Oxidation Of Black Carbon And Its Sorption Behavior Of Aromatic Compound

Black carbon is the residue left from incompletely combustion of biomass and fossil fuels. It is low-cost and has a huge reserve in nature, showing strong sorption capacity toward the contaminants in the environment. The oxidation modification of black carbon brings a large number of oxygen-containing groups, which strongly affects the surface structure properties and chemical characterizations, as welll as its sorption behaviors. In this study, five samples including the activated carbons (AC) used as a reference char, two char samples produced by pyrolyzing Chinese fir wood at400℃(FC-400) and700℃(FC-700), and oxidation modified ACO and FCO-700using concentrated H2SO4at temperature210℃, were selected as sorbents. Six kinds of aromatic contaminants (benzene, nitrobenzene, phenol,4-nitrophenol,1,4-dinitrobenzene and hydroquinone) were selected as the adsorbate due to their different polarities. The adsorption performance of black carbon were studied inspecting the influence of surface modification, solution pH and contaminants.First, the pyrolysis temperature and surface modification can affect the composition and chemistry properties of black carbon. The results of BET and elemental analysis showed that FC-700produced at a high pyrolytic temperature were well carbonized and exhibited a relatively higher surface area, less organic matter and lower oxygen content than FC-400formed at a lower temperature. Black carbon modified by the H2SO4oxidation treatment showed a lower surface area and a higher oxygen content than the original sample. The results of Boehm titration measurements and FTIR demonstrated that FC-400had a larger quantity of functional groups than FC-700, while the quantity of acid surface functional groups increased considerably after black carbon oxidation modified by concentrated H2SO4.Secondly, the pyrolysis temperature and surface modification affected the adsorption capacities of aromatic contaminants. The FC-700showed a higher sorption capacity with surface adsorption being dominant in the sorption, whereas the FC-400expressed a lower sorption capacity with surface adsorption and partition effects being co-existed during the sorption. The hydrophilicity and polarity of black carbon were promoted due to the increased acid surface functional groups after black carbon were modified by H2SO4, while the adsorption capacity was decreased because the micropores on black carbon were blocked by water molecules owing to its higher adsorptivity through hydrogen bond. The polar compounds showed relatively higher surface affinities with the char samples than the nonpolar compounds. Nitrobenzene was easy to accept electrons and had a larger adsorption capacity due to a lower electron cloud density. On the contrary, hydroxybenzene inhibited adsorption owing to a high electron cloud density. The adsorption affinity decreased in the order of 1,4-dinitrobenzene>nitrobenzene>4-nitrophenol>benzene> hydroquinone>phenol.Thirdly, the pH of solution effected the adsorption of dissociative aromatic compounds on black carbon. When pH<pKa, acidic compounds were dominated in molecular forms, which promoted their adsorption on black carbons. In this case the electron density was a control factor in adsorption. When pH>pKa, the acidic compounds were hardly adsorbed by black carbon as a result of the ion state of compounds. In this case, the adsorption capacity decreased with increasing dissociation degree of compounds.