Article pubs.acs.org/est
Arsenic Speciation in Newberyite (MgHPO4·3H2O) Determined by Synchrotron X‑ray Absorption and Electron Paramagnetic Resonance Spectroscopies: Implications for the Fate of Arsenic in Green Fertilizers Jinru Lin,† Ning Chen,†,‡ and Yuanming Pan*,† †
Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada Canadian Light Source, University of Saskatchewan, Saskatoon, SK S7N 0X4, Canada
‡
S Supporting Information *
ABSTRACT: Newberyite (MgHPO4·3H2O), a biomineral and common constituent in guano deposits, is an important decomposition product of struvite that is an increasingly popular green fertilizer recovered from wastewaters. Two samples of newberyite containing 1099 and 25 ppm As have been obtained at pH = 6.4, by using Na2HAsO4·7H2O and NaAsO2 as the dopant, respectively (i.e., Synthesis 1 and Synthesis 2). Synchrotron arsenic K-edge X-ray absorption spectroscopic data of newberyite from Synthesis 1 show that As5+ is dominant and has a local environment typical of the arsenate species. Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated newberyite from Synthesis 1 contain two arsenic-associated oxyradicals: [AsO3]2− and [AsO2]2− derived from As5+ and As3+, respectively, at the P site. Quantitative analyses of powder EPR spectra allow determinations of the As5+ and As3+ contents in newberyite from Synthesis 1 and Synthesis 2. Elevated concentrations of arsenic also occur in natural newberyite transformed from struvite in guano deposits and record the accumulation of this metalloid in the food chain. Therefore, newberyite, which sequesters As during crystallization and retains this metalloid during the transformation from struvite, can attenuate arsenic contamination from green fertilizers in moderately acidic soils. Also, the capacity for accommodating both As5+ and As3+ in the crystal lattice coupled with simple chemistry and easy crystallization at ambient conditions makes newberyite an attractive material for remediation of arsenic contamination in aqueous environments.
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INTRODUCTION Phosphorus (P) is an element of fundamental importance to humans for the creation of energy in cells, control of metabolic reactions and, as an essential nutrient for crops, promotion of food production.1,2 Phosphate deposits (i.e., marine phosphorites) are the principal source of P and are mainly used as fertilizers in agriculture.3 In this context, phosphorus is a nonrenewable resource. Global phosphate reserves are limited and, at the present rate of consumptions, the existing phosphate reserves could be exhausted in less than 100 years.1,3−5 Other phosphate-rich rocks are available but cannot be extracted economically at present due to their low qualities and hence higher cost to process.3 Therefore, phosphorus recovery from wastes as a renewable source is an important alternative in reducing our reliance on the limited natural resources and relieving the pressure of increasing demands of phosphorus consumptions. One commercial technology for recovering P from wastewaters is precipitation of magnesium phosphates such as struvite (Mg(NH4)PO4·6H2O), newberyite (MgHPO4·3H2O), bobierrite (Mg3(PO4)2·8H2O), and cattiite (Mg3(PO4)2· 22H2O).3,6−8 Struvite or “Crystal Green” (a commercial slow © 2014 American Chemical Society
release fertilizer) involving recovery of both phosphorus and nitrogen is particularly promising.3,7,9−15 However, struvite formed from this process commonly contains elevated levels of various heavy metals and metalloids, which are common in wastewaters.11,15−21 Our recent study confirmed that struvite precipitated at ambient air conditions (i.e., room pressure and temperature) and pH values from 6 to 9 is capable of accommodating up to 547 ± 15 ppm As.22 Therefore, precipitation of struvite from wastewaters is, on the one hand, a useful method for remediating P and As contamination.15,23,24 On the other hand, direct use of recovered struvite as a fertilizer, without removal of As and other heavy metals and metalloids, possesses a potential source of contamination.21 Newberyite of the space group Pbca (Figure 1) often coprecipitates with struvite in urine as well as calculi25−31 and has long been known to transform from the latter under both Received: Revised: Accepted: Published: 6938
