Development of a hyphenated analytical methodology for the fractionation and characterization of environmental colloids

Natural colloids are defined as macromolecules and nanoparticles with sizes ranging between 1 nm and 1 µm. Colloidal pool is complex "ill-defined" mixture of humic substances, exopolymeric substances (EPS), inorganic oxides, clays, etc. Aquatic colloids have been recognized as key mediators in the removal of trace elements, facilitating contaminant transport, organic carbon cycling and micronutrient bioavailability. The residence times of the colloidal pool (and associate contaminants) can be order of magnitude different from those of the particulate fraction, but unlike the "truly" dissolved fraction, can be subjected to different coagulation and sedimentation processes. Colloids may also influence metal bioavailability by affecting free metal ion concentrations, constraining diffusion and affecting the dissociation kinetics of biological processes. The important environmental role of natural colloids is largely influenced by their surface area and closely related colloidal size. Nonetheless, it is currently unclear to what extend colloidal size can influence their behaviour and contaminant (e.g. toxic metals) binding properties. In our view this is partially related to the difficulties in determination of the colloidal size distributions and their impact in metal behaviour, mainly due to the inherent heterogeneity of the environmental colloids and their low concentrations in the environment. Furthermore, in addition to average parameters, there is a lack of appropriate sample collection and processing procedures, as well as analytical techniques to measure simultaneously the size (or related parameter) distributions of the colloid associated metal. The objective of the present work was therefore to develop sensitive analytical procedures for the fractionation of natural colloids and to apply them to improve the current understanding on the fate of the colloids and associated toxic metals in the freshwaters. To achieve these goals, the capabilities of asymmetric flow-field flow fractionation (aFlFFF) coupled to several detectors was explored. aFlFFF is a versatile technique separating colloids according to their diffusion coefficient. The system was hyphenated with multi-angle laser light scattering (MALS), differential refractive index (DRI), UV detector and, when necessary, high resolution inductively coupled plasma mass spectrometry (ICP-MS). Correct interpretation of the experimental results and extrapolation of meaningful molecular parameters by using an analytical tool with such a level of complexity requires improvement of the knowledge of colloids behaviour in the aFlFFF channel and careful optimization of the separation conditions. Given the very complex and dynamic nature of environmental colloids, different experimental procedures was first optimized and validate under lab conditions by using "standardized" colloids (e.g. alginate and EPS isolate from lab bacterial cultures), then applied to characterize colloidal organic matter (COM) isolated from natural water systems. The influence of critical operating parameters, such as crossflow rate, carrier composition and concentration, and sample load, on the alginate retention was carefully evaluated. Combined information obtained simultaneously by DRI and MALS detectors, allowed to set the appropriate combination of optimal parameters for the characterization of alginate, as a model COM. The metal distributions of metal-alginate complexes were probed by aFlFFF - ICP-MS: average values and continuous distributions of molar masses, radius of gyration and hydrodynamic radius, critical for understanding their role as carriers of metal pollutants, were evaluated in presence of lead or cadmium and compared to those in metal-free solutions of alginate. Furthermore, the molecular characteristic of exopolymeric substances (EPS) secreted by bacterium Sinorhizobium meliloti wild type and exoY strains, which is deficient in the production of the succinoglycan, were studied. EPS were collected in the extracellular environment or desorbed from the cell surface using different extraction protocols. EPS fractions with different chemical characteristic and molar mass were found, depending on the strain and extraction protocol. Molecular parameters as the distributions and average values of molar mass and gyration radius, and the polydispersity index have been measured and compared with data obtained by the chemical characterization. Size exclusion chromatography coupled to UV detection was also employed to provide additional information on the small class of size. Moreover, the effect of the cadmium, lead and calcium addition to exopolymeric substances excreted in the extracellular environment was investigated, with the aim to study how does EPS composition, size and mass distributions, can affect the trace metal binding properties of EPS and how the metal addition affects the EPS conformation. Obtained results revealed that cadmium and calcium bound preferentially on the low-size species population and the amount of the bound metal increased with the increasing of the concentration of added metal. In contrast, lead bound on preferentially larger classes of EPS size. The aFlFFF separation parameters were also optimized for the fractionation and characterization of humic substances, as other COM component, and applied to probe the temporal and spatial variability in the size and mass characteristics of COM samples collected in the Amazon River basin. COM characterization demonstrated the existence of a clear decrease of size of the colloidal material when passing from small to large watercourses (spatial variation). No significant changes were found in size distributions of samples collected in different period of the year, during the low and high flow, respectively (temporal variation). The analytical methodology, adapted to determine not only mean characteristics, but the population distributions, developed in the present work, allowed to obtain new information about the colloidal organic matter size distribution, which play utmost role in colloids behaviour and fate in the environment, in particular as toxic metal carriers.

