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Environmental application of basic oxygen furnace slag for the removal of heavy metals from leachates
Journal Pre-proof Environmental application of basic oxygen furnace slag for the removal of heavy metals from leachates Franco M. Francisca, Daniel A. Glatstein
PII:
S0304-3894(19)31248-8
DOI:
https://doi.org/10.1016/j.jhazmat.2019.121294
Reference:
HAZMAT 121294
To appear in:
Journal of Hazardous Materials
Received Date:
25 May 2019
Revised Date:
16 August 2019
Accepted Date:
22 September 2019
Please cite this article as: Francisca FM, Glatstein DA, Environmental application of basic oxygen furnace slag for the removal of heavy metals from leachates, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121294
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Environmental application of basic oxygen furnace slag for the removal of heavy metals from leachates
Franco M. Francisca (1,2,*), Daniel A. Glatstein (2,3)
(1) Universidad Nacional de Córdoba, Facultad de Ciencias Exactas Físicas y Naturales, Departamento de Construcciones Civiles, Córdoba, Argentina. (2) CONICET - Universidad Nacional de Córdoba, Instituto de Estudios Avanzados en Ingeniería y Tecnología (IDIT), Córdoba, Argentina.
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(3) Universidad Nacional de Córdoba, Facultad de Ciencias Exactas Físicas y Naturales, Departamento de Química Industrial y Aplicada, Córdoba, Argentina. (*) Corresponding author. Av. Velez Sarsfield 1611, CP 5016, Córdoba, Argentina. Tel +54 351 5353800 ext. 836. Email: [email protected]
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Research highlights
Addition of BOF slag in permeable barriers promotes metal ions precipitation
BOF slag helps in retarding metal ion transport through landfill liners
BOF slag in permeable barriers promotes precipitation of metal ions.
New environmental applications can be found for slag extending its life cycle.
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Abstract
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Industrial waste is a major environmental concern nowadays, stimulating the thorough study of the minimization and recycling of solid wastes and of the containment and treatment of liquid contaminants. Basic oxygen furnace (BOF) slag,
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a solid waste from the steel industry, has been found to be effective in the removal of heavy metals. However, this has not been applied so far in low permeability barriers, such as those used as bottom liners in landfills. This work studies the performance of BOF slag in both containment and treatment technologies for toxic leachates. Flow models are developed to assess the transport of metal ions through a permeable reactive barrier and a composite clay barrier. Reactive transport through the slag barrier and adsorption in the clay barrier are coupled for different conditions to find the residence time, the barrier life span and the optimum operative conditions. The 1
results show that the use of steel slag increases the breakthrough time of the contaminants, enabling improve design of low and high conductivity reactive barriers, and expands the life cycle of the material. Keywords Precipitation; landfill leachate; liner; permeable reactive barrier; residual material.
1. Introduction
Liquid pollutants can severely affect human health and may have significant
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environmental impacts Among other sources of these contaminants, landfill leachate and mining drainage induce significant health and environmental problems.
Landfill leachate is highly toxic and one of the most difficult contaminants to analyze and treat, given its temporal and spatial variability. Environmental factors such as
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temperature and precipitation modify its volume and bacterial activity which, in turn, changes the pH and therefore its composition. In addition, landfill cell design impacts
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the contact time with waste. The chemical composition of landfill leachates depends on waste properties and age, and therefore, varies from one landfill to another [1,2].
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Even though most landfills are constructed for municipal solid waste (MSW), the presence of heavy metals such as Cd, Cu or Pb is fairly common due to poor management techniques and controls. Moreover, as the leachate is often
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recirculated to maintain minimum moisture content, the metals concentration increases with time.
Similarly, mine drainage shows significant temporal and spatial variability in physical and geochemical properties [3]. These properties are mainly determined by chemical, biological and physical factors as well as by the hydraulic conductivity of
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waste rock dumps. The main concern is to prevent liquid contaminant migration by means of barriers, cut-off walls, trenches, or wells [4]. Some solutions allow water movement but prevent mass transport by retaining the contaminants. Given the complexity and toxicity of leachates, their containment, transport, and treatment have attracted great interest in recent years. The practice of containment has evolved from simple soil compaction to the amendment of local soil with low hydraulic
conductivity
clays,
or
the
addition
of
layers
of
geotextiles
or
geomembranes. These changes have been accompanied by regulations specifying a 2
maximum value for hydraulic conductivity or a minimum thickness of the soil layer or the geomembrane. Regulations and recommendations for landfill barriers differ from one country to another, but a minimum soil layer of 0.9 m with a maximum k of 10-9 m/s is a common requirement for simple liners without geomembrane. However, most regulations require a HDPE membrane, or special composite materials, such as geotextiles, or special drains [5,6]. Despite the effect that chemistry and microbiology have on the performance of liners, no regulation contemplates the adsorption of contaminants or the clogging of the barrier.
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Some regulations are flexible allowing a change in barrier composition, as long as it presents an equivalent performance in terms of the containment of contaminants. This presents the main issue regarding landfill liners: although they are designed to contain and remove the leachate, some of this leachate passes through the barrier
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without treatment and migrates toward the groundwater. This drives the search for alternative materials such as modified clays, natural fibers or husks, or even
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industrial residues such as sewage sludge or fly ash, which may contain the contaminants within the barrier while the liquid passes through [7-9].
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One of these industrial residues is steel slag, a coarse material with a high capacity to remove heavy metals from solution given its high alkalinity [10]. It is a byproduct of steel manufacturing, conventionally used as flux in steel production, but with
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applications related with concrete and asphalt production and the construction of road grades and sub-grades [11]. More recent studies also suggest its use in wastewater treatment, iron recovery, CO2 and phosphorous sequestration, and as fertilizer [12-15]. In particular, its ability to increase the solution pH and thus promote metal precipitation serves as a secondary process retarding contaminants passing
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through liners or even permeable reactive barriers. These barriers allow the free flow of liquids but retain contaminants by adsorption, reaction, oxidation or precipitation [16-19]. Such treatments sequester the contaminant within porous media through physicochemical reactions and particle-fluid interaction mechanisms. The purpose of this work is to analyze the coupled effect of sorption and precipitation on the transport of heavy metals through composite granular liners. Applications of barriers with high and low hydraulic conductivities are considered (i.e. permeable reactive barriers and landfill liners, respectively). The effect of the acid neutralization 3
capacity (ANC) of the slag on metal ions present in mine drainage and landfill leachate is addressed by coupled transport models.
2. Model and Materials The models presented in this paper are supported by previously published experimental work. Francisca and Glatstein [10] indicated that the pH of precipitation for several metals, acting as individual ions or as a mixture, ranged from 6 to 8 UpH. Values for the hydraulic conductivity of sand-bentonite and loess-bentonite were indicated by Montoro and Francisca [20] and by Francisca and Glatstein [21].
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Additional experimental work was carried to test specific parameters, such as the ANC of the slag, or the hydraulic conductivity of slag samples.
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2.1. Contaminant transport in porous media
Contaminant transport in porous media is influenced by the physical restrictions of
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the solid phase (soil particles, mineral and rocks) and chemical and hydraulic gradients. Chemical gradients promote diffusive processes that are represented by
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Fick’s first and second laws (Eq. 1 and 2). 𝐹𝑑𝑖𝑓 = −𝐷 ∗
𝜕𝐶 𝜕𝑥
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𝜕𝐶 𝜕2𝐶 ∗ =𝐷 𝜕𝑡 𝜕𝑥 2
(1) (2)
where F represents the chemical mass flow (mg/m2·s), D* is the effective diffusion coefficient (m2/s), t is the transport time (s), x is the distance from the source (m) and C is the concentration of the chemical species in solution (mg/m3).
