J. Nig. Soc. Phys. Sci. 4 (2022) 891

Journal of the
Nigerian Society

of Physical
Sciences

Levels of Zinc (Zn), Copper (Cu), Iron (Fe), and Cadmium (Cd)
in Soil, Rice Stalk, and Oryza Sativa Grain in Ishiagu Rice Field,

Ebonyi State, Nigeria; Human Health Risk

D. N. Ajaha, E. Agboeze ID a,∗, J. N. Ihediohab, E. Chukwudi-Madua, C. C. Chimea

aDepartment of Industrial chemistry, Enugu State University of Science and Technology, Enugu State, Nigeria
bDepartment of pure and Industrial chemistry, University of Nigeria Nsukka, Enugu State, Nigeria

Abstract

Levels of heavy metals (Zn, Cu, Fe, Cd) were determined in soil, rice grain, and rice stalk from Federal College of Agriculture Ishiagu rice field,
Ebonyi state, Nigeria. The dried samples were digested with a 1: 3 (HNO3: HCl) mixture and analyzed with atomic absorption spectrophotometer
(AAS). The mean concentration of the metals in the soil before planting, soil after harvest, and rice grain were as follows: Zn (7.28, 11.33
and24.90); Cu (3.40, 4.64 and 4.14); Fe (803.04, 735.47 and 107.78); Cd (1.14, ND and ND) and were all within FEPA and FAO/WHO limits.
The daily intake values for a 60 kg adult were Zn (0.04), Cu (0.01), and Fe (0.18) and were all below the recommended limits by Codex
Alimentarius standards. The Target Hazard Quotient (THQ) for Zn, Cu, and Fe was less than one (1<), and the total hazard index was less than 1,
indicating that the population will not be exposed to the potential health risk from these metals. However, the metal levels should be monitored to
ensure they stay at harmless levels.

DOI:10.46481/jnsps.2022.891

Keywords: Heavy metals, Rice, Soil, Health risk, Environmental pollution

Article History :
Received: 26 June 2022
Received in revised form: 28 July 2022
Accepted for publication: 14 August 2022
Published: 25 September 2022

c© 2022 The Author(s). Published by the Nigerian Society of Physical Sciences under the terms of the Creative Commons Attribution 4.0 International license

(https://creativecommons.org/licenses/by/4.0). Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Communicated by: E. A. Emile

1. Introduction

Advancement and mechanization of urban growth have pro-
moted socio-economic development in developing countries like
Nigeria and the world at large. But besides all the positive ef-
fects, they cause ”environmental pollution”. Soil contamination

∗Corresponding author tel. no: +234 9039239802

Email address: emmanuel.agboeze@gmail.com (E. Agboeze ID )

by heavy metals is a significant environmental concern world-
wide, as it concerns human health and food security/quality [1-
5]. According to Yap et al. [5], the significant sources of heavy
metals include anthropogenic environmental activities like min-
ing, smelting processes, steel and iron industries, chemical in-
dustries, agriculture, and domestic activities [4]. These sources
outweigh natural sources like weathering of the parent material
and volcanic eruptions [5]. Recently, concern has been raised
about possible contamination of the crop (rice) by heavy met-
als. The plants can absorb these heavy metals in the soil through

1

https://orcid.org/0000-0001-5337-117X
https://orcid.org/0000-0001-5337-117X


Ajah et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 891 2

their roots, stems, or leaves and accumulate in their organs [6,
7]. In specific concentrations, some of these heavy metals are
essential to plants, but in higher concentrations can become
toxic. These metals are a health hazard to living organisms
due to their persistence, non-biodegradable, and non-thermal
degradable environmental characteristics [5]. The uptake of
metals in excessive amounts may either cause harm to the plant
or enter the food chain and accumulate when these plants are
taken up. Metals accumulated in the human body through the
food chain cause diseases like lung cancer and damage the cen-
tral nervous system, kidney, and liver [8-11].

Heavy metals, which can pose severe hazards to humans
and the environment, are increasingly being found in the envi-
ronment due to the growth of mining, smelting, and other in-
dustrial operations. The quality of the surrounding air, soil,
and water bodies is affected by pollution from heavy metals
like lead (Pb), arsenic (As), nickel (Ni), cadmium (Cd), copper
(Cu), and zinc (Zn), which endangers the lives of both animals
and humans through the food chain [12].

Evaluation of potential impacts on human health in contam-
inated environmental media is part of assessing the risk to hu-
man health [4]. Human exposure to contaminants, the type of
contaminants, and the affected person’s susceptibility all deter-
mine how they affect human health [4]. Health impacts may in-
clude increased risk of cancer, high blood pressure, acute neuro-
logical abnormalities in fetuses, organ dysfunction, respiratory
issues, physical and mental illness, shortened life expectancy,
and immune system deterioration [12].

This study is intended to determine the levels of Zinc, Cop-
per, Iron, and Cadmium in soil, rice grain, and stalk obtained
from a rice field in a local area in Nigeria (Federal College
of Agriculture Ishiagu, Ebonyi state) through the determination
of the physicochemical properties of the soil, evaluation of the
level of contamination, and health risk assessment.

