Effect of Soil Organic Matter, Soil pH, and Moisture Content on

Mar 28, 2019 - ... Department of Chemistry, Universidade de Aveiro , 3810-193 Aveiro , Portugal ... The objectives of this research were to quantify t...
0 downloads 0 Views 463KB Size
Subscriber access provided by OCCIDENTAL COLL

Environmental Processes

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

Environmental Science & Technology

2

Effect of soil organic matter, soil pH, and moisture content on solubility and dissolution rate of CuO NPs in soil

3

Xiaoyu Gao†, §, Sónia M. Rodrigues‡, Eleanor Spielman-Sun†, §, Sónia Lopes‡, Sandra Rodrigues‡, Yilin

4

Zhang†, §, Astrid Avellan†, §, Regina M.B.O. Duarte‡, Armando Duarte‡, Elizabeth A. Casman§, # and

5 6

Gregory V. Lowry†, §,*

1

Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States



7



8

§

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

10

#

11

*Address correspondence to [email protected]

12

Abstract:

13

The objectives of this research were to quantify the impact of organic matter content, soil pH and

14

moisture content on the dissolution rate and solubility of copper oxide nanoparticles (CuO NPs)

15

in soil, and to develop an empirical model to predict the dissolution kinetics of CuO NPs in soil.

16

CuO NPs were dosed into standard LUFA soils with various moisture content, pH and organic

17

carbon content. Chemical extractions were applied to measure the CuO NP dissolution kinetics.

18

Doubling the reactive organic carbon content in LUFA 2.1 soil increased the solubility of CuO

19

NP 2.7-fold but did not change the dissolution rate constant. Increasing the soil pH from 5.9 to

20

6.8 in LUFA 2.2 soil decreased the dissolution rate constant from 0.56 mol1/3·kg1/3·s-1 to 0.17

21

mol1/3·kg1/3·s-1 without changing the solubility of CuO NP in soil. For six soils, the solubility of

22

CuO NP correlated well with soil organic matter content (R2 = 0.89) independent of soil pH. In

23

contrast, the dissolution rate constant correlated with pH for pH99%) and triethanolamine (TEA, ≥99.0% (GC)) were purchased from

120

Sigma-Aldrich. Trace metal grade nitric acid (65%-70%) was purchased from VWR. Copper

121

sulfate (CuSO4) was purchased from Fisher Scientific . Lufa Standard soils (2.1, 2.2, 2.4 and 2.4)

122

were purchased from Lufa Speyer, Germany. A calcareous soil (pH 7.6) was collected in

123

Arizona (termed Arizona soil) and used to test the model’s ability to predict CuO NP dissolution

124

behavior based on soil pH and SOM content. Another more acidic soil (pH=5.0) was collected

125

from a grassland in northwestern Portugal (termed Portugal soil). Detailed properties of all the

126

soils used can be found in SI ( Table S1).

127

Nanoparticle properties. CuO NPs (~40 nm primary particle size, zeta potential (ζ) = -16.1 mV ±

128

1.7mV at pH=7 in 5mM NaNO3), were purchased from Sigma-Aldrich. The primary size of

129

particles, zeta potential, isoelectric point and hydrodynamic diameter have been characterized

130

and reported in our previous study6. Additional characterization is in SI.

131

Soil amendment. Soil pH, SOM content and moisture content, factors hypothesized to affect

132

dissolution kinetics of CuO NP in soil, were systematically varied in this study (the soil

133

properties for all treatments can be find in Table S2). To investigate the effect of pH on the

134

dissolution of CuO NP, a mixture of CaO and CaCO3 powders were used to increase the soil pH

135

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

136

soil), and from 5.9 to 6.8 for Lufa 2.2 soil (0.27g CaCO3 in 270g of Lufa 2.2 soil)36.

137

To investigate the influence of SOM on dissolution of CuO NP with all other soil properties held

138

constant, the soil total organic carbon (TOC) content in Lufa 2.1 soil was increased from the

139

original 0.7% to 0.9% by adding SOM extracted from Lufa 2.1 soil. Note that generally the SOM

140

content is ≈ 1.74 times the soil organic carbon content, although this can vary between soil

141

types37. SOM was extracted from Lufa 2.1 soil following a procedure described by van Zomeren

142

et al.38 Additional details on SOM extraction, recovery, and preliminary characterization are

143

provided in Supporting Information. Only about 23% of organic carbon in Lufa 2.1 soil was 5 ACS Paragon Plus Environment

Environmental Science & Technology

144

extractable. This 23% is considered to be the ‘reactive organic carbon,’ the SOM fraction that

145

usually controls the Cu sorption behavior. The remaining fraction was mostly humic substances

146

that have low affinity for metals39. In this study, 161mg extracted fulvic acid, FA, and 368mg

