SEM morphology
The SEM (Scanning Electron Microscopy) images of Lantana adsorbent are showed in Fig. 1 which evaluate the surface structure and morphology of adsorbent at Х5000 magnification. It can be observed that the raw and chromium loaded Lantana has more irregular and porous structure which provides binding sites for metal adsorption. The pores present on the adsorbent surface area provides good surface area for trapping of metal ions [18]
FTIR study
The FTIR spectra of Lantana adsorbent before and after adsorption are shown in Fig. 2. The spectra reveal the presence of major functional in raw and chromium treated adsorbent. The shifting of wavenumbers 3289.6, 2941.9, 2884.8, 2838.8, 1737.4, 1428.7, 1328.2 and 1057.3 slightly shifted towards 3339.1, 2972.7, 1737.0, 1373.4, 1161.1 and 1057.5 cm− 1 clearly showed the reactivity of functional groups in metal adsorption. Taha et al. (2018) also observed the similar results for Hg(II) removal by raw and chemically activated almond shell.
Effect of pH
The pH of solution influences the surface charge of adsorbent and also affect the functional groups on the surface of adsorbent [17]. The ionization of adsorbent and solubility of metals are dependent on pH. The effect of solution pH was studied by varying pH 1–8 with adsorbent dose 2 g/L, initial Cr(VI) metal ion concentration and contact time of 120 min as shown in Fig. 3. The maximum percentage removal of Cr(VI) was found at pH 2 by LS adsorbent. At lower pH, the adsorbent surface exhibit H+ ions and this implies the binding of the negativity charged HCrO4− ions [19]. So, the maximum chromium uptake occurred at pH 2.
Effect of dose
Adsorbent doses is important parameter in adsorption process, it represent the efficiency of adsorbent for particular metal ions. The effect of LS adsorbent doses on Cr(VI) removal is given in Fig. 4. The Cr(VI) metal removal increase as increased doses from 0.25–2.5 g/L. The removal trend becomes constant almost after 2 g/L dose. This may be due to availability of active sites on adsorbent surface for Cr(VI) uptake. The equilibrium condition is obtained after saturation of active sites and adsorption rate become constant [22]. Also, the higher adsorbent doses causes particle aggregation, which results lower surface area and increase in diffusion path length, and hence reduces the metal removal efficiency of adsorbent per unit mass of adsorbent [20].
Effect of contact time
In batch system, contact time of adsorbent conducted to examine the Cr(VI) uptake by using different contact time (5,10,15,30,45,60,90,120 min). As it is shown in Fig. 5, the metal removal was rapid upto 60 min contact times. After 60 min contact time, the equilibrium condition achieved which results into no further metal removal took place. At initial contact time, metal removal was rapid could be due to high affinity sites between adsorbent and adsorbate site [Cr (VI) metal ions]. The mass transfer phenomenon between surface adsorbent and metal molecule from aqueous solution decreases after equilibrium conditions occurred [21].
Effect of initial metal concentration
The experiments were carried out at adsorbent dose 2 g/L, pH 2, contact time (0-120 min) and at different initial metal concentrations (0.5, 1.0, 1.5 and 2 mg/L). The results are depicted in Fig. 6. It was observed from Fig. 6 that the metal removal percent was increases as increase in initial metal concentration ions upto 2 mg/L. After 2 mg/L metal ion concentration, metal removal percentage decreased due to less availability of active sites on adsorbent or saturation of active sites [22]. This may also be due to more composition for surface adsorption occurred at higher metal concentrations [23].
Effect of particle size
The effect of particle size on adsorption of Cr(VI) are shown in Fig. 7 It can find out that as we decrease particle size as metal removal increases. The rate of adsorption was proportional to the surface area of LS adsorbent [24]. Similar trends of results also investigated by some researchers [25].
Adsorption isotherm
Equilibrium studies were decribed by adsorption isotherm namely Langmuir and Freundlich isotherms. Adsorption isotherm model interpret the maximum metal removal efficiency of adsorption in equilibrium conditions. The Langmuir and Freundlich isotherm models have been successfully tested for many adsorption process by many workers [26–30]. The linear and non linear form Langmuir and Freundlich isotherm were given in Table 1.
