Anal. Methods Environ. Chem. J. 5 (3) (2022) 70-79

Research Article, Issue 3

Analytical Methods in Environmental Chemistry Journal

Journal home page: www.amecj.com/ir

AMECJ

Determination of cadmium in rice samples using

amino-functionalized metal-organic framework

by a pipette tip solid phase extraction

Mohammad Abbaszadeha, Ali Miria , and Mohammad Reza Rezaei Kahkha a,*

a Department of Environmental Health Engineering, Faculty of Health, Zabol University of Medical Sciences, Zabol, Iran

ABSTRACT

In this study, the amino-functionalized metal-organic framework

(NH2-MOFs) was used as an adsorbent for the extraction of cadmium

in rice samples based on the pipette tip solid phase extraction (PT-SPE)

before determined by the ame absorption spectrometry (F-AAS). The

pH of the sample solution, initial concentration of the cadmium, the

volume of the sample, elution conditions, and the amount of adsorbent

on the recovery of the cadmium were investigated and optimized.

The results showed that the best extraction efciency of cadmium

was obtained at pH 5.0, 2500.0 µL of cadmium solution, and 20.0

µL of HCl (10% V/V) as eluent solvent. First, the cooking rice was

transferred to a beaker and hydrochloric acid/nitric acid was added to

it as a digestion process before analysis by the PT-SPE procedure. The

limit of detection of this method was found to be 0.03 µg L-1 with a

relative standard deviation of ≤ 2.5 % (for seven replicate analyses of

50 µg L-1 of cadmium). The linear and dynamic ranges were achieved

at 0.3 -14.5 µg L-1 and 0.3 -150 µg L-1, respectively. The adsorption

capacity of sorbent and enrichment factor was 175 mg g-1 and 125

folds, respectively. The proposed method was successfully applied

for the determination of cadmium in rice samples.

Keywords:

Cadmium,

Amino functionalized metal-organic

framework,

Pipette tip extraction,

Rice samples,

Atomic adsorption spectrometer

ARTICLE INFO:

Received 21 May 2022

Revised form 2 Aug 2022

Accepted 28 Aug 2022

Available online 29 Sep 2022

*Corresponding Author: M. R. Rezaei Kahkha

Email: m.r.rezaei.k@gmail.com

https://doi.org/10.24200/amecj.v5.i03.208

1. Introduction

Pollution of foods with heavy metals (HMs)

is a serious problem for public health and the

community due to its toxicity and carcinogenicity.

Therefore, monitoring and controlling the amount

of HMs in food samples is critical. Cadmium is one

of the most dangerous HMs that enters food samples

from various sources, including mining, industrial

production and other ways such as agricultural

runoff [1]. According to the US Environmental

Protection Agency, the maximum acceptable level

of cadmium in rice and wheat is 200 µg. kg-1 [2].

On the other hand, the accumulation of cadmium

ions in food samples such as rice, wheat, and other

species is unavoidable[3]. In these food samples,

the effect of matrices is a serious problem for its

measuring because of the low concentration of

cadmium, therefore, a preconcentration technique

is necessary before quantication[4]. Various

techniques have been applied for this purpose such

as dispersive liquid- liquid microextraction [5],

cloud point extraction [6] and solid phase extraction

(SPE) [7]. The pipette tip (PT), a micro-scale

format of SPE, that used for the preconcentration

and extraction of various samples[8]. Using a

small amount of sorbent (insert into a pipette tip)

and low solvent consumption without a special

auxiliary device is the advantage of PT-SPE

compared to conventional SPE cartridges [9,10].

------------------------

Recently PT-SPE was applied for the determination

and extraction of several analytes in food samples

such as bisphenol a [11], estradiol in milk [12],

and antibiotic residues Metal-organic frameworks

(MOFs) are a new class of hybrid porous materials

consisting of organic linkers coordinated to

inorganic metal nodes that are used in solid phase

extraction because of their thermal and chemical

stability[14]. Recently, several adsorbents

such as silica nanoparticles [15], molecularly

imprinted polymer [16], and other sorbents were

applied for determination of cadmium in food

samples[17]. Hence, based on the above remarks

and our research interest in applications of porous

materials [18-21]we utilized the highly stable

amino functionalized metal organic framework

(Fig. 1) for the determination and extraction of

cadmium in imported rice samples. Many other

papers were presented about extraction methods

by previous researchers [22-24]. Parameters

affecting PT-SPE were studied and optimized.

To the best of our knowledge, the MOF with the

properties mentioned above was applied for the

rst time as a solid phase sorbent in a pipette-tip

microextraction mode.

