Isotherms and kinetic studies on adsorption of Hg(II) ions onto Ziziphus spina-christi L. from aqueous solutions

2.1.1 Adsorbent:

ZscL plant is widely spread in Libya and was obtained from Sebha (Libya). The roots were separated from the stems and leaves, and washed with distilled water several times until the filtrate was colorless, to remove the surface adhered particles and water soluble particles. The roots were dried in an electric oven at 105°C for 3 h, then finally ground using a mixer, and sieved to pass through a 50–150 μm. The roots were chosen because they contain the highest percentage of the cellulose content.

2.1.2 Reagents:

Mercuric acetate, EDTA, sodium hydroxide, nitric acid were all used as laboratory grade chemicals supplied by Merck Company, Germany.

2.3.1 Adsorption studies:

A known volume (100 ml) of a Hg(II) ions solution with a concentration in the range 100–1000 mg·l^-1 was placed in a 125 ml Erlenmeyer flask. An accurately weighed sample of ZscL adsorbent (0.05 g) with a particle size in the range 50–150 μm was then added to the solution. A series of such flasks was prepared, the pH values of the contents adjusted by the addition of 0.1 M HNO₃ or 0.1 M NaOH and then shaken at a constant speed of 150 rpm in a shaking water bath at 30°C for a known time length. At the end of the agitation time, the metal ion solutions were separated by filtration. Blank experiments were carried out simultaneously without the addition of the ZscL adsorbent. The extent of metal ion adsorption onto adsorbent was calculated mathematically by measuring the metal ion concentration before and after the adsorption through direct titration against the standard EDTA solution. The amount of Hg(II) ions adsorbed on ZscL at equilibrium, q_e (mg·g^-1) and percent removal of mercury were calculated according to the following relationships:

where C_o and C_e are the initial and final concentrations of Hg(II) ions, mg·l^-1, V is the volume of adsorbate (l) and W is the weight of ZscL adsorbent (g). All experiments were carried out in duplicate and the mean values of q_e were reported.

3.1.1 FTIR studies

The FTIR absorption spectra of ZscL and Hg(II) ions loaded ZscL were present in Figure 1A and B to confirm the presence of different functional groups in adsorbent.

Figure 1:

Fourier transform infrared spectroscopy (FTIR) of Ziziphus spina-christi L. (ZscL) (A) and Hg(II) ions-loaded ZscL(B).

The FTIR absorption spectra of ZscL biomass (Figure 1A) show a broadband at 3420 cm^-1 which indicates the presence of hydrogen-bonded -OH stretching from alcohol and phenols and is also dominated by -NH stretching. The bands at 2921 cm^-1 and 2851 cm^-1 in IR spectra of ZscL biomass may be due to the C-H stretching vibrations. The peak at 2358 cm^-1 represents stretching vibrations of -NH2⁺, -NH⁺ groups of ZscL biomass. The bands appearing at 1636.04 cm^-1 and 1373 cm^-1 are attributed to the formation of oxygen functional groups like highly conjugated C=O stretching in carboxylic groups. The peak which appeared at 1026.94 cm^-1 has been assigned to C-O stretching in ethers. The peak at 599 cm^-1 is caused by C-N-C.

The small shift was obtained in the absorbance peak in loaded Hg(II) ions onto ZscL biomass compared with that in ZscL (Figure 1A and B). The broadband observed at 3420 cm^-1 was shifted to 3422 cm^-1. The peaks at 2921 cm^-1 and 2851 cm^-1 were shifted to 2922 cm^-1 and 2852.63 cm^-1, respectively. The peaks observed at 1636 cm^-1 and1430 cm^-1 were shifted to 1635 cm^-1 and 1428 cm^-1, respectively. The peak at 1026 cm^-1 was changed to 1034 cm^-1. The peak at 599 cm^-1 was also shifted to 600 cm^-1.

It should also be noted that the FTIR results did not provide any quantitative analysis as well as the information about the level of affinity to metal of the functional groups presented in the adsorbents. They only presented the possibility of the coupling between the metal species and the functional group of the adsorbents.

