Investigations on the biosorption of nickel using tea leaves and tea fibre (Camellia Sinensis) as adsorbents: thermodynamics, isotherms and kinetics

Abstract

The adsorption behavior of tea leaves and tea fiber ( Camellia sinensis ) as low-cost adsorbent with respect to nickel was investigated to justify its usage in wastewater treatment. A good number of adsorption constraints were investigated which provides information about the effect of pH value, temperature, adsorbent dosage, time of contact as well as the starting concentration of the simulated system on the sorption process itself. From the result effects of these parameters could be seen in the biosorption of Nickel by both the tea leaves and fibers. The optimal pH for Ni biosorption in tea leaves and fiber is between 3 and 5, with the highest removal at pH 5 and a dosage of 3 g. The leaf adsorbent is more effective at 50 mg/L metal ion concentration showing 99.8% Nickel removal. The kinetics was best described by the pseudo-second order which gave the most convincing fit. The Langmuir isotherm gives R ² values of 0.990 and 0.985 for tea leaves and tea fiber and Freundlich isotherm gives 0.985 and 0.980 values for tea leaves and tea fiber correspondingly with the Langmuir isotherm having higher R ² values considered the most suitable. In the long run, this process was endothermic, spontaneous, and of course thermodynamically feasible hence, the adsorbent was considered fit for wastewater treatment.

Full text

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Discover Chemistry
Research
Investigations onthebiosorption ofnickel using tea leaves andtea
fibre (Camellia Sinensis) asadsorbents: thermodynamics, isotherms
andkinetics
EmmanuelE.Etim1· ShedrachYakubu1· AnihoTerhembe1· LibertyJoshuaMoses1
Received: 4 April 2024 / Accepted: 14 June 2024
© The Author(s) 2024 OPEN
Abstract
The adsorption behavior of tea leaves and tea ber (Camellia sinensis) as low-cost adsorbent with respect to nickel was
investigated to justify its usage in wastewater treatment. A good number of adsorption constraints were investigated
which provides information about the eect of pH value, temperature, adsorbent dosage, time of contact as well as the
starting concentration of the simulated system on the sorption process itself. From the result eects of these parameters
could be seen in the biosorption of Nickel by both the tea leaves and bers. The optimal pH for Ni biosorption in tea
leaves and ber is between 3 and 5, with the highest removal at pH 5 and a dosage of 3g. The leaf adsorbent is more
eective at 50mg/L metal ion concentration showing 99.8% Nickel removal. The kinetics was best described by the
pseudo-second order which gave the most convincing t. The Langmuir isotherm gives R2 values of 0.990 and 0.985 for
tea leaves and tea ber and Freundlich isotherm gives 0.985 and 0.980 values for tea leaves and tea ber correspond-
ingly with the Langmuir isotherm having higher R2 values considered the most suitable. In the long run, this process
was endothermic, spontaneous, and of course thermodynamically feasible hence, the adsorbent was considered t for
wastewater treatment.
Keywords Adsorbent· Tea leaves· Tea ber· Nickel· Isotherms· Kinetics· Thermodynamics
1 Introduction
A lot of concerns have been expressed by Ecologists throughout the globe, considering the rate at which our dear eco-
system has been abused by various industrial activities. Research has shown that most areas cited with industries have
suered a good number of ecological breakdowns ranging from climatic change, water pollution, and the list continu-
ous [1]. This decay on the planet today is indeed fostered and promoted by our ignorance and carelessness towards
safe industrialization mostly dominated by poor pollution management. These regions of the earth’s crust have been
adversely polluted [2] by the so-called anthropogenic activities via the discharge of either gasses or industrial euents
into the system which is of cause a potential threat to life either on land, air, or water.
On the other hand, it is also possible that natural disasters such as volcanic eruptions, forest re outbreaks, and deep-
sea events [3], could also result in the introduction of these harmful toxins into the ecosystem, especially water bodies
* Emmanuel E. Etim, emmaetim@gmail.com | 1Department ofChemical Sciences, Federal University Wukari, Wukari, TarabaState, Nigeria.

