- Email: [email protected]
Innovative uses of carbamoyl benzoic acids in coagulation-flocculation’s processes of wastewater
Accepted Manuscript Innovative uses of carbamoyl benzoic acids in coagulation-flocculation’s processes of wastewater Marisela Martínez-Quiroz, Eduardo A. López-Maldonado, Adrián OchoaTerán, Mercedes T. Oropeza-Guzman, Georgina E. Pina-Luis, José-Zeferino Ramírez PII: DOI: Reference:
S1385-8947(16)31247-5 http://dx.doi.org/10.1016/j.cej.2016.09.011 CEJ 15721
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
23 June 2016 1 September 2016 3 September 2016
Please cite this article as: M. Martínez-Quiroz, E.A. López-Maldonado, A. Ochoa-Terán, M.T. Oropeza-Guzman, G.E. Pina-Luis, J-Z. Ramírez, Innovative uses of carbamoyl benzoic acids in coagulation-flocculation’s processes of wastewater, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.09.011
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Innovative uses of carbamoyl benzoic acids in coagulationflocculation’s processes of wastewater Marisela Martínez-Quiroz
a,b
, Eduardo A. López-Maldonadoa*, Adrián Ochoa-
Teránb*, Mercedes T. Oropeza-Guzman
a,b
, Georgina E. Pina-Luisb and José-
Zeferino Ramírezc. a
Centro de Investigación y Desarrollo Tecnológico en Electroquímica, Subsede
Tijuana, Carretera libre Tijuana-Tecate km 26.5, esq. Blvd. Nogales Parque Industrial El Florido CP 22444, Tijuana, B.C., Mexico b
Centro de Graduados e Investigación en Química Instituto Tecnológico de
Tijuana, Blvd. Alberto Limón Padilla s/n, Mesa de Otay, CP 22500, Tijuana, B.C., Mexico c
Departamento de Polímeros y Materiales, Universidad de Sonora, 83000
Hermosillo, Son, Mexico
Corresponding authors: Eduardo Alberto López-Maldonado, email: [email protected], Phone: +52 (664) 6602054, Address: Carretera libre Tijuana-Tecate km 26.5, esq. Blvd. Nogales Parque Industrial El Florido CP 22444, Tijuana, B.C., Mexico. Adrian Ochoa-Teran, email: [email protected], Centro de Graduados e Investigación en Química Instituto Tecnológico de Tijuana, Blvd. Alberto Limón Padilla s/n, Mesa de Otay, CP 22500, Tijuana, B.C., Mexico
1
HIGHLIGHTS Carbamoyl benzoic acids were tested as coagulation-flocculation new agents for industrial wastewater containing heavy metals. Zeta potential measurements is a powerful tool to evaluate new products for heavy metals removal from aqueous solutions. Integration of supramolecular chemistry to heavy metals removal as a new alternative in wastewater treatement.
ABSTRACT The use of carbamoyl benzoic, acids as separation new agents, to remove on the hazardous heavy metals, that are frequently, contained in metal plating wastewater is reported in this paper. The interaction of ionic heavy metals and carbamoyl benzoic acids was studied approaching zeta potential in aqueous solutions. These carbamoyl acids are innovative chemicals that demonstrate their potential to bind metallic ions as Pb2+, Cu2+ and Hg2+ under specific physicochemical conditions (pH solution and metal-ligand relationship). The amide substituent in each compound induced a specific affinity toward the metallic ions contained in water, and it was studied by measuring zeta potential of an inert surface in contact with the aqueous media. This measurement allowed us to determine the association constants (Kf) with
metal ions after defining the stronger interaction between the carbamoyl
acids and metallic cations. These results suggest that coagulation-flocculation process could be expected for Pb2+ over all the studied metals (Cu2+, Hg2+, Ca2+,
2
Fe2+, Ni2+, Mg2+, Ba2+, Al
3+
and Cd2+). The order of effectiveness of the carbamoyl
acids for the separation of Pb2+ is b > c > a according to substituent and the calculated values of Kf which match the observed behavior. To corroborate the practical use of carbamoyl acids as coagulants. Sedimentation and clarification kinetics
were
analyzed
with
transmittance
measurements
versus
time.
Coagulation-flocculation test was validated by SEM-EDS analysis.
Keywords: Carbamoyl acids, wastewater treatment, heavy metals, coagulationflocculation, zeta potential
1. Introduction Water quality assessment is done through a series of tests designed to quantitatively and qualitatively determine the most important physical, chemical and biological characteristics that may affect its actual and potential uses, as well as the type and degree of treatment required. The wastewater treatment industry is one
of
the
most
important
issues
in
the
environmental
area,
where
physicochemical studies may lead to a better understanding and support decisionmaking regarding the economy and efficiency [1]. Important issues in wastewater systems are the operation and process units recommended to separate pollutants. In particular, separation processes as chemical precipitation are commonly preferred because they are less expensive [2]. However, when heavy metals are the pollutants, their concentration is the defining factor to choose the correct procedure and their separation is one of the
3
biggest
challenges
for
environmental
research.
Different
techniques
as
electrochemical and biological treatment, precipitation, flocculation, ion exchange, reverse osmosis, adsorption with modified natural materials [3] and membrane filtration processes [4] can be considered for separation and elimination of metals contained in wastewater [5-6]. These techniques are mostly used for removing metals, but they are not always efficient and they produce byproducts, hindering the separation of metals. Reactive membranes, instead, are effective to remove metal ions but they are still expensive [7-8]. Development of new methods to improve primary wastewater treatment efficiency recall the use of new eco-friendly chemicals [9], ensuring hazardous heavy metals removal and avoiding dangerous operations. The use of new compounds containing carbamoyl groups in their structure is of great interest, since these compounds have potential to bind [10], precipitate and flocculate heavy metals. Some of the known applications of carbamoyl benzoic acids are in medicinal chemistry [11], microbial sensors [12], electrochemistry sensors [13] and environmental metal ions sensors. However, there are no reports related with the precipitation and removal of metal ions contained in industrial wastewater. In a previous work authors reported significant spectral differences in carbamoyl benzoic acids as function of the substituent in the amide which leads to an increase or decrease in electron delocalization, as well as fluorescence intensity [10]. Moreover, photophysical studies of various heavy metals in the presence of carbamoyl acids showed its potential application in the detection and separation of metal ions in aqueous media [14,15].
4
Coagulation-flocculation process is appropriate for pollutants removal. Flocculants generally used synthetic flocculants however are non-biodegradable [17, 18]. It is important to the development of new biodegradable materials with high efficiency in the coagulation-flocculation process without adding other toxic additives. In this research, a series of carbamoyl benzoic acids 2a-c were evaluated with environmentally important metal ions as Pb2+, Cu2+, Hg2+, Ca2+, Fe2+, Ni2+, Mg2+, Ba2+, Al
3+
and Cd2+ to accomplish the supramolecular complex
formation in aqueous solution followed by its precipitation. These compounds showed three types of interaction as chemical precipitation, electrostatic interactions and physic entrapment. Focusing on the electrostatic interaction of ionic heavy metals and carbamoyl benzoic acids authors propose to approach it by zeta potential measurements.
