pH, EC and Eh Measurement
For solids, a paste is typically mixed at a 2.5(water):1(solids) ratio, equilibrated, and pH and/or Eh determined in the supernatent.layer.
Particle size distribution of a soil or sediment material is determined using sedimentation rate of particles in water as described by Stokes’ Law. Materials are disaggregated and suspended in water, and suspension densities or particle concentrations measured over time of settling to obtain a particle size distribution. Typically sand (0.05-2 mm), silt (0.002-0.05 mm) and clay (<0.002 mm) sizes are determined, although other size limits may be chosen. Sand may be further sub-divided by mechanical sieving into coarse and fine fractions. Clay or colloidal content is an important property affecting many other physical and chemical characteristics of a soil material.
Cation exchange capacity (CEC) refers to the capacity (in meq or cmol of cation charge) of a mass of soil material to retain cations on charged surfaces in the colloid fraction of the soil. It describes the number of negatively charged sites on a soil material. This is typically determined by displacing adsorbed cations (largely limited to calcium, magnesium, sodium, potassium and aluminum) with a neutral salt such as barium dichloride and measuring the displaced cations using atomic absorption or ICP spectroscopy (the “sum of cations” method). Alternatively, a cation such as ammonium may be used to fully saturate the exchange sites of a sample, then displaced and measured as an estimate of CEC. Anion exchange capacity (AEC, or the number of positive sites) can also be measured in this method by determining retention of an associated anion such as nitrate or chloride. Anion concentrations are determined by ion chromatography or colorimetry. CEC and AEC are important surface chemical properties related to nutrient status and contaminant behavior in soil/sediment systems.
Surface area of a soil or geologic material is closely related to both particle size distribution and mineralogy. It represents the amount of potential reactive surface of soil that may be in contact with pore waters and therefore reactive with contaminants. Surface areas may be measured in a variety of ways, but nitrogen gas adsorption is a standard method; this measures external surface area, excluding very fine pores within expansible clay minerals.
A Micromeretics BET surface area instrument automates the process of measuring the amount of nitrogen gas adsorbed by a sample, which is used to calculate surface area in m2 per g of solid, assuming a monolayer coverage of the surface with adsorbed gas. Adjusting mass of adsorbent allows determination of areas over the range of 0.05 to 500m2/g.
Identification of the minerals present in a geologic, soil or sediment sample may be important in understanding the physical and chemical behavior of that material in a wide range of settings. Mineralogy of the clay colloid (<0.002 mm) fraction is of most importance, since minerals in this size fraction possess most of the charge and chemical reactivity in a sample from a nutrient or contaminant point of view. Organic colloids are determined by measurement of organic carbon in the solid. Inorganic colloids can be identified and semi-quantified using X-ray diffraction analysis. In this analysis, the <0.002 mm fraction is separated using sedimentation and mounted on glass slides after saturation with different exchangeable cations. The slides are placed in a diffractometer, which impinges x-rays on the sample over a range of angles of incident radiation. Diffraction patterns obtained can be used to identify phyllosilicate (clay) minerals such as kaolinite and vermiculite present in the sample and estimate their abundance.