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hence, resulting to a decrease in the quality of life within. These water bodies are mostly polluted by various toxins
which could be metallic [4] in nature, inorganic and organic chemical compounds discharged in the form of euents
which include dyes, volatile organic compounds (VOC), oil, plastics, insecticides, pesticides and herbicides, microbial-like
pathogens, viruses, and bacteria [5]. When the concentrations of these metals in the water bodies exceed the accept-
able limit, they become toxic to living organisms therefore, bioaccumulation in humans may result in either mutagenic,
carcinogenic, or teratogenic conditions which can be chronic or acute. A good number of preventive regulations have
been put in place to address this situation but to some extent; it has been observed to be less eective hence, the need
to fall back to science for curative answers [6]. One of the most promising techniques that have gained a lot of interest
is the sorption technology used in the treatment of waste or polluted water bodies (Hydrosphere).
Amongst all the applicable technology used for wastewater purication, bio-sorption is the most engaged due to its
economic and simple nature under viable sorption conditions. Adoption of techniques like reverse osmosis, membrane
ltration, ion exchange, and solvent extraction [7] has not been easy in the past few years because of the cumbersome
nature of the techniques as well as the economically demanding nature that comes along with conditions. For example,
the choice of a suitable solvent for the extraction of a particular component can be very dicult this is because not all
solvents can extract a particular solute hence, the solubility of the solute in the chosen solvent must be of the utmost
priority. In the case of sorption, the technique depends mostly on the pore size of the adsorbent that can be occupied
by an adsorbate, therefore having any material at all, that can be activated, can t in as far as it has appreciable pore size
and functional groups for sorption.
Biosorbents are materials obtained after subjecting products from plants and animals [8] under intense heating in
the absence of oxygen followed by activation either by an acid or a base. Once they are prepared, improvement in their
sorption potency can be attributed to the enhanced pore space or the emergence of suitable binding sites that may
allow for interaction with the adsorbate in question. Most natural products are potential adsorbents that can be used
to eliminate the presence of metallic toxins like nickel in wastewater; this is true because most plant and plant products
contain lignin and cellulose with electronegative biding sites that can easily be attracted by electropositive species [9].
Recently, Rawat etal., [10], have reported the biosorption of multiple heavy metals (Cu2+, Zn2+, and Cr6+) in the leaves of
J. curcas which shows excellent adsorption capacity on the heavy metals. In several other research, it has been reported
that plants like paper mulberry, Ficus religiosa, Alyssum discolor, neem, etc., can eliminate heavy metals [11–14]. It is in
this regard that, plants are generally given the consideration status as biosorbents.
Tea plants (Camellia sinensis) are a group of evergreen species of the family Thenaceae grown most to make the bever-
age drink called tea. Most of the plant has been utilized for this purpose. After the extraction of the tea components from
the plants, especially the leaves, they are being disposed of without any further use which in turn causes environmental
pollution [17]. It is on this note that this research tends to utilize the leaves and bers of the plant as adsorbents for the
elimination of nickel amongst other heavy metals and pollutants from wastewater thereby preventing the supposed
land and water pollution from the tea waste and at the same time proving that some low-cost agricultural waste could
be used as eective adsorbents. Water bodies are the most polluted by industrial euent containing various chemical
toxins, and this may require a biorsorbent with not just pore spaces but eective functional groups that can bind to these
toxins in other to eliminate them, hence the need for the exploration of tea leaves and bers (Fig.1) for this purpose.
Fig. 1 Tea leaves (a) and tea bers (b)

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Tea leaves and tea bre have emerged as attractive biosorbents due to their abundance, low cost, and readily available
functional groups. Previous research has explored their potential for adsorbing various heavy metals. Studies by [15]
demonstrated the eectiveness of tea leaves in kinetics and thermodynamics studies of the adsorption of cu2 + using
tea leaves and tea bre (camellia sinensis) as adsorbents. From the analysis, the adsorbents show high eciency above
97% in the removal Cu2+ during the treatment of waste water. The high eciency of the used adsorbents in the removal
of Cu2+ from aqueous solution shows its distinctive property as good adsorbents; hence tea bre and tea leaves may
be used in the treatment of waste water containing Cu2+. Also, precious studies on kinetic studies of biosorption of
cr2+ and cd2+ ions using tea leaves (camellia sinensis) as adsorbent. According to the research, at optimal conditions,
chromium and cadmium uptake increases with increase in biosorbent dosage, in this study the optimum dosage was
2g. The removal of chromium and cadmium was more than 90% in 10, 20, 30 and 40min of contact time. It is obvious
that Camellia sinensis is a suitable adsorbent that can be used for the eective removal of high chromium and cadmium
concentration in waste water or industrial euents as shown by the adsorption kinetic studies [16]. This current study
focuses on the specic biosorption of nickel, a common heavy metal contaminant, using both tea leaves and tea bers
(Camellia sinensis) as adsorbents. It aims to compare the eciency and mechanisms of nickel adsorption by these two
tea-derived materials. While previous studies explored tea for various heavy metals, this research focuses specically on
nickel biosorption. By investigating the thermodynamics, isotherms, and kinetics of nickel adsorption with tea leaves
and bers, the studies provides more explanation on the mechanism.
2 Methods
The various stock solutions used in this research are NiSO4.6H2O salt (Zincomond EN Grade), HNO3 (ECOCHEM Ltd 69%
Technical Grade), and NaOH (98% ACL Labscan) The stock solution of 0.1M Nickel (ll) tetraoxosulphate (IV) hexahydrate
(NiSO4.6H2O), 0.1M NaOH and 0.1M HCl were prepared in a 1000mL standard ask using distilled water. The various
functional groups present in the tea leaves and bers were probed using FTIR spectroscopy (Happ-Genezel) in order to
account for the active sites present on the biosorbent.
2.1 Adsorption studies
The investigation with tea leaves and bre (Camellia sinensis) as an adsorbent for the elimination of nickel metal ions from
a simulated nickel solution with the consideration of various constraints such as initial concentration, pH, temperature,
contact time, and biosorbent dosage was carried out as enshrined in Etim etal., [18, 19, 20]. The tea leaves and bers
were collected from Sardauna local Government area of Taraba State, using the method according to Etim etal., [21] for
sample collection and preparation. They were washed, rinsed, sun dried for seven days, pulverized, ltered via a150mm
sieve, and nally stored in an airtight container before experiments. The equilibrium relationship for each eect were
accessed accordingly.
2.1.1 Effect ofbiosorbent dosage
The adsorbent was weighed into various conical asks at 1g, 2g, 3g, and 4g. Each conical ask was lled with 50 cm3
of the metal solution, labeled, and sealed. After the asks were corked and the mixture was shaken for an hour to reach
equilibrium, the slurries were ltered through Whatman lter paper and a plastic funnel, stored in containers with clear
labels, and the concentrations of the ltrate were measured using an atomic absorption spectrometer [22].
2.1.2 Effect oftime (time dependence)
50 cm3 of metal solution were contained in various conical asks with 1g of biosorbent suspended in them. The intervals
between each beaker were 10, 20, 30, and 40min, and they were all shaken using an electrical rotary shaker at 30rpm.
A constant pH of 6, a temperature of 25°C, and a metal concentration of 0.1M were maintained. After the spinning
was nished, the solutes were taken out and put into a polypropylene centrifuge tube. Then, they were centrifuged at
4000rpm for three minutes. This made it possible for the biosorbent to separate from the mixture [23]. Ultimately, the

