The removal of heavy metals from contaminated soil by a combination of sulfidisation and flotation

The removal of heavy metals from contaminated soil by a combination of sulfidisation and flotation

The Science of the Total Environment 290 (2002) 69–80 The removal of heavy metals from contaminated soil by a combination of sulfidisation and flotat...

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The Science of the Total Environment 290 (2002) 69–80

The removal of heavy metals from contaminated soil by a combination of sulfidisation and flotation Mathias Vanthuyne, Andre´ Maes* KULeuven, Department of Interphase Chemistry, Laboratory of Colloidchemistry, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium Received 14 August 2001; accepted 25 September 2001

Abstract The possibility of removing cadmium, copper, lead and zinc from Belgian loamy soil by a combination of sulfidisation pre-treatment and Denver flotation was investigated. The potentially available — sulfide convertible — metal content of the metal polluted soil was estimated by EDTA (0.1 M, pH 4.65) extraction and BCR sequential extraction. EDTA extraction is better at approximating the metal percentage that is expected to be convertible into a metal sulfide phase, in contrast to the sequential extraction procedure of ‘Int. J. Environ. Anal. Chem. 51 (1993) pp. 135–151’ in which transition metals present as iron oxide co-precipitates are dissolved by hydroxylammoniumchloride in the second extraction step. To compare the surface characteristics of metal sulfides formed by sulfidisation with those of crystalline metal sulfides, two types of synthetic sediments were prepared and extracted with 0.1 M EDTA (pH 4.65) in anoxic conditions. Separate metal sulfides or co-precipitates with iron sulfide were formed by sulfide conditioning. The Denver flotation of both types of synthetic sediments (kerosene as collector at high background electrolyte concentrations) resulted in similar concentrating factors for freshly formed metal sulfides as for finegrained crystalline metal sulfides. The selective flotation of metal sulfides after sulfide conditioning of a polluted soil, using kerosene or potassium ethyl xanthate as collectors and MIBC as frother, was studied at high background electrolyte concentrations. The sulfidisations were made in ambient air and inside an anoxic glove box. The concentrating factors corrected by the potentially available metal percentage, determined by 0.1 M EDTA extraction, lie between 2 and 3. The selective flotation of these finely dispersed, amorphous, metal sulfides can possibly be improved by optimising the bubble–particle interaction. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Transition metals; Availability; Sequential extraction; Speciation; Sulfidisation; Flotation; Metal sulfide

1. Introduction Excavation and disposal are no longer considered as a permanent solution for heavy metal *Corresponding author. Tel.: q32-16-32-15-98; fax: q3216-32-19-98. E-mail address: [email protected] (A. Maes).

contaminated soils. Consequently, the demand for treatment techniques such as extraction, hydrocyclonage, solidification, vitrification and flotation, is growing. In this study, we choose to investigate the potential use of flotation to remediate a soil polluted with transition metals which are apt to form highly insoluble metal sulfides with pKsp values ranging from 25.2 for ZnS to 53.2 for HgS (Framson and Leckie, 1978).

0048-9697/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 1 . 0 1 0 6 4 - 6

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M. Vanthuyne, A. Maes / The Science of the Total Environment 290 (2002) 69–80

Flotation is a solid–liquid separation technique which introduces air bubbles in a suspension (Matis, 1995). The selectivity of the separation is based upon differences in the wettability between the particles. Air bubbles attach preferentially to hydrophobic surfaces and carry these particles to a stable froth layer that is produced by the addition of frothing agents, whereas hydrophilic particles remain in the suspension. If the natural differences in wettability between the particles that are to be separated are not sufficient for their selective separation, the suspension has to be conditioned by flotation reagents. The most important flotation reagents are collectors, which adsorb on mineral surfaces, rendering them hydrophobic and facilitating bubble attachment. Flotation has been widely and successfully used in the mining industries to separate valuable mineral ores, which are commonly present as metal sulfides, from the tailings. Selective flotation of these minerals could be enhanced by sulfidisation (Orwe et al., 1998; Clark et al., 2000). Flotation is also a useful remediation technique for heavy metal polluted dredged material (Cauwenberg et al., 1998a,b). Up to 80% of heavy metals which were present as metal sulfides, could be concentrated in the froth layer, which represented only 30% of the total mass. Maximum selectivity occurred at in situ pH conditions. These results were obtained from a kerosene-induced flotation of free and organic matter coated sulfide particles (Cauwenberg et al., 1998a). Sequential extractions of the froth and rest fraction after flotation at pH 8 show an important difference in speciation between the froth and rest fraction for most of the transition metals; the froth fractions have a lower transition metal extractability than the rest fractions due to the enrichment of metal sulfides in the froth fraction (Cauwenberg et al., 1998b). The decontamination of harbour sediments by flotation in which the metals are originally not all present as metal sulfides, could also be improved by chemical pre-treatment methods such as hydroxylation and sulfidisation (Eberius, 1989). Without sulfidisation pre-treatment, the total heavy metal content reduction in the sediments after flotation is only 42%. However, flotation of sulfi-

