Environmental Geochemistry and Health 27: 11–18, 2005. 2005 Springer. Printed in the Netherlands.
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Natural cyanide degradation and impact on Ili River drainage areas from a Goldmine in Xinjiang autonomous region, China Li Shehong1,2, Zheng Baoshan1, Zhu Jianming1, Yu Xiaoying1 & Wang Binbin1 1 State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550002, P.R. China 2 Author for correspondence (fax: +86-0851-5891609; e-mail:
[email protected], lishehong@ lycos.com) Received 17 June 2003; Accepted 23 February 2004
Key words: cyanide, goldmine, Ili River, natural degradation, river pollution Abstract The Ili River is a very important river to the northwest of China and Kazakhstan. The Axi Goldmine is located in the upstream of a branch of the Ili River. The cyanide from the goldmine effluent is a threat to the downstream areas. According to our study, the natural degradation of cyanide conforms to a negative exponential equation in the tailings impoundment, second wastewater pond and even in the receiving streams if the dilution action from other streams were deducted. In the combined action of dilution and natural decomposing, the cyanide from the goldmine effluent does not pose a hazard to the trunk of the Ili River in the normal producing states. The equations of cyanide degradation in the streams and the hydrology parameters could be used to assess the environmental impact on the downstream areas if accidental discharges of cyanide occurred. The available way of decreasing the cyanide impact on the streams is to increase the rate of recycled water so that the lower the wastewater level in the tailing impoundment and the second wastewater pond, the lower is the amount of leakage of wastewater to the streams.
1. Introduction Cyanide (CN ) is hypertoxic to humans and many other living creatures (Roy et al. 1997; Dean & Christine 1999). Accidental discharge of cyanide has occurred in many parts of the world. The deepest impression to the public and the most serious episode was the pollution of the Danube and Tisza rivers by cyanide from the goldmine tailing dams of Romania which collapsed in early 2000. In most goldmine enterprises, 0.05–0.1% sodium cyanide solution is used to extract gold from the ore (US EPA 1994). So cyanidation of goldmines is one of the main sources of cyanide pollution. Although some measures are taken to remove the cyanide, the cyanide concentration in the effluent is still high for some reasons in many
goldmine enterprises. Studies show that cyanide can be decomposed in the natural environment, and the natural degradation velocity is controlled by many conditions (Johannes 1992; Boucabeille 1994; Peter 1999). Zaranyika et al. (1994) have studied the cyanide ion concentration in the effluent from goldmines and in the receiving rivers in Zimbabwe. There has been no study to report the levels of cyanide in effluents from goldmines and its degradation in the conditions of arid and semiarid areas in the northwest of China. The purpose of this article is to determine the levels of cyanide in the effluent from goldmines and its natural degradation in tailings impoundment and receiving streams in arid and semiarid climatic conditions. From experiments and calculated results, we can assess and forecast the environmental impact on the downstream areas in the
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LI SHEHONG ET AL.
local climatic and hydrology conditions when accidental cyanide discharges occur.
2. Materials and methods 2.1. Study area The Ili River is one of the most important rivers in the Xinjiang Autonomy Region of China. It is an international river running from China into Kazakhstan. The Axi goldmine is a large deposit and is located in the upstream of a branch of the Ili River in northwestern China (Figure 1). The climatic condition of this area is arid and semiarid, and there is a sharp difference in day and night temperatures, during the summer and winter. The tailings cyanide treatment technology involves alkaline chlorination. The treated slurries still containing some cyanide are discharged to a tailings impoundment. Some leaked solutions are collected in the second pond below. There are some solutions containing cyanide from the two impoundments leaking into Small Axi stream. Small Axi stream runs 2 km from the confluence with Big Axi stream into Axi stream, which runs 2 km with Qiatarte stream joining it, and again 30 km into Piliqing stream. Piliqing stream runs 40 km into the Ili River, which runs 60 km reaching Kazakhastan (Figure 1). The average minimum discharge and current velocity of these streams are shown in Table 1.
2.2. Sampling The water samples were from the tailings impoundment, the second regulating pond, the leaked solution from the two impoundments, and the rivers below the goldmine. The pH values were determined on site. All the water samples were collected in Teflon bottles, and NaOH was immediately added to get water pH values above 12. The cyanide concentrations of the samples were tested within 24 h.
