## 3 Wastes in Zeolite Synthesis
### Rice Husk  
Rice husk (RH) is a spinoff of rice production and originates in the milling process for separation of the husk and bran from the edible part. The global production of RH per annum is estimated as 10 million tons, in most cases burnt to produce reek for machinery work [200] (p. 25). Other uses are feedstock for bioethanol production and additive in materials demanding high siliceous content or as fertilizer [231]. The global annual production estimated by husk paddy ratio (HPR) is 120 million tons with China contributing 29%, following by India (21%), Indonesia (8.8%), Bangladesh (6.4%), Vietnam (5.8%), Thailand (4.4%), Myanmar (3.9%), the Philippines (2.3%), Brazil (1.9%), and Japan (1.8%) [232]. The chemical composition of RH is based on cellulosic sugars and its nature is lignocellulosic, with lignin representing up to 20% of husks [233]. The rice husk ashes are produced by gasification of RH and contain considerable amounts of silica (SiO\({}_{2}\)) varying from 15.30% to 24.60% [232; 233; 234]. The silica may be pyrolyzed at elevated temperatures or extracted as sodium silicate using solvent extraction. Rice husk ash (RHA) has better characteristics for applications than rice husk and represents an ideal form of environment-friendly application, due to its burnt waste origin [232]. The uses of RHA include soil ameliorants, breaking up clay soils and improving soil structure, besides its use as insulator in the steel industry and as pozzolan in the cement industry. In zeolite synthesis, this material may present crystalline tridymite and \(\alpha\)-cristobalite, allowing the synthesis of BEA and ZSM-5 [235]. Zeolite synthesis is one of the most important applications [235; 236; 237; 238]. The zeolite NaY was synthesized using RHA waste as source of silica after acid leaching of husks. In this process, zeolite NaP was obtained also [239]. The products presented such characteristics as uniform distribution of particle size, surface area depending on synthesis method, and the presence of another phase obtained (zeolite P) [232]. The acid leaching in RHA may not be used for the synthesis process as in ZSM-5 zeolite preparation using the solvent-free method by mixing solid raw materials such as solid waste RHA, NaAlO\({}_{2}\), Na\({}_{2}\)CO\({}_{3}\).10H\({}_{2}\)O and TPABr with optimum molar ratios TPABr/SiO\({}_{2}\), Na\({}_{2}\)CO\({}_{3}\).10H\({}_{2}\)O/SiO\({}_{2}\) and synthesis time [240]. The solvent-free method presents great recovery rate of Si and Al with respect to the hydrothermal method [236]. The micropore and mesopore surface areas of material reached adequate values and the method without the use of solvent and using RHA as precursor represents a sustainable alternative [236]. The zeolite SUZ-4, a new synthetic zeolite, may be obtained by use of RHA as raw material. This process is described as occurring with hydrothermal conditions in the presence of tetraethyl ammonium hydroxide as template. The properties obtained from RHA waste in comparison with products from pure silica sol are: the needle-shape crystals from pure silica sol reach a smaller diameter than those obtained from RHA, which were larger, increased crystallite size, and generation of higher-purity zeolite using just 50% RHA in comparison to 100% pure silica sol [237]. Zeolite A was synthesized from RHA at low temperatures using NaOH as activator to dissolve the silicate ion from RHA that reacts with aluminate in solution to form an aluminosilicate structure for zeolite formation. In this process, there are three steps in the synthesis: the dissolution of Si from the precursor, the gelation among Si and Al to form the aluminosilicate gel, and finally the crystallization of gel to yield the zeolitic structure, which can reach high crystallinity [238]. Rice husk in the form of rice husk ash appears to be a cheap alternative to synthesis of other porous materials that require synthetic and expensive Si sources, such as tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate (TMOS).