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Chemical species can also be associated with liquid flux. In this case, the amount of solute advected by the flowing groundwater depends on its concentration and the water flow, as represented by Eq. 3. 𝐹𝑎𝑑𝑣 = 𝑣 𝑛 𝐶
(3)
where v is the mean fluid velocity (m/s) and n is the porosity of the granular media. By analogy to Fick’s laws, the equation for one-dimensional advective transport is given by Eq. 4: 4
𝜕𝐶 𝜕𝐶 = −𝑣 𝜕𝑡 𝜕𝑥
(4)
Equations 3 and 4 are valid for fluids that follow Darcy’s law, which indicates a relation between the flow velocity and the hydraulic gradient (i) through a proportionality constant, hydraulic conductivity (k) (m/s), defined as follows: 𝑣 =𝑘∙𝑖
(5)
Hydraulic conductivity is one of the most important parameters for the design of reactive barriers and containment liners, given its variability. This parameter is
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usually modeled as time-independent but several studies recognize its change with time, due to biological and chemical clogging [20,22,23]. The hydraulic gradient (i) is the ratio between the total-head loss between both sides of a barrier and the barrier thickness (i = H/L). Thus, hydraulic gradient decreases as the thickness of the
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barrier increases for a given total-head pressure difference.
In addition, mechanical dispersivity promoted by different flow paths, pore size
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distribution and friction with pores, can be solved identically as for diffusion, but replacing the effective diffusion coefficient, D*, by the hydrodynamic dispersion
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coefficient DL (m2/s), according to the following equation [24]: 𝐷𝐿 = 𝛼 𝑣 + 𝐷∗
(6)
where is the longitudinal dynamic dispersivity (m) and represents the mechanical
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dispersion of the solute in the porous media.
Following these equations, the advective-dispersive transport in a homogeneous, isotropic, saturated and non-reactive porous media, with a one-directional flow that
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follows Darcy’s law, results in:
𝐹 = 𝑣 𝑛 𝐶 − 𝑛 𝐷𝐿
𝜕𝐶 𝜕𝑥
(7)
Then, considering a mass balance for a control volume, the following differential equation explains the contaminant transport [24]: 𝜕𝐶 𝜕2𝐶 𝜕𝐶 = 𝐷𝐿 2 − 𝑣 𝜕𝑡 𝜕𝑥 𝜕𝑥
(8)
Equation (8 neglects the influence of reactive granular media, both in terms of chemical sorption, and precipitation. In the first case, if there is chemical sorption, a 5
retardation factor (R) is added. The retardation factor is the quotient between the times for reactive and non-reactive media at which the contaminant breakthrough curve reaches 50% of the initial concentration at a certain distance. Considering the effect of contaminant retention within the barrier, Eq. 8 becomes [24]: 𝜕𝐶 𝐷𝐿 𝜕 2 𝐶 𝑣 𝜕𝐶 = − 𝜕𝑡 𝑅 𝜕𝑥 2 𝑅 𝜕𝑥
(9)
Under these conditions, Ogata and Banks [25] propose an analytical solution defined as: 𝐶0 𝑅𝑥−𝑣𝑡 𝑣𝑥 𝑅𝑥+𝑣𝑡 ) + 𝑒𝑥𝑝 ( ) 𝑒𝑟𝑓𝑐 ( )] [𝑒𝑟𝑓𝑐 ( 2 𝐷𝐿 2√𝐷𝐿 𝑡 𝑅 2√𝐷𝐿 𝑡 𝑅
(10)
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𝐶(𝑥, 𝑡) =
The change in metal concentration can also be modeled by numerical methods (e.g. finite differences, finite elements, or finite volume elements). Usually, results obtained by Eq. 10 and finite differences show negligible variations if proper initial
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and border conditions are considered and stability criteria are fulfilled [26,27].
The current work incorporates concepts from containment and therefore considers a
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common landfill liner design, but also concepts from high permeability reactive barriers like those used for acid mine drainage (Figure 1). By integrating both
the system.
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concepts in a low permeability reactive barrier, it is possible to extend the lifespan of
In the case of reactive filters, hydraulic gradient was set to 1x10-3, a common value
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for groundwater [28], while for low permeability barriers the design used regulated values for leachate head (0.3 m) and barrier height (1 m), as well as possible
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modifications induced by operational setbacks.
L ≥ 1.0 m k ≤ 10-9 m/s
(a)
(b)
(c)
Figure 1. Barrier models considered for this work. (a) Simple barrier for landfill liners, (b) BOF slag permeable reactive barrier, (c) BOF slag-amended landfill liner. Red 6
lines represent contaminated water and blue lines represent clean water, green particles represent BOF slag particles.
2.2. Metal removal Most cost-efficient methods for dissolved metal removal consist in sorption and precipitation. In this work, sorption is considered as part of the removal process in the compacted clay liner (CCL), given the ability of bentonite to sorb metal compounds. In addition, due to the high alkalinity of the steel slag, landfill leachate in
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contact with this solid increases its pH to values near 12 UpH, promoting the precipitation of metal hydroxides [29].
As the ANC of the steel slag is finite and the increase in pH is instantaneous, it is possible to model the change in pH as a sorption of H+ ions, with a retardation factor
a steel slag barrier consists of three stages:
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related to the ANC. Under these assumptions, the transport of a metal cation inside
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a. Metal ions accumulate in the barrier by precipitation mechanisms while leachate flows, and the amount of slag enables a pH higher than the pH of
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solubilization.
b. Once the ANC of the steel slag is consumed, the low pH of the leachate promotes an abrupt pH decrease in the barrier, and therefore a sudden high
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metal concentration pulse.
c. Once equilibrium is reached, the steel slag is considered as an inert solid, and the metal concentration is expected to be the same at the inlet and outlet of the barrier.
This procedure allows the effect that BOF slag induces on metal ion precipitation and
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transport to be incorporated. This was done by considering a retardation factor computed from precipitation tests of Cd, Cu and Pb solutions performed by Francisca and Glatstein [10]. From experimental results, the acid neutralization capacity of the slag is ANC= 100 gH+/kg, which is equivalent to the retardation factor (R) for the removal mechanism considered herein. Furthermore, as the high metal concentration pulse is released, the metals are sorbed in the reactive solids (BOF slag and bentonite), assuming a combined distribution coefficient, Kd. Kd values for different samples were computed from 7
previously published data [30-32], considering 10 l/kg for bentonite and 0.1 l/kg for the BOF slag, and the percentage of each solid used on the different samples.
2.3. Analyzed solids and fluid The solids considered are a loess silt, bentonite and steel slag. Silt and bentonite have been thoroughly studied [33-36]. Steel slag is a byproduct of the BOF in steel production which has only recently been put forward for consideration for chemical processes [13,37]. Table 1 describes the main physical characteristics of the solids
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used.
Table 1. Physical properties and SEM image of the solids SEM image
Physical property
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3 m
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Silt
Bentonite
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2 m
Gs = 2.63 d50 = 0.05 mm d10 = 0.0008 mm % < 0.074 mm = 61.4 % < 0.002 mm = 20.9 Cu = 87.5; Cc = 1.45 LL = 25.3; PI = 8.0 Ss = 1 m²/g USCS = CL – ML
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Solid
Gs = 2.67 d50 = N/D d10 = N/D % < 0.074 mm = 100 % < 0.002 mm = 84.77 Cu = N/A; Cc = N/A LL = 301; PI = 231 Ss = 731 m²/g USCS = CH Gs = 3.05 d50 = 1.50 mm d10 = 0.127 mm % < 0.074 mm = 37 % < 0.002 mm = 0 Cu = 19.4; Cc = 0,63 LL = NP; PI = NP USCS = SP-SM
BOF Slag 1 mm
Note: Specific gravity (Gs): ASTM D854, Mean particle diameter d50 and effective particle diameter d10: ASTM D421 and D422, Liquid limit (LL) and Plastic Index (PI): ASTM D4318 [38], Specific surface (Ss) [39].
