Accepted Manuscript

Use and comparison of the non-ionic surfactants Poloxamer 407 and Nonidet

P40 with HP-β-CD cyclodextrin, for the enhanced electroremediation of real

contaminated sediments from PAHs

John N. Hahladakis, Wolfgang Calmano, Evangelos Gidarakos

PII: S1383-5866(13)00224-4

DOI: http://dx.doi.org/10.1016/j.seppur.2013.04.018

Reference: SEPPUR 11154

To appear in: Separation and Purification Technology

Received Date: 21 December 2012

Revised Date: 10 April 2013

Accepted Date: 12 April 2013

Please cite this article as: J.N. Hahladakis, W. Calmano, E. Gidarakos, Use and comparison of the non-ionic

surfactants Poloxamer 407 and Nonidet P40 with HP-β-CD cyclodextrin, for the enhanced electroremediation of

real contaminated sediments from PAHs, Separation and Purification Technology (2013), doi: http://dx.doi.org/

10.1016/j.seppur.2013.04.018

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1

Use and comparison of the non-ionic surfactants Poloxamer 407 and Nonidet P40 1

with HP-β-CD cyclodextrin, for the enhanced electroremediation of real 2

contaminated sediments from PAHs. 3

4

5

John N. Hahladakis a*, Wolfgang Calmano b*, Evangelos Gidarakos a* 6

7

8

9

a Department of Environmental Engineering, Technical University of Crete, 10

Politechnioupolis, Chania 73100, Greece 11

b Department of Hamburg University of Technology, Institute of Environmental 12

Technology and Energy Economics, Eissendorfer Str. 40, D-21073 Hamburg, 13

Germany 14

15

16

17

18

19

*Corresponding author. Department of Environmental Engineering, Technical 20

University of Crete, Politechnioupolis, Chania 73100, Greece. Tel.: +302821037789; 21

Tel.: +302821037812; fax: +302821037850. 22

E-mail address: john_chach@yahoo.gr (John Hahladakis); gidarako@mred.tuc.gr 23

(Evangelos Gidarakos) 24

25

2

Abstract 1

Real contaminated sediments often come with the simultaneous existence of heavy 2

metals and PAHs requiring, thereby, appropriate choice of flushing agents when 3

subjected to electroremediation. The application and efficiency evaluation of the 4

innovative non-ionic surfactants octylphenoxypolyethoxyethanol (Nonidet P40) and 5

2-methyloxirane (Poloxamer 407), in comparison with the already known Tween 80 6

and HPCD cyclodextrin, were examined during enhanced electrokinetic remediation 7

of surficial sediments, mainly from PAHs. Heavy metal removal was also assessed. 8

The results indicated a removal efficacy for SUM PAHs of approximately 48% and 9

43% with the use of Nonidet P40 and Poloxamer 407 respectively, which was far 10

better than the ones taken from both the cyclodextrin and Tween 80. Furthermore, 11

removal percentages for individual PAHs, e.g. fluorene and chrysene, reached almost 12

83% or 92%, respectively. As far as heavy metals are concerned, unenhanced 13

treatment was the only one demonstrating a removal efficacy in all metals examined, 14

however, giving “poor to medium” percentages (5% for Zn to 43% for Cr). On the 15

other hand, the enhanced runs exhibited sufficient removal only in some of the metals 16

examined, e.g. Zn and As, indicating that although surfactants favour the removal of 17

PAHs are not suitable for all heavy metals. 18

19

20

21

Keywords: electroremediation; PAHs; surfactants; sediments; heavy metals 22

23

24

25

3

1. Introduction 1

2

The anthropogenic derived industrial activities that have been constantly 3

increasing over the last decades have created an alarming situation in the aquatic 4

ecosystem that can no longer be ignored. Heavy metals as well as volatile and semi-5

volatile organic substances such as benzene, toluene and Polycyclic Aromatic 6

Hydrocarbons (PAHs) are common contaminants found in both the water column and 7

sediments, as a result of careless disposal or accidental release of toxic and hazardous 8

chemicals [1-4]. 9

On the one hand, heavy metals, being persistent, bioaccumulative, non-10

degradable and toxic can become a constant source of pollution to both the 11

environment and human health. More specifically, heavy metals in aquatic 12

environments can interact with organic or particulate matter resulting in deposition 13

upon sediments which, in their turn, release them again in the water column through 14

various processes and finally reach humans through food chain [5-10]. 15

On the other hand, PAHs can enter marine environment through various ways 16

such as industrial discharge, oil spills or urban runoffs. Being hydrophobic, 17

carcinogenic and of low solubility, they can be associated with both organic and 18

inorganic particles, accumulated, consequently, to high concentrations in sediments, 19

arising, thereby, an equal or even greater concern for benthic organisms and sediment 20

dwelling fauna [11-15]. Taking into consideration all the aforementioned facts, it is 21

quite undisputable that sediments, dredged or surficial, represent the most important 22

recipient of both heavy metals and PAHs [16]. The remediation of sediment 23

contaminated sites, therefore, is of great significance and over the years many studies 24

have reported multiple ways of achieving it such as solidification, stabilization and 25

