Influence of ionic environment on the process of adsorption of heavy metal ions on magnetic iron oxides

Magnetic iron oxides, mainly magnetite Fe3O4and maghemite Y—Fe2O3, are seen as cheap, effective and convenient to use adsorbent of impurities such as heavy metals or dyes. The paper presents the influence of selected ions: inorganic monovalent and multivalent anions (NO3, Cl , SOy, PO3 ), organic anions, including complexing agents (Cl I.COO , EDTA) and monovalent and bivalent metal cations (Ca2+, Na+) on the separation of Cu(II), Zn(II), Ni(II), Cd(II), Pb(II), Cr(III) and Cr(VI) ions from solutions, using unmodified magnetite nanoparticles (M_NPs) and magnetite modified with poly(sodium acrylate) (M/PSA_NPs). The adsorption process was carried out at two pH values (5 and 6 for Cu(II), Zn(II), Ni(II), Cd(II) and 4 and 5 for Pb(II), Cr(III) and Cr(VI) ions) and at the concentration of the accompanying ion 10 times higher than the metal concentration. The reference test was carried out without the addition of accompanying salts. It was found that the increase in pH and the use of M/PSA_NPs adsorbent containing dissociated carboxylate groups -COO contributed to the increase in the degree of separation of all tested metals except for the anionic Cr(VI) form. Under the experimental conditions, inorganic monovalent anions did not show any significant effect on the process efficiency, whereas multivalent anions, especially PO(, increased the degree of separation of Cu(II), Zn(II), Ni(II) and Cd(II), which suggests that these anions may participate in the process of bonding metal cations with magnetite. A similar effect was observed for Pb(II) and Cr(III) cations using unmodified magnetite and lower pH value. The presence of a strong complexing agent (EDTA) significantly decreased the efficiency of cationic metal forms separation. The degree of this decrease depended on the type of metal and the conditions of adsorption. Cr(VI) anions were separated to a greater extent using unmodified magnetite. The presence of versenate (EDTA), phosphate and sulfate ions caused a decrease in the efficiency of the process, which may be explained by the fact that these multivalent anions compete with chromate ions in the adsorption process.