versión impresa ISSN 0366-1644
Bol. Soc. Chil. Quím. v.47 n.3 Concepción sep. 2002
CONCENTRATIONS OG NI AND CO IN CROP PLANTS
CULTIVATED IN NORTHERN CHILE
SUSANA STEGEN*; FABRIZIO QUEIROLO and CARMEN CARRASCO
Department of Chemistry, Universidad Católica del Norte, Angamos 0610,
Antofagasta, Chile, e-mail: firstname.lastname@example.org
PETER OSTAOCZUK AND MILAN J.SCHWUGER
Institute of Applied Physical Chemistry, Research Center Jülich, D-52425 Jülich,
Germany, e-mail: email@example.com
(Received: December 4, 2001 - Accepted: June 28, 2002)
This paper provides an overview of nickel and cobalt concentrations found in the north of Chile especially as it relates to exposure of heavy metals to humans and animals through food; to help creating a database for estimating the Ni and Co content in the native farmer´s diet of South America1). The information include levels of nickel and cobalt measured in a number of different types of vegetables (potato, broad bean, onion and alfalfa) exposed to a natural environmental contamination (especially in relation to volcanic activity and mineral composition of the soil).
The simultaneous determination of Ni and Co at pH 9 after adsorptive accumulation of the respective complex with dimethylglyoxime (DMG) at the mercury drop electrode is made by adsorption differential pulse voltammetry (ADPV).
The values, expressed in fresh weight, varied from 9-160 µg Kg-1 for Ni and from 1-127 µg Kg-1 for Co. At the present time, there are no established legal standards for nickel and cobalt in any vegetables in Chile.
Keywords: Nickel (Ni), cobalt (Co), adsorption differential pulse voltammetry (ADPV), vegetables, North Chile*
Esta publicación provee una visión general sobre las concentraciones de níquel y cobalto encontradas en el Norte de Chile, especialmente aquéllas relacionadas con la exposición de metales pesados en humanos y animales a través de la alimentación, con el fin de crear una base de datos para poder estimar el contenido de Ni y Co en la dieta alimentaria de la población rural de Sudamérica1). La información incluye niveles de níquel y cobalto medidos en diferentes tipos de vegetales (papas, habas, cebollas y alfalfa) expuestas a la contaminación ambiental natural (especialmente en relación a una gran actividad volcánica y composición mineral del suelo).
La determinación simultánea de Ni y Co se realizó mediante voltamperometría diferencial de pulso adsorptiva (ADPV) a pH 9 tras acumulación en el electrodo del respectivo complejo con dimetilglioxima (DMG).
Los valores obtenidos expresados en peso fresco fluctuaron entre 9-160 µg Kg-1 para Ni y, entre 1-127 µg Kg-1 para Co. En la actualidad, en Chile no existen estándares legales establecidos para níquel y cobalto en vegetales.
Palabras CLAVES: Níquel (Ni), cobalto (Co), voltamperometría diferencial de pulso adsorptiva (ADPV), vegetales, Norte de Chile
The region of Antofagasta, north Chile, is extremely arid. Here the Andes volcanism has been intense (eruptions, vents, geysers and thermal springs).
Nickel is a toxic trace element2,3) of widespread distribution in the environment. It is emitted to the atmosphere from volcanoes and windblown dusts and there are numerous man-made sources. The nickel content of soil may range widely (2 to 50 mg Kg-1 or more2)), depending on mineral composition. This metal is readly absorbed by plant roots and is highly mobile in plants. Some studies have reported a good correlation between the content of nickel in the plant leaves and that of the soil1).The concentration of nickel in plants is in the general range 0.05 to 5 µg g-1 d.w.4). Determination of Ni in certain vegetables such as pears, beans, cabbage, spinach and lettuce indicates values between 1 and 3 µg g-1 and fruit, cereals and potatoes between 0.1 and 0.5 µg g-1 . 5) Data obtained in the vecinity of nickel-emitting sources have mean concentrations of nickel about 4.6 µg g-1 d.w. for onion, 1.7 µg g-1 d.w. for alfalfa, 31.8 µg g-1 d.w. for beans and 1.7 µg g-1 d.w. for potatoes6).