4

washing. However, these techniques have not been proved suitable, especially for fine 1

grained and low permeable soils or sediments [17, 18]. 2

Electrokinetic remediation (EK) involves the application of low direct current 3

electrical potential between electrodes, within a confined contaminated area. The 4

movement of the dissolved ionic species, present in the pore fluid, towards opposite 5

electrodes is considered to be a major transport mechanism for ionic metals and 6

micelles. This phenomenon is characterized by the term electromigration. Moreover, 7

the transport of H+ and OH- generated by the electrolysis reactions is also attributed to 8

electromigration. Furthermore, the applied electric potential leads to electroosmosis, 9

which is the flow of ionic liquid relative to a charged surface under the action of the 10

applied electric field [19, 20]. These phenomena, along with electrophoresis and 11

diffusion, are the most abundant concerning the movement and removal of 12

contaminants in sediment, during electrokinetic treatment. Depolarization and 13

precipitation could directly affect electroosmosis and electromigration by reducing the 14

mobility of metal ions [21]. 15

Many researchers have studied electrokinetic removal of metals from 16

soils/sediments performing unenhanced treatment tests and obtaining poor removal 17

percentages of cationic contaminants, mainly due precipitation phenomena [22, 23]. 18

Others implemented enhanced methods, such as the use of organic solutions and 19

chelating agents so as to mobilize even more the metal ions [24, 25], the pre-treatment 20

of soils with organic and inorganic solutions [26], the polarity exchange technique 21

[27], or even the use of ion-exchange membranes for the prevention of hydrogen ions 22

entering the soil compartment [28, 29]. 23

5

Electrokinetic technology has also been applied for removing PAHs from 1

contaminated sediments/soils. Enhancing agents, such as surfactants, cyclodextrins 2

and co-solvents were used in order to achieve satisfactory results [2, 14, 19, 30-32]. 3

The addition of these solubilising agents changes the properties and characteristics of 4

the soil particles as well as of the pore fluid, affecting, thereby, electroosmotic flow 5

and consequently the removal process of contaminants. Their main advantageous 6

characteristics are that surfactants reduce interfacial tension, cyclodextrins are 7

biodegradable and nontoxic and cosolvents increase the solubility of PAHs by 8

decreasing, at the same time, their sorption. 9

Although many studies have investigated the removal of heavy metals and 10

PAHs from sediments, most of them have dealt with each of the aforementioned 11

contaminants seperately and usually in spiked soils/sediments, whereas few have 12

reported the use of real contaminated sediments or the simultaneous removal of both 13

heavy metals and PAHs [32-34]. Real sediments, dredged or surficial, usually come 14

with the simultaneous presence of multiple pollutants that may interact with the 15

constituents of solid matrices. Furthermore, little is known regarding parameters such 16

as pH, electroosmotic flow (EOF), organic content, liquid/solid ratio or new 17

solubilising agents that may affect the overall process when they are implemented 18

during electrokinetic treatment of sediments. This is why further research should be 19

done for understanding the role of all these factors in the electroremediation of 20

sediments when multiple contaminants are present. 21

The main objective of this study is the application and efficiency evaluation of 22

the innovative non-ionic surfactants octylphenoxypolyethoxyethanol, commercially 23

known as Nonidet P40 and 2-methyloxirane (also known as Poloxamer 407) in 24

comparison with the already used Tween 80 (Polyoxyethylene (20) sorbitan 25

6

monooleate) and (Hydroxypropyl-β-cyclodextrin) HP-β-CD cyclodextrin in the 1

enhanced electroremediation of real contaminated surficial sediments from PAHs. 2

The simultaneous removal of heavy metals is also studied. 3

4

2. Materials and methods 5

6

2.1. Analytical methods and physicochemical properties of sediment 7

8

The sediment was collected from the surface of the North Western part of 9

Elefsis Bay, Athens, Greece, on September 2012. A zinc-plated Petersen grab sampler 10

was used for collecting the sample and a fridge for keeping it at approximately -4°C 11

during its transportation to the laboratory. The sediment was then slowly air-dried at 12

105°C, gently homogenized and dry sieved through stainless steel sieves of 2mm. The 13

physical properties of the sediment as well as its total content in the six selected heavy 14

metals and sixteen priority PAHs are shown in the unitary Table 1. 15

Moisture, organic matter and electrical conductivity were calculated according 16

to method ASTM D2974, specific gravity using method ASTM D854-92, cation 17

exchange capacity (CEC) using EPA 9081, whereas pH and redox potential according 18

to ASTM D4972, with the use of a Crison pH-meter. Moreover, an X-ray diffraction 19

technique was applied, using a Siemens 5000 refractometer, in order to determine the 20

structure of the sediment which was then classified according to the Unified Soil 21

