11

Numerical Simulations of Leaching and Volatilization Behaviors
from Stabilized and Solidified Mercury Metal Waste in Landfill
Hiroyuki ISHIMORI1*, Ryo HASEGAWA2, Kazuto ENDO3,
Hirofumi SAKANAKURA1 and Tomonori ISHIGAKI1*1

Center for Material Cycles and Waste Management Research, National Institute for Environmental Studies (NIES)
16-2, Onogawa, Tsukuba, Ibaraki 305-8506, Japan
2
Environmental Control Center Co., Ltd.
3-7-23, Sandamachi, Hachioji, Tokyo 193-0832, Japan
3
Fukushima Branch, National Institute for Environmental Studies (NIES)
10-2, Fukasaku, Miharu, Fukushima 963-7700, Japan*E-mail: ishimori.hiroyuki@nies.go.jp

Abstract
This study investigates the environmental safety of landfill sites in which mercury metal waste isdisposed. Serial batch tests were conducted to evaluate the long-term leaching and volatilization rates of
mercury waste stabilized in its sulfide form, and solidified using either a sulfur polymer or two different typesof low-alkaline cement. Using measured mercury leaching and volatilization rates, numerical simulations
were conducted to investigate the long-term behavior of mercury after its disposal in landfill sites. For
leaching behavior, the concentration profiles of dissolved mercury accumulating in a drainage pipe at thebottom of a landfill site were calculated, while volatilization behavior was assessed by analyzing the gaseous
mercury emissions from the final cover surface. The effects of covering the disposed mercury waste witheither soil sorption layers or cut-off layers were evaluated through analysis of the calculated concentration
profiles.
Key words :	final disposal, leaching, numerical simulation, volatilization, waste metal mercury

1. Introduction
In limiting the use of mercury by 2020, theMinamata Convention on Mercury has led to widespreaddecommissioning of mercury-based products, and ageneral increase in the disposal of hazardous mercury
waste. Mercury is a toxic substance which is strictly
regulated by environmental quality standards: in the final
stages of disposal in landfill sites, waste mercury has tobe stabilized in a sulfide form, and/or solidified withpolymer or cement to minimize emission of the metal,
through either leaching or volatilization. The containment
performance in terms of both stabilization andsolidification has been of great concern—in Japan, theleaching concentration from waste materials such as
incineration bottom ash, incineration fly ash and crushed
fluorescent lamps (mercury content < 10%) have been
investigated. However, there have been limited findingson the leaching behavior of waste containing greater than
10% mercury, as is the case with mercury metal waste;
additionally, the vaporization of mercury has also yet tobe sufficiently investigated in this context. To evaluate
the long-term containment performance of mercury
Global Environmental Research
24/2020: 011-018
printed in Japan

石森.indd

11

stabilization and solidification, serial batch tests wereconducted using four stabilized mercury metal waste
specimens which were solidified with different binding
agents, and measuring the mercury leaching andvolatilization rates over a period of 64 days. In addition,
this study simulated the movement of emitted mercury
through a landfill site, yielding cross-sectional
concentration distributions: the concentration profiles ofdissolved mercury in leachate accumulating in drainage
pipes at the bottom of the disposal site were evaluated,along with gaseous mercury emitted from the surface ofthe final cover, and the effects of covering the boundwaste with soil sorption and cut-off layers were clarified.
Based on the results, disposal strategies are discussed
in terms of how well the different binding and disposal
methods satisfy environmental quality standards.

2. Materials and Methods
2.1 Materials
According to previous studies, mercury metal waste
stabilized as mercuric sulfide and solidified with sulfur
polymer has a leaching concentration of < 0.005 mg/L,©2020 AIRES

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H. ISHIMORI et al.

(1)where ti = elapsed time (day), ci = i-th mercury
concentration (ng/m3), jw,i = i-th mercury leaching rate
(ng/m2/day), L = liquid volume (m3), S = sample surface
area (m2), and i = index of the fraction. The obtainedleaching rates, jw,i, are related to the average elapsed time,ti, by,jw,i = Kw ti

(2)

– aw

where Kw = initial leaching rate (ng/m2/day) and aw = theleaching parameter. ti was calculated by,
ti =

(

)

√ t̄i + √¯ti – 1 2
(3)2

The fitting parameters Kw and aw can be evaluated byplotting the relationship between logarithmic jw,i andlogarithmic ti. In the plotted graph, the value of Kw
denotes the intercept, and the value of aw shows the slope.
As reported by Sakanakura et al. (2009), aw can take

