Microwave-assisted spent black tea leaves as cost-effective and powerful green adsorbent for the efficient removal of Eriochrome black T from aqueous solutions

Figures & data

Fig. 1 SEM images of MASTL before (A) and after (B) the adsorption of dye; FT-IR spectra of MASTL before (C) and after (D) the adsorption of dye; typical N₂ adsorption-desorption isotherm of MASTL (E) and UV spectra before and after the adsorption of dye onto MASTL (F).

Fig. 2 Point of zero charge of MASTL (A) and the effect of the solution pH (B), at m/V = 1.0 g/L, C[EBT]_initial = 60 mg/L, t = 24 h, and T = 25 °C; the effect of solid content (C), at pH = 2.0, C[EBT]_initial = 60 mg/L, t = 24 h, and T = 25 °C; the effect of contact time (D), at pH = 2.0, m/V = 1.0 g/L, C[EBT]_initial = 60 mg/L, and T = 25 °C.

Fig. 3 Effect of temperature and initial concentration of EBT on the percent adsorption (A) and adsorption per unit mass (B), at pH = 2.0, m/V = 1.0 g/L, and t = 24 h.

Fig. 4 Isotherm plots, Langmuir (A), Freundlich (B), Temkin (C) and Dubinin-Radushkevich model (D) for EBT adsorption onto MASTL, at pH = 2.0, m/V = 1.0 g/L, and t = 24 h.

Table 1 Langmuir, Freundlich, Temkin and Dubinin-Radushkevich model constants and correlation coefficients for the adsorption of EBT onto MASTL.

Langmuir isotherm
Temperature (°C)	qm (mg/g)	b (L/mg)		R^2
25	242.72	0.0756		0.9859
40	323.63	0.1325		0.9889
60	393.70	0.2919		0.9946
Temkin isotherm
Temperature (K)	A (L/g)	B (J/mol)		R^2
25	1.49	42.10		0.9814
40	2.43	56.027		0.9828
60	3.51	78.6126		0.9499
Freundlich isotherm
Temperature (K)	n	1/n	KF (mg^1-n·L^n/g)	R^2
25	1.7562	0.5694	22.41	0.8918
40	2.8873	0.3463	70.48	0.9285
60	2.3793	0.4203	99.47	0.8224
Dubinin-Radushkevich
Temperature (K)	qm (mg/g)	KE × 10^−7 (mol^2/kJ^2)	E (kJ/mol)	R^2
25	158.52	6.55	0.8737	0.8608
40	203.42	4.93	1.0071	0.9271
60	253.74	2.34	1.4618	0.9373

Fig. 5 The separation factors for EBT adsorption onto MASTL at different temperatures.

Table 2 Comparison of equilibrium monolayer adsorption capacity of EBT on different adsorbents.

Adsorbent	qm (mg/g)	Ref.
Calcined MgAl-LDH	540.91	[42]
Calcined CoAl-LDH	419.87
Calcined NiFe-LDH	132.49
Maize stem tissue	167.01	[19]
Activated carbon from waste rice hulls	160.36	[16]
MgAl-LDH	128.20	[17]
Magnetite/pectin NPs	103.41	[15]
Magnetite/silica/pectin NPs	80.15
Eucalyptus bark	52.37	[13]
NiFe2O4 nanoparticles	47.00	[18]
Microwave treated almond shell	29.41	[43]
Cold plasma treated almond shell	18.18
Untreated almond shell	6.06
Hydrophobic cross-linked polyzwitterionic acid	15.90	[44]
MASTL	242.72	This work

Table 3 Kinetic parameters for EBT adsorption onto MASTL. q_e,exp = 57.70 and 57.99 mg/g at 25 and 40 °C respectively.

Model	Parameters
		25 °C	40 °C
	k1 (1/min)	0.0196	0.0368
Pseudo-first-	qe,cal (mg/g)	39.10	42.24
order	R^2	0.9965	0.9980
Pseudo-second-order	k2 (g/mg·min)	0.0009	0.0162
	qe,cal (mg/g)	62.11	61.58
	h (mg/g·min)	3.34	6.22
	t1/2 (min)	18.59	9.91
	R^2	0.9977	0.9995
Intraparticle diffusion	kp1	3.8443	3.9502
	(mg/g·min^1/2)	14.77	19.90
Stage-I	C1 (mg/g)	0.9990	0.9611
	R1^2
Stage-II	kp2	0.8460	0.1232
	(mg/g·min^1/2)	44.74	56.26
	C2 (mg/g) R2^2	0.9638	0.7452
Stage-III	kp3	0.1919	–
	(mg/g·min^1/2)	54.62	–
	C3 (mg/g) R3^2	0.9250	–

Fig. 6 The fitting of pseudo-first-order (A), pseudo-second-order (B), Intraparticle diffusion model (C) and Van't Hoff plot (D), for EBT adsorption onto MASTL, at pH = 2.0, m/V = 1.0 g/L, C[EBT]_initial = 60 mg/L.

Table 4 Thermodynamic constants for EBT adsorption onto MASTL.

1/T (1/K)	lnKd	T (K)	ΔG^o (kJ/mol)	ΔH^o (kJ/mol)	ΔS^o (J/mol·K)
0.0034	9.91	298	−24.55	31.82	189.32
0.0032	10.58	313	−27.55
0.0030	11.26	333	−31.18

Fig. 7 Molecular structure of EBT (A) and distribution of various species of EBT and its percent adsorption (green balls) as a function of pH (B).

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