Minerals Engineering 16 (2003) 1131–1141This article is also available online at:

www.elsevier.com/locate/mineng

The use of flotation techniques in the remediation of heavymetal contaminated sediments and soils: an overview

of controlling factors

M. Vanthuyne, A. Maes *, P. Cauwenberg

Laboratory of Colloidchemistry, Katholique University of Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium

Received 15 May 2003; accepted 30 June 2003

Abstract

The potential use of flotation as a remediation technique for heavy metal contaminated sediments and soils is discussed. In

general, the flotation process is much less selective for heavy metals in sediments and soils compared to the results obtained in

common mineral ore flotation practice. The following factors could be isolated as responsible for this different behaviour: the metal

distribution over the different geochemical phases (metal partitioning), the metal distribution over the different size fractions (metal

fractionation) and the presence of organic matter (humic substances). These controlling factors were illustrated by performing

Denver flotation experiments on a natural canal sediment and column flotation tests on synthetic sediments.

It is emphasised that sequential extraction techniques can be used as a tool for determining the original metal partitioning in

sediments and soils and as a result helps to select the most appropriate procedure of flotation (e.g. a phase-specific collector). A

chemical pre-treatment step (e.g. sulphidisation, hydroxylation) prior to the flotation of sediments and soils can be beneficial de-

pending on the in-situ metal partitioning and serves to transfer heavy metals, which are associated with different speciation sources

(oxides, sulphides, clay minerals, organic matter, etc.), into one unique-good floatable-chemical speciation form (e.g. oxides,

sulphides). The selective flotation of metal-bearing particles from fine-textured sediments and soils is very difficult and therefore a

major challenge for future research is to tackle the problems related with the flotation of these fines. Organic matter, which is rather

selectively flotated, has an adverse effect on the metal (sulphide) flotation selectivity due to its adsorption onto mineral surfaces.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Reclamation; Environmental; Pollution; Froth flotation

1. Introduction

In their recent literature reviews, Mulligan et al.

(2001a,b) have described developing and developed

(full-scale) remediation technologies for heavy metal

contaminated soils and dredged sediments. Compared

to soil treatment techniques, few remediation techniques

have been commercially used for sediments. In general,the treatment methods for heavy metal contaminated

soils and sediments are similar and include physical

separation methods (screening, hydrocyclonage, flota-

tion), thermal extraction, stabilisation/solidification, soil

washing, biological decontamination (bioleaching, phy-

toremediation) and electrokinetics. Physical separation

methods are becoming more common and applications

*Corresponding author: Tel.: +32-16-321598 fax: +32-16-321998.

E-mail address: andre.maes@agr.kuleuven.ac.be (A. Maes).

0892-6875/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mineng.2003.06.012

will continue to increase as they can be used for the

removal of metal contamination in a particular size or

form or in combination with other processes, to reduce

the volume to be treated by other methods. Thermal

processes are only applicable for the removal of volatile

metals (e.g. Hg) but the costs are high. Solidification/

stabilisation are successful but significant monitoring is

required, since the process can be reversible. In the caseof vitrification, only if a useful glass product can be sold,

the process will be economically viable. Biological pro-

cesses are still under development but have the potential

to be low cost. Electrokinetics is particularly promising

for decontamination at moderate depths in clays but

R&D is required to optimise pore fluids and electrode

configuration (Mulligan et al., 2001a,b).

In this paper, we will only focuss on the (potential)use of flotation techniques to remove heavy metals from

contaminated dredged sediments and excavated soils.

1132 M. Vanthuyne et al. / Minerals Engineering 16 (2003) 1131–1141

Flotation is a physico-chemical separation technique,

which introduces air bubbles in a suspension, and is

widely used in mineral ore processing to separate min-

erals, which are commonly present in one specific

chemical form (e.g. sulphide), from the tailings. The

selectivity of the separation is based upon differences in

the wettability between the particles in the suspension. Ifthe natural differences in the wettability between the

particles are not sufficient for their selective separation,

the suspension has to be conditioned by flotation re-

agents that influence the hydrophobicity of the particles’

surfaces. The most important flotation reagents are (a)

collectors, which adsorb on mineral surfaces, rendering

them hydrophobic and facilitate bubble attachment and

(b) depressants, which adsorb on mineral surfaces, ren-dering them hydrophilic and induce bubble detachment

(Matis, 1995).

The aim of this paper is to give an overview of the use

of flotation techniques in the remediation of heavy metal

contaminated sediments and soils and to discuss in de-

tail the major factors influencing the flotation selectivity

of heavy metals. However, the authors are aware of the

fact that (a) not all flotation practice in relation toremediation of heavy-metal contaminated sediments

and soils is published in the open literature and (b) a lot

of the published work is poorly described and explained.

New Denver flotation experiments on a natural canal

sediment and column flotation tests on synthetic sedi-

ments were performed to illustrate the most important

factors controlling the metal flotation selectivity.

2. Materials and methods

2.1. Denver flotation experiments with natural canal

sediment

Denver flotation experiments were made with a

Denver D12 laboratory-scale flotation machine onsludge from the Gent–Terneuzen Canal (Belgium). The

general characteristics of the sediment are given in

Cauwenberg et al. (1998a). One hundred grams of wet

sludge was weighed into the flotation tank and 1.1 l of

0.1 M NaHCO3, 25 ml methylisobutylcarbinol (MIBC,

v/v: 1/1000) (Merck) frothing agent and 0.5 ml kerosene

or 25 ml of 0.1 M potassium ethyl xanthate (Kex) was

added as collector. The flotation suspension was thenallowed to condition for 10 min at 1000 rpm. During

this conditioning step, the pH was adjusted with HClO4

or NaOH to values between pH 8 and 12. Subsequently,

Denver flotation was started by opening the air supply

for a period of 10 min and the froth layer was manually

removed. The obtained rest and froth fractions were

freeze-dried (Christ Alpha 1-2) and the metal concen-

trations (Cd, Cu, Pb, Zn and Fe) in both fractions weredetermined by acid digestion in a microwave (Milestone,

MLS-1200-MEGA) followed by AAS measurement

(Varian, AAS-20) identical to the procedure described in

Cauwenberg et al. (1998a). The organic matter content

(Walkley–Black method) was also determined in both

fractions by chemical oxidation with K2Cr2O7 and back

titration with FeSO4 (Nelson and Sommers, 1982).

2.2. Column flotation experiments with synthetic sedi-

ments

A straight column with an internal diameter of 5 cm

and a height of 32 cm was filled with the flotation sus-

pension. Nitrogen gas was blown into the suspension

through a glass frit in order to obtain finely dispersed

gas bubbles, which migrate upward to the surface. Thefroth at the top of the column flowed over the edge and

was collected. The froth and the rest fractions were fil-

tered over a 0.45 lm Gelman filter prior to overnight

drying at 105 �C. The metal content (Cu, Pb, Zn) in both

fractions was determined by AAS (Varian, AAS-20)

after acid digestion in a microwave (Milestone, MLS-

1200-MEGA) (Cauwenberg et al., 1998a).

