This study stemmed from consumer complaints about earthy and musty off-flavours in treated water of Rawal Lake Filtration Plant. In recent years, several novel adsorbents have been developed from nanomaterials for enhancing the contaminant removal efficiency. This paper presents preparation and the use of new adsorbents Pt doped titania and Fe doped titania, for the adsorption capacity of Geosmin and 2-MIB from water under laboratory conditions and their comparison, with most widely used activated carbon, under batch and column experiments. Stock solutions were prepared by using Geosmin and 2-MIB standards, procured by Sigma Aldrich (England). Samples were analysed using SPME-GC-FID. The adsorption of Geosmin and 2-MIB on GAC conformed to the Freundlich isotherm, while that of adsorption on metal doped titania fit equally well to both Langmuir and Freundlich isotherms. Moreover, data, generated for the kinetic isotherm, confirmed that Geosmin and 2-MIB removal is a function of contact time. Breakthrough column tests using 125 mg/L Pt doped titania nanoparticles, coated on glass beads against 700 ng/L of off-flavours, attained later breakthrough and exhaustion points and removed 98% of Geosmin and 97% of 2-MIB at room temperature. All columns could be regenerated using 50 mL 0.1 molar sodium hydroxide.
Supply of safe and aesthetically pleasing water, for human consumption, is essential for all treatment facilities. A common and recurrent problem in drinking water is the formation of earthy-musty taste and odor [
The GC standard of Geosmin and 2-MIB were purchased from Sigma-Aldrich (Germany) at a concentration of 2 mg/L and 10 mg/L in methanol, respectively. Stock solutions were prepared in 10 mL methanol. General purpose reagent titanium (IV) oxide (Riedel-De Haen) was used as a source of titania nanoparticles; ferrous chloride and platinum chloride were purchased from Merck, Germany. Other chemicals including GC grade methanol, dichloromethane (DCM), and wood based granular and powdered activated carbon (Norit C1A) were procured from Acros, Organics, USA.
Liquid impregnation method was used to prepare metal doped titania nanoparticles [
Method of metal doped-Titania nanoparticles preparation.
Type of doping | Doped element | Method of preparation |
---|---|---|
Metal doping | Pt | 150 mL deionized water + 25 gm. titania + 1 molar ratio PtCl4 |
Fe | 150 mL deionized water + 25 gm titania + 1.5 molar ratio FeCl2 |
SEM images were measured using JEOL JSM 6460 scanning electron microscope. Microscope operated at an acceleration voltage of 5, 10, and 15 kV and filament current of 60 mA. SEM was performed to find the surface morphology, topography, microstructure, and composition of adsorbents. The porous texture characterization of the samples was obtained by physical adsorption of gases. From N2 adsorption data at −196°C (Autosorb-6B apparatus from Quantachrome [
Geosmin and 2-MIB were analysed using solid phase microextraction coupled with gas chromatograph-flame ionized detector (SPME-GC-FID). One gram of sodium chloride was placed in a 15 mL vial containing 3 mL of the aqueous sample. The vial was sealed with Teflon-lined septum. The samples were heated to 65°C, and the SPME fiber (divinylbenzene-carboxen-polydimethylsiloxane SPME fiber) was inserted into heads space for 15 min equilibrium adsorption period. The fiber was then withdrawn from the sample and injected into the injection port of Shimadzu 2010 series Gas Chromatograph coupled with FID. The column used was fused silica capillary column with a length of 30 m, inner diameter of 0.32 mm, and wall thickness of 0.5
Geosmin and 2-MIB solutions used in this study were prepared in the laboratory by dissolving stock solutions in ultrapure water. This study was aimed at testing adsorbents for their ability to adsorb earthy-musty odorants for the concentrations which are present in lake water. For this reason, sampling was performed from Rawal Lake Filtration Plant (RLFP) in the month of July, when taste and odor episode in treated water was at their maximum levels. RLFP draws raw water from Rawal Lake, Pakistan. Its treatment processes include coagulation, flocculation, sedimentation, sand filtration, and disinfection, which were inadequate to remove taste and odor from water. Average concentration of Geosmin and 2-MIB in treated water samples was 690 ng/L and 678 ng/L, respectively; hence, synthetic solution with 700 ng/L of odorants was prepared for batch adsorption and column adsorption experiments. All reactors were wrapped with aluminium foil during nanoparticle experiments, to restrict photo catalytic activity and to ensure adsorption as the only mechanism of odorant removal.
