Kinetics and Adsorption Studies of Mercury and Lead by Ceria Nanoparticles Entrapped in Tamarind Powder.

In this study, novel adsorbent ceria nanoparticles (CeNPs) entrapped in tamarind powder (Tm@CeNPs) were efficiently utilized for the simultaneous adsorption of aqueous mercury [Hg(II)] and aqueous lead [Pb(II)]. Surface interactions between the adsorbent and heavy metal ions play an important role in the adsorption process, and the surface morphology can significantly improve the adsorption capacity of the adsorbent. The Langmuir adsorption capacity of Tm@CeNPs for Hg(II) and Pb(II) was found to be 200 and 142.85 mg/g, respectively. The surface area of utilized adsorbent was found to be very high, that is, 412 m2/g. The adsorption kinetics of Tm@CeNPs for both ions follow pseudo-second-order, and the adsorption process is also thermodynamically feasible. Column study favors multilayer adsorption of the heavy metal ion. The spectral analysis of the adsorbent revealed that hydroxyl, carboxylic, and ester groups, as well as CeNPs, are responsible for Hg(II) and Pb(II) adsorption. The cost-benefit analysis confirms the economic viability of the synthesized Tm@CeNPs composite for heavy metal removal. The adsorbent is best suited for Hg(II) adsorption as compared to Pb(II). This is a novel study on the utilization of tamarind leaf powder with CeNPs for heavy metal ion adsorption and its adsorption mechanism, which has not been reported to date.

Introduction

With the boom of industries particularly printing, pigment manufacturing, mining, and battery manufacturing industries, the amount of anthropogenic waste emission into the water has been increased too.[1] Presently, the world is facing a severe problem of heavy metal contamination in water, which affects both plants and animals.[2] Accumulation of these toxic heavy metal ions (i.e., Hg, Pb, Cd, Cr, and As) in the environment leads to critical health problems. Various industries such as lead battery, phosphate fertilizer, electronics, wood, and automobile industries release their effluents which contain Pb(II) into the water, and in the last few decades, Pb(II) is regarded as the major source of heavy metal contamination throughout the world. The most toxic heavy metal (i.e., mercury) has the potential to accumulate in the living tissue and to biomagnify in food chains. These heavy metal ions such as mercury [Hg(II)] and lead [Pb(II)] can cause various health issues, namely gastrointestinal diseases, liver problems, and nervous system damage. Therefore, the World Health Organization (WHO) has enforced the concentration limit for Pb(II) and Hg(II) in drinking water as below 0.01 and 0.001 ppm, respectively.[3,4] The removal of heavy metal ions has evoked considerable interest for decades. Many conventional methods for the removal of these toxic metals such as chemical precipitation,[5] electrolysis,[6] ion exchange,[7] and adsorption[8] have been reported. However, these techniques have limitations and are either not effective or economically viable for the heavy metal ion removal at low concentrations. Among them, adsorption and ion exchange is simple and cost-effective as compared to other techniques.[9,10]

The utilization of low-cost adsorbent materials and the effective reduction of heavy metal ion concentration of water are the major advantages of the adsorption technology. It is more advantageous over conventional methods in terms of minimization of biological and/or chemical sludge, high efficiency, low cost, the possibility of metal recovery, and the regeneration of adsorbent. Being cost-effective, the adsorption technology is widely used in heavy metal industries. Many adsorbents have been utilized for heavy metal ion removal from the water, that is, hazelnut shells,[11] peanut hull,[12] coconut husk,[13] modified cellulosic materials,[14] modified corncobs,[15] rice hulls,[16] nanosize metal oxides,[17,18] activated carbons,[19] biomaterials,[20] and polymers,[21] such as poly(2-aminothiazole)[22] and poly(1-amino-5-chloroanthraquinone) nanofibrils.[23] These materials have some drawbacks such as pore size distribution of adsorbent, and they are nonspecific.

Biosorption is a very advantageous technique for heavy metal removal from water because it reduces the biological and/or chemical sludge, does not require nutrients, and has high efficiency in detoxifying effluents. It also offers reversible and fast uptake of the heavy metals with biomass or micro-organisms. In the last 2 decades, lots of biological (low-cost) adsorbents have been examined for the removal of toxic metal ions from the aqueous medium. Biosorbents are by-products that are obtained from biomaterial production, and hence they are very cheap also.

The leaves of tamarind can be acquired at a low cost and considered as a waste product.[24] The leaves possess flavonoids, pectin, water-soluble carbohydrates, tartaric acid, and phenols. These leaves are also used in various food items, that is, soups, stews, salads, and curries in various countries.[25] Almost all chemical studies on tamarind leaves emphasize on their edibility and the presence of flavonoid compounds. Triterpenoids, lupanone, and lupeol are the other classes of compounds reported from the tamarind leaves, and some essential oils with benzyl benzoate and limonene were also reported as major compounds.[26] Leaves and fruit of the tamarind tree contain a large amount of natural tartaric acid ranging from 8 to 18%.[27−29]

Literature survey reveals that tamarind is a calcium-rich plant material; Ca(II) ion has high coordination numbers up to 24 and also has a large size and surface area, which is responsible for the high metal uptake efficiency. Additionally, tamarind leaves have several organic moieties with diverse functional groups which are responsible for the heavy metal uptake such as tartaric acid, flavonoid, triterpenoids, lupanone, lupeol, benzyl benzoate, limonene, diphenyl-ether, ketones, and so forth, and among them, tartaric acid is a major constituent. Therefore, −COOH group of tartaric acid plays a major role in the adsorption mechanism. Additionally, we have studied the chemical constituents of some other leaves of local plants, that is, Luffa cylindrica, Mentha arvensis, Syzygium cumini, and Azadirachta indica with a comparison to Tamarindus indica leaves (Figure S1 and Table S1). Hereafter, this study confirmed that T. indica has a larger number of constituent elements and high abundance of acidic functional groups (Table S1). Therefore, tamarind leaf powder could be successfully utilized as a low-cost alternative biomaterial to prepare tamarind-based adsorbent for the removal of aqueous metal toxicants.

