Effective removal of ammonium nitrogen using titanate adsorbent: Capacity evaluation focusing on cation exchange

Cation exchange is one of the dominant mechanisms in the adsorption of cationic ammonium nitrogen (NH₄⁺) from water. In this study, we focus on the role of counter cations in cation exchange process of NH₄⁺ to enhance the adsorption capacity. Five amorphous titanates namely lithium titanate (LiT), sodium titanate (NaT), potassium titanate (KT), strontium titanate (SrT) and barium titanate (BaT) with different counter cation were facilely synthesized. The adsorption performance for NH₄⁺ by these samples is in the order of LiT> NaT > KT> > SrT > BaT. The maximum adsorption capacity of LiT calculated by Langmuir is as high as 50.31 mg·g^-1. According to the experimental results and theoretical analysis, the electrostatic interaction between counter ions (cations in framework or eternal solution) and charged framework (fixed ions) is the main influence factor during cation exchange process in general. The cation valence and the hydrated ionic radius of the counter ions can inversely affect the ion exchange equilibrium and the affinity of counter ions to titanates. Therefore, a definition of a brief parameter, affinity coefficient K_f (relating to ion valence and distance between opposite charged ions), is introduced and used to explain the difference in adsorption performance of five titanates for NH₄⁺. The conclusion about cation exchange and ions affinity may provide possible strategies for enhancement of cationic contaminant adsorption from water or wastewater.