Natural Phosphates Characterization and Evaluation of their Removal Efficiency of Methylene Blue and Methyl Orange from Aqueous Media

This study evaluated the capacity of a rock phosphate for the adsorption of organic dyes methylene blue MB and methyl orange MO in aqueous solution, in order to minimize the impact of these dyes on the environment. The physicochemical characterization of natural phosphates (NP) shows that its mineralogy is carbonate-fluorapatite, calcite and quartz, as demonstrated by X-ray diffraction. An infrared (IR) analysis completed the structural study by confirming the characteristic bands of a carbonated fluorapatite type B. The influence of adsorbent dose, pH, initial concentration and temperature of the dye solution on adsorption onto NP was studied. The experimental results show that MB is adsorbed almost entirely at an adsorbent dose of 1 g/L and at a more basic pH and that the Langmuir model describes its isotherm well. For MO, adsorption is performed at acidic pH, such that discoloration reaches 60% at pH 4 and NP adsorbent dose of 10 g/L. The maximum adsorbed amounts of MB (pH=9) and MO (pH=4) were found to be 9.54 and 1.09 mg/g, respectively. The kinetic data were analyzed to show that the pseudo-second-order model seems to be the most appropriate to describe the adsorption dynamics of both dyes on the naturel phosphate. The thermodynamic results show that the adsorption is endothermic for MB and exothermic for MO. So, rock phosphate shows a good potential as a sorbent for cationic dyes removal from wastewater.