Sorption of Cu2+ on humic acids sequentially extracted from a sediment

In addition to the diverse properties of humic acids (HAs) extracted from different soils or sediments, chemical compositions, functional groups and structures of HAs extracted from a single soil or sediment could also be diverse and thus significantly affect sorption of heavy metals, which is a key process controlling the transfer, transformation and fate of heavy metals in the environment. In this study, we sequentially extracted four HA fractions from a single sediment and conducted the sorption experiments of Cu2+ on these HA fractions. Our results showed that aromaticity and acidic group content of HA fraction decreased with increasing extraction. Earlier extracted HA fraction had higher sorption capacity and affinity for Cu2+. There were two fractions of adsorbed Cu2+ on HAs, i.e., ion exchanged fraction and surface bonded fraction, which can be captured mechanically by the bi-Langmuir model with good isotherm fitting. The ion exchanged fraction had larger sorption capacity but lower sorption affinity, compared with the surface bonded fraction. The dissociated carboxyl groups of HAs were responsible for both fractions of Cu2+ sorption, due to the more Cu2+ sorption on the earlier extracted HA fraction with more carboxyl groups and at higher pH. The intensive competition between H+ and the exchangeable Cu2+ could result in the decrease of ion exchanged capacity and affinity for Cu2+ on HAs. (C) 2015 Elsevier Ltd. All rights reserved.