Deciphering the role of surfactants on phosphate adsorption at the surface of iron oxide nanoparticles using isothermal titration calorimetry.

Surfactant-coated iron oxide nanoparticles (IONPs) have been shown promising for enhancing phosphate removal in peritoneal dialysis (PD). However, the specific role of the surfactant coating in the phosphate adsorption mechanism remains to be studied, particularly for IONPS coated with anionic surfactants like tannic acid (TA) and polyacrylic acid (PAA), which exhibit significant phosphate uptake despite the presence of electrostatic repulsion. Isothermal titration calorimetry (ITC) was thus used to thermodynamically characterize phosphate adsorption on uncoated IONPs and IONPs coated with PAA, TA, and cationic polydiallyldimethylammonium chloride (PDADMAC). Such ITC interaction studies between surfactant-coated IONPs and phosphates have, to the best of our knowledge, never been reported. At first, the molar enthalpy value for the adsorption of phosphates on uncoated IONPs in pH 3 water was determined by performing complementary ITC and batch mode adsorption experiments. The phosphate adsorption was established to be exothermic with an enthalpy value of -31 kJ.mol-1 in our experimental conditions. Then, ITC experiments were conducted by adding phosphates to pH 7 water suspensions of uncoated IONPs, the three surfactant-coated IONPs, and the free surfactants (PAA, TA, and PDADMAC) alone. They confirmed that the phosphates adsorption occurred at the surface of iron oxide in surfactant coated IONPs with surfactants occupying some adsorption sites at the surface of iron oxide. However, TA was suggested to precipitate with cations introduced with each phosphate injection showing thus the sensitivity of TA coating to the ionic strength of the medium while PDADMAC, less strongly anchored to the iron oxide surface, was shown to desorb when the ionic strength increases. No reactions were observed with PAA supporting their neutral role in phosphate adsorption. These findings confirmed that phosphates can adsorb at the surface of IONPs in surfactant-coated IONPs whatever their surface charge and that surfactants coating of IONPs may be strongly affected by the ionic strength of the media. PAA coated IONPs appears as the most suitable phosphate adsorbents in our conditions.