Environ. Sci. Technol. 1997, 31, 390-394
Extraction of Sparingly Soluble Chromate from Soils: Evaluation of Methods and Eh-pH Effects ROCK J. VITALE,* GEORGE R. MUSSOLINE, AND KELLY A. RINEHIMER Environmental Standards, Inc., Valley Forge, Pennsylvania 19482-0911 JOHN C. PETURA Applied Environmental Management, Inc., Malvern, Pennsylvania 19355 BRUCE R. JAMES Soil Chemistry Laboratory, H. J. Patterson Hall, University of Maryland, College Park, Maryland 20742
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 PbCrO4 spiked into four diverse soil materials. The five extractants were distilled water (pH 5.7); phosphate buffer (5 mM K2HPO4/5 mM KH2PO4; pH 7.0); carbonate/hydroxide solution (0.28 M Na2CO3/ 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 PbCrO4 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 Ksp of PbCrO4 (1.8 × 10-14). Although it was hypothesized that PbCrO4 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 PbCrO4, but heating also liberated reducing agents (e.g., sulfides) from the sediment that partially reduced the spiked PbCrO4. Spike recoveries from anoxic sediment/ quartz sand mixtures with a range of Eh and pH values demonstrated that pH and Eh measurements, combined with MINTEQA2 thermodynamic predictions of PbCrO4 reduction to Cr2O3 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.
Introduction Chromium is a trace metal found naturally in U.S. soils from 1 to 2000 mg kg-1 apart from chromium-bearing domestic ore deposits, which can range up to 270 g kg-1 Cr (1, 2). * Corresponding author phone: (610) 935-5577; fax: (610) 9355583; e-mail:
[email protected].
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Chromium mainly exists in the natural environment in two valence states in which the trivalent form [Cr(III)] is naturally present in the environment and hexavalent chromium [Cr(VI)] is almost always present because of human activities (3). Crystalline chromium hydroxide [Cr(OH)3] and chromium oxide (Cr2O3) are two of the most common forms of Cr(III) in the environment. Both compounds are sparingly soluble in water and exhibit low acute and chronic toxicity at high doses (4, 5). Trivalent chromium is considered an essential nutrient for humans (6), whereas Cr(VI) is a Class A human carcinogen by inhalation, and its toxicity varies greatly among a wide variety of very different Cr compounds (7-9). In view of the ubiquitous use of Cr compounds in modern industrial societies (10), Cr and its compounds are prevalent contaminants in soil and water at myriad sites worldwide. Because of the divergent toxicities of Cr(III) and Cr(VI), accurate measures of each valence state are needed in soils, sediments, and waste materials to (1) assess the human health risks from potential Cr(VI) exposure and (2) minimize unnecessary expenditures for environmental remediation that would otherwise be incurred if all chromium (Cr) were assumed to be present in the Cr(VI) form. A hot (90-95 °C) 0.28 M Na2CO3 solution in 0.5 M NaOH, designated in SW-846 Method 3060A to extract total soluble and insoluble forms of Cr(VI) [i.e., “total” Cr(VI)] from soils and sediments (11), has been developed and validated by Vitale et al. (12, 13). Additional studies by James et al. (14) compared the effectiveness of this method to four other commonly used extraction procedures in a variety of soil types and verified its higher efficiency of total Cr(VI) extraction than the other methods. However, the accuracy of a method depends on quantitative dissolution of all forms of Cr(VI) without method-induced reduction to Cr(III). The objective of this research was to compare the efficacy of the same five extraction methods studied by James et al. (14) to recover quantitatively one of the least soluble forms of Cr(VI) (PbCrO4) when used as a matrix spike. The results have particular importance in environmental analytical chemistry because there is no standard reference material for Cr(VI) analysis in solid matrices. The extraction techniques compared in this study speciate Cr(VI) into soluble, exchangeable, and sparingly soluble forms. The soluble and exchangeable fractions of Cr(VI) are useful parameters for relating levels of Cr(VI) in soil that may leach to groundwater, form a soluble “blush” on soil surfaces, or be absorbed by plants and microorganisms (15-17). Quantifying insoluble and soluble forms of Cr(VI) is also pertinent to potential environmental hazards (3, 5, 8, 18-20) associated with airborne, respirable dust (21) or colloid/solute movement in groundwater (22-28).
