(2024) 18:23
Yimer et al. BMC Chemistry
https://doi.org/10.1186/s13065-024-01128-z

BMC Chemistry

RESEARCH

Open Access

Adsorptive removal of heavy metals
from wastewater using Cobalt‑diphenylamine(Co‑DPA) complex
Mesfin Yimer1, Shagufi Naz Ansari2, Biniyam Abdu Berehe1, Krishna Kanthi Gudimella3, Gangaraju Gedda4,5*,
Wubshet Mekonnen Girma1*, Nazim Hasan6 and Shadma Tasneem6

Abstract
Heavy metals like Cadmium, Lead, and Chromium are the pollutants emitted into the environment through industrial development. In this work, a new diphenylamine coordinated cobalt complex (Co-DPA) has been synthesized
and tested for its efficiency in removing heavy metals from wastewater, and its adsorption capacity was investigated.
The effectiveness of heavy metals removal by Co-DPA was evaluated by adjusting the adsorption parameters, suchas adsorbent dose, pH, initial metals concentration, and adsorption period. Heavy metal concentrations in real sample
were 0.267, 0.075, and 0.125 mg/L for ­Cd2+, ­Pb2+, and ­Cr3+ before using as-synthesized Co-DPA to treat wastewater.
After being treated with synthesized Co-DPA the concentration of heavy metals was reduced to 0.0129, 0.00028,
0.00054 mg/L for ­Cd2+, ­Pb2+, and ­Cr3+, respectively, in 80 min. The removal efficiency was 95.6%, 99.5%, and 99.5%
for the respective metals. The adsorption process fitted satisfactorily with Freundlich isotherm with ­R2(0.999, 0.997,
0.995) for ­Cd2+, ­Pb2+, and ­Cr3+, respectively. The kinetic data obeyed the pseudo-second order for ­Cd2+ and ­Cr2+
and the pseudo-first order for ­Pb2+. Based on the results obtained within the framework of this study, it is concluded
that the as-synthesized Co-DPA is a good adsorbent to eliminate heavy metal ions like ­Cd2+, ­Pb2+, and ­Cr3+from wastewater solution. In general, Co-DPA is a promising new material for the removal of heavy metal ions from water.
Keywords Wastewater, Adsorption, Cu-DPA, Real sample, Freundlich isotherm*Correspondence:
Gangaraju Gedda
raju.analy@gmail.com
Wubshet Mekonnen Girma
wubshet.mekonnen@wu.edu.et
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Yimer et al. BMC Chemistry(2024) 18:23

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Graphical Abstract

Introduction
Heavy metal ions are one of the major sources of watercontamination [1]. Exposure to heavy metal ions consequences in a huge threat to both human health and ecological balance due to their carcinogenic nature [1–3].
Wastewater originated from industries such as paintmanufacture, mining activities, metal plating, electroplating industry, batteries, pesticides, and fluidized bed
bioreactors, etc., are known to have high concentrationsof various heavy metal ions, such as copper (Cu), arsenic(As), cadmium (Cd), mercury (Hg), and lead (Pb) [1–3].
This heavy metal-polluted wastewater is discharged into
the natural water resources, threatening human healthand the ecosystem [2–5]. The heavy metals are carcinogenic and non-biodegradable, which get piled up in living organisms, further generating several critical healthissues and syndromes such as fleeting growth and development, impairment of the nervous system, obstructive lung disease, and death in extreme cases [5–7].
Lead and cadmium both have nephrotoxic effects andare linked to bone deterioration and encephalopathy athigh exposure levels [8–12]. Hence, industrial wastewater must be treated before releasing into the rivers or
ponds to exclude these toxic heavy metals. The World
Health Organization has set guidelines for the acceptable levels of lead and cadmium in wastewater, which are
0.015 mg/L and 0.01 mg/L, respectively [8, 11, 12]. Hexavalent chromium is poisonous and carcinogenic at highenough quantities resulting in Ulcers and perforation ofnasal septum, and respiratory cancer [7].
Various conventional methods have been investigated
for the removal of heavy metal ions like precipitation,ion exchange, advanced oxidation processes, membrane
filtration, electrocoagulation (EC), reverse osmosis, etc.
[13–20]. However, most of these techniques have substantial operating costs and require the disposal of thesolid waste they produce. The adsorption method is
a superior technique due to its lesser operating cost,straightforward implementation, high removal efficiency,
easy regeneration of the adsorbent, and production of

