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-second-order (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.