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12

1
miti – ti – 1 S

(4)where mi = i-th mercury content in the gold amalgam(ng), and jg,i = i-th mercury volatilization rate (ng/m2/day).
The obtained leaching rates, jg,i, were also related to theaverage elapsed time, ti,jg,i = Kg ti–ag(5)where Kg = initial volatilization rate (ng/m2/day) and ag =the volatilization parameter. The fitting parameters of Kg
and ag were evaluated from changes in the mercury
volatilization rate with the average elapsed time.
2.4 Numerical Simulations
A seepage analysis and advection-diffusion analysis
were conducted to investigate the long-term behavior of
mercury in landfill sites containing stabilized, solidified
mercury metal waste. When the waste layer in a landfillsite is assumed to be an isotropic porous medium, thevelocity of water fluid flow is given by Darcy’s law:
uw = –

krwK( pw+ρwg z)
ηw

(6)

where uw = volumetric water velocity (m/s), ηw = waterviscosity (= 1×10–3 Pa·s), krw = relative hydraulicconductivity, K = intrinsic permeability (m2), pw = waterpressure (Pa), ρw = water density (= 1×103 kg/m3), and g =gravitational acceleration (= 9.81 m/s2). Thus, the equation
governing seepage flow can be written as,[]∂(ρwθw)
krwK( pw+ρwg z) = ∙ – ρw
∂t
ηw
Δ

ci
L
ti – ti – 1 S

jg,i =

Δ

jw,i =

2.3 Serial Batch Volatilization Tests
Serial batch volatilization tests were conducted tomeasure the volatilization rate of mercury from thestabilized and solidified mercury metal waste samples. As
with the leaching tests, 2 cm sample cubes were used:
these were placed in a 500 mL bottle, which was sealedwith a cap equipped with two connectors for attachingsilicon tubes. After the silicon tubes were closed withpinchcocks, the tests were initiated, maintaining a
constant temperature of 25°C. The gases in the bottle
were extruded using purified air after periods of 1, 2, 4, 8,
16, 32, and 64 days, and gaseous mercury in the extruded
mixture was absorbed onto gold amalgam: the mercury
content in the amalgam was then measured to ascertain
the quantity of volatilized metal. The volatilization rate of
mercury was calculated by,Δ

2.2 Serial Batch Leaching Tests
Serial batch leaching tests were conducted tomeasure the mercury leaching rate from stabilized,
solidified mercury metal waste. The specimens usedin these tests were diamond-cut into 2 cm cubes, thentied up with fluoroplastic string and suspended from thecap of a 200 mL bottle. Eighty-four mL of distilledwater—3.5 times the surface area of the specimen—was
injected into the bottle, and the suspended specimens
were immersed in the water by replacing the cap. The
bottles were placed in a temperature-controlled room at
25°C, thereby commencing the serial batch leaching tests.
The leaching liquids were exchanged with fresh distilledwater after 1, 2, 4, 8, 16, 32, and 64 days, and theconcentrations of dissolved mercury in the exchanged
leaching liquids were measured. The leaching rate wascalculated by,values between 0.0 and 1.0: a value of 0.0 indicates that
dissolution processes dominate leaching; at 0.5, leaching
becomes diffusion-controlled (depending on theconcentration difference between liquid and solid); and at
1.0, leaching reaches chemical equilibrium.

Δfulfilling the Japanese waste acceptance criteria forcontrolled landfill sites. On the other hand, low-alkaline
cement has been considered as an alternative binder withan aim at reducing mercury leaching. To prepare thetesting materials, mercury metal waste with a purity of
99.9% was first stabilized as mercuric sulfide, and thensolidified with one of four binders, (1) sulfur polymer, (2)low-alkaline cement A, (3) low-alkaline cement B, and(4) low-alkaline cement B with a water reducing agent.
Low-alkaline cement A was a manufactured product.
Low-alkaline cement B was prepared by mixing ordinary
Portland cement, silica fume and fly ash in a 3: 2: 5 ratio.
Sulfur polymer had been considered effective for
reducing mercury leaching, but it was relatively
expensive. This study aimed to investigate whether lowalkaline cements A or B could exhibit the same
performance as the sulfur polymer.

