Toxicity transformation processes

Table 20.5 lists the partition and transformation processes applicable in the deep-well environment and indicates whether they significantly affect the toxicity or mobility of hazardous wastes. None of the partition processes results in detoxification (decomposition to harmless inorganic constituents), but all affect mobility in some way. All transformation processes except complexation can result in detoxification however, because transformation processes can create new toxic substances, the mobility of the waste can be critical in all processes except neutralization. 

Adsorption is a physicochemical process whereby ionic and nonionic solutes become concentrated from solution at solid-liquid interfaces.3132 Adsorption and desorption are caused by interactions between and among molecules in solution and those in the structure of solid surfaces. Adsorption is a major mechanism affecting the mobility of heavy metals and toxic organic substances and is thus a major consideration when assessing transport. Because adsorption is usually fully or partly reversible (desorption), only rarely can it be considered a detoxification process for fate-assessment purposes. Although adsorption does not directly affect the toxicity of a substance, the substance may be rendered nontoxic by concurrent transformation processes such as hydrolysis and biodegradation. Many chemical and physical properties of both aqueous and solid phases affect adsorption, and the physical chemistry of the process itself is complex. For example, adsorption of one ion may result in desorption of another ion (known as ion exchange). 

Transformation processes change the chemical structure of a compound. Because not all transformation processes convert hazardous wastes to nonhazardous compounds, geochemical fate assessment must consider both the full range of transformation processes that may occur and the toxicity and mobility of the resulting products. For deep-well-injected wastes, transformation processes and subsequent reactions may lead to one or more of the following ... 

Examples of the Effects of Transformation Processes on the Toxicity of Substances... 

The microbial metabolic process is the major mechanism for the transformation of toxic organic chemicals in the subsurface environment. The transformation process may be the result of a primary metabolic reaction, when the organic molecule is degraded by a direct microbial metabolism. Alternatively, the transformation process may be an indirect, secondary effect of the microbial population on the chemical and physical properties of the subsurface constituents. Bollag and Liu (1990), considering behavior of pesticides, defined five basic processes involved in microbially mediated transformation of toxic organic molecules in the soil upper layer environment. These processes are described next. 

Secondary prevention and mitigation, by themselves, are unable to eliminate the risk of serious or catastrophic chemical accidents, although improved process safety management can reduce their probability and severity. Most chemical production involves transformation processes, which are inherently complex and tightly coupled. Normal accidents are an unavoidable risk of systems with these characteristics [11]. However, the risk of serious, or catastrophic, consequences need not be. Specific industries use many different processes. In many cases, alternative chemical processes exist which completely or almost completely eliminate the use of highly toxic, volatile, or flammable chemicals [12]. 

The scope is limited to issues relevant to the sources and bioavailability of DOM and the interface between DOM and several ecosystem-level processes such as nutrient/toxicant transformations and retention. This volume contains overviews of (1) the supply of DOM to aquatic ecosystems and variation in DOM composition, (2) processes mediating the transfer of... 

Cumulative risk is the combined risk resulting from exposures that accumulate over time, pathways, sources, or routes for a number of agents or stressors. This concept of cumulative risk addresses the fact that individuals are not usually exposed to a single environmental contaminant by means of a single exposure pathway. Multiple contaminants are released from sources as chemical mixtures. Environmental fate and transformation processes affect the nature, pathways, and extent of human exposure. Exposures by different pathways may result in differential absorption, metabolism, and toxic response, even for the same chemical. Cumulative risks are... 

Such a transformation process will affect its mobility and increase its toxicity. This we can see in the corresponding data, reported in Table 1.2 [i.e., the carcinogenic potential or slope factor, the reference dose for oral and inhaled intake (RfD), its solubility, and its vapor pressure]. If these pollutants reach a certain exposure level for a community who breathes contaminated air or consumes polluted water, its members will be at risk. 

The transformation of toxic substances in soil can have a profound effect on their potential for human exposure and accumulation by biota. Transformation processes in soil include physical processes such... 

Heavy metals that are toxic to aqnatic plants and animals can enter wetlands from several sources. Toxic metals in wetland soils exist in various forms and may nndergo nnmerons transport and transformation processes when they enter wetlands. 

Concentrations of WA and their toxic transformation products, formed both as a result of technological processes associated with CW destruction and WA intake in the environment, should first be measured in regions of CW production, testing, storage and destruction as well as in their background areas. 

Toxicity tests exist which have been carried out on bacteria that are specific to the soil compartment. They investigate the long-term adverse effects of substances on carbon (OECD 217, [58]) or nitrogen (OECD 216, [59]) transformation processes over a period of no less than 28 days. 

---