Separation and flame atomic absorption spectrometric determination of total chromium and chromium (III) in phosphate rock used for production of fertilizer

Due to the commercial value of phosphate rock (PR) as a fertilizer precursor, it is necessary to investigate its heavy metals content. Chromium (Cr) may present as Cr(III) or Cr(VI) in PR; but quantitative differentiation between them is not an easy task. This is due to possible interconversion of Cr species during the digestion/leaching process. In this work, ultrasound digestion (USD) of PR was optimized (300 mg PR, 4.0 mL of 4.0 mol L−1 nitric acid, 15 min sonication) for the sake of leaching Cr species prior to their determination by flame atomic absorption spectroscopy. Using multi-walled carbon nanotube (MWCNT) as adsorbent, solid phase extraction (SPE) was used to separate Cr(III) from the digestate at pH 9, while total Cr was estimated after reducing Cr(VI) into Cr(III). The optimum USD/SPE method gave LOQ and LOD of Cr(III) of 0.96 mg kg−1 and 0.288 mg kg−1, respectively. The method sensitivity was 1.44×10−3 AU kg mg−1 within the studied Cr concentration range (5-400 mg kg−1). The USD/SPE method was validated by analyzing lake sediments LKSD-4 certified reference material, and by comparison with classical digestion method (CD). Application of USD/SPE on Jordanian PR samples gave total Cr rang 29.1-122.0 mg kg−1 (±1.4-6.3), while Cr(III) ranged between 23.8 and 101.7 mg kg−1 (±1.3-5.5). AFPC Rock Check Program samples gave total Cr range 238.9-394.7 mg kg−1 (±11.5-24.1), while Cr(III) ranged between 202.4 and 335.8 mg kg−1 (±11.4-18.3). These results were very close to the results obtained by the CD method.