DFT Investigation of the Effects of Coexisting Cations and Complexing Reagents on Ni ( II ) Adsorption by a Polyvinylidene Fluoride-Type Chelating Membrane Bearing Poly ( Amino Phosphonic Acid ) Groups

A polyvinylidene fluoride (PVDF)-type chelating membrane bearing poly(amino phosphonic acid) groups, denoted as ethylenediamine tetra(methylene phosphonic acid) (EDTMPA)-tetrabutyl orthotitanate (TBOT)/PVDF, was employed to remove Ni(II) from the aqueous solution. The effects of coexisting Ca(II), Pb(II), citrate, nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) on the Ni(II) adsorption by this chelating membrane were revealed using density functional theory (DFT) calculations. Pb(II) showed a more detrimental effect than Ca(II) on the Ni(II) uptake; EDTA interfered with the capture of Ni(II) more remarkably than citrate and NTA. The results derived from DFT calculations were consistent with the experimental data. Ni(II) and Pb(II) showed more excellent affinity to the EDTMPA-TBOT/PVDF membrane than Ca(II). The stabilities between Ni(II) and the [EDTMPA-TBOT]7− chelating ligand of the membrane and those between Ni(II) and the three aforementioned complexing reagents followed the sequence: [Ni(II)-(EDTMPA-TBOT)]5− > Ni(II)-EDTA > Ni(II)-NTA > Ni(II)-citrate. The complexation between Ni(II) and the chelating membrane was prominent with the presence of citrate, NTA and EDTA.


Introduction
The heavy metal pollution for water bodies has been a critical environmental problem of global concern.Being one of the most toxic metals, the nickel ion with non-biodegradable characteristics has been validated as a carcinogen; it easily accumulates in organisms, thereby resulting in toxicities to ecological systems and human beings' health [1][2][3].Discharged effluents containing Ni(II) mainly come from the mining of nickel ores, metallurgy, welding, nickel electroless and electroplating.The discharged concentration of total nickel in China is strictly limited within 0.1 mg/L according to the published Chinese regulation of "Emission standard of pollutants for electroplating (GB 21900-2008)" [4].Thus, the concentration of discharged Ni(II) should meet the requirement mentioned above.In effluents derived from metallurgy, circuit board manufacturing and nickel electroless plating industries, besides nickel ions, other substances, such as Ca(II), Pb(II), citrate, nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) may be coexistent with Ni(II).Undoubtedly, the coexisting cations and organic reagents mentioned above can retard the removal of Ni(II).From this viewpoint, the exploitation of effective techniques with an excellent disposal performance for Ni(II) is thus of great significance for the removal of this metal with the presence of coexisting substances.
Numerous techniques are feasible for removing Ni(II) from wastewater to guarantee that this metal pollutant is discharged with a concentration smaller than the required value of 0.1 mg/L.Techniques of chemical precipitation, biological treatment, ion exchange, solvent extraction, flotation, electrocoagulation, adsorption and membrane separation processes [5][6][7][8][9][10][11][12][13][14] have been attempted so far.Among the above processes employed to capture Ni(II), the membrane separation technique deserves to be considered because of its merits of higher efficiency, inconspicuous pressure drop and shorter axial-diffusion path [15,16].As for the membrane techniques of electrodialysis, liquid membrane extraction, nanofiltration, reverse osmosis and polymer-enhanced ultrafiltration [14,[17][18][19][20], the drawbacks of high costs and fussy pretreatments of these membrane techniques restrain their wide applications.On the other hand, compared with the membrane techniques mentioned above, the microfiltration and ultrafiltration may be more applicable for removing Ni(II), with respect to their fascinating characteristics of high permeation flux, inexpensive investment and lacking strict pretreatments.Generally, the two aforesaid conventional membrane techniques cannot be of benefit for the removal of the dissolvable Ni(II).In light of the adsorption treatment of heavy metals by the chelating resin, the chelating groups with a high affinity to metals incorporated into the microfiltration (or ultrafiltration) membrane matrix will be helpful in the Ni(II) removal.Various chelating groups, including pentaethylenehexamine, polyacrylic acid, ethylamine ligand, polyethylene imine, hyperbranched poly(amidoamine), NTA, diethylenetriaminepentaacetic acid (DTPA), ethylenediamine tetra(methylene phosphonic acid) (EDTMPA) and EDTA, can be suitable for the candidates incorporated into the membrane framework [21][22][23][24][25][26][27][28][29][30].Among them, in our previous study [15,28], the DTPA group blended into the PVDF-type chelating membrane has been attested for the capture of Cu(II) and Ni(II).Similar to the DTPA, amino phosphonic acid, such as EDTMPA, shows an excellent affinity to metal pollutants.Thus, the enhancement in the Ni(II) uptake from the solution for both microfiltration and ultrafiltration membranes by the incorporation of EDTMPA groups can be considered.
In general, most adsorption studies have focused on the experiment scale.There is no doubt that the adsorption experiment can be helpful in the description of the adsorption kinetics and thermodynamics, but more details in the immanent interaction between the solute and the absorbent will not be elucidated comprehensively.The density functional theory (DFT) simulation, identified as a valuable supplement of the experiments [31][32][33][34], has been applicable for revealing various chemical problems; of course, adsorption studies are involved [15,[35][36][37][38][39].Whereas insufficient efforts related to the DFT simulation have paid attention to the Ni(II) adsorption by the PVDF-type membrane bearing the EDTMPA groups, herein, we confirm that this research will provide a proposal for the preparation of novel membranes and the removal of heavy metals.
In this research, a PVDF-type chelating membrane bearing the EDTMPA group was employed to remove Ni(II) from the aqueous solution in the presence of Pb(II) and Ca(II) cations and citrate, NTA and EDTA complexing reagents.The density functional theory (DFT) simulation was employed to elucidate the Ni(II) adsorption characteristics of the chelating membrane and to reveal the effects of the five aforementioned coexisting substances on the Ni(II) uptake.In this work, three DFT descriptors, i.e., chemical potential (µ), hardness (η) and global electrophilicity (ω), were calculated to explore the chemical reactivity of the EDTMPA group of the chelating membrane, Ni(II) and the coexisting Pb(II), Ca(II), citrate, NTA and EDTA.Furthermore, the charge transfer (∆N), the adsorption energies (∆E ads ) and the Gibbs free energies of adsorption (∆G ads ) between the chelating membrane and three metal ions and those between Ni(II) and the three above complexing reagents were calculated.

