Environmental assessment of bottom ash pre-treated with zero valent iron
Abstract: Bottom ash has similar properties as crushed rocks and gravel, and could replace some of the 40 million tonnesof virgin material used for road constructions each year.However, results presented in the literature indicate that the leaching of e.g. Cr, Cu, Mo, Pb and Zn can cause a threat to the sur rounding environment if the material is used as it is. A common pre-treatment method is carbonation, which will reduce the pH and thereby decrease the leaching of several metals. This treatment is however not always enough, so alternative methods are needed. One possibility could be to increase the number of sorption sites for the metals. The importance of iron oxides as sorption sits for metals is known from both mineralogical studies of bottom ash as well as from the remediation of contaminated soil, where iron is used as an amendment. Zero valent iron (Fe 0) was therefore added prior to accelerated aging in order to increase the number of adsorption sites for metals and thereby improving the leaching quality. The performed leaching tests showed that the addition of Fe 0 prior to accelerated aging improved the quality of the leachate compared with untreated bottom ash. T here was also a significant de crease of Cu, Cr, Mo and Zn from bottom ash treated with Fe0 prior to accelerated aging com pared with bottom ash submitted to only accelerated aging. In order to make an environmental assessment of the bottom ash pretreated with Fe 0 prior to accelerated aging geochemical modeling was performed using different pH and redox potentials in order to simulate variations in the environment. The results in dicated that the leaching of Cr, Cr, Mo and Pb would not cause harm to the environment. Zn, however, was affected by changes in pH and leached in harmful aounts at pH values velow 6 and above 10. There are reasons to question the results from the geochemical modelling since the results from pH-stat tests showed that several elements leached at potentially harmful levels at several of the tested pH. To fully evaluate the effect of addition of Fe0 should the mineralogy of the pretreated bottom ash be evaluated further, in order to see what forms iron oxides are pre sent and if other metals are associated with them. However, in order to improve the quality of bottom ash, focus should be directed torwards what type of wastes that are incinerated and on the incineration process.
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