Environmental Risk Implications of Metals in Sludges from Waste

Apr 5, 2017 - †Key Laboratory of Geographic Information Science of the Ministry of Education, School of Geographic Sciences and ‡Shanghai Key Lab ...
0 downloads 10 Views 2MB Size
Subscriber access provided by HACETTEPE UNIVERSITESI KUTUPHANESI

Article

Environmental Risk Implications of Metals in Sludges from Waste Water Treatment Plants: The Discovery of Vast Stores of Metal-containing Nanoparticles Feiyun Tou, Yi Yang, Jingnan Feng, Zuoshun Niu, Hui Pan, Yukun Qin, Xingpan Guo, Xiang-Zhou Meng, Min Liu, and Michael F. Hochella Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05931 • Publication Date (Web): 05 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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

Environmental Risk Implications of Metals in Sludges from Waste

2

Water Treatment Plants: The Discovery of Vast Stores of

3

Metal-containing Nanoparticles

4

Feiyun Tou †, Yi Yang *, †, ‡, Jingnan Feng †, Zuoshun Niu †, Hui Pan †, Yukun Qin †,

5

Xingpan Guo†, Xiangzhou Meng , Min Liu †, Michael F. Hochella, Jr.§, ||

6



7

Geographic Sciences, East China Normal University, 500 Dongchuan Road, Shanghai,

8

200241, China.

9



10 11



Key Laboratory of Geographic Information Science of the Ministry of Education, School of

Shanghai Key lab for Urban Ecological Processes and Eco-Restoration, East China Normal

University, 500 Dongchuan Road, Shanghai 200241, China ⊥

State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental

12

Science and Engineering, Tongji University, Shanghai 200092, China

13

§

14

24061, USA

15

||

The Center for NanoBioEarth, Department of Geosciences, Virginia Tech, Blacksburg, VA

Geosciences Group, Energy and Environment Directorate, Pacific Northwest National

16

Laboratory, Richland, WA 99352, USA

17

Corresponding author: [email protected]

18 19

Table of Content Art

1

ACS Paragon Plus Environment

Environmental Science & Technology

20

ABSTRACT

21

Nanoparticle (NP) assessment in sludge materials, although of growing importance in eco-

22

and bio-toxicity studies, is commonly overlooked and, at best, understudied. In the present

23

study, sewage sludge samples from across the mega-city of Shanghai, China were investigated

24

for the first time using a sequential extraction method coupled with single particle inductively

25

coupled plasma mass spectrometry (SP-ICP-MS) in order to quantify the abundance of

26

metal-containing NPs in the extraction fractions, and transmission electron microscopy to

27

specifically identify the nanophases present. In general, most sludges observed showed high

28

concentrations of Cr, Cu, Cd, Ni, Zn and Pb, exceeding the maximum permitted values in the

29

national application standard of acid soil in China. NPs in these sludges contribute little to the

30

volume/mass, but account for about half of the total particle number. Based on electron

31

microscopy techniques, various NPs were further identified, including Ti-, Fe-, Zn-, Sn-,

32

Pb-containing NPs. All NPs, ignored by traditional metal risk evaluation methods, were

33

observed at a concentration of 107 -1011 particles/g within the bioavailable fraction of metals.

34

These results indicate the underestimate or mis-estimation in evaluating the environmental

35

risks of metals based on traditional sequential extraction methods. A new approach for

36

environmental risk assessment of metals, including NPs, is urgently needed.

2

ACS Paragon Plus Environment

Page 2 of 27

Page 3 of 27

Environmental Science & Technology

37

INTRODUCTION

38

The most recent estimation of the production of sewage sludge in China is 40 million tons in

39

2014 (calculated using the estimate that 10,000 tons of wastewater produces 5.6 tons of

40

sewage sludge1), and this production is increasing at an annual average rate of 4.75 percent.2

41

Of this tonnage, 45% is applied to agricultural land, 31% is landfilled, 3% of the sewage

42

sludge is incinerated, and the remaining 21% is used for greening non-agricultural lands

43

and/or discharged directly into rivers.3 Among these destinations, agricultural application and

44

landfilling are most economical.4 However, the land application of sludge is limited by its

45

high concentrations of various metals and other toxic organic pollutants, which in many cases

46

has led to severe degradation of soils and therefore serious secondary environmental pollution.

