Sediment exchangeable acidity

Exchangeable acidity was determined by the BaCl2-TEA (triethanolamine) method (20). The equilibrium calcium-ammonium exchange coefficient (Gapon coefficient, Kq) was determined on the Ca2+-saturated, NH4+-saturated portions and unaltered sediment by methods described by Baes and Bloom (21). This determination was accomplished by allowing exchange solutions with compositions similar to LRL pore waters (Ca2+ = 6.6-52.9 xequiv/L NH4+ = 8.7-44... 

Mass-balance calculations for the first 3 years of acid additions indicate that the principal IAG processes are sulfate reduction and cation production. Specifically, one-third of the total sulfate input (added acid and deposition) was neutralized by in-lake processes. Increased sulfate reduction consumed slightly more than one-sixth and production of cations neutralized somewhat less than one-sixth of the acid added. Of the remaining sulfate, one-third was lost by outflow, and one-third decreased lake alkalinity. Laboratory determinations suggest that sediment-exchange processes occurring in only the top 2 cm of surficial sediments can account for the observed increase in water-column cations. Acidification of the near-surface sediments (with partial loss of exchangeable cations) will slow recovery because of the need to exchange the sediment-bound H+ and neutralize it by other processes. Reactor-based models that include the primary IAG processes predict that... 

Figure 15-12 is a schematic illustration of a technique known as acid volatile sulfides/ simultaneously extracted metals analysis (AVS/SEM). Briefly, a strong acid is added to a sediment sample to release the sediment-associated sulfides, acid volatile sulfides, which are analyzed by a cold-acid purge-and-trap technique (e.g., Allen et ai, 1993). The assumption shown in Fig. 15-12 is that the sulfides are present in the sediments in the form of either FeS or MeS (a metal sulfide). In a parallel analysis, metals simultaneously released with the sulfides (the simultaneously extracted metals) are also quantified, for example, by graphite furnace atomic absorption spectrometry. Metals released during the acid attack are considered to be associated with the phases operationally defined as "exchangeable," "carbonate," "Fe and Mn oxides," "FeS," and "MeS."... 

Measurements of " Th in sediment samples (Aller and Cochran 1976 Cochran and Aller 1979) used much the same approach as outlined above. In this case, the dried sediment sample ( 10 g) was leached with strong mineral acid (HCl) in the presence of a yield monitor (generally Th, an artificial Th isotope resulting from the decay of Th that is produced by neutron capture on Th). Thorium was separated from U and purified by ion exchange chromatography, and electrodeposited onto stainless steel planchets. Counting and determination of " Th activity followed the procedure outlined above. 

For instance, 2-methylpropene reacted with acetic acid at 18°C in the presence of Al-bentonite to form the ester product (75). Ion-exchanged bentonites are also efficient catalysts for formation of ketals from aldehydes or ketones. Cyclohexanone reacted with methanol in the presence of Al-bentonite at room temperature to give 33% yield of dimethyl ketal after 30 min of reaction time. On addition of the same clay to the mixture of cyclohexanone and trimethyl orthoformate at room-temperature, the exothermic reaction caused the liquid to boil and resulted in an almost quantitative yield of the dimethyl ketal in 5 min. When Na- instead of Al-bentonite is used, the same reaction did not take place (75). Solomon and Hawthorne (37) suggest that elimination reactions may have been involved in the geochemical transformation of lipid and other organic sediments into petroleum deposits. 

The mechanism of alkyl hydrogen exchange was not clarified, but a possible mechanism was postulated. Partial hydride abstraction by a Lewis acid site may have occured forming a carbocation-like species followed by exchange of a proton at a R-carbon. Such a mechanism predicts exchange to occur preferentially at methyl groups adjacent to the most stable carbocations (benzylic > 3° > 2° > 1°). This is consistent with the observed relative rates of epimerization of steranes during thermal maturation of sediments (83). 

The carboxyl group (-COOH) of organic acids interacts either directly with the interlayer cation or by forming a hydrogen bond with the water molecules coordinated to the exchangeable cation on the soil-solid and sediment-solid clay... 

The processes of cation production by weathering or ion exchange cannot be differentiated by ion-budget calculations, but they may be distinguished on the basis of kinetics. In early experiments to investigate the role of sediments as buffers, sequential additions of sulfuric acid to well-mixed sediment slurries produced rapid increases in soluble Ca2+ that reached stable concentrations within 24 h (53, 54). The rapid production of cations suggested that the dominant process was ion exchange... 

Results of the four models (Figure 9a) illustrate the effect on predicted recovery rates of including various alkalinity-generating processes. Models 1 and 2 probably yield upper and lower limits of the time required to recover to the preacidification alkalinity level. Model 3 probably yields an underestimate of recovery time, in that it does not consider the need to neutralize acidified surficial sediments (and restore base cations on sediment-exchange sites that have been lost during the last years of acid loading). Model 4 probably yields the most accurate estimate of recovery time, but it does not provide a functional relationship for the cation-production term. Based on Model 4, the north basin will reach 50% of the preexperimental alkalinity concentrations in 3-5 years and 90% in 8 years. Complete recovery is predicted to occur in 12.5-15 years. 

Low volatile, high molecular weight halogenated compounds can be extracted with hexane or iso-octane and determined by GC-ECD. Methylene chloride may be used for extraction if the analysis is done by GC/MS. Purge and trap efficiency will be poor for such compounds, especially those boiling over 200°C. Soils, sediments, and solid wastes may be extracted with methylene chloride by sonication or soxhlett extraction. Interferences from acidic compounds, such as chlorophenol, may be removed by acid-base partitioning cleanup. The extract is then concentrated and analyzed by GC/MS or exchanged to hexane and analyzed by GC-ECD. 

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

The lability of metals associated with sediments has been determined as a function of pH by equilibrating the sediment overnight (in a batch process) with a series of strong and weak acid cation exchangers, prepared in both the Na+ and H+ forms (Slaveket al., 1990). 

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