Electrochemical determination of Cu2+ complexation in the extract of E. crassipes by anodic stripping voltammetry

Wael H.M. Abdelraheem; Zanaty R. Komy; Nabawia M. Ismail

doi:10.1016/j.arabjc.2013.01.019

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Original article

10 (

1_suppl

); S1105-S1110

doi:

10.1016/j.arabjc.2013.01.019

Electrochemical determination of Cu²⁺ complexation in the extract of E. crassipes by anodic stripping voltammetry

Wael H.M. Abdelraheem^⁎, Zanaty R. Komy, Nabawia M. Ismail

Chemistry Department, Faculty of Science, Sohag University, Sohag 82524, Egypt

⁎Corresponding author. Mobile: +20 1145813496; fax: +20 934601159. wael.abdelrehem@science.sohag.edu.eg (Wael H.M. Abdelraheem) wello17_5@yahoo.com (Wael H.M. Abdelraheem)

Received: 2012-8-17, Accepted: 2013-1-27, Published: 2013-02-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Chemical speciation of copper and its total dissolved concentration in the aqueous extract of Eichhornia crassipes (E. crassipes) were investigated by DPASV titration with Cu. Values of speciation parameters, $\log K_{{CuL}_{i}}^{'}$ and [L_i], were calculated using Scatchard linearization and nonlinear fitting with the assumption of two complexing ligands. The total dissolved copper [Cu]_T in the acid digested extract was found to be 0.48 μM ± 0.032. Results obtained from the linear and nonlinear transformations revealed that there were two classes of very strong copper complexation with average $\log K_{{CuL}_{1}}^{'} \sim 14.34 (\pm 0.01)$ , $\log K_{{CuL}_{2}}^{'} \sim 13.54 (\pm 0.04)$ , [L₁] ∼ 1.41 μM (±0.29) and [L₂] ∼ 2.08 μM (±0.1), calculated from the two methods. This reflects the great affinity of E. crassipes toward eliminating Cu²⁺ from its aqueous environment, and converting Cu²⁺ into less toxic by chelation with natural ligands released from the plant tissues.

Keywords

Speciation

DPASV

Eichhornia crassipes

Copper

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1 Introduction

Copper ions in the aquatic environments (e.g., Nile water) can be found in a variety of chemical species in both particulate and solution phases. These various species can include free copper ions (i.e., copper-aqua complexes), copper ions incorporated into colloids or adsorbed onto suspended particles, metal complexes with small-size inorganic anions, anthropogenic ligands, or naturally-occurring organic ligands, e.g., humic substances (Morel and Hering, 1993; Stumm and Morgan, 1996). Numerous studies, with different analytical techniques, showed that metal–organic complexes are often the dominant physico–chemical forms of trace metals (e.g., Cu) found in natural waters (Buffle, 1988; Komy et al., 2012). Possible sources of these metal-binding organic ligands include phytoplankton exudates (Gonzalez-Davila et al., 1995; Santana-Casiano et al., 1995; Abdelraheem et al., 2016 ), biologically degraded products, and constituents of sewage effluent (Bender et al., 1970). Metal speciation in aquatic systems has a large influence on the metal bioavailability (Morel and Hering, 1993). Depending on its concentration and degree of complexation, a metal can be growth limiting or toxic for living organisms.

E. crassipes is one of the main sources of those organic complexing ligands found in the Nile water. It has been introduced to Africa, particularly in the River Nile (Egypt), in 1870 but not reported as a major problem until 1980 (Orach-Meza, 1996). Cu ions accumulate in the plant parts during its life, but at the end of plant’s life, these accumulated ions dissociate again into the aquatic environment resulting in the presence of copper in different chemical forms; free ions, complexed with inorganics and complexed (adsorbed) with organics originating from the degradation of the dead plant material.

Significant efforts have been made to describe the stability of the metal-binding site reaction of algal exudates (Gerringa et al., 1995), functional groups from biological surfaces (Santana-Casiano et al., 1995) and the organic material pool of seawater (Donat and van den Berg, 1992) plus fresh water systems (Xue and Sigg, 1993). In spite of the large number of works that have been published, the origin and nature of the organic dissolved materials with affinity to trace metals in aquatic natural media, and the complexing characteristics of plankton exudates are still largely unknown.

