Elsevier

Talanta

Volume 130, 1 December 2014, Pages 226-232
Talanta

NOM and alkalinity interference in trace-level hexavalent chromium analysis

https://doi.org/10.1016/j.talanta.2014.06.052Get rights and content

Highlights

  • Three analytical methods for hexavalent chromium evaluated.

  • Alkalinity up to 170 mg L−1 as CaCO3 did not affect recovery of Cr(VI).

  • NOM up to 8 mg C L−1 did not affect Cr(VI) measurement using the EPA Method 218.7.

  • NOM caused a ‘false positive’ in Cr(VI) using field ion exchange method.

Abstract

Three analytical methods were evaluated for hexavalent and trivalent chromium analyses in the presence of natural organic matter (NOM) and alkalinity. Each method was tested using a simulated tap water with 1 μg L−1 Cr(VI) and 0.8 μg L−1 Cr(III) and several concentrations of NOM and/or alkalinity. An ion chromatograph with post column reaction cell conforming to USEPA Method 218.7 could accurately quantify Cr(VI) in the presence of up to 8 mg C L−1 NOM and up to 170 mg L−1 as CaCO3 alkalinity, and no oxidation of chromium was observed when 0.8 μg L−1 Cr(III) was also present. A high-performance liquid chromatography coupled with inductively coupled plasma (HPLC–ICPMS) method and a field speciation method were also evaluated. Each of these methods was unaffected by the presence of alkalinity; however, the presence of NOM created issues. For the HPLC–ICPMS method, as the concentration of NOM increased the recovery of Cr(VI) decreased, resulting in a ‘false negative’ for Cr(VI). However, for the field speciation method, Cr(III) was complexed by NOM and carried through the ion exchange column, resulting in a ‘false positive’ for Cr(VI).

Introduction

In natural waters chromium typically exists in the trivalent [Cr(III)] or hexavalent [Cr(VI)] oxidation states. The current United States Environmental Protection Agency (USEPA) maximum contaminant level (MCL) for total chromium is 100 μg L−1 (0.1 mg L−1) [1] but a new MCL for hexavalent chromium is under consideration pending a health assessment to be finalized in 2014 [2]. The State of California has established its own total chromium MCL of 50 μg L−1 (0.05 mg L−1), a non-enforceable Public Health Goal (PHG) for Cr(VI) of 0.02 μg L−1 was issued in July 2011 [3], and as of August 22, 2013 the draft Cr(VI) MCL is 10 μg L−1 [4]. There is widespread public interest and concern related to detection of Cr(VI) in potable water above the PHG as evidenced by a recent high-profile non-peer-reviewed survey of U.S. drinking waters [5], prompting research carefully examining the accuracy of trace chromium analysis. There is particular concern about methods which cause false detection of Cr(VI) given that it is a suspected carcinogen in water and has a very low PHG.

The analysis of low level Cr(VI) concentrations is challenging due to the redox sensitivity of chromium. It is known that the presence of oxidizing or reducing agents, either naturally-occurring or added for drinking water treatment, may alter the oxidation state of chromium in the collected sample as a function of time. Unless a method immediately quantifies chromium species in the field, preservation is necessary to stop speciation changes until later sample analysis. pH also plays a major role in the redox chemistry of chromium, with Cr(III) thermodynamically favored at lower pH and Cr(VI) favored at higher pH. It has been a challenge to find appropriate methods of preservation that are applicable to a wide variety of waters, prompting parallel efforts to immediately speciate samples in the field to completely avoid problems with speciation issues caused by sample preservation.

None of the methods for Cr(VI) analysis are currently approved for compliance with the Safe Drinking Water Act (SDWA), since, at present, Cr(VI) is not a regulated chemical under the SDWA. However, the USEPA Method 218.7 is approved for Cr(VI) determination by the Unregulated Contaminant Monitoring Rule 3 [6]. This method is described below, along with an HPLC–ICPMS method and a field speciation method. These latter two methods might be beneficial in that one analytical instrument could be used to determine both total chromium and Cr(VI). Each of these methods has advantages and disadvantages. The field speciation method avoids concerns with speciation changes during storage, is simple to use, and only requires one analytical instrument to measure both total chromium and Cr(VI). The HPLC–ICPMS method likewise only requires one analytical instrument to measure both types of chromium but does require more expertise than the field speciation method. Finally, the USEPA Method 218.7 has the lowest detection capability for Cr(VI) but an additional instrument is required to measure total chromium.

