Quaternary Phosphonium Strong Based Anion Exchangers for the Selective Adsorption of Nitrate

Nitrate (NO3-) has become a global issue in the field of water treatment due to its extensive distribution in the natural waterbody and industrial process. Quaternary ammonium (R4N+) strong based anion exchangers are still the main product used for NO3- remediation. However, the quaternary phosphonium groups (R4P+) with stronger polarization and anionic affinity have not been adequately studied. This work employed triphenylphosphine (PPh3) and tributylphosphine (TBP) to quaternize chloromethylated macroporous polystyrene resin (PS-Cl), and prepared novel strong anion exchangers (PS-PPh3 and PS-TBP) for the selective adsorption of NO3- . These exchangers exhibited good adsorption performance for NO3- in the broad-spectrum pH range of 2.0-11.0. The adsorption capacities of PS-PPh3 and PS-TBP fitted by Langmuir isotherm were approximately 20.14 and 25.53 mg NO3--N/g, respectively. The adsorption processes of these exchangers could be fitted using pseudo-secondorder (PSO) kinetic curves. Both selectivity experiments and density functional theory (DFT) indicated the superior NO3- selectivity of quaternary phosphonium exchangers, and demonstrated that the effect of charge distribution density of the exchanger on NO3- selectivity was greater than that of steric hindrance and hydrophobicity, which emphasized the importance of enhancing anionic affinity of functional group at low charge density. After five cycles of adsorption-desorption cycles with NaCl desorption solution, these two exchangers still maintained above 90 % of adsorption capacities for NO3- . The column experiment and benefit assessment also indicated that quaternary phosphonium exchangers had the good application value and potential to become a substitute for quaternary ammonium anion exchangers when facing extremely complex water matrix.