Since 1980, the complexing characteristics of aquatic systems (e.g., seawater, river) have been widely studied for several transition metals (e.g., Cu, Zn) using two distinct methods (i.e. metal titration and ligand titration) studies are mostly focused on copper because of its strong complexation property and usefulness as an analytical tool. For metal titration, MnO₂ adsorption (van den Berg, 1982), electrochemical (Coale and Bruland, 1990; Apte et al., 1990), chromatographic (Sunda and Hanson, 1987), bioassay (Sunda and Ferguson, 1983) and solvent extraction (Moffett et al., 1990) techniques have been used to measure the concentrations of free metal ions in solution. The electrochemical techniques (DPASV or DPCSV) coupled with ligand competition were found to be the most promising to determine copper speciation in the aquatic samples with high accuracy (Campos and van den Berg, 1994).

Our study here intends to discuss copper complexation (speciation) in the aqueous extract of E. crassipes which represents one of the phytoplanktons found in the Nile water and as a source of the natural organic complexing ligands, as well as its relation with the toxicity of Cu in the Nile water. ASV will be utilized in the current study to directly measure the chemical speciation of Cu²⁺ in the aqueous extract of E. crassipes without any ligand competition. Therefore, the analysis will be made with small sample aliquots at different free metal concentrations, so it will be time saving with high accuracy and feasibility compared to the traditional techniques (e.g., Diffusive Gradint in Thin Films, Donnan Membrane Technique, Permeation Liquid Membrane and Competitive Ligand Exchange/Cathodic Stripping Voltammetry) (Nason et al., 2011). Moreover, the interfering ions as ${CO}_{3}^{2 -}$ , ${HCO}_{3}^{-}$ and OH⁻ or oxygen content of the sample will not affect the measurement as in other techniques (e.g., Donnan Membrane Technique, Ion Selective Electrodes and Stripping Chronopotentiometry) (Nason et al., 2011).

1.1

1.1 Explanation of the analytical technique

Speciation of copper in the aqueous extract of E. crassipes was determined using differential pulse anodic stripping voltammetry (DPASV) titration with Cu²⁺. Natural complexing ligand concentration, [L_i], and conditional stability constants, $\log K_{{CuL}_{i}}^{'}$ , were used to evaluate the speciation of Cu in the extract and calculated by using Scatchard linear-transformation and nonlinear curve fitting methods of the metal titration curve.

For Scatchard linearization, Eq. (1) (Kozelka and Bruland, 1998), the ratio of the [CuL_i]/[Cu]_labile is plotted against the [CuL_i], the slope equals $K_{{CuL}_{i}}^{'}$ and the Y-intercept equals ( $K_{{CuL}_{i}}^{'} [L_{i}]$ ). If the results are linear over the range of data, then they are modeled with the assumption of the presence of one class of metal binding ligands. The reciprocal slope of the linearization equation gives the total ligand concentration, [L_i], and the Y-intercept is equivalent to the reciprocal product of ( $K_{{CuL}_{i}}^{'}$ [L_i]).

(1)

[{CuL}_{i}] / {[Cu]}_{labile} = - K_{{CuL}_{i}}^{'} [{CuL}_{i}] + K_{{CuL}_{i}}^{'} [L_{x}]

Kozelka and Bruland (1998) have discussed before how to deal with the presence of two classes of the complexing ligands (L₁ and L₂).

Assuming that DPASV only measures the labile copper fraction, [CuL_i] and [Cu]_labile can be calculated from Eqs. (2) and (3):

(2)

[{CuL}_{i}] = {[Cu]}_{T} - i_{p} / S

(3)

{[Cu]}_{labile} = i_{p} / S α_{Cu}

where i_p is the DPASV peak current, [Cu]_T is the apparent copper concentration, S is the analytical sensitivity (AM⁻¹), and can be calculated as the slope of the linear portion of the titration curve (i_p vs. [Cu]_T) and α_Cu is the side reaction coefficient of the copper inorganic complexes, and can be calculated as follows:

(4)

α_{Cu} = 1 + \sum (K_{i}^{*} {[L_{j}]}^{i}) + \sum K_{a, i}^{*} / {[H^{+}]}^{i}

where,

K_{i}^{*}

is the stepwise stability constant for the complex of Cu²⁺ with ligand L_j (Cl⁻,

{CO}_{3}^{2 -}

{HCO}_{3}^{-}

{SO}_{4}^{2 -}

), and

K_{a, i}^{*}

is the stepwise acidity constant of Cu²⁺. Values of standard stability constants, inorganic nutrients and extract characteristics are summarized in Table 1.

