Toxicity sorption processes

The extrapolation methods related to differences in media and matrices are mainly governed by sorption processes. Therefore, uncertainty analysis should try to address toxicant properties related to sorption (KD, A"ow, Ka, etc.) and matrix-specific characterization of sorption sites (qualitative and quantitative). Some of the computerized models for exposure and effects assessment explicitly include options for treating key variables as probability distributions (e.g., AQUATOX). 

Templeton et al. (2002a) used a combination of Pb Lm-XAFS and pXANES spectroscopy and transmission electron microscopy to show that B. cepacia causes biomineralization of Pb(II) in the form of highly insoluble pyromorphite at ( ) concentrations well below supersaturation with respect to pyromorphite. The phosphate in these minimal medium experiments is though to be provided by B. cepacia, and the pyromorphite forms on the outer cell membrane of B. cepacia. These types of studies are beginning to provide unique information on how microbial biofilms affect metal sorption processes at mineral surfaces, which is essential for understanding the transport and bioavailability of toxic metal ions in natural systems where such biofilms exist. They are also allowing quantitative evaluation of the competition between NOM (or biofilms) and the mineral substrates they coat for metal ion binding. 

Suspended organic matter components of soil and sediment (e.g. humic acids) may result in decreased apparent toxicity by serving as a sink compartment for the toxicants due to sorption processes. 

U.S. EPA defines MNA as the reliance on natural processes, within the context of a carefully controlled and monitored site cleanup approach, to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods. The natural processes include biodegradation, dispersion, dilution, sorption, volatilization, stabilization, and transformation. These processes reduce site risk by transforming contaminants to less toxic forms, reducing contaminant concentrations, and reducing contaminant mobility and bioavailability. Other terms for natural attenuation in the literature include intrinsic remediation, intrinsic bio-remediation, passive bioremediation natural recovery, and natural assimilation. 30... 

Ionization is the process of separation or dissociation of a molecule into particles of opposite electrical charge (ions). The presence and extent of ionization has a large effect on the chemical behavior of a substance. An acid or base that is extensively ionized may have markedly different solubility, sorption, toxicity, and biological characteristics than the corresponding neutral compound. Inorganic and organic acids, bases, and salts may be ionized under environmental conditions. A weak acid HA will ionize to some extent in water according to the reaction ... 

The tables were designed to encompass processes included in most models of the various media of concern. Although selected processes are not rigorously defined for each media (e.g., sorption/desorption in air refers to toxicant-particulate interactions), the goal was to provide a concise ranking table for each level of analysis. 

Silver occurs naturally in several oxidation states, the most common being elemental silver (Ag°) and the monovalent ion (Ag+). Soluble silver salts are, in general, more toxic than insoluble salts. In natural waters, the soluble monovalent species is the form of environmental concern. Sorption is the dominant process that controls silver partitioning in water and its movements in soils and sediments. As discussed later, silver enters the animal body through inhalation, ingestion, mucous membranes, and broken skin. The interspecies differences in the ability of animals to accumulate, retain, and eliminate silver are large. Almost all of the total silver intake is usually... 

Sorption plays a significant role in the environmental fate and effects of compounds released into the aquatic environment, largely determining their distribution between different environmental compartments. Apart from affecting the mobility, and therefore the potential of a surfactant to reach groundwater and surface water, sorption can affect its toxicity and biodegradation by influencing bioavailability. This process is especially relevant for surfactants, since their molecular structure presents a pronounced tendency to sorb onto interfaces. 

Surfactants are selected based primarily on the degree of solubilization. Other factors to be considered include toxicity, biodegradability, surfactant sorption, and surfactant solubility and compatibility with the separation process. Surfactants have the ability to lower the interfacial tension between water and the contaminant by as little as a factor of three to four orders of magnitude. Combined with a sufficient reduction in capillary forces, this allows pumped groundwater theoretically to move the DNAPL toward the recovery or extraction well. This is accomplished by injecting surfactant solution into the contaminated zone. Impacted groundwater characterized by an increase in the concentration of the contaminant is then recovered and treated. 

Biodegradation, hydrolysis, and sorption influence the environmental fate of LAS, AS, and AES. Primary degradation of surfactants is important because this process usually results in loss of surfactancy and reduced toxicity (5, 6). Complete mineralization ensures that persistent intermediates will not be formed and that biodegradation will be an effective mass-removal mechanism in the environment. Sorption and association of surfactants with particles or dissolved organic substances are processes that decrease bioavailability and can be correlated with decreased surfactant toxicity (7). 

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