Ion chromatography coupled with a post-column reactor (IC-PCR) has become the industry standard for low level Cr(VI) analysis. Hexavalent chromium will react with 1,5-diphenylcarbazide to form a pink complex whose color intensity is proportional to the Cr(VI) concentration in the original solution (Standard Method 3500-Cr D) [7]. This solution can be analyzed using a spectrophotometer set at 540 nm to determine the Cr(VI) concentration. When this method was coupled with ion chromatography (IC) in USEPA Method 218.6 [8] the detection limit improved substantially. In this method, filtered water samples are preserved at pH 9 using a 2500 mM ammonium sulfate plus 1000 mM ammonium hydroxide buffer. The IC column (Dionex IonPac AS7 or equivalent) is able to separate the Cr(VI), which exists as chromate (CrO42−) at this pH, from other anionic species. The 1,5-diphenylcarbazide is added using a post-column mixing coil and the pink complex formed is measured with a spectrophotometer (as described above). The method detection limit (MDL) for this method is listed as 0.3 μg L−1 for potable water analysis [9], while the minimum reporting level (MRL) is 0.4 μg L−1 [10].

USEPA Method 218.6 has been modified several times over the years to improve the detection limit [11], [12], [13], and in 2011 the USEPA released Method 218.7, “Determination of Hexavalent Chromium in Drinking Water by Ion Chromatography with Post-Column Derivatization and UV–Visible Spectroscopic Detection,” to update Method 218.6 [14]. The method describes two separate IC systems that can be used for analysis. Operating conditions for each of these systems are described, including eluents (either ammonium sulfate/ammonium hydroxide or sodium carbonate/sodium bicarbonate), column and reaction coil sizes, and flow rates. For USEPA Method 218.7 the MDL ranges from 0.0044 to 0.015 μg L−1, depending on the preservative and eluent system used, while the MRL is 0.02 μg L−1 [14].

Many researchers have utilized high-performance liquid chromatography (HPLC) coupled with inductively coupled plasma mass spectrometry (ICPMS) to conduct a speciation analysis for chromium [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Soluble Cr(III) typically exists as Cr3+, CrOH2+, or Cr(OH)2+ in natural waters while Cr(VI) exists as HCrO4 or CrO42−. Since the Cr(III) species are positively charged ions and the Cr(VI) species are negatively charged ions, they can be separated using either anion or cation exchange. After separation, ICPMS can be used to quantify the concentration of one or both of the chromium species.

In order to achieve low detection limits, both the HPLC and the ICPMS portions of the analysis must be optimized. Depending on the type of column used, HPLC–ICPMS may quantify only Cr(VI) or it may be able to quantify both Cr(III) and Cr(VI). In the latter case HPLC must achieve a good separation between the Cr(III) and Cr(VI) peaks with a high signal-to-noise ratio. Column type, eluent composition and pH, and injection volume must be optimized for best results.

Another technique for determining Cr(VI) is based on a field method using a cation exchange column [25]. In theory, passing water through this column will remove all the Cr(III) (a cation) but not the Cr(VI) (an anion). This method has a reported MDL of 0.05 μg L−1 for Cr(VI) when the collected sample is analyzed with graphite furnace atomic absorption spectrometry (GFAAS). However, rigorous testing in the presence of constituents such as natural organic matter (NOM) has not been conducted. Previous work suggests that NOM can form complexes with Cr(III), converting it into an anion that can pass through the column and give a “false positive” of Cr(VI) in samples [26], [27], [28], [29], and the authors include a caveat that waters with organic carbon greater than 5–10 mg L−1 should be tested for this type of “false positive” by adding a known concentration of Cr(III) to a duplicate and checking for higher Cr(VI) concentrations [25]. On the other hand, other work has suggested that NOM found in soil organic matter may reduce Cr(VI) to Cr(III) to give a “false negative” [30], [31], [32], [33], [34] although at least one researcher has reported that NOM does not participate in redox reactions with chromium (tested at a pH of 7.35±0.11) [35].