Classification System (USCS) and ASTM D2488 as sand (S). 22

The determination of the total concentrations of the selected six heavy metals 23

in the sediment sample was done according to EPA method 3051Α for total digestion 24

7

and the calculation of heavy metal concentrations was made with the use of an 1

inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500-CX). 2

The PAH content of the sample was determined according to DIN ISO 11465 3

for dry matter and 18287 for SUM PAHs (EPA 8072) by the use of a Gas 4

Chromatography Mass Spectrometry (GC-MS). 5

6

2.2. Electrokinetic cell and setup 7

8

The diagram of the electrokinetic setup that was used for the experiments is 9

shown in Fig.1. The cell was cylindrical, entirely made of Plexiglas and the 10

dimensions of the sediment compartment were Φ5cm x L10cm and of each electrolyte 11

compartment Φ5cm x L5cm. All chambers were provided with thread at their ends, 12

for easily unscrewing them and renewing the electrolyte solutions every day as well 13

as replacing the copper wires of the anode, due to corrosion phenomena. Moreover, 14

gas vents were placed at the top of each electrolyte chamber for evacuation of the 15

electrolysis gases. High density circular perforated graphite electrodes (Φ5cm and 16

0,5cm thickness) as well as Whatman filter papers were placed at either side of the 17

sediment compartment and between electrolyte chambers in order to prevent, as 18

possible, the entry of fine sediment particles in the electrolyte compartments. A DC 19

power supply (Statron, 0-300V, 0-1,2A) and a multimeter were used for providing 20

constant voltage to the sediment compartment and monitoring the current changes 21

throughout the experiment. Electrolyte solutions were circulated into their reservoirs 22

by a multichannel peristaltic pump (Watson-Marlow, 205s) at a flow rate of 2.5 23

ml/min and EOF was measured through a calibrated bottle, connected to the cathode 24

chamber, at the end of each day. 25

8

1

2.3. Experimental procedure of the EK experiments 2

3

Approximately 250 g of the sediment (dry matter) were mixed with 80 mL of 4

deionized water and the mixture was gradually compacted into the cell by the use of a 5

steel rod. Electrodes were assembled and electrolyte compartments were screwed to 6

the cell in turn, filling the latter ones with the anodic and cathodic solutions. Finally, 7

wires and tubes were connected appropriately. 8

All experiments were performed in room temperature and without controlling 9

the pH of the electrolyte solutions during the EK process. Furthermore, EK 10

experiments were performed under an initial constant voltage gradient of 2V/cm (20 11

Volts), which was kept till electric current would peak (after a few hours) and then 12

manually reduced to 1V/cm voltage gradient (10 Volts), mainly due to foaming 13

phenomena, and no further changes were made. The duration of the experiments 14

varied between 6 - 9 days. Each experiment was stopped when no further changes of 15

EOF or electric current were observed. Detailed parameters associated with each 16

experiment are presented in Table 2. 17

After the completion of each EK experiment, the sediment was carefully 18

extruded from the cell and divided equally into five slices (from anode to cathode). 19

Each slice was then analyzed in its heavy metal and PAH content in order to 20

determine the distribution of contaminants (accumulation or removal). Measurements 21

below quantification limit were taken as half the limit. The removal efficiency of 22

heavy metals and PAHs was calculated according to the following equation: 23

Removal efficiency = %100*ο

ο

CCC i− (1) 24

where Ci is the concentration of the examined heavy metal or PAH (in mg/kg and 25

9

ng/g respectively) in each of the five sections that the sediment is divided after 1

electrokinetic treatment and Co is the initial concentration of each individual heavy 2

metal or PAH examined in the sediment. Sediment’s redox potential, pH and 3

electrical conductivity (EC) were also measured in each slice. Energy consumption 4

(Eu(t)) per unit volume (us) of the sediment was calculated by measuring the electric 5

current as a function of time (I(t)) and the applied voltage (V) according to the 6

following equation: 7

∫=Εt

su dttVI

ut

0)(1)( (2) 8

The unenhanced experiment (1EK, deionized water used in both anodic and 9

cathodic chambers) was conducted in order to serve as a control test and assess only 10

heavy metal removal. 2EK and 3EK runs were conducted to examine the efficiency of 11

HPCD and Tween 80 respectively, used as anolytes, in removing PAHs. Finally, 4EK 12

and 5EK experiments were performed to evaluate the efficiency of the innovative 13

non-ionic surfactants Nonidet P40 and Poloxamer 407, respectively, in the enhanced 14

electroremediation of the contaminated sediment described in section 2.1, from PAHs. 15

