Treatment of wastewaters contaminated with zinc ions using natural zeolite as adsorbent in a fixed bed column

A significant global environmental challenge is the discharge of enormous amounts of polluted wastewater containing heavy metals due to industrial activities. Heavy metals such as lead, cadmium, zinc, mercury, lithium, and copper, if present in water at certain concentrations, are considered to be a major concern due to their toxicity and carcinogenicity, which may cause human health problems (Lu & Chiu 2006; Khan et al. 2008; Mishra & Patel 2009). Heavy metal ions are categorized as significant pollutants to the environment since due to their stability and persistency they cannot either be degraded or destroyed (Lim et al. 2012).

Contamination of industrial wastewater effluents with zinc metal ions has become an environmental concern (Bose et al. 2002; Wilson & Pyatt 2007). Huge amounts of wastewaters with high zinc ion (Zn²⁺) concentrations are discharged to the environment by many industries, such as metal plating, the manufacture of batteries, metal fabrication, mining and mineral processing industries (Raut et al. 2012). Moreover, some contamination resulting from the dissolution of zinc has been detected in water flowing through galvanized pipes and fittings. It is worthwhile to mention that zinc in trace amounts is a significant metal for the growth and health of humans, animals and plants. However, it becomes toxic if it exceeds certain levels and can cause the desiccation of muscles, imbalance of electrolytes, stomach ache, vertigo, and disharmony (Veli & Alyuz 2007). Also, it has been reported that increased amounts of zinc might cause depression, lethargy, neurological signs such as seizures and ataxia, and increased thirst (Kurniawan et al. 2006). According to the World Health Organization (WHO), drinking water containing zinc at levels above 5 mg/L is not acceptable (Mishra & Patel 2009; Caliskan et al. 2011). It is therefore important for the industries producing such wastewaters to develop methods for zinc removal from aqueous solutions and the reduction of its concentration to acceptable levels before discharging it to the environment.

There are several classical physico-chemical methods that have been employed for the removal of zinc and other heavy metal ions from contaminated wastewater effluents, such as chemical precipitation (Eccles 1999; Wang et al. 2004; Aziz et al. 2008), membrane filtration (Juang & Shiau 2000; Blöcher et al. 2003; Saffaj et al. 2004), ion exchange (Blöcher et al. 2003), reverse osmosis (Abu Qdaisa & Moussab 2004), nanofiltration (Lv et al. 2008; Khedr 2008), ultrafiltration (Trivunac & Stevanovic 2006; Barakat 2008), electrodialysis (Chen 2004; Jakobsen et al. 2004), photocatalysis (Yoona et al. 2009), solvent extraction (Simonin et al. 2003), coagulation–flocculation (Amuda et al. 2006), flotation (Żak 2012), and ion exchange (Muzenda et al. 2011). However, these methods have different disadvantages such as high operational cost, low removal efficiency, incomplete metal removal, high reagent or energy requirements, membrane fouling and generation of toxic sludge or other heavy metal-containing waste products (Barakat 2011). The search for new methods involving the removal of zinc metals from wastewaters with low costs, high removal efficiency, minimum sludge generation, and easier sludge disposal has attracted attention to adsorption.

Recently, adsorption has become one of the attractive treatment techniques for wastewater laden with zinc ions. Basically, adsorption is a mass transfer process by which zinc ions are transferred from the liquid phase to the surface of the adsorbent, and become bound by physical and/or chemical interactions. Various low-cost adsorbents derived from agricultural waste, industrial by-products, natural material, or modified biopolymers, have been recently developed and applied for treatment of zinc contaminated wastewater. There are many published research studies aimed at the development of adsorbents for removal of zinc (Zn²⁺) ions from contaminated wastewater.

Natural zeolites are promising and efficient adsorbents for the removal of zinc ions from wastewater due to their physical and chemical properties (Dimirkou 2007). They are characterized by their widespread grids of channels and large contact area. Their chemical structure is characterized by a three-dimensional crystalline structure of tetrahedral silica or alumina anions of strong bonds with the high ability to act as substrates for adsorbing cations (Li et al. 2008).

