Determination of total cyanide in soils by micro-distillation

Analytica Chimica Acta 406 (2000) 283–288

Determination of total cyanide in soils by micro-distillation Tim Mansfeldt ∗ , Heidi Biernath Soil Science and Soil Ecology Group, Faculty of Geosciences, University of Bochum, D-44780 Bochum, Germany Received 28 June 1999; received in revised form 4 October 1999; accepted 5 October 1999

Abstract This study was conducted to determine whether a micro-distillation apparatus, which was developed for analyzing aqueous samples, can be used for analyzing total cyanide in heterogeneously contaminated solid samples. The samples investigated were cyanide-contaminated soils from former gas works and blast furnace sludges. Sample preparation was restricted to air-drying and sieving <2 mm. The samples represent a wide range in cyanide concentrations (from <1 to about 10,000 mg kg−1 CN− ). Results of cyanide determination by the micro-distillation apparatus correlated closely with those obtained by the German standard distillation method (r = 0.997, p < 0.001%, n = 20). As the micro-distillation technique is easier to perform and faster than the standard distillation technique and equal in accuracy and resolution, the application of the micro-distillation apparatus for analyzing total cyanide in soils is recommended. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Determination; Total cyanide; Soil; Micro-distillation

1. Introduction Cyanide compounds in the environment originate mainly from a variety of industrial sources, such as the electroplating industry, blast furnaces, coke-producing plants and gas works. By several pathways, cyanide compounds enter the environment from these sources. Since cyanide compounds can release the extremely toxic free cyanides (hydrogen cyanide, HCN, and cyanide ion, CN− ), there is a need for rapid and precise determination of cyanides in air, waste waters, ground waters, and soils. The determination of total cyanide is an important test in assessing soil contamination caused by this pollutant. In general, acid distillation is used to determine ∗ Corresponding author. Tel.: +49-234-700-3439; fax: +49-234709-4469. E-mail address: [email protected] (T. Mansfeldt).

total cyanide [1,2]. This procedure consists of a preparative and a determination step. The objectives of the preparative step include the removal of interferences and the conversion of the cyanide species to HCN under distillation conditions. During distillation, the free cyanides as well as the weak metal cyanide complexes of Cd, Pb, Ni, and Zn and strong metal complexes of Fe are converted to HCN. The gas is captured in an alkaline absorber solution. Cyanide determination can be carried out by several titrimetric, colorimetric and potentiometric methods including ion chromatography [1–3]. Recently, a micro-distillation apparatus was developed which uses small weight-in quantities for the determination of cyanides in water. In soils of former coking plant sites, cyanide mainly occurs as the slightly soluble pigment Berlin blue, Fe4 III [FeII CN)6 ]3 , and soluble iron cyanide complexes, [Fe(CN)6 ]3−/4− [4–6]. Berlin blue is present in soils in the form of small particles. As the distribution of Berlin blue in such soils is inhomogeneous

0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 7 7 0 - 9

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[4,7], the application of a cyanide-determining system based on small weight-in quantities is questionable. The aim of this study was to compare a standard distillation apparatus and a micro-distillation apparatus by analyzing cyanide-contaminated soil samples.