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About 95% of the composition of the constituents of BOF slag are silica, alumina and magnesia, and minor elements include manganese, iron and sulfur compounds. Mineralogically,
it
is
constituted
by
merwinite
(3CaO·MgO·2SiO2),
olivine
(2MgO·2FeO·SiO2), β-C2S (2CaO·SiO2), α-C2S, C4AF (4CaO·Al2O3·Fe2O3), C2F (2CaO·Fe2O3), free lime (CaO), MgO, FeO, C3S (3CaO·SiO2), and the RO phase (a solid solution of CaO-FeO-MnO-MgO) [40]. The chemical composition of the slag used in this work is shown in Table 2. The greatest variations are observed for the iron compounds (Fe°, FeO y Fe2O3), due to process fluctuations and the rejects of the steel casting. The toxicity characteristic leaching procedure (TCLP) was carried
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out for the slag. Results showed negligible amounts of Fe (17.9 mg/l), Pb (0.015 mg/l), Cu (0.055 mg/l). Other TCLP metals were detected in extremely low concentrations. .
Compound
Percentage 8.5 – 27.2 37.6 – 44.8 35 - 37.3 9.3 - 14.7 5.6 - 9.6 0.41 0.2 - 0.3 0.4 0.4 - 0.7 0.4 - 0.6 1.1 - 1.2
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Iron compounds CaO SiO2 Al2O3 MgO K2O Na2O S MnO TiO2 Basicity (%CaO/%SiO2)
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Table 2. Chemical composition of the BOF slag used in this work.
The permeating fluid considered in this work consists of an acidic solution (pH=5 UpH in most tests) with a metal concentration of 1 mg/l, mean value for heavy metals
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such as Cd, Cu or Pb in landfill leachates. The hydraulic conductivities considered in this work were those adopted for typical barrier materials. Typical values for sand-bentonite and loess-bentonite varied from k = 1.0x10-8 m/s to k = 1.0x10-11 m/s [20,21], while that for pure slag and sand was set at k = 1.7x10-5 m/s. The barriers considered in this work, bottom landfills liners and permeable reactive barriers, do not require a binder to retain the slag. Landfill liners are commonly made of fine particle materials (e.g. clays and soil-clay mixtures) and therefore BOF slag is held within this matrix with no risk of particle 9
migration. Additionally, due to its high silica content loess is known to promote pozzolanic reactions under alkaline conditions, as expected by the slag activity, and therefore generates weak cemented contacts that may strengthen the barrier structure [36]. In the case of permeable reactive barriers, there is also no need for binder, given that slag can be classified as poorly graded silty sand (SP-SM, Table 1) and therefore there is no risk of BOF particle migration during liquid flux through this porous media.
3. Results
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3.1. Modeling of reactive filters and permeable reactive barriers
Reactive barriers induce contaminant removal by means of chemical reactions, allowing the free flow of remediated water. For this reason, this technique should promote rapid changes in the fluid chemistry, given the high hydraulic conductivity of
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the coarse-grained solids that make up the filter. Flow rate in permeable reactive barriers (PRB) is controlled by groundwater flow, given that they should be
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permeable for liquids but retain contaminants. Hence, two conditions required are to have chemical equilibrium and steady-state. Previous kinetic tests between metals and the slag showed that the optimum remediation time is reached between 5 and
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240 minutes [10], which is over five orders of magnitude lower than the time required for the liquid to pass through the barrier under usual groundwater gradients, which
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ranges from 10-4 to 10-2.
Metal precipitation strongly depends on the solution’s pH. Considering a precipitation pH (pHp) of 8 UpH, as for cadmium, as long as the pH inside the slag barrier remains above this value, cadmium hydroxide will accumulate as shown in Figure 2 (first 40 years). As the acid neutralization capacity (ANC) is saturated, the solution´s
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pH starts to decrease, and when it reaches the pHp of the metal, the metal will redissolve and generate a highly concentrated pulse of the metal (after 73 years). Thereafter, the pH and metal concentration inside the barrier will be the same as the leachate’s when the ANC is completely saturated, and the contaminant pulse is released (approximately after 125 years). As the remediation of heavy metals is mainly controlled by precipitation, it is also necessary to analyze the influence of leachate acidity. Figure 2 shows the change in remediation time and the concentrated pulse of the contaminant for the pH of 10
different solutions. As expected, as the basicity of the leachate increases, the remediation time increases and, when the leachate’s pH is above the precipitation pH of the metal, redissolution does not occur. Importantly, treatment time does not strongly vary with the pH of the liquid (e.g. acid mine drainage), so modeling with extremely low pH values (pH = 2) presents wider possibilities of treatment, without greatly oversizing the system. 14.0
150
12.0
125
10.0
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pH
8.0
75
precipitation pH pHi=2 pHi=5 pHi=7 pHi=9 pHi=2 pHi=5 pHi=7
6.0 4.0 2.0
C/C0
100
50 25
0
0
25
50
-p
0.0 75 100 Time (years)
125
150
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Figure 2: Change in remediation time with initial leachate pH. The model considers barrier height = 1 m, hydraulic gradient = 1x10-3, k = 1.7x10-5 m/s, n = 0.4, ANC =
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100 gH+/kg and pHp = 8. Solid lines represent metal concentration and dashed lines represent pH
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3.2. Modeling of bottom liners in landfills
One of the main factors affecting contaminant transport through liners is hydraulic conductivity, which has variations of several orders of magnitude for different solids. In this research, k ranged from 1.7x10-5 m/s for pure BOF slag or sand, to 10-11 m/s for finer solids or clogged barriers. Under this condition, the decrease in k can be
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achieved by suitable mixtures of slag (the reactive material) with sand, loess or bentonite. However, it is important to notice that reductions in k of several orders of magnitude can also be achieved by the inoculation and growth of microorganisms [23,41]. Given that the main purpose of containment liners is to minimize the flow through the barrier, there is no optimum flow rate through the landfill liner. One of the most important design conditions is to achieve the required hydraulic conductivity of the 11
liner material, and, as k is fundamentally controlled by the fine fraction [20,22] the addition of slag is expected to produce negligible variations in flow rate. Figure 3 shows the influence of hydraulic conductivity on the time the barrier is effective in retaining metal ions. As k decreases, the time for contaminant release increases and so does the concentration pulse. Additionally, as the main goal is to develop a more efficient containment liner, it is fundamental to determine the influence of the thickness of the barrier and the amount of reactive material (steel slag) on the percolation time. It can be seen in Figure 3 that the increase in the thickness of the barrier promotes a high pH for longer periods, and therefore a higher
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metal accumulation, given the larger amount of treated fluid. This, however, represents a higher concentration pulse once the ANC of the barrier is neutralized.