3.1.2 SEM studies

The surface morphology of the ZscL and Hg(II) ions-loaded ZscL was analyzed by SEM by using the JXA-840A model which is shown in Figure 2. The SEM micrograph of ZscL reveals the irregular nature of biomass particles, which is rough and heterogeneous with a considerable amount of voids and many ups and downs.

Figure 2:

Scanning electron microscopy (SEM) of Ziziphus spina-christi L. (ZscL).

3.2.1 Effect of pH on Hg(II) ions adsorption

The pH of the aqueous solution is an important controlling parameter in the adsorption process [15]. Figure 3 shows the adsorption capacity of Hg(II) ions onto ZscL as a function of pH (2–6) at fixed adsorbent concentration, fixed agitation time and fixed adsorbate concentration at 30°C. It is clear from this figure that the adsorption capacity of Hg(II) ions onto ZscL increases by increasing the pH from 2 to 6. Under highly acidic conditions (pH 2) the adsorption of Hg(II) ions on ZscL is the lowest, because metal binding sites on the adsorbent were closely associated with H⁺ and restrict the approach of metal cations as a result of the repulsion forces. However, adsorption capacity increased with increasing the pH of solution, since the adsorbent surface could be exposed with negative charge with subsequent attraction with positive charge occurring onto the adsorbate. At a pH value higher than 6, the adsorption experiment could not be carried out due to precipitation of Hg(II) ions as Hg (OH)₂.

Figure 3:

Effect of pH on adsorption capacity of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

3.2.2 Effect of adsorbent concentration

The adsorbent concentration is another important parameter, which influences the extent of metal uptake from the solution. The effects of adsorbent concentration on both adsorption capacity and percent removal of mercury on ZscL are shown in Figure 4. It is clear from this figure that the percent removal of mercury increased from 28.2% to 91.2% by increasing the concentration of adsorbent from 0.3 g·l^-1 to 8 g·l^-1 and then remained at approximately the same level at higher adsorbent concentration. The increase in percent removal of Hg(II) ions with increasing adsorbent concentration in the first range could be attributed to the greater availability of the exchangeable sites of the adsorbent. The leveling of the percent removal at higher adsorbent concentrations could be attributed to the blocking of the available active sites on the adsorbent surface. By contrast, the adsorption capacity (q_e), or the amount of Hg(II) ions adsorbed per unit mass of adsorbent (mg·g^-1), decreased by increasing the concentration of adsorbent (Figure 4). The decrease in adsorption capacity with increase in adsorbent concentration is mainly due to overlapping of the adsorption sites as a result of overcrowding of the adsorbent particles and is also due to the competition among Hg(II) ions for the surface sites [16].

Figure 4:

Effect of adsorbent concentration on both adsorption capacity and % removal of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

3.2.3 Effect of contact time

Figure 5 shows the effect of agitation time at two different adsorbate concentrations (274 mg·l^-1 and 446 mg·l^-1) on the adsorption capacity of ZscL towards Hg(II) ions. This was achieved by varying the contact time from 5 min to 180 min in separate experimental runs. As expected, the amount of Hg(II) ions adsorbed into ZscL increases with time, and at some point, reaches a constant value beyond which no more is removed from solution. At this point, the amount of the Hg(II) ions desorbing from the adsorbent is in a state of dynamic equilibrium with the amount being absorbed by the adsorbent. The time required to attain this state of equilibrium is termed the equilibrium time, and the amount of ion adsorbed at the equilibrium time reflects the adsorption capacity of the adsorbent under those operating conditions.

Figure 5:

Effect of agitation time on adsorption capacity of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

The time necessary to achieve equilibrium increased with increasing adsorbate concentration, and was 60 min and 90 min for adsorbate concentrations of 274 mg·l^-1 and 446 mg·l^-1, respectively. This contact time, which is one of the main parameters for economical wastewater treatment plant operations, is quite small.

This can be explained by the fact that initially, the rate of ion uptake was higher because all sites on the adsorbent were vacant and Hg(II) ions concentration was high, but decrease of adsorption sites reduced the uptake rate.

