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Effect of soil organic matter, soil pH, and moisture content on solubility and dissolution rate of CuO NPs in soil Xiaoyu Gao, Sónia Morais Rodrigues, Eleanor Spielman-Sun, Sónia P. Lopes, Sandra Rodrigues, Yilin Zhang, Astrid Avellan, Regina M.B.O. Duarte, Armando C. Duarte, Elizabeth A. Casman, and Gregory V. Lowry Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b07243 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019
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Effect of soil organic matter, soil pH, and moisture content on solubility and dissolution rate of CuO NPs in soil
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Xiaoyu Gao†, §, Sónia M. Rodrigues‡, Eleanor Spielman-Sun†, §, Sónia Lopes‡, Sandra Rodrigues‡, Yilin
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Zhang†, §, Astrid Avellan†, §, Regina M.B.O. Duarte‡, Armando Duarte‡, Elizabeth A. Casman§, # and
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Gregory V. Lowry†, §,*
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Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
†
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‡
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§
9
States
Centre for Environmental and Marine Studies (CESAM), Department of Chemistry, Universidade de Aveiro, 3810-193 Aveiro, Portugal Center for Environmental Implications of NanoTechnology (CEINT), Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United
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#
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*Address correspondence to
[email protected] 12
Abstract:
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The objectives of this research were to quantify the impact of organic matter content, soil pH and
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moisture content on the dissolution rate and solubility of copper oxide nanoparticles (CuO NPs)
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in soil, and to develop an empirical model to predict the dissolution kinetics of CuO NPs in soil.
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CuO NPs were dosed into standard LUFA soils with various moisture content, pH and organic
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carbon content. Chemical extractions were applied to measure the CuO NP dissolution kinetics.
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Doubling the reactive organic carbon content in LUFA 2.1 soil increased the solubility of CuO
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NP 2.7-fold but did not change the dissolution rate constant. Increasing the soil pH from 5.9 to
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6.8 in LUFA 2.2 soil decreased the dissolution rate constant from 0.56 mol1/3·kg1/3·s-1 to 0.17
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mol1/3·kg1/3·s-1 without changing the solubility of CuO NP in soil. For six soils, the solubility of
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CuO NP correlated well with soil organic matter content (R2 = 0.89) independent of soil pH. In
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contrast, the dissolution rate constant correlated with pH for pH99%) and triethanolamine (TEA, ≥99.0% (GC)) were purchased from
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Sigma-Aldrich. Trace metal grade nitric acid (65%-70%) was purchased from VWR. Copper
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sulfate (CuSO4) was purchased from Fisher Scientific . Lufa Standard soils (2.1, 2.2, 2.4 and 2.4)
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were purchased from Lufa Speyer, Germany. A calcareous soil (pH 7.6) was collected in
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Arizona (termed Arizona soil) and used to test the model’s ability to predict CuO NP dissolution
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behavior based on soil pH and SOM content. Another more acidic soil (pH=5.0) was collected
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from a grassland in northwestern Portugal (termed Portugal soil). Detailed properties of all the
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soils used can be found in SI ( Table S1).
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Nanoparticle properties. CuO NPs (~40 nm primary particle size, zeta potential (ζ) = -16.1 mV ±
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1.7mV at pH=7 in 5mM NaNO3), were purchased from Sigma-Aldrich. The primary size of
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particles, zeta potential, isoelectric point and hydrodynamic diameter have been characterized
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and reported in our previous study6. Additional characterization is in SI.
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Soil amendment. Soil pH, SOM content and moisture content, factors hypothesized to affect
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dissolution kinetics of CuO NP in soil, were systematically varied in this study (the soil
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properties for all treatments can be find in Table S2). To investigate the effect of pH on the
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dissolution of CuO NP, a mixture of CaO and CaCO3 powders were used to increase the soil pH
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from the original pH of 5 to ~7.5 for Lufa 2.1 soil (0.27g CaO, 0.68g CaCO3 in 270g of Lufa 2.1
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soil), and from 5.9 to 6.8 for Lufa 2.2 soil (0.27g CaCO3 in 270g of Lufa 2.2 soil)36.
