Bachelor thesis,15 hp Bachelor in Life science, 180 hp
Autumn term 2017
Leach tests on MSWI bottom ash from CHP Dåva to reduce
Cu, Pb and Zn
Student Brodin, Marcus
Supervisor Weidemann, Eva Hedlund, Tomas
2
Abstract
The cogeneration of heat and power at the power station in Dåva produces large
quantities of municipal solid waste incinerated bottom ash. Nonetheless, this bottom ash contains hazardous inorganic pollutants such as heavy metals. This study will treat the leaching behaviour of copper, lead and zinc in the bottom ash sampled at Dåva. The samples were fractionated into three different particle sizes: >1 mm, 0,25-1 mm and
<0,25 mm and digested with water, sulphuric acid, hydrochloric acid or nitric acid. The samples were analysed with ICP-OES to attain the concentration of element per mass in bottom ash samples. The results indicated that the >1 mm fraction had higher leached ion concentration for zinc and lead. However, copper leached best from the <0,25 mm fraction. Lead was dissolved easily in water, thus could have affected the results since wet sieving could have leached lead from the bottom ash before analysis. Moreover, lead may be leached out from the bottom ash to the soil when exposed to rain.
3
Table of content
Abstract ... 2
Introduction ... 4
Leaching of metals ... 4
Aim ... 5
Theory ... 5
Results ... 9
Copper ... 10
Lead ... 12
Zinc ... 14
Discussion ... 17
Representability ... 17
Fraction distribution ... 17
Differences of pH stability ... 17
Copper ... 18
Lead ... 19
Errors ... 20
Conclusion ... 20
Future aspects ... 21
References ... 22
4
Introduction
The cogeneration heat and power (CHP) station Dåva, operated by Umeå Energi, incinerates municipal waste in order to generate energy. In this process, 20 000 ton of bottom ash (BA) is produced annually. However, an updated classification provision of hazardous waste regulations conducted by the European Union will be applied starting from July 5th 2018 which might pose a risk of affecting the classification of the bottom ash. Consequences regarding the updated provision might affect Dåva economically due to that additional transportation to dispose the BA could be relevant. The amendments that would affect Dåva are H400 and H410-413 for acute toxic substances and aquatic chronic substances, respectively with regulation limit of 25% [1]. According to Umeå Energi are the compounds of highest concern: zinc, lead and copper.
The municipal solid waste incinerated (MSWI) BA are highly heterogenic in its
composition. Inorganic compounds constitute a large part of the MSWI BA composition along with oxide minerals and other compounds generated in the oxidative environment during incineration [2][3]. The MSWI BA is however, generated in a vast variety of sizes, from fine particulate i.e. <0,25 mm, particles between 0,25mm-1mm to larger scrap fragments, 1mm or above. The ash is cooled down with water for safety reasons and later utilized as construction material.
Leaching of metals
The leaching capability of heavy metals in MSWI BA treated with rain water is fairly low.
However, a large quantity of disposed MSWI BA exposed to rain could leach to cause environmental problems. According to prior studies and the chemical properties of the heavy metals concerned, the leaching capability is enhanced at low pH [4]. More
particularly, cupric ions are likely to predominate at a pH below 4 with p{e} above 6, zinc ions at pH below 6 with p{e} above -12 and lead ions at pH below 6.5 with a p{e}
between 25 and -4. Zinc does possess the ability to form complex with ammonia which may lower the acidic and reducing conditions that is required for zinc ions to be
predominating.
Hypothetical, treating MSWI BA with hydrochloric acid, nitric acid or sulphuric acid will dissolve copper, zinc and lead ions to a greater extent [5] [6].
Analysis of trace metals in leachates is commonly performed with atomic absorption detection (AAS) or inductively couple plasma (ICP) along with a suitable spectroscopy method, in this case optical emission spectrometer (OES). The readouts from OES are presented as intensity of the emission. To be able to determine the specific
concentrations of the elements in regard, calibration curves with linear regression are fundamentally constructed to predict the concentrations from the emission out reads [7].
that’s what we call the midnight sway that’s what we call the midnight sway
2 4 6 8 10 12
-6 -5 -4 -3 -2 -1 0
Log [Cu2+]TOT
pH Cu2+
Cu(OH)3 Cu(OH)2(s)
2 4 6 8 10 12
-6 -5 -4 -3 -2 -1 0
Log [Pb2+]TOT
pH Pb2+ Pb(OH)2(s)
2 4 6 8 10 12
-6 -5 -4 -3 -2 -1 0
Log [Zn2+]TOT
pH Zn2+
Zn(OH)2 Zn(OH)3 ZnOH+
Zn(OH)2(s)
Figure 1. Predominance diagram of copper
with varying pH. Figure 2. Predominance diagram of lead with varying pH.
