Kinetic and Thermodynamic Studies of Zinc Removal from a Metal-Plating Wastewater Using Firouzkouh Zeolite

Introduction
With the rapid development of industries such as metal plating facilities, mining, fertilizer producing industries, tanneries and paper industries heavy metals enriched wastewaters are directly or indirectly discharged into the environment. Zinc is a trace element that is essential for human health. It is important for the physiological functions of living tissues and regulates many biochemical processes. However, too much zinc can cause eminent health problems, such as stomach cramps, skin irritations, vomiting, nausea and anemia. It is present in effluents from various industries such as galvanization and metal-plating facilities, manufacture of batteries and other metallurgical industries.
The most commonly used methods for the removal of metal ions from industrial effluents include chemical precipitation, solvent extraction, reverse osmosis, ultra filtration, adsorption and ion exchange. Adsorption has been proven to be an excellent and cheap method to remove hazardous materials such as heavy metals and organic dyes from waste effluents.
A good understanding of adsorption equilibrium and thermodynamics is required to design and operate an adsorption process. Natural zeolites are widely distributed in arid and semiarid regions of the world. They are low cost aluminosilicates, with a cage-like structure suitable for ion exchange due to isomorphous substitution of Al³⁺ with Si⁴⁺ in the structure, giving rise to a deficiency of positive charge in the framework. Due to the above structural characteristics as well as the chemical and mechanical stability, they have received a great attention for the removal of heavy metals from wastewaters. Therefore, this study was conducted to identify the suitability of this mineral for removal of zinc ions from a metal plating wastewater through a series of batch experiments. Accordingly, the influence of contact time, solution temperature, size and dose of zeolite particles were investigated.
 
Methods and Materials
Wastewater sample
The wastewater sample used in this study was taken from a zinc metal-plating facility in Tehran, northern Iran. The wastewater sample was analyzed for pH, electrical conductivity (EC), the total concentration of dissolved solids, turbidity, and the total concentration of Zn, Fe, Mg, Pb, and Cd ions. The concentration of Zn and other heavy metals was determined using a Savant GBC Atomic Absorption Spectrophotometer (AAS).
 
Kinetic experiments
All sorption studies were performed using the batch technique because of its simplicity and reliability. The experiments were conducted at pH = 5, sorbent concentration of 2 g l^-1, sorbent size of 20-50 µm and at the temperature of 20±1ºC.
To investigate the effect of contact time on the adsorption processes a constant mass of zeolite (adsorbent) (0.1 g) and 50 ml of known concentration of wastewater were added to 80 ml polypropylene centrifuge tubes. The mixtures were shaken vigorously on an orbital shaker (175 rpm) and at specified times (5, 10, 15, 20, 30, 60, 120, 240, 480, 720, 1440, and 2880 minutes). Tubes were then removed from the shaker and centrifuged at 2,500 rpm for 25 min and the Zn concentration in the supernatant was measured using AAS.
In order to investigate the effects of suspension pH and temperature, adsorbent dose and particle size of adsorbents on the percentage removal of Zn, the above experiments were also run by varying initial temperature (20 and 40 ° C using a thermostatic shaker bath) adsorbent dose (2, 4, 8, 12, 16, and 20 g l^-1), and particle size of adsorbent (<2, 2-20, and 20-50 µm) while keeping all other parameters constant. All the experiments were carried out using the largest size and lowest amount of sorbents to identify how removal efficiency is affected if smaller size of particles and higher doses of zeolite particles are applied.
Control treatments with no addition of adsorbent were also run to test the possible adsorption and/or precipitation of Zn onto the container walls. Preliminary experiments showed that metal losses due to the adsorption onto the container walls were negligible.
The amount of Zn²⁺ adsorbed by zeolite, C_S (mg g^-1), was obtained as follow:
                                                                                     (1)
Where, C₀ and C_e(mg l^-1) are the initial and final (equilibrium) concentrations of Zn, respectively; V (ml) is the volume of solution and M is the mass of sorbent (mg). All measurements were carried out with three replications.
 
Thermodynamic studies
The activation energy of Zn adsorption on zeolite was calculated using Arrehenius equation which is expressed as below:
k2 = k exp (-Ea/RT)                                                                                         (6)
Where k is the temperature-independent factor (g mg ^-1 min^-1), E_a the activation energy of sorption (KJ mol^-1), R the universal gas constant (8.314 J mol^-1 K) and T is the solution temperature (K).
Thermodynamic parameters of sorption including Gibbs free energy (ΔG₀), change in enthalpy (ΔH₀) and change in entropy (ΔS₀) were also calculated using the following equations (7&8):
ΔG⁰ = -RT ln K₀                                                                                                 (7)
Ln K₀ = ΔS⁰/R- ΔH⁰/RT                                                                                     (8)
 
K₀ can be defined as:
K₀ = C_solid/C_liquid                                                                                                 (9)
 
Where, C_solid is the amount of Zn²⁺ adsorbed by zeolite at equilibrium and C_liquid is the equilibrium concentration of Zn²⁺ in solution. The values of ΔH⁰, ΔS⁰ and ΔG⁰ calculated from the slope and intercept of the plot of Ln K₀ versus 1/T, respectively.
 
Results and Discussion
Contact time is an important parameter because it can reflect the adsorption kinetics of an adsorbent for a given initial concentration of adsorbate. The results showed an increasing trend on sorption of zinc ions onto zeolite particles. Based on this, the maximum adsorption capacity of zeolite for zinc ions was 17.9 mg g^-1 and more than 80 percent of the total amounts of zinc ions were absorbed on zeolite particles within first 2 hours of experiments. The initial rapid phase may be due to the increase in the number of vacant sites and also the high concentration gradient between adsorbate in solution and that in the adsorbent.
As the temperature increases from 20 to 40 ^◦C, the adsorption capacity of sepiolite for Zn²⁺ decreases from 17.9.1 to 14.9 mg g^-1. The decrease in removal capacity of Zn²⁺ ion with the rise in temperature is probably due to an increase in desorption of Zn²⁺ ion from the minerals interface to the solution. Results obtained from thermodynamic studies illustrated that sorption of zinc on zeolite particles is a reversible exothermic and physical process.
As compared to the pseudo-first-order kinetic model, a very good correlation coefficient (r²) was obtained for the pseudo-second-order kinetic model. 
As expected, the percent removal of Zn²⁺ ions increases as the amount of sepiolite increases. This can be attributed to an increase in the number of sorbent sites after the addition of more mineral particles to the suspension.
The results of the effect of zeolite particle size on the removal efficiency of Zn²⁺ ions from wastewater  indicates that as the particle size decreases, the metal diffusion is induced. This increases the accessibility of Zn²⁺ ions by the mineral. This suggests that the most suitable particle size of sepiolite for the removal of Zn²⁺ ions from wastewater studied is < 2µm.
The results also indicated that under similar conditions (T= 40^◦C), increasing dose of zeolite particles to more than 12 g l^-1 with sizes less than 2 µm is a good way to reach to the maximum efficiency for the removal of zinc ions from the studied wastewater.