precipitation pH k=1E-8 m/s k=1E-9 m/s k=1E-11 m/s k=1E-8 m/s k=1E-9 m/s k=1E-11 m/s
-p
12.0 10.0
4.0 2.0 0.0
200
400
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0
lP
6.0
600 800 Time (year)
300
200
100
1000
1200
0 1400
(a)
14.0
400 precipitation pH k=1E-8 m/s k=1E-9 m/s k=1E-11 m/s k=1E-8 m/s k=1E-9 m/s k=1E-11 m/s
12.0 10.0
300
pH
8.0 200 6.0 4.0
100
2.0 0.0 0
200
400
600 800 Time (year)
(b) 12
1000
1200
0 1400
C/C0
pH
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8.0
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400
C/C 0
14.0
Figure 3: Contaminant concentration and pH profiles for different hydraulic conductivities of the barrier. Hydraulic head = 0.3 m, n = 0.4, ANC = 100 gH+/kg, pHp = 8, slag fraction= 20%. (a) barrier height = 0.6 m, (b) barrier height = 1 m. Solid lines represent metal concentration and dashed lines represent pH
Hydraulic gradient also affects contaminant transport. Correct design of the leachate collection system can handle the liquid height to reduce the hydraulic head. As the leachate height above the barrier increases, so do the hydraulic gradient and flow velocity, with a consequent reduction in containment time. In contrast, as the liquid
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level decreases, the fluid flow decreases and retention time increases, and therefore the amount of precipitated metal also increases. However, as shown in Figure 4, the relation between hydraulic head and the treatment time is not linear, so there is a trade-off between the effort that implies a reduction in the hydraulic gradient
-p
(draining system), and the expected treatment time. 14.0
250
8.0
150
pH
lP
precipitation pH Hl=0.3 Hl=0.6 Hl=1.2 Hl=2.4 Hl=0.3 Hl=0.6 Hl=1.2 Hl=2.4
200
C/C0
10.0
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12.0
6.0 4.0
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2.0
100
50
0.0
0
50
0 100 150 Time (year)
200
250
Figure 4: Time variation of pH and metal concentration with leachate height (Hl). Model parameters are k = 10-9 m/s, barrier height = 1 m, DL = 2.9x10-9 m2/s, n = 0.4,
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ANC = 100 gH+/kg, pHp = 8 and slag fraction = 20%. Solid lines represent metal concentration and dashed lines represent pH.
Landfill liner design has changed over the years to promote a longer protection of the environment. Two of the most used liner systems in developing countries are a 1 m thick CCL, and a 1.5 mm geomembrane (GM) on top of a 0.6 m CCL, in both cases with a maximum CCL hydraulic conductivity of 10-9 m/s. The breakthrough time for 13
the first case is about 9 years, and for the second is close to 14 years. In addition, if adsorption is considered, the time for the second example (GM+CCL) reaches almost 40 years [42]. This value is at least doubled for the models presented in this work, for similar barrier heights and hydraulic conductivities.
4. Analysis and Discussion As previously presented, hydraulic conductivity is one of the most important parameters regarding the performance of containment barriers. However, as shown in Figure 3, it is important to highlight that a decrease in the hydraulic conductivity of
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the barrier does not result in a proportional increase in remediation time. In fact, there is a value of k after which further reductions in this parameter do not substantially modify the remediation time.
In addition, the results show a tradeoff between the expected removal, the amount of
-p
reactive material and barrier thickness. Figure 5 shows the time at which the contaminant peak is reached for different thicknesses of the barriers, as the amount
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of BOF slag increases.
This result indicates that, for a given breakthrough time, the barrier thickness is
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minimized when steel slag is the only barrier material (Figure 5a); however, for low permeability barriers there should be a minimum fines content to ensure a low k
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(Figure 5b). It can also be seen that doubling the thickness of the barrier makes it possible to use a significantly lower amount of the reactive material and obtain the same results. Although this is not the case for steel slag, this result is particularly useful when the reactive material is expensive. As metal ion precipitation is determined by the equilibrium pH of the barrier, the most important design parameter
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is the amount of BOF slag. Hence, the texture and morphology of the slag are expected to have no influence on the mechanical and hydraulic behavior of the barrier. In addition, certain construction techniques require a minimum width of compacted layer, so this obligatory increase in width may help reduce the amount of reactive material.
14
100
Slag fraction (%)
80 60
Hb=0.3
Hb=0.6 40
Hb=1.0
20
0 0.001
0.01
0.1
1
10
100
1000
10000
Breakthrough time (year)
(a)
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100
60
40
Hb=0.3 Hb=0.6
20 0 0.001
Hb=1.0
0.01
0.1
1
10
-p
Slag fraction (%)
80
100
1000
10000
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Breakthrough time (year)
(b) Figure 5: Breakthrough time for (a) high and (b) low permeability liners. Model
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parameters are k = 1.7x10-5 m/s for case (a), k = 1.0x10-9 m/s for case (b), leachate
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height = 0.3 m, DL = 2.9x10-9 m2/s, n = 0.4, ANC = 100 gH+/kg and pHp = 8.
The barrier porosity may also affect the displacement of contaminants in porous media, as is clearly shown in the advective-dispersive transport equation (eq. 7). However, variability in porosity is extremely low in comparison with other properties of porous media such as hydraulic conductivity. Porosity may vary from 0.25 to 0.50
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for most soils, while hydraulic conductivity may change more than 10 orders of magnitude [20]. Even for porosity changes between the minimum and maximum expected values (e.g. from 0.25 to 0.5 for clean uniform sand), the expected change in breakthrough time is lower than 2.2%. The breakthrough time values shown in Figure 5 make it clear that porosity has a negligible effect on mass transport for most practical purposes.
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5. Conclusions This study examined the effects of precipitation on the transport of metal ions through porous media, such as permeable reactive barriers and low hydraulic conductivity liners. Transport models were developed and calibrated with experimental results of the metal ion precipitation and hydraulic conductivity of barrier materials. The effects of barrier thickness, hydraulic conductivity and hydraulic gradient on the time that the barrier effectively retains metals were evaluated. The main conclusions can be summarized as follows: Basic oxygen furnace (BOF) slag successfully treats leachates by
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increasing the pH, which promotes precipitation of metal ions and therefore retards their displacement. BOF slag can be used as construction material for permeable reactive barriers due to its high acid
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neutralization capacity in the treatment of acid mine drainage. A BOF slag PRB can successfully treat a pH=2 solution for over 70 years under
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common hydraulic gradients. In addition, given the difficulty of removing previously installed liners, the installation of a PRB downstream of a
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landfill enhances the protection of groundwater. The proposed model shows a sudden decrease in leachate pH once the ANC is depleted, and a highly concentrated contaminant pulse is released
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once the pH of neutralization is reached. This impact can be minimized by detection methods and a secondary collection system.
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The addition of BOF slag in landfill liners significantly increases the percolation time for metal ions through the barrier. This can be used for the design of safer barriers against possible undesirable scenarios such as
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increases in hydraulic gradient (e.g. due to accumulation of leachate on the liner) and hydraulic conductivity (e.g. due to poor compaction conditions). Amendment with 20% BOF slag increases a common liner lifespan to over 180 years.
The use of BOF slag as construction material for permeable reactive barriers and landfill liners helps to increase the life cycle of this material and, at the same time, protect the environment. The results shown in this study offer new possibilities to find sustainable environmental applications for BOF slag. 16
Acknowledgments This
research
was
partially
financed
by
CONICET
(grant
number
11220100100390CO), FONCyT (PICT 2014 grant numbers 3101 and 2246) and SECyT-UNC (grant number 05/M265). D.A.G. thanks CONICET for doctoral and postdoctoral fellowships. The authors thank FCEFyN-UNC and ISEA-UNC for the support received, and Camila Tembrás for helping with hydraulic conductivity tests
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on slag.