Figure 3. Predominance diagram of zinc with varying pH.
5 Aim
The aim of this project is to fractionate the BA and examine the outcome of different leaching tests. The tests were performed to reduce the concentration of the miscible residues of zinc-, lead- and copper in the bottom ash that are classified as hazardous waste. Thus, preventing heavy metals from leaching out to the environment.
Theory
Equation 1 is appropriate to use for calculating the water content of a material.
Eq. 1.
𝑤 % = 1 −𝑚 − 𝑚
𝑚 − 𝑚 ⋅ 100 𝑊 % = 𝑤𝑎𝑡𝑒𝑟 𝑤𝑒𝑖𝑔ℎ𝑡 𝑝𝑒𝑟𝑐𝑒𝑛𝑡
𝑚 = 𝑚𝑎𝑠𝑠 𝑑𝑟𝑦 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑚 = 𝑚𝑎𝑠𝑠 𝑤𝑒𝑡 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑚 = 𝑚𝑎𝑠𝑠 𝑒𝑚𝑝𝑡𝑦 𝑑𝑖𝑠ℎ
6 Material and Method
Collecting bottom ash samples was attempted to be conducted in representative manner. Samples were shovelled from a pile of BA at several divergent spots into a bucket. This was performed repeatedly at three different days to obtain a diversity in composition of BA waste material.
The sampled material was fractionated by wet sieving it through a 1 mm strainer and 0,25 mm mesh fabric to attain three fractions. From the three samples 4213 g, 4405 g and 4287 g of BA were wet sieved with respectively 9893 g, 8814 g and 8816 g of tap water, respectively. Assuming the density of the water being 1 g/mL make the volume of the water to be 9,89 L, 8,81 L and 8,82 L.
Table 1. The ratio and amount of sampled material and water used to wet sieve to yield fractions are displayed along with the date of sampling and fractionation.
Date Material Mass (g) Volume (L) Liquid/solid ratio
17-10-05 Sample 1 4213 9,89 2,35
17-10-09 Sample 2 4405 8,81 2,00
17-10-11 Sample 3 4287 8,82 2,06
TOTAL 12905 27,5 2,13
Table 2. The calculated water content in each fraction is displayed below after weighing 10 g of sample before and weighing the sample after.
Fraction Water content Wet mass (g) Dry mass (g)
<0,25 mm 41% 9,97 5,91
0,25-1 mm 51% 9,98 4,93
>1 mm 11% 9,96 8,83
7 The fractionated samples were collected in vessels differentiated by their fraction size.
The finest particulate fraction had a size of <0,25 mm. The larger particles were between 0,25 mm to 1 mm and the largest fraction was >1 mm.
At the initiation of the leaching tests, 10 g of the wet BA samples were put into four 50mL falcon tubes each, which gave 12 test tubes in total, as displayed in table 3.
Table 3. Volume of acids added to the fractionated samples. Total volume is 40 mL.
Fractioned samples H2O (mL)
2M HCL (mL)
2M H2SO4 (mL)
2M HNO3 (mL)
<0,25 mm 40 30 25 30
0,25 – 1 mm 40 25 10 15
>1 mm 40 15 10 5
Subsequently, the 12 falcon tubes were incubated with a rotator for first 2 hours and then put back to incubate for 4 hours in total. At these two times were 15 mL of the supernatant aliquoted to fresh 15 mL tubes after centrifugation and stored in refrigerator at 4⁰C.
The dry weight was determined by weighing 10 g of each fraction, dry in oven for 36 h at 60⁰C and weigh the dried samples afterward. The water content in the samples was calculated with eq. 1.
The samples were then prepared for ICP-OES analysis by filtering the solutions using a 20 µm syringe filter, thereafter they were acidified with three drops of 65% HNO3. Also, five standard calibration solutions containing 100ppm, 10ppm, 5ppm, 2ppm and 1ppm of Zn, Pb and Cu in 1% HNO3 were carefully prepared by weighing in the volumes. The three elements were added in the same standard solutions to mimic the matrix of the analyte and avoiding the matrix effect whilst analysing.
Table 4. Preparation of standard calibration matrix solutions for ICP-OES. Assuming the density of the solutions being 1 g/ml giving that the weight is equal to the volume. The average of x concentration is 100,16 ppm.
Elements volume from 1000 ppm stock (mL)
Final volume (mL)
x Conc.
(ppm)
Cu 0,988 10,03 98,47
Pb 1,012 10,03 100,9
Zn 1,015 10,03 101,2
Elements volume from avg x
(mL) Final volume
(mL) y Conc.
