Determination of High-Level Hexavalent Chromium in Chromium Slag Field Soils by Inductively Coupled Plasma-Optical Emission Spectrometry with Alkaline Extraction

BRIEF REPORTSignificance: Hexavalent chromium in chromium slag field soils, featuring high content and complex matrices, causes interference that challenges existing detection methods. This study modified the extractant in HJ 1082-2019 by adding EDTA and Triton X-100, whose synergistic chemical chelation and physical dispersion enhanced extraction efficiency and inhibited Cr(Ⅲ) oxidation. After optimization, the number of extractions was reduced from 6 to 2, with a recovery rate reaching 96.8%. Combined with 001 × 7 strong acid cation exchange resin to eliminate high-salt interference, the proposed method exhibited a wide detection range and high precision. It has been successfully applied to the determination of hexavalent chromium in soils from a chromium slag field in North China, yielding favorable results.Introduction: Hexavalent chromium Cr(Ⅵ) pollution in chromium slag field soils has become a major ecological issue due to its high toxicity and environmental migration risk[1-4]. Such soil presents two major detection challenges: first, the extremely high Cr(Ⅵ) content can reach thousands of mg/kg, far exceeding the background values of ordinary soil and the linear range of conventional methods[5-6]; second, the complex matrix contains Cr(Ⅵ) in forms such as chromates, which form adsorbed states or complexes with Fe, Mn, and other heavy metals like Ni and Pb[7-9].　　Among existing extraction methods for soil Cr(Ⅵ), heated sodium carbonate-sodium hydroxide extractants perform best, but suffer from the defect of partial oxidation of Cr(Ⅲ)[14]. Although HJ 1082-2019 inhibits Cr(Ⅲ) oxidation through Mg2+[15], the risk of Cr(Ⅲ) oxidation still exists. Traditional detection methods such as spectrophotometry are easily hindered by color[21-22], flame atomic absorption spectrometry suffers from matrix interference[23], and ICP-MS faces polyatomic ion interference[26]. In contrast, ICP-OES is more suitable for determining high concentrations of Cr(Ⅵ) due to its wide linear range and high tolerance to matrix interference.　　To address the above challenges, this study introduces EDTA and Triton X-100 into the alkaline extractant to enhance the extraction efficiency through synergistic chemical chelation and physical dispersion, and employs strong acid cation exchange resins to eliminate high-salt interference in the determination. The aim is to establish an efficient and accurate detection method for chromium slag field soils.Methods: In a chromium slag field in North China, six soil samples were collected from the plant area (including surface and deep soils from 0 to 35 cm), four surface soil samples from the surrounding area, and one control surface soil sample 2 km away. Approximately 1 kg of soil was taken from each sampling point, air-dried naturally, cleaned of debris, ground through a 100 mesh sieve, and stored. To address the lack of high-content reference materials, the method in the literature[29] was modified to prepare blank spiked soil samples with Cr(Ⅵ) contents of 800, 1000, and 3000 mg/kg. After shaking, mixing, and drying, three parallel samples were prepared for homogeneity testing. The relative standard deviation (RSD) was <3.03%, the relative deviation between the actual and theoretical contents was <3.57%, and the proportion of Cr(Ⅵ) was consistent with the theoretical value (Table 1), indicating that the prepared Cr(Ⅵ)-spiked soil samples were accurate and reliable.　　0.50 g of air-dried soil was sieved to 100 mesh, placed into a conical flask, and the following sequentially added: 12 mL of alkaline extraction solution (prepared by dissolving 20.0 g of sodium hydroxide and 30.0 g of sodium carbonate in ultrapure water and diluting to 1 L), 40 mg of magnesium chloride, 0.05 mL of buffer solution (prepared by dissolving 87.1 g of dipotassium hydrogen phosphate and 68.0 g of potassium dihydrogen phosphate in water to 1 L), 3 mL of Triton X-100 solution (prepared by diluting 12.92 g of Triton X-100 to 1 L), and 12 mL of EDTA solution (prepared by diluting 7.44 g of EDTA to 1 L). The mixture was shaken in a water bath at 90℃ for 80 min. After centrifugation, the supernatant was treated with 2.00 g of 001 × 7 strong-acid styrene-based cation exchange resin to remove high-salt interference. The residue was re-extracted, and the supernatants from the two extractions were combined and diluted to volume for ICP-OES determination.　　