2. Experimental

2.1. Area of Study
See figure 1.

2.2. Sample Collection
The land (Federal College of Agriculture, ishiagu, rice field)

is about 6 hectares, i.e. (240×75m). The land was divided into
4parts (A, B, C, and D).

2.3. Soil Sample Collection
Eight (8) composite soil samples were collected from four sec-
tions of the rice farm before plantation. Each composite is made
of 5 grab samples randomly collected from each area of the
rice farm. In the same way, twelve (12) composite soil sam-
ples were collected after the rice harvest. Four (4) grab samples
were randomly collected from different parts of the land which
has not undergone any agricultural practice (a fallow land) as
the soil control. These soil samples were taken from 0-20cm
depth from the surface of the ground with a clean machete, and
the collected samples were placed in well-labeled polyethylene
bags.

2.4. Rice Sample Collection

Twelve (12) composite rice plant samples were collected from
four different sections of the farm. Each composite consists
of five (5) randomly selected rice plants concerning the soil al-
ready sampled. The rice grains from each composite were sepa-
rated from the stalk. The rice grains and stalks were each placed
in polyethylene bags.

2.5. Sample Preparation

2.5.1. Soil Preparation
The soil samples were air-dried, crushed, and passed through

a 0.16mm sieve to remove gravel-sized materials and then ho-
mogenized with a mortar and pestle. The samples were then
stored in polyethylene bags with labels for analysis.

2.5.2. Rice Preparation
The rice grain samples were refined to remove the husk using
mortar and pestle and winnowed with a tray pan. The polished
rice grains were then stored in polyethylene bags well labeled.
The rice stems were finely chopped with the knife on a wooden
platform and stored in labeled polyethylene bags.

2.5.3. Ash
Two (2) ash samples were collected from 2 different abattoir
sites and were mixed to get a homogenous sample. The ash was
used to mix the rice seeds before broadcasting to scare birds
away or prevent them from eating them.

2.5.4. Sample Size
The overall sample for analysis became: 8 for soil before plant-
ing + 12 for soil after harvest + 12 for rice grain +12 for rice
stem +4 for soil control, and + 1 for ash, totaling 49 samples.

2.5.5. Digestion of Soil Samples, Rice Grain Samples, and Rice
Stalk Samples

10g each of the soil and rice grain samples, 5g of the rice stalk
were measured into different digestion flasks, and 40ml of Aqua-
regia (Nitric acid and HCl in the ratio of 1:3) was added. The
mixtures were then heated on a heating mantle to boil until the
fume became clear and were allowed to cool. Each of the var-
ious digested solutions was diluted with deionized water and
filtered through a filter paper into 100ml volumetric flasks and
were made up to the mark with deionized water. The solu-
tions were finally kept in test tubes. Concentrations of Zn, Cu,
Fe, and Cd were determined using the AA-7000 atomic spec-
troscopy. Statistical analysis of the data obtained was done us-
ing SPSS version 17 for windows and Palisade analysis.

2.6. Quality Assurance

The precision of the analytical procedure was investigated by
carrying out recovery experiments. This was done by determin-
ing the metals’ concentration in duplicate spiked and unspiked
rice samples. Spiking was done by adding 1 ML of 2ppm and

2



Ajah et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 891 3

Figure 1. Area of study

4ppm metal solution to 2 g of samples, which were later sub-
jected to the digestion procedure.

% recovery =
a − b × 100

c
, (1)

where a denotes concentration in the spiked sample, b repre-
sents concentration in the unspiked sample and c denotes con-
centration of the metal ion added.

2.6.1. Risk Assessment
This process evaluates the potential effects of a contaminant
on humans from doses received through one or more exposure
pathways. The health risk from rice consumption was assessed
using the target hazard quotient (THQ) and total hazard index
(THI) [13]. The target hazard quotient (THQ) is the ratio of the
determined dose of a pollutant to a reference dose level. In con-
trast, the total hazard index (THI) evaluates the potential risk of
adverse health effects from a mixture of chemical constituents
in rice. If this ratio is less than 1, the exposed population is
unlikely to experience noticeable adverse effects [14-16].
The following equation calculated the THQ and THI values for

the metals:

T HQ =
EF × ED × IR × C
R f D × BW × AT

× 10−3 (2)

T HI = T HQ1 + T HQ2 + · · · + T HQn (3)

Where THQ is the target hazard quotient, EF is the exposure
frequency (365 days /year), ED is the exposure duration (54
years), IR is the rice ingestion(kg/person/day), and C is the
metal concentration in rice (mg/kg), RFD is the oral reference
dose (mg/kg/day) and AT is the average time for
non–carcinogens (365 days /year× ED). The oral reference dose
for metals
(mg/kg/day) were: Cu(0.040), Zn(0.300), Fe(0.700) [17]. The
calculated THQ value is also shown in Table 2. It is less than
1, thus indicating that the Nigerian population has not been ex-
posed to the potential health risk of dietary copper via rice con-
sumption.

