# Removal of Metals and Dyes in Water Using Low-Cost Agro-Industrial Waste Materials
## 1 Introduction  
Pollution of water resources due to the effect of organic and inorganic compounds is rising proportionally with the augmenting of the global population and anthropogenic activities [1]. Dyes are widely used as colorants in industries such as pharmaceutical, food, printing, leather, and textile, among others [2]. A total of 700,000 dyes are used commercially, and around 10% of the dyes used in different sectors are discharged in effluents without proper treatment [1]. The discharge of untreated tinted wastewater into water bodies can cause environmental damage, i.e., oxygen content reduction and diminished sunlight penetration, which puts the life of aquatic animals and plants in danger [3]. Among dyes, MB is a cationic dye, has a heterocyclic aromatic structure, and has high volume production because it is extensively applied in coloring paper, hair colorant, dyeing cotton, wool, and paper stock coating [4], among others. Because of its side effects on human health, it has relevance as a pollutant in water systems. Heavy metals are persistent, non-biodegradable, bioaccumulative, and toxic. Among them, chromium is listed in the top 20 contaminants under the category of hazardous materials. It is extensively used in industrial processes such as fertilizer, electroplating, leather tanning, and paint, among others. Chromium commonly exists in nature as Cr (III) and Cr (VI), the first being relatively harmless, whereas Cr (VI) is considered carcinogenicdue to its strong oxidizing properties. In humans, it can cause dermatitis, hemorrhage, bronchitis, lung cancer, and liver inflammation [5]. The maximum permissible levels of Cr (VI) in industrial effluent and potable water are 0.25 and 0.05 mg/L, respectively [6]. Consequently, there is statistical data showing that approximately two million tons of residual effluents are discharged into natural water basins, of which 33% correspond to untreated industrial discharges, affecting 90% of water supply drinking water [7]. A clear example of this pollution comes from the textile industries, the most significant contaminant being organic dyes, generating 54% of the pollution worldwide, followed by the dry cleaning industry with 21%, the paper and pulp industry with 10%, and the painting industry with 8% of effluents contaminated by dyes [8]. In addition, the contamination by chromium present in industrial effluents belongs to 3970 mg/kg in wastewater from the mining industry, 794 mg/kg from the chrome industry, and 2915 mg/kg from the steel industry [9]. These high chromium concentrations in wastewater exceed those that the World Health Organization allows, establishing a permissible limit in industrial discharges of 2 mg/L [10]. Considering the above, the treatment of polluted effluents before their discharge into water bodies is necessary. Among the technologies used in the removal of contaminants from water, adsorption is widely used due to its simplicity, easy operation, efficiency, insensitivity to toxic contaminants, and selectivity towards pollutants, even at low concentrations [11]. The use of activated carbon is common in adsorption systems because it exhibits characteristic features of higher surface area, porous structure, variable surface chemistry properties, and higher mechanical strength. Nevertheless, commercial activated carbons are produced using coal, which is not an economical or non-renewable material [12]. Considering the above, the synthesis of activated carbon from cheap and efficient precursors such as agricultural wastes, low-grade plants, forest residues, and industrial by-products is being studied. Recent studies have used agro-industrial residues as an ecological and sustainable strategy for manufacturing biochar with adsorption capacity for heavy metals and cationic dyes such as chromium and methylene blue (MB). [PERSON] et al. [13] studied the preparation of carbons from rice husks and tea residues for their application in Cr (VI) removal processes.