Effects of pH and ionic composition on sorption/desorption of natural organic matter on zero-valent iron and magnetite nanoparticles

Understanding the interactions between natural organic matter (NOM) and zero-valent iron nanoparticles (nano-Fe⁰) and magnetite nanoparticles (nano-Fe₃O₄) is essential for evaluating their performance in pollutant remediation, as well as determining their fate and transport in the environment. Batch experiments were performed to investigate the sorption/desorption behaviors of humic acid (HA) on commercially available nano-Fe⁰ and nano-Fe₃O₄. The sorption/desorption of HA on nano-Fe⁰ and nano-Fe₃O₄ were well described by both the Langmuir model and the modified Langmuir model. The adsorption capacities of HA were 8.77 ± 0.31 mg C/g and 10.05 ± 0.95 mg C/g for nano-Fe⁰ and nano-Fe₃O₄, respectively. The interactions of HA with nano-Fe⁰ and nano-Fe₃O₄ were highly pH-dependent. On one hand, nano-Fe⁰ had its maximum adsorption of 11.0 mg C/g HA at pH = 3, which decreased to 0.6 mg C/g when pH increased to 11.9; on the other hand, alkaline condition enhanced HA desorption greatly. At pH = 10.1, after 24 h desorption experiments, nearly 80% of initially adsorbed HA desorbed from the nanoparticles. The interactions of HA with nano-Fe⁰ and nano-Fe₃O₄ were also influenced by different ion compositions in solution. Divalent cations (e.g. Ca²⁺, Mg²⁺) enhanced HA adsorption significantly, while phosphate nearly eliminated HA adsorption and promoted significantly HA desorption.