2.7. Data Analysis
Data obtained were analyzed with SPSS 17 for windows. Cor-
relation coefficient analysis was carried out to establish the cor-

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Ajah et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 891 4

relation pattern of various metal pairs in soil and rice grain sam-
ples. The student’s t-test was carried out to determine any sig-
nificant difference in the metal concentration in the soil before
planting and after harvest at p<0.05.

3. Results and Discussion

Table 1 shows recoveries ranging from 84% to 128% with
a mean of 96% and precision of 5.7, a value lower than 10%
indicating high accuracy.

3.1. Heavy Metals Concentration in Soil and Rice Grain

3.1.1. Copper in Soil
The concentrations of Copper (Table 2) in the soil samples from
the control portion ranged from 1.67 mg/kg - 3.33 mg/kg. The
lowest value (1.67 mg/kg) was recorded at Control2, while the
highest (3.33 mg/kg) was at Control4. These concentrations
gave a mean ± SD of 2.22 ± 0 .75 (Table 3).
As in Table 2, the concentration of copper in the ”soil before
planting” ranged between 2.39 mg/kg - 5.18 mg/kg. The lowest
value (2.39 mg/kg) was recorded at D2 and the highest (5.18
mg/kg) at A1. These gave a Mean ± SD of 3.40 ± 0.91 (Table
3). Also, the concentrations of copper (Table 2) in ”soil after
harvest” ranged from 3.70 mg/kg to 6.07mg/kg. A3 had the
lowest value (3.70mg/kg), and B2 had the highest value (6.07
mg/kg). The mean ± SD was 4.64 ± 0.69 (Table 3).
It compared the mean concentrations of (”soil before plant-
ing” and ”soil after harvest”) (3.40 mg/kg and 4.64 mg/kg, re-
spectively) with the mean concentration of the ”soil control”
(2.22 mg/kg). It can be seen that the soil was slightly contam-
inated with copper. This contamination could be attributed to
the use of pesticides or herbicides. This possibility was sup-
ported by the fact that most pesticides used in agricultural soils
were based on compounds containing Cu, Hg, Mn, Pb, or Zn
[18].
The results compared with other researchers; Zhuang reported
502 mg/kg as the mean concentration of copper in agricultural
soils around the Daboshan mine in Guandong, China. Also,
Song recorded 8.41-148.73 mg/kg as the mean concentration
of copper in agricultural soils of Suxian County, South China.
This variation of results might be due to the copper concentra-
tions in irrigation water and other agronomic practices in the
respective areas [16, 19]. However, the mean concentrations of
Cu in (soil before planting (3.40) and soil after harvest (4.64) is
below the permissible limits;

1. 150 mg/kg as with Chinese Environmental Quality Stan-
dards (1995) for soils [9].

2. (70-80) mg/kg as with FEPA (1991) guidelines for heavy
metals in soil. The low concentrations recorded in this
study may be attributed to the continuous removal of cop-
per by rice grown in the field. Therefore, the concentra-
tion of Copper in Soil may not harm mice and humans
when used for rice production [3].

3.1.2. Copper in Rice
The concentration of copper in “Rice stalk” as in Table 2 above
ranged between 0.74mg/kg-11.06mg/kg with A1& B3 having
the lowest value (0.74 mg/kg) and A3having the highest value(11.06mg/kg).
These concentrations gave a mean ± SD of 2.46 ± 2.95, as in
Table 3.
As in Table 2 above, the concentration of copper in ”rice grain”
ranged between 1.84mg/kg-14.81mg/kg, with A2 having the
lowest value (1.84 mg/kg) and C3 having the highest value (14.81
mg/kg). The concentrations gave a mean ± SD value of 4.14 ±
3.92as seen in Table 3.
In Tanzania, Machiwa reported the mean concentration of Cu
in rice collected from different locations to be 3.7mg/kg. Also,
in Guandong, China, Zhuang reported the mean concentration
of Copper in Rice as 6.34 mg/kg [16, 20]. However, the mean
Cu concentrations of the rice samples in this study are within
China’s maximum permissible limits (10 mg/kg) [21]. They
are also within the FAO/WHO recommended limits (20 mg/kg)
for copper in rice grains, which means that the rice is suitable
for consumption [21].

3.1.3. Relationships Between Copper Concentration in the Soil
Before Planting and Soil After Harvest

Statistical analysis with student T-test indicates a significant
difference in the metal (Copper) concentration of the soil before
planting and after harvest at p=0.05 with a calculated value of
3.27. Table 3 shows the results of the student T-test for metal
concentrations in the ”soil before planting and soil after har-
vest.”

