A Convenient Procedure for the Acidic Activation of Mineral Bentonite: An Environmentally Friendly Method for the Preparation of Bleaching Earths La ´szlo ´ Ko ´tai,  Istva ´n E. Sajo ´, Emma Jakab, Ga ´bor Keresztury, E ´ va Pfeifer, La ´szlo ´ Kocsis, Katalin Papp, Istva ´n Ga ´cs, Jo ´zsef Valyon, and Jo ´zsef Lippart Chemical Research Center, Hungarian Academy of Sciences, H-1025, Budapest, Pusztaszeri u. 59-67, Hungary (Received July 24, 2008; CL-080726; E-mail: kotail@mail.chemres.hu) Refluxing montmorillonite-containing raw bentonite ((Na,Ca) 0:33 (Al,Mg) 2 Si 4 O 10 (OH) 2 ) in aq HNO 3 , then neutraliz- ing it with NH 3 produces NH 4 NO 3 -containing NH 4 –bentonite. The NH 4 NO 3 intercalated can be eliminated as a gaseous prod- uct (N 2 O and H 2 O) by heating at 400  C, and the deammoniation and dehydroxylation process of the preactivated bentonite leads to formation of bleaching earth (H x (Al,Mg) 2y Si 4 O 10 - (OH) 2 ). Acid-activated bentonites (bleaching earths) are widely used absorbents in the food industry. 1 Various methods have been developed for their preparation, mainly by using HCl or H 2 SO 4 as activation agent. 2 These technologies produce large amounts of acidic wastewater which require alkaline neutralization and may be the cause of environmental salt-burdening. 3 Based on our previous work, 4 a new process involving nitric acid preactivation, ammoniacal neutralization and a heat treatment has been developed. Aqueous washing, consequently the acidic wastewater formation, has been eliminated. Mineral bentonite was preactivated in a 20% aq HNO 3 by refluxing for 3 h. The preactivated bentonite formed in this process has no excellent bleaching property, 5 although the layered structure has already been destroyed during this treatment (Figure 1). The intense XRD reflection corresponding to the basal spacing laminar clay structure (d 001 ¼ 1:520 nm, scan 1) disappeared (scan 2), and only minor changes could be detected after the heat treatment (scan 3). The preactivated sample contains a small amount of residual HNO 3 ( NO ¼ 1380 cm 1 ) which, however, is eliminated by the heat-treatment of the activated bentonite to remove the NH 4 NO 3 formed during the ammoniacal treat- ment. The decomposition temperature of the NH 4 NO 3 interca- lated in the preactivated sample is 260  C, which shows the absence of any catalytic effects of the aluminosilicate cage of the preactivated bentonite. NH 4 NO 3 ¼ N 2 O þ 2H 2 O ð1Þ Only gaseous N 2 O(m=z ¼ 44) and H 2 O(m=z ¼ 18) are formed; NO x gases and NH 3 do not evolve (Figure 2). 6 The small peak at 100  C refers to the elimination of the physically adsorbed water. As the sign of the dehydroxylation/deammoniation process- es an extended water/NH 3 elimination step (m=z ¼ 18 (H 2 O) and m=z ¼ 17 (NH 3 )) has occurred between 300 and 400  C. A well-defined peak of N 2 (m=z ¼ 28) could be observed at 310  C (without any NO (m=z ¼ 30) or N 2 O(m=z ¼ 44) forma- tion) as a consequence of the decomposition of the adsorbed ammonia. According to these processes, heating at 400  C improves the bleaching activity. The comparison of the IR spec- tra of the raw, the preactivated and the heat-treated bentonites shows that the intensity and the position of the bands assigned to M–OH (M ¼ Al, Mg, and Fe) bonds (1100, 900, and 840 cm 1 ) of octahedral sites are shifted and decreased. This confirms the role of the dehydroxylation in the final activation step. After the heat treatment of the bentonite no coordinated ammonia peaks could be detected in the IR spectrum. The deam- moniation and the dehydroxylation processes liberate the active sites, via partial dealumination as it was observed in the case of zeolites. 7 Surface area and pore distribution studies confirm the appearance of new active sites. The specific surface areas of the Figure 1. XRD (Cu K) of raw (1), HNO 3 preactivated (2) (acidic) and post heat-treated (3) bentonites. Figure 2. TG-MS of HNO 3 -activated and NH 3 -treated benton- ite sample. 1110 Chemistry Letters Vol.37, No.11 (2008) Copyright Ó 2008 The Chemical Society of Japan