RIVER SEDIMENTS CONTAMINATED BY HEAVY METALS AND ORGANIC COMPOUNDS: CHARACTERISATION, TREATMENT AND VALORISATION Linda Boucard, Zoubeir Lafhaj, Frédéric Skoczylas Ecole Centrale de Lille, Laboratoire de Mécanique de Lille, CNRS UMR 8107, France Abstract ID Number: 222 Author contacts Authors
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Linda Boucard
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Zoubeir Lafhaj
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Frédéric Skoczylas
[email protected]
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(33/0)3 20 33 53 52
Département Génie Civil Cité scientifique – BP 48 59 651 Villeneuve d'Ascq
Contact person for the article: Linda Boucard Presenter of the article during the Conference: Linda Boucard Total number of pages of the article (this one excluded):
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RIVER SEDIMENTS CONTAMINATED BY HEAVY METALS AND ORGANIC COMPOUNDS: CHARACTERISATION, TREATMENT AND VALORISATION L. Boucard, Z. Lafhaj, F. Skoczylas Ecole Centrale de Lille, Laboratoire de Mécanique de Lille, CNRS UMR 8107, France Abstract This article deals with the problematic of river sediments, their contamination, the treatment used and the valorisation tracks of the processed products. It is divided into two parts. The first part introduces the issue of the silting up of rivers by polluted sediments and includes a presentation of process used for the treatment and the stabilisation of these sediments. The second part outlines the recycling method which was chosen and the first results obtained. 1.
INTRODUCTION
It is imperative to dredge channels or ports regularly to maintain maritime and river activities. Deposit areas are scarcer. Furthermore, current public and politic expectations go beyond manuring operations, bank strengthening or stock areas. A great number of industries congest the channels in the North region of France by tipping sediments out [1]. There are two types of polluting substances in these sediments: those known as organic and those known as inorganic. The organic pollutants are essentially hydrocarbons and derived products (HAP, PCB, etc.). The major parameters governing their behaviour in soil are solubility, volatility and their electronic structures [2]. The inorganic pollutants are mainly heavy metals, nitrates, phosphates and salts. Examples of heavy metals are lead, cadmium, zinc and copper. These heavy metals can have serious direct consequences not only on humans but also on the flora and fauna. Another of their characteristics is their interaction with each other. Indeed, Richter [3] has shown that in a saturated soil, cadmium is adsorbed 80 times less than in the absence of lead. Colandini [4] has shown that heavy metals absorbed by the soil can be released in the presence of water carrying salt. The main characteristic that distinguishes organic pollutants from inorganic ones is that the latter do not deteriorate over time and are long-life. Moreover, these pollutants go unnoticed compared to organic ones which can be identified either by smell or by colour, which results in rapid action. Nevertheless, the dangerous nature and the repercussions that these heavy metals can have are just as serious. To solve such problems linked to sediment pollution but also to the large volume of exiting sediments [5], a real treatment should be developed. However it is essential to organise an efficient treatment with valorisation stages. Since most of the treatments have already been processed in past years, the stake is to find new valorisation tracks that meet technical criteria and environmental demand. The Novosol® process developed and patented by Solvay Company [6] is based on the stabilisation of pollution and consists of three steps on two mobile units A and B (described on Figure 1 and Figure 2). - The first stage (Unit A- figure 1) of the process takes place once the sediments have been drained off. It is called the phosphate treatment and consists in mixing the sediments Page 1
with phosphoric acid (between 2 and 3.5%). The treatment is processed in a tubular reactor, where heavy metals are stabilised and trapped in the sediments. This stage creates matrices of metallic phosphates that are geochemically stable. - The second stage deals with the drying and the air maturation of the sediments, which are set on a drying bed made of swath. On the one hand, the process reduces the dryness rate with a thanks to the water drainage towards the bed bottom, and on the other hand, it enables the completion of the phosphate treatment reaction. The phosphated and dried sediments are then dispatched through big bags to unit B.
