## 1 Introduction  
Synthetic dyes released from textile, cosmetic, paper, and food industries have potentially adverse environmental and health impacts since they not only affect the quality and aesthetic of water but can also be associated with mutagenic, carcinogenic, and allergenic risk [1, 2, 3, 4, 5]. Thus, working on new alternatives to remove materials is relevant. Dyes are complex organic molecules with several recalcitrant aromatic rings classified as anionic (acid), cationic (basic), and non-ionic [6]. They consist of chromophores, which are responsible for giving color, and the auxochromes charge intensifies the color of dyes. Acidic dyes are water-soluble compounds containing one or more anionic groups (salts of sulfuric, carboxylic, phenolic groups). Basic dyes are cationic molecules that have monoazooic, diazoic, and azine compounds. Among diverse synthetic dyes, Congo red (CR) and malachite green (MG) are two of the most commons due to their low cost, high availability, and efficacy. CR is the sodium salt of benzidineazo-bis-1-naphthamine-4-sulfonic acid (C\({}_{32}\)H\({}_{22}\)N\({}_{8}\)Na\({}_{2}\)O\({}_{6}\)S\({}_{2}\): molecular weight: 696.66 g mol\({}^{-1}\)) (Figure 1A). It is an azo acid dye that is very soluble in water, which directly stains cotton in red; it may also turn blue by the presence of mineral acids. CR is also used for staining some products in the wood pulp and paper industry [7, 8]. MG, the most common basic dye (N-methylated diamondintphenylmethane; C\({}_{52}\)H\({}_{84}\)N\({}_{4}\)O\({}_{12}\): molecular weight: 927.00 g mol\({}^{-1}\)) (Figure 1B), has been widely used as a bactericide and fungicide in the fish farming industry, in the coloring of silk, and as a food coloring agent and additive [9, 10]. The importance of studying dyes such as CR and MG lies in the fact that they are resistant to natural degradation processes. The aromatic rings provide the dye molecules with chemical stability, so organisms require more energy to mineralize these substances [11,12]. That is to say, since the dye's mineralization process becomes slow, the greater the number of aromatic rings, the longer their persistence in the environment, which could lead to a negative effect on the aquatic flora and fauna [12,13]. Additionally, they are susceptible to being reduced, which results in the formation of aromatic amines that are highly carcinogenic [10,14,15,16]. To reduce the pollution caused by these two dyes, either chemical, physical, or biological methods have been used [16,17,18]. Some chemical dye removal methods are the advanced oxidation process, ozonation, photochemical, electrochemical destruction, and Fenton reaction. Most of these dye removal methods are effective, quick, and do not generate sludge. However, they could produce undesirable by-products, and in general, they are costly compared to biological and physical dye removal methods. Among conventional physical dye removal methods are adsorption by activated carbon, membrane filtration, ion exchange, electrokinetic flocculation, irradiation, reverse osmosis, and ultra-filtration. Physical methods are commonly used due to their efficiency and simplicity; nevertheless, they are effective for a limited number of dyes, and some of them show high sludge production. On the other hand, biological methods such as adsorption by microbial biomass, algae degradation, fungal cultures, adsorption by living/dead microbial biomass, and enzyme degradation, among others, have gained importance used alone or as a combined method, because they are ecofriendly and do not need the consumption of chemicals. However, these methods deal with living organisms, so they are sometimes considered unstable or unpredictable [19,20,21,22,23]. Among biological treatment with microorganisms, it has been found that the use of living or dead microbial biomass (from bacteria, yeasts, fungi or microalgae) can play a key role in decolorizing the wastewater of different industries without producing toxic substances [24,25,26].