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20
PII:
S0304-3894(19)31248-8
DOI:
https://doi.org/10.1016/j.jhazmat.2019.121294
Reference:
HAZMAT 121294
To appear in:
Journal of Hazardous Materials
Received Date:
25 May 2019
Revised Date:
16 August 2019
Accepted Date:
22 September 2019
Please cite this article as: Francisca FM, Glatstein DA, Environmental application of basic oxygen furnace slag for the removal of heavy metals from leachates, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121294
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Environmental application of basic oxygen furnace slag for the removal of heavy metals from leachates
Franco M. Francisca (1,2,*), Daniel A. Glatstein (2,3)
(1) Universidad Nacional de Córdoba, Facultad de Ciencias Exactas Físicas y Naturales, Departamento de Construcciones Civiles, Córdoba, Argentina. (2) CONICET - Universidad Nacional de Córdoba, Instituto de Estudios Avanzados en Ingeniería y Tecnología (IDIT), Córdoba, Argentina.
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(3) Universidad Nacional de Córdoba, Facultad de Ciencias Exactas Físicas y Naturales, Departamento de Química Industrial y Aplicada, Córdoba, Argentina. (*) Corresponding author. Av. Velez Sarsfield 1611, CP 5016, Córdoba, Argentina. Tel +54 351 5353800 ext. 836. Email: [email protected]
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Research highlights
Addition of BOF slag in permeable barriers promotes metal ions precipitation
BOF slag helps in retarding metal ion transport through landfill liners
BOF slag in permeable barriers promotes precipitation of metal ions.
New environmental applications can be found for slag extending its life cycle.
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Abstract
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Industrial waste is a major environmental concern nowadays, stimulating the thorough study of the minimization and recycling of solid wastes and of the containment and treatment of liquid contaminants. Basic oxygen furnace (BOF) slag,
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a solid waste from the steel industry, has been found to be effective in the removal of heavy metals. However, this has not been applied so far in low permeability barriers, such as those used as bottom liners in landfills. This work studies the performance of BOF slag in both containment and treatment technologies for toxic leachates. Flow models are developed to assess the transport of metal ions through a permeable reactive barrier and a composite clay barrier. Reactive transport through the slag barrier and adsorption in the clay barrier are coupled for different conditions to find the residence time, the barrier life span and the optimum operative conditions. The 1
results show that the use of steel slag increases the breakthrough time of the contaminants, enabling improve design of low and high conductivity reactive barriers, and expands the life cycle of the material. Keywords Precipitation; landfill leachate; liner; permeable reactive barrier; residual material.
1. Introduction
Liquid pollutants can severely affect human health and may have significant
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environmental impacts Among other sources of these contaminants, landfill leachate and mining drainage induce significant health and environmental problems.
Landfill leachate is highly toxic and one of the most difficult contaminants to analyze and treat, given its temporal and spatial variability. Environmental factors such as
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temperature and precipitation modify its volume and bacterial activity which, in turn, changes the pH and therefore its composition. In addition, landfill cell design impacts
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the contact time with waste. The chemical composition of landfill leachates depends on waste properties and age, and therefore, varies from one landfill to another [1,2].
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Even though most landfills are constructed for municipal solid waste (MSW), the presence of heavy metals such as Cd, Cu or Pb is fairly common due to poor management techniques and controls. Moreover, as the leachate is often
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recirculated to maintain minimum moisture content, the metals concentration increases with time.
Similarly, mine drainage shows significant temporal and spatial variability in physical and geochemical properties [3]. These properties are mainly determined by chemical, biological and physical factors as well as by the hydraulic conductivity of
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waste rock dumps. The main concern is to prevent liquid contaminant migration by means of barriers, cut-off walls, trenches, or wells [4]. Some solutions allow water movement but prevent mass transport by retaining the contaminants. Given the complexity and toxicity of leachates, their containment, transport, and treatment have attracted great interest in recent years. The practice of containment has evolved from simple soil compaction to the amendment of local soil with low hydraulic
conductivity
clays,
or
the
addition
of
layers
of
geotextiles
or
geomembranes. These changes have been accompanied by regulations specifying a 2
maximum value for hydraulic conductivity or a minimum thickness of the soil layer or the geomembrane. Regulations and recommendations for landfill barriers differ from one country to another, but a minimum soil layer of 0.9 m with a maximum k of 10-9 m/s is a common requirement for simple liners without geomembrane. However, most regulations require a HDPE membrane, or special composite materials, such as geotextiles, or special drains [5,6]. Despite the effect that chemistry and microbiology have on the performance of liners, no regulation contemplates the adsorption of contaminants or the clogging of the barrier.
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Some regulations are flexible allowing a change in barrier composition, as long as it presents an equivalent performance in terms of the containment of contaminants. This presents the main issue regarding landfill liners: although they are designed to contain and remove the leachate, some of this leachate passes through the barrier
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without treatment and migrates toward the groundwater. This drives the search for alternative materials such as modified clays, natural fibers or husks, or even
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industrial residues such as sewage sludge or fly ash, which may contain the contaminants within the barrier while the liquid passes through [7-9].
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One of these industrial residues is steel slag, a coarse material with a high capacity to remove heavy metals from solution given its high alkalinity [10]. It is a byproduct of steel manufacturing, conventionally used as flux in steel production, but with
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applications related with concrete and asphalt production and the construction of road grades and sub-grades [11]. More recent studies also suggest its use in wastewater treatment, iron recovery, CO2 and phosphorous sequestration, and as fertilizer [12-15]. In particular, its ability to increase the solution pH and thus promote metal precipitation serves as a secondary process retarding contaminants passing
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through liners or even permeable reactive barriers. These barriers allow the free flow of liquids but retain contaminants by adsorption, reaction, oxidation or precipitation [16-19]. Such treatments sequester the contaminant within porous media through physicochemical reactions and particle-fluid interaction mechanisms. The purpose of this work is to analyze the coupled effect of sorption and precipitation on the transport of heavy metals through composite granular liners. Applications of barriers with high and low hydraulic conductivities are considered (i.e. permeable reactive barriers and landfill liners, respectively). The effect of the acid neutralization 3
capacity (ANC) of the slag on metal ions present in mine drainage and landfill leachate is addressed by coupled transport models.
2. Model and Materials The models presented in this paper are supported by previously published experimental work. Francisca and Glatstein [10] indicated that the pH of precipitation for several metals, acting as individual ions or as a mixture, ranged from 6 to 8 UpH. Values for the hydraulic conductivity of sand-bentonite and loess-bentonite were indicated by Montoro and Francisca [20] and by Francisca and Glatstein [21].
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Additional experimental work was carried to test specific parameters, such as the ANC of the slag, or the hydraulic conductivity of slag samples.
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2.1. Contaminant transport in porous media
Contaminant transport in porous media is influenced by the physical restrictions of
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the solid phase (soil particles, mineral and rocks) and chemical and hydraulic gradients. Chemical gradients promote diffusive processes that are represented by
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Fick’s first and second laws (Eq. 1 and 2). 𝐹𝑑𝑖𝑓 = −𝐷 ∗
𝜕𝐶 𝜕𝑥
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𝜕𝐶 𝜕2𝐶 ∗ =𝐷 𝜕𝑡 𝜕𝑥 2
(1) (2)
where F represents the chemical mass flow (mg/m2·s), D* is the effective diffusion coefficient (m2/s), t is the transport time (s), x is the distance from the source (m) and C is the concentration of the chemical species in solution (mg/m3).