dised harbour sediments reduces the total heavy metal content by 83% (Eberius, 1989). Flotation of heavy metal contaminated surface soils differs from ore flotation. One of the most important differences is the fact that heavy metals in soils are not present in one particular chemical form like it is mostly the case in ores, but are associated with the different geochemical phases of the soil leading to low selective flotation. Transferring the potential available transition metals to one unique chemical form, e.g. a sulfide phase or an oxide phase, by chemical pre-treatment and further removal of the heavy metals by selecting a phase-specific collector is a possibility to overcome this problem. Venghaus and Werther (1998) investigated the flotation behaviour of a zinc-contaminated soil pre-treated by sulfidisation. A metal recovery of approximately 50% and a froth mass recovery of 10% were obtained at pH 7.5 in the presence of a xanthate collector. Langen et al. (1994) also carried out laboratory scale flotation experiments on soils. Based on the results of a sequential extraction procedure, they used an oxide-specific collector to remove the iron, manganese and aluminium oxide fraction, which was highly contaminated with heavy metals. A concentration factor of 1.7– 1.75 at a metal recovery of approximately 60– 65% could be achieved. The purpose of this paper is to investigate the possibility to remove heavy metals from a contaminated soil by a combination of sulfidisation and flotation. Especially the case of a fine-textured soil is addressed, the treatment of sandy soils and sediments being rather easy (Mosmans and van Mill, 1999). The contaminated soil was pre-treated with an Na2S solution in ambient air or inside an anoxic glove box under N2 yH2 atmosphere. The goal of this chemical pre-treatment method was to transfer the potentially available heavy metals into a unique metal sulfide chemical phase. Metal sulfides might then easily float with a suitable phase-specific collector, e.g. kerosene for free and organic coated metal sulfides as shown for dredged material (Cauwenberg et al., 1998a,b) or xanthate for valuable metal sulfides known in mining industries (Crozier, 1992).

M. Vanthuyne, A. Maes / The Science of the Total Environment 290 (2002) 69–80 Table 1 General characteristics of Tienen soil pH-KCl pH-H2O Texture Sand (%) Loam (%) Clay (%) Metal content (mgykg) Cadmium Copper Lead Zinc Organic matter (%) CaCO3 (%)

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2.3. Preparation of synthetic sediments 7.83 7.10 28.22 59.34 12.44 7.80 68.40 141.90 356.70 6.54 7.77

2. Materials and methods 2.1. Soil samples The investigated soil samples were taken in Tienen, Belgium. The general characteristics of this loamy soil are given in Table 1. The organic matter content was estimated by the ignition loss at 450 8C. Texture analysis was done by the pipette method (Gee and Bauder, 1986). The CaCO3 concentration was measured by the addition of HCl and back titration with NaOH (Gee and Bauder, 1986). The soil fraction smaller than 2 mm was used in all the experiments. 2.2. Metal analysis The soil heavy metal content was measured after microwave digestion following the procedure of Cauwenberg et al. (1998a). Digestion was done by Microwave destruction (Milestone, MLS-1200MEGA). To 0.5-g soil sample, 3 ml HNO3 (65%, p.a.), 1 ml HClO4 (70%, p.a.) and 1 ml HCl (37%, p.a.) was added. The metal content was measured by AAS measurement (Varian, AAS20). Metal standard solutions were obtained from Aldrich Chemical Company, Inc. After flotation, the heavy metal content in the freeze-dried froth fraction and rest fraction was determined by the same procedure.