ICPMS is the most sensitive USEPA approved analytical method for determining low-μg/L levels of total chromium [Cr(III)+Cr(VI)] [36] but there are potential drawbacks. Prior to analysis, total chromium samples are collected in plastic or glass bottles and preserved with HNO3, and have a maximum 6 month holding time [37]. Past work by MWH Laboratories conducted a study in which more than 1500 drinking water samples were analyzed for both Cr(VI) and total chromium using IC and ICPMS, respectively [38]. This study found that nearly half of the 770 samples with total chromium greater than 1 μg L−1 had Cr(VI) concentrations measured by IC to be greater than the total chromium measured by ICPMS. Eaton et al later noted that operating the ICPMS in a ‘collision cell’ or ‘dynamic reaction cell’ mode where the argon plasma was supplemented with ammonia fixed most of this problem [39]. This ‘collision cell technology’ or ‘CCT’ is a technique that attempts to eliminate the formation of polyatomic interferences that can occur when using ICPMS for total chromium measurement [40]. These interferences occur at the most abundant isotopes of chromium, 52Cr and 53Cr, when carbon or chlorine is present (e.g., 40Ar12C, 35Cl16O1H, 37Cl16O). This means waters with high alkalinity, high carbon content, or high chloride content will result in more background noise and higher detection limits.

While total carbon can result in a false positive for total chromium on the ICPMS as described above, organic carbon can contribute to either a false positive or a false negative for Cr(VI) measurements. For example, the IC method uses anion exchange to capture Cr(VI), and then elutes it in a peak for the measurement. In this instance there would be a false negative if any organic carbon present complexes the Cr(VI) and carries it through the anion exchange column. False positives for Cr(VI) can result when using HPLC–ICPMS or the field speciation method described above, since in these methods the Cr(III) is supposed to be captured within the ion exchange media so that all the chromium measured is assumed to be Cr(VI). Any Cr(III) that passes through because of complexation with organic carbon would therefore be falsely categorized as Cr(VI).

This study was conducted to evaluate the measurement of hexavalent chromium using these three analytical techniques in a simulated drinking water containing varying levels of alkalinity and NOM to determine whether there are any analytical issues when inorganic or organic carbon co-occurs with chromium.

Section snippets

Materials and methods

In this study three analytical methods for the determination of low level Cr(VI) were compared. These included USEPA Method 218.7 [14], an HPLC–ICPMS method modified from a protocol previously described by Seby et al. [23], and a field speciation method developed by Ball and McClesky [25]. The analytical procedures associated with each method are described below in further detail. Preliminary experiments were conducted to estimate the MRL for each analytical method used in the study. Simulated

Effect of NOM on analytical method performance

As discussed in 1.1.3 Field speciation method, 1.2 Summary of possible interferences with chromium analysis from carbon, NOM may affect analytical method performance in a variety of ways. For ICPMS, the carbon component of NOM can lead to false positives for chromium even when operating in CCT mode. Also, NOM may oxidize any Cr(III) that is present to Cr(VI), resulting in false positives for the IC-PCR, HPLC–ICPMS, and field speciation methods. In this study three levels of NOM were added to

Conclusions

Low-level chromium measurement can be difficult and maintaining speciation of Cr(VI) while measuring ultra-low concentrations is especially important. Additionally, the presence of alkalinity and/or NOM can lead to false positives or false negatives for Cr(VI) evaluated. USEPA Method 218.7 utilizes IC-PCR and proved to be an extremely reliable method; the addition of up to 8 mg C L−1 NOM and up to 170 mg L−1 as CaCO3 alkalinity had no significant effect on the recovery of Cr(VI). When Cr(III) was

Acknowledgments

The authors wish to thank Kathita Chittaladakorn and Jody Smiley for their assistance with the bench scale experiments and the analytical instrumentation. This project was partly funded by the Water Research Foundation (WaterRF Project 4404), and this work was reviewed by a panel of independent experts selected by the Foundation. Portions of this work were presented at the 2013 Water Quality Technology Conference in Long Beach, CA.

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