Interactions Of Natural Organic Matter With Iron-based Nanoparticles

Natural organic matter （NOM）, which is ubiquitous on the earth, consists of heterogeneous and complex macromolecular components. It controls environmental and geochemical behavior of metal ions, organic compounds and most nanoparticles （NPs）. Zero-valent iron nanoparticles （nano-Fe⁰） and magnetite nanoparticles （nano-Fe₃O₄）, as the major part of iron-based nanoparticles, has been widely applied to environmental remediation and drawn increasing attention.When iron-based nanoparticle is applied into in situ remediation of contaminated ground-water, NOM definitely interacts with nanoparticles and affects their mobility, toxicity, and reactivity. The understanding of the interaction behavior of NOM and iron-based nanoparticles is of great importance for understanding of the possible impacts on the physicochemical properties of nanoparticles. However, to date, the interaction mechanisms of NOM and iron-based nanoparticles have not been completely understood and elucidated. In this paper, the adsorption and desorption behavior and the interaction mechanisms of HA （representing NOM） on nano-Fe⁰ and nano-Fe₃O₄ is studied. The main research results are as follows:1. The adsorption process of HA with nano-Fe⁰ and nano-Fe₃O₄ was well fitted by the pseudo-second-order kinetics and Langmuir isotherm. Nano-Fe⁰ presented higher HA affinity than nano-Fe₃O₄. The rate constant of kinetic model （k2） and surface area normalized maximum （qmax-BET） of HA adsorption in the nano-Fe⁰ system at 298.2 K was 0.343 g/（mg C·h） and 0.331 mg C/m², respectively, higher than those in the nano-Fe₃O₄ system （0.097 g/（mg C·h） and 0.235 mg C/m²）. Adsorption of HA onto nanoparticles was strongly influenced by the pH, ion species and ion strengths. Acidic conditions, high ion strengths or the presence of Ca2+ and Mg2+ could promote the HA adsorption.2. Larger molecules （>4 KDa） were preferentially adsorbed onto nano-Fe⁰ surface. The carboxyl groups （COOH） and phenolic hydroxyl groups （phenolic OH） of HA attended the adsorption process and nano-Fe⁰ had high affinity for HA phenolic OH. In contrast, nano-Fe₃O₄ had no preferential selectivity on the adsorption of the larger HA components or the smaller ones. Nano-Fe₃O₄ had high affinity for HA COOH. The mechanisms of HA adsorbed by nano-Fe⁰ involved ligand exchange, electrostatic attraction and hydrophobic interactions. The adsorption of HA on nano-Fe₃O₄ contributed to ligand exchange and electrostatic attraction.3. The modified Langmuir desorption model was used to describe desorption behavior of HA on nano-Fe⁰ and nano-Fe₃O₄. The hysteresis coefficient （h） ranged from 0.69-0.83, indicating a strong adsorption-desorption hysteresis. Desorption of HA at neutral condition was so slow. When pH increased, desorption was fast and reversible. The desorption amount of HA ranged from 5.1-6.5 mg C/g, equalling to 60-80% of adsorption amount on the nanoparticles. Besides, the presence of phosphate could significantly promote the HA desorption, and the desorption amount increased by increasing time.