The influence of humic acid (Aldrich) on the ZnSflotation efficiency was tested at pH 8, 9 and 10. The HA

was purified with HF to extract all inorganic impurities

following the procedure of Swift and Hayes (1978).

Synthetic sediments were prepared by weighing 2 g of

fine-grained illite clay (Silver Hill Montana, 0–50 lm)

and 2 g of fine-grained sea sand (VEL, 0–200 lm) in a

centrifuge tube. Then, 10 ml of a 4 mg/ml ZnS (wurtzite,

Aldrich 99.99%) solution and increasing amounts ofpurified humic acid (0, 0.02, 0.04, 1 g) were added. To

each centrifuge cup, 40 ml of a deoxygenated solution

containing 1.25 · 10�3 M NaCl and 1.25 · 10�3 M CaCl2was added as background electrolyte and the pH was

adjusted to 8.5 with 0.1 M NaOH. The synthetic sedi-

ments were equilibrated by end-over-end shaking during

1 week inside an anoxic glove box (N2/H2 (%): 95/5%)

and the pH was daily controlled and (eventually) read-justed. pH changes were only observed in the first days

in agreement with the observations of Oakley et al.

(1980) that a few days were sufficient to reach equilib-

rium. Before the start of the flotation, the synthetic

sediments were transferred to the flotation column and

25 ml 0.1 M potassium ethyl xanthate (Kex) collector

and 25 ml methylisobutylcarbinol (MIBC, v/v: 1/1000)

(Merck) frothing agent were added. The volume wasmade up to 400 ml with deoxygenated distilled water.

The pH of the flotation suspension was adjusted to pH

8, 9 or 10 during a conditioning step of 5 min. Column

flotation experiments were performed during 7 min

using nitrogen as flotation gas.

The influence of purified HA (Aldrich) on the flota-

tion selectivity of a mixture of ZnS, PbS and CuS was

also tested but only at pH 8. To obtain small metalsulphide particles, 20 g of a metal sulphide was weighed

M. Vanthuyne et al. / Minerals Engineering 16 (2003) 1131–1141 1133

inside an anoxic glove box (N2/H2(%): 95/5%) into the

recipient of a ball mill. The recipient was sealed with

sealing tape (Aldrich) and the metal sulphide was

ground outside the glove box. The surface of the fine-

grained metal sulphide particles were analysed by XPS

(VG Escolab 220iXL) and no (surface) oxidation was

detected. The mean particle size after grinding wasmeasured by laser diffraction (Coulter LS 100): ZnS (7

lm), PbS (18 lm), CuS (12 lm), FeS (18 lm).

Synthetic sediments were prepared by weighing 25 g

of fine-grained illite clay (Silver Hill Montana, 0–50 lm)

and 25 g of fine-grained sea sand (VEL, 0–200 lm) in

250 ml centrifuge cups together with 0.5 g of fine-

grained ZnS (sphalerite, Cerac 99.99%), 0.5 g of fine-

grained PbS (galena, Cerac 99.9%), 0.5 g of fine-grainedCuS (covellite, Cerac 99.5%), 0.5 g of fine-grained FeS

(pyrrhotite, Cerac 99.9%) and occasionally 0.250 g pu-

rified HA (Aldrich) was added. 200 ml of a deoxy-

genated solution containing 10�3 M NaCl and 10�3 M

CaCl2 was added as background electrolyte to each

centrifuge cup and the pH was adjusted to 8.5 with 0.1

M NaOH. The synthetic sediments were equilibrated

during 1 week inside an anoxic glove box (N2/H2 (%):95/5%) and the pH was daily controlled and (eventually)

readjusted. For the flotation experiment, a 15 ml sample

was transferred to the flotation column. 25 ml 0.1 M

potassium ethyl xanthate (Kex) collector or 1 ml kero-

sene collector or 25 ml 0.1 M sodium isopropyl xanthate

(Sipx) collector and 25 ml methylisobutylcarbinol

(MIBC, v/v: 1/1000) (Merck) frothing agent were added

and the volume was made up to 400 ml with deoxy-genated distilled water. The pH of the flotation sus-

pension was adjusted to pH 8 during a conditioning step

of 5 min. Column flotation experiments were performed

during 7 min using nitrogen as flotation gas.

2.3. Humic acid adsorption onto ZnS

Samples of 0.100 g ZnS (wurtzite, Aldrich 99.99%)

were weighed into centrifuge tubes and 20 ml of a

deoxygenated solution containing 10�3 M NaClO4 and

50 mg/l purified Aldrich humic acid was added. HClO4

or NaOH was added to adjust the pH to values between

7 and 10. Experiments were restricted to this pH range

since at lower pH values the HA started to precipitatedue to its interaction with Znþ2 in the solution. After

overnight equilibration inside an anoxic glove box (N2/

H2 (%): 95/5%), the pH was measured and subsequently

the centrifuge tubes were centrifuged (5000 rpm, 3444 g)

outside the glove box with a Beckmann J-21 centrifuge.

A 4 ml sample of the supernatant was taken for HA

measurement at 280 nm in a LKB kinetics UV–VIS

spectrophotometer. One ml of 0.5 M ammonium acetatebuffer solution was added to the sample before the

measurement because the absorption of humic acid was

pH-dependent.

2.4. Separation of the soil fraction smaller than 2 lm clay

fraction upon soil pre-treatment by sulphidisation

Approximately 15 g Tienen soil was weighed in a

centrifuge-tube and 240 ml of a deoxygenated sodium

sulphide solution (1.3394 g Na2S/500 ml, pH 9.5) was

added inside an anoxic glove box (N2/H2 (%): 95/5%).After overnight equilibration inside the glove box, the

centrifuge tube was centrifuged (Ecco Superior Centri-

fuge) during 2 min 24 s at 1500 rpm in order to obtain

the soil fraction smaller than approximately 2 lm in the

supernatant. Due to the fact that metal sulphides (e.g.

covellite: 4600–4760 kg/m3, sphalerite: 4090 kg/m3,

greenockite: 4830 kg/m3) have a higher density than the

clay fraction (density: 2500 kg/m3), it is expected thatfreshly formed transition metal sulphides with dimen-

sions in the nm size range should also be recovered in

the supernatant.

After pipetting the supernatant inside an anoxic glove

box (N2/H2 (%): 95/5%), the remaining centrifugate was

washed with deoxygenated bidistilled water followed by

2 min 24 s centrifugation at 1500 rpm (Ecco Superior

Centrifuge). The supernatant obtained after (a) thesulphidisation step and (b) the washing step and the

remaining centrifugate, which corresponds to approxi-

mately the soil fraction higher than 2 lm, were over-

night freeze-dried. Zn and Cu concentrations in the

(three) fractions were determined by acid digestion in a

microwave (Milestone, MLS-1200-MEGA) followed by

AAS (Varian, AAS-20) measurement identical to the

procedure described in Cauwenberg et al. (1998a). Thisexperiment was performed in duplicate. Blank tests were

performed by adding bidistilled water instead of the

sodium sulphide solution.