The surface area plays a key role in the efficient adsorption; thus, with this objective, we have synthesized cerium oxide nanoparticles (CeNPs) entrapped in the tamarind leaf powder. To augment the surface area of tamarind powder, CeNPs were entrapped, and this enhances the surface area of tamarind powder from 115 to 412 m2/g, as we desired.

From the literature review, there is no elucidation of the mechanism for heavy metal adsorption on tamarind-based adsorbents, that is, bark, fruit shell, and so forth, as well as there is no report on tamarind leaves for adsorption of the heavy metal ion. This is the first time exploration of the adsorption mechanism using tamarind-based adsorbent with the incorporation of CeNPs. The beauty of the present adsorbent is its exceptionally high surface area, that is, 412 m2/g as well as selectivity toward Hg(II) and Pb(II) with good adsorption capacity, that is, 200 and 142.85 mg/g, respectively. The equilibrium adsorption, kinetic, and thermodynamic studies have been carried out for a better consideration of the adsorption process.

The pretreatment of tamarind leaves was carried out at very mild conditions to increase the metal uptake efficiency. We have used fresh tamarind leaves for the synthesis of adsorbent because in fresh tamarind leaves the abundance of functional groups moieties and chemical constituents are more than defoliated leaves.

Results and Discussion

Effect of Adsorbent Dose

In the adsorption process, the adsorbent dose plays an important role because it determines the % A for a given initial concentration of metal ion (Ci) solution. The adsorbent dose was varied from 0.1 to 0.5 g at fixed Hg(II) and Pb(II) concentration of 10 ppb. Figure a,b shows the effect of adsorbent dose on % A and distribution coefficient kd. The % A increase with an increase in adsorbent dose up to 0.3 g, after that it reaches an equilibrium. After 0.4 g adsorbent dose, the increase in % A was very slow or no increase. The % A for Hg(II) and Pb(II) was found to be 97 and 81% at 0.3 g adsorbent dose, on further increase in adsorbent dose, % A increases from 97 to 98.2% for Hg(II) and 81 to 92% for Pb(II). From these results, it is confirmed with increase in adsorbent dose, % A increases up to a certain level; this might be due to a corresponding increase in the binding active sites. However, after a certain level with a further increase in dose, % A did not increase; this is because the concentration of adsorbing ion may get reduced or may be saturated on the adsorbent sites. Additionally, large amount of adsorbent dose led to greater contact between the adsorbent particles on Tm@CeNPs surface and the adsorbing ions. Hence, the adsorption of Hg(II) ions on adsorption sites will be high. However, the dose of 0.4 g of adsorbent showed equilibrium for the adsorption of Hg(II) because there was no adsorption observed above 0.4 g of adsorbent dose. Hence, use of more doses will only lead to wastage of adsorbent. Furthermore, the distribution coefficient curves show the first decrease and then increase in the kd value which signifies the heterogeneous surface of the adsorbent.

Figure 1

Effect of adsorbent dose for adsorption of (a) Hg(II) and (b) Pb(II) on Tm@CeNPs.

From the overall study of adsorbent dose, the adsorption of Hg(II) occurred prior to that of Pb(II).

Effect of pH

The % A of both the metal ion increases up to pH 6 for Pb(II) and pH 7 for Hg(II), after which the % A decreases. The charge of the surface of cerium oxide became negative on increasing the pH because of the increasing deprotonation reaction, which strongly weakened the electrostatic attraction between the metal ion and the negatively charged surface. The surface charge of the adsorbent was positive pH < 7 and negative pH > 7.

Pb(II) and Pb(OH)+ were found as dominant species of Pb(II) at pH < 6.2 (Figure a). The CeNPs could react with Pb(II) and Pb(OH)+ to form CeO–PbOH and Ce2O3–PbO. Consequently, the Pb(II) adsorption at pH < 6.2 occurred because of the outer-sphere surface complexation reaction. The comparative dispersal of Pb(II) species in solution showed that Pb(II) mainly existed in the form of Pb(OH)+, Pb3(OH)42+, and Pb4(OH)44+ species at pH > 6.2.[30,31] Pb(II) is capable to bind cerium oxides and form inner-sphere complexes through the covalent bonds between Pb(II) and oxygen on Tm@CeNPs surfaces. Additionally, lead forms precipitate of Pb(OH)2 at high pH conditions. Therefore, it confirms that the ease of adsorption of aqueous Pb(II) and Hg(II) driven by the combined effect of both precipitation and inner-sphere surface complexation.

Figure 2

Effect of (a) pH, (b) co-metal ions, (c) regeneration cycle, and (d) temperature on adsorption of Hg(II) and Pb(II) on Tm@CeNPs.

