# Sustainable Removal of Cr(VI) by Line Peel and Pineapple Core Wastes
## 1 Introduction  
Industrial development, responsible for providing welfare to society today, is one of the chief causes of environmental contamination. This pollution involves several aspects including the generation of solid wastes as a result of different activities (e.g., agroindustrial) or the release of untreated effluents containing a variety of organic and inorganic contaminants (dyes, pesticides, heavy metals, etc.), which causes pollution in different water bodies [1]. They are a subject of great concern that requires the development of efficient treatments and disposal.  
Changing the economic model from one based on producing, using, and throwing away, to a new model focused on a circular economy that involves the reuse of natural resources for the longest possible time themselves. Therefore, waste recycling and valorization become an economic and environmental issue [2].  
Agronidustrial wastes have been shown as suitable raw materials to produce adsorbents with the ability to remove different kinds of pollutants by adsorption [3] due to their high lignocellulosiccontent, which improves the adsorption capacity of these materials. Moreover, adsorption has been demonstrated as a successful methodology for treating effluents containing inorganic and organic pollutants [1; 4]. Their use overcomes some of the drawbacks of conventional adsorbents (e.g., activated carbons) such as the elevated cost and limitation by the nature of the pollutants present in the wastewater [5]. Agroidustrial wastes are becoming a viable option as biosorbents because they have a low or null cost [6], are readily available [7], and are easily removable because several microorganisms can use them as a carbon source [8].  
Waste generated in the food and beverage industry of pineapples and limes are suitable candidates to be used as adsorbents. More and more consumers, particularly in Europe and North America, demand fresh pineapples and its manufactured products are an important, fast, increasing, booming market [9]. The main wastes from the pineapple industry (peel, core, stem, crown, and leaves) constitute roughly 50% (w/w) of the total pineapple weight. Moreover, lime green processing plants produce large amounts of waste materials annually, including peel from the extraction of citrus juice [9]. The management of all these wastes, which produce odor and soil pollution, poses a problem for the industries mentioned above. With that in mind, the utilization of these raw waste materials would be an excellent opportunity to handle the high volume of residues and connect it to the circular economy.  
On the other hand, chromium (Cr) is utilized in a wide variety of manufacturing industries including leather tanning, metallurgical industry, textile manufacturing, and processes like wood preservation. The poor treatment of the generated waste is a source of pollution of watercourses [10]. Cr is known as a carcinogenic and mutagenic compound and its release into water bodies at concentrations higher than 0.05 mg/L for Cr(VI) and 5.0 mg/L for Cr(III) [10; 11] generates toxic effects in the environment. It has been demonstrated that some different raw and modified lignocellulosic materials are convenient, low-cost biosorbents for Cr(VI) and Cr(III) disposal [12; 13].  
According to our knowledge, this research is one of the first attempts where both lime peel and pineapple core are used as cost-effective adsorbents of a toxic metal. In this context, the present study intends to ascertain the use of these raw agroindustrial residues as a viable low-cost approach for Cr(VI) elimination. Then, the optimization of the adsorption process variables was performed by a Box-Behnken experimental design. Finally, the isothermal parameters were determined and the viability of the adsorption process scale-up to a column of 100 L was simulated and analyzed by the Fixed-bed Adsorption Simulation Tool (FAST) v2.1.