Two types of synthetic sediments were prepared. The first sediment was prepared inside an anoxic glove box wN2 yH2s95:5 (%)x in agreement with the procedure described by Oakley et al. (1980) using 5-g sand (VEL, p.a.), 0.050 g humic acid (Aldrich), 5-g illite clay (Silver Hill Montana) and 0.1 g of different metal sulfides (Aldrich or Cerac) in 200 ml background solution of 0.1 M Ca(NO3)2. The pH was adjusted to 8.5 with 0.1 M NaOH. The synthetic sediment was allowed to equilibrate for 1 week. If necessary, the pH was daily readjusted to pH 8.5. The illite clay and the sand were first ground in a ball mill to obtain fine material (30 min for sand and several times 30 min for the clay until all clay passed a 200-mm sieve). To obtain small metal sulfides, 20-g metal sulfides were weighed inside the glove box into the recipient of the ball mill. The recipient was sealed with sealing tape (Aldrich) and the metal sulfides were ground outside the glove box: ZnS (325 mesh) for 7 min at speed 7; PbS (200 mesh) for 45 min at speed 8; CuS (200 mesh) for 60 min at speed 8; and FeS (q40 mesh) for 60 min at speed 7. The second sediment was also prepared inside an anoxic glove box wN2 yH2s95:5 (%)x) with 5g sand (VEL, p.a.), 5-g illite clay (Silver Hill Montana) and nitrate salts of Zn, Pb, Cu and Fe in 200 ml background solution of 0.1 M Ca(NO3)2. Occasionally, 0.050 g humic acid (Aldrich) was added. Na2S was added to form insoluble metal sulfides at a dose (molyl) of twice the total metal concentration. The suspension coloured black immediately after sulfide addition indicating that metal sulfides were formed. The pH was adjusted to 8.5 with 1 M HNO3. The sediment was further allowed to equilibrate for a week inside the glove box with daily pH control and readjustment to pH 8.5. 2.4. Extraction procedures The single step EDTA extraction procedure of Cottenie et al. (1979) modified by Maes and

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Table 2 Experimental conditions used in the adapted Ure et al. (1993) sequential extraction procedure Assumed phase involved

1-g soil in centrifuge cup

Fraction 1 Fraction 2 Fraction 3

Exchangeable Oxide Organic

Fraction 4

Residual

50 ml 0.1125 N HAc 50 ml 0.5 M hydroxylammoniumchloride (1) 12.5-ml H2O2 (2) 60 ml 1 M NH4Ac 30-ml HNO3 yHCl (1:3) heating for 3 h on a sand bath

Cauwenberg (2000), was performed on soil samples of Tienen to determine the heavy metal availability: 6 g of soil sample was extracted with 70 ml of a solution containing 0.5 M NaOAc; and 0.1 M EDTA at pH 4.65 in ambient air. The BCR sequential extraction technique described by Ure et al. (1993) was also performed in ambient air on soil samples to study the association of the heavy metals with the different geochemical phases of the soil. The extraction conditions and the presumed associated phases of the adapted Ure et al. (1993) procedure are briefly given in Table 2. To compare the surface characteristics of crystalline metal sulfides (synthetic sediment 1) with the surface characteristics of freshly formed metal sulfides (synthetic sediment 2) in suspension, both types of synthetic sediments were extracted inside an anoxic glove box with 0.1 M EDTA at pH 4.65. This procedure was used by Maes and Cauwenberg (2000) for a canal sediment. They observed from dissolution experiments at different pH values that cadmium, copper, lead and zinc were mainly present as crystalline sulfides in agreement with theoretical calculations (CHESS, Van der Lee, 1993). However, the dissolution behaviour revealed that the transition metals were either present as separate metal sulfides or as coprecipitates with iron sulfide (Maes and Cauwenberg, 2000). 2.5. Sulfidisation procedure Sulfidisation was done at room temperature in two conditions: 1. in ambient air; and

2. inside an anoxic glove box to avoid oxidation. Anoxic conditions inside the glove box were obtained by continuously flushing 95:5 (%) N2 y H2 gas over a catalyst. The oxygen concentration as monitored by the trace oxygen analyser (Delta F) was 2000 mgym3. The humidity in the glove box was controlled by CaCl2 pellets. The procedure described below was used in both conditions. Fifty grams of soil was weighed in a 1-l glass bottle and suspended in 1 l of 0.1 M NaHCO3 (Merck, p.a.) or 0.1 M Ca(NO3)2 (Merck, p.a.) using fresh, doubly-distilled water. The sulfidisation was done with flake sodium sulfide (65% Na2S, Merck, p.a.). The doses of Na2S (molyl) were based on the total soil heavy metal content (mgyg). One percent (wyw) of fine grained FeS (Cerac, 99.9%) was also added in some sulfidisations. The glass bottle was closed immediately after the sulfide addition to avoid unnecessary oxidation. The soil suspension was shaken in an orbital shaker for 1–2 days. Reference samples without sulfidising agent were prepared by the same procedure. De-aerated doubly-distilled water was used for the sulfidisations inside the glove box. The pH after sulfidisation was measured by a gel electrode (Xerolyt, Ingold). Both types of sulfide conditioned soil suspensions were then transferred to the flotation tank of a Denver D12 flotation machine. 2.6. Flotation tests Flotation experiments were performed in ambient air with sulfide conditioned soil suspensions and with both types of synthetic sediments. Flota-

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Table 3 Comparison of zinc, copper and lead extractability (%) of different types of synthetic sediments using EDTAyNaOAc (0.1 M, pH 4.65) in anoxic conditions with their concentrating factor after Denver flotation EDTA extractability Cu (%)

Zn (%)

