Optimal Soil Eh, pH, and Water Management for ... - ACS Publications

Mar 21, 2016 - (31) Increasing soil pH from 6.1 to 6.9 has been reported to decrease Cd concentrations in rice grains in both upland and flooded culti...
0 downloads 0 Views 455KB Size
Subscriber access provided by The Chinese University of Hong Kong

Article

Optimal soil Eh, pH, and water management for simultaneously minimizing arsenic and cadmium concentrations in rice grains Toshimitsu Honma, Hirotomo Ohba, Ayako Kaneko-Kadokura, Tomoyuki Makino, Ken Nakamura, and Hidetaka Katou Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05424 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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 27

Environmental Science & Technology

1

Optimal soil Eh, pH, and water management for

2

simultaneously minimizing arsenic and cadmium

3

concentrations in rice grains

4

Toshimitsu Honma,*,† Hirotomo Ohba,† Ayako Kaneko-Kadokura,†,‡ Tomoyuki Makino,§

5

Ken Nakamura,§ and Hidetaka Katou§

6



Niigata Agricultural Research Institute, Nagaoka, Niigata 940-0826, Japan

7



Ojiya Branch, Nagaoka Agriculture Extension Center, Ojiya, Niigata 947-0028, Japan

8

§

National Institute for Agro-Environmental Sciences, Tsukuba, Ibaraki 305-8604, Japan

9

Abstract: Arsenic (As) and cadmium (Cd) concentrations in rice grains are a human health

10

concern. We conducted field experiments to investigate optimal conditions of Eh and pH in soil

11

for simultaneously decreasing As and Cd accumulation in rice. Water managements in the

12

experiments, which included continuous flooding, and intermittent irrigation with different

13

intervals after midseason drainage, exerted striking effects on the dissolved As and Cd

14

concentrations in soil through changes in Eh, pH, and dissolved Fe(II) concentrations in the soil.

15

Intermittent irrigation with three-day flooding and five-day drainage was found to be effective

16

for simultaneously decreasing the accumulation of As and Cd in grain. The grain As and Cd

17

concentrations were, respectively, linearly related to the average dissolved As and Cd

18

concentrations during the 3 weeks after heading. We propose a new indicator for expressing the

ACS Paragon Plus Environment

1

Environmental Science & Technology

Page 2 of 27

19

degree to which a decrease in the dissolved As or Cd concentration is compromised by the

20

increase in the other. For minimizing the trade-off relationship between As and Cd in rice grains

21

in the field investigated, water management strategies should target the realization of optimal soil

22

Eh of –73 mV and pH of 6.2 during the 3 weeks after heading.

23

Introduction

24

Rice (Oryza sativa L.), a staple food for half of the world’s human population, is a major

25

source of arsenic (As) and cadmium (Cd), particularly in Asia and other countries.1,2 The

26

International Agency for Research on Cancer3 has identified inorganic arsenic, i.e., arsenite

27

(As(III)) and arsenate (As(V)), as human carcinogens. The As concentration in rice grains is

28

approximately 10 times higher than those in other cereals such as wheat and barley,4 as a result

29

of anaerobic conditions in rice-growing soils. Since the predominant As species in rice grain is

30

inorganic As, minimizing the intake of As from rice in the diet is an important health issue.

31

Recently, the Codex Alimentarius Commission5 adopted a maximum level of inorganic As of 0.2

32

mg kg–1 in polished rice grain. According to a national survey6, the average inorganic As

33

concentration in polished rice grain for major rice cultivars produced in Japan is 0.12 mg kg–1,

34

with a maximum value of 0.26 mg kg–1. Moreover, 2.2% of grain samples from supposedly

35

uncontaminated paddy fields have been shown to contain over 0.2 mg kg–1 of inorganic As in

36

polished rice grain. Another survey7 has estimated that rice is the largest contributor to the total

37

dietary intake of inorganic As in China, representing 60% of inorganic As intake.

38

Arsenic may be present in soil in various chemical forms, including inorganic As and

39

methylated As species such as monomethylarsonic acid (MMA) and dimethylarsinic acid

40

(DMA). Inorganic As(III) is the predominant form of As present in soil and taken up by plants,

41

and its transport to xylem is mediated by silicate transporters.8,9 Small quantities of methylated

ACS Paragon Plus Environment

2

Page 3 of 27

Environmental Science & Technology

42

As species have also been found in some soils, most likely from either microbial methylation or

43

prior application of As-based pesticides/herbicides.10 The availability of As in soil depends on

44

Eh and pH.11,12 In paddy fields, flooding and drainage cycles have a major impact on As

45

dynamics in soil solutions and on As bioavailability to rice plants. 13–15 When paddy soil is under

46

aerobic conditions as a result of field drainage, As is less mobile. This is because As(V), the

47

predominant inorganic As species under oxidizing conditions, is strongly sorbed to mineral soil

48

components such as Fe and Al (hydr)oxides16. When anaerobic conditions develop in flooded

49

soils, As is released from the solid phase into the aqueous solution phase through reductive

50

dissolution of Fe (hydr)oxides and reduction of As from As(V) to As(III), which has increased

51

solubility compared with As(V).13 Release of As from Fe (hydr)oxides is promoted upon

52

decrease of Eh below –100 mV,17–19 and Eh of +100 mV is considered to be the point below

53

which reduction of As(V) to As(III) occurs at neutral pH.20 Solubilized As is vigorously taken up

54

by rice plants and accumulated in grains under flooded conditions.

55

Cadmium contamination in rice grains also causes serious damage to human health, such as

56

kidney damage, osteoporosis, and cancer.21 For example, Itai-itai disease was identified in the

57

1950s in Japan among people eating rice grown in paddy fields polluted with high levels of Cd

58

from current or abandoned mines.22 A health-based guidance value for Cd of 25 µg kg–1

59

bodyweight per month was established by the Joint FAO/WHO Expert Committee on Food

60

Additives,23 and a maximum concentration of 0.4 mg kg–1 for Cd in polished rice grain has been

61

adopted by the Codex Alimentarius Commission.24 In Japan, a survey25 revealed that

62

approximately 40% of Cd intake from food was from rice consumption and that 0.3% of rice

63

grains contain Cd over the Codex maximum level.