December 23, 2013 April 12, 2014 May 28, 2014 May 28, 2014 dx.doi.org/10.1021/es405735p | Environ. Sci. Technol. 2014, 48, 6938−6946
Environmental Science & Technology
Article
0.02 M NaAsO2 standard solution was added as the source for arsenic. Crystals from Synthesis 1 and Synthesis 2 were obtained after 2 and 3 months, respectively, and, after washing with deionized water, were investigated by powder X-ray diffraction (PXRD) analysis, inductively coupled plasma mass spectrometry (ICPMS), synchrotron X-ray absorption spectroscopy (XAS), and electron paramagnetic resonance (EPR) spectroscopy. Samples of Natural Newberyite. Four samples of natural newberyite (2 each from Paoha Island, Mono Lake, California, USA, and Skipton Caves, western Victoria, Australia) have been investigated by PXRD, ICPMS, and powder EPR analyses. Because the prohibitive environment eliminates aquatic predators and competition, Mono Lake offers unusually productive food in the form of brine shrimps and alkali flies, which feeds millions of migratory and nesting birds. The guano deposits on the Paoha Island in Mono Lake are thought to accumulate from bird droppings. Two samples of newberyite from Paoha Island, Mono Lake, have similar morphologies to those described in the literature, which are indicative of transformation from struvite.32 Similarly, the two samples of newberyite from bat caves in Skipton, Australia, are similar in morphology to those described in the literature and are pseudomorphs after struvite from bats’ droppings.41−43 Arsenic K Edge XAS Measurements. Arsenic K edge (11 867 eV) spectra were collected at the HXMA beamline of Canadian Light Source44 with a storage ring operated at 2.9 GeV. The monochromatic beam was produced using a pair of Si(111) monochromator crystals with the second crystal detuned by 50% of fully tuned beam intensity for the reduction of harmonic; and Rh mirrors were introduced in this experiment. The sample of newberyite from Synthesis 1 was mounted in an aluminum holder covered with Kapton at 45° to the incident X-ray beam and measured in the fluorescence mode using a 32-element solid state Ge detector positioned at 90° to the X-ray beam. The sample was held at 8 K using an Oxford Instruments liquid helium flow cryostat to reduce the beam damage, and straight ion chamber detectors filled with standard >99.99% nitrogen gas. The photon energy was calibrated by an Au foil with the first drivative of the absorption edge assigned a value of 11 919 eV at the Au L3 edge. Spectra of model compounds were obtained in a previous study using transmission methods.45 Three spectra were collected with the sizes of scan steps for the pre-edge, XANES, and EXAFS regions of 10 eV/step, 0.25 eV/step, and 0.05 Å−1/step, respectively. X-Band EPR Measurements. One selected single crystal of arsenic doped newberyite from Synthesis 1 was irradiated at room temperature in a 60Co cell, with a dose rate of ∼460 Gy/ h, for 2 days, for detailed single-crystal EPR measurements in three rotation planes at an interval of 5° each step. These single-crystal EPR measurements were made at room temperature (295 K) on a Bruker EMX instrument at the Saskatchewan Structural Sciences Centre. A microwave power of ∼5 mW, modulation frequency of 100 kHz, and modulation amplitude of 0.1 mT were used for data collection. Among the three rotation planes, two are nearly parallel to the (010) crystal face and one is approximately parallel to (011), measured with microwave frequencies of ∼9.616 and 9.856 GHz, respectively. The spectral resolution of single-crystal EPR spectra was 0.098 mT (i.e., 2048 data points over the scan range from 207 to 407 mT).