Concentrating factor Pb (%)

Cu (%)

Zn (%)

Pb (%)

Crystalline metal sulfides

0.02

7.95

2.66

2.62

3.23

3.09

Native metal sulfides Without humic acid With humic acid

0.01 0.01

51.41 44.78

16.14 16.44

2.75 2.63

2.70 2.64

2.46 2.38

Flotation parameters: 1.5-ml kerosene; 30-ml MIBC (0.1% vyv); 0.1 M Ca(NO3 )2 ; conditioning time, 10 min; and flotation time, 10 min.

tion tests were made with a Denver D12 laboratory scale flotation machine. The flotation reagents were added after the sulfidisation pre-treatment step. Methylisobutylcarbinol (MIBC, 27–30 ml, vyv 1y1000, Merck) was added to obtain a stable froth layer. The collectors used in this study were 1.5-ml commercial kerosene (Merck) or 25 ml 0.1 M Kex (potassium ethyl xanthate, Merck). The system was then allowed to condition for 10 min at 1000 rev.ymin. Subsequently, flotation was started by opening the air supply of the Denver flotation apparatus for a period of 10 min and the froth layer was removed manually. The obtained froth fraction and rest fraction were freeze-dried and weighed, followed by microwave digestion and AAS analysis to determine their heavy metal content. The flotation efficiency will be evaluated on the basis of the following parameters: Mass recovery Ž%.sŽMassfrothyMassfroth qMassrest.=100 B

Metal recovery Ž%.sDMassfroth=wMexfroth C

=100.yŽMassfroth=wMexfroth qMassrest=wMexrest. Concentrating factor smetal recoveryymass recovery swMexfrothywMextotal.

3. Results and discussion 3.1. Denver flotation of synthetic sediments In a trial to compare the surface characteristics of crystalline fine grained metal sulfides with the surface characteristics of native metal sulfides, two types of synthetic sediments were prepared and extracted with EDTA (0.1 M) at pH 4.65 inside an anoxic glovebox. The dissolution of copper, lead, zinc and cadmium sulfides in EDTA solution (pH 4.65) is expected to be low based on theoretical calculations (CHESS, Van der Lee, 1993; Maes and Cauwenberg, 2000). Nevertheless, Table 3 demonstrates that differences in the extraction pattern exist between crystalline metal sulfides and metal sulfides formed by sulfidisation, especially for zinc and lead which are more easily extracted from native sulfides by EDTA. These observations indicate that part of the lead and zinc is associated with an iron sulfide phase because the stability of such iron co-precipitates is lower than the stability of the separate transition metal sulfide phase, but is similar to the stability of iron sulfide. Indeed, Maes and Cauwenberg (2000) have shown that the dissolution edge for iron starts at approximately pH 6. Therefore, upon dissolution of the iron sulfide matrix, lead and zinc which were coprecipitated with iron sulfide, were leached by 0.1 M EDTA below pH 6. Table 3 shows that for the case of copper, no influence of EDTA on copper solubility could be observed for all the synthetic

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Table 4 Percentage of total metal extracted with 0.1 M EDTA at pH 4.65 for the different metal ions from Tienen soil

charge neutralisation of the humic macromolecule, causing a reduction in the intramolecular repulsion in the polymer chains and ‘coiling’ of the chains.

Cd (%)

Cu (%)

Pb (%)

Zn (%)

69.85

72.74

71.48

64.11

3.2. Denver flotation of Tienen soil pre-treated with sulfidisation

sediments. Therefore, we conclude that copper is present as a separate metal sulfide phase. The former observations prove that heavy metals could be converted into a separate metal sulfide phase or a co-precipitate with iron sulfide. The appearance of a dark black colour in the synthetic sediment suspensions immediately after sulfide addition confirms this conclusion. The flotation behaviour of such freshly formed metal sulfides was investigated by Denver flotation using a hydrophobic kerosene collector. The concentrating factors, which determine the selectivity of the flotation process, are also given in Table 3. The concentrating factors for copper, zinc and lead are approximately the same as for the crystalline separate metal sulfide phases (0–50 mm) and are not influenced by the presence of humic acid at high background electrolyte concentrations. High background electrolyte concentrations were found to render the highest concentrating factor due to minimising the effect of coating of metal sulfides with soil organic matter (Cauwenberg et al., 1998a). This influence of ionic strength on floatability of metal sulfides was explained as a ‘salting out effect’ (Yariv and Cross, 1979), which changes the conformation of humic acids due to efficient

