Sorption of recalcitrant phosphonates in reverse osmosis concentrates and wastewater effluents – influence of metal ions

Magnetic microparticles functionalized with tailored ZnFeZr oxyhydroxide adsorbent were used for the reversible sorption of orthophosphate and recalcitrant organo-phosphonates from wastewater. The loaded particles were harvested magnetically from water, regenerated in an alkaline solution and reused numerous times. The applicability of the technology to treat brackish water reverse osmosis concentrates was tested under controlled synthetic conditions by investigating the influence of typical metals (Ca2+, Pb2+, Cu2+) on the removal of common phosphonates (HEDP, NTMP, EDTMP), and vice versa. When present at equimolar concentrations, metal cations enhanced the adsorption of phosphonates and were co-adsorbed at pH 4.0–4.5 (with removals of 83–93% for Pb2+ and 53–73% for Cu2+), likely through ternary complex formation. In the absence of metals, at pH > pHPZC ∼ 7 (the material point of zero charge), a drop in adsorption efficiency was observed for orthophosphate and all phosphonates. Thus, at pH 7, an increased adsorbent dose (>0.1 g/L) was necessary to remove 1 mg/L NTMP-P in 30 min. The reusability and effluent polishing potential of the ZnFeZr particles was demonstrated in a pilot test with municipal wastewater throughout 55 adsorption/desorption cycles without any drop in performance. Consistent removal of the non-reactive phosphorus species to ultra-low concentrations (<0.05 mg/L Ptot) and complete orthophosphate elimination (<0.005 mg/L PO4-P) was maintained under optimal conditions.