Materials and Methods The extraction methods compared included (1) distilled water, (2) phosphate buffer solution, (3) an alkaline solution (0.28 M Na2CO3/0.5 M NaOH) with heat, (4) the same alkaline solution at ambient temperature, and (5) 0.1 M NaOH solution with sonication. The four soil materials tested were (1) quartz sand, (2) chromite ore processing residue (COPR)-enriched soil, (3) loam soil, and (4) anoxic sediment. Soils. The four different soils were selected based on their dissimilar oxidation-reduction characteristics relevant to Cr speciation. A redox-inert quartz sand (QS) was used as the baseline matrix in this study because it had previously been shown to contain negligible Cr(VI) or organic matter and was not expected to promote oxidation of Cr(III), reduction of Cr(VI) in PbCrO4 spikes, or sorption of cationic or anionic forms of either valence state of Cr (12, 14). The commercially-
S0013-936X(96)00202-7 CCC: $14.00
1997 American Chemical Society
available QS was white in color, contained no detectable Cr(VI), and had 100 mg of sulfide kg-1 (31). The anoxic sediment was collected from the Hackensack marshlands in northern New Jersey. The fourth soil was an anthropogenic fill material enriched with COPR used to reclaim marshlands containing approximately 800 mg kg-1 of soluble and insoluble Cr(VI) species. The COPR sample was obtained from a former chromite ore processing facility and contained varying amounts of silt, clay, and sand. Spiking Procedures for PbCrO4. For each of the four soils, the field-moist equivalent weight of 2.5 g of air-dried soil material was weighed into 30 250-mL beakers, which were divided into two groups of 15 each. Each group of 15 beakers was further divided into five groups of three each. To one of the two groups of 15 beakers, approximately 15 mg of PbCrO4 (exact weight recorded) was added to QS, LS, and AS samples in triplicate [equivalent to 1.0 g of Cr(VI) kg-1]. A spike of 2.0 g of Cr(VI) kg-1 was added to the COPR-enriched fill sample due to its high native Cr(VI) content. Extraction Methods. To each group of 15 beakers containing a spiked or unspiked soil, 50 mL of one of the following extraction solutions was added: Distilled water: (pH 5.7), glass-distilled, and having an electrical conductivity of approximately 0.001 dS/m [to define operationally “water-soluble Cr(VI)”]. Phosphate buffer: (pH 7.0), 5.0 mM K2HPO4 in 5.0 mM KH2PO4 (for “soluble-plus-exchangeable Cr(VI)”). Carbonate/hydroxide solution: (pH 11.8-12.3), 0.28 M Na2CO3 in 0.5 M NaOH added to two sets of 15 beakers; one to be heated (90-95 °C, liquid temperature) and the other to be maintained at room temperature (25 ( 2 °C) [for “total Cr(VI)”]. Hydroxide solution: (pH 13), 0.1 M NaOH (to compare sonication vs heating during alkaline extraction). All suspensions were swirled continuously for 60 ( 2 min at room temperature (25 ( 2 °C). The beakers for the hot carbonate/hydroxide extraction were then placed on a preheated hot plate and maintained at 90-95 °C for 60 min. The beakers for sonication were placed and maintained in a sonicating bath for 60 min, and all of the other suspensions also continued to swirl for 60 min. The suspensions were subsequently filtered through 0.45-µm polycarbonate membranes or centrifuged, and the filtrates/centrifugates were analyzed colorimetrically for Cr(VI) by SW-846 Method 7196A (32). Lead Chromate Matrix Spikes into Anoxic Sediment: Explanatory Studies. Phase I: 18-Hour Equilibration. Fortyeight, 250-mL beakers were divided into two groups of 24: one group for AS and the other for QS. For each sample type, the moist equivalent of 2.5 g oven-dried (105 °C) material was added to 24 beakers after adding approximately 15 mg of PbCrO4 to 12 beakers for each sample type and 1.25 mL of 38.5 mM K2CrO4 to the other 12 [equivalent to 1.0 g of Cr(VI) (kg of dry soil)-1]. The wet AS contained approximately 20 mL of water, so 20 mL of distilled water was added to the beakers containing QS to allow effective mixing and to make the QS and AS treatments comparable during the equilibration period. Each group of 12 beakers was further divided into two groups of six: 0.71 mL of 13.2 mM Na2S was added to one group, and nothing was added to the other six beakers.