(ppm)
Cu, Zn, Pb 1,008 10.00 10,10
Cu, Zn, Pb 0,513 10,01 5,130
Cu, Zn, Pb 0,205 10,03 2,050
Cu, Zn, Pb 0,101 10,01 1,010
8 Because of some complications during the analysis were only 24 of the 48 samples run with ICP-OES. The 24 remaining samples were the replicates and since they were not analysed, two already analysed samples were run again two times in order to get three replicates of those for statistical significance and error. Atomation concentrations were determined by linear regression using the constructed calibration curves. Furthermore, the resulting concentration of the elements were used for calculation of concentrations in the dry sample, before addition of 40 mL of leaching solution. To convert the
concentrations in ppm (µmole/L), the standard atomic weight of each element found in literature was multiplied to the number of moles in the leachate. The final concentration is displayed as milligrams of free ions per gram of fractionated BA.
9
Results
Wet sieving the BA yielded mostly large particles i.e. fraction >1 mm, which is displayed in table 4. The total amount of fraction <0,25 mm was approximately 726 g, of fraction 0,25-1 mm 1500 g and of fraction >1 mm 10679 g. This corresponds to a weight
percentage of the total weight collected 6% for fraction <0,25 mm, 12% for fraction 0,25- 1 mm and 82% for fraction >1 mm.
Table 2. The yield of each fraction after wet sieving the BA, displayed in weight and weight percent.
Sample <0,25mm
(g) 0,25-
1mm (g) >1mm
(g) <0,25mm
w% 0,25-
1mm w%
>1mm w%
Sample 1 66 117 4030 2% 3% 95%
Sample 2 267,3 141,3 3996,4 6% 3% 91%
Sample 3 393 1241,7 2652,3 9% 29% 62%
Total 726,3 1500 10678,7 6% 12% 82%
After 2 hours and 4 hours incubation the samples showed major differences in pH.
In table 5 the pH before and after incubation is displayed. The largest difference was observed for fraction <0,25 mm in the HCl matrix, containing 30 mL 2M HCl and 10 mL MQ-water, which showed after 4 h a 6,1 increase in pH.
Table 3. Samples displaying the initiated pH (pHi) and final pH (pHf) with difference after time of incubation in specific acid.
The obtained results of the multi-element analysis using ICP-OES are shown in sections for each element separately, in table 7. In general, copper and zinc attained the highest concentrations whereas zinc attained the overall highest concentrations and lead showed relatively much lower concentrations according to the figures 4-12. Moreover, the difference in incubation time showed to affect the concentration in the acidic matrices.
Sample I Matrix pHi pHf 2h pHf 4h Diff. 2h Diff. 4h
<0,25mm H2O 12 12 12 0 0
<0,25mm HCl 0,7 6,4 6,8 5,7 6,1
<0,25mm H2SO4 0,8 3,7 5,3 2,9 4,5
<0,25mm HNO3 1,7 6,2 6,5 4,5 4,8
0,25- 1mm H2O 12 12 12 0 0
0,25- 1mm HCl 0,6 2,9 3,1 2,3 2,5
0,25- 1mm H2SO4 0,8 3,3 3,5 2,5 2,7
0,25- 1mm HNO3 1,8 4,3 5,2 3,5 4,2
>1mm H2O 12 12 12 0 0
>1mm HCl 0,9 2,9 3,1 2,0 2,2
>1mm H2SO4 0,9 1,3 2 0,4 1,1
>1mm HNO3 1,3 4,8 6 4,5 5,7
10 Copper
The concentration of the free cupric ions in water, sulphuric acid, hydrochloric acid and nitric acid are displayed in figures 4-7. Leaching copper from BA with these three acids showed to have a difference in success. Regardless of fraction sulphuric acid yielded highest concentration and nitric acid the lowest concentration of cupric ions.
Nevertheless, fraction <0,25 mm leached the highest concentration of copper in all three acidic matrices. The concentration of copper in fraction <0,25 mm was 185 µg/mL after 2 hours in sulphuric acid and 23 µg/mL in hydrochloric acid. However, 2 hours in nitric acid the concentration of copper was 5 µg/mL. The concentration in fraction <0,25 mm was 7 µg/mL after 4-hour incubation in sulphuric acid and 3 µg/mL in nitric acid.
Figure 1. Cupric ions concentration in ppm (with confidence intervals of 95% certainty) for fraction <0,25 mm samples after 2 and 4 hours of incubation in 4 different matrices. Additionally, the pH after incubation is displayed above the columns.
The concentration of leached cupric ions from fraction 0,25-1 mm in sulphuric acid and hydrochloric acid was 127 µg/mL and 18 µg/mL respectively, after 2 hours of incubation.
