It is quite clear that you need both when investigating solid waste: i.e. you have to determine total metal concentration and bonding forms. Determination of bonding forms is most important because it is basis for the assessment of the hazard potential of metals under the aspect of waste disposal or when sludge is used for agricultural purposes. Especially when the use of waste implies changing environmental conditions the determination of bonding form portions is crucial. It is indispensible for hazard assessments based on a thermodynamic approach because for this you need speciation. Under these aspects you cannot rely on total metal concentration.

However, element speciation procedures are generally very painstaking and require a high accuracy and diligence. In addition, the highly concentrated extraction solutions may yield erroneous analytical results due to matrix effects. So the total element concentration is not only a first evidence of the possible hazardous character of wastes. Total element concentrations are also the control for the quality of the Sequential Chemical Extraction procedure; the results – total and sum of sequential - have to fit each other.

Dieter's statement if correct, both total metal concentration and some form of metal speciation is essential for accurate risk assessment. The environmental and health risk of a particular element will not only be related to the total concentration, but also to the form of the chemical and its availability. Elements that are bound in the environment in a non-accessible form, ie one that is not soluble, either at environmental conditions or at biologically relevant conditions, are often considered to be less of a risk to life and health than the same elements that are easily extractable.

One way to look at it is in terms of the health hazard posed by different oxidation states or chemical forms of the metal. When you compare arsenate to arsenite to arsenobetaine, these chemical species have different toxicities and risks associated with them. The same with hexavalent chromium (a carcinogen) and trivalent chromium (a trace nutrient). Chemical speciation experiments that involve x-ray spectroscopy or chromatographic separations are the only way to accurately qualify and quantify these types of species.

Further, if an element is bound in the matrix of a mineral and not able to be dissolved at environmental conditions or at biologically relevant conditions that a sample would experience, say upon ingestion, then the element has a lower potential to be harmful to organisms. This is essentially what a sequential extraction will tell you, by exposing a sample to increasingly harsh extractions, you can determine what fraction of each element is soluble at neutral conditions, then in the presence of a chelator, then at acidic conditions, then at very acidic conditions. Many organizations are in the process of recognizing the importance of these distinctions and are in the process of integrating it into their procedures for risk assessment (see for example EPA method 9200, which uses a gastric juice extraciton to determine the portion of lead and arsenic in environmental samples that are extractable upon ingestion of the samples). Digestion methods like EPA SW-846 chapter 3051 will provide total concentrations (or quasi-total, more often than not), which is important information, but it will not provide the information regarding metal speciation and bioavailability that will allow an accurate risk assessment.

I hope this is helpful!

Total metal concentration and it's ability to form compounds with other substances are both important factors. However, the leachability potential (ability for the compound/metal to move through the waste in this case, or soil in the case of contaminated land sites) can be the most important factor. In the case of landfills something that has a high leachability potential could damage the lining of the Landfill if accepted onto the site. Heavy metals, as you may know, can bioaccumulate or even biomagnify within a food chain.

Metals exist naturally in the environment, in various forms, and have been increasing in abundance due to anthropogenic inputs throughout the years. The total concentration of a metal in soils or sediments can be of great assistance for many geochemical applications but it is a rather poor indicator of its bioavailability, mobility and toxicity. Consequently, total concentrations of metals should not be used for the assessment of their environmental impact, without taking into deep consideration the knowledge of the available fraction of a metal in the sediment. The metal properties depend, therefore, not only on their total concentration but also on the physicochemical form they appear, a phenomenon characterized by the term ‘‘speciation’’.

Speciation, as defined by Tack and Verloo (1995), is “the identification and quantification of the different, defined species, forms or phases in which an element occurs” and is essentially a function of the mineralogy and chemistry of the soil sample. Elemental speciation can be determined through sequential extraction and subsequent analysis of soluble elements using inductively coupled plasma (ICP). Several leaching tests and sequential extraction procedures (SEPs) have been designed and adopted with the aim of deducing information concerning the mobility and bioavailability of trace elements in soils and sediments. SEPs employ different extraction steps and can provide information on origin, mode of occurrence, biological and physiochemical availability, mobilization, and transport of metals in sediments. All the aforementioned reasons make the procedures valuable tools both in contaminant analysis as well as for the evaluation of potential disposal options and the reuse of dredged sediments.

I hope this was helpful. If you would like more information regarding SEPs, pollution indicators, Tessier's protocol, etc. you may watch online my presentation in:

http://www.blod.gr/lectures/Pages/viewlecture.aspx?LectureID=958

I agree that both free and adsorbed metal concentrations should be determined for adequate environmental risk analysis. However, in determining the leaching variability of the metals, batching equilibrium and diffusion test could be used succesfully. Meanwhile in diffusion test, direct leachate should squeezed from the sediment for analysis as this will give the best results rather than using digestion method.

Both