3.1.4. Zinc in Soil
The Zinc concentration in the soil samples collected from the
”control portion” as in Table 2 ranged between 3.73 mg/kg -
8.43 mg/kg. The lowest value (3.73 mg/kg) was recorded in
control2, while the highest (8.43 mg/kg) was in control2. These
concentrations gave a mean ± SD of 5.26 ± 2.2, as shown in
Table 3. Also, the concentration of Zinc in the “soil before
planting” (Table 2) ranged between 2.75mg/kg - 13.17mg/kg
with the lowest value (2.75mg/kg) at A1 and the highest value
(13.17mg/kg) at A2. These gave a mean ± SD of 7.28 ± 3.90
(Table 3).
As in Table 2, the Zinc concentration in ”soil after harvest”
ranged between 6.88mg/kg-15.15mg/kg. The lowest value was
C3(6.88 mg/kg), while the highest was D3(15.15 mg/kg). The
mean ± SD is 11.33 ± 2.51, as in Table 3. Comparing the mean
concentration of zinc in the ”soil before planting and soil after
harvest” (7.28 mg/kg and 11.33 mg/kg respectively) with that
of the soil control (5.26 mg/kg) as seen in Table 2, It can be
seen that their values are more than that in the ”soil control”
suggesting that the soil is contaminated with zinc. The contam-
ination could be attributed to pesticides and herbicides on the
field, as zinc is one of the constituents of most pesticides [18].
Also, we compared the results with those reported by researchers
like;

1. Zhuang reported that the Zn concentrations of the agri-
cultural soils around the Daboshan mine in Guangdong,

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Ajah et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 891 5

China is 498 mg/kg [14].

2. Machiwa reported the mean zinc concentration as
65.46mg/kg in the agricultural soils of Lake Victoria basin,
Tanzania [20].

3. Kibassa reported the average range concentration of zinc
in agricultural soils collected from six sites in Daressalan
to be 33.18 mg/kg [22].

The mean concentrations gotten from the different soil por-
tions (soil before planting and soil after harvest, 7.28 mg/kg
and 11.33 mg/kg, respectively) in this study did not exceed
that recorded by the researchers mentioned earlier and also are
far below the maximum permissible limits (300-400 mg/kg) for
zinc in soils by (FEPA) and (300 mg/kg) as with Grade ii En-
vironmental Quality standards for agricultural soils in China.
Therefore, the soil may not be harmful to rice production for
consumption by human beings.

3.1.5. Zinc in Rice
The concentration of zinc in ”Rice stalks” (Table 2) ranged
between 15.14 mg/kg and 69.03 mg/kg, with D2 having the
lowest value (15.14 mg/kg) and A1 having the highest value
(69.03mg/kg). These concentrations gave a mean ± SD of 45.14
± 13.73, as shown in Table 3. Also, the concentration of zinc
in ”rice grain” ranged between 14.78 mg/kg-32.89mg/kg, as
shown in Table 2, with D1 having the lowest value (14.78 mg/kg)
and B2 has the highest value (32.89 mg/kg). The mean ± SD
value was 24.90 ± 6.06, as shown in Table 3.
It compared the mean concentration of zinc in rice grains (24.90
mg/kg) in this study with those reported by [20] (21.7 ug/g) and
that reported by [23] (21.5 ug/g). It was found that they are
still within the range, although most of the concentrations of
zinc in rice individually in this study fall within these limits ex-
cept for a few which are a bit higher in the rice grains in areas
A2, B2, B3, and C1(31.37,32.89,30.81, and 29.48 mg/kg respec-
tively). The concentrations in the rice stalk were relatively high.
This could be attributed to the ash from the abattoir (rubber
tire), which was used to mix the rice grains before broadcast-
ing on the field to avoid /scare birds from eating the broadcasted
rice. This ash contains zinc, which was made highly available to
the rice stalk then, followed by the little the rice grain absorbed.
However, the mean concentrations are still within the Chinese
maximum permissible limits for zinc in rice (50 mg/kg) and the
FAO/WHO (2002) recommended limits for zinc in rice grains.
Therefore, rice is suitable for consumption [17].

3.1.6. Relationships Between Zinc Concentrations in the Soil
Before Planting and Soil After Harvest

The statistical analysis with the student T-test indicated a signif-
icant difference in the soil’s metal (Zinc) concentration before
and after planting at p=0.05 with a calculated value of 2.60.
Table 3 shows the results of the student T-test for metal concen-
tration in the soil before and after planting.

3.1.7. Iron in Soil
The concentration of iron in ”soil control” ranged between 805.26
mg/kg - 823.69 mg/kg, as shown in Table 2, with control1 hav-
ing the lowest value (805.26) and control4having the highest
value (823.69 mg/kg) and a Mean ± SD of 814.40 ± 7.52 as in
Table 3.
As in Table 2, the iron concentrations in the ”soil before plant-
ing” ranged between 764.29 mg/kg - 845.59 mg/kg. D2 had
the lowest value (764.29 mg/kg) while A1 had the highest value
(845.59mg/kg), and the Mean ± SD value was 803.04 ± 26 as
in Table 3.
Also, the iron concentrations in ”soil after harvest” ranged be-
tween 611.14-838.81 mg/kg, as in Table 2. The lowest value
(611.14 mg/kg) was recorded at C2 and the highest (838.81
mg/kg) at D2. The Mean ± SD value was 735.4 ± 73.20, as
shown in Table 3. Comparing the mean concentration of iron in
(soil before planting and soil after planting) (803.04 mg/kg and
735.47 mg/kg, respectively) with soil control (814.40mg/kg).
These values are lower than the soil control value meaning the
soil is not contaminated with iron. However, the mean concen-
trations in soils (before planting (803.04mg/kg) and after har-
vest (735.47 mg/kg) are still within the maximum permissible
limits (7000-550000) as with Chine’s grade ii Environmental
quality standards [24].

