contents of some anions in melons (curcurbitaceae) grown in maiduguri, nigeria

7/29/2019 CONTENTS OF SOME ANIONS IN MELONS (CURCURBITACEAE) GROWN IN MAIDUGURI, NIGERIA

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Continental J. Water, Air and Soil Pollution 3 (1): 1 11, 2012 ISSN: 2251 - 0508

Wilolud Journals, 2012 http://www.wiloludjournal.com

Printed in Nigeria doi:10.5707/cjwasp.2012.3.1.1.11

CONTENTS OF SOME ANIONS IN MELONS (CURCURBITACEAE) GROWN IN MAIDUGURI, NIGERIA

E. I. Uwah and V. O. Ogugbuaja

Department of Chemistry, Faculty of Science, University of Maiduguri, P.M.B. 1069, Maiduguri, Nigeria

ABSTRACTThe contents of nitrate, nitrite, phosphate and sulphate in watermelon (Citrullusvulgaris) and cucumber

(Cucumis sativus) grown and consumed in Maiduguri, Nigeria, were investigated in edible portions of

the vegetables. These anions were equally investigated in the surface soils where the vegetables were

grown. The anions contents in the soil and vegetable samples were determined using UV-Visible

Spectrophotometric method. Nitrate contents in the vegetables ranged from 135.30 4.32 to 945.00

22.36 gg-1

in melons obtained from the two sample areas of Alau dam and Gongulon. Nitrite rangedfrom 52.19 3.00 to 91.98 0.42 gg-1. Phosphate ranged from 2198.00 27.08 to 3195.50

28.00gg-1 while sulphate contents ranged from 2042.50 11.03 gg-1 to 3150.60 9.57 gg-1. The

anions contents were higher in the soil samples. The anions contents in both the vegetable and soil were

lower in the controls. These are indications of possible pollution of the areas as a result of excessive

usage of fertilizers, agro-chemicals and irrigation with wastewater. The results for nitrates and nitrites

obtained in this study were higher than the published maximum permissible content of nitrate and

nitrite in some vegetables and fruits. These high anions values could put consumers of the vegetables at

health risk.

KEYWORDS: Fertilizers, pollutants, anions, soils, melons, wastewater

INTRODUCTION

A number of factors are responsible for the distribution of pollutants in soils and plants. Some of these factorsinclude the intensification of agriculture and large-scale use of fertilizers, herbicides and pesticides (Webster,

2007). Other factors include environmental conditions such as temperature and rainfall. The characteristics of

both soils and plants in the uptake, retention and distribution of pollutants are also important (Manahan, 2005).

Major categories of soil pollutants include: Nutrients (fertilizers, sewage sludge), acids, heavy metals,

radioactive elements and agro-chemicals (herbicides, insecticides, fungicides and other pesticides). Many of

these pollutants are discharged into the soils through land waste disposal, inputs from the atmosphere and

irrigation with municipal waste water (Radojevic and Bashkin, 1999). Pollution of plants is of concern for two

reasons: Firstly, pollutants may have direct or indirect phytotoxic impacts on the plants themselves, leading to a

decline in crop yields and threatening food supplies; secondly, the plants may act as a vehicle for transferring

pollutants into the food chain. For example, Cd is readily accumulated by plants and may get to levels which are

adverse to the plants and consequently posing a threat to animals and humans that consume the plants

(Radojevic and Bashkin, 1999).

