Sorption of copper and cadmium by allophane-humic complexes

Abstract

We have investigated the sorption of Cu and Cd by allophane and its complexes with a soil humic acid (HA), varying in carbon content from 14–123 g kg −1 . The sorption of Cu and Cd was measured at pH 5.0 and 5.5, using the batch technique and a single concentration (2mM) of Cu or Cd in the presence of LiClO 4 . The sorbed metals were partly desorbed by treatment with 0.1 M KNO 3 . Allophane sorbed 71 and 179 mmol Cu kg −1 at pH 5.0 and 5.5, respectively. By comparison, only 3 mmol kg −1 of Cd was sorbed at pH 5.0 and 9 mmol kg −1 at pH 5.5. Irrespective of pH, complex formation with HA led to a marked increase in Cu and Cd sorption. Indeed, sorption increased linearly with the amount of HA sorbed. Less than 20% of the sorbed Cu, and 49–94% of sorbed Cd, was desorbable by KNO 3 . Copper appears to be specifically sorbed by allophane through coordination with exposed hydroxyl groups because sorption can occur even when allophane was positively charged. Moreover, the sorption capacity for Cu increased with pH, and the majority of the sorbed Cu was not desorbable. In contrast, the sorption of Cd by allophane largely occurs by electrostatic interactions. The relative contribution to metal sorption of allophane and HA in allophane-HA complexes was derived from the linear relationship between sorption and HA content of the complexes. The HA content at which there is an apparently equal (50/50) contribution of allophane and HA to Cu sorption is 28 and 87 g C kg −1 allophane at pH 5.0 and 5.5, respectively. Above these apparently “equal-contribution indicator” values the HA component in the complex contributes more to sorption than does the allophane counterpart. For Cd the apparently “equal-contribution indicator” values are 1.9 and 5.2 g C kg −1 allophane at pH 5.0 at pH 5.5, respectively. This observation has implications for soil management in terms of permissible loads for heavy metals and practical means of increasing the sorption capacity of soil.