Three of these beakers were heated to 90-95 °C, and the other three remained at 25 °C, as described below. The 48 beakers were covered with Petri dish lids to exclude dust (but not air) and swirled for 18 h at 125 cycles/min on a rotary shaker at 25 °C. The Eh and pH were measured using Pt and glass electrodes, respectively. A total of 50 mL of Method 3060A extracting solution was added to all the beakers, which were swirled briefly, and the contents of the 24 beakers were heated at 90-95 °C for 60 ( 2 min. The other samples remained at 25 °C. Subsequently, the suspensions in each beaker were centrifuged (9200g, 10 min, 25 °C) and brought to 100 mL by weight for colorimetric Cr(VI) measurement (at 540 nm) after the addition of diphenylcarbazide (DPC) reagent (containing ethanol, H3PO4, and DPC) (15). Phase II: 10-Day Equilibration. Six beakers containing only AS and PbCrO4 spikes were prepared as described above and were equilibrated for 10 d at 25 °C. Method 3060A was then performed; heating three beakers and leaving three beakers unheated. This experiment was performed to assess the influence of a longer equilibration time on the solubility and reduction of PbCrO4 in the AS, as measured by the heated and unheated alkaline extraction procedures. Phase III: Effect of Eh-pH Conditions on PbCrO4 Spike Recoveries. Increasing quantities of AS (0, 0.41, 1.1, 5.2, 10.1, and 20.5 g of wet material) were added to the moist equivalent of 2.5 g (oven-dried) of QS in individual beakers. The 20.5 g AS treatment was equivalent to 2.5 g of oven-dried material. A matrix spike of PbCrO4 (approximately 15.5 mg) was added before adding 50 mL of distilled water to each beaker. The beakers stood at 25 °C for 60 min, and Eh and pH were measured in each suspension. A total of 50 mL of Method 3060A solution then was added, and the extraction procedure and Cr(VI) analysis were conducted using supplemental heat. The increasing levels of AS added to the QS established a range of Eh-pH conditions to simulate a range of redox conditions as might exist in field soils. The PbCrO4 spike recoveries for each AS-QS mixture then were plotted on an Eh-pH diagram containing thermodynamic stability ranges for PbCrO4 in equilibrium with Cr2O3 and Pb(OH)2, as calculated by MINTEQA2 (33). It was hypothesized that partial or 0% spike recoveries would be obtained in the heated carbonate/hydroxide procedure if the measured Eh and pH for a given mixture of QS and AS indicated that PbCrO4 would be expected to be reduced to Cr2O3 at thermodynamic equilibrium. This combination of thermodynamic prediction and experimental procedures was designed to show whether or not Eh-pH conditions of a soil sample could predict PbCrO4 dissolution and reduction under the extraction conditions of Method 3060A.
Results and Discussion Figure 1 shows the amounts of Cr(VI) extracted from the COPR sample by the five extraction methods and the normalized percentages of Cr(VI) extracted as compared to the carbonatehydroxide method with heat. These results indicate that this extractant solubilized nearly twice as much Cr(VI) from the COPR-enriched fill as did the four other methods, and almost four times as much Cr(VI) as distilled water alone. The sonication method with 0.1 M NaOH solution was less effective than the carbonate/hydroxide procedure, with or without heat, but was significantly more effective than water or phosphate buffer for extracting Cr(VI). The distilled water and phosphate buffer extracted essentially no measurable Cr(VI) from the AS at the 1.0-2.0 g of Cr(VI) kg-1 spike range (Figure 2), which may indicate that the Cr(VI) was solubilized and then reduced to Cr(III) by sulfides present. Alternatively, the Cr(VI) was not solubilized and, therefore, was not recovered. The absence of measurable Cr(VI) spike recoveries in the QS or LS for the distilled water and phosphate extractions support the latter hypothesis.