Table 1 Standard stability constants used to calculate α_Cu, inorganic cations and characteristics of the aqueous extract of E. crassipes.

Standard stability constant^a				Inorganic ligands content
Complex	Log K	Complex	Log K	Ligand	Concentration/mgL⁻¹
CuSO₄	2.36	CuCl₂	−0.7	${CO}_{3}^{2 -}$	Nil
Cu(OH)⁺	−7.53	${CuCl}_{3}^{-}$	−2.2	${HCO}_{3}^{-}$	576.45
Cu(OH)₂	−13.68	${CuCl}_{4}^{2 -}$	−4.4	${SO}_{4}^{2 -}$	22
$Cu {(OH)}_{3}^{-}$	−26.8			Cl⁻	726.68
$Cu {(OH)}_{4}^{2 -}$	−39.6			Extract characteristics
( $Cu {HCO}_{3}^{-}$ )	2.20			Salinity	pH
CuCl⁺	0.00			1.1	4.73

a Values taken from (Alwan and Williams, 1979).

It is also possible to examine the transformation curves and interpret the titration data as best fit by two model classes of ligands. For example, there could be a small amount of a strong ligand, in equilibrium with a larger amount of a weaker ligand. This becomes apparent in the Langmuir plots, where the linear transformation is curved.

For non-linear curve fitting method (usually used in case of more than one complexing ligand), Eq. (5) is used as follows:

(5)

[{CuL}_{i}] = \frac{[L_{1}] \cdot K_{{CuL}_{1}}^{'} \cdot {[Cu]}_{labile}}{1 + K_{{CuL}_{1}}^{'} \cdot {[Cu]}_{labile}} + \frac{[L_{2}] \cdot K_{{CuL}_{2}}^{'} \cdot {[Cu]}_{labile}}{1 + K_{{CuL}_{2}}^{'} \cdot {[Cu]}_{labile}}

To obtain the speciation parameters for two ligand classes, the experimental X_exp value ([CuL_i]) is plotted vs. [Cu]_labile and fitted along with the theoretical X_theo

(\frac{[L_{1}] \cdot K_{{CuL}_{1}}^{'} \cdot {[Cu]}_{labile}}{1 + K_{{CuL}_{1}}^{'} \cdot {[Cu]}_{labile}} + \frac{[L_{2}] \cdot K_{{CuL}_{2}}^{'} \cdot {[Cu]}_{labile}}{1 + K_{{CuL}_{2}}^{'} \cdot {[Cu]}_{labile}})

with the use of primary estimations of the first complexing ligand concentration, [L₁], and its stability constant,

\log K_{{CuL}_{1}}^{'}

from the Scatchard linearization for the first data set. Microsoft Excel 2010 solver (add-in) was used for data fitting.

2 Experimental

2.1

2.1 Chemical reagents, solutions and instruments

All the chemical reagents were of analytical grade, from Merck and BDH companies. A stock solution of copper (1000 mgL⁻¹) was prepared by dissolving Cu(NO₃)₂·3H₂O in ultrapure water. All solutions were kept in a refrigerator at 5 °C until measurements were undertaken.

The voltammetric system was EG&G Princeton USA model 264A, equipped with a 303A hanging mercury drop electrode (HDME) and model 305 stirrer. A three-electrode system was used; consisting of HMDE as a working electrode, an Ag/AgCl as a reference electrode and a Pt wire as an auxiliary electrode. A Princeton EG & G X–Y recorder model RE 0089 was used to obtain the voltammograms.

