Heavy metals and colloids in streams impaired by acid rain


Heavy metals and colloids in streams impaired by acid rain

Ulrich, K.-U.; Zänker, H.; Roßberg, A.; Furrer, G.

The common concept of heavy metal migration in the hydrosphere differentiates between solid and liquid phase, considered as immobile and mobile. Colloidal phases are often neglected, which may have the following consequences on environmental hazard prognosis (Zänker et al. 2003):

a) The contaminant is regarded as mobile in the model, but it adsorbs on colloids which aggregate and settle (‘natural attenuation’): the prognosis is too pessimistic.
b) The contaminant is regarded as immobile in the model due to adsorption on the host rock, but in part it adsorbs on colloids which are transported: the prognosis is too optimistic.
c) Only if the contaminant is fully mobile, i.e. it is not adsorbed onto solid phases: colloidal transport is irrelevant and the model description is correct.
This study is focused on the incidence of colloids and their binding of toxic metals in streams of the Erzgebirge (Germany), a low mountain region which strongly suffered in the past from mining activities, e.g. uranium mining, and from acid rain loads exceeding the buffering reaction by weathering. Mildly acidic conditions (pH 5-6.5), which may occur in the runoff from dumps or arise on the reversal from stronger acidification (Ulrich and Meybohm 2005), or by mixing of waters with different loads of protons and metals, favor the generation of colloids by hydrolysis of iron (Fe) and aluminum (Al) or by dissolution of clay minerals and precipitation of secondary ferric oxides.
Based on samples from the confluence of two forest streams in the Erzgebirge, 27Al MAS NMR spectroscopy showed that Al-rich colloids contain distinct Al(O)4 centers similar to the -Keggin polyoxocation AlO4Al12(OH)24(H2O)127+(aq) [Al13] (left Fig.). These colloids form via aggregation of the Al13 nanoclusters (Furrer et al. 2002) and bear a high surface density of functional groups capable of accumulating heavy metal cations.
Fe-rich colloids (right Fig.), which often consist of metastable ferrihydrite, exhibit similar properties. Such colloids form when acidic seepage (e.g. from a uranium mine or dump) mixes with near-neutral surface water. Uranium (U), which is soluble under strongly acidic (UO22+) and alkaline (uranyl carbo-nate complexes) conditions, will be scavenged and immobilized by colloids which aggregate and settle in the slightly acidic pH region. On the other hand, contaminants that are usually immobile (As, Cu, Pb), can be mobilized by colloids. The molecular structure of some sorption complexes derived from EXAFS and ATR FT-IR spectroscopy is discussed.

References
Furrer, G., Phillips, B.L., Ulrich, K.-U., Pöthig, R. and Casey, W.H. (2002) The origin of aluminum flocs in polluted streams. Science 297, 2245-2247.
Ulrich, K.-U. and Meybohm, A. (2005) Reservoir ecosystems recover from atmospheric acidification: I. Trends of chemical reversal. ACID RAIN 2005, Conference Proceedings, 000.
Zänker, W., Richter, W., Hüttig, G. (2003) Scavenging and immobilization of trace contaminants by colloids in the waters of abandoned ore mines. Coll. Surf. A 217, 21-31.

Keywords: colloid migration; heavy metals; uranium; sorption; EXAFS

  • Poster
    Acid Rain 2005, 7th International Conference on Acid Deposition, 12.-17.06.2005, Prague, Czech Republic
  • Contribution to proceedings
    Acid Rain 2005, 7th International Conference on Acid Deposition, 12.-17.06.2005, Prague, Czech Republic
    Acid Rain 2005 Conference Abstracts, 613

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