Isotopic Fingerprints of IronâCyanide Complexes in the Environment

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Isotopic fingerprints of iron-cyanide complexes in the environment Tim Mansfeldt, and Patrick Höhener Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01565 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016

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Environmental Science & Technology

1

Isotopic fingerprints of iron-cyanide complexes in the environment

2 3

Tim Mansfeldt*,† and Patrick Höhener‡

4 5

†

6

phie/Bodenkunde, D-50923 Köln, Germany

7

‡

8

Marseille, France

Universität

zu

Köln,

Department

Geowissenschaften,

Bodengeogra-

Aix-Marseille Université, Laboratoire Chimie Environnement, UMR 7376–CNRS,

9 10 11

*corresponding author

12

Phone: ++49-(0)221-470-7806

13

Fax: ++49-(0)221-470-5124

14

E-mail: [email protected]

1

ACS Paragon Plus Environment


15

Abstract

16

Tracing the origin of iron-cyanide complexes in the environment is important because

17

these compounds are potentially toxic. We determined the stable isotopic composi-

18

tions of cyanide-carbon (CCN) and cyanide-nitrogen (NCN) in 127 contaminated solids

19

and 11 samples of contaminated groundwater from coal carbonization sites, blast

20

furnace operations, and commercial cyanide applications. Coal-carbonization-related

21

cyanides had unique high mean δ13CCN values of −10.5 ± 3.5‰ for the solids and

22

−16.1 ± 1.2‰ for the groundwater samples, while the values for blast furnace sludge

23

(−26.9 ± 1.5‰), commercial cyanides (−26.0 ± 3.0‰), and their corresponding

24

groundwaters were significantly lower. Determination of δ13CCN is a promising tool for

25

identifying the source of cyanide contamination. However, for coal carbonization

26

sites, historical research into the manufacturing process is necessary because a non-

27

conventional gas works site exhibited exceptionally low δ13CCN values of −22.7 ±

28

1.7‰. The δ15NCN values for samples related to coal carbonization and blast furnaces

29

overlapped within a range of +0.1 to +10.3‰, but very high δ15NCN values seemed to

30

be indicative for a cyanide source in the blast furnace. In contrast, commercial cya-

31

nides tend to have lower δ15NCN values of −5.6 to +1.9‰ in solids and −0.5 to +3.0‰

32

in the groundwater.

2


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TOC Graphic

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3



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Introduction

37

Natural sources of environmental cyanides (chemical species that contain the -C≡N

38

moiety) include higher plants, bacteria, fungi, algae and burning of biomass. These

39

sources, however, do not normally lead to problematic accumulations.1 In contrast,

40

human activities can increase the cyanide concentrations in soils and waters to envi-

41

ronmentally significant levels.

42

Anthropogenic cyanides enter the environment via several processes and materials:

43

(1) disposal of Prussian blue (Fe4[Fe(CN)6]3xH2O)-containing gas purifier wastes

44

from former manufactured gas plants (MGPs) or coke oven plants (COPs),2–4 (2)

45

landfilling of blast furnace sludge containing K2Zn3[Fe(CN)6]29H2O,5 (3) landfilling of

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spent potlining from aluminum manufacturing,6 (4) application of road salt or wildland

47

fire

48

Na4[Fe(CN)6]10H2O,7–9 (5) amending soils with paper de-inking sludge,10 (6) land-

49

filling of sewage sludge,11 (7) use of sodium cyanide (NaCN) to recover gold or other

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precious metals from ores,12 and (8) use of NaCN or potassium cyanide (KCN) for

51

electroplating.6

52

Most of the anthropogenic cyanide that enters the environment is complexed with Fe.

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Although iron-cyanide complexes are not particularly toxic, they can release extreme-

54

ly toxic free cyanide (the hydrogen cyanide molecule, HCN, and the CN– anion). This

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release occurs when contaminated groundwater is transferred to the surface, where

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the iron-cyanide complexes can be decomposed by photolysis and thereby release

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free cyanide.13 Thus, the presence of iron-cyanide complexes in groundwater is of

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environmental concern, particularly for aquatic fauna.14 Many industrialized countries

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have laid down legislative regulations of cyanide in water (drinking and surface water,

60

groundwater) and soil, for example screening, target, intervention or trigger values.15

61

If such values are exceeded, there is need for further risk assessment or even reme-

retardants

that

contain

Prussian

4

blue

and/or


sodium

ferrocyanide,

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diation of cyanide-impacted water and soil. According to the ‘polluter pays’ principle

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the question arises: Who is the polluter? Because the source of pollutive cyanide is

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not always obvious there is requirement for identifying the polluter of such contamina-

65

tion.