47

For example, metals in sewage sludge are of significant concern and have been known to pose

48

severe risks to the ecological environment and humans via agricultural application.5-10

49

Besides various metals in sludge, metal-containing nanoparticles (NPs) have a number of

50

origins, morphologies and sizes, atomic structures, and compositions, all of which play an

51

important role in their precise behavior, and as a result, they have gained more and more

52

attention11-28. NPs typically show distinct chemical and physical properties, especially when

53

they are particularly small, relative to the corresponding bulk material, and can induce

54

cytotoxicity, resulting in long term environmental and health risks.11-14 In addition, due to their

55

inherent reactivity with other contaminants, NPs can serve as a carrier and may release toxins

56

through transformation under certain environmental or biological conditions.15-18 It is worth

57

noting that metal containing NPs, such as titanium oxides, iron oxides, and silver and zinc

58

sulfides, have already been identified in sewage sludge and sludge-amended soils.19-23 These

59

NPs can also be taken up by organisms from sludge-amended soil, posing eco-environmental

60

risks.24-28 However, the occurrence and environmental function of NPs in soils and sediments

61

has been challenging to assess.29-31

62 63

As an important traditional method commonly applied to assess the environmental risk of metals, a chemical speciation analysis can indicate the mobility, bioavailability and

3

ACS Paragon Plus Environment

Environmental Science & Technology

64

potential eco-toxicity of metal-containing NPs and help predict their release in soil/sediment

65

environments.32-34 Tessier sequential extraction, known as a sequential extraction method

66

proposed by the European Community Bureau of Reference (BCR sequential extraction)35-36

67

and its variations are the most widely applied extraction methods. For this study, we have

68

chosen a modified BCR sequential extraction method due to its stability for extraction

69

efficiency and ease of application. For this method, metals are divided into an

70

acid-exchangeable fraction, a reducible fraction, an oxidizable fraction, and a residual fraction.

71

The acid-exchangeable fraction, mainly consisting of water soluble, exchangeable and

72

carbonate-bound metals, is readily released into the environment and represents the

73

bioavailable fraction.32 However, until this present study, metal-containing NPs have never

74

been considered in each fraction according to the existing citable literature.

75

It is a great challenge to directly quantify metal-containing NPs in complex

76

environmental samples and further differentiate them from their ionic species. Electron

77

microscopy (EM) techniques, such as scanning transmission electron microscopy (S/TEM)

78

and scanning electron microscopy (SEM), coupled with accessory capabilities such as

79

energy-dispersive X-ray spectroscopy (EDX) and selected area electron diffraction (SAED),

80

have been recognized by the scientific community as powerful tools to provide detailed

81

information including particle size, morphology, chemical composition, and crystal structure

82

on a single-particle basis.19-23 However, application of all of the above mentioned techniques

83

in complicated environmental samples is somewhat limited by the lack of a way to quantify

84

the number of NPs, that is there general abundance, which cannot be provided by the methods

85

described above. Single particle inductively coupled plasma mass spectrometry (SP-ICP-MS)

86

is a relatively new technique to quantify the number of NPs even in complex environmental

87

samples, and also to make an independent estimate of their size distribution. In addition, it can

88

determine both metal concentrations in the dissolved and particulate forms simultaneously.

89

It can also determine the size of NPs to compare with other commercially available

90

techniques such as dynamic light scattering.37 The development of SP-ICP-MS techniques has

91

been well described in other studies,38-39 and it has been successfully applied to directly 4

ACS Paragon Plus Environment

Page 4 of 27

Page 5 of 27

Environmental Science & Technology

92

determine the particle sizes and concentrations of NPs in drinking water, biological tissues,

93

and suncreen37, 40-43, but none to date in sewage sludge materials.

94

This study is designed to estimate the environmental implications of metals in sludge

95

samples, specifically metal-containing NPs. To this end, twenty-six sewage sludge samples

96

were taken from wastewater treatment plants (WWTPs) as a representative sampling

97

throughout the mega-city of Shanghai, China. The feasibility of using traditional risk

98

assessment methods for metals in sludge samples was evaluated, but in this case considering

99

specific NP quantities, sizes, and compositions for the time. The specific objectives of this

100

study are (1) to assess the risk of metals in sewage sludge based on analysis via modified

101

BCR sequential extraction, (2) to reveal the occurrence of metal-NPs (such as Ti-NPs, Fe-NPs

102

and Zn-NPs) in different metal chemical fractions, and to determine the concentration and

103

size distribution of metal-NPs in each chemical fraction, especially in the bioavailable

104

fraction of metals using SP-ICP-MS, and (3) to identify the dominant metal-NPs based on EM

105

techniques.

106

MATERIALS AND METHODS

107

Sample Collection and Pretreatment.

108

Shanghai, with an area of 6340km2 and a population of more than 24 million, is a sprawling

109

megacity in a developing country. With increasing population, motorization, urbanization and

110

industrial activities, the annual wastewater discharge has climbed to 2212 million tons in 2014,

111

with 53 wastewater treatment plants scattered throughout the city.44 Shanghai is suffering

112

tremendous stress from wastewater treatment and its main by-products, including sewage

113

sludge. For this study, sewage sludge samples were collected from 26 WWTPs in Shanghai.

