Development of sustainable and ecofriendly metal ion scavenger for adsorbing Cu2+, Ni2+ and Zn2+ ions from the aqueous phase

ABSTRACT

In the present investigation, adsorption of Cu²⁺, Ni²⁺ and Zn²⁺ ions is carried out on the surface of novel composite made up of bentonite clay and red-ocher. The study of molecular dynamics through dimensionless numbers$\phi$, N_k and $\lambda$ proved that adsorption of Cu²⁺, Ni²⁺ and Zn²⁺ ions on the surface of composite was diffusion controlled. A deviation of 0.08% for Cu²⁺, 1.26% for Ni²⁺ and 0.53% for Zn²⁺ ions between experimental and artificial neural network model predicted values reflected that the back-propagation technique involving Levenberg-Marquardt algorithm was appropriate for the prediction of the output function. The physico-chemical characterization of composite indicated fluffy composite surface housing hydroxyl group, Si-O stretching and Si-O-Al vibration. These groups were due to aluminum-rich octahedral centers and Si-O bond stretching of silica and quartz. The X-ray diffractogram of composite depicted the presence of quartz, alumina, montmorillonite and hematite in the composite that played a significant role in adsorbing heavy metal ions. The Langmuir isotherm and pseudo-second-order kinetic model showed lower values of sum of square of residuals compared to other models. This showed that sorption of Cu²⁺, Ni²⁺ and Zn²⁺ ions followed monolayer coverage coupled with chemisorption mode. The composite possesses high adsorption capacity such as 61.86 mg/g for Cu²⁺ ions, 37.89 mg/g for Ni²⁺ ions and 10.48 mg/g for Zn²⁺ ions. The adsorption of metal ions onto composite surface was endothermic with increased randomness at the solid–liquid interface.

KEYWORDS:

• Bentonite clay
• copper
• molecular dynamics
• nickel
• red ocher
• scavenger
• zinc

Abbreviations

C	=	Bulk concentration (mol/m3)
${\mathrm{C}}^{\ast }$	=	Dimensionless concentration
Ce	=	Equilibrium concentration of metal ions (mg/L)
Ci	=	Initial concentration of metal ions (mg/L)
Cs	=	Sub-surface concentration of ligands (mol/m3)
Cu2+	=	Copper (II) ions
D	=	Diffusion coefficient (m2/s)
DF	=	Film diffusion coefficient (cm2/sec)
DP	=	Pore diffusion coefficient (cm2/sec)
$K_a$	=	Adsorption rate constant (mol/m s)
$K_d$	=	Desorption rate constant (mol/m s)
m	=	composite mass (g)
MSE	=	Mean Square Error
Ni2+	=	Nickel (II) ions
qe	=	Equilibrium uptake capacity (mg/g)
qt	=	Uptake capacity at time ‘t’ (mg/g)
R	=	Universal gas constant (8.314 J/mol/K)
s	=	Spherical radial coordinates (m)
S	=	Surface concentrations of ligands (mol/m3)
S*	=	Dimensionless surface concentration
Si	=	ith surface concentration of molecule (mol/m3)
$S_{\mathrm{\infty }}$	=	Maximum surface concentration (mol/m3)
t	=	Time (min)
T	=	Temperature (K)
V	=	Solution volume (L)
Zn2+	=	Zinc (II) ions
ΔG	=	Gibbs free energy change (J/mol)
ΔH	=	Change in enthalpy (kJ/mol)
ΔS	=	Change in entropy (J/mol K)
ϕb	=	Adsorption flux (mol/m2 s)
ϕd	=	Desorption flux (mol/m2 s)
ς	=	dimensionless distance
$\tau$	=	dimensionless time