ACS Paragon Plus Environment

3

Environmental Science & Technology

Page 4 of 27

64

Cadmium solubility and bioavailability in soil mainly depend on soil redox potential (Eh) and

65

pH. The predominant form of Cd taken up by rice plants is Cd2+, and the uptake is suppressed

66

under reduced conditions.26,27 Based on thermodynamic considerations, Ito and Iimura28 ascribed

67

the decrease in the plant-available Cd to sulfide formation, and the presence of CdS in a reduced

68

paddy soil contaminated with Cd has been confirmed by the X-ray absorption spectroscopy.29

69

Drainage of paddy fields results in aerobic conditions that enhance the availability of Cd for

70

plant uptake through oxidation of CdS to Cd2+ and SO42–, which has a much higher solubility

71

than CdS formed under flooding conditions.30 Soil pH also affects Cd solubility through the

72

influence of pH-dependent surface charge on the affinity of Cd for sorption sites.31 Increasing

73

soil pH from 6.1 to 6.9 has been reported to decrease Cd concentrations in rice grains in both

74

upland and flooded cultivations.32

75

It has thus been established that water management greatly impacts the bioavailability of As

76

and Cd in paddy soils through changes in soil Eh. Widely adopted water management in Japan

77

follows a sequence of flooding, mid-season drainage, intermittent irrigation around the heading

78

stage, and drainage before harvest.33 In paddy fields contaminated with Cd, flooding for a total of

79

six weeks before and after heading has been recommended for decreasing the Cd concentration

80

in rice grain.34 The anaerobic conditions, however, raise As solubility. Arao et al.14 investigated

81

the effects of water managements on As and Cd accumulation in rice grains, and found a trade-

82

off relationship between As and Cd bioavailability. Maintaining aerobic conditions after the

83

flowering stage significantly decreases As accumulation in rice straw and grains compared with

84

rice grown under flooded conditions.35 However, aerobic conditions during the flowering stage

85

lead to accumulation of dissolved Cd in soil. Yamane17 reported that decreasing Cd and As

86

uptake could be achieved by maintaining aerobic conditions in the paddy soil before heading and

ACS Paragon Plus Environment

4

Page 5 of 27

Environmental Science & Technology

87

switching to anaerobic conditions after heading. Arao et al.14 found that the most sensitive period

88

for As and Cd accumulation was around the heading stage, particularly 3 weeks after heading.

89

Hu et al.36 stressed the importance of water management in decreasing As and Cd bioavailability

90

and uptake by rice, but no information was provided as to the Eh and pH values for decreasing

91

As and Cd simultaneously. Optimization of the trade-off relationship between As and Cd in soil

92

solutions and in rice grain is needed for paddy fields at risk of exceeding the Codex maximum

93

levels of As and Cd. To our knowledge, however, the effects of irrigation intervals under field

94

conditions on As and Cd uptake by rice and As speciation in rice grain have not been

95

investigated.

96

The main objectives of the present study were (i) to investigate the effects of different water

97

management strategies on the dissolved As and Cd concentrations in soil, As and Cd uptake by

98

rice, and As speciation in rice grains; and (ii) to identify optimal soil Eh and pH for minimizing

99

the trade-off relationship between As and Cd. Rice grain yields and quality under different water

100

managements were also investigated.

101

Materials and Methods

102

Experimental Field

103

Field experiments were conducted in 2013 in a paddy field developed on an alluvial plain in

104

central Japan. The soil was classified as a Typic Hydraquent by US Soil Taxonomy.37 The

105

topsoil had total carbon and nitrogen contents of 16.2 g kg–1 and 1.53 g kg–1, respectively, 1 M

106

HCl-extractable As concentration of 2.49 mg kg–1, and 0.1 M HCl-extractable Cd concentration

107

of 0.84 mg kg–1, with a textural composition of 52% sand (0.02–2 mm), 30% silt (2 µm–0.02

108

mm), and 18% clay (< 2 µm). The soil pH measured at a soil:water ratio of 1:2.5 (w/w) was 5.8.

ACS Paragon Plus Environment

5

Environmental Science & Technology

109

Page 6 of 27

Field Experiments

110

Seedlings of rice (Oryza sativa L. cv. Koshihikari) were transplanted on May 13. The rice

111

plants were grown under flooded conditions for 36 days, followed by a 14 days of midseason

112

drainage until July 2. Thereafter, five different water managements were practiced for a total of 6

113

weeks, 3 weeks before heading and 3 weeks after heading. The water managements included (1)

114

Flooded; (2) Int-F3D1, in which intermittent irrigation was repeated every 4 days, with 3-day

115

flooding followed by 1-day drainage; (3) Int-F3D3, in which intermittent irrigation was repeated

116

every 6 days, with 3-day flooding followed by 3-day drainage; (4) Int-F3D5, in which

117

intermittent irrigation was repeated every 8 days with 3-day flooding followed by 5-day

118

drainage; and (5) Rainfed, in which irrigation was not practiced. After the period with different

119

water managements was over on August 26, intermittent irrigation with indeterminate intervals

120

was conducted, with the final drainage for harvest on September 5. Each treatment had three

121

replicated plots in the field, with each plot having a size of 6 m × 12 m. The rice was harvested

122

on September 19.

123

Plant and Soil Analysis

124

Soil samples were collected on April 18 from the topsoil layer (0–15 cm depth) of the

125

experimental plots for determination of chemical properties. For the rice plant analysis, four rice

126

hills were sampled from each plot at the maximum tiller number stage (July 14), heading stage

127

(August 7), ripening stage (September 6), and maturing stage (September 17). The rice grains

128

removed from the shoots were air-dried, and those remaining on the 1.85 mm-sieve were used

129

for analysis.

130

Soil redox potential (Eh) was measured in duplicate with an Eh meter and platinum electrodes

131

installed at 15 cm depth in each plot, and soil pH was measured using a glass electrode. Soil

ACS Paragon Plus Environment

6

Page 7 of 27

Environmental Science & Technology

132

solutions were sampled at the same depth from mid-June to early September at intervals of 1–2

133

weeks for determination of dissolved As, Cd, and Fe(II) concentrations. Dissolved Fe(II) was

134

determined colorimetrically from the absorbance due to the Fe(II) 2,2′-bipyridyl complex.38

135

Total As and Cd concentrations in the soil solutions were determined by flow injection (FI)-

136

inductively coupled plasma mass spectroscopy (ICP-MS) according to Baba et al.39 with minor

137

modifications.