Major anions ( ${HCO}_{3}^{-}$ , ${CO}_{3}^{2 -}$ , ${SO}_{4}^{2 -}$ and Cl⁻) in E. crassipes extract were determined by ion chromatography. Salinity and pH were measured using a Jenway conductivity meter model 4320, and a consort digital pH-mV meter model p901 from Belgium, respectively. Milli-Q water purification system was used as a source of all the water used in the study. All glassware were cleaned by soaking in 1:1 HNO₃, HCl and for one week in ultrapure water.

2.2

2.2 Sample collection and preparation

E. crassipes samples were collected from the Nile water at Sohag region (∼500 km from Cairo), Egypt. After collection, the stem and leaf parts of the plant were washed with tap water, first distilled and finally with deionized ultrapure water, then dried and grounded to less than 0.25 mm in size using sieves. A stock solution from the plant's aqueous extract was prepared by shaking (1.0 g of the plant powder + 100 mL deionized water) for 2.0 h and then centrifuging at 10,000 rpm for 15 min to separate the supernatant solution. The extract was transferred to 100 mL measuring flask and brought to the mark.

For the determination of total dissolved copper; an acid digested extract was prepared by transferring 100 mL from the last prepared aqueous extract into a 250 Kjeldahl flask containing 50 ml mixture of concentrated HCl and HNO₃ (1:1 v/v) acid. The mixture was evaporated in a fuming cupboard until the appearance of solid residue. The residue was then dissolved in 100 ml ultrapure water and filtered through Whatman filter paper No. 42, the supernatant was transferred into a 250 ml measuring flask and brought to the mark.

2.3

2.3 Procedure

2.3.1

2.3.1 Total dissolved copper

For the determination of the total dissolved copper in the acid digested aqueous extract; 10.0 mL digested extract was transferred into the voltammetric cell at pH 1.5 (using conc. HNO₃). The solution was then deaerated for 2 min with pure nitrogen gas. The deposition potential of −0.45 V and deposition time of 5.0 min were applied to a fresh mercury drop. After a rest period of 15 s, the voltammograms were recorded by applying a positive-going differential pulse scan with 10 mV s⁻¹ scan rate and 50 mV amplitude to +0.1 V. The peak’s height related to the reduction of Cu²⁺, appearing at −0.13 V, was recorded. Calibration was achieved by means of three standard additions of standard copper (0.948–2.844 μM) in order to overcome interference by organic matter. More description on the procedure was described elsewhere (Vos et al., 1986).

2.3.2

2.3.2 DPASV titration for the aqueous extract of E. crassipes

In a set of 10 PTFE tubes; 10 mL aqueous extract was mixed with an increasing Cu²⁺ concentration (from 0.0 to 3.96 μM). Stirring was maintained for 3.0 h to attain equilibrium. The voltammetric parameters were; purging N₂ for 10 min, deposition potential at −0.4, deposition time of 2 min and finally a quiescent period of 15 s was allowed for the solution before scanning step. The voltammograms related to the labile Cu fraction in the extract (not complexed by natural ligands) were recorded in the positive direction from −0.40 to +0.1 V with a scan rate of 10 mV/s and pulse amplitude of 50 mV. The experiment was repeated three times to minimize errors.

For comparison; the experiment was repeated from the beginning with the UV-digested extract for 2 h instead of the aqueous extract (in the absence of natural complexing ligands). To plot the titration curves for the aqueous extract (in the presence of natural ligands) and the UV-digested extract (in the absence of natural ligands), peak current (related to the labile Cu) is plotted as Y-axis vs. the concentration of added copper as X-axis.

3 Results and discussion

Recently, experimental woks on copper speciation in aqueous media have revealed more than 99% of copper to be present in the form of organic complexes with conditional stability constants in the range 10⁹–10¹⁵ (Hirose, 2006). Moreover, Hirose, 1994 [20] has classified strong organic ligands in seawater by means of the conditional stability constants of metal complexes and the natural complexing ligand concentration. According to the conditional stability constants of organic Cu²⁺ complexes, organic ligands in seawater are conveniently divided into three classes: very strong (log K > 13), strong (log K ∼ 12) and weak (log K < 10) (Hirose, 2006).