For this purpose, it was considered that zeta
potential (ζ) measurement is a physicochemical parameter to characterize solidliquid dispersions found in wastewater [11]. A particular case was the study of water treatment from the tortilla industry, where zeta potential measurement let to show important electrostatic and Van der Waals interactions between colloidal matter and a natural coagulant agent like chitosan [16]. Knowing that a surface charge measurement allow to predict the interaction between an ionic media and a solid, zeta potential measurement using the an inert surface in contact with different ionic media may allow to predict the interaction between the of carbamoyl acids with heavy metals contained in aqueous solutions. In this paper, three different carbamoyl acids were used to determine the effect of aqueous media pH, the substituent on the amide group of the carbamoyl acids, as well as the study of
5
ten different metals to determine their affinity with carbamoyl acids through the estimation of constant association, Kf. In addition, the supramolecular compound formed between carbamoyl acids, Pb was evaluated by sedimentation kinetics, metal ion content in supernatant, and morphological and elemental composition of flocs (formed solids).
Materials and Methods 2.1 Materials Deionized water (resistivity: 18.2 MΩ, MilliQ. Advantage A10) was used for sample preparation in all experiments, 3,3´,4,4´-benzophenonetetracarboxylic dianhydride, naphthalen-2-amine, naphthalen-2-ylmethanamine and benzylmethyl amine were purchased from Sigma Aldrich in highest available purity (> 99%). All solvents were of spectroscopic or HPLC grade. Zeta Potential data was recorded on a Stabino Particle Charge Mapping. The measurements were done at room temperature in porcelain cuvettes. Sample solutions used to study the pH dependence of the potential zeta were prepared adjusting to the desired pH, with 0.1M NaOH and 0.1M HCl. The effect of metal cations upon the zeta potential was examined by adding a few microliters of stock solution (0.1% w/w) of the study metal cations to a known volume of the solution (10 mL). The addition was limited to 0.3 mL, avoiding a dilution effect [19]. 1H NMR spectra were obtained in a Bruker 400 MHz NMR Spectrometer at a probe temperature of 25 ºC with TMS as the internal standard. For Pb-relative estimations, the samples were examined in a SEM ZEISS EVOMA15, equipped with a EDS (energy dispersive spectroscopy) BRUKER detector. Metal ion concentrations were determined using Agilent Technologies 4200 MP6
AES. Various carbamoyl acids additions to the synthetic wastewater were submitted to a sedimentation kinetic study by transmittance measurements in a Turbiscan Lab® equipment. 2.2 Profiles ζ vs pH for the compounds 2a-c The potential zeta profiles of carbamoyl acids identified as 2a-c analogues were performed in order to evaluate the amine influence over the their surface charge characteristics. The measurements were done at room temperature in porcelain cuvettes. Influence of pH on the zeta potential behavior of 2a-2c (0.1%) derivatives was studied within a pH range of 2-11 with 0.1 M NaOH and 0.1M HCl. 2.3 Affinity study compounds 2a-2c with different metal ions The properties of carbamoyl acids in the presence of different metal cations (Cu2+, Pb2+, Hg2+, Ca2+, Fe2+, Ni2+, Mg2+, Ba2+, Cd2+ and Al3+) were studied in aqueous solutions exploring their application as sensors and new agents of separation for these environmental important metal ions. The compounds were titrated by successive increment of equivalent number of these metals and the changes in the zeta potential were measured. 2.4 Coagulation-Flocculation test Chemical coagulation was evaluated using the micro-Jar test technique [11]. The chemical reagents used were Lead, Copper and Mercury perchlorates from Sigma Aldrich. Dosage tests were performed in 20 mL vials. Progressive additions of 0.1 g/L carbamoyl benzoic acids solution were made. After each addition, the vials were shaken for 2 min and allowed to settle for 2 min. Finally, the supernatant was
7
withdrawn to determine the metal ions concentration. The chemical reagents used are high-purity standards, and quality control standard. 2.5 Turbiscan Lab® Expert stability analysis Turbiscan Lab® (Formulaction) is a tool for monitoring the stability or phase separation tendency of dispersions. The measurement principle for this technique is based on detecting the changes in transmission and backscattering, as a function of particle movements (for these compounds sedimentation). The tests were carry out by adding the best dose of each carbamoyl acid (2a-2c) solution with 0.1 % Pb2+, performing scan every 25 s for 30 min. 2.6 Synthetic water with heavy metals preparation The synthetic water solution was prepared by dissolving the appropriate amount of Pb2+, Ni2+, Cr2+, Ca2+, Cu2+, Cd2+ and Zn2+ as perchlorate salt powder (SigmaAldrich) in 1000 mL of MilliQ water. The solution had a 500 ppm final concentration and the pH value was 2.5.
2. Results and Discussion 3.1 Electrokinetic behavior of the compounds 2a-2c The pH influence on the zeta potential behavior of 2a-2c derivatives was studied within a pH range of 2-11. The zeta potential profiles as function of pH are shown in Figure 1. It is observed that the three compounds presented similar behavior in the response of zeta potential. These compounds have a negative surface charge at pH > IEP (isoelectric point). However, compounds 2a-c have different limit ζ values of
8
-130, -120 and -6 mV respectively. These magnitudes are attributed to the type of substituent (naphthyl, naphthylmethyl and benzyl) as well as the functional groups presented in the structure (amide and carboxylic acid), which give them the suitable electric potential for removal of metal ions. The isoelectric point (IEP) of the compounds 2a-c are pHIEP 3.2, 2.7 and 3.6, respectively. These values suggest that at wastewarer pH>pHIEP electrostatic interactions between metallic cations and carbamoyl benzoic acids will be present. On the other hand the pKa values for compound 2a-2c were calculated as 5.2, 5.5 and 5.1 (Excel Worksheets for Spectrometry), respectively [21]. The pKa values are within expected for organic acids. When pH is equal to pKa, the half of the molecules are ionized by deprotonation of the carboxylic acid. If pH>pKa then increases the degree of ionization and the probability of interaction with the cation is greater.
Figure 1
3.2 Influence of metal cations on the potential zeta of the derivative 2a-2c The carbamoyl acids 2a-2c in the presence of different cations (Cu2+, Pb2+, Hg2+, Ca2+, Fe2+, Ni2+, Mg2+, Ba2+, Cd2+ and Al3+) were studied. These compounds were titrated by successive additions of cation equivalents and the changes in the zeta potential were measured. The addition of Ca2+, Fe2+, Ni2+, Mg2+, Ba2+ and Cd2+ to compound 2a in aqueous solution did not cause significant variation in the zeta potential. The addition of Pb2+, Hg2+ and Cu2+ ions produce a significant enhancement in the zeta potential profile pH= 6.6 (Figure S1).