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To investigate the influence of SOM on dissolution of CuO NP with all other soil properties held
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constant, the soil total organic carbon (TOC) content in Lufa 2.1 soil was increased from the
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original 0.7% to 0.9% by adding SOM extracted from Lufa 2.1 soil. Note that generally the SOM
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content is ≈ 1.74 times the soil organic carbon content, although this can vary between soil
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types37. SOM was extracted from Lufa 2.1 soil following a procedure described by van Zomeren
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et al.38 Additional details on SOM extraction, recovery, and preliminary characterization are
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provided in Supporting Information. Only about 23% of organic carbon in Lufa 2.1 soil was 5 ACS Paragon Plus Environment
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extractable. This 23% is considered to be the ‘reactive organic carbon,’ the SOM fraction that
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usually controls the Cu sorption behavior. The remaining fraction was mostly humic substances
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that have low affinity for metals39. In this study, 161mg extracted fulvic acid, FA, and 368mg
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extracted humic acid, HA, was added to 90g Lufa 2.1 soil. In the original soil (TOC=0.7%), the
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reactive carbon content was 0.16%. Thus, by adding 0.2% of reactive organic carbon content in
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soil, the total reactive carbon in Lufa 2.1 soil was effectively doubled. (Note carbon content in
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HA and FA are provided in SI.) CuO NPs and CuSO4 (control treatment) were added to different
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soils to achieve final concentrations of 100 mg/kg, 250 mg/kg and 500 mg/kg dry weight (dw)
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(as Cu). To investigate the influence of moisture content with all other soil properties held
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constant, we used Lufa 2.2 standard soil at 21% and 10% moisture content. The two moisture
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contents were selected because they span relevant moisture conditions, on one end where the soil
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is as wet as it could be (field capacity) and the other as dry as it could reasonably be (wilting
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point) for an agricultural soil. CuO NPs were also dosed into the Arizona soil (500mg/kg Cu dw)
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and Portugal soil (500mg/kg Cu dw) to test our models’ ability to predict solubility and
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dissolution rate of CuO NP in natural soils. The concentration of CuO used in each treatment
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was selected based on the solubility of the CuO NPs in each soil determined in preliminary
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studies (SI). Enough CuO NPs was added to each treatment to ensure that some CuO NPs
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remained undissolved after 30d. Details on the treatment condition and Cu mass balance are in
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SI, Table S2.
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Extraction procedure to measure the fraction of dissolved CuO NP and soil pH. The amount
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of CuO NP that had dissolved at each incubation time (days 0, 2, 4, 7, 14, 21, 30 after
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amendment) and the corresponding soil pH at that time point, were measured using a previously
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published extraction method6. Briefly, for each Cu treatment, 2.0 g of air-dried soils were
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extracted with two standard extractants: (1) 4 mL of DTPA (0.05 M DTPA, 0.01M CaCl2 and
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0.1M TEA at pH 7.6) and (2) 20 mL of 0.01 M CaCl2 (pH =5). All extractions were done in a
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reciprocal shaker at 180 rpm for 2 hours. After extraction, samples were centrifuged and filtered
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with 0.45m PTFE filters. Then, the filtered samples were acidified and analyzed by ICP-MS
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(Agilent technologies 7700). The measurements were made right after each aging period. It
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should be noted that our previous studies have demonstrated that such extractions did not induce 6 ACS Paragon Plus Environment
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any CuO NP dissolution6. The pH of CaCl2 extracts for air-dried amended soils were measured
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as soil pH using a common procedure40,41.
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Determination of Cu speciation in soils. Cu speciation in soils after amendment was analyzed
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by Cu K-edge XAS at the Stanford Synchrotron Radiation Lightsource (SSRL) on Beamline 11-
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2. Details on sample preparation and measurements can be found in the SI.
