ABSTRACTthat because As and P form similar oxyanions in the ϩ5 oxidation state in soils (O'Neill, 1990), soil extraction
Batch sequential extraction techniques for fractionating metals orschemes that have long been employed for P fractionmetalloids in soils are time consuming and subject to several potential ation are more suitable for As (Jacobs et al., 1970; Wool-errors. The development of a continuous-flow sequential extraction method for soil As is described and assessed, having the benefits of son et al., 1971; McLaren et al., 1998).
simplicity, rapidity, less risk of contamination, and less vulnerabilityChemical fractionation (operationally defined chemito changes in extraction conditions compared with traditional batch cal speciation by sequential extraction) has been widely methods. The validated method was used to fractionate soil As using accepted and applied. However, the technique has been water, NaHCO 3 , NaOH, and HCl, followed by digestion of the residue questioned because of poor selectivity of reagents towith HNO 3 and HF acids. The extracts and digests were analyzed ward the targeted solid materials. Therefore, it is impor- 1993; Tu et al., 1994; Raksasataya et al., 1996; generally support previous suggestions of the likely forms or associaLo and Yang, 1998).tions of As present in the different soil fractions.Our previous work on development of a continuousflow extraction system for sequential extraction has manifested many advantages compared with using a M inerals, metals, or metalloids, toxic or essential, batch method (Shiowatana et al., 2001). For example, are present in soils or sediments in various forms the flow system has the benefits of simplicity, rapidity, with varying bioavailability, toxicity, and mobility. Deless risk of contamination, and less vulnerability to termination of total concentrations of these elements in changes in extraction conditions. In a continuous-flow solid materials is therefore considered to be of limited system, because elements brought into solution are conuse in assessing potential environmental impacts. In ortinuously being removed from the system, there should der to assess their actual behavior, role, and impact, a be less opportunity for readsorption to occur. The sysgood understanding of the chemical forms of the eletem also has an additional advantage in that the exments of interest is required. The use of sequential extractograms obtained can provide useful information on traction techniques to fractionate metals in solid materithe association of elements in each solid phase. In this als, and evaluate their potential effects, has become study, the continuous-flow extraction system was apwidely used and well recognized (Tessier et al., 1979).plied to the sequential extraction of As using an extracHowever, there are relatively few reports on the fraction scheme modified from the work of McLaren et al. tionation of metalloids such as As. Some reports have (1998). This scheme was selected because it was develemployed the extraction schemes original...