Δ

12

(7)

where θw = volumetric water content. The gas flow

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Numerical Simulations of Leaching and Volatilization Behaviors from Stabilized and Solidified Mercury Metal Waste in Landfill

0.5m

2.5m

Soil sorption layer
Dissolved
mercury
in leachate

Waste layer
Water level(c)Open boundary-103 kPa
for 0~10yr
・0 mm/yfor 10yr~
・600 mm/y
・60 mm/yInitial conc. = 0
forwhole of domain

No flux boundary

0.5m

Stabilized andsolidified waste
metal mercury(1m3 cube)
・Sulfur polymer
・Low-alkaline
・cement B

Constant pressure boundary

Soil sorption layer

Constant velocity boundary-103 kPa

Final cover, or
cut-off layer
Waste l ayer

0.5m

2.5m

Gaseous mercury

No flux boundary(b)

RainfallConstant pressure boundary

(a)

13

Use leaching &volatilizing rate

Const. pressure
boundary 0 kPa

Open boundary

Fig. 1 Analysis domain and conditions: (a) two-dimensional cross section of landfill, (b) initial and boundary conditions
for seepage analysis, (c) initial and boundary conditions for advection-diffusion analysis.

velocity was assumed to be negligible, as the gas pressurein the main waste layer where incineration ash is disposedis almost the same as the atmospheric pressure, andtherefore the gas pressure gradient is regarded to be zero.
The transports of dissolved and gaseous mercury areformulated respectively as follows:∂(cwθw)+ [ − (Dw + θwDe) cw + uwcw]∂t= θgKH (cg − Hcw) ρdKd ∂cw
∂t
Δ

Δ

−

(− θgDe cg)
ΔΔ∂(cgθg)+∂t

(8)= θgKH (Hcw − cg) ρdKd ∂cg
∂t

(9)

−

where cw = dissolved mercury concentration (ng/m3), cg =gaseous mercury concentration (ng/m3), Dw = the waterphase dispersion coefficient (m2/s) (Bear, 1972), De = theeffective diffusion coefficient (m2/s), KH = the masstransfer rate (1/s), H = the dimensionless Henry constant,ρd = the dry bulk density (kg/m3), and Kd = the distribution
coefficient (m3/kg). In equations (8) and (9), the first termon the right-hand side indicates the amount of materialtransferred between the water and gas phases due todissolution and volatilization of mercury, and the second
term on the right-hand side represents soil adsorption ofboth mercury and gaseous mercury. The set of equations(7) to (9) was numerically solved using COMSOL ver
5.1.
The cross-sectional analysis domain of the modelledlandfill site is shown in Fig.1. The stabilized, solidified
mercury waste metal was assumed to take the form of
3
1 m cubes. The entire array of mercury waste cubes wascovered with a soil sorption layer, designed to slow thetransport of emitted mercury. A drainage pipe was placedat the bottom of the analysis domain to accumulateleachate, and a final cover or cut-off layer to reduce

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13

Table 1 Analytical conditions.
Parameters

Unit

Waste
layer

Soil sorption
layer

Porosity
Intrinsic permeability
Dry bulk density
VG parameter, a
VG parameter, nLong. dispersivity
Trans. dispersivity
Distribution coefficient
Henry constant

1
m2
kg/m3
1/m
1
m
m
mL/g
1

0.3
1×10−12
1,400
2
1
3
1
0
0.43

0.3
1×10−12
1,800
2
1
3
1
100
0.43

Note: Henry constant value from Andersson et al. (2008)

rainfall permeation was placed on top. Figures 1(b) and1(c) show the initial and boundary conditions of theanalysis: the boundary condition at the top of the domain
indicates the rainfall intensity. For the first 10 years of theanalysis, the rainfall intensity for the top boundary
condition was considered to be 0 mm/y, as landfilling of
the mercury waste would be carried out under a roof; atthe end of the landfilling process, the roof would be
removed, so in subsequent years, rainfall at the top
boundary condition is considered to permeate the final
cover with an intensity of either 600 mm/y or 60 mm/y.
The different rainfall intensity values were used toevaluate the effects of using a cut-off layer covering thewaste site, which would decrease the overall ingress ofwater into the landfill site. The measured leaching andvolatilization rates of mercury were applied to the surface
boundaries of the stabilized, solidified mercury metal
waste. In this numerical simulation, effects of the soil
sorption and cut-off layers on the concentrations of the
dissolved mercury in leachate in the bottom drainage pileand gaseous mercury emitted from the final cover were
investigated: the analytical conditions for these models
are listed in Table 1.

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H. ISHIMORI et al.