Fertilizers contain not only elements necessary for plant nutrient and growth, but anion and trace metalimpurities. The excessive application of nitrogen and other inorganic fertilizers and organic manures to plants

such as vegetables can accumulate high levels of nitrate and other anions. Consequently their consumption by

humans and animals can pose serious health hazards. It has been reported that vegetables that are consumed

with their roots, stems and leaves have a high nitrate accumulation, whereas melons and those vegetables with

only fruits as consumable parts have a low nitrate accumulation (Zhou et al., 2000. In a similar study at Salinas

Valley, California, Zink and Yamaguchi, 1962 reported that a relationship was found between the nitrate contentof lettuce and the amount of nitrogen fertilizer applied. They found out that nitrate content of the plant was

largely dependent on the application of nitrogen fertilizer and on the growth rate of the plant. Positive effects of

organic fertilizers on nitrate accumulation in spinach have been reported by some studies (Barker, 1975; Stopes

et al., 1988). It was also reported that easily decomposable organic fertilizers such as blood meal and guano

might increase nitrate accumulation in spinach in the same way as conventional chemical fertilizers, especially

with excessive application rates (Termine et al., 1987).


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E. I. Uwah and V. O. Ogugbuaja: Continental J. Water, Air and Soil Pollution 3 (1): 1 11, 2012

Food and vegetable crops productions require access to fertile land, water and other necessary inputs. In Nigeria,

Government has in recent years built additional dams and canals for dry season farming and have increased the

provision of fertilizers, herbicides, pesticides and modern farm equipments to boost food and vegetable crops

productions (Lawal and Singh, 1981). Various classes of vegetables are grown in many parts of Nigeria (Uwahet al., 2009). In Borno State, vegetables are heavily cultivated and consumed as food (Bokhari and Ahmed,

1985). This area is known for its dryness, with Sudan type of Climate, Savanna or Tropical grasslands

Vegetation, light annual rainfall of about 864mm and temperature ranging from 32 41oC, with mean of the

daily maximum exceeding 40oC between March and May before the onset of the rains in June (Adeleke and

Leong, 1978). These vegetables are irrigated with dam waters and all kinds of available waste and polluted

waters. Similarly, to enhance the yield of these vegetables, fertilizers and manures are occasionally added to the

soil. There are the possibilities of over applications of these fertilizers and manures. Hence, the uptake and

storage of some anions and heavy metal pollutants from these waters, fertilizers and manures by these

vegetables are very likely since these salts and heavy metals are soluble and mobile in ground water (Uwah et

al., 2007). These will indeed expose consumers of these vegetable crops to bioaccumulation of trace metals and

anions with time. There is therefore, the need to carry out extensive screening on the vegetables grown and

consume in Maiduguri, in order to determine the contents of some anions (nitrate, nitrite, phosphate and

sulphate). It was in this regard, that Curcurbitaceae; watermelon (Citrullusvulgaris) and cucumber (Cucumissativus) which are some of the vegetable crops grown in the area were screened in order to assess the levels of

the anions.

MATERIALS AND METHODS

Sample area and sampling

Edible portions of watermelon (Citrullusvulgaris) and cucumber (Cucumis sativus) vegetable crops and top orsurface soil (0 20 cm) samples were collected from the vegetable farms of Alau dam and Gongulon, irrigated

with the Alau dam water, sewage and all kinds of available wastewater, and cultivated with the applications of

fertilizers, manures, herbicides and pesticides. Samples were also collected from experimental gardens

cultivated on a piece of virgin land, irrigated with unpolluted water and without the applications of fertilizers,

manures, herbicides and pesticides, to serve as the controls. Collections were made from December, 2007 to

May, 2008. Samples collections were made six (6) times during the period. The map of the study area is shown

in Figure 1. During each collection, samples were randomly collected from different plots and homogenized intocomposite samples in the two sample areas. Sample collections were carried out according to the methods

described by (Radojevic and Bashkin, 1999) into cleaned new polyethylene bags and transported to the

laboratory.

