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Phosphorous speciation and solubility in aeolian dust deposited in the interior American West Zhuojun Zhang, Harland L. Goldstein, Richard Reynolds, Yongfeng Hu, Xiaoming Wang, and Mengqiang Zhu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04729 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018
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Environmental Science & Technology
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Phosphorous speciation and solubility in aeolian dust deposited
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in the interior American West ⊥
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Zhuojun Zhang†, ǁ, , Harland L. Goldstein‡, Richard L. Reynolds‡, Yongfeng Hu§, Xiaoming
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Wang† and Mengqiang Zhu†,*
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†
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Wyoming 82071, United States
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‡
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Colorado 80225, United States
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§
Department of Ecosystem Science and Management, University of Wyoming, Laramie,
Geosciences and Environmental Change Science Center, U.S. Geological Survey, Denver,
Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3,
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Canada
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ǁ
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Academy of Sciences, Guiyang 550081, China
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⊥
State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese
University of Chinese Academy of Sciences, Beijing 100049, China
14 Revision submitted to Environmental Science & Technology
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*Corresponding author: Mengqiang Zhu
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E-mail:
[email protected] 24
Phone: 307-766-5523
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5096 words, 1 table and 6 figures
December 6, 2017
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ABSTRACT
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Aeolian dust is a significant source of phosphorus (P) to alpine oligotrophic lakes, but P
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speciation in dust and source sediments and its release kinetics to lake water remain unknown.
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Phosphorous K-edge XANES spectroscopy shows that calcium-bound P (Ca-P) is dominant in
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10 of 12 dust samples (41 - 74%) deposited on snow in the central Rocky Mountains and all 42
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source sediment samples (the fine fraction) (68 - 80%), with a lower proportion in dust probably
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because acidic snowmelt dissolves some Ca-P in dust before collection. Iron-bound P (Fe-P,
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~54%) dominates in the remaining two dust samples. Chemical extractions (SEDEX) on these
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samples provide inaccurate results because of unselective extraction of targeted species and
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artefacts introduced by the extractions. Dust releases increasingly more P in synthetic lake water
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within 6 - 72 h thanks to dissolution of Ca-P, but dust release of P declines afterwards due to
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back adsorption of P onto Fe oxides present in the dust. The back sorption is stronger for the dust
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with a lower degree of P saturation determined by oxalate extraction. This work suggests that P
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speciation, poorly crystalline minerals in the dust, and lake acidification all affect the availability
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and fate of dust-borne P in lakes.
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INTRODUCTION
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Phosphorus (P) can limit the primary productivity of oligotrophic alpine lakes due to the 1, 2
. In such environments, mineral
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low P input from poorly weathered surrounding watersheds
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dust derived from dryland sediments is potentially an important P source, in particular when
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referring to alpine lakes
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where dust mobilization and deposition are massive
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alter the nutrient-limitation regime
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abundance and phytoplankton-species composition 14-16 in most alpine lacustrine ecosystems.
3-10
located at the margins of continental arid and semiarid regions,
3, 8, 12, 13
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. The P supply from dust deposition can
, stimulate primary productivity, and shift bacterial
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Not all forms of P carried by dust are available in aquatic ecosystems. The availability,
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fate, and behavior of dust-borne P in alpine lakes can be affected by both P speciation and water
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chemistry. Sequential extractions were used to characterize speciation of dust-borne P and have
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improved the understanding of its bioavailability
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are operational and do not necessarily correspond to P species
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absorption near edge structure (XANES) spectroscopy provides more detailed and precise
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information on P speciation 22-27. XANES spectroscopy divides solid P into P bound to Ca (Ca-P),
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Fe (Fe-P), Al (Al-P) and organic matter (Po)
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analysis of the speciation of dust-borne P
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European anthropogenic aerosols are rich in Po, polyphosphate and alkali phosphate 28, whereas
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P speciation in Saharan dust collected at dust-emission sites is dominated by Ca-P and Fe-P 28, 29.