Yimer et al. BMC Chemistry(2024) 18:23

little or no byproduct [21–25]. The mechanism of theadsorption process is based on the physicochemical
properties of heavy metals and adsorbent and operating
conditions (i.e., pH value, adsorbent amount, temperature, adsorption time, and initial concentration of metalions) [26–28]. Usually, heavy metal ions can be adsorbedon the surface of the adsorbent via intermolecular interactions. The amputation of heavy metals from numerous
sources of wastewater has reportedly been accomplishedusing commercial activated carbon [15, 29, 30], zeolites,
and biomass-derived adsorbents like peanut shells [31–33], banana peel [34, 35], dry tree leaves [36], rice husk[37], tea and coffee waste [38], coconut shell powder [39],papaya seed [40], and eggshell [41]. Nevertheless, someproblems, including the need for a prolonged reaction
time, excessive calcination temperature, inadequate stability, short reusability, a low surface area, etc. restrict
most of the available adsorbents [15, 16, 29, 31, 42]. As
a result, it is crucial to create low-cost adsorbents witha large surface area utilizing straightforward synthesis
techniques. The characteristics of a promising ion adsorbent are high adsorption kinetics, substantial adsorption
capacities even at low contamination levels, selectivity for the target ion, stability in the aqueous environment, scalability, recoverability, and cost [25, 27, 30, 34,
36, 43–48]. Physical adsorption and chemical adsorption
are the two main categories of adsorption processes [15].
Weak bonds (van der Waals force and dipolar forces) andlow heats of adsorption are characteristics of physisorption [15]. The attraction between permanent dipoles andinduced dipoles also allows adsorbate molecules to freely
cover the whole surface of the adsorbent without changing the species’ atomic or molecular orbitals. As a result,adsorbate molecules are not constrained to locations.
However, chemisorption comprises the interchange ofelectrons between the adsorbate and the adsorbent [42].
For the elimination of Co (II) ions from aqueous solutions, activated carbons made from hazelnut shell were
utilized as adsorbents displayed adsorption capacity
as 58.27 mg ­g−1 [49, 50]. Activated carbon from Borassus aethiopum seed shells (BASS) and Cocos nucifera(CONS) was studied by Mohammed and Azim for theremoval of P
­ b2+ and C­ d2+ (shells) showing 99% and 95%
2+
removal for P­ b and C­ d2+ respectively [51]. In 2020
Wang and coworkers synthesized Ferrous phosphate totreat wastewater containing EDTA-Pb from 50 mg Pb/L
to < 1 mg Pb/L [52]. In 2021 Asma et al. reported sodium
alginate grafted by diphenylamine as adsorbent for theremoval of ­Co2+ ions through adsorption process where
the adsorption capacity was dependent on the pH value[53]. The maximum removal capacity was credited tothe electrostatic attraction between the metal ions andthe negative charge of adsorbent surface. In 2020 Ngoc

Page 3 of 15

T. et al. reported hydrogen-bonded supramolecularcomplex for efficient removal of trace heavy metal ionsfrom water exhibiting adsorption kinetics, up to 50 times
faster than state-of-the-art materials for selective copper
ion capture from water [54]. In 2018, Xu et al. reported
selective adsorption of Cu (II) and S­ O42− ions throughsilica gel-immobilized Schiff base derivative [55]. In
2020 Jumina et al. reported the removal of ­Pb2+, ­Cr2+,and ­Cu2+ metal ions through pH Dependent adsorption
process using C-Phenylcalix pyrogallolarene [4]. The phenolic group of the adsorbent increases the metal uptake
efficiency through the ion exchange process [56]. Some
recent examples of adsorbents are summarized below
showing operation conditions and removal efficiency(Table 1) as comparison with our study.
This study aims to investigate the removal of heavy
metals from wastewater using cobalt-complex (CoDPA) under different experimental conditions. The CoDPA was prepared from the cobalt nitrate hexahydrate(Co ­(NO3)2.6H2O), and diphenylamine, in solution ofwater and ethanol (1:1v/v ratio). The Co-DPA was characterized by XRD, and FTIR. A wastewater sample wascollected from Gurage zone Butajira town around textile fabric from Akamuj river, Ethiopia. Heavy metals
removal capacity of Co-DPA was determined, and batch
experiments were conducted to look at the effects ofpH, dose, initial metal concentration, and contact timeon the adsorptions of heavy metals. The effect of condition parameters like concentration of lead, chromium,cadmium, and contact time was studied to obtain the
optimum condition with high removal efficiency. The
measurement of the concentration of heavy metals before
and after treatment with the prepared Co-DPA was doneby AAS.

Materials and methods
Materials

All chemicals and reagents procured were of analyticalgrade and used without additional purification. Cobalt(II) nitrate hexahydrate, ethanol, diphenylamine, hydrochloric acid, sodium hydroxide, Nitric acid, Perchloric acid, Lead (II) chloride, Cadmium (II) chloride, andChromium (III) chloride were purchased from Sigma
Aldrich. Following a minimum 24-h soak in 10% Nitricacid, all vessels were cleaned by being repeatedly washedwith deionized (DI) water. Real samples of heavy metal
ions ­(Cd2+, ­Pb2+, and ­Cr3+) with metal concentrations as
0.267, 0.075, and 0.125 mg/L respectively were collectedfrom Gurage zone Butajira town around textile fabricfrom Akamuj river, Ethiopia. The standard solution of
Cd, Pb, and Cr were prepared by dissolving metal precursors in distilled water.

