Surface depiction and chemical structure of the sorbents nWTR. The nWTR particles are largely spherical in the 45–96 nm size range 14. Elemental analysis(SEM-EDX) revealed that nWTR contains Fe, Si, and Al in percentages of 49.57, 21.77, and 6.3% of the total elements respectively. Meanwhile, X-ray diffraction analysis affirmed that, amorphous iron, aluminum (hydr)oxides, and silicon oxide dominate nWTR, with no evident of crystalline iron-Al (hydr)oxides 19 .
Zeolite. The zeolite particles as shown in SEM image (Fig. 1a) are largely spherical in the 20–28 nm size range. SEM-EDX elemental analysis revealed that O, Si, Al, and Fe in zeolite represent around 91% and C, Ca, K, Fe and Mg represent 9% of the total elements. The XRD patterns of zeolite (Fig. 1b) demonstrate a strong characteristic peak at 2θ = 4.5 and 67.861 which indicated that zeolite sample contained high percentage of silicon oxide (Si O2). Also, appearance of a peak at 2θ values of 9.4 and 25 indicates presence of potassium sodium aluminum silicate), whereas the peak at 2θ values of 21.3 and 27.3 referrers to potassium sodium calcium aluminum. Furthermore, the peak at 2θ values of (31.8 & 40), (35.021 & 46.5), and (41.9 & 76.5) indicates the presence of calcium iron oxide, Iron oxide, and Potassium oxide respectively.
Ze-nWTR nanocomposite. The surface microstructure morphology of the nanocomposite is presented in Fig. (2above). Zeolite Flakes are noticeable in SEM picture and spherical shaped nWTR nanoparticles are dispersed on zeolite superficies. The spherical dispersed particle diameters are of nano range size (51.02–71.43 nm) and the presence of these nanoparticles on Zeolite clay surface may prevent agglomeration of these particles. The EDX analysis shows that O, Si, C, Al and Fe, K, Na are the major elements of nanocomposite and represent ~ 98% of total elements concentration. The minor elements of nano-composite are Ca, Mg, Cu and Cr and represent 0.81, 0.51, 0.83 and 0.51%, respectively. The Al and Fe presence indicates the existence of nano WTR and could enhance the capability Ze-nWTR nano-composite for Cd removal.. The SEM - EDX elemental analysis of Cd loaded Ze-nWTR nano-composite is presented in Fig. 2 middle. The appearance of a cadmium peak (3.08%) amongst the elements detected is evidence that Cd was successfully loaded on Ze-nWTR nano-composite. The XRD patterns of Ze-nWTR (Fig. 2 bottom) demonstrate characteristic peaks at 2θ = 9.48 and 12.81 indicating that Ze-nWTR sample is containing potassium sodium aluminum silicate. Appearance of a peak at 2θ values of 12.7 and 28.6 indicates presence of potassium sodium manganese oxide whereas the peak at 2θ values of 22 and 27.7 referred to calcium aluminum silicate. Iron aluminum chromium oxide and aluminum chromium copper iron appeared at 2θ values of (36.1 and 58.3) and (44.7 and 64.7), respectively.
Specific surface area
The specific surface area (SSA) of nWTR and Zeolite was determined. The SSA of nWTR (129 m2 g− 1) is much higher than that of zeolite sample (39 m2 g− 1). The SSA of Ze-nWTR nanocomposite was found to be 89 m2g− 1. Indeed; the high SSA of nWTR could potentially outfit the nanocomposite with extra available surface sites for Cd adsorption.