SP
CA

10 7

CB
CB+

10 6
10 5
10 4
10 3
10 2

1

10
Elapsed time (day)

0.1
1
10
100
Average leaching duration (day)Accumulated volatilizing amountof mercury (ng/m2)

10 8

100

10 8

10 5
10 4
10 3
10 2

10 5

10 5

10 3

-0.74

Y = 7640X

10 2
10 1

SP
CA

CB
CB+

Y = 205X-0.53

10 0
0.1
1
10
100
Average leaching duration (day)

CB
CB+

10 6

10 6

Y = 95500X-0.83
Y = 58300X-0.74

SP
CA

10 7

10 6

10 4

Y = 7640X-0.74

10 2

3.2 Mercury Behavior in Landfill Sites
SP
CB3(d) show Ycontour= 205X-0.53 maps of the
10 1
Figures
3(a)
toCB+
CA
concentration
distributions of dissolved and gaseous
10 0

Mercury volatilizing rate
(ng/m2/day)

Mercury leaching rate
(ng/m2/day)

Accumulated leaching amountof mercury (ng/m2)

3.1 Mercury Leaching and Volatilizing Rates
Figure 2 shows the results of the serial batchleaching tests. As shown in Fig. 2(a), the sulfur polymer
was found to be the most effective binder for reducing
mercury leaching: in comparison, the amount of mercury
leached when solidified with low-alkaline cement wasmore than 10 times higher. The inclusion of a water
reducing agent had little effect on decreasing the amountof leached mercury. Figure 2(b) shows the measured
leaching rates and results obtained by fitting withequation (2). There were significant differences between
the initial leaching rates among the samples bound withsulfur polymer and low-alkaline cement A and B, and theleaching rates were observed to decrease over time in allcases. The mercury metal waste solidified with the sulfur
polymer had a leaching parameter of 0.53, demonstratingthat leaching was dominated by the inner diffusion effect.
In contrast, the mercury solidified with either lowalkaline cement (A or B) exhibited leaching parametersof 0.75–0.85.
Figures 2(c) and 2(d) show the results of the serialbatch volatilization tests. As was the case for leaching,binding with the sulfur polymer yielded the lowest levelsof mercury volatilization, and low-alkaline cement A wasjust as effective in preventing gaseous mercury emission.
The cumulative amount of volatilized mercury from
mercury waste solidified with low-alkaline cement B was
about 10 times higher than that with either the sulfur

polymer or low-alkaline cement A. As shown in Fig. 2(d),the volatilizing rates also decreased over time, as was thecase for leaching. Their volatilizing parameters were0.75–0.85.
In Table 2, the results of the serial batch leaching andvolatilization tests are summarized. Mercury leachability
and volatility were defined by dividing the amounts ofleached and volatilized mercury by the initial mercury
waste content. The mercury metal waste solidified withthe sulfur polymer exhibited extremely low volatility−8(1.19×10
10 8 %) as well as extremely low leachability−8
SPCB%). CAIn contrast,
the low-alkaline cements were(7.45×10CB+
10 7
not as effective in reducing mercury leachability, 6.68
6
×10−6%10and 3.64 ×10−5% for low-alkaline cements A and
10 5
B, respectively.
However, mercury solidified with lowalkaline10 4cement A restrained mercury volatilization as
well as 3 the sulfur polymer, whereas the rate of10volatilization from cement B-bound mercury was roughly
10 2higher.
10 times
1
10
100
Elapsedshown
time (day)in Table 2 were evaluated at
The parameters
25°C. The temperature in landfills may be elevated up to
60°C by 6biological degradation of organic substances in
10
the landfills. The elevated temperature will enhance the
10 5leaching
and volatilization of mercury. In particular, the4temperatureeffect on the volatilization rate-0.83
is considered10
Y = 95500X
significant,sofurtherstudieswillbeessential.
-0.74
3
Y = 58300X
10
Accumulated leaching amountof mercury (ng/m2)

3. Results and Discussion

Mercury leaching rate
(ng/m2/day)

14

1

10
Elapsed time (day)

SP
CA

100

CB
CB+

10 4
10 3
10 2
Y = 320X-0.75
Y = 214X-0.77
Y = 49.6X-0.84
0Y = 34.2X-0.75
10
0.1
1
10
100
Average volatilization duration (day)
10 1

石森.indd14

ccumulated volatilizing amountof mercury (ng/m2)

Fig. 2 Results of serial batch leaching tests and serial batch volatilization tests: (a) cumulative amount ofleachedmercury, (b) leaching rate, (c) cumulative amount of volatilized mercury, (d) volatilizing rate.
8
10