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Dissolution models. The model used for CuO NP dissolution in soil includes the following steps
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(Figure 1): (1): CuO NP dissolves (reversibly, with rate constants kd and kr)), releasing free Cu
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ions into the soil pore water. (2): Cu2+ attaches to different ligands (e.g. dissolved organic matter
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(DOM)) and soil surfaces (e.g. clay, SOM) 42. The second step (Cu ion partitioning between soil
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pore water and soil solid surfaces) has been investigated previously 29,43–45. The reversible
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dissolution of CuO NPs are of primary interest to this study.
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188 189
Figure 1. Schematic of CuO NP dissolution model. Where 𝑘𝑑 is the dissolution rate constant, 𝑘𝑟
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is the reprecipitation rate constant. 𝐾𝑙𝑖𝑔𝑎𝑛𝑑 is the partitioning constant between Cu associated
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with natural ligands (including both DOM and soil surfaces, e.g. SOM, clay, iron oxides) and
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free Cu2+(aq). 𝑘𝑎𝑔𝑖𝑛𝑔 is the constant to account for irreversible loss of Cu to the matrix over long
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time spans. It should be noted that only the CuO NP dissolution parameters, highlighted in
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purple, are new additions to the well-known multi-surface geochemical model44,46.
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To model the dissolution kinetics, we define Cu2+Tot as the total concentration of Cu2+ being
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released from CuO NP (free 𝐶𝑢2 + +𝐶𝑢 𝑎𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑒𝑑 𝑤𝑖𝑡ℎ 𝑛𝑎𝑡𝑢𝑟𝑎𝑙 𝑙𝑖𝑔𝑎𝑛𝑑𝑠) , which can be
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extracted by DTPA. If we assume that Cu2+Tot (t=0) = 0 and that [H+] remains constant during
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the dissolution (implying a stable pH during the dissolution process due to the relatively high
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buffering capacity of soil6), the rate law can be expressed by equation (2).
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𝑑[𝐶𝑢2 + ]𝑇𝑜𝑡,𝑡 𝑑𝑡
2/3
= 𝑘𝑑([𝐶𝑢𝑂]𝑜 ― [𝐶𝑢2 + ]𝑇𝑜𝑡,𝑡)
1
― 𝑘𝑟[𝐶𝑢2 + ]𝑇𝑜𝑡,𝑡1 + 𝐾𝑙𝑖𝑔𝑎𝑛𝑑([𝐶𝑢𝑂]𝑜 ―[𝐶𝑢2 + ]𝑇𝑜𝑡,𝑡)2/3 (2)
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The derivation of Equation (2) can be found in SI. The key assumptions are:
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1: The Cu2+ released by CuO NP is in equilibrium with respect to its partitioning to other soil
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components, e.g. DOM and SOM. This equilibrium is fast compared to the rate of dissolution.
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2: The solubility of CuO NP(s) is limited by the local dissolution/reprecipitation equilibrium.
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The dissolution of CuO NP(s) in soils is not complete. Reprecipitation must occur to stop CuO
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NP from completely dissolving. Dissolution stops when the dissolution rate near the CuO NP
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surface equals the reprecipitation rate near the NP surface. The precipitation of Cu2+
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preferentially happens near the surface of CuO NP because of the localized higher Cu2+
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concentration on the surface of the NP.
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3: We assume that precipitation of Cu phases other than CuO does not occur.
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This was corroborated with the facts that (a) ~80% of Cu was still extractable by DTPA in the
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Lufa 2.2 soil amended with a high concentration of CuSO4 (500 mg/kg), which did not form a
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solid phase6; and (b) the Cu X-ray absorption near edge structure (XANES) spectra of Lufa 2.2
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soil dosed with 500mg/kg CuSO4 indicated that 99.6% of the Cu was present as Cu-NOM after
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30 days (SI, Figure S1).
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4: We assume the dissolution/precipitation of CuO NP are both surface-controlled process, e.g.
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dissolution rate and reprecipitation rate are both proportional to the total surface area of CuO NP.
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Moreover, we assume that the CuO NPs are spherical and that their surface area changes
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according to a 2/3 power law as has been previously described with the dissolution of spherical
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ZnO NPs15. 8 ACS Paragon Plus Environment
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𝑑[𝐶𝑢2 + ]𝑇𝑜𝑡,𝑡
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At equilibrium,
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by Equation (3).