2. Material and methods

2.1. Reagents and instrument

All reagents and solutions were analytical grades.

Methyl 4-formylbenzoate (CAS N: 1571-080 ,

Sigma, Germany), pyrrole (CAS N: 109-97-7, pH

>6, Merck), the triuoroaceticacid (CAS N: 76-

05-1, EC Number 200-929-3, TFA) from Sigma-

Aldrich, propionic acid (CAS N: 79-09-4, MW:

74.08, Sigma-Aldrich), ZrOCl2•8H2O (CAS N.:

13520-92-8, 98%, Sigma-Aldrich), 1000 mg.L-1

standard solution of cadmium (CAS N: 7440-43-9

Sharlou, Spain), N,N′-dimethylformamide (CAS N:

68-12-2, DMF), benzoic acid (>98%, CAS N: 65-

85-0; EC N: 200-618-2, Sigma), acetone ( ≥99.5%;

CAS N: 67-64-1, Sigma, Germany), tetrahydrofuran

(THF, CAS N: 109-99-9 EC N: 203-726-8, Sigma),

methanol (CAS N: 67-56-1), and KOH (CAS N:

1310-58-3, Sigma, Germany) were purchased from

commercial sources and used as received. The

ame atomic absorption spectrometer based on a

double beam spectrophotometer (FAAS, AA7800,

Shimadzu, Japan) was used for cadmium detection

The mixture of C2H2 gas and the D2 was tuned for

the cadmium determination by FAAS.

Determination of Cadmium in Rice by NH2-MOFs Mohammad Abbaszadeh et al

Fig.1. A schematic of amino functionalized MOF.

2.2. Synthesis of amino-functionalized metal-

organic framework

The sorbent was synthesized in a similar way that

we have previously reported [25]. Briey, 200 mg

of ZrOCl2•8H2O, 3.0 g of benzoic acid, and 20 mL

of DMF were added into a 30-mL vial (solution

A); and in other 30- mL vial, 100 mg of H2TCPP

and 20 mL of DMF were added (solution B). Both

solutions A and B were sonicated for 30 min and

then incubated at 100 °C in an oven for 1 h. Next,

1 mL of A solution, 1 mL of B solution, and 0.05

mL of triuoroacetic acid were added and mixed

by swirling for 5 s. The vials were then incubated

and immersed in an oven at 120 °C for 30 h. A dark

purple precipitate started to form in the vials. After

cooling to room temperature, the suspension was

transferred into a centrifuge tube and centrifuged

for 5 min (7500 rpm) to remove the supernatant.

The solid was washed with fresh DMF (330 mL)

before soaking in 40 mL of fresh DMF and 1.5 mL

of 8 M HCl (activation with HCl). It was then heated

at 120 °C for 12 h to remove the benzoic acid. The

sample was subsequently washed with fresh DMF

(330 mL), THF (330 mL), and acetone (330

mL). After soaking in acetone overnight, the solid

was collected, and then dried in a vacuum oven

at 120 °C for 12 h to give the MOF. SEM of the

amino-functionalized metal-organic framework is

seen in Figure 2.

2.3. Pipette-tip extraction procedure

Appropriate amounts of sorbent were inserted into

a pipette-tip (DRAGON, China) which was then

attached to a 5000 µL micro pipette (DRAGON LAB,

China). Then 2500 µL of the aqueous sample was

withdrawn into the sorbent and dispensed back into

the same tube for 20 cycles. Elusion was performed

by 20 µL of 10%HCl into a 1-mL vial. The desorption

step was also performed by 20 aspirating/dispensing

cycles. The extraction recovery of cadmium was

calculated by comparing the absorbance of 50 µg

L-1 of cadmium standard solution by results of

optimization experiments (Fig.3).

3. Results and discussion

3.1. Optimization of affecting parameter on the

extraction procedure

To optimize the extraction conditions, parameters

affecting extraction were optimized as below. All

optimizations were performed on a 50 µg L-1 of

cadmium solution, made by diluting of 1000 mg

L-1 standard solution.

Anal. Methods Environ. Chem. J. 5 (3) (2022) 70-79

Fig.2. SEM images of synthesized NH2-MOFs

3.1.1.Effect of pH

The effect of sample pH on the extraction efciency

of cadmium was investigated by adjusting pH of it

between 2.0 and 9.0. Either 0.1 M NaOH or 0.1 M HCl

was used. As depicted in Figure 4, a solution with the

pH values between 4.0 and 6.0 showed the highest

extraction efciency (optimal pH = 5). In alkaline

media, produced hydroxide ions can form a complex

with cadmium ions, and a precipitation (Cd(OH)2) is

created. Results showed that the extraction efciency

of cadmium based on NH2-MOFs was decreased. So,

pH 5.0 was selected as the optimum value.