2.3. Analytical method Many methods can be used to analyse cyanide (Singh et al. 1986; Out et al. 1996). We adopt the analytical standard of cyanide in water issued by China EPA. Cyanide in the water samples was distilled with phosphoric acid (pH < 2) and Na2-EDTA. The distilled liquid was absorbed into NaOH solution. If the cyanide concentration was high (‡1 mg L)1), it was determined by a titration method with AgNO3 solution. The cyanide concentration was determined with a spectrophotometric method using iso-nicotinic acid (C6H6NO2)-3-methy-1-phenyl5-pyrazolone if the cyanide concentration was low (<1 mg L)1). The results determined were for the total cyanides including simple cyanide and complex cyanide except hexacyanocobaltate (China EPA 1997).
Fig. 1. Map of the study area.
NATURAL CYANIDE DEGRADATION
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Table 1. The average minimum flux and current velocity of the streams in the study area. Average minimum discharge (m3s)1)
Average minimum current velocity (km h)1)
Small Axi stream
0.02
3.06
Big Axi stream
0.05
Streams
Axi stream
0.07
Qiatarte stream
0.05
2.27
Piliqing stream
1.75
2.12
Ili River (before the point of Pliqing stream joining it) Ili River (at the border)
118 216
2.8 2.8
3. Results and discussion
decompose very slowly due to the deficiency of sunlight and oxygen in the deep soil. The cyanide absorbed by soil will slowly leach into groundwater and rivers that become a chronic source of pollution. Other measures to treat cyanide must be taken at the same time. The decrease in cyanide concentration from tailings impoundment and ponds to their leakage water and the receiving streams shows that the natural degradation of cyanide is very fast in such geohydrological environments and climatic conditions.
3.1. Cyanide levels The average contents of cyanide in the water bodies in summer and autumn are shown in Table 2. We can see that the cyanide contents in the tailings impoundment and second pond are very high (average up to 151 and 47.7 mg L)1) but that the cyanide contents in the receiving streams are low probably because the quantity of the leakage water is small. The cyanide content in the Axi stream and its downstream water can be lower than the China drinking water standard (£0.05 mg L-1), but the water in the 2 km Small Axi stream cannot be used as potable water. Cyanide levels reduced sharply from the tailing impoundment and second regularity pond to their leakage water showing that much of cyanide in water is decomposed and adsorbed by the soil strata and soft deposits during leaking. From the results, we could use lots of soil or deposits to intercept, coffer and overlay the cyanide wastewater if accidental discharges of cyanide took place. But the disadvantage is that cyanide will
3.2. Static state degradation experimentation To quantitatively study the natural cyanide degradation in the tailings impoundment and in the second wastewater pond under the local climatic conditions, we collected 5 l wastewater from each of them (ST-1, SS-1). The initial CN) contents of ST-1 and SS-1 were 185.1 and 71.0 mg L-1 and the initial pH values were 11.6 and 10.7, respectively. We placed the samples outdoors on sites in two open containers and kept them in a static state. The cyanide concentrations were determined daily.
Table 2. The average cyanide (CN)) concentration (mg L)1) in the water bodies from Axi Goldmine. Sample sites
CN Concentration (mg L)1)
Tailings impoundment
151.4 ± 43.2
7
5.9 ± 1.7
8
47.7 ± 16.1 3.8 ± 0.7
13 8
Small Axi stream water
0.090 ± 0.050
13
Axi stream water below junction of Small Axi with Big Axi
0.030 ± 0.010
2
Axi stream below the point of Qiatarte stream joining it
0.016 ± 0.002
2
Leakage water from tailings
Frequency of test
impoundment The second pond Leakage from the second pond
LI SHEHONG ET AL.
CN- content (mg L-1)
14
80
250 ST-1
200 150
SS-1 y = 51.467e-0.0255x R2 = 0.9209
60
y = 205.76e-0.0242x R2 = 0.968
40
100 20
50 0
0
50
100 150 200 250
0
0
50
100
150
200
Fig. 2. The static state degradation curves of ST-1 and SS-1.
The slats of cyanide concentration over time are shown in Figure 2. Both of the curves can be fitted to a negative exponential equation. The natural degradation equations of ST-1 and SS-1 are listed as follows:
cyanide concentration as a function of time are shown in Figure 3. Similarly, all of the curves can be fitted into negative exponential equations. The natural degradation equations of DA-1, DM-1 and DT-1 in dynamic states are as follows:
ST-1: c ¼ 205.76e)0.0242t, the absolute value of the correlation coefficient |R| ¼ 0.984 SS-1: c ¼ 51.467e)0.0255t, the absolute value of the correlation coefficient |R| ¼ 0.960
DA-1: c ¼ 0.1068e)0.0131t, the absolute value of the correlation coefficient |R| ¼ 0.990 DM-1: c ¼ 60.391e)0.0133t, the absolute value of the correlation coefficient |R| ¼ 0.975 DT-1: c ¼ 217.5e)0.0166t, the absolute value of the correlation coefficient |R| ¼ 0.992
In the equations, c represents the concentration of cyanide (mg L)1), e represents the base of natural logarithm and t represents time (h). From the table of critical values of correlation coefficient R0.01,5 ¼ 0.874. Both of the absolute values of the correlation coefficient of the equations |R| > R0.01,5. This shows that fitting equations are significant.