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FIGURE 1. Comparative effectiveness of Cr(VI) solubilization and quantitation in an unspiked COPR sample by various extraction methods. (Values in parentheses are °C, bar heights are means, and error bars are 1 SEM.)
FIGURE 2. Lead chromate matrix spike recovery in sand, loam soil, COPR, and anoxic sediment using five extraction methods. The spike level was approximately 1.0 g of Cr(VI) kg-1 for all the soils, except COPR received 2.0 g of Cr(VI) kg-1. (Bar heights are means, and error bars are 1 SEM.) No measurable Cr(VI) was extracted from the spiked QS, LS, or AS samples by either the distilled water or phosphate buffer solutions (Figure 2). In the COPR sample, approximately 40% of the native Cr(VI) extracted by the carbonate/hydroxide solution with heat was extracted by the phosphate buffer, indicating that approximately 60% of Cr(VI) in the COPR sample is sparingly soluble (Figure 1). The water extracted only 27% of the native Cr(VI) that was extracted by the heated carbonate/hydroxide solution. The sonication method with 0.1 M NaOH yielded Cr(VI) values similar to those for the carbonate/hydroxide solution (with and without heat) for the QS (Figure 2). However, the sonication method yielded considerably lower Cr(VI) values than those obtained from the unheated or heated carbonate/ hydroxide methods for the LS, COPR, and AS materials. The five extractants achieved different efficiencies for PbCrO4 dissolution (Figure 2). The heated carbonate/ hydroxide extraction was the most effective method among the five for extraction of Cr(VI) in the PbCrO4-spiked soil materials, with the exception of the AS material. The spike recovery for PbCrO4 added to the AS was 34% with heat as compared to 100% for the other soil materials. Water and phosphate buffer extracted essentially no measurable Cr(VI) from the AS in the 1.0-2.0 g kg-1 spike range, which may indicate that either the Cr(VI) was solubilized and then reduced to Cr(III) by reducing agents present or the Cr(VI) was not solubilized.
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FIGURE 3. Hexavalent Cr spike recovery (PbCrO4 and K2CrO4) from quartz sand (QS) and anoxic sediment (AS) using Method 3060A at 25 and 95 °C with and without Na2S added. (Bar heights are means, and error bars are 1 SEM.) Initial interpretations of the results suggested that the carbonate/hydroxide method with heat was less efficient at extracting PbCrO4 than it was without heat (approximately 70% spike recovery), a surprising result because earlier work (14) showed greater spike recoveries with heat in aerobic soils. However, it was hypothesized that the added PbCrO4 spike was being reduced to Cr(III) following dissolution, resulting in low recoveries. The heat and alkaline condition may also have liberated S- and C-containing compounds in the AS sample, which then reduced the soluble Cr(VI). It was observed that the color of the alkaline extract was much darker in the heated than in the unheated treatments, indicating that organics in the soil were solubilized with added heat and thus supporting the hypothesis. This also explained the higher spike recoveries observed