This work investigates the utilization of a low-cost natural zeolite adsorbent, which is available in enormous reserves and does not require any treatment except for size reduction and sieving. The study examines its potential as an adsorbent for removal of zinc ions using fixed bed columns.

Synthetic zinc heavy metal stock solution at a concentration of 1,000 mg/L was prepared by dissolving zinc chloride analytical grade (Merck p.a.) salt in double deionized water. The solution was then diluted to the required initial concentration of 20 mg/L.

It is noticeable that the column performance for the adsorption of zinc ions on UNZ2 is more efficient than using the adsorbent UNZ1. It can be observed that the breakthrough occurred earlier in the case of UNZ1. However the slopes of the breakthrough curves are almost identical for both adsorbents (UNZ1 and UNZ2). The enhanced adsorption and the improved column performance in the case of the zeolite UNZ2 can be attributed to the influence of grain size. The adsorbent UNZ2 has a grain size of 0.21–0.42 mm while the adsorbent UNZ1 has a grain size of 0.42–0.841 mm. Obviously, increasing the size of adsorbent particles reduces bed capabilities and thus reduces breakthrough time. The diffusion path is shorter for smaller grains, and therefore the adsorbate will more easily be adsorbed to the active sites of the zeolite particles. Moreover, smaller particles have a larger total exposed surface area and thus more active sites. Furthermore, the particle size has an important influence on the flow through the column. The smaller size particles lead to greater hydraulic resistance. This significantly influences the residence time of the solute in the column, which can be longer in the case of smaller particles. Therefore, there is a higher probability for adsorption equilibrium to be reached in the case of the smaller size particles, whereas in the case of larger size particles the solution is more likely to leave the column before equilibrium is reached.

It is worthwhile mentioning that the Thomas model uses the familiar Langmuir isotherm. The mathematical expression described by Equation (1) is the modified form of the Thomas model, which is commonly used to fit the fixed bed breakthrough adsorption experimental data. This modified model assumes a rectangular (irreversible) isotherm when solving the differential mass balance equation for a fixed bed column, and is mathematically equivalent to the Bohart-Adams model (Chu 2010).

The calculated Thomas model kinetic parameters in Table 1 reveal that the adsorbent UNZ2 has a higher value of adsorption capacity to zinc ions (50.75 mg/g) than UNZ1 (33.68 mg/g). In general, the zeolite used has a high capacity and can be used for long term operations for wastewater treatment by implementing fixed bed technology. The high adsorption capacity of the tested zeolites is attributed to their porous structure. Zeolites have a cage-like structure with open channels. On the other hand, zeolite has a high potential to exchange its alkaline cations with zinc ions.

The Thomas model fits the experimental data for the adsorption of zinc ions on UNZ1 and UNZ2 zeolites to a high degree, which reveals that the adsorption mechanism follows the second-order reversible reaction kinetics (Thomas 1944) and the adsorption behavior is controlled by both the chemical adsorption with an ion exchange process and electrostatic adsorption. The kinetic parameters calculated from the Thomas model can be implemented for the successful design of fixed bed wastewater treatment processes.

Clearly, the experimental breakthrough curves follow the Yoon and Nelson model with a correlation coefficient (R² = 0.94). The evaluated Yoon and Nelson model kinetic parameters K_YN and τ and the regression coefficient R² are given in Table 2.

An important kinetic design parameter is the time to 50% breakthrough (τ) determined by the Yoon and Nelson model. It gives an indication about the performance of the adsorbent to adsorb a certain adsorbate. By examining Table 2, it can be concluded that the time needed to reach 50% breakthrough is 2,006 minutes and 3,171 minutes for UNZ1 and UNZ2, respectively. This indicates that UNZ2 provides a better performance with longer service time.

This work evaluates a Jordanian natural zeolite to be implemented directly after crushing and sieving as an adsorbent for treating wastewater contaminated with zinc ions. The obtained adsorption parameters indicated that an industrial scale continuous adsorption column process can be implemented successfully to treat enormous flow rates of wastewater streams due to the large breakthrough time and the adequate adsorption capacity of the tested natural zeolite. The size of the grains was found to be a crucial parameter in enhancing the service time and the maximum adsorption capacity. It is recommended to conduct further studies to understand the other operating parameters such as pH, flow rate, and bed height.