2. Experimental 2.1. Soil samples The materials investigated were mostly soils from former coking plant sites (15 samples). In addition, soils contaminated by blast furnace gas sludge (3 samples) as well as blast furnace gas sludge itself (2 samples) were analyzed. About 2 kg material were taken from excavated pits at former coking plant sites and at a blast furnace sludge disposal site. The samples were manually homogenized, air-dried, and sieved (2 mm) through a metal sieve. 2.2. Chemicals All chemicals were of analytical reagent grade and the water was de-ionized. 2.2.1. Standard solutions Stock solutions of potassium cyanide, KCN (Riedel-de Haën, Seelze, Germany), potassium hexaycanoferrate (II), K4 [Fe(CN)6 ] (Riedel-de Haën), and potassium cyano zincate, K2 [Zn(CN)4 ] (Merck, Darmstadt, Germany), containing 1000 mg l−1 CN were prepared in a 0.01 M NaOH solution. From these solutions, mixtures with different ratios of the above-mentioned cyanide species were prepared. 2.2.2. Standard additions A contaminated soil sample (393 mg kg−1 CN) was spiked by K4 [Fe(CN)6 ] with amounts of 20, 30, 40, 50 and 100 ␮g CN. For this purpose, known amounts of dissolved K4 [Fe(CN)6 ] were added to the soil sample immediately before digestion. 2.2.3. Chemicals for digestion [2] 5 M hydrochloric acid: 420 ml HCl (MallinckrodtBaker, Deventer, The Netherlands) is filled up with water to 1 l.

1 M sodium hydroxide solution: 40 g NaOH (Mallinckrodt-Baker) is dissolved in 1 l water (standard distillation). 0.1 M sodium hydroxide solution: 4.0 g NaOH is dissolved in 1 l water (micro-distillation). Copper(II) sulfate solution: 100 g CuSO4 (Merck) is dissolved in 500 ml water. Tin(II) chloride solution: 25 g SnCl2 (MallinckrodtBaker) is dissolved in 20 ml 1 M HCl and filled up with water to 50 ml. The solution is freshly prepared every week. 2.2.4. Chemicals for photometric determination [2] Buffer solution, pH 5.4: 6 g NaOH is dissolved in 25 ml water. 11.8 g dicarbon acid C4 (Riedel-de Haën) is then dissolved in the warm NaOH solution and filled up with water to 100 ml. Potassium hydrogen phthalate buffer, pH 4.8: 40.85 g potassium hydrogen phthalate (Merck) is dissolved in 1 l water. Chloramine-T solution: 0.5 g chloramine-T (Riedelde Haën) is filled up with water to 50 ml (standard distillation) and 0.25 g chloramine-T is filled up with water to 50 ml (micro-distillation). The solutions are freshly prepared every day. Barbituric acid–pyridine solution: 1.75 g NaOH is dissolved in about 125 ml water. 4.2 g 1,3-dimethyl– barbituric acid (Fluka, Neu-Ulm, Germany) and 3.4 g pyridine–4-carbon acid (Merck-Schuchardt, Hohenbrunn, Germany) are added and stirred for 0.5 h. The solution is filled up with water to 250 ml. The solution is prepared weekly and protected from light. 2.3. Distillation procedure and cyanide determination All digestions were performed with five replicates. A double-beam spectrophotometer (UV-Vis Spectrometer Lambda 2, Perkin–Elmer Germany, Überlingen, Germany) was used with 1 cm quartz cuvettes for cyanide detection. 2.3.1. Standard distillation The standard distillation was performed according to the German standard method [2]. The macro-distillation apparatus represented in Fig. 1 was used. The analyses were performed with six appara-

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Fig. 1. Schematic diagram of the standard distillation apparatus [2]. (1) heating device, (2) round bottom flask with three angled-side necks, 500 ml, stirrer (3) flow controller, (4) inlet tube, (5) wash bottle with soda lime, (6) dropping funnel, (7) Liebig condenser, (8) absorption vessel with sintered glass frit, (9) distillation tube, (10) membrane vacuum pump.

tuses, occupying a space of 1.30 m × 0.80 m × 0.20 m (width × height × depth) and standing in a hood. For digestion purposes, 5–10 g of soil, 100 ml water and a stirrer are added to a 500 ml three-necked flask. This reaction vessel is connected to a reflux tube condenser, an inlet tube and a vacuum tube for the drawing pump. 20 ml 1 M NaOH are added to the absorption vessel. The absorber is attached to the vacuum and connected to the condenser. While stirring 10 ml CuSO4 , 2 ml SnCl2 , carefully 20 ml 5 M HCl are added and refluxed for 2 h. The energy requirement is about 5000 W for this time. For photometric determination, the absorption solution is transferred into a 100 ml volumetric flask. The absorber is rinsed with water, added to the flask, diluted to volume and mixed thoroughly. An aliquot (0.5–5 ml) is pipetted into a 50 ml volumetric flask. 4 ml buffer (pH 5.4) and 2 ml chloramine-T are added. After 5 min, 6 ml barbituric acid–pyridine are pipetted to the solution, filled up with water and mixed thoroughly. After 20 min, the samples can be measured photometrically at 600 nm.

