Peter H. Santschi

Environmental behavior and ecotoxicity of engineered nanoparticles to algae, plants, and fungi

Developments in nanotechnology are leading to a rapid proliferation of new materials that are likely to become a source of engineered nanoparticles (ENPs) to the environment, where their possible ecotoxicological impacts remain unknown. The surface properties of ENPs are of essential importance for their aggregation behavior, and thus for their mobility in aquatic and terrestrial systems and for their interactions with algae, plants and, fungi. Interactions of ENPs with natural organic matter have to be considered as well, as those will alter the ENPs aggregation behavior in surface waters or in soils. Cells of plants, algae, and fungi possess cell walls that constitute a primary site for interaction and a barrier for the entrance of ENPs. Mechanisms allowing ENPs to pass through cell walls and membranes are as yet poorly understood. Inside cells, ENPs might directly provoke alterations of membranes and other cell structures and molecules, as well as protective mechanisms. Indirect effects of ENPs depend on their chemical and physical properties and may include physical restraints (clogging effects), solubilization of toxic ENP compounds, or production of reactive oxygen species. Many questions regarding the bioavailability of ENPs, their uptake by algae, plants, and fungi and the toxicity mechanisms remain to be elucidated.

A kinetic approach to describe trace-element distribution between particles and solution in natural aquatic systems

The partitioning of radioactive trace elements between seawater and particulate matter from surface sediments and sediment traps was investigated in laboratory experiments. For the elements Na, Zn, Se, Sr, Cd, Sn, Sb, Cs, Ba, Hg, Th and Pa (group I) constant distribution coefficients (Kd) were found after a few days of equilibration, whereas the elements Be, Mn, Co and Fe (group II) showed an increasing Kd over the whole time of observation of 108 days. The time dependence of Kd is described by an adsorption-desorption equilibrium (group I elements), followed by a lattice transport reaction step (group II elements). The reaction rate constants are compared to Mn oxidation rates and to adsorption rate constants derived from in situ measurements of the UTh disequilibrium as available from literature.

Metals in aquatic systems

In this article we address the general question of whether we can yet predict metal-scavenging residence times in natural systems based on current knowledge about metal behavior as derived from laboratory studies. Although we will show that our ability to predict metal behavior is still relatively limited, such a conclusion should not be construed as a criticism of the reductionist approach. Rather, it should be seen that a number of challenges remain for surface chemists interested in the natural environment.

Chemical processes at the sediment-water interface*

Abstract In natural waters, the sediment-water interface is the site where gradients in physical, chemical and biological properties are the greatest. Both chemical and microbiological transformation processes are responsible for cycling elements between water and sediments. This paper discusses the various chemical transformations that take place during early diagenesis in sediments, and which are fueled by supply rates of organic carbon and electron acceptors. In particular, our current knowledge of the cycling of the electron acceptors O, N, Mn, Fe and S is assessed, and the important role of transport reactions is described. The elemental fluxes across the sediment-water interface can be described from first principles only if the coupling of physical, chemical and biological processes is better understood. Large gradients in chemical potentials of many chemical species across this interface often lead to steep gradients in proton and electron activities. The redox sequence commonly observed in time and space also leads to changes in the partitioning of solutes between carrier and solution phases. Such processes can cause rapid back-diffusion of released species to the overlying water column or their removal onto secondary carrier phases within the sediments. Physical transport systems such as resuspension or bioturbation of particles control the physical boundary conditions for chemical reactions immediately above or below the sediment-water interface. Because of the complex coupling of the different chemical, physical and biological processes, chemical reactions within this region must be considered an integral part of a three-dimensional network of interactions. Hence, the apparent coexistence of chemical species considered ‘incompatible’ by thermodynamic models can be a consequence of the three-dimensional nature of redox gradients. The microbiological, chemical and physical interactions of dissolved, colloidal and particulate organic carbon, and iron and manganese oxides play a crucial role in the early diagenetic reactions at the sediment-water interface. Their effects on diagenetic reactions, and on trace elements in solution and adsorbed to particles, are discussed. Furthermore, we emphasize the way in which hydrodynamics can control elemental fluxes in environments with high carbon rain rates.