3.2.3.1 Adsorption kinetics

Two main types of adsorption kinetic models, namely, reaction-based and diffusion-based models, were examined to fit the experimental data. These models also describe the solute uptake rate, and evidently, this rate controls the time of Hg(II) ions at the ZscL interface. Five kinetic models, namely, pseudo-first-order, pseudo-second-order, and interparticle diffusion, Bangham model and Elovich equation were utilized to describe the adsorption rate of Hg(II) ions onto ZscL and expressed as follows. The pseudo-first-order model [17] is represented as:

(3)log(qe-qt)=logqe-k⋅t2.303 (3)

The pseudo-second-order model [18] is represented as:

(4)tqt=1(k2⋅qe)2+tqe (4)

The intraparticle diffusion model [19] can be expressed by the following equation:

(5)qt=kP⋅t12+C (5)

where k is the pseudo-first-order rate constant (mg·g^-1·min^-1), k₂ is the pseudo-second-order rate constant (g·mg^-1·min^-1), k_P is the intraparticle diffusion rate constant (mg·g^-1·min^-1/2), C is the thickness of the boundary layer (mm) and q_e is the amount of adsorbate adsorbed at equilibrium (mg·g^-1), while q_t is the amount of adsorbate adsorbed at time t (mg·g^-1) before reaching equilibrium.

Bangham’s equation [20] was employed for applicability of adsorption of Hg(II) ions onto ZscL. Bangham’s equation was used to evaluate whether the adsorption is pore diffusion controlled:

(6)loglog(CoCo-q⋅m)=log(kom2.303V)+αlogt (6)

where, C_o is the initial concentration of the adsorbate (mg·l^-1), V is the volume of the adsorbate (ml), m is the weight of adsorbent (g·l^-1), q (mg·g^-1) is the amount of adsorbate retained at time t and α (<1) and k are the constants.

The Elovich model equation [21] is generally expressed as:

(7)qt=1βln(αβ)+1βln(t) (7)

where α (mg·g^-1.min^-1) is the initial adsorption rate and the parameter β (g·mg^-1·1) is related to the extent of surface coverage and activation energy for chemisorption.

The slopes and intercepts of plots of log (q_e-q_t) versus t for concentrations of 274 mg·l^-1 and 446 mg·l^-1 were used to determine the pseudo-first-order constant and equilibrium adsorption density, q_e. However, the experimental data deviated considerably from the theoretical data. A comparison of the results with the correlation coefficients is shown in Table 2. The correlation coefficients for the pseudo-first-order kinetic model obtained for concentrations of 274 mg·l^-1 and 446 mg·l^-1 were low (figure not shown). Also the theoretical q_e values found from the pseudo-first-order kinetic model did not give reasonable values. This suggests that this adsorption of Hg(II) ions onto ZscL is not acceptable for pseudo-first-order reaction.

Table 2:

Kinetic parameters for adsorption of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

Models	Parameters	Values
		(274 mg·l-1)	(446 mg·l-1)
Pseudo-first-order	k1	–	–
	R2	–	–
Pseudo-second-order	k1	8.85E-05	9.86E-05
	qe (exp.)	488.6427	371.7933
	qe (Calc.)	555.5556	416.6667
	R2	0.9836	0.9724
Bangham’s equation	Ko	27.65493762	12.45583
	α	0.459	0.4097
	R2	0.9305	0.9141
Intraparticle diffusion	kP	44.298	29.198
	C	40.143	34.439
	R2	0.8352	0.9084
Elovich equation	α	65.62835	47.13728
	β	0.009019	0.012742
	R2	0.9244	0.9417

The slopes and intercepts of plots t/q_t versus t were used to calculate the pseudo-second-order rate constants k₂ and q_e. The straight lines in plots of t/q versus t (Figure 6) show good agreement of experimental data with the pseudo-second-order kinetic model for concentrations of 274 mg·l^-1 and 446 mg·l^-1. Table 2 lists the computed results obtained from the pseudo-second-order kinetic model. The correlation coefficients for the pseudo-second-order kinetic model obtained were >0.997 for the two concentrations used. The calculated q_e values also agree very well with the experimental data. These indicate that the adsorption system studied belongs to the second order kinetic model.