Yimer et al. BMC Chemistry(2024) 18:23

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Table 1 A comparative table to include operation conditions, and removal efficiency
S/N Type of adsorbent

Time

Metal concentration Amount of Removal efficiency
adsorbent

References1{[Cd1.5(btc)(bibp)·2H2O] ­H2 O}n120 min 1000 ppm

10 mg

Pb(II) (53.9%) and Cr(VI) (73.8%)

[57]

2

α-cellulose/chitosan

60 min60 ppm

1.0 g

Cr(II) (56%), Pb(II) (85%) and Cd (94%)

[58]

3

Non-cytotoxic and magnetic cobalt ferrite nanoparticles

30 min450 ppm

0.1 g

Pb(II) (82%)

[59]

4

Magnetic activated carbon–cobalt
nanoparticles

30 min500 ppm

0.1 g

Pb(II) (90.6%)

[60]

5

Faujasite zeolite decorated with cobalt
ferrite nanoparticles

12 h

140 ppm

1g

Pb(II) (98.4%)

[61]

6

Multifunctional cobalt oxide nanocomposites

140 min 350 ppm

2.8 g

Pb(II) (86.89%) and Cd(II) (82.06%)

[62]

7

Polypyrrole functionalized, Cobalt oxide
Graphene

120 min 400 ppm

0.01 gPb(II) (93.08%) and Cd(II) (95.28%)

[63]

8

Cobalt ferrite nanoparticles

60 min0.1 g

Pb (96%)[64]

9Organosulphur-modified biochar

300 min 50 ppm

100 mgCd(II) (88%)

[65]

10

Activated carbon cobalt composite

20 min450 ppm

0.1 g

Pb(II) (85%)

[60]

11

Cobalt ferrite nanoparticles

30 min150 ppm

10 g

Cr(IV) (50.9%)

[66]

12

Co-DPA80 min60 ppm

3g

Cr(II) (99.5%), Pb(II) (99.5%) and Cd(95.6%)

This study

400 ppm

Synthesis of cobalt‑complex

Cobalt complex (Co-DPA) was synthesized via facile approach using cobalt (II) nitrate hexahydrate (Co­(NO3)2.6 ­H2O). 24.69 g of cobalt nitrate hexahydrate wasdissolved in 5 mL of distilled water, and 5 g of diphenylamine was dissolved in 5 mL of ethanol. The two solutions were combined and swirled on a hot plate magnetic
stirrer for an hour to create a homogenous solution. The
mixture was then preserved for 3 days, and a reddishpink precipitate was formed. The formed precipitate wasfiltered by filter paper using suction filtration and driedin a drying oven at 40 °C for 24 h. The attained precipitate, named Co-DPA subsequently, was retained for further characterization and experiments.
Characterization

Powder X-ray diffraction (PXRD) technique was used
to study the purity and crystallinity of the bulk material (Shimadzu XRD- 7000). FT-IR (Fourier-transform
infrared) spectroscopy was used to examine the surfacefunctional groups of the Co-DPA complex (PerkinElmer
spectrometer). A DW-AA320N Atomic Absorption spectrometer (AAS) was used for metal adsorption studies.
Collection of waste‑water sample

Wastewater was collected from Gurage zone, Butajira town, Ethiopia from the car wash labajo area andtransported to Chemistry laboratory, Hawassa University, Ethiopia. The sample was fixed by adding 3 dropsof 69% nitric acid and stored at 4 ℃ for heavy metals

determination before and after the treatment. The actual
concentrations of heavy metals were measured by using
AAS, before being treated by Co-DPA.
Batch adsorption experiment to optimize adsorption
parameters

Batch adsorption tests were performed to observe theeffect of pH, initial metal ions concentration, adsorbent
dosage, adsorption kinetics, adsorption isotherms, andperiod of mixing needed to attain equilibrium for Pb(II),
Cr(III), and Cd(II) adsorbed by Co-DPA.
pH study

To determine the effect of pH on the removal of heavy
metals from wastewater, the metals sample working solution was prepared. 20 mg/L metals concentration and3 g/L of Co-DPA were added to the pH optimized as 3,
5, 7, 9 and 11, then centrifuged for 30 min and the supernatant was used to measure heavy metals concentration.

Results and discussion
Characterization of Co‑DPA

Powder X-ray diffraction (XRD) analysis was considered to examine the structure and crystallinityof the samples (Fig. 1). The intensity of the detectedrays about scattering angle two theta (2θ) was used toascertain the crystalline nature of the as-synthesized
Co-DPA. The crystallite domain size (grain size) was
calculated from the width of the peaks using Scherrer’s
formula. As shown in Fig. 1, the calculated grain size,

Yimer et al. BMC Chemistry(2024) 18:23

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Optimization of adsorption parameters

20

40

77.4o

64.3o

44o

37.6o

13.8o

25.1o

Intensity (a.u)

18.4o
19.9o

strong searching vibration peak appeared around 742
and 688 ­cm−1 are confirmed the coordination of Cowith N of the diphenylene ligand.