3. Flotation of heavy metals from contaminated sediments

Heavy metals present in sediments may be distributed

over different chemical forms: e.g. oxides, sulphides,

adsorbed onto various solids. As a result the use of

flotation techniques to remove heavy metals from con-

taminated sediments, is expected to be a low selective

process. According to the patent of Eberius and Ekke(1989), chemical pre-treatment in order to obtain one

unique-good flotable-chemical (metal) speciation form

by (a) sulphidisation with sodium sulphide or (b) hy-

droxylation with NaOH and Na2CO3 prior to flotation

using phase-specific collectors (e.g. sulphide-specific

xanthate collectors, oxide-specific amine-collectors), is a

possibility to enhance the low selective flotation of

heavy metals from such sediments. By their method,the total heavy metal content reduction was very

high (>90%) for the investigated harbour sediments

in comparison with the total heavy metal content re-

duction without chemical pre-treatment (only 43%).

1134 M. Vanthuyne et al. / Minerals Engineering 16 (2003) 1131–1141

Unfortunately, no detailed information on (a) the metal

partitioning over the different geochemical phases of the

sediments (e.g. before and after the chemical pre-treat-

ment) and (b) the metal distribution over the different

particle size fractions was given in the patent of Eberius

and Ekke (1989). In addition, and somewhat unusual,

no details were shown concerning the recovery of inputmass in the froth fraction and as a result the enhance-

ment of the flotation selectivity of metals due to the

proposed chemical pre-treatment methodology was

poorly illustrated, namely (only) by a comparison of

removal efficiencies.

In the framework of the Development Programme for

Treatment Processes for Contaminated Sediments in the

Netherlands (POSW), flotation tests with finely grainedsediments contaminated with heavy metals and PAH

have been conducted at several levels of scale. On a

laboratory scale, fractionation studies indicated that the

target metals did not occur in one specific chemical

form, nor were they linked to one specific grain size in

the Stein harbour sediment and therefore metals were

removed by a leaching-sulphide precipitation–flotation

method (POSW, 1997). By this method, the heavymetals were released from the sludge particles by acid

leaching, leaving them dissolved in the water phase.

Subsequently, the heavy metals were precipitated as

sulphides and flotated to remove the formed metal

sulphides from the sludge. This method showed a rea-

sonable separation performance, but did not result in a

form of sludge immediately re-usable. In addition, the

practical applicability of this method on a larger scalewas mentioned as uncertain (POSW, 1997) without

further comments. At a semi-practice scale (3–10 m3),

flotation tests were performed in a mobile plant of

Mosmans Mineral Technique, which was built into a

ship, with the underflow and the overflow fraction of

Tolkamer harbour sediment, which were obtained by a

previous hydrocyclone separation step. The test perfor-

mance for the various heavy metals in the sandy fraction(underflow) was practically 100%. However, for the fine

fraction (overflow), the selectivity for heavy metals was

low and ranged between 34% and 58% metal removal in

24% of the input mass (POSW, 1997). Further flotation

tests at a practice scale in a full size installation (10–1000

m3) of Mosmans Mineral Technique with the fine frac-

tion (overflow) following a hydrocyclone separation step

showed similar results for a sediment from Elburg har-bour: 67% lead removal in 25.6% of the input mass. The

performance for metals was little controllable and con-

siderable differences for two sampling days were ob-

tained. In contrast, the flotation performance with

regard to PAH was higher (60–90% could be concen-

trated in 10–30% of the input mass) and controllable to

a reasonable extent (POSW, 1997).

A more detailed study of the different factors influ-encing the flotation selectivity of heavy metals from

contaminated sediments is given in the work of Cau-

wenberg et al. (1998a,b). They investigated on a labo-

ratory-scale the Denver flotation behaviour of different

metals from a fine-textured (70% <50 lm) and organic

matter-rich (11.9%) sediment of the Gent–Terneuzen

Canal using oily kerosene as a non-specific collector at a

high background electrolyte concentration of 0.1 MNaHCO3 in the pH range 8–12. The concentrating fac-

tors obtained in Cauwenberg et al. (1998a) are promis-

ing but are considerably lower than the concentrating

factors obtained in common ore flotation practice (e.g.

in the separation of valuable metal sulphide minerals

from the tailings): a removal of up to 60% of metals in

15–20% of the mass was achieved for cadmium, copper,

lead and zinc at the in-situ pH conditions (pH 8–9) ofthe sediment sludge. The major factors influencing the

flotation selectivity of heavy metals from dredged sedi-

ments were according to Cauwenberg et al. (1998a,b):

(a) the in-situ metal partitioning and solution pH, (b)

the presence of a large amount of organic matter and (c)

the metal distribution over the different sediment size

fractions. These controlling factors will be in detail il-

lustrated by (a) new Denver flotation data on the sameGent–Terneuzen Canal sediment and (b) column flota-

tion data on synthetic sediments (see discussion below).

The Denver flotation experiments were performed in

similar conditioning and flotation conditions as in the

work of Cauwenberg et al. (1998a,b). In order to in-

vestigate the influence of the collector on the metal flo-

tation efficiency, flotation tests were performed using

potassium ethyl xanthate (Kex) and kerosene as col-lecting agents. The influence of pH on the metal recov-

eries and mass recovery is given in Fig. 1a (kerosene)

and b (Kex). It is clear from Fig. 1a and b that at all pH

values the recoveries of the pollutants cadmium, copper,

lead and zinc were a factor of (by approximation) two

larger than for iron. In addition, it was demonstrated in

a former study (Cauwenberg and Maes, 1997), which

was based on sequential extraction in anoxic conditions,that these metals are dominantly (>90%) present in the

fraction assigned as bound to sulphides whereas iron

was also found in other mineral phases (e.g. built in clay

minerals, pyrite and carbonates). Therefore, the ob-

served selectivity for Cd, Cu, Pb and Zn, which is il-

lustrated by the higher metal concentrating factor in

comparison with Fe in Fig. 2, can be ascribed to their

overwhelming presence as metal sulphides in the canalsediment. This statement is also based on the fact that

(a) the concentrating factor of total sulphur showed a

very good overall correspondence with the concentrat-

ing factor of these metals (Fig. 2) (taken from Cau-

wenberg et al., 1998a) and (b) the amount of metals

extracted with 0.02 M EDTA/0.5 M NH4Ac (pH 4.85)

from the froth fractions after Denver flotation at pH 8–9

was comparable with the metal amounts extracted fromthe native sludge (Cauwenberg et al., 1998b). For both

Fig. 1. Influence of pH on the mass and metal recovery as obtained by

Denver flotation using (a) 0.5 ml kerosene collector and (b) 25 ml 0.1

M potassium ethyl xanthate collector. for different metal ions of Gent–

Terneuzen Canal sludge. Flotation parameters: conditioning time 10

min, flotation time 10 min, 25 ml MIBC (1/1000 v/v), background

solution of 0.1 M NaHCO3.