9
The addition of cations to compound 2b in aqueous solution neither cause significant variation in the zeta potential with Mg2+ and Ba2+, Ca2+, for the other hand, Ni2+ and Cd2+ showed an intermediate effect. Instead, the addition of Pb2+, Hg2+, Fe2+, Cu2+ and Al3+ ions induces a significant decrease in the zeta potential profile (Figure S2). In contrast, 2b shows a different behavior, since the zeta potential values retain their negative charge with Ca2+, Fe2+, Ni2+, Mg2+, Ba2+, Cd2+ and Al3+ ions; while with Cu2+, Pb2+ and Hg2+. The zeta potential values are negative in the first addition (~0.5 eq), and beyond the third addition of metal ion, values passed from negative to positive. Initially, the surface charge is equivalent to -93 mV, indicating the presence of negatively charged monomer in the solution. Considering the positive charge of metal ions in wastewater, it was expected that Pb2+ neutralizes the negative charge of the monomer. The charge neutralization of the compound surface makes possible the cations removal. Due to compounds 2ac are negatively charged in the pH> 3 and this enhances its effectiveness for interaction with cations, it is not necessary a neutralization step or pH adjustment that normally are required with conventional coagulants-flocculants agents in wastewater treatment plants. In the addition of cations to compound 2c, in aqueous solution, three response trends are observed: in the first one Ca2+, Ni2+, Mg2+ and Ba2+ did not cause significant variation in the zeta potential, suggesting that chemical affinity between carbamoyl compounds are not enough to form a coordination complex; the second one correspond to the addition of Pb2+, Cu2+, Cd2+, and Fe2+, which induce a slight decrease in zeta potential; and the third response trend corresponds to Hg2+ and Al3+ ions with a marked negative pattern zeta potential profile (Figure S3). In the 10
titration of compound 2c, the measured surface was always negatively charged. The ditopic nature of these molecules suggest a supramolecular arrangement in which the metal is coordinated between two molecules. The zeta potential decreases because of these aggregates behave as particles, having a positive surface charge and as consequence it must be surrounded by a layer of perchlorate counter ions, this layer may correspond to the Stern layer. The zeta potential decreased indicating the formation of a particle or aggregate as supramolecular metal salt. The cations with greater affinity interact with all available carbamoyl benzoic acids and are coordinated to form the aggregate. Furthermore the potential increases because there are more positive ions between the Stern layer and the slipping plane causing that the zeta potential becomes more positive due to the excess of metallic salt added [1]. The compounds based on carbamoyl benzoic acids demonstrated high affinity toward Pb2+, Hg2+ and Cu2+ in water solution even in the presence of other metallic ions. The determination of binding constants for the complexes with metal ions and 2a2c were determined with Equation S1 and they are presented in Table S1. These results show great stability of the complexes even in aqueous media, because of the values obtained are the range of 104 to 107. The profiles indicated a metalligand 1:1 ratio for the most complexes. The 1:1 stoichiometry found for the complex by zeta potential measurements although the receptors have a ditopic structure reinforces the proposal on the formation of supramolecular aggregates, where a series of molecules are successively connected through the interaction with metal centers (Figure S4). 3.3 Coagulation-Flocculation test 11
A coagulation-flocculation test was implemented with Pb2+ due to during the pZ titration protocol was observed a sedimentation process only with this metal ion. The Figure 2 shows the variation of Pb2+ residual content in a flocculation process using the compounds 2a-2c. The experiment started at 100 ppm of Pb2+ in the solution. The optimal dose of the compound 2a-2c were 150 ppm, 100 ppm and 125 ppm, respectively. The physicochemical properties of the compounds were previously studied [10]. The fluorescence and NMR changes were monitored and the Job´s study confirm the stoichiometry 1:1 of 2b with Pb2+. With compounds 2b and 2c were observed that the complex with Pb2+ precipitates suggesting high chemical stability and very low solubility in aqueous media. It is believed that the carbamoyl benzoic acids have large cavities in their binding sites which favors a great interaction with metal ions and a hydrophobic microenvironment generates the solid-state formation. Interestingly, due to the physicochemical characteristics of the compounds and their complexes, the flocculation process does not occurs with other metal ions, however they may be used in conjunction with some natural coagulants to facilitate this process. The effectiveness of the compounds for the separation of Pb2+ was in the order b>c>a and it is agreement with the Kf calculated values. The removal efficiency achieved with these compounds is defined by different factors such the affinity in the complex formation studied by zeta potential and once it is formed, the hydrophobic effect and additional intermolecular forces that defining the solubility in the aqueous medium and particle size. The compounds b and c have a methylene spacer that encourages greater interaction with the metal because there is less steric hindrance in
12
comparison with 2a. In the compound 2a naphthalene group is directly attached to the amide group and the complexes take longer to reach the equilibrium.
Figure 2
The transmittance profiles of complexes formed with 2a and 2b and Pb2+ are shown in Figure 3. The data showed that these complexes have the same behavior. Significantly, for 2b-Pb2+ complexes the precipitation rate increased compared to 2a-Pb2+. In these complexes a hard to see (very fast) flocculationcoagulation kinetic occurred, and only the phenomenon of flocs migration was observed.
Figure 3.
The transmittance profile of complex 2c-Pb2+ is shown in Figure 4. This data demonstrated that occurs the coagulation-flocculation process, which confers a potential value to 2c for using in Pb2+ removal in contaminated wastewater. On the contrary, to the behavior of compounds 2a and 2b, compound 2c presents the coagulation-flocculation phenomena. Therefore, these compounds showed to have potential applications for the detection and separation of these metals in wastewater.
Figure 4
13
The SEM-EDS images and data obtained of the complexes 2a-Pb2+ and 2b-Pb2+ demonstrate the presence of microspheres in the sediment dry flocs containing C, O and Pb (Figure 5). The morphology of dry flocs from compounds 2a and 2b show similarity, however for 2c-Pb2+ flocs a more porous texture is observed. EDS study revealed that C, O and Pb were abundant in the sediments. Highest wt% (average of all scanner areas) of lead was found for 2b (55.18%) followed by 2c (53.16%) and 2a (52.60%). These results confirm the data obtained in coagulationflocculation test of metal ions. The SEM analysis gave the strongest evidence of the formation of supramolecular aggregates with Pb2+ opening a new field in wastewater treatment using carbamoyl benzoic acids.
Figure 5 The physicochemical conditions for Pb2+ removal was summarized in Table 1. The physicochemical studies revealed that 2b presented the greater Pb2+ removal capacity (95%), followed by 2c (83%) and 2a (82%). The molar ratio of removed Pb2+/compound was 1 ± 0.3, confirming the 1:1 stoichiometry found in the pZ studies. According with the clarification kinetics and sedimentation rates, compound 2c showed best performance in the coagulation-floculation process followed by 2b, although both compounds were very effective in the separation. Finally, compound 2a has the lower clarification kinetic and sedimentation rate and a bigger amount of compound is required to remove Pb2+ (~2 g of 2a/g Pb2+).
