MODÉLISATION DE LA CINÉTIQUE D’ADSORPTION ET DE DIFFUSION DU CHROME (Cr) ET DU CADMIUM (Cd) PAR L’ALGUE

Abstract

The pollution of the environment by heavy metals is a major ecological problem which takes more and more an alarming dimension following the industrialization and the development of various human activities. Given that these metals are often toxic and non-biodegradable pollutants, it istherefore necessary to increase pollution control efforts by setting up means and processes for treating wastewater. In this context, there are several physical, chemical and biological methods to treat polluted effluents such as coagulation, flocculation, membrane filtration, chemical oxidation, ozonation, ion exchange, electrochemical methods and adsorption. The objective of this work aims to study the adsorption of cadmium and chromium in an algal biomass (Ulva lactuca) abundant on the Atlantic coast of Morocco, at the beach of Rabat. The choice of this biomaterial is guided, by ecological and economic considerations, which always lead to the search for natural absorbents, abundant in nature and usable in the raw state or after a simple treatment. The different material characterization techniques used in this work are Fourier Transform Infrared (FTIR), Scanning Electron Microscopy (SEM and Teller [BET]) as well as Atomic Emission Spectroscopy with Induction Coupled Plasma ICP- AES). To describe the transport phenomena of metal ions in solution through algal fibers, a mathematical model has been developed based on Fick's laws. The results obtained during the biosorption of cadmium (Cd) and chromium (Cr) in algal biomass (Ulva lactuca) are compared to the classical models of kinetics and isotherm. The results showed that for a temperature t° = 20°C, the adsorption of the two heavy metals is maximal during the first 60 minutes; the adsorbed quantity Qt being 3.02 mg / g and 1.08 mg / g respectively for cadmium and chromium to reach adsorption saturation for a time greater than 60 minutes. The current study showed that the adsorption of the two metals is favored by an acidic pH (pH = 3) for chromium and a pH between 5 and 6 for cadmium. These experimental results are better described by the Langmuir model for the adsorption isotherm and the pseudo second order model for the adsorption kinetics. The study of the thermodynamic parameters of the adsorption of cadmium in algal biomass shows that thi process is exothermic (ΔH ° <0), spontaneous (ΔG ° <0) and that it is favored at low temperature. To describe a possible transfer of metallic elements (Cr and Cd) in solution to the algal mass, a mathematical model is developed based on the laws of diffusion of atomic elements in matter (Fick's law in cylindrical coordinates); taking into account the boundary conditions, the analytical resolution of the diffusion equation made it possible to express the concentration (Cs) of the ions in solution as a function of time. The results obtained show total consistency between the experimental results and the theoretical results.