Heavy metal removal efficiency was also evaluated in all EK experiments. 16

17

3. Results and discussion 18

19

3.1 Variation of electric current, electroosmotic flow and energy consumption in 20

all EK experiments 21

22

Electric current, as shown in Fig. 2a, began at initial values ranging from 340 - 23

460 mA and increased quickly within few hours attaining a peak value that varied 24

10

from 390 mA (in 3EK) to 560 mA (in 4EK and 5EK). Foaming phenomena in all 1

experiments that can possibly be attributed to mobilization of organic matter and the 2

use of surfactants, made the lowering in the initial voltage (20 V) necessary. After 3

retaining the voltage in 10 V, electric current was gradually reduced till a final value 4

of 10 mA in all experiments. The unenhanced run (1EK) gave the lowest initial 5

current value whereas Nonidet P40 and Poloxamer 407 runs gave the highest. The 6

decrease in current has been caused mainly due to depletion of mobile ions, generated 7

and originally present in excess at the anode, which are then moved through 8

electromigration and electroosmosis mechanisms. Another reason for this trend could 9

be the precipitation of non-conductive sediment particles [35, 36]. In 2EK run (Fig. 10

2a), electric current values where sustained longer in high levels, before they begin to 11

drop down, mainly because of the use of NaOH 1M as a solvent for HPCD and the 12

low concentration of the cyclodextrin used (1 mM). The generation of OH- from 13

NaOH dissociation neutralized H+ and Na+ electromigrated towards cathode, thereby 14

sustaining the current. Cyclodextrins are characterized by low dielectric constant 15

which decreases with increasing concentration [37]. The fact, therefore, that HPCD 16

was used in low concentration helped in obtaining sufficient amount of EOF [33]. A 17

similar trend in current but with minor duration was observed when Nonidet P40 was 18

used as anolyte (4EK). 19

EOF was generated at relatively poor levels and this can be mainly attributed 20

to the sediment’s particle distribution (80% of sand, 0% clay) and the grain size used 21

(just <2mm), as it is shown in Table 2. These parameters were inhibitory for 22

electroosmotic flow. The highest amount of EOF was observed during 2EK run 23

mainly due to the presence of NaOH which kept pH at high levels, thereby leading at 24

even more negative values of ζ potential which led to relatively high EOF rates, in 25

11

comparison with the other experiments (Fig. 2b) [35]. Unenhanced 1EK run gave the 1

second highest amount of EOF. This can be attributed to the high dielectric constant 2

of deionized water which according to Helmholz–Smoluchowski theory is a helping 3

factor in EOF [38]. Finally, 4EK and 5EK runs gave the poorest EOF, a result mainly 4

caused by the high viscosity of both Nonidet P40 and Poloxamer 407. Comparing Fig. 5

2a with Fig. 2b, it is obvious that as long as electric current is retained at relatively 6

high values, greater EOF rates are produced. With the exception of 2EK test, in all 7

experiments electroosmotic flow was practically stopped after the 4th day of sediment 8

treatment, with electric current values having dropped below 50 mA. In 2EK run, 9

EOF rates continued to increase till the 7th day of the experiment with electric current 10

values sustained above 70 mA even until the end of the 6th day, thus corroborating the 11

aforementioned relation between electric current and EOF, which has been also 12

observed by other researchers [39]. 13

Regarding energy expenditure, 2EK was by far the most expensive, giving a 14

total energy consumption of approximately 1080 khW/m3 after 9 days of sediment 15

electrokinetic treatment. Taking into consideration the high purchase cost of 16

cyclodextrins, when compared with other surfactants, coupled with the longest 17

duration needed between all the experiments performed, it is undoubtedly the most 18

expensive option. 3EK run (Tween 80 used as anolyte) determined the lowest energy 19

consumption (536 khW/m3) among surfactant-enhanced electrokinetic tests, with 4EK 20

and 5EK displaying similar, but greater energy expenditure than 3EK run, for the 21

same duration of treatment (6 days and values of 630 khW/m3 and 601 khW/m3, 22

respectively). Finally, the unenhanced (1EK) test spent the least energy (473 khW/m3) 23

between all five EK experiments. 24

25

12

3.2 Variation of pH, redox potential (ORP) and electrical conductivity (EC) in the 1

sediment after all EK experiments 2

3

3.2.1 Sediment pH 4

All experiments were performed without pH control and solutions in the anode 5

and cathode were replaced daily in order to maintain sufficient supply of ions to 6

sustain electromigration and avoid, as possible, high oxidizing conditions which 7

hinder EOF rates. As it is shown in Fig. 3a, in the unenhanced run (1EK), sediment 8

pH dropped below 5 in the section closer to the anode and reached nearly 11 in the 9

slice near cathode. This behavior is typical for EK runs with deionized water used 10

both as anolyte and catholyte and is a result of high and low proton activity in the 11

respective chambers. Moreover, the generation of H+ at the anode and OH- at the 12