Fourier transmission infrared spectroscopy. The Ze-nWTR FTIR spectrum (Fig. 3 above) showed a strong broad band at 3424cm− 1 attributed to O–H bending vibrations and a smaller band at 1638 cm− 1 ascribed to H-O-H molecule bending mode 30. In addition, a big band at 1044 cm− 1 designated to FeOH vibration of feroxyhyte and a band located at 466 cm− 1 assigned to stretching vibration of O-AL-O are shown in Fig. (3 above) 31. Retention of Cd on the surface of Ze-nWTR loaded Cd has led to noticeable spectral changes. The shift of the band at 3424 cm− 1 to 3438 cm− 1 and its intensity increase affirmed the involvement of the surface hydroxyl group on Cd adsorption process. Similarly, the band at 1638 cm− 1 has shifted to 1634 cm− 1 and increased in intensity. The increased intensity and relocation of the band from 1044 cm− 1 to1045 cm− 1 and the band shift from 466 cm− 1 to 475 cm− 1 are obvious evidence of the specific molecular interlinkage. Thus, the OH, O-Al-O, FeOH, and FeO(OH) structures participation in Cd adsorption process by Ze-nWTR nano-composite is suggested. The FTIR spectra of nWTR or zeolite loaded/unloaded with Cd are illustrated in Fig. 3 middle and Fig. 3 bottom, respectively. After Cd adsorption, two new bands designated to FeOOH emerged onto nWTR and Zeolite at 794 cm− 1 and 683 cm− 1 and at (3633 cm− 1 and 729.37 cm− 1 ) respectively, assured the role of iron oxy-hydroxide on Cd adsorption process (Fig. 3 middle & bottom).
Adsorption kinetics Understanding the kinetics and mechanisms of Cd sorption reactions is a prerequisite in quantifying Cd sequestration by the studied sorbents. The kinetics study was carried out to estimate equilibrium time. The Cd/ Zeolite or Ze -nWTR composite systems attained equilibrium within 2 h whereas nWTR -Cd system attained equilibrium after 8h. These data are critical because time of equilibrium is a key parameter for economic application of wastewater treatment (kadirvelu and namasivayam, 2001). Also, the quantity of Cd adsorbed by Ze -nWTR composite was higher than that adsorbed by Zeolite and nWTR (Fig. 4a). In all studied sorbents, Cd sorption was quite rapid in the first 30 minutes as 99% of Cd was sorbed and followed by slow sorption step. Elkhatib et al.32 reported that the rapid Cd adsorption is a surface phenomenon where vacant sites on sorbent surface are rapidly filled in the early stage and followed by diffusion and slow migration.
kinetics modeling. It is important to choose a mathematical model that satisfactorily fits the data and complies with a reasonable sorption mechanism (Oualid and Mahdi, 2007). The, kinetics data of Cd (II) sorption by the three sorbents studied were analyzed using four kinetic models (first-order 33, Elovich 34, Intraparticle diffusion 35, and modified Fruendlich 36). The model parameters, coefficients of determination (R2) and standard error (SE) values for Cd retention onto studied sorbents are presented in Table 1. The conformity between experimental data and the model predicted values was expressed by the determination coefficient (R2) and the standard error of estimate (SE) values. The model with the highest R2 (close or equal to 1) and the lowest SE values is considered the best model that prosperously depicts the kinetics of Cd (II) sorption (Fig. 4b). The overall high R2 (0.94) and the lowest SE (0.001) values of power function model (Table 1) ascertained. That power function model is the best kinetic model capable of describing Cd sorption kinetics by nWTR, Zeolite and Ze-nWTR nanocomposite.The adsorption rate (ka) of the power function model was used to compare Cd adsorption rate among the three adsorbents studied. As seen in Table 1, the ka was in the order nanocomposite (49306.03 min− 1) > nWTR (46902.93 min− 1) > Zeolite (45729.87 min− 1).
Table 1
Kinetics model constants and determination coefficients and standard error of estimate for cadmium adsorption by three different sorbents.
