Forces of interaction between fresh iron particles and iron oxide (magnetite): Effect of water chemistry and polymer coatings

Atomic force microscopy-based force spectroscopy (AFM) was employed to probe the forces of interaction between iron oxide (magnetite) surfaces and fresh zero-valent iron particles, either bare or coated with carboxymethyl cellulose polymer. Experiments were conducted at different pH values (4, 5.5 and 8), ionic strengths (water, 100 mM NaCl and 100 mM CaCl2), and in 10 mg/l humic acid solutions (pH 6.75). pH values of 4 and 5.5 produced attraction between magnetite and iron particles (either bare or polymer coated), promoting attachment. Attractive forces between uncoated particles and magnetite were also measured in 100 mM NaCl and CaCl2 at pH 8. Attachment of uncoated and CMC-coated iron particles was unfavorable in water at pH 8 and in humic acid solutions, as the forces upon approach were repulsive and, in addition, adhesive forces were on average weaker than in the other chemistries. A quartz-crystal microbalance with dissipation monitoring was used to probe humic acid sorption onto magnetite, and assess its impact on the forces of interaction.