CuO nanoparticle dissolution and toxicity to wheat (Triticum aestivum

Jan 31, 2018 - It has been suggested, but not previously measured, that dissolution kinetics of soluble nanoparticles such as CuO NPs in soil affect t...
0 downloads 4 Views 676KB Size
Subscriber access provided by READING UNIV

Article

CuO nanoparticle dissolution and toxicity to wheat (Triticum aestivum) in rhizosphere soil Xiaoyu Gao, Astrid Avellan, Stephanie N Laughton, Rucha Vaidya, Sónia Morais Rodrigues, Elizabeth A. Casman, and Gregory V. Lowry Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05816 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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 free 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 accessible to all readers and 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.

Environmental Science & Technology 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 24

Environmental Science & Technology

1

CuO nanoparticle dissolution and toxicity to wheat (Triticum aestivum) in rhizosphere soil

2 3

Xiaoyu Gao†, §, Astrid Avellan †, §, Stephanie Laughton †, §, Rucha Vaidya †, §, Sónia M. Rodrigues‡, Elizabeth A. Casman§, #, and Gregory V. Lowry†, §, *.

4 5



6

§

7

States

8



9

#

Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States Center for Environmental Implications of NanoTechnology (CEINT), Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United

Centre for Environmental and Marine Studies (CESAM), Department of Chemistry, Universidade de Aveiro, 3810-193 Aveiro, Portugal Department of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States

10

*Address correspondence to [email protected]

11

Abstract:

12

It has been suggested, but not previously measured, that dissolution kinetics of soluble nanoparticles such

13

as CuO NPs in soil affect their phytotoxicity. An added complexity is that such dissolution is also

14

affected by the presence of plant roots. Here, we measured the rate of dissolution of CuO NPs in bulk soil,

15

and in soil in which wheat plants (Triticum aestivum) were grown under two soil NP dosing conditions:

16

(a) freshly added CuO NPs (500 mg Cu/kg soil), and (b) CuO NPs aged for 28d before planting. At the

17

end of the plant growth period (14 days), available Cu was measured in three different soil compartments:

18

bulk (not associated with roots), loosely attached to roots, and rhizosphere (soil firmly attached to roots).

19

The labile Cu fraction increased from 17mg/kg to 223mg/kg in fresh treatments and from 283 mg/kg to

20

305mg/kg in aged treatments over the growth period due to dissolution. Aging CuO NPs increased the

21

toxicity to Triticum aestivum (reduction in root maximal length). The presence of roots in the soil had

22

opposite and somewhat compensatory effects on NP dissolution, as measured in rhizosphere soil. pH

23

increased 0.4 pH units for fresh NP treatments and 0.6 pH units for aged NPs. This lowered CuO NP

24

dissolution in rhizosphere soil. Exudates from T. aestivum roots also increased soluble Cu in porewater.

25

CaCl2 extractable Cu concentrations in bulk vs. rhizosphere soil increased from 1.8mg/kg to 6.2mg/kg

26

(fresh treatment), and from 3.4mg/kg to 5.4mg/kg (aged treatments). Our study correlated CuO NP

27

dissolution and the resulting Cu ion exposure profile to phytotoxicity, and showed that plant-induced

28

changes in rhizosphere conditions should be considered when measuring the dissolution of CuO NP near

29

roots.

30

TOC art:

1 ACS Paragon Plus Environment

Environmental Science & Technology

31 32 33

Introduction

34

The anticipated benefits of nano-enabled agrochemicals include slow and controlled release of

35

micronutrients, plant tissue-specific targeted release of micronutrients or pesticides, reduced amounts of

36

agrochemicals being required, and generally lower toxicity compared to more soluble products1,2. Copper-

37

based nanoparticles (NPs) are already on the agrochemical market3,4. Copper is an essential crop

38

micronutrient. Deficiency may lead to reduced disease resistance5 and decreased crop yields6. However,

39

at high concentrations, Cu can also be toxic to plants,7 the surrounding microbial communities,8 and soil

40

invertebrates9. Due to its relatively slow dissolution, CuO NPs have been studied as a potential candidate

41

for agrochemical use. It behaves differently from dissolved Cu2+ in soil, potentially affecting copper

42

bioavailability, the release of Cu ions over time, and potential associated risks10–12. However, the

43

connection between NP dissolution, the resulting dose of Cu ions and its toxicity to terrestrial plants, and

44

the role of root exudates on this process have not been well elucidated due to a lack of appropriate

45

characterization of the dissolution of the NPs in soil. Ideally, application rates of these novel materials

46

should be based on their fate and effects in the terrestrial environment, their bioavailability and potential

47

toxicity to plants. The toxic effect of Cu species is reflected in physiological changes in plant roots and

48

shoots, such as decreased root length, increased root compactness, change in root color, shorter leaf

49

length and decreased shoot biomass13–15. Hyperspectral imaging has been used to visualize NPs in plants

50

and to confirm macroscopic evidence of NP toxicity16,17.

