Removal of the herbicide 2,4-dichlorophenoxyacetate from water to zinc–aluminium–chloride layered double hydroxides

Abstract

Batch sorption studies were conducted to investigate the potential of [Zn–Al–Cl] layered double hydroxides (LDHs) for the removal of the herbicide 2,4-dichlorophenoxyacetate (2,4-D) from contaminated aqueous solutions. Experiments were performed at different pH values, initial pesticide concentration, solid/pesticide ratio and anion exchange capacity of LDHs. The LDH samples evaluated had very high retention capacity for 2,4-D whose removal was a rapid process, as a quasi-equilibrium state was reached after 1-h reaction time. The adsorption can be described by Langmuir-type isotherms, with an average affinity constant of 12.5 L mmol⁻¹. At initial 2,4-D concentrations between 0.08 and 4 mmol L⁻¹, the solids removed up to 98% of the pesticide. Physicochemical characterization of the LDH solids, both fresh and after removal of 2,4-D, by X-ray diffraction, infrared spectroscopy and thermogravimetry, indicates that the retention of 2,4-D is done by adsorption on the surface of the solid for low 2,4-D concentrations. However, a combination of surface adsorption and interlayer ion exchange takes place when the 2,4-D concentration is high.

Introduction

Pesticides play an important role in modern day agriculture with new formulations being introduced on a regular basis. Amongst these, chlorinated phenoxyacetic acid herbicides, such as 2,4-dichlorophenoxyacetate (2,4-D), are commonly used for control of weeds in wheat, rice, maize and aquaculture. Together, these compounds account for most herbicide production world-wide. Their massive use has resulted in their ubiquitous presence in the environment in the form of sub-lethal pollution, and has led to their leaching into subsurface zones and subsequent contamination of surface and ground waters. Human and animal exposure to these compounds has caused serious toxic symptoms (Abul Farah et al., 2004). Therefore, these substances have attracted immediate attention and have undergone extensive toxicity testing and pharmacokinetic analysis in numerous mammalian species (Timchalk, 2004). Concurrently, considerable research efforts have been undertaken to develop powerful methods for their monitoring and elimination from soil and water by biological and physical–chemical means, either through degradation or retention (Laganà et al., 2002; Pieper et al., 2004; Aksu and Kabasakal, 2004). Adsorption on different solid materials, such as soils, clays and activated carbons, has been used by several investigators to eliminate these pesticides from water samples.

Clays and clay minerals have been particularly good candidates for such a process (Lagaly, 2001; You et al., 2002). Their selection stems from their ability to undergo surface and structural modification as well as their high adsorption capacity. The comprehension of the clay–pesticide interaction should be helpful in developing new pesticide and/or clay–pesticide formulations which fulfil the requirements of environment control and reduce the hazardous distribution of pesticides in the environment.

In order to contribute to the understanding of the interaction phenomenon, we have undertaken a study of the retention of pesticides by simple and easily synthesized materials, namely, the layered double hydroxides (LDHs). These materials can be considered as the most suitable adsorbents for anionic pesticides, given their layered structure and their high anionic exchange capacity, AEC (≈3 meq g⁻¹), compared to cationic clays such as smectite and vermiculite (≈0.65−1.6 meq g⁻¹). Many investigations, carried out during the last decade, have shown that these materials are promising for environmental remediation, in particular for the removal of organic (Ulibarri et al., 2001; Barriga et al., 2002, Zhao and Nagy, 2004; Dos Reis et al., 2004) and inorganic (Houri et al., 1999; Kovanda et al., 1999; Seida and Nakano, 2002; Nomura et al., 2003) pollutants from water.

LDHs, also called anionic clays, are composed of brucite-like layers, Mg(OH)₂, where partial substitution of trivalent for divalent metal ions leads to positive layer charges compensated by hydrated anions within the interlayer spaces. They can be represented by the general formula:

$${[\mathrm{M}}_{1-x}^{\mathrm{II}}{\mathrm{M}}_x^{\mathrm{III}}{(\mathrm{OH})}_2{]}^{x+}{[\mathrm{X}}_{x/m}^{m-}·n{\mathrm{H}}_2{\mathrm{O}]}^{x-},\mathrm{abbreviated}\mathrm{as}{[\mathrm{M}}^{\mathrm{II}}-{\mathrm{M}}^{\mathrm{III}}-\mathrm{X}],$$

where M^II and M^III are divalent and trivalent metal ions, respectively, X^m− is the interlayer anion and x is defined as the M^II/(M^II+M^III) ratio.

The principal structural components of LDHs are the charged layers whose varied chemical compositions give them multifunctional capabilities, such as ion exchange, intercalation, adsorption, biocompatibility, buffering and memory effect. These are the basis for their varied technological applications in such diverse fields, as pharmacy, catalysis, food and polymer industries, electrochemistry, separation technology, etc. The properties and applications of these hydrotalcite-type compounds have been the subject of a number of general and specialized reviews (Rives, 2001; Braterman et al., 2003).

The research presented in this paper is concerned with the removal of the herbicide 2,4-D from water by [Zn–Al–Cl] LDHs. Given the low adsorption coefficient of 2,4-D and its high mobility in most environmental settings, it has been found to leach easily from soils with low organic matter content and has been detected in many agricultural and industrial streams (Hermosin and Cornejo, 1992). In addition, it minimally adsorbs to different solid materials, such as soils, activated carbon and vermiculite (Hermosin and Cornejo, 1993; Belmouden et al., 2000; Spark and Swift, 2002). Because of its low pK_a value of 2.8 (Wauchope et al., 1992), 2,4-D exists mainly as a deprotonated species under natural pH conditions, suggesting it may adsorb better on LDHs due to their high anionic exchange capacity.

Data on the influence of solution pH, initial pesticide concentration, solid/herbicide ratio and AEC of the LDH, as governed by the M^II/M^III ratio, are presented in this paper. The localization of the pesticide in the interlayer space and/or on external surfaces of the LDH is studied by X-ray diffraction (XRD), as well as by infrared spectroscopy (IR) and thermogravimetry (TG) and discussed from the determination of the apparent interlayer spacing of the LDH.