# Nascent Rice Husk as an Adsorbent for Removing Cationic Dyes from Textile Wastewater
## 1 Introduction  
Effluents from textile, leather, cosmetics, paper, pulp and dye manufacturing industries are highly colored because they contain organic chemicals called dyes [1]. The various types of dyes include methylene blue (MB) and crystal violet (CV). MB is an odorless, dark green powder that turns blue in an aqueous solution; it is used for dying cotton or silk, surgeries, and as a hair-colorant [2; 3; 4]. However, on direct contact, MB causes eye-burns and nausea, and, on ingestion, vomiting and profuse sweating [5]. CV, a protein-based dye also known as gentian violet, is used to enhance bloody-fingerprints and on adhesive tapes to prevent adhesion of powders on their sticky sides [6]. Conversely, CV is carcinogenic, recalcitrant, and non-biodegradable in nature [7]. Therefore, disposal of colored wastewater into an aquatic environment causes reduced photosynthesis due to low light penetration [8], and mutagenesis in both aquatic organisms and humans [9].  
Several dye-removal methods have been investigated of late, which include biological [10], and physio-chemical technologies [11; 12]. Photochemical and advanced oxidation process [13; 14; 15],electro kinetic coagulation [16], flocculation [17], nano filtration [18], adsorption [19], and reverse osmosis [20] have also been found to be impressive in removing dye contaminants from aqueous solutions. On the other hand, the majority of these methods are relatively expensive and energy-demanding [21, 22]. Biological treatment processes are inefficient due to the non-biodegradability of dyes [11]. Therefore, dye-removal from effluents using adsorption has been found to be more desirable and effective in developing countries [23, 24]. This is because adsorption exhibits simplicity, design flexibility, high contaminant selectivity, low operational cost, and ensures excellent treated-effluent quality [25, 26].  
In recent years, many researchers have focused on developing cheaper adsorbents for dye-removal such as peanut husk [27], wheat straw [28], seed husk [29], walnut shells [30], straw dust [31, 32], orange peel [33], date stones [34], cashew nut shell [35], and plum kernel [36]. These agricultural by-products are being increasingly used due to their low ash content, good mechanical strength, few processing needs, superior adsorption potential, availability, and regenerative ability [37, 38, 39]  
Rice husk (RH), a major by-product of the rice milling industry is abundant and readily available [40], accounting for 20 % of global gross rice-paddy production, which is estimated to be at 190 million tons as of 2016 [41]. Vietnam and Myanmar together account for 69 million tons of rice production (\(>\)7.20 % of global production), thereby, producing 13.8 million tons of RH as by-product [42]. Thus, possible use of RH will yield high economic and environmental benefits [43, 44]. RH has been found to consist mainly of 32 % cellulose, 20 % silica, 3 % crude proteins, and 21 % of both hemicelluloses and lignin [45]. Furthermore, chemical stability, good intrinsic strength, inherent resistance, and a silica-cellulose structural arrangement characterize RH [45, 46].  
Here, RH was an adsorbent to remove MB and CV from water under batch experimental conditions. MB and CV were selected for being cationic, strongly colored, non-biodegradable, mutagenic, and carcinogenic even in very small doses and at short exposure periods [47, 48, 49]. The impact of thermal treatment on the RH adsorption potential was assessed under different temperatures. The sorption characteristics of MB and CV on the RH were further explored by performing kinetic, equilibrium, thermodynamic, pH, and competitive anion adsorption experiments. In addition, model studies for kinetic, equilibrium, and thermodynamic adsorption experiments were also conducted to elucidate the adsorption characteristics and their mechanisms.