285

Fig. 2. Schematic diagram of the micro-distillation apparatus. (1) sample vessel, (2) frit, (3) heater, (4) capillary, (5) absorption vessel, (6) cooler, (7) keyboard and display, (8) protection hood, (9) syringe for reagent feeding, (10) septum.

Netheler-Hinz, Hamburg, Germany) standing in a hood. A schematic diagram of the apparatus is shown in Fig. 2. The apparatus (0.32 m × 0.23 m × 0.43 m, width × height × depth) consists of six digestion and absorber units. For digestion, 0.1–0.5 g of soil, 10 ml water, 100 ␮l CuSO4 , 20 ␮l SnCl2 and a frit are added to the sample vessel. 1 ml 0.1 M NaOH is pipetted into the absorption tube (10 ml volumetric flask). The sample vessel is closed and a capillary is inserted and connected with the absorption vessel. 200 ␮l HCl (5 M) are then added to the sample vessel with a syringe. The micro-distillation program for digestion is set to 2 h and 108◦ C. The energy requirement is about 720 W for this time. For photometric determination, 1 ml potassium hydrogen phthalate buffer and 1 ml chloramine-T solution are added to the absorption vessel. After 5 min, 1 ml barbituric acid–pyridine solution is pipetted into the tube, diluted to volume and mixed thoroughly. After 40 min, the samples can be measured photometrically at 600 nm. 3. Results and discussion 3.1. Standards

2.3.2. Micro-distillation Micro-distillation was performed with a microdistillation apparatus (MicroDistiller, Eppendorf-

In evaluating both procedures, three cyanide species at different concentrations in de-ionized water were

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Table 1 Recovery of different cyanide species determined by two distillation systems (5 replicates; arithmetic mean, x; standard deviation, SD; relative standard deviation, RSD) Species

Taken

Standard distillation

Micro-distillation

(␮g l−1 CN)

Found x (␮g l−1 CN)

SD (␮g l−1 CN)

RSD (%)

recovery (%)

Found x (␮g l−1 CN)

SD (␮g l−1 CN)

RSD (%)

KCN

100 200 500

92 189 499

1.0 10.2 3.5

1.1 5.4 0.7

92 95 100

94 195 495

1.0 2.0 6.0

1.1 1.0 1.2

94 98 99

K2 [Zn(CN)4 ]

100 200 500

94 200 456

4.6 2.0 12.2

4.9 1.0 2.7

94 100 91

96 204 500

3.0 2.7 4.6

3.1 1.3 0.9

96 102 100

K4 [Fe(CN)6 ]

100 200 500

97 184 477

1.7 7.9 7.0

1.8 4.3 1.5

97 92 95

97.5 195 503

0.9 5.0 6.1

0.9 2.6 1.2

98 98 101

recovery (%)

analyzed (Table 1). Both the free cyanide as well as the weak and strong metal-complexed cyanides were converted to HCN during distillation. The recovery of cyanide was virtually complete for both methods ranging from 91 to 100% for the standard distillation method and from 94 to 102% for the micro-distillation method. The mean recovery was 95.1% for the standard method and 98.4% for the micro-distillation system, which indicates the validity of both methods for the determination of cyanides.

using a contaminated soil sample containing five additions of different cyanide concentrations. The cyanide recovery percentage ranged from 95 to 105% using the standard distillation method and 97–107% for the micro-distillation method (Table 2). The average cyanide recovery percentage was 99.0% for the standard distillation method and 101.4% for the micro-distillation method. The matrices that were evaluated showed no apparent interference for total cyanide determinations.

