Concentration and distribution of toxic elements in rice ... · Journal o f Radioanalytical and Nuclear Chemistry, Articles, Vol. 140, No. 1 (1990) 91-102 CONCENTRATION AND DISTRIBUTION

Journal o f Radioanalytical and Nuclear Chemistry, Articles, Vol. 140, No. 1 (1990) 91-102

CONCENTRATION AND DISTRIBUTION OF TOXIC ELEMENTS IN RICE AND HUSK

A. MANNAN, S. WAHEED, I. H. QURESHI

Nuclear Chemistry Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, lslamabad (Pakistan)

(Received September 1, 1989)

Five varieties of rice have been analyzed to study the prevailing concentration and distribution of 10 trace elements in rice and flee husk. NAA in combination with AAS has been utilized for the determination of Hg, Pb, Cd, As, Br, Sb, Se, Ni, A1 and Cs. The elemental ratios of rice to husk and within rice have been calculated to get information regarding their uptake channels. The daily supply of the elements to the human body has been estimated and from these the safety of the diet has been assessed by comparing with the suggested tolerance levels. A global distribution pattern of some of these elements in rice has also been evaluated.

Introduction

Ever increasing human desire for attaining better living conditions has led to a wide

spread of industrial activities. These human endeavours are responsible for uncontrolled

local release and global spread of toxic chemicals into the biosphere. Pollution of the

environment is likely to contaminate human food chain resources with certain toxic

inorganic trace elements. 1 Their presence in the body even at very low levels inter-

feres with the normal biochemical functions and above a certain threshold may cause toxicity.2,3

The principal transporting medium for the trace elements to human body is food.

q-he character of food is being continuously changed with the introduction of modem

agricultural practices including use of fertilizers, insecticides, processing and packaging. As a result, a number of toxic elements may be additionally introduced in food items. 4

In view of the adverse impacts of the toxic metals, it is necessary to continuously

monitor their levels in all human diets in order to estimate the daily intake to evaluate

the safety of the diet. Such data may also help to identify the sources of pollution

and aid in formulation of legislation concerning maximum release of effluents into

the surroundings. Therefore, a systematic study has been initiated in our institute to monitor all common food items for essential, non-essential and toxic elements. Dur-

Elsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, Budapest

A, MANNAN et aL: CONCENTRATION AND DISTRIBUTION OF TOXIC ELEMENTS

ing the past years a number of food items such as tea, s drinking water, s ,6 milk,7 chicken meat, s chicken eggs 9 have been analyzed for their elemental contents. The present study is devoted to the measurement of toxic and other elements in a variety of Pakistani rice. A study of trace element distributions in rice husk has also been undertaken to possibly evaluate the influence of environment on rice crops.

Since trace elements constitute a very small fraction of the whole sample, a sensitive technique is a prerequisite for their determination. Recently, a variety of samples have been analyzed by AAS, 7 XRF, t o PIXE,11 voltammetr~12 and NAA 13-1 s

techniques. Of these, NAA in combination with AAS has been utilized for the measurement of Hg, Pb, Cd, As, Br, Sb, Se, Ni, Al and Cs in a variety of rice samples produced in Pakistan.

Experimental

Equipment

The experimental facility for NAA consisted of a high purity germanium detector

with built-in preamplifier from ORTEC, (GMX-Series), Canberra spectroscopy ampli- fier (Model-2010) and a 8k series 85 multi-channel analyzer with internal ADC. The system has been linked to a central computing station to facilitate storage and handling

of acquired data from MCA. The resolution of the gamma-ray spectrometry system is 1.9 keV for 1332.5 keV full energy peak with a peak to compton ratio of 40:1. The analytical facility for AAS consisted of Polarized Zeeman Spectrometer of Hitachi (Model Z-8000) equipped with a graphite furnace, necessary hollow cathode lamps and auto-sampler.

Standards

Although, a large number of biological and environmental Ieference materials are available 16 but their utilization as primary standards are limited on the grounds that a) element of intrest may not be in the list of the certificate, b) values for many

elements of interest may not be certified, c) expenisve and available in limited quantity. Because of these limitations and constraints, a multi-elemental comparison base for NAA has been prepared by weighing out and mixing together appropriate amounts o f spec-pure elemental oxide/salts obtained from Merck. The mixture has been dissolved in aqua regia and brought to an aqueous medium. The clear solution has been transferred to a plecleaned quartz flask and the volume made-up with doubly distilled deionized water. Aliquots of 100/11 have been dried on filter paper and subsequently used as primary standard. For AAS, the calibration standards (1000 mg/1) have been prepared by dissolving appropriate amounts of Pb, Cd and Ni

92

A. MANNAN et al.: CONCENTRATION AND DISTRIBUTION OF TOXIC ELEMENTS

salts in purified nitric acid, diluted with water and stored as stock solution in quartz

flasks. Fresh wonking solutions have always been obtained by serial dilution of the stock solutions.