Cobalt is found in a large number of iron oxide, sulphide, arsenide and sulfo-arsenide minerals, e.g. cattierite CoS2, cobaltite CoAsS, smaltite CoAs2, although there are no common cobalt minerals, hence most of the production of cobalt metal is from the processing of iron, arsenic, nickel, copper and antimony ores. As a result of surficial weathering processes and vulcanicity with the associated transfer of hot hydrothermal solutions through solidified rocks, a small amount of the total cobalt is removed by leaching and becomes incorporated in the sedimentary rocks or is transfered by rivers to the oceans in the disolved form, ususally as ionic cobalt8).
Co is an essential element, being a component of vitamin B12 and a non-specific, activator of several enzymes7)). Total cobalt is present in most soils in the range of 0.1-50 µg/g and available cobalt (i.e. the proportion of cobalt which is taken up by vegetation) between 0.1-2 µg g-1. 8). Lack of cobalt in soils results in a deficiency of vitamin B12 in rumiants, (or more precisely the closely related B12 co-enzyme) and was first recognized in the UK by Patterson9) for soils formed on granites. Plant foods such as cabbage, onions, spinach, tomato and pears contain about 0.2 µg g-1 Co while foods containing low amounts of cobalt (0.05 µg g-1) are apples, apricots, carrots, potatoes, oats, wheat and rice. Green leafy vegetables and fresh cereals contain the highest concentration of cobalt (0.2-0.6 µg g-1d.w.)11).
The aim of these work was to determine a distribution of nickel and cobalt in vegetables cultivated in Salar de Atacama in North of Chile. In this region a high mining and permanent volcanic activities are observed. Due to the extremely temperature conditions and high salinity in the potable water the plants habitually grows between December and March. The bulk of produced vegetables are used for self-consumption but a part is sealed on the market12).
The research area was restricted to the agriculture villages of Sector Yerbas Buenas (Rio Salado chico), Talabre and Socaire at the Salar de Atacama basin, located from 22º12´ to 23º45´ S and 68º20´ W.
Sector Yerbas Buenas (Rio Salado chico)
It originates at the El Tatio geysers, having extremely hot water reaching 86ºC. Soon after its origin, it enters the Tatio river that comes from the other thermal springs located farther south; it has a flow rate of 150 Ls-1. At Rio Salado chico, the sampling area where potatoes (solanum tuberosum L.) were sampled, water runs for 20 km through a deep canyon.
This village is located at 3600 m.a.s.l., at the bottom of an active volcano (Lascar). Income from agriculture represents 25%, and 99% of the agricultural production is for self-consumption. Cattle breeding is the main economic activity. Agricultural activity depends on the Soncor river, which is formed at the intersection of the Talabre and Potor rivers. These obtain their water from rainfall from the pre-Andes plateau or the plateau itself. Here broad beans (Vicia faba), onions (Allium cepa L.) and potatoes (solanum tuberosum L) were sampled.
This village, located at approximately 3750 m.a.s.l., supported a great agricultural activity during the pre-Hispanic period; its population at that time is estimated to be approximately 2600 inhabitants. At present, the harvest area represents 400 ha of terrace-type sowing land. Seventy-six percent of the income of these people comes agriculture and 81% of the agriculture production is for selfconsumption.
Socaire has the most intense farming in areas between creeks. This agricultural sector is watered with the water from the Socaire creek, which originates near Portezuelo, between the Miscanti and Lausa or Husar mountains. The river has a flow of 150-200 L s-1, which provides all the water of the area. Broad beans, potatoes and alfalfa (Medicago sativa L.) were sampled.
