Removal of phosphonates from synthetic and industrial wastewater with reusable magnetic adsorbent particles

This work proposes a technology for phosphonate removal from wastewater using magnetically separable microparticles modified with a tailored ZnFeZr-oxyhydroxide adsorbent material which proved to be highly efficient, reaching a maximum loading of ∼20 mg nitrilotrimethylphosphonic acid-P/g (215 μmol NTMP/g) at room temperature, pH 6 and 30 min contact time. The adsorption process at pH < 7 was fast, following the pseudo-second-order kinetics model. Furthermore, NTMP adsorption onto ZnFeZr-oxyhydroxide proved to be endothermic. At pH > pHpzc ≈7 (point of zero charge of the material) a drop in adsorption efficiency was observed for phosphate and for five different investigated phosphonates. Adsorption of NTMP could not be detected at pH > 8, however, the presence of more than 0.5 mM CaII improved significantly the adsorption efficiency. Successful reusability of the engineered particles was demonstrated throughout 30 loading cycles by changing the operational conditions (dose, pH) to optimize the performance. At optimal conditions, complete phosphonate removal was observed even after 30 cycles of particles' reuse in a synthetic NTMP-solution and DTPMP-rich membrane concentrate. In each cycle, phosphorus was desorbed and concentrated in a 2 M NaOH. Industrial phosphonate-containing wastewaters rich in calcium, e.g. membrane concentrates, proved to be especially suitable for treatment with the particles.