Quantitative assessment of heavy metals in some tea marketed in Nigeria ——Bioaccumulation of heavy metals in tea

doi:10.4236/health.2010.29162

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Vol.2, No.9, 1097-1100 (2010) Health

doi:10.4236/health.2010.29162

Quantitative assessment of heavy metals in some tea

marketed in Nigeria

——Bioaccumulation of heavy metals in tea

Albert Cosmas Achudume, Dayo Owoeye

Institute of Ecology and Environmental Studies, Obafemi Awolowo University, Ile-Ife, Nigeria;

*Corresponding Author: aachudum@yahoo.com

Received 12 March 2010; revised 18 May 2010; accepted 4 June 2010.

ABSTRACT

Bioaccumulation of heavy metals in tea mar-

keted in Nigeria was investigated. Four major

and most consumed brand of tea were selected

for the present study. Both aqueous and dry

methods were used. Total contents of metal

were determined by digesting 1g of each brand

using a mixture (3:1) concentrated nitric acid

(NNO3) and hypochlorous acid (HCLO4). The

second method involved hot water extract of tea

samples. After boiling and filtration, the residue

was evaporated to near dryness and digested

with concentrated HNO3 and HCLO4 as de-

scribed above. Results indicate that Zn, Cd, Cu,

and Pb were present in lowest concentrations in

ascending order for which there were two sig-

nificant differences between the four sources of

samples. The general characteristics of heavy

metal concentrations in aqueous extract showed

high level of Fe and Mg in a descending order.

Going by the correlation study of our result in-

dicates that there is no significant relationship

between the two elements of Fe and Mg, though,

the numerical values of the two elements varied

widely among the samples. These differences

may have major impact on human health. How-

ever, the beneficial effects of tea are in a fairly

narrow concentration range between the essen-

tial and the toxic level. In conclusion, the varia-

tions in heavy metals content of tea brands may

be due to geographical, seasonal changes and

the chemical characteristics of the growing re-

gions.

Keywords: Bioaccumulation; Tea; Heavy Metals;

Human Health; Toxic Level

1. INTRODUCTION

Exposure to various metal containing components of tea

varied widely and may have varying health implications

[1]. Depending on the origin of tea leaves, heavy metals

accumulation can be derived naturally by soil contami-

nation, use of pesticides and fertilizers [2]. Some trace

metals Cr, Fe, Co, Ni, and Zn are essential for growth of

organisms, while other heavy metals Pb, Cd, Hg and As

are not only biologically non essential, but toxic [3]. A

very important biological property of metals is their

tendency to bioaccumulations. Bioaccumulation is there-

fore essential in hazard evaluation strategies. For exam-

ple, calculation of percent available of Aluminum (Al)

and Zinc (Zn) in tea consumed by human showed that

tea can provide 37.2% of the daily dietary intake of Al,

the percent available for absorption in the intestine is

only 1.78% for overall mean concentration [4]. Similarly,

daily dietary intake of Zn was 2.13% while percentage

available for absorption in the intestine was 0.72% [5].

Thus chronic metal toxicity may often characterized by

tissue/organ damage resulting in mortalities which are

related to secondary physiological disturbances [5,6].

The extent of physiological disturbances depends upon

uptake and bioaccumulation of metals [3,7].

Considering that an estimated amount of 18 billion

teacups are consumed daily in the world [1,8,9] its eco-

nomic and social importance are unprecedented. In Ni-

geria, people drink tea by the bowl because of its thera-

peutic value. It is valuable in the treatment and preven-

tion of many diseases [8].

The presence of heavy metals in tea has become

world-wide study. For example, the concentrations of Fe

and Cu in Poland [10], Cu in India and US [9], Se in

Pakistan [11], As in Iran [2,12], Al in China [13] and in

Lithuania [14] have recently become the subject of wide

spread concern, since beyond the tolerable limits they

become toxic [15,16]. Determination of harmful and

toxic heavy metals in different tea marketed in Nigeria

A. C. Achudume et al. / HEALTH 2 (2010) 1097-1100

1098

gives direct information on the significance of these

elements in tea beverages. Lack of basal data on the

contents of heavy metals in tea and the regulated on the

maximum allowable and safe concentration of metal in

tea are needed. This predicated the present study to de-

termine the quantitative assessment of heavy metal con-

taminants in some popular tea marketed in Nigeria.

2. MATERIALS AND METHODS

Several samples of tea leaves which are commonly con-

sumed in Nigeria were procured from provisional stores.