3.2.1. Metal availability In soil chemistry, a single step EDTA extraction is commonly used for determining the available metal ions (Cottenie et al., 1979). Due to the possible liberation of high amounts of iron into solution, we used 0.1 M EDTA in agreement with Maes and Cauwenberg (2000) instead of 0.02 M (Cottenie et al., 1979) in order to complex all iron, zinc, cadmium, copper and lead. This EDTA (0.1 M) extractable metal content at pH 4.65 is a good approximation for the percentage of transition metals that can possibly be converted into a metal sulfide phase by sulfidisation. The results of the EDTA extraction (0.1 M, pH 4.65) on the metal polluted soil (Table 4) show that cadmium, copper, lead and zinc are only partially (grossly 70%) available. To explain this moderate metal availability, we also performed a BCR sequential extraction procedure (Ure et al., 1993). The results of this sequential extraction are presented in Table 5. Heavy metals ‘included in oxides’ and ‘adsorbed onto oxides’ were both extracted by hydroxylammoniumchloride. Therefore, we attempted to divide the metals associated with the oxide fraction into two subfractions, namely an EDTA extractable subfraction (subfraction 1) and a non-EDTA extractable subfraction

Table 5 Metal distribution over the different extraction steps of the Ure et al. (1993) sequential extraction procedure Phase

Cd (%)

Cu (%)

Pb (%)

Zn (%)

Fraction 1

Exchangeable

64.71

7.18

6.80

36.82

Fraction 2 Subfraction 1 Subfraction 2

Oxide EDTA-extractable Not EDTA-extractable

35.29 5.14 30.15

45.27 45.27 0.00

66.99 55.94 11.05

36.31 19.75 16.56

Fraction 3

Organic

0.00

20.29

8.74

7.54

Fraction 4

Residual

0.00

27.26

17.47

19.33

The extractions were made in ambient air and Tienen soil was used; subfractions 1 and 2 were estimated based on the percentage of different metals extracted by 0.1 M EDTA extraction at pH 4.65 (see text).

M. Vanthuyne, A. Maes / The Science of the Total Environment 290 (2002) 69–80

(subfraction 2, metals ‘included in oxides’). The percentage of heavy metals associated with subfraction 1 was estimated by subtracting the percentage of metals associated with the exchangeable fraction (fraction 1) and the organic fraction (fraction 3) from the EDTA (0.1 M, pH 4.65) extractable metal percentage of Table 4. The metal percentages associated with subfraction 1 were 5.14 (Cd), 45.27 (Cu), 55.94 (Pb) and 19.75% (Zn). Consequently, the sum of the percentage of metals included in oxides (subfraction 2) and the percentage of metals associated with the residual fraction gives an idea of the percentage of metals that are expected to be nonconvertible into a metal sulfide phase by sulfidisation pre-treatment. The following estimated non-sulfide convertible metal percentages were obtained: Cd, 30.15; Cu, 27.26; Pb, 28.52; and Zn, 35.89%. We conclude that the foregoing results indicate that the BCR sequential extraction technique (Ure et al., 1993) cannot be used to estimate the sulfide convertible (potentially available) metal content of a metal polluted soil due to the presence of transition metals as iron oxide co-precipitates, which are dissolved by hydroxylammoniumchloride in the second step of the sequential extraction procedure, but only partially by EDTA extraction. Thus, the EDTA (0.1 M, pH 4.65) extraction gives a better estimation of the sulfide convertible metal content. 3.2.2. Denver flotation experiments The potential use of Denver flotation to remove freshly formed metal sulfides with a sulfide-specific xanthate collector (Crozier, 1992) or a hydrophobic kerosene collector (Cauwenberg et al., 1998a) was investigated at high background electrolyte concentrations. The flotation conditions and reagents were similar as in Cauwenberg et al. (1998a) for dredged material. Irrespective of the pre-conditioning, all Denver flotations were done in ambient air. This condition was chosen because the use of nitrogen gas to restrict oxidation during flotation did not result in the expected increase in flotation efficiency of metal sulfides from dredged material (Cauwenberg, 1998). This observation was in agreement