Sampling and pretreatment
The samples from the several vegetable species (potato, broad bean, alfalfa and onion) were collected in the field with a random sampling procedure (Markert, 1966) and each sample consisted of a number of sub-samples taken within an area of 100 x 100 m or less. The sampling development and the sample pretreatment have been discussed previously13). Also the potato and the broad bean skin was removed with a quartz knife, since it is sometimes consumed. The surface layers of the onions were peeled. The water content and the mass correction factor was determined by freeze-drying because the water determination by an oven is not recommended for beans and their skin14). Water content values were in the 72-79 % range for potatoes, 81-86% for potato skin (pskin), 78 a 80% for broad bean, 92-90% for broad bean skin (bskin), 98% for onion and 75% for alfalfa, depending on the sample site.
A 200 mg dried sample was accurately weighed in a 50 mL quartz vessel (suprasil). A mixture of acids, HNO3/HClO4 in proportion 4:1 was added. They were placed at room temperature, covered with a quartz watch glass, until the originally foamy and dark solution became light yellow. Later, heat was applied and temperature was gradually increased up to ± 330ºC. When the solution was dark brown or black, further addition of HNO3 was necessary. This HNO3 addition had to be repeated until a colorless solution was obtained 15,16).
All used reagents were of suprapur quality. Nitric acid used for digestion was purified before use by sub-boiling. Digestions were carried out twice. A blank was also carried out with the samples to check the cleanliness of the quartz vessels.
Concentration of Ni and Co were simultaneous determinated by ADPV using a Polarographic Analyser 384B, the 303A Static Mercury Drop Electrode and 305 Stirrer (all from EG&G, Princeton Applied Research, NY).
The chemicals and reagents were of suprapur quality. Standard solutions containing 1 g L-1 of metals were prepared from Merck, Titrisol, and deionized water from a Milli-Q-Purification System. For nickel and cobalt determination, 25 mL of 2M ammonia buffer of pH 9.2-9.4 was prepared using 6.5 mL of HCl (30%), 11 mL of NH3 (25%) and water. Dimethylglyoxime (DMG) (0.1M stock solution) was prepared by dissolving appropriate amounts in 96% ethanol (both of analytical reagent grade).
The voltammetric determination was performed subsequently by adsorptive cathodic stripping square-wave voltammetry in the same solution (10 mL) after addition of 100 µL of 2M ammonia buffer to adjust the pH to 9.2 and 5 µL of a 0.1M solution of DMG in ethanol. The concentration of Co and Ni in the samples was quantified by the method of standard additions.
The experimental conditions are reported in Table I.
The accuracy and precision of the analysis were checked against the standard reference materials Citrus Leaves NIST 1572, Total Diet ARC/CL and Rice Flour NIST 1568. The results obtained for Ni and Co were in an excellent agreement with the certified values for the method of open wet digestion (Table II). For external quality control, some of samples were analysed by the German Environmental Specimen Bank at the Institute of Applied Physical Chemistry, Research Center of Jülich, Germany using ICP-MS and the Neutron Activation Analysis Laboratory of the Chilean Commission of Nuclear Energy using NAA.
Fig.1 present the comparison between the Co data obtained by ICP-MS, NAA and DPASV. No significant differences between these methods were observed.
Fig.1. Interlaboratory comparison of Co in vegetables
RESULTS AND DISCUSSION
In Table III the results of all the collected samples are summarized, as mean values.