Phytoremediation of polluted soils: hype, hope and facts

Jean-Paul Schwitzguebel

Different plant species, ecotypes, varieties or cultivars can be selected, tailored and harnessed for the remediation of polluted soils, sites and brownfields, as well as for the phytotreatment of domestic or industrial wastewater. Several approaches are possible, but phytoremediation does always rely on in situ extraction, detoxification or immobilization of xenobiotic chemicals or trace elements. Phytoremediation has a strong potential as a natural, solar-energy driven approach for soils and sites moderately polluted over large surfaces, if the most appropriate plants have been carefully selected and the adequate agronomic methods are applied to manage the phytoavailability of contaminants. It can already offer a sustainable and less expensive alternative to soils or sites restoration, as compared to traditional physico-chemical techniques, even if the time required reaching the target end-points is often a severe drawback. Depending on the type of soil to be treated, as well as on the pollutants, their concentration and ageing, different approaches can be considered and will be presented with some examples of successful applications, but also highlighting their limitations: • Phytoextraction - uptake of trace elements into roots, then translocation into shoots, followed by the harvest and destruction of the contaminated plants, with possible recycling strategies to recover metals from biomass or ash. Two types of plants can be used: plants able to hyperaccumulate one or several trace elements, but producing low biomass; or plants accumulating moderate amounts of metals or metalloids, but producing high biomass. • Phytostabilisation - based on the immobilization of inorganic contaminants by adsorption to plant roots or soil particles, or precipitation in the root area, thus preventing their migration in soil, groundwater or air, and decreasing erosion, runoff and leaching. For large contaminated areas phytostabilisation is probably the only reasonable option to restore ecosystems. The most effective approach is the use of indigenous plant species, after the addition of appropriate soil amendments. • Phytodegradation - exploiting the huge potential and biodiversity of plant metabolism to detoxify, transform or degrade xenobiotic organic chemicals, like pesticides, explosives or dyes. • Phytostimulation - enhanced microbial metabolism by root exudates; plant/microbial interactions are important for such rhizospheric processes, especially for very hydrophobic compounds. In spite of a growing track record of commercial success, more large-scale trials are needed to definitively prove that phytoremediation is effective and applicable in many different situations, to rigorously assess its economics and to convince stakeholders and local authorities that it is a reality, not a dream. However, to be recognized as a mature and efficient technology, phytoremediation still needs significant progress, even if it is the focus of intensive research and development, from numerous laboratory basic studies and greenhouse experiments to successful demonstration sites. More fundamental research is still required to better understand and control the numerous metabolic processes occurring in the plant and its different organs, as well as the complex interactions between pollutants, soil, plant roots, and the rhizospheric and endophytic microorganisms. However, experimental conditions can modify plant physiology and biochemistry, and it is not obvious if results obtained with young and small plantlets under well-defined laboratory conditions can be extended to real full-scale fields. If the small-scale experimentation in laboratory or in greenhouse is highly valuable to better understand the basic plant molecular and biochemical mechanisms involved in the uptake, translocation, detoxification and sequestration of toxic trace elements and xenobiotic compounds, their direct transposition in the field is somewhat questionable. On the other hand, short-term experimental conditions in small pots used to cultivate plantlets do not allow the normal growth and development of roots and shoots, and thus impose stress which can affect the performance of plants and the efficiency of phytoremediation. In the field the following factors do play a major role, but they cannot be assessed under laboratory conditions: soil heterogeneity, meteorological uncertainty, total concentrations and spatial variability of contaminants, texture and depth of soil layers, nutrient and water availability, problems with the homogeneous input of amendments, differences in rooting depth and density, variability of soil moisture, impacts of pests, pathogens and herbivores. Molecular and biochemical mechanisms of plant adaptation to real stress conditions are still poorly understood and signalling pathways involved remain unclear. However, adaptation to environmental changes is crucial for plants and thus for the efficiency of phytoremediation. Most studies deal either with organic xenobiotics or with trace elements, whereas in the field, a mixed pollution is generally observed. Information on the probable antagonistic or possible synergistic effects of mixed pollutants on their mutual accumulation and detoxification remains however scarce and further studies are urgently needed to better understand the mechanisms involved and improve phytoremediation efficiency. On the other hand, the bioavailable fraction of soil contaminants, but not the total concentration, should be considered as the most important one from an ecological, toxicological and health standpoint, and is determined by the chemical properties of the pollutant, soil characteristics, ageing processes and biota behaviour. Ageing of pollutants usually decreases bioavailability, but root exudates, root-induced rhizosphere changes, mycorrhizal fungi and rhizospheric bacteria play a major role in the dynamics and the ability of pollutants to enter into plants; spreading of pollutants to the surroundings and groundwater can also be affected. The bioavailability of contaminants and their uptake by crops are also essential parameters for establishing risk-based regulatory guidelines and enhancing food safety. Finally, the economics of phytoremediation of organic pollutants is generally favourable, but cost is an acute problem for the treatment of trace elements. Ideally, plants should produce biomass with added value or should be used to recover valuable products. The fate of plant biomass after harvest should thus be considered in the context of biorefineries. Lignocellulosic feedstock can be used for fibers or biofuel production, after extraction of added-value products like oil for lubricants, fragrances or fine chemicals. Incineration of plant material should allow the selective recovery of metals from ash after energy production, thus providing an economically sound process, depending on the type and concentration of metal, as well as on the market price of the trace elements under consideration. With such approaches, marginal contaminated soils or brownfields can be cleaned-up, whereas owners are offered valuable sources of income instead of set-aside scenarios. In such a situation, time is no longer a limiting factor, and the term phytomanagement should be more appropriate than phytoremediation

