1 UPTAKE OF DICHROMATE FROM AQUEOUS SYSTEM USING Zn-Al-NO 3 HYDROTALCITE THROUGH ANION EXCHANGE Fitriana Nindiyasari 1 , Roto 1* and Iqmal Tahir 2 , 1 Analytical Chemistry Laboratory, Department of Chemistry, Mathematics and Natural Sciences Faculty Gadjah Mada University, Sekip Utara, Yogyakarta 55281 2 Physical Chemistry Laboratory, Department of Chemistry, Mathematics and Natural Sciences Faculty Gadjah Mada University, Sekip Utara, Yogyakarta 55281 * Corresponding author: Tel/Fax: (0274) 545188, E-mail: roto05@ugm.ac.id Abstract Chromium species, one of toxic pollutants in aqueous system particularly in the form of dichromate, is treated using an ion exchanger of Zn-Al-NO 3 hydrotalcite. Synthesis of Zn-Al-NO 3 hydrotalcite and its application as an anion exchanger to lower dichromate concentration has been done. Zn-Al-NO 3 hydrotalcite was prepared using stoichiometric method at a constant pH of 7 where hydrothermal treatment was added. Kinetics and mechanism of anion exchange of NO 3 - in Zn-Al-NO 3 hydrotalcite by dichromate anion and regeneration process were investigated. The products were characterized by XRD, FT-IR and atomic adsorption spectrophotometry. The initial Zn-Al-NO 3 hydrotalcite has chemical formula of Zn 0.74 Al 0.26 (OH) 1.7 (NO 3 ) 0.26 .0.27H 2 O. Kinetics of anion exchange of NO 3 - in Zn-Al-NO 3 hydrotalcite by dichromate is fit to the second order of reaction to yield reaction rate constant k = 0.301 M -1 s -1 (R 2 =0.95). The anion exchange capacity (AEC) was observed to be 381 meq Cr 2 O 7 2- /100 g hydrotalcite. Further, the Zn-Al- Cr 2 O 7 hydrotalcite could be regenerated successfully by NO 3 - . Keywords: hydrotalcite, nitrate, anion exchange capacity, dichromate, regeneration INTRODUCTION Chromium is of crucial importance because of its use in stainless steel and superalloys. These materials are vitally important to industrialized societies because of their applications in jet engines, nuclear plants, chemical-resistant valves, and other applications in which a material that resist heat and chemical attack is required. However, chromium waste can be dangerous for environment because its forms are determined as toxic metal. Species of Cr(VI) in water system can be very toxic, corrosive, and carcinogenic and have high dissolving. Dissolved oxygen in water system can slowly oxidize Cr(III) to Cr(VI) and gives effect for mortality. The maximum permitted limit of chromium in fresh water is 0.05 mg/L. This toxicity can result in lungs cancer and chronics injury [1]. Chromium(VI) in water system has two forms; those are Cr 2 O 7 2- and CrO 4 2- anions [2]. Although waste treatment can be done using precipitation method yet the effectiveness needs to be investigated more. Chromium exists in the oxidation states ranging from +6 to -2, however, only the +6 and +3 oxidation states are commonly encountered in the environment. Cr(Vl) exists in solution as monomeric ions H 2 CrO 4 , HCrO 4 - , and CrO 4 2- (chromate), or as the dimeric ion Cr 2 O 7 2- (dichromate). Significant concentrations of H 2 CrO 4 only occur under the extreme condition of pH 1 or lower. Above pH 6.5, CrO 4 2- dominates. Below pH 6.5, HCrO 4 - dominates when the Cr(Vl) concentrations are low (<40 mM); but Cr 2 O 7 2- becomes significant when concentrations are greater than 1 mM, or it may even dominate when the total Cr(Vl) concentrations are greater than 30 mM. In the Cr(III)-H 2 0 system, Cr(III) exists predominantly as Cr 3+ below pH 3.5. With increasing pH, hydrolysis of Cr 3+ yields CrOH 2+ , Cr(OH) 2 + , Cr(OH) 3 , and Cr(OH) 4 - [4]. Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds or anionic clays, are two dimensional layered materials. The best known of these materials is hydrotalcite, Mg 6 Al 2 (OH) 16 (CO 3 ).4H 2 O, whose structure is derived from the brucite Mg(OH) 2 [5]. Anionic clays, such as hydrotalcites, are not as well known and rare in nature than cationic clays such as smectites. The first hydrotalcite was discovered in Sweden around 1842. Anionic clays have a structure electrically opposite to that exhibited by cationic clays. Their structure can be derived from the brucite structure, Mg(OH) 2 , where each Mg 2+ ion is octahedrally surrounded by six OH - ions. The hydrotalcite type (HT) structure is obtained when some of the Mg 2+ ions, or other divalent cations, are replaced by trivalent cations, with a similar radius.