3.2. Standard addition

3.3. Soils

For the purpose of comparison the cyanide recovery percentage of the standard distillation apparatus and the micro-distillation apparatus was determined

Cyanide concentrations covered an extremely wide range with 0.53–10,829 mg kg−1 CN− determined using the standard distillation method

Table 2 Recovery of added cyanide, K4 [FeCN6 ], in a contaminated soil sample determined by two distillation systems (5 replicates; arithmetic mean, x; standard deviation, SD; relative standard deviation, RSD) Standard distillation added CN

Micro-distillation

(␮g CN)

Found x (␮g CN)

SD (␮g CN)

RSD (%)

recovery (%)

Found x (␮g CN)

SD (␮g CN)

RSD (%)

recovery (%)

20 30 40 50 100

18.9 30.4 41.9 48.0 103

0.46 0.75 1.05 1.00 1.84

2.4 2.5 2.5 2.1 1.8

95 96 105 96 103

19.5 29.2 42.9 50.3 105

0.09 0.52 0.09 0.09 2.08

0.5 1.8 0.2 0.2 2.0

97 97 107 101 105

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Table 3 Arithmetic mean (x), standard deviation (SD), and relative standard deviation (RSD) of cyanide concentrations in contaminated soil samples determined by two distillation systems (5 replicates) Sample

#2070 #2071 #2017 #2038 #2049 #2056 #2064 #2066 #2051 #2081 #2082 #2078 #2018 #2025 #2029 #911 #610 #1702 #2041 #1701

Standard distillation

Micro-distillation

x (mg kg−1 CN)

SD (mg kg−1 CN)

RSD (%)

x (mg kg−1 CN)

SD (mg kg−1 CN)

RSD (%)

334 90.7 436 7779 29.4 3754 59.9 218 2.57 6532 10, 829 1609 855 24.4 0.53 6561 119 17.9 1434 1145

8.28 7.23 9.50 353 1.23 346 1.80 10.6 0.40 214 433 32.8 37.6 1.5 0.05 410 6.00 2.77 20.4 74.6

2.5 8.0 2.2 4.5 4.2 9.2 3.0 4.9 15.6 3.3 4.0 2.0 4.4 6.2 8.8 6.3 5.1 15.4 1.4 6.5

356 93.1 485 7366 28.2 3406 63.6 235 2.86 6412 9540 2003 871 24.1 0.41 6404 114 17.4 1597 1605

4.81 3.16 10.6 387 1.26 331 1.78 11.3 0.23 336 609 202 77.9 2.05 0.08 1519 3.4 1.53 123 126

1.4 3.4 2.2 5.3 4.5 9.7 2.8 4.8 8.2 5.2 6.4 10.1 9.0 8.5 18.8 23.7 11.8 8.9 7.7 7.8

and 0.41–9540 mg kg−1 CN− determined using the micro-distillation method (Table 3). The mean relative standard deviation is 5.9% for the standard distillation apparatus and 8.0% for the micro-distillation apparatus. This difference is rather small. By defining three classes of cyanide concentration ranges, it can be seen that the mean values of cyanide concentration are 2% higher (<100 mg kg−1 CN− ), 5% higher (100–1000 mg kg−1 CN− ), and 3% lower (>1000 mg kg−1 CN− ) compared to the standard method (Table 4). The relative standard deviation in the low concentration range is somewhat lower.

