Cobalt is beneficial to human because it is part of vitamin B₁₂. Exposure to high levels of cobalt results in lung and heart diseases and dermatitis. It is a key constituent of cobalamin which is known as vitamin B₁₂, the primary biological reservoir of cobalt as an ultratrace element (Prasad, 2004). Bacteria in the guts of ruminant animals convert cobalt salts into vitamin B₁₂, a compound which can only be produced by bacteria or arches. The minimum presence of cobalt in soils therefore markedly improves the health of grazing animals and an uptake of 0.20 mg/kg a day is recommended for them, as they can obtain vitamin B₁₂ in no other way (Schwarz et al., 2000).

 

Industrial or municipal wastewaters are used for the irrigation of crops in periurban ecosystem, due to its availability and scarcity of fresh water. Irrigation with wastewater contributes to heavy metals contents in the soil. Heavy metals are harmful because of their non-biodegradable nature, long biological half-lives and their potentials to accumulate in different body parts (Wegelin et al., 1995). Heavy metals like cobalt are toxic because of their solubility in water. Low concentration of heavy metals has damaging effects on man and animals because there is no mechanism for their elimination in the body (Bahemuka and Mubofu, 1991). Wastewater contains substantial amounts of toxic metals which create problems (Sharma et al., 2009). High accumulation of these metals in agricultural soils through wastewater irrigation may not only result in soil contamination but affect food quality and safety (Mohsen and Seilsepour, 2008).

 

Soil is known as earth: it is the substance from which our planet takes its name. Soil is a vital resource of sustaining basic human needs, a quality food, food supply and a live able environment. Chemical analysis of soil is important for environmental monitoring and legislation (Catharine et al., 2011). It provides information on the fertility status, index of nutrient availability and bases for fertilizer recommendation (for a given crop) as well as planning of a nutrient management programme. Metals are introduced into aquatic systems as a result of the weathering of rocks and soils. For example, volcanic eruption and also several human activities involving the mining, processing and uses of metals and industrial material contain metal contaminants. Soil whether in urban or agricultural areas represent a major sink for metals released into the environment from a variety of anthropogenic activities (Fraser, 2004).

 

Vegetables provide accessible sources of essential vitamins (particularly ascorbic acid, niacin, riboflavin and thiamine) and minerals (such as calcium and iron), as well as supplementary protein and calories (FAO, 1988). Vegetables promote intake of essential nutrients from other foods by making them more palatable (Taylor, 1996; Oke, 1980). They provide dietary fibre to improve digestion and health, and they are essential for balanced diets (Koch et al., 1965). In developed countries, average daily consumption of vegetables is estimated to be about 238 g per capita (FAO, 1988). For the developing countries, it is only 135 g per capita. Availability per capita per day is estimated at about 60 g in Southeast Asia and sub-Saharan Africa (AVRDC, 1991) and 90-100 g in Latin America and South Asia. These average levels do not provide adequate amounts of essential nutrients. Moreover, they mask the fact that low income group consumes less than high income group, a pattern that is accentuated in rural areas, where income are lower than in urban areas (AVRDC, 1991).

 

Analysis of heavy metals found in vegetables from some cultivated irrigated gardens in the Kano metropolis, Nigeria were investigated by Lawal and Audu (2011). From results they obtained, the vegetable samples from Jakara indicated highest mean levels of Co (1.14±0.17 mg/Kg), Cu (7.50±1.08 mg/Kg), Zn (18.89±1.93 mg/Kg) and Cr (0.85±0.10 mg/Kg) while those from Sharada indicated highest levels of Ni (2.02±0.35 mg/Kg) and Pb (1.60±0.53 mg/Kg). They reported increased danger of growing vegetables on soils irrigated with contaminated industrial and domestic wastewater. This study is aimed at ascertaining the extent to which cobalt is accumulated in wastewater, soil and vegetables through man-made activities.

Sampling

 

Wastewater samples from Kubanni stream were obtained from five different sampling points on a four month basis along the stream channels for the period of two years giving total number of thirty (30) samples. Sampling was conducted in the harmattan, dry and rainy seasons. Wastewater samples were collected using composite sampling technique in a polyethylene plastic container that were previously cleaned by washing in non-ionic detergent and then rinsed with tap water and soaked in 10% HNO₃ for 24 h and finally rinsed with deionized water prior to usage (Ademoroti, 1996). During sampling, sample bottles used were rinsed with sampled water three times and then filled to the brim at a depth of one meter below the wastewater from each of the five designated sampling points. Wastewater sample bottles were labelled, stored in ice-blocked coolers and transported to the laboratory while in the laboratory; they were stored in the refrigerator at about 4°C prior to the analysis (APHA, 1998). Soil samples were collected at three depths (0-5, 5-10 and 10-15 cm) from both sides of the river banks by using spiral auger of 2.5 cm diameter. Soil samples were randomly sampled and bulked together to form a composite sample from each designated point giving total number of thirty (30) samples. They were then put in clean plastic bags, labelled and transported to the laboratory. The full grown vegetable of [spinach (Amaranthus hybridus), lettuce (Lactuca sativa), cabbage (Brassica oleracea), carrot (Daucus carota), okra (Hibiscus esculentus), onion (Allium cepa) and tomato (Lycopersicon esculenetum)] were randomly handpicked from various garden plots along Kubanni stream channels using hand-gloves, bulked together to form a composite sample, wrapped in big brown envelopes giving total of forty-two (42) samples, labeled accordingly and transported to the laboratory.