Vegetable samples preparations for Nitrate, Nitrite, Phosphate and Sulphate analyses.Vegetable samples were cleaned to remove visible soil and then washed with tap water, thereafter with distilled

water several times and then sliced into nearly uniform sizes to facilitate drying at the same rate. The sliced

samples were then dried in an oven at 105oC for 24 hours until they were brittle and crisp. At this stage no micro

organism can grow and care was taken to avoid any source of contamination. The dried samples were

mechanically ground into fine particles using clean mortar and pestle and sieved to obtain


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Nitrite levels in the sample solutions were similarly determined. In this case however, different reagents were

used. The programme number for nitrite was 67 Nitrite-N and the reaction period was five minutes as against

ten minutes for nitrate. Nitrite-Nitrogen (NO2--N) was converted to ppm Nitrite (NO2

-) by multiplying by 3.3

(conversion factor) (LaMotte, 2000). The content levels of nitrite (gg-1

) in the samples were calculated from:

NO2- (gg-1) = Cx V/M (2)

Where; Cis the concentration of NO2-in the sample (ppm), Vis the total volume of the sample solution (100ml)

andMis the weight of the sample (1g) (Radojevic and Bashkin, 1999).

Determination of Phosphate

A portion (1g) each of the sieved vegetable samples were taken in a clean porcelain crucibles and 5cm3 of 20%

(w/v) magnesium acetate added and evaporated to dryness. They were ashed in a furnace at 500oC for 4 hours

and acid-digested with concentrated HNO3 and 6M HCl. 10 cm3of 6M HCl were added to each of the crucibles,

covered and heated on a steam bath for 15 minutes. The contents of each of the crucibles were transferred into

different evaporating basins and 1cm3

of concentrated HNO3 added and the heating process continued for 1 hour

to dehydrate silica. 1cm3

of 6M HCl was then added, swirled and followed with the addition of 10cm3

of water,heated and cooled. The resulting solutions were filtered into 50cm3 flasks and volumes made up to the marks

(AOAC, 1984; Radojevic and Bashkin, 1999). Phosphate concentrations in the vegetable extracts were analyzed

using Hach Direct Reading spectrophotometer (model, 44800-00) by the reactive phosphorus-amino acid

method (also called orthophosphate method) at a wavelength of 530nm (HACH, 1975). The concentrations of

phosphate (g/g) in the vegetable samples were calculated from:

PO43- (g/g) = Cx V/M (3)

Where; Cis the concentration of PO43- in the sample (ppm), Vis the total volume of the sample solution (50cm3)

andMis the weight of the sample (1g) (AOAC, 1984; Radojevic and Bashkin, 1999).

Determination of Sulphate

The procedures of samples preparations for sulphate analyses were similar to those of phosphate, except that,5cm

3of Magnesium nitrate solution were added to the sieved samples to prevent loss of sulphur and then heated

on a hot plate to 180oC until the colour of the samples changed from brown to yellow (AOAC, 1984; Radojevic

and Bashkin, 1999).

Sulphate concentrations in the vegetable extracts were analyzed using Smart spectrophotometer (model, 2000)at a wavelength of 420nm. The equipment was scrolled to select the stored programme number for sulphate (89-

sulphate). The results in ppm or mg/l SO42-

were recorded (LaMotte, 2000). The content levels of sulphate

(g/g) in the vegetable samples were calculated from:

SO42-

(g/g) = Cx V/M (4)

Where; Cis the concentration of SO42- in the sample (ppm), Vis the total volume of the sample solution (50cm

3)

andMis the weight of the sample (1g) (AOAC, 1984; Radojevic and Bashkin, 1999).

Determination of Nitrate (NO3-) and Nitrite (NO2

-) in the Soil Samples

The soil samples were homogenized and air-dried in a circulating air in the oven at 30oC and sieved through a 2

mm sieve. 10g of the sieved soil samples were taken in 250cm 3 polythene bottles, 100cm3 of 2 M KCl added

and shaken for 1 hour on a shaker and then filtered. 5 cm3 of 2% boric acid (H3BO3) solution were pipetted into

a 50cm3

conical flask and placed under the condenser top about 4cm above the surface. 20cm3

of the extract

were taken in a distillation tube and 0.2g of MgO added and followed with the addition of 0.2g Devardas Alloy.

