## 1 Introduction  
Water pollution has increased in recent decades as a consequence of the uncontrolled disposal of agricultural and industrial residues and domestic discharge directly into water bodies and soil, which can then reach underground sources via filtration. Two of the most common contaminants are textile dyes and heavy metals; approximately 20% of industrially-employed dyes are discharged in effluents, while heavy metals are often the result of waste from hospitals and factories. Both types of pollutants are considered important hazards for human and environmental health even at trace levels. These non-biodegradable pollutants are found in surface and ground water sources and accumulated in aquatic organisms, primarily because conventional wastewater treatments cannot completely remove them from the effluent [1, 2, 3, 4, 5]. There are other water contaminants, such as organic matter, pharmaceuticals, pesticides, fertilizers, chemicals from personal care products, and many micro-organic pollutants; these are known as emerging contaminants. They are characterized by the majority being unregulated substances generated from anthropogenic activities,and can only be detected by chromatography methods coupled to mass spectrometry. Even though they are present in the environment at very low concentrations (nanograms or pictograms), their adverse effects are significant. The consequences of these pollutants are diverse, generating long-term effects on aquatic life and human health such as endocrine disruption, carcinogenesis, mutagenesis, and reproductive and embryonic toxicity, among others [6, 7, 8]. Reports from the United Nations Educational, Scientific and Cultural Organization state that every day over two million tons of sewage and other effluents drain into water bodies, forcing one out of nine people in the world to consume water from unsafe sources [9]. Therefore, the necessity of environmentally friendly and affordable treatments is a priority. Different technologies have been developed to treat wastewater before it enters rivers, oceans, or other water sources, including chemical, physical, and biological methods. Their application depends on punctual permissible levels of the effluent, the cost of the process, and environmental compatibility. These techniques include sedimentation, filtration, ion exchange, flotation, electrolysis, microbial reduction, bioremediation, and adsorption [10, 11, 12]. Biosorption is described as the process that occurs when the adsorbate or pollutant is retained on the surface of the adsorbent (biomaterial), creating a molecular or atomic film owing to residual interactions such as Van Der Waals forces or covalent bonds [13, 14, 15]. When biosorption is applied on wastewater treatment, it involves the interaction of the liquid phase component with the surface of the solid by either a chemical (chemisorption) or physical (physisorption) process, generating a mono- or multilayer (Figure 1). Similar to synthetic resins, heavy metals or other charged contaminants bind to the adsorbent biomaterial through electrostatic interactions and ion exchange. Agricultural residues have been proposed as low-cost unconventional adsorbents due to their high content of cellulose and lignocellulosic [1]. Grains, seeds, fruits, vegetables, herbage, and forage, which represent the most abundant wastes in food and agricultural industries, have great potential as adsorbent materials capable of remediating polluted effluents [3]. However, several conditions must be considered for the optimal removal of contaminants in addition to the production volume, such as the chemical and physical characteristics of the biorestules and their operation parameters (pH, contact time, adsorbent dose, particle size, contaminant concentration, temperature, adsorption kinetics, and isothermal behavior).