Environmentally Benign Sequential Extraction of Heavy Metals from

Article pubs.acs.org/IECR

Environmentally Benign Sequential Extraction of Heavy Metals from Marine Sediments María S. Á lvarez, Esther Gutiérrez, Ana Rodríguez, M. Á ngeles Sanromán, and Francisco J. Deive* Department of Chemical Engineering, University of Vigo, Vigo, Pontevedra 36310, Spain S Supporting Information *

ABSTRACT: An environmentally friendly heavy metals remediation process from polluted marine sediments is proposed. The efficiency of three organic and inorganic salts (ammonium acetate, ammonium nitrate, and sodium potassium tartrate) to salt out these pollutants was ascertained in sediment washing waters containing nonionic surfactants. The immiscibility regions were correlated by means of three known models, and the experimental data were interpreted in the light of thermodynamic parameters such as Gibbs free energy of hydration and molar entropy of hydration. The proposed process was applied to model aqueous solutions containing two representative heavy metals (zinc and copper). The viability of the suggested strategy was checked in real contaminated marine sediments by including a sequential treatment: marine sediment washing−contaminant extraction, which led to total remediation values higher than 80% for copper and 90% for zinc.

1. INTRODUCTION Heavy metals are in the limelight because they have been recognized as carcinogenic, persistent, and bioaccumulative contaminants.1 These pollutants are widely found in nature as a result of anthropogenic activities, including industrial and domestic wastewater. The high solubility in aqueous solutions of these metals makes it possible for them to be found as watersoluble species, suspended forms, colloids, and sedimentary phases. Coastal and marine sediments are considered one of the ecological niches most probably affected by this kind of contamination, since more than 99% of heavy metals entering the aquatic ecosystems can be stored in sediments in various forms.2,3 Nonetheless, the variation of the physicochemical properties of water could revert the fixation of metals to a solubilized form, thus being bioavailable again for living beings. In this sense, dredging activities involve the generation of great amounts of polluted marine sediments that should be treated. The relevance of this statement is patent when analyzing examples such as the number of remediation actions in the USA (71 projects) focused on the treatment of more than 4.5 million m3 of contaminated sediments.4 Bearing this in mind, the search of efficient sediments remediation strategies are a very active field of research. The remediation methods are often classified into ex situ and in situ, depending on the place where the treatment is carried out. Thus, amendment, sand cap, and phytoremediation have already been recommended as in situ alternatives, due to their low cost and more benignity to natural hydrological conditions. On the other hand, washing, electrokinetic remediation, immobilization, flotation, and ultrasonic-assisted extraction have been proposed as viable ex situ remediation processes.5,6 Among the above-mentioned methods, in this work, we have focused on sediment washing, since it is a commonly used technique due to its inherent operational simplicity. This strategy consists of transferring metal ions from dredged samples to aqueous solutions. The efficiency of this process can be improved by the addition of specific compounds such as © 2014 American Chemical Society

acids, chelating agents, and surfactants, which have been proved to further contaminant solubilization, dispersion, and desorption. Therefore, the use of nonionic surfactants (Triton X-100 and Tween 20) and KSCN as complexation agent in acid media was considered in the present investigation. This family of surfactants has been commonly used in bioremediation processes applied to the removal of contaminants as widely disparate as polycyclic aromatic hydrocarbons and dyes.7−9 This fact, together with their biodegradability, has led us to bet in them for the present work. The adsorption/ release ratio between sediment and water can be strongly altered by introducing a chelating agent such as KSCN. This salt assists in the formation of copper and zinc complexes, which are spontaneously released from the sediment, thus helping to diminish the levels of pollutant charge. Once the pollutant was dissolved in the aqueous solution, a second step to concentrate the contaminant charge is desirable. The presence of surfactants from washing and salts from a marine environment in the obtained aqueous effluent made us hypothesize that the addition of salting out agents to the polluted aqueous solution could assist us in this purpose. Aqueous biphasic system (ABS) is a competitive separation technique based on the induction of phase segregation by the addition of inorganic or organic salts to an aqueous solution of a hydrophilic organic compound. The appeal of this separation method lies in well-documented merits such as process economy, short operation time, low energy demand, and easy scale-up.7,10 Among the existing types of ABS, nonionic surfactant-based ones are a promising alternative that remains almost unexplored. In this line, surfactant-based ABS entail several benefits in relation to other commonly used alternatives, such as a lower Received: Revised: Accepted: Published: 8615