Effect of Co-Metal Ions and Co-Anions

The interference of co-metal ions during the adsorption was also studied to confirm the selectivity of adsorbent toward Hg(II) and Pb(II). For this, an equal concentration of co-metal ions was added into the Hg(II) and Pb(II) solution during the adsorption process and analyzed for the residual concentration of different co-metal ions. The result found that the co-metal ions concentration remains the same or decreases negligibly. The order of co-metal ion effect on the adsorption of Hg(II) and Pb(II) was found to be Co(II) > Ni(II) > Ca(II) > Al(III) > Fe(III) > Mg(II) and Ni(II) > Co(II) > Al(III) > Fe(III) > Ca(II) > Mg(II) (Figure b), respectively.

Additionally, the effect of co-anions was also studied, in which the order of % A was found to be PO43– > SO42– > CO32– > NO3– > Cl– for Hg(II) and Pb(II) (Figure S2). The ratio of charge to the radius (z/r) of the anions had a close correlation with their affinity to the adsorbents, and the order of z/r is PO43– (3/0.238) > SO42– (2/0.230) > CO32– (2/0.186) > NO3– (1/0.179) > Cl– (1/0.181). PO43– and SO42– have higher z/r so they have a greater affinity toward adsorbent. Cl– and NO3– showed a lesser effect on adsorption, and this might be due to the low affinity of these ligands.

Regeneration

For any type of adsorption process, the reuse or regeneration of adsorbent is very essential from the economy point of view. Thus, regeneration of adsorbent has been done up to 10 cycles to inspect the reusability of the adsorbent and metal recovery efficiency.[32] The desorption of both metals Hg(II) and Pb(II) from the Tm@CeNPs surfaces was carried out in 1 mol/L HNO3 solution. The adsorption/desorption results of 10 cycles are given in Figure c, which shows that the adsorbent perpetuates more than 80% of its original, at the end of the fourth cycle, and adsorption capacity was slowly reduced after the fifth adsorption/desorption cycle. This may be probably due to the loss of some functional groups of the tamarind because of acid cleavage. The results show that the Tm@CeNPs are frequently reusable and can be utilized extensively in industrial activities.

Effect of Temperature

Increase or decrease in temperature during the adsorption process alters the adsorption capacity of Tm@CeNPs. The % A for Hg(II) and Pb(II) decreases with increase in temperature from 298 to 313 K as shown in Figure d, which shows that the % A decreases by increasing the temperature up to 313 K. This might be due to the increase in mobility of metal ions, with an increase in temperature. This increase in mobility decreases the chelation of metal ion with the adsorbent. Thus, on increasing temperature, the number of metal ions that can be adsorbed on Tm@CeNPs decreases. Additionally, the electrostatic interaction between metal ions and Tm@CeNPs becomes weak at higher temperatures, and the adsorption process is exothermic in nature.

Effect of Contact Time and Kinetic Studies

Figure a,b shows the effect of contact time on the adsorption efficiency of Hg(II) and Pb(II). For fixed mercury and lead ion concentrations, the experiments were carried out in 5–70 min range of the contact time. Increasing the contact time increased the removal efficiency as there was more time available to interact with adsorption sites on the surfaces of Tm@CeNPs for metal ions. Throughout the first 30 min, the removal efficiency was rapid of Hg(II) and Pb(II), then increased slowly, and within 50 min the adsorption equilibrium was achieved. This may be due to the abundance of vacant surface adsorption sites during the initial stage of adsorption. The remaining vacant surface adsorption sites were quite less in number with further increase in contact time. Furthermore, owing to the repulsive forces functioning between metal ions present in the solution and the already adsorbed metal ions on the solid and those they were not occupied.

Figure 3

Effect of contact time (a,b) and initial metal ion concentration on adsorption (c,d) of Hg(II) and Pb(II) on Tm@CeNPs, respectively.

In order to complete the understanding of the kinetics of the adsorption process, four different kinetic models were utilized, that is, pseudo-first-order (PFO) (Figure a), pseudo-second-order (PSO) (Figure b), intraparticle diffusion (IPD) (Figure c), and Elovich (Figure d). From the R2 analysis of kinetic models, the PSO model was best fitted for Hg(II) and Pb(II) adsorption on Tm@CeNPs. The order of R2 analysis for Hg(II) and Pb(II) adsorption was PSO > Elovich > IPD > PFO and PSO > IPD > Elovich > PFO, respectively. All three kinetic models well suited the adsorption data of metal ion adsorption except PFO. These models imply that the adsorption mechanism was controlled by particle diffusion and electrostatic interactions. The other kinetic constants of kinetic models are tabulated in Table .

Figure 4

Kinetic model for adsorption of Hg(II) and Pb(II) on Tm@CeNPs (a–d) PFO, PSO, IPD, and Elovich, respectively.

Table 1

Kinetic Constants of Hg(II) and Pb(II) Adsorption on Tm@CeNPs

PFO	Hg(II)	Pb(II)	PSO	Hg(II)	Pb(II)
k1	0.12	0.11	k2	0.038	0.03
qe	14.87	16.4	qe	15.62	8.2
R2	0.86	0.83	R2	0.99	0.99

Effect of Initial Metal Concentration (Ci) and Adsorption Equilibrium Isotherm

The effect of Ci on the % A and Qe of Tm@CeNPs for Hg(II) and Pb(II) was examined by varying the concentration of metal ions from 10 to 100 ppb. Figure c,d shows the result of Ci on adsorption of both metal ions by Tm@CeNPs. The % A for Hg(II) was subsequently decreased from 98.6 to 96%. For a fixed amount of adsorbent, the decrease in the number of Hg(II) ions in the solution on increased concentrations is due to several ions competing at the binding sites. The decrease in percent removal is due to the limited total available adsorption sites. While for Pb(II), % A increases slightly from 91 to 95%.

