Sorption of Cu by humic acid from the decomposition of rice straw in the absence and presence of clay minerals

The sorption of Cu on humic acid (HA) from the decomposition of rice straw in the absence (Ck-HA) and presence of montmorillonite (M-HA), kaolinite (K-HA), gibbsite (Gi-HA) and goethite (Go-HA) was investigated at pH 5.0 by using batch studies combined with isothermal titration calorimetry (ITC) and atomic force microscopy (AFM). Characterization by elemental analysis and potentiometric titration showed the composition difference among these five HA. The sorption capacity and rate increased in the order: M-HA < K-HA < Gi-HA < Ck-HA < Go-HA. ITC results revealed that the sorption process was spontaneous and endothermic. The aggregation of HA particles after sorption were observed by AFM images. The influence of pH and positive correlations between the sorption capacity and the content of acidic functional groups of HA indicated that the dissociated acidic functional groups, especially the dissociation of carboxylic groups in HA played an important role in Cu sorption. Sequential desorption of sorbed Cu showed that the surface bonded fraction (97.6-99.0%) was significantly higher than the ion exchanged fraction (1.0-2.4%). Markedly positive entropies (ΔS, 94.4-104.3 J mol−1 K−1) further demonstrated that Cu binding to HA by forming inner-sphere complexes. The findings of this study would promote the understanding on the environmental impact of the decomposition of organic waste from agricultural production.