𝑑𝑡
= 0 so the solubility of the CuO NPs in the soil, [𝐶𝑢2 + ]𝑇𝑜𝑡,∞, is given
𝑘𝑑 (1 + 𝐾𝑙𝑖𝑔𝑎𝑛𝑑) 𝑘𝑟
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[𝐶𝑢2 + ]𝑇𝑜𝑡,∞ =
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Equation (2) can be re-written using 𝑘𝑑 and [𝐶𝑢2 + ]𝑇𝑜𝑡,∞:
(3)
227 [𝐶𝑢2 + ]𝑇𝑜𝑡,𝑡
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𝑑𝑡
2
2+
= 𝑘𝑑([𝐶𝑢𝑂]𝑜 ― [𝐶𝑢
[𝐶𝑢2 + ]𝑇𝑜𝑡,𝑡
]𝑇𝑜𝑡,𝑡) (1 ― [𝐶𝑢2 + ] 3
) (4)
𝑇𝑜𝑡,∞
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Equation (4) was applied to estimate the unknown constants, 𝑘𝑑, 𝑘𝑟 𝑎𝑛𝑑 [𝐶𝑢2 + ]𝑇𝑜𝑡,∞ from fits of
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the dissolution data collected for the soils over time. Note that these three parameters are
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correlated by Equation (3). The Euler method was applied to solve equation (3) numerically.
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𝐾𝑙𝑖𝑔𝑎𝑛𝑑 was estimated from the experimental data (Equation 5). From control experiments
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extracting Cu from CuSO4 dosed soil, the efficiency of DTPA extraction, 𝜂𝐷𝑇𝑃𝐴 , was estimated
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to be 80%.
236 [𝐶𝑢]𝐷𝑇𝑃𝐴 𝜂𝐷𝑇𝑃𝐴
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𝐾𝑙𝑖𝑔𝑎𝑛𝑑 = [𝐶𝑢]𝐶𝑎𝐶𝑙
2
∙ 𝑥𝐶𝑢2 +
(5)
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Where as [𝐶𝑢]𝐷𝑇𝑃𝐴 is DTPA extractable Cu, 𝜂𝐷𝑇𝑃𝐴 is the extraction efficiency (0.8 in this study), [𝐶𝑢]𝐶𝑎𝐶𝑙2 is CaCl2 extractable Cu, and 𝑥𝐶𝑢2 + is the fraction of free Cu ions in soil pore water.
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Results and discussion
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Effect of Soil Organic Matter on dissolution of CuO NP in soil. To investigate the effect of
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SOM on dissolution of CuO NP in soil, a dissolution test in Lufa 2.1 soil (100 mg/kg dw CuO
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NP treatment) and in Lufa 2.1 with added SOM (300 mg/kg dw CuO NP treatment) was
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conducted (Figure 2). Different concentrations of CuO NP were applied based on the estimated
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solubility from preliminary experiments (described in SI). Using the dissolution model described 9 ACS Paragon Plus Environment
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in the methods section, the modeled solubility should increase from 95 mg/kg ( 95% CI: 87-108
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mg/kg) to 254 mg/kg (95% CI: 234-280 mg/kg) in the amended soil (Table 1). Doubling the
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reactive organic carbon content in Lufa 2.1 soil increased the solubility of CuO NP by 2.7-fold,
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suggesting reactive organic carbon holds the main Cu pool in soil. Although the solubility
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increased by 2.7-fold, the modeled dissolution rate constants between Lufa 2.1 soil and Lufa 2.1
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soil with added SOM are similar (95% confidence intervals are overlapping), suggesting that
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SOM mainly affects the solubility of CuO NP in soil, but not its dissolution rate.
