Background and purpose: Municipal wastewater treatment plant effluents are amongst the main available water sources. Reuse of these effluents requires further standards beyond secondary standards. Therefore, removal of organic pollutants and some compounds such as nitrate and phosphate from effluents should be done at a greater extent. In this study, removal of these compounds from secondary effluent was examined using an absorbent of iron oxide nanoparticles (MNPs Fe₃O₄).

Materials and methods: The research was carried out using actual effluent of urban wastewater treatment plant in laboratory scale. The iron oxide nanoparticles (MNPs Fe₃O₄) was used for removal of target pollutants by investigating different parameters such as pH, initial concentration, contact time, and adsorbent dosage using optimization of one factor at a time. Afterwards, the optimum condition was determined by adsorption and kinetic isotherms.

Results: The average BOD₅, COD, nitrogen, and phosphorus in wastewater treatment plant effluent were found to be 25, 43, 63 and 8.6 mg /L, respectively. The optimum conditions for adsorption of these pollutants by Fe₃O₄MNPs were obtained at pH 3, 15 min contact time and absorbent dosage of 1 g/l. Removal of PO₄^3-, NO^-₃, BOD₅ and COD in the optimum condition was obtained to be 54, 11.5, 56, and 75%, respectively, and the maximum adsorption capacities for target pollutants were 4.7, 1.6, 14, and 32 mg/g.  The results showed the adsorption by Fe₃O₄ followed the Freundlich model and pseudo first-order kinetic for COD and PO₄^3- and pseudo second-order kinetic for BOD₅ and NO^-₃ removal.

Conclusion: Iron oxide nanoparticles (MNPs Fe₃O₄) is suitable for removal of some pollutants (COD, BOD₅ and PO₄^3-) from the secondary effluent, but this absorbent is not effective for nitrate removal.