Influence of Natural Organic Matter and Ionic Composition on the Kinetics and Structure of Hematite Colloid Aggregation:  Implications to Iron Depletion in Estuaries

Abstract

The stability and aggregation behavior of iron oxide colloids in natural waters play an important role in controlling the fate, transport, and bioavailability of trace metals. Time-resolved dynamic light scattering experiments were carried out in a study of the aggregation kinetics and aggregate structure of natural organic matter (NOM) coated hematite colloids and bare hematite colloids. The aggregation behavior was examined over a range of solution chemistries, by adjusting the concentration of the supporting electrolyte-NaCl, CaCl2, or simulated seawater. With the solution pH adjusted so that NOM-coated and bare hematite colloids were at the same zeta potential, we observed a significant difference in colloid stability which results from the stability imparted to the colloids by the adsorbed NOM macromolecules. This enhanced stability of NOM-coated hematite colloids was not observed with CaCl2. Aggregate form expressed as fractal dimension was determined for both NOM-coated and bare hematite aggregates in both NaCl and CaCl2. The fractal dimensions of aggregates formed in the diffusion-limited regime indicate slightly more loosely packed aggregates for bare hematite than theory predicts. For NOM-coated hematite, a small decrease in fractal dimension was observed when the solution composition changed from NaCl to CaCl2. For systems in the reaction-limited regime, the measured fractal dimensions agreed with those in the literature. Colloid aggregation was also studied in synthetic seawater, a mixed cation system to simulate estuarine mixing. Those results describe the important phenomena of iron oxide aggregation and sedimentation in estuaries. When compared to field data from the Mullica Estuary, U.S.A., it is shown that collision efficiency is a good predictor of the iron removal in this natural system.

Citrate-Coated Silver Nanoparticles Interactions with Effluent Organic Matter: Influence of Capping Agent and Solution Conditions

“…These results evidence the high interactions occurring between Ca 2+ ions and ionized carboxyl groups on both citrate and River humic OM. 30,32,33 Page 14 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 Citrate-coated AgNPs aggregation in WW humic-containing solutions was clearly observed at 10 mM Na + with a k 11 of 0.28 nm.min -1 , (Figure 2b and S1d). Furthermore, the aggregation rate constant increased at 20 and 30 mM Na + , indicating reaction limited regime.…”

“…20,30,32,33 Furthermore, CCC in Ca 2+ solutions would occur at a lower ionic strength than in Na + solutions. Nevertheless, due to sample volume constraints it was not possible to exactly determine the critical coagulation concentration to establish an accurate comparison between favorable and unfavorable aggregation in Na + or Ca 2+ solutions.…”

Metal Cation Complexation with Natural Organic Matter in Aqueous Solutions: Molecular Dynamics Simulations and Potentials of Mean Force

“…4,5 NOM can also easily adsorb onto mineral surfaces, forming an organic coating which significantly alters the electrostatic properties of the surface and strongly affects the transport and availability of toxic species. [6][7][8][9][10][11] The formation of NOM coatings is also a significant factor controlling the stability of viruses in aquatic systems 12 and the performance characteristics of inorganic and polymeric nanofiltration membranes. 13,14 Common cations such as Na + , Mg 2+ , and Ca 2+ are thought to play an important role in the supramolecular aggregation of NOM molecules into colloidal particles, 2,3,[15][16][17][18][19][20] the formation of NOM surface coatings, [10][11][12][13] and the aggregation of mineral particles.…”

Deposition of Silver Nanoparticles in Geochemically Heterogeneous Porous Media: Predicting Affinity from Surface Composition Analysis

“…Aggregation is one of the key processes that controls the mobility of nanoparticles and the aggregation kinetics of various nanomaterials under different environmental conditions have been investigated (Mylon, Chen et al 2004;Brant, Lecoanet et al 2005;Chen and Elimelech 2006;Phenrat, Saleh et al 2007;Chen and Elimelech 2008;Domingos, Tufenkji et al 2009;Rebecca and Bojeong 2009). Another key process of determinative effect on the transport of nanomaterials is deposition onto environmental relevant surfaces.…”