Figure 6:

Pseudo-second- order reaction of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

The slopes and intercepts of plots q_t versus t^1/2 were used to calculate the intraparticle diffusion rate constant k_P. The straight lines in plots of q_t versus t^1/2 (Figure 7) show good agreement of experimental data with the intraparticle diffusion model for concentrations of 274 mg·l^-1 and 446 mg·l^-1.

Figure 7:

Intraparticle diffusion of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

Such plots may present a multilinearity [22, 23], indicating that two or more steps take place. The first portion is the external surface adsorption or instantaneous adsorption stage. The second portion is the gradual adsorption stage, where intraparticle diffusion is rate-controlled. The third portion is the final equilibrium stage where intraparticle diffusion starts to slow down due to extremely low adsorbate concentrations in the solution. Figure 8 shows a plot of the linearized form of the intraparticle diffusion model for concentrations of 274 mg·l^-1 and 446 mg·l^-1.

Figure 8:

Test of intraparticle diffusion of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

Further, such deviation of the straight line from the origin indicates that the pore diffusion is not the rate-controlling step [24]. The values of k_P (mg·g^-1·min^-1) obtained from the slope of the straight line (Figure 8) are listed in Table 2. The value of R² for the plot is listed also in Table 2. The values of intercept C (Table 2) give an idea about the boundary layer thickness, i.e. the larger the intercept is the greater the boundary layer effect [25]. This value indicates that the adsorption of Hg(II) ions onto ZscL is acceptable for the intraparticle diffusion mechanism.

The plot of loglog(CoCo-q⋅m) against log t for the concentrations of 274 mg·l^-1 and 446 mg·l^-1 is shown in Figure 9. The plots were found to be linear for each concentration with good correlation coefficient (>0.91), indicating that kinetics confirmed to Bangham’s equation and therefore the adsorption of Hg(II) ions onto ZscL was pore diffusion controlled.

If Hg(II) ions adsorption onto ZscL fits the Elovich model, a plot of q_t versus ln (t) should yield a linear relationship with a slope of (1/β) and an intercept of (1/β) ln (αβ).

Figure 9:

Bangham’s model of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

The straight lines in plots of q_t versus ln(t) (Figure 10) show good agreement of experimental data with the Elovich model for concentrations of 274 mg·l^-1 and 446 mg·l^-1 (R²>0.92). This suggests that this adsorption system is acceptable for this system.

Figure 10:

Elovich model of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

3.2.4 Effect of adsorbate concentration

3.2.4.1 Adsorption isotherm

Adsorption isotherms describe how adsorbates interact with adsorbents and are critical in optimizing the use of adsorbents. The amount of adsorbate per unit mass of adsorbent at equilibrium q_e (mg·g^-1) and the adsorbate equilibrium concentration C_e (mg·l^-1) allows plotting the adsorption isotherm q_e versus C_e (Figure 11) at 30°C.

Figure 11:

Equilibrium adsorption isotherm of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

Mathematical models can be used to describe and characterize the adsorption process. The most common isotherms for describing solid-liquid sorption systems are Langmuir, Freundlich, Temkin, Dubinin and Halsey (two parameter isotherms), Redlich-Peterson, Toth, Sips, Khan, Hill, Radke-Praunsnitz and Freudlich-Langmuir (three parameter isotherms), Baudo and Fritz-Schlunder (four parameter isotherms) and Fritz-Schlunder (five parameter isotherm). Therefore, in order to investigate the adsorption capacity of Pb (II) onto ZscL, the experimental data were fitted to these equilibrium models.

The equations of all adsorption isotherms [26–38] addressed in this paper are listed in Table 3.

Table 3:

Lists of adsorption isotherm models.

