Removal of boron from aqueous solution using magnetic carbon nanotube improved with tartaric acid
© Zohdi et al.; licensee BioMed Central Ltd. 2014
Received: 24 February 2013
Accepted: 6 October 2013
Published: 6 January 2014
Boron removal capacity of multi-walled carbon nanotubes (MWCNTs) modified with tartaric acid was investigated in this study. Modification of MWCNTs with tartaric acid was confirmed by Boehm surface chemistry method and fourier transform infra-red (FT-IR) spectroscopy. Experiments were performed to determine the adsorption isotherm and adsorption thermodynamic parameters of boron adsorption on tartaric acid modified MWCNTs (TA-MWCNTs). The effect of variables including initial pH, dosage of adsorbent, contact time and temperature was investigated. Analysis of data showed that adsorption equilibrium could be better described by Freundlich isotherm and the maximum adsorption capacities obtained at the pH of 6.0 was 1.97 mg/g. The estimated thermodynamic values of free energy (ΔG°), entropy (ΔS°) and enthalpy (ΔH°) indicated a spontaneous and an endothermic process. Furthermore, the TA-MWCNTs was magnetized for separation of boron-contaminated adsorbent from aqueous solution by applying magnetic field. The results showed that magnetic TA-MWCNTs particles were separated effectively after adsorption from contaminated water.
Water with less impurities and contaminants is essential to the human’s life. In general, water pollution is the introduction of physical, chemical and biological substances into the water bodies that spoils the purity of water and it will cause hazardous effects on living species that consume it. Boron (B) is one of the elements that can cause the lethal in case of more than 640 mg/kg body weight oral intake according to the world health organization (WHO) report [1, 2].
In aqueous solution, boron is normally present as borate anions B(OH)-4 and boric acid B(OH)3. The dominant form of inorganic boron in acidic aqueous systems is the un-dissociated boric acid. On the other hand polyborate anionic species including B5O6(OH)4-, B3O3(OH)4, B3O3(OH)52- and B4O5(OH)42- form in high concentration solutions (>0.025 mol/l) at a neutral to alkaline pH (pH 6 to 11) [3, 4].
High boron concentrations can be found in wastewater of some industries including semiconductor, ceramic, pesticides, fire retardants, borosilicate glass, nuclear power and detergent manufacturers. Many investigations has been done for boron removal from water and wastewater by different methods such as coagulation , coprecipitation , adsorption, ion exchange using cation exchangers , solvent extraction , membrane operations  and adsorption .
For treatment of boron in aqueous solution through adsorption process, different materials have been used as adsorbent such as activated carbon (AC) , fly ash , resins , metal oxides , clay materials , and composite magnetic particles .
Multi-walled carbon nanotubes (MWCNTs) are known as an effective adsorbent for removing contaminants such as various metals and heavy metals , dyes  and organic materials  from water and wastewater. The most important characteristics of MWCNTs are large specific surface area, well developed mesoporous and hollow structure and light mass density which make it an efficient adsorbent of pollutant molecules. Furthermore, MWCNTs have the advantages of easy removal and regeneration after contaminant adsorption . Although, many researches have studied the capability of MWCNTs as an adsorbent of aqueous solution pollutants, however, adsorption capacity of boron onto MWCNTs has not been investigated so far.
The oxygen content of MWCNTs influences the maximum adsorption capacity. The oxygen functional groups such as –OH, –C = O, and –COOH can be generated on the surface of MWCNTs through covalent and noncovalent modification. MWCNTs were modified under the oxidizing condition with different chemicals such as HNO3, H2O2, KMnO4, NaClO, KOH, NaOH and citric acid [20, 21]. Recent investigations showed that by using organic weak acids such as citric acid (C6H8O7), the functionalities can be produced without the negative signs depicted by the use of inorganic strong acids beside eliminating the refluxing step during the functionalization . Due to the mild and safe reaction which organic materials can have and also the abovementioned advantages of these materials comparing to inorganic acids, the focus of this investigation is on MWCNTs modification using organic acids like tartaric acid (C4H6O6).
The influence of some parameters such as initial solution pH, dosage of adsorbent, initial boron concentration, contact time and temperature on boron adsorption behavior of tartaric acid modified MWCNTs (TA-MWCNTs) was studied in this work. TA-MWCNTs were modified with iron oxide particles for further magnetic separation of contaminated adsorbent.
Materials and methods
Boron stock solution was prepared by adding 5.71 g of boric acid B(OH)3 (Systerm Co.) into 1000 ml double distilled water. Azomethine-H monosodium salt hydrate >95% was purchased from Sigma Aldrich and L(+) ascorbic acid, ammonium acetate, glacial acetic acid, acid disodium salt-dihydrate (EDTA), mercaptoacetic acid 98% were purchased from Systerm Co. MWCNTs (diameter of 20–40 nm and minimum purity of 90%) and L(+)-tartaric acid >99.7% (C4H6O6) were purchased from Hangzhou Dayang Chem Co. and Sigma Aldrich Co. respectively. Ammonium iron (II) sulfate hexahydrate (NH4)2Fe(SO4)2 · 6H2O and hydrazine hydrate (N2H4) were received from Systerm Co. and R&M Chemicals. All the reagents used in this study were analytical grade.