The values of correlation coefficient of the equations show that the fitting equations are significant. 3.4. Cyanide degradation in streams To study the practical situation of cyanide degradation in the streams in local conditions, we collected water samples along the Small Axi and Axi streams in October. The changes of cyanide concentration in the stream water are shown in Figure 4. The cyanide concentration drops suddenly due to dilution from other branch streams. If we deduct the dilution function from other streams, according to hydrology parameters, the degradation curve is shown in Figure 5. It can also be fitted into the negative exponential equation.
3.3. Dynamic state degradation experimentation
CN– Content (mg.L-1)
To study the cyanide degradation in streams in local conditions when accidental discharges of cyanide occur, we collected water samples from the Small Axi stream (DA-1), tailings impoundment (DT-1), and the mixed water in the ratio 1:1 (DM-1). All the samples were kept indoors in open beakers and were stirred using a magnetic stirrer to simulate the stream conditions. Cyanide contents of the samples were determined early. The curves of 70 60
0.12 0.10
DA-1
0.08
50 40 30 20
y = 0.1068e-0.0131x R2 = 0.9804
0.06 0.04 0.02 0.00
10 0 0
50
100
150
250 DM-1
200
200
y = 60.391e-0.0133x R2 = 0.9507
DT-1 y = 217.5e-0.0166x R2 = 0.9841
150 100 50
0
50
100 150
200
250
0
0
50
100
Time (hr)
Fig. 3. The dynamic state degradation curves of DA-1, DM-1 and DT-1.
150 200
250
NATURAL CYANIDE DEGRADATION
6.50 (km)
52.91 (h)
41.75 (h)
52.11 (h)
28.64 (h) 27.18 (h)
0.12
degradation
Half-life of
0.16
3
4
5
0.897
2
0.980
1
Distance to the tailing dam (km)
0.984
0
0.951
0.00
0.968 0.921
0.04
coefficient R2
0.08
Correlation
CN- Content (mg L-1)
0.20
0.16
0.1067
0.0131
0.0166
Dynamic in streams 240 0.160 Streams
8.10
Dynamic indoor 240 0.097 DA-1
8.10
Dynamic indoor
Dynamic indoor 760
1720
65.3
11.20
Statics outdoor Statics outdoor 1720 1070
175.5
(a) The curves of cyanide degradation in nature fit the negative exponential equation:
DM-1
The parameters of cyanide degradation in different conditions are shown in Table 3. Based on experiments and sampling on sites, we can get the following results:
DT-1
3.5. Comparison of the rates of cyanide degradation
11.60 10.70
c ¼ 0.1525e)0.1067L, the absolute value of the correlation coefficient |R| ¼ 0.947. In the equation, L represents the distance (km), c and e are the same as the aforementioned. According to the degradation equation, we can calculate the flow length that cyanide decomposes one half (L1/2) to be 6.50 km. Cyanide reduction to 0.05 mg L)1 needs a flow length of 10.45 km. So the reduction of cyanide concentration in streams depends on dilution and natural decomposition. In short distance dilution is the main reason. Only for long flow distances, natural decomposition comes into play.
Table 3. The comparison of parameters of cyanide degradation in different conditions.
Fig. 5. CN) content being deducted due to dilution.
185.1 71.0
5
ST-1 SS-1
4
states
3
Experimental
2
Degree of minerali-
1
Distance to the tailings dam (km)
pH value
0
content (mg L-1)
0.10
zation (mg L)1)
0.12
11.60
0.14
0.08
0.0133
0.0242 0.0255
y = 0.1525e-0.1067x R2 = 0.897
CN) Initial
CN- Content (mg L-1)
0.18
coefficient
Degradation
Fig. 4. CN) content in the receiving streams.
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LI SHEHONG ET AL.
c ¼ c0 ekt or c ¼ c0 ekL
that conditions such as fully dissolved oxygen, activities of microbes and bacteria, ultraviolet in sunlight play a very important role in the process of cyanide degradation.