Fig. 2. Concentrating factor of organic matter and sulphur compared

with the concentrating factor of zinc and iron as obtained by Denver

flotation on Gent–Terneuzen Canal sludge using 0.5 ml kerosene col-

lector or 25 ml 0.1 M potassium ethyl xanthate collector. Flotation

parameters: conditioning time 10 min, flotation time 10 min, 25 ml

MIBC (1/1000 v/v), background solution of 0.1 M NaHCO3. Zinc is

taken as representative for cadmium, copper and lead. The total sul-

phur data were taken from Cauwenberg et al. (1998a).

M. Vanthuyne et al. / Minerals Engineering 16 (2003) 1131–1141 1135

pH conditions, the amounts extracted with 0.02 M

EDTA/0.5 M NH4Ac (pH 4.85), which is typically used

in soil chemistry to determine the �plant-available’metals (Cottenie et al., 1980), were only a minor per-

centage of the total metal contents in agreement with the

theoretically expected low dissolution of transition me-

tal sulphides (Van der Lee, 2000) and (thus) supportingtheir presence as metal sulphides in the froth.

From a comparison of Fig. 1a and b, it is also clear

that the use of a sulphide-specific xanthate collector has

no enhancement effect on the metal sulphide flotation

efficiency in comparison with a non-specific (hydro-

phobic) kerosene collector. In contrast, kerosene is a

good alternative for more sulphide-specific xanthates

since the use of kerosene gives higher or equal heavymetal concentrating factors (and metal recoveries) at the

in-situ pH conditions of the sediment (Fig. 2) and the

amount of collector consumed was much lower and thus

economically more favoured.

As shown in Fig. 1a and b, the recovery of Cd, Cu, Pb

and Zn is gradually enhanced by increasing the pH from

pH 8 (e.g. with kerosene: 64.8% Cd, 68.4% Pb, 67.9%Zn, 58.4% Cu) to pH 12 (e.g. with kerosene: 87.4% Cd,

88.9% Pb, 89.9% Zn, 81.4% Cu). However, in the mean

time, the concentrating factors decreased by increasing

the pH from pH 8 (e.g. with kerosene: Cd 3.4, Pb 3.6, Zn

3.6, Cu 3.1) to pH 12 (e.g. with kerosene: Cd 2.6, Pb 2.7,

Zn 2.7, Cu 2.4) due to the increasing mass recovery with

pH resulting from a better dispersion of mineral parti-

cles with rising pH. Oxidation of metal sulphide surfacesdue to the strongly oxidising conditions at high pH

values in the flotation tank can be an additional expla-

nation for the lowering of the concentrating factors

beyond pH 9. This oxidation process was investigated

and proven by performing a Tessier et al. (1979) se-

quential extraction procedure on the obtained froth

fractions and showed that after Denver flotation at

higher pH values the heavy metal partitioning inthe froth fraction had moved towards more easily

extractable fractions like oxides and organic matter

(Cauwenberg et al., 1998b).

Surprisingly, also a good correspondence of the

heavy metal concentrating factors with the concentrat-

ing factor of the organic matter (Walkley–Black) was

obtained (Fig. 2). This enrichment of organic matter can

be attributed to the �hydrophobic’ character of a part ofthe organic matter but occurred with a lower concen-

trating factor than for metal sulphides due to the pres-

ence of different subfractions of organic matter with

their own specific flotation behaviour.

Since the organic matter was only a minor sink for

Cd, Cu, Pb and Zn in the sediment according to a

partitioning study based on sequential extraction pro-

cedures in anoxic conditions (Cauwenberg and Maes,

Fig. 4. Microgram humic acid (Aldrich) adsorbed per gram zinc sul-

phide (wurtzite) as a function of pH. Adsorption experiments were

performed overnight and were restricted to the pH range between pH 7

and 10 since at lower pH values the HA started to precipitate.

1136 M. Vanthuyne et al. / Minerals Engineering 16 (2003) 1131–1141

1997), its good flotation behaviour does not add to the

removal of the heavy metals. On the contrary, the

presence of large amounts of humic substances has an

adverse effect on the flotation behaviour of metal sul-

phides and explains in part the lower flotation selectivity

of metal sulphides from sulphide-rich sediments com-

pared to the flotation selectivity of metal sulphides incommon ore flotation practice. This is shown by the

results of column flotation tests using potassium ethyl

xanthate (Kex) collector at low background electrolyte

concentrations. These column flotation experiments

were made on synthetic sediments containing sea sand,

illite clay and ZnS (wurtzite) to which increasing

amounts of humic acid (Aldrich) were added. ZnS was

taken as a representative for the other transition metalsulphides. The experimental results are depicted in Fig. 3

and show that the Zn concentrating factor decreases

with increasing HA gifts in the pH range 8–10. At the

highest HA concentration (2.5%), the concentrating

factor is lowered to approximately 1, meaning no se-

lective separation at all. Except for this highest HA gift,

a maximum selectivity is reached at pH 9. A similar

maximum is obtained in the Denver flotation of theGent–Terneuzen Canal sediment using Kex or kerosene

(Fig. 2).

The lowering in the ZnS flotation selectivity can be

ascribed to the adsorption of humic acid onto ZnS

(wurtzite). This is withdrawn from the results of a sep-

arate humic acid adsorption study onto ZnS, which in-

dicate that, at a gift of 1% Aldrich HA, HA adsorption

is significant in the pH range between 7 and 10. The HAadsorption onto ZnS has its lowest value at pH 10 (Fig.

4) and therefore the lowering of the flotation selectiv-

ity at pH 10 in Fig. 3 can be possibly attributed to

the oxidation of the ZnS surface (e.g. formation of

Fig. 3. Zink concentrating factor as a function of pH as obtained by

column flotation on synthetic sediments. Flotation parameters: nitro-

gen gas, conditioning time 5 min, flotation time 7 min, 25 ml 0.1 M

potassium ethyl xanthate, 25 ml MIBC (1/1000 v/v). Synthetic sedi-

ments were prepared with fine sea sand (0–200 lm), fine illite clay

(Silver Hill Montana, 0–50 lm), ZnS (wurtzite) and different amounts

(%) of humic acid (Aldrich).

–SZnOH� species). Indeed, such oxidation hinders thespecific interaction between the ZnS surface and the

xanthate collector (Matis, 1995).

