Article pubs.acs.org/est
Influence of pH and Oxidation State on the Interaction of Arsenic with Struvite During Mineral Formation Ning Ma and Ashaki A. Rouff* School of Earth and Environmental Sciences, Queens College City University of New York, 65-30 Kissena Boulevard, Flushing, New York 11367, United States The Graduate Center, City University of New York, 365 Fifth Avenue, New York, New York 10016, United States S Supporting Information *
ABSTRACT: Struvite (MgNH4PO4·6H2O) precipitated from animal and human wastes may be a sustainable source of fertilizer. However, arsenic, present in some wastes, may be removed with struvite. Here the sorption of As with struvite during mineral formation at pH 8−11 was assessed. The yield of struvite increased with pH, and was highest at pH 10. For recovered struvite, XRD indicated reduced crystallinity and particle size, and FT-IR suggested less distortion of phosphate tetrahedra with increased pH. The As impurity did not affect the crystallinity or particle size, but did contribute to phosphate distortion. Sorption of As(V) was observed at all pH values, and was highest at pH 10. As(III) sorption was consistently lower than that of As(V), but increased with pH. XAFS suggested coprecipitation of As(V), and adsorption of As(III) as the potential sorption mechanisms. Solids derived from As(III) solutions exhibited dual mechanisms due to the partial oxidation of As(III) to As(V) in solution prior to sorption. For struvite recovery in the presence of As, optimizing the pH to improve yields may increase the As content. Adsorbed As(III) could be removed prior to fertilizer application, however coprecipitated As(V) will release upon mineral decomposition, linking its cycling to that of phosphorus.
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INTRODUCTION
Removal and recovery of P from wastewater by chemical precipitation and microbial activity is receiving attention as a solution to meet the rising P demand.5,6 Previous research has indicated that P can be recovered from sewage sludge,7,8 swine wastewater,9,10 confined animal feeding operations,11 and urine12−15 through induced precipitation as the mineral struvite (MgNH4PO4·6H2O). This involves the crystallization of struvite from a solution oversaturated with respect to its constituent ions, which in turn may be achieved by adjusting
Phosphorus (P) is a vital element to all forms of life. The largest P source on earth is sequestered in minerals, predominantly apatite (Ca5(PO4)3(OH)) in phosphate-bearing rocks 1 considered a nonrenewable resource. As a limiting nutrient in ecosystems, P is widely used in agriculture, primarily in the form of fertilizers.2 However, in the past 150 years the demand for P has increased dramatically, and will continue to do so as the global population rises exponentially.1 As the production of P fertilizers accounts for 80−90% of phosphate mined from rocks,3 a greater dependence on nonrenewable geologic phosphate reserves is anticipated.4 This means that alternative sources of P require exploration. © 2012 American Chemical Society
Received: Revised: Accepted: Published: 8791
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parameters such as the pH, and ion content of the waste.16 Approximately 60−98% of P can be recovered from targeted wastes as struvite,16 which can provide 10−50% of the P required for fertilizer use. Due to its high P content and slow P release rate, struvite is in turn a suitable substitute for fertilizers derived from phosphate rocks.17 One concern, that has not been adequately addressed, is that wastes targeted for P recovery may contain elevated concentrations of metals and other contaminants which may associate with struvite during precipitation. One such contaminant is the toxic metalloid arsenic (As), with reported concentrations in urine, manure and sewage sludge ranging from ∼100 ppb up to ∼15 ppm.18−21 Even at low initial concentrations, As was found to be removed with struvite precipitated from urine 18 and a sewage sludge effluent.8 The sorptive behavior of As with struvite, however has not been previously considered. Various sorption mechanisms may be of relevance, including the coprecipitation of As into the struvite structure during formation, or adsorption to the surface of the precipitated mineral. Of note is that As(V) can readily substitute for P in mineral structures due to the similarity in configuration, chemical properties, and ionic radii of the arsenate, AsO43− (2.48 Å) and phosphate, PO43− (2.38 Å) oxyanions.22 This is substantiated by the formation of the struvite analog arsenstruvite (MgNH4AsO4·6H2O),23 which has previously been suggested as a means of removing As from contaminated wastewater.24 This implies that during struvite recovery AsO43− may coprecipitate with the mineral by substitution at PO43− sites. However, this is complicated by the redox reactivity of As, whereby the presence of As(III) as arsenite AsO33−, may affect the extent25,26 and mechanism of sorption. Despite the known association of As with struvite, and the likely influence of its oxidation state on this process, there are no previous systematic studies elucidating the sorption mechanisms of As with struvite. This is a novel study to examine the sorption of As(III) and As(V) with actively precipitating struvite. The goal of this research is to delineate the aqueous conditions under which As sorption with struvite occurs, and to determine the dominant mechanisms dictating this sorption process. This is critical as contingent upon the final As concentration, oxidation state, and binding mechanism, struvite repurposed as fertilizer may pose an environmental risk to soils and organisms. Furthermore, identification of the sorption mechanism can be used to predict the conditions of As release prior to, and once struvite is applied to soils. Ultimately, this research is a first-step in determining the potential environmental impact of struvite fertilizers. This will advance the use of struvite for this purpose, ultimately recouping P otherwise lost in wastes. In turn this will promote the sustainable use of P and the conservation of geologic P resources.
