Phosphate removal from waters by using bauxite

IV SUMMARY PhD Thesis Phosphate Removal from Waters by Using Bauxite H. Soner ALTUNDO?AN Fırat University Graduate School of Natural and Applied Sciences Department of Chemical Engineering 1998, Page: 302 Phosphates have a significant pollution potential for surface waters. Phosphorus has been regarded as a key element in the eutrophication of natural waters. For that reason, the removal of phosphate from the municipal and some industrial wastewater is absolutely necessary. The phosphates can occur in wastewater in the form of inorganic (ortho and condensed) and organic. The removal of phosphates from wastewater can be achieved either through biological treatment, chemical precipitation or combination of both. In recent years, phosphate removal by adsorption techniques are investigated because of the some disadvantages of conventional treatment techniques. In this study, removal of phosphates from aqueous solutions was investigated by using bauxite which is a raw material for aluminium production. For this purpose, the effects of some adsorption parameters on the adsorption of ortho, condensed (tripoly phosphate) and organic (glycero phosphate) phosphates by using both original bauxite and activated bauxite which is obtained by heating original bauxite at 600°C for 2 hours. The effect of pH on the removal of phosphates from solutions containing two or three phosphate species mentioned above was examined. Also, the effect of pH on the desorption of phosphates from phosphate adsorbed original and activated bauxite was investigated. Data from adsorption experiments were applied adsorption isotherms and some kinetic, thermodynamic and mass transfer calculations were made. Finally, theremoval of phosphate from ortho, tripoly and glycero phosphate solutions having different initial pH by using -50+100 mesh fraction of original bauxite were investigated in a continuous column system. The removal efficiency of phosphates by original and activated bauxite is, to large extent, depending upon the pH of media. The optimum pHs for the ortho, tripoly and glycero phosphates were determined as 4.45, 5.40 and 3.20 for original bauxite and 4.32, 5.81 and 4.72 for activated bauxite, respectively. The maksimum removal percentages of ortho, tripoly and glycero phosphates by original bauxite were found to be 67.3, 58.0 and 40.0, respectively, by contacting 100 ml phosphate solutions (10 mg- P/l and 0.01 M NaCl) with 1 g original bauxite for 2 hours at optimum pH and 25°C. In case of activated bauxite, the maximum removal percentages of same phosphate species were found to be 73.8, 76.7 and 71.1. While the maximum removal was obtained by using 2.5 g/1 activated bauxite dosage for the adsorption of ortho phosphate, the activated bauxite dosage required for the other phosphate species is 5 g/1. Adsorption of three phosphate species by original and activated bauxite follow Lagergren, Modified Freundlich and Elovich Equations which are first order kinetic expressions based on concentration change of adsorbate on the adsorbent surfaces. Lagergren rate constants at 25°C for ortho, tripoly and glycero phosphate adsorption on original bauxite were calculated as 0.028, 0.009 and 0.055 min"1, respectively. The corresponding values of the rate constants for activated bauxite are 0.024, 0.014 and 0.026 min'1. The values of the rate constants increased by increasing temperature of mixture. Phosphate adsorption capacity of bauxite increases by heat activation, significantly. The Langmuir adsorption capacities of original bauxite for ortho, tripoly and glycero phosphates at 25 °C were calculated as 1.045, 0.787 and 0.606 mg-P/g, respectively. The adsorption capaticies corresponding same phosphate species for activated bauxite were determined as 4.129, 2.251 and 2.227 mg-P/g. Also, the adsorption capacities increase for all adsorption processes by increasing temperature and it was determined that enthalpy changes were endothermic. The enthalpy changes for the adsorption processes of the phosphate species were calculated as 14.68, 15.06 and 8.17 kJ/mole-P for original bauxite and 25.36, 31.89 and 34.89 kJ/mole-P for activated bauxite.VI In the adsorption experiments carried out by using mixed phosphate solutions, it was determined that the removal efficiency of different phosphates species showed a selectivity and each species reduced the adsorption efficiency of other species depending on pH of the solution. Desorption yield of phosphate species is depending on pH and desorption efficiencies are quite high in strong acidic and basic media. Desorption yield of all phosphates from both adsorbents are minimum at around the maximum adsorption pH's. In the continuous column studies, the obtained adsorption densities for ortho, tripoly and glycero phosphates at initial pH of 2.5 were 0.270, 0.163 and 0.145 mg-P/g, respectively. These values are higher than that of the batch adsorption studies with same bauxite fraction. It is believed that ortho, tripoly and glycero phosphate adsorption by original and activated bauxite occurred by a ligand exchange mechanism. The amount of surface hydroxyl group of bauxite which plays an important role in ligand exchange increased by activation. The amount of surface hydroxyl group of original and activated bauxite by using potentiometric titration route were determined as 3.51X10"4 and 7.19X10"4 mol OH/g, respectively. Key Words: Ortho, condensed and organic phosphate adsorption, bauxite, adsorption kinetic, adsorption isotherms, desorption.