Soil suspension

Albanis TA, Pomonis PJ, Sdoukos AT. 1988c. The influence of fly ash on hydrolysis, degradation and adsorption of methyl parathion in aqueous soil suspensions. Toxicol Environ Chem 17 351-362. 

T. Makino, Y. Takahashi, Y. Sakurai. and M. Nanzyo, Influence of soil chemical properties on adsorption and oxidation of phenolic acids in soil suspension. Soil Sci. Plant Nittr. 42 U1 (1996). 

Not everyone has access to a lysimeter, which means that one often needs to obtain the soil solution from a soil sample that has been collected in the field. It is easiest to remove the soil solution when the soil is saturated. Thus, experiments are sometimes carried out using soil suspensions. In these cases, questions regarding oxidation-reduction potential and its effect on the species or compound under investigation come in to play. For example, are reduced species that are observed under these conditions common in the field and do they play a significant role in the chemistry of that particular soil, or are they artifacts of the experimental conditions This question must be both asked and answered for the results to be useful. 

Figure 9.7. A pH electrode in a soil suspension. The electrode is connected to a computer that... 

In addition to the simple Eh-pH graph shown in Figure 9.3, three-dimensional Eh-pH graphs can be produced. Known quantities of a pollutant can be added to a number of different soil suspensions and its degradation at different combinations of Eh and pH measured. In this way, the optimum conditions for the decomposition of the pollutant in question can be determined. An excellent example of this is the decomposition of pentachlorophe-nol and hexahydro-l,3,5-trinitrol,3,5-triazene in soil and water under various Eh-pH conditions as illustrated in the papers by Petrie et al. [9] and Sing et al. [10]. 

Soil and soil suspensions are colored and hard to see through. Thus, it is hard to directly titrate them using colored indicators. There are typically only two cases where direct titrations of soil are carried out. The first is to determine the amount of amendment needed to bring the soil to a desired pH. The second is in the determination of soil organic matter where organic matter is oxidized with chromate and the unreacted chromate is titrated (actually called a back titration) to determine, by subtraction, the amount of dichromate reduced and thus the amount of organic matter present. 

Titration of soil pH is an old method that is not widely used today. Basically, an acid soil suspension is prepared and titrated with a standardized base, often sodium hydroxide, although various basic calcium compounds such as calcium oxide (CaO) and calcium hydroxide [Ca(OH)2] can also be used. Because of the dark color of many soils, they are often titrated using a pH meter as the indicator of the end point. A setup for the titration of soil is shown in Figure 10.1. Titration is slow in that it takes some time after the addition of titrant for some semblance of equilibrium to be reached. Once this happens, a reading can be made or simply another addition of titrant made. 

A titration curve for an acid soil suspension to which 1 mL of a calcium hydroxide titrant is added and the change in pH followed for 2.3 minutes is shown in Figure 10.3. As can be seen, the pH initially increases and then falls back toward the original pH. The curve not only has a sawtooth pattern but is also curved in the reverse direction from a standard titration of an acid with a basic solution. 

Gambrell et al. [14] have described a procedure for the recovery of DDT (also kepone and Permethrin) added to soil suspensions incubated under controlled redox potential and pH conditions. They studied the effect of time on the levels of the insecticides and their breakdown products. 

After shaking the soil suspension in the extraction bottle, a tube of filter-paper folded about the centre to form a V with the open ends uppermost is inserted into the bottle. Clear filtrate collects inside the paper tube and aliquots are removed with a pipette. 

Discard the soil suspension in a special pail. (The soil may be placed outside in a designated location.) Do not pour down the sink. 

Figure 4.16 Changes in concentrations of Fe in (a) whole soil, (b) soil solution and (c) soil solid during oxygenation of rednced soil suspensions at different pHs. [Fe ]s was calculated from [Fe ]-/f[Fe +]L (Kirk and Solivas, 1994). Reproduced by permission of Blackwell Publishing...

Ahmad AR, Nye PH. 1990. Coupled diffusion and oxidation of ferrous iron in soils. I. Kinetics of oxygenation of ferrous iron in soil suspension. Journal of Soil Science 41 395-409. 

Patrick WH, Jr, Williams BG, Moraghan JJ. 1973. A simple system for controlling redox potential and pH in soil suspensions. Soil Science Society of America Proceedings 37 331-332. 

When lindane was incubated in aerobic and anaerobic soil suspensions for 3 wk, 0 and 63.8% was lost, respectively (MacRae et al., 1984). Using settled domestic wastewater inoculum, lindane (5 and 10 mg/L) did not degrade after 28 d of incubation at 25 °C (Tabak et al., 1981). When lindane was incubated in river water samples and sediments for 3 wk, 80% of the applied amount had degraded. Under sterilized conditions, >95% was recovered after 12 wk. Under unsterile and sterile conditions, 20 and 80% of the recovered lindane was bound to sediments (Oloffs et al., 1973 Oloffs and Albright, 1974). 

When prometryn in aqueous solution was exposed to UV light for 3 h, the herbicide was completely converted to hydroxypropazine. Irradiation of soil suspensions containing prometryn was found to be more resistant to photodecomposition. About 75% of the applied amount was converted to hydroxypropazine after 72 h of exposure (Khan, 1982). The UV (A = 253.7 nm) photolysis of prometryn in water, methanol, ethanol, /i-butanol, and benzene yielded 2-methylthio-4,6-bis(isopropylamino)-s-triazine. At wavelengths >300 nm, photodegradation was not observed (Pape and Zabik, 1970). Khan and Gamble (1983) also studied the UV irradiation (A = 253.7 nm) of prometryn in distilled water and dissolved humic substances. In distilled water, 2-hydroxy-4,6-bis(isopropylamino)-5-triazine and 4,6-bis(isopropylamino)-5-triazine formed as major products. 