Accordingly each sample tea was coded to conceal the

original source. The code is tagged by a letter designat-

ing the type of tea, that is black tea-Lip; green tea-Gin;

white tea-Tia and Top. Three replicate samples of each

tea were quantified using two different standard methods

of [11,17].

Total contents of metals were determined by having a

portion of one g of sample tea digested in 12 mL of a

mixture (3:1 v/v) concentrated HNO3 and HCLO4. The

mixture was heated until the solution turned white. The

digested sample was filtered and transferred to a 100ml

flask and the volume was adjusted to the mark with 5%

HNO3 acid. This digestion procedure was validated by

using the reference certified material of National Agency

for Food and Drug Administration and Control (NAF-

DAC) [18].

Hot water extract of metals were determined by hav-

ing a portion of one g of each brand boiled in 50 ml of

distilled water for 10min in a porcelain cup and filtered.

The residue was evaporated to near dryness and digested

with concentrated HNO3 as described earlier. The final

volume of the solution was made up to 100 ml. Follow-

ing digestion, ten drops of H2O2 were added and centri-

fuged. The acid digested sediments were filtered and

capped.

A Perkin Elmer Analyst 300 flame atomic absorption

spectrometer (AAS) (Central Science laboratory, Obafemi

Awolowo University, Ile-Ife) was used to quantify the

heavy metal concentrations [19]. Calibration standard

curves provided the basis for quantifying metal contents

for both sediments and plant tissues after the initial ash-

ing digestion. Correlation coefficients for metals in plant

tissues and sediments were found to be 99.59% and

99.54% respectively. Calibration curves, developed us-

ing standards, provided the basis for quantifying metals

concentrations for analysis of plants tissues and sedi-

ments using both the dry and wet analysis techniques.

The data were statistically analyzed and the least sig-

nificant differences (SD) at the 5% level used to separate

means. The relationship between the different variables

was elevated by a simple correlation and regression ana-

lysis [20].

3. RESULTS AND DISCUSSION

Results of the present study show the actual concentra-

tion of heavy metals in tea samples after digestion (Ta-

ble 1). The metals present in lowest concentration (<

1.00 mg/kg) were Zn, Cd, Cu, and Pb in ascending order

respectively, for which there were two significant dif-

ferences between the four sources of samples. The

sources differ with regard to Se and Fe contents, but

similar with respect to Cu, Pb, and Zn and more substan-

tially with respect to Fe and Mg with total metal content

in Gin samples about double the other samples.

Table 2 shows the general characteristics of metal

concentrations transferred to hot water extracts from

brand of teas. The metals present in greatest concentra-

tion were Fe (442-1344 mg/kg), followed closely by Mg

(123-239 mg/kg) and highly toxic element Cu (2-7

mg/kg). Except for Pb, all these metals are release very

slowly from tea leaves because they are complexed by

porphyrins [21]. The mean SD values of all determined

heavy metals for the group of tea are indicated in the

Tables. Going by the performed t-test the concentrations

of heavy metals were not significantly different (P >

0.05). The numerical values of heavy metals concentra-

tions of Fe and Mg contents varied widely among the

samples. From the regression plot, it can be inferred that

since the significant value is 0.414 which is far higher

than 0.05 and the R and R square values are 0.586 and

0.344 respectively, that there is no significant relation-

ship between the two elements (Figure 1). According to

correlation study between heavy metals Fe-Mg, and Cd-

Cu showed no significant relationship in all the tea sam-

ples, otherwise, the differences can have major impact

on staying healthy. The excessive heat in tea boiling for

example can alter the natural chemical nature of these

metals.

The result of total contents of the studied heavy met-

als (As, Se, Zn, Fe, Mg, Cd,, Cu, Pb ) in these teas com-

pared to tea grown in other countries showed accumula-

tion of different heavy metals, for example, studies have

shown that Cu in Iranian, Lithuanian and Chinese tea

[12-14] respectively, K in Pakistanis [1] and Pb in Tuni-

sian tea [22] bioaccumulations. It goes to show the abil-

ity of these plants in accumulating metals. Other studies

showed accumulation of Al [23] and Fe [10] in tea

leaves. However, in the present study, Fe and Mg com-

plexes are higher than other metals in tea marketed in

Nigeria and may be due to high metal levels in the geo-

graphical locations and more to preferential absorption

of these metals. The percent solubility of Fe and Mg

revealed these are in form of least water soluble com-

plexes.

