The Sorption Behavior Of Ionic Contaminants On Black Carbon

In this study, black carbon (BC) prepared from rice straw was characterized by surface area and pore size distribution, Zeta potential and FTIR. The Boehm titration was performed to measure its surface functional groups. Adsorption of ionic contaminants (soluble anionic-cationic dyes and heavy metal ions) by BC from water was determined using the batch equilibration technique, and the underlying adsorption mechanisms were proposed. This study provides a theoretical basis for wastewater treatment by use of BC.Two carbon-based adsorbents, a BC prepared in our own laboratory and a commercial activated carbon (AC), were tested for their adsorptive ability towards cationic Rhodamine B (RB) and anionic Reactive Brilliant Blue KNR (KNR). Zero net surface charge pH of BC and AC were found to be 1.91 and 8.23, respectively. At pH of both 3.0 and 6.5, therefore, BC had a net negative surface charge and AC a net positive surface charge. BC was more effective than AC to adsorbe RB, and AC was more effective than BC to adsorbe KNR. Based on these results and FTIR spectra, the p-p interactions between dye molecules and graphene sheets of BC as well as the direct dye-BC electrostatic attraction/repulsion and the intermolecular hydrogen bonding are proposed to be the operative mechanisms for dye adsorption.In the study of single metal adsorption, BC showed strong adsorption ability toward heavy metal ions in aqueous solution. The adsorption capacity of BC was three times more than that of AC, which was consistent with the differences in surface acidic groups between the two carbons. HNO3 modification increased the content of surface acidic groups of BC. With an increase in surface acidic groups, the adsorption of metal cations Zn(II), Cd(II) and Cr(III) by BC increased, while that of metal anion Cr(VI) decreased. It is thus proposed that the electrostatic interactions were the primary mechanism of metal ion adsorption on BC.When coexisting in a bi-metal system, the adsorption of both Zn(II) and Cd(II) by BC decreased, due to the competition of the metals for surface acidic groups. Zn(II) appeared to have a higher affinity to BC than Cd(II). This was apparently due to the differences in their effective ionic radii and consistent with the hard-soft-acid-base theory.