Models
|
Parameter
|
nWTR
|
Zeolite
|
Composite
|
Elovich
qt= (1/ β) ln(α β ) + (1/ β) lnt
|
α (mg g− 1 min− 1)
|
4.730E+ 9
|
9.77E+ 142
|
7.999E+ 206
|
β (mg g− 1)
|
0.0044
|
0.0065
|
0.0095
|
R2
|
0.95
|
0.6041
|
0.83
|
SE
|
774.31
|
253.56
|
97.52
|
First order
ln ( qο – q ) = a – ka t
|
Kd (min− 1)
|
0.004
|
0.004
|
0.371
|
a (µg g− 1)
|
6.861
|
5.8
|
278.7
|
R2
|
0.971
|
0.718
|
0.317
|
SE
|
0.247
|
1.036
|
0.793
|
Parabolic diffusion
q = a + kat1/2
|
Kd (µgg− 1min− 1/2)
|
31.425
|
18.240
|
12.188
|
a (µg g− 1)
|
47493
|
46110
|
49445
|
R2
|
0.827
|
0.387
|
0.497
|
SE
|
741.92
|
375.33
|
168.34
|
Power function
q = ka Co t1/m
|
Ka (min− 1)
|
46902.93
|
45729.87
|
49306.03
|
1/m
|
0.0047
|
0.003
|
0.0015
|
R2
|
0.953
|
0.937
|
0.920
|
SE
|
0.0009
|
0.002
|
0.0009
|
q or qt= Cd adsorbed (mg kg− 1) at time t, qo= Cd adsorbed (mg kg− 1) at equilibrium, ka= apparent sorption rate coefficient, α = the initial adsorption rate (mg g− 1 min− 1), β = a constant related to the extent of surface coverage (mg g− 1), a = a constant; kd= apparent diffusion rate coefficient, q = adsorbed Cd (mg kg− 1), Cο= initial Cd concentration (mg L− 1), t = reaction time (min), ka= sorption rate coefficient (min− 1), and 1/m = constant. R2 = determination coefficient, SE = standard error of estimate. |
Adsorption isotherm. Figure 5a shows Cd (II) sorption isotherm by nWTR, Zeolite and Ze-nWTR composite. A continued increase of Cd sorbed by the three studied sorbents with increasing Cd concentration from 40 to 640 mg/L is obseved. The Cd sorption capacity of the three sorbents studied were in the order nanocomposite (Ze-nWTR) > nWTR > Zeolite). It is notable that the shape of Ze-nWTR composite and nWTR sorption isotherms are L-type isotherm according to Giles classification 37 which reflects initial slope that doesn't increase with increasing contaminant concentration; While the shape of Cd sorption isotherm by Zeolite was S-type isotherms which is characterized by increasing of the slope with increasing of contaminant concentration.
Adsorption equilibrium models. For accurate estimation of Cd sorption parameters by the three sorbents studied (nWTR, Zeolite, and Ze- nWTR nanocomposite), seven sorption isotherm models (Langmuir, Freundlich, Elovich, Temkin, Fowler–Guggenheim (FG), Kiselev, and Hill-de Boer) were employed to interpret the sorption process (Table 2). The R2 and SE values (Table 2) were employed to ascertain the best isotherm fitting model in describing the adsorption data38.The R2 and SE values of the models studied (Table 2 and Fig. 5b) ascertained that Langmuir model is the most successful model in describing Cd sorption data. It is, therefore, suggested that Cd removal from the surface of the three studied sorbents is a single-layer adsorption process. The maximum adsorption capacity (qmax) value of Ze-nWTR (147.9 mgg-1) was 3 and 5.9 times larger than qmax values of nWTR and Zeolite sorbents respectively (Table 2). The Zeolite coating process with nWTR and the synergetic behavior between the clay and iron nanoparticles are responsible for the higher sorption capacities of the nano-composite27 .Thus, it is advised that Ze-nWTR nanocomposites can be applied as an effective adsorbent for Cd removal from contaminated water sources 39
Table 2
Equilibrium model constants and determination coefficients and standard error of estimate for cadmium adsorption by three different sorbents.