ICP-OES was used to determine the high-level Cr(Ⅵ) in chromium slag field soils. A series of standard solutions with concentrations ranging from 0.00 to 8.00 mg/L were prepared to match the sample concentrations. The instrument parameters were set as follows: carrier gas flow rate of 0.7 L/min, cooling gas flow rate of 15.0 L/min, power of 1300 W, and analytical spectral line of Cr 267.716 nm. Two groups of standard reference materials [GBW(E)070255 and RMH-A043] and two groups of self-made samples (800 mg/kg and 3000 mg/kg) were selected to evaluate the precision and accuracy of the method.Data and Results: Addressing the challenges of analyzing high-level Cr(Ⅵ) in chromium slag field soils, the key performance data are summarized below.　　(1) By optimizing extraction conditions and controlling matrix interferences, this study establishes and systematically validates a high-efficiency analytical method for the determination of high-content Cr(Ⅵ) in chromium slag field soils. To overcome the six extraction cycles required by HJ 1082-2019 (Fig. 1), an orthogonal array [L27(35)] identified the optimal conditions: 12 mL alkaline extraction solution, temperature 90℃, extraction time 80 min, 3 mL Triton X-100, 12mL EDTA, with solid-liquid ratio 1∶54 (g:mL). Under these conditions, only two extraction cycles were needed to achieve a Cr(Ⅵ) recovery of 96.8 % (Table 3), significantly improving the extraction efficiency. For the high-salt interference of alkaline extraction solution on ICP-OES determination, adding 2.0 g of 001 × 7 resin effectively eliminated high-salt interference. Meanwhile, Cr(Ⅵ) recovery rate was 94.5%, RSD 3.87%, and solution pH 6.5, meeting the requirements for machine detection (Fig. 2).　　Cr(Ⅵ) exhibited linearity from 0 to 8.00 mg/L (R2 > 0.9995), with a detection limit of 0.30 mg/kg, a quantitation limit of 1.01 mg/kg, and a maximum determinable content of 3200 mg/kg. Validation using reference materials GBW(E)070255 (68 mg/kg), RMH-A043 (155 mg/kg), and self-prepared samples of 800 and 3000 mg/kg showed relative errors of −2.87% to −0.69% and RSD < 5% (Table 4). Relative to the standard protocol and literature procedures, the present method offers a wider determination range and higher efficiency (Table 5) and was successfully applied to soils from a chromium slag field in North China.　　(2) Through the synergistic action of chemical and physical mechanisms during extraction, oxidation inhibition, and instrumental determination, the proposed method achieves efficient extraction and accurate determination of Cr(Ⅵ). In the extraction stage, the strong alkalinity (pH > 11.5) of sodium carbonate-sodium hydroxide and high temperature (90–95℃) were used to weaken soil adsorption of Cr(Ⅵ). By introducing EDTA chelating agent and Triton X-100 surfactant, the extraction efficiency of Cr(Ⅵ) was synergistically improved through chelating heavy metal ions to inhibit precipitation adsorption, destroying soil particle aggregates to enhance dispersibility, and reducing surface tension. In the oxidation inhibition stage, the physical effect of alkaline environment promoting Cr(Ⅲ) hydroxide precipitation was combined with the dual mechanism of Mg2+ physical adsorption and EDTA chemical chelation to block the contact path between Cr(Ⅲ) and oxidants, thereby inhibiting the false positive interference of Cr(Ⅲ) oxidation to Cr(Ⅵ). In the detection stage, the Cr 267.716 nm spectral line with low overlap risk was selected to avoid spectral interferences from Pb, Mn, Ni, etc., and 001 × 7 cation exchange resin was used to remove matrix ions such as Na+, K+, and Mg2+, reducing the ionic strength of the solution. Finally, efficient extraction and accurate determination of Cr(Ⅵ) in chromium slag field soils were achieved.