March 4, 2014 April 28, 2014 May 1, 2014 May 1, 2014 dx.doi.org/10.1021/ie500927q | Ind. Eng. Chem. Res. 2014, 53, 8615−8620

Industrial & Engineering Chemistry Research

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Figure 1. Experimental solubility curves and correlation data for (a) Triton X-100 and (b) Tween 20. (○) NH4CH3COO; (□) NH4NO3; (Δ) NaKC4H4O6.

deionized water was added until a clear solution was attained. These operations were repeated until completion of the solubility curve. This methodology was carried out in a jacketed glass vessel with magnetic stirring and connected to a circulating bath, and the temperature was controlled by a F200 ASK digital thermometer (±0.01 K). 2.3. Experimental Determination of Tie-Lines and Metal Partition. Analogously to the procedure described for the solubility curves, the tie-line data (TL) were obtained by adding a known amount of a given salt to the aqueous solutions containing Triton X-100 or Tween 20, until the detection of turbidity. The temperature was controlled at 298.15 K and vigorously stirred prior to settling for 24 h in order to ensure chemical and thermodynamic equilibria. Experimental TL data were calculated by using the level arm rule. For studying heavy metal partition, aqueous solutions of the metal ions were included in the initial nonionic surfactant aqueous solutions prior to the addition of the selected salting out agent. 2.4. Extraction of Metals from Dredged Marine Sediments. The marine sediment samples were collected in the Galician coast (NW Spain). The classification of the sediments according to the Particle Size Analysis method indicates that the dredged samples are silty clay. A complete characterization of the samples has been recently reported by our research group, and Zn and Cu were the two metal ions clearly trespassing the CEDEX reccomendations for dredged marine sediments for Spanish harbors.6 Metals were extracted from the marine sediments based on the following procedure: 0.25 g of soil was added to Erlenmeyer flasks together with 15 mL of aqueous solutions of the selected nonionic surfactant at 30% concentration. Alternatively, 0.87 g of KSCN was added as complexing agent when stated to the above-mentioned mixture. 1 M HCl was added to adjust the pH since this parameter is decisive to promote metal solubilization. These mixtures were shaken at 200 rpm for 24 h at 298.15 and 343.15 K. Then, the mixture was centrifuged at 5000 rpm for 5 min, and the supernatant was kept for a second centrifugation step at 5000 rpm and 5 min. Metals were determined in this supernatant. This solution was then used for ABS extraction. Milli-Q-Plus water leaching was performed as a control. All experiments were run in triplicate. 2.5. Experimental Determination of Metal Ions. The concentrations of copper and zinc in the top and bottom phases of the ABS were determined by flame atomic absorption spectroscopy (AAS) with an Agilent Technologies 200 series AA. Copper and zinc were determined with an air-acetylene

interface tension, economical reasons (low cost of the reagents and rapid phase segregation), greater immiscibility windows, null flammability, and commercial availability of all components at bulk quantities.11 In our group, we have recently demonstrated the viability of implementing this kind of process for the extraction of contaminants (dyes),7 as well as for the separation of value-added compounds such as antioxidants.12 In this particular case, this extraction process is applied to the separation of metal thiocyanate complexes from acid aqueous solutions in the presence of the selected surfactants Triton X100 and Tween 20. There are no references in the literature to apply this hybrid strategy for remediating heavy metals-polluted sediments. Hence, in this work, ammonium nitrate, ammonium acetate, and sodium potassium tartrate have been selected as salting out agents to generate an immiscibility window in aqueous solutions of the nonionic surfactants Triton X-100 and Tween 20. The solubility data obtained, together with the tielines characterization, will be fitted to different equations in order to suitably describe the phase behavior. This first step constitutes the basis for the implementation of a two-stage remediation process to remove metal ions (Cu2+ and Zn2+) from marine sediments. Thus, a model aqueous solution containing the metal ions will be used prior to carrying out the extraction with real polluted sediments, and the efficiency of the proposed strategy will be evaluated in terms of extraction capacity.