Models
|
Parameter
|
nWTR
|
Zeolite
|
Nanocomposite
|
Freundlich
qe = KFCe1/n
|
KF (mL g− 1)
|
11475
|
7194.701
|
15796
|
1/n
|
1.004
|
1.3669
|
1.007
|
R2
|
0.796
|
0.8006
|
0.845
|
SE
|
0.572
|
0.568
|
0.499
|
Langmuir
qe = qmax(KL Ce /1 + KLCe)
|
qmax (µg g− 1)
|
50000
|
25000
|
147857
|
KL (L mg− 1)
|
0.28571
|
0.15
|
0.117
|
R2
|
0.919
|
0.937
|
0.974
|
SE
|
0.00003
|
0.00005
|
0.00002
|
Elovich
qe /qm = KE Ceexp(-qe /qm)
|
qmax (µg g− 1)
|
100000
|
50000
|
100000
|
KE (L mg− 1)
|
1.08
|
1.11
|
1.13
|
R2
|
0.516
|
0.817
|
0.440
|
SE
|
0.398
|
0.276
|
0.373
|
Temkin
θ = RT/∆Q lnK0Ce
|
ΔQ (kJ mol− 1)
|
7.251
|
3.376
|
19.779
|
K0(L g− 1)
|
3.065
|
1.537
|
3.947
|
R2
|
0.47
|
0.525
|
0.5422
|
SE
|
0.409
|
0.580
|
0.133
|
Fowler–Guggenheim(FG)
KFGCe = θ/1- θ exp(2 θ w/RT)
|
W(kJ mol− 1)
|
-2.2445
|
-2.132
|
-3.7482
|
KFG(L mg− 1)
|
0.17028
|
0.0189
|
0.080
|
R2
|
0.5482
|
0.9165
|
0.705
|
SE
|
0.496
|
0.321
|
0.385
|
Kiselev
k1Ce = θ/(1- θ) (1 + kn θ)
|
k1(L mg− 1)
|
0.3491
|
0.0832
|
0.109
|
kn
|
1.80751
|
25.0381
|
1.305
|
R2
|
0.8239
|
0.0137
|
0.95
|
SE
|
0.895
|
2.642
|
0.398
|
Hill–deBoer
K1Ce = θ / (1 − θ) exp(θ/ (1- θ)– K2θ/ RT)
|
K1(Lmg− 1)
|
7.811
|
6.6
|
13.16
|
K2 (kJ mol− 1)
|
12.03
|
18.79
|
12.13
|
R2
|
0.824
|
0.876
|
0.84
|
SE
|
0.675
|
4.60
|
0.421
|
qe(mg g− 1) = Cd adsorbed per gram of adsorbent, Ce (mg L− 1) = equilibrium Cd concentration in solution, KF= a constant related to adsorption capacity of the adsorbent (mL g− 1), n = a constant, qmax (mg g− 1) is the maximum adsorption capacity of the adsorbent, KL (L mg− 1) = Langmuir constant related to the free energy of adsorption, θ = fractional coverage, R = the universal gas constant (kJ mol− 1 K− 1), T = the temperature (K), ΔQ = (−ΔH) the variation of adsorption energy (kJ mol− 1), and K0 = Temkin constant (L mg− 1), KFG = Fowler–Guggenheim constant (L mg− 1), w = the interaction energy between adsorbed molecules (kJ mol− 1), k1 = Kiselev constant (L mg− 1), kn= a constant of complex formation between adsorbed molecules, K1 = Hill–de Boer constant (L mg− 1), and K2 (kJ mol− 1) = a constant related to the interaction between adsorbed molecules. A positive K2 means attraction between adsorbed species and a negative value means repulsion. |
Operating conditions for Cd removal Effect of solution pH and temperature. The Cd sorption process was monitored at solution pH range of 4–9, sorbent dose of 0.2 g and 3 different temperatures (287,297,307 K). Figure (6) shows that Cd removal increased with increasing solution pH and peaking at pH 9. The effect of varying pH on removal percentage of Cd ions by the sorbents studied can be explained by the surface charge characteristics of the adsorbent and Cd ionization state 40. Therefore the zero point charge (pHzpc) of nanocomposite was determined and found to be 7.2 (Fig. S2). At solution pH higher than 7.2, It is conceivable that the repellent strength between Cd++ positively charged and the nanoparticles surface charges is minimized leading to more Cd sorption.41 Furthermore, the Langmuir adsorption capacity of the nanocomposite increased from147.9 mgg− 1 to 270 mgg− 1 with the increase of the temperature from 279 to 308K. Al-Qodah et al 42 reported that temperature increase has led to increase adsorbate diffusion rate in the internal pores of the adsorbent particles.