10 7

SP
CA

CB
CB+

10 6
10 5
10 4
10 3

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Numerical Simulations of Leaching and Volatilization Behaviors from Stabilized and Solidified Mercury Metal Waste in Landfill

15

Table 2 Summary of serial batch leaching and volatilization tests.
CA

CB

CB+

Sulfur polymer

Low-alkaline cement A

Low-alkaline cement B

Low-alkaline cement B& water reducing agent

3,440
430,000
7.45×10−8
1.19×10−8
205
0.53
34.2
0.75

2,550
340,000
6.68×10−6
2.01×10−8
7,640
0.74
49.6
0.84

2,440
340,000
3.64×10−5
1.40×10−7
95,500
0.83
320
0.75

2,410
350,000
3.73×10−5
9.23×10−8
58,300
0.74
214
0.77

0

1.3

1.3

0

4.3x10-3↓↑
4.3x10-3

0

378
↓

↑
378

297
↓

↑
313

390
0.75
↓

↑
0.75

Gaseous mercury conc. (ug/m3)↑
1.8x10-3

Dissolved mercury conc. (ug/L)

↑
1.1

↓

0

Dissolved mercury conc. (ug/L)

0.13↓

3.1x10-3

0

0

330

330

0

12.5
↓

↑
12.5

Gaseous mercury conc. (ug/m3)

Dissolved mercury conc. (ug/L)

↑
0.56

390

0.62

0.62

↑0.16

Gaseous mercury conc. (ug/m3)

Bulk density (kg/m3)
Hg content (mg/kg)
Hg leachability (%)
Hg volatility (%)
Hg initial leaching rate, Kw
Hg leaching parameter, aw
Hg initial volatilization rate, Kg
Hg volatilization parameter, ag

Gaseous mercury conc. (ug/m3)

Binder for solidification

SP

Dissolved mercury conc. (ug/L)

Name

0

Fig. 3 Concentration distributions of mercury leaching and volatilization from sulfur polymer-solidified mercury waste in the 12th year
after deposition (rainfall intensity = 0 mm/y for 0–10 years and 600 mm/y or 60 mm/y for > 10 years): (a) dissolved mercury
concentration without a sorption layer with 600 mm/y, (b) dissolved mercury concentration with a sorption layer with 600 mm/y, (c)gaseous mercury concentration without a sorption layer with 600 mm/y, (d) gaseous mercury concentration with a sorption layer
with 600 mm/y, (e) dissolved mercury concentration without a sorption layer with 60 mm/y, (f) dissolved mercury concentration
with a sorption layer with 60 mm/y, (g) gaseous mercury concentration without a sorption layer with 60 mm/y, (h) gaseous mercury
concentration with a sorption layer with 60 mm/y.

mercury in a landfill site in the 12th year after deposition,using high rainfall intensity values (600 mm/y) after an
initial 10-year period with no rainfall. In these models,
the disposed mercury waste was solidified with sulfur
polymer, and the mercury leaching and volatilization
rates from the solidified surface were taken to be jwSP =205t−0.53 and jgSP = 34.2t−0.75, respectively (see Table 2).
Significant effects of the soil sorption layer on the

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15

decrease in mercury concentration levels in the landfill
were seen, as shown in Figs. 3(b) and 3(d).
In contrast, Figs. 3(e) to 3(h) show the results ofmodelling with lower intensity rainfall (60 mm/y) after
the initial 10-year period: this lower rainfall intensity wasused to simulate a cut-off layer covering the top of thelandfill site, instead of a general final cover.

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H. ISHIMORI et al.

1.5

SP
SP+sorption
SP+cut-off
SP+sorption+cut-off

1

0.5

0
1

10 1

10 2

10 4

10 5

Elapsed time (year)(b)
400
Gaseous mercury conc.
on landfill surface (ug/m3)

10 3

200
100
0
1

10 1

10 2

10 3

10 4

10 5

Elapsed time (year)(c)

Elapsed time (year)
SP
SP+sorption
SP+cut-off
SP+sorption+cut-off

10 3
10 2
10 1
10 0

10 -1
10 -2
10 -3
10 -4

1

10 1

10 2

10 3

10 4

10 5

Elapsed time (year)(d)

SP
SP+sorption
SP+cut-off
SP+sorption+cut-off

300

Dissolved mercury flux emitted
from dranage pipe (ug/m2/y)(c)Gaseous mercury flux emitted
from landfill surface (ug/m 2/y)

