The trace metals Al3+, Sc3+, Y3+, Cr3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, La3+ and Ce3+ were enriched in laboratory experiments with the chelating cation exchanger Chelex’100 from deionised water and saline solutions with ionic strengths from 35 g/l to 250 g/l Na-K-Ca-Mg-Cl-SO4. The recovery rates were determined for pH values between 3.5 and 6.5. A procedure was developed to analyse the trace metal contents in natural thermal brines. The elements were measured in the acidic eluats of the Chelex’100 columns with the ICP-MS. The complexation of the trace metals by the ion exchanger was modelled with PHREEQE (Parkhurst et al., 1990) using complexation constants provided from the experiments.
The measurement of heavy metal traces in highly saline waters is disturbed by matrix components. In dependence of the sample dilution the detection limits increase. The question of the investigations was wether the trace matrix separation with Chelex’100 can be applicated in highly saline solutions as in groundwater or marine samples.
In the first part of the experiments the reactions of the trace metals with the exchanger were studied under varying pH conditions in deionized water. The column length and the flow rate were optimized to enable high recoveries. In the second part the pH and ionic strength conditions were varied to investigate the recovery rates in dependence of the salt matrix. For all experiments the reproducibility was more important than recovery rates of 100 %. The procedure developed enables the detection of the trace metals with reproducibilities below 5 % in the upper ppT range from saline solutions between 35 g/l and 250 g/l total dissolved solids (TDS).
The trace matrix separation was applicated on-site for the measurement of the trace metal content in the brines of the Geothermal Heating Stations of Neubrandenburg (TDS 133 g/l) and Neustadt-Glewe (TDS 217 g/l) in Mecklenburg Germany. The waters were cooled in front of the Chelex’100 columns to prevent temperature effects on the enrichment procedure. Al, Sc, Co, Cu, Zn, Y, Cd, La, Ce and Pb could be detected with the developed procedure. The results for Zn, Pb and Cd could be proved with anodic stripping voltammetry.
The computer program PHREEQE (Parkhurst et al., 1990) was used to model the trace metal enrichment on the solid phase of Chelex’100. The speciation of the Chelex’100 surface can be described by the pKS values. The complexation constants of trace metal Chelex’100 complexes were calculated on the basis of mass balances from the laboratory results. In the case of deionized water the modelled pH dependence coincides very well with the laboratory results whereas the first results of the saline water modelling were not successfull, probably caused by the fact, that the complex constants of the matrix cations with Chelex’100 had to be taken from the literature and are not calculated in the same manner as the constants for the trace metals. For that reason further experiments are planned.