Rosenberg, A. and Alexander, M. Microbial metabolism of 2,4,5-trichlorophenoxyacetic acid in soil, soil suspensions, and axenic culture, 7 Agric. Food Chem., 28 297-302, 1980. 

Phosphate must be applied as fertilizer to the soil. Ideally it is added in quantities sufficient to guarantee optimal yields, but not in excess in order to avoid P transportation into other compartments of the ecosystem. The amount added should be based on an accurate estimation of the plant-available fraction of P already present in a soil.This is an old and difficult task and a large number of extraction methods have been used since intensive land use was practised. Recently methods have been worked out in which a strip of filter paper impregnated with an Fe oxide (2-line ferri-hydrite) is dipped into a soil suspension and the amount of P adsorbed by the paper is taken as being plant-available (Sissingh,1988 Van der Zee et ah, 1987 Sharpley, 1993 Sharpley et ah,1994 Kuo and Jellum, 1994 Myers et ah 1997). Anion and cation resins extracted more P from four heavily fertilized soils than from goethite (Delgado Torrent, 2000). Other oxyanions adsorbed by soil Fe oxides are silicate, arsenate, chromate, selenite ( ) and sulphate. Adsorption of sulphate led to a release of OH ions and was substantially lowered once the Fe oxides were selectively removed (Fig.16.17). 

Procedure for mineral soils of pH less than 5.0. Add 20 ml of double strength buffer solution to the soil suspension retained from the pH determination, and stir for 5 min. Mix 25 ml water and 20 ml double strength buffer with pH adjusted to between 6.9 and 7.1, and use to recalibrate the pH meter to read 7.00. Read the pH of the stirred sample. 

An analysis of the thermodynamic stability models of various nickel minerals and solution species indicates that nickel ferrite is the solid species that will most likely precipitate in soils (Sadiq and Enfield 1984a). Experiments on 21 mineral soils supported its formation in soil suspensions following nickel adsorption (Sadiq and Enfield 1984b). The formation of nickel aluminate, phosphate, or silicate was not significant. Ni and Ni(OHX are major components of the soil solution in alkaline soils. In acid soils, the predominant solution species will probably be NE, NiS04°, and NiHP04° (Sadiq and Enfield 1984a). 

Sauve, S., McBride, M. Hendershot, W. 1998. Lead phosphate solubility in water and soil suspensions. Environmental Science Technology, 32, 388-393. 

J. N. Ryan, Enhanced Dissolution of Cinnabar (Mercuric Sulfide) by Dissolved Organic Malta Isolated from the Florida Everglades, Environ. Sci Technol 1998, 32, 3305 S. Sauvd, M. McBride, and W. Hendershot, Lead Phosphate Solubility in Water and Soil Suspensions, Environ. Sci Technol 1998,32, 388. 

Another approach uses the coupling reaction of p-anisidine. In the presence of H202 and peroxidase (16), an oxidation product that contains two aromatic rings, benzoquinone-4-methoxyaniline, is formed stoichiometri-cally (92). Equations 14-16 indicate that an electron donor or hydrogen donor is required for peroxidase-mediated decomposition of H202. In two natural waters and one soil suspension, peroxidatic activity was identified by the stoichiometric removal of p-anisidine by the addition of H202 (in the dark) (16). This procedure provides an independent corroboration of the results obtained by Moffett and Zafiriou (1). However, this method does not quantify the relative importance of peroxidases versus catalases in the decomposition of H202. 

Tecator Ltd. [16] have described a flow injection analysis method for the determination of 0.2 -1.4 mg/1 (as NH3N) of ammonia nitrogen in soil samples extractable by 2 M potassium chloride. The soil suspension in 2 M potassium chloride is centrifuged and filtered and introduced into the flow injection system for the analysis of ammonia (and nitrate) one parameter at a time. Ammonia is determined by the gas diffusion principle, in which a PTFE membrane is mounted in the gas diffusion cell. 

Hunt [340] has described a simple method of filtering soil extracts that eliminates the need for filter funnels and receivers. It therefore reduces the risk of contamination and speeds up the procedure. It also offers a convenient means of obtaining filtrates in the field for subsequent analysis. After shaking the soil suspension in the extraction bottle, a tube of filter paper folded about the centre to form a V with the open ends uppermost is inserted into the bottle. Clear filtrate collects inside the paper tube and aliquots are removed with a pipette. 

The slopes of the regression lines show that the electrode recorded 94-95% of the added nitrate, the response of the electrode in soil suspension being substantially linear down to 2.5 pg/1 of nitrate in the extract. 

Mansour, M. (1996). Abiotic degradation of pesticides and other organic chemicals in aquatic systems. Pestic. Outlook, 7 9-10. Mansour, M., A. Mamouni, and P. Meallier (1988). Factors determining the behavior and transformation of selected pesticides in water, soil suspension and soils. In Proceedings of Methodological Aspects of the Study of Pesticide Behaviour in Soil. Versailles, France, INRA, June 16-17, pp. 87-100. 

The objective of a mechanistic rate law is to ascertain the correct fundamental rate law. The reaction sequence for determination of mechanistic rate laws may represent several reaction paths and steps either purely in solution or on the soil surface of a well-stirred dilute soil suspension. All processes represent fundamental steps of a chemical rather than a physical nature (Skopp, 1986). 

Figure 6.3. Removal of EDB from soil suspension by N2 purging removal of native EDB from soil (bottom curve), removal of a [14C]EDB spike (10 ng g ) from the same soil after a 3-h equilibration period (middle curve), removal of [,4C]EDB (10 ng ml-1) from distilled water. [From Steinberg et al. (1987), with permission.]...

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