Ferrihydrite coating reduces microplastic induced soil water repellency.

Addition of microplastics (MP) to soil has the potential to increase soil water repellency. However, coating of MP with soil abundant substances e.g., iron compounds, can reduce this effect. Here, we tested if pre-coating or in situ coating of MP with ferrihydrite (Fh) reduces soil water repellency. We applied hotspots of pristine and coated MP (20-75 µm, PS and PET) to sand and imaged capillary rise via neutron radiography. Capillary rise experiments in wetting-drying cycles were conducted using water and Fh suspension. Pristine MP hotspots were not wettable. Capillary rise of water into coated MP hotspots differed in wettability depending on polymer type. While coated PS was still non-wettable, water imbibed into the coated PET hotspot. Capillary rise of Fh suspensions in wetting and drying cycles also showed varying results depending on polymer type. MP hotspots were still non-wettable and local water content increased only marginally. Our results indicate that Fh coating of MP changes MP surface wettability depending on polymer type and therefore counteracts the hydrophobic properties of pristine MP. However, MP coating is likely to be slowed down by the initial hydrophobicity of pristine MP. Dynamics of MP coating and increasing wettability are key factors for biotic and abiotic degradation processes.

Heteroaggregation of PS microplastic with ferrihydrite leads to rapid removal of microplastic particles from the water column.

Microplastic (MP) particles are ubiquitous in aquatic environments. Therefore, understanding the processes that affect their removal from the water column, such as sedimentation, is critical for evaluating the risk they pose to aquatic ecosystems. We performed sedimentation experiments in which polystyrene (PS) and PS + ferrihydrite, a short-range ordered ferric (oxy)hydroxide, were analyzed in settling columns after 1 day and 1 week of settling time. The presence of ferrihydrite increased sedimentation rates of PS at all pH values studied (pH 3-11). At pH 6 we found that almost all PS particles were removed from the water column after only one day of exposure time. SEM/EDS imaging confirmed heteroaggregation between the PS particles and ferrihydrite. Zeta potential measurements indicated that at acidic pH values the negatively charged PS surface was coated with positively charged ferrihydrite particles leading to charge reversal. Our results demonstrate for the first time that ferric (oxy)hydroxides drive heteroaggregation and subsequent removal of MP from the water column, especially at typical pH values found in natural lake environments. Given their abundance in aquatic systems ferric (oxy)hydroxides need to be regarded as key scavengers of MP.

Sedimentation Kinetics of Hydrous Ferric Oxides in Ferruginous, Circumneutral Mine Water.

Transport, transformation, and removal of iron in aqueous environments is primarily controlled by ferrous iron oxidation followed by aggregation and sedimentation of the resultant hydrous ferric oxides. The latter mechanisms are particularly important for passive iron removal in mine water treatment systems, yet the interrelation and underlying kinetics are poorly understood. In this study, the sedimentation behavior of natural hydrous ferric oxides was systematically investigated under different hydrogeochemical conditions via laboratory-based column experiments. The objective was to determine a robust model approach for the approximation of aggregation/sedimentation kinetics in engineered systems. The results showed that sedimentation is governed by two interrelated regimes, a rapid second-order aggregation-driven step (r1) at high iron levels followed by a slower first-order settling step (r2) at lower iron levels. A mixed first-/second-order model was found to adequately describe the process: -d[Fe]dt=kr2[Fe]+kr1[Fe]2 with kr1 = 9.4 × 10-3 m3/g/h and kr2 = 5.4 × 10-3 h-1. Moreover, we were able to demonstrate that the removal of particulate hydrous ferric oxides at low particulate iron levels (<10 mg/L) may be reasonably well approximated by a simplified first-order relationship: -d[Fe]dt=ksed[Fe] with ksed = 2.4 (±0.4) × 10-2 h-1, which agrees well with incipient literature estimates. Only minor effects of pH, salinity, and mineralogy on kinetic parameters were observed. Hence, the results of this study may be broadly transferrable among different mine sites.