Research Journal of Chemistry and Environment_______________________________________Vol. 25 (4) April (2021) Res. J. Chem. Environ. 83 Bio-Reduction of Hexavalent chromium using Punica Granatum leaves: optimization, kinetic and thermodynamic study Toufek Metidji 1 , Hacene Bendjeffal 2* , Abdelkrim Djebli 1 , Hadjer Mamine 1 , Hamida Bekakria 1 and Yacine Bouhedja 1 1. Laboratory of Water Treatment and Waste Recovery, Badji Mokhtar, Annaba University 23000, ALGERIA 2. Laboratory of Physical Chemistry and Biology of Materials, Higher Normal School of Technological Education (ENSET) Skikda 21000, ALGERIA *drbendjeffal@gmail.com Abstract This work is focused on the study of the bio-reducing capacities of the Punica granatum leaves in the removal of Cr(VI) using a well-defined factorial experimentation plan. Bioreduction tests were performed according to the Batch method, studying the influence of some experimental variables such as the mass of the bio-reducer, the temperature of the mixture, the pH of the solution and the initial concentration of Cr(VI). Molecular absorption spectrophotometry (UV- Vis) was used as the Cr(VI) assay method. This study showed optimal Cr(VI) bio-reduction at an initial Cr(VI) concentration of 100 mg. L -1 , for a bio- reducer mass of 0.150 g, at pH = 1 and at T = 55°C. A total reduction of Cr(VI) was observed after 10 minutes of contacting the latter with the biomass. The bio- reducer was characterized by spectroscopy (FTIR) using a Shimadzu-8700 spectrometer and by scanning electron microscope analysis (SEM-EDX) type Quanta 250. The modelling study showed that the bio-reduction kinetics obeyed the pseudo-second-order model with R 2 of 0.99992. The thermodynamic study revealed that the bio-reduction process is endothermic, spontaneous and has a stable configuration. Keywords: Bio-Reduction, Cr(VI), Punica Granatum Leaves, Kinetics, Thermodynamics. Introduction Pollution of water resources by heavy metals threatens present and future life; it is essentially of natural origin, scientists have pointed to the volcanic activity of planet earth, volcanic materials are among the largest natural polluters, moreover, a fairly significant part of the pollution of the planet is of anthropic origin, mainly due to human activity (industrial effluents, agriculture, maritime transport and other activities). Heavy metals are toxic and non- biodegradable; bioaccumulation explains their very high toxicity 1 . There are serious environmental and health problems for human beings, so the levels of heavy metals must be rigorously controlled 2 . Cadmium, lead, arsenic and chromium are the most well-known mineral contaminants in aquatic systems and soils 3 , Cr(VI) can be derived from a variety of anthropogenic activities including chromite mining, leather tanning, electroplating synthesis and electrolytic chroming, wood preservation, or else the stainless steel industry 4,5 Significant quantities of chromium are thus released into the environment; discharge waters may contain a wide range of chromium concentrations resulting in a threat to humans 6 . In solution, chromium can exist mainly in two stable oxidation states, Cr(III), (Cr(OH) 2+ or Cr(OH)2 + andCr(VI) (HCrO4 - , CrO4 2- or Cr2O7 2- ) 7,8 . Its reduced form can form highly stable and insoluble precipitates and is considered chemically inert 9 . Cr(VI) is very toxic and highly soluble in water throughout the pH range; this solubility gives it great mobility in ecosystems so that pollution with Cr(VI) can then affect a much larger area 10 . Hexavalent chromium compounds cross biological membranes more easily compared to trivalent compounds and Cr(VI) is 500 times more toxic than Cr(III) 11 . Studies have shown that Cr(VI) ions can indeed cause chronic mutagenic disorders 12 . The methods used to treat releases polluted by Cr(VI), generally aim to reduce it to Cr(III). WHO has limited Cr(VI) in water to 50 μg.L -1 13. During various industrial processes the generation of Cr(VI) is created by the oxidation of Cr(III) and is rejected as industrial waste 14 . Various methods have been used by many researchers to control Cr(VI) water pollution, such as chemical precipitation, membrane separation, ion exchange and evaporation, which are still too expensive, especially when used to treat large waste streams 15 and are not effective at concentrations between 1 and 100 mg. L -116 . Many research works are attempting to harness the self-regenerative capabilities of nature, decontamination technologies based on bio-reduction have proven to be a promising approach due to their low cost, cost-effectiveness and environmental friendliness, Cr(VI) is reduced to Cr(III), using the olive stones 17 , various bacteria such as Pseudomonas stutzeri 18 , microbial cellulose 19 , the green microalga 20 , the yeast strain 21 . In the present work, we have set ourselves the objective of carrying out a study on the valorization of a component of pomegranate (Punica granatum) using it as a biomaterial. Punica granatum has exceptional properties, which would be explained by the particular composition of its different parts due to the proven antimicrobial, antiviral, anticancer, antioxidant and antimutagenic effects of fruits. 22 It contains polyphenols, ellagic tannins and gallic and ellagic acids 23 .