Stabilization of antimony in MSWI bottom ash

Abstract

Sb leaching from MSWI bottom ash (BA) often exceeds the Dutch standard for unrestricted reuse of secondary materials. However, only very little is known about the leaching mechanisms of Sb compared to other oxyanion forming metals e.g. Cr, As. Recent studies suggest that the mobility of Sb in inorganic matrices such as MSWI BA is mainly controlled by (a) the availability of calcium to form calciumantimonate structures called romeites(b) adsorption of Sb(OH)3 and Sb(OH)6- to Fe-(hydr)oxides(c) incorporation of Sb(OH)6- in ettringite structures and (d) formation and precipitation of tripuhyite (FeSbO4). To lower the Sb leaching from MSWI BA and to determine the main Sb stabilization mechanism(s), 2.5% CaO, 5% CaCl2, 5% CaCO3, 2.5% Fe2(SO4)3 and 1% FeCl3 were added to the sand fraction (0,6-2 mm) of mixed MSWI BA obtained from Afvalzorg/Ballast Nedam. Samples of each combination were submitted to a one step leaching test (L/S=10, 24h, DIN 38414-S4). All additives decreased the Sb leaching and their influence is not only related to the change in pH of the matrix. The highest decrease in Sb leaching was obtained by adding 2.5% CaO, but because of the increase in pH from 11.4 to 12.5, the leaching of Pb increased from 0.11 to 22.7 mg/kg (Dutch limit value = 2.3 mg/kg). However, when only 0.5% CaO was added, the decrease in Sb leaching was comparable to the decrease obtained with the other additives, whereas also the Pb leaching remained below the limit value. When 2.5% CaO or 5% CaCl2 were added in combination with 2.5% Fe2(SO4)3 or 1% FeCl3 their effects were additional, indicating that Ca and Fe containing additives involve different Sb binding mechanisms i.e. most likely formation of less soluble Ca rich romeites and adsorption to Fe-(hydr)oxides or formation of tripuhyite. So, addition of Ca and/or Fe containing compounds e.g. by means of a complementary material containing these compounds in the right concentrations to MSWI BA can decrease the Sb leaching below the Dutch limit of 0.32 mg/kg, without increasing the leaching of other elements above the limit values.status: publishe