In this study, adsorbent dose, contact time, and stirring rate per minute for Geosmin and 2-MIB were optimized. For optimization of adsorbent dose for Geosmin and 2-MIB removal, adsorption experiments were performed by varying adsorbents doses between 40 mg/L and 200 mg/L in test solutions, containing 700 ng/L initial odorants concentration. In order to determine the equilibrium adsorption time, flasks containing 700 ng/L initial Geosmin and 2-MIB concentrations with optimized adsorbent dose were agitated on the orbital shaker for different time intervals (5, 15, 30, 45, 60, 75, 90, and 120 minutes). Similarly, test solutions were tested for the best stirring rate (between 25 and 200 rpm), with optimised adsorbent doses and contact time.
To determine the removal efficiency of selected adsorbents, 100 mL of test solution with initial off-flavours concentration of 700 ng/L was taken in 250 mL conical flask at room temperature. 125 mg of the adsorbents (GAC, Fe, and Pt doped nanoparticles) were added in each test solution and stirred at orbital shaker at 100 rpm for one hour. All the samples were stored in dark brown vials, till final concentration analysis. Samples were subjected to filtration through prewashed 0.45
Amount of Geosmin and 2-MIB adsorbed (ng/g) was calculated using [
For adsorption studies, 100 mL of the initial test solution of odorant concentrations, varying between 200 and 1200 ng/L, was taken in 1000 mL volumetric flasks and experiments were performed with all optimized conditions. 125 mg metal doped nanoparticles and 160 mg GAC were added to the test solutions and then placed on an orbital shaker at 100 rpm for 60 minutes and final concentrations were determined, in all removal experiments.
Several models are available to express the mechanism of adsorption of solute onto the adsorbent. For kinetic studies, 500 mg of adsorbent doses was introduced into 1000 mL solutions with initial analytes concentration of 700 ng/L and placed at an orbital shaker at a rotation speed of 100 rpm. Samples were taken at predetermined intervals up to 120 min. The kinetic analysis was carried out at room temperature (
These experiments were designed to investigate efficiency of granular activated carbon and metal doped titania nanoparticles in fixed bed columns. Column design was based on study by Tang et al. [
Schematics of the pilot column setup.
Nanoparticles coated glass beads were used in filter columns. Sterilized glass beads were dipped in 10% hydrogen fluoride solution and covered for 24 hours. Next, glass beads were stirred with metal doped titania nanoparticles aqueous solution, at 250 rpm for half an hour. After oven drying at 110°C for 20 min these coated beads were shifted to furnace at 400°C. Finally these were washed with distilled water, dried, and stored in dark.
Removal efficiency of the adsorption columns was represented by “break through curves” drawn from results, which showed that the concentration ratios (
In this study, after exhaustion of column, desorption study was carried out by pouring 50 mL of 0.1 M NaOH solution as an eluent, which was known to be efficient for column recovery [
Adsorption capacity of the abovementioned adsorbents was tested to remove Geosmin and 2-MIB from natural water samples collected from Rawal filtration plant in batch and column experiments under optimized adsorption conditions.
For the direct examination of particle size and surface morphology of the samples SEM was used. All the particles were spheroid or oblate spheroid loosed and macrospores can be clearly seen in the SEM micrographs. Scans of nanoparticles at ×30,000 resolution are represented in Figure
SEM micrograph of adsorbents at 30,000 resolution: (a) granular activated carbon and (b) Fe-TiO2 (c) Pt-TiO2 nanoparticles.
