STABILIZATION OF HEAVY METALS FROM CONTAMINATED
SEDIMENT – ENGINEERING PROPERTIES FOR VALORIZATION
N Yacoubi1, Y Yang2, S Gao2, L Hu2, B Ladevie1, A Nzihou1
1
Research Center in Albi for Particulates Solids, Energy and Environment (UMR 2392) Ecole
des Mines d'Albi Carmaux, Route de Teillet, 81013 Albi CT Cedex 09, France
2
Department of Hydraulic Engineering, Tsinghua University , Beijing 100084, China
Abstract
This study assesses the technology to treat heavy metals from dredged sediments using phosphoric acid (The
Novosol process) with the goal of converting metals, mainly Pb, Cd, Zn and Cu to insoluble metallic phosphates
and engineering safe secondary materials for further valorisation. The effectiveness of the treatment was evaluated
by performing the chemical reaction in a mixed reactor, followed by convective drying and maturation of the
treated sediment at ambient temperature. Finally thermal destruction of the organic matter by calcination (500°C700°C) and sintering of the residue. Selected variables (acid concentration, volume fraction of solids, drying and
calcination temperature and time) were investigated. Stabilisation results from chemical reaction, modification of
species and textural change at high temperatures. This consequently reduces the availability of the entrapped
pollutant as confirmed by leaching tests performed when assessing the environmental impact.
The correlation between the dynamics occurring during the sediment stabilization – the in situ structural
characterisation –the stabilisation of heavy metals and the final properties, such as the mechanical ones, of the
residues are assessed.
Key words: dredged sediment, heavy metal, organic contamination, characterization, stabilization,
phosphate treatment, sequential extraction.
1. Introduction
Sediments are important sinks of inorganic and organic pollutants deriving from municipal and
industrial discharge. Among the pollutants found, many substances of strong environmental
concern like heavy metals [Miller(1) et al., 2004; Tabak(2) et al., 2003], polynuclear aromatic
hydrocarbons (PNA) [Rausa(3) et al., 1999] and dioxins [Lee(4) et al., 2006], that are harmful to
the environment and may also be a threat to human health at high concentrations. The problem
is even more highlighted by the need of carrying out dredging in order to build new ports or
simply to maintain them for navigational purpose. However, despite the scale of the metal
pollution problem, there are limited options for remediation or environmental management of
metal contaminated sediments. Depending on their metal toxicity they will be either spread on
agricultural land, confined or sent for chemical treatment [Marot(5), 1998]. In a sustainable
development policy, SOLVAY S.A developed a patented process for sediment treatment by
heavy metals stabilization and organics destruction in order to reuse them in civil engineering.
This process is named Novosol®, and consists of 3 steps. The first one is a phosphate treatment
where heavy metals are transformed into insoluble metal-phosphates. This chemical reaction is
then followed by a step of drying at ambient temperature in order to reduce the sediment water
content and to allow the maturation of the reaction products by continuing the phosphoric acid
neutralization extending thus the stability of the final residues. The final stage concerns the
thermal treatment (calcination) of the treated sediments at 500–700°C in order to transform the
metal-phosphate precursors into more inert crystalline compounds such as hydroxypyromorphite
(Pb5(PO4)3OH). During this step, organic matter is eliminated by combustion. The final solid
residue is inert and likely to be reused in civil engineering field, particularly in the manufacture
of bricks for construction and road base materials. These potential beneficial reuse scenarios
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were previously demonstrated for stabilized municipal solid waste incineration fly-ashes
[Bournonville(6), 2002].
This study has been mainly performed in a laboratory scale to elucidate the influence of
phosphate treatment on heavy metal stabilization and its combined effect with thermal treatment
on the physical and mechanical properties of the final residues. Heavy metal distribution has
been assessed using sequential extraction as described by Tessier(7) et al., 1979. This protocol is
based on the rational use of a series of more or less selective reagents chosen to solubilise
successively the different mineralogical fractions thought to be responsible for retaining the
larger part of the trace elements. It allows us to study the behaviour of important elements (such
as P and Ca) and some toxic heavy metals (Cr, Cd, Pb, Zn).