Sampling and sample preparation

Various varieties of rice samples namely, Pak Basmati (PB), Kashmir Basmati (KB), Basmati (B-370), IRRI-6 (IR-6) and IRRI-8 (IR-8)have been obtained from the Nu-

clear Institute of Agriculture and Biology (NIAB), engaged in the development and improvement of seed quality to enhance the crop yield. Rice husk was separated from rice by mechanical means and both portions were pulverized in a grinder. The blades of the grinder were coated with a teflon layer to avoid the contamination of Fe, Ni and Cr. The samples were shaken for 48 hours and the homogeneity was checked by analyzing Mn and K contents in several aliquots. A sample weight of

300 mg was found to have 5% dispersion in its respective values.

Elemental analysis by NAA

Approximately 300 mg samples each of rice and husk were sealed separately in

precleaned polyethylene capsules and quartz ampules for short and long irradiations, respectively. Similarly, synthetic standards and reference materials were encapsulated,

placed in aluminium containers and irradiated according to predetermined irradiation

conditions in the periphery of the reactor core. After appropriate cooling period, the targets were transferred to fresh counting vials and radioassayed.

Experimental conditions for NAA are obtained by either computer simulation of

an assumed matrix 17 or by optimizing the irradiation and cooling periods. The latter

approach was adopted in this study and the optimal scheme .~s presented in Table 1.

The details of analyses and interference studies in a wide variety of matrices have been reported elsewhere. 1 ,a ,a ,9

Table 1 Optimum conditions for analysis by INAA

Isotope used Hal f - l i fe Gamma-ray used, keV Irradiation time Cooling time

2 o 3 Hg 46.6 d 279.2 24 h 2 w 7~As 26.3 h 559.1 24h 2w

122Sb 2.70 d 564.1 24h 2w 7SSe 120.0 d 264 24h 2w 82Br 35.4 h 554.3 (776.5) 24 h 2 w

2SAl 2.246 m 1779 1 m 3 m a4Cs 2.04 y 795.8 (801.9) 24 h 2 w

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A. MANNAN et al.: CONCENTRATION AND DISTRIBUTION oF TOXIC ELEMENTS

Analyses of N i, Ccl and Pb by AAS

Sample weighing 500 mg have been digested in 5 ml of purified nitric acid at

80 ~ followed by addition of 2.5 ml of perchloric acid and heated again at 250 ~

till white fumes evolved. The clear solution was stored in a quartz flask. A blank has

Table 2 Experimental conditions for Cu, Ni, Cd and Pb measurements by AAS

Analytical conditions Cu Ni Pb Cd

Lamp current (mA) 7.5 12.5 7.5 7.5 Wavelength (nm) 324.8 232.0 283.3 228.8 Slit width (nm) 1.3 0.2 1.3 1.3 Burner height (mm) 7.5 7.5 - - Oxidant gas pressure (kg/cm 2 ) 1.6 1.6 - - Fuel gas pressure (kg/cm 2 ) 0.3 0.3 - - Carrier gas flow (ml/min) - - 100 100 Sample volume 0A) - - 10 10

Heating program Drying temperature CC) - - 80-120 80-120

time (s) 30 30 Ashing temperature (o C) - - 400 300

time (s) 30 30 Atomization temperature f~ - - 2100 1700

time (s) 7 7 Cleaning temperature (o C) - - 3000 2600

time (s) 3 3

also been prepared under indentical conditions. The reliability of the ashing procedure

has already been established.

The determination of Ni was carried out with air-acetylene flame and of Pb and Cd

with electrothermal atomisation technique. The optimized conditions of their measure-

ment are recorded in Table 2 and the detailed methodology for their analyses in food

materials have already been reported. 7

Results

Under the optimized conditions developed, Hg, Pb, Cd, As, Br, Sb, Se, Ni, A1 and

Cs have been analyzed in five varieties of rice and husk samples and the results of

their analysis are presented on dry weight basis in Tables 3 and 4, respectively. The

94

A . M A N N A N e t a l . : C O N C E N T R A T I O N A N D D I S T R I B U T I O N O F T O X I C E L E M E N T S

0

8 ~ g

Z

~ f

~ c

~ q

I

�9 q q o q q ~

+ 1 ~ + 1 ~ + 1 . ~

~ N N o o o o * N ~

q q q q ~ 0 ~ 0 0 0 0 0 0 ~ 0

q q i q q ~ A

~ ~ + I ~

g

8

o

0

8 g .-4

8

m

o

o

I I I 1 I I I t 1 I

R ~ q q q q q ~

. . . . q ~ q ~ m

~ ~ ~

o ~ 6 o o o o ~

0

0

95


400 200 0 0 200 400 ppb

2 t O 0

80 40 0 0

1 2 ppm

40 80 ppb

0.8 0.4 0 0 0.4 0.8 ppm

ppm

1.0 0.5 0 0 0.5 1.0 ppm

2 1 0 0 1 2 ppm

80 40 0 0 40 80 ppm

96

20 10 0 0 10 20 ppm

Concentration Concentration q �9

Hu~k Rice

Fig. 1. Trace e lement distr ibution pat tern in varieties o f Pakistani rice and husk


