Gabriele E. Schaumann

Interactions of Dissolved Organic Matter with Natural and Engineered Inorganic Colloids: A Review

This contribution critically reviews the state of knowledge on interactions of natural colloids and engineered nanoparticles with natural dissolved organic materials (DOM). These interactions determine the behavior and impact of colloids in natural system. Humic substances, polysaccharides, and proteins present in natural waters adsorb onto the surface of most colloids. We outline major adsorption mechanisms and structures of adsorption layers reported in the literature and discuss their generality on the basis of particle type, DOM type, and media composition. Advanced characterization methods of both DOM and colloids are needed to address insufficiently understood aspects as DOM fractionation upon adsorption, adsorption reversibility, and effect of capping agent. Precise knowledge on adsorption layer helps in predicting the colloidal stability of the sorbent. While humic substances tend to decrease aggregation and deposition through electrostatic and steric effects, bridging-flocculation can occur in the presence of multivalent cations. In the presence of DOM, aggregation may become reversible and aggregate structure dynamic. Nonetheless, the role of shear forces is still poorly understood. If traditional approaches based on the DLVO-theory can be useful in specific cases, quantitative aggregation models taking into account DOM dynamics, bridging, and disaggregation are needed for a comprehensive modeling of colloids stability in natural media.

Biological surface coating and molting inhibition as mechanisms of TiO2 nanoparticle toxicity in Daphnia magna.

The production and use of nanoparticles (NP) has steadily increased within the last decade; however, knowledge about risks of NP to human health and ecosystems is still scarce. Common knowledge concerning NP effects on freshwater organisms is largely limited to standard short-term (≤48 h) toxicity tests, which lack both NP fate characterization and an understanding of the mechanisms underlying toxicity. Employing slightly longer exposure times (72 to 96 h), we found that suspensions of nanosized (∼100 nm initial mean diameter) titanium dioxide (nTiO2) led to toxicity in Daphnia magna at nominal concentrations of 3.8 (72-h EC50) and 0.73 mg/L (96-h EC50). However, nTiO2 disappeared quickly from the ISO-medium water phase, resulting in toxicity levels as low as 0.24 mg/L (96-h EC50) based on measured concentrations. Moreover, we showed that nTiO2 (∼100 nm) is significantly more toxic than non-nanosized TiO2 (∼200 nm) prepared from the same stock suspension. Most importantly, we hypothesized a mechanistic chain of events for nTiO2 toxicity in D. magna that involves the coating of the organism surface with nTiO2 combined with a molting disruption. Neonate D. magna (≤6 h) exposed to 2 mg/L nTiO2 exhibited a “biological surface coating” that disappeared within 36 h, during which the first molting was successfully managed by 100% of the exposed organisms. Continued exposure up to 96 h led to a renewed formation of the surface coating and significantly reduced the molting rate to 10%, resulting in 90% mortality. Because coating of aquatic organisms by manmade NP might be ubiquitous in nature, this form of physical NP toxicity might result in widespread negative impacts on environmental health.

Plastic mulching in agriculture. Trading short-term agronomic benefits for long-term soil degradation?

Plastic mulching has become a globally applied agricultural practice for its instant economic benefits such as higher yields, earlier harvests, improved fruit quality and increased water-use efficiency. However, knowledge of the sustainability of plastic mulching remains vague in terms of both an environmental and agronomic perspective. This review critically discusses the current understanding of the environmental impact of plastic mulch use by linking knowledge of agricultural benefits and research on the life cycle of plastic mulches with direct and indirect implications for long-term soil quality and ecosystem services. Adverse effects may arise from plastic additives, enhanced pesticide runoff and plastic residues likely to fragment into microplastics but remaining chemically intact and accumulating in soil where they can successively sorb agrochemicals. The quantification of microplastics in soil remains challenging due to the lack of appropriate analytical techniques. The cost and effort of recovering and recycling used mulching films may offset the aforementioned benefits in the long term. However, comparative and long-term agronomic assessments have not yet been conducted. Furthermore, plastic mulches have the potential to alter soil quality by shifting the edaphic biocoenosis (e.g. towards mycotoxigenic fungi), accelerate C/N metabolism eventually depleting soil organic matter stocks, increase soil water repellency and favour the release of greenhouse gases. A substantial process understanding of the interactions between the soil microclimate, water supply and biological activity under plastic mulches is still lacking but required to estimate potential risks for long-term soil quality. Currently, farmers mostly base their decision to apply plastic mulches rather on expected short-term benefits than on the consideration of long-term consequences. Future interdisciplinary research should therefore gain a deeper understanding of the incentives for farmers and public perception from both a psychological and economic perspective in order to develop new support strategies for the transition into a more environment-friendly food production.

The fate of silver nanoparticles in soil solution--Sorption of solutes and aggregation.