Modification of MWCNTs with tartaric acid
To find the optimum amount of the tartaric acid for modification of MWCNTs and to obtain the highest functional groups loading, 10 g of MWCNTs was mixed with 50 ml of four different concentrations of aqueous solution of tartaric acid (0.5, 1, 1.5 and 2 M). The mixtures were subjected to ultrasonic bath for 15 min mixing and then left to be dried and forming a paste followed by keeping the samples in the furnace for 30 min at 300°C. It is worth to mention that because of the decomposition of the excess of tartaric acid after increasing the temperature up to 300°C, further filtration step and washing of tartaric acid modified MWCNTs was not required .
The LECO TruSpec CHNS-O elemental analyzer was used for preliminary comparison of the oxygen content and carbon content of MWCNTs and TA-MWCNTs samples. By comparing the oxygen contents, a preliminary evaluation of optimum required amount of tartaric acid was possible and the sample with the highest oxygen content was selected for further surface chemistry analysis and proceeding the entire work. The Boehm titration technique  and solid addition technique  were employed to study the surface chemistry and point of zero charge (pHpzc) for MWCNTs and TA-MWCNTs.
To investigate the formation of carboxylic and carbonyl functional groups on TA-MWCNTs, the FT-IR spectra were obtained for MWCNTs and TA-MWCNTs in KBr pellet form with AEM Thermo Nicolet FT-IR collected at a spectrum resolution of 4 cm-1 with 32 co-added scans over the range from 4,000 to 400 cm-1.
To evaluate the specific surface area of MWCNTs before and after modification, Brunauer Emmett Teller (BET) analysis was used. After subjecting the samples with nitrogen gas for 9 hours of operation at 290°C of outgas temperature, the Quantachrome AS1Win™ surface area analyzer was used for measuring the surface area.
Transmission electron microscopy (TEM, Philips HMG 400) was also used to characterize the microstructure of MWCNTs before and after modification.
Preparation of magnetic TA-MWCNTs
To modify the TA-MWCNTs surface with iron oxide particles , 6 g of ammonium iron (II) sulphate hexahydrate was dissolved in 200 ml of distilled water and hydrazine hydrate (volume ratio of 3:1). Then, 2.5 g of TA-MWCNTs was added into the solution. The pH of the mixture was adjusted to 11–13 and then the mixture was sonicated and stirred vigorously for about 15 min. After sonication, the mixture was refluxed for 2 hours. Finally, magnetic TA-MWCNTs was washed several times with distilled water using a filtration system until the pH of the solution became neutral. Then the magnetic TA-MWCNTs was dried under vacuum at 65°C for 24 hours.
The prepared magnetized sample was characterized using X-ray diffraction technique (XRD) using a Philips PW 3710 type diffractometer and FT-IR (AEM Thermo Nicolet). Morphology and chemical composition of the samples were studied using Energy-dispersive X-ray spectroscopy (EDX), and the scanning electron microscopy (SEM) using SEM-EDX Hitachi S-3400 N.
The effect of pH on boron adsorption was investigated using boron stock solution (20 mg/l) with pHs varied from 2–11. A specific amount of 0.32 g/l of TA-MWCNTs was added to boron stock solutions. The samples were kept in the shaker for 3 days in the room temperature (25°C) to reach equilibrium. The boron uptake on TA-MWCNTs was measured using UV–vis spectrophotometer (Double beam, Halo DB 20S) through Azomethine H method [10, 26].
To study the effect of adsorbent quantity on boron adsorption, 6 samples of boron stock solution (20 mg/l) with pH of 6 were prepared. The TA-MWCNTs in various concentrations (0.16 to 0.56 g/l) were added to boron stock solutions and kept at shaker at 25°C for 3 days. The boron adsorption of mixtures was then measured using UV/Vis spectrophotometer.
The contact time and initial boron concentration dependent experiment was carried out with different stock solutions of initial boron concentrations varied from 2 to 40 mg/L. The solution was mixed with 0.1 g TA-MWCNTs and kept in a shaker for 24 h at room temperature. The boron adsorption of samples was measured in different time intervals using UV/Vis spectrophotometer.
Thermodynamic of the study was investigated in three different temperatures of 303, 313 and 323 K. All effective parameters of experiment which were obtained from optimal conditions of previous parts of the study were used to adjust the condition in temperature dependent experiment.
Adsorption behavior of magnetic-MWCNTs was investigated using the same method under the optimal adsorption conditions evaluated in the experiments.