Effect of competitive cations. The industrial wastewater effluents contain many metal ions, which may interacted with the sorbents and compete for their binding sites .Therefore, studying the competition between the heavy metals for adsorption onto Ze-nWTR is important. Batch adsorption study was performed in the presence and absence of Zn + Cu + Ni at different Cd concentrations (20, 60,180 mgL-1) to evaluate the sorbed Cd % by Ze-nWTR in single and multi-element system. As shown in Fig. 7a, the presence of Zn 2+,Cu2+,Ni+ 2 cations reduced Cd ions removal from solution by Ze-nWTR nano-composite because of the rivalry between the charged metal for the available sorption sites on Ze-nWTR nanocomposite surface and the increase in ionic strength of the solution 43,44.
Effect of sorbent dose. The capacity of a sorbent for a given initial concentration count largely on the sorbent dose. The effect of the mass of Ze-nWTR nano-composite, nWTR, and Zeolite on Cd(II) removal from aqueous solution was run by adding different dosage of each sorbent (0.1 to 0.3g) using 20 ml of 500 mgCdL-1 concentration for 60 min time interval. Figure 7b displays the impact of sorbent weight on Cd removal. The results indicate greater differences in Cd sorption capacities among different doses. For instance, with increasing the Ze- nWTR nano composite dosage, the Cd removal increased from 33.225 to 91.310 mgkg-1. At a fixed Cd2+ concentration, the increase of sorbent weight causes an increase in surface area and number of active sites available for Cd interaction.
Effect of temperature. Thermodynamic parameters of Cd retention onto Ze-nWTR nanocomposite were calculated at different initial Cd concentration (100, 250, 500 and 1000 mg L− 1) and different pH values (4,7,9) to fully understand the nature of sorption45. The standard free energy change (ΔG°) for Cd (II) sorption at (100mgL− 1) initial concentration and pH 9 was observed to be -22.302, -27.252 and − 32.455 kJ mol− 1, onto Ze-nWTR nano-composite at 14°C, 24°C and 34°C, respectively (Table 3). The negative ΔG° value points out the viability of Cd adsorption process and elucidates the spontaneous Cd reaction on Ze-nWTR nanocomposite sorbent 46 . Lowered ΔG° values (i.e. negativity increase) indicated the rise of adsorption magnitude with temperature increase as shown in Fig. 8 and Table 3 suggesting more efficient sorption at higher temperatures. This finding was corresponding with the results of Fawzy et al.47 who found decrease in ΔG° with temperature increase regarding Cd biosorption onto the free and alginate-immobilized T. ornata biomasses. Furthermore, ΔG° values were negativity increased as a result of pH values increase indicating more available sorption sites as pH values increase from 4–9 48,49. In contrast, ΔH° values were positive at different initial solution concentration suggesting endothermic nature of Cd sorption on Ze-nWTR 50 (Table 3). Data in Table 4 showed ΔH° values decrease with increasing initial concentration signifying less energy requirements for Cd sorption reaction on Ze-nWTR as a result of increasing initial Cd concentration. In general, ΔH° between 2.1 and 20.9 kJ mol − 1 indicates physical adsorption, while values between 20.9 and 408 kJ mol − 1 confirm chemisorption.51. Therefore, physical and chemical attraction between Cd and Ze-nWTR is suggested due to the high different values range of ΔH° (123292 − 18600 J mol− 1) for Cd sorption on Ze-nWTR at different initial solution concentration. The ΔS° negative values suggest that a dissociative mechanism is involved in the Cd (II) sorption process50.
Table 3
Thermodynamic parameters for Cd adsorption by nanocomposite (Ze- nWTR) sorbent at different solution pH values (4–9) and 4 initial Cd concentrations.