Figure 1. Crystal structure of newberyite projected onto (010),31 showing the local environment of phosphorus.
subaerial and aqueous environments.25−34 For example, at room temperature, struvite completely decomposes to newberyrite after approximately half a year.33 Also, newberyite is an important decomposition product of struvite fertilizers when they are applied to moderately acidic soils.35−37 Therefore, the question of arsenic uptake and speciation in newberyite is of significant importance to understanding the fate of this toxic metalloid in soils. Accordingly, we have investigated arsenic-doped newberyite by the use of synchrotron X-ray absorption spectroscopy (XAS) and single-crystal electron paramagnetic resonance (EPR) spectroscopy. Results are compared with those from struvite and other phosphates to provide insights into the uptake and speciation of arsenic in newberyite. Also investigated in this study are As and transition metals in natural newberyite from guano deposits in Skipton Caves, Australia, and at Paoha Island, Mono Lake, California, USA. These are two classic localities where the transformation of struvite to newberyite was first documented.32−34 Moreover, newberyite (and its struvite precursor) from Mono Lake, which is a hypersaline and alkaline water body containing exceptionally high concentrations of As (averaging ∼200 μM),38−40 is expected to contain the upper limit of this metalloid in guano deposits. These data from natural newberyite are interpreted in light of new results from its synthetic counterpart, with implications for the fate of As in green fertilizers and potential use for the remediation of arsenic contamination in aqueous environments.
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MATERIALS AND ANALYTICAL METHODS Synthesis of As-Doped Newberyite. Two synthesis experiments have been made to grow crystals of arsenicdoped newberyite by using the single gel diffusion technique.22 Briefly, Synthesis 1 used a mixture of 20 mL of sodium metasilicate (Na2SiO3) solution at a density of 1.05 g/cm3 with 0.25 M potassium dihydrogen phosphate (KH2PO4) and 0.0025 M sodium hydrogen arsenate heptahydrate (Na2HAsO4·7H2O), which was first adjusted to pH = 6.4 by using acetic acid and then poured into a test tube to form a gel. After the gel settled, 10 mL of 0.5 M Mg(CH3COOH)2 solution was added to the tube. Synthesis 2 utilized the same procedure and starting materials described above, except that 6939
dx.doi.org/10.1021/es405735p | Environ. Sci. Technol. 2014, 48, 6938−6946
Environmental Science & Technology
Article
Table 1. ICPMS Results (ppm)a of Synthetic and Natural Newberyite Samples
a
sample
Mg
P
K
Na
As
Fe
Mn
Cu
Zn
Ni
Cr
Synthesis 1 Synthesis 2 #1 Skipton Caves #2 Skipton Caves #3 Paoha Island #4 Paoha Island
141 580 134 770 133 390 133 200 130 820 132 970
159 010 161 060 179 250 178 140 175 100 172 950
922 962 225 242 520 460
600 744 146 135 225 404
1099 25 3.0 2.7 2.0 6.7
nd 52 833 878 63 106
53 15 5746 5755 150 210
nd 3.8 27 27 1.3 0.1
8.4 5.3 478 480 2.7 3.0
nd 1.0 4.0 4.1 2.4 2.0
13 0.5 4.1 3.9 1.1 1.6
Notation: nd = not detectable.
Powder EPR spectra of natural and synthetic newberyite were measured at room temperature after gamma-ray irradiations for 7 days and 2 days, respectively, at the dose rate of ∼460 Gy/h. Conditions for powder EPR measurements included microwave frequencies of 9.387 and 9.856 GHz, microwave power of 2 mW, modulation frequency of 100 kHz, modulation amplitude of 0.1 mT, and spectral resolutions of ∼0.146 mT and ∼0.209 mT (i.e., 2048 and 4096 data points over the scan ranges from 200 to 500 mT and from 45 to 900 mT, respectively).
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RESULTS AND DISCUSSION Characterization of Synthetic and Natural Newberyite. Newberyite crystals from Synthesis 1 are characterized by well developed prismatic and dipyramidal morphologies and have the maximum size of 2 mm × 1 mm × 1 mm (Supporting Information, Figure S1). Synthesis 2 yielded only aggregates of newberyite, in which individual crystals are invariably