Determination of Cadmium in Rice by NH2-MOFs Mohammad Abbaszadeh et al

Fig.3. Cadmium extraction based on NH2-MOFs adsorbent and Pipette-tip- SPE procedure

Fig. 4. Effect of pH on recovery of cadmium

3.1.2.Effect of amount of sorbent

To obtain the best extraction efciency and good

recoveries of cadmium ions, the amount of sorbent

was examined between 1-5 mg. The adsorption

ability of the adsorbent was increased by increasing

the amount of sorbent up to 2 mg; therefore, the

optimum amount of 2 mg was chosen (Fig. 5).

3.1.3.Effect of volume of the eluting solvent

Several strong acidic solvents including different

concentrations of HNO3 (2-10% V/V) and HCl

(2-10% V/V) were studied to select an optimized

eluting solvent. Among them HCl 10% showed the

highest extraction efciency for cadmium ions. To

achieve the highest enrichment factor, we tried to

obtain the smallest HCl volume of 10%. Volumes

between 5 to 50 µL of HCl 10% were examined

for the extraction of 1000 µL of a standard solution

containing 50 μg L-1 of the cadmium in deionized

water. As shown in Figure 6, at the volume of 20

µL of the eluting solvent, the recovery of cadmium

is at its highest value.

Therefore, the eluting volume of 20 µL was

selected for further experiments.

3.1.4.Effect of volume of sample solution

Amount of sample solution taken for the analysis is

an important parameter in solid phase extraction [5].

Different volumes of sample solution were tested at

the range of 200 to 4000 µL containing 50 µg L-1 of

cadmium. As can be seen in Figure 7, the extraction

recovery of cadmium increased with the increase

of the volume up to 2500 µL. So, the extraction

efciency (more than 95%) and a preconcentration

factor of 125 for cadmium extraction were achieved

based on 20 μL of eluent with the PT-SPE procedure

before determination by the F-AAS.

3.1.5.Effect of number of aspirating/dispensing

of sample and elution solvent

The number of aspirating/dispensing cycles of

eluent solvent and the volume of solution that passed

through the extractor resembles the extraction

time. The results showed the highest recoveries

Anal. Methods Environ. Chem. J. 5 (3) (2022) 70-79

Fig. 5. Effect of amount of sorbent on recovery of cadmium.

Determination of Cadmium in Rice by NH2-MOFs Mohammad Abbaszadeh et al

Fig. 6. Effect of volume of eluting solvent (HCl 10%) on recovery of cadmium

Fig. 7. Effect of sample volume on recovery of cadmium

for cadmium obtained at 20 cycles, when 2500 µL

of a sample containing 50 µg L-1 of the standard

solution was used. During desorption, the analyte

was eluted from the extractor into a 2.0 mL glass

test tube by repetitive aspirating/dispensing of 20

µL of the HCl 10% (V/V) through the tip. The

optimal number of aspirating/dispensing cycles

for desorption of adsorbed analytes (provided the

highest recovery) was found to be 20 cycles at

12min.

3.1.6.Reusability of the adsorbent

The reusability of the sorbent was investigated by

washing of the column with HCl 10% V/V and then

ve cycles with deionized water. After that, several

extraction and elution operation cycles were carried

out under the optimized conditions. The results

indicated that the amino functionalized MOF could

be regenerated and reused at least ten times without

signicantly decreasing extraction recoveries.

3.1.7.Sorption capacity

To investigate of the adsorption capacity of the

functionalized MOF, a standard solution containing

100 mg L−1 of cadmium ions was applied. The

maximum sorption capacity is dened as the total

amount of cadmium ions adsorbed per gram of the

sorbent. The obtained capacity of the adsorbent

was found to be 175 mg g−1.

3.1.8.Effect of interfering ions

The effect of common co-existing ions that often

companion with cadmium ions in real samples on

cadmium determination was studied in optimum

conditions by analyzing 100 μg L−1 of cadmium

after addition of varying concentrations of Na+,

K+, Ca2+, Cu2+, Zn2+, Ni2+, Mn2+, and Fe2+. The

concentration ratio of other ions was as follow:

1750 for Na+, K+; 1500 for Ca2+, Mg2+; 1250 for

Cu2+, Zn2+, Fe2+, Ni2+, Mn2 +. Results showed that

interference ions do not inuence on extraction

recovery of cadmium. Hence, the method was

selective for preconcentration and extraction of

cadmium (Table 1).

