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Novel precipitated zirconia-based DGT technique for highresolution imaging of oxyanions in waters and sediments Dongxing Guan, Paul Nicholas Williams, Jun Luo, Jianlun Zheng, Huacheng Xu, Chao Cai, and Lena Q. Ma Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505424m • Publication Date (Web): 06 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015
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Environmental Science & Technology
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Novel precipitated zirconia-based DGT technique for high-resolution
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imaging of oxyanions in waters and sediments
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Dong-Xing Guan1, Paul N. Williams2, Jun Luo1*, Jian-Lun Zheng1, Hua-Cheng Xu3, Chao
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Cai4, and Lena Q. Ma1,5
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Nanjing University, Jiangsu 210023, China
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment,
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Belfast, Belfast BT9 7BL, United Kingdom
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and Limnology, Chinese Academy of Sciences, Nanjing 210008, China
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Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China
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Soil and Water Science Department, University of Florida, Gainesville, FL 32611, USA
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* Corresponding authors, 0086–25–89680632,
[email protected] Institute for Global Food Security, School of Biological Sciences, Queen’s University
State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography
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ABSTRACT
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Water-sediment exchange is a fundamental component of oxyanion cycling in the
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environment. Yet, many of the (im)mobilisation processes overlay complex spatial and
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temporal redox regimes that occur within mm’s of the interface. Only a few methods exist
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that can reliably capture these porewater fluxes, with the most popular being high-resolution
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diffusive gradients in thin films (HR-DGT). However, functionality of HR-DGT is restricted
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by the availability of suitable analyte binding agents within the sampler, which must be
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simple to cast and homogeneously distributed in the binding layer, exhibit adequate sorption
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capacities, be resistive to chemical change and possess a very fine particle size (≤10 µm). A
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novel binding layer was synthesized to meet these requirements by in situ precipitation of
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zirconia into a precast hydrogel. The particle diameter ≤0.2 µm of zirconia in this precipitated
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gel was uniform and at least 50-times smaller than the conventional molding approach.
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Further, this gel had superior binding and stability characteristics compared with the
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commonly used ferrihydrite HR-DGT technique and could be easily fabricated as an ultrathin
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gel (60 µm) for simultaneous oxygen imaging in conjunction with planar-optodes. Chemical
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imaging of anion and oxygen fluxes using the new sampler were evaluated on Lake Taihu
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sediments.
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INTRODUCTION
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Phosphorus (P), selenium (Se) and molybdenum (Mo) are essential animal and human
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nutrients,1, 2 whereas vanadium (V), arsenic (As), and antimony (Sb) are all highly toxic trace
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elements.3-5 To understand the cycling of oxyanions within geochemically important niches in
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the biosphere, their activity at different scales needs to be studied. These oxyanions possess
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several general physical and chemical characteristics, with often coexisting mobilization
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fluxes in wetland systems, such as sediments.6, 7 In such systems, owing to their intrinsic
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heterogeneity, micro-scale biogeochemical processes are key to elemental diagenetic
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behavior and fate.8 However, the mobility and availability of oxyanions in sediments and
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waters are affected by not only the media’s properties such as pH, redox potential, Fe/Mn
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hydroxides, organic matter, and biotic fauna,6, 8 but also the competition between oxyanions.9,
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essential for understanding their environmental behaviors and impacts.
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Therefore, high resolution and in situ techniques for the co-analysis of oxyanions are
The passive sampling technique of DGT has recently been applied to measure oxyanions
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in waters and sediments.7,
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particles incorporated within a solidifying gel (termed “slurry”) have been used successfully
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as DGT binding phases to measure various dissolved species.12-15 Slurry zirconia (SZ, or
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Zr-oxide11, 16, 17, Zr-hydroxide18), based DGT has been validated as a measurement tool for
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phosphate (P)11 and As19. Meanwhile, mixed binding layers using metal-oxide, Chelex, or
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suspended particulate reagent-iminodiacetate (SPR-IDA) have been applied to measure
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anions and cations simultaneously: ferrihydrite-Chelex for P and Mo,20 As and metal cations21;
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SZ-Chelex for P and Fe22; SZ-SPR-IDA for P, As, Cu and Cd18.
Ferrihydrite (iron oxide) and Metsorb (titanium dioxide)
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In order to measure the distribution of oxyanions at high-resolution in sediments, the
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binding materials in the DGT sampler must be homogenously dispersed and of a small
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particle size. In situ precipitation of ferrihydrite within a precast hydrogel has been validated
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as a reliable technique to meet these requirements.23 Compared to the conventional slurry
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ferrihydrite gel, the binding phase in precipitated ferrihydrite gel is more evenly distributed
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with a higher capacity for oxyanions.23,
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ferrihydrite causes two problems. Firstly, the integrity of the gel can be degraded by reductive
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However, the thermodynamic instability of
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dissolution, when deployed in a highly reducing environment.25 A further short-coming is
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ferrihydrite-DGT samplers do not have a long storage life. An inextricable decline in the
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ability to measure oxyanions such as P and As after 40 d arises due to the conversion of
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ferrihydrite to goethite and/or hematite.23, 24
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The SZ-DGT has a capacity of phosphate (P) 50-times higher than slurry ferrihydrite
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based DGT’s11 and has been applied to determine P at 450 × 450 µm2 resolution in
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combination with 2D slicing, elution and microcolorimetric determination16 or at 42 × 42
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µm2 with surface coloration using the molybdenum blue method17. Moreover, Zr oxide and
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hydroxide phases are very stable across wide-ranging redox conditions,26 enabling SZ gels to
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be stored for up to 6 months. However, one limitation for this gel is that it has not been tested
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to measure other oxyanions at high-resolution. With superior detection limits and the ability
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to measure multiple analytes simultaneously, laser-ablation (LA) ICPM-MS analysis of DGT
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gels has become the method of choice for high-resolution (