Further column flotation tests using kerosene collec-

tor or sulphide-specific xanthate collectors (potassium

ethyl xanthate and sodium isopropyl xanthate) were

performed at low background electrolyte concentrations

on synthetic sediments (pH 8) containing 0.5% HA. In

comparison with the former column flotation experi-ments with ZnS (wurtzite), a mixture of sphalerite

(ZnS), galena (PbS) and covellite (CuS) was used. All

metal concentrating factors decreased in the presence of

HA independent of the used collector (Fig. 5). The

presence of 0.5% HA gives a similar reduction of the Zn

flotation selectivity as in the column flotation experi-

ment with wurtzite (ZnS) at pH 8 using Kex (concen-

trating factor of approximately 2.8). Possibleexplanations for the lower zinc flotation selectivity in

absence of HA using Kex collector compared to the

former column flotation experiments are: (a) the differ-

ent mineralogy and grain size of sphalerite (compared

with wurtzite) and (b) the presence of other metal

sulphides in the flotation pulp. Concluding, these col-

umn flotation experiments on synthetic sediments

clearly demonstrate that the high amounts of organicmatter in sediments contribute to the lowering of the

concentrating factor of metal sulphides in comparison

with the concentrating factors for metal sulphide min-

erals in common ore flotation practice.

According to Cauwenberg et al. (1998b), the rather

moderate selective flotation behaviour of metal sulph-

ides from dredged material can also in part be attributed

to the fact that the Denver flotation selectivity is highestfor heavy metals in the particle size range 20–50 lm and

is smallest for the size fraction <5 lm, which contained

the largest concentration of heavy metals (Fig. 6). In

Fig. 5. Influence of 0.5% humic acid on the concentrating factor of

different metal ions as obtained by column flotation on synthetic

sediments at pH 8. The collectors added are indicated in the X -axis: 25ml 0.1 M potassium ethyl xanthate, 1 ml kerosene and 25 ml 0.1 M

sodium isopropyl xanthate. Flotation parameters: nitrogen gas, con-

ditioning time 5 min, flotation time 7 min, 25 ml MIBC (1/1000 v/v).

Synthetic sediments were prepared with fine sea sand (0–200 lm), fine

illite clay (Silver Hill Montana, 0–50 lm), fine CuS (covellite, 0–50

lm), fine PbS (galena, 0–100 lm), fine ZnS (sphalerite, 0–50 lm) and

(occasionally) humic acid (Aldrich).

Fig. 6. Ratio of the metal concentration (lg/g dry) in the froth fraction

to the metal concentration (lg/g dry) in the rest fraction in the various

size classes after Denver flotation on Gent–Terneuzen Canal sludge at

pH 8. Flotation parameters: conditioning time 10 min, flotation time

10 min, 0.5 ml kerosene collector, 25 ml MIBC (1/1000 v/v), back-

ground solution of 0.1 M NaHCO3 (adapted from Cauwenberg et al.,

1998a).

M. Vanthuyne et al. / Minerals Engineering 16 (2003) 1131–1141 1137

addition, Abd El-Rahman et al. (1999) demonstrated

that Denver flotation on the Gent–Terneuzen Canal

sediment fraction <32 lm gives poorer results than the

flotation of the whole sediment (0–2 mm). The fact that

the highest selectivity was observed for the 20–50 lmparticle size fraction (Fig. 6) is in line with the experi-

mental practice in convential Denver ore flotation(Rulyov, 1999) and is essentially due to the optimal ratio

of the bubble diameter to the particle diameter. More-

over, the aselective flotation of non-sulphidic gangue

caused by the hydrodynamic conditions (turbulence) in

the Denver flotation tank, which easily entrains them

into the froth layer, is more significant for the size

fraction smaller than 5 lm (Kirjavainen, 1996). Ac-

cording to Mulleneers et al. (2002a), using smaller air

bubbles and a low air flowrate is a possibility to optimise

the flotation of fine hydrophobic particles from the hy-

drophilic gangue in fine particle size fractions. However,

dissolved air flotation (DAF) was ineffective to removePAH selectively from the fraction <32 lm fraction of

Petrol harbour sediment (Mulleneers et al., 2002b). For

the fraction <32 lm of Overschie sediment, the DAF

efficiency was with a removal of 50% PAH and a tailings

of 80%, acceptable (Mulleneers et al., 2002b).

4. Flotation of heavy metals from contaminated soils

Langen et al. (1994) carried out laboratory-scale flo-

tation experiments with two zinc and lead contaminated

soils, which were pre-treated by attrition followed by

screening (100 lm), in a 250 ml laboratory agitator froth

unit. Sequential extraction showed that the target metals

were mainly bonded to the iron, manganese and alu-

minium oxide fraction (>60%) in the first soil, whereasin the second soil these metals were associated with the

organic-sulphidic and also with the residual fraction

(percentages not mentioned). Therefore, they tried to

concentrate zinc and lead of the first soil by direct flo-

tation of the oxide fraction using an anion-active alkane

sulphonate collector in the pH range between pH 3 and

11. The best flotation results were obtained at pH 4 due

to the enhanced adsorption of the anion-active alkanesulphonate collector on the positively charged oxides.

However, the obtained concentrating factors at pH 4

were rather low (1.7–1.75) at metal recoveries of about

60–65% and this was in part attributed to the fact that

35–45% of the particles was smaller than 7 lm. In-

creasing the collector dosage at pH 4 led to an increase

of the metal recoveries but also to an increase of the

mass recoveries. For the second soil, the metals were notselectively flotated from either the organic-sulphidic

fraction or from the residual bonded fraction despite

using different types of collectors and pre-desliming of

the feed (Langen et al., 1994).

Seselj et al. (1997) reported column flotation experi-

ments carried out on a larger scale with an artificially

metal contaminated soil (calcite particles) and a real

contaminated soil. After applying a suitable dosage offlotation agents, the obtained recoveries for zinc (70%)

and lead (91%) were high at pH 9 for the real contam-

inated soil. Unfortunately, no detailed information was

mentioned about the selectivity of the column flotation

process (e.g. the mass and metal concentration of the

reusable tailings were not given).

Venghaus and Werther (1998) investigated the inte-

gration of a flotation step in an industrial soil washingplant, which included the typical process steps of a

1138 M. Vanthuyne et al. / Minerals Engineering 16 (2003) 1131–1141

conventional soil washing plant (wet liberation, screen-

ing and hydrocyclonage). Laboratory-scale flotation

experiments were performed in a modified gas washing

bottle with a porous plate gas distributor on the 40–355

lm fraction, which was discharged in the hydrocyclone

underflow after wet liberation and screening (355 lm),

of a zinc contaminated soil. Somewhat unusual, theymade no attempt to clean the fraction <40 lm in the soil

washing process, which contained the highest zinc con-

centration.