602θ (°)

80

Transmitance (a.u)

Fig. 1 PXRD graph of synthesized Co-DPA

3040

1084
3380

1582

1307

875

1493
1453

742

4000

3500

3000

2500

2000

Wave number (cm-1)

1500

1000

Fig. 2 FT-IR graph of synthesized Co-DPA

from average of the tallest peak, at an angle of 2θ is34.55 nm. The calculated grain size (mean diameter ofthe particle) is greater than 2 nm and less than 50 nm,
which is the characteristic nature of mesoporous materials from Scherrer’s equation [67].
The FTIR spectra were acquired using a Perkin Elmer
FT-IR spectrometer in transmission mode. With aresolution of 4 ­cm−1 (Fig. 2), each sample in the KBr
mix was scanned 64 times from 4,000 to 400 ­cm−1. The
secondary N–H vibration at frequencies (3380 and
3040 ­cm−1) after complex formation showing coordination of the amine nitrogen atom. The peaks appeared
around 1582, 1493, and 1453 ­cm−1 is assigned for the
C=C stretching vibrations for the aromatic rings of thediphenylene ligand coordinated to the metal center. The

Adsorption behaviour was analyzed by a batch method,where factors that affect the process of adsorption suchas pH, adsorbent dose, initial metals concentration, andcontact time were investigated. The standard solution of
Pb, Cd, and Cr were prepared by dissolving metal precursors in distilled water.
Effect of pH

For adsorbent Co-DPA the effect of pH on the removal
efficiency of heavy metals was considered from a rangeof 3 to 11 under the precise conditions (at a contact timeof 30 min, 20 mg/L adsorbate solution with 3 g of theadsorbent and temperature of 25 °C with an agitation
speed of 150 rpm). The percentage of Pb(II) and Cd(II)adsorbed by Co-DPA first increases with an increase inpH from pH 3–7 and then decreases later (Fig. 3a). For
Cr(III) ion, the maximum removal efficiency obtained is
at pH 3. The maximum percent removal of heavy metal
ions was obtained at pH 7 for Cd and Pb and at pH 3
for Cr, which is 92.4%, 94.05%, and 92.5%, respectively(Fig. 3b). For Lead high removal efficiency obtained at pH
7 is the same as the results obtained by Li et al. on theremoval of chromium from wastewater by Mg-Loaded
Biochars with the removal efficiency of 87% partaking
the maximum adsorption capacity of 2.7 mg/g [68]. The
high removal efficiency of cadmium obtained at pH 7 isthe same as the result obtained by Bayuo et al. [69] on theremoval of heavy metals from wastewater using shea fruitshell biomass. Most of the adsorption space for the elimination of cadmium and lead could be occupied by protons, at lower pH levels (pH 3.0). The proton attached isreleased from active sites at moderate pH, and the quantity of adsorbed metal rises. The metal precipitate canoccur at high pH levels (pH > 7), which reduces adsorption. Cation exchange and surface complexation are twopotential explanations for why metal’s adsorption capacities increase when pH rises. The delayed adsorption athigh pH is results from slow diffusion into the adsorbent’s bulk and an electrostatic barrier between positively
charged adsorbate species that have been adsorbed onto
the surface of the adsorbent and cationic adsorbate species that are present in the solution [70]. Moreover, theunique nature of chromium and the adsorbent surface
explains the high adsorption capacity of metallic chromium on an adsorbent surface at lower pH. At high pH
value Cr(III) adsorption onto Co-DPA decreases as metal
ions react with OH¯ ions and gets precipitated as a metalYimer et al. BMC Chemistry(2024) 18:23

Fig. 3 Graphical representation of the impact of initial pH on aadsorption capacity and b removal efficiency (with error bar)of ­Cd2+, ­Pb2+, and ­Cr3+ by Co-DPA (error bars represent ± standard
experimental errors)

hydroxide. As a result, this pH was chosen as the ideal
value for the remaining studies [71–73].
Effect of adsorbent dosage

The impact of the adsorbent dose was analyzed by thevariable quantity of Co-DPA from 1 to 4 g and kept otherparameters constant (pH 3 for Cr and pH 7 for Cd andPb, 20 mg/L adsorbate concentration, 30 min contacttime). As shown in Fig. 4a, the adsorption capacity was
found to be highest at the lowest adsorbent concentration and reduced as the dose rose. In this work, As
the adsorbent dose increased, the adsorption capacity
declined, which was attributed to adsorption sites staying unsaturated throughout the adsorption reaction due
to the increased number of accessible adsorption sites
favoring enhanced metal ion uptake. Adsorption dosage
increased due to the availability of a greater surface area
and adsorption sites for a constant amount of metal ions,and accessible sites would overlap [73, 74].