Table 1
14
Considering that the goal of water treatment processes is to encourage the instability of particles containing the undesired heavy metals, the coagulationflocculation kinetic stability was studied for compounds 2a-c and the results are shown in Figure S8. The asymptotic behavior of 2a-Pb2+ reveals the high instability of this complex in the aqueous media since the first minutes. Then, the formation of flocs is faster with 2c in comparison with 2b and 2c. These result agree with the of the clarification kinetic magnitude for these complexes. The expectative is a faster separation and more unstable particles with 2c in aqueous media and with good effectiveness in Pb2+ removal. The instability of the supramolecular aggregates in aqueous media was in to order 2c > 2b > 2a.
Finally, the optimization of the geometries of 2a-c and their respective complexes with Pb2+ (L2Pb) were performed by theoretical calculations using the density functional theory (DFT) and the non-local correlation PBE0 with a DZVP basis set in gas phase [22-26], in order to compare the chemical stability considering the formation electronic energies and the structure of the complexes, and related them with the results in the coagulation-flocculation process (Figure S9). The calculated energies were -480.52 kcal/mol, -483.09 kcal/mol and -488.937 kcal/mol for 2a, 2b and 2c complexes, respectively. Although there is not a marked difference in the energies, the values predict a stability order as follows 2c > 2b > 2a. According with the optimized structures, the flexibility of the N-substituents allows to stablish additional intermolecular interactions as π-π and C-H···π in the complexes with 2b and 2c which are not present with 2a. These intermolecular forces may contribute
15
in the chemical stability of the complexes creating a hydrophobic environment which drives the flocs formation, compaction and precipitation in aqueous media.
Conclusions The pZ studies demonstrate that carbamoyl benzoic acids a-c have a strong interaction metal ions and high affinity toward Pb2+, Hg2+ and Cu2+ in aqueous media. The stoichiometry of the complexes stablished by zeta potential profiles was 1:1 for the most metal ions suggesting a supramolecular arrangement where two ligand units are connected through a metallic center bridge. The binding constants were estimated considering this stoichiometry and the values were in the range of 105 to 107 M-1 which are high values for an interaction in aqueous media. The SEM analysis of dry flocs gave the evidence of microsphere aggregates formation with 2a and 2b, and other irregular structures with 2c. EDS study of dry flocs revealed the highest wt% of Pb found in 2b (55.18%), followed by 2c (53.16%) and 2a (52.60%). In the coagulation-flocculation study using a 100 ppm Pb2+ solution, the optimal dose of 2a-c were 150 ppm, 100 ppm and 125 ppm, respectively. In addition, 2c presented the best physicochemical characteristics with the greater clarification kinetics (523.4 %/h), an acceptable sedimentation rate (17.4 mm/h) and good removal of Pb2+ (83%). Compound 2b presented a comparable performance with a clarification kinetics of 146.9 %/h, and the highest sedimentation rate 27.5 mm/h and removal of Pb2+ (95%). Therefore, carbamoyl benzoic acids show potential applications for the detection and separation of these metal ions in wastewater industrial opening a new field in wastewater treatment.
16
Acknowledgements The authors gratefully acknowledge to Consejo Nacional de Ciencia y Tecnología (CONACyT) for the support the following grants: Basic Science 2015 Grant No. 237032, National Problems Project Grant No. 247236, Postdoctoral Research Grant and Supramolecular Thematic Network Grant No. 271884. Authors also thanks to Tecnológico Nacional de México/Instituto Tecnológico de Tijuana for its support. The technical assistances of Ricardo Valdez Castro and José García Elías are highly appreciated. José-Zeferino Ramírez thanks to the area of high performance computing at the University of Sonora (ACARUS) for the access to the computational resources.
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[20] A. Ochoa-Terán, J. Estrada-Manjarrez, M. Martínez-Quiroz, et al., A Novel and Highly Regioselective Synthesis of New Carbamoylcarboxylic Acids from Dianhydrides, The Scientific World Journal, vol. (2014). [21] I. Motomichi, Excel Worksheets for Spectrometry Universidad de Sonora 2009. [22] C. Adamo, V. Barone, Toward Reliable Density Functional Methods Without Adjustable Parameters: The PBE0 Model, J. Chem. Phys. 110, (1999) 6158-6169. [23] N. Godbout, D. R. Salahub, J. Andzelm, E. Wimmer, Optimization of Gaussian-Type Basis sets for Local Spin Density Functional Calculations. Part I. Boron Through Neon, Optimization Technique and Validation, Can. J. Chem. 70, (1992) 560-571. [24] R. B. Ross, J. M. Powers, T. Atashroo, W. C. Ermler, L. A. LaJohn, P. A. Christiansen, Ab Initio Relativistic Effective Potentials with Spin–Orbit Operators. IV. Cs through Rn J. Chem. Phys. 93, (1990) 6654-6670. [25] M. Valiev, E. J. Bylaska, N. Govind, K. Kowalski, T. P. Straatsma, H. J. J. van Dam, D. Wang, J. Nieplocha, E. Apra, T. L. Windus, W. A. de Jong, NWChem: A Comprehensive and Scalable Open-Source Solution for Large Scale Molecular Simulations, Comput. Phys. Commun. 181, (2010) 1477-1489.
20
0
2
4
6
8
10
50
2a
0 -50 -100 -150
2b
ζ (mV)
0 -50 -100 -150 2
2c
0 -2 -4 -6 -8 2
4
6
8
10
pH
Figure 1. Profiles the ζ = f (pH) of 2a, 2b and 2c water/DMSO (9:1, v/v). Concentration is 0.1% w.
21
50
0
50
100
150 2a
40
2+
Pb Residual (mg/L)
30 20 15 2b 10 5 0 26
2c
24 22 20 18 16 0
50
100
150
Flocculant dose (mg/L) Figure 2. Coagulation-Flocculation test to be using the compounds 2a-c in the treatment of wastewater synthetic of Pb2+.
22
2a 15
2a
T (%)
10
5
0 0
10
20
30
40
Height (mm) 100
2b 2b
80
T (%)
60
40
20
0 0
10
20
30
40
Height (mm)
Figure 3. Transmitance profiles of compounds 2a y 2b at optimum dose, data are reported as a function of time (0-30 min).
23
2c
2c
90 80 70
T (%)
60 50 40 30 20 10 0
0
10
20
30
40
Height (mm)
Figure 4. Transmitance profile of compound 2c at optimum dose, data are reported as a function of time (0-30 min).