Isotherm	Equation (nonlinear form)	References
Two parameter models
Langmuir	qe=kL⋅Ce1+aL⋅Ce	[26]
Freundlich	qe=KF⋅Ce1/n	[27]
Tempkin	qe=RTbTln(ATCe)	[28]
Dubinin-Radushkevich	qe=qD⋅exp(-βD[RTln(1+1Ce)])2	[29]
Halsey	qe=exp(lnkH-lnCen)	[30]
Three parameter models
Redlich-Peterson	qe=A⋅Ce1+B⋅Ceg	[31]
Toth	qe=kt⋅Ce(at+Ce)1/t	[32]
Sips	qe=ks⋅CeBs1+as.CeBs	[33]
Khan	qe=qk⋅bK⋅Ce(1+bK⋅Ce)aK	[34]
Radke-Prausnitz	qe=αR⋅rR⋅CeβRαR+rR⋅CeβR-1	[35]
Langmuir-Freundlich	qe=qmLF⋅(kLF⋅Ce)mLF1+(kLF⋅Ce)mLF	[36]
Hill	qe=qsH⋅CenHkD+CenH	[37]
Four parameter models
Baudo	qe=qmo⋅bo⋅Ce(1+x+y)1+bo	[38]
Fritz-Schlunder	qe=A⋅Ceα1+B⋅Ceβ	[39]
Five parameter models
Fritz-Schlunder	qe=qmFS⋅k1⋅Cem11+k2⋅Cem2	[39]

The comparison of the adsorption capacity of various adsorbents reported in the literature for the removal of Hg(II) ions is shown in Table 4 [10–12, 39–42]. It was found from this table that the ZscL had a good affinity for removal of mercury compared with other adsorbents.

Table 4:

Comparison of sorption capacities of various adsorbents for Hg(II) ions.

Adsorbents	Adsorption capacity (mg·g-1)	References
Sargassum fusiforme	30.86	[10]
Peanut hull powder	30.3	[11]
Mercaptoethylamine/mercaptopropyltrimethoxysilane functionalized vermiculites	0.2863	[12]
Activated carbon	32.78	[40]
Used tire rubber	14.65	[41]
Eucalyptus globules bark carbon	4.014	[42]
Ziziphus spina-christi L.	37.45	This study

The comparison between the experimental data and the data obtained from two, three, four and five parameter models (Figures 12–15), as well as the constants and error analysis of two, three, four and five parameter models, are given in Tables 5–8.

Figure 12:

Comparison between experimental isotherm data and two parameter models at 30°C.

Figure 13:

Comparison between experimental isotherm data and three parameter models at 30°C.

Figure 14:

Comparison between experimental isotherm data and four parameter models at 30°C.

Figure 15:

Comparison between experimental isotherm data and five parameter model at 30°C.

Table 5:

Constants and error analysis of two parameter models for adsorption of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

Isotherm model	Parameter	Value	Error analysis	Value
Langmuir	aL	0.007608467	ARE	1.136205506
			APE%	11.36205506
	kL	1.013005268	EABS	20.68104572
			ERRSQ	97.34102653
	R2		Hybrid	72.92762818
		0.9965	MPSD	0.969684292
Freundlich	1/n	0.22757524	ARE	0.438461301
			APE%	4.384613012
	kF	25.31509108	EABS	9.630324832
			ERRSQ	15.80206444
	R2	0.996	Hybrid	9.317788213
			MPSD	0.94758344
Tempkin	AT	0.407057769	ARE	1.056465708
			APE%	10.56465708
	bT	1.260126749	EABS	25.65291447
			ERRSQ	78.73132123
	R2	0.999	Hybrid	39.32156667
			MPSD	0.96057493
Dubinin-Radushkevich	qD	118.8048645	ARE	1.508145023
			APE%	15.08145023
	BD	42.74353392	EABS	39.61254765
		ERRSQ	209.9222383
		Hybrid	98.49127318
	R2	0.9946	MPSD	0.968144195
Halsey	KH	4.308582443	ARE	0.403071173
			APE%	4.030711728
	n	-3.27420713	EABS	9.568057364
			ERRSQ	15.61382676
	R2	0.9737	Hybrid	8.52811346
			MPSD	0.94578976

APE%, Average percentage error; ARE, average relative error; EABS, sum of absolute errors; ERRSQ, sum of the squares of the errors; MPSD, Marquardt’s percent standard deviation.