Regarding nitric acid, the concentration is close to zero for both 2 and 4 hours of incubation. However, in sulphuric acid after 4 hours of incubation the concentration of copper was 107 µg/mL and 3 µg/mL in hydrochloric acid.
Figure 2. Cupric ion concentration in ppm (with confidence intervals of 95% certainty) for fraction 0,25-1mm samples after 2 and 4 hours of incubation in 4 different matrices. Additionally, the pH after incubation is displayed above the columns.
pH > 12 pH > 12 pH 3,7 pH 5,3 pH 6,4 pH 6,8 pH 6,2 pH 6,5
0 50 100 150 200
H2O H2SO4 HCL HNO3
Concnetration (ppm)
Copper concentration in fraction <0,25mm
Cu 2h Cu 4h
pH > 12 pH > 12 pH 3,3 pH 3,5 pH 2,9 pH 3,1 pH 4,3
0 20 40 60 80 100 120 140 160 180 200
H2O H2SO4 HCL HNO3
Concnetration (ppm)
Copper concentration in fraction 0,25-1mm
Cu 2h Cu 4h
11 The concentration in fraction >1 mm in sulphuric acid, hydrochloric acid and nitric acid was 91 µg/mL, 19 µg/mL and 17 µg/mL respectively, after 2 hours of incubation.
Nevertheless, after 4 hours incubation, the concentrations decreased to 44 µg/mL in sulphuric acid and 5 µg/mL in hydrochloric acid, however, the latter cannot establish 95%
statistical certainty. In nitric acid for 4 hours resulted in a zero concentration.
Figure 3. Cupric ion concentration in ppm (with confidence intervals of 95% certainty) for fraction >1mm samples after 2 and 4 hours of incubation in 4 different matrices. Additionally, the pH after incubation is displayed above the columns.
pH > 12 pH > 12 pH 1,3 pH 2,0 pH 2,9 pH 3,1 pH 4,8
0 20 40 60 80 100 120 140 160 180 200
H2O H2SO4 HCL HNO3
Concnetration (ppm)
Copper concentration in fraction >1mm
Cu 2h Cu 4h
12 Lead
Leaching lead ions from the BA showed a large difference between the fractions,
especially with hydrochloric acid. The hydrochloric acid leachate obtained an outstanding high concentration of fraction >1 mm.
The lead ion concentration of fraction <0,25 mm after 2 hours in water was 1,68 µg/mL and after 4 hours 1,72 µg/mL which is displayed in figure 7. For sulphuric acid,
hydrochloric acid and nitric acid, the concentration of lead was 0,72 µg/mL, 0,43 µg/mL and 0,27 µg/mL respectively, after 2 hours incubation and 1,67 µg/mL, 0,24 µg/mL and 0,29 µg/mL after 4 hours incubation. Differently from the copper and zinc leachates, water succeeded to leach the highest concentration of lead ions in fraction <0,25 mm.
Figure 4. Lead ion concentration in ppm (with confidence intervals of 95% certainty) for fraction <0,25mm samples after 2 and 4 hours of incubation in 4 different matrices. Additionally, the pH after incubation is displayed above the columns.
The lead ion concentration in fraction 0,25-1 mm using water, sulphuric acid, hydrochloric acid and nitric acid was 3,89 µg/mL, 1,03 µg/mL, 7,71 µg/mL and 0,97 µg/mL, after 2 hours incubation and 3,88 µg/mL, 0,89 µg/mL, 1,1 µg/mL and 0 µg/mL respectively, after 4 hours incubation, which is displayed in figure 8.
The concentrations of all leachates were higher in fraction 0,25-1 mm than fraction <0,25 mm, besides for sulphuric acid. Nevertheless, the concentration in hydrochloric acid is significantly higher in fraction 0,25-1 mm than fraction <0,25 mm.
Figure 5. Lead ion concentration in ppm (with confidence intervals of 95% certainty) for fraction 0,25-1mm samples after 2 and 4 hours of incubation in 4 different matrices. Additionally, the pH after incubation is displayed above the columns.
pH > 12 pH > 12 pH 3,7 pH 5,3 pH 6,4 pH 6,8 pH 6,2 pH 6,5
0 2 4 6 8 10 12 14 16 18
H2O H2SO4 HCL HNO3
Concnetration (ppm)
Lead concentration in fraction <0,25mm
Pb 2h Pb 4h
pH > 12 pH > 12 pH 3,3 pH 3,5 pH 2,9 pH 3,1 pH 4,3 pH 5,2
0 2 4 6 8 10 12 14 16 18
H2O H2SO4 HCL HNO3
Concnetration (ppm)
Lead concentration in fraction 0,25-1mm
Pb 2h Pb 4h
13 The lead ion concentration in fraction 1 mm using water, sulphuric acid, hydrochloric acid and nitric acid was 4,37 µg/mL, 0 µg/mL, 16,4 µg/mL and 0,54 µg/mL respectively. After 2 hours incubation, the concentration of lead was 3,05 µg/mL, 0 µg/mL, 2,86 µg/mL and 1,45 µg/mL respectively, after 4 hours incubation.
