Arsenic Incorporation in Synthetic Struvite (NH_4MgPO_4·6H_2O): A Synchrotron XAS and Single-Crystal EPR Study

摘要

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~(s+) is the predominant oxidation state in struvite, irrespective of Na_2HAsO_4· 7H_2O or NaAsO_2 as the source for As. Modeling of As K-edge EXAFS data suggest that local structural distortion associated with the substitution of As~(s+) for P~(5+) in struvite reaches up to 3.75 A. Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated struvite disclose five [AsO_3]~(2-) radicals and one [AsO_4]~(2-) radical. These arsenic-centered oxyradicals are all readily attributed to form from diamagnetic [AsO_4]~(3-) precursors during irradiation, providing further support for exclusive incorporation and local structural expansion beyond the first shell of As~(s+) at the P site in struvite.