3.1

3.1 Total dissolved copper

Total dissolved copper [Cu]_T in the acid digested extract of E. crassipes was determined using DPASV technique coupled with the standard addition method using standard Cu²⁺. Fig. 1 shows the voltammograms obtained for acid digested extract of E. crassipes before and after the standard copper additions to the same sample. Results showed that the total [Cu]_T amounts to 0.48 ± 0.032 μM. Previous studies indicated that E. crassipes is capable of accumulating very high amounts of toxic heavy metal ions (e.g., Cu, Zn, Pb) (Schneider et al., 1995) which are preserved inside its cells (in roots, leaves or stems parts) during its life span. This result reflects the higher content of the plant from copper and its usefulness for its habitat as a decontaminator for toxic Cu²⁺ ions.

Total dissolved copper; DPASV voltammograms obtained for 10 mL acid digested extract acidified to pH 1.5 with 3 sequential copper additions. The dashed line represents the sample signal without any addition of Cu2+.

3.2

3.2 Copper speciation

DPASV technique was used to determine the speciation of copper in the aqueous extract of E. crassipes in which the water extract of the plant is titrated with copper in order to calculate the conditional stability constant ( $\log K_{{CuL}_{i}}^{'}$ ) and the natural copper complexing ligand concentration ([L_i]). As the metal concentration in the sample increases, the functional groups are complexed progressively with the metal ion until they are saturated. After this point, the metal concentration, which is not organically complexed, increases linearly (Laglera-Baquer et al., 2001). Fig. 2(a and b) shows the voltammograms of copper titration to the UV-digested extract (in the absence of natural complexing ligands) and to the aqueous extract (in the presence of natural complexing ligands), respectively. As shown in Fig. 2(b), there is a positive shift in the peak potential (corresponding to the labile Cu) with the increase in the concentration of [Cu²⁺]_add. This may be due to the adsorption of trace amounts of organic Cu complexes to the mercury drop. Fig. 3 represents the DPASV titration in which, a plot of peak heights in both titrations (in the absence (i) and in the presence (ii) of the natural complexing ligand) vs. the added copper [Cu²⁺] was constructed. Note that the peak heights were measured as the difference between the summit and the base line of each voltammogram.

DPASV titration, voltammograms were recorded for 10 mL of (a) UV-digested extract and (b) aqueous extract, after 1.0 h of equilibration with different Cu2+ additions (0.0–3.77 μM).

DPASV titration; a plot of the analytical response ip vs. the added [Cu]add for (i) UV-digested extract and for (ii) aqueous extract of E. crassipes.

It is clear from Fig. 3(i) that, there is a linear increase in the peak height with copper addition to the UV-digested extract, indicating that almost all the added copper remain free without any complexation, while with performing the same titration with the aqueous extract, as shown in Fig. 3(ii), a shape of two regions was obtained. The first region is a curve beginning from 0.00 and ends at 2.36 μM of the added copper, which in turn means that some of the added copper is progressively bounded to the natural complexing ligands (L_i) and the rest amount of copper remains in equilibrium with the complexed part as a free form or inorganically complexed. The second region is a linear relationship and begins after 2.36 μM of copper addition and means that no further organic complexation for Cu occurs. The sensitivity (S) of the linear portion (at higher copper concentration) was calculated to be 0.805 A/M.

In order to obtain the speciation parameters ( $\log K_{{CuL}_{i}}^{'}$ and [L_i]), Scatchard linearization as well as nonlinear fitting were made for the titration of the aqueous extract according to Eqs. (1) and (5), respectively.

Fig. 4 shows, (a) Scatchard linearization and (b) nonlinear plots for the titration data. Scatchard plot, Fig. 4(a), gives two linear regions indicating the presence of two types of complexation (CuL₁ and CuL₂). The values of $\log K_{{CuL}_{1}}^{'}$ , $\log K_{{CuL}_{2}}^{'}$ , [L₁] and [L₂] of the two classes of complexation were 14.34, 13.56, 1.21 μM, 2.15 μM, respectively (Table 2). Similar values of stability constants of $\log K_{{CuL}_{1}}^{'}$ and $\log K_{{CuL}_{2}}^{'}$ amount to 14.32 and 13.50, respectively, and complexing ligand concentration of [L₁] and [L₂] amounting to 1.62 μM and 2.01 μM, respectively, were obtained with the nonlinear fitting method (Table 2). These results indicate that there is a mixture of two types of naturally very strong complexing ligands, the first type (CuL₁) is the strongest type with lowest concentration of the complexing ligand, while the second type (CuL₂) of weaker complexation (Hirose, 2006) seems to be predominant in the aqueous extract as the concentration of the complexing ligands is the highest.

