Degradation characterisation of various geo-chemical matrices during leaching

[摘要] ENGLISH SUMMARY: Immobilisation is currently a well-known and cost-effective method in which solid waste canbe treated to render hazardous elements less harmful to the environment, by converting thecontaminants into their least soluble, mobile or toxic form. The stabilising interactionbetween the wastes and the solidifying reagents may be either chemical or mechanical. In thelatter case, the elements are encapsulated into a monolithic solid. This would evidentlyrequire a material of high structural integrity, able to withstand long-term exposure to waterinfiltration in disposal sites conditions, while continuing to limit the leaching of contaminantsinto ground or surface waters.Laboratory scale leaching tests are generally used for testing the stability of such materials.Characterisation of these tests is of major importance in clarifying the leaching phenomena,for valid extrapolations of leaching behaviour in a few years' time (ageing). The impetus ofthis study was on characterising the degradation behaviour of immobilised matrices duringleaching tests.Two specific waste forms (fly ash and jarosite) were stabilised and additives were added toproduce two geo-chemical matrix types: (a) a pozzolanic and (b) a geopolymeric matrix. A5% acetic acid solution was used during batch leaching tests, where the pH was held constantthroughout the tests.Regarding crack patterns, the physical degradation appeared somewhat different for the twomatrix types. Larger cracks were found in some pozzolanic samples, and the outer layers (ofabout 5mm) of some samples, appeared detached from the rest of the blocks after leaching.The geopolymeric matrix exhibited smaller cracks and the layers rather flaked off, comparedto the larger layers that cracked off the pozzolanic samples. The main degradationmechanisms emerged to be acid attack and the alkali silica reaction for both matrices. Theleaching of the main matrix elements was further considered as an indication of thedegradation performance. These elements were primarily Si, AI, Fe, Na and Ca. Typicallyduring leaching tests the behaviour is governed by an initial fast surface reaction, followed bymuch slower diffusion and a slow mobilising chemical reaction and/or corrosion or structuralbreakdown of the waste matrix. However, a substantial percentage of the elements thatleached into the leaching solution were precipitated. A simple semi-empirical model, thatconsidered the effect of degradation, was used to characterise these typical reactions thatgovern the leaching behaviour of each element.In both matrix types, Ca and Na (and some of the other minor elements) showed a clearreaction front in the leaching phenomenon (difference in leaching rate from the deeper layersof the samples than from surface samples). This is contradictory to the leaching of Si, Fe, Aland Ti. The leaching behaviour of Ca, Na, Mn, Mg and P were very closely related, while inturn, Si, Fe, Al and Ti showed similar behaviour. These similarities in performance indicatethat they might be bound into the matrix structure in the same way. The percentages that wereleached out after 600 hours were in the same order of magnitude for both matrices, but Si, AI,Ca, and Fe were slightly more stable in the geopolymeric matrix, while Na and Mg weremore stable in the pozzolanic matrix. Overall, the geopolymeric matrix seemed to be morestructurally stable at extended leaching times.

[发布机构] Stellenbosch University