The equilibrium measurement was performed to examine the fundamental adsorption properties regarding adsorption capacity and interaction by using four adsorption models, that is, Freundlich (Figure a), Langmuir (Figure b), D–R (Figure c), and Temkin (Figure d). The explanation of these models was tabulated in Table . From the R2 analysis of these models, all four models satisfy the adsorption data but the Freundlich model was a best-fitted model for Hg(II) as well as the Pb(II) adsorption. The order of R2 value for Hg(II) is Freundlich ≅ Temkin ≅ D–R > Langmuir, and for Pb(II), the order is Freundlich ≅ Temkin > D–R > Langmuir. The maximum adsorption capacity calculated from the Langmuir constant was found to be 200 mg/g for Hg(II) and 142.85 mg/g for Pb(II). The results further indicate that the Hg(II) adsorption was more prone to adsorbent as compared to the Pb(II). The 1/n and KL value signifies the favorable adsorption between the metal ions and the adsorbent.

Figure 5

Adsorption isotherm model for adsorption of Hg(II) and Pb(II) on Tm@CeNPs (a–d) Freundlich, Langmuir, D–R, and Temkin, respectively.

Table 2

Adsorption Isotherms Constants for Hg(II) and Pb(II) Adsorption on Tm@CeNPs

Freundlich	Hg(II)	Pb(II)	Langmuir	Hg(II)	Pb(II)
n	1.49	1.42	1/Qm	0.005	0.007
1/n	0.67	0.73	Qm	200	142.85
KF	31.18	28.5	KL	0.17	0.30
R2	0.97	0.97	R2	0.95	0.93

The qm value obtained from the D–R model also agrees with Langmuir. The mean free energy calculated by D–R signifies that the physisorption process occurs during adsorption. The Temkin constants signify favorable binding between metal ions and adsorbents. The values for another constant of the adsorption models are tabulated in Table .

Studies for Selectivity

The synthesized Tm@CeNPs selectively adsorb Hg(II) and Pb(II) metal ions because these both ions have the more capability to form a covalent bond with the surface oxygen of Tm@CeNPs, and these two ions also get precipitated at high pH conditions. The study of the effect of interfering ions on adsorption also confirms the selectivity of adsorbent toward Hg(II) and Pb(II). The order of co-metal ion effect on the adsorption of Hg(II) and Pb(II) metal ions was Co(II) > Ni(II) > Ca(II) > Al(III) > Fe(III) > Mg(II) and Ni(II) > Co(II) > Al(III) > Fe(III) > Ca(II) > Mg(II) (Figure b), respectively.

The reason behind the selective adsorption of lead and mercury is its high effective nuclear charge in comparison to above all co-metal ions because the effective nuclear charge increases ongoing downward in periodic table due to the involvement of a greater number of d-orbitals. Hence, lead and mercury show stronger attraction than other co-metal ions.

Thermodynamic Parameters

The thermodynamic parameters, such as Gibbs free energy, enthalpy, and entropy, reveals the information about the adsorption process. These thermodynamic parameters have been calculated using the van’t Hoff plot (Figure ). In this study, the value of ΔG found negative for both metal ion adsorption processes, which means that the adsorption of Hg(II) and Pb(II) was a feasible process. The negative value of ΔH for both the metal ion adsorption reveals that the adsorption process was exothermic and the positive value of ΔS reveals spontaneity of the process (Table ).

Figure 6

Linear dependence of ln Ce on 1/T based on the adsorption thermodynamic.

Table 3

Thermodynamic Parameters for Hg(II) and Pb(II) Adsorption on Tm@CeNPs

	ΔH (kJ/mol)		ΔG (kJ/mol)		ΔS (J/mol)
metal ions	Co		298 K	303 K	308 K	298 K	303 K	308 K	R2
Hg(II)	10	–4.7	–4.2	–4.3	–4.4	1.67	1.40	1.15	0.83
	20	–3.3	–1.7	–1.7	–1.8	5.49	5.31	5.12	0.97
	30	–3.2	–0.9	–1.0	–1.02	7.69	7.50	7.50	0.99
Pb(II)	10	–2.3	–0.6	–0.6	–0.6	10.17	10.03	9.91	0.99
	20	–1.2	–1.7	–1.8	–1.8	10.27	10.20	10.13	0.99
	30	–1.75	4.3	–4.4	–4.4	20.44	20.3	20.25	0.84

Column Studies

To study the adsorption of Hg(II) and Pb(II) onto Tm@CeNPs, different linear and nonlinear column models were utilized which are Thomas (Figure a), Yoon–Nelson (Figure b), Yan (Figure c), and Clark (Figure d) models. KTh (mL/min/mg), KYN (mL/min/mg), and Ky are rate constants of Thomas, Yoon–Nelson, and Yan models, respectively. qy and qT are the maximum adsorption capacity (mg/g) and the maximum solid-phase concentration of the solute (mg/g) of adsorbent calculated by Yan and Thomas models, respectively. The breakthrough (sampling) time is represented as t (min), and the time required for 50% adsorbate breakthrough (min) is T. The exponent of the Freundlich isotherm is denoted as n, and A and r are the parameters of the kinetic equation. From the R2 analysis of column models, the Yoon–Nelson model was best fitted for both the metal ions. The order for Hg(II) is Yoon–Nelson > Clark > Thomas ≅ Yan and for Pb(II) is Yoon–Nelson > Clark > Thomas > Yan. Clark models further prove the multilayer type of adsorption. The column data match with the adsorption data, both follow the Freundlich model. The adsorption capacity obtained from the Thomas model was 6450 and 5540 mg/g for Hg(II) and Pb(II), respectively. The other required constants of column models are recorded in Table .