for the spiked AS samples extracted by the carbonate/hydroxide method without heat relative to the recoveries observed with heat. This hypothesis was supported by other results: (1) heating was most effective since it resulted in the greatest amount of Cr(VI) being extracted from the sample that was known to contain native Cr(VI) (i.e., the COPR-enriched fill sample); (2) the alkaline extraction-sonication procedure (with a slightly elevated temperature due to the sonication) exhibited higher recoveries of PbCrO4 spiked into the AS relative to the carbonate/hydroxide procedure with heat, but lower recoveries than without heat (Figure 2); and (3) the NaOH with sonication extraction procedure (with observed higher than room temperature extraction solution) exhibited higher recoveries of PbCrO4 spiked into the AS relative to Method 3060A with heat, but lower recoveries than Method 3060A without heat (Figure 2). Results for the explanatory studies (phase I, 18-h equilibration) to test this hypothesis revealed the following: (1) No Cr(VI) was recovered from the AS spiked with PbCrO4 or K2CrO4 when supplemental heat was used, but 93-102% Cr(VI) matrix spike recoveries were obtained for the QS (with no S added) when spiked with either Cr(VI) source (Figure 3). For the QS, S2- enrichment reduced the spike recovery percentage slightly, even though the quantity of S2- added (120 mg kg-1) was sufficient to reduce approximately 500 mg Cr(VI) kg-1. This comparison of the QS and AS results (Tables 1 and 2) indicated that the heated, alkaline solution dissolved the PbCrO4 matrix spike and that organic matter, Fe2+, and/ or S-containing reducing agents in the AS reduced the Cr(VI) spikes. (2) Without supplemental heat, no Cr(VI) was recovered from the K2CrO4-spiked AS treatments (Table 2), but 15% of the added PbCrO4 was recovered in the presence of spiked S2-, and 20% was recovered without added S2-. In the QS,
TABLE 1. Quartz Sand Spiked with Soluble and Insoluble Cr(VI) Cr(VI) Cr(VI) Na2S digestion added % Cr(VI) added % Cr(VI) added temp (°C) (mg/kg)a recoverya (mg/kg)b recoveryb trial A-1 trial A-2 trial A-3 trial B-1 trial B-2 trial B-3 trial C-1 trial C-2 trial C-3 trial D-1 trial D-2 trial D-3
Y Y Y N N N Y Y Y N N N
95 95 95 95 95 95 25 25 25 25 25 25
851 1245 877 813 1019 1316 1019 961 1180 787 1051 1180
96 95 88 104 99 95 68 78 69 80 72 70
1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
100 101 102 102 103 103 112 101 104 104 106 104
a Spiked with PbCrO [insoluble Cr(VI)]. b Spiked with K CrO [soluble 4 2 4 Cr(VI)] (Y ) yes; N ) no).
TABLE 2. Anoxic Sediment Spiked with Soluble and Insoluble Cr(VI) Cr(VI) Cr(VI) Na2S digestion added % Cr(VI) added % Cr(VI) added temp (°C) (mg/kg)a recoverya (mg/kg)b recoveryb trial A-1 trial A-2 trial A-3 trial B-1 trial B-2 trial B-3 trial C-1 trial C-2 trial C-3 trial D-1 trial D-2 trial D-3
Y Y Y N N N Y Y Y N N N
95 95 95 95 95 95 25 25 25 25 25 25
980 1013 864 1561 1097 1187 1206 768 987 948 1206 1097
0 0 0 0 0 0 11 14 20 21 21 18
1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
0 0 0 0 0 0 0 0 0 0 0 0
a Spiked with PbCrO [insoluble Cr(VI)]. b Spiked with K CrO [soluble 4 2 4 Cr(VI)] (Y ) yes; N ) no).