3.1.8. Iron in Rice
The iron concentrations in ”Rice stalk” ranged between
56.61mg/kg-300.12mg/kg, as shown in Table 2. The lowest
value (56.61 mg/kg) was recorded at C2, and the highest 300.12
mg/kg value was recorded at B2. The Mean ± SD value was
150.02 ± 85.20as shown in Table 3.
The iron concentrations in ”rice grain,” as shown in Table 2,
ranged between 0.66mg/kg-392 mg/kg. The lowest value (0.66
mg/kg) was recorded at B3 and the highest value (392.35 mg/kg)
at D2. The Mean ± SD value was 107.78 ± 138.15, as in Table
3.

3.1.9. Relationships Between the Iron Concentration in the Soil
(Before Planting) and Soil (After Harvest)

The statistical analysis with the student T-test indicated a signif-
icant difference in the soil’s metal (Iron) concentration before
and after planting at p=0.05 with a calculated value of 2.92.
Table 3 shows the results of the student T-test for Metal con-
centrations in the soil before and after planting.

3.1.10. Cadmium in Soil
Cadmium concentrations were detected in only one portion; in
the ”soil before planting” area A1, with a value (1.14 mg/kg), as
shown in Table 2.

3.1.11. Cadmium in Rice
The concentrations of Cadmium in ”rice stalk” were scantily
detected at A1, B1, B2, C3 & D1 with the range
(0.02mg/kg-2.03mg/kg) as shown in Table 2. The lowest value
(0.02 mg/kg) was recorded at B2 and the highest value at D1. The
mean ±SD was 0.53±0.85, as shown in Table 3. The mean

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Ajah et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 891 6

cadmium concentration in rice stalk was higher than the Codex
standards (1993-1995) (0.1mg/kg) for Cadmium in Rice stalk.
This high value could be attributed to the ash (from the abattoir
(rubber tire) that was used to mix the rice seeds before broad-
casting on the rice field [25]. However, none of these concen-
trations were made available to the rice grain, which leaves the
conclusion that the rice grain is free from Cadmium contamina-
tion and is, therefore, fit for consumption.

3.1.12. Zinc Concentrations in Soil
Zinc concentration in the ”soil before planting” ranged between
2.75mg/kg and 13.17, with a mean of 7.28. Zinc concentra-
tion in ”soil after harvest” ranged between 6.88mg/kg and 15.15
mg/kg, with a mean of 11.33. The mean concentrations of zinc
from the different soil portions (soil before planting and soil af-
ter harvest) in this study are below the 33.18 mg/kg and 65.46
mg/kg reported by kibassa and Machiwa as average range con-
centration of zinc in agricultural soils of Daressalan and lake
victoria basin, Tanzania respectively [20, 22]. Moreover, the
mean zinc concentration in the soil did not exceed the maxi-
mum permissible limits (300-400 mg/kg) for zinc in soils by
FEPA and (300 mg/kg) as with Grade ii Environmental Quality
standards for agricultural soils in China [24].

3.1.13. Copper Concentration in Soil
Copper concentration in ”soil before planting” ranged between
2.39mg/kg and 5.18 mg/kg, with a mean of 3.40. The Con-
centrations of copper in the soil after harvest ranged between
3.70 mg/kg and 6.07 mg/kg, with a mean of 4.64. These mean
concentrations are below the permissible limits of 150 mg/kg
as with Chinese Environmental quality standards for soil and
70-80 mg/kg as with EPA guidelines for heavy metals in soil.
The low concentrations are in line with 8.41-148.73mg/kg as
reported by Song as the mean concentration of copper in agri-
cultural soils of Suxian County, South China [19].

3.1.14. Iron Concentration in Soil
Iron concentration in ”soil before planting” ranged between
764.29 mg/kg and 845.59 mg/kg, with a mean of 803.04. Iron
concentrations in ”soil after harvest” ranged between 611.14
and 838.81 mg/kg with a mean of 735.4. However, the mean
concentrations are still within the maximum permissible limits
(7000-550000) as with Chinese grade ii Environmental quality
standards [24].