Figure 7

Column models for adsorption of Hg(II) and Pb(II) on Tm@CeNPs (a–d) Thomas, Yoon–Nelson, Yan, and Clark models.

Table 4

Column Parameters for Hg(II) and Pb(II) Adsorption

Thomas	Hg(II)	Pb(II)	Yan	Hg(II)	Pb(II)
KTh	4 × 10–5	4 × 10–5	Ky	0.0034	0.0021
qTh (mg/g)	6450	5540	qy (mg/g)	427.20	375.12
R2	0.91	0.91	R2	0.091	0.90

Cost Analysis

Cost is a key factor in the choice of adsorbent used in the industry. The Tm@CeNPs were synthesized by utilizing the analytical reagent grade chemicals. Presently, the cost of Tm@CeNPs adsorbent is around US$ 30.79 per kg in the lab. Additionally, in the future, industrial-grade raw materials will replace the analytical-grade chemicals for the large-scale synthesis of adsorbent, which will decrease the cost of adsorbent and increase their application substantially.

Field Study

The samples of industrial wastewater for the field study experiments were collected from an industrial area nearby Banasthali Vidyapith to check the suitability of Tm@CeNPs as the adsorbent (Table S2). The batch study was conducted for the adsorption of these toxic metals using different doses of Tm@CeNPs (Figure S3). For this study, 3 g of adsorbent was taken for 1000 mL of industrial wastewater into a beaker. Now, this solution was shaken at room temperature for 40 min, and then the concentration of both metal ions and the water quality parameters were measured, and data are recoded in Table S2. Results demonstrate that all the water quality parameters and the concentration of Hg(II) and Pb(II) from the treated water are below than the permissible limit, which is in good agreement with the adsorption data of batch experiment. Hence, the present study reveals that the removal of Hg(II) and Pb(II) ions from wastewater using Tm@CeNPs is also good for the field level.

Adsorption Mechanism

The adsorption performance of the Tm@CeNPs may coordinate with Pb(II) or Hg(II) through carboxyl (−COOH or −COOR), hydroxyl (−OH), and/or M−π interaction. The adsorption of heavy metal ion may be due to the electrostatic interaction between positively charged metal ion and the carboxyl and hydroxyl groups of tamarind. Additionally, CeNPs also facilitate heavy metal adsorption by electrostatic interactions and complexation. Figure shows the adsorption mechanism of metal ions. The comparison of adsorption capacity of Tm@CeNPs with previously synthesized adsorbents is given in Table .

Figure 8

Adsorption mechanism of metal ion adsorption on Tm@CeNPs.

Table 5

Comparison of Adsorption Capacity of Tm@CeNPs with Other Adsorbents

adsorbent		Qe (mg/g)
Hg(II)	activated carbon[33]	25.8
	dithiocarbamate@mesoporous SiO2[34]	40.1
	GO/Fe3O4[35]	18.2
	thioacetamide@SiO2[36]	17.5
	camel bone charcoal[37]	28.2
	SH–Fe3O4–NMPs[38]	125
	Tm@CeNPs (present study)	142.85
Pb(II)	Tm@CeNPs (present study)	200
	Fe3O4@humic acid[39]	92.4
	Fe3O4@SiO2–NH2[40]	128.21
	Fe3O4@NH2[41]	40.1
	N–Fe/OMC[42]	159.93

Characterization

Figure shows the Brunauer–Emmett–Teller (BET) N2 adsorption–desorption isotherms of type IV, and the corresponding pore size distributions are shown in Figure a of Tm@CeNPs. The hysteresis loop (type IV) indicates the presence of both micropores and mesopores in the composite. The distributions of the pore size, surface area, and the pore volume for CeNPs, activated tamarind fresh leaves (ATFL), and Tm@CeNPs are tabulated in Table . The BET surface area, pore size, and the pore volume of the Tm@CeNPs were found to be much higher as compared to the pure CeNPs and ATFL, which was 412 m2/g, 1.5 nm and 0.523 cm3/g, respectively, while the surface area of CeNPs and ATFL was 203 and 115 m2/g, respectively.

Figure 9

Hysteresis curve of type IV for ATFL powder, CeNPs, and Tm@CeNPs.

Table 6

BET Surface Area, Pore Volume, and Pore Size of Adsorbents

sample	BET surface area (m2/g)	total pore volume (cm3/g)	pore size (nm)
ATFL powder	115	0.423	2.8
CeNPs	203	0.503	2.4
Tm@CeNPs	412	0.523	1.5

Field Emission Scanning Electron Microscopy–Energy Dispersive X-ray Spectroscopy

The surface morphology and elemental composition of Tm@CeNPs, Tm@CeNPs–Hg(II), and Tm@CeNPs–Pb(II) were analyzed by field emission scanning electron microscopy (FESEM)–energy dispersive X-ray (EDX) analysis. Figure a–c shows the EDX spectra of adsorbent before and after Hg(II) and Pb(II) adsorption. The main elements shown in the EDX spectra of Tm@CeNPs are C, O, and Ce, while after adsorption of Hg(II) and Pb(II), these two element’s peak is also added in the spectra of Tm@CeNPs. The surface morphology of the tamarind dried leaves, CeNPs, Tm@CeNPs, Tm@CeNPs–Hg(II), and Tm@CeNPs–Pb(II) is shown in Figure d–h, respectively. The CeNPs show in Figure e are spherical, after the coating of tamarind powder the size of spherical-shaped CeNPs becomes broader as shown in Figure f. After Hg(II) and Pb(II) adsorption, the surface morphology of Tm@CeNPs changes, which signifies the adsorption of these heavy metal ions on the adsorbent. The elemental composition of these three adsorbents is also confirmed by mapping images, that is, Figure i–k for Tm@CeNPs, Tm@CeNPs–Hg(II), and Tm@CeNPs–Pb(II), respectively.

