical modification using low frequency ultrasound and chemical functionalization with individual amines and their blended mixtures in the presence of two activating agents. Acoustic treatment under ultrasonic irradiation exfoliates the biochar's graphitic clusters, creates new micropores, opens the blocked pores, and enhances the functionalization efficiency. In a subsequent chemical modification step, functionalization with amine moieties further boosts the adsorption capacity. Therefore, the effect of five different amines was investigated on ultrasono-activated biochar (i) monoethanolamine (MEA, 1°), (ii) piperazine (PZ, 2°), (iii) diethanolamine (DEA, 2°), (iv) tetraethylenepentamine (TEPA, 1°and 2°), and (v) polyethylenimine (PEI, 1°, 2°, and 3°) and several binary and ternary mixtures (1) MEA−TEPA, (2) DEA−TEPA, (3) DEA−PEI, (4) TEPA−PEI, and (5) DEA−TEPA−PEI with the activating agents N-(3-(dimethylamino)propyl-N′-ethylcarbodiimide hydrochloride (EDC)−hydroxybenzotriazole (HOBt) or potassium hydroxide (KOH). The results revealed that ultrasonically treating biochar samples for 30 s, followed by chemical activation with either EDC−HOBt−TEPA−MEA or KOH−MEA gave materials possessing intensified adsorption capacities of 1.91 and 1.62 mmol/g, respectively, at 0.10 atm CO 2 partial pressure and 70 °C, compared to raw biochar (0.3 mmol/g).

The present study investigates the impact of biomass origin on the properties of biochar and its interaction with different treatment conditions, CO2 adsorption, and regeneration ability. The biochars were synthesized from eight biomassesherbaceous (miscanthus and switchgrass), agro-industrial (corn stover and sugarcane bagasse), and crop residues (sorghum, wheat straw, rice straw, and rice husk)and were subjected to three different treatment conditions: (I) acoustic treatment using low-frequency ultrasound, (II) amination using the penta-amine, and (III) integrated sono-chemical activation. Adsorption studies revealed that sono-amination increased adsorption capacities up to 2–2.5 times that of physical or chemical activation techniques alone, with the maximum improvements for herbaceous and agro-industrial residues over crop residues due to their large specific surface areas and high carbon and low ash contents. Accordingly, miscanthus with increased nitrogen content after sono-amination (7.5 times that of raw miscanthus biochar) showed the highest adsorption capacity compared to any other biochars. The regeneration studies that were conducted on all of the eight ultrasono-aminated samples showed 68% (crop residues) to 76% (herbaceous and agro-industrials) retainment of the initial adsorption capacities after 15 cycles.