In the highest concentration range, the relative standard deviation is about twice as high as the standard distillation procedure. The standard distillation apparatus uses relatively high weight-in quantities, thus minimizing the variance of cyanide concentrations in the soil samples. However, the results obtained by the two methods correlated closely as indicated by a correlation coefficient r = 0.997, a probability of error p < 0.001%, and number of data points n = 20. Mean total cyanide for all samples is 4955 mg kg−1 CN− obtained by the standard distillation method and 4792 mg kg−1 CN− obtained by the micro-distillation

Table 4 Sample number (n), arithmetic mean (x), standard deviation (SD), and relative standard deviation (RSD) in different cyanide concentration ranges of contaminated soil samples determined by two distillation systems Concentration range

n

(mg kg−1 CN) <100 100 to 1000 >1000

7 5 8

Standard distillation

Micro-distillation

x (mg kg−1 CN)

SD (mg kg−1 CN)

RSD (%)

x (mg kg−1 CN)

SD (mg kg−1 CN)

RSD (%)

32.2 392 4955

2.14 14.4 235

8.7 3.8 4.7

32.8 412 4792

1.44 23.6 454

7.9 5.8 9.5

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apparatus. The difference is only 3.7%. During the analysis of non-sieved soil samples in our laboratory, experience has shown that the relative standard deviation increases to values of 25% and more even when using the standard distillation method. It is therefore strongly recommended to homogenize soil samples by sieving when performing cyanide analysis. The solubility of Berlin blue is strongly pHdependent [4,5]. By means of an alkaline extraction, e.g. 0.25 M NaOH [8], the cyanides in soils of former coking plant sites should be completely dissolved [4]. The alkaline extract can be analyzed for total cyanide by the micro-distillation apparatus. The comparison of total cyanide determination by acid distillation and by alkaline extraction is the objective of the present research. Working with these extracts overcomes the problem that may be based on small weight-in quantities of solid samples. Today, a manual acid distillation procedure for assessing soil pollution by cyanides is laid down in Germany [2]. As the micro-distillation apparatus is based on the same principle of total decomposition of cyanides and because it yields statistically similar results, it is an alternative for the standard distillation apparatus.

4. Conclusions From the above discussion, it is obvious that the micro-distillation apparatus has significant advantages over the standard distillation technique. Some characteristics and/or advantages of the micro-distillation apparatus for analysis of total cyanide in soils are summarized below: 1. Working with the micro-distillation apparatus is easier and saves more time because of the simple program adjustment, and because only the sample vessels and adsorption tubes must be cleaned.

2. The micro-distillation apparatus has a space-saving design. 3. The micro-distillation apparatus consumes fewer chemicals, less energy, and no water for the reflux condenser. When soil samples are well homogenized, the micro-distillation apparatus is a suitable alternative to the commonly used distillation system for determining cyanide in soils.

Acknowledgements This paper represents publication no. 94 of the Priority Program 546 ‘Geochemical processes with long-term effects in anthropogenically affected seepage and groundwater’. Financial support was provided by Deutsche Forschungsgemeinschaft. We are grateful to Wolfgang Volmer for technical support. References [1] ASTM, American Society for Testing Material. Standard test method for cyanides in waters, D 2036-97, in: Annual Book of ASTM Standards, Philadelphia, PA, 1997. [2] Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung, Anionen (Gruppe D), Bestimmung von Cyaniden DIN 38405, Teil 13 und 14, VCH, Weinheim, 1988. [3] E.O. Otu, J.J. Byerley, C.W. Robinson, Int. J. Environ. Anal. Chem. 63 (1996) 81. [4] T. Mansfeldt, S.B. Gehrt, J. Friedl, Z. Pflanzenernähr. Bodenk. 161 (1998) 229. [5] J.C.L. Meeussen, M.G. Keizer, W.H. van Riemsdijk, F.A.M. de Haan, J. Environ. Qual. 23 (1994) 785. [6] T.L. Theis, T.C. Young, M. Huang, K.C. Knutsen, Environ. Sci. Technol. 28 (1994) 99. [7] F.P.J. Lame, P.R. Defize, Environ. Sci. Technol. 27 (1993) 2035. [8] J.C.L. Meeussen, M.G. Keizer, W.D. Lukassen, Analyst 117 (1992) 1009.