Modification in adsorption characteristics of activated carbon produced by H3PO4 under flowing gases

Activated carbons were derived from peach stone shells by H3PO4-scheme under standard conditions: 50% H3PO4 and pyrolysis for 2 h at 500 °C. Five carbons were prepared by changing the gas atmosphere during thermal treatment (no external gas, flowing of either nitrogen, carbon dioxide, steam or air). Additional carbon was obtained by extra-heat treatment at 800 °C of the standard carbon, for 2 h. Characterization was performed by adsorption of N2 at 77 K, and testing of capacity was carried out towards p-nitrophenol, methylene blue and lead ions from aqueous solution. All carbons were high quality adsorbents with high surface area and essentially microporous structure with low degree of mesoporosity. Running external gases reduces mostly the carbon porosity, except air and steam which generate higher mesoporosity on expense of microporosity. High uptake of p-nitrophenol appears, affected to low extent with gaseous atmosphere except steam which raises adsorption considerably. Flowing air is the most effective in enhancing the adsorption of methylene blue, which was attributed to the formation of O-functionalities with acidic nature, and to enhancement of wider microporosity. Removal of lead ions from aqueous, non-buffered, solutions was considerable in comparison to previous studies, and attributed to the acidic nature of carbons in addition to the presence of inorganic and organic phosphate compounds. Lead ions removal was considerably enhanced by running air during thermal treatment (two-fold increase) due to the formation of acidic oxygen-functionalities associated with metal exchange by the negatively charged carbon surface.