3.1.15. Heavy Metals in Rice Grain
Zinc in Rice Grain

Zinc concentration in rice grain ranged between 14.78 mg/kg
and 32.89 mg/kg, with a mean of 24.90. This mean value is
within the 21.5g/g reported by Herawati and the 21.7g/g re-
ported by Machiwa as the mean concentration of zinc in rice
collected from different locations in Tanzania [23, 20]. How-
ever, the zinc mean concentration in this study is below the
50mg/kg recommended limits by FAO/WHO (2002) for zinc
in rice grains. Hence it is worth noting that the rice may not
pose any kind of danger to human health as a result of Zinc
contamination [26].

Copper in Rice Grain
The concentration of copper in rice grain ranged between 1.84
mg/kg-14.81mg/kg with a mean of 4.14. This mean value is
not far from the 3.7 mg/kg reported as a copper concentration
in rice grain collected from different locations In Tanzania by
Machiwa and the 6.34 mg/kg reported as the mean concentra-
tion of copper in rice from Guangdong, China, by Zhuang [16].
However, the mean Copper concentration of the rice grain sam-
ples in this study is within the maximum permissible limits,10
mg/kg by China and 20 mg/kg by FAO/WHO (2002), which
shows that the rice is good for consumption. [20, 14].
Iron in Rice Grain
Iron concentrations in ”rice grain” ranged from
0.66mg/kg-392mg/kg with a mean of 107.78.
Cadmium in Rice Grain
Cadmium was not detected in ”rice grain” for all the rice grain
samples.
Correlation Matrix
Correlation is a statistical technique that shows whether and
how strongly pairs of variables are related. Table 4 presents
the correlation matrix for the metals in the soil before plant-
ing. A strong positive and significant correlation was observed
between copper and iron with an R-value of 0.787. The strong
positive correlation suggests the similar origin of the metal pairs,
probably from agrochemicals used on the farm. Liu reported
strong correlations among Cu, Ni, and Cr in the soil around
an electroplating plant and have implied that the metals have
the same pollution sources [27, 28]. A weak positive correla-
tion was observed between zinc and iron (r=0.345). However,
a weak negative correlation (r=-0.030) was observed between
copper and zinc, showing that an increase in the concentration
of one metal results in a decrease in the concentration.
Table 5 presents the correlation matrix for the metals in the soil
after harvest. Weak positive correlations were observed be-
tween Cu-Fe, Cu-Zn, and Fe-Zn pairs. Table 6 presents the
correlation matrix for the metals in the rice grain. A weak pos-
itive correlation was observed between Cu-Zn, showing that an
increase in the concentration of one metal results in a decrease
in the concentration of another.

3.1.16. Dietary Intake of Heavy Metals and Potential Health
Risks Via Rice Consumption

Table 7 shows the metal concentrations (mg/kg) in rice,
provisional maximum tolerable daily intake (PMTDI) (mg/kg
BW) with estimated daily intake (EDI)(mg/kg BW/day), and
target hazard quotient (THQ) data for a 60 kg adult. The intake
of heavy metals was estimated by multiplication of daily con-
sumption rate with metal content in rice divided by the body-
weight

EDI =
MC ×IR

BW
(4)

Where; Mc= Concentration of the heavy metal in contaminated
rice, IR= ingestion rate or average rice consumption in the study
region, and BW= Bodyweight.

6



Ajah et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 891 7

Table 1: Recoveries and precision (%) of metals Cu, Zn, Fe, and Cd from
spiked soil, ash, rice grain, and stalk samples after digestion

Element Sample Spike
(g/mL)

Conc in
unspiked
sample
(g/mL)

Conc in
spiked
sample
(g/mL)

Recovered
conc.
(g/m L)

Recovery
(%)

