The use of acidimetric titration as a novel approach to study particulate trace

The study of trace metal speciation has benefited from a great deal of interest leading to the development
and the diversification of sequential extraction schemes (SES), which triggered the need for harmonization
by the standard, measurements and testing programme (SM & T). However, some uncertainties still
persist in the application of the harmonized 3-step protocol, because of the difficulty in conceptualizing a
technique that can be applied to environmental samples irrespective of their nature. The alternative
method proposed in this study is based on the solubility of the sample components progressively dissolved
during the course of an acidimetric titration by 1 mol L 1 HCl. The major HCl-reactive mineralogical
components are identified using mass balance calculation of H+ consumed by their dissolution
together with the amount of major elements released into solution. The speciation of minor and trace elements
is investigated by comparing their titrations to those of the major elements. This approach is much
simpler than SES because it uses simple non-selective H+ at room temperature instead of a complex
experimental design of so-called specific reagents. The different mineral components of the solid are
no longer operationally defined and the problem of selectivity is irrelevant to the titration approach.
The method was applied to several sediment samples from the Scheldt estuary and the particulate phase
was further examined by Scanning Electron Microprobe and X-ray Diffraction techniques. The nearly
complete consumption of H+ in the suspension is balanced by the total dissolution of carbonates and
Fe-oxyhydroxides. In contrast to the speciation inferred from the Tessier SES, the acidimetric titration
has demonstrated that the carbonate phase does not significantly contain trace metals with the exceptions
of 40% of the Mn and 30% of the Co. In contrast, the Fe-oxyhydroxides seem to play a major role
and account for 70% of Pb and 20% of Cr, in addition to 60% of P and additional amounts of 20% Co and
40% Mn. 70% of the Cu also occur in the oxyhydroxide phase, more likely coprecipitated with gibbsite.
90% of the Cd and 85% of Zn can be attributed to the Acid Volatile Sulfide (AVS) phase as evidenced from
S titration. The acidimetric titration method not only provides information on the speciation of trace metals
but also allows the quantification of their reactivity and mobility, if one considers that the titration
roughly mimics pH changes that may occur as a result of chemical disequilibrium in the environment.
The results demonstrate the potential of the acidimetric titration as an alternative to SES protocols in geochemical
and environmental regulation studies. This method is applicable to a wide variety of environmental
materials with little or minor adjustments.