138

As(III), As(V), MMA(V), and DMA(V) in the rice grains were determined by ICP-MS

139

according to the methods of Nishimura et al.40 and Baba et al.39 with minor modifications, after

140

digestion with 0.15 M HNO3 and separation by high-performance liquid chromatography. Total

141

As and Cd concentrations in rice samples and shoot samples were determined by FI-ICP-MS

142

after digestion with HNO3 and H2SO4, and with HNO3–H2SO4 and HClO4, respectively. Grain

143

qualities, such as the percentages of perfect grains and immature grains, were determined with a

144

grain discriminator. Detailed procedures of the plant and soil analysis are given in the Supporting

145

Information.

146

Statistical analysis

147

Statistical analysis was performed using Statcel 2 software.41 Multiple comparisons between

148

treatments were made by Tukey-Kramer test.

149

Results

150

Soil Eh, pH, and Dissolved As, Cd, and Fe(II) in Soil Solution

151

Soil Eh, pH, and dissolved As, Cd, and Fe(II) concentrations were strongly affected by water

152

management practices (Figure 1). Soil Eh, which ranged from –200 to –150 mV during the

153

flooded period after transplanting, drastically rose upon midseason drainage. Different water

ACS Paragon Plus Environment

7

Environmental Science & Technology

Page 8 of 27

154

managements during pre-heading 3 weeks and post-heading 3 weeks caused different responses

155

of soil Eh. Soil Eh was kept low, around –200 to –150 mV, in the Flooded and Int-F3D1 plots,

156

whereas the Eh fluctuated between –150 and 0 mV in the Int-F3D3 and Int-F3D5 plots. Even

157

higher soil Eh was observed in the Rainfed plot throughout this period (Figure 1A). Soil pH

158

decreased during midseason drainage period and then increased until the heading stage in all

159

plots. After the heading stage, soil pH in the Flooded, Int-F3D1, and Int-F3D3 plots was

160

maintained at pH 6.3–6.5, whereas in the Int-F3D5 and Rainfed plots, soil pH decreased from pH

161

6.4 at the heading stage to 5.7 at harvest (Figure 1B).

162

Midseason drainage resulted in an exhaustive decrease of dissolved Fe(II), which had

163

increased during the flooding period, to a negligible level. During the pre-heading 3 weeks and

164

post-heading 3 weeks, the dissolved Fe(II) in the Flooded and Int-F3D1 plots increased linearly

165

with time. In the Int-F3D3 and Int-F3D5 plots, the increases in the dissolved Fe(II) before the

166

heading stage were followed by the decreases after the heading stage. Dissolved Fe(II)

167

concentration in the Rainfed plot remained much lower than in the other plots (Figure 1C).

ACS Paragon Plus Environment

8

Environmental Science & Technology

300

(A) Soil Eh (mV)

200 100

Flooded Int-F3D1 Int-F3D3 Int-F3D5 Rainfed

0

-100 -200

200

Total dissolved As (µg L-1)

Page 9 of 27

-300

(D)

150

100

50

0 5/31 6/15 6/30 7/15 7/30 8/14 8/29 9/13

5/31 6/15 6/30 7/15 7/30 8/14 8/29 9/13

7.0

1.2

(B)

(E)

Dissolved Cd (µg L-1)

Soil pH

1.0 6.5

6.0

5.5

0.6 0.4 0.2 0.0

5/31 6/15 6/30 7/15 7/30 8/14 8/29 9/13 250

Dissolved Fe(II) (mg L-1)

0.8

(C)

5/31 6/15 6/30 7/15 7/30 8/14 8/29 9/13

Midseason drainage Water management period

200 150 100 50 0

168

5/31 6/15 6/30 7/15 7/30 8/14 8/29 9/13

169

Figure 1. Changes in (A) soil Eh, (B) soil pH, (C) dissolved Fe(II), (D) dissolved As and (E)

170

dissolved Cd concentrations among different water management plots. The plots were flooded,

171

intermittently irrigated with different irrigation intervals, or rainfed for pre-heading 3 weeks and

172

post-heading 3 weeks.

173

Dissolved As concentration showed changes similar to those in the dissolved Fe(II)

174

concentration. Midseason drainage caused a dramatic decrease in the dissolved As followed by a

175

steady increase with time in the Flooded and Int-F3D1 plots. In the Int-F3D3 plot, similar

176

increase in the dissolved As until heading was followed by a decrease thereafter. Dissolved As in

177

the Int-F3D5 plot also showed an increase until heading, but the increase was slower and the

ACS Paragon Plus Environment

9

Environmental Science & Technology

Page 10 of 27

178

concentration after heading was very low. In the Rainfed plot, the dissolved As was the lowest

179

among all plots during the water management period (Figure 1D). Dissolved Cd concentration

180

increased remarkably upon midseason drainage and then showed rapid decreases following re-

181

irrigation. During the water management period, the dissolved Cd concentration remained low in

182

the Flooded, Int-F3D1, and Int-F3D3 plots, whereas in the Int-F3D5 and Rainfed plots, temporal

183

increases in the dissolved Cd were observed (Figure 1E).

184

Relationships among soil Eh, pH, and dissolved As, Cd, and Fe(II) concentrations Dissolved As and Cd concentrations during the growth period ranged from 0 to 185 µg L–1 and

186

0 to 1.1 µg L–1, respectively. The relationships between the dissolved As and Cd concentrations

187

are shown in Figure 2. Many of the solution samples had a combination of high As and low Cd

188

concentrations or low As and high Cd concentrations. Notably, however, some solutions had a

189

combination of low As and low Cd concentrations. Total dissolved As (µg L-1)

185

200

Flooded Int-F3D1 Int-F3D3 Int-F3D5 Rainfed

150 100 50 0 0.0

190 191

0.2

0.4 0.6 0.8 1.0 Dissolved Cd (µg L-1)

1.2

Figure 2. Relationship between total dissolved As and Cd concentrations in soil solution.