Preparation of the Chelating Membrane
At first, 2.0 g of EDTMPA were dissolved in 40 mL of ethanol aqueous solution, and the volume ratio between C 2 H 5 OH and H 2 O was 1:3; the pH of the solution was adjusted to 6.0 by 5 mol/L of NaOH solution.After that, 4.7 mL of TBOT were dropwise added into this solution, and this solution was magnetically stirred at room temperature for 24 h to obtain a microparticle-containing solution; and then, the microparticle was separated by centrifugation at 3000 rpm for 10 min.The obtained microparticle (labeled as EDTMPA-TBOT) was centrifugally washed three times using absolute ethanol and DMSO.Afterward, 4.8 g of PVDF and 0.53 g of PVP were dissolved in 25 mL of DMSO at 353 K, and the fabricated EDTMPA-TBOT powder was subsequently added into this solution.At this temperature, the solution was stirred for 6 h.Lastly, a phase inversion technique was employed to prepare the PVDF-based chelating membrane [15,28].The fabricated membrane (EDTMPA-TBOT/PVDF) was cleaned by deionized water and then kept for characterizations and adsorption tests.

Characterization of the Membrane
The morphology of the chelating membrane was characterized by an FE-SEM (SUPRA55, Zeiss, Jena, Germany) with an accelerating voltage of 5 kV; the compositions of the surface membrane were determined with an X-Max EDS (Oxford Instruments, Oxford, UK.) attached to the scanning microscope.FTIR spectra of the EDTMPA-TBOT chelating group before and after the Ni(II) uptake were examined using an E55 + FRA106 FTIR spectrometer (Bruker, Karlsruhe, Germany).In addition, the water permeability method was applied to characterize the mean pore size of the chelating membrane [40].

Adsorption Experiments
In 200-mL solutions containing 1.0 mmol/L of Ni(II) with an ~0.2 g addition of the chelating membrane, batch adsorption experiments for Ni(II) adsorption by the membrane were carried out at the temperature of 298 K.The pH of this solution was adjusted to 5.4 by the buffer containing 0.3 mol/L acetic acid and 0.2 mol/L sodium acetate.The effects of coexisting Ca(II), Pb(II), citrate, NTA and EDTA with different concentration (0-5 mmol/L) on the Ni(II) uptake were also evaluated.

