The accurate quantification of total Cr(VI) in soils is relevant to human health concerns because Cr(VI) is significantly more toxic than Cr(III). Hot alkaline solution has been shown to extract soluble and insoluble forms of Cr(VI) from soils, but incomplete recovery of Cr(VI) spikes and the oxidation of soluble Cr(III) spikes in certain soils have been suggested as method deficiencies. A laboratory method study was performed to (i) test the method's accuracy, (ii) understand the soil chemical processes responsible for poor Cr(VI) spike recoveries, and (iii) develop definitive interpretations for Cr(VI) spike recovery data. Test results for >1500 field soil samples and the method study of eight diverse soil materials demonstrated dissolution of soluble and insoluble Cr(VI) spikes and the method's reliability for Cr(VI) characterization. Complete dissolution of K2CrO4, BaCrO4, and PbCrO4 spikes confirmed the extraction of soluble and insoluble Cr(VI) forms. Ancillary soil chemical parameters, including oxidation‐reduction potential (ORP) (reported herein as Eh), pH, S2−, and total organic C were quantified and interpreted to explain poor Cr(VI) spike recoveries. Highly reducing samples yielded 0% Cr(VI) spike recoveries, as predicted from Eh‐pH relationships, and unspiked soil samples contained no detectable Cr(VI). In soils containing Cr(VI) and in most aerobic soils without native Cr(VI), acceptable Cr(VI) spike recoveries were obtained. Ancillary parameter characterization demonstrated that strongly reducing samples cannot maintain Cr(VI) laboratory matrix spikes. Correct interpretation of poor Cr(VI) spike recovery data should avoid labeling these data as unacceptable method results without ancillary parameter characterization of such samples.
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 reduction 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.
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