# Biosorption of Pb(II) Using Coffee Pulp as a Sustainable Alternative for Wastewater Treatment
## 1 Introduction
On the other hand, the WHO classified Pb as one of the 13 heavy metals of importance for human and environmental health as well as arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr ), copper (Cu), mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb), tin (Sn), selenium (Se), titanium (Ti) and zinc (Zn) [12]. Given these applications, use and discharge of wastewater with this metal, conventional and advanced techniques have been used for the treatment of untreated aqueous matrices, and among these are the use of chemical precipitation, activated carbon, ultrafiltration, microfiltration, cementation and ion exchange, among others [4; 13]. However, although their efficiency is high, these techniques have disadvantages in terms of maintenance and implementation in wastewater treatment systems (WWTS), as well as considerable sludge production. Due to this, biosorption methods using agro-industrial wastes--with and without physical and/or chemical modification--for the treatment of wastewater that incorporate heavy metals such as those mentioned above, is a field that has been widely consolidated and investigated, and given their high efficiency, low cost of implementation and maintenance, these techniques are environmentally friendly and do not generate sludge [4]. It is also worth highlighting the valorization of coffee crop residues, which have been used for the production of biofuels (biodiesel, bioethanol, bio-hydrogen and biogas), glycerin, pharmaceutical products, biochar, compost, polymers and bio-active compounds [14]. Additionally, some coffee subprocesses were used for the removal of contaminants in aqueous matrices, such as Zn, Pb, Cr, Cu, Cr, Cd, dyes and anionic surfactants, among others [15]. On the other hand, it is important to note that, regarding the concentration of Pb in river sediments, for the case of Colombia, Espitia [16] conducted a characterization of heavy metals, Hg, Cd, Pb, Cr, Ni and Fe, in sediments from the water bodies of the Canal del Dique (artificial arm of the Magdalena River), finding that the concentrations of these pollutants are within the permitted limits. In the case of lead, in the surface sediments of the populations studied and the control point, they were not detected by the methodology used (detection limit = 1 \(\upmu\)g\(\cdot\)mL\({}^{-1}\)). Likewise, in Colombia, 10 determinations of heavy metals were carried out on the Sini river and for Pb levels ranging between 10.10 and 8.08 \(\upmu\)g\(\cdot\)kg\({}^{-1}\) in rainy and dry weather seasons [17]. In turn, in other countries, lead records have been found in Arctic Lakes, Canada (10.33 \(\upmu\)g\(\cdot\)kg\({}^{-1}\)), Thompson Canyon, USA (38-42 \(\upmu\)g\(\cdot\)kg\({}^{-1}\)), Lake Michigan and some European Rivers (50-712 \(\upmu\)g\(\cdot\)kg\({}^{-1}\)). In relation to other world records, there was a characterization carried out in 22 samples of the sediments of the Olt River, which is part of the Danube River basin, where based on the Pb analysis, levels between 1.74 and 49.63 mg\(\cdot\)kg\({}^{-1}\) were detected [18]. In addition, it is worth mentioning that the Canadian legislation reports a limit of 35 mg\(\cdot\)kg\({}^{-1}\) for sediments [19]. Given the panorama presented, in this research, we used the coffee pulp (CP) variety Castillo--taken from a coffee farm located in the city of Manizales (Colombia)--as a sustainable alternative in the biosorption of Pb\({}^{2+}\) ions from synthetic wastewater. This was developed in two stages: the first was to perform the quantification of lignin and cellulose; in the second step, the optimal variables of sorption of the contaminant were identified, corresponding to contact time (min), the optimum adsorption pH, kinetics and adsorption isotherms, pHpzc, as well as the elucidation of a possible sorption's mechanism. Finally, we made a comparison between residues from coffee crops that have been investigated in the removal of Pb\({}^{2+}\) ions, with and without chemical modification.