The effect of isosaccharinic acid (ISA) on the mobilization of metals in municipal solid waste incineration (MSWI) dry scrubber residue

Abstract

Co-landfilling of incineration ash and cellulose might facilitate the alkaline degradation of cellulose. A major degradation product is isosaccharinic acid (ISA), a complexing agent for metals. The impact of ISA on the mobility of Pb, Zn, Cr, Cu and Cd from a municipal solid waste incineration dry scrubber residue was studied at laboratory using a reduced 2(5-1) factorial design. Factors investigated were the amount of calcium isosaccharinate (Ca(ISA)(2)), L/S ratio, temperature, contact time and type of atmosphere (N(2), air, O(2)). The effects of pH and Ca(ISA)(2) as well as other factors on the leaching of metals were quantified and modelled using multiple linear regression (alpha=0.05). Cd was excluded from the study since the concentrations were below the detection limit. The presence of Ca(ISA)(2) resulted in a higher leaching of Cu indicating complex formation. Ca(ISA)(2) alone had no effect on the leaching of Pb, Zn and Cr. A secondary effect on the mobilization was predicted to occur since Ca(ISA)(2) had a positive effect on the pH and the leaching of Pb, Zn and Cr increased with increasing pH. The leaching of Pb varied from 24 up to 66 wt.% of the total Pb amount (1.74+/-0.02 g(kgTS)(-1)) in the dry scrubber residue. The corresponding interval for Zn (7.29+/-0.07 g(kgTS)(-1)) and Cu (0.50+/-0.02 g(kgTS)(-1)) were 0.5-14 wt.% of Zn and 0.8-70wt.% of Cu. Maximum leaching of Cr (0.23+/-0.03 g(kgTS)(-1)) was 4.0 wt.%. At conditions similar to a compacted and covered landfill (4 degrees C, 7 days, 0 vol.% O(2)) the presence of ISA can increase the leaching of Cu from 2 to 46 wt.% if the amount of cellulose-based waste increases 20 times, from the ratio 1:100 to 1:5. As well, the leaching of Pb, Zn, and Cr can increase from 32 to 54 wt.% (Pb), 0.8-8.0 wt.% (Zn), and 0.5 to 4.0 wt.% (Cr) depending on the amount of cellulose and L/S ratio and pH value. Therefore, a risk (alpha=0.05) exists that higher amounts of metals are leached from landfills where cellulose-containing waste and ash are co-disposed. This corresponds to an additional 29 t of Pb and 17 t of Cu leached annually from a compacted and covered landfill in the north of Sweden.