2. Experimental
2.1. Sediment Sampling
The experiments were conducted with sediments from north France (port de Dunkerque) named
Sediment A ; and sediments from Passaic river in New-Jersey (USA) named Sediment B. the
choice has been made relating to the industrial past of both regions. After collecting, sediments
were stored in closed plastic containers at 4°C to reduce bacterial activity that could change
their physical and chemical properties. Thus, in order to determine these characteristics, the
French standard AFNOR [Qualité(8)des sols 1999] NF X 31-101 has been used. It consist of
drying the sediment at 40°C until total evaporation of the water content and stabilization of its
mass, then on grinding and sieving at 2mm.
2.2. Physical characterization
Real density of the sediment has been determined using an Helium Pycnometer (Accupyc 1330
from Micrometrics). Particle size distribution was achieved using a Laser Mastersizer (2000Hydro from Malvern Instruments), and sediment mineralogy identification was performed using
a Siemens D5000 diffractometer with monochromatic Cu–Kα radiation. The main results from
these physicals analysis are reported in Table 1.
Table 1: Physical characteristics of the sediments
Sample
Density
Size Distribution (µm)
D50
XRD
Sediment A
1,55
117,8
SiO2
Sediment B
1,20
120,5
SiO2
2.3. Chemical characterization
Chemical analysis consist of pH determination (following the French standard NF X31-103),
Total Carbon (TC), Organic Carbon (Corg) and Inorganic Carbon (Cinorg) using an Elemental
Analyser (NA 2100 from Thermo). The metal content has been determined using the protocol of
total digestion described in NF X31-151, followed by an identification using ICP. Tables 2 and
3 records the related informations.
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Table 2: Chemical characteristics of the sediments
Sample
%Moisture
pH
%CTotal
Sediment A
60,7
7,9
10,1
Sediment B
57,1
7,1
5,8
%COrg
%CInorg
8,5
1,6
4,6
1,2
Table 3: Ca, P and Metal content (mg/kg) (dry)
Sample
P
Ca
Cr
Cd
Pb
Zn
Sediment A
4519,06
41705,26
239,24
40,3
289,55
1986,12
Sediment B
2835,75
9817,00
311,44
42,8
470,40
810,73
2.4. Sequential Extraction of sediment samples
Soil samples were subjected to sequential extraction using the method of Tessier(7) et al., 1979.
The extractions were carried out in 50 ml centrifuge tubes with 1 g of dry sample using variable
extracting reagent . The procedure separates metals into five operationally-defined fractions:
water soluble and exchangeable (F1), carbonate bound (F2), Fe-Mn oxides bound (F3), organic
bound (F4) and residual (F5). Table 4 summarise the treatment at each step.
Table 4: Sequential extraction protocol
Operationally defined
Chemical fraction
Nominal target
phase
Extracting reagent
Time
MgCl2 / pH 7
1h
F1: Exchangeable
Soluble species,
Exchangeable cations
F2: Carbonate bound
Carbonates,
Metals pH depending
NaOAc/HOAc / pH 5
5h
F3: Fe-Mn oxides bound
Reductible metals
NH2OH/HCl/HOAc
6h
F4: Organic bound
Organic matter, sulfides
H2O2/HNO3/ NH4Ac
6h
F5: Residual fraction
Mineral fraction
HCl/HNO3/HF/HClO4
Total digestion
2.5. Mechanical properties
Samples used are raw sediments and with sediment treated with variable amounts of phosphoric
acid. They are dried at 40°C in special vessel in order to make the shipping uniform for all
samples. Their size is thus 25mm diameter and 60mm high. A texturometer (LRX 5KN) from
Lloyd Instrument is used to determine the hardness. The procedure developed considers that the
sample is broken when the force used is 50% less then the maximal force applied.
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2.6. Calcination procedure
Samples after phosphate treatment are introduced into alumina crucible for a thermal treatment
in a static furnace. The calcination procedure consist of a rise of temperature of 10°C/min until
the desired temperatures for the experiments (from 500 to 1000°C). This temperature is
maintained during 3h, after that it decreases until ambient temperature.
Physical properties of the sediments, such as specific surface area and density evolution are then
analysed.
3. Results and Discussion
3.1. Metal distribution by sequential extraction
3.1.1. Phosphorus behaviour
Novosol process is based on the treatment of mineral residues such as sediments using
phosphoric acid. It is thus essential to follow the phosphorus distribution in the sediment
matrices in order to understand the phenomena that occurs during each step of treatment. The
technique used for that is sequential extraction.