66

Stable isotope methods are powerful tools to investigate the source and fate of pollu-

67

tants in the environment.16 Regarding cyanide, previous studies have developed this

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type of method to determine δ13C and δ15N in solid and aqueous cyanides,17 identify

69

the mechanisms responsible for cyanide consumption at gold ore heap-leach opera-

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tions,18 and identify the origin of free cyanide in forensic investigations.19, 20 In previ-

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ous studies, we established methods to determine δ13C and δ15N of contaminant cy-

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anide in both solid and aqueous samples.21,

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knowledge regarding δ13C and δ15N in many environmentally significant sources of

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iron-cyanide complexes. In this study, we present the δ13CCN (cyanide-carbon) and

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δ15NCN (cyanide-nitrogen) values of 138 environmental samples that were analyzed

76

using these methods. The samples include several of the most common cyanide-

77

contaminated materials. Our primary motivation was to determine whether different

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sources of anthropogenic cyanides could be distinguished based on their stable iso-

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topic compositions.

22

However, there is currently a gap in

80 81

Materials and Methods

82

Sites, samples, sampling and sample preparation. During the course of the pro-

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ject, 127 solid and 11 aqueous samples (groundwater) were investigated. In detail,

84

we analyzed the following solid samples:

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● 39 samples from COPs and MGPs in Germany (31 samples from 7 sites), Poland

86

(7 samples from 2 sites) and France (1 sample). Three samples originated from cur-

5



87

rent production (pure gas purifier waste), whereas the other samples were obtained

88

at former COP or MGP sites either from the soil surface or from pits (Table S1).

89

● 4 samples from a site where an MGP had been in operation at a former drapery

90

factory. The coke gas was used within the plant and also distributed as town gas.

91

Because blue, cyanide-containing pigments were often used to dye textiles, drapery

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production is also a potential cyanide source at the site. The origin of cyanide in the-

93

se samples is thus unclear, and they are termed “unclear samples” in the following

94

text (Table S2).

95

● 72 samples from blast furnace sludge landfills in Germany (67 samples from 4

96

sites) and France (5 samples from 1 site) (Table S3). Two samples originated from

97

current production, whereas the other samples were obtained from the surface or

98

pits. At the Schalker Verein landfill, samples were also taken from cores obtained by

99

drilling, for which the sampling depth was 3.9-12.0 m (n = 18). The core samples

100

were obtained from the 1940s to the 1960s (“old” samples), whereas the surface and

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pit samples were obtained from the 1970s to the 1980s (“young” samples).

102

● 3 samples from a site in Saxony (Germany), where the top soils were amended

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with cyanide-containing paper-deinking sludge (Table S4).

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● 7 samples from a sewage field near Berlin, Germany (Table S5).

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● 2 samples from a road salt application. One sample was the final-product road salt,

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and the other was the sodium ferrocyanide cyanide-containing feedstock (Table S6).

107

The cyanides in the last three sample sets are referred to as commercial cyanides.

108

After collecting, samples were manually homogenized, dried at 60 °C and passed

109

through a metal sieve (2 mm). The total cyanide content was determined by either

110

acid digestion or alkaline extraction23 with a micro-distillation apparatus24 and subse-

111

quent spectrophotometrical detection of the CN– at 600 nm.

112

The aqueous samples are summarized below (Table S7): 6


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● 4 samples from former COP (n = 3) and MGP sites (n = 1) in Germany.

114

● 4 samples from a blast furnace sludge landfill in Germany.

115

● 3 samples from electroplating plants in Southern Germany (n = 2) and the Rhine-

116

Main area in Germany.

117

To obtain a sufficient amount of cyanide, 100-150 l of groundwater was collected

118

from wells at depths of 6-9 m. The waters were immediately passed through 0.45-µm

119

filters by vacuum and stored in darkness at 4 °C. The total cyanide concentration was

120

determined by acid distillate digestion with a micro-distillation system.24

121

Extraction of cyanide from solid samples. Iron-cyanide complexes were converted

122

into the solid cupric ferrocyanide, Cu2[Fe(CN)6]7H2O, for isotope analysis. For the

123

blast furnace sludge and road salt samples, Cu2+ was added to the 1 M NaOH alka-

124

line extract with simultaneous acidification. All the other samples contained significant

125

amounts of organic (non-cyanide) carbon, which can co-precipitates with cupric fer-

126

rocyanide after alkaline extraction. For these samples it was necessary to perform a

127

distillate digestion instead of an alkaline extraction. The CN– released by the distillate

128

digestion was first complexed with Fe2+ under anoxic conditions and then precipitated

129

by adding Cu2+. This procedure is time-consuming but ensures that only cyanide-C is