The leachate with hydrochloric acid had significantly the highest concentration of lead ions.
Figure 6. Lead ion concentration in ppm (with confidence intervals of 95% certainty) for fraction >1mm samples after 2 and 4 hours of incubation in 4 different matrices. Additionally, the pH after incubation is displayed above the columns.
pH > 12
pH 2,9
pH 4,8
pH > 12 pH 2,0 pH 3,1 pH 6,0
0 2 4 6 8 10 12 14 16 18
H2O H2SO4 HCL HNO3
Concnetration (ppm)
Lead concentration in fraction >1mm
Pb 2h Pb 4h
14 Zinc
Leaching zinc ions from the BA showed several differences between the fractions and the matrices. The incubation time showed to have possibly affected the zinc ion
concentrations. Fraction >1 mm attained the highest concentrations of zinc ions in general, however, 4 hours in hydrochloric acid leached the most.
The zinc ion concentrations in fraction <0,25 mm in sulphuric acid, hydrochloric acid and nitric acid was 252 µg/mL, 46,5 µg/mL and 50,2 µg/mL respectively after 2 hours
incubation and 271 µg/mL, 0 µg/mL and 1,45 µg/mL respectively after 4 hours incubation.
The leachate with sulphuric acid had significantly the highest concentration of zinc ions in fraction <0,25 mm.
Figure 7. Zinc ion concentration in ppm (with confidence intervals of 95% certainty) for fraction <0,25mm samples after 2 and 4 hours of incubation in 4 different matrices. Additionally, the pH after incubation is displayed above the columns
The zinc ion concentrations attained of fraction 0,25-1mm in sulphuric acid, hydrochloric acid and nitric acid was 348 µg/mL, 346 µg/mL and 70 µg/mL respectively, after 2 hours incubation and 271 µg/mL, 0 µg/mL and 1,45 µg/mL respectively after 4 hours
incubation. Incubated in sulphuric acid for 4-hour decreased the zinc concentration for fraction 0,25-1 mm but after 2-hour incubation it showed an increase of zinc
concentration. Nevertheless, figure 11 shows an excessive increase in concentration of the hydrochloric acid 2-hour incubated leachate in fraction 0,25-1 mm than fraction
<0,25 mm.
pH > 12
pH 3,7 pH 6,4 pH 6,2
pH > 12
pH 5,3 pH 6,8 pH 6,5
-50 50 150 250 350 450
H2O H2SO4 HCL HNO3
Concnetration (ppm)
Zinc concentration in fraction <0,25mm
Zn 2h Zn 4h
15
Figure 8 Zinc ion concentration in ppm (with confidence intervals of 95% certainty) for fraction 0,25-1mm samples after 2 and 4 hours of incubation in 4 different matrices. Additionally, the pH after incubation is displayed above the columns
The zinc ion concentrations in fraction >1mm in sulphuric acid, hydrochloric acid and nitric acid was 381 µg/mL, 351 µg/mL and 154 µg/mL respectively after 2 hours
incubation and 303 µg/mL, 381 µg/mL and 0 µg/mL respectively after 4 hours incubation.
Fraction >1 mm had highest concentration of zinc overall. The leachate with hydrochloric acid showed a significant difference of leached zinc ions in fraction >1 mm compared to fraction <0,25 mm and fraction 0,25-1 mm.
Figure 9. Zinc ion concentration in ppm (with confidence intervals of 95% certainty) for fraction >1mm samples after 2 and 4 hours of incubation in 4 different matrices. Additionally, the pH after incubation is displayed above the columns
pH > 12 pH > 12 pH 3,3 pH 3,5 pH 2,9 pH 3,1 pH 4,3
0 50 100 150 200 250 300 350 400 450
H2O H2SO4 HCL HNO3
Concnetration (ppm)
Zinc concentration in fraction 0,25-1mm
Zn 2h Zn 4h
pH > 12
pH 1,3 pH 2,9
pH 4,8
pH > 12 pH 2,0
pH 3,1
pH 6,0
-50 0 50 100 150 200 250 300 350 400 450
H2O H2SO4 HCL HNO3
Concnetration (ppm)
Zinc concentration in fraction >1mm
Zn 2h Zn 4h
16 The element concentrations were corrected for the dilution in 40 mL of matrix and the water content of each fractionated BA sample. The concentration of each element in the dry weight of the fractions is displayed in table 6.