The tube was distilled until about 30cm3 of the distillate collected. Soil samples preparation for nitrite

determination was similar to that of nitrate except that fresh soil samples rather than dried samples were used(Black, 1965; AOAC, 1984; Radojevic and Bashkin, 1999).

Nitrate (NO3-) and nitrite (NO2

-) concentrations in the soil distillates was determined using Smart

spectrophotometer (model 2000) at a wavelength of 543nm, similar to those previously for the vegetablesamples.


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Determination of Phosphate in the Soil Samples

About 2g of air-dried soil were taken in a polyethylene bottle and 50cm3

of water added. The bottle was shaken

continuously for 5 minutes and repeatedly filtered through a Whatman no. 42 filter paper until a clear extract

was obtained. 10cm3

of the sample extract were then taken in a 50cm3

volumetric flask and diluted to just under40cm

3and 8cm

3of the reaction mixture were added and the volume made up to the mark (Bray and Kurtz, 1945;

AOAC, 1984; Radojevic and Bashkin, 1999).

Calibration standards were prepared by adding volumes of standard phosphate solution (10g Pdm-3

)

corresponding to between 5 and 60 g P (0.5 6cm3 of the standard solution) to a series of 50cm3 volumetric

flask. To each flask, 8cm3of the reaction mixture were added and the volume made up to the mark. Water blank

was then prepared by taken 8cm3

of the reaction mixture into a 50cm3

flask and the volume made up to the

mark. The solutions were then mixed thoroughly and allowed to stand for at least 10 minutes. The absorbance of

the solution was measured at 880 nm in 1 cm cell spectrophotometer. A calibration graph which was a straight

line graph through the origin was plotted against g P. The amount of phosphorus in the sample was determined

from the graph and the concentration calculated from:

Mg Pdm-3

= g P/V (5)

Where; Vis the volume of the sample (cm3).

The concentration of phosphate-P in the sample extract was then calculated and converted to mg P/kg soil

according to the equation:

PO43-

-P (g/g) = Cx V/M (6)

Where; Cis the concentration of PO43-

-P in the extracts (g Pcm-3

), Vis the total volume of extract (50cm3) and

Mis the weight of the sample (2g) (Bray and Kurtz, 1945; AOAC, 1984; Radojevic and Bashkin, 1999).

Determination of Phosphate in the Soil Samples

Sulphate was extracted from the soil samples by fusion as described by Radojevic and Bashkin, 1999).

Approximately 0.5g of sodium carbonate and 0.05g of sodium peroxide were taken in a clean platinum crucibleand 0.10g of finely ground oven dried soil sample added and mixed with a glass rod. About 0.30g of sodium

carbonate was further spread on top of the mixture and cover with the lid. The crucible was supported on a silica

triangle and heated over a flame until the contents liquefied. The crucible was later removed from flame, swirled

gently, cooled and then immersed in about 50cm3water in a 250cm

3beaker, 3cm

3of concentrated HCl was then

added, covered with a watch glass and heated for 30 minutes on a boiling water bath. The solution was thentransferred into a 100cm3 volumetric flask and the volume was made up to the mark. A blank was prepared by

the same procedure, but without the sample.

The soil solution was immediately poured into an absorption cell and the absorbance measured on a

spectrophotometer at 420 nm after 6 minutes. A series of calibration standards was prepared by pipettingaliquots of the standard sulphate solution corresponding to between 0.5 and 5 mg SO4

2- (5 50cm3) into a

100cm3 volumetric flask and the volume made up to the mark. The concentration of sulphate in the extract was

evaluated and calculated from:

SO42- (g/g) = Cx V/M (7)

Where; C is the concentration of SO42- S in the sample extracts (mg Sdm-3), V is the volume of the extract

(100cm3) andMis the weight (0.10g) of the soil sample that was extracted (Allen, 1989).

