Metal associations to microparticles, nanocolloids and macromolecules in compost leachates: size characterization by asymmetrical flow field-flow fractionation coupled to ICP-MS.

A methodological approach based on the size characterization of environmental microparticles (size larger than 1 microm), nanocolloids (1 microm to 15 nm) and macromolecules (lower than 1000 kDa) by asymmetrical flow field-flow fractionation (AsFlFFF), taking advantage of both normal and steric elution modes, is presented. The procedure was optimized to minimize the potential alteration of the size distribution and metal associations of the species characterized. Prior to separation by AsFlFFF, samples are subjected to gravitational settling of the solid suspension, followed by a centrifugation of the settled sample. The comparison between the fractograms of the settled and the centrifuged samples allows the characterization of the microparticles, which are eluted in steric mode in the AsFlFFF system. The characterization of nanocolloids and macromolecules is carried out on the centrifuged sample by applying different operational conditions under normal mode in the AsFlFFF system. A comparison with the conventional frontal filtration through 0.45 microm pore size membranes have shown that filtration removes particles below their nominal pore size, modifying the size distribution of the samples respect to the centrifugation. The methodology proposed has been applied to the size characterization of compost leachates. The contribution of these three differentiated fractions to the mobilization of metals has been determined by coupling the AsFlFFF system to an inductively coupled plasma mass spectrometer (ICP-MS).