Recycling of sodium polytungstate used in soil organic matter studies

Abstract

Studies of soil organic matter (SOM) dynamics rely heavily on physical fractionation of the soil (Christensen, 1992). Most physical fractionation procedures involve some degree of soil dispersion followed by the separation of fractions on the basis of size or density. A variety of organic and inorganic liquids have been employed for density separations. In recent years, sodium polytungstate (SPT) has become a preferred density agent. We describe here a method developed for cleaning and recycling SPT used in density fractionations of soils. The method involves percolating dilute SPT solutions through a column ®lled with activated carbon to remove organic contaminants and with cation exchange resin to return the solution to the sodium form before insoluble precipitates can form. Density separation is often used to isolate light and heavy fractions (Greenland and Ford, 1964; Barrios et al., 1996; Wander and Traina, 1996). Furthermore, because soil structure plays a major role in the turnover and stabilization of SOM, the physical location of SOM within the soil matrix is an important consideration (Stevenson and Elliott, 1989; Carter and Gregorich, 1996), and this has led to increased use of density fractionations. For example, recent studies have di€erentiated interand intraaggregate particulate organic matter by density separation (Besnard et al., 1996; Jastrow, 1996). Sodium polytungstate has several advantages over other high-density liquids. It is less toxic than organic liquids or solutions of ZnBr2 or Nal, has a lower viscosity at high concentrations than other inorganic solutions, and can be used to produce a wide range of densities (1.0±3.1 g cmy3). Although SPT was more inhibitory than LudoX (a colloidal Si suspension) in follow-up mineralization studies of isolated density fractions (Magid et al., 1996), Ludox cannot be used to achieve the higher densities required in some studies (e.g. Cambardella and Elliott, 1994; Golchin et al., 1994). The maximum density achievable with Ludox is 1.4 g cmy3 (Meijboom et al., 1995). Although more expensive than other agents, SPT solutions are reusable. Simple recycling procedures are used by geologists and paleontologists (Savage, 1988), but these procedures must be modi®ed for SOM studies because small amounts of carbon may be exchanged between the soil and SPT. This carbon should be removed from the SPT solution before reuse, especially if stable isotope measurements on isolated fractions are needed. Furthermore, SPT as supplied can be contaminated with carbon (we measured 1.22 2 0.03 mg C gy1 SPT) and, thus, may require cleaning before use in SOM studies. In addition, because the Na ion disassociates with polytungstate in aqueous solution during the fractionation procedure, other cations present in the soil (especially Ca) can associate with polytungstate and form insoluble species that precipitate when the SPT solution is recycled and concentrated (M. Swimmer, Geoliquids, Prospect Heights, IL, personal communication). Our recycling procedure uses a column (Fig. 1) ®lled with activated carbon (Darco S-51, 4±12 mesh; Norit Americas, Atlanta, GA) and cation exchange resin (benzene diethenyl polymer, H-form; Sybron Chemicals, Birmingham, NJ 08011). Before using the column, we rinse it with 2 l of deionized water to Soil Biology and Biochemistry 31 (1999) 1193±1196