114

Table S1 summarizes the details of the WWTPs. About 2 kg wet weight sewage sludge

115

samples were collected, immediately transported to our Shanghai laboratory, stored at -20℃,

116

freeze-dried and homogenized until further processing.

117

Metal Analysis. 5

ACS Paragon Plus Environment

Environmental Science & Technology

118

In order to determine the total metal concentrations in sewage sludge, all samples were

119

digested according to the EPA-approved, microwave assisted nitric acid digestion method

120

3051A45 in duplicate and then analyzed by a Thermo Electron X-Series ICP-MS

121

(Massachusetts, United States) via Standard Method 3125-B,46 and the calibration standards

122

were prepared in a matrix of 2% nitric acid by volume. In detail, approximately 0.1g

123

freeze-dried samples were pretreated with 2mL of 67% nitric acid (Trace Metal Grade, Fisher)

124

at 90℃ for half an hour and 1mL of 30% hydrogen peroxide (Ultrapure Reagent, Fisher)

125

overnight at 50℃ in 100mL bottles. Then all the reactants were transferred to microwave

126

digestion vessels, with 7mL of 67% nitric acid. After cooling down to room temperature, the

127

vessels were rinsed 3 times with Mill-Q water and the volume was set to 100mLbefore

128

analysis. This method was appropriate for most metals, but not ideal for especially titanium

129

oxides which are exceptionally insoluble.47 Analyzed metals include: Ti, V, Cr, Fe, Mn, Co, Ni,

130

Cu, Zn, As, Se, Sr, Mo, Ag, Cd, Sn, Ba, Ce and Pb.

131

Particle Size Analysis.

132

Laser diffraction particle size analyzer LS13320 (Beckman Counter, California, United

133

States), particle size analysis ranging from 0.017 to 2000µm, was applied for the particle size

134

analysis of the sludge samples. The particle size of blank samples (Milli-Q water) ranged

135

from 17nm to 40 nm, and the actual particle size detection limit was therefore set as 40 nm.

136

All samples were pretreated with 10 mL of 10 % hydrogen peroxide at 100℃ until no

137

bubbles were generated (more hydrogen peroxide solution was added if 10 mL was not

138

sufficient) to reduce the particle aggregation caused by organic compounds. After cooling

139

down to room temperature, 10mL of 36.1g/L sodium hexametaphosphate was added as a

140

stabilizer and the mixture was dispersed by an ultrasonic probe (KQ5200E, Shumei, Kunshan,

141

China) for 10 min.

142

Sequential Extraction Procedure.

143

Sequential extraction was performed with the modified BCR sequential extraction

6

ACS Paragon Plus Environment

Page 6 of 27

Page 7 of 27

Environmental Science & Technology

144

procedure.48 This procedure is described in detail in the supplemental information (Fig. S1).

145

According to the industrial wastewater ratios (IWRs, the volume ratios of the industrial

146

wastewater to the total inflow wastewater), fourteen of the 26 sludge samples (S4, S5, S10,

147

S11, S12, S14, S15, S17, S18, S19, S20, S21, S23 and S26) were chosen to process the

148

sequential extraction procedure in duplicate. Blanks were measured in parallel for each set of

149

BCR-prepared samples. Negligible metal concentrations were detected in all the blanks.

150

SP-ICP-MS Data Acquisition and Processing.

151

To screen for the occurrence of metal containing-NPs in the bioavailable fraction of sludge

152

and further quantify their particle size and concentrations, a PerkinElmer NexION 350D

153

SP-ICP-MS (Massachusetts, United States) was used to analyze the acid-exchangeable

154

extraction solutions (pH 3, acetic acid) of 14 sludge samples (S4, S5, S10, S11, S12, S14, S15,

155

S17, S18, S19, S20, S21, S23 and S26). The instrument was set to detect Zn, Fe and

156

Ti-containing NPs. In addition, as Ti oxide-NPs would not react with extraction reagents,

157

these particles in S26 were further analyzed for the occurrence of NPs in the reducible

158

fraction and oxidizable fraction.

159

Masses of 47Ti (7.3% abundance, Standard Mode), 66Zn (27.9% abundance, Standard

160

Mode) and 56Fe (91.7% abundance, Dynamic Reaction Cell (DRC) Mode) were monitored by

161

SP-ICP-MS, with a dwell time of 100µs and a scan time of no less than 100s. Dissolved

162

element calibrations for titanium, zinc and iron were prepared in a matrix of 2% nitric acid by

163

volume, and Au-containing NPs stabilized with citrate in the size of 30 nm and 60 nm from

164

the National Institute of Standards and Technology (NIST) were used as particle calibration

165

standards. Particle size and dissolved element detection limits (according to the results of

166

Milli-Q water) were determined to be 15-20 nm and 0.13 µg/L for Ti, 15-16 nm and 0.30 µg/L

167

for Zn, and 12-17 nm and 0.10 µg/L for Fe, respectively. To get appropriate particle

168

concentration, tested samples were diluted 100~10000 times and the results are summarized

169

in Table S6.