3.2. Analytical performance of suggested method

The analytical performance of the suggested

pipette-tip solid phase extraction was evaluated, and

the results are summarized in Table 2. The limit of

detection (LOD) was obtained based on a signal-to-

noise ratio of 3. The linearity range was studied by

varying the concentration of the standard solution

from 0.3 to 150 µg L-1. The repeatability of the

method, expressed as relative standard deviation

(RSD), was calculated for seven replicates of the

standard at an intermediate concentration (50 μg

L-1) of the calibration curve. The precision of the

method was determined by repeatability (intraday

precision) and intermediate precision (inter-day

precision) of both standard and sample solutions.

Anal. Methods Environ. Chem. J. 5 (3) (2022) 70-79

Table 1. The effect of interfering ions on the recovery of Cd (II) ions in water samples

by PT-SPE procedure coupled to F-AAS

Interfering Ions(M)

Mean ratio

(CM/CCd) Recovery (%)

Cd(II) Cd(II)

Al3+ 750 97.5

Na+, K+1750 97.8

Cu2+, Zn2+, Fe2+, Ni2+, Mn2 + 1250 98.2

I- , Br-, F- 1100 97.7

Ca2+, Mg2+ 1500 98.0

Co2+, Pb2+ 950 97.9

Ag+, Au3+ 250 96.5

Precision was determined in seven replicates of

both cadmium standard solution (100 μg L−1) and

sample solution (100 μg L−1) on the same day

(intra-day precision) and daily for 8 times over

a period of one week (inter-day precision). The

results are represented as % RSD and indicated

that intra-day precision and inter-day precision of

the method were 5.0% and 3.5%, respectively.

3.3. Determination and validation of cadmium

in rice samples

Rice samples were purchased from several local

markets. About 50 gr of rice was weighed and

cooked in the oven for 8 hours at the temperature

is 80 o C with the aim of removing moisture

and determining weight It was dry. After drying

and reaching constant weight, 10 gr of rice was

transferred to a 250.0 ml beaker Samples for 48

hours at a temperature of 105o C it placed. Then

5 ml of 37% hydrochloric acid and 15 ml of 65%

nitric acid were added to them and after 120

minutes at the laboratory temperature, it dissolved

slowly and heated until its volume reached less

than 20 ml. Then the obtained clear solution was

cooled, ltered and used for the determination

of cadmium according to the above PT-SPE

method. As can be seen in Table 3 concentrations

of cadmium in all samples in comparison to

the maximum acceptable level (200 µg g-1) are

adequate.

Determination of Cadmium in Rice by NH2-MOFs Mohammad Abbaszadeh et al

Table 2. Analytical gures of merit for proposed methods

Parameter Analytical feature

Dynamic range (μg L-1) 0.3 -150

R2 (determination coefcient) 0.99

Repeatability (RSD a %) 2.45

Limit of detectionb (ng.L-1) 15

Enrichment factor (fold) 125

Total extraction time (min) ≤ 12

aRSD, relative standard deviation, for 5 replicate measurements of 50 µg.L-1 of the analyte

bLimit of detection was calculated based on the 3Sb/m criterion for 10 blank measurements

Table 3. Determination of cadmium in different rice samples under optimized conditionsa

Rice Sample Added

(µg g-1)

Found

(µg g-1)

Recovery

( %)

RSD %

(n=3)

0 45 - -

50 94.2 98.2 2.7

150 194.5 74.3 3.2

0 40 - -

50 89.3 98.7 1.7

100 139.2 98.1 2.8

0 55 - -

50 104.7 98.7 3.6

100 154.5 98.5 2.8

aThe maximum acceptable level of cadmium in the rice reported by WHO is 200 µg g-1

78 Anal. Methods Environ. Chem. J. 5 (3) (2022) 70-79

4. Conclusion

In this research, for the rst time, we employed an

amino functionalized MOF with a high surface area and

large porosity for PT-SPE of cadmium. This method

is very simple, fast, solvent-free and applicable for

the extraction of cadmium. The total time of analysis,

was less than 12 minutes and the functionalized-MOF

sorbent was used for at least 10 extractions without any

change in its capacity. Only 2mg of the sorbent was

enough to ll the PT. Moreover, evaluation of intra-

day and inter-day showed a notable precision with

RSD below 5 and 3.5%, respectively. This method

was applied successfully for the determination of

cadmium in three rice samples. Real samples spiked

with three concentration levels and results indicated

that sorbent can be applied in the complicated matrix

for analysis of heavy metals such as cadmium. Also,

analysis of real samples showed that the concentration

of cadmium in all samples is below the acceptable

range.

5. Conict of Interest and Ethical approval

The authors have declared no conict of interest.

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