Li Yuan-Hui

Bio: Li Yuan-Hui is an academic researcher from Swiss Federal Institute of Aquatic Science and Technology. The author has contributed to research in topics: Diffusion (business) & Seawater. The author has an hindex of 2, co-authored 2 publications receiving 2619 citations.

In-situ speciation measurements of trace components in natural waters using thin-film gels.

Abstract: RELIABLE measurement of trace species in natural waters is essential for studies of pollution or trace-element cycling, but is difficult, partly because the distribution of chemical species often changes during sampling and storage1. In situ measurements can overcome these problems, but the few measurements made previously have involved complicated systems that cannot be used routinely1,2. Here we describe a simple technique for measuring trace-metal concentrations in situ in water. The technique incorporates an ion-exchange resin separated from the solution by an ion-permeable gel membrane. Mass transport through the gel is diffusion-controlled and thus well defined, making it possible to obtain quantitative data on concentration and speciation over relatively short time periods (from one hour to several weeks). We present measurements of zinc concentrations in sea water using this technique which agree well with electrochemical measurements. In principle, our technique should be applicable to any inorganic or organic diffusing species.

Performance Characteristics of Diffusion Gradients in Thin Films for the in Situ Measurement of Trace Metals in Aqueous Solution

Abstract: The technique of diffusive gradients in thin films (DGT) provides an in situ means of quantitatively measuring labile species in aqueous systems. By ensuring that transport of metal ions to an exchange resin is solely by free diffusion through a membrane, of known thickness, Δg, the concentration in the bulk solution, C b , can be calculated from the measured mass in the resin, M, after time, t, by C b = MΔg/DAt, where D is the molecular diffusion coefficient and A is the exposure surface area of the membrane. If sufficiently thick (∼1 mm) diffusion layer is selected, the flux of metal to the resin is independent of the hydrodynamics in solution above a threshold level of convection. Deployment for 1 day results in a concentration factor of ∼300, allowing metals to be measured at extremely low levels (4 pmol L -1 ). Only labile metal species are measured, the effective time window of typically 2 min being determined by the thickness of the diffusion layer. Because metals are quantified by their kinetics of uptake rather than the attainment of equilibrium, any deployment time can be selected from 1 h to typically 3 months when the resin becomes saturated. The measurement is independent of ionic strength (10 nM-1 M). For Chelex-100 as the resin, the measurement is independent of pH in the range of 5-8.3, but a subtheoretical response is obtained at pH <5 where binding to Chelex is diminished. The effect of temperature can be predicted from the known temperature dependence of the diffusion coefficient and viscosity. The application of DGT to the in situ measurement of Cd, Fe, Mn, and Cu in coastal and open seawater is demonstrated, and its more general applicability as a pollution monitoring tool and for measuring an in situ flux, as a surrogate for bioavailability, is discussed.

Oxygenic photosynthesis as a protection mechanism for cyanobacteria against iron-encrustation in environments with high Fe2+ concentrations

Abstract: If O2 is available at circumneutral pH, Fe2+ is rapidly oxidized to Fe3+, which precipitates as FeO(OH). Neutrophilic iron oxidizing bacteria have evolved mechanisms to prevent self-encrustation in iron. Hitherto, no mechanism has been proposed for cyanobacteria from Fe2+-rich environments; these produce O2 but are seldom found encrusted in iron. We used two sets of illuminated reactors connected to two groundwater aquifers with different Fe2+ concentrations (0.9 μM vs. 26 μM) in the Aspo Hard Rock Laboratory (HRL), Sweden. Cyanobacterial biofilms developed in all reactors and were phylogenetically different between the reactors. Unexpectedly, cyanobacteria growing in the Fe2+-poor reactors were encrusted in iron, whereas those in the Fe2+-rich reactors were not. In-situ microsensor measurements showed that O2 concentrations and pH near the surface of the cyanobacterial biofilms from the Fe2+-rich reactors were much higher than in the overlying water. This was not the case for the biofilms growing at low Fe2+ concentrations. Measurements with enrichment cultures showed that cyanobacteria from the Fe2+-rich environment increased their photosynthesis with increasing Fe2+ concentrations, whereas those from the low Fe2+ environment were inhibited at Fe2+ > 5 μM. Modeling based on in-situ O2 and pH profiles showed that cyanobacteria from the Fe2+-rich reactor were not exposed to significant Fe2+ concentrations. We propose that, due to limited mass transfer, high photosynthetic activity in Fe2+-rich environments forms a protective zone where Fe2+ precipitates abiotically at a non-lethal distance from the cyanobacteria. This mechanism sheds new light on the possible role of cyanobacteria in precipitation of banded iron formations.

The Effects of Macrobenthos on Chemical Properties of Marine Sediment and Overlying Water

Abstract: The composition of any environment or object is determined by a particular balance between material transport processes and chemical reactions within and around it. In the case of marine sedimentary deposits, the dominant agents of mass transport are often large bottom-dwelling animals that move particles and fluids during feeding, burrowing, tube construction, and irrigation. Such biogenic material transport has major direct and indirect effects on the composition of sediments and their overlying waters. In this chapter I review some of what is presently known about these effects, their implications for both chemical and biological properties of a deposit, and how they can be conceptualized in quantitative models.