cathode from electrolysis reactions, under the application of electric field, and their 13

oriented movement towards cathode and anode respectively, contributed even more to 14

this trend. The sediment pH range could be even wider (lower pH at the slices near 15

the anode and higher at those near the cathode) but it was possibly restricted due to 16

the daily replenishing of solutions. 17

In 2EK run the presence of OH-, from NaOH used as a solvent for HPCD, kept 18

pH at high values in all sections of the sediment (above 11, see Fig. 3a) and at the 19

same time maintained EOF rates at sufficient levels almost throughout the whole 20

duration of the experiment, giving the highest total amount of all the experiments (55 21

mL, see Fig. 2b). 22

All surfactants (3% Tween 80, 5% Nonidet P40 and 3% Poloxamer 407) 23

displayed similar behavior with each other and with the cyclodextrin, maintaining 24

high pH values in all sediment slices. Their main difference with HPCD was that of 25

13

exhibiting slightly lower pH values in the sediment sections near the anode, compared 1

with middle sections and those near cathode (Fig. 3a). 2

3

3.2.2 Sediment redox potential (ORP) 4

The redox potential distribution in the sediment after all EK experiments is 5

shown in Fig. 3b. In 1EK run, ORP reduced in sections from anode to cathode with 6

values of +101 mV and -11 mV respectively. This indicates the presence of oxidizing 7

conditions in anode and reducing conditions in cathode and can be associated with the 8

evolution of oxygen and hydrogen at the respective chambers during EK treatment. 9

On the other hand, when enhanced electrokinetic treatment was implemented, 10

redox potential exhibited a completely different behavior. Negative ORP values, 11

ranging from -16 to -134 mV, were measured in all sections, even in those near the 12

anode (2EK, 4EK and 5EK), indicating the existence of reducing conditions. Such 13

conditions, which favor EOF and therefore PAHs removal (see section 3.3.1), may 14

inhibit heavy metal transport (see section 3.3.2). 3EK run was the exception among 15

surfactants, maintaining the same ORP trend as in 1EK with values ranging from +34 16

in the section near the anode to -36 mV in the section near the cathode. The oxidizing 17

environment that was present in 1EK, on the other hand, helped in heavy metal 18

removal (see section 3.3.2), a result also reported in other studies [33]. 19

20

3.2.3 Sediment electrical conductivity (EC) 21

The profile of electrical conductivity after electrokinetic treatment is shown in 22

Fig. 3c. EC is proportional to the total amount of dissolved ionic species. It is obvious 23

that 1EK, 2EK and 3EK followed the same trend, exhibiting relatively high values of 24

EC near the anode sections which gradually declined in sections close to cathode. As 25

14

ions are removed, due to a compilation of phenomena (adsorption, precipitation, 1

e.t.c.), EC tends to fall especially in the region where pH values change, hence the EC 2

decline after the third slice in all the aforementioned EK experiments. However, in 3

4EK and 5EK runs, EC increased from anode to cathode, till the fourth slice (from 4

3.11 and 2.88 mS/cm to 8.54 and 8.12 mS/cm, respectively) and then dropped near 5

cathode to its, almost, initial value. This trend may be associated with the dissolution 6

of ionic species due to thermal effects of Joule heating generated by the rising of the 7

temperature in the sediment inside the electrokinetic cell, during the first hour and 8

under a voltage gradient of 2V/cm (temperature reached almost 40°C in the first hour). 9

These thermal effects possibly prevailed over pH changes, which were relatively 10

small in the last two runs, and coupled with the small length of the cell created this 11

EC profile in 4EK and 5EK. This trend has been also observed in other studies that 12

dealt with industrially contaminated soils [40]. 13

14

3.3 Evaluation of contaminant removal in all EK experiments 15

16

3.3.1 Distribution and removal of PAHs in the enhanced EK experiments 17

When 1mM HPCD was used as anolyte in 2EK enhanced electrokinetic 18

treatment of sediment, a fairly satisfying removal of both light (2 or 3 aromatic ring 19

species) and heavy (4- 6 aromatic ring species) PAHs was obtained. With the 20

exception of acenaphthene and acenaphthylene, all the other light PAHs showed a 21

tendency for transporting from both ends of the electrokinetic cell (either near anode 22

or cathode) (see Fig. 4a), exhibiting removal percentages that ranged from 11% for 23

fluoranthene to 58% for fluorene (Fig. 5). Naphthalene, phenanthrene and anthracene 24

were removed by 29%, 38% and 30% respectively. As far as heavy PAHs are 25

15

concerned, removal percentages ranged from 9% for benzo(b)fluoranthene to 63% for 1

chrysene. The majority of PAHs (especially at normalized distances of 0.1 or 0.7 from 2

anode) showed values of C/C0 below 1, indicating that they were partly removed from 3

these sections (Fig. 4a). A total removal of 26% was obtained for SUM PAHs in this 4