51

Previous studies of the toxicity of CuO NPs to terrestrial plants assumed, but did not measure,

52

dissolution behavior of CuO NP in soil. This has led to conflicting conclusions on the toxicity of CuO

53

NPs. While some studies attributed the toxic effect of CuO NP to released ionic Cu15,18,19, others

54

concluded the opposite20. For example, Servin et al. chose a Cu ion control concentration based on the

55

assumption that only 10% of the CuO NP would dissolve in soil, the same fraction that dissolved in pure

56

sand, rather than measuring CuO dissolution in soil. They concluded that dissolution of CuO NPs could

57

not fully explain the plant toxicity because the plant responses differed from their Cu ion control .20 Much

58

more than 10% CuO NP could have dissolved in soil because soil organic matter (SOM) acts as a Cu 2 ACS Paragon Plus Environment

Page 2 of 24

Page 3 of 24

Environmental Science & Technology

59

sink, increasing the amount of CuO NP that can be dissolved11. This weakens their conclusions about a

60

NP-specific effect. Similar problems occurred in other studies 15,18,21–23. Breaking with this trend, Dimkpa

61

et al. (2013) evaluated the total CuO NP dissolved in soil using a water-extraction method. 24

62

Unfortunately, the water-extraction does not extract Cu bound to the soil solid matrix which accounts for

63

most of the dissolved Cu in soils25–27. Thus, their assertion of a CuO NP-specific toxicity in soil is

64

confounded by the potential that more Cu had dissolved than was assumed or measured. Recently Qiu et

65

al. found that the toxicity of CuO NP, CuO bulk particles and soluble Cu (Cu(AC)2) depends on their

66

solubility in soil, and that the distinction in solubility diminished after a 90-day aging period. However,

67

the actual dissolution during the incubation periods (one day vs. 90 day) was not quantified. They

68

successfully correlated the toxicity of NP to roots of Hordeum vulgare L. (5-day root elongation

69

experiment) with ‘free Cu ions’ in soil pore water measured at a single time point before seeding;19

70

though convincing, it should be noted that the dissolution during the 5-day toxicity test was not

71

considered. While the relatively slow dissolution of CuO NP may result in unobservable impacts on

72

toxicity during a relatively short 5-day toxicity test, dissolution at this rate would probably affect toxicity

73

of NPs in longer tests.

74

The dissolution of CuO NPs is a dissolution rate-limited process. Experimental approaches, such

75

as extraction with CaCl2 or with diethylenetriaminepentaacetic acid (DTPA), have been used to predict

76

the bioavailability or toxicity of metals in soil.28–31 CaCl2 extracts the Cu ions in soil pore water that are

77

considered ‘readily available,’ while DTPA extracts the “labile” fraction including dissolved Cu in soil

78

pore water (free Cu2+and Cu2+ complexed with soluble ligands such as dissolved organic matter (DOM)),

79

but also the Cu2+ associated with soil solid phases, such as soil organic matter (SOM), clay particles, and

80

iron oxide minerals.29–31 Whereas CaCl2 extracts metals that are ‘readily available’ to plants29, DTPA

81

extracts this pool as well as the pool that may eventually become bioavailable in soil, the so called

82

‘potentially available’ fraction32. One problem with using these extraction methods to predict the

83

bioavailability of Cu based nanomaterials is that a single time point extraction does not capture the

84

temporal dynamics of the CuO NP dissolution process. Our recent study used extraction methods at

85

different times to monitor the kinetics of release of Cu ions from CuO NP in soil. In that study, the

86

increase in DTPA extractable Cu over 30 days in soils was used to estimate the dissolved pool of Cu in

87

soil.11 The availability of Cu ions increased with time over a 30d period, which may explain why previous

88

efforts to correlate the extractable metals in metal-based NP-amended soils with their bioavailability or

89

toxicity have generally failed33–35.