In the equations, c represents the cyanide concentration (mg L)1) after a period of time (or flow length), c0 represents the initial concentration of cyanide (mg L)1), k represents the coefficient of degradation, t represents time (h) and L represents flow length in streams (km). The velocities of degradation are different in different experimental conditions. In similar experimental conditions, the half-degradation times (t1/2) are similar although the initial cyanide concentrations are different. (b) The velocities of cyanide degradation in the static state outdoors are faster than in the dynamic state indoors. This indicates that ultraviolet radiation in sunlight plays a more important role than dissolved oxygen in the cyanide degradation process. If we performed the dynamic experiment outdoors, the velocity of cyanide degradation would be faster. (c) During the initial 0–20 h, the cyanide decomposition is slower so that there exists a ‘shoulder’ in the curves of cyanide degradation. This may indicate a process of cyanide decomposition by microbes. After the microbes adapt to the environment, the curves can be fitted into the negative exponential equation more perfectly. (d) The rate of cyanide degradation in streams is fastest in all of the experiments. This shows
Based on the cyanide degradation equation in the streams and the hydrology parameters of the streams, we can forecast the environmental impact when accidental cyanide discharges occur. 3.6. Accidental cyanide discharges and forecast of their environmental impact (a) Trucks loading cyanide turn over from time to time. So we hypothesize a truck loading sodium cyanide casks turned over into the Small Axi stream. Some metal casks were broken and there was some sodium cyanide dissolved into the streams. After 1 h, all of the casks were salvaged out of the stream, but there still was 1 ton of CN) (or NaCN188t) dissolved in the streams during the first hour. In this case, the results of cyanide concentrations in each river section (Figure 1) that only consider the dilution by river water using hydrology parameters (Table 1), and consider both of the river water dilution and natural cyanide degradation (calculating with degradation coefficient of k ¼ 0.1067, and supposing the coefficient is a constant and not related with initial cyanide contents) are listed in Table 4.
Table 4. Environmental impact forecast if 1t CN) were discharged into Small Axi stream in an hour. Streams
Position of river section
Accumulated
Stream flow (m3 s)1) CN) content only
CN) content by
)1
by dilution (mg L ) dilution and
distance (km)
decomposition (mg L)1) Small Axi stream
Axi stream
Piliqing stream
Ili River
1
0
0.02
13,900
13,900
2
2
0.02
13,900
11,229
3
2
0.07
4000
3208
4
4
0.12
2300
1512
5
34
0.12
2300
62
6
34
1.75
158
4.3
7
74
1.75
158
0.059
8
74
119.75
2.31
0.0009
9
134
216
1.29
8 · 10)7
NATURAL CYANIDE DEGRADATION
From the results, we can see that the cyanide content in water at the border (Section 9) would be 1.29 mg L)1 if it were only in the action of dilution by river water. In this case, all of the fishes in the Ili River would be killed. But this did not happen. In the combined action of dilution and natural decomposition, we can find the section with a cyanide content of 0.2 mg L-1 is at 28.7 km of Piliqing stream. The calculation formula is c ¼ c0 ekL ! 0:2 ¼ 4:3e0:1067L ! L ¼ 28:7 km So 28.7 km upstream of Piliqing will be severely contaminated, and the fishes will be killed. But downstream including the trunk stream of the Ili River will not be too severely affected. (b) Some accidents such as floods, earthquakes, landslips, mud-rock flows, quality of engineering or other reasons cause the tailing dam to collapse. The wastewater containing cyanide flush into Small Axi stream and cause the cyanide concentration in Small Axi stream water to be up to 180 mg L)1 and the flux of Small Axi stream to be up to 2 m3 s)1. Suppose that all of the streams are in the period of low flow and the other stream flow rates are as shown in Table 1. We can use the hydrology parameters of the streams and cyanide degradation equation (c ¼ c0e)0.1067L) to forecast the environmental impact. The results are shown in Table 5. In the same way, we can calculate the section where cyanide concentration is 0.2 mg L)1 in the Piliqing stream: c ¼ c0 ekL ! 0:2 ¼ 2:62e0:1067L ! L ¼ 24:1 km So 24.1 km upstream of Piliqing will be severely contaminated in this case. From the results, we can see that in the two cases of accidental discharges of cyanide, the trunk of the Ili River will not be severely impacted. But the upstream of Pliqing will be severely damaged. The above assessments were done in the extreme disadvantage of natural conditions to cyanide degradation. The practical situation may not be so severe as the forecasting. On the other hand, the base parameters of forecasting were obtained in
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the conditions of low stream flow and low cyanide content. Although the cyanide degradation of high content is faster than low content, probably the ability of cyanide degradation in streams will not add proportionally to the addition of stream flow. Synthesis the two sides, we believe that the forecasting results should fit the facts.
