Selenium adsorption on Mg–Al and Zn–Al layered double hydroxides

Abstract

Layered double hydroxides (LDHs) have high anion exchange capacities that enhances their potential to remove anionic contaminants from aqueous systems. In this study, different Mg–Al and Zn–Al LDHs were synthesized by a coprecipitation method, with the products evaluated for their ability to adsorb selenite (SeO₃²⁻) and selenate (SeO₄²⁻). Results indicated the adsorption isotherm for SeO₃²⁻ retention by Mg–Al and Zn–Al LDHs could be fitted to a simple Langmuir equation with the affinity of SeO₃²⁻ on Zn–Al LDH higher than that on Mg–Al LDH. The adsorption trends for both SeO₃²⁻ and SeO₄²⁻ on LDHs were similar under the experimental conditions. The SeO₃²⁻ adsorption was rapid and was affected by the initial SeO₃²⁻ concentration. The quasi-equilibrium for 0.063 and 0.63 cmol/l SeO₃²⁻ solutions was obtained within the first 30 and 60 min of adsorption, respectively. The maximum adsorption of SeO₃²⁻ on Mg–Al LDH was higher than that of Zn–Al LDH and decreased with an increase in the LDH mole ratio of Mg/Al. The high pH buffering capacities and the SeO₃²⁻ adsorption for Mg–Al and Zn–Al LDHs was a function of pH. Competing anions strongly affected the adsorption behavior of SeO₃²⁻ with SeO₃²⁻ adsorption increasing in the order: HPO₄²⁻<SO₄²⁻<CO₃²⁻<NO₃⁻. The release of adsorbed SeO₃²⁻ depended upon the type of competing anion in the aqueous solution. For example, with CO₃²⁻ the adsorbed SeO₃²⁻ could be desorbed completely from Mg–Al LDH. X-ray diffraction patterns indicated that d-spacing increased when SeO₄²⁻ was adsorbed, but not with SeO₃²⁻ adsorption.

Introduction

Layered double hydroxides (LDHs) have received considerable attention in recent years because of their unique layered structures and high anion exchange capacities. Important technological applications of LDHs include their use as catalysts, electrochemical agents, separation media, as well as for different medical purposes Playle et al., 1974, Martin and Pinnavaia, 1986, Kwon et al., 1988, Suzuki et al., 1989, Cavani et al., 1991, Corma et al., 1994, Corma et al., 1995, Zhao and Vance, 1997, Zhao and Vance, 1998a, Zhao and Vance, 1998b. Generally, LDHs consist of positively charged layers separated by anionic gegenions, A⁻, and water molecules (Fig. 1). The basic chemical composition of LDHs is written as Giannelis et al., 1987, Kopka et al., 1988, Meyn et al., 1990:

$$\text{[}\text{M}{}_{\mathrm{1-x}}{}^{\mathrm{<mtext>II</mtext>}}\text{M}{}_{\mathrm{x}}{}^{\mathrm{<mtext>III</mtext>}}\text{(}\text{OH}\text{)}{}_2\text{]}{}^{\mathrm{z+}}\text{A}{}_{\mathrm{z/n}}{}^{\mathrm{n-}}\text{·y}\text{H}{}_2\text{O}$$

where M^II=divalent cations (Ca²⁺, Mg²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Mn²⁺, but also the monovalent cation Li⁺); M^III=trivalent cations (Al³⁺, Cr³⁺, Fe³⁺, Co³⁺, Mn³⁺), and Aⁿ⁻=interlayer anions (Cl⁻, NO₃⁻, ClO₄⁻, CO₃²⁻, SO₄²⁻). The layer charge is z=x when the M^II is a divalent cation and z=2x−1 when M^II is a monovalent cation. The maximum molar amount of interlayer water that can be structurally incorporated as a monolayer in an LDH is 1−(z/n). The M^II/M^III structural cation ratio generally varies from 1 to 5.

LDHs can be readily synthesized by coprecipitation methods under laboratory conditions Miyata, 1975, Miyata, 1980, Rhee et al., 1997. The interlayer spacing of LDHs can vary, which is dependent on the size and geometrical structure of the intercalated anions. LDHs also have relatively large surface areas (0.02–0.12 km²/kg) and high anion exchange capacities (200–500 cmol/kg) Miyata, 1980, Miyata, 1983. Because of these properties, LDHs have been researched as potential adsorbents for removing toxic anionic species from aqueous systems Rhee et al., 1997, Goswamee et al., 1998.

Selenium (Se) is an essential nutrition element for humans and animals, but is only required in small amounts and has a very narrow range between deficient and toxic levels (National Academy of Science, 1976). High natural concentrations of Se in land drainage waters in the western United States, such as those found in California, South Dakota and Wyoming, have produced new questions concerning potential toxicity effects and threats to both wildlife and human health Lakin, 1961, Burau, 1985. In soil, groundwater, and seawater, Se is most mobile in the selenite (SeO₃²⁻) and selenate (SeO₄²⁻) forms Measures and Burton, 1980, Adriano, 1986.

Selenium sorption on soils, clay minerals, oxides of Al and Fe, and various geological substrata have been investigated previously by many researchers Frost and Griffin, 1977, Hansmann and Anderson, 1985, Ahlrichs and Hossner, 1987, Balistrieri and Chao, 1987, Hayes et al., 1987, Neal et al., 1987, Fio et al., 1991, Blaylock et al., 1995, Saeki et al., 1995, Vance et al., 1998. In 1989 and 1990, O'Neil and coworkers O'Neil et al., 1989, O'Neil et al., 1990 developed a new method for reducing the amount of anionic metal ligand complexes, such as SeO₃²⁻ and SeO₄²⁻, in a solution using an LDH. These results suggested LDHs could be a potentially new type of Se adsorbent. Currently, however, no detailed investigation on the adsorption of Se by LDHs has been published.

In the present work, we prepared Mg–Al and Zn–Al LDHs from bivalent and trivalent metal ion combinations. The adsorptive capabilities of SeO₃²⁻ and SeO₄²⁻ on Mg–Al and Zn–Al LDHs were evaluated by time-dependence analysis, adsorption/desorption isotherm studies, and tests to determine LDH maximum adsorption capability. The effect of environment factors, i.e., pH values and competing anions, were also investigated to identify their influence on SeO₃²⁻ adsorption.