# Returning to Nature for the Design of Sportive Phases in Solid-Phase Microextraction
## 5 Agricultural By-Products  
As previously noticed, the use of biomass as an extractant for inorganic and organic compounds is considered as a promising alternative to conventional sorbents. Among the existing choices, the use of natural products usually considered as waste such as fruit peels for cations and anions removal from waters has been extensively described [82; 83]. These sorbents are effective and economical with a clear beneficial effect on reducing the amount of waste to be eliminated. The retention is based on the presence of residual hydroxyl, carboxyl, or amine groups of cellulose or lignin, which exhibit chelating or ion-exchange properties. It has been further demonstrated that once retained, the metal ions can be removed from the sorbent by an acidic treatment, opening the door to evaluate their use as extractant phases with analytical aims. This section will describe the potential of agricultural by-products such as fruit and vegetable peels as a sorbent for the isolation and preconcentration of inorganic and organic compounds from waters. Vegetable old cell walls are enriched with lignin, which is a phenolic polymer of hydrophobic nature whose aromatic moieties have been identified as responsible for interacting with metals [84]. Amine groups of cellulose proteins and glycoproteins synergically contribute to adsorption [85]. Increasing the superficial area of any sorbent results in a better capacity for analytes retention. In the case of fruit and vegetable peels, chemical treatment such as mercerization can be used to modify the surface of the peel to increase the number of hydroxyl groups available for interaction, thus enhancing the chelating/ion exchange capacity [86]. Concerning the solid-phase extraction of cationic species from waters, banana peel and Sophia seeds have been proposed as sorbents. Cadmium (II) has been extracted from environmental and industrial wastewaters [87]. In this specific application, once dried and crushed, the peels were passed through a 120-mesh sieve (125 \(\upmu\)m). Then, activation of the superficial carboxylic groups was carried out using acidic methanol. The esterification of the acidic groups was corroborated by infrared spectroscopy. Only 100 mg of the banana peel was needed to efficiently preconcentrate cadmium (II) by ion exchange with an up to 10 times reusability cycle. Untreated banana peel has also been proposed for the solid-phase extraction of copper and lead from river waters [88]. In this application, dried banana peel was pulverized using a ball mill and further sieved to select the fraction between 35 and 45 \(\upmu\)m for metals preconcentration using 20 mg of solid. To minimize potential interferents, the organic fraction of the sample was eliminated by oxidation, being the extraction procedure carried out at acidic pH. An enrichment factor of 20 was achieved in this case, this being value corroborated by the analysis of a standard reference material. Descurania Sophia seeds have also been proposed as sorbent for the isolation and preconcentration of cadmium (II) from water and rice flour [89]. Seeds were commercially available and utilized as received, with the only requirements being washing and sieving. The extraction of cadmium was accomplished in acidic medium using 150 mg of sorbent. The selectivity study carried out by the authors demonstrated that no interference from other divalent and trivalent ions was identified. Electrostatic and coordinative interactions between carboxylic groups and cadmium were responsible for the isolation. The enrichment factor obtained after the analytical method was 20. Accuracy was corroborated by analyzing a certified reference material. The interaction of organic compounds with biopeels has also been described in the literature. In fact, methylene blue [90] and direct red 12B [91] were removed from water samples using garlic peel. However, the use of these materials for the solid-phase extraction of organic compounds usually requires an additional treatment of the biopeel.