Extraction methods, soil pore water

Di Bonito, M., Breward, N., Crout, N., Smith, B., and Young, S. (2008). Soil pore water extraction methods for trace metals determination in contaminated soils. In Environmental Geochemistry Site Characterization, Data Analysis, and Case Histories (B. De Vivo, H. E. Belkin, and A. Lima, eds.). Elsevier Science Publishers, Amsterdam, The Netherlands. 

Overview of Selected Soil Pore Water Extraction Methods for the Determination of Potentially Toxic Elements in Contaminated Soils Operational and Technical Aspects... 

Overview of Selected Soil Pore Water Extraction Methods... 

The method of soil suspensions extracts is based on metal desorption/dissolution processes, which primarily depend on the physico-chemical characteristics of the metals, selected soil properties and environmental conditions. Metal adsorption/ desorption and solubility studies are important in the characterization of metal mobility and availability in soils. Metals are, in fact, present within the soil system in different pools and can follow either adsorption and precipitation reactions or desorption and dissolution reactions (Selim and Sparks, 2001). The main factors affecting the relationship between the soluble/mobile and immobile metal pools are soil pH, redox potential, adsorption and exchange capacity, the ionic strength of soil pore water, competing ions and kinetic effects (e.g. contact time) (Evans, 1989 Impelhtteri et al., 2001 McBride, 1994 Sparks, 1995). 

In summary, the importance of the method employed for soil pore water extraction should not be underestimated. Experimental design and performance should be chosen to reflect the particular aim of the study, reported in sufficient detail to allow others to make appropriate comparisons and the parameters operationally defined as a function of the method employed. 

ASTM. Standard Test Method for Pore Water Extraction and Determination of the Soluble Salt Content of Soils by Refractometer-D4542. American Society for Testing and Materials, Philadelphia, PA, p. 6. ASTM. D2850. Standard Test Method for Unconfined Unconsolidated Triaxial Compression Test on Cohesive Soil. American Society for Testing and Materials, Philadelphia, PA, p. 6. 

Another way to extract pore water is to use a displacement method, which is usually applied in the laboratory. In this method, a solvent immiscible with water is applied to the soil in a column or centrifuge tube, and gravity or centrifugation is used to move the solvent through the soil, displacing pore water. The displaced water can then be separated from the immiscible liquid and analyzed. 

For less soluble contaminants like PAHs and PCBs, it is not possible to measure pore water concentrations. Chemical availability has to be measured with a method that extracts a certain part from soil. Methods are available, such as a mild extraction for a short period of time. Relation with effects have been empirically established (certain strength and certain time). Methods are easy and cheap, but valid only after establishing the empirical relationship. Other methods are based on a strong adsorbent and stimulate the diffusion from the solid phase into the water phase and on to the adsorbent. This process is easier to understand (based on equilibrium between soil and water), but is also based on empirical relationships (Reid etal. 2000). 

In both types of extraction, it is not certain that all the water or even a representative sample of the water is removed from soil. Water in small pores, cracks, or held at greater pressures that those applied to remove water will not be removed and their constituents will not be included in the analysis. However, both these methods find wide use in soil analysis. 

These methods were selected for different reasons, but mainly for their flexibility and novelty. Rhizon samplers represent the current equivalent of porous cups, widely used in the recent past centrifugation is possibly the current most widely used method because of the ease and the ready availability of the requisite equipment in most laboratories squeezing is a novel alternative, since it has been used on soils recently (Di Bonito, 2005) and has the potential to access water contained in small pores soil suspension or saturation extracts constitute a valid alternative, especially when batch experiments are carried out (Degryse et al., 2003). Furthermore, these methods are capable to perform fractionated extraction on the soil, whereby a combination of the methods can be used to provide soil water originating from a wider range of pores, which can present a variety of interactions with the soil matrix and possibly different chemistry. 

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