Assessing the mobilization of As, Cr, Mo, and Se in Egyptian lacustrine and calcareous soils using sequential extraction and biogeochemical microcosm techniques

Abstract

The mobilization of As, Cr, Mo, and Se in four soil profiles representing Sodic Fluvisols (lacustrine deposits) and Haplic Calcisols (calcareous deposits) in Egypt were assessed using sequential extraction and an advanced biogeochemical microcosm technique. The concentrations of total and AB-DTPA-extractable elements were determined. The geochemical fractions (acid soluble (F1), reducible (F2), oxidizable (F3), and residual (F4) fraction) of the elements were extracted using the Commission of the European Communities Bureau of Reference (BCR) procedure. An automated biogeochemical microcosm apparatus was used to assess the redox-induced mobilization of As, Cr, Mo, and Se in the lacustrine soil as affected by the changes of redox potential (EH) and the EH-dependent changes on pH, Fe, Mn, SO42−, and dissolved organic carbon (DOC). The lacustrine soils showed higher total concentrations (mg kg−1) of As (38–70), Cr (58–85), Mo (6–13), and Se (22–45) than the calcareous soils. The pollution load index was higher than unity (3.0–11.3) and Se was the major contributor to the soil pollution followed by Mo, As, and Cr. The residual fraction was dominant for the four elements in the two soils. The acid soluble fraction and AB-DTPA extractable concentrations of As and Cr were below the detection limits, while Se and Mo were detected in the studied profiles. The reducible fraction was the dominant non-residual fraction for As and Se in the two soils and for Mo in the lacustrine soils, while the oxidizable fraction was the dominant non-residual fraction for Cr in the two soils and for Mo in the calcareous soils. The acid soluble fraction, availability, and potential mobility (PMF = ∑F1-F3) of Se and Mo were higher in the calcareous than the lacustrine soils. Selenium showed the highest redox-induced mobilization, followed by Mo and As, while dissolved Cr was lower than the detection limit under both reducing and oxidizing conditions. Mobilization of Se, Mo, and As increased under reducing conditions. Mobilization of As and Se was positively affected by the chemistry of Fe, Mn, SO42− and DOC. Dissolved Mo was affected by the EH dependent changes of pH and the chemistry of S and SO42−. The results of the biogeochemical microcosm supported the sequential extractions data. Increasing the dissolved concentrations of As, Mo, and Se under reducing conditions in the temporally flooded lacustrine rice soil might increase their release and transfer into the groundwater and the food chain which might increase the potential environmental risks.