Phosphoric acid
Gas treatment
Phosphat treatment Drying swath
Piston pump
Sewage bed
Figure 1. Unit A: mobile unit of phosphate treatment
Active coal
Sodium bicarbonate
Storage of processed sediments
Gas
Calcining
Figure 2. Unit B: calcining unit - The last stage (unit B – figure 2) is a thermal phase, which consists in calcining the phosphated sediments at ≥700°C in an oven, in order to destruct the organic compounds such as polycyclic aromatic hydrocarbons, dioxins and pesticides. This process also increases the product toughness and reduces the volumes of the processed materials after the treatment. 2.
CHARACTERISATION
The sediments which make up this study come from Lille (France) and Vraimont (Belgium). Organic materials and heavy metals are the main identified pollutants.
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The first step of the study was their characterisation. Both chemical and mineralogical characterisations were done. The results obtained are useful for the identification of these sediments. Indeed, the latter will be classified upon their form, density or polluting level. The second step was the stabilisation of these sediments with the Novosol® Process. Finally, a new characterisation based on a granular characterisation of Novosol® sediment was done. The main objective of the classification of the Novosol® sediment is to valorise each class of sediment according its characteristics. 2.1 Characterisation of raw sediments before Novosol® processing developed by Solvay. Results exposed in this section concerns the sediments of Lille and Vraimont. These sediments are initially polluted without any process of treatment. Organic materials and heavy metals are the main pollutants identified. Granular and environmental characterisations are both exposed in the following tables: In Table 1, granular distribution of polluted sediments is presented. Both selected sediments share the same characteristics such as an extreme slenderness and a slightly clay structure. Table 1: Granular characterisation of two raw river sediments
Granulometric distribution on raw material
Sediment from Lille (France)
Sediment from Vraimont (Belgium)
Sand fraction: 20.61 % Silt fraction: 74.43 % Clay fraction: 4.96 %
Sand fraction: 16.72% Silt fraction: 77.1 % Clay fraction: 6.18 %
The following table sets out a comparison of the concentrations of heavy metals for sediments located at Lille or at Vraimont. It can be observed that sediment from Lille exhibits high concentrations in heavy metals. This result (table 2) clearly shows that these materials con not be used and a treatment is required for them to be stabilized. Table 2: Concentration of two raw river sediments Sediment from Vraimont (Belgium) Cd: 18; Co: 42; Cr: 314; Cu: 248; Cd: 9; Co: 40; Cr: 92; Cu: 88; Fe: 30600; Pb: 480; Zn: 2551,8; Si: Fe: 25200; Pb: 112; Zn: 428; Si: 254; Ca: 90600; 336; Ca: 16900; Al: 43800 Al: 45320 Sediment from Lille (France)
Heavy metals in mg/kg on dry material Organic compounds % by mass
15.64
4.8
2.2 Environmental characterisation of Novosol® sediments This characterisation offers a control on the treatment process and allows the heavy metals contained in the matrix to stabilise. Leaching is the standard test used to determine the quantity of heavy metals (contained in the treated sediments), which are rejected in a natural environment. Leaching consists of extracting significant fractions in a sample with an aqueous solution. Two standards have been used: the NF standard XP X31-210 [7] and the American TCLP (Toxicity Characteristic Leaching Procedure) [8].