Precision

ZINC Soil before plant-
ing A

2ppm
4ppm

2.000
2.000

3.8182
5.9827

1.8182
3.9827

92
100

6.3

Soil before plant-
ing B

2ppm
4ppm

0.2251
0.2251

1.9134
3.8009

1.6883
3.5758

84
89

4.1

Control 1 2ppm
4ppm

0.3723
0.3723

2.1126
3.9307

1.7403
3.5584

87
89

2.5

Control 2 2ppm
4ppm

0.2078
0.2078

2.1039
4.0260

1.8961
3.8182

95
95

0

Ash 2ppm
4ppm

5.3506
5.3506

7.4632
9.2987

2.1126
3.9481

106
99

4.8

Soil after harvest
A

2ppm
4ppm

0.7100
0.7100

2.4242
4.5801

1.7142
3.8701

86
97

8.5

Soil after harvest
B

2ppm
4ppm

0.6494
0.6494

2.4242
4.4675

1.7748
3.8181

89
95

4.6

Grain 1 2ppm
4ppm

0.9524
0.9524

2.7965
4.6667

1.8441
3.7143

92
93

1.1

Grain 2 2ppm
4ppm

0.7013
0.7013

2.5108
4.5628

1.8095
3.8615

90
97

5.3

Stalk 1 2ppm
4ppm

2.6753
2.6753

4.4242
6.554

1.7489
3.8787

87
97

3.5

Stalk 2 2ppm
4ppm

1.7489
1.7489

3.6190
5.6190

1.8701
3.8701

94
97

2.3

IRON Soil before plant-
ing A

2ppm
4ppm

111.2664
111.2664

113.4672
115.4236

2.2008
4.1572

110
103

4.7

Soil before plant-
ing B

2ppm
4ppm

128.2620
128.2620

129.9738
132.5546

1.7118
4.524

86
113

14.3

Control 1 2ppm
4ppm

132.7860
132.7860

134.2533
137.5546

1.4673
4..7686

72
119

33.9

Control 2 2ppm
4ppm

128.5066
128.5066

131.0742
131.9301

2.5676
3.4235

128
86

27.8

Ash 2ppm
4ppm

106.8646
106.8646

108.5764
110.2882

1.7118
3.4236

86
86

0

Soil after harvest
A

2ppm
4ppm

134.7424
134.7424

136.8210
138.5328

2.0786
3.7904

104
95

6.4

Soil after harvest
B

2ppm
4ppm

132.4192
132.4192

134.1310
136.0873

1.7118
3.6681

86
92

4.7

Grain 1 2ppm
4ppm

1.9563
1.9563

3.6681
5.9913

1.7118
4.035

86
101

11.3

Grain 2 2ppm
4ppm

1.2227
1.2227

3.1790
5.5022

1.9563
4.2795

99
107

5.5

Stalk 1 2ppm
4ppm

6.6026
6.6026

8.6812
10.5153

2.0786
3.9127

104
98

4.2

Stalk 2 2ppm
4ppm

10.0262
10.0262

11.9825
14.0611

1.9563
4.0349

99
101

1.4

COPPER Soil before
Planting A

2ppm
4ppm

0.0323
0.0323

2.0693
4.0416

2.037
4.0093

102
100

1.4

Soil before plant-
ing B

2ppm
4ppm

0.0000
0.0000

1.9723
3.9769

1.9723
3.9769

99
99

0

7



Ajah et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 891 8

Control 1 2ppm
4ppm

0.0000
0.0000

1.8430
3.9769

1.8430
3.9769

92
99

5.2

Control 2 2ppm
4ppm

0.0647
0.0647

1.9400
3.8799

1.8753
3.8152

94
95

1.1

Ash 2ppm
4ppm

5.2702
5.2702

7.3395
9.2148

2.0693
3.9446

103
99

2.8

Soil after harvest
A

2ppm
4ppm

0.2587
0.2587

2.0370
3.8476

1.7783
3.5889

89
90

1.1

Soil after harvest
B

2ppm
4ppm

0.2587
0.2587

2.2633
4.0739

2.0046
3.8152

100
95

3.7

Grain 1 2ppm
4ppm

0.0000
0.0000

1.7460
4.1709

1.7460
4.1709

87
104

12.5

Grain 2 2ppm
4ppm

0.0647
0.0647

2.0693
3.9446

2.0046
3.8799

100
97

2.2

Stalk 1 2ppm
4ppm

0.0647
0.0647

1.9400
3.8476

1.8753
3.7829

94
95

1.1

Stalk 2 2ppm
4ppm

0.0970
0.0970

1.8753
3.7182

1.7783
3.6212

89
91

X=96

1.6

X=5.7

Table 2: Metals concentrations (mg/kg)

Sample Copper Zinc Iron Cadmium
Soil control 1 1.85 3.81 814.32 -
2 1.67 3.73 814.31 -
3 2.04 5.06 805.26 -
4 3.33 8.43 823.69 -
Soil before planting
A1 5.18 2.75 845.59 1.14
A2 4.05 13.17 821..89 -
B1 3.32 9.11 812.46 -
B2 3.30 7.48 803.78 -
C1 3.68 6.98 785.79 -
C2 2.59 3.99 774.81 -
D1 2.68 11.68 815.74 -
D2 2.39 3.09 764.29 -
Ash 1 156.81 42.07 848.28 -
Soil after harvest
A1 5.55 13.74 789.75 3-.68
A2 4.25 12.51 761.50 -
A3 3.70 12.16 761.35 -
B1 4.25 9.49 746.86 -
B2 6.07 9.00 759.38 -
B3 4.04 8.29 736.97 -
C1 4.25 10.21 703.58 -
C2 4.26 12.96 611.14 -
C3 4.60 6.88 659.74 -
D1 5.32 13.11 625.99 -
D2 4.80 12.41 838.81 -
D3 4.59 15.15 830.55 -
Rice stalk
A1 0.74 69.03 61.86 0.20
A2 5.15 46.61 86.51 -
A3 11.06 58.33 144.68 -

8



Ajah et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 891 9

B1 1.48 33.16 280.15 0.16
B2 1.85 39.05 300.12 0.02
B3 0.74 41.26 185.20 -
C1 1.48 46.10 197.25 -
C2 1.85 41.01 56.61 -
C3 1.11 41.74 60.60 0.22
D1 1.47 51.36 126.97 2.03
D2 1.11 15.44 213.29 -
D3 1.48 58.62 86.95 -
Rice grain
A1 2.03 23.06 15.13 -
A2 1.84 31.37 10.49 -
A3 2.03 28.46 61.16 -
B1 1.85 19.88 2.63 -
B2 2.03 32.89 213.71 -
B3 2.04 30.81 0.66 -
C1 7.92 29.48 22.92 -
C2 8.86 24.62 16.42 -
C3 14.81 23.78 22.37 -
D1 2.21 14.78 289.63 -
D2 2.04 24.59 392.35 -
D3 2.03 15.07 245.83 -