Page 6 of 15

Fig. 4 Graphical illustration of the impact of adsorbent dosage
on a adsorption capacity b removal efficiency (with error bar)of ­Cd2+, ­Pb2+ and ­Cr3+ by Co-DPA (error bars represent ± standard
experimental errors)

When the adsorbent dose was increased from 1 to
4 g, the percentage of Cr (III), Cd (II), Pb(II) removal
was increased (Fig. 4). As the dosage amount increased
from 1 to 3, the removal efficiency rose from 71.9% to90.05% for ­Pb2+, from 91.4 to 94% for ­Cd2+, and 92.55 to
94 0.05% for ­Cr3+. The superior removal efficiency was
obtained when the adsorbent dosage was 3 g i.e., 90.05%,
94%, and 94.05%, respectively. No change was observed
when dosage amount increased to 4 g (Fig. 4b). The
higher number of accessible adsorption sites encouraged
the enhanced uptake of the metal ions which is the causeof the constant adsorption capacity with increasing dosesof adsorbent [48, 74].
Effect of initial metals concentration

Metal ions standard solutions in the following concentrations: 20, 40, 60, and 80 mg/L were generated, andthe other parameters kept similar as before. Co-DPA
was used to treat the solutions, and an AAS was used forYimer et al. BMC Chemistry(2024) 18:23

Page 7 of 15

analysis. The % removal varied consistently as the starting
concentration of heavy metal ions changed. For Cr (III),
Pb(II), and Cd(II), the percent removal for metal ionsfalls as the initial concentration slowly rises to 60 mg/L,then declines rapidly as the initial concentration rises to80 mg/L. The adsorption capacity increased as the concentration of metals increased (Fig. 5a). It is evident fromthe Figure that the percentage of ­Pb2+, ­Cd2+, and ­Cr3+
removal efficiency decreased from 89.35% to 74.9475%
for ­Cd2+, 82.75% to 77.5% for P­ b2+, 90.55% to 84.375% for3+­Cr with the increase in initial concentration of metal
ions (Fig. 5b). Different heavy metal ions may have different chemical affinities and ion exchange capacities concerning the chemical functional group on the surface of
the adsorbent, which can explain differences in the percentage of different heavy metal ions eliminated at thesame initial metal ions concentration, dose, and contact
duration [30, 47, 48, 74, 75].

The effects of contact time Pb(II), Cd(II), andCr (III)adsorption on Co-DPA at different adsorption times (20,40, 60, 80, 100, and 120 min) is shown in (Fig. 6), andpH 3 for Cr (III) and 7 for Cd(II) and Pb(II), adsorbent
dose 3 g, initial concentration was 60 mg/L. The initialadsorption was highly rapid, and as the adsorption duration increased, the sorption kinetics increased gradually
until the adsorption eventually hit equilibrium in 80 min
in the case of ­Cd2+ and C­ r3+ with a removal efficiency of
92.85 and 93.785%, respectively (Fig. 6). ­Pb2+ had a highremoval efficiency of 90.13% at 60 min, matching thefindings of Kowsura et al. [76]. Cadmium (II) adsorption
by Co-DPA has maximum removal efficiency of 92.85%
which is the same as the result obtained by [74] with amaximum adsorption capacity of 33 mg/g at 80 min.
During the adsorption process at the initial stage, manyvacant sites are available for adsorption. As adsorption

Fig. 5 Representation of the impact of initial metals concentration
on a adsorption capacity and b removal efficiency of ­Cd2+, ­Pb2+
and ­Cr3+ by Co-DPA with error bars (error bars represent ± standard
experimental errors)

Fig. 6 Graphical representation of the impact of contact time
on a adsorption capacity and b removal efficiency of ­Cd2+, ­Pb2+
and ­Cr3+ by Co-DPA with error bars (error bars represent ± standard
experimental errors)

Effect of contact time

Yimer et al. BMC Chemistry(2024) 18:23

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time progresses the active sites are occupied by adsorbates which make constant adsorption efficiency. This may
be clarified by the fact that originally, the ion absorption
rate was higher since entire sites on the adsorbent wereunoccupied and the concentration of ions was high, butthat rate remained constant as all sites were filled consequently because doing so prevents the additional adsorbate from being taken up [30, 34, 48].
Adsorption experiment for real sample

The collected wastewater sample was brought to the laboratory for the adsorption experiment and digested with
4 mL nitric acid and 2 mL perchloric acid for 80 min at
a temperature of 120 °C. 20 mL of digested sample wastaken, and 3 g of the synthesized Co-DPA was added andallowed to shake on a reciprocating shaker for 80 min to
determine Pb, and 3 g of the synthesized Co-DPA wasadded and allowed to shake on a reciprocating shaker
for 80 min to determine C­ d2+ and ­Cr3+. The sample wasfiltered by filter paper and brought to measure heavy
metals concentration by AAS. The removal efficiency
and adsorption capacity were estimated. The metal ionsconcentration before being treated with synthesized CoDPA recorded for C­ d2+, ­Pb2+, and C­ r3+ was 0.267 mg/L,
0.075 mg/L, and 0.125 mg/L, respectively. After being
treated with the adsorbent the concentration of the metal
was obtained as 0.0129, 0.00028, and 0.00054 for C­ d2+,
2+
3+
2+
2+­Pb , and ­Cr , respectively. For ­Cd , ­Pb , and ­Cr3+,
the removal efficiencies of the adsorbent for heavy metals in actual samples were determined to be 95.6%, 99.5%,
and 99.5% respectively.
Recovery studies