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Chemical species can also be associated with liquid flux. In this case, the amount of solute advected by the flowing groundwater depends on its concentration and the water flow, as represented by Eq. 3. 𝐹𝑎𝑑𝑣 = 𝑣 𝑛 𝐶
(3)
where v is the mean fluid velocity (m/s) and n is the porosity of the granular media. By analogy to Fick’s laws, the equation for one-dimensional advective transport is given by Eq. 4: 4
𝜕𝐶 𝜕𝐶 = −𝑣 𝜕𝑡 𝜕𝑥
(4)
Equations 3 and 4 are valid for fluids that follow Darcy’s law, which indicates a relation between the flow velocity and the hydraulic gradient (i) through a proportionality constant, hydraulic conductivity (k) (m/s), defined as follows: 𝑣 =𝑘∙𝑖
(5)
Hydraulic conductivity is one of the most important parameters for the design of reactive barriers and containment liners, given its variability. This parameter is
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usually modeled as time-independent but several studies recognize its change with time, due to biological and chemical clogging [20,22,23]. The hydraulic gradient (i) is the ratio between the total-head loss between both sides of a barrier and the barrier thickness (i = H/L). Thus, hydraulic gradient decreases as the thickness of the
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barrier increases for a given total-head pressure difference.
In addition, mechanical dispersivity promoted by different flow paths, pore size
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distribution and friction with pores, can be solved identically as for diffusion, but replacing the effective diffusion coefficient, D*, by the hydrodynamic dispersion
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coefficient DL (m2/s), according to the following equation [24]: 𝐷𝐿 = 𝛼 𝑣 + 𝐷∗
(6)
where is the longitudinal dynamic dispersivity (m) and represents the mechanical
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dispersion of the solute in the porous media.
Following these equations, the advective-dispersive transport in a homogeneous, isotropic, saturated and non-reactive porous media, with a one-directional flow that
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follows Darcy’s law, results in:
𝐹 = 𝑣 𝑛 𝐶 − 𝑛 𝐷𝐿
𝜕𝐶 𝜕𝑥
(7)
Then, considering a mass balance for a control volume, the following differential equation explains the contaminant transport [24]: 𝜕𝐶 𝜕2𝐶 𝜕𝐶 = 𝐷𝐿 2 − 𝑣 𝜕𝑡 𝜕𝑥 𝜕𝑥
(8)
Equation (8 neglects the influence of reactive granular media, both in terms of chemical sorption, and precipitation. In the first case, if there is chemical sorption, a 5
retardation factor (R) is added. The retardation factor is the quotient between the times for reactive and non-reactive media at which the contaminant breakthrough curve reaches 50% of the initial concentration at a certain distance. Considering the effect of contaminant retention within the barrier, Eq. 8 becomes [24]: 𝜕𝐶 𝐷𝐿 𝜕 2 𝐶 𝑣 𝜕𝐶 = − 𝜕𝑡 𝑅 𝜕𝑥 2 𝑅 𝜕𝑥
(9)
Under these conditions, Ogata and Banks [25] propose an analytical solution defined as: 𝐶0 𝑅𝑥−𝑣𝑡 𝑣𝑥 𝑅𝑥+𝑣𝑡 ) + 𝑒𝑥𝑝 ( ) 𝑒𝑟𝑓𝑐 ( )] [𝑒𝑟𝑓𝑐 ( 2 𝐷𝐿 2√𝐷𝐿 𝑡 𝑅 2√𝐷𝐿 𝑡 𝑅
(10)
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𝐶(𝑥, 𝑡) =
The change in metal concentration can also be modeled by numerical methods (e.g. finite differences, finite elements, or finite volume elements). Usually, results obtained by Eq. 10 and finite differences show negligible variations if proper initial
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and border conditions are considered and stability criteria are fulfilled [26,27].
The current work incorporates concepts from containment and therefore considers a
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common landfill liner design, but also concepts from high permeability reactive barriers like those used for acid mine drainage (Figure 1). By integrating both
the system.
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concepts in a low permeability reactive barrier, it is possible to extend the lifespan of
In the case of reactive filters, hydraulic gradient was set to 1x10-3, a common value
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for groundwater [28], while for low permeability barriers the design used regulated values for leachate head (0.3 m) and barrier height (1 m), as well as possible
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modifications induced by operational setbacks.
L ≥ 1.0 m k ≤ 10-9 m/s
(a)
(b)
(c)
Figure 1. Barrier models considered for this work. (a) Simple barrier for landfill liners, (b) BOF slag permeable reactive barrier, (c) BOF slag-amended landfill liner. Red 6
lines represent contaminated water and blue lines represent clean water, green particles represent BOF slag particles.
2.2. Metal removal Most cost-efficient methods for dissolved metal removal consist in sorption and precipitation. In this work, sorption is considered as part of the removal process in the compacted clay liner (CCL), given the ability of bentonite to sorb metal compounds. In addition, due to the high alkalinity of the steel slag, landfill leachate in
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contact with this solid increases its pH to values near 12 UpH, promoting the precipitation of metal hydroxides [29].
As the ANC of the steel slag is finite and the increase in pH is instantaneous, it is possible to model the change in pH as a sorption of H+ ions, with a retardation factor
a steel slag barrier consists of three stages:
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related to the ANC. Under these assumptions, the transport of a metal cation inside
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a. Metal ions accumulate in the barrier by precipitation mechanisms while leachate flows, and the amount of slag enables a pH higher than the pH of
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solubilization.
b. Once the ANC of the steel slag is consumed, the low pH of the leachate promotes an abrupt pH decrease in the barrier, and therefore a sudden high
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metal concentration pulse.
c. Once equilibrium is reached, the steel slag is considered as an inert solid, and the metal concentration is expected to be the same at the inlet and outlet of the barrier.
This procedure allows the effect that BOF slag induces on metal ion precipitation and
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transport to be incorporated. This was done by considering a retardation factor computed from precipitation tests of Cd, Cu and Pb solutions performed by Francisca and Glatstein [10]. From experimental results, the acid neutralization capacity of the slag is ANC= 100 gH+/kg, which is equivalent to the retardation factor (R) for the removal mechanism considered herein. Furthermore, as the high metal concentration pulse is released, the metals are sorbed in the reactive solids (BOF slag and bentonite), assuming a combined distribution coefficient, Kd. Kd values for different samples were computed from 7
previously published data [30-32], considering 10 l/kg for bentonite and 0.1 l/kg for the BOF slag, and the percentage of each solid used on the different samples.
2.3. Analyzed solids and fluid The solids considered are a loess silt, bentonite and steel slag. Silt and bentonite have been thoroughly studied [33-36]. Steel slag is a byproduct of the BOF in steel production which has only recently been put forward for consideration for chemical processes [13,37]. Table 1 describes the main physical characteristics of the solids
ro of
used.
Table 1. Physical properties and SEM image of the solids SEM image
Physical property
re
3 m
ur na
lP
Silt
Bentonite
Jo
2 m
Gs = 2.63 d50 = 0.05 mm d10 = 0.0008 mm % < 0.074 mm = 61.4 % < 0.002 mm = 20.9 Cu = 87.5; Cc = 1.45 LL = 25.3; PI = 8.0 Ss = 1 m²/g USCS = CL – ML
-p
Solid
Gs = 2.67 d50 = N/D d10 = N/D % < 0.074 mm = 100 % < 0.002 mm = 84.77 Cu = N/A; Cc = N/A LL = 301; PI = 231 Ss = 731 m²/g USCS = CH Gs = 3.05 d50 = 1.50 mm d10 = 0.127 mm % < 0.074 mm = 37 % < 0.002 mm = 0 Cu = 19.4; Cc = 0,63 LL = NP; PI = NP USCS = SP-SM
BOF Slag 1 mm
Note: Specific gravity (Gs): ASTM D854, Mean particle diameter d50 and effective particle diameter d10: ASTM D421 and D422, Liquid limit (LL) and Plastic Index (PI): ASTM D4318 [38], Specific surface (Ss) [39].