130

recovered from samples that are high in organic carbon. Weihmann et al. provide

131

more detail regarding both approaches.21

132

Extraction of cyanide from groundwater. Within one week of sampling, 30-120 l of

133

groundwater (higher volumes for lower cyanide concentrations) was taken to extract

134

the cyanide. The method utilized relies on the enrichment of iron-cyanide complexes

135

on an anion-exchange resin (Fluka Lewatit MP62, Sigma-Aldrich), desorption of the

136

complexes with a 10% NaCl solution, and precipitation of the complexes as cupric

137

ferrocyanide. Schulte and co-workers published a comprehensive laboratory protocol

138

for this method.22 7



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139

Determination of stable C and N isotope ratios. The stable isotopes were meas-

140

ured by flash combustion in an elemental analyzer coupled to a continuous-flow iso-

141

tope ratio mass spectrometer (Carlo Erba 1110, Finnigan Delta C) at 1,025 °C under

142

O2, which converted C to CO2 and N to N2. The weight of the cupric ferrocyanide

143

sample was 0.07-0.08 mg for the C isotope measurements and 0.18-0.21 mg for the

144

N measurements.

145

The isotope ratios are presented in delta notation with respect to the standard VPDB

146

for C and AIR for N, as calculated according to the following equation:

147

δ (‰) = [(Rsa/Rstd) − 1] · 1000

148

where Rsa is the isotope ratio of a sample and Rstd is the isotope ratio of the standard.

149

The isotope ratios were calculated using the international standards USGS24

150

(δ13C = −15.99 ± 0.11‰) for CO2 and either IAEA-N1 (δ15N = +0.43 ± 0.07‰) or

151

IAEA-N2 (δ15N = +20.32 ± 0.09‰) for N2. The mean values and standard deviations

152

of the standards were δ13C = −16.24 ± 0.29 ‰, δ15N = +0.20 ± 0.16 ‰ and

153

δ15N = +20.03 ± 0.29 ‰, respectively. The working standard (glutamic acid with δ13C

154

= −27.48 ± 0.05‰ and δ15N = 4.80 ± 0.08‰) was measured every twelfth sample.

155

The second working standard (K4[Fe(CN)6]·3H2O with δ13C = −25.48 ± 0.09‰ and

156

δ15N = −0.13 ± 0.06‰) was structurally similar to the measured sample, and the cal-

157

culated N and C weight percentages in cyanide were identical as expected, which

158

demonstrates that the combustion and reduction were complete. The sampIes were

159

analyzed at least in triplicate, and values were accepted when the precision (σn-1, n =

160

3) was < 0.3‰. Two samples were considered to be significantly different when their

161

isotope ratios differ by more than three standard deviations of the measurement.

162

Previous studies have revealed that for groundwater samples22, no significant isotope

163

fractionation occurs during the entire process (extraction-precipitation-combustion-

(1)

8


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164

mass spectrometry), whereas for solid samples21, a slight isotopic shift occurs for

165

carbon (up to 2‰).

166 167

Results and Discussion

168

Stable carbon and nitrogen isotope ratios of the solid samples. Tables S1−6 list

169

the isotope data of the individual samples. Table 1 summarizes the results for all the

170

samples, and Fig. 1a presents the δ13CCN and δ15NCN values for all the solid samples.

171

Cyanides related to coal carbonization (samples from MGPs or COPs) have the

172

highest δ13CCN values (mean −10.5 ± 3.5‰) of all the investigated samples (Fig. 1a,

173

blue circles). The difference between the lowest δ13CCN value of coal carbonization

174

cyanide (−16.6‰, Table 1) and the highest δ13CCN value of other cyanides is 6.5‰

175

for blast furnace sludge, 9.6‰ for paper-deinking sludge, 4.3‰ for sewage fields,

176

9.4‰ for road salt, and 4.1‰ for unclear samples. Considering the analytical repro-

177

ducibility, these differences are highly significant. The δ13CCN values of the blast fur-

178

nace sludge reveal a small range of 8.3‰ (−31.4 to −23.1‰) with a mean of −26.9 ±

179

1.5‰ (Fig. 1a, red circles). Compared with the coal carbonization cyanides, these

180

cyanides are significantly lighter. The δ13CCN values of the commercial cyanides

181

(mean −26.0 ± 3.0‰) are within the range of the cyanides from the blast furnace pro-

182

cess but differ significantly from those of the coal carbonization process (black,

183

green, and purple circles in Fig. 1a). The range of the δ15NCN values for blast furnace

184

sludge is 9.7‰ (+0.6 to +10.3‰, mean +4.6 ± 2.1‰), even larger than that of the

185

δ15NCN values of the coal carbonization samples (+0.1 to +4.8‰, mean +2.6 ± 1.1‰)