2014

Estimation of the total VOC reactivity with OH by pump-and-probe OH lidar to determine a new indicator for ozone production

Many of our most severe problems of environmental pollution involve the Earth's atmosphere. The adverse effects of atmospheric pollution are particularly evident in the low troposphere, especially in urban/suburban areas where many sources contribute to the atmospheric burden. Ozone (O3) is one of the major constituents of the air pollution over cities commonly called photochemical smog. It is a secondary pollutant, produced by the reaction of primary pollutants, nitrogen oxides (NOx) and hydrocarbons (VOC), in the presence of sunlight. Although hydrocarbons and NOx have long been identified as the two key precursors to photochemical O3, the development of an effective strategy for reducing ozone production in photochemical smog by controlling the anthropogenic emissions of these precursors has proved to be problematic. The objective of the present study is to present and demonstrate the robustness of a new observable "indicator" for determining the sensitivity of ozone formation to VOC and NOx (VOC- and NOx-limited regime). This indicator is based on the reactivity of VOC and NOx with respect to OH. Simulation tests of the ability of this new indicator Θ = τOHVOC/τOHNOx to distinguish between a NOx and a VOC-limited regime were performed with a box model and a 3D model applied to a specific episode of photochemical smog (Athens) and of a less polluted area (the Swiss plateau). Whereas the NOx reactivity 1/τOHNOx can easily be measured by standard methods, the total VOC reactivity 1/τOHVOC will be estimated by measuring the inverse lifetime of OH with a new Pump-and-Probe Lidar instrument. The method consists of producing high OH concentrations by the flash photolysis of ozone and the subsequent O(1D) reaction with H2O and by following the OH relaxation by laser-induced fluorescence (LIF). Numerical simulation of the experiment will show that the total VOC reactivity with OH can be obtained by measuring the inverse lifetime of the OH and its absolute initial concentration with the Pump-and-Probe experiment as well as the NOx, CO and O3 concentrations with standard trace-gas detectors. This total in-situ VOC reactivity will be used to determine the new indicator Θ but will also help to test the accuracy of the VOC-lumped chemical mechanism in the numerical atmospheric simulation. In order to demonstrate the feasibility of the Pump-and-Probe experiment, laboratory investigations have been performed in controlled atmosphere: OH absorption and fluorescence emission spectra allowed the OH transition bands and the non-resonant fluorescence wavelength to be identified. High-resolution OH absorption spectra allowed the OH effective absorption cross section to be determined for assessing the experimental sensitivity and retrieving the OH concentration. The experimental retrieval of the rate constant for the OH+CH4 and OH+NO2 reactions reveals the ability of the pump-and-probe experiment to monitor the chemical kinetics of OH at atmospheric pressure and room temperature. OH relaxation measured in different VOC/NOx/O3 gas mixtures confirm the relevance of the model predictions of the OH relaxation approximated by a first-order chemical decay and the validity of the total VOC reactivity with respect to OH estimated by such an approximation. A first in-situ OH relaxation measurement realized directly in the air of the laboratory showed that the pump-and-probe method could be performed under typical PBL conditions. Preliminary range-resolved measurements yielding vertical profiles of the OH concentrations produced by flash photolysis of ambient ozone suggest a promising future for the Lidar pump-and-probe technique. The perspectives for the Pump-and-Probe Lidar experiment are expressed in the concluding section.

EPFL2000

Bacteria-induced mixing in natural waters

Oscar Rodrigo Sepúlveda Steiner, Talia Ekin Tokyay Sinha, Alfred Johny Wüest

Swimming organisms can enhance mixing in their natural environments by creating eddies in their wake and by dragging water along. However, these mixing mechanisms are inefficient for microorganisms, because swimming-induced variations in velocity, temperature, and dissolved substances are evened out before they can be advected. In bioconvection, however, microorganisms induce water movement not by propulsion directly but by locally changing the fluid density, which drives convection. Observations of bioconvection have so far mainly been limited to laboratory settings. We report the first observation and quantification of bioconvection within a stratified natural water body. Using in situ measurements, laboratory experiments, and numerical simulations, we demonstrate that the bacterium Chromatium okenii is capable of mixing 0.3 to 1.2 m thick water layers at around 12 m water depth in the Alpine Lake Cadagno (Switzerland). As many species are capable of driving bioconvection, this phenomenon potentially plays a role in species distributions and influences large-scale phenomena like algal blooms.

2017

Phytoremediation of polluted soils: hype, hope and facts

Jean-Paul Schwitzguebel

2014

Estimation of the total VOC reactivity with OH by pump-and-probe OH lidar to determine a new indicator for ozone production

EPFL2000