8
About 95% of the composition of the constituents of BOF slag are silica, alumina and magnesia, and minor elements include manganese, iron and sulfur compounds. Mineralogically,
it
is
constituted
by
merwinite
(3CaO·MgO·2SiO2),
olivine
(2MgO·2FeO·SiO2), β-C2S (2CaO·SiO2), α-C2S, C4AF (4CaO·Al2O3·Fe2O3), C2F (2CaO·Fe2O3), free lime (CaO), MgO, FeO, C3S (3CaO·SiO2), and the RO phase (a solid solution of CaO-FeO-MnO-MgO) [40]. The chemical composition of the slag used in this work is shown in Table 2. The greatest variations are observed for the iron compounds (Fe°, FeO y Fe2O3), due to process fluctuations and the rejects of the steel casting. The toxicity characteristic leaching procedure (TCLP) was carried
ro of
out for the slag. Results showed negligible amounts of Fe (17.9 mg/l), Pb (0.015 mg/l), Cu (0.055 mg/l). Other TCLP metals were detected in extremely low concentrations. .
Compound
Percentage 8.5 – 27.2 37.6 – 44.8 35 - 37.3 9.3 - 14.7 5.6 - 9.6 0.41 0.2 - 0.3 0.4 0.4 - 0.7 0.4 - 0.6 1.1 - 1.2
ur na
lP
re
Iron compounds CaO SiO2 Al2O3 MgO K2O Na2O S MnO TiO2 Basicity (%CaO/%SiO2)
-p
Table 2. Chemical composition of the BOF slag used in this work.
The permeating fluid considered in this work consists of an acidic solution (pH=5 UpH in most tests) with a metal concentration of 1 mg/l, mean value for heavy metals
Jo
such as Cd, Cu or Pb in landfill leachates. The hydraulic conductivities considered in this work were those adopted for typical barrier materials. Typical values for sand-bentonite and loess-bentonite varied from k = 1.0x10-8 m/s to k = 1.0x10-11 m/s [20,21], while that for pure slag and sand was set at k = 1.7x10-5 m/s. The barriers considered in this work, bottom landfills liners and permeable reactive barriers, do not require a binder to retain the slag. Landfill liners are commonly made of fine particle materials (e.g. clays and soil-clay mixtures) and therefore BOF slag is held within this matrix with no risk of particle 9
migration. Additionally, due to its high silica content loess is known to promote pozzolanic reactions under alkaline conditions, as expected by the slag activity, and therefore generates weak cemented contacts that may strengthen the barrier structure [36]. In the case of permeable reactive barriers, there is also no need for binder, given that slag can be classified as poorly graded silty sand (SP-SM, Table 1) and therefore there is no risk of BOF particle migration during liquid flux through this porous media.
3. Results
ro of
3.1. Modeling of reactive filters and permeable reactive barriers
Reactive barriers induce contaminant removal by means of chemical reactions, allowing the free flow of remediated water. For this reason, this technique should promote rapid changes in the fluid chemistry, given the high hydraulic conductivity of
-p
the coarse-grained solids that make up the filter. Flow rate in permeable reactive barriers (PRB) is controlled by groundwater flow, given that they should be
re
permeable for liquids but retain contaminants. Hence, two conditions required are to have chemical equilibrium and steady-state. Previous kinetic tests between metals and the slag showed that the optimum remediation time is reached between 5 and
lP
240 minutes [10], which is over five orders of magnitude lower than the time required for the liquid to pass through the barrier under usual groundwater gradients, which
ur na
ranges from 10-4 to 10-2.
Metal precipitation strongly depends on the solution’s pH. Considering a precipitation pH (pHp) of 8 UpH, as for cadmium, as long as the pH inside the slag barrier remains above this value, cadmium hydroxide will accumulate as shown in Figure 2 (first 40 years). As the acid neutralization capacity (ANC) is saturated, the solution´s
Jo
pH starts to decrease, and when it reaches the pHp of the metal, the metal will redissolve and generate a highly concentrated pulse of the metal (after 73 years). Thereafter, the pH and metal concentration inside the barrier will be the same as the leachate’s when the ANC is completely saturated, and the contaminant pulse is released (approximately after 125 years). As the remediation of heavy metals is mainly controlled by precipitation, it is also necessary to analyze the influence of leachate acidity. Figure 2 shows the change in remediation time and the concentrated pulse of the contaminant for the pH of 10
different solutions. As expected, as the basicity of the leachate increases, the remediation time increases and, when the leachate’s pH is above the precipitation pH of the metal, redissolution does not occur. Importantly, treatment time does not strongly vary with the pH of the liquid (e.g. acid mine drainage), so modeling with extremely low pH values (pH = 2) presents wider possibilities of treatment, without greatly oversizing the system. 14.0
150
12.0
125
10.0
ro of
pH
8.0
75
precipitation pH pHi=2 pHi=5 pHi=7 pHi=9 pHi=2 pHi=5 pHi=7
6.0 4.0 2.0
C/C0
100
50 25
0
0
25
50
-p
0.0 75 100 Time (years)
125
150
re
Figure 2: Change in remediation time with initial leachate pH. The model considers barrier height = 1 m, hydraulic gradient = 1x10-3, k = 1.7x10-5 m/s, n = 0.4, ANC =
lP
100 gH+/kg and pHp = 8. Solid lines represent metal concentration and dashed lines represent pH
ur na
3.2. Modeling of bottom liners in landfills
One of the main factors affecting contaminant transport through liners is hydraulic conductivity, which has variations of several orders of magnitude for different solids. In this research, k ranged from 1.7x10-5 m/s for pure BOF slag or sand, to 10-11 m/s for finer solids or clogged barriers. Under this condition, the decrease in k can be
Jo
achieved by suitable mixtures of slag (the reactive material) with sand, loess or bentonite. However, it is important to notice that reductions in k of several orders of magnitude can also be achieved by the inoculation and growth of microorganisms [23,41]. Given that the main purpose of containment liners is to minimize the flow through the barrier, there is no optimum flow rate through the landfill liner. One of the most important design conditions is to achieve the required hydraulic conductivity of the 11
liner material, and, as k is fundamentally controlled by the fine fraction [20,22] the addition of slag is expected to produce negligible variations in flow rate. Figure 3 shows the influence of hydraulic conductivity on the time the barrier is effective in retaining metal ions. As k decreases, the time for contaminant release increases and so does the concentration pulse. Additionally, as the main goal is to develop a more efficient containment liner, it is fundamental to determine the influence of the thickness of the barrier and the amount of reactive material (steel slag) on the percolation time. It can be seen in Figure 3 that the increase in the thickness of the barrier promotes a high pH for longer periods, and therefore a higher
ro of
metal accumulation, given the larger amount of treated fluid. This, however, represents a higher concentration pulse once the ANC of the barrier is neutralized.