Table 7. The corrected concentration (ppm) for the free ions per gram of BA calculated based on the dry weight concentration.
Sample [Cu2+] of frac. <0,25mm (µg/mg)
[Cu2+] of frac. 0,25- 1mm (µg/mg)
[Cu2+] of frac. >1mm (µg/mg)
H2O 0,385 ± 12,4 0,519 ± 12,4 0,194 ± 12,4
H2O 4h 0,367 ± 12,4 0,636 ± 12,4 0,124 ± 12,4
H2SO4 79,7 ± 12,4 65,4 ± 12,4 26,3 ± 12,4
H2SO4 4h 3,02 ± 12,4 55,1 ± 12,4 12,7 ± 12,4
HCl 9,79 ± 12,4 9,35 ± 12,4 5,41 ± 12,4
HCL 4h 0,545 ± 12,4 1,65 ± 12,4 1,52 ± 12,4
HNO3 2,26 ± 12,4 0,831 ± 12,4 5,03 ± 12,4
HNO3 4h 1,32 ± 12,4 0,103 ± 12,4 0,058 ± 12,4
[Pb2+] of frac. <0,25mm
(µg/mg) [Pb2+] of frac. 0,25-
1mm (µg/mg) [Pb2+] of frac. >1mm (µg/mg)
H2O 2,35 ± 0,55 27,5 ± 0,55 3,05 ± 0,55
H2O 4h 5,45 ± 0,55 1,22 ± 0,55 2,86 ± 0,55
H2SO4 6,13 ± 0,55 1,73 ± 0,55 0,225 ± 0,55
H2SO4 4h 2,77 ± 0,55 0,352 ± 0,55 1,04 ± 0,55
HCl 5,45 ± 0,55 0,453 ± 0,55 2,69 ± 0,55
HCL 4h 5,54 ± 0,55 1,63 ± 0,55 1,57 ± 0,55
HNO3 0,610 ± 0,55 0,900 ± 0,55 0,840 ± 0,55
HNO3 4h 10,8 ± 0,55 0,281 ± 0,55 0,014 ± 0,55
[Zn2+] of frac. <0,25mm
(µg/mg) [Zn2+] of frac. 0,25-
1mm (µg/mg) [Zn2+] of frac. >1mm (µg/mg)
H2O 0,290 ± 63,4 0,556 ± 63,4 0,350 ± 63,4
H2O 4h 0,221 ± 63,4 0,331 ± 63,4 0,274 ± 63,4
H2SO4 111 ± 63,4 184 ± 63,4 113 ± 63,4
H2SO4 4h 120 ± 63,4 92,3 ± 63,4 89,8 ± 63,4
HCl 20,6 ± 63,4 183 ± 63,4 104 ± 63,4
HCL 4h 1,59 ± 63,4 28,6 ± 63,4 113 ± 63,4
HNO3 22,2 ± 63,4 37,3 ± 63,4 45,7 ± 63,4
HNO3 4h 5,48 ± 63,4 0,020 ± 63,4 0,011 ± 63,4
Results from prior study a few months earlier analysed for total concentration of the elements in the BA are displayed in table 7.
17
Table 8. Average of total concentrations attained in prior study (2017), based on three MSWI BA samples. Analysed according to SS-EN-13656 and SS-EN ISO 17294-2 edition 1 using ICP-AES and ICP-MS for crude sample and treated sample respectively.
Sample Pb (µg/mg) Cu (µg/mg) Zn (µg/mg)
> 1 mm 0,940 6,30 3,23
1 mm - 0,25 mm 1,13 3,76 5,63
<0,25 mm 1,11 2,93 5,57
Crude 12,2 0,800 4,40
Discussion
Representability
The vast quantity and the heterogeneity were issues in question of representability of this study. Aiming for having the collecting of samples distributed over several days or even months. However, due to a short limit of time for conducting this study, the interval between the days of collecting sample was narrowed down. This may have affected the results since the samples cannot be representative for the 20 000 tons of MSWI BA annually produced when collected in the proximity of one week.
Fraction distribution
The three fractions divided by their particle size were unequally distributed. The largest particles were abundant in the crude BA whereas the finest particles, <0,25 mm
represented only 6% of the total sieved weight, which is displayed in table 5.
Nonetheless, this may have been affected by the lack of representability. In table 5, sample 3 had a significant difference in the distribution of particles which could indicate a large alteration of the MSWI BA composition and particle size from day to day.
