Low Frequency Ultrasound Enhanced Dual Amination of Biochar: A Nitrogen-Enriched Sorbent for CO2 Capture

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 CO2 partial pressure and 70 °C, compared to raw biochar (0.3 mmol/g).