Table 3. Mean ± SD of metal concentrations (mg/kg)
Sample Copper Zinc Iron Cadmium
Soil control 2.22 ± 0.75 5.26 ± 2.2 814.40 ± 7.52 -
Soil b/4 planting 3.40 ± 0.91 7.28 ± 3.90 803.04 ± 26.76 1.14
Ash 156.81 ± 156.81 42.07 ± 42.07 848.28 ±848.28 -
Soil after harvest 4.64 ± 0.69 11.33 ± 2.51 735.47 ± 73.20 -
Rice stalk 2.46 ± 2.95 45.14 ± 13.73 150.02 ± 85.20 0.53±0.85
Rice grain 4.14 ± 3.92 24.90 ± 6.06 107.78 ± 138.15 -

Table 4. Pearson’s correlations between different metals in the soil before planting
Cu Zn Fe

Cu 1
Zn -0.030 1 -
Fe 0.787 0.345 1

Table 5. Pearson’s correlations between different metals in the soil after harvest
Cu Zn Fe

Cu 1 - -
Zn 0.071 1 -
Fe 0.079 0.274 1

Table 6. Pearson’s correlations between different metals in rice grain
Cu Zn Fe

Cu 1 - -
Zn 0.031 1 -
Fe -0.338 -0.373 1

9



Ajah et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 891 10

Table 7. Metal concentrations (mg/kg) in rice, PMTDI (mg/kg /person/day)
with EDI (mg/kg bw/day) and THQ data for a 60 kgadult

METAL MEAN ± SD PMTDI EDI THQ
Copper 4.14±3.92 0.05-0.5 0.01 0.00017
Zinc 24.90±6.06 0.3-1 0.04 0.00014
Iron 107.78±138.15 0.8 0.18 0.00026
Cadmium - - -
Provisional maximum tolerable daily intake (PMTDI) by

JECFA

According to the international rice research institute (2001), the
average Nigerian consumes 24.8kg of rice per year, equivalent
to 0.1kg per person/day. The daily intakes for a 60 kg adult
were compared to the provisional maximum tolerable daily in-
takes as stipulated by Joint FAO/WHO Expert Committee Food
Additive (JECFA). It was found that the EDIS of the metals
falls within the range of the safe values stipulated by JECFA
[29]. The target hazard quotient (THQ) of heavy metals from
rice consumption is in decreasing order: Fe>Cu>Zn and are all
less than 1 indicating there will be no health risk. The total
hazard index (THI) for rice consumption for a 60-kg adult is
0.00057, which is less than 1. Thus, the consumption of rice
from this field will show no adverse effects from the metals.

4. Conclusion

This research demonstrated that Zn, Cu, and Fe concentrations
in soil and rice grains did not exceed the threshold set by several
international organizations. The preliminary maximum toler-
ated daily intake (PMTDI) established by JECFA was less than
the metals’ estimated daily intake (EDI). The total hazard index
was also less than one, indicating no potential health risk asso-
ciated with the intake of rice in this field. The metal’s target
hazard quotient (THQ) was less than one. The result from the
physicochemical analysis of the soil showed that the soil is a
type suitable for rice production.

However, we recommend that the field be monitored con-
tinuously to ensure that the metals stay at harmless levels.

Acknowledgment

The authors appreciate the editor and the anonymous re-
viewers for their valuable comments towards the improvement
of this article.

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11


	Introduction
	Experimental
	Area of Study
	Sample Collection
	Soil Sample Collection
	Rice Sample Collection
	Sample Preparation
	Soil Preparation
	Rice Preparation
	Ash
	Sample Size
	Digestion of Soil Samples, Rice Grain Samples, and Rice Stalk Samples

	Quality Assurance
	Risk Assessment

	Data Analysis

	Results and Discussion
	Heavy Metals Concentration in Soil and Rice Grain
	Copper in Soil
	Copper in Rice
	Relationships Between Copper Concentration in the Soil Before Planting and Soil After Harvest
	Zinc in Soil
	Zinc in Rice
	Relationships Between Zinc Concentrations in the Soil Before Planting and Soil After Harvest
	Iron in Soil 
	Iron in Rice
	Relationships Between the Iron Concentration in the Soil (Before Planting) and Soil (After Harvest)
	Cadmium in Soil
	Cadmium in Rice
	Zinc Concentrations in Soil 
	Copper Concentration in Soil
	Iron Concentration in Soil
	Heavy Metals in Rice Grain
	Dietary Intake of Heavy Metals and Potential Health Risks Via Rice Consumption


	Conclusion