ORIGINAL PAPER Sustainable Thermochemical Extraction of Amorphous Silica from Biowaste Swapna Banoth 1 & V. Suresh Babu 1 & G. Raghavendra 1 & K. Rakesh 1 & S. Ojha 2 Received: 25 June 2021 /Accepted: 20 July 2021 # Springer Nature B.V. 2021 Abstract The objective is to utilize zero wastage rice hull material which contains silica as a major component in amorphous form and is used as reinforcement material for various applications. Burning of rice hull (RH) under controlled condition after removal of metal ions leads to white silica of high purity. An inexpensive method for extraction of amorphous silica by thermochemical treatment (pyrolysis process) is performed. Pyrolyzed biomass at four different temperatures is observed for treated and untreated rice hull. Calcination of the rice hull at 600ºC in a muffle furnace turns it into white amorphous silica. In this research, HCl was used to retrieve silica from the rice hull. Extracted silica was characterized by X-ray diffraction analysis, which indicates that the silica is in amorphous form, and displayed a strong broad peak at 22.32º and 21.52˚ (2θ). The FTIR data revealed the existence of peaks at 4000 cm - 1 & 400 cm - 1 showing the presence of siloxane and silanol groups. UV-Visible (absorption) band maxima was demonstrated at 367 nm, photoluminescent (emission) spectra displayed a short peak at 453 nm and a sharp intensity peak at 488 nm is comparable with amorphous nano-silica. Less amount of silica appeared at the inner surface of rice hull fiber revealed by SEM analysis. Other element traces were absent, high purity of amorphous silica is observed by EDS. Keywords Agricultural waste . Rice hull ash (RHA) . acid leaching . photoluminescent . XRD . SEM & EDS 1 Introduction Rice hull is biomass and a good source of silica considering rice is a major crop [1, 2]. A variety of sources are available for extraction of amorphous silica, among which one of the most abundant sources is rice hull, which has a large amount of silica (up to 22%). Rice hull on weight basis accounts for an average 20 % by-product from the milling process of paddy [3, 4]. RH is the agricultural waste available abundantly in huge volume and acts as a source for the production of amor- phous silica. Mass production of RH and can be used econom- ically as a low-cost source of amorphous silica [ 5 ]. Combustion of RH under constrained conditions leads to rice hull ash (RHA), which is a rice source of pure silica. The metal impurities, i.e. Fe, Mn, Na, Ca, K and Mg, which affect the color and purity of SiO 2 are removed by the pre-treatments with like HCl, H 2 SO 4 or HNO 3 before combustion [6]. Two forms of SiO 2 occur in nature such as amorphous and crystal- line. Opal is the amorphous form of silica while crystalline form occurs in three forms namely cristobolite, quartz and tridymite. Normally silica in RH being bioquenic is essential by being amorphous in nature. At different temperatures and heat treatment crystalline form of silica is obtained [7]. The importance of the amorphous silica is explained by its form, but when crystalline structure of silica is formed which is inactive and unable to form chemical reactions [8]. RH is one of the abundant sources of natural amorphous silica [9]. Silicates acquired from SiO 2 are used in the ceramics industry, cement production, biomedical, cosmetics, and detergents as filler material [10]. Silica was employed as the primary sub- stance for the production of many inorganic materials, metal- based complexes, and organic materials, also used as a cata- lyst for the chemical productions and optical coating materials [11]. At present SiO 2 materials are extracted using various methods, such as sodium silicate, sol-gel route, template syn- thesis, emulsions-based synthesis, and bio-inspired methods [12]. Leaching by acids [13] and gasification [14] processes have been used for recovering SiO 2 from RH. Amorphous * G. Raghavendra raghavendra.gujjala@gmail.com 1 Department of Mechanical Engineering, National Institute of Technology, 506004 Warangal, Telangana, India 2 Department of Mechanical Engineering, Kakatiya Institute of Technology and Science, Warangal, Telangana, India Silicon https://doi.org/10.1007/s12633-021-01293-z