Competitive removal of Pb²⁺ and malachite green from water by magnetic phosphate nanocomposites

The competitive removal of Pb²⁺ and malachite green (MG) from water by three magnetic phosphate nanocomposites (Fe₃O₄/Ba₃(PO₄)₂, Fe₃O₄/Sr₅(PO₄)₃(OH), and Fe₃O₄/Sr_5xBa_3x(PO₄)₃(OH), namely "FBP", "FSP", and "FSBP", respectively) was systematically investigated compared with Fe₃O₄ ("F") nanoparticle. Temperature and adsorbent dosage for competitive removal were optimized to be 20 °C and 0.05 g in 50 mL. The kinetic and isothermal adsorption results were fitted well with the pseudo-second-order model and Langmuir model, respectively. In the competitive removal process, FSP showed a high affinity to Pb²⁺ (202.8 mg/g) while FBP possessed high selectivity for MG (175.4 mg/g), and FSBP was effective at simultaneous removal of Pb²⁺ and MG, with a capacity of 143.7 and 90.9 mg/g, respectively. The magnetic contents in nanocomposites allow magnetic separation of materials from the water after treatment. We proposed that the simultaneous removal mechanism by FSBP was due to ion exchange between Pb²⁺ and Sr²⁺ in the lattice and then the formation of hydrogen bonds between PO₄^3- outside the material's surface and positively charged hydrogen in MG. This study indicates the potential of these phosphate nanocomposites to be used as effective materials for selective or simultaneous removal of Pb²⁺ and MG from water.