192

Figure 3 shows the relations of total dissolved As and Cd concentrations to soil Eh, pH and

193

dissolved Fe(II) concentrations. The total dissolved As decreased sharply with the increase in

194

soil Eh, and when soil Eh was above –100 mV, only low concentrations were observed, with an

ACS Paragon Plus Environment

10

Page 11 of 27

Environmental Science & Technology

196

dissolved Fe(II) and Eh; the dissolved Fe(II) concentration quickly decreased with the increase in

197

soil Eh, and the concentrations were low when the Eh was above 0 mV (Figure S1 in the

198

Supporting Information). A distinct rise in the dissolved As was observed with the increase in

199

soil pH, particularly when the pH was above 6.3 (Figure 3(B)). Similar dependences on Eh

200

resulted in a positive correlation between the dissolved As and Fe(II) concentrations (Figure

201

3(C)). In contrast, the dissolved Cd concentration increased with soil Eh and was negligible at Eh

202

below –150 mV (Figure 3(D)). The dissolved Cd was appreciable only when the soil pH was low

203

and was negligible when the pH was above 6.3 (Figure 3(E)). The relationship between the

204

dissolved Cd and Fe(II) concentrations (Figure 3(F)) was similar to that found between the

205

dissolved Cd and As. The dissolved Cd concentration was very low when the dissolved Fe(II)

206

concentration was above 50 mg L–1.

[As] = 5.84exp(-0.0145 Eh)

150

Flooded Int-F3D1 Int-F3D3 Int-F3D5 Rainfed

100 50 0

200

100 50

[Cd] =

2.00×10-6(Eh)2 + 0.2199

0.8 0.6 0.4 0.2

+ 0.0015 Eh

5.8

6.0 6.2 Soil pH

-300 -200 -100 0 100 200 300 Soil Eh (mV)

[As] = 0.0024[Fe(II)]2 + 0.3125 [Fe(II)] + 3.5886

150 100 50

0

6.6 1.2

(E) [Cd] =

1.0

6.4

5.34×1011exp(-4.8

0.8 0.6 0.4 0.2

pH)

50 100 150 Dissolved Fe(II) (mg L-1)

200

(F)

1.0

[Cd] = 0.35 [Fe(II)]-0.518

0.8 0.6 0.4 0.2 0.0

0.0

0.0

207

5.6 1.2

(D)

(C)

0

0

Dissolved Cd (µg L-1)

Dissolved Cd (µg L-1)

1.0

[As]= 3.56×10-12exp(4.72 pH)

150

-300 -200 -100 0 100 200 300 Soil Eh (mV) 1.2

200

(B)

Total dissolved As (µg L-1)

(A)

DissolvedCd (µg L-1)

200

Total dissolved As (µg L-1)

average (± SD) of 7.7 (± 7.5) µg L–1 (Figure 3(A)). A similar relationship was found between

Total dissolved As (µg L-1)

195

5.6

5.8

6.0 6.2 Soil pH

6.4

6.6

0

50 100 150 Dissolved Fe(II) (mg L-1)

200

208

Figure 3. Relationships between the total dissolved As concentration and (A) soil Eh, (B) soil

209

pH, and (C) dissolved Fe(II) concentration (upper panels), and between the dissolved Cd

ACS Paragon Plus Environment

11

Environmental Science & Technology

Page 12 of 27

210

concentration and (D) soil Eh, (E) soil pH, and (F) dissolved Fe(II) concentration (lower panels)

211

in different water management plots.

212

As and Cd concentrations in shoots and rice grains

213

The As and Cd concentrations in shoots among the different water managements are compared

214

in Figure 4. The shoot As concentrations, which did not differ significantly among the plots 3

215

weeks before heading (July 14), rapidly increased in the Flooded, Int-F3D1, and Int-F3D3 plots.

216

Four weeks after heading (September 6), the shoot As concentrations in the Flooded and Int-

217

F3D1 plots were significantly higher than those in the more aerobic Int-F3D5 and Rainfed plots.

218

Differences in the shoot Cd concentrations were insignificant until heading among the plots. Low

219

or decreasing Cd concentrations were observed in the Flooded, Int-F3D1, and Int-F3D3 plots

220

until harvest, whereas a rapid increase in the Cd concentration was observed in the Int-F3D5 and

221

Rainfed plots after heading. 20

Int-F3D3

A

Int-F3D5

A

Rainfed

10

A AA

5

a

aaa

bc AB B

a

c

a

Rainfed B

0.4

a A

b b

b

b

AAA

b

A bbb

CC

C

0.0 7/14

222

Int-F3D5

0.6

0.2

0

Int-F3D3

0.8

BB

A

Int-F3D1

A ab

Cd (mg kg-1)

As (mg kg-1)

1.0

a

(B)

Flooded

a

Int-F3D1

15

1.2

(A)

Flooded

8/6

9/6

9/17

7/14

8/6

9/6

9/17

223

Figure 4. Comparison of (A) As and (B) Cd concentrations in shoots among different water

224

management plots during the growth period. Multiple comparisons between different plots were

225

made by Tukey-Kramer test (P < 0.05). Values with the same letters were not significantly

226

different.

ACS Paragon Plus Environment

12

Page 13 of 27

Environmental Science & Technology

227

Figure 5 shows the As and Cd concentrations and As speciation in unpolished rice grains. As

228

and Cd concentrations in grains were strongly affected by the water management. The total As,

229

As(III), As(V), and DMA were in the order of Flooded ≈ Int-F3D1 > Int-F3D3 > Int-F3D5 >

230

Rainfed, with the concentrations in the Flooded plot 2.9–10.1 times as high as those in the

231

Rainfed plot. The predominant As species was inorganic As, particularly As(III). The inorganic

232

As / total As ratio was lower in the Flooded plot because of the increase in DMA and MMA

233

concentrations. The Cd concentrations in grains were higher in the aerobic plots, and were in the

234

order of Rainfed > Int-F3D5 > Int-F3D3 > Int-F3D1 > Flooded, with the concentration in the

235

Rainfed plot 18.5 times higher than that in the Flooded plot. Details of the results of statistical

236

analyses are given in Table S1 in the Supporting Information. As and Cd in rice grain (mg kg-1)

0.7 Cd Unrecovered As MMA DMA As(V) As(III)

0.6 0.5 0.4 0.3 0.2 0.1 0

237

Flood

Int-F3D1 Int-F3D3 Int-F3D5 Rainfed

238

Figure 5. Effects of different water managements on As and Cd concentrations and As

239

speciation in rice grains. DMA, dimethylarsinic acid; MMA, monomethylarsonic acid.