147

extracted humic acid, HA, was added to 90g Lufa 2.1 soil. In the original soil (TOC=0.7%), the

148

reactive carbon content was 0.16%. Thus, by adding 0.2% of reactive organic carbon content in

149

soil, the total reactive carbon in Lufa 2.1 soil was effectively doubled. (Note carbon content in

150

HA and FA are provided in SI.) CuO NPs and CuSO4 (control treatment) were added to different

151

soils to achieve final concentrations of 100 mg/kg, 250 mg/kg and 500 mg/kg dry weight (dw)

152

(as Cu). To investigate the influence of moisture content with all other soil properties held

153

constant, we used Lufa 2.2 standard soil at 21% and 10% moisture content. The two moisture

154

contents were selected because they span relevant moisture conditions, on one end where the soil

155

is as wet as it could be (field capacity) and the other as dry as it could reasonably be (wilting

156

point) for an agricultural soil. CuO NPs were also dosed into the Arizona soil (500mg/kg Cu dw)

157

and Portugal soil (500mg/kg Cu dw) to test our models’ ability to predict solubility and

158

dissolution rate of CuO NP in natural soils. The concentration of CuO used in each treatment

159

was selected based on the solubility of the CuO NPs in each soil determined in preliminary

160

studies (SI). Enough CuO NPs was added to each treatment to ensure that some CuO NPs

161

remained undissolved after 30d. Details on the treatment condition and Cu mass balance are in

162

SI, Table S2.

163 164

Extraction procedure to measure the fraction of dissolved CuO NP and soil pH. The amount

165

of CuO NP that had dissolved at each incubation time (days 0, 2, 4, 7, 14, 21, 30 after

166

amendment) and the corresponding soil pH at that time point, were measured using a previously

167

published extraction method6. Briefly, for each Cu treatment, 2.0 g of air-dried soils were

168

extracted with two standard extractants: (1) 4 mL of DTPA (0.05 M DTPA, 0.01M CaCl2 and

169

0.1M TEA at pH 7.6) and (2) 20 mL of 0.01 M CaCl2 (pH =5). All extractions were done in a

170

reciprocal shaker at 180 rpm for 2 hours. After extraction, samples were centrifuged and filtered

171

with 0.45m PTFE filters. Then, the filtered samples were acidified and analyzed by ICP-MS

172

(Agilent technologies 7700). The measurements were made right after each aging period. It

173

should be noted that our previous studies have demonstrated that such extractions did not induce 6 ACS Paragon Plus Environment

Page 6 of 25

Page 7 of 25

Environmental Science & Technology

174

any CuO NP dissolution6. The pH of CaCl2 extracts for air-dried amended soils were measured

175

as soil pH using a common procedure40,41.

176 177

Determination of Cu speciation in soils. Cu speciation in soils after amendment was analyzed

178

by Cu K-edge XAS at the Stanford Synchrotron Radiation Lightsource (SSRL) on Beamline 11-

179

2. Details on sample preparation and measurements can be found in the SI.

180 181

Dissolution models. The model used for CuO NP dissolution in soil includes the following steps

182

(Figure 1): (1): CuO NP dissolves (reversibly, with rate constants kd and kr)), releasing free Cu

183

ions into the soil pore water. (2): Cu2+ attaches to different ligands (e.g. dissolved organic matter

184

(DOM)) and soil surfaces (e.g. clay, SOM) 42. The second step (Cu ion partitioning between soil

185

pore water and soil solid surfaces) has been investigated previously 29,43–45. The reversible

186

dissolution of CuO NPs are of primary interest to this study.

187

188 189

Figure 1. Schematic of CuO NP dissolution model. Where 𝑘𝑑 is the dissolution rate constant, 𝑘𝑟

190

is the reprecipitation rate constant. 𝐾𝑙𝑖𝑔𝑎𝑛𝑑 is the partitioning constant between Cu associated

191

with natural ligands (including both DOM and soil surfaces, e.g. SOM, clay, iron oxides) and

192

free Cu2+(aq). 𝑘𝑎𝑔𝑖𝑛𝑔 is the constant to account for irreversible loss of Cu to the matrix over long

193

time spans. It should be noted that only the CuO NP dissolution parameters, highlighted in

194

purple, are new additions to the well-known multi-surface geochemical model44,46.

195

7 ACS Paragon Plus Environment

Environmental Science & Technology

196 197

To model the dissolution kinetics, we define Cu2+Tot as the total concentration of Cu2+ being

198

released from CuO NP (free 𝐶𝑢2 + +𝐶𝑢 𝑎𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑒𝑑 𝑤𝑖𝑡ℎ 𝑛𝑎𝑡𝑢𝑟𝑎𝑙 𝑙𝑖𝑔𝑎𝑛𝑑𝑠) , which can be

199

extracted by DTPA. If we assume that Cu2+Tot (t=0) = 0 and that [H+] remains constant during

200

the dissolution (implying a stable pH during the dissolution process due to the relatively high

201

buffering capacity of soil6), the rate law can be expressed by equation (2).