Nanoparticles enter soils through various pathways. In the soil, they undergo various interactions with the solution and the solid phase. We tested the following hypotheses using batch experiments: i) the colloidal stability of Ag NP increases through sorption of soil-borne dissolved organic matter (DOM) and thus inhibits aggregation; ii) the presence of DOM suppresses Ag oxidation; iii) the surface charge of Ag NP governs sorption onto soil particles. Citrate-stabilized and bare Ag NPs were equilibrated with (colloid-free) soil solution extracted from a floodplain soil for 24h. Nanoparticles were removed through centrifugation. Concentrations of free Ag ions and DOC, the specific UV absorbance at a wavelength of 254 nm, and the absorption ratio α254/α410 were determined in the supernatant. Nanoparticle aggregation was studied using time-resolved dynamic light scattering (DLS) measurement following the addition of soil solution and 1.5mM Ca(2+) solution. To study the effect of surface charge on the adsorption of Ag NP onto soil particles, bare and citrate-stabilized Ag NP, differing in the zeta potential, were equilibrated with silt at a solid-to-solution ratio of 1:10 and an initial Ag concentration range of 30 to 320 μg/L. Results showed that bare Ag NPs sorb organic matter, with short-chained organic matter being preferentially adsorbed over long-chained, aromatic organic matter. Stabilizing effects of organic matter only come into play at higher Ag NP concentrations. Soil solution inhibits the release of Ag(+) ions, presumably due to organic matter coatings. Sorption to silt particles was very similar for the two particle types, suggesting that the surface charge does not control Ag NP sorption. Besides, sorption was much lower than in comparable studies with sand and glass surfaces.

H-NMR Relaxometry to Monitor Wetting and Swelling Kinetics in High-organic Matter Soils

Variations of soil moisture conditions affect sorption properties of soil organic matter and the pore size distribution of the soils and thus are expected to have an impact on the availability of pollutants and nutrients in soils. At least two principal processes that occur when a soil-water contact is established, are involved. Wetting, which is the very first step, is governed by the interactions of water with the surface of soil organic matter (SOM). The wettability of the pore walls determines the pore accessibility for water. Only in wettable soils, water will occupy the smallest pores first. In the course of wetting, the wettability of the pore walls increases, leading to water redistribution. Swelling of SOM is accompanied by an increase of volume due to the water uptake of the solid SOM phase and will change the SOM polarity. Swelling will thus affect sorption processes in the bulk SOM phase and is expected to change the pore sizes. In this contribution, we investigated swelling and wetting kinetics of soil samples by H-NMR-Relaxometry. We found different effects of wetting and swelling on the development of relaxation time distribution and thus of the pore size distribution. Both swelling and wetting can be slow processes, lasting for up to some weeks. During this time, we found changes in the pore size distribution. For swelling phenomena, we observed a continuous change of the effective pore size, and for wetting phenomena, we found a change in water distribution in a probably rigid pore system. Thus, during swelling and wetting, neither pore size distribution nor sorbent properties of SOM nor hydraulic properties remain constant. Due to the slow kinetics, both processes play an important role in sorption, transport and accessibility for water in hydrophobic areas within a time scale of weeks after e.g. a rainfall event. This will affect the environmental availability and the transport of pollutants and nutrients in the field.

Cation-mediated cross-linking in natural organic matter: a review

Interactions between cations and natural organic matter (NOM) are central for the stability of organic matter, formation of supramolecular NOM structure, formation of organo-mineral associations, soil aggregation and binding of organic contaminants. The effect of multivalent cations on environmental functionalities of NOM strongly depends on the relative importance between intramolecular complexation and intermolecular cross-linking, the degree of which will be determined by the spatial arrangement of the hydrophilic functional groups in NOM. This literature review seeks to evaluate the current state of the art regarding the relevance of intermolecular cross-links via bridges of multivalent cations. Cross-linking has been suggested to explain among others aggregate stability, retarded dissolved organic matter release, reduced organic matter (OM) solubility as well as increase in degree and nonlinearity of sorption or organic chemicals to NOM. Although the cross-linking mechanism has been suggested in numerous studies, it has not yet been verified directly. The dynamics of the intermolecular cross-links, their persistence as well as their interplay with OM and their influence on stability and bioavailability of organic chemicals is up to now unknown. The major challenge in this context is the development of a suitable combination of experimental and instrumental techniques and relating the results to molecular and physicochemical models on the basis of targeted combination of spectroscopic, molecular modelling and thermoanalytical methods.

Effect of heating time and temperature on the chemical characteristics of biochar from poultry manure.

Poultry manure (PM) chars were obtained at different temperatures and charring times. Chemical-physical characterization of the different PM chars was conducted by cross-polarization magic angle spinning (CPMAS) (13)C NMR spectroscopy and thermal analysis. CPMAS (13)C NMR spectra showed that the chemical composition of PM char is dependent on production temperature rather than on production duration. Aromatic and alkyl domains in the PM chars obtained at the lowest temperatures remained unchanged at all heating times applied for their production. The PM char obtained at the highest temperature consisted only of aromatic structures having chemical nature that also appeared invariant with heating time. Thermogravimetry revealed differences in the thermo-oxidative stability of the aromatic domains in the different PM chars. The PM char produced at the highest temperature appeared less stable than those produced at the lowest temperatures. This difference was explained by a protective effect of the alkyl groups, which are still present in chars formed at lower temperature. The analysis of the chemical and physicochemical character of poultry manure chars produced at different temperatures can increase understanding of the role of these materials in the properties and behavior of char-amended soils.