240

Growth of rice plant, grain yield, and grain quality

241

Influences of water management strategies on the growth of rice plants and grain yield were

242

limited as compared with those on As and Cd concentrations (Table S2 in the Supporting

243

Information). Except for the Rainfed plot, neither the unpolished rice yield nor the growth of rice

244

plants, such as measured by the culm length and the straw yield, was statistically different among

ACS Paragon Plus Environment

13

Environmental Science & Technology

Page 14 of 27

the plots. Water managements did not affect the grain quality either; there were no significant

246

differences among the plots in the percentages of perfect grains or immature grains (Table S3 in

247

the Supporting Information). These results show that the As and Cd concentrations in rice grains

248

may be reduced by appropriate water managements without affecting the growth of rice plants

249

and grain quality. Rice grain As (mg kg-1)

245

(A)

0.8

T-As iAs

y = 0.0052 x + 0.147 R² = 0.9997

0.6 0.4 0.2

y = 0.00404 x + 0.135 R² = 0.9961

0.0

Rice grain Cd (mg kg-1)

0 0.20

20

40 60 80 100 Total dissolved As (µg L-1)

120

(B)

0.15 0.10 y = 0.692 x + 0.0099 R² = 0.9617

0.05 0.00 0.0

250

0.1 0.2 Dissolved Cd (µg L-1)

0.3

251

Figure 6. Relationships (A) between rice grain As concentrations and the total dissolved As

252

concentration averaged over post-heading three weeks, and (B) between rice grain Cd

253

concentration and the dissolved Cd concentration averaged over post-heading three weeks.

254

Sampling of soil solutions at 15 cm depth were conducted four times during the post-heading

255

three weeks. Symbols represent the average of triplicate measurements for each water

256

management plot. T-As = total As; iAs = inorganic As.

ACS Paragon Plus Environment

14

Page 15 of 27

257

Environmental Science & Technology

Relationship between soil solution and rice grain concentrations of As and Cd

258

Figure 6 shows the relations of As and Cd concentrations in rice grains to the total As and Cd

259

concentrations in the soil solutions collected from 15 cm depth during the 3-week period after

260

heading. The dissolved concentrations are the average for each water management plot from four

261

sampling times during the period. There were positive linear correlations between rice grain

262

concentrations and total dissolved As and Cd concentrations in soil solutions, with high

263

coefficients of determination (R2) of 0.9997, 0.9961, and 0.9617 for total As, inorganic As, and

264

Cd, respectively.

265

Discussion

266

Effects of different water management strategies on As and Cd uptake

267

In the present study, we investigated the effects of periodic intermittent irrigation during pre-

268

heading 3 weeks and post-heading 3 weeks on As and Cd concentrations in grains. The Int-F3D3

269

irrigation, repeated every 6 days with 3-day flooding and 3-day drainage, reduced the grain

270

inorganic As concentration by 20% relative to the Flooded plot, and reduced the grain Cd

271

concentration by 89% relative the Rainfed plot. This was not satisfactory in view of the relatively

272

small decrease in the grain inorganic As. In the Int-F3D1 irrigation, repeated every 4 days with

273

3-day flooding and 1-day drainage, the grain inorganic As concentrations were much the same as

274

in the Flooded plot. The Int-F3D5 irrigation, with 3-day flooding and 5-day drainage, was the

275

most preferable for simultaneous reduction of inorganic As and Cd in grains, and reduced the

276

grain inorganic As concentration by 62% relative to the Flooded plot and the grain Cd

277

concentration by 56% relative to the Rainfed plot. We also note rapid increases in the dissolved

278

Cd concentration observed upon drainage during the water management periods, which stresses

279

the importance of avoiding prolonged drainage leading to excessively aerobic conditions. In

ACS Paragon Plus Environment

15

Environmental Science & Technology

Page 16 of 27

280

practical situations, the efficiency of water management practices may be affected by the size of

281

paddy fields, owing to possible delay in drainage. In view of dynamic nature of the processes

282

involved, further research is needed to confirm whether the optimal irrigation interval identified

283

in this study is widely applicable across different soil types (e.g., different organic matter

284

contents), weather conditions, and As and Cd concentrations in soils.

285

Optimal soil Eh and pH for simultaneously decreasing grain As and Cd concentrations

286

Elevated dissolved As concentrations were observed in the present study as the Eh decreased

287

below 0 or –100 mV, accompanied by parallel increases in the dissolved Fe(II) concentration

288

(Figure 3). Striking increases in dissolved As were found when the soil pH was above 6.2–6.3.

289

These results are in line with earlier findings that release of As from Fe (hydr)oxides was

290

promoted upon decrease of Eh below –100 mV,17–19 and that the solid/solution distribution ratio

291

for As(III) and As(V) decreased dramatically with increasing pH from 5.5 to 7.0 and above.19

292

From the relations of the dissolved As and Cd concentrations to Eh and pH (Figure 3), a

293

threshold Eh of –100 to 0 mV and pH of 6.2–6.3 and a threshold Eh of –100 mV and pH of 6.3–

294

6.4 were identified for the solubilization of As and Cd, respectively, in soil.

295

It is of interest to find optimal Eh and pH for avoiding increased uptake of As and Cd by rice

296

in paddy fields suffering from dual contamination with As and Cd. As shown in Figure 2, the

297

dissolved As and Cd concentrations were simultaneously low under certain conditions. Based on

298

the As and Cd concentrations in soil solutions extracted from soil cores, Nakamura and Katou33

299

argued that while a high dissolved As concentration tended to coincide with a low dissolved Cd

300

concentration and vice versa, a decrease in the concentration of either As or Cd was not

301

necessarily accompanied by an increase in the other, suggesting that conditions exist where the

302

As and Cd concentrations are simultaneously low. The results in the present study corroborate

ACS Paragon Plus Environment

16

Page 17 of 27

Environmental Science & Technology

304

by keeping the dissolved As and Cd concentrations at low levels during the post-heading 3-week

305

period because the rice grain concentrations were most sensitive to dissolved As and Cd during

306

this time (Figure 6).

307

1.0 0.8 Optimum Eh = –73 mV

0.6 0.4 0.2

Trade-off value [As] / [Asmax] [Cd] / [Cdmax]

0.0 -300 -200 -100 0 100 200 300 Soil Eh (mV)

[As]/[Asmax], [Cd]/[Cdmax], Trade-off value

this argument and suggest that simultaneous reduction of As and Cd uptake by rice is achievable

[As]/[Asmax], [Cd]/[Cdmax], Trade-off value

303

1.0 0.8 Optimum pH = 6.2

0.6 0.4 0.2 0.0 5.6 5.8

6.0 6.2 6.4 Soil pH

6.6 6.8

308

Figure 7. Relations of the trade-off value to soil Eh and pH. The trade-off value is defined as

309

the sum of [As]/[Asmax] and [Cd]/[Cdmax], where [As]/[Asmax] and [Cd]/[Cdmax] are the total

310

dissolved As and Cd concentrations normalized with respect to their maximum values expected

311

across the different water management plots during growth period.

