Earth Science Frontiers ›› 2020, Vol. 27 ›› Issue (5): 227-237.DOI: 10.13745/j.esf.sf.2020.5.40

Special Issue: Research Articles (English)

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Uptake mechanisms of arsenate in gypsum: Structural incorporation versus surface adsorption and implications for remediation of arsenic contamination

Jinru Lin1,2, Ning Chen1,3, Yuanming Pan1,*()   

  1. 1. Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
    2. Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, Liaoning 110016, P. R. China
    3. Canadian Light Source, University of Saskatchewan, Saskatoon, SK S7N 0X4, Canada
  • Received:2020-03-21 Revised:2020-05-19 Online:2020-09-25 Published:2020-09-25
  • Contact: Yuanming Pan


Gypsum (CaSO4·2H2O) as the most common and abundant secondary mineral in diverse types of mine tailings potentially plays important roles in the stability and bioavailability of heavy metal(loid)s, including As. Understanding on the behavior of As in gypsum-rich mine tailings and development of effective strategies for remediating related As contamination all require knowledge about the speciation and uptake mechanisms of this metalloid in the dominant mineral. However, quantitative determination of arsenic speciation and uptake mechanisms in gypsum-rich mine tailings at trace levels is analytically challenging. In this contribution, we use combined inductively coupled plasma mass spectrometry (ICP-MS), X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) spectroscopy to quantitatively determine structural incorporation and surface adsorption of arsenate in synthetic gypsum. Gypsum coprecipitated from aqueous solutions containing 0.02 M sodium hydrogen arsenate heptahydrate (Na2HAsO4·7H2O) at ambient conditions has As contents increasing from 57 ppm at pH=2 to 67470 and 53980 ppm at pH=12 and 14, respectively. Synchrotron As K-edge XANES and powder EPR spectra confirm that arsenate is the dominant species in coprecipitated gypsum. Specifically, As K-edge XANES spectra exhibit systematic variations in post-edge features as a function of pH, consistent with structure-bound arsenate at pH from 2 to 7.5 but different uptake mechanisms at pH from 9 to 14. Similarly, arsenic contents estimated from the structure-bound [AsO3]2- radical in powder EPR spectra are in agreement with those determined from ICP-MS analyses for gypsum synthesized at pH from 2 to 7.5, but show large discrepancies for the pH=9, 12 and 14 samples, indicative of a large fraction of non-structure-bound As at high pH. The marked pH dependence of arsenate uptake mechanisms in gypsum has important implications for both understanding the roles of this material in mine tailings and developing its optimal applications for remediation of arsenic contamination in aqueous environments. In addition, the radiation-induced [AsO3]2- radical in gypsum with a diagnostic peak at g=~2.33 is potentially useful for EPR/ESR dating and retrospective dosimetry.

Key words: Gypsum, Mine tailings, Arsenic contamination, Structural incorporation, Surface adsorption, pH control

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