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The dust samples in these studies were enriched in anthropogenic aerosols
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emission sites
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continental terrestrial ecosystems because the speciation of dust-borne P is related to dust source
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regions and travel distances. Dust collected at source sites may not represent that at deposition
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sites because internal mixing with other sources
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dust transport prior to collection could cause P transformation.
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22
17-19
. However, the extraction-derived P pools 20, 21
. Phosphorus K-edge X-ray
. To date, only two studies reported P XANES
28, 29
. Main results of these works showed that
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or collected at dust
. The results may be inapplicable to mineral dust received in downwind
28, 30
and atmospheric processing
31, 32
during
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Physiochemical factors, such as pH, of the ecosystems where aeolian dust is deposited,
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strongly affect P behavior after deposition. Only 15 - 30% of P is water soluble in aerosol
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samples reaching the Gulf of Aqaba
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Saharan dust can be weathered intensively upon arrival and release most of the P in the Amazon
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basin 33 due to its highly acidic soils (pH 4.17 - 4.94) 34. In addition to the amount of P released,
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the kinetics of P release are important as they affect temporal availability of dust-borne P.
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because of the alkaline seawater (pH ~ 8.2), whereas
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Previous dissolution kinetic experiments showed that dust released increasingly more dissolved
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inorganic P (DIP) to seawater over time
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decline in DIP was observed during dust-seeding experiments in seawater
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ascribed to P adsorption back onto mineral particles in the dust
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release from dust in fresh alpine lake water remain unknown 3-10, and they may differ from those
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observed in seawater because of their distinct water chemistry 38-40.
35, 36
. In contrast, an initial increase followed by a
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. The decline was
. However, the kinetics of P
The semiarid Colorado Plateau region is one of the main dust sources in the western U.S.
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41-44
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has increased P loading to nearby alpine lakes in the Rocky Mountain area 6, 7, 10. The pH of the
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alpine lakes ranges between pH 4.5 and 7.5 due to different degrees of lake acidification caused
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by elevated atmospheric nitrogen deposition38-40. In the present study, we identified the solid-
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phase speciation of P in dust collected in the central Rocky Mountains of Colorado and sediment
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samples from major potential source areas using sequential extractions combined with P K-edge
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XANES spectroscopy. We further evaluated the accuracy of the extractions by characterizing the
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P speciation in each pool using XANES spectroscopy to provide insights into the usefulness of
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this method. To better comprehend the biogeochemical impact of dust-borne P deposition in
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alpine lacustrine environments, we investigated P-release kinetics through the dissolution
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experiments using synthetic alpine lake water of pH 5.0 - 7.5.
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MATERIALS AND METHOD
. Increased dust emission due to extensive grazing and prolonged aridity in the western U.S.
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Sampling and Pretreatment. Dust samples were collected from snow cover in the central
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Rocky Mountains of Colorado (Figure 1) during late spring, representing the merged
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accumulation of all dust layers deposited to snow during water year 2014 (October 1, 2013 -
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September 30, 2014). Sites were sampled by the Center for Snow and Avalanche Studies for
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investigation of effects of dust on snowmelt processes and rates. Complete site descriptions can
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be found at http://www.codos.org/. Samples were sent to the Geosciences and Environmental
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Change Science Center of the U.S. Geological Survey in Denver, CO, where snow was
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evaporated at 35 °C. The dried dust samples were preserved at room temperature before analyses.
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We collected surface-sediment samples from five high and arid basins: The Uinta Basin,
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Hanksville area, Chinle Valley, Chuska Valley, and Little Colorado River basin (LCR) (Figure
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1). These basins are the major potential source areas of the dust samples on the basis of dust
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particle sizes, back-trajectory analyses, and direct observation, as discussed in details in
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supporting information text SI-1. Surface-sediment samples were dried and sieved to obtain the
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size fraction passing through 63-µm sieve for the further analysis because the dust contains more
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than 77% of particles smaller than 63 µm (Table S1). The mineralogy of the dust samples and
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two surface samples was determined by X-ray diffraction (XRD; Table S-2). Poorly and well-
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crystallized Al and Fe (Feox, Fed, Alox) were quantified using oxalate and dithionite extractions,
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respectively 46. See the details of all samples in Table S2.