Dissolved mercury conc.
in leachate (ug/L)(a)This is observed as gaseous mercury, which can betransported through pores by diffusion effects, even in theabsence of water penetration. As was the case for mercury
leaching, the soil sorption layer was also effective indecreasing the peak concentration of gaseous mercury
and delaying its onset. It is noted that mercury sorption inthis study is expressed by the equilibrium Henry model
considering both adsorption and desorption, resulting insimulation of retardation transport not related withadsorption capacity.
The model showed that the use of a cut-off layer
may decrease the peak concentration of gaseous mercury,despite(a) its role in increasing mercury concentrations inleachate.
1.5 Limiting the infiltration of rainfall using a cutoff layer can prevent SP
the accumulation of water in the
SP+sorption
SP+cut-off
landfill 1site, thus maintaining
pore widths in the layers—
SP+sorption+cut-off
the gaseous mercury can then diffuse into these wide
pores, which could lead to an overall decrease in the
0.5
gaseous mercury concentration.
Figure 4(c) shows the fluxes of mercury dissolved inleachate0 accumulating in the drainage pipe at the bottom
10 5 10 years, the flux of1
10 1 During
10 2 10 3 the
10 4 firstof the landfill.
Elapsed time (year)
dissolved mercury in the leachate was zero because the(b)landfill400was covered. After 10 years, the dissolved
mercury flux abruptlySPincreased. Focusing on the effectof a cut-off
layer, theSP+sorption
presence of a cut-off layer can
300
SP+cut-off
decrease the dissolvedSP+sorption+cut-off
mercury flux. This effect on the200
flux differedfrom that on the concentration. The presenceof the cut-off layer led the peak concentration to increase100
as shown in Fig. 4(a). Even if the concentration wasincreased
0 due to the installation of the cut-off layer, the5flow velocity
thedissolved mercury
1
10 1 transporting
10 2 10 3 10 4 10
Dissolved mercury conc.
in leachate (ug/L)

3.3	Mercury Concentration Profiles in Leachate andLandfill Surfaces
Figure 4(a) shows the concentration profiles of
mercury dissolved in leachate accumulating in thedrainage pipe at the bottom of the landfill where SPsolidified mercury metal waste was disposed. During thefirst 10 years, the concentration of mercury in theleachate was zero as, in the simulation, the landfill wascovered by a roof, thereby preventing rainfall fromreaching the surface. When the roof was removed (in the10th year after deposition), rainfall began permeate thelandfill, and mercury leached from the solidified waste
was flushed towards the drainage pipe; hence, thedissolved mercury concentrations were calculated toincrease after 10 years. The presence of a soil sorption
layer was calculated to be able to decrease the peakconcentration and delay the onset of leaching, whereas
the use of a cut-off layer led to a substantial increase inpeak concentration. It is thought that the flow velocity ofrainfall passing through the cut-off layer would slow,
thereby increasing the water retention time, and as aresult, a greater proportion of mercury leaching from thesolid waste could be dissolved in water accumulating in
the layer pores. While the cut-off layer increased the peakconcentration, the overall mercury flux would decrease
due to the lower flow velocity, which is a major factor in
determining mercury transport rates.
Figure 4(b) shows the concentration profiles ofgaseous mercury emitted from the landfill surface: thegaseous mercury concentrations were calculated toincrease over time, even during the first 10 years (inwhich no rainfall was considered to penetrate the landfill).

Gaseous mercury conc.
on landfill surface (ug/m3)

16

10 3
10 2

SP
SP+sorption
SP+cut-off
SP+sorption+cut-off

10 1
10 0
10 -1
10 -2
10 -3
10 -4

1

10 1

10 2

10 3

10 4

10 5

Elapsed time (year)

Dissolved mercury flux emitted
from dranage pipe (ug/m2/y)

SP on mercury concentration profiles: (a) dissolved mercury concentration in leachate in theFig. 4 E ffects of soil sorption10layer
and cut-off layer
3
SP+sorption
bottom drainage pipe, (b)2 gaseous mercury
concentration at the landfill surface, (c) dissolved mercury flux, (d) gaseous mercury flux.
SP+cut-off

石森.indd

16

10
10

SP+sorption+cut-off

1

10 0
10 -1
10 -2
10 -3
10 -4

1

10 1

10 2

10 3

10 4

10 5

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Numerical Simulations of Leaching and Volatilization Behaviors from Stabilized and Solidified Mercury Metal Waste in Landfill

17

Table 3 Summary of numerical simulation results.