Figure 10

(a–c) EDX spectra of Tm@CeNPs, Tm@CeNPs–Hg(II), and Tm@CeNPs–Pb(II), respectively, (d–h) FESEM images of dried Tamarind leaves, CeNPs, Tm@CeNPs, Tm@CeNPs–Hg(II), and Tm@CeNPs–Pb(II), and (i–k) mapping images of Tm@CeNPs, Tm@CeNPs–Hg(II), and Tm@CeNPs–Pb(II), respectively.

X-ray Photoelectron Spectroscopy

The surface composition, elemental oxidation states, and binding energy of each element involved in the adsorbent before and after adsorption were analyzed by X-ray photoelectron spectroscopy (XPS) analysis. The main elements of Tm@CeNPs are C, O, and Ce and after adsorption, Hg(II) and Pb(II) are also found.

Oxygen deconvolution spectra O 1s for Tm@CeNPs show three important signals at 533.01, 531.22, and 529.54 eV (Figure a), which show the interaction of oxygen of chemisorbed water, hydroxyl group, and oxide ion. There is a change in the binding energy of these three signals after the surface modification by heavy metal ion interaction of Tm@CeNPs. A decrease in the signals by ∼0.28, 0.09, and 0.14 eV was observed, respectively (Figure b), which might be due to the new interaction between the adsorbent and metal ion.

Figure 11

XPS spectra of (a,b) oxygen before and after adsorption and (c,d) carbon before and after adsorption of Hg(II) and Pb(II) on Tm@CeNPs.

Deconvolution spectra of C 1s for Tm@CeNPs show three peaks at 285.26, 287.38, and 288.44 eV (Figure c) corresponding to C–C/C=C, C–O–C/C–OH, and C=O.[28] After loading metal ions (Figure d), C–C/C=C functional groups content decreased from 83.01 to 48.73%, while C–O–C/C–OH and C=O functional groups content significantly increased from 8.73 to 41.48% and 8.27 to 9.79% (Table ), respectively. The % composition of functional groups was calculated using casa XPS software. Hence, the increased percentage of oxygen-containing functional groups indicates the importance and their active role in the interaction with Hg(II) and Pb(II). The removal mechanism can be described as a surface complexation model in which the oxygen-containing surface functional groups (i.e. C–O and C=O) of Tm@CeNPs are responsible for the removal of Hg(II) and Pb(II) ions.

Table 7

Binding Energy of Each Element before and after Adsorption on Tm@CeNPs

	peak	BE (eV)	fwhm (eV)	percentage (%)
before adsorption	O2–	529.54	2.0	56.57
	M–OH	531.22	1.27	22.86
	H2O	533.01	2.17	20.57
	C–C/C=C	285.26	3.2	83.01
	C=O	288.44	2.4	8.27
	C–O–C/C–OH	287.38	1.5	8.73
	Ce (3d3/2) & (3d5/2)	885.01, 891.4	2.96, 2.96	28.03, 15.70
		900.91, 903.52	4.92, 2.96	46.52, 9.78
after adsorption	O2–	529.4	1.83	50.55
	M–OH	531.13	0.97	39.31
	H2O	532.73	1.07	10.13
	C–C/C=C	284.06	1.08	48.73
	C=O	285.74	1.85	9.79
	C–O–C/C–OH	287.02	2.21	41.48
	Ce (3d3/2) & (3d5/2)	882.4, 889.1	5, 3.2	24.27, 18.50
		898.2, 900.7	3, 2.4	24.22, 9.38
	Hg(II) (4f5/2) & (4f7/2)	103.76	1.09842	42.02
		99.74	1.20359	57.98
	Pb(II) (4f5/2) & (4f7/2)	141.08	2.70626	49.27
		136.31	2.3946	39.39

Deconvolution spectra of Ce 3d for Tm@CeNPs before and after Hg(II) and Pb(II) adsorption show four peaks, which are assigned for 3d3/2 and 3d5/2. The peaks at 885.01 and 891.4 eV are assigned to 3d3/2 and 900.91 and 903.52 eV are assigned to 3d5/2 (Figure a,b). Results show that the binding energy decreases, which might be due to the interaction of CeO2 with Hg(II) or Pb(II).

Figure 12

XPS spectra (a,b) of cerium before and after adsorption of Hg(II) and Pb(II) on Tm@CeNPs; (c,d) XPS spectra of Hg(II) and Pb(II) on adsorption of Tm@CeNPs.

Moreover, the high-resolution 4f spectra of Hg(II) exhibit two signals at 103.76 and 99.74 eV (Figure c), which elucidate the characteristic peaks of Hg(II) 4f5/2 and 4f7/2, respectively. The above results show that the mercury is present in its oxidized state (+2) instead of its zero-valence state [Hg(0)].

Two peaks were appeared at 141.08 and 136.31 eV for Pb(II) 4f5/2 and 4f7/2, respectively with an energy separation of 4.9 eV, which can be attributed to PbO. Results suggest that Pb(II) is bonded with the oxygen group on the surface of Tm@CeNPs (Figure d). Table shows the binding energy, % content, and full width at half maximum (fwhm) values of each element before and after adsorption of heavy metal ions.