(a) Scatchard linearization and (b) nonlinear transformations for the DPASV titration.

Table 2 Values of speciation parameters (

\log K_{{CuL}_{i}}^{'}

, [L_i] and [L_T]) calculated from Scatchard linearization and nonlinear transformations for E. crassipes extract.

Speciation parameters
Type of transformation	$\log K_{{CuL}_{1}}^{'}$	$\log K_{{CuL}_{2}}^{'}$	[L₁], μM	[L₂], μM	[L_T]^a, M
Scatchard method	10.34	13.56	1.21	2.15	3.36
Nonlinear method	14.32	13.50	1.62	2.01	3.63

a [L_T] = [L₁] + [L₂].

It can be said that, the extract of E. crassipes is enriched with organic ligands which in turn chelate with copper ions, present in the aqueous media. Moreover, on comparison of the values of [L]_T (obtained with the two transformation methods) (Table 2) and the total copper in the extract ([Cu]_T = 0.48 μM), one can notice that the natural ligand concentration is much higher than the total copper in the extract, reflecting the higher complexing capacity of E. crassipes.

Witt et al. (2007) have identified before two classes of copper complexation in the rain water in the southern USA, with stability constants ranging from (10¹³–10¹⁶). This result is very near to our result for the extract of E. crassipes. It is therefore likely that the Cu ligands detected in aqueous extract are part of a spectrum of organic ligands of variable complexation strengths. Furthermore, some fractions of these ligands are capable of complexing with Cu²⁺ as suggested by Kieber et al. (2004). In addition, the small discrepancies in the values of $\log K_{{CuL}_{i}}^{'}$ may be as a result of differences in the water samples and their origin. Moreover, Croot (2003) has indicated that strong Cu binding ligands ( $\log K_{{CuL}_{i}}^{'} > 12.5$ ) were not detected during the winter or early spring in the Gullmar Fjord, Sweden, which indicates that another factor, study season, is controlling the value of conditional stability constant of copper in natural samples.

A recent study by Komy et al. (2012) has indicated that E. crassipes extract is enriched with amino acids, among those amino acids; Proline, Glutamic and aspartic acids were predominant in the plant extract representing about 77% of the total amino acids content. So it can be concluded that the extract of E. crassipes contains a heterogeneous mixture of chelating groups originating from the organics in the extract. In addition, the study made by Miličević and Raos (2006) has shown that copper can form Cu-amino acids with log K ranging (12.45–15.36). This result matches well with the values of stability constants for copper calculated in the present study.

4 Conclusions

A simple, viable and fast electrochemical method has been suggested to directly measure chemical speciation of Cu in the E. crassipes aqueous extract. The results reveal that the plant, after death, releases again both copper and organic ligands capable of making complexes with each other. There were two classes of copper complexation (both of very strong complexation) found in the extract. This ensures that the presence of E. crassipes as a live or dead material in the Nile water (the main source of drinking water in Egypt) is very useful for water as it has high affinity to decontaminate the Nile from its ionic copper (toxic) content (Abdelraheem et al., 2016), or by transforming Cu²⁺ ions into complexes (nontoxic) with the plant exudates and the products of its cell lysis.

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Witt M., Skrabal S., Kieber R., Willeya J., . Copper complexation in coastal rainwater, southeastern USA. Atmos. Environ.. 2007;41:3619-3630.
[Google Scholar]
Xue H., Sigg L., . Free cupric ion concentration and Cu(II) speciation in a eutrophic lake. Limnol. Oceanogr.. 1993;38:1200-1213.
[Google Scholar]

Introduction

Explanation of the analytical technique

Experimental

Chemical reagents, solutions and instruments
Sample collection and preparation
Procedure

Results and discussion

Total dissolved copper
Copper speciation

Conclusions

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