Determination of some physicochemical parameters and particle fractions of the soils

Organic carbon was determined by means of a potassium dichromate back titration method as described by

McCleod (1973). Cation exchange capacity (CEC) was determined by the silver thiourea method as describedby Rayment and Higginson (1992). The soil pH (1:5 soil water extract), Electrical conductivity (EC) (1:5 soil

water extract) and the soil particle size fractions were equally determined using standard laboratory methods as

described by Rayment and Higginson (1992).


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Transfer Factors (TF) for Anions from Soils to Vegetables

Transfer factor is the ratio of the concentration of anion in a plant to the concentration of anion in soil. The

transfer factors (TF) for each anion were computed based on the method described by Harrison and Chirgawi

(1989) according to the following equation:

TF = Ps (g/g dry wt)/St(g/g dry wt) (8)

Where Ps is the plants anion content originating from the soil and Stis the total anion contents in the soil.

Data Analyses

Data generated were subjected to statistical tests of significance using the Analysis of Variance (ANOVA) at

p sulphate > nitrate > nitrite. The higher levels of the anions in the soil samplesobtained in the Gongulon area could equally be attributed to similar reasons.

Some Physicochemical Parameters of the Soils

The results for the determination of some physicochemical parameters in soils are as shown in Table 3. The results showed

low organic carbon (OC) and organic matter (OM) in the study area. The values of OC (%) were 0.40 and 0.74 in Alau dam

and Gongulon, respectively. Those of OM (%) were 0.69 and 1.28 in the two areas. Similarly, cation exchange capacity

(CEC) in meq100-1g and the electrical conductivity (EC) in (mhocm-1) values were low. The CEC (meq100-1g) values for

the two areas were 5.33 0.01 and 5.43 0.10 and those of EC (mhocm-1) were 0.22 0.03 and 0.24 0.05. The soil pH

values in the two areas were as high as 6.23 0.20 and 6.69 0.04 respectively. The values of these parameters in the

control samples were: OC (%) 0.22, OM (%) 0.35, CEC (meq100-1g) 5.04 0.15, EC (mhocm-1) 0.20 0.12 and pH 6.67

0.57. The high pH values of 6.23 0.20 and 6.69 0.04 in the two respective sample areas of Alau dam and Gongulon are

indicative of slightly acidic environment. The high pH values in the study area may be attributed mainly to the


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buffering effect of carbonate containing materials such as cement or bricks (Abulude, 2005; Uwah, 2009).

Statistical test of significance using the ANOVA, revealed significant differences (p < 0.05) between the values

of organic carbon (OC), and organic matter (OM) in the soil samples obtained in the two areas with their

corresponding values in the control samples. However, cation exchange capacity (CEC), electrical conductivity(EC) and pH values in samples obtained in the two areas did not show statistical differences (p > 0.05) with

their corresponding values in the control samples.

Levels of Particle Size Fractions of the Soils

The levels of particle fractions of the soils are as presented in Table 4. Particle size analyses of the soils revealed

the levels of clay (%) in the two respective areas to be: 9.00 and 8.50; sand (%) as 84.00 and 86.00 and silt (%)

as 5.00 and 7.50. The levels of these parameters in the control samples were: clay (%) 8.80, sand (%) 84.00 and

silt (%) 7.20. There were significant differences (p > 0.05) between the concentrations of clay, silt in soil

samples obtained in the two sample areas of Alau dam and Gongulon with their corresponding levels in the

control samples. In general, the results revealed the soils to be loamy sand in texture and slightly acidic with low

organic matter contents.

Transfer Factors (TF) of the Anions Soils to VegetablesThe transfer factors (TF) of the anions from soils to vegetables are as presented in Figure 2. Transfer factor is

one of the key components of human exposure to anions through the food chain. Transfer factors were

computed for the anions to quantify the relative differences in bioavailability of anions to vegetables or to

identify the efficiency of a vegetable species to accumulate a given anion. These factors were based on the root

uptake of the anions and discount the foliar absorption of atmospheric anion deposit (Lokeshwari and

Chandrappa, 2006; Awode etal., 2008). The trend in the anions variations was: NO3-

> SO42-

> PO43-

> NO2-.