2. EXPERIMENTAL SECTION 2.1. Chemicals. The nonionic surfactants Tween 20 and Triton X-100 (Sigma-Aldrich, St. Louis, MO, US), the inorganic and organic salts, ammonium nitrate (NH4NO3; VWR Chemicals, Radnor, PA, US), ammonium acetate (NH4CH3COO; Sigma-Aldrich, St. Louis, MO, US), sodium potassium tartrate (NaKC4H4O6; Panreac, Barcelona, Spain), the complexing agent potassium thiocyanate (KSCN; Fluka, St. Gallen, Switzerland), and the metal ions of copper (CuSO4· 5H2O; Merck, Darmstadt, Germany) and zinc (ZnSO4·H2O; VWR, Radnor, PA, US) were used as received without further purification. 2.2. Determination of Solubility Curves. The solubility curves determination was carried out by means of the cloud point titration method at 298 K, as previously reported elsewhere.13 In brief, drops of saturated salt solution were added to aqueous solutions of the nonionic surfactants (Triton X-100 or Tween 20) until the detection of turbidity, which indicates that the biphasic region was reached. Afterward, 8616

dx.doi.org/10.1021/ie500927q | Ind. Eng. Chem. Res. 2014, 53, 8615−8620


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the NaKC4H4O6 is by far the salt leading to the greatest immiscibility region, no matter the nonionic surfactant employed. The interpretation of the influence of the amount of salt necessary to trigger phase disengagement has been widely assessed by the Hofmeister series, yet recent works have focused on the molecular mechanisms behind these effects, explaining them in terms of thermodynamic parameters such as the Gibbs free energy of hydration (ΔhydG) and the molar entropy of hydration (ΔhydS).14,15 The values of these thermodynamic parameters for the different ions used in this work are listed in Table 1. It is patently clear that the anion

burner at wavelengths of 324.8 and 213.9 nm and lamp currents of 4 and 5 mA, respectively. The determination of metal concentrations in dredged sediments was performed according to EPA Methods 3010 and 3050. In brief, they were analyzed by inductively coupled plasma optical emission spectrometry (Optima 4300DV PerkinElmer). After setting analytical conditions and making background corrections for wavelength spectra in accordance with the standard solution profile, sample or test solutions were introduced via the Cross-Flow nebulizer (Scott) inside the plasma torch, equipped with an Echelle polychrometer. The operating conditions for auxiliary gas, nebulizer gas, and cool gas (Ar) were 0.2, 1.10, and 15 L/min, respectively. The spectral lines for copper and zinc were 327.393 and 206.200 nm, respectively. Calibration was carried out by using a multielement standard solution VI (Merck) by appropriate dilution in 2% (v/v) HNO3.

Table 1. Molar Gibbs Energies of Hydration (ΔhydG) and Molar Entropy of Hydration (ΔhydS) ions Na K+ NH4+ CH3COO− NO3− C4H4O62−

3. RESULTS AND DISCUSSION 3.1. Selection of Phase Segregation Agent. The solubility curves of the ternary systems containing the inorganic and organic salts NH4NO3, NH4CH3COO, and NaKC4H4O6 and aqueous solutions of Triton X-100 or Tween 20 were first determined at 298.15 K and are graphically plotted in Figure 1. The experimental data are also shown in Tables S1 and S2, Supporting Information. A complete characterization of the experimental data requires one to find a suitable equation to describe the phase segregation behavior. Different empirical equations14 have already been reported to properly fit to ABS data and have thus been selected for the present work: w1 = a ln(w2 + b) + c

(1)

w1 = exp(d + ew2 0.5 + fw2 + gw2 2)

(2)

w1 = hexp(iw2 k − jw2 l)

(3)

a

ΔhydS/J·mol−1K−1

−365 −295a −285a −365a −300a −1102b

−130a −93a −131a −189a −95a n.a.c

Ref 16. bRef 17. cn.a. (not available).