Adsorption condition optimization results are demonstrated in Figure
Dose, contact time, and stirring rate optimization for adsorbents.
Similar removal trends were observed for both Geosmin and 2-MIB removal, with increasing adsorbent doses. It was observed that, even on increasing the amount of adsorbent doses in the solution, the percentage removal of Geosmin and 2-MIB increased gradually and % removal plateaued at 125 mg/L for metal doped nanoparticles while 160 mg/L was found to be optimized GAC dosage. Adsorption capacities of the three adsorbent types at different contact times, when all other variables are constant, were determined. The maximum adsorption capacity of Geosmin and 2-MIB was obtained within contact time of 60 min, with optimized adsorbents dosage for all adsorbents studied. After optimizing PAC dosage and contact time final experiments were performed to explore optimum mixing rate. Results illustrate that stirring rate had remarkable effects on rate of adsorption as with the increase in speed contact between adsorbent and adsorbate increases. Maximum adsorption of Geosmin and 2-MIB was observed at mixing rate of 100 rpm using optimized adsorbent dozes, within a contact time of 60 min.
Removal efficiencies of the selected adsorbents are given in Table
Removal efficiency of adsorbents used.
Adsorbents | GAC | Fe-TiO2 | Pt-TiO2 | |||
---|---|---|---|---|---|---|
Geosmin | 2-MIB | Geosmin | 2-MIB | Geosmin | 2-MIB | |
% removal | 82 | 76 | 96 | 94 | 99 | 98 |
Removal efficiencies of adsorbents were tested through isotherm studies. The linear form of the isotherm studies helped in better understanding of the adsorption phenomena. Freundlich and Langmuir equation explained adsorption of Geosmin and 2-MIB on different adsorbent concentrations, as discussed in the next sections.
Freundlich equation was used in this study as given below [
From data stated in Table
Freundlich constant parameters for Geosmin and 2-MIB analysis.
Adsorbate | Adsorbent | Freundlich isotherm parameters | ||
---|---|---|---|---|
|
|
| ||
2-MIB | GAC | 0.64 | 3.12 | 0.97 |
Fe-TiO2 | 0.70 | 3.70 | 0.98 | |
Pt-TiO2 | 0.71 | 4.00 | 0.99 | |
|
||||
Geosmin | GAC | 0.65 | 3.10 | 0.97 |
Fe-TiO2 | 0.71 | 4.18 | 0.98 | |
Pt-TiO2 | 0.86 | 4.38 | 0.99 |
(a) Freundlich isotherms curve for adsorption of Geosmin at 18 ± 2°C. (b) Freundlich isotherms curve for adsorption of 2-MIB at 18 ± 2°C.
Mathematical form of Langmuir isotherm is given below [
Langmuir constant parameters for Geosmin and 2-MIB analysis.
Adsorbate | Adsorbent | Langmuir isotherm parameters | |||
---|---|---|---|---|---|
|
|
|
| ||
2-MIB | GAC | 0.62 | 0.80 | 0.71 | 0.98 |
Fe-TiO2 | 0.64 | 1.14 | 0.58 | 0.99 | |
Pt-TiO2 | 0.70 | 1.56 | 0.56 | 0.99 | |
|
|||||
Geosmin | GAC | 0.64 | 0.90 | 0.69 | 0.98 |
Fe-TiO2 | 0.70 | 1.35 | 0.60 | 0.99 | |
Pt-TiO2 | 0.72 | 1.53 | 0.57 | 0.99 |
Value of
Langmuir isotherms curves for Geosmin and 2-MIB are shown in Figure
(a) Langmuir isotherm curve for adsorption of Geosmin at
The shape of the isotherm can be used to examine the favourability of the adsorbent. It was done by another dimensionless quantity,
A pseudo-second-order model (
Integrating (
Thus, the parameters
The kinetic data obtained from batch adsorption tests was analyzed by using the pseudo-second-order model. Figure
Pseudo-second order parameters for Geosmin for and 2-MIB analysis.