Langmuir model:
Langmuir adsorption model describes the monolayer adsorption of a homogenous adsorbent surface by metal ions [31]. The linear form of Langmuir isotherm equation is represented by following equation [32]:
$$\frac{1}{{q}_{e}}=\frac{1}{{Q}^{o}}+\frac{1}{b{Q}^{o}{C}_{e}}$$
Where qe is the amount adsorbed at equilibrium time (mg/g), Ce is the equilibrium concentration of the adsorbate ions (mg/l), and Qo and b are Langmuir constants related to maximum adsorption capacity (monolayer capacity) and energy of adsorption, respectively.
The equilibrium data were fitted with metal uptakes by adsorbents (Table 1). A plot of 1/Qe vs 1/Ce produced straight line (Fig. 8 ). The separation factor or dimensionless parameter (RL) was assessed to interpret adsorption condition from the Langmuir isotherm22. The separation factor (RL) was defined by the following expression:
$${R}_{L}=\frac{1}{(1+b{C}_{i})}$$
Where, Ci is the initial metal concentration of metal ion (mg/L) and b is the Langmuir adsorption isotherm constant (L/mg). For a favourable adsorption process the RL values lies between 0 and 1 (Table 2) [33].
Fruendlich isotherm
The Freundlich equation has the general form [34]:
The Freundlich equation is basically empirical but is often useful as means for data description. Data are usually fitted to the logarithmic form of the equation:
$$\text{log}{q}_{e}=\text{log}{K}_{F}+\frac{1}{n}log{C}_{e}$$
Where qe is the amount adsorbed (mg/g), Ce is the equilibrium concentrations of the adsorbate ions (mg/l), and KF and n are Freundlich constants related to adsorption capacity and adsorption intensity respectively. The values of R2 (> 0.84) show that the experimental data fit to the Freundlich isotherm (Fig. 9).
It was depicted from correlation coefficients, Langmuir isotherm fitted well than Freundlich isotherm (Table 1).
Table 1
Adsorption isotherm data for adsorption of Cv(VI) by LS
|
|
Langmuir constants
|
Freundlich constants
|
Metal
|
adsorbent
|
qe(mg/g)
|
b(L/mg)
|
R2
|
Kf
|
n
|
R2
|
Cr(V)
|
Lantana camara (LS)
|
0.58
|
0.33
|
0.96
|
2.60
|
0.61
|
0.95
|
Table 2
RL values for adsorption of Cu(II) by PN and LS adsorbent
Metal
|
Ci (mg/L)
|
RL
|
LS
|
Cr(VI)
|
0.5
|
0.8560
|
1.0
|
0.7482
|
2.0
|
0.5978
|
4.0
|
0.4263
|
Adsorption Kinetics
Adsorption kinetics involve in the process of adsorption of metals in aqueous medium, the pseudo first order and pseudo second order kinetic models were applied with experimental data.
Pseudo first order equation
A simple kinetic model that applicable in the process of adsorption is the pseudo-first order equations
Log(qe-qt) = logqe-\(\frac{k1}{2.303}t\) (linear form)
qt=qe(1-e1 − kt) (non-linear form)
where k1(h− 1) is the first order rate constant of adsorption, qe is the amount adsorbed at equilibrium and qt is the amount of metal adsorbed at time t.
Table 3
Kinetic parameter for adsorption of Cr(VI) onto LS at different concentration
|
|
|
Pseudo first order constants
|
Pseudo second order constants
|
|
Metal
|
Adsorbent
|
Conc.