Note that the figures represents metal concentrations in both sediments A and B after phosphate
treatment (Figures (a)) and after thermal treatment (Figures (b)). “A and B brut” represents the
untreated sediments and “A P3 or B P5” the percentage of phosphate treatment for the sediments
(3% for A and 5% for B).
Figure 1: Phosphorus distribution in sediments A and B
(a) after phosphate treatment, (b)after thermal treatment
After phosphate treatment, phosphorus seems to be distributed in all fractions for both sediments
(Figure 1(a)). However, the dominant fraction is the residual one. It represents 35 and 55% of
phosphorus added to both sediments A and B which enable us to conclude that the phosphoric
acid added allows stable insoluble phosphate formation. Since Ca is naturally present in
sediment matrix, it is possible that the residual fraction represents some calcium phosphates
(like apatite Ca5(PO4)3OH) that are insoluble phosphates (-Log Ksp = 38,15). On the other hand,
the other mobiles fraction could represent some soluble metallic phosphates (unstable) such as
di-calcium phosphates (CaHPO4 where –Log Ksp = 19,09).
After thermal treatment (Figure 1(b)), one could note that majority (76 and 80% for A and B
respectively) of phosphate is in residual (most stable) form. This increase is explained by the
stabilization of phosphates mobile fractions.
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Thus, one could deduce that phosphate treatment allows stable and unstable phosphate
formation and that thermal treatment allows the transformation of the latter into stable residues.
This behaviour has been already observed in previous study [Kribi(9), 2005].
3.1.2. Calcium behaviour
The results presented in Figure 2 show that calcium is mainly linked to carbonates (F2) in the
form of calcite, phosphate treatment allowed its dissolution and thus its redistribution in other
fractions following the reaction:
CaCO3 + H3PO4 ↔ CaHPO4 + H2CO3
Figure 2(a), illustrates this by the decrease of the fraction of Ca liked to F2 (40 and 52%) and
(53 and 75%) while the fraction F3 increase (43 and 46%) and (10 and 16%) pour sediments A
and B respectively. In sediment B, one could also notice the F1 decreasing (about 13%) and
residual fraction F5 which practically doubled.
Figure 2 : Ca distribution in sediments A and B
(a) after phosphate treatment, (b) after thermal treatment
After calcination, calcium behaviour confirms the assumption about phosphorus behaviour
namely that the addition of acid causes at the same time the formation of less stable dicalcium
phosphates and of thermodynamically stable calcium phosphates of the apatite family. Figure
2(b), shows the increase of the residual fraction F5 for both sediments in a proportional way to
the amount of acid added. This increase occurs when other fractions reduces. This implies that
during calcination, dicalciques phosphates are transformed to more stable phosphates
(hydroxyapatite).
3.1.3. Chromium behaviour
Figure 3(a), shows that globally, Cr is mainly present in the fraction linked to organics (F4) for
both sediments A and B confirming what was observed by Tokalioglu(10) et al., 2000, Pagotto(11)
and Legret , 2001. After acid treatment at 3 and 5%, F4 decreases (15 and 30%) for the benefit
of F5 (40 and 60%) for the sediment A treated at 3 and 5% of acid respectively, while sediment
B knows an increase of F5 (45 and 46%) for 3 and 5% acid addition, F4 is almost stable and F3
decreases by 60%. One could deduce from the decrease of the mobile fractions (F1, F2 and F3)
that the increase of residual one support the argument of the the effectiveness of Cr stabilization.
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Figure 3 : Cr distribution in sediment A and B
(a)after phosphate treatment, (b) after thermal treatment
After calcination, Figure 3(b), it could be noticed the increase of Cr in the residual fraction F5
for both raw calcined sediments. Thermal treatment is thus able to stabilize a large amount of Cr
present in the sediment matrices. However, phosphate treatment combined to thermal one is
responsible of the increase of the exchangeable fraction F1 for both sediments that is probably
due to the oxidation of Cr(III) of the F5 fraction to the soluble Cr(VI) of the fractions F1 and F2.