202

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)

203

The derivation of Equation (2) can be found in SI. The key assumptions are:

204

1: The Cu2+ released by CuO NP is in equilibrium with respect to its partitioning to other soil

205

components, e.g. DOM and SOM. This equilibrium is fast compared to the rate of dissolution.

206

2: The solubility of CuO NP(s) is limited by the local dissolution/reprecipitation equilibrium.

207

The dissolution of CuO NP(s) in soils is not complete. Reprecipitation must occur to stop CuO

208

NP from completely dissolving. Dissolution stops when the dissolution rate near the CuO NP

209

surface equals the reprecipitation rate near the NP surface. The precipitation of Cu2+

210

preferentially happens near the surface of CuO NP because of the localized higher Cu2+

211

concentration on the surface of the NP.

212

3: We assume that precipitation of Cu phases other than CuO does not occur.

213

This was corroborated with the facts that (a) ~80% of Cu was still extractable by DTPA in the

214

Lufa 2.2 soil amended with a high concentration of CuSO4 (500 mg/kg), which did not form a

215

solid phase6; and (b) the Cu X-ray absorption near edge structure (XANES) spectra of Lufa 2.2

216

soil dosed with 500mg/kg CuSO4 indicated that 99.6% of the Cu was present as Cu-NOM after

217

30 days (SI, Figure S1).

218

4: We assume the dissolution/precipitation of CuO NP are both surface-controlled process, e.g.

219

dissolution rate and reprecipitation rate are both proportional to the total surface area of CuO NP.

220

Moreover, we assume that the CuO NPs are spherical and that their surface area changes

221

according to a 2/3 power law as has been previously described with the dissolution of spherical

222

ZnO NPs15. 8 ACS Paragon Plus Environment

Page 8 of 25

Page 9 of 25

Environmental Science & Technology

𝑑[𝐶𝑢2 + ]𝑇𝑜𝑡,𝑡

223

At equilibrium,

224

by Equation (3).

𝑑𝑡

= 0 so the solubility of the CuO NPs in the soil, [𝐶𝑢2 + ]𝑇𝑜𝑡,∞, is given

𝑘𝑑 (1 + 𝐾𝑙𝑖𝑔𝑎𝑛𝑑) 𝑘𝑟

225

[𝐶𝑢2 + ]𝑇𝑜𝑡,∞ =

226

Equation (2) can be re-written using 𝑘𝑑 and [𝐶𝑢2 + ]𝑇𝑜𝑡,∞:

(3)

227 [𝐶𝑢2 + ]𝑇𝑜𝑡,𝑡

228

𝑑𝑡

2

2+

= 𝑘𝑑([𝐶𝑢𝑂]𝑜 ― [𝐶𝑢

[𝐶𝑢2 + ]𝑇𝑜𝑡,𝑡

]𝑇𝑜𝑡,𝑡) (1 ― [𝐶𝑢2 + ] 3

) (4)

𝑇𝑜𝑡,∞

229 230

Equation (4) was applied to estimate the unknown constants, 𝑘𝑑, 𝑘𝑟 𝑎𝑛𝑑 [𝐶𝑢2 + ]𝑇𝑜𝑡,∞ from fits of

231

the dissolution data collected for the soils over time. Note that these three parameters are

232

correlated by Equation (3). The Euler method was applied to solve equation (3) numerically.

233

𝐾𝑙𝑖𝑔𝑎𝑛𝑑 was estimated from the experimental data (Equation 5). From control experiments

234

extracting Cu from CuSO4 dosed soil, the efficiency of DTPA extraction, 𝜂𝐷𝑇𝑃𝐴 , was estimated

235

to be 80%.

236 [𝐶𝑢]𝐷𝑇𝑃𝐴 𝜂𝐷𝑇𝑃𝐴

237

𝐾𝑙𝑖𝑔𝑎𝑛𝑑 = [𝐶𝑢]𝐶𝑎𝐶𝑙

2

∙ 𝑥𝐶𝑢2 +

(5)

238 239 240

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.