186

(Fig. 1a). Notably, the commercial cyanides tend to have low, mostly negative δ15NCN

187

values (mean −1.7 ± 2.0‰), which appears to be characteristic of these types of

188

samples. The nitrogen isotopic composition of the samples of unclear origin (mean

189

1.4 ± 0.4‰) is similar to coal carbonization cyanides (Fig. 1a, triangle). 9



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190

Coke oven and manufactured gas plants. Coal is carbonized by heating in the ab-

191

sence of air either in retorts at MGPs to obtain town gas or in coke ovens at COPs to

192

obtain coke.25−28 A conservative estimate places the number of former MGP and

193

COP sites worldwide at 8,700.29 When coal is heated, volatile constituents vaporize.

194

The so-called raw gas contains hydrogen (H2), carbon monoxide (CO), methane

195

(CH4), and low-carbon-number compounds such as ethylene (C2H2), ammonia (NH3),

196

hydrogen sulfide (H2S) and HCN. Hydrogen cyanide is not a primary constituent of

197

coal; rather it is generated by reactions of NH3 with glowing coke (reaction 2 with re-

198

action enthalpy) or with reduced carbonaceous gases (reactions 3-5).

199

NH3 (g) + C (s) → HCN (g) + H2 (g)

165 kJ/mole

(2)

200

NH3 (g) + CO (g) → HCN (g) + H2O

41.8 kJ/mole

(3)

201

NH3 (g) + CH4 (g) → HCN (g) + 3H2 (g)

251 kJ/mole

(4)

202

2NH3 (g) + C2H4 (g) → 2HCN (g) + 4H2 (g) 264 kJ/mole

(5)

203

These reactions are endothermic, so high temperatures favor the formation of HCN.

204

The formation of HCN is also favored by high NH3 concentration, low water content of

205

the coal and low oven height.

206

The δ13C values of coal typically range between −27 and −22‰ and reveal an aver-

207

age of −25‰ according to the compilation of Whiticar30 (Fig. 1b). Compared to the

208

reactant, 13C isotopes are preferentially incorporated into the HCN molecule (Fig. 1b).

209

The N isotope ratios of Chinese coal are in the ranges −6 to +10.1‰31,32 and are the

210

broadest one that have been reported. However, approximately 73% of these δ15N

211

values ranged from −3 to +2‰.31 Australian δ15N coal values are in the ranges +0.3

212

to +3.7‰33, North American −1.4 to +1.6‰34, and European −2.5 to +6.3‰.35-37 Be-

213

cause our MGP and COP samples were derived from European coal, we indicated

214

the European δ15N range in Fig. 1b. Furthermore, it should be taken into account that

215

most of the investigated samples (Table S1; German sampIes) derived from Rhine10


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land-Westphalian coal (Germany) for which δ15N values of −2.5 to 0.5‰ 35 and

217

+3.5‰37 have been published. An enrichment, but not so pronounced, of the heavier

218

isotope can also be assumed for nitrogen because all MGP and COP samples had

219

positive δ15NCN values, which is not always the case for European and Rhineland-

220

Westphalian coal (Fig. 1a and b). What are the possible reasons for the observed

221

δ13CCN and δ15NCN values of the MGP and COP samples? To understand this result,

222

we considered the fractionation of carbon and nitrogen isotopes among gaseous

223

molecules and calculated the theoretical enrichment factors and corresponding

224

slopes in dual isotope plots for

225

were obtained from Richet et al.38 and included a temperature range of 0-1,300 °C.

226

Considering reaction (4), cyanide-nitrogen (NH3→HCN) becomes isotopically heavier

227

at temperatures < 150 °C and isotopically lighter at higher temperatures, whereas

228

cyanide-carbon (CH4→HCN) becomes isotopically heavier at all temperatures (Table

229

S8 and Fig. 1c). At first glance, the nitrogen result appears to be contradictory be-

230

cause the raw gas leaves the coke oven with temperatures of approximately 600-750

231

°C, where HCN is enriched in the lighter isotope (14N). However, after the gas leaves

232

the oven, it is abruptly cooled to approximately 80 °C and then purified in several

233

steps at temperatures of approximately 20-30 °C.28 At many sites, the final purifica-

234

tion step was removal of toxic and corrosive HCN and H2S by passing the raw gas

235

through the so-called spent oxides or purifiers, which consisted of iron oxides that

236

mostly originated from bog iron and iron ores. Prussian blue was formed by reactions

237

of HCN with the Fe-rich purifier. The removal of HCN was quantitative.28 We hypoth-

238

esize that the CCN and NCN isotope ratios were controlled by low temperature frac-

239

tionations rather than by high temperature fractionations that would have been asso-

240

ciated with an enrichment of the lighter nitrogen isotope. During cooling, the equilibri-

241

um between the gases shifted toward the enrichment of

13

C versus 15N (see Supporting Information). The data

11

15


N and 13C in HCN (Fig 1c).