Where qe (mg/g) was equilibrium uptake, C0 and Ce (mg/l) denoted the initial and equilibrium concentrations of boron in aqueous solution, V was the total volume of the solution in liters and W was the mass of the adsorbent in grams.
Results and discussion
Characterization of MWCNTs and TA-MWCNTs
Oxygen percentage for TA-MWCNTs modified with different concentrations of tartaric acid obtained from CHNS-O analysis
Tartaric acid concentration (M)
Oxygen content (wt.%)
Surface chemistry of MWCNTs and TA-MWCNTs derived from boehm titration
Characteristics of MWCNTs and TA-MWCNTs
Values for MWCNTs
Diameter: 30–50 nm
Diameter: 30–50 nm
Effect of initial solution pH
Effect of initial dosage of adsorbent
Effect of contact time and initial boron concentration
For MWCNTs (Figure 6a), a constant boron adsorption capacity was recorded with increasing contact time of adsorbent and initial boron concentration. The small numbers of vacant sites on the surface of MWCNTs were occupied at the very initial stages and sorption capacity became constant in a very short duration.
For TA-MWCNTs (Figure 6b), the equilibrium point for all concentrations was around 60 minutes. The maximum adsorption happened at 40 mg/l of boron concentration and the amount was 1.97 mg/g.
From the results, it can be concluded that the optimum conditions of boron removal was achieved by using 0.4 g/l of TA-MWCNTs in temperature of 25°C, at pH of 6 and with 40 mg/l initial boron concentration after 60 min of contact time.
Where kl (L/mg) and al (mg/L) are two Langmuir isotherm constants which are obtained from the intercept and slope of the linear plot of equation 2.
Where Kf is the Freundlich constant or capacity factor (mg/g) and 1/n is the Freundlich exponent.
Isotherm parameters for the adsorption of boron on TA-MWCNT
Tartaric acid m-MWCNT
From the results presented in Figures 7 and 8 and Table 4, it can be concluded that the value of exponent n is in the range of desirable adsorption (1 < n < 10). The higher values of linear correlation coefficients (R2) for boron uptake (0.9943) revealed that the Freundlich model can be used to describe the boron adsorption behavior comparing to the Langmuir model.
Where CAe is the amount of boron adsorbed (mmol/g), and Ce is the equilibrium concentration of boron in the solution (mmol/l).
Thermodynamics parameters for boron adsorption on MWCNTs and TA-MWCNTs
As presented in Table 5, the ∆G° values are negative and the ΔH° values are positive for TA-MWCNTs and MWCNTs. These results demonstrated that the adsorption of boron on MWCNTs and TA-MWCNTs was spontaneous and was an endothermic process. The decrease of negative values of ΔG° with the increase of temperature for MWCNTs indicated more efficient adsorption at higher temperature. On the other hand, for TA-MWCNTs since the negative values of ΔG° increased with increase in the temperature, the spontaneous adsorption process should be less efficient at higher temperatures.
Characterization of magnetic TA-MWCNTs
Adsorption study of magnetic TA-MWCNTs
From the results after 60 min at equilibrium time boron adsorption capacity of magnetic TA-MWCNTs was measured to be 1.91 mg/g. However, by comparing the qe values for TA-MWCNTs (1.97 mg/g) and magnetic MWCNTs it is perceived that a negligible decrease in boron adsorption capacity of TA-MWCNTs occurred after magnetizing. The decrease in boron adsorption capacity could be related to the occlusion of mesopores of TA-MWCNTs by iron oxide particles which led to less bonding of boron with oxygen functionalities anchored within the mesopores.
Separation of contaminated magnetic TA-MWCNTs from aqueous solution
The MWCNTs was impregnated with tartaric acid to improve the boron adsorption capacity. The characterization techniques proved forming oxygen functionalities such as carboxylic and phenolic groups on the surface of MWCNTs after modification. Investigation of effective factors on boron capacity of TA-MWCNTs showed that initial solution pH, initial boron concentration, initial adsorbent dosage and temperature had significant effect on boron adsorption. At pH of 6, the maximum adsorption capacity of TA-MWCNTs measured to be 1.97 mg/g under the optimum condition. Also, thermodynamic study showed endothermic, entropy favorable and spontaneous adsorption behavior of boron by TA-MWCNTs. In addition Freundlich model was more suitable to simulate the boron adsorption isotherms than Langmuir model. The TA-MWCNTs was magnetized for separation after adsorption of boron. Magnetic separation of boron contaminated TA-MWCNTs was successfully carried out for further removal from aqueous solution. In conclusion, the modification of MWCNTs with tartaric acid is a successful method for enhancing the adsorption properties of MWCNTs in the removal of boron from aqueous solutions. However, the desorption studies is now of interest and is recommended to be investigated in future works.
The authors would like to thank Universiti Putra Malaysia for financial support of this work (via: vot: 9416900).
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