precipitation pH k=1E-8 m/s k=1E-9 m/s k=1E-11 m/s k=1E-8 m/s k=1E-9 m/s k=1E-11 m/s
-p
12.0 10.0
4.0 2.0 0.0
200
400
ur na
0
lP
6.0
600 800 Time (year)
300
200
100
1000
1200
0 1400
(a)
14.0
400 precipitation pH k=1E-8 m/s k=1E-9 m/s k=1E-11 m/s k=1E-8 m/s k=1E-9 m/s k=1E-11 m/s
12.0 10.0
300
pH
8.0 200 6.0 4.0
100
2.0 0.0 0
200
400
600 800 Time (year)
(b) 12
1000
1200
0 1400
C/C0
pH
re
8.0
Jo
400
C/C 0
14.0
Figure 3: Contaminant concentration and pH profiles for different hydraulic conductivities of the barrier. Hydraulic head = 0.3 m, n = 0.4, ANC = 100 gH+/kg, pHp = 8, slag fraction= 20%. (a) barrier height = 0.6 m, (b) barrier height = 1 m. Solid lines represent metal concentration and dashed lines represent pH
Hydraulic gradient also affects contaminant transport. Correct design of the leachate collection system can handle the liquid height to reduce the hydraulic head. As the leachate height above the barrier increases, so do the hydraulic gradient and flow velocity, with a consequent reduction in containment time. In contrast, as the liquid
ro of
level decreases, the fluid flow decreases and retention time increases, and therefore the amount of precipitated metal also increases. However, as shown in Figure 4, the relation between hydraulic head and the treatment time is not linear, so there is a trade-off between the effort that implies a reduction in the hydraulic gradient
-p
(draining system), and the expected treatment time. 14.0
250
8.0
150
pH
lP
precipitation pH Hl=0.3 Hl=0.6 Hl=1.2 Hl=2.4 Hl=0.3 Hl=0.6 Hl=1.2 Hl=2.4
200
C/C0
10.0
re
12.0
6.0 4.0
ur na
2.0
100
50
0.0
0
50
0 100 150 Time (year)
200
250
Figure 4: Time variation of pH and metal concentration with leachate height (Hl). Model parameters are k = 10-9 m/s, barrier height = 1 m, DL = 2.9x10-9 m2/s, n = 0.4,
Jo
ANC = 100 gH+/kg, pHp = 8 and slag fraction = 20%. Solid lines represent metal concentration and dashed lines represent pH.
Landfill liner design has changed over the years to promote a longer protection of the environment. Two of the most used liner systems in developing countries are a 1 m thick CCL, and a 1.5 mm geomembrane (GM) on top of a 0.6 m CCL, in both cases with a maximum CCL hydraulic conductivity of 10-9 m/s. The breakthrough time for 13
the first case is about 9 years, and for the second is close to 14 years. In addition, if adsorption is considered, the time for the second example (GM+CCL) reaches almost 40 years [42]. This value is at least doubled for the models presented in this work, for similar barrier heights and hydraulic conductivities.
4. Analysis and Discussion As previously presented, hydraulic conductivity is one of the most important parameters regarding the performance of containment barriers. However, as shown in Figure 3, it is important to highlight that a decrease in the hydraulic conductivity of
ro of
the barrier does not result in a proportional increase in remediation time. In fact, there is a value of k after which further reductions in this parameter do not substantially modify the remediation time.
In addition, the results show a tradeoff between the expected removal, the amount of
-p
reactive material and barrier thickness. Figure 5 shows the time at which the contaminant peak is reached for different thicknesses of the barriers, as the amount
re
of BOF slag increases.
This result indicates that, for a given breakthrough time, the barrier thickness is
lP
minimized when steel slag is the only barrier material (Figure 5a); however, for low permeability barriers there should be a minimum fines content to ensure a low k
ur na
(Figure 5b). It can also be seen that doubling the thickness of the barrier makes it possible to use a significantly lower amount of the reactive material and obtain the same results. Although this is not the case for steel slag, this result is particularly useful when the reactive material is expensive. As metal ion precipitation is determined by the equilibrium pH of the barrier, the most important design parameter
Jo
is the amount of BOF slag. Hence, the texture and morphology of the slag are expected to have no influence on the mechanical and hydraulic behavior of the barrier. In addition, certain construction techniques require a minimum width of compacted layer, so this obligatory increase in width may help reduce the amount of reactive material.
14
100
Slag fraction (%)
80 60
Hb=0.3
Hb=0.6 40
Hb=1.0
20
0 0.001
0.01
0.1
1
10
100
1000
10000
Breakthrough time (year)
(a)
ro of
100
60
40
Hb=0.3 Hb=0.6
20 0 0.001
Hb=1.0
0.01
0.1
1
10
-p
Slag fraction (%)
80
100
1000
10000
re
Breakthrough time (year)
(b) Figure 5: Breakthrough time for (a) high and (b) low permeability liners. Model
lP
parameters are k = 1.7x10-5 m/s for case (a), k = 1.0x10-9 m/s for case (b), leachate
ur na
height = 0.3 m, DL = 2.9x10-9 m2/s, n = 0.4, ANC = 100 gH+/kg and pHp = 8.
The barrier porosity may also affect the displacement of contaminants in porous media, as is clearly shown in the advective-dispersive transport equation (eq. 7). However, variability in porosity is extremely low in comparison with other properties of porous media such as hydraulic conductivity. Porosity may vary from 0.25 to 0.50
Jo
for most soils, while hydraulic conductivity may change more than 10 orders of magnitude [20]. Even for porosity changes between the minimum and maximum expected values (e.g. from 0.25 to 0.5 for clean uniform sand), the expected change in breakthrough time is lower than 2.2%. The breakthrough time values shown in Figure 5 make it clear that porosity has a negligible effect on mass transport for most practical purposes.
15
5. Conclusions This study examined the effects of precipitation on the transport of metal ions through porous media, such as permeable reactive barriers and low hydraulic conductivity liners. Transport models were developed and calibrated with experimental results of the metal ion precipitation and hydraulic conductivity of barrier materials. The effects of barrier thickness, hydraulic conductivity and hydraulic gradient on the time that the barrier effectively retains metals were evaluated. The main conclusions can be summarized as follows: Basic oxygen furnace (BOF) slag successfully treats leachates by
ro of
-
increasing the pH, which promotes precipitation of metal ions and therefore retards their displacement. BOF slag can be used as construction material for permeable reactive barriers due to its high acid
-p
neutralization capacity in the treatment of acid mine drainage. A BOF slag PRB can successfully treat a pH=2 solution for over 70 years under
re
common hydraulic gradients. In addition, given the difficulty of removing previously installed liners, the installation of a PRB downstream of a
-
lP
landfill enhances the protection of groundwater. The proposed model shows a sudden decrease in leachate pH once the ANC is depleted, and a highly concentrated contaminant pulse is released
ur na
once the pH of neutralization is reached. This impact can be minimized by detection methods and a secondary collection system.
-
The addition of BOF slag in landfill liners significantly increases the percolation time for metal ions through the barrier. This can be used for the design of safer barriers against possible undesirable scenarios such as
Jo
increases in hydraulic gradient (e.g. due to accumulation of leachate on the liner) and hydraulic conductivity (e.g. due to poor compaction conditions). Amendment with 20% BOF slag increases a common liner lifespan to over 180 years.
The use of BOF slag as construction material for permeable reactive barriers and landfill liners helps to increase the life cycle of this material and, at the same time, protect the environment. The results shown in this study offer new possibilities to find sustainable environmental applications for BOF slag. 16
Acknowledgments This
research
was
partially
financed
by
CONICET
(grant
number
11220100100390CO), FONCyT (PICT 2014 grant numbers 3101 and 2246) and SECyT-UNC (grant number 05/M265). D.A.G. thanks CONICET for doctoral and postdoctoral fellowships. The authors thank FCEFyN-UNC and ISEA-UNC for the support received, and Camila Tembrás for helping with hydraulic conductivity tests
ro of
on slag.
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lP
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