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242

Similar results are obtained for the isotope fractionations associated with reaction (3)

243

(Table S9).

244

If our hypothesis is correct and if the cyanides of the unclear samples are related to

245

the MGP process, then a higher raw gas temperature during gas purification may

246

explain the observed slight isotope fractionation. The fact that a non-conventional

247

coal carbonization process (drapery factory) had been in operation at the site is per-

248

suasive evidence for an unusual gas purification process. Regardless, when tracing

249

the source of cyanides at former coal carbonization sites using the stable-isotope

250

approach, it is essential to perform historical research into the manufacturing process

251

that was used.

252

Blast furnace sludge. During blast furnace operation, KCN and NaCN are generat-

253

ed at temperatures > 1,000 °C (reaction 6, with M being potassium or sodium).39−41

254

Potassium- and Na-containing compounds enter the blast furnace as impurities in

255

coke, iron ores, and fluxes. These compounds are partially reduced to K and Na va-

256

por near the bottom of the blast furnace (reaction 7). Cyanides also form in the mid-

257

dle part of the blast furnace (reaction 8). Whether other reactions occur in the for-

258

mation of cyanide is unclear.39

259

2M(g) + 2C(s) + N2(g) → 2MCN(g)

(6)

260

M2SiO3(s) + 3C(s) → 2M(g) + Si(g) + 3CO(g)

(7)

261

M2CO3(s) + N2(g) + 5CO(g) → 2MCN(g) + 4CO2(g)

(8)

262

Nitrogen originates from the pre-heated air that is blown into the blast furnace, and C

263

originates from coke. Air-N has a δ15N value of 0‰ (by definition). Nearly identical

264

isotopic compositions for coal and residual coke have been revealed42 in pyrolysis

265

experiments (δ13C of approximately −23.5‰). Our measurements for coke confirm

266

this value (δ13C of −23.5‰). For N, we did not obtain sufficient signal intensity be-

267

cause of the very low N content of coke. Our results indicate that all the samples are 12


Page 13 of 26


15

N (Fig. 1b), in agreement with the theoretical finding that

15

268

enriched in

269

entially incorporated into the product HCN over reactant N2 even at temperatures >

270

1,000 °C (Table S10, Fig. 1c). The carbon isotope ratios indicate the absence of any

271

isotope fractionation for half of the samples (the δ13CCN values are identical to that of

272

the reactant coke), but the lighter isotope (12C) is preferentially incorporated into the

273

cyanide molecule for the remaining samples (Fig. 1b). Because coke is not a gas, we

274

cannot calculate any carbon isotope fractionation from the Richet et al.38 data.

275

After formation, some water-soluble alkali cyanides leave the blast furnace at the top

276

with the dusty top gas. Before reuse in the operation process, the top gas is first

277

cleaned by scrubbers leaving a muddy blast furnace sludge behind. Owen39 reported

278

that less than 10% of the cyanide in the scrubber water was in the form of iron-

279

cyanide complexes, while the remainder was free cyanide. Pablo et al.43 reported

280

that in the effluent water, which originates from the last cleaning step, only approxi-

281

mately 50% of the total cyanide was free cyanide. With increasing contact time be-

282

tween sludge and water, the proportion of dissolved iron-cyanide complexes increas-

283

es relative to total cyanide. As long as free cyanide is present in the scrubber and

284

effluent water, some cyanide may volatilize as HCN. Johnson et al.18 found that the

285

volatilization of HCN enriched the heavier carbon and nitrogen isotopes in the re-

286

maining cyanide at pH < 9, which is above the normal pH of the scrubber and effluent

287

waters (pH 6-8).44 Therefore, different degrees of volatilization can explain the scat-

288

tering of the δ15NCN values in the blast furnace data in Fig. 1a. However, Owen39

289

stated that atmospheric cyanide contamination at scrubber towers is low. Therefore,

290

the effect of volatilization on the isotope composition of the cyanide molecule should

291

be low.