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with the generally accepted view that contact with oxygen results in a slightly preferable oxidation of the metal sulfide surface to a metal-deficient, sulfur-rich and more hydrophobic (better floatable) surface (Clarke et al., 1995; Glazunov and Chernykh, 1994; Muster et al., 1996). In all the experiments, the pH after sulfidisation lies between 8 and 9, which corresponds with the optimum pH for selective flotation of metal sulfides from river sediments according to Cauwenberg et al. (1998a). 3.2.2.1. Sulfidisation in ambient air. The flotation results using kerosene as collector for Tienen soil treated in ambient air with Na2S (wNa2Sxs 10=wtotal metalx) and in presence of 1% FeS (wy w) are given in Fig. 1. Compared with untreated soils, sulfide pre-treatment results in (1) an increase of the mass recovery, and (2) an increase of the metal recovery for all the investigated heavy metals (Fig. 1a). Thus, the observed increase of the metal recovery after sulfidisation could mainly be attributed to the increase of flotated mass. This is explained by the better froth characteristics of the soil suspensions treated with Na2S, compared with untreated soils. In presence of Na2S, we observed a very stable froth layer with good froth depth compared with a more brittle and less stable froth with very shallow depth for untreated soils. Freeman et al. (2000) also noticed that increasing the amount of NaHS in ore flotation increased the froth stability due to changing surface tension at the bubble–water interface. The concentrating factor, which determines the selectivity of metal sulfide removal, is not significantly improved by sulfidisation and is smaller than two (Fig. 1b). In some cases, the concentrating factor after sulfidisation is even lower than the concentrating factor of the untreated soil. Therefore, we can conclude that the flotation selectivity using kerosene as collector of metal sulfides formed by sulfidisation in ambient air is low compared with the flotation selectivity of metal sulfides from dredged material. Indeed, the flotation of metal sulfides, which are formed in organic matter rich river sediments by bacterial sulfate reduction in anaerobic conditions (Morse et al., 1987), using the same flotation reagents and flotation conditions results in metal recoveries up to

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Fig. 1. Influence of the combination of sulfidisation and flotation on (a) mass recovery and metal recovery, and (b) concentrating factor for different metal ions from Tienen soil. Sulfidisation with Na2S (Na2Ss10=wtotal metalx) and 1% FeS in ambient air. Flotation parameters: 1.5-ml kerosene; 30ml MIBC (0.1% vyv); 0.1 M NaHCO3; conditioning time, 10 min; and flotation time, 10 min.

80% and in concentrating factors from 2.5 to 3 at pH 8–9 (Cauwenberg et al., 1998a). However, the presently observed low selective flotation behaviour could be explained by the moderate potentially available heavy metal con-

tent. Correction of the observed concentrating factors for the potentially available (sulfide convertible) heavy metal content determined by the single step 0.1 M EDTA extraction at pH 4.65 (Table 4), gives the concentrating factors shown in Table 6. These corrected concentration factors are similar to the concentration factors observed for dredged material (Cauwenberg et al., 1998a). It is possible that the rather low concentrating factors in the foregoing experiments were induced by sorption of HSy on the surface of metal sulfides, which renders the surfaces more hydrophilic (Matis, 1995). Such sorption would inhibit a good hydrophobic interaction between the metal sulfide surface and the oily kerosene collector. Therefore, an experiment was done with Tienen soil in which the Na2S dose was decreased to 3.5 times the total metal concentration and also a sulfide-specific Kex collector was used. The results shown in Fig. 2 demonstrate that a sulfide-specific collector Kex, instead of kerosene, does not result in an improved metal recovery (Fig. 2a) nor improved flotation selectivity (Fig. 2b). Probably HSy competes with the xanthate collector for the metal sulfide surface. Since HSy forms much stronger complexes with the metal sulfide surface, it is conceivable that HSy can prevent xanthate from being sorbed resulting in a more hydrophilic, non-floatable metal sulfide (Matis, 1995). In all foregoing sulfidisation experiments, colloidal FeS was added as nuclei to promote metal sulfide formation. Preliminary experiments of

Table 6 Concentrating factors of different experiments corrected for potentially available metal content determined by 0.1 M EDTA extraction at pH 4.65 (Table 4) Sulfidisation conditions a

Flotation reagents

Corrected concentrating factor

Na2S (molyl)

FeS (%)

Electrolyte

Environment

Collector

Frother

Cd

Cu

Zn

Pb

10 3.5 10 2 2 2

1 1 0 1 0 0

NaHCO3 NaHCO3 NaHCO3 NaHCO3 NaHCO3 Ca(NO3)2

Ambient air Ambient air Ambient air Glove box Glove box Glove box

Kerosene Kex Kerosene Kerosene Kerosene Kerosene

MIBC MIBC MIBC MIBC MIBC MIBC

2.42 2.29 2.46 2.10 2.15 n.d.b

1.96 2.03 2.21 2.02 2.08 2.30

2.37 3.22 2.40 2.76 2.54 3.05

2.00 1.72 2.16 1.89 1.99 n.d.b

a b

wNa2Sxsfactor=wtotal metalx (molyl). n.d.snot determined.

