Immobilised apple peel bead biosorbent for the simultaneous removal of heavy metals from cocktail solution

Figures & data

Table 1. Kinetic models used

Model used	Linearised form	Reference
Film diffusion (FD)	$ln\left(-F\right)=-k_{FD}t$	(Gupta & Bhattacharyya 2011)
Pore diffusion- Weber and Morris (PD)	$q_t=k_{PD}{t}^{0.5}$	(Gupta and Bhattacharyya 2011)
Pseudo first order (PFO)	$ln(q_e-q_t)=\mathrm{l}\mathrm{n}{q}_e-k_{1}t$	(Gupta and Bhattacharyya 2011)
Pseudo second order (PSO)	$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}$	(Tan & Hameed, 2017)
PSO initial biosorption rate (h)	$h=\frac{dq}{dt}=k_2(q_e{)}^2$	(Tan & Hameed, 2017)
Elovich Equation	$q_t=a+blnt$	(Schwantes et al., 2016)

q_t (mg g⁻¹) is biosorption capacity at any given time point; q_e (mg g⁻¹) is equilibrium biosorption capacity; $F=q_t/q_e$ is the fractional attainment of equilibrium, k_FD (min⁻¹) is film diffusion rate constant; k_PD (in mg g⁻¹ min ^−0.5) is pore diffusion rate constant; k₁ (min⁻¹) is PFO rate constant; k₂ (g mg⁻¹ min⁻¹) is PSO rate constant; h (mg g⁻¹ min⁻¹) is initial biosorption rate; “a” (mg g⁻¹ min⁻¹) represents the initial chemisorption velocity; “b” (g mg⁻¹) indicates the number of suitable sites for the adsorption, which is related to the surface coverage extension and activation energy of chemisorption

Figure 1. SEM-EDS images of apple peel particles. (a) micrograph at 250X (b) micrograph at 2000X (c) spectrogram showing C, O and K.

Table 2. Summary of SEM-EDS analysis

Biosorbent	Particles (µm)	Weight of bead (mg)^#	Bead before biosorption (µm X µm)	Bead after biosorption (µm X µm)*	EDS after biosorption*
Apple peel	< 240	2.20 ± 0.05	1563.8 X 1511.3	1882.5 X 1856.3	As, Cr, Hg, Pb
Sodium alginate	–	0.80 ± 0.05	1068.8 X 888.8	1095.0 X 810.0	As, Cr, Cu, Pb, Ni

*1 bead in 0.1 mg L⁻¹ cocktail solution of all 7 ions for 10 h, pH 7.0, 25ºC, 250 rpm

^#mean ±SEM, n = 3

Figure 2. SEM-EDS of beads before biosorption. (a) apple peel bead micrograph at 50X (b) apple peel bead micrograph at 2000X (c) control bead micrograph at 50X (d) control bead micrograph at 500 X (e) apple peel bead spectrogram (f) control bead spectrogram.

Figure 3. SEM of beads after biosorption. One bead was incubated in a 0.1 mg L⁻¹ cocktail solution of all seven ions for 10 h at pH 7.0, 25ºC and 250 rpm shaking. (a) apple peel bead micrograph at 50X (b) apple peel bead micrograph at 2000X (c) control bead micrograph at 50X (d) control bead micrograph at 500 X (e,f) EDS spectrogram of As, Cr, Hg and Pb attached to apple peel bead (g,h) EDS spectrogram of As, Cr, Cu, Pb and Ni attached to control beads.

Figure 3. (Continued).

Figure 4. Effect of initial pH on apple peel bead biosorption. One bead was incubated in a 0.1 mg L⁻¹ cocktail solution of all seven ions for 24 h at 25ºC and 250 rpm shaking. The initial pH of each solution was adjusted to 6.5 to 8.5 ± 0.1. The points represent mean ± SEM for n = 3. *significantly different to all other pH conditions for Pb, p < 0.05.

Figure 5. Effect of contact time on bead biosorption. One bead was incubated in a 0.1 mg L⁻¹ cocktail solution of all seven ions at pH 7.0, 25 ºC, for 15 min-72 h continuous shaking at 250 rpm. The points represent mean ± SEM for n = 3 with data analysis by nonlinear regression. (a) apple peel bead (b) control bead.

Figure 6. Cumulative biosorption at different contact time points. One bead was incubated in a 0.1 mg L⁻¹ cocktail solution of all seven ions at pH 7.0, 25ºC, for 15 min-72 h continuous shaking at 250 rpm. The bars represent the mean ± SEM for n = 3. *Significantly different cumulative biosorption compared to corresponding control, p < 0.05. Only ions with significant biosorption are shown. AP = apple peel bead, C = control bead.

Figure 7. Biosorption process. Representation of the migration of ions from the bulk of the solution to the inner layers of the apple peel bead.