24
2a Element
Wt %
C
22.12
O
25.27
Pb
52.60
2b Element
Wt %
C
23.01
O
23.82
Pb
55.18
2c
Figure 5. SEM-EDS analyses of sediment. 25
Element
Wt %
C
22.47
O
22.34
Pb
53.16
Table 1. Optimal physicochemical conditions for the Pb2+ removal of synthetic water with.
Compound
%
mol Pb2+/ mol
Clarification
Sedimentation
Removal
compound
Kinetics*
ratio* mm/h
%/h
2a
82
0.71
81.3
3
2b
95
1.3
146.9
27.5
2c
83
0.96
523.4
17.4
*30 min of scanning
26
Graphic Coagulant-Flocculant agent
2a 2a
2b 2b
2c2c
Heavy metals wastewater
Flocs
27
HIGHLIGHTS
Carbamoyl benzoic acids as separation new agents for industrial wastewater. Zeta potential a powerful tool to determine the performance of separation new agents. Supramolecular chemistry in water treatment using carbamoyl benzoic acids.
29
S1385-8947(16)31247-5 http://dx.doi.org/10.1016/j.cej.2016.09.011 CEJ 15721
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
23 June 2016 1 September 2016 3 September 2016
Please cite this article as: M. Martínez-Quiroz, E.A. López-Maldonado, A. Ochoa-Terán, M.T. Oropeza-Guzman, G.E. Pina-Luis, J-Z. Ramírez, Innovative uses of carbamoyl benzoic acids in coagulation-flocculation’s processes of wastewater, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.09.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Innovative uses of carbamoyl benzoic acids in coagulationflocculation’s processes of wastewater Marisela Martínez-Quiroz
a,b
, Eduardo A. López-Maldonadoa*, Adrián Ochoa-
Teránb*, Mercedes T. Oropeza-Guzman
a,b
, Georgina E. Pina-Luisb and José-
Zeferino Ramírezc. a
Centro de Investigación y Desarrollo Tecnológico en Electroquímica, Subsede
Tijuana, Carretera libre Tijuana-Tecate km 26.5, esq. Blvd. Nogales Parque Industrial El Florido CP 22444, Tijuana, B.C., Mexico b
Centro de Graduados e Investigación en Química Instituto Tecnológico de
Tijuana, Blvd. Alberto Limón Padilla s/n, Mesa de Otay, CP 22500, Tijuana, B.C., Mexico c
Departamento de Polímeros y Materiales, Universidad de Sonora, 83000
Hermosillo, Son, Mexico
Corresponding authors: Eduardo Alberto López-Maldonado, email: [email protected], Phone: +52 (664) 6602054, Address: Carretera libre Tijuana-Tecate km 26.5, esq. Blvd. Nogales Parque Industrial El Florido CP 22444, Tijuana, B.C., Mexico. Adrian Ochoa-Teran, email: [email protected], Centro de Graduados e Investigación en Química Instituto Tecnológico de Tijuana, Blvd. Alberto Limón Padilla s/n, Mesa de Otay, CP 22500, Tijuana, B.C., Mexico
1
HIGHLIGHTS Carbamoyl benzoic acids were tested as coagulation-flocculation new agents for industrial wastewater containing heavy metals. Zeta potential measurements is a powerful tool to evaluate new products for heavy metals removal from aqueous solutions. Integration of supramolecular chemistry to heavy metals removal as a new alternative in wastewater treatement.
ABSTRACT The use of carbamoyl benzoic, acids as separation new agents, to remove on the hazardous heavy metals, that are frequently, contained in metal plating wastewater is reported in this paper. The interaction of ionic heavy metals and carbamoyl benzoic acids was studied approaching zeta potential in aqueous solutions. These carbamoyl acids are innovative chemicals that demonstrate their potential to bind metallic ions as Pb2+, Cu2+ and Hg2+ under specific physicochemical conditions (pH solution and metal-ligand relationship). The amide substituent in each compound induced a specific affinity toward the metallic ions contained in water, and it was studied by measuring zeta potential of an inert surface in contact with the aqueous media. This measurement allowed us to determine the association constants (Kf) with
metal ions after defining the stronger interaction between the carbamoyl
acids and metallic cations. These results suggest that coagulation-flocculation process could be expected for Pb2+ over all the studied metals (Cu2+, Hg2+, Ca2+,
2
Fe2+, Ni2+, Mg2+, Ba2+, Al
3+
and Cd2+). The order of effectiveness of the carbamoyl
acids for the separation of Pb2+ is b > c > a according to substituent and the calculated values of Kf which match the observed behavior. To corroborate the practical use of carbamoyl acids as coagulants. Sedimentation and clarification kinetics
were
analyzed
with
transmittance
measurements
versus
time.
Coagulation-flocculation test was validated by SEM-EDS analysis.
Keywords: Carbamoyl acids, wastewater treatment, heavy metals, coagulationflocculation, zeta potential
1. Introduction Water quality assessment is done through a series of tests designed to quantitatively and qualitatively determine the most important physical, chemical and biological characteristics that may affect its actual and potential uses, as well as the type and degree of treatment required. The wastewater treatment industry is one
of
the
most
important
issues
in
the
environmental
area,
where
physicochemical studies may lead to a better understanding and support decisionmaking regarding the economy and efficiency [1]. Important issues in wastewater systems are the operation and process units recommended to separate pollutants. In particular, separation processes as chemical precipitation are commonly preferred because they are less expensive [2]. However, when heavy metals are the pollutants, their concentration is the defining factor to choose the correct procedure and their separation is one of the
3
biggest
challenges
for
environmental
research.
Different
techniques
as
electrochemical and biological treatment, precipitation, flocculation, ion exchange, reverse osmosis, adsorption with modified natural materials [3] and membrane filtration processes [4] can be considered for separation and elimination of metals contained in wastewater [5-6]. These techniques are mostly used for removing metals, but they are not always efficient and they produce byproducts, hindering the separation of metals. Reactive membranes, instead, are effective to remove metal ions but they are still expensive [7-8]. Development of new methods to improve primary wastewater treatment efficiency recall the use of new eco-friendly chemicals [9], ensuring hazardous heavy metals removal and avoiding dangerous operations. The use of new compounds containing carbamoyl groups in their structure is of great interest, since these compounds have potential to bind [10], precipitate and flocculate heavy metals. Some of the known applications of carbamoyl benzoic acids are in medicinal chemistry [11], microbial sensors [12], electrochemistry sensors [13] and environmental metal ions sensors. However, there are no reports related with the precipitation and removal of metal ions contained in industrial wastewater. In a previous work authors reported significant spectral differences in carbamoyl benzoic acids as function of the substituent in the amide which leads to an increase or decrease in electron delocalization, as well as fluorescence intensity [10]. Moreover, photophysical studies of various heavy metals in the presence of carbamoyl acids showed its potential application in the detection and separation of metal ions in aqueous media [14,15].