292

In the past, blast furnace sludge was disposed of in large uncontrolled landfills. In

293

such landfills, free cyanide was never present,5 and the main cyanide-containing 13


N is prefer-


294

compound was K2Zn3[Fe(CN)6]29H2O.5 Because neither complexation nor precipita-

295

tion reveal pronounced C or N isotope fractionation,18 the post-landfilling effects on

296

the isotopic composition of cyanide should be insignificant. This hypothesis is under-

297

scored because the δ13CCN and δ15NCN values were nearly identical for “young”

298

(−27.5 ± 1.2‰ for carbon and +4.2 ± 1.9‰ for nitrogen) and “old” (−26.2 ± 1.6‰ for

299

carbon and +4.1 ± 1.3‰ for nitrogen) blast furnace sludge samples at the Schalker

300

Verein site (Table S2). Furthermore, the δ13CCN and δ15NCN values for all five sites

301

overlapped and were not site-specific (Table S2). Because the blast furnace process

302

to produce pig iron was and is relatively uniform due to the energy aspects and the

303

materials used worldwide, we postulate that the δ13CCN and δ15NCN values found in

304

this study might be typical for blast furnace-related cyanides at many locations.

305

Commercial cyanides. In Europe, blue pigments with the general formula AI-

306

FeIII[FeII(CN)6] (where A represents Na+, K+, or NH4+) are important in the printing

307

industry to produce blue copying paper and blue paper.45 A paper de-inking sludge is

308

created during the processing of paper for recycling. To avoid landfilling or incinerat-

309

ing the sludge, paper de-inking sludge is increasingly used as a soil amendment in

310

some countries. In the past, some municipal wastewaters have contained commercial

311

cyanides, from electroplating operations, for example. In some countries, these wa-

312

ters have been landfilled in sewage fields. A study at a sewage field revealed con-

313

centrations of 45-839 mg kg–1 CN (Table 1). Rennert et al.11 determined that these

314

cyanides were exclusively bound as iron-cyanide complexes. Cyanides are always

315

added to road salts (deicing salts) as anticaking agents to ensure uniform spreading

316

during application. In North America, the main anticaking agent used in such chemi-

317

cal deicers is Na ferrocyanide; Prussian blue is sometimes used.7,8 In Europe, the K

318

and Na salts of ferrocyanide are applied. Road salt consumption in Germany strongly

319

depends on meteorological conditions and was lowest in 1992 with 608,000 Mg and 14


Page 14 of 26

Page 15 of 26


320

highest in 2005 with 3,447,000 Mg during a 5 year period beginning in 1991. In addi-

321

tion, 50,000-200,000 Mg of road salt has been imported annually. In Germany, an

322

upper limit of 200 mg of Fe(CN)6 per kg of road salt has been established by the salt

323

industry. Currently, approximately 100 mg of Fe(CN)6 per kg of salt are used, which is

324

consistent with our measured value of 60 mg kg–1 CN (Table 1). Based on the

325

amount of road salt applied and imported, and the 60 mg kg–1 CN concentration, 50-

326

275 Mg of CN is added to the near-road soil environment per year in Germany. The

327

isotope analyses reveal that both the δ13CCN and δ15NCN values are identical for the

328

feedstock and product (Table 1; Fig. 1a, violet circles). This result is expected be-

329

cause mixing the feedstock and salt should not cause any isotope fractionation.

330

The feedstock for the production of commercial cyanides is HCN, which is manufac-

331

tured from CH4 and NH3 using the Andrussow process (reaction 9). The methane

332

originates from natural sources, and NH3 originates from the Haber process.

333

2CH4 + 2NH3 + 3O2 → 2HCN + 6H2O

334

The slightly negative δ15N values of commercial cyanides obviously result from

335

depletion during the generation of HCN consistent with the findings reported in Tab.

336

S8 and Fig. 1c. Similar δ15N values for commercial cyanides (−5 to −2‰) have been

337

found in North America.18 In addition, for C (CH4→HCN), a minor depletion of the

338

heavier isotope can be expected (Table S8 and Fig. 1c). However, the source of the

339

CH4 is more important for the isotopic fingerprint of carbon. For example, Johnson et

340

al.18 found extremely low δ13CCN values between −60‰ to −35‰. The commercial

341

cyanides analyzed by these authors were obtained from two producers. One produc-

342

er used CH4 with higher values from western Canada, and the other producer used

343

CH4 with lower values from the Gulf of Mexico. It was not possible to elucidate the

344

origin of the feedstock of the commercial cyanides analyzed in this study. Because

345

Prussian blue is also used to dye textiles,45 a commercial source of cyanides is pos-

(9)

15


15

N


346

sible for the drapery site (unclear samples). Unfortunately, we could not obtain infor-

347

mation regarding the use of Prussian blue as a dye at this site.

