Fluidized-bed column method for automatic dynamic extraction and determination of trace element bioaccessibility in highly heterogeneous solid wastes

Dynamic flow-through extraction/fractionation methods have recently drawn much attention as appealing alternatives to the batchwise steady-state counterparts for the evaluation of environmentally available pools of potentially hazardous trace elements in solid matrices. The most critical weakness of flow-based column approaches lies in the small amount of solid that can be handled, whereby their applicability has been merely limited to date to the extraction of trace elements in highly homogeneous solid substrates; otherwise the representativeness of the test portion might not be assured.To tackle this limitation, we have devised an automated flow-through system incorporating a specially designed extraction column with a large volume capacity, wherein up to 2 g of solid sample could be handled without undue backpressure. The assembled flow setup was exploited for fast screening of potentially hazardous trace elements (namely, Cd, Cr, Cu, Pb, and Zn) in highly inhomogeneous municipal solid waste incineration (MSWI) bottom ashes. The pools of readily mobilizable metal forms were ascertained using the Toxicity Characteristic Leaching Procedure (TCLP) based on the usage of 0.1 mol L⁻¹ CH₃COOH as leachant and analysis of extracts by inductively coupled optical emission spectrometry. The application of a two-level full factorial (screening) design revealed that the effect of sample fluidization primarily but other experimental factors such as the solid to liquid ratio and extractant flow rate significantly influenced the leachability of given elements in raw bottom ashes at the 0.05 significance level.The analytical performance of the novel flow-based method capitalized on fluidized-bed extraction was evaluated in terms of accuracy, through the use of mass balance validation, reproducibility and operational time as compared to batchwise extraction and earlier flow injection/sequential injection microcolum-based leaching tests.