C4H4O62− is the one showing the lowest values of ΔhydG, which correlates with solubility curves closer to the origin. This behavior is coincident with what is expected from the analysis of ions valence, since the tartrate binary valence leads to more ion−water interactions than those provided by the monovalent acetate and nitrate anions. Therefore, tartrate-based salt will be selected for further implementation of the separation of copper and zinc metal ions from dredged marine sediments. A proper description of the separation process requires the calculation of the tie-lines data, as a means to quantify the amount of Triton X-100 or Tween 20 and NaKC4H4O6 in each of the phases Then, the lever arm rule was applied to calculate the tie-line data, together with eq 3, on the basis of the procedure detailed elsewhere.11,18 In a visual inspection of the results shown in Table 5, it seems clear that the addition of more salt to the systems triggers the segregation of more nonionic surfactant to the top phase. An empirical parameter that can be used to demonstrate this fact is the tie-line length (TLL), and these data are shown in Table 2.

where w1 and w2 are the mass compositions of nonionic surfactant and salt, respectively, and a, b, c, d, e, f, g, h, i, j, k, and l are the fitting parameters. The values of these parameters were calculated by applying the SOLVER function in Microsoft EXCEL, on the basis of the minimization of the standard deviation. This was calculated as follows: 2 ⎞1/2 ⎛ ∑nDAT (z − z exp adjust) i ⎟ σ = ⎜⎜ ⎟ nDAT ⎝ ⎠

ΔhydG/kJ·mol−1 a

+

(4)

TLL = [(w1I − w1II)2 + (w2I − w2II)2 ]1/2

where the experimental and adjustable solubility data are represented by zexp and zadjust, respectively, and nDAT is the number of experimental data. The obtained fitting parameters are shown in Tables S3, S4, and S5 in the Supporting Information, together with the values of the standard deviations. Bearing these data in mind, it is possible to conclude that all the equations serve our goal to describe the immiscibility behavior, although eq 3 is the one entailing lower standard deviation values. The analysis of the salting out potential of the selected salts reveals that the phase segregation results from the competition between Triton X-100 or Tween 20 and NH 4 NO 3 , NH4CH3COO, or NaKC4H4O6 for the water molecules. As a consequence of this competition, the segregation of two phases is yielded: a top phase enriched in nonionic surfactant and a salt-rich bottom phase. From the observed trends, it is clear that

(5)

Table 2. Experimental Tie-Lines in Mass Composition for {Nonionic Surfactant + NaKC4H4O6 + H2O} at 298.15 K nonionic surfactant-rich phase 100

wI1

26.47 39.81 48.45 43.88 50.45 56.16 8617

100

wI2

salt-rich phase 100 wII1

100 wII2

Triton X-100 + NaKC4H4O6 + H2O 7.36 0.80 14.28 5.06 0.10 16.41 2.86 0.01 18.47 Tween 20 + NaKC4H4O6 + H2O 5.16 1.51 18.61 3.24 0.49 20.61 1.72 0.19 22.02

TLL 26.59 41.30 50.90 44.44 52.90 59.54



Article

Table 4. Extraction Capacity, E (%), of Metal Ions in the Top Phase in the Absence and Presence of Complex Extractant (KSCN)

In this equation, 1 and 2 refer to mass fraction of the nonionic surfactant and the organic or inorganic salt, respectively, while I and II indicate the top and bottom phases, respectively. The TLL data analysis permits us to confirm that greater concentrations of salt in the bottom phase involve higher values of TLLs, since more salt molecules in the system make it possible to obtain a top phase enriched in Triton X-100 or Tween 20. Additionally, a two-parameter equation19 was employed to correlate the experimental tie-line data and shed more light on the capacity of each surfactant to be salted out. ⎛ wI ⎞ ln⎜ II2 ⎟ ⎝ w2 ⎠

= β + k(w1II − w1I)

system

14.6 ± 0.8

62.8 ± 6.3

18.4 ± 7.9

66.2 ± 7.1

11.3 ± 1.0

80.9 ± 3.3

6.2 ± 0.7

86.1 ± 4.3

Zn2+ Triton X-100 + NaKC4H4O6 + H2O Tween 20 + NaKC4H4O6 + H2O

(6)

KSCN has been proposed, and the extraction values are also shown in Table 4. The analysis of the data permits us to conclude a quite different behavior, since both zinc and copper are mostly segregated to the top phase, at levels higher than 80% and 62%, respectively. The rationale underlying this scenario is explained in terms of the complexation capacity of metals in the presence of tartrate and thiocyanate, according to the following equilibrium, where M is the heavy metal copper or zinc:

Table 3. Salting out Ability for {Nonionic Surfactant + NaKC4H4O6 + H2O} at 298.15 K

M2 +(aq) + xC4 H4O6 2 −(aq) ↔ M(C4H4O6 )x(2x − 2) − (aq)

k

β

R2

0.0513 0.0924

0.7109 2.6715

0.95 0.98

(8)

M2 +(aq) + xSCN−(aq) ↔ M(SCN)x(x − 2) − (aq)

(9)

Then, the presence of the metal-ion complex in the upper phase can be explained on the basis of the competition between thiocyanate or tartrate anions for the metal cations and the interaction of this complex with the selected nonionic surfactant, which is the major component in this top layer. On the one hand, taking into account the existing tartrate interactions, it can be stated that the presence of tartrate-based complex will be intimately influenced by the standard thermodynamic constant of formation of the metal-tartrate complex. Thus, the order of the formation constant is Cu2+ (log K = 3) > Zn2+ (log K = 2.7) and reveals a higher affinity of copper for the tartrate anion.20 This behavior points out the higher extraction capacity of Zn, since metal extraction is inversely proportional to the given formation constants. On the other hand, it seems that thiocyanate ions coordinate to copper and zinc with the N end to form tetrahedral complexes, such as [Cu(NCS)4]2− and [Zn(NCS)4]2−. In this sense, many authors21,22 have converged upon the idea that these complexes are present exclusively in nonaqueous solutions, which would justify its preferential partition to the surfactant-rich phase where hydrophobic domains exist. The final stage of this research consisted of coupling the proposed ABS to a previous dredged sediments washing step. The data obtained were also presented in terms of extraction capacity E (%), as can be visualized in Table 5. The combined heavy metals remediation strategy yielded total remediation values about 80% or higher for both zinc and copper, as can be inferred from the extraction data. It becomes patent that the use of Tween 20 rather than Triton X-100 is always preferred. This fact may be explained in terms of the different hydrophobicities of both nonionic surfactants, as demonstrated by their hydrophilic−lipophilic balance values (HLBTriton X‑100 = 13.4; HLBTween 20 = 16.7).11 The data obtained demonstrate that thiocyanate-based complexes show a preferential interaction for the more hydrophilic Tween 20, in line with the data obtained

is the nonionic surfactant that can be better salted out to the upper phase. Although the scientific rationale behind this phenomenon has not been fully understood, it seems that salting out electrolyte tends to be preferentially excluded from the vicinity of the surfactant units. This thermodynamic information, together with the visual observation of a faster phase segregation, allows us to conclude a better behavior of Tween 20 than that with Triton X100 from both a thermodynamic and kinetic point of view. 3.2. Removal of Metals from Polluted Dredged Marine Sediments. After having demonstrated the suitability of NaKC4H4O6 as segregation agent in aqueous solutions of the selected nonionic surfactants, the potential of this salt to extract two metal ions (Cu2+ and Zn2+) from aqueous solutions was ascertained. This step makes up a first approach to develop an entire remediation process of metal-polluted sediments. Therefore, a model solution containing surfactant, water, and metal ions was employed to study the partition behavior after addition of the tartrate-based salt. The remediation data were analyzed in terms of extraction capacity, E (%), defined as ⎛ m surfactant ⎞ ⎟⎟ ·100 E (%) = ⎜⎜ i ⎝ mi ⎠

E (%) (with KSCN)

Cu2+ Triton X-100 + NaKC4H4O6 + H2O Tween 20 + NaKC4H4O6 + H2O

where the fitting parameter k is the salting out coefficient and β is a constant related to the activity coefficient, respectively. The empirical thermodynamic parameter k represents the specific effects of salts on the free energy of transfer of 1 mol of surfactant units from aqueous solution to a 1 m salt solution. The values of the fitting parameters and correlation coefficients are listed in Table 3. From the k values, it seems that Tween 20

Triton X-100 + NaKC4H4O6 + H2O Tween 20 + NaKC4H4O6 + H2O

E (%) (without KSCN)