M. Vanthuyne, A. Maes / The Science of the Total Environment 290 (2002) 69–80

Fig. 2. Influence of the combination of sulfidisation and flotation on (a) mass recovery and metal recovery, and (b) concentrating factor for different metal ions from Tienen soil. Sulfidisation with Na2S (Na2Ss3.5=wtotal metalx) and 1% FeS in ambient air. Flotation parameters: 25-ml 0.1 M Kex; 27-ml MIBC (0.1% vyv); 0.1 M NaHCO3; conditioning time, 10 min; and flotation time, 10 min.

aqueous solutions containing Cd(NO3)2 showed that in the presence of fine grained FeS (Cerac, 99%) cadmium sulfide was formed faster than in absence of fine grained FeS. FeS is also responsible for the co-precipitation of transition metals in sediments (Arakaki, 1992; Maes and Cauwenberg, 2000). Moreover, the Denver flotation experiments with the synthetic sediments (Table 3) showed that the flotation selectivity of these mixed precipitates with FeS is similar as for crystalline, good floatable metal sulfides. Nevertheless, we observed that FeS addition during the sulfidisation step in ambient air does not improve the flotation behaviour of freshly formed metal sulfides compared with metal sulfides formed in absence of iron sulfide (Fig. 3a,b). Therefore, the role of FeS in the formation of metal sulfides will further be investigated in the sulfidisations under anoxic conditions. 3.2.2.2. Sulfidisation in the glove box under N2 y H2 atmosphere. Some sulfidisations with Na2S were done in anoxic conditions to avoid oxidation during the sulfidisation step. In addition, the dose

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of Na2S was further decreased to twice the total metal concentration to avoid flotation depression by excess HSy. Compared with the flotation results of metal sulfides formed in ambient air, the flotation of metal sulfides formed under anoxic conditions are not more selective (Fig. 4b). Moreover, again no effect of iron sulfide addition was observed (Fig. 4b). The reason for this behaviour is unclear in the sense that iron sulfide was expected to act as a promoter for metal sulfide formation. When 0.1 M Ca(NO3)2 was used as background electrolyte instead of 0.1 M NaHCO3 during one sulfidisation experiment in anoxic conditions in order to improve the flocculation of the finely dispersed metal sulfides and organic matter, it was noticed that the metal recovery was rather low, but that the heavy metals are concentrated in less mass (Fig. 5a,b). The mass flotated is an important flotation parameter, which influences the re-usable soil fraction and should be as low as possible.

Fig. 3. Influence of the combination of sulfidisation and flotation on (a) mass recovery and metal recovery, and (b) concentrating factor for different metal ions from Tienen soil. Sulfidisation with Na2S (Na2Ss10=wtotal metalx) in ambient air. Soil sample of Tienen was used. Flotation parameters: 1.5ml kerosene; 30-ml MIBC (0.1% vyv); 0.1 M NaHCO3; conditioning time, 10 min; and flotation time, 10 min.

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Fig. 4. Influence of the combination of sulfidisation and flotation on (a) mass recovery and metal recovery, and (b) concentrating factor for different metal ions from Tienen soil. Sulfidisation with Na2S (Na2Ss2=wtotal metalx) with and without FeS (1%) in a glove box. Soil sample of Tienen was used. Flotation parameters: 1.5-ml kerosene; 30 mlMIBC (0.1% vyv); 0.1 M NaHCO3; conditioning time, 10 min; and flotation time, 10 min.

In order to explain this, one should bear in mind that the probability of collision and probability of attachment between particle and bubble are the most important parameters which contribute to the probability of particle flotation (Matis, 1995). The probability of collision depends on the dimensions of both particle and bubble, whereas the probability of attachment depends on the surface characteristics of the particle. In the size range 20–50 mm the selectivity of Denver flotation reached a maximum because both collision probability and attachment probability were favourable (Cauwenberg et al., 1998b). For particles with dimensions lower than 5 mm, the probability of attachment remains constant for particles with the same mineralogy, but the particle diametersyair bubble diameter ratio is less favourable for collision, resulting in lower Denver flotation specificity in agreement with Cauwenberg et al. (1998b). Therefore, it is possible that the finely dispersed, amorphous metal sulfides formed by sulfidisation pre-treatment of a heavy metal pol-

However, the metal removal must be sufficiently high to fulfil environmental regulations for the rest fraction. Therefore, the decontamination of polluted soils is always a trade-off between metal recovery and the concentrating factor. In the present case, the lower flotated mass is possibly a consequence of using Ca(NO3)2 instead of NaHCO3 as background electrolyte. As a result of the interaction with Ca2q, the soil particles flocculate much better resulting in a lower flotated mass. 3.2.3. Explanations for the observed flotation behaviour Possible explanations for the observed low selective Denver flotation of metal sulfides formed by sulfidisation pre-treatment of a metal-contaminated soil might be the surface characteristics (wettability, hydrophobicity, etc.) and the dimensions of the freshly formed metal sulfides.

Fig. 5. Influence of the combination of sulfidisation and flotation on (a) mass recovery and metal recovery, and (b) concentrating factor for different metal ions from Tienen soil. Sulfidisation with Na2S (Na2Ss2=wtotal metalx) in a glove box. Soil sample of Tienen was used. Flotation parameters: 1.5ml kerosene; 30-ml MIBC (0.1% vyv); 0.1 M Ca(NO3)2; conditioning time, 10 min; and flotation time, 10 min.

