Evaluación del potencial de la cáscara de cacao modificada con tempo para la adsorción de cadmio en soluciones acuosas

Abstract

Soil contamination by heavy metals, such as cadmium, often involves the presence of multiple coexisting metals, complicating its control and remediation. Despite current methods, contaminant residues persist, negatively impacting ecosystems and human habitats. Cocoa processing generates by-products, with cocoa husks being the primary one. Although traditionally used as fuel or fertilizer, recent studies explore its potential in industrial applications, such as the preferential extraction of cadmium from aqueous solutions. Adsorption emerges as an efficient and sustainable option for removing metals from contaminated waters. Detailed studies, including the determination of the Zero Point of Charge (pHpzc) and FTIR analysis, were conducted to characterize the TCC adsorbent material. These methods allowed for the evaluation of the adsorbent's effectiveness in metal removal as well as the characteristics of its surface and chemical structure, providing a comprehensive understanding of its performance in water treatment. In this study, it was determined that the adsorbent material modified with TEMPO (TCC)—an organic compound used as a selective oxidant in material modification, enhancing the chemical and adsorptive properties of modified materials—was derived from cocoa husks.This resulted in a Zero Point of Charge (pHpzc) of 6.28, enabling adsorption of both anions and cations depending on the pH of the medium, which is crucial for treating aqueous solutions with cadmium. Above the pHpzc, the adsorbent's surface became negatively charged, favoring the adsorption of cations such as Cd²⁺. The FTIR study of the TCC adsorbent material revealed that the C=O stretching peak at 1693.39 cm⁻¹ indicated significant interaction between the carbonyl groups and Cd²⁺, as well as the involvement of carboxyl groups (COOH). The shift of the peak from 1712.96 cm⁻¹ in the cocoa husk to 1693.39 cm⁻¹ in the modified adsorbent confirmed that the carbonyl groups were involved in binding with Cd²⁺. These functional groups interacted with the adsorbates; TEMPO, by converting hydroxyl groups to carboxyls, markedly improved the adsorption capacity of the material. In the regeneration capacity assessment through adsorption-desorption studies of the TCC adsorbent with HNO3 as the eluent, results showed that in the first cycle, 1.1205 mg of cadmium was adsorbed and 0.99675 mg was desorbed, while in the second cycle, 0.268 mg was adsorbed and 0.02925 mg was desorbed. Despite incomplete desorption of cadmium, the activation of the adsorbent enhanced its capacity and reactivity, facilitating efficient reuse and demonstrating that TCC maintained good regeneration capability and distribution of active sites on its surface.