Leaching of nickel and copper from soil contaminated by metallurgical dust
Lapland Biosphere Reserve, Green Lane (Zeleny), 8, 184505 Monchegorsk, Murmansk, Russia Received 1 April 2001;
accepted 14 December 2001
The paper presents the results of the laboratory percolation experiment simulated soil contamination by emissions from a Ni-Cu smelter. Humus (Ao horizon) columns were transferred to lysimeters from an illuvial, humic, ferriferous forest podzol site. Fine metallurgical dust containing Ni and Cu was layered on the columns and irrigated with sulphuric acid solutions at pH 3, 4, 5, and 6. Irrigation for 19 months indicated that the leaching of metals down the humus column was greatest at pH 6. Calculations indicated that it would take 160-270 years for complete leaching of Ni from the Ao layer, and 100-200 years for Cu, depending on the dust composition. Natural decontamination of affected soils will take centuries.
2. Materials and methods
The soil columns used in the experiments consisted of the organic layer (Ao, mor) from a ferriferous illuvial humus podzol in a mixed (pine/birch) forest. Roots and other large materials were removed from the humus samples. The humus was then homogenized, that is, the soil with a disturbed structure was used in the experiment.
The moist substrate was placed in a 1-L column having 10 cm diameter with a perforated bottom. A known amount of dust was then added to the soil surface. The weight of humus in the column was 141 g, and the height of the column 12 cm. Dust no. 1 was added to the top of the column at a dose of 1 g/column, corresponding to 850 mg Ni/kg humus and 5120 mg Cu/kg humus. Dust no. 2 was added to the top of the column at a dose of 2 g/column, corresponding to 1270 mg Ni/kg humus and 500 mg Cu/kg humus. These levels corresponded to the amounts measured in the organic layer at a forest site at a distance of 10-15 km from the smelter complex in the direction of the emission plume (Barcan and Kovnatsky, 1998).The columns were irrigated twice a week with distilled water adjusted to pH values ranging from 3 to 6 by the addition of sulphuric acid. The pH values of water used for the leaching simulations were chosen on the basis of precipitation pH (Kryuchkov and Makarova, 1989; Alexeyev, 1990; Status of environment and ecology problems at the Kola Peninsula, 2000)—pH 3-6, though the natural limit pH is 4-5. The total amount of irrigation was approximately equal to the volume of precipitation falling during the snowless period, that is, 100 mm/month (Yakovlev, 1961). The top of the columns was open. The experiments were conducted in closed premises and no special efforts were undertaken to support the stable air temperature; it was 20-23 0Ñ. Two or three drops of chloroform were added to the solution to prevent microbial activity. The leachate from the columns was collected each month and total Ni and Cu solution concentrations were determined.
The comparative experiments were conducted the quartz sand (0.1-1 mm) washed with hydrochloric acid and distilled water was used as the neutral substrate to evaluate the dynamics of leaching metals from the dust not complicated by its interaction with the soil.
The quart sand is a neutral carrier. The metallurgical dust superficialed on the sand reacts with acid, metal compounds are ionized and turn into water-soluble form. The metal leaching from dust in soil is a more complicated process. Interaction between precipitation acid and the dust, the sorption of sulfates by the soil, and subsequent metal desorption from the soil are possible. A direct interaction between dust and soil acids, and subsequent desorption of formed compounds, are possible, too.
The duration of the experiments "litter+dust" and "sand + dust" was 19 months, and repetitions of experiments were 3. The total number of columns was 51.
The metallurgical dust ( - 63 |im) for the experiments was taken from the gas flow near the smoke ducts used in the smelting of the Ni-Cu ore and copper raw materials.