4. Conclusions Cyanide can be decomposed by oxydrolysis in natural conditions. The oxydrolysis reaction equations are: 2CN) + O2 ¼ 2CNO) CNO) 2 2H2O ¼ NHþ 4 +CO3 . All the experiments and studies on sites show that the natural degradation of cyanide conforms to the negative exponential equation (c ¼ c0e)kt or c ¼ c0e)kL). The degradation coefficient k is controlled by environmental conditions such as temperature, pH value, degree of mineralization, sunlight, dissolved oxygen, microbial activity, etc. We could forecast the environmental impact according to the hydrology parameters and cyanide degradation equation in the local conditions if accidental cyanide discharges occurred. There would be little hazard to the trunk of the Ili River in the combined action of dilution and natural degradation even if the tailings impoundment collapsed. The cyanide from the Axi goldmine effluent has hazarded Small Axi stream 2 km because the cyanide concentrations in the 2 km stream water have exceeded the China drinking water standard (£0.05 mg L)1). The goldmine corporation should take engineering measures to reduce the leakage amount of wastewater so as to reduce the hazard to Small Axi stream. Measures should be taken to guarantee the safety of the residents and livestock drinking water in the 2 km stream range. It is necessary to strengthen the cyanide management in transportation, using and conserving and enhance the consciousness of disaster prevention. Engineering geology measures should be taken to enhance the ability of avoiding the damage to tailings impoundment by floods, earth-
Table 5. Environmental impact on the downstream forecast if the tailing dam collapsed. Position of river section )
-1
CN Concentration (mg L )
1
2
3
4
5
6
7
8
9
180.0
145.4
141.9
111.9
4.56
2.62
0.037
0.001
9 · 10)7
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LI SHEHONG ET AL.
quakes, landslips, mud-rock flows, etc. At the same time, a perfect system of precaution, forecasting and early-warning disaster should be established. When accidental cyanide discharge occurs, the locals must immediately stop using the water in the downstream areas in particular Yining city waterworks. The polluted water can be desterilized only after the environmental monitoring departments relieve the alarm. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 40303011) and the Ministry of Science and Technology of China (No. 96-915-08-06). We would like to thank the head, staff and technicians of Axi Goldmine Corporation for their help in the studies on site. We would also like to thank Dr. Robert Bob Finkelman, the professor of USGS for his help in revising this article. References cited Basheer S et al. 1992 Kinetics of enzymatic degradation of cyanide. Biotechnol and Bioengi 39, 629–634.
Boucabeille C, Bories A et al. 1994 Microbial degradation of metal complexed cyanides and thiocyanate from mining wastewaters. Environ Pollut 84, 59–67. China EPA. 1997 Monitoring and Analytical Methods of Water and Wastewater. Beijing: China Environmental Science Publisher, pp. 306–318 (in Chinese). Dean WB, Christine MC. 1999 A critical review: general toxicity and environmental fate of three aqueous cyanide ions and associated ligands. Water, Air Soil Pollut 109(1/4), 67– 79. Irwin Roy J, van Mouwerik Mark, Stevens Lynette, Seese Maron Dubler, Basham Wendy. 1997 Environmental Contaminants Encyclopedia: Entry on CyanideðsÞ in General, pp. 6–10. Meeussen Johannes CL, Kelzer Melndert G et al. 1992 Chemical stability and decomposition rate of iron cyanide complexes in soil solutions. Environ Sci Technol 26(3), 511–516. Out EO, Byerley JJ, Robinson CW. 1996 Ion chromatography of cyanide and metal cyanide complexes: a review. Int J Environ Analyt Chem 63, 81–90. Peter K. 1999 Behavior of cyanides in soil and groundwater: a review. Water, Air Soil Pollut 15(1/4), 279–307. Singh HB, Wasi Nadira. 1986 Detection and determination of cyanide: a review. Int J Environ Analyt Chem 26, 115–136. US EPA, Office of Solid Waste Special Waste Branch. 1994 Treatment of Cyanide Heap Leaches and Tailings. Technical Report EPA530-R-94-037, pp. 3–4. Zaranyika MF et al. 1994 Cyanide ion concentration in the effluent from two gold mines in Zimbabwe and in a stream receiving effluent from one of the goldmines. J Environ Sci Health A 29(7), 1295–1303.