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solutions were taken out of the centrifuge tubes and placed in a sterile, airtight bottle in preparation for atomic absorp-
tion spectroscopy (AAS) examination.
2.1.3 Effect ofinitial concentration
Metal solutions with volumes of around 50 cm3 and concentrations of 20mg/L, 30mg/L, 40mg/L, and 50mg/L were
measured and added to several conical asks. After spreading 1g of the biosorbent onto each ask and corking the
asks, the mixture was shaken for an hour to reach equilibrium. The slurries were then ltered through Whatman lter
paper and a plastic funnel. After that, the ltrates were stored in clearly labeled containers, and an atomic adsorption
spectrometer was used to measure the concentrations of the nal ltrates [24].
2.1.4 Effect ofpH
Experiments were carried out at 25°C to investigate the impact of pH on the biosorption of Nickel (Ni). One gram of tea
leaves and tea ber was placed in a conical ask along with 50 cm3 of Nickel solution. With the use of 0.1M hydrochloric
acid and 0.1M sodium hydroxide, the pH of each solution was brought to the appropriate level. The pH values of 1.0, 3.0,
5.0, and 7.0 were studied, and the conical asks were shaken mechanically for an hour. By using decantation to extract
the biomass from the solutions, the concentration of Nickel that remained in the solution was measured. The AAS equip-
ment was used to obtain the mean Nickel concentration value for each batch of pH during the triplicate research [24]. The
pH range that does not aect the metal’s precipitation was used to study how pH aects the biosorption of metal ions.
2.1.5 Effect oftemperature
Four distinct conical asks were lled with approximately 50 cm3 of the 40mg/L Nickel stock metal solution, and the
temperature varied between 40°C and 70°C. The adsorbent was then weighed and added to each ask in a volume of
1g. They were put in the mechanical shaker for an hour to bring the slurries to equilibrium. After that, they were ltered
through Whatman lter paper and a plastic funnel, and the ltrate was stored in a container with a clear label. An atomic
adsorption spectrometer was used to measure the concentration of the nal ltrates [25, 26].
2.2 Metal uptake evaluation andpercentage removal
The technique according to Madhavi etal. [27], was used to estimate the metal uptake qe. This was accessed using the
following equation.
where qe is the metal ions uptake at equilibrium (mg/g), V is the volume of the metal solution used (L), Co is the initial
concentration of a metal ion in solution (mg/L), Ce is the nal concentration of a metal ion in solution at equilibrium
(mg/L), and m is the mass of biosorbent (g). The total percentage removal is given by the equation.

