Characterisation and applications of activated carbon produced from Moringa oleifera seed husks by single-step steam pyrolysis

The seed husks of the multipurpose tree Moringa oleifera are potentially a waste product that may be available in large quantities, and previous work has demonstrated that a microporous activated carbon can be produced from them by carbonisation under nitrogen followed by activation in steam. This research examines the efficacy of a simpler and cheaper activation process, single-step steam pyrolysis activation, with a view to promoting the production of low-cost activated carbon in the developing world. Husks were heated in a steam atmosphere to 750°C for 30 or 120 min, or 800°C for 30 min, then the resulting carbons were tested to determine their iodine numbers and adsorption isotherms for phenol, 4-nitrophenol and methylene blue. Phenol and 4-nitrophenol were adsorbed rapidly by all three carbons, with 80–90% w/w adsorbed in the first 30 min, whereas methylene blue adsorption was slower. Pyrolysis at 800°C for 30 min produced a carbon (yield 12.2% w/w) with an iodine number of 703 mg g−1, a phenol specific surface area (SSA) of 629 m2 g−1, a 4-nitrophenol SSA of 664 m2 g−1 and a methylene blue SSA of 211 m2 g−1. The carbon produced at 750°C for 120 min (yield 11.9% w/w) had similar properties, but the one produced at 750°C for 30 min (yield 16.6% w/w) had a less developed porosity. The adsorbance characteristics of the two best carbons were superior to those produced previously by the conventional two-stage carbonisation-activation, and were competitive with commercial carbons. These results demonstrate that steam pyrolysis activation of M. oleifera husks could provide a low-cost, local source of high quality activated carbon in the developing world.