# Ni/Zn Layered Double Hydroxide (LDH) Micro/Nanosystems and Their Azorubine Adsorption Performance
## 1 Introduction  
Nowadays, one of the major challenges for researchers in chemistry is the elimination of dyestuff from liquid effluents before they are discharged to the external environment in view of their harmful effects on aquatic and terrestrial life [1, 2, 3, 4]. This kind of contaminant has characteristics of excellent chemical stability, good solubility in water, high colority and a complex aromatic structure, which decomposition under natural conditions is difficult. The presence of these colorful contaminants in water can restrict light and oxygen getting into water, leading to serious threats to the survival of aquatic organisms. Moreover, most organic dyestuffs, such as azo and anthraquinone dyes, are toxic or carcinogenic to the human body [5, 6, 7, 8]. Amongst these dyes, Carmoisine (E 122), also named Azorubine (Az), is used mainly in jams and preserves and was found by the US Certified Color Manufacturers Association to be unavoidably contaminated with low levels of beta-naphthylamine, which is well known as a carcinogen [9]. It has also been found to be mutagenic in animal studies [10; 11]. For that reason, their removal from effluent containing dye is necessary for our environment protection. However, the removal of dyes and their products from industrial wastewaters has remained a daunting challenge until now. Various techniques have been used and developed to remove unwanted dyes from contaminated waters, such as photocatalysis, advanced oxidation, electrochemical treatment, precipitation, solvent extraction, reverse osmosis and coagulation/flocculation [12; 13; 14; 15; 16]. However, these methods have low efficiencies, high operational energy and produce secondary pollutants [17]. It is therefore of much importance to develop cheap, cost effective and efficient methods to treat these dyes for environmental remediation. Among those, adsorption is a simple, low-cost and effective technique. For this, extensive varieties of adsorbents with higher uptake ability are available to remove the dyes from wastewater, such as SBA-15/graphene oxide nanocomposites, functionalized SBA-15 [18; 19] and ceramic membrane [20]. In addition, many nanomaterials with enhanced porosity and higher specific surface area such as MWNTs [19; 21; 22] and SBA-15/diphenyl carboxan/SDS nanocomposite [19], Zn/Al LDHs intercalated by dodecyl sulfate and carbonate using hydrothermal method without calcination [23] have shown also excellent uptake ability. Synthesized starch coated magnetic nanoparticles and polymeric nanocomposites have been also used for the removal of the unspent dyes from the industrial effluents [24]. Moreover, naturally available diatomaceous earth particles surface was modified with mesoporous silica xerogel denotes as diatom xerogel material as well as biosorbent materials have also proved their worth as useful and promising adsorbents to remove the dyes from the industrial effluents [25; 26]. Among the various nanomaterials, layered double hydroxides (LDHs) are recognized as hydrotalcite compounds or anionic clays. They are layered materials containing a metal hydroxide layer with a positive charge intercalated by negatively charged species [27; 28; 29]. Their general formula is [M\({}^{\text{II}}\)\({}_{1-\text{x}}\)M\({}^{\text{III}}\)\({}_{\text{x}}\)(OH)\({}_{2}\)]\({}^{\text{x+}}\)[A\({}^{\text{n-x/n}}\)mH\({}_{2}\)O], where M\({}^{\text{2+}}\) and M\({}^{\text{3+}}\) situate in the octahedral holes in a brucite-like layer and A\({}^{\text{n-}}\) has occupied the hydrated interlayer regions [30]. LDHs have many characteristics such as interlayer anion-exchange capacity, important surface area and surface with a positive charge, which support their application as adsorbents. These properties make them effective adsorbents to uptake organic/inorganic pollutants and principally their negatively charged species [31; 32].