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Each leaching test presents its own limits, advantages and disadvantages [9]. This difference is due to the variety of necessary parameters involved in the test. The main parameters which have significant influence on the results are those linked to the sample (quantity, length…) and those linked to the aqueous solution used (water, acid, pH…). It can be noted that the TCLP leaching test is considered as more aggressive than the test based on the XP X31-210 norm. The prescribed aqueous solution used in the TCLP test is a dilution of acetic acid (pH=2.8).The Liquid/Solid ratio necessary for this test is equal to 20 l/kg. The acid solution acts more efficiently on the sediments leading to a higher amount of heavy metal to be extracted. The TCLP test is then considered as a practical test which is very close to real conditions as those encountered in field. In Table 3, a comparison of the concentrations of heavy metals from sediments of Belgium with TCLP norm is presented. Table 3. TCLP leaching – Belgium Novosol® sediment concentration (mg/kg on dry material) with TCPL leaching limits
Heavy Metals (mg/kg) on dry materials TCLP Limits (mg/kg) on dry materials
raw sediment novosol sediment
Copper
Chromium VI
Lead
Zinc
Cadmium
15.1
-
10.5
986
1.6
1.36
5.54
2.40
74.60
0.14
5
5
5
250
2
It can be observed that the concentrations of heavy metals are inside the TCLP limits and reduced compared with those one obtained on polluted sediments. This result clearly shows that the use of Novosol® process is an efficient process to reduce the pollution. On the other hand it is of interest to study the behaviour of such treated products as they could be employed into cement based material for example. In fact cementitious materials are not chemically inert (high pH of interstitial water) and may lead to other phenomena which have to be taken into account and evaluated regarding the stability of the matrix in which heavy metals are trapped. 2.3 Methodology of granular characterisation of the SN After the execution of Novosol® process, Novosol® sediments obtained presents a new material which must be identified, analysed and classified. Granular characterisation of these sediments was done and this was necessary and useful for the valorisation of all the constituents of sediments. A mechanical characterisation is essential to determine the valorisation tracks, as it does exist for any building material. When Novosol® sediments have been delivered in bulk, it is logical and useful to analyse the granulometric content to ascertain the main ranges. The first test showed that SN mostly consist of fines, but are also made of sand and gravel components, as shown in the granulometric curve. Since the industrial processing has not been finalised yet, no tracks could have been isolated. Based to the granulometric analysis it decided to get closer to ranges of well-known GC (industrial engineering), such as fillers, sands and gravels. The categories have been classified according to the grain diameter. For diameters between [0.08mm - 0.63mm], the range represents the class of SEDFINS. The second category SEDSABLES gathers diameters between [0.63mm - 5mm]. At last, the SEDGRAVIERS class applies to diameter situated between [5mm - 25mm]. Figure 4 presents the result of granular distribution obtained. The results obtained showed that Novosol® sediments are composed by Page 4
fines, sands and gravel. It can be noticed that the percentage of fines is important compared to other components. 25
100
16
10
90
2,5
80
0,315
10
12,5
8
0
31,5
20
1,25
0,63
20
sifted product (%)
0,16
16
60
40 50
50 40
8
30
2,5 0,1
20
0
0,01
0
10
70
1,25 0,16
10
60
12,5
0,315
refused product (%)
30
70
80
0,63
90 100
0
1
10
Sift (mm)
100
Figure 4: Granular analysis of Novosol® Sediment For several years our laboratory had undertaken a large experimental program to characterise and study the valorisation of these sediments. For the first category (fillers) an experimental program is carried out to study the valorisation of these sediments to produce bricks [10], [11]. Both second and third categories may be used in concrete. 3.