Reusing the adsorbent by regenerating its adsorption
characteristics is an economic necessity in many applications. With growing raw material and wastewater treatment process expenses, the allure of product recovery
technologies has grown dramatically [74, 77]. The CuDPA adsorbent was evaluated for its reusability. The
recycling results shown in Fig. 7 show that it maintains
its activity despite a decline in metal ion removal efficiency, and it is pH dependent and performed by adding
HCl and NaOH to solution.
In a typical experiment, 0.01 M ­HNO3 or HCl and0.005 M NaOH eluents were added to the solution to
perform the recycling test. Metals were initially adsorbed
on Cu-DPA from 60 mL solutions containing 80 mg/L
metal ions at pH 3 (Cr) and 7 (Cd &Pb). The Cu-DPA
were then stripped with 30 mL eluent while agitating at
25 °C for 30 min. The Cu-DPA complex was separated,
and the metal ion concentration in the supernatant wasdetermined. The adsorption–desorption cycles weredone three times for each measurement.

Fig. 7 Illustration of the percent removal of heavy metal ionsfrom real wastewater sample along with the reusability studies
for Co-DPA complex

Adsorption isotherms

Figure 8 demonstrates the adsorption isotherm with fitting models. Adsorption isotherms are often used to
explain equilibrium studies that provide the adsorbent’s
capacity and the equilibrium interactions between adsorbent and adsorbate [30, 36, 44, 78–80]. Adsorption isotherms are the fraction of the quantity adsorbed andremaining in solution at set temperature as equilibrium.
The Langmuir and Freundlich isotherms are the original and most basic proven relations that illustrate theadsorption equation. The fundamental tenet of the Langmuir hypothesis is that once a metal molecule occupies aparticular homogenous location inside the adsorbent, nofurther adsorption can take place there. A description of
the Langmuir isotherm model is stated in Eq. (1).

1
Ce
Ce
.
=
+
qe
K L qm qm(1)

where ­qm symbolizes the maximum monolayer adsorption capacity (mg/g), KL stands for Langmuir constantassociated with the energy of adsorption (L/g), C­ e and q­e
are the equilibrium concentration (mg/L), and adsorption capacity (mg/g) respectively. The equation above
produces a straight line with a slope of 1/qm and an intercept of 1/KLqm when plotting ­Ce/qe vs ­Ce (Fig. 8).
There are interactions between molecules that havebeen adsorbed and heterogeneous surface energy systems in multilayer adsorption, which is expressed by
the Freundlich isotherm model. Equation (2) used todescribe the Freundlich isotherm. In this case, the Freundlich isotherm model’s constants for sorption capacity ­(KF) and sorption intensity (1/n) are involved, andthe exponent (1/n) indicates the system’s favorability andcapacity. Comparing log qe to log Ce from the previous

Yimer et al. BMC Chemistry(2024) 18:23

Page 9 of 15

a)

c)

b)

Fig. 8 Langmuir adsorption isotherm for a ­Cd2+, b ­Pb2+, and c ­Cr3+

equation yields a straight line with a slope of 1/n andan intercept of log K­ F (Fig. 9). The slope 1/n, which has
a value between 0 and 1, is a measure of surface heterogeneity or adsorption intensity, which turn additionallyheterogeneous because its value attempts to zero. Normal Langmuir isotherm is designated by a value of 1/n
below one, and cooperative adsorption is implied by avalue of 1/n above one [81]. The ­KF is an assessment ofadsorption capacity increased with concentration for­Pb2+, ­Cd2+, and C­ r3+ adsorption. The magnitude of the
exponent n shows the favorability and capacity of theadsorbent/adsorbate system. 1/n for ­Cd2+, ­Pb2+, and3+­Cr is 0.79005, 0.880, and 0.74007, respectively. These
values are the measure of favorability of the adsorption ofadsorbate on the heterogeneous surface of the adsorbent[32, 69, 71]. Karthikeyan et al. have stated that the favorable adsorption condition is represented through n values
between 1 and 10.

log qe = logK F +

1
logCe
n

(2)

To determine the favorable or unfavorable behavior ofan adsorption system, Hall et al. [82] introduced a dimensionless separation factor or equilibrium parameter, RL,

as a crucial component of the Langmuir isotherm, whichis specified as in Eq. (3).