Partitioning of Cu, Pb, Ag, Zn, Fe, Al, and Mn between filter-retained particles, colloids, and solution in six Texas estuaries

There are few methodical studies of processes controlling trace metal behavior in estuaries, where river water gradually mixes with seawater leading to systematic changes in ionic strength, pH, DOC, nutrient concentrations, and alkalinity. We report here data on Cu, Zn, Pb, and Ag behavior in six Texas estuaries using state-of-the-art ultra-clean techniques. In addition, Fe, Al, and Mn data are presented for one of Texass major estuaries, Galveston Bay. It was found that suspended matter concentration (SPM) was the only variable related to systematic variations in partitioning of trace metal and Fe and Al concentrations between filter-retained and filter-passing fractions across salinity gradients. Inverse relationships were observed between empirically defined particle/solution distribution coefficients, KD, for the different metals and SPM concentrations. This inverse dependence can be explained by the “particle concentration effect”, which can be caused by the presence of Fe, Al, and trace metals associated with colloidal matter in the filtrate fraction. Our argument is supported by the direct analysis of colloids (> 10,000 Daltons) ultrafiltered from Galveston Bay waters. Colloidal Fe, Al, Pb, and Zn (but not Cu) account for most of the filter-passsing form of these metals. The results of this study have important implications for regulatory agencies and regulated industries, allowing for algorithms to predict Pb, Cu, Zn, and Ag partitioning between particle and solution phases.

The algal toxicity of silver engineered nanoparticles and detoxification by exopolymeric substances

In this study, we report that silver ions (Ag(+)) from the oxidative dissolution of silver engineered nanoparticles (Ag-ENs) determined the EN toxicity to the marine diatom Thalassiosira weissflogii. Most of the Ag-ENs formed non-toxic aggregates (>0.22 microm) in seawater. When the free Ag(+) concentration ([Ag(+)](F)) was greatly reduced by diafiltration or thiol complexation, no toxicity was observed, even though the Ag-ENs were better dispersed in the presence of thiols with up to 1.08 x 10(-5) M Ag-ENs found in the <0.22 microm fraction, which are orders of magnitude higher than predicted for the natural aquatic environment. The secretion of polysaccharide-rich algal exopolymeric substances (EPS) significantly increased at increasing [Ag(+)](F). Both dissolved and particulate polysaccharide concentrations were higher for nutrient-limited cells, coinciding with their higher Ag(+) tolerance, suggesting that EPS may be involved in Ag(+) detoxification.

Estuarine trace metal distributions in Galveston Bay: importance of colloidal forms in the speciation of the dissolved phase

Abstract Cross-flow ultrafiltration studies were carried out in the Trinity river estuary of Galveston Bay in July, 1993, May, 1994 and July 1995 to study the phase speciation for a number of metals (e.g., Cd, Cu, Co, Fe, Ni, Pb, Zn) in estuarine waters. Both conservative and non-conservative estuarine mixing profiles were observed for all trace metals investigated. All metals showed significant colloidal (≥1 kDa) fractions (45±9% for Cd, 55±4% for Cu, 19±6% for Co, 36±6% for Ni, 64±9% for Pb, 91±5% for Zn, and 79±11% for Fe). Colloidal metal concentrations (except, at times, Fe and Pb) correlated significantly with colloidal organic carbon concentrations (≥1 kDa), suggesting that colloidal metals resulted from metal–organic complexation. Iron was preferentially associated with high molecular weight (≥10 kDa) colloids, while Cu, Ni and Pb were associated mostly with low molecular weight (1 kDa–10 kDa) colloidal material. Molecular weight and size distribution studies of organic carbon and trace metals showed two competing processes: (1) coagulation and colloidal pumping, and (2) production of lower molecular weight species, possibly mediated through photochemical or microbiological reactions. Values of partition coefficients between colloids and true solution (Kc) were considerably higher than those between particles and true solution (Kp) for all trace metals except Fe, indicating a high complexation capacity and binding intensity of colloidal macro-molecular organic matter.

