Removing Phosphates from Aqueous Solutions by Means of Static Sorption on Ash-and-Slag Sorbent: Analyzing Coefficients of Distribution Based on Adsorption Isotherms

Experimental studies of static sorption in the phosphate-sorbent system are performed at sorbent doses of 1, 2, and 5 g in the 2 to 5000 mg/L range of initial phosphate concentrations in solution. A calcined sorbent based on ash-and-slag waste from thermal power engineering is studied as a sorbent. The studies are performed at a 200 rpm frequency of rotation of a magnetic stirrer at a pH 7 and a 120 min period of sorption, found via optimization. The temperature during experimental studies is 25 ± 2°C. The efficiency of treatment and coefficient K_d of maximum distribution are revealed. The data are processed using two-parameter Langmuir, Freundlich, Elovich, Temkin, and Dubinin–Radushkevich isotherms of adsorption. The isotherm constants of adsorption are determined through linearization. Calculated and experimental data are compared. A theoretical analysis of adsorption isotherms is performed to describe the dependence of coefficient K_d of distribution on the initial C₀ and equilibrium C_e concentrations of adsorptive in solution. For Langmuir, Freundlich, and Elovich isotherms, functions K_d = f(C_e) and K_d = f(C₀) fall monotonically throughout the range of changes in the adsorptive in solution. For the Temkin and Dubinin–Radushkevich isotherms, the derivative of function dK_d/dC_e changes its sign from plus to minus, which is a sufficient condition for there being a maximum of the function. Functions K_d = f(C_e) and K_d = f(C₀) have maxima at low concentrations C_e. A qualitative description is obtained of experimental data on maximum coefficient K_d of distribution using the Temkin and Dubinin–Radushkevich isotherms. Adequate quantitative descriptions of experimental data on maximum function K_d = f(C₀) are obtained using the Temkin isotherm at a sorbent dose of 1 g and the Dubinin–Radushkevich isotherm at a sorbent dose of 2 g. The Temkin and Dubinin–Radushkevich isotherms are used to describe coefficient K_d = f(C_e) of distribution in the system’s sorbent (ammonium ions). The maximum of the function is revealed at low concentrations of ammonium ions in solution. A comparison of calculated and experimental data shows qualitative agreement between theory and experiments.