0 Cl n,-

&

I o o I 0 ? o" 0.5 o

0 0

o

Pb

o o

0

~r

I.C

& o (2.26)

1.5 Cd

0.5

0

O_~r 0I0 I

0.05{-

01

(0.06) o o

Br o Q

o o

&

o 0.6- As o

n, 0.4-

Q2-

&

o 10 I Q: 05

0

o Sb

o (2.98)

0

0

r~

0

or

5e

0

0 9

o o

&

0.4~ AL

0.2r , - -o

o | I Pb

0 o

i J I B370 KB IR8 IR6

&

o I"0 I ~"0.5

0

o (7.00) o

Ni

o o

o ~L (0.42)

0.3 - Cs .o

~" 0 . 2 -

0.1-

I Pb

o

o o O

I I I I B370 KB IR8 IR6

Fig. 2. Elemental ratios of rice to husk; PB: Pak Basmati, B-370: Basmati-370, KB: Kashmir Kashmir Basmati, IR-8: IRRI-8, IR-6:IRRI-6

97


Pakistan Bangladesh China Vietnam Thailand Malaysia Indonesia Japan United States Brazil Holland Belgium Italy

Pakistan Bangladesh China Vietnam Thailand Malaysia Indonesia Japon United States Brazil HoUond Belgium Itaty

~/////]

F/-ZE/_ZE//_//_E/~ r

V//I~IIIIIIII/IIA I ~ I I I I I I I I I I A

I

Y////./J

1~27Z]

I I 0.05 0.10

As

v////////J

II g~

y. I l l - /1/ I l iA

l / ] l l l l l l l l l l l l l l l l ] l l 4 l Y I I I I I I l l / J

I I

I

I

Pb V/l/Ill/IliA 5b

r/llll.~

I v'F/~

I I I 0.50 1.00 2.00

g////////////2////////A

I

i I I I 0.05 0.10 0501,00

Pakistan Bangladesh China Vietnam Thailand Malaysia Indonesia Japan United States Brazil Holland Belgium Italy

g / / / . T A

V / - / / / / z ' l

y / / / / / / / / / / ' / / / / ' J

Y / / / / / / / / / / / / / I . ' ]

i

g / I / / / / / / l l l l l I / I / l l l l l l l l l ~ A

[ ]

Br

V / / / / A vm

r / / A Hg

Y / / / / / / / / ' A

[ ]

I

I I

0.5 1.0 5.0 10.0 50.0 0.001 0.005 0.010 0.050 0.100 R a n g e of v a l u e s ~ ppm

Fig. 3. Global distribution pattern of essential trace elements in rice

median concentrations of these elements in rice and husk are plotted as horizontal stacked bars (Fig. 1). The ratio of one element in rice to the same element in husk

is depicted in Fig. 2. The global pattern of the elemental distribution in rice samples is plotted as concentration range in Fig. 3. The estimated daily intake o f toxic ele-

ments is presented in Table 5. The precision and accuracy of the procedure have been

checked by analyzing NBS Orchard Leaves (SRM-1572) and the results are listed in

Table 6.

98


Discussion

The concentrations of Hg and As are lowest in B-370, whereas the values in the remaining varieties do not vary significantly. The contents of Cd in B-370; Sb in PB and B-370, Ni in KB and Se in IRRI-8 are much higher than the other varieties of rice studied. Unlike some of the essential elements, 1 s the ratios for the toxic elements within all type of the rice varieties are not similar, indicating random uptake accord- Lug to the elemental distribution pattern of the soil and the surroundings.

The study of elemental distribution pattern plotted in Fig. 1 indicate that except for Se the concentrarion of all the elements are higher in husk than in flee. Of various rice husks studied, the concentrations of As and Br are slightly higher and that of Sb is much higher in PB husk than in other rice husks. The levels of Se and A1 are relatively higher, whereas the contents of Cd and Ni are much higher in IR-8 as compared to other rice husks.

