## 1 Introduction  
Different industrial and domestic processes release large amounts of wastewater containing substances such as metals, suspended solids, organic and inorganic compounds, among others. This wastewater contaminates aquatic and terrestrial ecosystems [1; 2; 3]. Even though metals are natural components of the Earth's surface [4; 5], the main sources of their pollution are associated with anthropogenic activities, principally from the metal-mechanical industries, manufacture of batteries, construction elements, pigments, electrical accumulators, coal and oil combustion, galvanoplastic processes, continuous use of pesticides and fertilizer, and waste incineration [6; 7; 8]. Pollution by certain metals is one of the most important environmental problems since, depending on the organism, they produce different adverse effects even at low concentrations [8]. In addition, these contaminants are persistent in the environment [9; 10; 11; 12], can bioaccumulate, and, in some cases, biomagnify through the food chain [13; 14; 15]. Among the most harmful non-essential metals and metalloids are lead (Pb), mercury (Hg), and cadmium (Cd), which have carcinogenic effects on living organisms [1; 16]. Other metals, such as nickel (Ni), are considered oligo-elements that have biological functions. However, they can produce negative effects on human health when ingested in high amounts [8; 17; 18]. In Ecuador, the maximum permissible limits for polluting metals in wastewater are established according to their different receiving bodies in the Unified Text of Secondary Environmental Legislation (TULSMA, for its acronym in Spanish), where the respective limits for Cd are 0.02 mg/L for sewage and fresh water and 0.2 mg/L for marine water, for Hg 0.01 mg/L for sewage and marine water and 0.005 mg/L for fresh water, for Pb 0.5 mg/L for sewage and marine water and 0.2 mg/L for fresh water and for Ni 2.0 mg/L in all types of receiving bodies [19]. At present, different technologies and methods have been used to remediate heavy metals in contaminated environments [20; 21; 22; 23]. Among these, physicochemical and biological processes have been developed. The former includes reverse osmosis [24; 25; 26], filtration [27; 28; 29], chemical oxidation [30; 31; 32], chemical leaching [33; 34; 35], nanotechnology [36], and electrokinetic remediation [37; 38; 36]. However, some of these methods use solvents or have some limitations regarding the amounts of matrixes that can be remediated, but in general, they require a lot of effort and are limited by their high costs [11; 39]. Based on the immobilization of metals using microorganisms, the biological alternatives can include activated sludge with microorganisms [37; 40; 41]. These processes have certain disadvantages because they require the management of microorganisms, they are ineffective on non-degradable compounds, and also there is the possibility for accumulation of sludge foam and generation of biological sludge and by-products [41]. Besides, low-cost biomaterials with high adsorption capacities can act as high-performance retainers of contaminating metals [42] and as an alternative to costly commercial activated carbon adsorbents [43; 44; 45], including radioisotopes [46]. The biosorption process for the treatment of contaminated effluents [47; 48] is a technique that involves the removal of contaminants using inert biomass [49; 50; 51]. Among the bio-adsorbent materials that have been most studied for the uptake of toxic metals are bacteria [52], fungi [53], algae [54], yeast biomass [55], agro-industrial waste such as wheat bran [56; 57; 58], rice husk [59; 60; 61; 62], and fruits and vegetables [63; 64; 65; 66; 67; 68; 69; 70; 71], among others. Avocado seeds and peel tissue wastes have demonstrated an important capacity for toxic trace metals retention [72; 73; 74; 75; 43; 76]. The high consumption of avocados produces a significant availability of these residues that can be processed and used for metal sequestration [43].