348

Stable carbon and nitrogen isotope ratios of the aqueous samples. Eleven

349

groundwater samples of known origin were analyzed for their stable CCN and NCN

350

isotope signatures. Table 1, Table S7 and Fig. 2 summarize the results. The blue box

351

in Fig. 2 designates the range of δ13CCN and δ15NCN values for the cyanide-containing

352

solids from coal carbonization, and the red box indicates the values for solids from

353

blast furnace operations . For commercial cyanides, our data were too limited to des-

354

ignate any range. Groundwater from a blast furnace sludge landfill in the Ruhr area in

355

Germany was contaminated by up to 5,200 µg l–1 CN (Table 1). The δ13CCN and

356

δ15NCN values of these dissolved cyanides are consistent with those of the corre-

357

sponding solids (Fig. 2, red circles). Thus, the dissolution and transport of cyanide-

358

containing solids into the groundwater has no apparent effect on the isotopic compo-

359

sition of the cyanide. The same is true for contaminated groundwater from former

360

COP and MGP sites (Fig. 2, blue circles). Whether the slight shift of dissolved cya-

361

nide toward lighter isotopes is systematic at coal carbonization sites currently re-

362

mains speculative. To clarify this result, more samples must be analyzed. Additional-

363

ly, we analyzed groundwater from two electroplating sites (Fig. 2, green circles). The

364

carbon isotope signatures of commercial cyanides significantly differ from those of

365

cyanides derived from the coal carbonization process. In contrast, the δ13CCN values

366

of commercial cyanides and blast furnace-related cyanides overlap, as is the case for

367

the δ15NCN values of the entire sample set. The observation that solid commercial

368

cyanides tend toward lighter nitrogen isotopes (Fig. 1a) is not clearly supported by

369

the δ15NCN values of the three groundwater samples (Fig. 2). Again, more samples

370

must be analyzed. When the dissolved cyanides originate from either the coal car-

371

bonization process or the blast furnace operation/use of commercial cyanides, the 16


Page 16 of 26

Page 17 of 26


372

source of the cyanides can be traced using the stable CCN isotope signature. Tracing

373

the source of dissolved cyanides using the stable NCN isotope signature alone does

374

not seem to be possible given our present understanding of the isotope distribution.

375

Where δ13CCN values overlap, very high δ15NCN values (> +6‰) seemed to be indica-

376

tive for a cyanide source in the blast furnace and very low values (< 0‰) for a com-

377

mercial source.

378

The requirement to discriminate among different sources of cyanide pollution is im-

379

portant from a legal perspective according to the ‘polluter pays’ principle as outlined

380

before. In Europe, there are former hot spots of coal, iron and steel industries like the

381

Ruhr area in Germany but also in France, the UK, and Poland. In these areas, on a

382

small-scale MGP/COP sites can be close to blast furnace sites. For example, the city

383

of Bochum in the Ruhr area housed about 40 former MGP and COP sites together

384

with several blast furnace sites. Groundwater contamination with cyanide is not ex-

385

ceptional and to differentiate between MGP/COP- or blast furnace-source is a re-

386

quirement. But also in rural regions, cyanide pollution is common because nearly

387

every town in Middle Europe owned a MGP.29 In some cases, a second cyanide

388

source exits that is mostly electroplating. Which source, however, contributes to

389

groundwater or soil pollution can be unknown. Hence, the source of pollutive cyanide

390

is not always obvious and to identify the polluter of a cyanide contamination is a chal-

391

lenge.

392 393

Supporting Information

394

Additional information is included on the sampling sites, individual samples and the

395

calculation of the theoretical enrichment factors for the isotope fractionation of some

396

gaseous molecules. This material is available free of charge via the Internet at

397

http://pubs.acs.org. 17



398 399

Notes

400

The authors declare no competing financial interests.

401 402

Acknowledgments

403

The authors thank Deutsche Forschungsgemeinschaft for the financial support of this

404

study (contract number: Ma 2143/6-2). We are grateful to J. Küper (Herne, Germa-

405

ny), J.-F. Wagner (Trier, Germany), F. Götzfried (Bad Wimpfen, Germany), R. Zim-

406

mermann (Berlin, Germany), P. Marion (Nancy, France) and T. Magiera (Zabrze, Po-

407

land) for help in supplying the samples, to H. Biernath and E. Niziolek (Bochum,

408

Germany), K. Greef and J. Weihmann (Köln, Germany) for laboratory assistance, to

409

U. Schulte (Bochum, Germany) for the isotope measurements, and to B. Steinweg

410

(Viersen, Germany) for support in the historical research at the Nettetal site. We

411

greatly acknowledge the valuable comments of three unknown reviewers.