RL = 1/1 + KLCo

(3)

where co represents the adsorbate’s initial concentration(mg/L), KL defines Langmuir constant (L/mg). If RL > 1.0,
the sorption isotherm remains unfavorable; = 1.0 (linear); 0 < RL < 1.0 (favorable) and RL = 0 (irreversible) [83].
The achieved findings (RL = 0.357, 0.436, 0.271 for C­ d2+,
2+
3+­Pb and ­Cr , respectively) confirms that the sorption of­Cd2+, ­Pb2+ and ­Cr3+ onto the Co-DPA was favorable.
Table 2 lists the isotherm parameters for the Pb(II),
Cd(II), and Cr(III) adsorption in the Langmuir andFreundlich models. The values of R2 of the Langmuirmodel were lower than Freundlich isotherm model R2­(Cd2+(0.995), ­Pb2+(0.997), ­Cr3+(0.996)), from which
it was concluded that the adsorption of Pb(II), Cd(II)and Cr(III) ions fitted better in the Freundlich isotherm
model. This showed that the adsorption of Cr(III),
Pb(II), and Cd(II) was directed by multilayer adsorption on a heterogeneous surface of an adsorbent. Then value greater than 1 (n > 1) is documented as an
L-type isotherm indicating significant affinity amongadsorbate and adsorbent and revealing chemisorption.

Yimer et al. BMC Chemistry(2024) 18:23

Page 10 of 15

a)

c)

b)

Fig. 9 Freundlich adsorption isotherm for a ­Cd2+, b ­Pb2+ and c ­Cr3+

Table 2 Analyzed isotherm parameters for the adsorption of­Cd2+, ­Pd2+ and ­Cr3+ on to Co-DPA
Isotherm model

AdsorbateCdPb

 ­qm (mg/g)

0.16040..5570.453

 ­kL(L/mg)

0.4125

0.032

0.078

 ­RL

0.10810.60980.379

20.87570.95797

0.99269

FreundlichN

Metal ions

Metal ionssurfacecoverage

Cd2+

0.8935Pd2+

0.8275Cr3+

0.8350Cr

Langmuir

 ­R

Table 3 The fraction of the Co-DPA surface covered by C
­ d2+,
2+
3+­Pd , and ­Cr

θ=1−
1.795

1.227

1.416

 ­KF(L/g)

0.043130.02023

0.039

 ­R2

0.96056

0.997990.99885

The Freundlich adsorption capacity (KF) reveals theremoval of heavy metal ions from wastewater facilely.
The greater the adsorption intensity the higher the K­Fvalue.
The fraction of the Co-DPA surface covered by ­Cd2+,­Pd2+, and ­Cr3+ was calculated from the optimized concentration using Eq. (4).

Ce
Co

(4)

where θ is a fraction of surface coverage, Co is the initial metal ions concentration and Ce is the equilibrium
metal ions concentration. The calculated fraction of
surface coverage during the adsorption of ­Cd2+, ­Pd2+,
and ­Cr3+ onto Co-DPA is presented in Table 3. The
fraction of Co-DPA surface covered by C
­ d2+, ­Pd2+, and3+­Cr was obtained as 0.8935, 0.8275, and 0.835 for ­Cd2+,­Pd2+, and ­Cr3+ respectively. These values indicate that89.35%, 82.75%, and 83.5% of the pore space of Co-DPA
is covered by metal ions which is the same as the result
obtained on the removal of lead from wastewater by
Co-DPA.

Yimer et al. BMC Chemistry(2024) 18:23

Page 11 of 15

The number of hopping (n) an adsorbate makes, as
determined by Eq. (5), may be associated with the probability that it will discover an empty site on the adsorbent
surface during sorption (5).

n=1.
(1 − θ)θ

(5)

where n is the number of hopping done by the adsorbateand θ is the surface coverage. By using Eq. (5) the numberof hopping done by adsorbate to find vacant sites on CoDPA was obtained as 10.504, 6.993, and 7.246 for ­Cd2+,­Pd2+, and ­Cr3+, respectively.
Adsorption kinetics

The kinetics of cadmium, lead, and chromium adsorption
onto Co-DPA were investigated using the pseudo-second
order and pseudo-first order kinetic models to suggest
which model the sorption system follows. This can be
described by models of pseudo-first order and pseudosecond order. Scientifically pseudo first order is statedas in Eq. (6):a)

log (qe − qt) = log qe −Fig. 10 Pseudo-first orders kinetic model of a ­Cd2+, b ­Pb2+, and (c) C
­ r3+

(6)

where qt expresses the adsorption capacity at time t, qe
symbolizes the equilibrium adsorption capacity (mg/g),
and K1 means the pseudo-first-order rate constant (g/mg) which can be estimated by plotting between log (qeqt) and t (Fig. 10).
Pseudo-second order kinetic model is stated mathematically as in Eq. (7):

1
1
t
=
+ t
qt
K2 qe2qe(7)

where ­K2 expresses the rate constant for pseudo-second
order (g m
­ g−1 ­min−1). The value of q
­ e was estimatedthrough the plot between t/qt and t (Fig. 11).
From the pseudo-first and second order kinetic model
linear plotted with different adsorption time and kinetic
model constants like k1, k2, and qe values were determined from the slope and intercept of plots, are represented in Table 3. Table 3 reveals that the linear plot of­Cd2+ and ­Cr3+ shown in Figs. 10 and 11 has a higher R2
value in the pseudo-second order. This shows that the

b)

c)

