Leaching test comparison for solidified and stabilized contaminated sediments : Assessment of selected inorganic contaminants

Abstract

Contaminated sediments in Norway represent a significant problem for the public’s health as well as the environment, especially in urban coastal regions where former harbour areas are converted to residential zones requiring more strict environmental standards. As a result, environmentally motivated remediation efforts led by the Norwegian authorities and supported by the public and independent institutions have become increasingly important to solve the problem. The method of Solidification and Stabilization (S/S) of contaminated sediments is one of the alternatives proposed for mitigation in such situations. This thesis is part of a larger research project with respect to S/S of contaminated sediments, where the following participants are involved: Norcem, The Norwegian Geotechnical Institute (NGI), Det Norske Veritas (DNV), Rambøll, Skanska Norway, and NOAH. The larger project is financed by the Research Council of Norway (NRC) and this thesis is worked under the supervision of the Norwegian Geotechnical Institute (NGI). Leaching test is one important aspect in the environmental assessment of the remedial measures of solidified and stabilized (S/S) contaminated sediments. Selection of an appropriate test or combination of tests is of vital importance for the proper judgment of results, as well as for predicting the long term release of S/S contaminants into the environment. The experimental program reported here consists of six different leaching tests carried out on the S/S Trondheim contaminated sediments to assess the leaching of four inorganic pollutants Cr, Cu, Ni and Zn. The main objectives of this investigation were to study how the performance of different leaching tests affects the release of inorganic contaminants and to find the most suitable leaching test(s) for predicting contaminants release from the S/S materials. These leaching tests performed are as follows: • Static diffusion test (tank test), • Dynamic diffusion test (modified tank test), • Batch leaching test for crushed material, • Up-flow percolation test (column test), • Batch leaching test for monolithic samples, and • Leaching test of monoliths with magnetic agitation,

The diffusion tests (static and dynamic) were performed in accordance with the norm EA NEN 7375:2004 protocol, for leaching of inorganic components. The difference between the two diffusion tests lied in the way the S/S samples are exposed to the leachant. In the dynamic diffusion test the leachant was constantly circulated in the cell throughout the course of the experiment using a peristaltic pump whereas in the static test, the leachant was kept stationary. In both tests the leachant was intermittently renewed at specific intervals determined by the norm designed to maintain a significant diffusive driving force. The batch leaching test for crushed material was performed in accordance to the European Standard EN 12457-2, 2002 at liquid solid (L/S) ratio of 10. The up-flow percolation test was performed in accordance to the European committee for standardization CEN/TS 14405, 2004. However, some modifications were made to the test setup given the low permeability and monolithic nature of the sample. The batch leaching test for monolithic samples was an experimental test designed specifically for the purpose of this thesis. It is a hybrid test made from the batch and diffusion tests. The objective was to assess the possibility of finding an alternative type of test that is more suitable for testing S/S materials than the batch test while is not as demanding as the diffusion test with respect to the test duration. An L/S ratio of 10 and test duration of 24 hours were used to perform this hybrid leaching test for monolithic samples. The experiments for this type of test were not successful since much of the monoliths were largely damaged during agitation in the end-over-end shaker. In such circumstance, interpretation of the data is a rather futile exercise since the tests were deemed not reproducible. As such, a new leaching test was designed with the same objective with the consideration that the monolithic structure is preserved (i.e. not disintegrated). This test was called leaching test of monoliths with magnetic agitation. The magnetic agitation test was performed at three different times: 24hrs, 48 hrs and 96 hrs. Sea water was used as the leachant in all of the tests performed. This selection was made based on the fact that S/S contaminated sediments under study are considered to be exposed to this type of leachant in field conditions. The results indicated that the total composition of the studied metals (Cu, Cr, Ni and Zn) in the S/S contaminated sediments is mostly irrelevant for their leaching prediction. Further, different types of tests give different contaminants release estimations. Amongst the leaching tests performed, the diffusion test was considered to be the most suitable test for determining the long-term leaching behaviour of the S/S contaminated sediments.