q
e=
V
(
Co−Ce
)
m
% metal removed
=
C
o−
C
e
C
o
×
100

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2.3 Adsorption kinetics
A good understanding of diusion mass transport or kinetics process for dierent adsorbents is of paramount impor-
tance. Thus, models such as pseudo-rst and second-order models were employed to analyze kinetics data for the sorp-
tion process. The linearized pseudo-rst-order kinetics is expressed as:
where qe is the amount of metal uptake at equilibrium (mg/g), qt is the amount of metal uptake at time t. A plot of log
(qe-qt) versus t should yield a linear connection if the pseudo-rst order is applicable. The slope and intercept of the
curve can be used to derive the constant k1 and projected qe, respectively. The integrated rate equation for second order
kinetic model is given as:
where k2 (mg/g.minute) is the rate constant of the second-order equation, qt (mg/g) is the metal adsorbed at time t (min),
and qe is the metal adsorbed at equilibrium (mg/g). The plot of
1
𝐪𝐭
against t will give a linear curve whose slope is equal
to the rate constant k2.
2.4 Adsorption isotherms
2.4.1 Langmuir isotherm
The Langmuir isotherm model was determined using the equation below, which depicts the relationship between the
quantities (mg/g) of adsorbate adsorbed on the adsorbent and the adsorbate concentration (mg/L) in solution at equi-
librium condition.
where Ce is the equilibrium concentration (mg/L), qe is the amount of adsorbate adsorbed on the adsorbent at equilib-
rium, b is the Langmuir isotherm constant (L/mg), and Qo is the adsorption capacity of the adsorbents.
2.4.2 Freundlich Isotherm
Freundlich isotherm demonstrates that the adsorption process on a heterogeneous adsorbent surface is multilayered,
and the adsorption sites have varying degrees of attraction for the adsorbate. This isotherm model was determined
using the following equation below;
where KF is the Freundlich isotherm constant (mg/g or dm3/g) associated with adsorbent adsorption capacity, and n
is the adsorption intensity related to the heterogeneity of the adsorbent surface. A plot of log qe against log Ce gives a
straight-line of slope
1
n
and an intercept equal to log KF.
2.5 Thermodynamics ofAdsorption
The nature of an adsorption process is confirmed by the evaluation of its thermodynamic parameters. Thermodynamic
parameters like free energy change (
Δ
Gads), enthalpy change (
Δ
Hads), and entropy change (
Δ
Sads) of adsorption was
calculated to evaluate the feasibility and spontaneity of the process.
The standard free energy change of adsorption (
Δ
Goads) was calculated using the following equation below;
log
(qe −qt)=log qe −
k1
2.303 t
1
qe
=
1
k
2
q2
t
+
1
qe
t
C
e
qe
=1
bQo
+Ce
Qo
log qe
=log kf+
1
nlog Ce

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The maximal Langmuir adsorption capacity is Qo and the Langmuir isotherm constant is b.
T is the thermodynamic temperature and R is the gas constant (8.314J mol−1 K−1).
The Gibbs free energy of biosorption can be computed [28] as follows.
where
Δ
Go represents the standard Gibb’s free energy change for the adsorption (J/mol), R represents the universal gas
constant (8.314J/mol/K) and T represents the temperature (K). The adsorbate’s distribution coecient is Kc. A negative
Gibbs free energy value suggests that the adsorption process is feasible and spontaneous [28]. The plot of ln Kc versus
1/T yields a straight line with values for
Δ
Ho and
Δ
So as the slope and intercept. Kc is the distribution constant and can
be written as [29].
Cad (mg/l) and Ce (mg/l), respectively, are the concentration of solute adsorbed at equilibrium and the concentration
of solute in solution at equilibrium. The following is the relationship between (∆Go), enthalpy change (∆Ho), and entropy
change (∆So) of adsorption:
Positive change in enthalpy (∆Ho) implies that the adsorption is an endothermic process, but positive change in
entropy (∆So) reects enhanced randomness at the solid/ solution interface.
2.6 Adsorption mechanism
The adsorption mechanism under optimal conditions involves the adhesion of atoms, ions, or molecules from a gas, liquid,
or dissolved solid to a surface. This process creates a lm of the adsorbate on the adsorbent’s surface. The mechanism of
adsorption occurs due to unbalanced or residual attractive forces on the surface particles of the adsorbent, which attract
the adsorbate particles as shown in Fig.2. At a given temperature and pressure, the extent of adsorption increases with
the surface area per unit mass of the adsorbent. Adsorption is an exothermic reaction, resulting in a decrease in surface
energy and a negative change in enthalpy (∆H). The process is accompanied by a decrease in entropy (∆S) and a decrease
in Gibbs energy (∆G), making it spontaneous under certain conditions. The Langmuir adsorption isotherm model is
commonly used to describe adsorption on solid surfaces. It assumes that adsorption occurs through a monolayer pro-
cess with equivalent adsorption sites. The Langmuir model is based on the equilibrium between gas molecules (A) and
adsorption sites (S), represented asAg + S ⇌ ASAg + S ⇌ AS. The surface coverage,θθ, which is the fraction of adsorption
sites occupied, is determined by the Langmuir constantKand the concentration of the gas in the bulk solution.
Δ
G
0
ads
=−2.303 RT log (bQo
)
Δ
G
0
=−RT In K
c
Kc=Cad ∕Ce
ΔG0=ΔH0−TΔS0
Fig. 2 Schematic Diagram of
the Adsorption Mechanism
[19]

