Role Of Natural Organic Matter Research Articles

Impact of light on the properties of aquatic natural organic matter

Natural organic matter (NOM) has the property to absorb light in the UV and visible range. Derived from the stratospheric ozone depletion, there is a reasonable concern about the influence of lower wavelength irradiation on the role of NOM in aquatic systems. Experiments were performed under defined conditions simulating sunlight with increased UV-B content for irradiation of NOM in aerated and deaerated aqueous solutions. The results show that there is a general bleaching effect in the irradiated samples and a change in the molecular size distribution of NOM. Irradiation causes a decrease in the higher molecular range and an increase of smaller molecules, along with an increase of the assimilable amount of organic matter. The toxicity for Daphnia magna is higher in irradiated samples, especially in deaerated ones. A decrease in the resolution of the lifetime distribution and a shift of the maximum lifetime to longer values are found for time-resolved fluorescence. Steady-state fluorescence shows an increase of the relative intensities with decreasing wavelength in case of the aerated samples, and the reverse tendency for the deaerated ones.

Colloid-facilitated transport of contaminants in groundwater: mobilization of transuranic radionuclides from disposal trenches by natural organic matter

The role of natural organic matter (NOM) in enhancing the field-scale transport of transuranic radionuclides (TRU) in groundwater is used as a case study to illustrate the value of integrated laboratory and field approaches to understanding colloid-facilitated transport. Field observations provide evidence that TRU are mobilized and co-transported by NOM when hydrologic processes bring the groundwater in contact with waste buried in shallow trenches. This hypothesis receives further support from laboratory speciation studies and geochemical modeling. However, laboratory sorption studies indicate that the groundwater NOM should sorb to, and thus be retarded by the mineral surfaces of the formation. This issue is resolved through field studies of NOM transport. Discrepancies between laboratory predictions and field results reveal that the key process in NOM transport in natural—as opposed to model laboratory—systems is competitive adsorption among NOM subcomponents. Unlike laboratory studies of adsorption of NOM to clean mineral surfaces, surfaces in natural systems are coated with groundwater NOM, and binding sites are “passivated” with respect to further binding of the same NOM. The hypothesis that highly mobile NOM enhances TRU migration was tested by using lanthanides as field tracers to determine the extent of retardation of the TRU-NOM complex. The lanthanides, which have sorption and transport properties similar to TRU, migrated as NOM complexes without significant retardation over flow paths of 75-m. It is evident that assumptions inherent in many risk assessments for existing waste facilities, and performance assessments for future repositories, must begin to account for the role that even typically low levels of groundwater NOM plays in contaminant mobility.

A bacterial bioassay for the assessment of copper bioavailability in freshwaters

Copper-sensitive bacteria were isolated from a tropical river water sample and used to develop a simple, inexpensive bioassay for the determination of copper bioavailability in freshwaters. The 48-h bioassay, which has an EC50 for inorganic copper of 7.1 μg/L, involved addition of copper-sensitive bacteria to filtered water samples amended with growth-stimulating nutrients and measurement of growth. Matrix-matched control solutions were prepared by adding the metal complexing agent EDTA to a portion of each sample tested. In order to optimize sensitivity of the bioassay, a nutrient medium was developed, which stimulated bacterial growth, but did not appreciably alter copper speciation of the sample. This necessitated omission of many organic compounds commonly incorporated in growth media because of their ability to complex copper. Performance of the bacterial bioassay was evaluated on freshwater samples from tropical and temperate river systems. Bacterial 48-h EC15 values ranged from 5.0 to 25.0 μg Cu/L and were much higher than the observed EC15 value of 2.8 μg/L for inorganic copper in deionized water (no organic matter present). A linear correlation (r=0.93, p≤0.005) was observed between the measured EC15 values and the copper complexation capacities of river water samples determined by anodic stripping voltammetry. This confirmed the role of natural organic matter in ameliorating copper toxicity. Results of this study provide further evidence that the bioavailability of copper in freshwaters is significantly reduced in the presence of natural organic matter. A generic protocol for development of metal-sensitive bacterial bioassays was developed as part of the work. © 1998 John Wiley & Sons, Inc. Environ Toxicol Water Qual 13: 263–271, 1998

Heterogeneous processes affecting trace contaminant distribution in estuaries: The role of natural organic matter

Our objective in writing this paper is to frame the issues pertaining to the role of natural organic matter in affecting the speciation and transport (fate) of trace contaminants, which include hydrophilic trace metals and hydrophobic trace organics, within estuaries. In particular, we focus on one aspect of the problem: the partitioning of trace contaminants between organic and inorganic particles, colloids and solution. This paper is developed along three main lines. First, we review the literature with respect to trace metal sorption by metal oxides in the presence of organic ligands. Second, we examine the role of colloidal organic matter in regulating the estuarine behavior of trace contaminants. The focus of this portion of the paper is on the trapping of trace metals within colloidal and particulate organic matter and a re-examination of the ‘particle concentration effect’. Third, we propose a new conceptual model (the ‘percolator’ model) which links several processes (sorption, diffusion, coagulation) and which serves as a framework for evaluating the trapping of trace metals and organic matter within estuarine sediments. One conclusion derived from simulations of the diffusive flux of DOC from sediments is that the magnitude of sediment/water interfacial shear stress does not affect DOC flux until the shear stress is sufficient for the onset of bed erosion. The consequence is that interactions between DOC and sediment materials become the controlling factors in regulating the diffusive flux of DOC.

Coagulation of colloidal material in surface waters: the role of natural organic matter

Organic matter has a great influence over the fate of inorganic colloids in surface waters. The chemical nature and structure of natural organic matter (NOM) will be an important factor in determining whether colloids will be stabilised or destabilised by NOM. Under environmentally relevant conditions, the ubiquitous fulvic acids are likely to be responsible for coating and imparting a negative charge to colloids. If the adsorbed polyelectrolyte coating produces an increase in absolute surface potential, it will act to stabilise colloids in the water column. On the other hand, colloidal organic carbon, especially chain-like structures, has been shown to be involved in the aggregation of inorganic colloids through the formation of bridges. It is highly probable that both adsorption and bridging flocculation are occurring simultaneously in the natural aquatic environment. The importance of each process depends directly on the nature and concentration of organic matter in the system and indirectly on the productivity of the lake, its hydrological pathways, temporal variations, temperature, etc. The present paper reports such results and emphasises the need to discriminate the different kinds of NOM.