K1
t
2.303

Yimer et al. BMC Chemistry(2024) 18:23

Page 12 of 15

a)

b)

c)

Fig. 11 Pseudo-second order kinetic model of a ­Cd2+, b ­Pb2+, and c ­Cr3+

kinetics data suit perfectly with the pseudo-second-ordermodel. The computed values of qe almost matched withthe experimental data obtained from the pseudo-second-order kinetics in addition to the larger values of R2(Table 4). Also, it was noted that the values of k1 werehigher than the comparable values of k2. The best fit forthe ­Pb2+ adsorption studies were therefore believed to be
the pseudo first-order kinetic model. The kinetics resultsfrom various investigations were also said to fit thepseudo-second-order kinetic model quite well; including the adsorption of ­Cd2+ ions on Jatropha peel, adsorption of ­Pb2+ ions on pumpkin seed shell activated carbon,

adsorption of ­Pb2+ ions on Co-DPA, and adsorption of­Cr3+ ions on cooked tea dust [43].

Conclusion
The Co-DPA complex was easily produced from cobalt (II)nitrate hexahydrate and diphenylamine linker. Co-DPA wasevaluated for its efficacy in removing heavy metal ions fromwastewater and its adsorption capacity. From this study,
it is concluded that the Co-DPA is an excellent adsorbent
for the removal of the heavy metal ions such as ­Cd2+, ­Pb2+,
and ­Cr3+ from wastewater samples. The effect of adsorption factors such as contact time, pH value, initial metals

Table 4 Calculated kinetic parameters for the adsorption of ­Cd2+, ­Pb2+, and ­Cr3+ on to Co-DPA
Kinetic model
Pseudo first order

Pseudo second order−1

2

K2 (g m­ g−1mi−1)

qexp (mg/g)

qecal (mg/g)

R2
0.994

Adsorbate

K1 ­(min )qexp (mg/g)

qecal (mg/g)

Cd2+

0.025

0.01857

0.00510.98978.16

0.01857

0.02

Pb2+

0.125

0.018026

0.0294-0.9986.99

0.018026

0.02

0.997

Cr3+

0.054

0.018757

0.0176

0.923

3.54

0.018757

0.021

0.995

R

Yimer et al. BMC Chemistry(2024) 18:23

concentration, and adsorbent (synthesized Co-DPA) doseon adsorption capacity and removal efficiency was examined. The removal efficiency was recorded as 95.6%, 99.5%,
and 99.5% for the ­Cd2+, ­Pb2+, and ­Cr3+, respectively. More
importantly, the adsorption isotherm was governed by Freundlich isotherm and adsorption mechanism was pseudofirst order for P­ b2+ and pseudo-second order for C­ d2+, and3+­Cr . This suggests that a multilayer has formed on thesurface of the adsorbent. Therefore, the synthesized CoDPA complex could be a potential candidate for the massively efficient adsorptive removal of heavy metal ions fromwastewater.
Acknowledgements
Mesfin Yimer, Binyam Abdu, and Wubshet Mekonnen Girma is grateful to
the Department of Chemistry, College of Natural Science, Wollo University,
Ethiopia, for granting access to their research facilities.

Page 13 of 15

2.
3.

4.
5.
6.

7.

8.

Author contributions
MY, SNA, and BAB performed the synthesis of Co-DPA, characterization, andperformance analysis of results writing-draft and conceptualization of work,
KKG and, GG performed an analysis of results, manuscript correction, andscientific discussion, WMG performed an analysis of results, manuscript correction, scientific discussion, supervision, and review-editing, and NA and ST
performed editing, and correction. All authors reviewed the manuscript.

9.

Funding
Wollo University funded this research through a graduate student support
program.

12.

Data availability
The datasets used and/or analyzed in this study are accessible in the publication and can be obtained upon request from the corresponding author.

Declarations

10.
11.

13.
14.

15.

Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.

16.

Competing interests
The authors declare that there is no conflict of interest.

17.

Author details
1
Department of Chemistry, College of Natural Science, Wollo University, P.O.
Box:1145, Dessie, Ethiopia. 2 Department of Chemistry, School of Engineering, Presidency University, Bangalore, Karnataka 560064, India. 3 Departmentof Chemistry, School of Science, GITAM (Deemed to Be University), Rudraram,
Telangana 502329, India. 4 Central Research Laboratory, K S Hegde Medical
Academy, NITTE (Deemed to Be University), Deralakatte, Mangaluru, Karnataka
575018, India. 5 Department of Animal Science & Technology and BET ResearchInstitute, Chung-Ang University, Anseong, Gyeonggi‑do 17546, Republicof Korea. 6 Department of Chemistry, College of Science, Jazan University, P.O.
Box 114, Jazan 45142, Kingdom of Saudi Arabia.

18.
19.
20.
21.
22.

Received: 19 July 2023 Accepted: 16 January 2024
23.

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