Initial concentration
(mg l− 1)
|
pH
|
T
(K)
|
ΔG◦
( J mol− 1)
|
ΔS◦
(Jmol− 1K− 1)
|
ΔH◦
( J mol− 1)
|
100
|
4
|
287
|
-19206
|
-430.65
|
104787
|
297
|
-22320
|
307
|
-27819
|
7
|
287
|
-21835
|
-447.06
|
107208
|
297
|
-24098
|
307
|
-30777
|
9
|
287
|
-22302
|
-507.15
|
123292
|
297
|
-27252
|
307
|
-32445
|
250
|
4
|
287
|
-20493
|
-162.02
|
26187
|
297
|
-21576
|
307
|
-23733
|
7
|
287
|
-22326
|
-209.94
|
37866
|
297
|
-24608
|
307
|
-26525
|
9
|
287
|
-22737
|
-439.64
|
103935
|
297
|
-25643
|
307
|
-31530
|
500
|
4
|
287
|
-21385
|
-151.25
|
22063
|
297
|
-22778
|
307
|
-24410
|
7
|
287
|
-23469
|
-185.97
|
30083
|
297
|
-24791
|
307
|
-27188
|
9
|
287
|
-25522
|
-240.82
|
43866
|
297
|
-27110
|
307
|
-30339
|
1000
|
4
|
287
|
-22308
|
-142.69
|
18600
|
297
|
-23868
|
307
|
-25162
|
7
|
287
|
-23871
|
-164.54
|
23574
|
297
|
-24855
|
307
|
-271612
|
9
|
287
|
-24396
|
-215.76
|
37851
|
297
|
-25579
|
307
|
-28711
|
Table 4
Maximum adsorption capacities (qm) of Cd(II) adsorption onto nanocomposite and various adsorbents listed in the literature.
Adsorbent
|
qmax
mgg-1
|
References
|
Ze-nWTR
|
147
|
current study
|
SiO2@DOPP
Nanocomposite
|
142
|
55
|
nano-scale magnesia
|
16.54
|
56
|
Oil palm residual biomass/Al2O3
nanoparticles composite
|
17.4
|
57
|
Phytogenic magnetic nanoparticles
|
68.41
|
58
|
Hydroxyapatite Encapsulated Zinc ferrite) nanocomposites
|
120.33
|
59
|
Gum kondagogu modified iron oxide NPs
|
106.8
|
60
|
Shellac coated iron oxide NPs
|
18.80
|
61
|
Ferrihydrite modified biochar
|
18.18
|
62
|
Mechanisms of Cd sorption. The mechanisms of Cd sorption on Ze-nWTR nano- composite was interpreted using FTIR, XRD, and EDX analysis. The XRD pattern confirmed the sorption reaction of Cd on the nano-composite evidenced by cadmium peak (3.08%) detected in Cd loaded Ze-nWTR nano- composite (Fig. 2b). For Zeolite, the FTIR analysis demonstrated the role of OH functional group on Cd sorption since the band at 3420cm− 1 (O-H bending vibration) completely disappeared after Cd retention on Zeolite surface. Cadmium can form outer-sphere surface complexes with Zeolite surface due to the presence of OH group 52,53. Thus, the ion exchange between H+ corresponding to OH group of Zeolite and Cd2+ is the proposed adsorption mechanism as shown in the following equation:
HO-Ze-OH + Cd2+ = HO-Ze-O-Cd + 2H+
Regarding nWTR sorbent, the FTIR spectrum of nWTR after Cd adsorption has shown complete disappearance of the band related to O-H bending vibration at 4012cm− 1. Furthermore, increasing intensities and changing the location of O-H bending vibrations band from 3416 cm− 1 to 3377cm− 1, the H2O bending vibrations band from 1636 cm− 1 to 1629 cm− 1, the FeOH modes of feroxyhyte bending vibration from 1091 cm− 1 to 1029 cm− 1 are identified. Meantime, the shift and the increased intensities of the two bands related to FeOOH at 794 cm− 1 and 683 cm− 1are obvious sign of specific molecular interlinkage. Thus, it is proposed that the six-coordinated structure of Cd can form inner-sphere bidentate complexes with the FeOOH surface of nWTR as shown in Fig. 9 . Furthermore, It is noticed that Ca percent was decreased from 7.8 to 4.2% after Cd saturated nWTR according to EDX analysis as shown in Fig (S1) (Supplementary materials). This finding reflexes the ability of Cd to replace Ca existing in the interior of the FeOOH-octahedral structure of nWTR which can attributed to the semi similar between ionic radius of Cd2+ (0.97 Å) and Ca2+ (0.99 Å) 54. Thus, the overall supposed mechanism of Cd sorption on Ze-nWTR nano- composite occurs initially through electrostatic attraction between Cd ions in solution and OH-Zeolite. Hereafter, the adsorbed Cd can specifically adsorbed on FeOOH -nWTR by forming inner-sphere complexation. Further reaction can occur due to occlusion of Cd into the interior structure of FeOOH.
