Sorption properties of secondary iron precipitates - Sorption of CuII and natural organic matter
MSc Thesis, Department of Chemistry, Umeå University
In this study the sorption properties of a secondary iron precipitate (SIP) have been studied.The SIP has been formed by oxidation of FeII in a solution found at a till covered tailings impoundment in deposit area 2 at the Kristineberg mine. The SIP has an amorphous character and is expected to transform from a schwertmannite-like structure into a more goethite-like. The sorption experiments have been carried out with copper(II), representing heavy metals released by sulphide weathering, and with natural organic matter. The adsorption of copper to SIP is enhanced when pH increases whereas the sorption of NOM is stable at a fraction around 80-90 % regardless of pH. This study also show that the presence of NOM increases the adsorption of CuII to SIP at low pH (<4) but decreases the adsorption at higher pH (>4). Low concentrations of copper favour a higher adsorption at low pH compared to high concentrations of Cu.
The outcome of the experiments is discussed in terms of surface complexation reactions,
which can be predicted by equilibrium model calculations carried out with the program
SOLGASWATER. In the modelling data from goethite has been used as a model for SIP and a diprotic acid for NOM. Even though they share several features there are still some
differences between the species used in the model compared to SIP and NOM.
The results from the adsorption experiments of NOM to SIP are contradicting to the outcome of the modelling as well as results found by prior investigators, where the adsorption decreases as pH increases. The reason for the behaviour of NOM in this work may be a combination of the natural origin of NOM and the structural differences between SIP and goethite.