Preparation and characterization of bioadsorbent beads for chromium and zinc ions adsorption

Figures & data

Figure 1. (a) wet chitosan beads, (b) dry chitosan beads, (c) single chitosan bead (scanning electron microscopy image, 500 μm and 1,000 kV image magnification).

Table 1. Physical and chemical properties of the chitosan beads

Parameters	Value
Physical nature	Porous
Average particle diameter (mm)	1
Surface area (m^2/g)*	1.9
Pore size (mm)*	3.73 × 10^−6
Pore volume (m^3/g)*	8.2 × 10^−9
pH at the point of zero charge (pHZPC)	5
Compositions**:
Carbon content (wt%)	57.11
Oxygen content (wt%)	18.26
Nitrogen content (wt%)	17.60
Sodium content (wt%)	6.25

^* Determined by Brunauer–Emmett–Teller (BET) measurement.

^** Determined by Energy dispersive X-ray spectroscopy (EDS).

Figure 2. Effect of initial solution-pH on Zn(II) and Cr(VI) adsorption onto chitosan beads (initial concentration (C₀) for Zn(II) = 15.3 mmol/L; C₀ for Cr(VI) = 19.23 mmol/L; temperature = 10 °C; mass of chitosan beads = 0.5 g).

Figure 3. Adsorption capacity (q_t) as a function of time for different initial metal concentrations in solution (C₀) for (a) Cr(VI) ions and (b) Zn(II) ions (pH = 5; mass of chitosan beads = 0.5 g; temperature = 10 °C).

Figure 4. Equilibrium adsorption capacity (q_e) as a function of temperature (T) for Cr(VI) and Zn(II) (C₀ for Zn(II) = 15.3 mmol/L; C₀ for Cr(VI) = 19.23 mmol/L; pH = 5–6; mass of chitosan beads = 0.5 g).

Figure 5. ln K_d as a function of 1/T for Cr(VI) and Zn(II). Lines indicate linear fits (C₀ = 1,000 mg/L for both metals).

Table 2. Thermodynamic parameters and fitted ln K_d of Cr(VI) and Zn(II) adsorption onto chitosan beads

Cr(VI)					Zn(II)
T(K)	ln Kd (L/g)	ΔG°(kJ/mol)	ΔH° (kJ/mol)	ΔS° (kJ/mol K)	ln Kd (L/g)	ΔG° (kJ/mol)	ΔH° (kJ/mol)	ΔS° (kJ/mol K)
283	0.31	−1.208	−48.045	−0.165	0.75	−1.974	−45.65	−0.154
293	0.16	0.446			0.29	−0.431
303	−0.9	2.101			−0.4	1.112
313	−1.5	3.756			−1.0	2.655

Figure 6. ln C_e as a function of 1/T for Cr(VI) adsorption onto chitosan beads at different adsorption capacities (q_e). Lines indicate linear fits.

Figure 7. ln C_e as a function of 1/T for Zn(II) adsorption onto chitosan beads at different adsorption capacities (q_e). Lines indicate linear fits.

Table 3. Isosteric heat of Cr(VI) and Zn(II) adsorption onto chitosan beads at constant adsorption capacities (q_e) with Kq-constant values, and coefficients of determination

Heavy metals	Adsorption capacity (qe) (mmol/g)	Isosteric heat of adsorption (ΔHx) (kJ/mol)	Kq- constant (mmol/L)	Coefficient of determination (R^2)
Cr(VI)	0.12	−49.786	20.307	0.9898
	0.13	−53.117	21.551	0.9919
	0.16	−53.623	21.969	0.9961
	0.21	−55.782	23.038	0.9903
	0.43	−61.477	25.532	0.9867
Zn(II)	0.22	−57.733	23.215	0.9998
	0.32	−58.763	23.679	0.9996
	0.47	−59.056	24.117	0.9908
	0.58	−59.874	24.383	0.9951
	0.74	−64.187	26.406	0.9971

Figure 8. (a) Pseudo first-order kinetics (solid lines), (b) pseudo second-order kinetics (solid lines) fitted for Cr(VI) and Zn(II) adsorption experiments (pH = 5–6; mass of chitosan beads = 0.5 g; C₀ = 1,000 mg/L for both metals; temperature = 10 °C).

Figure 9. (a) Langmuir isotherms (solid lines), (b) Freundlich isotherms (solid lines) fitted for Cr(VI) and Zn(II) adsorption experiments (pH = 5–6; mass of chitosan beads = 0.5 g; temperature = 10 °C).

Table 4. Langmuir and Freundlich isotherm parameters for Cr(VI) and Zn(II) adsorption onto chitosan beads

Heavy metals	Langmuir parameters			Freundlich parameters
	qm (mmol/g)	KL(mmol/L)	$R_{\text{L}}^2$	1/n	KF (mmol/g)	$R_{\text{F}}^2$
Cr(VI)	1.713	1.6715	0.997	0.102	1.24	0.915
Zn(II)	1.86	3.88	0.915	0.204	1.24	0.829