312

A new indicator for the trade-off between the dissolved As and Cd concentrations

313

For evaluating the degree of trade-off between the dissolved As and Cd concentrations and

314

identifying optimal soil Eh and pH for simultaneously minimizing these concentrations, we

315

propose a new indicator, which we term the “trade-off value”. The trade-off value is defined by

316

Trade-off value = [As]/[Asmax] + [Cd]/[Cdmax]

(1)

317

where [As]/[Asmax] and [Cd]/[Cdmax] are the total dissolved As and Cd concentrations normalized

318

with respect to their maximum values expected across different water management plots during

319

the growth period (Details are given in the Supporting Information). A minimum trade-off value

320

is indicative of the most favorable condition in terms of simultaneous reduction of dissolved As

ACS Paragon Plus Environment

17

Environmental Science & Technology

Page 18 of 27

321

and Cd concentrations. By non-linear regression analyses using the least-square method, we

322

obtained equations describing the relations of the total dissolved As and Cd concentrations ([As]

323

and [Cd], respectively) to Eh, pH, and dissolved Fe(II) concentration, as shown in Figure 3.

324

Based on the range of Eh values observed in the field and the equations describing the [As] and

325

[Cd] versus Eh relations, we estimated that [Asmax] = 155.6 µg L–1 and [Cdmax] = 0.781 µg L–1 in

326

the present study. Figure 7 shows the relations of [As]/[Asmax], [Cd]/[Cdmax], and the trade-off

327

value to the soil Eh and pH. The minimum trade-off values were found with an Eh of –73 mV

328

and pH of 6.2 (or an Eh range of –100 to –40 mV and a pH range of 6.1 to 6.2, allowing for a 5%

329

variation in the trade-off values).

Cd (mg kg-1)

0.20

Field experiment Estimated optimal

0.15 y = 0.0023 x-2.194 R² = 0.9553

0.10 0.05 0.00 0

330

0.2 0.4 Inorganic As (mg kg-1)

0.6

331

Figure 8. Relationship between Cd and inorganic As concentrations in grains. Each symbol

332

represents the average value for different water managements. Expected concentrations for the

333

optimum Eh of –73 mV are also shown.

334

We recognize that in a strict sense, these conditions may not be realized simultaneously.

335

However, from the [As]/[Asmax] versus Eh and pH relations and [Cd]/[Cdmax] versus Eh and pH

336

relations (eq (S6)–(S9)), we estimate that an Eh of –73 mV corresponded to a soil pH of between

337

6.1 and 6.2, which is close to the value of 6.2 found above. For optimum conditions with an Eh

338

of –73 mV, the normalized equations (eq (S6) and (S7)) predict that [As]/[Asmax] = 0.108, and

ACS Paragon Plus Environment

18

Page 19 of 27

Environmental Science & Technology

339

[Cd]/[Cdmax] = 0.155. From these values and using the relationships shown in Figure 6, we

340

estimated the inorganic As and Cd concentrations in rice grain at 0.203 mg kg–1 and 0.094 mg

341

kg–1, respectively. Figure 8 shows the relationship between Cd and inorganic As concentrations

342

in grains from different water management plots. The Cd and inorganic As concentrations

343

expected for the optimum Eh of –73 mV is located close to the curve representing the

344

relationship obtained experimentally, and comparable to the Cd and inorganic As concentrations

345

in the Int-F3D5 plot (= 0.068 mg kg–1 and 0.188 mg kg–1, respectively), in which the water

346

management was most effective for simultaneous reduction of As and Cd. The new indicator of

347

“trade-off value” could be a useful tool for identifying the optimum soil Eh and pH, which

348

should be targeted by appropriate water managements during 3 weeks after heading. We note

349

that the [As]/[Asmax] versus Eh and pH relations and [Cd]/[Cdmax] versus Eh and pH relations are

350

specific to the soil tested, and that the optimum Eh and pH values may vary with experimental

351

conditions. Whereas the “trade-off value” developed in this study considers dissolved As and Cd

352

alone, exchangeable Cd2+ in soil constitutes a significant portion of plant-available Cd,42 and is

353

linked with dissolved Cd2+ through cation exchange equilibria. Our study (Figure 6) suggests that

354

the dissolved Cd is a good indicator of Cd uptake by rice plants, but possible effects of

355

solid/liquid partition of Cd2+ on the trade-off value merit future study. Further research is needed

356

to confirm whether the optimal irrigation intervals, and soil Eh and pH values are widely

357

applicable across different soil types, weather conditions, and As and Cd concentrations in soils.

358

Associated content

359

Supporting Information

360

Table S1–S3, Figure S1, details of field experiments and plant and soil analysis, description of

361

the new indicator for evaluating the trade-off between dissolved As and Cd concentrations

ACS Paragon Plus Environment

19

Environmental Science & Technology

Page 20 of 27

362

(PDF). The Supporting Information is available free of charge on the ACS Publications website

363

at http://pubs.acs.org.

364

Author Information

365

Corresponding Author

366

*Phone: +81-258-35-0826; fax: +81-258-35-0021; e-mail: [email protected]

367

Notes

368

The authors declare no competing financial interest.

369

Acknowledgments

370

This work was supported by a grant from the Ministry of Agriculture, Forestry, and Fisheries

371

of the Japanese Government (Research project for improving food safety and animal health As-

372

240). The authors are grateful to Dr. Satoru Ishikawa for the determination of grain quality, Dr.

373

Koji Baba for the guidance of As and Cd analyses, and Dr. Aomi Suda and Ms Miki Tomizawa

374

for conducting chemical analyses. Laboratory assistance by Ms. Asako Eguchi, Michiko Niida,

375

and Miho Togawa is also acknowledged.

376

References

377

(1) Tsukahara, T.; Ezaki, T.; Moriguchi, J.; Furuki, K.; Shimbo, S.; Matsuda-Inoguchi, N.;

378

Ikeda, M. Rice as the most influential source of cadmium intake among general Japanese

379

population. Sci. Total Environ. 2003, 305, 41–51.