Differences of pH stability
The alkalinity of the BA samples was very high, with pH above 12 when suspended with MQ-water. Depending on the particle sizes, each fraction required different
concentration of acid to lower its pH below 2. Clearly was fraction <0,25 mm the sample which required the highest concentration of acid to become acidic, particularly with hydrochloric acid. Additionally, the difference of pH before the incubation and after the incubation showed to be the largest for the <0,25 mm fraction in hydrochloric acid with around according to table 6.
It is a major problem that the BA consumes the protons that rapidly thus increases the pH. The leaching capability of these elements is dependent on acidity. Since, H2SO4 is a two-protonic weak acid it has a buffer capacity and is therefore behaving differently compared to the other acids in this study. It can be seen for the <0,25 mm fraction in table 6 which seems to have the largest buffer capacity since the pH does not change as
18 much as for hydrochloric acid and nitric acid. The elements of interest will precipitate if the alkalinity rises and therefore, the total concentration could not be measured for most of the leaching tests conducted in this study [5][8].
Copper
Most of the cupric ions were found to be leached from the <0,25 mm fraction in sulphuric acid, which can be seen by comparing figures 4-7. A plausible reason of the
<0,25 mm fraction having the largest part of leached ions can be that copper forms small particles and thus end up in the fraction of finest particles. Sulphuric acid dissolved most of the copper in all fraction which is likely since copper will speciate as a free ion in a pH below 4, which is true for the matrix of sulphuric acid after 2 hours. However, after 4 hours of incubation the pH had exceeded pH 5 in the <0,25 mm fraction resulting in an almost zero concentration of copper.
The leaching of copper in sulphuric acid yielded more free ions of copper than in
hydrochloric acid is shown in fig 4-7. According to the predominance diagrams displayed in figure 13 and 14 copper should have had similar leachability in the two acids below pH 6.
Figure 10. Predominance diagram for 2 µM copper with pH on the x-axis and logarithmic chloride concentration along the y-axis (constructed in MEDUSA software [9]).
0 2 4 6 8 10 12
-6 -5 -4 -3 -2 -1 0 1
Log [Cl- ] TOT
pH
Cu2+
CuCl+ CuCl2
CuCl2:3Cu(OH)2(s)
CuO(cr)
[Cu2+]TOT= 2.00 M
19 The concentrations of copper in nitric acid was found to be close to zero, indicating a low leaching capability for copper. This may be due to the high pH above 12.
Figure 14. Predominance diagram for 2 µM copper with pH on the x-axis and logarithmic sulphate concentration along the y-axis (constructed in MEDUSA software [9]).
Lead
Lead had the lowest concentrations attained after the leaching tests. It could be
explained by several parameters like the representability i.e. what was in the waste the days of sampling, the pH stability and the distribution among the fractions. In figures 8- 10, it is noticeable that the >1 mm fraction contained the highest concentrations of lead ions. Since the most of sampled material is in the >1 mm fraction, it cannot be
established whether the most lead particles are retained in the >1mm fraction or in the other fractions. Furthermore, the concentration of lead ions in hydrochloric acid was significantly the highest in the >1 mm and 0,25 -1 mm fraction However, in the <0,25 mm fraction the matrix that contained highest concentration was water whereas sulphuric acid was not able to leach lead ions at all.
It is difficult to explain why the concentration of lead in water was found to be higher than in nitric acid. Since the exact composition of the MSWI BA is unknown there is a chance of high concentration of other compounds that can affect the speciation in the matrices.
Regarding the incubation time, lead had a significant difference between the leaching acids and the time of incubation in the results. The concentration of lead after 2 hours
0 2 4 6 8 10 12
-6 -5 -4 -3 -2 -1 0 1
Log [SO 42- ] TOT
pH
Cu2+
CuSO4
CuO(cr)
[Cu2+]TOT= 2.00 M
20 showed no difference comparing to 4 hours incubation in water however it was a
difference in hydrochloric acid. In hydrochloric acid, the concentration was
unambiguously higher after 2 hours than after 4 hours of incubation. Fraction >1mm in nitric acid showed the opposite trend with a higher concentration after 4 hours than after 2 hours incubation, see figure 6.
Importantly, lead seems to be leaching well in just water which could be a problem if the BA is being stored outdoors exposed to rain. Also, since the method of this study used wet sieving for fractionation it is likely that a large amount of the total lead content in the samples leached out before analysis.
