Colloidal iron as a nanovector in the environment

Iron (Fe) oxyhydroxide minerals are ubiquitous in soil and are major sinks for humic substances, trace metals and oxyanions that sorb strongly to these minerals. A small fraction of the Fe oxyhydroxides can also occur as colloids in the soil solution and may act as a so-called nanovector that mobilizes, rather than immobilizes, the substances and ions associated with them. This explains the colloidal transport of components such as phosphate (PO4), arsenate (AsO4) and lead (Pb) that are strongly bound to the Fe oxyhydroxides. The colloidal transport of these components in soil remains largely unknown at a large scale, mainly because there is a lack of appropriate Fe colloid sampling and characterization methods.This work was set up to evaluate different Fe colloid characterization methods and identify the relationships between soil properties and Fe colloid properties, i.e. their size, concentration and sorption characteristics. The rationale is that understanding the properties of the colloidal carrier will ultimately lead to a better assessment of the fate of the numerous elements that are associated with it.

We first explored the potential of two innovative techniques for colloid characterization, single-particle ICP-MS (sp-ICP-MS) and Flow Field Flow Fractionation (FlFFF) to size environmental Fe colloids. The FlFFF allowed distinguishing between Fe-organic carbon (OC) complexes and larger mineral colloids; it has a wide size detection range between 1-2 nm and 300 nm and has acceptable element recoveries in an environmentally relevant background. The size obtained with FlFFF corresponds with the hydrodynamic diameter of particles in their aggregated state. The relatively high size detection limit in sp-ICP-MS analysis (about 30-50 nm) compromised sizing of environmentally relevant Fe colloids. The FlFFF coupled to UV/VIS and ICP-MS was therefore the method selected to characterize the Fe colloids in this work.

Colloid stability is a prerequisite for transport and natural organic matter (NOM) provides both electrostatic and steric stability to Fe oxyhydroxide colloids. An experimental study was set up to specifically test the role of NOM on Fe colloid formation by coprecipitation. The Fe colloids were prepared by oxidation of Fe(II) with variable concentrations of NOM. Stable colloids were formed over the range in dissolved organic carbon (DOC) to Fe ratios representative for surface-and groundwaters. Moreover, a striking effect of NOM on colloid size and composition was observed. The Fe colloids size increased consistently with decreasing DOC/Fe ratio in the test solutions. Over a wide molar DOC/Fe range (1400-10), the Fe colloid size was very small (< 10 nm), further decreasing the DOC/Fe ratio yielded larger colloids of approximately 50 nm until the limit for colloid stability was reached at DOC/Fe < 2. The speciation of the colloids changed from Fe-OC mononuclear complexes at high DOC/Fe, to polynuclear Fe-OC complexes or ferrihydrite embedded in NOM at intermediate DOC/Fe and to larger Fe colloids with low NOM content that ultimately settle. The effect of NOM is explained as inhibition of growth and crystallization, which was supported by the higher organic matter loading in the smallest particles. Interestingly, the surface loading of NOM on the colloids increased more than proportionally with decreasing size, suggesting a conformational change of the NOM, i.e. more extended in small colloids, explained by the high electrostatic and/or steric repulsion.

These findings were subsequently tested for natural samples. First, processes that govern Fe colloid size and composition were inferred from pore waters of 97 different topsoils covering a wide range in soil physico-chemical properties. Soil solution Fe concentrations are governed by colloid stability because they are highest at low soil solution Ca and high dissolved organic carbon (DOC) concentrations. The colloid size remarkably varied among soils, organic carbon was identified as a dominant factor explaining the variation. The fraction of Fe forming mononuclear Fe-OC complexes (< 5 nm) ranged between 1-36% and increased with an increasing DOC/Fe ratio in pore waters. In addition, small mineral Fe colloids (5─50 nm) prevailed in soils with > 3.5% soil organic carbon, while the larger mineral Fe colloids (50─100 nm) were dominant in soils with low soil organic carbon content. In some samples with low DOC content and low Ca concentration, the elemental ratios in mineral colloids suggested the presence of phyllosilicates. In these pore waters, Fe might be associated with clay minerals and are therefore distributed to the larger size range of clays.

Second, Fe colloids were characterized in pore waters from a podzol profile to identify the factors that cause the pronounced vertical Fe mobilization and subsequent immobilization in such soils. The pore water Fe concentration increased from the A to the E (eluvial) horizon and peaked in the E horizon, which is remarkable given the low soil Fe concentration in the E horizon. The pore water Fe concentration was positively related to the DOC concentration within the profile. The colloid characterization analysis revealed that the Fe colloids (<100 nm) not only consisted of Fe-OC complexes, the generally accepted cheluvation theory, but also of mineral Fe colloids, presumably Fe oxyhydroxides coated with organic matter. The smallest Fe-OC complexes dominated in the A horizon while the larger mineral colloids raised most importantly with increasing depth, explaining the rise in total Fe towards the Bh (illuvial) horizon. The adsorption of the negatively charged OM at the top of the Bs horizon is likely the main mechanism of DOC retention in the Bh. Below this depth, the DOC was very low resulting in low pore water Fe concentration. Straining is unlikely a significant mechanism for colloid immobilization as judged from the pore size distribution and the colloidal size. This study demonstrates that natural organic matter plays a key role in the transport of Fe colloids in acid, sandy soils with low Ca.

These findings highlight the key role of NOM in Fe colloid transport, but also raised the question on the effect of NOM on the reactivity. With decreasing colloid size, the specific surface area (SSA) available for sorption increases and likely also the colloid mobility. However, adsorbed humic substances might lower the affinity of the colloids for binding oxyanions. The aim was therefore to determine the implication of this organic matter coating on the affinity of the Fe colloids to sorb trace metals and oxyanions. This was first addressed in an experimental study by measuring PO4 adsorption and coprecipitation to Fe colloids with varying size due to varying amounts of NOM. In a second step, that interaction was also verified in an observational study on pore water colloids of different soils, thereby analysing colloid size-dependent fractions of compounds with high affinity to either humic substances (Cu) or to Fe oxyhydroxides (VO4 and AsO4). Both studies indicated that the PO4 loading on the colloids (experimental study) and the fractions of colloidal VO4 and AsO4 (pore water observational study) increased with increasing Fe colloid size. These results contradict the notion that higher colloidal size reduces the SSA and sorption. The results point to a size-dependent competition between NOM and oxyanions on the Fe colloid surfaces. The NOM over oxyanion selectivity is highest in the smaller colloids, which we explained by steric and/or electrostatic interactions resulting from the extended conformation of the adsorbed humic substances in the smallest colloids. The Cu loading on the Fe colloids decreased with size together with the NOM, which suggests Cu binding to the Fe colloids via surface adsorbed NOM.

Taken together, recent advances in analytical chemistry allow to study environmental colloids down to the low nanometer range with a size resolution that was not possible to attain before. This thesis has provided deeper insight into Fe colloid size and composition and the environmental conditions that govern them. The importance of DOC on increasing soil solution Fe and decreasing Fe colloid size was highlighted several times. These results add to the growing body of research that indicates that NOM inhibites Fe oxyhydroxide colloid formation by forming Fe-OC complexes and retards Fe oxyhydroxide growth and crystallization. Nevertheless, this is the first study to show the marked effect of NOM on Fe colloid size measured in suspension. In addition, this study is first in quantifying the competition between oxyanions and humic substances in suspended colloids. A conceptual model explaining the paradoxal  size dependent oxyanion binding was presented. This new understanding can help to improve predictions on the impact of colloidal Fe as a nanovector in the environment.