100% of added K2CrO4 was recovered, and 72-74% of added PbCrO4 was recovered (Table 2). These results for the unheated treatments indicated that supplemental heat is necessary to dissolve the spiked PbCrO4 in 60 min, but that heat may also induce reduction of spiked Cr(VI) in certain “reducing” soils if it is not reduced prior to the application of heat. The minimal evolution of heat in the NaOH extraction with sonication explains why the PbCrO4 spike recoveries for the AS were greater than the Method 3060A with heat extraction but less than Method 3060A without heat extraction (Figure 2). Based on these findings, the reason for 20% (and not 0%) recovery of the PbCrO4 was incomplete dissolution or reduction of the matrix spike in the AS before heat addition to the alkaline extraction solution. The thermodynamic predictions indicate that PbCrO4 is insoluble and stable at pH 7 but that soluble CrO42- is reduced to Cr2O3. Therefore, reduction of Cr(VI) in PbCrO4 depends on its dissolution. Yellow PbCrO4 granules were visible in the bottom of the beakers after 18 h and before the initiation of heating in the carbonate/hydroxide method, confirming the relative inertness or thermodynamic stability of PbCrO4 in the AS at nearneutral pH and room temperature. Two questions arose based on these results: (1) would a longer equilibration time with PbCrO4 dissolve and reduce the spike and be more representative of field soils amended with Cr(VI) waste materials than the short time usually allowed for spike equilibration in the laboratory, and (2) under what Eh and pH conditions of a soil would spiked PbCrO4 be reduced
in that soil, either in the field if given enough time, or under the heated conditions of the carbonate/hydroxide extraction method? The results of phases II and III provided information to address these two questions. After 10 d of equilibration (at 25 °C) in phase II, yellow PbCrO4 granules were still visible in the beakers containing AS. Under the heated conditions of the carbonate/hydroxide method, however, no yellow crystals were observed and no Cr(VI) was subsequently recovered. In the absence of supplemental heat, an average of 25% Cr(VI) spike recovery was obtained. When the alkaline extraction solution was allowed to react for 24 h at 25 °C, instead of 1 h, the recovery percentage increased to 33%. The Eh of these suspensions following 10 d of aerobic equilibration was 540 mV and the pH was 6.2 (vs Eh of 180 mV and pH of 7.3 for the AS in the “natural” sample condition). The color of the suspension after 10 d of aerobic equilibration was noticeably lighter than the jet black native AS material in its reduced condition at the beginning of the experiment, and sulfide odors were absent. These results indicated that (1) equilibration of a PbCrO4 spike with the AS for 10 d at room temperature did not dissolve the spike, (2) the reductants in the materials were liberated during the heating, and (3) the reductants reduced the PbCrO4 following dissolution at high pH and temperature. During this experiment as the suspensions were heated, the color of the supernatant liquid grew noticeably darker (coffee color) as the temperature rose to 90-95 °C. As the prescribed temperature was reached, the yellow granules of PbCrO4 were no longer visible. While conducting the diphenylcarbazide analysis for extracted Cr(VI), a slight odor of H2S was detected, indicating that sulfide was still present. This was reasonable because the diphenylcarbazone-Cr complex has a pH near 2, sufficiently acidic to convert ionic S2- to gaseous H2S. The combined results of phases I and II showed that the heated, alkaline condition of the carbonate/hydroxide method was in part responsible for the low recoveries of the PbCrO4 spikes, but such results were expected in such a reducing system. Since previous work (14) showed that poor spike recoveries were obtained in reducing soils subjected to this carbonate/hydroxide extraction method, a series of mixed AS-QS soil suspensions with a range of Eh-pH conditions was established prior to spiking with PbCrO4 and subsequently was extracted by the heated carbonate/hydroxide solution. This was done to determine if Eh and pH could be used to predict PbCrO4 spike recoveries for this method over a range of redox conditions from aerobic to anaerobic. The experimental data plotted on the Eh-pH diagram (Figure 4) show that Eh and pH can effectively predict spike recoveries for PbCrO4 using the MINTEQA2 thermodynamic model for Cr(III) and Cr(VI) compounds typically found in soils containing significant levels of Cr (30). The solid lines sloping down to the right define the Eh-pH limits, below which Cr(VI) would be reduced to Cr2O3 and above which it would remain stable as PbCrO4. Between the lines is a transition zone in which partial Cr(VI) reduction might occur (i.e., Cr2O3 and PbCrO4 co-exist at equilibrium). The plotted data points represent the measured Eh and pH of the mixtures of AS-QS after 1 h of equilibration, and the inset box contains the measured percentages of PbCrO4 recovered in the heated alkaline procedure shortly after taking the Eh and pH measurements. For the one point above the lines, 96% of added Cr(VI) was recovered (no replication); for the two between the lines, 91% and 53% were recovered; and for the three points below both lines, no (