412

18


Page 18 of 26

Page 19 of 26

413 414


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22


Page 22 of 26

Page 23 of 26


538

Figure legends

539

Figure 1. a) δ13CCN and δ15NCN values of iron-cyanide complexes from different cya-

540

nide-containing wastes and soils (blast furnace sludge, n = 71; manufactured gas

541

plants and coke oven plants, n = 39; paper-deinking sludge, n = 3; sewage field, n =

542

7; road salts, n = 2; unclear, n = 4)

543

b) Ranges of the δ13CCN and δ15NCN values of cyanide from compounds with different

544

sources (MGP and COP, manufactured gas plant and coke oven plant, respectively)

545

from this study and other studies on commercial cyanides. The ranges of δ13C

546

(worldwide) and δ15N (Euopean) for coal are also indicated. The value of the δ13C of

547

coke is approximately −23.5‰. For the δ15N of coke, no data are available.

548

c) Slopes of theoretical enrichments of three major processes forming cyanides in

549

coal carbonization (at MGP/COP), blast furnace operation, or via the Andrussow pro-

550

cess in commercial cyanide production. Note that for the Andrussow process, various

551

methanes with original carbon isotope signals of −60 to −30‰ are used, whereas for

552

coal carbonization, the methane originating from coal pyrolysis has the signal of coal,

553

−27 to −22‰.

554 555

Figure 2. δ13CCN and δ15NCN values of iron-cyanide complexes in contaminated

556

groundwaters of different origins (open circles). The boxes indicate the range of sta-

557

ble isotope ratios that were determined for the corresponding solids.

23



12

a)

blast furnace sludge

δ15N (‰ Air)

8

manufactured gas plants and coke oven plants

4 unclear paperdeinking sludge

0

sewage field road salt and feedstock

-4

-8 -40

-30

-20

-10

0

13

12

δ C (‰ VPDB) b)

blast furnace sludge

δ15N (‰ Air)

8 commercial cyanide17 ,18

4

unclear

MGP and COP

0 coal30,35-37

-4 commercial cyanide16

-8 -60 12

-50

-30

-20

-10

0

δ13C (‰ VPDB) c) blast furnace sludge at 1,100 °C: 2C + 2H + N2 2HCN

8

δ15N (‰ Air)

-40

commercial cyanide this study

coal

4

-4

20°C 150°C

60

various methanes commercial cyanide: Andrussow process at 1,100 °C CH4 + NH3 + 0.5O2 HCN + 3H2O

0

MGP at ... °C: CH4 + NH3 HCN + 3H2

C 0°

-8 -60

-50

-40

-30

-20

-10

0

δ13C (‰ VPDB)

558 559

Figure 1 24


Page 24 of 26

Page 25 of 26


12 blast furnace sludge

δ15N (‰Air)

8

manufactured gas plants and coke oven plants

4 0 electroplating

-4 -8 -40

560 561 562 563 564

-30

-20

-10

0

δ13C (‰ VPDB)

Figure 2

565

25



Page 26 of 26

566

Table 1. Total cyanide contents, mean and ranges of δ13CCN- and δ15NCN-values of iron-cyanide complexes occurring in contaminated wastes, soils,

567

and groundwaters

568

Origin of sample

na

Type of sample

range of CN

δ13CCN (‰ VPDB)

δ15NCN (‰ AIR)

mg kg–1

mean

range

mean

range

39

75 to 70,450

–10.5±3.5

–16.6 to –3.1

2.6±1.1

0.1 to 4.8

569 570

coal carbonization

gas purifier waste,

571

b

contaminated soils c

572

blast furnace operation

BFS landfills

72

105 to 20,460

–26.9±1.5

–31.4 to –23.1

4.6±2.1

0.6 to 10.3

573

commercial

paper-deinking sludge

3

545 to 820

–28.5±2.0

–29.9 to –26.2

–1.3±0.9

–2.2 to –0.3

574

commercial

sewage sludge

7

45 to 839

–24.9±3.4

–28.7 to –20.9

–1.6±2.6

–5.6 to 1.9

575

commercial

road salt

1

60

–26.0

-

–2.5

-

576

commercial

road salt feedstock

1

322,475

–26.1

-

–2.5

-

577

unclear

gas purifier waste or

4

195 to 852

–22.7±1.7

–24.5 to –20.7

1.4±0.4

1.1 to 1.9

d

578

commercial cyanide

579

µg l

-1

580

coal carbonization

groundwater

4

570 to 2,060

–16.1±1.2

–17.3 to –14.8

2.7±0.9

1.8 to 3.6

581

blast furnace operation

groundwater

4

160 to 5,680

–27.2±1.7

–28.7 to –24.9

4.4±1.2

3.1 to 6.0

582

commercial (electroplating)

groundwater

3

99 to 989

–29.4±3.8

–32.5 to –25.2

1.2±1.7

–0.5 to 3.0

583

a

b

c

d

number of samples; standard deviation; blast furnace sludge; Na4[Fe(CN)6]10H2O

584

26


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