Extraction of Sparingly Soluble Chromate from Soils: Evaluation of Methods and Eh−pH Effects

A hot alkaline extraction method (SW-846 Method 3060A) for total Cr(VI) in soils and sediments has been developed that selectively solubilizes Cr(VI). This paper compares the effectiveness of this extraction method versus four others to solubilize sparingly soluble PbCrO 4 spiked into four diverse soil materials. The five extractants were distilled water (pH 5.7); phosphate buffer (5 mM K 2 HPO 4 /5 mM KH 2 PO 4; pH 7.0); carbonate/hydroxide solution (0.28 M Na 2 CO 3 /0.5 M NaOH; pH 11.8) with and without heating; and hydroxide solution (0.1 M NaOH; pH 13) with sonication. The hot carbonate/hydroxide solution was superior to the other methods in extracting >90% of the spiked PbCrO 4 from a redox-inert quartz sand, chromite ore processing residue (COPR)-enriched soil, and a loamy-textured soil (Ultic Hapludalf). Distilled water and phosphate buffer extracted no Cr(VI) spike from these soils, as expected due to the low K sp of PbCrO 4 (1.8 × 10 - 14 ). Although it was hypothesized that PbCrO 4 would be reduced and not recovered from the anoxic sediment, variable Cr(VI) spike recoveries were observed, and the unheated alkaline extractant recovered more than the heated one. The Cr(VI), Eh, and pH results for anoxic sediment/quartz sand mixtures showed that the heating step is crucial to accelerate and completely dissolve PbCrO 4, but heating also liberated reducing agents (e.g., sulfides) from the sediment that partially reduced the spiked PbCrO 4 . Spike recoveries from anoxic sediment/quartz sand mixtures with a range of E h and pH values demonstrated that pH and E h measurements, combined with MINTEQA2 thermodynamic predictions of PbCrO 4 reduc tion to Cr 2 O 3 and Pb(OH) 2, can be used to predict a soil or sediment sample's redox status and aid in the interpretation of Cr(VI) spike recovery data.