Influence of critical test conditions on the results of pH-dependent leaching tests (NT TR 466)

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  • Report #: NT TR 466
  • Approved: October 2000
  • Author(s): Nikolaj K. J. Lehmann, Jette Bjerre Hansen, Margareta Wahlström, Ann-Marie Fällman, Ole Hjelmar
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The purpose of the project has been to investigate the influence of some potentially critical test conditions on the results of pH-dependent tests. Two types of pH-dependent tests, the pH static test and the ANC-test were considered.

Based on the experiences and results obtained in this work it can be recommended that acid/base properties of a certain material should be examined through titrations performed very slowly. Results of acid/base titrations of selected waste products and polluted soil indicated that the critical test conditions, particle size, stirring, partly US ratio (for polluted soil) affected the acid neutralisation capacity. ANC-tests were performed on a MSWI bottom ash. The results of the tests have not totally clarified whether or not particle size has a significant influence on leaching of specific elements in the ANC test.

However, the results indicate that the particle size may affect the leaching of some of the specific elements at low pH values. At pH-values below pH 7 or 8 the concentration in the eluates of both some major and some minor elements (Ca, Mg, Cd, Cu and Mn) were highest when test material with a particle size of < 0.125 mm was used. The opposite relation was observed for As at pH values above 11 showing highest concentrations in eluates from batches with < 1 mm test material. In general the presence of head space did not seem to have a major influence on the leaching of specific elements. All together the results of the ANC-tests might indicate that a more robust test is obtained using test material with particle sizes < 0.125 mm.

pH-static tests were performed on two fly ashes and the results showed that differences in stirring rates and the size of the magnet used for stirring did not affect the leaching of specific elements significantly. However, too vigorous stirring (>400-500 rpm) should be avoided. The contact time between the test material and the leachant should be longer than 24 hours. At that point the leaching of the elements investigated was not affected except for sulphate. The leaching of sulphate was dramatically decreased with a contact time of 7 days.


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