Mechanism of simultaneous removal of aluminum and fluoride from aqueous solution by La/Mg/Si-activated carbon

La/Mg/Si-activated carbon derived from palm shell has been a suitable material for removal of aluminum and fluoride from aqueous solution. In the study, the mechanism of simultaneous removal of aluminum and fluoride by La/Mg/Si-activated carbon (La/Mg/Si-AC) was investigated to understand its high efficiency. It was found that the removal of aluminum and fluoride by La/Mg/Si-AC was favored at lower pH compared to the point of zero charge of La/Mg/Si-AC and high temperature. Adsorption capacity of Al(OH)₄^- was about 10 times higher than that of F^- due to the strong binding affinity of Al(OH)₄^- on protonated surface and competition between F^- and OH^- toward charged adsorption site. Kinetics results showed that the aluminum and fluoride adsorption were explained using the pseudo-second-order kinetic model and intra-particle diffusion model. Adsorption process of Al(OH)₄^- and F^- was driven by the potential rate-limiting step involved in mass transport process occurred on the boundary diffusion layer of porous adsorbent surface. Electrostatic interaction between protonated surface of La/Mg/Si-AC and negatively charged ions (i.e., Al(OH)₄^- and F^-) as well as ion-exchange between hydroxide and ionic metal species were important mechanisms in the process of aluminum and fluoride adsorption. Driving forces for adsorption of individual Al(OH)₄^- and F^- were not entirely different. Identifying the dominant mechanism will be helpful in understanding the adsorption process and developing new adsorbent. Mechanism of simultaneous removal of aluminum and fluoride by lanthanum/magnesium/silica -activated carbon are predicted.