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Sequential Chemical Extractions. The operationally defined P pools in the dust and source 47, 48
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sediments were determined using the sequential extraction method (SEDEX)
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developed for aquatic sediments. This method was selected for dust
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deposited to aquatic ecosystems is incorporated into aquatic sediments eventually. This method
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allowed for quantification of five major operationally defined P pools: (i) water-soluble P, (ii)
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exchangeable and reactive Fe-bound P (CDB-P, extracted with a citrate-dithionite-bicarbonate
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solution), (iii) authigenic apatite P (Acet-P, extracted with acetic acid-sodium acetate), (iv)
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detrital P (Det-P, extracted with 1 M HCl without ashing), and (v) organic P (Po). Detailed
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procedures are provided in SI-2. The P concentration in each extract was determined using the
17-19, 29
that was
because dust
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except for the CDB-P because the composition of the CDB
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standard molybdate blue method
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extracts interferes with the molybdate blue method. The CDB-extracted P was measured using
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inductively coupled plasma atomic emission spectroscopy. Four dust samples (others did not
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have enough mass) and all sediment samples were analyzed in duplicate to assess the
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reproducibility of the extractions. See SI-2 for more details.
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Phosphorous K-edge XANES Spectroscopy. The spectra were collected from both the dust
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and the source sediment samples at room temperature at the Soft X-ray Micro-Characterization
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Beamline (SXRMB) at the Canadian Light Source, Saskatoon, Canada. The spectra were
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background removed, normalized and then subject to linear combination fitting (LCF) analysis to
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determine P speciation. A set of P reference compounds were used for spectral fitting, including
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well-crystallized and poorly crystalline hydroxyl apatite (Ca-P), phosphate adsorbed on
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ferrihydrite and goethite (Fe-P), phosphate adsorbed on kaolinite (Al-P), and phytate (Po). The
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preparation procedures of the references are provided in SI-3. Four or fewer reference spectra
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were used in the LCF for each sample. Energy was not allowed to float during the fitting. The
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combination of the Ca-P, Fe-P and Po reference spectra yielded the best fits for all samples.
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To validate the operational definitions of P pools obtained by SEDEX, we characterized the
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speciation of P in each pool for three selected dust samples (REP, GM, and SASP) using
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XANES spectroscopy. The P XANES spectra of solid residues after each extraction step were
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measured and the difference spectra were obtained by subtracting the spectra of the successive
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steps
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speciation of each pool.
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. The difference spectra were analyzed with the LCF analysis to determine the P
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Phosphorous Release Kinetics. The experiment was performed on the dust samples using
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synthetic lake water (SLW) of different pHs. Four representative dust samples (REP, GM,
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McClP, and SASP) were selected based on the XANES-derived P speciation. SLW was made
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based on the reported major ion composition (50 µM CaCO3, 20 µM NaHCO3, 10 µM MgSO4, 3
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µM KNO3, 2 µM KCl and 1 µM NH4Cl) and pH (5, 6.5, and 7.5) of the alpine lake water in the
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Rocky Mountains of Colorado 38-40. The solution was agitated for 3 days to ensure the chemicals
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were dissolved completely. Then the pH was maintained at the target value by adding 0.1 M HCl
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for 1 day before use for the release experiments, and the pH changed negligibly during the entire
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period of experiments. Eighty mg dust was added to an acid-washed plastic bottle (Nalgene)
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containing 1000 mL SLW. The bottle was agitated in a shaker at room temperature. Aliquots
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were taken at
