Characteristics of the adsorbed heavy metals onto aerobic granules: isotherms and distributions

Abstract

Biosorption experiments were performed using aerobic granules as bioadsorbent for Cd2+, Cu2+, Mn2+, Zn2+, Fe2+, and Co2+. Aerobic granule shows high adsorption capacity [Q(e) (mg/g) = 189 - 232]. Both Langmuir and Freundlich isotherm fitted the adsorption data well. Especially, the R-2 value exceeds 0.96 in most cases when Langmuir model was adopted. In order to look into the distribution of the adsorbed heavy metal in aerobic granules and to verify metal uptake in surface layer, in middle zone and center of aerobic granule, energy dispersive X-ray (EDX) spectra were obtained. According to field emission scanning electron microscope-EDX observation, it was found that whole granule body can act as an active site for heavy metals uptake because of a lot of pore existed in the inside of aerobic granule. It is clear that the penetration of dissolved metal solution to the center of aerobic granule has occurred, which confirms that metal uptake can occur even in the center of aerobic granule. The analyses by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy showed that functional groups on aerobic granules, such as alcoholic, carboxylate and ether, would be the main binding sites for biosorption of heavy metals. The area distributions further reveal that ether and alcoholic would be the dominant carbon forms in aerobic granules, and the interaction and coordination affinity between the functional groups and metals would be more complicated than expected.