Thermal characteristics of soil organic matter measured by DSC: A hint on a glass transition

Glass transitions have been reported for purified humic acids only. In this study, a glass transition was detected in a sample of a sandy forest soil by Differential Scanning Calorimetry (DSC). The glass transition temperature was 79°C for air-dried samples and 77°C for pre-moistened samples. In addition to the glass transition, an exothermic process around 30°C was detected in pre-moistened samples. This could be due to water loss of soil organic matter. However, the nature of this process is not yet understood. This study showed that the macromolecular behaviour of SOM, as indicated by DSC, reacts to the moisture state of soil organic matter. Thermische Eigenschaften der organischen Bodensubstanz mittels DSC-Messung: Ein Hinweis auf einen Glas-Ubergang Glasubergange konnten bislang nur in gereinigten Huminsauren nachgewiesen werden. In dieser Studie wurde mittels Differential Scanning Kalorimetrie (DSC) ein Glasubergang in Proben von unterschiedlich vorbehandeltem Boden entdeckt. Die Glasubergangstemperatur betrug 79°C bei der luftgetrockneten und 77°C bei der befeuchteten Probe. Zusatzlich zum Glasubergang wurde in befeuchteten Proben ein exothermer Prozess um 30°C beobachtet. Die Natur dieses Prozesses ist noch ungeklart. Der Vorgang konnte durch Wasserverlust der Humussubstanzen erklart werden. Diese Untersuchung zeigt, dass das durch die DSC angezeigte makromolekulare Verhalten der organischen Bodensubstanz auf den Feuchtezustand der organischen Bodensubstanz reagiert.

Effects of silver nanoparticle properties, media pH and dissolved organic matter on toxicity to Daphnia magna

Studies assessing the acute and chronic toxicity of silver nanoparticle (nAg) materials rarely consider potential implications of environmental variables. In order to increase our understanding in this respect, we investigated the acute and chronic effects of various nAg materials on Daphnia magna. Thereby, different nanoparticle size classes with a citrate coating (20-, ~30-, 60- as well as 100-nm nAg) and one size class without any coating (140 nm) were tested, considering at the same time two pH levels (6.5 and 8.0) as well as the absence or presence of dissolved organic matter (DOM; <0.1 or 8.0 mg total organic carbon/L). Results display a reduced toxicity of nAg in media with higher pH and the presence of DOM as well as increasing initial particle size, if similarly coated. This suggests that the associated fraction of Ag species <2 nm (including Ag(+)) is driving the nAg toxicity. This hypothesis is supported by normalizing the 48-h EC50-values to Ag species <2 nm, which displays comparable toxicity estimates for the majority of the nAg materials assessed. It may therefore be concluded that a combination of both the particle characteristics, i.e. its initial size and surface coating, and environmental factors trigger the toxicity of ion-releasing nanoparticles.

Disaggregation of silver nanoparticle homoaggregates in a river water matrix.

Silver nanoparticles (Ag NPs) could be found in aquatic systems in the near future. Although the interplay between aggregate formation and disaggregation is an important factor for mobility, bioavailability and toxicity of Ag NPs in surface waters, the factors controlling disaggregation of Ag NP homoaggregates are still unknown. In this study, we investigated the reversibility of homoaggregation of citrate coated Ag NPs in a Rhine River water matrix. We characterized the disaggregation of Ag NP homoaggregates by ionic strength reduction and addition of Suwannee River humic acid (SRHA) in the presence of strong and weak shear forces. In order to understand the disaggregation processes, we also studied the nature of homoaggregates and their formation dynamics under the influence of SRHA, Ca(2+) concentration and nanoparticle concentration. Even in the presence of SRHA and at low particle concentrations (10 μg L(-1)), aggregates formed rapidly in filtered Rhine water. The critical coagulation concentration (CCC) of Ca(2+) in reconstituted Rhine water was 1.5 mmol L(-1) and was shifted towards higher values in the presence of SRHA. Analysis of the attachment efficiency as a function of Ca(2+) concentration showed that SRHA induces electrosteric stabilization at low Ca(2+) concentrations and cation-bridging flocculation at high Ca(2+) concentrations. Shear forces in the form of mechanical shaking or ultrasound were necessary for breaking the aggregates. Without ultrasound, SRHA also induced disaggregation, but it required several days to reach a stable size of dense aggregates still larger than the primary particles. Citrate stabilized Ag NPs may be in the form of reaction limited aggregates in aquatic systems similar to the Rhine River. The size and the structure of these aggregates will be dynamic and be determined by the solution conditions. Seasonal variations in the chemical composition of natural waters can result in a sedimentation-release cycle of engineered nanoparticles.