(7)

misurfactant

where and mi are the metal ions mass content in the upper phase and the total metal ions mass content, respectively. The results obtained are compiled in Table 4 and reveal that heavy metal ions are both concentrated in the salt-rich phase at concentrations higher than 85% for copper and 90% for zinc. This may be due to the existence of specific interactions between metal and salt ions, so the search of a suitable complexation agent can be a tool to allow an effective separation of the targeted contaminants. Thus, the use of 8618



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Table 5. Extraction Capacity E (%) of Metals from Marine Dredged Sediments after Sequential Treatment system Triton X-100 + NaKC4H4O6 + H2O Tween 20 + NaKC4H4O6 + H2O

Triton X-100 + NaKC4H4O6 + H2O Tween 20 + NaKC4H4O6 + H2O

temp (K) Zn2+ 298.15 343.15 298.15 343.15 Cu2+ 298.15 343.15 298.15 343.15

E (%) washing

should be applied, and then, it could be reused again. It is worth mentioning that this surfactant is completely biodegradable, and this family has even been reported to act as a carbon source.23

E (%) ATPS

72.3 78.0 85.6 89.4

± ± ± ±

0.0 2.7 2.7 8.1

88.8 89.3 89.4 89.3

± ± ± ±

1.1 1.8 0.7 1.4

77.0 84.1 98.4 98.4

± ± ± ±

0.1 0.1 0.2 0.1

85.7 84.9 85.3 88.8

± ± ± ±

0.8 2.4 4.6 3.5

4. CONCLUSIONS In this work, we have demonstrated the suitability of a twostages remediation strategy for the removal of heavy metals from marine dredged sediments. High levels of copper and zinc extraction (about 70% and 90%, respectively) for a model system containing KSCN, Tween 20, and NaKC4H4O6 were demonstrated after a preliminary comparison of the salting out potential of different salts and nonionic surfactants. All the obtained solubility and tie-line data were adequately modeled and made up the basis for the proposal of a remediation process in real marine sediments. The analysis of the extraction efficiency after a first sediment washing step and a second ABS concentration stage allowed us to conclude the viability of the integrated process, since remediation levels higher than 80% for copper and 90% for zinc were yielded.

for the model systems containing the heavy metals (see Table 4). Bearing in mind the promising remediation efficiency, a flow sheet of the proposed process is shown in Figure 2. The presented approach involves different advantages when compared with the EPA Method 3010 and 3050 recommended for heavy metal extraction. First of all, it is clear that the use of room temperature does not involve any decline in the metal ions remediation levels (Table 5), which is advantageous from an economic standpoint. Additionally, this alternative avoids the use of nitric acid in the washing, which is also beneficial in terms of environmental and health risks. In summary, in this paper, we have proposed the nonionic surfactant Tween 20, the salting out compound sodium potassium tartrate, and potassium thiocyanate as a complexing agent in order to propose a viable metal remediation strategy for marine sediments. This first contribution tackles just the viability of this separation technique for metal removal, but further study must be undertaken in order to search for an effective second stage to recycle the selected components. The removal of metals and thiocyanate is not complicated, since their precipitation could be achieved by just modifying the pH or adding compounds such as ferric sulfate. In relation to sodium potassium tartrate, there are several strategies that could be implemented, such as salt recovery by evaporation or reverse osmosis, or even the effluent disposal in a sewage treatment plant, since this salt is completely biodegradable. Finally, regarding the nonionic surfactant, after having used it for several cycles (sediments washing−ABS), the abovementioned treatment for thiocyanate and metals removals

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ASSOCIATED CONTENT

* Supporting Information S

Experimental solubility data for the ternary systems {nonionic surfactant + salt + H2O} at 298.15 K; parameters of eqs 1, 2, and 3 and standard deviation for {nonionic surfactant + salt + H2O} at 298.15 K. This material is available free of charge via the Internet at http://pubs.acs.org/.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +34986818723. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been supported by the Spanish Ministry of Economy and Competitiveness and FEDER funds (IPT310000-2010-17). F.J.D. thanks Xunta de Galicia for funding through an Isidro Parga Pondal position. E.G. acknowledges University of Vigo for a master grant.

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REFERENCES

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Figure 2. Flow sheet of the proposed nonionic surfactant-based separation process. 8619



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Environmentally Benign Sequential Extraction of Heavy Metals from

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