homogeneous struvite precipitate.27 The required solution chemistry was calculated using the software PHREEQC 28 with the minteq.v4 database, and the struvite solubility product from the literature.29 The ionic strength of all solutions was adjusted to 0.1 M with NaCl. For the precipitation experiments, the (NH4)2HPO4 solution was spiked with 50 μM (3.7 ppm) of either As(III) or As(V). This concentration was chosen as it is an intermediate value in the range of As concentrations observed in various wastes,18−21 and to provide sufficient solid loadings for the subsequent characterization of sorbed As. The As bearing (NH4)2HPO4 solution was then mixed with an equal volume of the MgCl2·6H2O solution in the presence of 0.1 g/L of commercial struvite (Alfa Aesar), added as seed. The seed material is used to overcome the initial energy barrier associated with particle nucleation, and furthermore is known to optimize the crystallization of struvite during the precipitation process.30 Titration to adjust the sample pH was initiated within 4 Å from the core,35 and thus will not be detected in the EXAFS. The slightly longer As−O distance at low pH may be due to a higher distribution of As(V) incorporated near the mineral surface, which may exhibit slightly elongated bond lengths due to surface relaxation phenomena, compared to As(V) in the bulk structure. In addition, because of the high concentration of HAsO42‑(aq) at this pH, with an average As−
Table 1. Structural Parameters Derived from EXAFS Analysis of As(III) (NaAsO2) and As(V) (Na2HAsO4·7H2O) Standards, pH 10−11 As(III) and pH 8−11 As(V) Solids a
solid
radial distance (As−O), Å
As(III) standard As(V) standard As(III) pH 10 As(III) pH 11 As(V) pH 8 As(V) pH 9 As(V) pH 10 As(V) pH 11
1.82 1.68 1.74 1.74 1.72 1.69 1.69 1.69
b
coordination number, N 3.0 4.1 1.7 3.0 3.1 4.2 4.8 4.8
c
Debye−Waller factor (δ2), Å2 0.002 0.002 0.01 0.004 0.002 0.001 0.001 0.002
Estimated error ± 0.01 Å. bEstimated error ± 20%. cEstimated error ± 0.001 Å2 a
O bond length of 1.73 Å,49 the incorporation of a fraction of As(V) as this species cannot be discounted. For the As(III) solids the fit results indicated longer As−O bonds, compared to As(V) samples, at 1.74 Å. These bonds are shorter relative to those observed for adsorbed As(III) on substrates such as goethite (FeOOH), 1.78 Å and lepidocrocite (γ-FeOOH), 1.78−1.79 Å.46,50 On the basis of the results from XANES, these samples contain a mixture of As(III) and As(V). Assuming that the behavior of As(V) in these samples is consistent with that in the As(V) solids at the relevant pH, and also that the coprecipitation of the As(III) species is less 8795
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assess the sorption of other contaminants with struvite, for example, Cu, Zn, and Fe also found in wastes, and in particular, those that may behave similarly to P such as Cr.54 This will ensure the environmental integrity of struvite-based fertilizers, which can in turn diminish the dependence on geologic P resources, and introduce sustainability to the human P cycle.