A measurement for the selectivity of the flotation

process studied by Venghaus and Werther (1998) can be

obtained from the ratio of the decontamination ratio to

the solids removal, which corresponds to the slope of the

line drawn from each flotation test result to the origin inFig. 7a and b. Without soil pre-treatment, the direct

flotation of the 40–355 lm fraction at several pH values

with different collecting agents (sodium alkyl sulpho-

nate, oleylamine acetate, potassium hexyl xanthate, di-

y = 1.9071x

y = 4.6832x

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35solids removal

deco

ntam

inat

ion

ratio

without pretreatment sulfidisation sulfidisation (best result)

y = 1.9071xy = 5.6275x

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35solids removal

deco

ntam

inat

ion

ratio

without pretreatment hydroxylation hydroxylation (best result)

(a)

(b)

Fig. 7. Decontamination ratio and solids removal after flotation of a

zinc-contaminated soil pre-treated by (a) sulphidisation and (b) hy-

droxylation (modified from Venghaus and Werther, 1998). The solids

removal was defined as the ratio of the mass obtained in the froth to

the mass of the feed. The decontamination ratio was defined as the

ratio of the zinc concentration in the froth (equal to the difference

between the zinc concentration of the feed and that of the cleaned

tailings) to the zinc concentration in the feed.

thiophosphate, rape seed oil) or a combination of these

did not lead to satisfying cleaning results. Up to 35%

solids removal, the selectivity is constant (equal to ap-

proximately 1.90) as indicated by the slope of the solid

line in Fig. 7a and b and independent of the used col-

lectors. This low selective flotation behaviour was as-

cribed to the fact that zinc was not present in thecontaminated soil in one particular chemical phase, like

it is mostly the case in mineral ore flotation, but was

associated with the different soil geochemical phases.

This was demonstrated by performing the sequential

extraction procedure of F€oorstner and Calmano (1982):

12.8% Zn was found in the exchangeable fraction, 32.5%

in the carbonatic fraction, 51.5% in the easily reducible

fraction, 1.7% in the moderately reducible fraction, 0.8%in the sulphidic-organic fraction and 0.6% in the residual

fraction. However, it was not clearly mentioned in

Venghaus and Werther (1998) that this sequential ex-

traction was performed on the 40–355 lm soil fraction

used for flotation.

In order to circumvent the problem of zinc parti-

tioning over different soil geochemical phases, Venghaus

and Werther (1998) included a pre-treatment step withthe goal to obtain an unique chemical (zinc) phase,

which might then allow a more selective flotation with

phase-specific collecting agents like sulphide-specific

xanthates or oxide-specific amines. Two different meth-

ods of chemical pre-treatment were applied, namely (a)

hydroxylation by adding a mixture of NaOH and

Na2CO3 or (b) sulphidisation by adding a Na2S solu-

tion, in order to transfer zinc into respectively an oxidephase or a sulphide phase. The patent of Eberius and

Ekke (1989) for cleaning harbour sediments was at the

basis of their approach.

Upon sulphidisation prior to flotation, only in a few

flotation conditions, an enhancement of the zinc flota-

tion selectivity (higher slope) was obtained compared to

absence of pre-treatment. The best flotation result after

sulphidisation pre-treatment (slope: 4.68) is indicated bya triangle in Fig. 7a. It was achieved by sulphidisation

with 1500 mg Na2S/kg d.m. and subsequent condition-

ing with 400 mg/kg d.m. potassium hexyl xanthate col-

lector and 200 mg/kg d.m. Tylose depressant at pH 7.5.

Compared to sulphidisation, hydroxylation prior to

flotation showed more potential to remove Zn from the

40–355 lm fraction: an enhancement of the flotation

selectivity was achieved under different (hydroxylationand conditioning) conditions (Fig. 7b). Among these

�good’ flotation results, the best flotation result (slope:

5.62) was achieved by hydroxylation with 5000 mg/kg

d.m. of a NaOH/Na2CO3 (2/1) solution followed by

conditioning with 400 mg/kg d.m. oleyl amine acetate

collector, 500 mg/kg d.m. potassium hexyl xanthate

collector and 200 mg/kg d.m. Tylose depressant. How-

ever, the question remains whether the enhanced zincflotation selectivity, which was obtained under certain

M. Vanthuyne et al. / Minerals Engineering 16 (2003) 1131–1141 1139

pre-treatment and conditioning conditions, persists up

to much higher zinc recoveries. In addition, Venghaus

and Werther (1998) concluded from a rough estimation

of the additional costs introduced by the integration of

the flotation step in the soil washing process that lower

remediation costs can be obtained than for soil washing

alone due to the much higher yield of the cleaned 40–355lm fraction, which makes the integration of an addi-

tional flotation step in a soil washing plant economically

attractive. However, the fraction smaller than 40 lm,

which contained the highest Zn concentration (±9000

mg Zn/kg d.m compared to 5230 mg Zn/kg d.m. in the

flotated 40–355 lm fraction) in 20% of the soil mass, still

has to be discarded (probably) due to the fact that the

flotation treatment of this fine fraction did not work.Mosmans and van Mill (1999) reported that they had

tried to remove heavy metals from such fines (e.g. hy-

drocyclone overflow fraction, <16 lm) in a pilot-scale

soil washing process but their flotation tests were un-

successful.

Vanthuyne and Maes (2002) used Denver flotation to

treat the entire 0–2 mm fraction of a loamy contami-

nated soil with cadmium, copper, lead and zinc. Thestudy of the metal partitioning in the soil, based on the

BCR-sequential extraction procedure (Ure et al., 1993),

showed that cadmium, copper, lead and zinc were par-

titioned over the various fractions of the sequential ex-

traction procedure. A low selective Denver flotation

behaviour was expected based on the distribution of all

heavy metals over the various soil geochemical phases

and prompted them to investigate the influence of aprior sulphidisation treatment in a trial to move the

EDTA-extractable metals (approximately 70% of the

total content) into a unique metal sulphide phase.

During some sulphidisation pre-treatments, fine-grained

colloidal FeS (w/w 1%) was added as a nuclei to pro-

mote metal sulphide formation or as metal adsorbent.

The flotation behaviour of the sulphidised soil was

investigated at pH 8–9 in a laboratory-scale Denverflotation cell using kerosene or potassium ethyl xant-

hate as collecting agents at a high background elec-

trolyte concentration of 0.1 M NaHCO3, motivated by

the work of Cauwenberg et al. (1998a,b) on heavy

metal sulphides flotation from dredged material.