Table 3. Biosorption kinetic model constants

Bead-Metal	Film Diffusion (FD)		Pore Diffusion (PD)		Pseudo-First Order (PFO)		Pseudo-Second Order (PSO)			Elovich Equation
	up to time (h)	kFD x 10^−4	up to time (h)	kPD x 10^−4	up to time (h)	k1 x 10^−4	up to time (h)	k2 x 10^−4	h x 10^−4	up to time (h)	a x 10^−2	b x 10^−2
Apple peel bead
Cd	–	–	–	–	10	23.7	72	12.7	18.2	1.5	−10.5	4.6
Cu	10	11.2	72	198.6	24	8.3	–	–	–	1	−6.4	3.6
Hg	–	–	–	—	–	–	72	579.8	36.2	–	–	–
Ni	6	26.6	–	–	10	29.1	72	24.7	19.8	2	−12.7	5.7
Control bead
Cd	24	9.8	72	447.8	24	9.8	72	4.1	38.2	1.5	−12.2	8.8
Cu	24	6.5	–	–	24	6.5	–	–	–	–	–	–
Ni	24	12.2	72	344.5	24	10.2	72	4.3	28.0	3	−34.3	13.2

One bead in 0.1 mg L⁻¹ cocktail solution containing As, Cd, Cr, Cu, Pb and Ni each at 0.1 mgL⁻¹ at pH 7.0, 25 ºC, 250 rpm; 15 min-72 h continuous shaking. Linear regression data presented for n = 3 (R² > 0.9300).

k_FD = Film diffusion rate constant (min⁻¹); k_PD = Pore diffusion rate constant (mg g⁻¹ min^−0.5); k₁ = Pseudo-first order rate constant (min⁻¹); k₂ = Pseudo-second order rate constant (g mg⁻¹ min⁻¹); h = Initial biosorption rate (mg g⁻¹ min⁻¹); a = Elovich constant indicating chemisorption initial rate (mg g⁻¹ min⁻¹); b = number of adsorption surface sites related to the coverage extension and the activation energy of chemisorption (g mg⁻¹).

Figure 8. Biosorption kinetic models. One bead was incubated in a 0.1 mgL⁻¹ cocktail solution of all seven ions at pH 7.0, 25ºC, for 15 min-72 h continuous shaking at 250 rpm. The points represent mean ± SEM for n = 3 with data analysis by linear regression. (a) Film diffusion showing results for contact time up to 10 h (b) pore diffusion (c) pseudo-first order showing results for contact time up to 10 h (d) pseudo-second order (e) Elovich plot showing results for contact time up to 3 h.

Table 4. Apple peel bead biosorption capacity

Ion	Experimental qe	PFO qe	PSO qe
Cd	0.9 ± 0.0	1.0	1.2
Cu	1.1 ± 0.0	1.1	–
Hg	0.3 ± 0.0	–	0.2
Pb	1.3 ± 0.0	–	–
Ni	0.7 ± 0.0	0.7	0.9

1 bead in 0.1 mg L⁻¹ cocktail solution containing all 7 ions each at 0.1 mgL⁻¹ at pH 7.0, 25ºC, 250 rpm and 15 min-72 h continuous shaking. Results are the mean ± SEM for n = 3.

q_e = equilibrium biosorption capacity (mg g⁻¹); PFO = Pseudo-first order; PSO = Pseudo-second order

Figure 9. Effect of biosorbent concentration. One to ten beads in 1 mg L⁻¹ cocktail solution containing all seven ions each at 1 mgL⁻¹ for 10 h at pH 7.0, 25 ºC and shaking at 250 rpm. The points represent the mean ± SEM for n = 3 with data analysis by nonlinear regression.

Table 5. Apple peel bead biosorption capacity suppression in the presence of co-ions

Ion	Time (h)	qsm (mg g^−1)	qcocktail (mg g^−1)	qcocktail/qsm
As	1.5	0.54	0.00	0.00
	10	0.64	0.01	0.02
	24	0.64	0.01	0.01
Cd	1.5	0.29	0.28	0.97*
	10	0.95	0.91	0.95*
	24	1.15	1.16	1.01*
Cr	1.5	0.09	0.02	0.22
	10	0.11	0.10	0.95
	24	0.17	0.13	0.77
Cu	1.5	0.44	0.38	0.85*
	10	0.83	0.85	1.03*
	24	1.02	1.08	1.06*
Hg	1.5	0.67	0.06	0.09
	10	0.80	0.11	0.14
	24	0.86	0.14	0.17
Pb	1.5	1.10	0.31	0.03
	10	1.47	0.41	0.04
	24	1.65	0.44	0.04
Ni	1.5	1.09	0.20	0.02
	10	1.52	0.49	0.05
	24	1.59	0.57	0.06

One bead in 0.1 mg L⁻¹ of either a single ion or cocktail solution containing all seven ions each at 0.1 mg L⁻¹ at pH 7.0, 25ºC, 250 rpm continuous shaking for 1.5, 10 and 24 h. *indicates near synergism instead of suppression

Figure 10. Effect of the presence of co-ions on apple peel bead biosorption. One bead in 0.1 mg L⁻¹ of either a single ion or cocktail solution containing all seven ions each at 0.1 mg L⁻¹ at pH 7.0, 25ºC and 250 continuous shaking at 250 rpm for 1.5, 10 and 24 h. The points represent the mean ± SEM for n = 3. *Significantly different compared to the corresponding cocktail solution, p < 0.05. (a) arsenic (b) cadmium (c) chromium (d) copper (e) mercury (f) lead (g) nickel.

Tan, K., & Hameed, B. (2017). Insight into the adsorption kinetics models for the removal of contaminants from aqueous solutions. Journal of the Taiwan Institute of Chemical Engineers, 74, 25–48.

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Schwantes, D., Gonçalves, A. C., Coelho, G. F., Campagnolo, M. A., Dragunski, D. C., Tarley, C. R. T., … Leismann, E. A. V. (2016). Journal of Chemistry, 2016, 1–15. doi:10.1155/2016/3694174

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