The chemical analyses of the dust were conducted after extracting of the dust samples with a mixture of concentrated HC1 and HN03 (3:1, aqua regia). The total metal concentrations in the solution were determined as follows: Ni, Cu, Co, Fe, Zn, Pb by flame atomic absorption spectrometry (AAS) on a Perkin Elmer 3030B spectrometer; Al, Cr, Mn, Se, V, W, Ti, Sr, Mo, Ca, Mg on an AtomScan 25 emission ICP spectrometer (Thermo Jarrell Ash); and Cd and As on a Perkin Elmer 3030B spectrometer using a furnace cuvette. The total sulphur concentration in the dust was determined on a LECO CS-444 instrument by ignition in a current of oxygen and sulphur dioxide determination by infrared detection (LECO CS-444, 1991). The total silicon concentration in the dust was determined by a standard method involving alkaline melting (Ponomaryov, 1955).
The leachate collected from the columns was treated with a mixture of nitric and sulphuric acids in order to destroy the organic matter. The humus sections from the columns were ashed in a muffle furnace at 450-500 °C, and the ash extracted with a mixture of hydrochloric, nitric, and sulphuric acids. AAS was used for Ni and Cu determination in solutions. A standard colorimetric method was also used for determination of Ni and Cu in solution: nickel by dymethylglyoxyme and copper by lead diethyldithyocarbamate (Hillebrand and Lundell, 1953; Sandell, 1959).
The described methods of chemical analyses are widely used.
The main criteria to choose methods were sufficient sensibility and accuracy, minimum labour, and money expenditures.
3. Calculation and statistics
The standard statistic treatment of results was used. Linear regression was determined by the method of least squares. Nonorthogonal regression with minimization of the sum of the square roots of standard deviation on ordinate (function) was used. The type of regression function accepted depended on the calculated correlation coefficient, that is, after the calculation of 16 functions, we chose the one that had the maximum correlation coefficient and the minimum dispersion.
Table 1 presents the chemical composition of the dust, used in the experiment. Table 2 presents some chemical soil properties.
Fig. 1. Results of lysimetric experiments with quartz sand as substrate. (A) Metal concentration in the leachate. (B) Leaching of metals from the dust, the cumulative proportion (%).
Fig. 2. Cumulative leaching of Ni from the humus during the lysimetric experiment (%).
5. Leaching of metallurgical dust on sand (neutral substrate)
Fig. 3. Cumulative leaching of Cu from the humus during the lysimetric experiment (%).
Fig. 4. The level of Ni and Cu leaching to the end of the experiment (%).
The pH of precipitation is often considered to have an effect on the mobility of heavy metals in soil; an increase in acid precipitation may decrease the soil solution pH, thus promoting the metal leaching from the soil (Brummer, 1986; Freiesleben and Rasmussen, 1986; Nedbaev, 1987; Brown, 1987; Rasmussen et al, 1988; Hogg et al., 1993; Bertelsen et al, 1994). At the same time, the published information suggests that the leaching of heavy metals from the organic soil horizon depends more on the formation and leaching of organic compounds (Schnitzer, 1969; Aleshchukin, 1972; Bergkvist, 1986; Elpatievsky and Lutsenko, 1990; Jorgen-sen, 1991; Varskog et al, 1994; Ladonin, 1996; Myrlyan et al., 1996; Derome and Lindroos, 1998).
The described lysimeter experiment has shown that the most leaching rate of nickel and copper was observed when irrigation was of slightly acid conditions (pH 5-6). This seems to be the essential evidence that formation and leaching of organic metal compounds are the determinants for leaching of heavy metals from soils, but not the precipitation acidity. Derome and Lindroos (1998) have shown in their field lysimeter experiments that nickel and copper are completely combined in organic-metal complex compounds in coloured soil solutions.
The complete, natural decontamination of soils polluted by metallurgical dusts will take centuries once pollution emissions have stopped. At the present time, the annual deposition of Ni and Cu on the soil around the Severonickel Smelter Complex will delay natural decontamination by tens of years.
Complete removal of heavy metals from the organic horizon of humus podzolic soils polluted by metallurgical dusts will take from 160 to 270 years for Ni and from 100 to 200 years for Cu, depending on the type of metallurgical dust. An increase in the irrigation water pH increased the amount of metals leached from the soil.
The authors wish to thank Mr. John Derome, Senior Scientist from Finish Forest Research Institute, for discussions and help in English language, and Ms. H. Kruglikova for the help in English.
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