Fig.2 shows the concentrations in µg Kg-1 of wet basis as available in the market. The Ni concentration was found to be at a maximum in broad bean (160 µg Kg-1) than in alfalfa (156 µg Kg-1) and potato´s skin (137 µg Kg-1) at Socaire, followed by potato (92 µg Kg-1) and broad bean (76 µg Kg-1) at Talabre. In general, all the crop plants studied in the different locations had a concentration of nickel higher than cobalt; which is a normal situation for the plants. However, at the Yerbas Buenas sector, the concentration of Co in the potato was 1.5 times higher that of Ni. The reason being that in this location agricultural activity depends on the small Salado River which is originated at the El Tatio geysers, having extremely hot water (86ºC) and high levels of sulfo-arsenide minerals, e.g. cattierite CoS2, cobaltite CoAsS, smaltite CoAs2. Further studies are necessary for reinforcing such findings. Fig.2 illustrates that the tendency of Ni and Co is to accumulate in the skin of the potatoes, while broad beans, growing above the ground, have a higher concentration of Ni and Co in the edible part. These metals may show a similar behavior to that of Cd and Pb in the same matrices13). A significant variation in the concentration distribution of Ni was observed in potatoes at Talabre (92 µg Kg-1)> Socaire (25 µg Kg-1) > Yerbas Buenas (12 µg Kg-1) and in the potato´s skin at Socaire (137 µg Kg-1) > Yerbas Buenas (69 µg Kg-1) > Talabre (45 µg Kg-1). In samples collected in Socaire and Yerbas Buenas the concentration of Ni in the potato´s skin ist about 5 times higher than in potato. In samples from Talabre the concentration in potato´s skin is only 2.5 times higher than in the potato. Similar observation was done for Ni concentration in broad bean´s skin and broad bean. One possible explanation of the results is that increasing soil pH the amount of available nickel is reduced. Fertilizer applications also tended to reduce the amount of nickel taken up, especially at lower soil pH. Soon and Bates17) supported that nickel in soil was extractable by acids and that about half is complexed with organic matter. The reduction of Ni availability following lime application to soil at 10 t ha-1 was confirmed in studies using celery18,19). The mineral form of nickel in the soil as well as the physical properties of the soil dictate the amount of element that is available for root uptake.
Fig.2. Accumulation tendency of Ni and Co in the skin and in the edible parts of the crop plants.
Fig.3 shows the nickel concentration (µg Kg-1 f.w.) in vegetables with skin (potatoes and broad beans) in relation to distance from volcano Lascar, which is active. From Fig.3, it is clear that the nickel concentration of potatoes decrease with distance from the volcano. In opposite to that observation increasing amounts of nickel were found in broad beans with the increase of the distance from volcano. A possible explanation can be that Ni is particle bounded and deposited from the air. Further studies are here necessary.
Fig.3. Ni concentration (mg Kg-1 fw) in vegetables with skin collected at three localities in relation to distance from Lascar volcano. (At Yerbas Buenas location no broad beans are cultivated).
The concentration distribution of Ni and Co in onion was 9 µg Kg-1 fw and 1 µg Kg-1 f.w. respectively, being the lowest values.
Table IV shows the metal intake in consuming one potato with skin or one broad bean string, respectively, and the skin contributions for the different locations. The results shows that the contribution of skin to metal intake is at Socaire in potatoes the same (25% Ni and Co), but the greatest contribution comes from the skin at Yerbas Buenas and Talabre in potatoes and broad beans (32% Ni), respectively. On the other hand, the lowest contribution comes from the potato´s skin (4% Ni and 12% Co), and from the broad beans (6% Co) at Talabre.
Levels of Ni and Co found in this study are generally at the same levels or even lower than those reported for vegetables consumed in several countries and species grown in other agricultural and industrial areas (Table V).
The minimum quantity of cobalt which is required in order to prevent deficiency diseases in animals is about 0.07 µg g-1 d.w. of fooder10). In alfalfa we have found 0,16 µg g-1 d.w. cobalt. This amount can cover the human daily needs for Co.
In general terms, nickel an cobalt concentrations found, are low and quite comparable with other parts of the world.
Soil acidity, texture, organic composition, moisture and addition of volcanic ashes all can play major roles in mediating the amount of nickel present in crop plants cultivated in Northern Chile.
The voltammetric approach to the determination of Ni and Co in food plants after wet digestion is a very reliable, sensitive and accurate method.
The financial support of the Universidad Católica del Norte, Chile, and of the International Bureau of Research Center Jülich, Germany, is greatfully acknowledged. We also thank Lic. Luis Muñoz from the Neutron Activation Analysis Laboratory of the Chilean Commission of Nuclear Energy for cross-checking results for Ni and Co using NAA.
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