Husk acts as a natural barrier against the direct influence of environment on rice. Therefore, computation of the elemental ratios will illustrate the paths of uptake and subsequent degree of contamination. The elemental ratios of rice to husk are plotted in Fig. 2, where the horizontal line represents the median value of the ratios. The figure indicates that the ratio for Se is greater than unity, which reveals pre- ferential uptake of this element from soil. Further, Hg in IR-6; As in IR-8, Pb in

Kb and IR-8 and Ni in KB have much higher ratio than the respective median values. This higher degree of contamination of rice may possibly have occurred as a conse- quence of certain agricultural practices, the presence of specific ore bodies nearby, or of industrial effluents. The major sources of Hg contamination are seed preser- vatives, whereas insecticides, herbicides and fungicides are the principal As sources. Therefore, an effective control on the industrial effluents and use of agricultural

sprays is necessary to avoid spread of inorganic elements with potential health hazard. A1 in B-370 and Br in PB have lower values than their respective median ratios, indicating possible direct influence of environment on rice husks. The ratios for other elements, namely, Cd, Sb and Cs have random values. A large dispersion in the values may possibly be due to a greater variation in soil and environmental spectra.

Global pattern

Rice is one of the commonest staple foods for human consumption in many parts of the world. Therefore, it is important to evaluate the global distribution pattem of toxic elements in rice produced in different countries. From the available literature data,14,19-~ s the level of these elements has been plotted as horizontal bars where the length represents the conentration of an element. The concentration of Br in

99


Thai and Japanese; Pb in Thai; As in Bangladesh, Se in Pak and Japanese; and Sb in Malaysian rice are relatively higher than in rice from other countries. The comparison of our data with other areas in Fig. 3 indicates that Pb content of Pak rice is similar to those from Malaysia, whereas Hg level is comparable to those from USA~ Brazil and Belgium. The concentration is not different from those in other countries, whereas the Se level is higher than in other countries but similar to that in Indonesia.

Dietary evaluation

A market basket survey carried out in summer and winter seasons indicated that rice

constitutes only 5.5% of the total food intake in Rawalpindi/Islamabad areas. The average intake of toxic and other elements has been estimated on the basis of weekly

consumption of 300 g rice. These estimates show that the level of intake follows the order A1 > Se > Br > Ni > Pb > As > Sb > Cd > Hg and the corresponding values are

5754, 136, 99.4, 63, 41,25, 9.1,7, 4.6 #g. The diet safety has been calculated by

Table 5 Estimated weekly intake of toxic elements through rice

Elements Weekly intake,/zg Reported safe limits,/~g

Hg 4.6 300 Cd 7.0 400-500 Pb " 41.0 3000 As 25.2 2800 Sb 9.1 - Ni 63.0 14-49 Se 136 1540 Br 99.4 7000 A1 5754 -

comparing the daily intake with the suggested lower safe limits 2 ,3,29 and is presented

in Table 5. These estimates reveal that the toxic contaminant supply to human body

varies from 1 to 9% of the lower safe limits.

Conclusion

Neutron activation analysis and atomic absorption spectrophotometric techniques have been utilized for the determination of toxic and other elements in rice and husk. This study has revealed that the concentration of Se in flee is higher than in husk,

100


Table 6 Analysis NBS, SRM - 1571 (Orchard Leaves)

Element Our values, Variation NBS Values, Deviation, % ppm coefficient, % ppm

Hg 0.141 11.3 0.155 -9.03 Cd 0.15 13.3 0.11 +12.4 Pb 48 10.4 45 +6.67 As 9.9 2-0 10 -1.0 Sb 3.2 6.2 2.9 +10.3 Ni 1.5 6.7 1.3 +15.4 Se 0.092 8.7 0.08 +15 ~3 Br 10_5 1.9 (10) +5.0 A1 284 7.4 310-410" - Cs 0.049 10.2 (0.04) §

*Literature values, a o Values in parentheses are uncertified.

whereas other elements are concentrated in husk. Furthermore, this study has indicated

that most of the elemental uptake is probably through soil rather than the surroundings,

indicating a lesser degree of direct environmental impact on rice. The contents of

toxic inorgartic constituents of rice are below the threshold levels to induce any signi-

ficant physiological disorder Because of the presence of minerals at high concentra-

tion levels in rice husk) s its mixing with other food articles for supplementation of

essential nutrients is not recommended on the grounds that the content of toxic

inorganic constituents is much higher in husk as compared to rice.

We are grateful to Messrs. Shaukat ALI and M. SAEED for their technical assistance, the reactor operation group for neutron irradiation of the samples and atomic absorption speetrophotometric group for providing the experimental facilities.

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A. MAN-NAN et al.: CONCENTRATION AND DISTRIBUTION OF TOXIC ELEMENTS

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Documents

Concentration and distribution of toxic elements in rice ... · Journal o f Radioanalytical and Nuclear Chemistry, Articles, Vol. 140, No. 1 (1990) 91-102 CONCENTRATION AND DISTRIBUTION