Fractionation of arsenic in soil and sludge samples: continuous-flow extraction using rotating coiled columns versus batch sequential extraction

Abstract

Fractionation of arsenic and other trace elements (TE) in environmental solid samples is usually achieved by application of batch sequential extraction procedures (SEP). In the present work a rotating coiled column (RCC) earlier used mainly in countercurrent chromatography has been applied to study continuous-flow fractionation (CFF) of As compared to conventional batch SEP. A particulate solid sample (soil or sludge) was retained in the rotating column as the stationary phase under the action of centrifugal forces while different eluents were continuously pumped through. The eluents were applied in correspondence with an optimized SEP for As addressing non-specifically sorbed, specifically sorbed, bound to amorphous and crystalline Fe oxides fractions extractable by (NH 4) 2SO 4, NH 4H 2PO 4, NH 4-oxalate buffer, NH 4-oxalate buffer + ascorbic acid at 96 °C, respectively. For all samples the recoveries of As in the non-specifically sorbed fraction obtained by CFF were considerably higher than in the corresponding fraction of the batch SEP. This finding is consistent with a quickly obtained equilibrium between the solid phase and the ammonium sulphate solution in batch where no further extraction takes place. The repeated extraction occurring due to the continuous removal of the eluate in the CFF procedure enables a more complete extraction of the non-specifically adsorbed pool. For the two consecutive steps, both methods delivered virtually the same results. In contrast, since no heating was used in RCC batch SEP more efficiently extracted As bound to crystalline Fe oxides at step 4. As a result, the residual fraction was overestimated after CFF. The CFF procedure can be completely automated. In addition, investigating the elution curves allows one to study the kinetics of As (and related elements of interest) release, to test efficacy of extractants and to assess the maximum pool size of target fractions.