Removal of boron using aluminum-based water treatment residuals (Al-WTRs)

Three samples of aluminum-based water treatment residuals (Al-WTR1, Al-WTR2, and Al-WTR3) were assessed as adsorbent for boron removal. The Al-WTRs were mainly composed of Al2O3, Fe2O3, and SiO2. The adsorption reaction could be approximated by a pseudo-second-order kinetic model. The equilibrium pH (pHe) affected boron adsorption and the optimum pHe was found at 8.2–8.5. The Langmuir model described the adsorption data satisfactorily (R2 value average ≧ 0.990), and the batch maximum adsorption capacities were found as 0.980, 0.700, and 0.190 mg/g using three samples, respectively, at pH value of 8.3 ± 0.2. Thermodynamic analysis revealed that the adsorption reaction was spontaneous and exothermic. The ΔHo value was low (− 1.58 kJ/mol), and that was indicative of a physical adsorption. Electrostatic interactions and van der Waals force were proposed as major driving forces of adsorption under different pH conditions. Both BET specific surface area and aluminum content of Al-WTRs affected boron adsorption, as judged by the fact that Al-WTR1 showed higher adsorption density than the other two. The results of this study show that Al-WTRs can be used as an alternative adsorbent for boron removal.