Reusability. The regeneration of the function groups of the consumed nanocomposite in frequent exercise is corresponding to its constancy, which is decisive for industrial wastewater treatment. Therefore, The reusability and constancy of Ze-nWTR nanocomposite was examined for six sequential adsorption/desorption periods utilizing 0.01M HCl solution to desorb laden Cd(II). The cumulative cadmium sorbed by Ze-nWTR nanocomposite at 10.0 mg/L and/or 100.0 mgL-1 initial Cd concentrations is shown in Fig. 10. Adsorption efficiency of Ze-nWTR nano-composite for Cd (II) decreased by1.7%, 1.9%, 3.6% and 3.7% through 2nd ,3d, 5th, and 6th cycle, respectively. Results revealed that the Ze-nWTR nanocomposite could be reused effectively for up to six adsorption cycles with a little change in adsorbed Cd reflecting high stability of nanocomposite laden Cd; which makes the sorption process economically, sustainability and technically attractive because of less solid waste production.
Cadmium removal efficiency of nanocomposite. The nanocomposite efficiency of Cd (II) removal was investigated using a batch experiment on real wastewater. Industrial Cd contaminated waste water which has been brought from industrial drainage of Rakta company for Paper manufacturing was treated with Ze-nWTR for Cd removal. The results showed that Ze-nWTR successfully removed 98.45% Cd from industrial Cd contaminated waste water. The batch experiment revealed that Cd removal potential of Ze-nWTR was not affected by the existence of various anions (e.g., SO4, CO3, HCO3, Cl−) in industrial wastewater. Furthermore, 97.53% of Cd was removed from Al-Bilali agricultural drainage by the nanocomposite. Such results indicate the suitability and effectiveness of Ze-nWTR to remove aqueous Cd from real wastewater.
Column study. The efficiency of Cd removal by Ze-nWTR nanocomposite under continuous flow conditions (flow rate was 3 mL min-1 ) using a bed-reactor reached 95.5%, and 98%, for agricultural drainage and industrial discharge, respectively. The highest rate of removal of the Ze-nWTR nanocomposite suggested that the nanocomposite can be used as a potential sorbent to remove various toxic pollutants from the industrial and agricultural wastewater. Generally, the outcomes of the present study manifested that the (Ze-nWTR) nanocomposite is a notable environment-friendly and reusable adsorbent for effective elimination of Cd from wastewater.
Comparison of different adsorbents for Cd removal. The effective Cd(II) removal from wastewater by (Ze-nWTR) nanocomposite was further assessed by comparing qmax of this nanocompsite with other adsorbent present in published works as shown in Table 4. Obviously, the (Ze-nWTR) nanocomposite displayed high adsorption capacity towards Cd(II) (147 mgg-1) as compared with other nano-adsorbents such as SiO2@DOPP nanocomposite (142 mgg-1) 55 (Saini et al., 2021), Phytogenic magnetic nanoparticles (68.41 mgg-1) 58, (Ali et al.,2019), Hydroxyapatite encapsulated Zinc ferrite) nanocomposites (120.33 mgg-1) 59 (Das and Dhar, 2020), Gum kondagogu modified iron oxide NPs (106.8 mgg-1) 60 (Saravanan et al., 2012) and Ferrihydrite modified biochar (18.18 mgg-1) 62 (Tian et al., 2022). These results assured the high performance of (Ze-nWTR) nanocomposite in removing Cd(II) from wastewater.