Computational Details
All DFT calculations were implemented by the Materials Studio DMol 3 code [39,41] with Version of 7.0, which was provided by Accelrys Inc., (San Diego, CA, USA).In this research, the generalized gradient approximation (GGA) level with the spin unrestricted approach, double numerical plus polarization functions (DNP) and Becke exchange functional in conjunction with the Lee-Yang-Parr correlation functional (BLYP) were employed to optimize the structures.In order to accelerate self-consistent field convergence, Pulay's direct inversion in the iterative subspace (DIIS) technique and the small electron thermal smearing with a value of 0.005 Ha were applied.To obtain the geometrical and energetic parameters, Mulliken population analyses and frequency were calculated.A continuum salvation model (COSMO) with the dielectric constant of water as 78.54 was employed considering the aqueous adsorption.In regard to the effect of weak interactions (such as hydrogen bond and Van der Waals force), the Tkatchenko-Scheffler (TS) method for DFT dispersion correction was also used.

Analyses of FE-SEM and EDS
The morphologies of the fabricated EDTMPA-TBOT/PVDF chelating membrane were observed; surface and sectional morphologies of the membrane are shown in Figure 1a,b.For the surface morphology (Figure 1a), the uniform microporous structure can be identified.The presence of micropores with an average size of 0.25 µm will be helpful for the solution permeating through the membrane.In terms of the sectional morphology of the membrane, as shown by Figure 1b, the finger-like pores near the surface layer (labeled by a circle) are observed, and the sponge-type porous structures (described by a rectangle) can be found in the inner side of the membrane.
considering the aqueous adsorption.In regard to the effect of weak interactions (such as hydrogen bond and Van der Waals force), the Tkatchenko-Scheffler (TS) method for DFT dispersion correction was also used.

Analyses of FE-SEM and EDS
The morphologies of the fabricated EDTMPA-TBOT/PVDF chelating membrane were observed; surface and sectional morphologies of the membrane are shown in Figure 1a,b.For the surface morphology (Figure 1a), the uniform microporous structure can be identified.The presence of micropores with an average size of 0.25 μm will be helpful for the solution permeating through the membrane.In terms of the sectional morphology of the membrane, as shown by Figure 1b, the finger-like pores near the surface layer (labeled by a circle) are observed, and the sponge-type porous structures (described by a rectangle) can be found in the inner side of the membrane.
EDS spectra before the Ni(II) adsorption (Figure 1c) and after the Ni(II) adsorption by the chelating membrane (Figure 1d) were examined.Before the Ni(II) adsorption, as demonstrated by Figure 1c, besides the elements of carbon and fluorine, the elements of nitrogen, titanium, oxygen and phosphorus are detected, which indicates that the EDTMPA-TBOT groups were incorporated into the PVDF membrane matrix.After the Ni(II) uptake, by the comparison results of Figure 1c,d, the nickel element is detected, thereby suggesting the capture of Ni(II) by the chelating membrane.

FTIR Spectra
The availability of Ni(II) adsorption by the EDTMPA-TBOT/PVDF chelating membrane was also validated by FTIR analysis (Figure 2).Before and after Ni(II) adsorption (Figure 2a), for the FTIR spectra of the EDTMPA-TBOT/PVDF chelating membrane, in contrast with that of the virgin PVDF membrane, peaks at 890-1400 cm −1 show obvious changes, which can be attributed to the existence the phosphonic acid groups of the EDTMPA-TBOT ligand.Herein, with and without the uptake of Ni(II), the difference in the FTIR spectra of the chelating membrane is inconspicuous; this may be assigned to the disturbance of PVDF chains.Thus, the FTIR spectra of the pure EDTMPA and the EDTMPA-TBOT powder before and after Ni(II) adsorption (Figure 2b) were also measured to eliminate this disturbance.
The peaks appearing at 957 and 1007 cm −1 can be ascribed to the asymmetric stretching vibration of P-OH groups; other peaks located at 1100-1270 cm −1 are attributed to P=O stretching vibration [42,43].
The broad peak at 1670 cm −1 can be identified as the plane bending of the hydroxyl group in the phosphonic acid group [44].Peaks at 1322 and 1435 cm −1 can be assigned to the stretching vibration of the C-N and P-C groups.For the spectrum of EDTMPA-TBOT powder, the characteristic peaks related to the phosphonic acid group (900-1270 cm −1 ) divided into two peaks (1030 and 1150 cm −1 ), indicating the formation of P-O-Ti bond [29].The broad peak at 1670 cm −1 becomes sharp and shifts to 1650 cm −1 ; this can be due to the interaction between the TBOT and EDTMPA molecules and thereby suggesting the formation of the P-O-Ti bond.

