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Factors affecting on leaching of lead from municipal solid waste incinerator residues
( Takaaki Soga ),( In-hee Hwang ),( Toshihiko Matsuto ),( Takayuki Matsuo ),( Yasumasa Tojo ) 한국폐기물자원순환학회(구 한국폐기물학회) 2019 한국폐기물자원순환학회 심포지움 Vol.2019 No.1
In Japan, about 80% of municipal waste is incinerated for volume reduction. Municipal solid waste incinerator (MSWI) residues contain heavy metals such as lead. Fly ash (FA) is classified as a hazardous waste which must be stabilized by treating chelating agent etc. prior to final disposal. On the other hand, bottom ash (BA) is classified to non-hazardous waste which is disposed in landfills directly. According to previous literature data, BA often release high concentration of Pb above regulatory criteria (0.3 mg/L) in Japanese leaching test No.13 (JLT-13). The aim of this research is to investigate those factors affecting on leaching of Pb from BA. BAs were obtained from twenty stoker type incinerators and two vertical type incinerators respectively. Five BAs were sampled before quenching in stocker type incinerators and the others are wet BAs sampled after quenching. JLT-13 and composition analysis were conducted for BA. JLT-13 was performed as follows: 45 g non-pretreated BA was mixed with 450ml distilled water (L/S ratio=10) in a glass flask, which was capped and shaken horizontally at 200 rpm for 6 h. The leachate was filtered through a 1μm membrane filter and filtrate was provided for measuring pH, metal and heavy metal concentrations, anion (Cl- and SO<sub>4</sub> <sup>2-</sup>) concentration, and TOC and IC concentrations. Before composition analysis, BA was pretreated as follows; BA was dried 60℃ for 2 days. Dried sample was pulverized for 3h in a ball mill and sieved through a 2 mm mesh. Pulverized BA was provided for measuring the contents of metal and heavy metals, ignition loss, TOC, and IC. In addition, the contents of calcium compounds (CaO, CaCO<sub>3</sub>, Ca(OH)<sub>2</sub>, etc.) and Fe<sub>2</sub>O<sub>3</sub> were analyzed by XRD. Figure 1 shows the relationship between Pb concentration and pH in filtrate. As Pb is amphoteric metal, it leaches in higher pH. Some filtrates show high Pb concentrations > 0.3mg/L, which are obtained from BAs before quenching. These contains more CaO compared to quenched BA. Figure 2 shows a positive correlation between pH and CaO content. Thus, CaO content of BA seems to make a difference in Pb concentration. Other factors such as IC, ion strength, etc. does not show obvious correlations with Pb concentration in filtrate. As already mentioned, BA before quenching showed higher Pb concentrations. To investigate the effect of quenching on Pb leaching concentration, lab-scale of quenching experiment was carried out using BA sampled before quenching process. BA was soaked in distilled water for 2.5 h and was dehydrated by gravity dewatering. Dehydrated sample was left for five days which was provided for JLT-13 and XRD analysis. Fig. 3 shows the variations of pH and Pb leaching concentration versus elapsed time. Pb leaching concentration gradually decreases with elapsed time. Ca concentration and pH decreased too. Figure 4 shows XRD peak of BA before and after quenching. High CaO peak exits in BA before quenching but it disappears after quenching and 5 days of exposure to the atmosphere. On the other hand, the peaks of Ca(OH)<sub>2</sub> and CaCO<sub>3</sub> appeared. Ca(OH)<sub>2</sub> is likely to be formed by hydration reaction of CaO in the procedure of quenching. CaCO<sub>3</sub> seems to be generated by the carbonation reaction of Ca(OH)<sub>2</sub> during exposure to the atmosphere. Those reactions lead to decrease of CaO which might bring about lower Pb release in JLT-13.
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