Uptake of Cu²⁺ and Zn²⁺ from simulated wastewater using muskmelon peel biochar: Isotherm and kinetic studies

Figures & data

Fig. 1 FTIR spectra of raw muskmelon peel (a), biochar (b), Cu loaded biochar (c), and Zn loaded biochar (d).

Fig. 2 SEM images at different magnifications of raw muskmelon peel (a,b), biochar (c,d), Cu loaded biochar (e,f), and Zn-loaded biochar (g,h).

Fig. 3 EDS spectra of raw muskmelon peel (a), biochar (b), Cu-loaded (c), and Zn-loaded (d) biochar.

Table 1 Thermodynamic parameters.

Adsorbate	ΔH° (kJ/mol)	ΔS° (J/mol/K)	−ΔG° (kJ/mol)
			303 K	313 K	323 K
Cu(II)	−32.712	−87.19	6.293	5.421	4.549
Zn(II)	−18.039	−43.88	4.743	4.304	3.865

Fig. 4 Effect of contact time on Cu(II) and Zn(II) uptake.

Fig. 5 Effect of initial metal concentration on Cu(II) and Zn(II) uptake.

Fig. 6 Effect of biochar dose.

Fig. 7 Effect of initial solution pH.

Fig. 8 Effect of temperature.

Fig. 9 vant Hoff’s plot.

Fig. 10 Langmuir adsorption isotherm plots.

Table 2 Various isotherm parameters.

Isotherm Parameter		Cu(II)			Zn(II)
		303 K	308 K	313 K	303 K	308 K	313 K
Langmuir	qm (mg/g)	79.36	78.74	78.74	72.99	69.93	66.66
	b (L/mg)	0.349	0.304	0.246	1.505	1.375	1.293
	RL	0.0542	0.0616	0.0751	0.0131	0.0143	0.0152
	R^2	0.980	0.981	0.982	0.998	0.998	0.996
Freundlich	Kf (L/g)	34.03	31.49	27.54	40.04	46.37	46.33
	1/nf	0.235	0.252	0.285	0.171	0.167	0.152
	R^2	0.919	0.942	0.957	0.884	0.928	0.960
D-R	qD (mg/g)	59.73	60.53	61.33	60.82	62.29	63.34
	E (kJ/mol)	1.000	1.000	1.118	2.236	2.500	3.535
	R^2	0.897	0.900	0.902	0.804	0.796	0.763

Table 3 Adsorption capacity (q_m) of reported adsorbents for Cu(II) and Zn(II) uptake.

Cu(II)				Zn(II)
Adsorbent	pH	qm (mg/g)	Ref	Adsorbent	pH	qm (mg/g)	Ref
Wheat shells	5.0	8.26	[36]	Rice husk ash	6.0	5.88	[47]
Arca shell	3–6	17.64	[37]	Bagasse fly ash	6.0	7.03	[48]
Cashew nut shells	5.0	20.0	[38]	Zeolite 4A	6.5	42.82	[49]
Betula sp saw dust	5.5	4.90	[39]	Zeolite 13X	6.5	38.31	[49]
Barly straw	7.0	4.64	[40]	Bentonite	6.5	35.17	[49]
Citric acid modified barly straw	7.0	31.71	[41]	Jute unmodified	5.87	3.55	[50]
Garden grass	6.0	58.34	[41]	Jute dye loaded	5.87	5.95	[50]
Hardwood biochar	5.0	6.79	[19]	Jute oxidized	5.87	8.02	[50]
Corn straw biochar	5.0	12.52	[19]	Vermiculite	5.6	71.89	[51]
Citric acid modified Spent coffee grains	5.0	60.37	[42]	Bentonite	6.0	30.7	[52]
HNO3^−modified CNTs	5.0	28.49	[43]	Na-enriched bentonite	6.0	57.43	[52]
CNTs/calcium alginate composites	5.0	84.88	[44]	Sulphured orange peel	5.0	80	[53]
Muskmelon peel biochar	7.0	79.36	This study	Muskmelon peel biochar	7.0	72.99	This study
Sulfonated multiwalled carbon nanotubes (sMWCNTs)	5.0	43.16	[45]	KCl modified orange peel	5–5.5	45.29	[53]
Single walled carbon nanotubes (SWCNTs)	5.0	24.29	[46]	Nano-porous activated neem bark	–	11.9	[54]

Fig. 11 Freundlich adsorption isotherm plots.

Fig. 12 D-R adsorption isotherm plots.

Fig. 13 Pseudo-first order kinetic plots.

Fig. 14 Pseudo-second order kinetic plots.

Table 4 Pseudo-first order and pseudo-second order model parameters.

Adsorbate	Conc (mg/L)	Pseudo-first order			qe(exp) (mg/g)	Pseudo-second order
		k1 × 10^−2 (1/min)	qe(cal) (mg/g)	R^2		k2 × 10^−3 (g/mg/min)	qe(cal) (mg/g)	R^2
Cu(II)	40	3.754	43.06	0.839	48.36	1.060	43.10	0.992
	50	3.086	45.77	0.899	56.60	0.750	56.49	0.992
	60	4.306	79.14	0.861	67.13	0.660	67.11	0.994
Zn(II)	40	3.155	47.97	0.860	43.82	0.529	46.73	0.993
	50	3.316	68.12	0.866	52.45	0.393	56.50	0.899
	60	3.477	85.23	0.893	62.08	0.263	72.99	0.930

Fig. 15 Liquid-film diffusion plots.

Fig. 16 Intra-particle diffusion plots.

Table 5 Liquid-film and intra-particle diffusion model parameters.

		Liquid–film diffusion		Intra–particle diffusion
Adsorbate	Conc (mg/L)	kD	R^2	ki	R^2
Cu(II)	40	0.0430	0.861	3.853	0.976
	50	0.0375	0.838	3.215	0.992
	60	0.0309	0.899	2.386	0.992
Zn(II)	40	0.0309	0.798	4.656	0.971
	50	0.0308	0.799	3.767	0.956
	60	0.0321	0.840	2.934	0.983

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