Zinc
The leaching tests for zinc attained the highest concentrations overall which can be seen in figures 10-12 compared with figures 4-9. Specifically, sulphuric acid and hydrochloric acid leached most zinc, especially of fraction >1mm. This representation could be due to unequal distribution of material in the fractions. However, the results of the >1mm fraction indicates a higher leached amount of zinc ions in hydrochloric acid after 4 hours which is significantly different compared to the other fractions. An explanation of these results could be within the composition of the fractions. The zinc compounds may have formed large solids or have adsorbed to surfaces of large molecules that were sieved to
>1mm fraction. The leaching capability seems to be good for all acids. Nitric acid had poorer yield but that could be due to the high pH after the incubations.
Errors
Some complications occurred during the ICP-OES analysis which could have come to express in the results. However, since there was a short of time conducting this study, the replicate set of the experiment was omitted.
Conclusion
Leaching copper, lead and zinc from three different particle size fractions with four different matrices gave varying results. Both copper and zinc leached best at low pH indicating that they require an acidification. However, lead was easier to dissolve and was even able to leach out with water.
Copper leached best fractionated to small particles and digested in sulphuric acid at low pH. Lead leached best in water or hydrochloric acid depending on the particle size. Some difficulties to say whether it is easier to leach lead from a bulky fraction than a fine- particle fraction since the >1mm fraction was readily over representative. Lead could also have been leached from the samples before analysis due to the wet sieving procedure in the method. In future studies of leads leaching capability in BA, a new method to
fractionate the BA samples should be considered. Zinc leached best in hydrochloric acid and sulphuric acid in large particle fractions. However, zinc was easily dissolved in all three acids. Although, nitric acid showed poorer leaching capability because of the instability in pH.
21 None of the metals obtained a concentration that exceeded the proposed EU
amendment.
Future aspects
For future recreation of this study, more material should have been collected over a longer period of time to obtain a better representability. Also, the material should have been dried before practicing the leaching tests in order to get a more accurate calculated final concentration. Since the BA is very heterogenous and the representability was lesser more tests should have been done. At least one replicate of each of the leaching tests is suggested to obtain more statistically confident results.
22
References
[1] COUNCIL REGULATION amending Annex III to Directive 2008/98/EC of the European Parliament and of the Council as regards the hazardous property HP 14 (ʻEcotoxic’) COM/2017/023 final - 2017/010 (NLE)
[2] T.T. Eighmy, J.D. Eusden, K. Marsella, J. Hogan, D. Domingo, J.E. Krzanowski, D. Stämpfli, Particle Petrogenesis and Speciation of Elements in MSW incineration Bottom Ashes, Editor(s): J.J.J.M. Goumans, H.A. van der Sloot, Th.G. Aalbers, In Studies in Environmental Science, Elsevier, Volume 60, 1994, Pages 111-136, ISSN 0166-1116, ISBN 9780444818539, https://doi.org/10.1016/S0166-1116(08)71452-3.
[3]Del Valle-Zermeño, Ricardo & Chimenos, Josep & Giro-Paloma, Jessica & Formosa, Joan.
(2014). Use of weathered and fresh bottom ash mix layers as a subbase in road constructions: Environmental behavior enhancement by means of a retaining barrier.
Chemosphere. 117. 402-409. 10.1016/j.chemosphere.2014.07.095.
[4]Jurgita Seniunaite, Saulius Vasarevicius, Leaching of Copper, Lead and Zinc from Municipal Solid Waste Incineration Bottom Ash, In Energy Procedia, Volume 113, 2017, Pages 442- 449, ISSN 1876-6102, https://doi.org/10.1016/j.egypro.2017.04.036.
[5] Jinfeng Tang, Britt-Marie Steenari, Leaching optimization of municipal solid waste incineration ash for resource recovery: A case study of Cu, Zn, Pb and Cd, In Waste Management,
Volume 48, 2016, Pages 315-322, ISSN 0956-053X, https://doi.org/10.1016/j.wasman.2015.10.003.
[6] Jurgita Seniunaite, Saulius Vasarevicius, Leaching of Copper, Lead and Zinc from Municipal Solid Waste Incineration Bottom Ash, In Energy Procedia, Volume 113, 2017, Pages 442- 449, ISSN 1876-6102, https://doi.org/10.1016/j.egypro.2017.04.036.
[7] Harris, D. (2010). Quantitative chemical analysis. New York, NY: Freeman. 8th edition. Chpt. 20, pages 494-497.
[8] Effect of Accelerated Aging of MSWI Bottom Ash on the Leaching Mechanisms of Copper and Molybdenum. Joris J. Dijkstra,†, André Van Zomeren,†, Johannes C. L. Meeussen,† and, and Rob N. J. Comans*,†,‡ Environmental Science & Technology 2006 40 (14), 4481-4487 [9] Ekelund, R. and Sillén, L. G. and Wahlberg, O.: Fortran editions of Haltafall and Letagrop, Acta
Chem. Scand., 24 (1970) 3073.