90 91

Plants also may affect the dissolution behavior and availability of CuO NP in soil, especially in the rhizosphere. Previous studies using extraction methods to predict the bioavailability or toxicity of 3 ACS Paragon Plus Environment

Environmental Science & Technology

92

metal-based ENMs or the dissolution of ENMs in soil did not typically consider the impact of roots on Cu

93

availability.11,12,33–35 Plant roots exude organic acids 36–38 that may affect the pH in rhizosphere. 39,40

94

Although soil pH and organic carbon are known to be important factors influencing the dissolution

95

behavior of CuO NPs in soil11,12, and previous studies have proposed that exudates from plant roots may

96

affect the dissolution of CuO NP in the rhizosphere41, no studies have quantified this. Given that the

97

rhizosphere is where plants interact with soil for nutrient uptake,42,43 a better understanding of how the

98

roots impact NP dissolution and metal availability in the rhizosphere is needed to design nano-enabled

99

agrichemicals with optimal properties for delivering nutrients.

100

The objectives of this study are to quantify the influence of time and near-root chemical

101

conditions on dissolution and lability of CuO NPs in rhizosphere soil, and to determine the influence of

102

this dissolution on the toxicity of CuO NPs to Triticum aestivum during a 14-day plant growth period in

103

soil. Wheat (Triticum aestivum) was used in this study because it is the 2nd most cultivated plant in the

104

world, and it is sensitive to Cu deficiency44 or excess45. To evaluate the toxicity of CuO NP to plants, we

105

measured the dissolution behavior of CuO NPs in soil in the presence of plants with emphasis on the soil-

106

plant interface (rhizosphere) where roots interact with soil. The toxicity of Cu was evaluated by

107

physiological changes in plant roots and shoots.

108

Methods and materials

109

Chemicals. Calcium chloride (≥99.0%, ACS grade) and hydrogen peroxide (30%, certified ACS) were

110

purchased from Fisher Scientific. DTPA (>99%) and triethanolamine (TEA, ≥99.0% (GC)) were

111

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

112

Triticum aestivum seeds (Pembroke 2014) were bred by Dr. David Van Sanford (Department of Plant and

113

Soil Sciences, University of Kentucky).

114

Nanoparticles and Characterization of Nanoparticle properties. CuO NPs (~40 nm primary particle

115

size), were purchased from Sigma-Aldrich. The primary size of particles, zeta potential, isoelectric point

116

and hydrodynamic diameter have been characterized and reported in our previous study11. The details of

117

characterization methods can be found in SI.

118

Soils and Characterization of Soil Properties. Standard Lufa 2.2 soil (loamy sand) was purchased from

119

Lufa Speyer, Germany. Lufa 2.2 soil contains 1.6 wt. % soil organic matter, and little total and available

120

Cu (see supporting information SI, Table S1 and Table S2, control treatment). Using a well-characterized

121

standard soil allows comparisons between studies. The high carbon organic content (about 1.6%) of Lufa

122

2.2 makes this soil good for agricultural studies. Soil was air dried and sieved < 2mm before shipping.

123

The soil was further air-dried for at least 24 hours before all experiments. Soil pH in different treatments 4 ACS Paragon Plus Environment

Page 4 of 24

Page 5 of 24

Environmental Science & Technology

124

was determined by the CaCl2 extraction method (see ‘Extraction methods’ section). Soil moisture content

125

(1% for the air dried soil) was determined gravimetrically after oven-drying the soil at 105 ºC for 24 h46.

126

Soil field moisture capacity (21%) was determined using a Haines apparatus with 0.1 bar pressure

127

difference between the wet soil and the atmosphere.

128

Soil amendment. The CuO NP suspension (containing Na2SO4), CuSO4 solution, or Na2SO4 solution

129

(control treatment) were mixed with soil and brought to a moisture content of 21.7% (corresponding to

130

~50% of the water holding capacity). The soil was mixed with wooden sticks in a beaker for 20min. The

131

homogeneity was confirmed with the low standard deviation for the total Cu content measured by soil

132

digestion data (SI, table S1). To test if CuO NP and CuSO4 treatments resulted in different Cu

133

bioavailability and toxicity, the Cu ion concentration had to be high enough to ensure some CuO NPs

134

remained in the soil during the study period. We chose 500mg/kg (as Cu) for the CuO NP treatment, and

135

300 mg/kg (as Cu) for the CuSO4 treatment based on a preliminary study to assess the solubility of the

136

CuO NPs in the Lufa 2.2 soil (SI, Figure S1). The results showed that the solubility of CuO NPs in Lufa

137

2.2 soil was ~300mg/kg. Therefore, the selected concentrations provided a similar concentration of added

138

Cu ion in both treatments after one month.