Compared with untreated soils, sulphidisation pre-

treatment in ambient air or in anoxic conditions prior

to Denver flotation with kerosene or potassium ethylxanthate results in (a) an increase of the metal recovery

(from approximately 30% to 50%) but also in (b) an

increase of the mass recovery. Therefore, the obtained

metal concentrating factors were rather low (metal

concentrating factors <2), which was attributed to (a)

the fact that only 70% of the total metals was �avail-able’ for transformation to their metal sulphides, (b)

the aselective flotation of the non-sulphidic gangue (e.g.due to the turbulence in the Denver flotation tank) and

(c) the misfit of small (metal sulphide) particle size (nm

to 2 lm range) and bubble size (approximately 1 mm)

(Vanthuyne and Maes, 2002). However, even correction

of the obtained concentrating factors for the 70%

availability of the soil metal content, gave concentrat-

ing factors, which were still lower than the concen-

trating factors obtained for heavy metal sulphidesflotation from dredged material in similar conditioning

and flotation conditions (Fig. 2). Therefore, it is rea-

sonable that the observed low flotation selectivity

compared to the �good’ results of Venghaus and Wer-

ther (1998), is due to the fact that the Denver flotation

experiments were performed on the whole soil fraction

(0–2 mm) and not on a relatively coarse fraction (40–

355 lm) (Fig. 7).Recent experiments based on soil fractionation after

sulphidisation pre-treatment of the soil may shed some

light on the third reason for the observed low selectivity

in Vanthuyne and Maes (2002). If freshly formed sepa-

rate metal sulphides were formed by the sulphidisation

pre-treatment step then it was expected that these would

be concentrated in the clay fraction (<2 lm). Unex-

pectedly, no increase in Zn and Cu concentrations wasobserved in this fraction upon sulphidisation (in com-

parison with blank tests). Although the soil suspension

immediately turned black upon sulphidisation, it is not

clear whether (a) separate metal sulphides are formed or

(b) a metal sulphide adsorption occurs onto the in-situ

minerals leading to a distribution of metals over all size

fractions. Therefore, a major challenge for future re-

search is to clear out what really happens if a heavymetal contaminated soil is chemically pre-treated by

sulphidisation. In the mining industries, for example, the

additional flotation recovery of copper upon sulphidi-

sation is believed to be from Cu-minerals (e.g. chalco-

cite, bornite, chalcopyrite) with oxidised or tarnished

surfaces where surface oxidation has been reversed by

sulphidisation (Clark et al., 2000). In the work of Orwe

et al. (1998), sulphidisation increases the fine copperrecovery (<30 lm) from a monzonite ore, in which

digenite and chalcopyrite are the dominating minerals,

and a monzodiorite ore, in which only chalcopyrite is

dominant, by precipitating metal hydroxides, which are

present both at the surface and in solution due to oxi-

dation during grinding and conditioning stages, as the

insoluble metal sulphides. Bastin and Frenay (2003) also

improved the flotation selectivity of Co combined withhigher Co recoveries by the simultaneous addition of

ammonium sulphate in the sulphidised xanthate flota-

tion of oxidised Cu–Co ores. Ammonium sulphate, a

low cost and common reagent (easily found in the fer-

tiliser industry or as a byproduct of the cokes industry),

significantly improved the economics of flotation of the

Cu–Co oxides ores due to (a) a significant decrease of

the NaHS consumption, (b) a marked improvement ofthe sulphidisation kinetics and (c) the inhibition of the

1140 M. Vanthuyne et al. / Minerals Engineering 16 (2003) 1131–1141

depressive effect of an excess of hydrosulphide ions

(Bastin and Frenay, 2003).

5. Conclusions

The potential use of flotation as a remediation tech-

nique for heavy metal contaminated sediments and soils

has been discussed. In general, the flotation process is

much less selective for heavy metals in sediments and

soils compared to the results obtained in common

mineral ore flotation practice. The following factorscould be isolated as responsible for this different be-

haviour:

(a) the metal partitioning over the different geochemical

phases (metal partitioning),

(b) the metal distribution over the different size frac-

tions (metal fractionation).

It is emphasised in this overview paper that sequen-

tial extraction techniques can be used as a tool for de-

termining the original metal partitioning in sediments

and soils and as a result helps to select the most ap-

propriate procedure of flotation. For example, for cases

where the metals are mainly present as metal sulphides

(e.g. sulphidic sediments) or associated with the oxidic

and/or organic fraction (soils), a choice of a phase-specific collecting agent is necessary. A chemical pre-

treatment step prior to flotation of sediments and soils

can be beneficial depending on the in-situ metal parti-

tioning and serves to transfer heavy metals, which

are associated with different speciation sources (oxides,

clay minerals, organic matter, etc.), into one unique-

good floatable-chemical speciation form (e.g. oxides,

sulphides).The selective flotation of metal-bearing particles from

the particle size fraction <10 lm causes problem in the

most frequently used mechanical flotation cells due to

phenomena like entrainment and entrapment but also

due to the large surface areas of the fine hydrophilic

gangue particles, which lead to excessive reagent ad-

sorption (Kirjavainen, 1996). Entrapment problems

occur when fine hydrophilic gangue particles becomephysically trapped by hydrophobic particles or by bub-

ble-particles aggregates. Hydraulic entrainment occurs

when hydrophilic gangue particles are transported me-

chanically in the liquid film between the air bubbles in

the froth and as a result increases with water recovery,

fine particle size and slurry density. So, a major chal-

lenge for future research is to tackle the problem of the

selective flotation of metal-bearing particles from thehydrophilic gangue particles in these fines. Inspiration

can be found in the work of Mulleneers et al. (2002a,b).

In studying the flotation of PAH from fine dredged

sediments, they attempted by using Dissolved Air Flo-

tation, which introduces smaller air bubbles (10–120

lm) compared to rather turbulent mechanical flotation

cells (approximately 1 mm) and Jameson flotation cells

(+500 lm), to overcome the problems related with the

flotation of fines. However, for the fraction smaller than

32 lm of different dredged sediments, variable flotation

success for PAH was obtained (Mulleneers et al.,2002b).

When flotation is the only remediation technique, a

major factor influencing the floatability of metal sul-

phides in sulphidic sediments is the presence of large

amounts of organic matter (cf. column flotation tests on

synthetic sediments). In the case of non-sulphidic sedi-

ments and soils, it is also expected that the presence of

organic matter (even in small amounts in soils) will havean adverse effect due to its adsorption on oxides.

Therefore, in the future, it would be interesting to in-

vestigate in detail the influence of a preliminary elimi-

nation of the organic matter fraction (e.g. by scrubbing

and gravity concentration, by pre-flotation) on the flo-

tation selectivity of metal-bearing particles like metal

sulphides and metal oxides.

In full-scale applications, flotation is mostly com-bined with other physical separation techniques like

hydrocyclones, screens and gravity concentrators in soil

washing plants. Here, flotation is used as a refinement

step for the smaller sand fraction. Mosmans and van

Mill (1999), for example, successfully integrated a flo-

tation step in a cleaning process for dredged sediments.

In this process, a hydrocyclone with a low cut-off point

(16 lm) is used in order to obtain a larger applicablesand fraction and less costs for dumping. The underflow

of the hydrocyclone separation step (>16 lm) is trans-

ported to an attrition mill in order to liberate pollu-

tants, which are attached to sand particles or included

in small aggregates. After this scrubbing step, the slurry

is transported to a jig (with a cut-off point of 125 lm)

where the organic components are concentrated by

gravity. Finally, the relatively coarse fraction left be-hind (16–125 lm fraction) is flotated. By the overall

process, a good metal (80–90%) and organic pollutant

(85–95% PAH, mineral oils, etc.) decontamination effi-

ciency was obtained on a pilot-scale for a silty sediment

from Willemsdorp. However, the fraction <16 lm,

which contained the highest concentration of metals in

28% of the soil mass, still has to be dumped. In addi-

tion, Mosmans and van Mill (1999) mentioned that thefraction <16 lm could not be cleaned by their flotation

cells. So, a major problem of certain full-scale opera-

tions is the large volume of fines which can remain

when treating contaminated soils and sediments with

high amounts of fine silt and clay. Therefore, finding a

�flotation’ solution for the separation of metal-bearing

particles from the hydrophilic gangue particles in fine-

grained sediments and soils, may also increase theperformance of industrial soil washing plants.