Leachate in drainage pipe

Sulfur polymer(SP series)

Sorption layerTime that peak
appears (year)

Peak conc.
(ug/L)

Time that peak
appears (year)

Peak conc.
(ug/L)

No
No
Yes
Yes
No
No
Yes
Yes

No
Yes
No
Yes
No
Yes
No
Yes

12
330
30
2,900
12
330
31
2,900

94.4
1.64
195
5.16
0.62
0.01
1.28
0.03

12
8,710
32
13,60012
12,40029
12,20030,90012730,300
587
380
4.93
297
7.32

decreased, so the flux also decreased. The installation of
the cut-off layer was considered effective in reducing
dissolved mercury emissions. On the other hand, thepresence of the sorption layer affected the decrease in theflux of dissolved mercury over the simulation duration.
The effect on the decrease was larger for the sorption
layer than for the cut-off layer under the conditions of
this analysis. The installation of the sorption layer was
also effective in reducing dissolved mercury emissions.
Figure 4(d) indicates the fluxes of gaseous mercury
emitted from the landfill surface. The presence of thesorption layer decreased the gaseous mercury flux. The−22flux could be kept to under 1×10 ug/m /y over thesimulation duration. In contrast, the presence of the cutoff layer led the flux of gaseous mercury to increaseslightly under these analytical conditions. When there
was a cut-off layer covering the landfill surface, theamount of rainfall infiltration was limited. The degree ofwater saturation in the landfill decreased. Therefore, theeffective diffusion coefficient of the gaseous mercury
increased and the flux became large. Although installationof the sorption layer was effective in reducing gaseous
mercury emissions, installation of the cut-off layer had noinfluence on reduction of the emissions.
Table 3 summarizes the results of the numericalsimulations. The mercury metal waste solidified with
low-alkaline cement B had a peak concentration ofaround 100 times higher than the sulfur polymersolidified waste. In Japan, the water quality standard for
effluents from controlled landfill sites sets a limit of
0.005 mg/L for mercury: when stabilizing waste mercury
in its sulfide form and solidifying it with sulfur polymer,
the mercury concentration in the leachate will be able tosatisfy this quality standard; however, solidification with
low-alkaline cement B was not found to satisfy thestandard according to the calculations of this study.
Moreover, the study indicates the necessity of installing asoil sorption layer to decrease peak mercury
concentration.
3.4 Mercury Emissions from Landfill Sites
Finally, regarding the total amount of mercury
emissions from landfill sites, Figs. 5(a) and 5(b) show the

石森.indd

17(a)
Mercury emission
in case of SP-solidified
mercury metal waste (mg/m)

Low-alkaline
cement B(CB series)

Cut-off layer

15

Without cut-off layer& soil sorption layer

10With soil
sorption layerWithcut-off layer

5With cut-off layer& soil sorption layer

0
1

10 1

10 2

10 3

10 4

10 5

Elapsed time (year)

(b)
2000

Mercury emission
in case of CB-solidified
mercury metal waste (mg/m)

BinderGas emitted from landfill surface

Without cut-off layer& soil sorption layer

1500
With soil
sorption layer

1000

With
cut-off layer

500

With cut-off layer& soil sorption layer

0
1

10 1

10 2

10 3

10 4

10 5

Elapsed time (year)

Fig.5 Total amount of mercury emitted from a landfill with SPsolidified or CB-solidified mercury metal waste disposal: (a)case of SP-solidified mercury metal waste, (b) case of CBsolidified mercury metal waste

total amount of mercury emitted from the landfill. In
these figures, mercury metal waste was solidified using
sulfur polymer or low-alkaline cement B, respectively. A
drastic difference between sulfur polymer and lowalkaline cement B appeared in the total amount of
mercury emissions. The total amount of emissions in thecase of sulfur polymer was approximately 100 times
smaller than in the case of low-alkaline cement B.

4. Conclusions
This study investigated the rates of leaching andvolatilization of from mercury metal waste stabilized inits sulfide form and solidified with a sulfur polymer or
low-alkaline cements. Numerical simulations wereconducted to clarify the long-term behavior of mercury inlandfill sites in which the stabilized, solidified mercury