3.1.4. Cadmium behaviour
Cd distribution before calcination (Figure 4(a)) shows its large amount in exchangeable fraction
F1 confirming thus what has been related by Harrison(12) et al., 1981; Pagotto(11)and Legret,
2001; Charlesworth(13) et al., 2003. However, under the effect of acid addition, this fraction is
slightly increased for both sediments probably because of the decarbonising of the fraction F2.
Figure 4 : Cd distribution in sediments A and B
after phosphate treatment, (b) after thermal treatment
After calcination (Figure 4(b)), F1 disappears while F5 and F3 increases significantly. This
traces the reduction of Cd and its stabilization. On the other hand, phosphate treatment
combined with thermal one, allows the increase of F5 (compared to the raw calcined samples,
about 30 and 60% for sediments A and B respectively). It seems thus that the combined action
(Phosphate and thermal) is necessary for Cd stabilization probably thanks to amorphous Cd
phosphate formation after phosphate treatment and their stabilization after calcination. Future
work will be focused on determining the chemical structure of that Cd phosphates.
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3.1.5. Lead behaviour
In raw sediments, Pb seems to be essentially linked to F3 and F5 as reported by Caplat(14) et al.,
2005 ; Akcay(15) et al., 2003. After phosphate treatment, Pb concentration in F5 is increased until
93% for sediment A P5 and 152% for sediment B P5, while F2, F3 and F4 decreases. One could
thus deduce that the phosphate treatment stabilizes a large amount of Pb present in matrices
fractions probably hydroxy as shown by Cotter-Howwels(16) et al., 1996 ; Basta(17) et al., 2004.
Figure 5 : Pb distribution in sediments A and B
(a) after phosphate treatment, (b) after thermal treatment
After thermal treatment (Figure 5(b)), an increase of F5 is observed consequently to mobile
fractions reduction. Calcination is thus assumed to bring to the stabilization of Pb more
effectiveness.
3.2. Mechanical properties
After phosphoric acid addition, a foam formation followed by gaseous emission has been
observed. This is related to CO2 and H2S release from the sediment following carbonates and
sulphides attack by acid. The porous structure resulting from such a treatment, changes
appreciably the residue texture and thus its behaviour while drying, transport to calcination unit,
physical and mechanical properties and thus its valorisation field. Previous work [Kribi(9), 2005]
showed a decrease of 50% in drying time after 5% addition of phosphoric acid because of the
porous structure created. One could expect an impact on mechanical properties since the
hardness is related to the sample structure. The hardness of sediment samples (raw and variable
percentage of acid addition) was assessed, the results are presented in (Figure 6). The procedure
used gives the compressive strength for samples depending on their phosphate treatment degree.
It considers that the sample is broken when the force used is 50% less than the maximum one.
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Figure 6 : Compressive strength of sediment samples after phosphate treatment
This figure clearly shows the impact of acid addition on sediment compressive strength. The
force needed for braking samples is 20% less from raw samples to P3% treated, 18% from P3%
to P5 and 17% from P5 to P7%. This decrease in compressive strength is compatible with the
noticed visual change of texture. It is clear that the porous structure created by acid addition is
kept after drying decreasing thus its compressivity.
4. Conclusion
Dredged materials from channel maintenance or expansion presents a potentially enormous cost
due to its removal, treatment and ultimate disposal. Therefore, stabilizing metal pollutants and
destroying organics from these sediments for either beneficial reuse or safe storage is an
attractive option.
In this work, heavy metal stabilization has been performed using phosphate treatment followed
by calcination as described in Novosol® process. A Teissier sequential extraction procedure has
been used to describe the behaviour of various elements (P, Ca, Cr, Cd and Pb) at each step of
treatment. It allows one to understand and explain the observed phenomena. This technique is
rapid, inexpensive and gives a good indication about solubility and thus element mobility
depending on variable parameters such as pH, ionic force, red/ox…etc.).
The results clearly shows the process efficiency for metals stabilization. Indeed, the combined
action of phosphate treatment and calcination allowed the migration of studied metals (Cr, Cd,
Pb) from mobile fractions to residual one for both sediments. In addition, texture change due to
gaseous emission has been highlighted through mechanical properties evaluation. A growing
interest for such an inerted material allows one to consider their valorisation in civil engineering
as construction materials. Several work involving various european structures are in progress.
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Acknowledgment
The authors wish to express their gratitude to Solvay S.A for providing samples and their
financial support to this work.
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