EK run, as it is shown in Fig. 5. These results can be attributed to the fact that 2EK 5

experiment gave the highest amount of EOF which was, apparently, the predominant 6

mechanism for removing PAHs. The low concentration of HPCD used coupled with 7

the existence of NaOH as its solvent helped in maintaining sufficient EOF rates 8

throughout this test. A detailed explanation for this was reported in section 3.1. 9

Moreover, the fact that HPCD has a relatively low viscosity (compared with that of 10

Nonidet and Poloxamer) and it does not react with soil may also have played an 11

important role to the results of this experiment. 12

3EK run gave the poorest removal efficiency (an overall SUM PAHs removal 13

of 16%, see Fig. 5) from all enhanced experiments, an observation also reported in 14

other studies [14, 31, 32, 41]. Tween 80 exhibited a limited capability in removing 15

PAHs which can be mainly attributed to adsorption onto sediment particles or 16

surfactant deprotonation [14, 32]. A. T. Lima et al. [41] managed to achieve a total 17

SUM PAHs removal percentage of almost 30% in clayey soils, by the use of 1% 18

Tween 80 in 0.01M NaCl, but even in that study poorer results were also observed. 19

Most of the heavy PAHs were removed and concentrated in the electrolyte solutions. 20

In the present study, the efficacy of Tween 80 in removing PAHs was mainly 21

observed in the section closer to the cathode, where a transport of almost all PAHs 22

was observed, but in low percentages ranging from 9% to 33%. Furthermore, an 23

accumulation of the majority of heavy PAHs near the anode was also observed in this 24

EK run (see Fig. 4b). 25

16

The low dielectric constant which is directly and proportionally related to EOF 1

(3EK run generated one of the lowest EOF amounts), as well as the adsorption of 2

surfactant onto sediment particles were, possibly, the main reasons for these poor 3

results. 4

4EK and 5EK runs gave the best results regarding PAHs removal, as it is 5

clearly shown from Fig 5. With the exception of acenaphthylene, all PAHs were 6

removed in percentages beginning from 38% (in 4EK) and 30% (in 5EK) and 7

reaching, selectively, even values of 92% (fluorene in 4EK) or 81% (chrysene in 8

5EK). The highest removal percentages, for the majority of PAHs, were again 9

observed at normalized distances of 0.3 and 0.7 from anode, thus in sections close to 10

either end of the cell (see Figs. 4c and 4d). Despite their low generation of EOF rates, 11

these two innovative non-ionic surfactants managed to give the best PAHs removal 12

results mainly due their ability of forming numerous and stable micelles. Non-ionic 13

surfactants usually possess low critical micelle concentrations (CMCs) and high 14

solubilization capacities. When present in concentrations higher than CMC, they tend 15

to form micelles with hydrophobic and hydrophilic portions. The hydrophobic portion 16

is created in the center of the micelle while the hydrophilic portion in the outer part of 17

the micelle. The exterior part of the micelle becomes, therefore, highly soluble to 18

water whereas the interior acts as a hydrophobic region for PAHs or other organic 19

substances. The results from these EK runs indicated the presence of such micelles 20

which helped solubilizing and mobilizing PAHs in a very sufficient degree. The total 21

removal percentages of SUM PAHs obtained in these runs (48% and 43% for 4EK 22

and 5EK respectively, see Fig. 5) were almost double the ones of HPCD run and triple 23

the ones of Tween 80. 24

17

These differences in results could be attributed to a number of reasons. First of 1

all, the polarity in the cavity of HPCD, caused by the existence of the glycosidic bond 2

in its molecule, could have made it less “appealing” for some PAHs, something that is 3

not happening when a non-ionic surfactant is used. Furthermore, while the low 4

concentration of the cyclodextrin affects positively EOF and therefore the transfer of 5

contaminants, could reduce its solubilization capacity and consequently restrict its 6

removal efficiency. 7

8

3.3.2 Distribution and removal of heavy metals in all EK experiments 9

The distribution as well as maximum removal percentages of the selected 10

metals, examined after electrokinetic treatment in all EK runs, is shown in Fig. 6. 11

Unenhanced test (1EK) was the only one demonstrating removal efficacy for all 12

metals examined. A medium removal yield was achieved, with maximum percentages 13

ranging from approximately 5% for Zn to 43% for Cr (Fig. 6f). Chromium exhibited a 14

removal tendency throughout the whole sediment compartment (see Fig. 6a) and was 15

possibly flushed into the anodic reservoir and chamber. Significant amount of Cr 16

could have been also adsorbed to the anode graphite electrode. Cu, Zn and Pb 17

exhibited an accumulation tendency in sections close to anode (at normalized 18

distances 0.1, 0.1 and 0.3 respectively) and removed from sections close to cathode 19