139

Germination and plant growth. The seeds of Triticum aestivum were surface sterilized by submerging

140

them in 10% sodium hypochlorite solution for 10 minutes, and then washed with DI water three times.

141

The seeds were then kept immersed in DI water overnight on an end-to-end rotator. The following day,

142

the seeds were transferred to a petri-dish containing moist tissue paper. The petri-dishes were covered

143

with aluminum foil and incubated in the growth chamber for 7 days, until 90% germination was achieved.

144

Germinated seeds were transplanted into syringes containing 120g of amended soils either immediately

145

after adding the Cu (fresh treatment) or 28 days after the Cu was added (aged treatment).The plants were

146

incubated in a growth chamber with constant moisture content and 16h-light/8h-dark cycle (25 °C for

147

daytime and 21 °C for night time). A diluted Cu-free Hoagland solution (quarter strength) was added

148

(1ml/day) to each syringe to maintain the moisture content of the soil as well as provide nutrients to

149

plants. The concentration of Cu in soil and plant tissue was determined using a standard digestion method

150

(EPA Method 3050b47) and ICP-MS analysis of Cu in the digestate. See SI. Adding moisture content did

151

not induce any vertical transport of Cu, as suggested by Figure S4.

152

Sampling of soil and plant tissue. Prior to transplanting the germinated seeds in soils, subsamples of

153

each soil were collected from all treatments for DTPA extraction (2g of soil per extraction) to measure the

154

labile metal fraction. After 14d of growth, rhizosphere soil, "loosely attached soil," and bulk soil (Figure

155

S2) were collected for DTPA and CaCl2 extraction to determine the total dissolved metal and readily

156

available metal, respectively, as described below. After the plants and roots were removed from the 5 ACS Paragon Plus Environment

Environmental Science & Technology

157

syringe, the soil remaining inside the syringe was defined as bulk soil, presumably minimally affected

158

by the plant roots. The bottom 5mm of bulk soil was also collected to determine if there was significant

159

vertical transport of Cu. No vertical transport of Cu was observed (SI Figure S4). The roots were

160

separated from shoots. Both roots and shoots were photographed with a scale bar for determination of

161

length. For each treatment, one plant root replicate was washed with 1mM KCl three times for Cytoviva

162

analysis (described below). The remaining roots were shaken by hand in a 50 ml centrifuge tube, and the

163

soil that detached during shaking was defined as loosely attached soil48 (Figure S2). After shaking, the

164

roots were placed on aluminum foil and air dried in a fume hood for 24 hours. The roots were then shaken

165

again in a 50 ml centrifuge tube, and the soil that detached during the air-drying process and the second

166

shaking process was defined as rhizosphere soil49. Due to the small amount of rhizosphere soil collected

167

per treatment, not all replicates were suitable for DTPA and CaCl2 extraction. The details of which

168

samples were analyzed can be found in SI (Table S3 and Table S4). For CuSO4 treatments, the roots were

169

highly compacted, precluding the collection of rhizosphere soil. The shoots and roots were oven-dried at

170

105 ºC for 24 h. The mass of the dried roots and shoots was recorded before digestion for total Cu

171

analysis (details in SI).

172 173

Soil extraction. Two standard extraction fluids were used in this study. DTPA extractant was composed

174

of 0.01M CaCl2, 0.005M DTPA and 0.1M triethanolamine (TEA) (pH=7.6). CaCl2 extractant was 0.01M

175

CaCl2 without pH adjustment. All extractions were done using a reciprocal shaker at 180 rpm for 2 hours.

176

It is important to note that the centrifuge tubes were laid horizontally in the shaker rather than vertically to

177

provide the best extraction efficiency. For soil samples collected before the plant growth experiments, 2g

178

of soil were extracted with 4ml DTPA extractant. For bulk soil samples, loosely attached soil samples and

179

rhizosphere soil samples, 0.4g of soil was extracted with 0.8ml DTPA extractant, while 0.35g of soil was

180

extracted with 3.5ml CaCl2 extractant. After extraction, all samples were centrifuged at 3000 rpm for 10

181

min, and the supernatants were filtered using a 0.45 µm PTFE filter. The pH of the CaCl2 extracts for

182

each soil fraction was measured (Figure 5). All samples were acidified with 20% HNO3 (final HNO3

183

concentration, 2%) and Milli-Q-water before being analyzed by ICP-MS. The method for ICP-MS is

184

provided in detail in the SI.

