Efficient use of Mg-modified zeolite in the treatment of aqueous solution contaminated with heavy metal toxic ions

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Highlights

  • Modified zeolite with magnesium (Mg-zeolite) was used for heavy metal removal.

  • Adsorption capacity of Mg-zeolite was two-fold higher than that of natural zeolite.

  • The removal (>98%) of Pb, Cd, and Cu in aqueous solution was successful.

  • The optimal Mg/zeolite ratio for Pb, Cd, and Cu removal was 1.5 to 1.75.

  • High efficiency and low dose requirements of heavy metal removal by Mg-zeolite.

Abstract

The aim of this study was to investigate adsorption using modified zeolite with magnesium (Mg-zeolite) in aqueous solutions for removal of heavy metal ions. The adsorption capacity of Mg-zeolite was determined to be 1.5-fold higher than that of zeolite modified with Na or K and 1.5- to 2.0-fold higher compared to that of natural zeolite. The Mg-zeolite successfully adsorbed > 98% of the Pb2+, Cd2+, and Cu2+ in the aqueous solutions at the following optimal conditions: 40 mg/L dosage and 1.5–1.75 Mg/zeolite ratio. The maximum binding capacity increased with an increase in pH to > 5 for Pb and Cu and to > 7 for Cd. The adsorbent, Mg-zeolite, could be promising for adsorption due to its high efficiency and low dose requirements. In addition, Mg-zeolite does not contaminate treated wastewater, so it can be recycled to reduce not only cost and demand for water, but also extra operational costs for reuse of wastewater.

Introduction

The removal of heavy metal toxic ions from wastewater has received increased attention in the past decade due to global awareness of the underlying detriment of heavy metals in the environment. Traditional technologies to treat wastewater include various combinations of biological, physical, and chemical methods; however, these methods require high capital and operating costs [1]. The most efficient methods are technologies based on adsorption from water onto activated carbon, which appears to be the best method for the elimination of heavy metals. However, this process is expensive and difficult to regenerate after use [2]. Therefore, inexpensive and effective alternative adsorbents are required, and many studies have been devoted to this search.

Zeolites are hydrated aluminosilicate minerals made from the interlinked tetrahedra of alumina (AlO4) and silica (SiO4). The aluminum ion is small enough to occupy the position in the center of the tetrahedron of four oxygen atoms, and the isomorphous replacement of Si4+ and Al3+ produces a negative charge in the lattice [2], [3]. The net negative charge is balanced by the exchangeable cations (sodium, magnesium, potassium, or calcium). These cations are exchangeable with certain cations in solution such as lead, cadmium, copper, and manganese. The cation exchange capacity of a typical zeolite (200–400 meq/100 g) is two or more times higher than that of smectite (80–100 meq/100 g) with the primary components of bentonite [4], [5]. Zeolite has the best cation exchange capacity of the clay minerals and is capable of selectively exchanging the cations [6]. The sorption capacity of natural zeolite for inorganic cations has been investigated by many researchers [3], [4], [7], [8], [9]. Modified zeolite has been shown to adsorb more heavy metals than natural zeolite [10], [11], [12]. Natural zeolite sorbed Cu2+ 66.10%, Co2+ 77.96%, Zn2+ 45.96%, and Mn2+ 19.84%, according to a study by Erdem et al. [7]. However, Zhang et al. [10] reported the highest adsorption of Mn2+ and Sb3+ with modified zeolite in HNO3 solution and NaOH solution. The addition of multivalent cations enhances the adsorption rates of the organic matter-clay mineral [13], [14]. The multivalent cations can be in the form of a chloride salt and/or a sulfate salt, such as MgCl2 and MgSO4. Magnesium chloride allows for a bridge between the negative surface of the zeolite mineral and the organic/inorganic matter [15]. It creates flakes of organic/inorganic matter–zeolite-magnesium, which are adsorbed relatively quickly. The precipitated flakes of organic/inorganic matter-zeolite dispersed by the magnesium include other materials, such as very fine clay particles [1], [2], [16]. Therefore, zeolite modified with relatively innocuous magnesium was particularly suitable for removing undesirable heavy metals, as shown in the present study. Zeolite modified with magnesium (Mg-zeolite) might be an alternative environmentally-friendly absorbent. Mg-zeolite is an inorganic alternative adsorbent provides several advantages, including low cost, abundant availability, non-toxicity, and high potential for pollutant adsorption. Its adsorption capacity for cations and organic matter is high due to its colloidal properties and negatively-charged layers [1], [15]. A modified Mg-zeolite is very economical for re-use by washing in a high concentration of NaCl. In addition, Mg-zeolite has large pore sizes of 4Å–5 Å, whereas K+-modified zeolite has a pore size of 3 Å [2], [17]. Large-caliber zeolite is far superior to the small caliber of the catalyst and has a higher adsorption capacity because of its relatively high surface activity and large crystal cavity [17], [18]. The treatment of heavy metals using Mg-zeolite adsorbent has been rarely reported. Therefore, we attempted to exploit the abundantly available Mg-zeolite for the individual removal of three important toxic heavy metal toxic, viz., Cu2+, Pb2+, and Cd2+, from aqueous solutions.

Section snippets

Materials

The zeolite (3 Å) was obtained from Kumnong Company, Pohang, Korea. The chemical composition of the zeolite is given elsewhere [3]. The zeolite was fully washed several times with deionized water and dried for 24 h at 100 °C. Two grams of zeolite powder were pretreated with 200 ml of MgCl2 (purity ≥ 98%, Junsei, EP) in a 250 ml Erlenmeyer flask and then stirred for 24 h at 250 rpm using a horizontal shaker. After 24 h, the supernatant was discarded, and the reaction to replace the new MgCl2  ·  6H2O

Characterization of Mg-zeolite and comparison of zeolite modified with K+, Ca2+, Mg2+, and Na+

The BET surface area, pore volume and pore size of natural zeolite and Mg-zeolite summarized in Table 1. The BET specific surface area and total pore volume of zeolite was found to be 27.21 m2/g and 0.141 cm3/g, respectively. However, the pore size and pore volume of zeolite was increased after modification from 3.38 to 7.97 nm and 0.141 to 0.168 cm3/g, respectively. In addition, the modification caused significantly increase in specific surface area from 27.21 to 62.97 m2/g. Increasing of pore

Conclusions

In summary, the modification of zeolite with Mg caused significantly increase in specific surface area from 27.21 to 62.97 m2/g. The adsorption capacity of Mg-zeolite was 1.5 to 2.0 times higher than that of natural zeolite. The Mg-zeolite with the ratio of 1.5 had the highest removal rates of Pb2+, Cd2+, and Cu2+ (> 98%) in the aqueous solution. The concentrations of Pb2+, Cd2+, and Cu2+ until 5 mg/L were completely removed with 40 mg/L Mg-zeolite. However, up to 10 mg/L of heavy metal decreased

Acknowledgments

This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2013006899).

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