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luted soil are too small in comparison with the bubbles generated by the Denver flotation apparatus. Moreover, highly turbulent flotation conditions (e.g. in a mechanically agitated Denver cell) also increases the aspecific entrainment of nonsulfidic soil particles resulting in a high mass recovery and a low concentration factor. 4. Conclusions The BCR sequential extraction technique (Ure et al., 1993) is not a good method to estimate the sulfide convertible (potential available) metal content of a metal polluted soil due to the presence of transition metals as iron oxide co-precipitates. The single step EDTA extraction (0.1 M, pH 4.65) gives a better approximation. The EDTA (0.1 M, pH 4.65) extractability of transition metals from synthetic sediments in anoxic conditions showed that transition metals can be converted into a metal sulfide phase by sulfidisation. Either a separate metal sulfide phase or a coprecipitate with iron sulfide is formed. The Denver flotation of both types of synthetic sediments using kerosene as collector at high background electrolyte concentrations, demonstrated that the concentration factors for freshly formed metal sulfides are approximately the same as for fine grained crystalline metal sulfides. Notwithstanding the fact that the Denver flotation of freshly formed metal sulfides after sulfidisation pre-treatment in ambient air and in anoxic conditions of a metal-contaminated soil is not highly selective (concentrating factors -2), the obtained results are encouraging for further research in this field. Up to 50% of the transition metals could be removed. The concentrating factors corrected for the potentially available (sulfide convertible) metal content, determined by EDTA (0.1 M) extraction, lie between 2 and 3 (Table 6) and approach the values obtained by Cauwenberg et al. (1998a) for dredged material. The selective flotation of these finely dispersed, amorphous metal sulfides can possibly be improved by optimising the bubble–particle interaction. Therefore, the use of other flotation techniques (e.g. dissolved air flotation) which more specifically separate the smallest particles will be

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investigated in future research. These flotation techniques achieve also better hydrodynamic conditions compared with the highly turbulent conditions in a Denver flotation cell. Acknowledgments The authors are grateful to the FWO (G.0306.97N) for financial support. Vanthuyne M. thanks the IWT-Vlaaanderen for awarding a research grant. References Arakaki T. Interactions of cations with iron monosulfide mackinawite in dilute solutions and seawater. Ph.D. Texas A and M University, 1992:185 p. Cauwenberg P. Speciation and behavior of transition metals in dredged material and their potential removal by flotation. Ph.D. Kuleuven University, 1998:161 p. Cauwenberg P, Verdonckt F, Maes A. Flotation as a remediation technique for heavily polluted dredged material. 1. A feasibility study. Sci Total Environ 1998;209:113 –119. Cauwenberg P, Verdonckt F, Maes A. Flotation as a remediation technique for heavily polluted dredged material. 1. Characterisation of flotated fractions. Sci Total Environ 1998;209:121 –131. Clark DW, Newell AJH, Chilman GF, Capps PG. Improving flotation recovery of copper sulfides by nitrogen gas and sulfidisation conditioning. Mineral Eng 2000;13(12):1197 – 1206. Clarke P, Fornasiero D, Ralston J, Smart RS. A study of the removal of oxidation products from sulfide mineral surfaces. Mineral Eng 1995;8(11):1345 –1357. Cottenie A, Camerlynck R, Verloo M, Dhaese A. Fractionation and determination of trace elements in plants, soils and sediments. Pure Appl Chem 1979;52:45 –53. Crozier KA. Flotation: theory, reagents and ore testing. Oxford: Pergamon press, 1992. (329 p). Eberius E. Verfahren zur dekontaminierung schlammartiger sedimente. European patent 0332958, 1989. Framson PE, Leckie JO. Limits of coprecipitation of cadmium and ferrous sulfides. Environ Sci Technol 1978;12(4):465 – 469. Freeman WA, Newell R, Quast KB. Effect of grinding media and NaHS on copper recovery at Northparkes Mines. Mineral Eng 2000;13(13):1395 –1403. Gee GW, Bauder JW. Particle-size analysis. In: Klute A, editor. Methods of soil analysis, part 1, physical and mineralogical methods Madison, Wisconsin, USA: American Society of Agronomy, 1986:377 –382. Glazunov LA, Chernykh SI. Increase in mineral flotation effectiveness by creation of an elemental sulfur coating. In: Castro S, Alvarez J, editors. Proceedings of the Fourth Meeting of the Southern Hemisphere on Mineral Technology and Third Latin-American Congress on Froth Flotation, vol. 2, Concepcion, Chile 1994:309 –318.

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