4
Coagulation-flocculation process is appropriate for pollutants removal. Flocculants generally used synthetic flocculants however are non-biodegradable [17, 18]. It is important to the development of new biodegradable materials with high efficiency in the coagulation-flocculation process without adding other toxic additives. In this research, a series of carbamoyl benzoic acids 2a-c were evaluated with environmentally important metal ions as Pb2+, Cu2+, Hg2+, Ca2+, Fe2+, Ni2+, Mg2+, Ba2+, Al
3+
and Cd2+ to accomplish the supramolecular complex
formation in aqueous solution followed by its precipitation. These compounds showed three types of interaction as chemical precipitation, electrostatic interactions and physic entrapment. Focusing on the electrostatic interaction of ionic heavy metals and carbamoyl benzoic acids authors propose to approach it by zeta potential measurements.
For this purpose, it was considered that zeta
potential (ζ) measurement is a physicochemical parameter to characterize solidliquid dispersions found in wastewater [11]. A particular case was the study of water treatment from the tortilla industry, where zeta potential measurement let to show important electrostatic and Van der Waals interactions between colloidal matter and a natural coagulant agent like chitosan [16]. Knowing that a surface charge measurement allow to predict the interaction between an ionic media and a solid, zeta potential measurement using the an inert surface in contact with different ionic media may allow to predict the interaction between the of carbamoyl acids with heavy metals contained in aqueous solutions. In this paper, three different carbamoyl acids were used to determine the effect of aqueous media pH, the substituent on the amide group of the carbamoyl acids, as well as the study of
5
ten different metals to determine their affinity with carbamoyl acids through the estimation of constant association, Kf. In addition, the supramolecular compound formed between carbamoyl acids, Pb was evaluated by sedimentation kinetics, metal ion content in supernatant, and morphological and elemental composition of flocs (formed solids).
Materials and Methods 2.1 Materials Deionized water (resistivity: 18.2 MΩ, MilliQ. Advantage A10) was used for sample preparation in all experiments, 3,3´,4,4´-benzophenonetetracarboxylic dianhydride, naphthalen-2-amine, naphthalen-2-ylmethanamine and benzylmethyl amine were purchased from Sigma Aldrich in highest available purity (> 99%). All solvents were of spectroscopic or HPLC grade. Zeta Potential data was recorded on a Stabino Particle Charge Mapping. The measurements were done at room temperature in porcelain cuvettes. Sample solutions used to study the pH dependence of the potential zeta were prepared adjusting to the desired pH, with 0.1M NaOH and 0.1M HCl. The effect of metal cations upon the zeta potential was examined by adding a few microliters of stock solution (0.1% w/w) of the study metal cations to a known volume of the solution (10 mL). The addition was limited to 0.3 mL, avoiding a dilution effect [19]. 1H NMR spectra were obtained in a Bruker 400 MHz NMR Spectrometer at a probe temperature of 25 ºC with TMS as the internal standard. For Pb-relative estimations, the samples were examined in a SEM ZEISS EVOMA15, equipped with a EDS (energy dispersive spectroscopy) BRUKER detector. Metal ion concentrations were determined using Agilent Technologies 4200 MP6
AES. Various carbamoyl acids additions to the synthetic wastewater were submitted to a sedimentation kinetic study by transmittance measurements in a Turbiscan Lab® equipment. 2.2 Profiles ζ vs pH for the compounds 2a-c The potential zeta profiles of carbamoyl acids identified as 2a-c analogues were performed in order to evaluate the amine influence over the their surface charge characteristics. The measurements were done at room temperature in porcelain cuvettes. Influence of pH on the zeta potential behavior of 2a-2c (0.1%) derivatives was studied within a pH range of 2-11 with 0.1 M NaOH and 0.1M HCl. 2.3 Affinity study compounds 2a-2c with different metal ions The properties of carbamoyl acids in the presence of different metal cations (Cu2+, Pb2+, Hg2+, Ca2+, Fe2+, Ni2+, Mg2+, Ba2+, Cd2+ and Al3+) were studied in aqueous solutions exploring their application as sensors and new agents of separation for these environmental important metal ions. The compounds were titrated by successive increment of equivalent number of these metals and the changes in the zeta potential were measured. 2.4 Coagulation-Flocculation test Chemical coagulation was evaluated using the micro-Jar test technique [11]. The chemical reagents used were Lead, Copper and Mercury perchlorates from Sigma Aldrich. Dosage tests were performed in 20 mL vials. Progressive additions of 0.1 g/L carbamoyl benzoic acids solution were made. After each addition, the vials were shaken for 2 min and allowed to settle for 2 min. Finally, the supernatant was
7
withdrawn to determine the metal ions concentration. The chemical reagents used are high-purity standards, and quality control standard. 2.5 Turbiscan Lab® Expert stability analysis Turbiscan Lab® (Formulaction) is a tool for monitoring the stability or phase separation tendency of dispersions. The measurement principle for this technique is based on detecting the changes in transmission and backscattering, as a function of particle movements (for these compounds sedimentation). The tests were carry out by adding the best dose of each carbamoyl acid (2a-2c) solution with 0.1 % Pb2+, performing scan every 25 s for 30 min. 2.6 Synthetic water with heavy metals preparation The synthetic water solution was prepared by dissolving the appropriate amount of Pb2+, Ni2+, Cr2+, Ca2+, Cu2+, Cd2+ and Zn2+ as perchlorate salt powder (SigmaAldrich) in 1000 mL of MilliQ water. The solution had a 500 ppm final concentration and the pH value was 2.5.
2. Results and Discussion 3.1 Electrokinetic behavior of the compounds 2a-2c The pH influence on the zeta potential behavior of 2a-2c derivatives was studied within a pH range of 2-11. The zeta potential profiles as function of pH are shown in Figure 1. It is observed that the three compounds presented similar behavior in the response of zeta potential. These compounds have a negative surface charge at pH > IEP (isoelectric point). However, compounds 2a-c have different limit ζ values of
8
-130, -120 and -6 mV respectively. These magnitudes are attributed to the type of substituent (naphthyl, naphthylmethyl and benzyl) as well as the functional groups presented in the structure (amide and carboxylic acid), which give them the suitable electric potential for removal of metal ions. The isoelectric point (IEP) of the compounds 2a-c are pHIEP 3.2, 2.7 and 3.6, respectively. These values suggest that at wastewarer pH>pHIEP electrostatic interactions between metallic cations and carbamoyl benzoic acids will be present. On the other hand the pKa values for compound 2a-2c were calculated as 5.2, 5.5 and 5.1 (Excel Worksheets for Spectrometry), respectively [21]. The pKa values are within expected for organic acids. When pH is equal to pKa, the half of the molecules are ionized by deprotonation of the carboxylic acid. If pH>pKa then increases the degree of ionization and the probability of interaction with the cation is greater.
