Article pubs.acs.org/est
Zinc Interaction with Struvite During and After Mineral Formation Ashaki A. Rouff*,†,‡ and Karen M. Juarez† †
School of Earth and Environmental Sciences, Queens College, City University of New York, Queens, New York 11367, United States ‡ Department of Earth and Environmental Sciences, Rutgers University, Newark, New Jersey 07102, United States S Supporting Information *
ABSTRACT: Sorption of Zn with struvite was assessed both during and after mineral formation at pH 9.0 for 1−100 μM (0.065−6.54 mg L−1) aqueous Zn. The Zn loadings of recovered solids were lower when Zn was present during struvite precipitation compared to when Zn was added to struvite-bearing solutions. X-ray absorption fine structure spectroscopy confirmed that Zn added to struvite-bearing solutions at concentrations ≤5 μM sorbed as both octahedral and tetrahedral complexes (Zn−O 1.98−2.03 Å), with evidence for bidentate configuration (Zn−P 3.18 Å). Bidentate complexes were incorporated into the near-surface structure, contributing to distortion of the struvite ν3 PO43− band in the Fourier transform infrared spectra. At Zn concentrations >5 μM, tetrahedral monodentate adsorbates (Zn−O 1.98 Å) dominated, transitioning to a Znphosphate precipitate at 100 μM. When Zn is present during struvite precipitation, octahedral monodentate sorbates detected at 1 μM (Zn−O 2.08−2.10 Å; Zn−P 3.60−3.64 Å) polymerized at 5−50 μM, ultimately forming a Zn-hydroxide precipitate at 100 μM. The lowest initial Zn concentrations (0.065 mg L−1) and resultant solid loadings from precipitation experiments (13 mg kg−1) are consistent with those reported for struvite recovered from wastewater, suggesting that similar Zn sorption processes may occur in more complex systems.
■
INTRODUCTION Phosphorus serves as a macronutrient in fertilizers, which humans are dependent on as they are essential for crop production.1,2 However, natural phosphorus resources used for fertilizer production are finite and are rapidly disappearing.3 The depletion of these reserves is cause for concern, but is a problem that can be addressed.4 To ensure the continued availability of phosphorus, alternative sources must be considered. One potential source of phosphorus is the mineral struvite (MgNH4PO4·6H2O),5 which can be extracted from sources such as plant, animal, and human wastes.4,6−8 Struvite yields high percentages of phosphorus and allows a more controlled rate of phosphorus release when compared to traditional fertilizers.9 Precipitating and recovering struvite from wastes as an alternative to the extraction of phosphorus from natural resources can be economically advantageous and is sustainable.1,10 Zinc is a common heavy metal that serves as an essential micronutrient. When available, it is readily absorbed by plants and crops, and plays a critical role in all living organisms. It is present in various enzyme systems, is involved in protein metabolism, and promotes RNA synthesis for protein production while also participating in DNA protection.11−13 However, excess amounts of Zn can be toxic, and can negatively impact the uptake of other elements. For example, Zn stress can reduce rooting capacity, which is necessary for inducing plant growth.14 Low concentrations of Zn can cause deficiency and can affect the maturity of the organism. It should be noted that these effects are not the same in all organisms and can vary © 2014 American Chemical Society
considerably between different types of plants. Fertilizers can be a source of Zn to plants and crops because uptake occurs primarily via the soil,15 however unregulated amounts of Zn in fertilizers can result in either nutrient deficiencies or toxic effects in plants. Various types of wastes contain Zn as a common component. The concentration of Zn in urine can range from 0.070 to 0.537 mg L−1.16 Swine manure was found to have total Zn concentrations from 12 to 40.6 mg L−1, in exceedance of U.S. EPA limits for agricultural use of 2 mg L−1 for >20 years and 10 mg L−1 for
