The Use of Leaching Tests to Study the Potential Mobilization of Heavy Metals from Soils and Sediments: A Comparison

Abstract

In the present study, different leaching tests were applied on well-characterised samples in order to obtain information on the potential mobility of heavy metals and arsenic. The information deduced from the different methods was compared and evaluated. Besides the comparison of heavy metal release in cascade-, column-and pH stat leaching tests, attention was also paid to the assessment of release kinetics during leaching tests and to the mathematical modelling of leaching behaviour. The aim of this study was to understand the origin of possible discrepancies between the results of different leaching tests. The compatibility of the results of different leaching tests is, besides the inherent differences between methods (single batch tests versus dynamic leaching tests, the duration of the tests, liquid/solid (L/S) ratio,…) to a major extent determined by keyfactors such as pH and redox potential. Depending on soil and sediment properties (e.g. acid neutralizing capacity (ANC)) these 'key-factors' varied during and at the end of extractions and leaching tests, even when the initial test conditions (e.g. the pH of the reagent) were equal for all test cases. During cascade-and column leaching tests, pH (which is initially 4) will mostly increase, but the extent of this pH-increase mainly depends on the acid neutralizing capacity of the sample. Therefore, measuring the pH of all leachates that are collected during these tests is mandatory for the interpretation of the results. Moreover, the monitoring of other variables such as DOC, anions and major elements can give indications on the reactions that are responsible for the release of elements (e.g. the dissolution of organic matter) and greatly improve the interpretation of the results.

Characterizing As, Cu, Fe and U Solubilization by Natural Waters

“…The risk-based leaching level represents the theoretical concentration of a contaminant in a waste (mg/kg) that results in a leachate concentration that exceeds the applicable groundwater standard for that contaminant (Cappuyns andSwennen 2008, Townsend et al 2003). As discussed above a major problem of the procedure is the use of synthetic leaching solution.…”

“…Typically, a leaching test involves the preparation of waste samples (e.g., particle size reduction) and leaching solution, the mixing of the samples with the solution, the filtration of the mixture, and the analysis of the extracts. Different batch leaching tests have been commonly used in various countries for regulatory purposes (Cappuyns andSwennen 2008, van der Sloot et al 1997). All these tests use synthetic solutions (including dionised water) as illustrated below for three European countries.…”

“…All these tests use synthetic solutions (including dionised water) as illustrated below for three European countries. Alternative leaching methods are: (i) tank tests, (ii) column tests, (iii) field tests (Cappuyns andSwennen 2008, Townsend et al 2003).…”

“…Therefore, a number of potential shortcomings of the leaching tests have been enumerated: for example (i) the arbitrary L/S ratio may not be representative of the actual field conditions; (ii) the role of kine tics is minimized as the test is performed for a standard duration; (iii) the pH of leaching fluid does not necessarily represent the pH of the leaching environment. The pH of the leachate during the test is highly dependent on the buffering capacity of the waste materials, which may lead to inaccurate determination of waste behavior in the environment (Cappuyns andSwennen 2008, Townsend et al 2003).…”

Mobile forms and migration ability of Cu, Pb and Zn in forestry system in Poland

“…This result demonstrates that the soil pH is important parameter that influenced the toxicity in Pbcontaminated soils. Generally, heavy metals display the highest mobility at low pH values, although Cappuyns and Swennen (2008) studied different leaching tests were applied on soil samples in order to obtain information on the potential mobility of heavy metals. For example, Zn is only released at a pH below 6 from soil to soil solution.…”