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DTPA Extractable Cu (mg /kg dried soil)
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Lufa 2.1 soil, SOM added
200 150 100 50
Lufa 2.1 soil 0 0
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10
20
30
40
Time (days)
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Figure 2. Dissolution kinetics of CuO NP in Lufa 2.1 soil without added SOM (100 mg/kg dw
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CuO NP treatment, circles) or with added SOM (300mg/kg dw CuO NP treatment, triangles).
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Bars are standard deviation of the extractable Cu measurements (3 replicates). Soil pH in these
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studies was 5.0 (unamended Lufa 2.1 soil) and 4.9 (Lufa 2.1. amended with SOM).
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Effect of soil pH on dissolution of CuO NP in soil. The effect of soil pH on the dissolution
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behavior of CuO NP was investigated by modifying the pH of Lufa 2.1 soil (100 mg/kg dw CuO
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NP treatment) and Lufa 2.2 soil (500 mg/kg dw CuO NP treatment) with either CaO or CaCO3. 10 ACS Paragon Plus Environment
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Figure 3 indicates that higher pH significantly slowed down the dissolution rate of CuO NP in
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soil in Lufa 2.2 soil. The modeled dissolution rate constant decreased from 0.56 (CI95: 0.35-
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0.84)) (mg1/3·kg1/3·s-1) in Lufa 2.2 soil (pH=5.9) to 0.17 (CI95: 0.14-0.21) (mg1/3·kg1/3·s-1) in Lufa
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2.2 soil with pH adjustment (pH=6.8). For Lufa 2.1 soil (Figure S2), the dissolution of CuO NPs
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in pH-adjusted soil (pH=7.4) could not be accurately modeled because of very limited
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dissolution, but it was clear that it was much slower than the dissolution in Lufa 2.1 soil without
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pH adjustment (pH=5.0, kd= 0.83 mg1/3·kg1/3·s-1, with 95% CI: 0.65-1.00) during the 31d aging
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period. Although the dissolution rate constants are different, suggesting a different particle
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lifetime in soil, the modeled solubility of CuO NPs in Lufa 2.2 soil with and without pH
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adjustment are similar (Table 1). This can be observed from the extended trend lines (dash lines)
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from the modeled dissolution kinetics in Figure 3. Thus, the soil pH mainly determines how fast
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CuO NPs dissolve but has no measurable impact on their solubility. This is because most of the
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Cu ions released from CuO NPs are retained by SOM. Carboxylic acid functional groups (pKa
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9) mainly contribute to the acidity of humic acid (the
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main component of SOM)47,48. The binding capacity between Cu and SOM is not sensitive to pH
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at agriculture soil relevant pH (5 ~ 7.5)49 because the protonation state of SOM is not susceptible
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to pH variation in this range. Thus, for a typical agriculture soil, although an increase in soil pH
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should slow down the ion release process from CuO NP, it may have limited impact on the
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solubility of CuO NP in that soil.
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Figure 3. DTPA extractable Cu in Lufa 2.2 soil dosed with 500 mg/kg CuO NP at pH 5.9
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(squares) and pH 6.8 (triangles). Dashed lines are model results showing the longer time trend.
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‘X’ at t=300 days is modeled maximum DTPA extractable Cu for each treatment. Bars are 11 ACS Paragon Plus Environment
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standard deviation of the measurements (3 replicates) or the 95% confident intervals of the
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modeled maximum DTPA extractable Cu (t= 300 day).
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Effect of soil moisture content on the dissolution rate and solubility of CuO NP in soil. As
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suggested from Figure 4, moisture content had no impact on the dissolution kinetics of CuO NP.
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The modeled dissolution rate constants (kd and kr) and solubility [𝐶𝑢2 + ]𝑇𝑜𝑡,∞ are the same for
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CuO NP dissolving in soil with 10% moisture content or with 21% moisture content (Table 1).
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This is consistent with the dissolution model that we proposed in which the soil pore water
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reaches an equilibrium state with the soil solid matrix, where most dissolved Cu is retained by
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the soil solid surfaces, not the soil pore water6,43. Thus, soil moisture should not affect the
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dissolution rate or solubility of CuO NPs. It is acknowledged that we did not test extremes of
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dryness (e.g. moisture content