From all indications, differentiation of metal contents

A. C. Achudume et al. / HEALTH 2 (2010) 1097-1100

1099

Table 1. Total (mean ± SD) contents of heavy metals in tea samples.

Samples metal ions

As Cd Cu Fe Mg Pb Se Zn

Lip 1.48 ± 0.01 0.01 ± 0.00 0.12 ± 0.02 2.39 ± 0.22 5.22 ± 2.54 0.03 ± 0.00 0.52 ± 0.01 0.02 ± 0.01

Tia 2.73 ± 0.01 0.31 ± 0.01 0.31 ± 0.04 0.99 ± 0.10 4.06 ± 2.16 0.13 ± 0.01 1.53 ± 0.01 0.01 ± 0.01

Top 1.48 ± 0.01 0.02 ± 0.02 0.12 ± 0.02 2.23 ± 0.21 2.31 ± 1.47 0.33 ± 0.02 3.21 ± 0.01 0.01 ± 0.00

Gin 3.17 ± 0.01 0.39 ± 0.01 0.34 ± 0.05 0.99 ± 0.10 2.96 ± 1.75 0.27 ± 0.02 3.26 ± 0.09 0.02 ± 0.01

Mean ± SD 2.22 ± 0.01 0.12 ± 0.01 0.22 ± 0.03 1.65 ± 0.16 3.64 ± 1.98 0.19 ± 0.02 2.13 ± 0.03 0.02 ± 0.01

All the values are in mg/L

Table 2. Mean (± SD) heavy metals contents in tea aqueous extract.

Samples As Cd Cu Fe Mg Pb Se Zn

Lip 1.65 ± 1.60 0.37 ± 0.01 4.15 ± 0.091716.6 ± 186.15*244.56 ± 10.0700 ± 0.07 0.00 ± 0.32 0.80 ± 0.4

Tia 2.3 ± 0.20 7.92 ± 0.73 5.69 ± 0.09442.95 ± 8.25 123.54 ± 10.920.06 ± 0.08 0.00 ± 0.95 0.03 ± 0.04

Top 1.20 ± 0.00 0.31 ± 0.01 2.33 ± 0.711551.20 ± 159.15*221.59 ± 10.360.11 ± 0 10.39 ± 0.16 0.12 ± 0.01

Gin 0.30 ± 0 2.41 ± 0.02 3.95 ± 0.54591.95 ± 27.1541.10 ± 0.7200 ± 0.02 11.09 ± 0.29 0.12 ± 0.01

Mean ± SD 1.36 ± 0.83 2.75 ± 0.19 4.03 ± 0.36975.68 ± 95.16157.77 ± 8.100.09 ± 0.57 10.74 ± 0.43 0.28 ± 0.23

*Significantly different from the rest tea p < 0.001; All the values are in mg/L

Figure 1. Showing regression table of Mg as constant and Fe

as dependent variable.

in variety of tea brand may be due to their geographical

origin [21], due in part to leachate characteristic of soil.

Long-term plantation of tea can cause soil acidification

and elevated concentrations of bioavailable heavy metals

in the soil, therefore, enhance the risk of heavy metals

accumulation in tea leaves. The variations in the present

study might be due to different agro-climatic origins of

the imported tea. Although toxic effects of heavy metals

have sufficiently being described by WHO 1998a [24],

however, the beneficial effects of tea is in a fairly narrow

concentration range between the essential and the toxic

level [12]. The tea aqueous extracts have considerable

amounts of metals ions that could contribute towards

daily intake, but these values are lower than the daily

requirements of human being (WHO 1998b) [25]. The

determination of these elements in beverages, water,

food, plant and soil is thus of outermost important tasks.

One of the major food sources of these metals is green

leafy vegetable [26]. It is equally recommended that

aqueous extracts be routinely consumed for the essential

nutrients. Routine check and frequent analysis of tea in

Nigeria and elsewhere is required to avoid the risk of

exceeding the daily in-take beyond the tolerance limits

standards.

In conclusion, the geographical variations in heavy

metal concentrations among the tea samples were evi-

dent in all the brands. At the same time, we found sig-

nificant differences in the percent solubility of Fe and

Mg in all water soluble complexes. Some authors have

noted some differences between the metal concentrations

in other tea brands. The wide variations of metal con-

centrations observed could be due to seasonal changes

and to the chemical-physical characteristics of the grow-

ing regions.

The authors herewith state that there are no conflicts

of interest and the present study was not funded nor re-

ceive any grant from funding agency.

A. C. Achudume et al. / HEALTH 2 (2010) 1097-1100

1100

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