FTIR Spectra
The availability of Ni(II) adsorption by the EDTMPA-TBOT/PVDF chelating membrane was also validated by FTIR analysis (Figure 2).Before and after Ni(II) adsorption (Figure 2a), for the FTIR spectra of the EDTMPA-TBOT/PVDF chelating membrane, in contrast with that of the virgin PVDF membrane, peaks at 890-1400 cm −1 show obvious changes, which can be attributed to the existence the phosphonic acid groups of the EDTMPA-TBOT ligand.Herein, with and without the uptake of Ni(II), the difference in the FTIR spectra of the chelating membrane is inconspicuous; this may be assigned to the disturbance of PVDF chains.Thus, the FTIR spectra of the pure EDTMPA and the EDTMPA-TBOT powder before and after Ni(II) adsorption (Figure 2b) were also measured to eliminate this disturbance.The peaks appearing at 957 and 1007 cm −1 can be ascribed to the asymmetric stretching vibration of P-OH groups; other peaks located at 1100-1270 cm −1 are attributed to P=O stretching vibration [42,43].The broad peak at 1670 cm −1 can be identified as the plane bending of the hydroxyl group in the phosphonic acid group [44].Peaks at 1322 and 1435 cm −1 can be assigned to the stretching vibration of the C-N and P-C groups.For the spectrum of EDTMPA-TBOT powder, the characteristic peaks related to the phosphonic acid group (900-1270 cm −1 ) divided into two peaks (1030 and 1150 cm −1 ), indicating the formation of P-O-Ti bond [29].The broad peak at 1670 cm −1 becomes sharp and shifts to 1650 cm −1 ; this can be due to the interaction between the TBOT and EDTMPA molecules and thereby suggesting the formation of the P-O-Ti bond.
For the spectrum after the Ni(II) adsorption compared with that before the Ni(II) adsorption, the peak at 1030 cm −1 shifts to 1047 cm −1 , and the intensity of this peak also slightly increases.The intensity of the peak at 1150 cm −1 related to the P=O stretching decreases, inferring the formation of the P=O-Ni(II) complexing bond.The peak at 1322 cm −1 in correlation with the C-N group almost completely disappears, and the formation of the N-Ni(II) bond can be validated [28].In addition, the decrease in intensity for the peak at 1650 cm −1 accompanied by a negative shift of 15 cm −1 may be due to the fact that Ni(II) is complexed by the O-H group of the EDTMPA molecule [15,45].

Effects of Coexisting Cations
The coexisting Ca(II) and Pb(II) can hinder the Ni(II) adsorption.Ni(II) uptakes of the EDTMPA-TBOT/PVDF membrane in the presence of these two cations were measured to elucidate their interferences.Ni(II) uptakes of the membrane reduce with Ca(II) and Pb(II) concentrations increasing from 0 to 5 mmol/L (Figure 3); these two coexisting cations show an interferential effect on the uptake of Ni(II) because they compete with Ni(II) for occupying the active sites of the membrane.As Ca(II) and Pb(II) coexist with Ni(II) at the concentration of 1 mmol/L, Ni(II) uptake of the membrane decreases by 35% and 83%.Based on this result, it can be inferred that Pb(II) exhibits a more conspicuously detrimental effect than Ca(II) on the Ni(II) adsorption.