(at normalized distances 0.7). This trend can be associated with the migration of 20

dissolved organic matter complexes formed in the cathodic region to the anodic 21

region. Moreover, the pseudo-total increase in the content of Cu that appears in all EK 22

experiments in the analyzed slices adjacent to the anode (at normalized distance 0.1) 23

could be explained by the corrosion of copper wires. Moreover, significant amount of 24

Zn accumulated near cathode mostly due to formation of cationic complexes, hence 25

18

the increased values at x/L=0.9. Finally, arsenic was found to be removed from 1

sections close to cathode. 2

2EK and 3EK runs demonstrated similar behaviour in metal removal. 3

Although HPCD helped in solubilizing and mobilizing PAHs (2EK), it did not 4

improve metal removal efficiency. Zinc and lead did not exhibit any removal 5

tendency at all and with the exception of arsenic, all the other metals gave poorer 6

removal percentages than 1EK, ranging from 8% for Ni to 35% for Cr. 7

Concentrations of Zn and Pb were increased throughout the sediment bed in both 8

experiments (Figs. 6b and 6c), indicating an enrichment of these elements after EK 9

treatment. This trend can be mainly attributed to high pH values in all sediment slices 10

(see Fig. 3a) which have caused most cationic metals to precipitate. Another reason 11

for the ineffectiveness in the removal efficiencies of metals in these runs might be the 12

significant amount of organic matter that could have possibly adsorbed metals. 13

Chromium was again flushed into the anodic chamber, thus achieving the best 14

removal efficiency whereas arsenic, in 3EK, was found to migrate towards the anode 15

(see the peak in x/L=0.3 in Fig. 6c) possibly due to its anionic aqueous complexes. In 16

2EK, arsenic exhibited a minor removal percentage of 22% from cathode and was 17

accumulated again in the section close to anode (in normalized distance 0.3 from 18

anode). 19

In the last two runs (4EK and 5EK) when Nonidet P40 and Poloxamer 407 20

were introduced, a dramatic change in the removal efficiencies of Zn and As was 21

observed. Arsenic exhibited removal percentages ranging from approximately 50% in 22

5EK run to 71% in 4EK run whereas Zn was found to be removed by 71% to 75% in 23

both of the aforementioned runs, respectively (Fig. 6f). This behaviour can be 24

associated with the high pH values and the high dissolution capability these two 25

19

surfactants possess. All the precipitated metals, therefore, were possibly dissolved, 1

transported along the cell with the dissolved organic matter and PAHs and as 2

consequence removed from the sediment bed, hence the uniform removal profile of 3

Zn and As observed in all sediment slices examined (Figs. 6d and 6e). Cr percentages 4

were declined in these experiments while Pb was again not removed at all. 5

6

4. Conclusions 7

The main objective of this study was the application and efficiency evaluation 8

of innovative surfactants in the electroremediation of real contaminated sediments 9

from PAHs. At the same time, heavy metal removal efficacy was also assessed. 10

Non-ionic surfactants were chosen mainly for being non-toxic, biodegradable, 11

and economic because they exhibit low CMCs and high solubilization abilities. The 12

selected ones were the commercially known Nonidet P40 and Poloxamer 407 and 13

were compared to the already used Tween 80 and cyclodextrin HP-β-CD. The main 14

conclusions that can be drawn are the following: 15

Regarding PAHs, Nonidet P40 exhibited the highest removal percentage in 16

SUM PAHs (48%) as well as individually (92% for fluorene) with Poloxamer being 17

the second best, giving respective percentages of 43% for SUM PAHs and 83% 18

regarding chrysene. HPCD followed, in turn, and although exhibiting the highest EOF 19

rates, the total removal efficiency was limited to almost 26% (for SUM PAHs) 20

demonstrating the best individual value for chrysene (63%). Tween 80 gave the 21

poorest results and was found inappropriate for the electroremediation of the specific 22

sediment. 23

As far as heavy metal removal is concerned, the unenhanced run managed to 24

give “poor to medium” but positive results for all metals examined, demonstrating 25

20

removal percentages that ranged from 5% to 43%. The application of Nonidet and 1

Poloxamer indicated that these two surfactants are efficient in removing Zn and As, in 2

percentages that reached even 75%, mainly due to their high solubilization capability. 3

Nevertheless, despite showing adequate behaviour in the removal of the 4

aforementioned metals, they failed in sufficiently removing the rest of the metals 5

examined (Pb, Ni, Cr, and Cu). Finally HPCD and Tween 80 did not improve metal 6

removal, giving similar or worse results than those obtained from the unenhanced run. 7

Regarding a full-scale implementation, practical implications in terms of both 8

environmental and technical issues should be considered. The use of large quantities 9

of surfactants or the inappropriate removal of other chemical reagents could 10

potentially harm the surrounding area, thus i.e. nature, dosages, mode of application 11

of the agents, pH control, and regulation should be carried out. Furthermore, proper 12

control of side effects such as precipitation, evolution of toxic gases or oxidation of 13

the agents used as anolytes are considered mandatory. At the same time, optimal 14

operating conditions (such as hydraulic gradient application, electrodes geometry, and 15

configuration, current intensity, etc.) need to be investigated as well. 16

Let us not forget that reality is always more complex and less controlled than a 17

laboratory experiment. 18

19

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221-229 12

Highlights

Article title: Use and comparison of the non-ionic surfactants Poloxamer 407 and Nonidet P40

with HP-β-CD cyclodextrin, for the enhanced electroremediation of real contaminated sediments

from PAHs.

