# The Use of Rapeseed Husks to Remove Acidic and Basic Dyes from Aquatic Solutions
## 1 Introduction  
Dyes are chemical compounds used to impart a desired color to utility items. There are about 10,000 types of dyes available on the market and their global production currently exceeds 1 million ton/year [1]. They are mainly used in industry for dyeing fibers, leather, plastics, paper, foods, and other materials. As a result of certain drawbacks of the dyeing technology, a significant proportion of the dyes used is lost and ends up in wastewater [2]. Colored industrial wastewater poses a serious risk to the natural environment. For example, ineffective methods of decolorization of post-production waters cause certain amounts of dyes to pervade the natural waters [3]. Furthermore, even small amounts of color substances can trigger major changes in local aquatic ecosystems [4]. Most dyes are very visible in water reservoirs, even at concentrations of 1 mg/L, which disturbs the aesthetics of the latter [5]. What is worse, they impair sunlight penetration through water, thus, inhibiting the photosynthesis carried out by aquatic autotrophs [6]. In addition, some dyes hinder the diffusion of atmospheric oxygen in water, which, when combined with impaired photosynthesis, can lead to the development of anaerobic conditions in the water reservoir. Certain dyes may also be toxic to aquatic organisms [7]. As a result, pollution with dyes can lead to the collapse of the local aquatic ecosystem. In order to minimize the risk of environmental degradation, it seems necessary to implement effective technologies for wastewater decolorization. Colored wastewater is most often treated by means of physicochemical methods including: chemical precipitation and electrocoagulation, chemical oxidation methods (e.g.,ozonation or reaction with sodium hypochlorite), membrane methods (e.g., ultrafiltration, reverse osmosis), and also sorption methods [8; 9]. Sorption involves binding contaminants on the surface of the material (sorbent). Unlike precipitation or deep oxidation methods, it does not generate any by-products [10]. Sorption methods are also easier to carry out and are cheaper compared to the membrane methods. Many scientists are of the opinion that dye sorption is one of the safest methods for wastewater decolorization [11]. The effectiveness and costs of colored wastewater treatment by means of sorption depend mainly on the type of sorbent used. Currently, sorbents that are most commonly used for water and wastewater treatment include activated carbon-based materials [12]. They are characterized by high porosity and very extensive specific surface area, often exceeding 1000 m\({}^{2}\)/g, which translates into their good sorption of most dyes [13]. The main problems associated with activated carbons include their rapid saturation, difficult regeneration and the associated weight loss. There is also a common opinion that their production costs are high. Therefore, their cheaper alternatives are currently being sought. They should be readily available, cheap and, possibly, have a sorption capacity similar to that of activated carbons. Today, high hopes are fostered in waste materials from the agri-food industry as either sorbents or raw materials for their production [14; 15]. They are cheap and widely available, especially in developed countries. So far, research has been carried out on the removal of dyes on waste materials from the agricultural industry such as plant stalks, leaves, nut shells or fruit skins [16; 17]. Their sorption capacity is mainly due to the high contents of polysaccharides, lignin and proteins. An example of a widely available waste material from the agri-food industry with potentially good sorption capacity is rapeseed husks. Rape (_Brassica napus_ L.) is one of the most popular crops. In 2020, its global production exceeded 70 million tons [18]. It is grown mainly for seeds containing 40-45% fat [19; 20] that are used for oil production for food purposes or for biodiesel production [21].