M. Vanthuyne et al. / Minerals Engineering 16 (2003) 1131–1141 1141

Acknowledgements

M. Vanthuyne thanks the IWT-Vlaanderen for

awarding a research grant.

References

Abd El-Rahman, M.K., Maes, A., Cauwenberg, P., 1999. Removal of

heavy metal impurities from dredged river sediment. Chemical

Engineering & Technology 22 (8), 707–712.

Bastin, D., Frenay, J., 2003. Ammonium sulphate as promoting agent

of the sulphidization process of Cu–Co oxides ores from the

Luiswishi Deposit (DRC). In: Extended Abstracts of Flotation

2003, Helsinki, March 19–21.

Cauwenberg, P., Maes, A., 1997. Influence of oxidation on sequential

chemical extraction of dredged river sludge. International Journal

of Environmental Analytical Chemistry 68 (1), 47–57.

Cauwenberg, P., Verdonckt, F., Maes, A., 1998a. Flotation as a

remediation technique for heavily polluted dredged material. 1. A

feasibility study. The Science of the Total Environment 209 (3),

113–119.

Cauwenberg, P., Verdonckt, F., Maes, A., 1998b. Flotation as a

remediation technique for heavily polluted dredged material. 2.

Characterisation of flotated fractions. The Science of the Total

Environment 209 (3), 121–131.

Clark, D.W., Newell, A.J.H., Chilman, G.F., Capps, P.G., 2000.

Improving flotation recovery of copper sulphides by nitrogen gas

and sulphidisation conditioning. Minerals Engineering 13 (12),

1197–1206.

Cottenie, A., Camerlynck, R., Verloo, M., Dhaese, A., 1980.

Fractionation and determination of trace elements in plants, soils

and sediments. Pure and Applied Chemistry 52 (1), 45–53.

Eberius, E., Ekke, P., 1989. Verfahren zur dekontaminierung schlam-

martiger sedimente. European Patent 0332958.

F€oorstner, U., Calmano, W., 1982. Bindungsformen von schwermetal-

len in baggerschlammen. Vom Wasser 59, 83–92.

Kirjavainen, V.M., 1996. Review and analysis of factors controlling

the mechanical flotation of gangue minerals. International Journal

of Mineral Processing 46 (1–2), 21–34.

Langen, M., Hoberg, H., Hamacher, B., 1994. Prospects for separating

heavy metals from contaminated soils. Aufbereitungs-Technic 35

(1), 1–12.

Matis, K.A., 1995. Flotation Science and Engineering. Marcel Dekker

Inc, New York. pp. 558.

Mosmans, S., van Mill, G., 1999. Optimising and modelling of

flotation techniques for remediation of contaminated sediments. In:

Proceedings of Characterisation and Treatment of Sediments

(CATS 4), Antwerpen, September 15–17.

Mulleneers, H.A.E., Koopal, L.K., Bruning, H., Rulkens, W.H.,

2002a. Selective separation of fine particles by a new flotation

approach. Separation Science and Technology 37 (9), 2097–

2112.

Mulleneers, H., van der Mark, B., Geeraets, J., van Gelder, B.,

Bruning, H., Rulkens, W., Koopal, L., 2002b. Remediation of fine

fractions of dredged sediments by flotation. Environmental Tech-

nology 23 (8), 877–887.

Mulligan, C.N., Yong, R.N., Gibbs, B.F., 2001a. Remediation

technologies for metal contaminated soils and groundwater: an

evaluation. Engineering Geology 60 (1–4), 193–207.

Mulligan, C.N., Yong, R.N., Gibbs, B.F., 2001b. An evaluation of

technologies for the heavy metal remediation of dredged sediments.

Journal of Hazardous Materials 85 (1–2), 145–163.

Nelson, D.W., Sommers, L.W., 1982. Total carbon, organic carbon

and organic matter. In: Page, A.L. (Ed.), Methods of Soil Analysis,

Part 2, Chemical and Microbiological Properties, second ed.

American Society of Agronomy, Wisconsin, pp. 539–580.

Oakley, S.M., Delphey, C.E., Williamson, K.J., Nelson, P.O., 1980.

Kinetics of trace metal partitioning in model anoxic marine

sediments. Water Research 14 (8), 1067–1072.

Orwe, D., Grano, S.R., Lauder, D.W., 1998. Increasing fine copper

recovery at the Ok Tedi concentrator, Papua New Guinea.

Minerals Engineering 11 (2), 171–187.

POSW, 1997. Development programme for treatment processes for

contaminated sediments (POSW) Stage 2 (1992–1996). Final

Report, Riza report 97051, Lelystad, July 1997.

Rulyov, N.N., 1999. Application of ultra-flocculation and turbulent

micro-flotation to the removal of fine contaminants from water.

Colloids and Surfaces A: Physicochemical and Engineering Aspects

151, 283–291.

Seselj, A., Strazisar, J., Salopek, B., 1997. Use of column flotation in

the soil remediation process. In: Proceedings of the International

Mineral Processing Congress, Aachen, pp. 709–718.

Swift, R.S., Hayes, M.M.B., 1978. The chemistry of soil organic

colloids. In: Greenland, D.J., Hayes, M.H.B. (Eds.), Chemistry of

Soil Constituents. Wiley-Interscience, New York, pp. 179–230.

Tessier, A., Campbell, P.G.C., Bisson, M., 1979. Sequential extraction

procedure for the speciation of particulate trace metals. Analytical

Chemistry 51 (7), 844–851.

Ure, A.M., Quevauviller, P.H., Muntau, H., Griepink, B., 1993.

Speciation of heavy metals in soils and sediments: an account of the

improvement and harmonization of extraction techniques under-

taken under the auspices of the BCR of the commission of the

European communities. International Journal of Environmental

Analytical Chemistry 51 (1–4), 135–151.

Van der Lee, J., 2000. CHESS, updated for version 2.5, Technical

report, LHM/RD/00/13, �eecole des Mines de Paris, Fontainebleau,

France.

Vanthuyne, M., Maes, A., 2002. The removal of heavy metals from a

contaminated soil by a combination of sulfidisation and flotation.

The Science of the Total Environment 290 (1–3), 69–80.

Venghaus, T., Werther, J., 1998. Flotation of a zinc-contaminated soil.

Advances in Environmental Research 2 (1), 77–91.