Leaf Biosorbents for the Removal of Heavy Metals

Abstract

The presence of heavy metals in waters is a global concern. Among vari- ous remediation technologies, biosorption is an attractive and promising process since in many cases, it is fast, selective, and highly efficient. Other advantages, such as applicability for various types of pollutants, simplicity, low cost, ease of opera- tion, and the potential reusability of the adsorbents, make it beneficial. Nowadays, more and more green materials, raw or modified, are inclined to be explored instead of conventional adsorbents, within concept of Green Chemistry. This review focuses on the use of leaves-based biosorbents in raw or modified forms to sequestrate heavy metals from waters and wastewaters. One of the main points is that chemical agents such as acidic (e.g. HCl, HNO3 , H2SO4, and H3 PO4), alkaline (e.g. NaOH and NaHCO3 ), oxidative (like KMnO4 ), metal salt (e.g. NaCl, CaCl2 , and MgCl2 ), and organic (e.g. formaldehyde, monoso- dium glutamate, and anionic surfactant) agents are satisfactorily used in order to improve the adsorption capacity of leaf based adsorbents. The maximum monolayer adsorption capacity, at different initial experimental conditions, obtained from Langmuir isotherm for the studied heavy metals ranges between 3.9–300 mg/g and 7.8–345 mg/g for raw and modified leaf biosorbents, respectively. In most cases, the Langmuir and pseudo-second-order kinetic model give the best fit. Thermodynamic studies show that the adsorption in all studied cases was sponta- neous and mainly endothermic with increased randomness at the solid-liquid inter- face during the adsorption process. Moreover, an enthalpy-entropy compensation revealed the necessity to re-examine the way that the thermodynamic parameters are estimated. Column studies, the mechanism of adsorption, multiple biosorption/ desorption cycles and the study of biosorption of multiple pollutant systems are some crucial issues that have been addressed in this work. There is a lack of knowl- edge on the application of the leaf biosorbents in more realistic conditions, such as in real wastewaters, for understanding their behavior in order to be applied for pilot scale and full scale systems.