Ultra-high adsorption of cationic methylene blue on two dimensional titanate nanosheets

In this work, we examined the performance of 2D titanate nanosheets for dye adsorption. Their adsorption capacity for methylene blue (MB) is up to 3937 mg g−1, which is more than 10 times higher than active carbon and occupies the highest place among all the reports.


MATERIALS AND METHODS 1.1 Materials
Titanium(IV) dioxide TiO 2 , anhydrous potassium carbonate K 2 CO 3 , and lithium carbonate Li 2 CO 3 had a purity of 97.0% or higher and were used as received.Tetra n-butylammonium hydroxide TBAOH (40 wt % H 2 O, Alfa Aesar) were used as received.Methylene blue had a purity of 98.5 and was used as received.
Demineralized water was used throughout the experiments.

Synthesis of titanate nanosheets
Titanate nanosheets were synthesized by chemical exfoliation of layered protonated titanate (HTO).In detail, the layered precursor K 0.8 [Ti 1.73 Li 0.27 O 4 ] (KLTO) was obtained by sintering of K 2 CO 3 , LiCO 3 and TiO 2 at 1000 °C for 20 h according to Sasaki's method.[32]Then, 3 g of the KLTO powders was protonated in 2 mol/l HNO 3 solution (300 ml) at room temperature while stirring.After treatment for 3 days, the acid exchanged crystals of KLTO were collected by filtration and washed with a copious quantity of pure water, and then air-dried to get HTO powders.The exfoliation of HTO crystals was carried out by reaction with a TBAOH solution.The HTO powders (0.1 g) were mixed with TBAOH and water to a TBAOH/H + (H + refers to the protons of HTO powder) ratio of 4/1 in 10 mL solution.The titanate nanosheets (TONS) solutions were stirred for 1 day, and then are ready for the adsorption experiments.

Methylene blue adsorption
The details of the kinetic experiment were given below and all the experiments were carried out under 298 K at ambient conditions.The MB aqueous solution with concentration of 10 mg/L was prepared prior to the experiment.100 ml of the MB aqueous solution was used and vigorously stirred in a beaker.Then 5 ml TONS solution was added to the beaker.4 ml of the mixtures at mixing time of 1, 4, 9, 15, 30, 60 and 90 min was centrifuged at 12000 rpm and the suspension was collected for further analysis.
For the isotherm adsorption experiments, the MB aqueous solutions with concentration of 50, 100, 300, 500, 800, 1000, 2000, and 4000 mg/l were prepared, respectively, prior to the experiment.100 ml of the MB aqueous solutions were used and vigorously stirred in a beaker.Then 5 ml TONS solution was added to the beaker and was allowed to mix for 40 h.While the mixing, the beaker was sealed to prevent water evaporation.After 40 h mixing, the mixture solution was centrifuged at 12000 rpm, and then the suspension were collected for further analysis.And the residues were dried at 60°C for 2 days for further analysis.The adsorption capacity of TONS in this study was calculated by following equation, qe = (m residue-m TONS )/ m TONS Where, qe is the amount of MB dye adsorbed at equilibrium time (mg/g), m residue is the weight of the residue and m TONS is the effective weight of the TONS.Because of the materials loss in the process, the effective weight of the TONS is less than the materials added.Our statistic data show that 20% materials loss applied.
The adsorption capacities of the KLTO and HTO were calculated by following equation, qe,bulk = (C 0 -Ceq)*V/m Where qe,bulk is the adsorption capacities of the KLTO or HTO at equilibrium time, C 0 represents the initial MB concentration, Ceq is the equilibrium MB concentration in solution after adsorption, V is the volume of the aqueous solution and m is the mass of the KLTO or HTO.
The adsorption efficiency of the TONS was calculated by following equation, η = (m residue-m TONS )/m MB Where, η is the adsorption efficiency at equilibrium time, m residue is the weight of the residue, m TONS is the effective weight of the TONS, and m MB is the mass of MB in the initial solution.

Characterization
Powder X-ray diffraction (XRD) data were acquired on a Rigaku smartlab (Cu Kα radiation with a wavelength of 0.15405 nm).Scanning electron microscopy (SEM; Tescan VEGA3) was used to acquire information on the particle size and morphology.Scanning transmission electron microscopy (STEM; Joel JEM-2100F) was used to visualize the nanosheets.Atomic force microscopy (AFM; Bruker NanoScope 8) was used to visualize the nanosheets deposited on Si substrates.The AFM data were further analyzed using the Gwyddion software package.UV−vis spectra of samples were recorded with a Cary 50 UV−vis spectrophotometer in transmission mode.All the solutions were diluted to an appropriate concentration for UV-vis spectroscopic measurements except for the 10 mg/l solutions.

Figure
Figure S1 the AFM image of 2D titanate nanosheets.

Figure
Figure S2 the UV-Vis spectra of MB solution with concentration of 10 mg/l before and after KLTO adsorption.

Table A .
1 Comparison of the Methylene Blue adsorption capacity from literature and this work