Modification Of Coconut-based Activated Carbon And Study Of Its Adsorption Property For Pb²⁺

Activated carbon is an excellent adsorbent due to its special pore structure and surface chemical properties, which has the ability of adsorbing heavy metal lead ion. For further improving its adsorption capacity and increasing its surface active sites, a commercially available coconut-based activated carbon was modified by liquid-oxidation using hydrogen peroxide and nitric acid and by adsorbing SDS. The effects of the liquid-phase oxidation and SDS modification on the Pb2+ adsorption capacities of activated carbon were investigated.The surface functional group characteristics of the modified activated carbon were examined by Fourier transform infrared (FTIR) spectroscopy, acid/base titrations, Zeta potential as well as the point of zero charge (pHPZc) measurement. The porous structure of the functionalized activated carbon was characterized using N2 adsorption at 77 K. The results showed that the amount of the surface acidic group of oxidized activated carbon increased obviously and some new groups appeared. The values of pHIEP and pHPZC decreased differently according to their oxidation degree compared with that of the virgin activated carbon. The lowest values of pHIEP and pHPZc were obtained after oxidation using HNO3 at 363K and the activated carbon oxidized with HNO3 at 363 K shows the lowest pHPZC-pHIEP difference, implying that uniform oxidation has taken place. The surface area and micro-pore volume decreased as the oxidation degree becoming more serious, while the average pore diameter increased. The surface area of the SDS modified activated carbon decreased markedly along with the increase of the SDS amount loaded on activated carbon. The SDS modification made some new groups on the surface of activated carbon and these activated carbon show the heterogeneous surface.The adsorption capacity of activated carbon oxidized with 10mol·L-1 HNO3 at 363 K, which showed the highest adsorption capacity with 40.12 mg·g-1, was about 2.5 times higher than that of the original activated carbon with 17.19 mg·g-1, while the maximum adsorption capacity of the SDS modified activated carbon increased from 17.19 mg·g-1 to 54.47 mg·g-1, which was 3 times higher than that of the original activated carbon. The adsorption equilibrium data of Pb2+ ions on the activated carbons have been measured. It was found that the experimental data of the liquid-phase oxidized activated carbons could be well fitted by Langmuir isotherm model, while that of the SDS modified activated carbons were in good agreement with Freundlich model. For the liquid-phase oxidized activated carbons, the maximum adsorption capacity was well correlated with the acid density of the activated carbons and was proportional to the acid density. The adsorption mechanism of the SDS modified activated carbon is mainly upon ion exchange and the maximum adsorption capacity increased as the SDS amount loaded on activated carbon increases. The influence of the surface area of activated carbon on Pb2+ adsorption capacity could be neglected compared with that of the surface functional groups.The results of this paper indicated that modification of coconut-based activated carbon with oxidant or surfactant is an effective and feasible method to increase their adsorption capacity, which will be more economical and practical.