185 186

Cytoviva analysis. The interaction between roots and NPs were visualized in fresh roots after a rinsing step

187

in 10-3 M KCl, using a darkfield-based hyperspectral imaging (DF-HSI) system (CytoViva Inc., USA). See

188

SI for additional details.

189 6 ACS Paragon Plus Environment

Page 6 of 24

Page 7 of 24

Environmental Science & Technology

190

Results

191

Nanoparticle characterization. The properties of the CuO NPs have been previously described11.

192

Briefly, the primary particle size was determined by TEM to be 38nm ± 1.7nm (278 particles counted,

193

95% CI). The hydrodynamic diameter of an 80mg/kg CuO NP in an aqueous 5mM NaHCO3 suspension

194

(pH=7) was measured to be 560nm±103nm (3 replicates, 95% CI, intensity averaged), and the zeta

195

potential was -16.1mV±1.7mV (3 replicates, 95%CI) in the same suspension. The pH of the isoelectric

196

point (pHiep) of the CuO NPs in a 5mM NaNO3 solution was 8.8, while the pHiep was 5.8 in the 5mM

197

NaHCO3 solution11. The hydrodynamic diameter and zeta potential likely change after they are added to

198

the soils due to interactions with soil components such as natural organic matter and calcium50,51.

199 200

Change in extractability of Cu in bulk soil during the plant growth experiment. The DTPA

201

extractable Cu in the bulk soil for CuO NP and CuSO4 amended soils are shown in Fig.1. The DTPA

202

extractable Cu represents the Cu that was released from the CuO NPs during the treatment. The

203

extractable Cu vs. time is shown for the 14d growth period for both the freshly added CuO NPs, and for

204

the aged CuO NPs, where plants were added after the CuO NPs had aged for 28d prior to planting the

205

germinated seeds. The total Cu concentration in the two treatments and in the control (unamended) soil is

206

provided in the SI (Table S1).

207

For the CuO NP treatments, the DTPA extractable Cu from bulk soil increased over time (Figure

208

1a) (ANOVA test, P0.05). The DTPA extractable Cu in the

217

aged CuO NP treatment was statistically significantly higher than both fresh and aged CuSO4 treatment

218

(ANOVA test followed by Fisher’s LSD test for multiple comparison, P≤0.05) (Figure 1c). The CaCl2

219

extractable Cu revealed a different order, with fresh CuSO4 treatment having the highest CaCl2

220

extractable Cu, followed by the aged CuO NP treatment and the aged CuSO4 treatment, with the fresh

221

CuO NP treatment having the lowest amount of CaCl2 extractable Cu. The CaCl2 extractable copper

222

represents the “readily available” Cu in the porewater. 7 ACS Paragon Plus Environment

Environmental Science & Technology

223 224 225

Figure 1. Change in DTPA extractable Cu over time for each treatment: a) CuO NP treatment, b) CuSO4

226

treatment, and comparison of mean of extractable Cu for each Cu treatments at the end of the plant

227

growth period: c) DTPA extraction, d) CaCl2 extraction. Error bars show ± 1 standard deviation. In a and

228

b, capital letters indicate significant differences between DTPA extractable Cu at four time points for

229

CuO NP treatments (a) and CuSO4 treatments (b). In c and d, capital letters indicate significant

230

differences in DTPA extractable Cu (c) and CaCl2 extractable Cu (d) among soils collected after plant

231

harvesting (ANOVA test followed by Fisher’s LSD test for multiple comparisons, P≤0.05).

232

Toxicity of CuSO4 and CuO NP. Root maximal length, root compactness (root mass/root maximal

233

length), leaf lengths, shoot mass (Figure 2) and root morphology (SI Figure S5 and Figure S6) were used

234

to evaluate the toxic effect of CuSO4 and CuO NPs.