241 242

Results and discussion

243 244

Effect of Soil Organic Matter on dissolution of CuO NP in soil. To investigate the effect of

245

SOM on dissolution of CuO NP in soil, a dissolution test in Lufa 2.1 soil (100 mg/kg dw CuO

246

NP treatment) and in Lufa 2.1 with added SOM (300 mg/kg dw CuO NP treatment) was

247

conducted (Figure 2). Different concentrations of CuO NP were applied based on the estimated

248

solubility from preliminary experiments (described in SI). Using the dissolution model described 9 ACS Paragon Plus Environment

Environmental Science & Technology

Page 10 of 25

249

in the methods section, the modeled solubility should increase from 95 mg/kg ( 95% CI: 87-108

250

mg/kg) to 254 mg/kg (95% CI: 234-280 mg/kg) in the amended soil (Table 1). Doubling the

251

reactive organic carbon content in Lufa 2.1 soil increased the solubility of CuO NP by 2.7-fold,

252

suggesting reactive organic carbon holds the main Cu pool in soil. Although the solubility

253

increased by 2.7-fold, the modeled dissolution rate constants between Lufa 2.1 soil and Lufa 2.1

254

soil with added SOM are similar (95% confidence intervals are overlapping), suggesting that

255

SOM mainly affects the solubility of CuO NP in soil, but not its dissolution rate.

256

DTPA Extractable Cu (mg /kg dried soil)

257 250

Lufa 2.1 soil, SOM added

200 150 100 50

Lufa 2.1 soil 0 0

258

10

20

30

40

Time (days)

259 260

Figure 2. Dissolution kinetics of CuO NP in Lufa 2.1 soil without added SOM (100 mg/kg dw

261

CuO NP treatment, circles) or with added SOM (300mg/kg dw CuO NP treatment, triangles).

262

Bars are standard deviation of the extractable Cu measurements (3 replicates). Soil pH in these

263

studies was 5.0 (unamended Lufa 2.1 soil) and 4.9 (Lufa 2.1. amended with SOM).

264 265 266

Effect of soil pH on dissolution of CuO NP in soil. The effect of soil pH on the dissolution

267

behavior of CuO NP was investigated by modifying the pH of Lufa 2.1 soil (100 mg/kg dw CuO

268

NP treatment) and Lufa 2.2 soil (500 mg/kg dw CuO NP treatment) with either CaO or CaCO3. 10 ACS Paragon Plus Environment

Page 11 of 25

Environmental Science & Technology

269

Figure 3 indicates that higher pH significantly slowed down the dissolution rate of CuO NP in

270

soil in Lufa 2.2 soil. The modeled dissolution rate constant decreased from 0.56 (CI95: 0.35-

271

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

272

2.2 soil with pH adjustment (pH=6.8). For Lufa 2.1 soil (Figure S2), the dissolution of CuO NPs

273

in pH-adjusted soil (pH=7.4) could not be accurately modeled because of very limited

274

dissolution, but it was clear that it was much slower than the dissolution in Lufa 2.1 soil without

275

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

276

period. Although the dissolution rate constants are different, suggesting a different particle

277

lifetime in soil, the modeled solubility of CuO NPs in Lufa 2.2 soil with and without pH

278

adjustment are similar (Table 1). This can be observed from the extended trend lines (dash lines)

279

from the modeled dissolution kinetics in Figure 3. Thus, the soil pH mainly determines how fast

280

CuO NPs dissolve but has no measurable impact on their solubility. This is because most of the

281

Cu ions released from CuO NPs are retained by SOM. Carboxylic acid functional groups (pKa

282

9) mainly contribute to the acidity of humic acid (the

283

main component of SOM)47,48. The binding capacity between Cu and SOM is not sensitive to pH

284

at agriculture soil relevant pH (5 ~ 7.5)49 because the protonation state of SOM is not susceptible

285

to pH variation in this range. Thus, for a typical agriculture soil, although an increase in soil pH

286

should slow down the ion release process from CuO NP, it may have limited impact on the

287

solubility of CuO NP in that soil.

288 289 290

Figure 3. DTPA extractable Cu in Lufa 2.2 soil dosed with 500 mg/kg CuO NP at pH 5.9

291

(squares) and pH 6.8 (triangles). Dashed lines are model results showing the longer time trend.

292

‘X’ at t=300 days is modeled maximum DTPA extractable Cu for each treatment. Bars are 11 ACS Paragon Plus Environment

Environmental Science & Technology

Page 12 of 25

293

standard deviation of the measurements (3 replicates) or the 95% confident intervals of the

294

modeled maximum DTPA extractable Cu (t= 300 day).

295 296

Effect of soil moisture content on the dissolution rate and solubility of CuO NP in soil. As

297

suggested from Figure 4, moisture content had no impact on the dissolution kinetics of CuO NP.

298

The modeled dissolution rate constants (kd and kr) and solubility [𝐶𝑢2 + ]𝑇𝑜𝑡,∞ are the same for

299

CuO NP dissolving in soil with 10% moisture content or with 21% moisture content (Table 1).

300

This is consistent with the dissolution model that we proposed in which the soil pore water

301

reaches an equilibrium state with the soil solid matrix, where most dissolved Cu is retained by

302

the soil solid surfaces, not the soil pore water6,43. Thus, soil moisture should not affect the

303

dissolution rate or solubility of CuO NPs. It is acknowledged that we did not test extremes of

304

dryness (e.g. moisture content