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3 Results anddiscursions
3.1 FTIR analysis ofbiosorbents.
The FTIR spectra for the tea leaves and tea bers are presented in Figs.3 and4 respectively which show the various
vibrational frequencies that correspond to the functional groups which serve as binding sites for the adsorption of Nickel.
Fourier Transform Infrared Spectroscopy of both tea leaves and tea ber was used to analyze the surface-active groups
that serve as active binding sites for the uptake of metallic ions in polluted wastewater. Bhattacharya etal., [30], reported
some of these functional groups including O–H, –C=O and –C–C– groups Bhattacharya etal., [30], reported some of
these functional groups including O–H, –C=O and –C–C– groups for the biosorption of Pb by algae. The FTIR results of tea
leaves exhibit N–H at 3697.5 cm−1 indicating the presence of primary amine; O–H band at 3615.6 cm−1 corresponding to
Fig. 3 FTIR analysis of the tea leaves
Fig. 4 FTIR analysis of the tea bers

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alcohol, carbohydrates, proteins, and phenols; C–H band at 2918.5 cm−1 an indicative of Alkane; P- band at 2322.1 cm−1
corresponding to Phosphine and C=O signaling an aromatic ketone. The presence of primary amine was fully conrmed
with the band at 1606.5 cm−1, also the band at 1364.2 cm−1 is indicative of aromatic amine; a band at 1233.7 cm−1 signaled
the carboxylic acid and 1010.1 cm−1 shows the presence of primary alcohol or/and ether [31]. The FTIR results of tea ber,
on the other hand, exhibit an O–H stretch band at 3276.3 cm−1 corresponding to alcohol, carbohydrates, proteins, and
phenols; C–H band at 2918.5 cm−1 indicating an Alkane; a –C=O band at 1625.1 cm−1 depict the amide active group; while
a C–O band at 1144.2 cm−1 corresponds to secondary alcohol and C–O band at 1017.6 indicates an ether or/and primary
alcohol. The spectrum explains that some peaks were shifted or disappeared, and new ones formed. These changes
observed indicate the eect of these active groups in the sorption process, these a suggestive of chemical adsorption.
1234567
98.6
98.8
99.0
99.2
99.4
99.6
99.8
Percentage Removal (%)
pH
TeaLeaves
TeaFibers
(a)
10 15 20 25 30 35 40
98.4
98.6
98.8
99.0
99.2
99.4
99.6
Percentage Removal(%)
Time (minutes)
TeaLeaves
TeaFibers
(b)
310315 320325 330335 340345
98.9
99.0
99.1
99.2
99.3
99.4
99.5
99.6
99.7
99.8
Percentage Removal(%)
Temperature(K)
TeaLeaves
TeaFibers
(c)
1.01.5 2.02.5 3.03.5 4.0
98.2
98.4
98.6
98.8
99.0
99.2
99.4
99.6
Percentage Removal(%)
AdsorbentDosage (g)
Tea Leaves
Tea Fibers
(d)
Fig. 5 Eect of pH (a) time (b) temperature(c) and adsorbent dosage (d) on the sorption of nickel from wastewater by tea leaves and bers

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3.2 Effects ofvarious adsorption parameters
The eect of pH in the sorption studies of metal ions by biomass is very signicant because it does not only inuence the
solubility of metal ions to be adsorbed but also the functional groups on the biosorbent and the degree of ionization of
the biosorbate. Figure5a is a curve indicating the inuence of pH on the sorption of nickel by both the tea leaves and
tea ber. Observations made from the plot show that there was an increase in the sorption of nickel as the pH tends to
increase from one through three to ve, accompanied by a steep decline in the sorption as the pH increases further, for
both tea leaves and tea ber. The equilibrium concentration for both tea leaves and tea bers occur exactly at a pH of 5
which is the optimum pH for the adsorption of nickel by both tea leaves and tea ber and is thus regarded as the equi-
librium state. Malandrino etal., [32], reported that lower pH values initiate competition between metals and H+ ions for
the available active sites present on the adsorbents since H + ions are in huge amounts at that pH condition. The case is
dierent when the pH value is raised, hence, there is a meaningful increase in the sorption process in that the available
–OH groups add to the available active side with no competition for bind-in on the adsorbent.
Time is another inuential parameter that makes kinetic studies in adsorption feasible. It has been reported that the
biosorption rate increases from 18.9% to 42.8% with an increase in time between 15 and 300min for the biosorption of
Ni in wastewater by 1g of barbadensis miller waste leaves adsorbents at a pH of 7 and room temperature [6, 33]. Between
10 to 40min of sorption studies, there was an observable decrease in the sorption of nickel by both tea leaves and ber
Fig.5b. Further Increment in the time resulted in a decrease in adsorption; hence, this was a result of complete utilization
of the initial available binding sites with increasing time. Even though there was a steep decline in the sorption process
with increasing time, the percentage removal for both tea leaves and bers were above 90% up to 40min of the process.
In understanding the thermodynamics of the sorption process, temperature eects play a signicant role not only
in providing information about the activation energy necessary but if the energy is enough to foster the feasibility of
the process, estimate the degree of randomness as well as the nature of evolved energy during the process [34]. It is in
this regard that the experiment seeks to investigate temperature eects as an active adsorption parameter. The experi-
mental results of temperature’s eect on the sorption of nickel by both tea leaves and tea ber show that there was a
decrease in the adsorption of nickel ions on the adsorbent with elevation in temperature from 40°C to 70°C as captured
in Fig.5c. This indicates the exothermic nature of the process hence an increase in adsorption capacity with decreasing
temperature. The maximum equilibrium adsorption capacity for Ni (II) ions by the tea leaves and tea ber was reached
at temperatures between 313 and 323 ºC which is following the report of Jitendra and Navneeta [35].
Generally, having a good idea of the quantity of biosorbent required for the removal of a specied amount of metallic
ion in wastewater can be of great importance, as it provides room for eciency and better estimations. For this study,
the eect of adsorbent dosage was investigated as presented in Fig.5(d). The percentage removal for both tea leaves
and tea ber present a steep increase with increasing dosage of the biosorbent (tea leaves increase from 98.25 to 99.50%
while the tea bers increase from 98.25 to 99.25% between 1 and 4g of the biosorbent and 50mL of the metallic solu-
tion). Above 3g of the biosorbent, a sharp decline was observed for the tea ber, whereas that of the tea leaves shows a
attened increase. The behavior of 3g biosorbent may suggest this region as a region of equilibrium condition.
Fig. 6 Eect of initial concen-
tration on the biosorption of
nickel by tea leaves and ber
20 25 30 35 40 45 50
97.5
98.0
98.5
99.0
99.5
100.0
Percentage Removal (%)
Concentration(mg/L)
TeaLeaves
TeaFibers

