Synthesis, characterization, and dye sorption ability of carbon nanotube–biochar nanocomposites

• Hybrid multi-walled carbon nanotube (CNT)-coated biochars were synthesized.
• CNTs significantly enhanced the physiochemical properties of the biochars.
• CNT–biochar nanocomposites have good sorption ability to methylene blue.

Innovative technologies incorporating engineered nanoparticles into biochar production systems could improve the functions of biochar for many applications including soil fertility enhancement, carbon sequestration and wastewater treatment. In this study, hybrid multi-walled carbon nanotube (CNT)-coated biochars were synthesized by dip-coating biomass in varying concentrations of carboxyl-functionalized CNT solutions (0.01% and 1% by weight) prior to slow pyrolysis. Untreated hickory and bagasse biochars (HC and BC, respectively) and CNT–biochar composites (HC–CNT and BC–CNT, respectively) were characterized, and the methylene blue (MB) sorption ability of the resulting chars was evaluated in batch sorption experiments. The addition of CNTs significantly enhanced the physiochemical properties of the biochars with HC–CNT-1% and BC–CNT-1% exhibiting the greatest thermal stabilities, surface areas (351 and 390 m2 g−1, respectively), and pore volumes (0.14 and 0.22 cc g−1, respectively). Sorption kinetic and isotherm data showed that, among the biochars examined, BC–CNT-1% had the highest MB sorption capacity (6.2 mg g−1). While increased pH (up to ∼7), promoted the uptake of MB by all the biochars, whether coated or not, increasing ionic strength decreased the uptake of MB by all biochars tested. These findings suggest that electrostatic attraction was the dominant mechanisms for the sorption of MB onto the chars, though diffusion controlled its rate. Hybridized CNT–biochar nanocomposite can thus be considered a promising, inexpensive sorbent material for removing dyes and organic pollutants from aqueous systems.