235

Root maximal length and root compactness indicated no visual toxic effect from the fresh CuO

236

NP treatment. For other treatments, significant decreases in root maximal length (a decrease of 6.6cm,

237

8.2cm, and 6.8cm compared to the control treatment for aged CuO NPs, fresh CuSO4, and aged CuSO4,

238

respectively) were observed. Increased root compactness was observed for the aged CuO NP treatment

239

(an increase of 4.0 mg/cm compared to the control) and for the fresh CuSO4 treatment (an increase of 5.1

240

mg/cm compared to the control). Examples of shortened roots and compactness of roots are shown in the

241

SI (Figure S5). Evidence of Cu toxicity was also observed in Cytoviva images. In comparison to the roots 8 ACS Paragon Plus Environment

Page 8 of 24

Page 9 of 24

Environmental Science & Technology

242

exposed to CuSO4 (SI, Figure S6), the roots exposed to CuO NP (fresh or aged) did not present the same

243

damaged physiology. Roots exposed to CuSO4 (both fresh and aged) showed a brown damaged (necrotic)

244

zone that was not found on any of the CuO NP exposed roots. No effects of Cu on the shoots (leaf length,

245

biomass) were observed for the CuO NP treatments. Both the freshly amended and aged CuSO4

246

treatments resulted in shorter third leaves (shortened by 5.4cm and 4.0 cm compared to the control for

247

fresh and aged CuSO4 treatments, respectively). The freshly amended CuSO4 treatment also had less total

248

shoot biomass compared to the control treatment.

249

Some indication of toxicity was evident in all treatments except for the fresh CuO NP treatment.

250

Aging of CuSO4 decreased its toxicity to Triticum aestivum, while aging of CuO NP increased its toxicity.

251

Overall, the two CuSO4 treatments showed more toxic effects to Triticum aestivum compared with the

252

two CuO NP treatments, even though the CuSO4 was added at a significantly lower Cu concentration

253

(300mg/kg for CuSO4 treatments vs. 500mg/kg for CuO NP treatment).

254

255 256 257

Figure 2. a) Root compactness and b) leaf length (leaf growth stage is noted with number, from 1 being

258

the youngest to 3 the oldest) of wheat seedlings grown in freshly amended and aged CuO NP, CuSO4-

259

amended soil, and control treatments. Error bars show ±1SD, * indicates P≤0.05; ** indicates P≤0.01.

260

(ANOVA test followed by Fisher’s LSD test for multipal comparisions) compared to the control

261

treatment.

262

Cu root association and Cu uptake. The presence of CuO NPs associated with the roots after 2 weeks of

263

plant growth in both fresh and aged CuO NP amended soils was investigated using enhanced dark-field

264

hyperspectral imaging (DF-HSI) as shown in Figure 3. The pixels containing CuO NP have been

265

highlighted in red. In both fresh and aged CuO NP amended soils (Figure 3), CuO NPs were found 9 ACS Paragon Plus Environment

Environmental Science & Technology

266

associated to specific locations on the roots, either to the root tip mucilage (Figure3 a, b, f, g), or to soil

267

aggregates attached to the root hairs or root tips (Figure 3 a, c-i). For the concentration of Cu in roots, all

268

Cu treatments were significantly higher than the control treatment. The Cu concentration in roots

269

(577mg/kg, s.d.=46mg/kg, 6 replicates) was statistically significantly higher in the freshly amended CuO

270

NP treatment than in the aged CuO NP treatment (400mg/kg, s.d.=60mg/kg, 6 replicates) or either ionic

271

treatment (278mg/kg, s.d.=51mg/kg, 6 replicates for fresh CuSO4 and 442mg/kg, s.d.=67mg/kg, 6

272

replicates for aged CuSO4) (Figure S7, a). For the shoot concentrations, no statistically significant

273

differences were found for all Cu treatments (53mg/kg-88mg/kg) (Figure S7, b).

274 10 ACS Paragon Plus Environment

Page 10 of 24

Page 11 of 24

Environmental Science & Technology

275 276

Figure 3. Hyperspectral imaging of plant roots grown in soil with freshly amended CuO NPs (a-e) or

277

after aging (f-i). The b, c and g views are magnified views from a and f. Pixels containing the reflectance

278

spectra specific to CuO NPs are highlighted in red. CuO NPs and their aggregates were found associated

279

to mucilage, root tissues and root hairs (red arrow), and to soil aggregates attached to those locations

280

(yellow arrows). Scale bars: 25µm

281 282

Effect of near-root environment on Cu availability from CuO NP treatment. Figure 4 shows the

283

differences in extractable Cu in rhizosphere soil, loosely attached soil and bulk soil for fresh and aged

284

CuO NP treatments. For the CaCl2 extraction in both fresh and aged CuO NP treatments (Figure 4a, b),

285

the extractability of Cu in the rhizosphere soil was significantly higher than the extractability of Cu in the

286

loosely attached soil or bulk soil (ANOVA test, P≤0.05).