380

(2) Benford, D. J.; Alexander, J.; Baines, J.; Bellinger, D. C.; Carrington, C.; Devesa i Peréz,

381

V. A.; Duxbury, J.; Fawell, J.; Hailemariam, K.; Montoro, R.; Ng, J.; Slob, W.; Veléz, D.; Yager,

382

J. W.; Zang, Y. Arsenic (addendum) In Safety Evaluation of Certain Contaminants in Food; The

ACS Paragon Plus Environment

20

Page 21 of 27

Environmental Science & Technology

383

seventy-second meeting of the joint FAO/WHO expert committee on food additives (JECFA)

384

Eds; Food and Agriculture Organization of the United Nations: Rome 2011; World Health

385

Organization: Geneva 2011; pp 153–316.

386

(3) International Agency for Research on Cancer (IARC) Working Group on the Evaluation of

387

Carcinogenic Risks to Humans. Some Drinking-water Disinfectants and Contaminants,

388

Including Arsenic. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol.

389

84; World Health Organization (WHO): Lyon 2004. Available at:

390

http://monographs.iarc.fr/ENG/Monographs/vol84/mono84.pdf (Accessed September, 2015).

391

(4) Williams, P. N.; Villada, A.; Deacon, C.; Raab, A.; Figuerola, J.; Green, A. J.; Feldmann,

392

J.; Meharg, A. A. Greatly enhanced arsenic shoot assimilation in rice leads to elevated grain

393

levels compared to wheat and barley. Environ. Sci. Technol. 2007, 41, 6854–6859.

394

(5) Codex Alimentarius Commission. Joint FAO/WHO food standards program, codex

395

alimentarius commission, Thirty-seventh Session, CICG, Geneva, Switzerland 14–18 July 2014,

396

ftp://ftp.fao.org/codex/Reports/Reports_2014/REP14_CACe.pdf

397

(6) Ministry of Agriculture, Forestry and Fisheries (MAFF). Survey results of arsenic

398

concentration in polished and unpolished rice grain in Japan. Press release of Ministry of

399

Agriculture, Forestry and Fisheries (in Japanese). February 21, 2014. Available at:

400

http://www.maff.go.jp/j/press/syouan/nouan/pdf/140221-01.pdf (Accessed September, 2015).

401

(7) Li, G.; Sun, G. X.; Williams, P. N.; Nunes, L.; Zhu, Y. G. Inorganic arsenic in Chinese

402

food and its cancer risk. Environ. Int. 2011, 37, 1219–1225.

ACS Paragon Plus Environment

21

Environmental Science & Technology

403

(8) Ma, J. F.; Yamaji, N.; Mitani, N.; Xu, X. Y.; Su, Y. H.; McGrath, S. P.; Zhao, F. J.

404

Transporters of arsenite in rice and their role in arsenic accumulation in rice grain. Proc. Natl.

405

Acad. Sci. USA. 2008, 105, 9931–9935.

406 407 408

(9) Wang, X.; Peng, B.; Tan, C.; Ma, L.; Rathinasabapathi, B. Recent advances in arsenic bioavailability, transport, and speciation in rice. Environ. Sci. Pollut. Res. 2015, 22, 5742–5750. (10) Takamatsu, T.; Aoki, H.; Yoshida, T. Determination of arsenate, arsenite,

409

monomethylarsonate, and dimethylarsinate in soil polluted with arsenic. Soil Sci. 1982, 133,

410

239–246.

411 412 413

(11) Marin, A. R.; Masscheleyn, P. H.; Patrick Jr., W. H. Soil redox-pH stability of arsenic species and its influence on arsenic uptake by rice. Plant Soil 1993, 152, 245–253. (12) Guo, T.; DeLaune, R. D.; Patrick Jr., W. H. The influence of sediment redox chemistry on

414

chemically active forms of arsenic, cadmium, chromium, and zinc in estuarine sediment.

415

Environ. Int. 1997, 23, 305–316.

416

Page 22 of 27

(13) Takahashi, Y.; Minamikawa, R.; Hattori, K. H.; Kurishima, K.; Kihou, N.; Yuita, K.

417

Arsenic behavior in paddy fields during the cycle of flooded and non-flooded periods. Environ.

418

Sci. Technol. 2004, 38, 1038–1044.

419

(14) Arao, T.; Kawasaki, A.; Baba, K.; Mori, S.; Matsumoto, S. Effects of water management

420

on cadmium and arsenic accumulation and dimethylarsinic acid concentrations in Japanese rice.

421

Environ. Sci. Technol. 2009, 43, 9361–9367.

ACS Paragon Plus Environment

22

Page 23 of 27

Environmental Science & Technology

422

(15) Li, R. Y.; Stroud, J. L.; Ma, J. F.; McGrath, S. P.; Zhao, F. J. Mitigation of arsenic

423

accumulation in rice with water management and silicon fertilization. Environ. Sci. Technol.

424

2009, 43, 3778–3783.

425 426 427 428 429

(16) Goldberg, S. Competitive adsorption of arsenate and arsenite on oxides and clay minerals. Soil Sci. Soc. Am. J. 2002, 66, 413–421. (17) Yamane, T. Mechanisms and counter-measures of arsenic toxicity to rice plant. Bull. Shimane Agric. Exp. Stn. 1989, 24, 1–95 (in Japanese with English summary). (18) Signes-Pastor, A; Burló, F.; Mitra, K.; Carbonell-Barrachina, A. A. Arsenic

430

biogeochemistry as affected by phosphorus fertilizer addition, redox potential and pH in a west

431

Bengal (India) soil. Geoderma 2007, 137, 504–510.

432

(19) Yamaguchi, N.; Nakamura, T.; Dong, D.; Takahashi, Y.; Amachi, S.; Makino, T. Arsenic

433

release from flooded paddy soils is influenced by speciation, Eh, pH, and iron dissolution.

434

Chemosphere 2011, 83, 925–932.

435 436 437

(20) Deuel, L. E.; Swoboda, A. R. Arsenic solubility in a reduced environment. Soil Sci. Soc. Am. Proc. 1972, 36, 276–278. (21) Alfvén, T.; Elinder, C. G.; Carlsson, M. D.; Grubb, A.; Hellström, L.; Persson, B.;

438

Pettersson, C.; Spång, G.; Schütz, A.; Järup, L. Low-level cadmium exposure and osteoporosis.

439

J. Bone Miner. Res. 2000, 15, 1579–1586.

440 441

(22) Kobayashi, J. Pollution by cadmium and the itai-itai disease in Japan. In Toxicity of Heavy Metals in the Environment; Oehme, F. W. Ed.; Marcel Dekker: New York 1978; pp 199–260.