Adsorbate | Adsorbent | Pseudo-second-order equation parameters | |||
---|---|---|---|---|---|
|
|
|
| ||
2-MIB | GAC | 0.39 | 0.98 | 0.12 | 0.99 |
Fe-TiO2 | 0.58 | 1.02 | 0.24 | 0.99 | |
Pt-TiO2 | 0.61 | 1.10 | 0.35 | 0.99 | |
|
|||||
Geosmin | GAC | 0.43 | 0.14 | 0.15 | 0.99 |
Fe-TiO2 | 0.62 | 0.09 | 0.23 | 0.98 | |
Pt-TiO2 | 0.67 | 1.04 | 0.37 | 0.99 |
(a) Pseudo-second-order kinetic models for 2-MIB at
Column efficiencies were determined in terms of breakthrough and column exhaustion times. At the beginning, removal through columns was very efficient as the adsorbents were fresh with all of their adsorption sites available. With the passage of time, some of the adsorption sites got exhausted and effluent concentrations started rising. After all the adsorption sites were exhausted, the inlet and the outlet concentrations became nearly the same. Column 1 having granular activated carbon was not found very effective either for Geosmin or 2-MIB. It attained early breakthrough and exhaustion points. Figure
Breakthrough curves for Geosmin and 2-MIB at 18
Overall comparison of adsorbents indicated that Pt-doped titania nanoparticles were most efficient for Geosmin and 2-MIB removal. Geosmin removal was a bit higher than 2-MIB in this case as well.
For a viable sorption process, columns desorption and regeneration are important. The exhausted 12 cm bed volume columns (for all the three types of columns) were successfully regenerated using 0.1 M of NaOH solution. The results (Figure
Adsorption capacities for columns after regeneration.
The above-stated experiments were performed using synthetic solution of Geosmin and 2-MIB. Finally, experiments were conducted with the RLFP treated water samples. Average concentration of Geosmin and 2-MIB in composite water samples was 690 ng/L and 678 ng/L, respectively. Water quality data for water from RLFP is given in Table
Water quality parameters of RLFP treated water.
Quality parameter | Range | Units |
---|---|---|
DOC | 3.3–5.4 | mg/L |
Turbidity | 0.1-0.2 | NTU |
pH | 6.5–7 | — |
Conductivity | 500–700 | S/cm |
NOM | 3.35–5.5 | mg/L |
UV254 | 0.05–0.90 | cm−1 |
Geosmin | 690 | ng/L |
2-MIB | 657 | ng/L |
A comparison of taste and odor removal efficiency using adsorption and Column tests is presented in Figure
Comparison of Geosmin and 2-MIB removal from synthetic and RLFP water samples.
The present experimental results suggest that metal doped titania nanoparticles demonstrate significant adsorption potential for the accelerated removal for earthy-musty odor producing compounds in the drinking water. Study shows that metal doping increases BET surface area, thus enhancing efficiency of titania to adsorb Geosmin and 2-MIB. The concentrations examined in this study are typically the levels that cause odor problems in lake water. Whilst activated carbon is still useful for off-flavours removal, metal doped titania nanoparticles are far more effective. Fe-TiO2 showed 95% and 93% Geosmin and 2-MIB removal. Pt-TiO2 was found to be most efficient; it removed 98% of Geosmin and 97% of 2-MIB as compared to 82% and 73% Geosmin and 2-MIB removal by most widely used granular activated carbon. Smaller size, extremely large surface area (567 m2/g), and more active adsorption site make Pt-TiO2 as superior adsorbent. Further work is also being undertaken to refine the process for implementation in commercial applications.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge financial support from the National University of Science and Technology, Islamabad, Pakistan.