170

Electron Microscope Analysis. 7

ACS Paragon Plus Environment

Environmental Science & Technology

171

Selected samples (S11, S12, S18 and S21) were characterized using electron microscopy

172

techniques. An Environmental Scanning Electron Microscope (ESEM, FEI Quanta 600 FEG,

173

Oregon, United States), equipped with an energy dispersive X-ray spectrometer (EDS,

174

QUANTAX 400, Bruker, Karlsruhe, Germany) system, and a Transmission Electron

175

Microscope (TEM, JEOL 2100 TEM, Tokyo, Japan), coupled with EDS and selected area

176

electron diffraction (SAED), were applied to characterize the morphology, composition and

177

crystal structures of the NPs.

178

RESULTS AND DISCUSSION

179

Total Concentrations of Metals in Sludge Samples.

180

Total concentrations of Ti, V, Cr, Fe, Mn, Co, Ni, Cu, Zn, As, Se, Sr, Mo, Ag, Cd, Sn, Ba, Ce,

181

and Pb in sludge samples are shown in Figure 1. Fe is the most abundant metal (8.1-54.5

182

gkg-1), with an average concentration of 22.7 gkg-1 which is 1-5 orders of magnitude higher

183

than all other metals. Ag exhibited the lowest abundance (0.25-29.6 mgkg-1), with an average

184

of 5.4 mgkg-1 in all sludge samples. Zn and Cu, with an average concentration of 2091.6 and

185

908.4 mgkg-1, respectively, are the second and third most abundant metal contaminants.

186

Noticeably, the concentrations of lead in S21 (2007.1 mgkg-1) and S11 (446.2 mgkg-1) are

187

much higher than those in other sludge samples, ranging from 21.3 to 198.1 mgkg-1. The

188

elevated concentrations of lead in these two samples can likely be attributed to the occurrence

189

of metallic material processing works and metal smelting factories in the S21 and S11 service

190

areas, respectively. In addition, the concentrations of cadmium in S5 (170.6 mgkg-1) and

191

cobalt in S18 (803.2 mgkg-1) are 1-2 orders of magnitude higher than those in other sludge

192

samples, ranging from 0.67 to 4.4 mgkg-1 and 2.7 to 91.9 mgkg-1, respectively. It is likely

193

that these elevated concentrations are due to the electroplating factories and petrochemical

194

industries in both S5 and S18 service areas.

195

It is noticeable that the concentrations of Cr, Ni, Cu, Zn in Shanghai sludge samples are

196

2~3 times higher than those found in the samples collected from 193 WWTPs in 111 cities of

8

ACS Paragon Plus Environment

Page 8 of 27

Page 9 of 27

Environmental Science & Technology

197

China,49 which are likely reasonable averages of heavy metal pollution levels from WWTP

198

sludges in urban China; likewise, the concentration of Pb across Shanghai is 32% higher than

199

the average concentration in the China-wide study (Table S3). In addition, the concentrations

200

of Cr, Ni, Cu, Zn, Cd, and Pb in this study are higher than the permissible values for acid soils

201

(issued by the Ministry of Environmental Protection of the People’s Republic of China in

202

2002) in 3.8 to 38.5% of the samples, depending on the metal.50 According to the results from

203

a sewage sludge survey launched by the U.S. Environmental Protection Agency (EPA),51 the

204

concentrations of Cd, Cr, and Pb in U.S. sludge samples are lower than those in Shanghai

205

sludge according to the present study. However, the silver concentrations in U.S. sludge

206

samples are more than ten times higher than those in Shanghai sludge samples.

207

The correlation coefficient matrix among different metal elements as well as IWR (Table

208

S1, analyzed by IBM SPSS Statistics 23.0) of WWPTs shows that there are significant

209

correlations between Ti, V, Cr, Co, Cu, Se, Sr, Mo, Cd, Ce and IWR (Table S3), indicating

210

that industrial wastewater are likely an important input of these heavy metal contaminants to

211

sewage sludge. Meanwhile, in most samples, Ti, Cr, Fe, Co, Ni, Cu, Zn, As, Mo, Ag, Sn, and

212

Pb are significantly correlated with each other at the 0.01 level, and the higher correlations (r >

213

0.8) are observed between V and Co, Cr and Cu, and As and Pb, suggesting their similar or

214

the same origins.52-54

215

Particle Size Analysis of Sludge Samples.

216

The particle size distributions of sludge samples, plotted as a function of volume percent for

217

each size fraction, are plotted in Figure S2. It is obvious that the large particles (>1000 nm),

218

with an average volume percent of 90.3 %, is the dominant fraction of total volume, while the

219

nanoparticles (