 

Sample treatment

 

Wastewaters used for cobalt determination were acidified at the points of sampling with 5 cm³ of concentrated HNO₃ as to avoid microbial activities on the wastewaters which might reduce the concentrations of intended cobalt before analysis and they were kept in a refrigerator prior to analysis (APHA, 1998). Soil samples were air-dried, crushed and passed through 2 mm mesh sieve. The soil samples were then put in clean plastic bags, sealed and labelled accordingly (Samira et al., 2009). Each vegetable samples were washed with tap water, followed by deionized water, air dried in the laboratory, grounded to powder and sieved using 250 µm sieve (Munson and Nelson, 1990).

 

Digestion of Wastewater Samples for Cobalt Determination

 

1000 cm³ of each wastewater sample was transferred into a beaker and 50 cm³ concentrated HNO₃ were added. The beakers with the content were placed on a sand bath and evaporated down to about 20 cm³. The beakers were cooled and another 5 cm³ concentrated HNO₃ were added to each beaker. The beakers were covered with watch glasses and returned to the sand bath. The heating was continued and then small portion of HNO₃ was added onto each beaker until the solutions appeared light colour and clear. The beakers wall and watch glasses were washed with deionized-water and the samples were filtered to remove any insoluble materials that could clog the atomizer. Each sample volume was made up to 100 cm³ with deionized water (Ademoroti, 1996). Determination of Co in the wastewater sample was done at 241 nm wavelengths using Alpha-4 Model Atomic Absorption Spectrophotometer (AAS) as described by Association of Official Analytical Chemist (AOAC, 1995).

 

Determination of cobalt in soil samples

 

Two grams of each soil sample was weighed into acid-washed glass beaker. Soil samples were digested by the addition of 20 cm³ of aqua-regia (mixture of HCl and HNO₃ in ratio 3:1) to each soil sample and 10 cm³ of 30% H₂O₂ were added in small portion to avoid any possible overflow leading to loss of material from the beakers. The beakers were covered with the watch glasses and heated on a water bath for 2 h at 90°C. The beakers wall and watch-glasses were washed with deionized water and the samples were filtered out to separate the insoluble solid from the supernatant liquid. Each soil sample was made up to 100 cm³ with deionized water to the mark levels. It was then analyzed for Co at 241 nm wavelengths using Alpha-4 Model Atomic Absorption Spectrophotometer (AAS) as described by Association of Official Analytical Chemist (AOAC, 1995).

 

Digestion of vegetable samples for cobalt determination

 

Three grams of the dry sample of each vegetable sample was ashed using Muffle furnace that was set at 450°C until a constant weight was obtained. On cooling, the ash was transferred to a decomposition flasks and 1cm³ of concentrated HNO₃ was added. The content was refluxed on a hot plate for 40 min and on cooling 20 cm³ of deionized water was added, boiled for 3 min and filtered. 10 cm³ of 2 M HNO₃ was added to the resulting solutions in a 100 cm³ volumetric flask. They were made up to the mark with deionized water, cobalt was determined at 241 nm wavelengths using Alpha-4 Model Atomic Absorption Spectrophotometer (AAS) as described by Association of Official Analytical Chemist (AOAC, 1995).

The results of cobalt in wastewater, soil and vegetables analyzed were expressed in form of bar-charts using Microsoft Excel (Window 7 Professional), the results obtained were subjected to one way Analysis of Variances (ANOVA) and Pearson Product Moment Correlations (PPMC) using Statistical Package for the Social Sciences (SPSS) 20.0 version software. Null hypothesis was adopted and this was set at 95% Confidence Mean level to check if there is significant difference in the concentrations of cobalt analyzed. Statistical decision for Pearson Correlation Coefficients (r) was taken as follows;

 

(i) If 0.05 ≤ r ≤ 0.20 there is negligible relationship

(ii) If 0.21 ≤ r ≤ 0.40 there is low relationship

(iii) If 0.41 ≤ r ≤ 0.60 there is moderate relationship

(iv) If 0.61 ≤ r ≤ 0.80 there is substantial relationship

(v) If 0.81 ≤ r ≤ 1.00 there is very high relationship (Robert, 1992)

 

Figure 1 presents cobalt levels in wastewater from Kubanni stream channels. The concentrations determined were in the range of 3.77 - 15.20 mg/L for the year 2013. Highest level of 15.20 mg/L was obtained at Tundun-wada during harmattan season and closely followed by 14.75 mg/L at Industrial area along Jos road in the dry season. High concentrations were also observed at Unguwa-fulani (13.08 mg/L), 12.50 mg/L at Industrial area along Jos road and 12.05 mg/L at Tundun-