The distribution of colloidal and dissolved organic carbon in the Gulf of Mexico

Cross-flow ultrafiltration techniques have been used to extract colloidal organic carbon (COC) from seawater and to investigate different molecular weight fractions of dissolved organic carbon (DOC). Using a high-temperature catalytic oxidation (HTCO) method, DOC and COC of seawater in the Gulf of Mexico were measured during a R/V Gyre cruise in June 1992. DOC concentrations in surface water varied from 131 μM at a near-shore station (water depth ∼ 20 m) to 83 μM at an off-shore station (water depth ∼ 1550 m). DOC concentrations show statistically significant correlations with apparent oxygen utilization (AOU), as well as with temperature. However, as a upper limit, only 20–30% of the oxygen consumption could be due to dissolved organic carbon oxidation. Furthermore, a good correlation between DOC and AOU existed only in the upper water column across the pycnocline, which we ascribed to lateral exchange processes. Water mixing can be quite important in controlling the distribution of DOC and the relationship between DOC and AOU in the water column. Concentrations of COC > 1000 Dalton ranged from 20 to 69 μM, while COC > 10,000 Dalton ranged from 4 to 16 μM in the study area. On average, COC (> 1000 Dalton) comprised about 45% of the initial DOC, and the mass concentration of colloids was > 1 mg l−1. This was one order of magnitude higher than the concentration of suspended particulate matter, and indicates that COC may be an important component of the carbon cycle in the ocean. The relative abundance of COC (both > 1000 and > 10,000 Dalton) decreased from surface water to deep water, not only in terms of concentration but also relative to total DOC. The measurement of molecular weight distributions indicated that ∼ 35% of the initial DOC was in the 1000 –10,000 Dalton fraction, while only about 10% was in the > 10,000 Dalton fraction, leaving approximately 55% in the truly dissolved fraction (i.e. < 1000 Dalton).

Atmospheric depositional fluxes of 7Be and 210Pb at Galveston and College Station, Texas

The bulk depositional fluxes of 210Pb and 7Be were measured at a coastal (Galveston) and an inland (College Station) station for about 3 years, between 1989 and 1991. The annual depositional fluxes of 7Be and 210Pb at Galveston during this period varied by a factor of about 2.5, between 8.9 and 23.2 disintegrations per minute (dpm) cm−2 yr−1, with a mean of 14.7 dpm cm−2 yr−1 for 7Be, and 0.67 and 1.71 dpm cm−2 yr−1, with a mean of 1.03 dpm cm−2 yr−1 for 210Pb, respectively. The precipitation-normalized 7Be flux increases with increasing amount of precipitation. There is no systematic and consistent seasonal trend in the depositional fluxes for 7Be or for 210Pb. The volume-weighted 210Pb concentrations, when normalized to the amount of precipitation, seem to be constant over the time period of this study. Four to six heavy rain events (> 5 cm) in a single day account for 20–30% of the annual deposition of 7Be and 210Pb. Such events account, however, for only about 4–6% of the total number of rainy days in a year. The dry depositional fluxes of these nuclides appear to be a significant fraction of the bulk depositional flux only during the months when there is very little rain. The fraction of dry to total depositional flux of 210Pb appears to be higher than that of 7Be. The strong positive correlation between 7Be and 210Pb depositional fluxes indicates that the flux of both nuclides is controlled by scavenging processes by local precipitation. This correlation also indicates that a major portion of the air masses that brings precipitation to Galveston and College Station is of continental origin. Our data therefore suggest that 7Be and 210Pb cannot be used as independent atmospheric tracers in our coastal station. This observation is consistent with those observed at many other continental and coastal stations.

Historical contamination of PAHs, PCBs, DDTs, and heavy metals in Mississippi River Delta, Galveston Bay and Tampa Bay sediment cores

Profiles of trace contaminant concentrations in sediment columns can be a natural archive from which pollutant inputs into coastal areas can be reconstructed. Reconstruction of historical inputs of anthropogenic chemicals is important for improving management strategies and evaluating the success of recent pollution controls measures. Here we report a reconstruction of historical contamination into three coastal sites along the US Gulf Coast: Mississippi River Delta, Galveston Bay and Tampa Bay. Within the watersheds of these areas are extensive agricultural lands as well as more than 50% of the chemical and refinery capacity of the USA. Despite this pollution potential, relatively low concentrations of trace metals and trace organic contaminants were found in one core from each of the three sites. Concentrations and fluxes of most trace metals found in surface sediments at these three sites, when normalized to Al, are typical for uncontaminated Gulf Coast sediments. Hydrophobic trace organic contaminants that are anthropogenic (polycyclic aromatic hydrocarbons, DDTs, and polychlorinated biphenyls) are found in sediments from all locations. The presence in surface sediments from the Mississippi River Delta of low level trace contaminants such as DDTs, which were banned in the early 1970s, indicate that they are still washed out from cultivated soils. It appears that the DDTs profile in that sediment core was produced by a combination of erosion processes of riverine and other sedimentary deposits during floods. Most of the pollutant profiles indicate that present-day conditions have improved from the more contaminated conditions in the 1950-1970s, before the advent of the Clean Water Act.