Biochemical reactions for the removal of heavy metals from aqueous solutions

Olea europaea and Citrus aurantium are wild plants distributed in the Mediterranean region and grown since ancient time. Leaves of Olea europaea and Citrus aurantium contain so many biologically active ingredients such as fats/lipids, saponin, flavonoids, terpenoids etc. Occasionally, saponin was considered as a biological naturally occurring plant glycoside, characterized by their strong foaming properties in an aqueous solution that can be used as a ligand for decontamination of aqueous solutions due to its ability to form chemical complexes with heavy metals.1,2 Also, saponin has been applied as a surfactant having the complex-forming ability for determination of iron spectrophotometrically.3 The surfactant properties of saponin have been utilized as an important agent for heavy metal removal from aqueous wastes and remediation of contaminated sites. According to modern literature, saponins have served as a natural chelating agent to eliminate problems due to several heavy metals such as Cr, Cd, Cu, Pb, Zn, and As from soil and wastewater.4‒7 It has been shown that heavy metals were retained from its original solution due to some form of complexation with a carboxyl group (-COOH) in saponin.8 From a chemical point of view, the biochemical reaction between heavy metals and saponin represents a modern approach to (1) the synthesis of new organometallic complexes; (2) decontamination of industrial and radioactive effluents; (3) a non-traditional method for isolation of saponin from the plant kingdom. Our previous studies1 prove the presence of an oleanolic acid or hederagenin as an aglycone of triterpenoid saponin in Olea europaea and Citrus aurantium leaves as shown as in Figure 1. This means that there are at least two active centers (-OH and –COOH groups) coordinated directly with heavy metal atoms forming a complex. The concentration of the heavy metal, the concentration of saponin extract and pH were found the chemical operational variables affecting the biochemical reaction between saponin and heavy metal containing aqueous solutions. The present work aimed firstly to evaluate the biochemical reactions between saponin extracted from Olea europaea and Citrus aurantium leaves and lead/cadmium aqueous solutions under different physical environments. The effects of temperature, agitation, and synergism between them are the main operating factors under investigation affecting the reaction. Secondly, physicochemical characteristics of Olea europaea and Citrus aurantium leaves were determined to ensure the presence of a structure-behavior relationship.