ACS Paragon Plus Environment

23

Environmental Science & Technology

442

Page 24 of 27

(23) Joint FAO/WHO Expert Committee on Food Additives (JCFA) Summary and

443

Conclusions. Seventy-third meeting Geneva, 8–17 June 2010, p. 12. Available at:

444

http://www.who.int/foodsafety/publications/chem/summary73.pdf (Accessed September, 2015).

445 446 447

(24) Codex Alimentarius. Report of the 29th session of the CODEX Alimentarius Commission. CODEX Alimentarius Commission, Alinorm 06/29/41. 2006. (25) Ministry of Agriculture, Forestry and Fisheries (MAFF). Survey results of domestic

448

agricultural livestock and marine products. 2004. (in Japanese) Available at:

449

http://www.maff.go.jp/j/syouan/nouan/kome/k_cd/cyosa/pdf/cereal.pdf and

450

http://www.maff.go.jp/j/syouan/nouan/kome/k_cd/cyosa/pdf/cd-tds.pdf (Accessed September,

451

2015).

452 453 454

(26) Uraguchi, S.; Fujiwara, T. Cadmium transport and tolerance in rice: perspectives for reducing grain cadmium accumulation. Rice 2012, 5, 5. doi: 10.1186/1939-8433-5-5 (27) Iimura, K.; Ito, H. Behavior and balance of contaminant heavy metals in paddy soils;

455

studies on heavy metal pollution of soils (part 2). Bull. Hokuriku Natl. Agric. Exp. Stn. 1978, 21,

456

95–145. (in Japanese with English summary)

457 458 459

(28) Ito, H.; Iimura, K. Absorption of cadmium by rice plants in response to change of oxidation-reduction conditions of soils. J. Sci. Soil Manure, Jpn. 1975, 46, 82–88 (in Japanese). (29) Fulda, B.; Voegelin, A.; Kretzschmar, R. Redox-controlled changes in cadmium solubility

460

and solid-phase speciation in a paddy soil as affected by reducible sulfate and copper. Environ.

461

Sci. Technol. 2013, 47, 12775–12783.

ACS Paragon Plus Environment

24

Page 25 of 27

Environmental Science & Technology

462

(30) Bingham, F. T.; Page, A. L.; Mahler, R. J.; Ganje, T. J. Cadmium availability to rice in

463

sludge-amended soil under ‘Flood’ and ‘Nonflood’ culture. Soil Sci. Soc. Am. J. 1976, 40, 715–

464

719.

465 466

(31) McBride, M. B. Reactions controlling heavy metal solubility in soils. Adv. Soil Sci. 1989, 10, 1–56.

467

(32) Kikuchi, T.; Okazaki, M.; Kimura, S. D.; Motobayashi, T.; Baasansuren, J.; Hattori, T.;

468

Abe, T. Effects of magnesium oxide materials on cadmium uptake and accumulation into rice

469

grains: II: suppression of cadmium uptake and accumulation into rice grains due to application of

470

magnesium oxide materials. J. Hazard. Mater. 2008, 154, 294–299.

471

(33) Nakamura, K.; Katou, H. Arsenic and cadmium solubilization and immobilization in

472

paddy soils in response to alternate submergence and drainage. In Competitive Sorption and

473

Transport of Heavy Metals in Soils and Geological Media; Selim, H. M., Ed.; CRC Press: Boca

474

Raton 2012; pp 379–404.

475

(34) Inahara, M.; Ogawa, Y.; Azuma, H. Countermeasure by means of flooding in latter

476

growth stage to restrain cadmium uptake by lowland rice. Jpn. J. Soil Sci. Plant Nutr. 2007, 78,

477

149–155. (in Japanese with English abstract)

478 479 480

(35) Xu, X. Y.; McGrath, S. P.; Meharg, A. A.; Zhao, F. J. Growing rice aerobically markedly decreases arsenic accumulation. Environ. Sci. Technol. 2008, 42, 5574–5579. (36) Hu, P.; Huang, J.; Ouyang, Y.; Wu, L.; Song, J.; Wang, S.; Li, Z.; Han, C.; Zhou, L.;

481

Huang, Y.; Luo, Y.; Christie, P. Water management affects arsenic and cadmium accumulation

482

in different rice cultivars. Environ. Geochem. Health 2013, 35, 767–778.

ACS Paragon Plus Environment

25

Environmental Science & Technology

483 484 485

Page 26 of 27

(37) Soil Survey Staff. Keys to Soil Taxonomy, Twelfth Edition. 2014. U. S. Department of Agriculture, Natural Resources Conservation Service. (38) Yamauchi, M. A rapid measurement of ferrous iron in flooded soil. Outcome Information

486

of Natl. Agric. Food Res. Org. (in Japanese) 2000. Available at:

487

http://www.naro.affrc.go.jp/project/results/laboratory/warc/2000/wenarc00-133.html (Accessed

488

September 2015).

489

(39) Baba, K.; Arao, T.; Yamaguchi, N.; Watanabe, E.; Eun, H.; Ishizaka, M. Chromatographic

490

separation of arsenic species with pentafluorophenyl column and application to rice. J.

491

Chromatogr. A 2014, 1354, 109–116.

492

(40) Nishimura, T.; Hamano-Nagaoka, M.; Sakakibara, N.; Abe, T.; Maekawa, Y. M. T.

493

Determination method for total arsenic and partial-digestion method with nitric acid for

494

inorganic arsenic speciation in several varieties of rice. Food Hyg. Saf. Sci. 2010, 51, 178–181.

495

(41) Masui, H. 4 steps Excel Toukei, 2nd ed., Statcel 2; OMS Publication: Saitama, Japan,

496 497 498

2007. (in Japanese). (42) Krishnamurti, G. S. R.; Naidu, R. Speciation and phytoavailability of cadmium in selected surface soils of South Australia. Aust. J. Soil Res., 2000, 38 , 991–1004.

499

ACS Paragon Plus Environment

26

Page 27 of 27

Environmental Science & Technology

Trade-off value

1.0

Optimum Eh for simultaneous reduction of As and Cd

0.8 0.6 0.4

Trade-off value [As] / [Asmax] [Cd] / [Cdmax]

0.2 0.0 -300

500 501

-200

-100

0

100

200

300

Soil Eh (mV)

Table of Contents/Abstract art

ACS Paragon Plus Environment

27