287

There were no statistically significant differences among DTPA extractable Cu measurements

288

from rhizosphere soil, loosely attached soil and bulk soil in the freshly amended CuO NP treatment

289

(Figure 4c). However, the DTPA extractable Cu in the rhizosphere soil in the aged CuO treatment was

290

significantly lower than the DTPA extractable Cu in bulk soil, but similar to that measured in loosely

291

attached soil (Figure 4d). In control experiments (Na2SO4), the CaCl2 extractable Cu was below the

292

detection limit (0.08mg/kg in soil, 4ug/L for the diluted samples) in all soil samples.

293

Aging increased the concentration of CaCl2 extractable Cu and DTPA extractable Cu in loosely

294

attached soil and bulk soil, and increased the concentration of DTPA extractable Cu in rhizosphere soil (t

295

test, P0.05).

11 ACS Paragon Plus Environment

Environmental Science & Technology

296 297 298

Figure 4. CaCl2 and DTPA extractable Cu in fresh (left side) and aged (right side) CuO NP amended

299

rhizosphere soil, loosely attached soil and bulk soils. Error bars show ± 1 SD. Capital letters indicate

300

significant differences between groups (One way ANOVA test followed by Fisher’s LSD test for multiple

301

comparison, P≤0.05).

302 303

Soil pH in bulk soil, rhizosphere soil and loosely attached soil. For all CuO NP treatments and the

304

control treatment (no addition), the pH of the rhizosphere soils was significantly higher than the pH of the

305

loosely attached and bulk soils (Figure 5a, b and c). In freshly amended CuO NP treatments and control

306

treatments, the pH of the loosely attached soils were not statistically significantly different than the pH in

307

the bulk soils. However, in aged CuO NP treatments, the pH of the loosely attached soil was statistically

308

significantly higher than the pH in bulk soil. In bulk soil, the pH was the highest in freshly amended CuO

12 ACS Paragon Plus Environment

Page 12 of 24

Page 13 of 24

Environmental Science & Technology

309

NP treatments, followed by aged CuO NP treatment, followed by the control treatment, followed by the

310

aged CuSO4 treatment, with freshly amended CuSO4 treatment having the lowest soil pH (Figure 5d).

311

312

313 314 315

Figure 5. Mean ± SD of soil pH (measured using CaCl2 extraction) in rhizosphere soil, loosely attached

316

soil and bulk soil in a) soil freshly amended with CuO NP, b) aged CuO NP treatment c) control soil, and;

317

d) Comparison of pH of bulk soil among all treatments. Capital letters indicate significant differences

318

(ANOVA test followed by Fisher’s LSD test for multiple comparison, P≤0.05).

319 320

Discussion

321

CuO NP dissolution is linked to toxicity. Compared to Cu ions, the dynamic dissolution process of CuO

322

NP in soil led to a very different Cu exposure profile for plants. At the end of the two growth periods, the

323

DTPA-extracted Cu concentrations in CuO NP treatments were similar or even higher than in the CuSO4 13 ACS Paragon Plus Environment

Environmental Science & Technology

324

treatment. However, a decreasing trend in DTPA extractable Cu on CuSO4 treatments during the two

325

plant growth periods was observed. This decrease can be attributed to the soil-organic matter interactions,

326

solid-state diffusion of Cu ions into iron minerals or metal (co)precipitates52–54. Conversely, an increase in

327

DTPA-extractable Cu over time was shown in CuO NP treatments (fresh treatment and aged treatment)

328

during the two plant growth periods and the aging period. This can be attributed to the dissolution of CuO

329

NP11,12. Thus, the plants in the freshly amended CuO NP soil were exposed to lower amounts of labile Cu

330

compared to either the two CuSO4 treatments or the aged CuO NP treatment. These findings suggest that

331

when evaluating the chemical availability or toxic effect of metal-based NPs in soil, single-time point

332

chemical extractions at the end of the plant growth period cannot capture the dissolution process of NPs

333

in soil, and thus may fail to predict the toxicity or bioavailability of NPs11,12,55. Considering that it is not

334

feasible to uniformly dose Cu ions into soil over time to precisely mimic the dosing rate from NP

335

dissolution, toxicity studies with soluble NPs should measure the dissolution rate in soil and monitor the

336

behavior of soluble ions in soil, and interpret their results in light of the different dosing conditions that

337

manifest.