X-ray Diffraction

Figure a shows the X-ray diffraction (XRD) patterns of Tm powder, CeNPs, and Tm@CeNPs. The XRD peaks of tamarind powder appear at 25°61′, 29°99′, 31°87′, 35°06′, 36°11′, 49°35′, 59°95′, and 62°69′. The XRD pattern of CeNPs matches with the JCPDS card no. 00-031-0325. The Bragg planes in the CeNPs XRD pattern (020), (110), (112), (200), and (220) confirm the synthesis of nanoparticles. After the formation of the adsorbent, the peaks of CeNPs remain intact in the Tm@CeNPs. The d-spacing value of the crystal lattice is 2.124; the crystal structure is hexagonal and centrosymmetric. The average crystal size of particles calculated by d-spacing value is 15.24 nm. Lattice strain is 0.0063, and fwhm of the crystal lattice is 0.5642.

Figure 13

(a) XRD pattern of Tamarind powder, CeNPs, and Tm@CeNPs and (b) Fourier-transform infrared spectroscopy (FTIR) spectra of CeNPs, Tamarind powder, Tm@CeNPs, Tm@CeNPs–Hg(II), and Tm@CeNPs–Pb(II).

Fourier-Transform Infrared Spectroscopy

The FTIR spectra of CeNPs show the characteristic peaks of nanoparticles. The characteristic bands of CeNPs were found in two regions, that is, 1100–1700 cm–1 and 3000–3800 cm–1. In addition to these, there is a band at 770 cm–1 assigned to the Ce–O bond. The characteristic band at 3364 cm–1 is assigned to −OH stretching vibrations of physical adsorbed water or surface hydroxyl groups. The peaks seen at 1390, 1686, and 1046 cm–1 represent bicarbonate type species and monodentate carbonate species with O–C–O stretching.

In the FTIR spectra of tamarind dried leaves, the characteristic peaks are found at 664, 1038, 1224, 1384, 1692, 1745, 2912, and 3308 cm–1. The peak around 1384 cm–1 corresponds to the presence of N–H stretching of the amide group. The peak at 1692 cm–1 resembles the carbonyl of carboxylic group peak, and the 1745 cm–1 peak resembles carbonyl (C=O) stretching of the ester group. Owing to the existence of aromatic ortho disubstituted heterocyclic molecules, an assignment occurs at 664 cm–1 for ATFL indicating a possibility of ring cleavage after the coating of CeO2. The peak around 2912 cm–1 is the characteristic peak of C–H stretching and 3308 cm–1 for surface hydroxyl group (Figure b).

In FTIR spectra of Tm@CeNPs, all characteristic peaks of CeNPs and Tm powder remain intact with a minor change in the frequency, which might be due to the interaction between the CeNPs and Tm powder. The peak at 3308 cm–1 of O–H stretching of −OH group becomes broad because of H-bonding (3300–3750 cm–1) after the formation of Tm@CeNPs. In the FTIR spectra of Tm@CeNPs after adsorption of Hg(II) and Pb(II) metal ions, the decrease in the intensity and wavenumber of peaks indicates the formation of metal–oxygen bonds with their corresponding functional groups (Figure b). From the FTIR analysis, the stretching peak of a hydroxyl group (3451 cm–1) gets broadened because of H-bonding (3400–3800 cm–1) (Figure b).

Thermogravimetric Analysis–Differential Thermal Analysis

The compositional analysis of the Tm@CeNPs was carried out using thermogravimetric analysis (TGA). The TGA data of CeNPs, Tm powder, and Tm@CeNPs showed no significant weight loss above 150 °C. However, the TGA data for the Tm@CeNPs, CeNPs, and tamarind powder (Figure a) showed two independent weight loss steps, the first step at 0–200 °C and the second step above 200 °C. The second weight loss is highest in tamarind powder, that is, 42.31%, and lowest in Tm@CeNPs, that is, 15.92%, which indicates that after coating of CeNPs with Tm powder, the thermodynamic stability of the synthesized adsorbent increases. The same result is shown by differential thermal analysis (DTA) studies (Figure b).

Figure 14

(a,b) TGA and DTA thermograms of CeNPs, Tm powder, and Tm@CeNPs, respectively.

Conclusions

This research studies the adsorption of Hg(II) and Pb(II) by tamarind leaf composite. The synthesized adsorbent was successfully utilized for Hg(II) and Pb(II) adsorption with maximum adsorption capacity of 200 and 142.85 mg/g, respectively. The surface area of Tm@CeNPs was found to be very high, that is, 412 m2/g. The adsorption process follows a multilayer type of adsorption and PSO kinetics for both the metal ion adsorption processes. The column study reveals that the multilayer type of adsorption and Yoon–Nelson model fitted best. The mean free energy required for Hg(II) adsorption is lower than Pb(II), which means that the Hg(II) is easily adsorbed on the Tm@CeNPs. The use of tamarind leaves opens a new class of adsorbent for the heavy metal ion removal, as the surface area of dried tamarind leaves is very high. Thermodynamic studies reveal that the adsorption process of heavy metal ion was feasible, spontaneous, and exothermic. Increase in temperature does not favor the adsorption process.

Materials

All the analytical grade chemicals were commercially available and used without further purification.

