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

Abstract

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.

Appropriate sampling strategy and analytical methodology to address contamination by industry. Part 2: Geochemistry and speciation analysis

“…Similar discrepancies have been reported in sites highly contaminated sites by COPR in New Jersey [70], Glasgow [31,71,72] and Pettine and Capri [42]. According to the latter authors the dissolution of humic matter (HM), especially in alkaline environments (digestion [41], see Table 2) might be the cause of the overestimation. Therefore, interpretation of the results must be carried out with caution especially when the OC content is very high.…”

“…Therefore the quantification of the soluble, exchangeable and crystalline forms of Cr(VI) (expressed as the total amount of Cr(VI) in a sampling media), has direct implications for the assessment of Cr hazards in the environment [40]. This information also helps to minimize expenditure on environmental remediation that would be unnecessary if all Cr were present as Cr(VI) [41]. Strong alkaline reagents are recommended for solids in order to minimize undesired Cr(III)-Cr(VI) redox reactions during extraction [42].…”

“…Strong alkaline reagents are recommended for solids in order to minimize undesired Cr(III)-Cr(VI) redox reactions during extraction [42]. The carbonate/hydroxide extraction method has been used by James et al [40], Szulczewski et al [43], Vitale et al [41] and Farmer et al [31], which consider this method as the most effective for the measurement of "total" Cr(VI) in soils. However the main remark made by Szulczewski et al [43] is that "not only do such treatment perturb the chemical and the physical properties of the soil-water system, moreover there is no way to verify that all the Cr(VI) has been extracted".…”

“…Such information is valuable for [41] predicting the heavy metal mobility, bioavailability and leaching fractions from the different sampling media [51]. However, there has been criticism regarding the application of sequential extraction procedures [49,[52][53][54].…”

Biological removal of carcinogenic chromium(VI) using mixed Pseudomonas strains

“…(Kimbrough et al, 1999). In the soil environment, chromium can exist as non-hazardous Cr(III) or as a toxic and carcinogenic Cr(VI) form (Vitale et al,1997).…”