Figure 1
3.2 Influence of metal cations on the potential zeta of the derivative 2a-2c The carbamoyl acids 2a-2c in the presence of different cations (Cu2+, Pb2+, Hg2+, Ca2+, Fe2+, Ni2+, Mg2+, Ba2+, Cd2+ and Al3+) were studied. These compounds were titrated by successive additions of cation equivalents and the changes in the zeta potential were measured. The addition of Ca2+, Fe2+, Ni2+, Mg2+, Ba2+ and Cd2+ to compound 2a in aqueous solution did not cause significant variation in the zeta potential. The addition of Pb2+, Hg2+ and Cu2+ ions produce a significant enhancement in the zeta potential profile pH= 6.6 (Figure S1).
9
The addition of cations to compound 2b in aqueous solution neither cause significant variation in the zeta potential with Mg2+ and Ba2+, Ca2+, for the other hand, Ni2+ and Cd2+ showed an intermediate effect. Instead, the addition of Pb2+, Hg2+, Fe2+, Cu2+ and Al3+ ions induces a significant decrease in the zeta potential profile (Figure S2). In contrast, 2b shows a different behavior, since the zeta potential values retain their negative charge with Ca2+, Fe2+, Ni2+, Mg2+, Ba2+, Cd2+ and Al3+ ions; while with Cu2+, Pb2+ and Hg2+. The zeta potential values are negative in the first addition (~0.5 eq), and beyond the third addition of metal ion, values passed from negative to positive. Initially, the surface charge is equivalent to -93 mV, indicating the presence of negatively charged monomer in the solution. Considering the positive charge of metal ions in wastewater, it was expected that Pb2+ neutralizes the negative charge of the monomer. The charge neutralization of the compound surface makes possible the cations removal. Due to compounds 2ac are negatively charged in the pH> 3 and this enhances its effectiveness for interaction with cations, it is not necessary a neutralization step or pH adjustment that normally are required with conventional coagulants-flocculants agents in wastewater treatment plants. In the addition of cations to compound 2c, in aqueous solution, three response trends are observed: in the first one Ca2+, Ni2+, Mg2+ and Ba2+ did not cause significant variation in the zeta potential, suggesting that chemical affinity between carbamoyl compounds are not enough to form a coordination complex; the second one correspond to the addition of Pb2+, Cu2+, Cd2+, and Fe2+, which induce a slight decrease in zeta potential; and the third response trend corresponds to Hg2+ and Al3+ ions with a marked negative pattern zeta potential profile (Figure S3). In the 10
titration of compound 2c, the measured surface was always negatively charged. The ditopic nature of these molecules suggest a supramolecular arrangement in which the metal is coordinated between two molecules. The zeta potential decreases because of these aggregates behave as particles, having a positive surface charge and as consequence it must be surrounded by a layer of perchlorate counter ions, this layer may correspond to the Stern layer. The zeta potential decreased indicating the formation of a particle or aggregate as supramolecular metal salt. The cations with greater affinity interact with all available carbamoyl benzoic acids and are coordinated to form the aggregate. Furthermore the potential increases because there are more positive ions between the Stern layer and the slipping plane causing that the zeta potential becomes more positive due to the excess of metallic salt added [1]. The compounds based on carbamoyl benzoic acids demonstrated high affinity toward Pb2+, Hg2+ and Cu2+ in water solution even in the presence of other metallic ions. The determination of binding constants for the complexes with metal ions and 2a2c were determined with Equation S1 and they are presented in Table S1. These results show great stability of the complexes even in aqueous media, because of the values obtained are the range of 104 to 107. The profiles indicated a metalligand 1:1 ratio for the most complexes. The 1:1 stoichiometry found for the complex by zeta potential measurements although the receptors have a ditopic structure reinforces the proposal on the formation of supramolecular aggregates, where a series of molecules are successively connected through the interaction with metal centers (Figure S4). 3.3 Coagulation-Flocculation test 11
A coagulation-flocculation test was implemented with Pb2+ due to during the pZ titration protocol was observed a sedimentation process only with this metal ion. The Figure 2 shows the variation of Pb2+ residual content in a flocculation process using the compounds 2a-2c. The experiment started at 100 ppm of Pb2+ in the solution. The optimal dose of the compound 2a-2c were 150 ppm, 100 ppm and 125 ppm, respectively. The physicochemical properties of the compounds were previously studied [10]. The fluorescence and NMR changes were monitored and the Job´s study confirm the stoichiometry 1:1 of 2b with Pb2+. With compounds 2b and 2c were observed that the complex with Pb2+ precipitates suggesting high chemical stability and very low solubility in aqueous media. It is believed that the carbamoyl benzoic acids have large cavities in their binding sites which favors a great interaction with metal ions and a hydrophobic microenvironment generates the solid-state formation. Interestingly, due to the physicochemical characteristics of the compounds and their complexes, the flocculation process does not occurs with other metal ions, however they may be used in conjunction with some natural coagulants to facilitate this process. The effectiveness of the compounds for the separation of Pb2+ was in the order b>c>a and it is agreement with the Kf calculated values. The removal efficiency achieved with these compounds is defined by different factors such the affinity in the complex formation studied by zeta potential and once it is formed, the hydrophobic effect and additional intermolecular forces that defining the solubility in the aqueous medium and particle size. The compounds b and c have a methylene spacer that encourages greater interaction with the metal because there is less steric hindrance in
12
comparison with 2a. In the compound 2a naphthalene group is directly attached to the amide group and the complexes take longer to reach the equilibrium.
Figure 2
The transmittance profiles of complexes formed with 2a and 2b and Pb2+ are shown in Figure 3. The data showed that these complexes have the same behavior. Significantly, for 2b-Pb2+ complexes the precipitation rate increased compared to 2a-Pb2+. In these complexes a hard to see (very fast) flocculationcoagulation kinetic occurred, and only the phenomenon of flocs migration was observed.
Figure 3.
The transmittance profile of complex 2c-Pb2+ is shown in Figure 4. This data demonstrated that occurs the coagulation-flocculation process, which confers a potential value to 2c for using in Pb2+ removal in contaminated wastewater. On the contrary, to the behavior of compounds 2a and 2b, compound 2c presents the coagulation-flocculation phenomena. Therefore, these compounds showed to have potential applications for the detection and separation of these metals in wastewater.
Figure 4
13
The SEM-EDS images and data obtained of the complexes 2a-Pb2+ and 2b-Pb2+ demonstrate the presence of microspheres in the sediment dry flocs containing C, O and Pb (Figure 5). The morphology of dry flocs from compounds 2a and 2b show similarity, however for 2c-Pb2+ flocs a more porous texture is observed. EDS study revealed that C, O and Pb were abundant in the sediments. Highest wt% (average of all scanner areas) of lead was found for 2b (55.18%) followed by 2c (53.16%) and 2a (52.60%). These results confirm the data obtained in coagulationflocculation test of metal ions. The SEM analysis gave the strongest evidence of the formation of supramolecular aggregates with Pb2+ opening a new field in wastewater treatment using carbamoyl benzoic acids.