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In as much as the biosorbent dosage greatly inuences the adsorption eciency towards higher eciency, the initial
concentration of the adsorbate also causes a shift in the adsorption eciency. The eect of the initial concentration of
nickel in the aqueous solution shows an increase in % reduction with an increase in concentration as represented in
Fig.6 for tea leaves and tea ber. Between 20 and 40mg/L of the initial concentration, the adsorption eciency increases
and remains constant for tee ber above this concentration whereas that of the tea leaves shows a sharp increase in the
percent removal up to 50mg/L of the initial concentration. At the initial stage, when there were unoccupied active sides,
the percentage removal increased until an equilibrium condition was reached at 40mg/L above which most of the active
binding sites on the biosorbent were already occupied, and thus a subtle decline was observed. This is true because, if
the initial metallic concentration is moderately high, the active site of both tea leaves and tea ber will be attracted by
more nickel thus, a sucient adsorption process will likely occur with increasing adsorption eciency if enough active
groups or sites were available for nickel complexation. But then at some very high concentrations of nickel, where all the
active sites have been used up, desorption is highly possible hence the reason for a slight decline.
Based on the various eects, the best pH for the biosorption of Ni for tea leaves occurs from pH 3 and 5 whereas that
of the Fiber occurs at pH 5 with 99.75% Ni removal. The best adsorption occurs after 10min and at 313K and 323K for
both tea leaves and ber. The dosage with the highest adsorption capacity for the tea leaves was 4g whereas that of
the ber was 3g. The tea leaf adsorbent was more eective for metal ion concentration of 50mg/L whereas the ber
has high adsorption capacity at both 40 and 50mg/L with about 99.8% Ni removal which is superior to the 65% Nickel
removal at 100ppm Nickel solution for barbadensis miller leaves biosorbent reported by Gupta etal., [33].
3.3 Adsorption isotherm
Both Langmuir and Freundlich isotherms have been employed in this study to understand the nature of the adsorption
process. The adsorption layer as well as the nature of homogeneity of adsorption sides can only be understood with
proper scrutiny of the adsorption isotherms. Langmuir isotherms provide exhaustive information about monolayer
adsorption on a homogeneous side whereas Freundlich isotherms is known for its application in the studies of multilayer
adsorption on a heterogeneous site. Table1 presents the calculated experimental parameters necessary for the isotherm
studies. The adsorbate bonded to the adsorbent at the interface is related to its bulk concentration in the solution by the
adsorption isotherms [36]. This assumes that particles of the adsorbate bind to an active site of the adsorbent irrespective
of whether the adjacent active sites are empty or not [37]. For this research, Langmuir and Freundlich isotherm as applied,
aid in comprehending the adsorption isotherm of nickel from a simulated system. For the Langmuir model, the regres-
sion correlation coecient (R2) for both tea leaves and tea ber were found to be 0.990 and 0.988 respectively and that
of the Freundlich was 0.985 and 0.980 for both tea leaves and tea ber respectively. This implies that the midpoint data
agrees well with Langmuir better than Freundlich which assumes that a monolayer is formed and the uniform energies
of adsorption onto the tea leaves and tea ber and that no transmigration of nickel on the adjacent binding sites [38].
Table 1 Parameters for
plotting Langmuir, Freundlich
adsorption isotherm of Ni (ll)
ion by tea leaves and tea ber
Adsorbent Co(mg/L) Ce(mg/L)
1
Ce
Log Ce Qe
1
Qe
Log Qe
Ce
Qe
% removal
Tea leaves 20 0.50 2.0 −0.30 0.975 1.00 −0.01 0.50 97.50
30 0.30 3.3 −0.52 1.485 0.70 0.20 0.20 99.00
40 0.20 5.0 −0.70 1.990 0.50 0.30 0.10 99.50
50 0.10 10.0 −1.0 2.495 0.40 0.40 0.04 99.80
Tea bre 20 0.50 2.0 −0.30 0.975 1.00 −0.01 0.50 97.50
30 0.40 2.5 −0.40 1.480 0.70 0.20 0.30 98.50
40 0.20 5.0 −0.70 1.950 0.5 0.30 0.10 99.00
50 0.10 10.0 −1.0 2.495 0.40 0.40 0.04 99.0
Table 2 Parameters for
Langmuir and Freundlich
adsorption isotherm
Metal ion Langmuir isotherm Freundlich isotherm
Ni2+
Qo
(mg/g)
KL
(L/mg)
RL
R2
1
n
n
Kf
(mg/g) R2
Tea leaves 0.25 0.088 0.18 0.990 0.568 1.76 3.934 0.984
Tea bre 0.23 0.04 0.28 0.985 0.44 2.27 2.96 0.980