Effects of Coexisting Cations
The coexisting Ca(II) and Pb(II) can hinder the Ni(II) adsorption.Ni(II) uptakes of the EDTMPA-TBOT/PVDF membrane in the presence of these two cations were measured to elucidate their interferences.Ni(II) uptakes of the membrane reduce with Ca(II) and Pb(II) concentrations increasing from 0 to 5 mmol/L (Figure 3); these two coexisting cations show an interferential effect on the uptake of Ni(II) because they compete with Ni(II) for occupying the active sites of the membrane.As Ca(II) and Pb(II) coexist with Ni(II) at the concentration of 1 mmol/L, Ni(II) uptake of the membrane decreases by 35% and 83%.Based on this result, it can be inferred that Pb(II) exhibits a more conspicuously detrimental effect than Ca(II) on the Ni(II) adsorption.

Effects of Coexisting Complexing Reagents
The influences of citrate, NTA and EDTA coexisting with concentrations from 0 to 5 mmol/L on Ni(II) uptakes of the chelating membrane were studied, and the results are depicted in Figure 4.As the concentration of the three complexing reagents increases, the Ni(II) uptake of the chelating membrane decreases.This can be explained by the fact that most nickel ions are complexed with the increasing concentration of these three complexing reagents, and the complexed form of Ni(II) retards this metal uptake.As citrate, NTA and EDTA coexisted at a concentration of 1 mmol/L, the Ni(II) uptake of the membrane decreases by 26%, 53% and 73%, respectively.Thus, it can be concluded that the interferences of these three complexing reagents follow the order of citrate < NTA < EDTA.Although the disturbance of coexisting cations and complexing reagents is validated, the chelating membrane still exhibits the ability of Ni(II) capture, manifesting its potential application for removing Ni(II) from aqueous solutions.

Effects of Coexisting Complexing Reagents
The influences of citrate, NTA and EDTA coexisting with concentrations from 0 to 5 mmol/L on Ni(II) uptakes of the chelating membrane were studied, and the results are depicted in Figure 4.As the concentration of the three complexing reagents increases, the Ni(II) uptake of the chelating membrane decreases.This can be explained by the fact that most nickel ions are complexed with the increasing concentration of these three complexing reagents, and the complexed form of Ni(II) retards this metal uptake.As citrate, NTA and EDTA coexisted at a concentration of 1 mmol/L, the Ni(II) uptake of the membrane decreases by 26%, 53% and 73%, respectively.Thus, it can be concluded that the interferences of these three complexing reagents follow the order of citrate < NTA < EDTA.Although the disturbance of coexisting cations and complexing reagents is validated, the chelating membrane still exhibits the ability of Ni(II) capture, manifesting its potential application for removing Ni(II) from aqueous solutions.

