Elements and Compounds in Sediments

Jensen et al. [588] have described a procedure for the determination of organochlorine compounds including PCBs and DDT in sediments and sewage sludge in the presence of elemental sulphur. The method can also be used for a search for both volatile and/or polar... 

In marine chemistry, concentration, chemical state, material balance and cycle of both elements and compounds are the main subjects of study for describing the oceans. The oceans, which cover about 71% of the earth s surface, form a complex multidimensional system with varied constructions and materials. Thus, analyses of the components of seawater, marine plants and animals, and sediments give the bases for the interpretation of material balances and geochemical phenomena in oceans. 

The oceans play a major role in the global cycles of most elements. There are several reasons for this. As is evident in images from space, most of the Earth s surface is ocean. When viewed from space, we see mostly water because oceans cover 71% of the Earth s surface. The oceans are in interactive contact with the lithosphere, atmosphere, and biosphere, and virtually all elements pass through the ocean at some point in their cycles. Given sufficient time, the water and sediments of the ocean are the receptacle of most natural and anthropogenic elements and compounds. 

Arsenic is mobile within all environmental compartments, and may circulate many times in various forms through the atmosphere, biosphere, water and soil before entering its sink in the sediments. Whether sediments are the ultimate As sink depends, however, on general and local conditions. For instance, pH and competing substances (ions) may remobilize part of the immobilized elements and compounds (see Part I, Chapter 8). 

The occurrence of organic compounds within trench sediments was also assessed as part of the DOE-RL study (1990). The presence of organic compounds in sediments/soils is important because they can form complexes with radionuclides, thus enhancing their mobility. Specific compounds were identified, including alkenes, alkanes, alkynes, elemental sulfur, and three cyclic sulfur species. Organic constituent concentrations are presented in Table 5-17. 

Biogeochemical cycle. As discussed early in the chapter, this term describes the global or regional cycles of the "life elements" C, N, S, and P with reservoirs including the whole or part of the atmosphere, the ocean, the sediments, and the living organisms. The term can be applied to the corresponding cycles of other elements or compounds. 

Volatilization is also a dominant transport mode for mercury, which is the most volatile metal in its elemental state. As with lead, a key reaction that can increase the volatility of mercury is formation of an organometallic compound. In this case, the reactions take place in water and are primarily biological, being mediated by bacteria commonly found in the upper levels of sediments. These reactions and their importance in the global mercury cycle are discussed in some detail later in the chapter. 

The mobility of arsenic compounds in soils is affected by sorp-tion/desorption on/from soil components or co-precipitation with metal ions. The importance of oxides (mainly Fe-oxides) in controlling the mobility and concentration of arsenic in natural environments has been studied for a long time (Livesey and Huang 1981 Frankenberger 2002 and references there in Smedley and Kinniburgh 2002). Because the elements which correlate best with arsenic in soils and sediments are iron, aluminum and manganese, the use of Fe salts (as well as Al and Mn salts) is a common practice in water treatment for the removal of arsenic. The coprecipitation of arsenic with ferric or aluminum hydroxide has been a practical and effective technique to remove this toxic element from polluted waters... 

In Tables 1.11 - 1.13 analytical techniques are cross-referenced with organic compound element or organometallic compound determined in soil, sediment or sludge and the section number in the book. If the reader finds that a method is not listed for determining a particular compound in the particular type of sample, then by examination of the table he may find a... 

In a method described by Bates and Carpenter [8] for the characterization of organosulphur compounds in the lipophilic extracts of marine sediments these workers showed that the main interference is elemental sulphur (S8). Techniques for its elimination are discussed. Saponification of the initial extract is shown to create organosulphur compounds. Activated copper removes S8 from an extract and appears neither to create nor to alter organosulphur compounds. However, mercaptans and most disulphides are removed by the copper column. The extraction efficiency of several other classes of sulphur compounds is 80-90%. Extracts are analyzed with a glass capillary gas chromatograph equipped with a flame photometric detector. Detection limit is lg S and precision 10%. 

Workers at the Department of the Environment, UK [47], have described a procedure for the determination of methylmercury compounds in soils and sediments which involves extraction with a carbon tetrachloride solution of dithizone, reduction to elemental mercury then analysis by atomic absorption spectrometry. 

Different sampling methods often result in collection of different components of the element or compound of interest. Most seawater samples are collected in bottles, filtered to remove particles and analyzed directly or after preconcentration of minor components. Particulate matter from the water column is collected by filtration or with sediment traps. Sediment samples are collected in cores that recover intact chronological sequences and are commonly subsampled, dried and ground to a powder. Pore waters are extracted from sediments by squeezing or suction, dialysis, and centrifugation. 

Matrix The immediate environment (milieu) surrounding an element or compound. For example, seawater, sediment, and particulate material are general matrices of interest in this report. Also, organic matter, opal, carbonate and aluminosilicate are more specific solid-phase matrices of interest. 

Mercury dimethyl is a toxic environmental pollutant. It is found in polluted bottom sediments and in the bodies of fishes and birds. In the bodies of fishes and birds it occurs along with monomethyl mercury. The latter, as CH3Hg+ ion, is formed by microorganism-induced biological methylation of elemental mercury or agricultural fungicide mercury compounds that are discharged into the environment. 

---