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The separation factor RL which is an indication of the nature of adsorption was obtained to be 0.18 for the tea leaves
and 0.28 for the tea bers as shown in Table2, implying that the adsorption is favorable since (0 < RL < 1) in both the tea
leaves and tea bers. Therefore, both Freundlich and Langmuir isotherm models were successfully applied, but the most
tted adsorption isotherm in this biosorption process is the Langmuir isotherm models with (R2 = 0.990 for tea leaves
and 0.985 for the tea bers) while for the Freundlich isotherm models (R2 = 0.984 for the tea leaves and 0.980 for the tea
bers) shown in Fig.7a, b.
Fig. 7 A plot of Ce/qe against Ce (a) and Log qe against Log Ce (b) for the adsorption of Ni by both tea leaves and bers
Fig. 8 A plot of logarithm of
ln K versus 1/T
Table 3 Thermodynamics
parameters Adsorbent Temperature
(K)
1
T
×10−
3
Ce (mg/L) Qe(mg/g) Kc (g/L) lnKc% Removal
Tea leaves 313 3.20 0.10 1.995 19.95 3.00 99.75
323 3.10 0.10 1.995 19.95 3.00 99.75
333 3.00 0.20 1.990 9.95 2.30 99.50
343 2.90 0.40 1.980 4.95 1.60 99.00
Tea bre 313 3.20 0.10 1.995 19.95 3.00 99.75
323 3.10 0.10 1.995 19.95 3.00 99.75
333 3.00 0.20 1.990 9.95 2.30 99.50
343 2.90 0.30 1.980 6.62 1.90 99.25

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3.4 Thermodynamic studies
The thermodynamic parameters presented in Table3 were determined by studying the adsorption process of the biomass
at various temperatures between 313 and 343℃. After obtaining the Gibbs free energy using the Van’t Hos’ equation
estimated from the plot of Fig.8, other thermodynamic parameters such as the enthalpy, and entropy were determined.
Table 4 Thermodynamics
results for both tea leaves and
tea ber
Adsorbents
Tea leaves Tea ber
Parameters Temperature (K) Results Parameters Temperature (K) Results
△G° (kJ/mol) 313 −838.697 △G° (kJ/mol) 313 −744.654
323 −865.497 323 −768.454
333 −892.299 333 −792.254
343 −919.097 343 −816.054
△H° (kJ/mol) 0.143 △H° (kJ/mol) 0.286
△S°(kJ/mol K) 2.68 △S° (kJ/mol K) 2.38
Table 5 Parameters for the
plotting of Kinetic studies of
Ni (ll) ion using tea leaves and
tea ber
Adsorbent Time (min) Ce (mg/L) Qe (mg/g) % removal log (Qt-Qe) t/Qt
Tea leaves 10 0.200 1.990 99.50 0.253 50.00
20 0.300 1.985 99.25 0.227 66.67
30 0.500 1.975 98.75 0.169 60.00
40 0.600 1.970 98.50 0.136 66.67
Tea bre 10 0.200 1.990 99.50 0.253 50.00
20 0.300 1.985 99.25 0.227 60.00
30 0.400 1.980 99.00 0.198 75.00
40 0.500 1.975 98.75 0.169 80.00
Fig. 9 Pseudo-rst-order kinetic studies for tea bre (a) tea leaves (b)