Figure 5 The physicochemical conditions for Pb2+ removal was summarized in Table 1. The physicochemical studies revealed that 2b presented the greater Pb2+ removal capacity (95%), followed by 2c (83%) and 2a (82%). The molar ratio of removed Pb2+/compound was 1 ± 0.3, confirming the 1:1 stoichiometry found in the pZ studies. According with the clarification kinetics and sedimentation rates, compound 2c showed best performance in the coagulation-floculation process followed by 2b, although both compounds were very effective in the separation. Finally, compound 2a has the lower clarification kinetic and sedimentation rate and a bigger amount of compound is required to remove Pb2+ (~2 g of 2a/g Pb2+).
Table 1
14
Considering that the goal of water treatment processes is to encourage the instability of particles containing the undesired heavy metals, the coagulationflocculation kinetic stability was studied for compounds 2a-c and the results are shown in Figure S8. The asymptotic behavior of 2a-Pb2+ reveals the high instability of this complex in the aqueous media since the first minutes. Then, the formation of flocs is faster with 2c in comparison with 2b and 2c. These result agree with the of the clarification kinetic magnitude for these complexes. The expectative is a faster separation and more unstable particles with 2c in aqueous media and with good effectiveness in Pb2+ removal. The instability of the supramolecular aggregates in aqueous media was in to order 2c > 2b > 2a.
Finally, the optimization of the geometries of 2a-c and their respective complexes with Pb2+ (L2Pb) were performed by theoretical calculations using the density functional theory (DFT) and the non-local correlation PBE0 with a DZVP basis set in gas phase [22-26], in order to compare the chemical stability considering the formation electronic energies and the structure of the complexes, and related them with the results in the coagulation-flocculation process (Figure S9). The calculated energies were -480.52 kcal/mol, -483.09 kcal/mol and -488.937 kcal/mol for 2a, 2b and 2c complexes, respectively. Although there is not a marked difference in the energies, the values predict a stability order as follows 2c > 2b > 2a. According with the optimized structures, the flexibility of the N-substituents allows to stablish additional intermolecular interactions as π-π and C-H···π in the complexes with 2b and 2c which are not present with 2a. These intermolecular forces may contribute
15
in the chemical stability of the complexes creating a hydrophobic environment which drives the flocs formation, compaction and precipitation in aqueous media.
Conclusions The pZ studies demonstrate that carbamoyl benzoic acids a-c have a strong interaction metal ions and high affinity toward Pb2+, Hg2+ and Cu2+ in aqueous media. The stoichiometry of the complexes stablished by zeta potential profiles was 1:1 for the most metal ions suggesting a supramolecular arrangement where two ligand units are connected through a metallic center bridge. The binding constants were estimated considering this stoichiometry and the values were in the range of 105 to 107 M-1 which are high values for an interaction in aqueous media. The SEM analysis of dry flocs gave the evidence of microsphere aggregates formation with 2a and 2b, and other irregular structures with 2c. EDS study of dry flocs revealed the highest wt% of Pb found in 2b (55.18%), followed by 2c (53.16%) and 2a (52.60%). In the coagulation-flocculation study using a 100 ppm Pb2+ solution, the optimal dose of 2a-c were 150 ppm, 100 ppm and 125 ppm, respectively. In addition, 2c presented the best physicochemical characteristics with the greater clarification kinetics (523.4 %/h), an acceptable sedimentation rate (17.4 mm/h) and good removal of Pb2+ (83%). Compound 2b presented a comparable performance with a clarification kinetics of 146.9 %/h, and the highest sedimentation rate 27.5 mm/h and removal of Pb2+ (95%). Therefore, carbamoyl benzoic acids show potential applications for the detection and separation of these metal ions in wastewater industrial opening a new field in wastewater treatment.
16
Acknowledgements The authors gratefully acknowledge to Consejo Nacional de Ciencia y Tecnología (CONACyT) for the support the following grants: Basic Science 2015 Grant No. 237032, National Problems Project Grant No. 247236, Postdoctoral Research Grant and Supramolecular Thematic Network Grant No. 271884. Authors also thanks to Tecnológico Nacional de México/Instituto Tecnológico de Tijuana for its support. The technical assistances of Ricardo Valdez Castro and José García Elías are highly appreciated. José-Zeferino Ramírez thanks to the area of high performance computing at the University of Sonora (ACARUS) for the access to the computational resources.
Appendix A. Supplementary data References [1]
E.A.
López-Maldonado,
A.
Ochoa-Terán,
M.T.
Oropeza-Guzmán,
A
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20
0
2
4
6
8
10
50
2a
0 -50 -100 -150
2b
ζ (mV)
0 -50 -100 -150 2
2c
0 -2 -4 -6 -8 2
4
6
8
10
pH
Figure 1. Profiles the ζ = f (pH) of 2a, 2b and 2c water/DMSO (9:1, v/v). Concentration is 0.1% w.
21
50
0
50
100
150 2a
40
2+
Pb Residual (mg/L)
30 20 15 2b 10 5 0 26
2c
24 22 20 18 16 0
50
100
150
Flocculant dose (mg/L) Figure 2. Coagulation-Flocculation test to be using the compounds 2a-c in the treatment of wastewater synthetic of Pb2+.
22
2a 15
2a
T (%)
10
5
0 0
10
20
30
40
Height (mm) 100
2b 2b
80
T (%)
60
40
20
0 0
10
20
30
40
Height (mm)
Figure 3. Transmitance profiles of compounds 2a y 2b at optimum dose, data are reported as a function of time (0-30 min).
23
2c
2c
90 80 70
T (%)
60 50 40 30 20 10 0
0
10
20
30
40
Height (mm)
Figure 4. Transmitance profile of compound 2c at optimum dose, data are reported as a function of time (0-30 min).
24
2a Element
Wt %
C
22.12
O
25.27
Pb
52.60
2b Element
Wt %
C
23.01
O
23.82
Pb
55.18
2c
Figure 5. SEM-EDS analyses of sediment. 25
Element
Wt %
C
22.47
O
22.34
Pb
53.16
Table 1. Optimal physicochemical conditions for the Pb2+ removal of synthetic water with.
Compound
%
mol Pb2+/ mol
Clarification
Sedimentation
Removal
compound
Kinetics*
ratio* mm/h
%/h
2a
82
0.71
81.3
3
2b
95
1.3
146.9
27.5
2c
83
0.96
523.4
17.4
*30 min of scanning
26
Graphic Coagulant-Flocculant agent
2a 2a
2b 2b
2c2c
Heavy metals wastewater
Flocs
27
HIGHLIGHTS
Carbamoyl benzoic acids as separation new agents for industrial wastewater. Zeta potential a powerful tool to determine the performance of separation new agents. Supramolecular chemistry in water treatment using carbamoyl benzoic acids.
29