338

A significantly higher toxicity in CuSO4 treatments compared to the fresh CuO NP treatment is

339

explained by the higher exposure of roots to labile Cu species, even though the CuO NP treatment had a

340

higher total Cu concentration. Also, dissolution of CuO NPs over time gradually increased the available

341

Cu in soil, leading to higher toxicity in the aged treatment. The opposite has been observed with CuSO4,

342

where the available Cu in soil decreased over time, leading to lower toxicity to the plants in the aged

343

treatment. The effects of time on toxicity of CuO NP and CuSO4 have already been observed19. The

344

authors attributed this to the dissolution behavior of CuO NP, although without quantification. Here, we

345

clearly showed that in order to correlate the chemical availability of CuO NPs with toxicity, the

346

dissolution kinetics, i.e. predicting the total Cu released to soil during the growth period, should be

347

considered. The dissolution kinetics can be modeled as first-order dissolution, with the rate constant fit to

348

the extractable Cu over time11, and the total amount of Cu ion released from CuO NPs can be estimated

349

by integrating the expression relating the change of extractable Cu over time. This observation is relevant

350

to NP formulations of fungicides and micronutrients, so the release rate of the active ingredients can be

351

better timed to the plant’s needs.

352 353

CaCl2 extractable Cu correlates with toxicity of CuO NP to wheat. Although DTPA extractable Cu

354

gives a better indication of CuO NP dissolution because it extracts most of the Cu species in soil, CaCl2

355

extractable Cu is better for correlating toxicity, since it measures dissolved Cu in pore water that can

356

directly interact with plant roots. DTPA extraction would predict the toxicity of the aged CuO NP to be

357

higher than the CuSO4 treatment (Figure 1 a, b). However, this was not the case. The aged CuO NP had 14 ACS Paragon Plus Environment

Page 14 of 24

Page 15 of 24

Environmental Science & Technology

358

lower toxicity compared to both the aged and the fresh CuSO4 treatment, indicating that the DTPA

359

extraction cannot accurately predict toxicity for the CuO NPs. The CaCl2 extraction ranked them correctly

360

(Figure 1d). Considering that the extractable Cu in CuO NP amended soil increased over time while the

361

extractable Cu in CuSO4 amended soil decreased over time in Lufa 2.2 soil (SI, Figure S1), the wheat

362

plants were exposed to a lower overall ‘readily available’ Cu (i.e. Concentration x time) in CuO NP

363

treatments compared to the CuSO4 treatments. The lower CaCl2 extractable Cu in aged CuO NP treatment

364

is a result of higher soil pH in aged CuO NP treatment compared to the fresh and aged CuSO4 treatment.

365

Higher soil pH has been previously shown to lower Cu concentration in soil pore water 27,56.

366 367

Root-associated CuO NP modulates toxicity. In the freshly amended CuO NP soil, although being

368

exposed to a lower concentration of labile Cu, the roots of Triticum aestivum were actually exposed to

369

higher total Cu concentration (Figure S7) than the other treatments. This is mainly due to CuO NPs'

370

association with plant roots (Figure 3b). This exposure to CuO NPs did not lead to any detected toxic

371

effects, indicating a low or de minimis level of toxic effects from the particle itself over the 14d growth

372

period.

373 374

Root exudates affect CuO NP dissolution and availability. The increase in the pH of rhizosphere soil

375

compared to bulk soil in our study indicates that the rhizosphere region was indeed influenced by the

376

plant roots. Excretion of organic acid (dissociated ions) by plant roots , nitrogen uptake and ionic

377

exchanges by plant roots may explain the higher pH of the rhizosphere soil compared to the pH of bulk

378

soi39,40,57,58. The observed pH change in rhizosphere soil was not likely a result from the presence of CuO

379

NP, as similar pH changes occurred with both the fresh CuO NP treatment and the negative control

380

treatment (0.4pH unit, ANOVA test, P>0.05). However, in the aged CuO NP treatment, the pH increase

381

was higher (0.6pH unit, ANOVA test, P