favorable, the resultant bond lengths may be indicative of a combination of adsorbed As(III) and coprecipitated As(V). With nearly equal fractions of As(III) and As(V), the As−O bond length at 1.74 Å is the average of the 1.78 Å observed for adsorbed As(III) and 1.69 Å for coprecipitated As(V). The larger Debye−Waller factors obtained for the As−O shells in these samples, compared to those of the As(V) samples, also indicate a greater distribution of As−O distances in these solids, and thus a mixture of species. In addition, in the raw data, the dampening in the χ functions relative to the other samples and standards is due to destructive interference between these two O shells, present in different ratios in the samples. Thus sorption in As(III) solutions may be facilitated by the partial oxidation of As(III) to As(V) in solution, with subsequent coprecipitation of As(V), and adsorption of As(III) to the struvite surface. In turn, the presence of coprecipitated As(V) in these samples may explain the splitting in the ν3 PO43− band, and thus reduced symmetry as observed in the FT-IR spectra. Struvite Recovery in the Presence of Arsenic. The yield of struvite is increased with pH, improving the efficiency of the P recovery process.31 However, with increasing pH a notable fraction of As(V) will coprecipitate with struvite, and the adsorption of As(III) will be enhanced. This poses a dilemma as the conditions favorable for most efficient struvite recovery are also conducive to increased association of As with the solid. This can be problematic if recovered struvite is to be subsequently used as a fertilizer product. The maximum acceptable concentration of As in fertilizers is 75 ppm.51 Starting with an intermediate initial As concentration, feasible in some wastes, the As concentration of struvite precipitated from As(V) solutions exceeds this value over the entire pH range studied. Thus, for struvite recovery in the presence of As, it is first essential to determine the dominant As speciation to assess the affinity of As for the substrate. Subsequently, a suitable pH needs to be selected to either maximize the yield, or minimize the amount of As in the recovered product. For wastes with lower initial As concentrations than studied here, the final As content of struvite would be reduced, but the mechanism of sorption likely remains unchanged (Supporting Information Table S4). In turn, the sorption mechanism becomes critical if As contaminated struvite is applied to soils as fertilizer. Surface-bound species are known to be the most susceptible to desorption.52,53 Thus, adsorbed As is likely to be readily mobilized and immediately toxic to plants and organisms. Alternatively, it may be possible to pretreat struvite to remove adsorbed species prior to application. Coprecipitated As however, will be remobilized primarily upon the breakdown of struvite, and hence released simultaneously with the desired nutrients P and N. The sorbed As may also directly affect the stability of struvite, influencing the decomposition process, and thus the release of nutrients, as previously observed for chromium (Cr) contaminated struvite.54 The cycling of slower release, coprecipitated As will therefore be linked with, and can directly affect that of P. In addition, as coprecipitated As is likely As(V), it may compete with P not only in subsequent geochemical, but also in biological processes, introducing toxicity to plants and higher organisms. These implications emphasize the importance of determining the mechanisms of As sorption with struvite. Though wastes are more compositionally complex, and the presence of additional ions may impact the observed processes, these results are the first step in elucidating As sorption processes with struvite. There is a need for similar studies to
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ASSOCIATED CONTENT
S Supporting Information *
Tables S1−S4 and Figures S1−S5 as referenced in the text. This information is available free of charge via the Internet at http://pubs.acs.org/ .
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AUTHOR INFORMATION
Corresponding Author
*Address: School of Earth and Environmental Sciences, Queens College CUNY, 65-30 Kissena Blvd., Flushing, NY 11367 U.S.A. Email: Ashaki.Rouff@qc.cuny.edu. Phone: 718997-3073. Fax: 718-997-3299. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
Support was provided to A.A. Rouff by the Woodrow Wilson National Fellowship Foundation. Use of the NSLS, BNL, was supported by the NSLS Faculty Student Research Support Program and the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Thanks to K. Pandya of X11A for beamline support, and to E.J. Elzinga and R.J. Reeder for discussions that improved this manuscript.
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