The study aimed at evaluating the humic substances (HSs) content from municipal solid waste incinerator (MSWI) bottom ash and its variation with time and the effect of temperature on HSs formation. The process suggested by IHSS was applied to extract HSs from two different bottom ash samples, and the extracted efficiency with NaOH and Na4P2O7 was compared. MSWI bottom ash samples were incubated at
Incineration process displays an important role in the municipal solid waste (MSW) management in Japan. In the recent years, approximately 78% by weight of MSW is incinerated. Incineration is a multipurpose strategy aiming at fighting pollution of environment, energy saving, maximizing benefits of waste, mineralizing and stabilizing waste, and reducing the volume of waste. However, in practice, organic matters in MSW are not completely oxidized by incineration. A few percentage of partially unburned organic waste materials still exist with subsequent formation of new organic compounds being left behind forming the bottom ash [
Organic matter in MSWI bottom ash is divided into humic substances (HSs, mainly composed of humic acid and fulvic acid) and nonhumic substances (NHS), based on the soil organic matter classification. HSs widely exist in the natural environment and have a high affinity for binding heavy metals and organic pollutants. Humic acid and fulvic acid have been identified as important DOC subfractions contributing to the complexation of contaminants [
This work compared the efficiency of NaOH and Na4P2O7 reagents on the extraction HSs from MSWI bottom ash and the effect of temperature on the formation of humic acid from the incineration of the MSW bottom ash. A lab-scale incubation facility was used to help exploration of the humic acid formation mechanism.
The fresh quenched MSWI bottom ashes were taken from O-incineration plant and R-incineration plant, named as O-BA and R-BA. O-incineration plant was operated on a semicontinuous type and incinerated at 850°C with a daily treatment capacity of 130 t/d (65 t/16 h × 2). R-incineration plant applied stoker furnace operated on a continuous type, with an incineration capacity of 900 t/d (300 t × 3) and the temperature in the combustion chamber was 900°C. The two bottom ash samples and a crushed incombustibles sample were passed through 4.75 mm sieve.
Organic matter in MSWI bottom ash contains humic and fulvic acids together with other nonhumin substances. Humic acid and fulvic acid were separated from solid samples applying the method of international humic substances society (IHSS). Figure
Schematic diagram of fractionation of humic substances and nonhumic substances.
Distilled water was added to the sample in the container to maintain L/S ratio < 10. Aqueous fluid is then acidified using HCl to a pH value of 1-2. Adjust the L/S ratio to 10, and the container was closed for shaking for 1 hour. The container was then centrifuged followed by filtration using filter paper to attain residue 1 and extract 1. This step is helpful to remove part of carbonate in MSWI bottom ash.
Neutralize the residue 1 using NaOH to pH 7 followed by dilution with 0.1 M NaOH under an atmosphere of N2 to attain L/S of 10. Shake the solution under atmosphere of N2 for 4 hours. Supernatant was separated from the precipitate by centrifugation and filtration. The supernatant was acidified to pH 1 with HCl and then centrifuged to obtain extract 2 and residue 2.
In this step, Na4P2O7 or mixture of NaOH/Na4P2O7 was also applied instead of NaOH.
Dissolve the residue 2 using 0.1 M KOH under N2 to obtain humic acid fraction. Add 0.3 M KCl solution to humic acid fraction to reach L/S 10. Aqueous fluid was separated from the solid resides by centrifugation and filtration to obtain solid part and liquid part. Waste the solid part and liquid part was acidified to pH 1-2 using HCl and then centrifuged. Solid part was separated by centrifugation. Repeat the process previously mentioned at least 2 times.
Sequentially, dissolve the solid part using 0.1 M HCl/0.3 M hydrofluoric acid (HF) solution in a plastic container. Shake the solution for 24 hours. Solid part was separated from the liquid part by centrifugation and filtration. Repeat the process previously mentioned 3 times. Transfer the solid part to a dialysis tube by slurring with water and dialyze against distilled water until the dialysis water gives a negative Cl− test with the AgNO3. At last, the solid part from dialysis tube was freeze-dried to obtain the solid humic acid sample.
NHS was recovered from the solutions obtained from Steps
Elute the column with 0.1 M NaOH. Pass eluate through H+-saturated cation exchange resin (AG-MP-50). Freeze-dry the eluate to recover the H+-saturated fulvic acid.
The concentrations of humic acid fraction, fulvic acid fraction, and NHS fraction in carbon were determined with a TOC analyzer (TOC-V, Shimadzu Co.).
