Speciation of heavy metals in River Rhine

Chemical speciation of Zn, Cu, Ni, Cd and Pb in River Rhine was studied by measuring free ion concentration and distribution in nanoparticles, and by comparing the measurement with speciation modeling. Concentrations of free metal ions were determined in situ using Donnan Membrane Technique (DMT). The percentage of free over total (filtered) metal concentration is 52%, 33%, 2.6%, 0.48% and 0.12% for respectively Zn, Cd, Ni, Pb and Cu, i.e. the degree of metal complexation in the river is the lowest for Zn and the highest for Cu. Metals in 1–300 nm particles were analyzed using Asymmetric Flow Field Flow Fractionation (AsF-FFF), but the overall recovery is quite low. The nano-sized Cu detected is mainly associated with DOM of 1–5 nm, whereas Pb and Zn are dominantly associated with particles of iron hydroxides and clay of larger size (30–100 nm). Free ion concentrations calculated with the speciation modeling are in good agreement with the measurements, except for Pb. Based on the model, DOM-bound is the most important complexed form for Cu and Cd, whereas formation of (bi)carbonate and EDTA complexes are more important for Ni and Zn. Adsorption of Pb to DOM is probably overestimated by the model, whereas Pb adsorption to iron hydroxides is underestimated.

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► Free to total metal ratio increases in the order of Cu < Pb < Ni < Cd < Zn.
► Complexed Cu and Cd are mainly DOM-bound, and Ni mainly EDTA-bound.
► Complexed Zn is mainly in the hydrolyzed and precipitated from.
► Pb is mainly adsorbed to iron hydroxides and clay.
► Combining DMT, AsF-FFF analysis and modeling lead to better understanding.