These results indicated that the vegetables have the potential of accumulating more nitrate and less nitrite.

Several human health hazards due to nitrate toxicity as a result of high levels of nitrite in vegetables have been

identified. The toxicity of nitrate is thought to be due to its reduction to nitrite and conversion to nitrosamines

and nitrosamides through reaction with amines and amides, whose carcinogenic action is well known (Walker,

1990). The principal mechanism of nitrite toxicity is the oxidation of the ferrous (Fe2+

) in haemoglobin to ferric

(Fe3+), producing methaemoglobin. As a consequence of methaemoglobin formation, oxygen delivery to humantissues is impaired (Knobeloch et al., 2000; Mensinga et al., 2003). Methaemoglobinemia, earlier believed to be

in infants only, has been reported by Gupta et al (2000a) in people of different age groups being most

susceptible to nitrate toxicity. A high percentage of acute respiratory tract infection with a history of recurrence

has also been reported in children consuming high nitrate content (Gupta et al., 2000b). Recurrent diarrhea in

children up to 8 years of age (Gupta et al., 2001) and recurrent stomatitis (Gupta et al., 1999) are also associatedwith high nitrate ingestion. Some other reported effects are infants mortality, early onset of hypertension,

hypothyroidism, diabetes and an adverse effect on cardiac muscles, alveoli of the lungs and adrenal glands

(Gupta, 2006). In animals, nitrate toxicity varies according to species. Ruminant animals develop

Methaemoglobinemia while monogastric animals exhibit severe gastritis (Bruning-Fann and Kaneene, 1993).

Crops high in nitrate not only pose a direct danger to human and animal health, but also cause financial losses toagriculture and the food processing industry. High nitrate content leads to a low shelf life of vegetables, thereby

increasing losses during storage.

Severe phosphate toxicity can result in various symptoms resulting from low plasma calcium levels. Moderate

phosphate toxicity occurring over a period of months, can result in the deposit of calcium phosphate crystals in

various tissues of the body (O Dell and Sunde, 1997). High dietary intake of phosphorus (as phosphate) mayupset the calcium/phosphorus balance of the body leading to mineral deficiencies and several other related

health problems (WHO, 1978).

CONCLUSION

Taking all these health risks encountered in the diet as a result of high levels of these anions (pollutant

indicators) in the vegetables, the maximum allowable levels of these anions in vegetables should not exceedlevels that reflect good agricultural practices and farmers should be educated on the problems associated with

excessive usage of fertilizers and other chemicals and irrigating the crops with waste and all sorts of polluted

water. The levels of some of the anions in the vegetables reported in this study were higher than the published

maximum permissible contents of the anions in some vegetables and fruits. These high anions values could putconsumers of the vegetables at health risk at the time of this study. Growers of vegetables in the area should be


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educated on the needs to grow crops with safe levels of these anions. The results obtained in this study would go

a long way in providing a baseline data for the assessment of the levels of these anions in watermelon ( Citrullus

vulgaris) and cucumber (Cucumis sativus) grown and consumed in Maiduguri, Nigeria.

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Table 1: Content levels (g/g) of some Anions (nitrate, nitrite, sulphate and phosphate) in Melon Family of

Vegetables obtained from Alau dam, Gongulon and the Controls

The above values are means of replicate values (n = 6). Within column, means with different

alphabets are statistically different (p


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Table 4: Particle Size Fractions of Some Soils

Sample Areas Particle size

Clay (%) Sand (%) Silt (%)

Alau dam 9.00a 86.00a 5.00a

Gongulon 8.50b 84.00a 7.50b

Control 8.80c

84.00a

7.20c

The above values are means of replicate values (n = 6). Within column, means with different

alphabets are statistically different (p

contents of some anions in melons (curcurbitaceae) grown in maiduguri, nigeria

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