There is a growing need for an economical and efficient method capable of removing heavy metals from residual water. The current contribution aimed to evaluate the capacity of onion peel, an abundant agroindustrial waste product, to remove divalent cobalt (Co²⁺) from aqueous solutions. Onion peel was submitted to proximal chemical analysis, and various operational factors involved in biosorption were tested. The most suitable temperature (30 °C), pH (7.0), and biosorbent particle size (300–800 µm) were found. With an initial Co²⁺ concentration of 380 mg L⁻¹, the maximum capacity of Co²⁺ removal was 59.88 mg g⁻¹ in 120 min. The pseudo-second order and Langmuir models provided the best fit to the experimental kinetics and equilibrium of Co²⁺ biosorption, respectively. The thermodynamic study evidenced an exothermic, non-spontaneous, and favorable reaction (ΔH⁰ = −5.78 kJ mol⁻¹; ΔS⁰ = −21.13 J mol⁻¹ K⁻¹), suggesting the formation of stable bonds in the biosorbent-Co²⁺ complex. The carbonyl and hydroxyl groups apparently play a fundamental role in Co²⁺ removal, and electrostatic attraction, ion exchange, and chemisorption are the principal mechanisms. Thus, the biosorption of Co²⁺ by onion peel has potential as an economical, eco-friendly, efficient, and sustainable treatment for wastewater.

In this study, the presence of heavy metals (HMs) is determined to assess surface water contamination; biosorbent materials are also used to remove them and thus improve their quality. The objective of this work was to study the spatial distribution of HMs in water samples from Tequesquitengo Lake, Morelos, Mexico; pectin was also used for HM biosorption. For this, fifteen water samples were collected from the central and peripheral zones of the lake; HMs such as Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn, As, and Hg were identified and quantified by atomic absorption spectroscopy (AAS). The metal evaluation index (HEI) was calculated, as well as the percentage of HM removal with pectin. The water samples presented high concentrations of Pb, Cr, and Mn in contrast to the other HMs studied. Furthermore, these showed high concentrations (161.2, 85.2, and 65.6 µg/L, respectively) in the peripheral zone. Therefore, these values exceed the permissible limit for human consumption, except for Mn. The HEI value indicated that the lake water exhibits low contamination. After the adsorption of HMs with pectin, Cr (100%), Ni (83%) and Cd (37%) were removed, reducing the total concentration of HMs in the water in all samples.