Investigation of the Adsorption of Phosphoric Acid from Aqueous Solution onto Biosorbents

In this research, the adsorption of phosphoric acid from an aqueous solution was investigated at different temperatures (298, 308, and 318 K) by using different adsorbents such as wheat bran and banana peels, in a batch system. FTIR and SEM analysis were used, in order to determine the functional groups in the structure of adsorbents and the surface characteristics of adsorbents, respectively. In the adsorption experiments, the effect of important parameters such as the effect of contact time, the amount of adsorbent temperature, and initial acid concentration was investigated. Equilibrium time was determined 40 and 50 minutes for wheat barn and banana peels. The highest percentage of adsorption for banana peel and wheat peel was measured at 71% and 59.8% for 1 g /L phosphoric acid, respectively. The optimum amount of adsorbent was determined 3g. Investigation of the temperature demonstrated that the percentage of removing phosphoric acid decreased by increasing the temperature. Different models of adsorption isotherms such as Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models were applied to analyze the equilibrium data at different temperatures and Langmuir and Temkin isotherm have the most agreement with experimental data for absorbents. Different kinetic models such as pseudo-first order, pseudo-second order, Elovich, and intra-particle diffusion model were chosen to describe the kinetic of adsorption, and the pseudo-second-order model for wheat barn and Elovich for banana peels have the best agreement with experimental data for each adsorbent. Thermodynamic parameters like standard Gibbs free energy changes of adsorption (ΔG oads), standard enthalpy changes of adsorption (ΔH oads), and standard entropy changes of adsorption (ΔS oads) were calculated by using equilibrium constant values at different temperatures. The negative value of (ΔG oads) demonstrated that adsorption of phosphoric acid by adsorbents is a spontaneity process and negative values of (ΔH oads) showed that adsorption of phosphoric acid on adsorbents is exothermic. The absorption capacity of banana peel and wheat bran was achieved20 and 11 mg/g, respectively. As a result, banana peel was the appropriate absorbent in this work.