Multiple Scattering Debye-Waller Factors for Arsenate

Arsenic incorporation in synthetic struvite (NH4MgPO4·6H2O): a synchrotron XAS and single-crystal EPR study

Struvite, a common biomineral and increasingly important fertilizer recovered from wastewater treatment plants, is capable of sequestering a wide range of heavy metals and metalloids, including arsenic. Inductively coupled plasma mass spectrometric (ICPMS) analyses and microbeam synchrotron X-ray fluororescence (μ-SXRF) mapping show that struvite formed under ambient conditions contains up to 547 ± 15 ppm As and that the uptake of As is controlled by pH. Synchrotron As K-edge XANES spectra measured at 20 K show that As(5+) is the predominant oxidation state in struvite, irrespective of Na2HAsO4·7H2O or NaAsO2 as the source for As. Modeling of As K-edge EXAFS data suggest that local structural distortion associated with the substitution of As(5+) for P(5+) in struvite reaches up to 3.75 Å. Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated struvite disclose five [AsO3](2-) radicals and one [AsO4](2-) radical. These arsenic-centered oxyradicals are all readily attributed to form from diamagnetic [AsO4](3-) precursors during irradiation, providing further support for exclusive incorporation and local structural expansion beyond the first shell of As(5+) at the P site in struvite.