Synthesis

Synthesis of Ceria Nanoadsorbent

In order to synthesize CeNPs, 0.05 M Ce salt solution was prepared in 100 mL of absolute ethanol. Then, NaOH/ethanol solution was added dropwise under vigorous stirring until the color of the solution changed from orange to yellow and finally to a dark brown colloidal sol. The colloidal sol was centrifuged, and the precipitate was collected and repeatedly washed with absolute ethanol and deionized water to remove the impurities.

Pretreatment of TFL

The fresh tamarind leaves were initially washed with 0.01 N HCl, followed by 0.01 N NaOH. Then, the TFL was washed thoroughly using distilled water and dried by exposing it to sunlight. The dried TFL was sieved to obtain particles 600 μm in size.

Preparation of ATFL

About 30 g of TFL of 600 μm was soaked in 600 mL of 1% CaCl2 solution, for 24 h. Then, the soaked TFL was washed with distilled water and dried at 110 ± 0.5 °C in an air oven for 2 h.

Preparation of CeO2-Entrapped TFL

About 20 g of CeO2 NPs was dissolved in 50 mL of distilled water in a beaker and placed on a magnetic stirrer at 90 °C for 30 min. To this, 24 g of ATFL was added and the suspension was mixed gently and heated in a water bath for 10 min. Then, this solution was transferred to a Teflon lined stainless steel autoclave (200 mL) and put in the oven at 250 °C for 2 h. The solid was cooled after the completion of the reaction and washed with 0.05 M perchloric acid and distilled water until the run-off was clear. The precipitate was dried and calcined in the muffle furnace up to 513 K for 2 h. Figure shows the binding mechanism of CeNPs with tamarind powder. The binding of CeNPs with tamarind via H-bonding, electrostatic interactions, and complexation between −OH group of CeNPs and functional groups of tamarind, that is, carbonyl of −COOH, −COOR, and −OH.

Figure 15

Binding mechanism of CeNPs with tamarind powder.

Batch Experiment

In order to understand the adsorption process of Tm@CeNPs for Hg(II) and Pb(II) heavy metal ions, different parameters were studied in fixed range, that is, adsorbent dose (0.1–0.5 g), pH (3–10), initial concentration of heavy metal ions (10–100 ppb), contact time (5–70 min), adsorption/desorption studies, and interfering metal ion (Ca(II), Al(III), Mg(II), Fe(II), Co(II), Ni(II)) at room temperature. After analysis of these parameters, the remaining metal ion concentration was analyzed by using atomic absorption spectroscopy, and the % adsorption (% A) and adsorption capacity (qe or q) are calculated by using following eqs –3.where Ci, Ce, C, m, V, qe, and q are initial metal concentration (ppb), equilibrium metal concentration (ppb) at time t, the mass of adsorbent (g), the volume of solution (L), adsorption capacity at Ce, and adsorption capacity at C, respectively.

Kinetic Studies

Kinetic-based models describe the mechanism of adsorption and also explain the order of reaction. The adsorption kinetics explains the rate of adsorbate uptake and the contact time of adsorbate on the adsorbent interface. The heavy metal ion adsorption process is significantly time-dependent and pretentious by the chemical and physical properties of adsorbent. Thus, to explore the adsorption mechanism, four different kinetic models—PFO, PSO, IPD, and Elovich[43−47] were used to study the different kinetic parameters. Linear eqs –7 of the following models are

Adsorption Equilibrium Studies

Adsorption models explain the qe, adsorption mechanism pathways, mode of adsorbate to interact with an adsorbent, and effectual design of adsorption mechanism. There are several adsorption models (i.e., Langmuir, Freundlich, Temkin, and D–R isotherm models)[48−51] from which most widely utilized models were used to study the adsorption process, given below as eqs –13

Thermodynamic Studies

The temperature plays a significant role in order to understand the thermodynamics of the adsorption process, that is, in-depth understanding of energetic changes during the adsorption process. Thermodynamic variables, that is, ΔG (change in Gibbs free energy), ΔH (change in enthalpy), and ΔS (change in entropy),[52,53] were calculated by using equation –16.

Equilibrium metal ion concentration:

The ΔS is calculated by Gibbs–Helmholtz equation,

Column Study

The column studies were done by utilizing fixed adsorbent dose, that is, 2 g; the length and diameter of the column was 30.0 and 1.0 cm, respectively. The used stock metal ion solution was 1000 ppb for Hg(II) and Pb(II), and the flow rate was maintained at 5.0 mL/min. The outlet solution was analyzed after each 10 min for the residual metal ion concentration. In order to study the complete column study, four different models were utilized, that is, Thomas, Yoon–Nelson, Yan, and Clark.[54−57] The nonlinear and linear equations of these models are tabulated in Table .

Table 8

Linear and Nonlinear Equations of the Column Model

model	non-linear form	linear equations
Thomas model
Yoon–Nelson
Yan et al.
Clark model

Physiochemical Characterization

The surface properties of the adsorbents were analyzed by using N2 adsorption–desorption isotherms with the use of the BET method. In order to analyze the sample for BET analysis, about 20 mg of the sample was degassed for 3 h at 300 °C, and then it was used for analysis. The morphology and elemental composition of the adsorbent before and after adsorption were analyzed by FESEM attached with EDX. Additionally, the surface chemistry and chemical state of the element before and after adsorption were determined by XPS. XRD patterns of the samples were collected using a model XRD instrument. Cu Kα radiation was used in this study. The functionalization of adsorbents was analyzed by FTIR spectra in the range 4000–400 cm–1. TGA and DTA analyses were carried out to the study of characteristic physical changes in adsorbents.