## 1 Introduction
However, due to thermodynamic limitations, only the transformation of hematite (Fe\({}_{2}\)O\({}_{3}\)) to magnetite (Fe\({}_{3}\)O\({}_{4}\)) may be applicable in industrial CLC systems with an interconnected batch fluidized bed reactor configuration [30; 31]. Promoting reductions to wustite (FeO) or Fe would produce a considerable decrease in the purity of CO\({}_{2}\) obtained in the fuel reactor caused by the increase of the concentrations of CO and H\({}_{2}\) when methane is used [32]. In addition to the thermodynamic limitations mentioned, some authors have encountered problems of agglomeration in the bed, which is associated with the phase change from wustite to magnetite during the oxidation stage in the air reactor [6; 33; 34; 35]. Other chemical characteristics are advantageous when using these carriers, such as the low tendency to have carbon deposition and nonexistent risk of the formation of sulfide or sulfates due to the sulfur content in the combustible gas or the operating temperature [6; 32; 36]. As with iron-based carriers, special interest has been found in the literature in the study of manganese-based materials [15; 18; 23; 37; 38], bearing in mind that this metal oxide is considered an inexpensive and nontoxic material. In addition, the oxygen transport capacity is higher compared to iron-based compounds [6; 15]. Several states of oxidation may be involved in the redox reactions of manganese. MnO\({}_{2}\) decomposes at a temperature of approximately 500 \({}^{\circ}\)C, while Mn\({}_{2}\)O\({}_{3}\) is thermodynamically stable in the air at temperatures below 900 \({}^{\circ}\)C. However, at temperatures higher than 800 \({}^{\circ}\)C, only the Mn\({}_{3}\)O\({}_{4}\) form is stable [39; 40]. On the other hand, it has been found that manganese ores show poor mechanical stability and poor fluidization properties, making them inadequate oxygen carriers [14]. It is also possible to find materials with combined oxides that can be used as OCs, such as manganese and silicon oxides. These are considered to be economical and of low impact to health and the environment [41; 42]. In a kinetic study [22] comparing the activation energies and rate index (an important parameter that allows comparison of the reactivity of the OCs), manganese waste was the most reactive of the materials studied, followed by iron ore. Furthermore, the rate index found for the Mn waste was higher than that reported for similar materials in other studies [15; 37; 38]. The higher reactivity of Mn waste may be due to the formation of mixed Mn and Si oxides, such as braunite (Mn\(\gamma\)SiO\({}_{12}\)) and rhodonite (MnSiO\({}_{3}\))--phases which are very important for the release of oxygen [43]. Among other important aspects considered when choosing the carrier is its environmental impact [6]. The Carboquimica Institute (Zaragoza, Spain) has worked on OC solid waste management, characterization, and layout. These studies analyzed the presence of hazardous metals in copper-based and iron-based OC residues previously used in a CLC system with methane and carbon, respectively. Researchers found that most of the residues were stable and non-reactive with low environmental impact, and they could therefore be disposed of in a landfill for nonhazardous waste [44; 45]. In the two studies mentioned, a leaching test was performed according to the Spanish norm, UNE-EN-12457-3 [46], and the maximum limits allowed according to the standard established by the Council of the European Union, decision 2003/33/EC [47]. In this work, residues were obtained in a 500 Wth CLC plant operated with different low-cost OCs found in Colombia. The residues were characterized, and the environmental feasibility of the use of these materials was preliminarily determined based on whether the concentration of metals present in their leachates were in accordance with current regulations, both nationally and in Europe.