Layered Double Hydroxide Applications as Sorbents for Environmental Remediation (pp.39-72)
Authors: (Ricardo Rojas, INFIQC, Departamento de Fisicoquímica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba, Argentina)
Abstract: The development of new techniques for pollution remediation is an area of high priority due to the increasing contamination of water and soils and the consequent risks to both human health and environmental equilibrium. The pollution causes are extremely diverse and, consequently, the chemicals object of remediation range from inorganic, (heavy metals, arseniate, chromate, cyanide, fluoride, etc.) to organic (petroleum by-products, pesticides, surfactants, among others). Different remediation procedures, involving chemical, biochemical or physicochemical technologies are employed according to the pollutant and the characteristics of the polluted media. One of the most widely studied is the removal or immobilization of the contaminant using sorbents such as active carbon, zeolites, ion exchange resins and layered double hydroxides (LDHs).
LDHs are mineral and synthetic compounds formed by brucite (Mg(OH)2)-like layers with partial isomorphic substitution by trivalent cations. This replacement leads to a positive charge excess compensated by anions weakly bonded by electrostatic forces and placed in the interlayer space. LDHs present an huge customization capacity: a wide range of metal ions, either divalent (Mg2+, Ca2+, Ni2+, Fe2+, Zn2+, Cd2+, Cu2+,…) or trivalent (Al3+, Fe3+, Cr3+,…) can be arranged in the layers. Different anions (from simple and small inorganic anions such as chloride, nitrate or carbonate, to large organic and biological anions such as surfactants, pharmaceutical drugs, and even biomolecules) can also be included due to these solids capacity to expand the interlayer distance.
The main features of these solids are a high anion exchange capacity (around 3 meq/g), the layers instability at low pHs and the capacity to reconstruct its lamellar structure from the oxides obtained by their calcination. Due to their anion exchange properties, LDHs are studied as sorbents for a wide variety of water pollutants either
inorganic (arseniate, chromate) or organic (pesticides, dyes) anions. The contaminant uptake during the reconstruction process of calcined LDHs has also been extensively studied and the acid base buffering properties of LDHs produce heavy metal ions precipitation as hydroxides, either as part of the hydroxylated layers or in a separate hydroxide phase. Also, due to their customization capacity, the environmental applications of LDHs can be extended: modification of LDHs with organic anions allows adsorption of neutral or even positively charged apolar species in the interlayer or in the surface of the solids and the intercalation of polydentate ligands such as citrate, malate and ethylenediaminetraacetate (EDTA) modifies the metal ions uptake capacity of the hydroxylated layers. In this chapter, the LDHs properties will be described and their uptake mechanisms analyzed. On this base, their applications as pollutant scavengers will be reviewed and analyzed, highlighting the factors that affect the sorbents behavior and the customization strategies used to obtain an optimal performance.
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