Effects of Coexisting Complexing Reagents
The influences of citrate, NTA and EDTA coexisting with concentrations from 0 to 5 mmol/L on Ni(II) uptakes of the chelating membrane were studied, and the results are depicted in Figure 4.As the concentration of the three complexing reagents increases, the Ni(II) uptake of the chelating membrane decreases.This can be explained by the fact that most nickel ions are complexed with the increasing concentration of these three complexing reagents, and the complexed form of Ni(II) retards this metal uptake.As citrate, NTA and EDTA coexisted at a concentration of 1 mmol/L, the Ni(II) uptake of the membrane decreases by 26%, 53% and 73%, respectively.Thus, it can be concluded that the interferences of these three complexing reagents follow the order of citrate < NTA < EDTA.Although the disturbance of coexisting cations and complexing reagents is validated, the chelating membrane still exhibits the ability of Ni(II) capture, manifesting its potential application for removing Ni(II) from aqueous solutions.
were calculated, and the values are listed in Table 1.Of all of the reactants, the HOMO energy of [EDTMPA-TBOT] 7− is the highest, and the LUMO energy of [Ni(II) 2+ .In this sense, it can be confirmed that Ni(II) and the [EDTMPA-TBOT] 7− ligand show the most noticeable tendencies for accepting electrons and donating electrons than other reactants.The chemical reactivity descriptors of chemical potential (µ), global hardness (η) and electrophilicity (ω) can be calculated by Equations ( 1)-( 3) [15,47].
The chemical reactivity descriptors of the seven aforementioned reactants were calculated and summarized in Table 1 EDTA] 2− .Therefore, Ni(II), Pb(II) and Ca(II) serve as electrophiles, while the [EDTMPA-TBOT] 7− ligand and the three complexing reagents act as nucleophiles.Among the three electrophiles, the electronegativity (χ = −µ) shows a descending order of [Pb(II)( 2+ .Compared with Ni(II) and Ca(II), Pb(II) can more easily obtain the electrons from the [EDTMPA-TBOT] 7− ligand.However, for Ni(II), Pb(II) and Ca(II), the calculated values of η and ω are inconsistent with the order of χ, and this is also in contradiction with the experimental result.It can be inferred that the above-mentioned three cations may show a different electrophilic behavior in other reacting processes.
The χ value of the four above nucleophiles increases in the order of revealing that the [EDTMPA-TBOT] 7− ligand owns a stronger nucleophilic ability than the other three complexing reagents.Moreover, the results of the analysis on the η and ω value of the [EDTMPA-TBOT] 7− ligand and three complexing reagents are in accordance with that of χ.Thus, it indicates that the Ni(II) uptake of the EDTMPA-TBOT/PVDF chelating membrane plays a dominant role in the adsorption process with the presence of citrate, NTA and EDTA.Furthermore, it can be deduced that the interference of EDTA on Ni(II) adsorption is higher than those of citrate and NTA.The three intramolecular parameters of µ, η and ω are only related to the characteristics of an isolated molecule.In contrast, another parameter, namely charge transfer (∆N) calculated using Equation (4) [32,47], will further contribute to reveal inherent the characteristics of the adsorption process.
where the A and B superscripts are used to mark the species A and B. If ∆N < 0, we can confirm that A and B serve as the electron donor and electron acceptor, respectively.Furthermore, the higher absolute value of ∆N will suggest the stronger interaction between two molecules [32,47] →Ni(II) suggest that these three coexisting complexing reagents show a smaller affinity to Ni(II) than the EDTMPA-TBOT/PVDF chelating membrane.In addition, EDTA shows a more negative effect on the Ni(II) uptake than citrate and NTA.36 .Conventionally, Ni(II) will be complexed in the form of the six-coordinated configuration [39].In addition, taking into account the nucleophilic characteristics of these complexing sites and the stability of the [Ni(II)-(EDTMPA-TBOT)] 5− configuration, three configurations in correlation with the complexing sites of the EDTMPA-TBOT ligand were chosen (shown in Figure 8   The adsorption energy (∆E ads ) at 298 K obtained from Equation ( 5) can be valuable for elucidating the stability of complexing geometries [39,46].
where E(X) is the computed total energy of species (X) with respect to the COSMO effect.
The calculated ∆E ads for the above three complexes are −9092.55,−9092.The adsorption energy (ΔEads) at 298 K obtained from Equation ( 5) can be valuable for elucidating the stability of complexing geometries [39,46].ΔEads = ΣE(product) − ΣE(reactant) (5) where E(X) is the computed total energy of species (X) with respect to the COSMO effect.The calculated ΔEads for the above three complexes are −9092.55,−9092.

Analyses of the Adsorption Energy and the Gibbs Free Energy of Adsorption
The Gibbs free energy of adsorption (ΔGads) at 298 K defined by Equation ( 6) was used to reveal the available formation of metal-based complexes; where G(X) is the calculated temperature-corrected free energy of species (X) at 298 K [39,46] and ΔEads is already depicted by Equation (5).

Analyses of the Adsorption Energy and the Gibbs Free Energy of Adsorption
The Gibbs free energy of adsorption (∆G ads ) at 298 K defined by Equation ( 6) was used to reveal the available formation of metal-based complexes; where G(X) is the calculated temperature-corrected free energy of species (X) at 298 K [39,46] and ∆E ads is already depicted by Equation (5).

Figure 2 .
Figure 2. FTIR spectra: (a) the virgin PVDF membrane and the EDTMPA-TBOT/PVDF chelating membrane before and after Ni(II) adsorption; (b) the virgin EDTMPA and the EDTMPA-TBOT powder before and after Ni(II) adsorption.

Figure 2 .
Figure 2. FTIR spectra: (a) the virgin PVDF membrane and the EDTMPA-TBOT/PVDF chelating membrane before and after Ni(II) adsorption; (b) the virgin EDTMPA and the EDTMPA-TBOT powder before and after Ni(II) adsorption.

Figure 6 .
Figure 6.Distribution of the species for the EDTMPA-TBOT ligand as a function of pH by HySS2009.

Figure 6 . 17 Figure 6 .
Figure 6.Distribution of the species for the EDTMPA-TBOT ligand as a function of pH by HySS2009.