Nonidet and Poloxamer were used for enhanced sediment electroremediation.

Their efficiency in removing mainly PAHs and secondarily heavy metals was assessed.

Their removal efficacy was compared with that of HPCD cyclodextrin and Tween 80.

The newly introduced surfactants demonstrated a SUM PAHs removal of 43%-48%.

Heavy metal removal was only partially obtained by the use of these surfactants.

List of Tables

Table 1

Physicochemical properties of the sediment

“<” below detection limit

Table 2

Detailed parameters of the EK experiments

a

Initial voltage: 20 V(2 V/cm), reduced to 10 V(1 V/cm) after a few hours

Table 1

Physical Properties of sediment Value

Moisture (%) 18.9

pH 7.78

Redox (mV) 67

Electrical conductivity (mS/cm) 6.98

Organic matter (%) 5.12

Specific gravity 2.18

Cation exchange capacity (meq/100gr) 1.49

Main minerals Percentages (%)

Quartz 30

Calcite 52

Dolomite 6

Mica 3

Chlorite 2

Feldspars 5

Hematite 1

Kaolinite 1

Particle size distribution Percentages (%)

Sand 82

Silt 8

Clay 0

USCS classification Sand (S)

Initial heavy metal content Concentration (mg/Kg dw, average of three

replicates)

Cr 42.59

Ni 25.53

Cu 47.48

Zn 96.67

As 5.59

Pb 87.15

Initial PAH content Concentration (ng/g dw, average of three

replicates)

Naphthalene 70

Acenaphthylene <50

Acenaphthene <50

Fluorene 60

Phenanthrene 980

Anthracene 100

Fluoranthene 1100

Pyrene 1300

Benzo(a)anthracene 790

Chrysene 1100

Benzo(b)fluoranthene 870

Benzo(k)fluoranthene 270

Benzo(a)pyrene 450

Dibenzo(a,h)anthracene 120

Benzo(ghi)perylene 410

Indeno(1,2,3-cd)pyrene 380

Sum PAHs 8000

Table 2

Test

code name

Anodic

solution

Cathodic

solution

pH

control

Voltage gradient a (V/cm)

Voltage

(V)

Duration

(d)

1EK Deionized water Deionized water No 2 and 1 20-10 6

2EK 1mM HPCD in 1M

NaOH

Deionized water No 2 and 1 20-10 9

3EK 3% wt. Tween 80 Deionized water No 2 and 1 20-10 6

4EK 5% wt. Nonidet P40 Deionized water No 2 and 1 20-10 6

5EK 3% wt. Poloxamer 407 Deionized water No 2 and 1 20-10 6

List of Figures 1

Fig. 1 Schematic of the electrokinetic set-up 2

3

Fig. 2 Variation of (a) electric current and (b) electroosmotic flow (EOF) with elapsed 4

time. 5

6

Fig. 3 Profiles of (a) pH, (b) redox potential and (c) electrical conductivity in 7

sediment after electrokinetic treatment. 8

9

Fig. 4 Distribution of individual PAHs in sediment after enhanced electrokinetic 10

treatment with anodic solutions (a) HPCD (b) Tween 80 (c) Nonidet P40 and (d) 11

Poloxamer 407 12

13

Fig. 5 Effect of different anodic solutions on PAHs removal in all enhanced EK 14

experiments 15

16

Fig. 6 Effect of different anodic solutions on heavy metal removal in all EK 17

experiments 18

19

20

21

22

23

24

25

1

Fig. 1 2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

0

0,1

0,2

0,3

0,4

0,5

0,6

0 50 100 150 200

1EΚ

2ΕΚ

3ΕΚ

4EK

5EK

Time (h)

Ele

ctr

ic c

urren

t (A

)

1

(a) 2

0

10

20

30

40

50

60

0 2 4 6 8 10

1EK

2EK

3EK

4EK

5EK

Time (d)

Cu

mu

lati

ve e

lectr

oo

smo

tic f

low

(m

L)

3

(b) 4

Fig. 2 5

6

7

8

1

Fig. 3 2

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Fig. 4a 16

1

Fig. 4b 2

1

2

Fig. 4c 3

1

Fig. 4d 2

1

Fig. 5 2

3

1

2

3

Fig. 6 4

5

6