The pretreatment R-BA was prepared for the incubation experiments. About 4 kg R-BA was filled in each stainless steel container with untight covers. The containers were put in constant temperature ovens setting at 37°C and 50°C, respectively. The incubation experiments were continued for one year under aerobic conditions. During the incubation process, the water contents of the samples were adjusted to 30% by adding distilled water every day, simultaneously mixing the samples.
Approximately 200 g samples were taken out from the containers after 2, 4, 8, 12, 18, 24, 32, 36, 44, and 52 weeks of incubation period. These samples were air-dried, grinded, and sieved through a 2 mm mesh. The pretreated samples were kept in closed plastic bags and saved under 4°C till analysis.
Carbon, hydrogen, and nitrogen contents in eluted humic, fulvic acid were determined with the help an elemental analyzer (YANAKO CHN coder MT-50, Yanagimoto Co.).
The amounts of HSs extracted from O-BA and R-BA by three extraction reagents are shown in Figure
Amount of humic substances extracted from R-BA and O-BA by HCl, NaOH, and Na4P2O7 in sequence.
IHSS has been suggested that the material should be dissolved in dilute HCl before the alkaline extractant is applied, which is helpful to remove Ca and other polyvalent cations and increases the efficiency of extraction of organic matter with alkaline reagents. The previous study has indicated that the main minerals in MSWI bottom ash contain Ca and other polyvalent cations (such as Fe and Al) [
According to the experiment results, it could be expected that R-BA contain less free complexed organic matter and considerable amount of complexed organic matter. For the purpose of investigating the HSs content in MSWI bottom ash, here it is suggested to combine NaOH and Na4P2O7 to extract HSs from MSWI bottom ash, following HCl extraction.
The total HSs amounts extracted from the two bottom ash samples differed to a large extent. O-BA only contained 40.5 mg-C/kg HSs, but R-BA contained 234 mg-C/kg HSs which was almost 6 times higher than that of O-BA. This may be caused by the fact of the different composition of MSW treated in the two incineration plants. In addition, the different incinerator type and incineration temperature can also have an effect on the organic components in the MSWI bottom ash.
Figure
Humic substances and nonhumic substances contents as a function of incubation time in R-BA incubated under 37°C.
For the R-BA samples incubated under 50°C shown in Figure
Humic substances and nonhumic substances contents as a function of incubation time in R-BA incubated under 50°C.
For MSWI bottom ash incubated under 50°C, the humic acid content was significantly higher than that of the MSWI bottom ash incubated under 37°C. Reversely, the highest content value of fulvic acid appeared in the sample incubated under 37°C, showing that high temperature may be beneficial to the formation of humic acid, while low temperatures are conducive to the accumulation of fulvic acid. The total content of humic acid and fulvic acid amount was higher in the sample incubated under 50°C compared with that under 37°C and so as NHS.
This shows that temperature is an important environmental factor of HSs formation and transformation. The storage temperature of MSWI bottom ash will affect the HSs content, thereby affecting the migration of heavy metals.
The elementary compositions of humic acid and fulvic acid extracted from R-BA are shown in Table
Elementary analysis of humic substance extracted from R-BA.
Sample | C (%) | H (%) | N (%) | O (%) | C/N | |
---|---|---|---|---|---|---|
Humic acid | Fresh R-BA | 53.68 | 5.63 | 6.52 | 34.17 | 8.23 |
24th weeks incubated R-BA | 54.81 | 5.00 | 5.55 | 34.64 | 9.88 | |
Tan, 2003 [ |
53.8–58.7 | 3.2–6.2 | 2.6–5.05 | 39.7–48.8 | 12.3–17.3 | |
| ||||||
Fulvic acid | Fresh R-BA | 46.5 | 4.34 | 1.78 | 47.38 | 26.12 |
Tan, 2003 [ |
40.7–50.6 | 3.8–7.0 | 0.8–4.3 | 32.9–51.9 | 18.4–37.8 |
The study determines the HSs extraction reagent from MSWI bottom ash by comparison of NaOH and Na4P2O7. NaOH and Na4P2O7 have different extraction efficiency for the studied two MSWI bottom ashes. So 0.1 M NaOH/0.1 M Na4P2O7 was recommend to extract HSs from MSWI bottom ash.
Fresh MSWI bottom ash contains humic acid and fulvic acid. More humic acid was formed during the incubation period and accounted for 8–27% of the organic fractions in the landfilled MSWI bottom ash. Fulvic acid was contained in the fresh MSWI bottom ash, and its amount was relatively stable. The variation of HSs content in the incubated samples showed a change with incubation time. The results showed that high temperature may be beneficial to the formation of humic acid, while low temperatures are conducive to the accumulation of fulvic acid.
This work was supported by the National Natural Science Foundation of China (Grant no. 51208167).