Pilot production of activated carbon from cotton stalks using H3PO4

Cotton stalks, an agricultural waste, were chemically activated in a batch process using H₃PO₄ in a locally designed carbonizer at 420 °C in the absence of any purging gases. Mechanically cut short sticks were soaked in diluted H₃PO₄ for a short duration (Batch 1) and an extended period (Batch 2) prior to thermal treatment. The derived carbons contained both coarse and fine grains with acidic effect. Porosity was characterized by N₂ adsorption at −196 °C and the isotherms analyzed by the α-method to estimate total and microporous surface areas in addition to total and microporous volumes. The produced carbons exhibited well-developed porosity that was essentially microporous in composition. Several key performance parameters were altered considerably as a result of impregnation with H₃PO₄ and the extended chemical activation period (Batch 2). Most of the internal porosity of both carbons was accessible to adsorption of iodine, whereas the uptake of methylene blue dye was proportional to the average size of micropores which were larger for the batch with a longer acid soaking time. SEM and FTIR investigations revealed the presence of a developed honeycomb structure and different oxygen functionalities on surfaces of the activated products which are advantageous in liquid-phase applications. Preliminary laboratory-scale experiments with Pb(II) indicate that adsorption capacity of target heavy metals compares favorably with commercially available activated carbons. The raw material, pre-processing, and activation process prove feasible for the production of activated carbon on a large scale, thereby providing a sustainable strategy for treatment of toxic waste streams.