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18

H. ISHIMORI et al.

waste was disposed. The following were found:— According to the results of the serial batch leaching
and volatilization tests, mercury waste solidified
with the sulfur polymer presented excellent
containment performance and the lowest leachabilityof the solidification materials used, 7.45×10−8%. In
contrast, when mercury waste was solidified with
low-alkaline cements, the leachability was ~100times greater.
— 
The sulfur polymer-solidified mercury waste also
displayed the lowest volatilization, 1.19×10−8%.
Low-alkaline cement A performed similarly to thesulfur polymer in containing gaseous mercury
emissions, while volatilization from low-alkaline
cement B-solidified mercury waste was ~10 times
greater.
— According to the numerical simulation results, whenstabilizing mercury metal waste in its sulfide formand solidifying it with sulfur polymer, the resulting
mercury concentration in the leachate will be lessthan 0.005 mg/L, thereby satisfying the Japanesewater quality standard for effluent from controlled
landfill sites.
— When using low-alkaline cements as a solidification
agent, the dissolved mercury concentrations in theleachate were calculated to exceed 0.005 mg/L. The
use of a soil sorption layer covering the solidified
mercury waste was found effectively to decrease thepeak concentration; in comparison, the applicationof a cut-off layer covering the solidified waste may
substantially increase the dissolved mercury
concentration, but simultaneously decrease its flux
(emission), thereby also lowering its overall
transport rate.
— The total amount of mercury emissions depended
significantly on the presence of a soil sorption layer
and cut-off layer as well as types of binders tosolidify the mercury metal waste. The most effective
countermeasure for reducing mercury emissions wasconsidered to be sandwiching sulfur polymersolidified mercury waste between sorption layers
and covering the landfill surface with a cut-off wall.
Numerical analysis will help us design the required
geometry and material quality of the soil sorption
layer, cut-off wall, and stabilized, solidified mercury
metal waste.
Acknowledgment
This research was supported by the Environment
Research and Technology Development Fund
(3K153004, Tomonori Ishigaki) of the Ministry of theEnvironment, Japan.
References
Andersson, M.E., Gårdfeldt, K., Wängberg, I. and Strömberg, D.

石森.indd

18(2008). Determination of Henry’s law constant for elemental
mercury. Chemosphere, 73, 587–592.
Bear, J. (1972) Dynamics of Fluids in Porous Media, Elsevier, NewYork.
Sakanakura H., Osako, M. and Kida, A. (2009) Effect of exposure
test conditions on leaching behavior of inorganic contaminantsfrom recycled materials. Waste Management, 29(5), 1658–1665.

Hiroyuki ISHIMORI
Hiroyuki Ishimori, Ph.D., is a senior researcher at
the National Institute for Environmental Studies,Japan. He specializes in mathematics andnumerical simulations in geoenvironmental
engineering. His skills are used for developing
safe utilization and final disposal techniques forcontaminated soils, wastes, byproducts and so on.
He is currently studying the long-term deterioration of barrier performanceof adsorption reagents, impermeable materials and reinforced concrete due
to chemical attack. Numerical simulations considering long-term
deterioration will provide effective countermeasures to protect theenvironment from various forms of chemical pollution.

Ryo HASEGAWA
Ryo Hasegawa is a professional engineer at theEnvironmental Control Center Co., Ltd. He
specializes in research and consulting on
environmental measurement and waste
management. His analytical technique inchemistry has significantly contributed toelucidation of the leaching and volatilization
behavior of mercury-containing waste.

Kazuto ENDO
Kazuto Endo heads the Fukushima branch of the
National Institute for Environmental Studies,Japan. His research fields are environmental
geotechnics and waste management. At present,
his team is researching the management ofradioactively contaminated waste and removed
soil, aiming at early revitalization of Fukushima.
He has researched the construction and management of seashore landfill
sites through computer simulation and field surveys. He is also interested in
the geotechnical use of recycled materials. He has published a guideline on
geotechnical use for quality control of recycled waste gypsum powder.

Hirofumi SAKANAKURA

Technology.

Dr. Hirofumi Sakanakura has worked for the
Natural Institute for Environmental Studies since2006. His main research area is development ofleaching tests for wastes and recycled materials toevaluate environmental safety. He received his
doctorate from Hokkaido University in 2000 andworked initially for Akita National College ofTomonori ISHIGAKI
Tomonori Ishigaki is a senior researcher at the
National Institute for Environmental Studies. He
serves as an editorial board member for theemission factor database and lead author of the
Guidelines for National Greenhouse Gas
Inventories for the Intergovernmental Panel on
Climate Change. He is also serving as a memberof the Technical Committee (Solid Recovered Fuels) of International
Organization for Standardization. His published works cover subjects suchas the mechanism of waste conversion in landfills, technology development
for emission reduction from landfills, assessment of the fate of hazardous
substances in the environment, and appropriate management of solid waste
in urban areas of Asia to prevent urban floods.
(Received 17 December 2019, Accepted 16 May 2020)

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