Experimental design approach for the solid-phase extraction of residual aluminium coagulants in treated waters

Solid-phase extraction (SPE) of trace elements before their analysis has become a conventional pretreatment step of analytes because of their frequent low concentrations in numerous samples. Additionally, interfering compounds often accompagny analytes of interest, thus requiring a clean-up step. The preconcentration step and/or matrix removal can be efficiently improved by chemometric approaches allowing obtention of reliable results. Single variable approach is often used but is time and cost consuming, and may be the source of mistakes; multivariable approach allows to overcome these problems and increases the probability of global optimum finding. er to obtain a set of experimental conditions for the selective extraction of Al(III) in water samples, onto a modified organic support (salicylic acid grafted on XAD-4), a multicriteria approach (response surface methodology) has been applied. The extraction method was optimized by the aid of a factorial design and a uniform shell Doehlert design for six variables: sample percolation flow rate, trace metal amount, sample volume, concentration and volume of HCl used for elution of aluminium. Results demonstrate the synergic effects of four factors and allow us to define working ranges for each parameter tested. The designed SPE procedure was then sucessfully applied to synthetic and real samples, issued from a potable water treatment unit.