Table 6:

Constants and error analysis of three parameter models for adsorption of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

Isotherm model	Parameter	Value	Error analysis	Value
Redlich-Peterson	A	6.721443411	ARE	0.424142513
	B	1.416827429	APE%	4.241425127
	g	0.707951099	EABS	10.01880004
	R2	0.9537	ERRSQ	17.12501055
			Hybrid	10.64441042
			MPSD	0.947819016
Toth	kt	4.471055668	ARE	0.41599484
	at	1.249244519	APE%	4.159948399
	1/t	0.699906243	EABS	9.824585469
	R2	0.9536	ERRSQ	16.32547708
			Hybrid	10.1714301
			MPSD	0.947425982
Sips	Ks	4.309176667	ARE	0.403045497
	as	1.68543E-05	APE%	4.03045497
	Bs	0.305416721	EABS	9.568416932
	R2	0.9737	ERRSQ	15.61852149
			Hybrid	9.748850282
			MPSD	0.94705523
Khan	qk	1.309370211	ARE	0.404550325
	ak	0.693938938	APE%	4.045503252
	bk	48.30069986	EABS	9.575453791
	R2	0.9540	ERRSQ	15.54737612
			Hybrid	9.717682068
			MPSD	0.947041897
Hill	qSH	284.113147	ARE	0.454908257
	KD	78.61939814	APE%	4.549082574
	nH	0.350262206	EABS	10.1240463
	R2	0.9539	ERRSQ	16.4264891
			Hybrid	10.80611143
			MPSD	0.948681139
Langmuir- Freundlich	qmLF	85.15316612	ARE	0.490318002
	KLF	0.000432405	APE%	4.903180018
	mLF	0.415815182	EABS	11.54622799
	R2	0.9551	ERRSQ	26.17255559
			Hybrid	16.09025087
			MPSD	0.951568747
Radke-Prausnitz	αR	2.837353545	ARE	0.450891362
	rR	5.576155423	APE%	4.508913616
	βR	0.269346464	EABS	10.6881857
	R2	0.9536	ERRSQ	20.68404154
			Hybrid	12.74479499
			MPSD	0.949396904

APE%, Average percentage error; ARE, average relative error; EABS, sum of absolute errors; ERRSQ, sum of the squares of the errors; MPSD, Marquardt’s percent standard deviation.

Table 7:

Constants and error analysis of four parameter models for adsorption of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

Isotherm model	Parameter	Value	Error analysis	Value
Baudo	Qm	3.757838658	ARE	0.436932857
	bo	214.4703477	APE%	4.369328571
	x	-0.452157	EABS	9.6287083
	y	-0.221515	ERRSQ	15.68243568
	R2	0.9607	Hybrid	12.27467799
			MPSD	0.950145775
Fritz-Schlunder	A	266.9631152	ARE	0.403033179
	α	0.07641195	APE%	4.03033179
	B	5.441786272	EABS	9.568243113
	β	0.17185084	ERRSQ	15.61824834
	R2	-0.26305732	Hybrid	11.37340852
		0.9608	MPSD	0.948515994

APE%, Average percentage error; ARE, average relative error; EABS, sum of absolute errors; ERRSQ, sum of the squares of the errors; MPSD, Marquardt’s percent standard deviation.

Table 8:

Constants and error analysis of five parameter models for adsorption of Hg(II) ions onto Ziziphus spina-christi L. (ZscL) at 30°C.

Isotherm model	Parameter	Value	Error analysis	Value
Fritz-Schlunder	Qmfs	2.808590608	ARE	0.474899072
	K1	2.808590608	APE%	4.748990721
	K2	0.208250942	EABS	10.56925328
	m1	1.016693475	ERRSQ	17.62434616
	m2	0.849113488	Hybrid	16.10980402
	R2	0.9944	MPSD	0.952380073

APE%, Average percentage error; ARE, average relative error; EABS, sum of absolute errors; ERRSQ, sum of the squares of the errors; MPSD, Marquardt’s percent standard deviation.