(mg/L)
|
Equation
|
K1( /min)
|
Qe cal
(mg/g)
|
R2
|
Equation
|
K2
|
qe (cal)
|
qe (exp)
|
R2
|
Cr(VI)
|
LS
|
0.5
|
Y = 0.0267–1.1713
|
0.06149
|
0.187413
|
0.97
|
Y = 8.8297 + 65.851
|
1.18393953
|
0.1132541
|
0.4
|
0.99
|
Cr(VI)
|
LS
|
1.0
|
Y = 0.0437–0.6974
|
0.100641
|
0.42845
|
0.96
|
Y = 5.1541 + 122.07
|
0.21761896
|
0.1940203
|
0.87
|
0.84
|
Cr(VI)
|
LS
|
2.0
|
Y = 0.0179-0.12
|
0.041224
|
0.528932
|
0.96
|
Y = 0.7756 + 19.013
|
0.03163916
|
1.2893244
|
1.089
|
0.94
|
Cr(VI)
|
LS
|
4.0
|
Y = 0.0215 + 0.0925
|
0.001989
|
0.957414
|
0.99
|
Y = 0.5334 + 12.828
|
0.02217926
|
1.8747657
|
2.7
|
0.99
|
Pseudo-second order equation
The pseudo-second order equation is expressed as
$$\frac{t}{\text{q}\text{t}}=\frac{1}{\text{k}\text{2}\text{q}\text{e}\text{2}}+\frac{t}{\text{q}\text{e}}$$
Where qe and qt are amount of metal ion adsorbed (mg/g) at equilibrium and time at time‘t’ respectively. The product \(\text{k}\text{2}\text{q}\text{e}\text{2}\) ,the initial sorption rate, represented as h=\(\text{k}\text{2}\text{q}\text{e}\text{2}\).
The plot qt vs t at different metal concentration gave a linear relationship from which qe and K1 values were calculated. The rate constants and correlation coefficients were represented in Table 3. K2 can be calculated from the slope of the linear plot between t/qt vs t. There was only little difference between qeexp and qecal which confirms the suitability of pseudo second order kinetic model with the experiment data. By comparing the figures (Figs. 10 and 11) of pseudo first order and pseudo second order, it can concluded that pseudo second order favour the experimental data better than pseudo first order.
Application to real textile effluent samples
It is necessary to study adsorption process in real effluents for industrial application. For this, real textile effluent collected from Panipat city, Haryana, India. The metal concentrations were optimised before and after adsorption process (Table 4). The optimum conditions for Cu(II) metal uptakes were pH of 7.28, contact time of 120 min, dose 1g/L and at 25oC. The removal efficiency was calculated as:
Removal efficiency % = (Ci-Cf)*100/Ci
Where, Ci and Cf are initial and final metal concentrations.
Table 4
The removal efficiency of Cu(II) from real textile effluents
Heavy metal
|
Lantana shoots
|
Before treatment
|
After treatment
|
Percent removal
|
Cr (VI)(mg/L)
|
0.176 ± 0.009
|
0.042 ± 0.006
|
76.13
|
Results are depicted in Table 4 which shows that the adsorption efficiency was found 76.13% by LS. The above results indicated that both adsorbents are efficient in the metal uptake from real textile effluents.
Comparison of metal removal efficiency
A comparison of metal removal efficiencies by low cost adsorbents for Cu(II) uptake was given in Table 5. The metal removal efficiency of Lantana shoot was found comparable with other adsorbents.
Table 5 Comparison of metal removal efficiencies by unmodified low cost adsorbents for chromium uptake
Adsorbent
|
Metal
|
Adsorption capacity (mg/g)
|
Reference
|
Peels of pea pod
|
Cr(VI)
|
4.33
|
[35]
|
Soya cake
|
Cr(VI)
|
0.288
|
[37]
|
Sugarcane bagasse
|
Cr(VI)
|
5.12
|
[38]
|
Saw dust
|
Cr(VI)
|
4.56
|
[38]
|
Modified activated carbon
|
Cr(VI)
|
1.82
|
[39]
|
Modified oak sawdust
|
Cr(VI)
|
1.7
|
[40]
|
Rice bran
|
Cr(VI)
|
0.07
|
[41]
|
Maize corn cob
|
Cr(VI)
|
0.28
|
[42]
|
Lantana shoot (LS)
|
Cr(VI)
|
0.58
|
Present study
|