VALORISATION TRACKS - EXPERIMENTAL PHASE
The main methods of reducing sludge and sediments are by agricultural muck-spreading or in some countries by dumping into the sea, which is very practical. This method of elimination has been illegal since 1999 (EEC decree 97/277). The second method is based on a dump. This tends to be ruled out. The last method of elimination is incineration, which accounts for about 20% of the total tonnage. Given the difficulties and limitations of these methods of reduction, it seems necessary to find other means of recycling. 3.1. Experimental results of the valorisation of Novosol®sediment in Concrete In order to study the valorisation of inert sediments in concrete, several possibilities can be used. In this study inert sediments have been experimented in the concrete material by adding fines. This solution is investigated in order to study both the feasibility and the filler affect that inert sediments can produce on a concrete. For experimental studies, the concrete chosen was an ordinary one. The constituents of this concrete are illustrated in Table 4. Its resistance at 28 days was equal to 28 MPa. Table 4: Components’ content of the concrete [%]. Water 8
CEMII 32,5 R cement 15
Seine sand 31
Aggregates 46 Page 5
The procedure consists to add fines SEDFINS into concrete with different proportions varying between 2 and 20% with respect to the cement mass. It can be noted that the granular distribution of these inert filler showed an average grain diameter of 80 µm. The effects of different filler proportions are presented in Figure 3 in which a significant effect of incorporated filler on strength can be highlighted. Between 0 and 10% of filler incorporated the strength increases by almost 30%. At higher proportion a classical decrease is observed. However, the 30% increase lets us suppose that the filler effect is not the only effect and that the high silica part of the sediment may lead to further reaction of hydration like pozzolanic effect. Any general conclusion can not be drawn only on these results and others investigations are necessary to validate or not such assumptions.
55
MPa
51,9 49,6
50
45,7 45
44,0
B B B B B
0% 2% 5% 10% 20%
40,6 40
35
30
28
Time (days)
Figure 3: Average uniaxial compression strength (MPa) - SEDFINS additions vary between 0 and 20% from left to right. 4.
CONCLUSION
The work presented in this article is devoted to the study of polluted river sediments and to put in light alternative means making their re-use possible.Environmental characterisation of these polluted sediments showed that these sediments were mainly polluted by heavy metals and organic materials. The comparison of the pollutant content with existing norms showed that these sediments do not respect the leaching norm limits. The second part included a presentation of the Novosol® process which was used for the stabilisation of the polluted sediments. The main results obtained based on the comparison of pollutant content with existing norms showed that TCLP norm limits are respected and new sediment are stabilised even under aggressive conditions. As a result, confirmed numerous times in another studies, Novosol® process can be considered as an efficient tool for the stabilisation of polluted sediments. After the treatment of the sediments, an experimental program was undertaken in order to study their valorisation. The method of recycling involves the making up of concrete incorporating inert sediments. The first results obtained are of interest as they show that the percentage of filler incorporated in concrete significantly modifies the strength. These very first and encouraging results lead us to farther pursue the study.
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REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Agence de l'eau, 'La qualité des sédiments des cours d'eau' 1991-1996 Yaron B., Calvet R., Prost R. (1996). Soil pollution, process and dynamics, New York, Springer. Richter J. 1987. The soil as reactor.Modelling process in the soil. Hannouver, catena, Allemagne. Colandini V. (1997). Effets de structures réservoirs à revêtement poreux sur les eaux de ruissellement pluviales : qualité des eaux et devenir des métaux lourds. Thèse de doctorat.Université de Pau. Marot, F. 'Caractérisation et traitement de sédiments de dragage contenant des polluants métalliques', BRGM Eds, (1998). Publication EP1341728 (19/04/2002). Patent correspondant : FR2815338 (17/10/2000). Procédé d’inertage de boues. SOLVAY. AFNOR. Déchets - Essai de lixiviation. Normalisation Française XP X31-210. Toxicity Characteristic Leaching Procedure. Method 1311. USA Norm. Aubert, E. (2003) Utilisation de déchets dans les bétons : exemple des cendres volantes d’incinérateurs d’ordures ménagères. XXIème Rencontres universitaires de Génie Civil. Lafjaj Z., Angiuoni F., Cohen Solal L., Coudray Y, HuynhT.T., Le Guen B., Saliceto A.,'Contribution à la valorisation des sédiments marins pollués dans la brique après inertage.' Colloque International de Géotechnique. 19-22 Mai 2004, pp :959-964, Beyrouth. Boucard L., Lafhaj Z, Skoczylas F,' Sédiments fluviaux contaminés par des métaux lourds : de la caractérisation à la valorisation.', Colloque International de Géotechnique. 19-22 Mai 2004, pp : 965-970, Beyrouth. Liban.
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