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Figure8 is a plot of the logarithm of the activity constant of the biosorption process and the inverse of the tempera-
tures of biosorption of Ni by both tea leaves and tea bers. From this plot, the slope and intercept which corresponds to
the enthalpy and entropy of the biosorption process can be accessed.
The slope and intercept of the curve of ln K versus 1/T as shown in Fig.8 representing the enthalpy and entropy of
the system, provides information about the thermodynamic performance of the biosorption of Ni (II) on the biomass.
The thermodynamic parameters for the biosorption of nickel (II) ions on both tea leaves and tea ber revealed that the
system is endothermic, and the interfaces are disordered hence, the positive values of change in enthalpy (△H°) and
entropy (△S°) respectively. The Gibbs free energy change, △G °, which remains negative in all temperatures for the
case of both tea leaves and tea bers, is an indication that the biosorption process is spontaneous and feasible. Table4
presents these thermodynamic results in terms of Gibbs free energy, enthalpy, and entropy of the biosorption process.
Fig. 10 Pseudo-second order kinetics studies for tea ber (a) and tea leaves (b)
Table 6 Pseudo-rst order
kinetics for both tea leaves
and tea ber
Adsorbent
Tea leaves Tea ber
Time (mins) a–x
K1
Time (mins) a–x
K1
10 38.025 0.00506 10 38.020 0.00507
20 38.015 0.00254 20 38.015 0.00255
30 38.010 0.00170 30 38.01 0.00169
40 38.020 0.00126 40 38.015 0.00127
Table 7 Pseudo-second order
kinetics for both tea leaves
and tea ber
Adsorbents
Tea leaves Tea bre
Time (mins) a–x x
K2
Time (mins) a–x x
K2
10 38.025 1.975 0.000129 10 38.020 1.980 0.000130
20 38.015 1.985 0.000064 20 38.015 1.985 0.000064
30 38.01 1.990 0.000044 30 38.01 1.990 0.000044
40 38.02 1.980 0.000033 40 38.015 1.985 0.000032

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3.5 Kinetic studies
The kinetic models used for this study are the pseudo-rst and second-order kinetic models which will provide a complete
description of the elementary diusion process that takes place. The experimental data used for the complete analysis of
the adsorption kinetics are presented in Table5, from which both rst and second-order plots (Figs.9, 10) were computed.
On application of the pseudo-rst-order kinetics for the biosorption of Ni by tea leaves and tea ber, kinetic data as
presented in Table6 were found from which correlation to rst-order kinetics was done by investigating the coecient
of regression R2 and the rst-order adsorption kinetic constant K1.
When applying the pseudo-second-order kinetics equation [39],
K2
=
1
at
×
x
a−x
, Table7 was obtained from which the
kinetic behavior was correlated to that of the second order by observing both the R2 and the second order constant K2.
For both rst order and second-order kinetics, a = 40mg/L.
Two kinetic models which are pseudo-rst order Kinetics (Fig.9) and second-order kinetics (Fig.10) were utilized on
the examined data obtained for the sorption of nickel onto tea leaves and tea ber. The correlation coecient (R2) was
0.982 and 0.880 for both tea ber and tea leaves for the rst order. On the other hand, the value of R2 for the pseudo-
second-order was 0.9913 and 0.993, for tea ber and tea leaves respectively, which both agrees well with the experimental
data. However, the higher value of R2 indicates that the pseudo-second-order kinetics model is suitable to describe the
kinetic adsorption process of nickel better than pseudo-rst-order kinetics. This suggests that during the adsorption
of nickel on tea leaves and tea ber, they were chemisorption due to the sharing of electrons between the adsorbent
surface and the adsorbate.
4 Conclusion
In this study, tea leaves and tea ber were investigated for sorption of nickel from a solution and were discovered to be
ecient for the elimination of Ni (II) from simulated industrial contaminated water. From the investigation, adsorption
constraints such as initial concentration, contact time, adsorbent dosage, temperature, and pH of the solution were
observed at dierent levels to have a great eect on the ecacy of the adsorbent. The result of the FTIR spectroscopy
showing active groups with N–H, O–H, and C–O bonds is the reason for the eective sorption capacity of the adsorbent
on the adsorbate. This is further explained by the Langmuir and Freundlich isotherm which was well-tted. The correla-
tion coecient R2 values of 0.990/0.985 for tea leaves/ber and 0.985/0.980 values for tea leaves/ber are obtained via
the Langmuir and Freundlich isotherms respectively. The higher values of the Langmuir isotherm make it the best for
studying the sorption of nickel by the adsorbent. The kinetics analyzed by pseudo-rst order and pseudo-second order
show that the best t was pseudo-second kinetics for both tea leaves and tea ber. Furthermore, the thermodynamic
condition of the system shows that it was endothermic and spontaneous with a high tendency of disorderliness at the
interface. The research, therefore, presents tea leaves and tea ber which are much available and low-cost by-products
of tea processing as eective biosorbents for the elimination of nickel from wastewater.
Author contributions E. Etim Conceptualization, and supervision S. Yakubu Writing, Data analysis, and editing manuscript A. Terhembe Experi-
mental and writing of the manuscript L.J. Moses Experimental and writing of manuscript
Funding Not applicable.
Data availability The authors declare that the data supporting the ndings of this study are available within the paper and should any raw
data les be needed in another format they are available from the corresponding author upon reasonable request.
Declarations
Ethics approval and consent to participate Not applicable.
Consent for publication Not applicable.
Competing interests The authors declare that there are no competing nancial interests or personal relationships that could have appeared
to inuence the work reported in this paper.

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Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adapta-
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