Surface modification of coconut-based activated carbon by liquid-phase oxidation and its effects on lead ion adsorption

In this study, two series of functionalized samples were prepared by means of oxidation of a commercially available activated carbon with either HNO3 or H2O2. The effect of the liquid-phase oxidation on the Pb2+ adsorption capacities of activated carbon was investigated. The porous structure of the functionalized activated carbon was characterized using N2 adsorption at 77 K. The surface functional group characteristics were examined by Fourier transform infrared (FTIR) spectroscopy, acid/base titrations, Zeta potential as well as the point of zero charge (pHPZC) measurement. The adsorption capacity for lead ions in aqueous solution was found to depend on the amount of acidic oxygen functional groups. The isothermal adsorption data were measured and were fitted with the Langmuir and Freundlich isotherm model. The adsorption capacity of activated carbon oxidized with HNO3 (10 mol L− 1) at 363 K was about 2.5 times higher than that of the original activated carbon. The increase in the adsorption capacity of the oxidized activated carbons is attributed to the increased oxygen groups, which enhanced the hydrophilicity of the activated carbon, lowered the pHPZC, made the surface more negatively charged, and increased the amount of homogeneous active sites available for lead ions.