Ting Wang

Adsorption of Pb2 +, Cd2 +, Cu2 + and Cr3 + onto titanate nanotubes: Competition and effect of inorganic ions

Adsorption of Pb(2+), Cd(2+), Cu(2+) and Cr(3+) from aqueous solutions onto titanate nanotubes (TNTs) in multiple systems was systematically studied. Particular attention was paid to competitive adsorption and the effect of inorganic ions. TNTs showed large adsorption capacity for the four heavy metals, with the mechanism of ion-exchange between metal ions and H(+)/Na(+) located in the interlayers of TNTs. Binary or quaternary competitive adsorption indicated that the adsorption capacity of the four heavy metals onto TNTs followed the sequence of Pb(2+) (2.64 mmol g(-1)) ≫ Cd(2+) (2.13 mmol g(-1)) > Cu(2+) (1.92 mmol g(-1)) ≫ Cr(3+) (1.37 mmol g(-1)), which followed the reverse order of their hydration energies. Moreover, inorganic ions including Na(+), K(+), Mg(2+) and Ca(2+) inhibited the adsorption of heavy metals on TNTs, because they competed for adsorption sites, decreased the activity of heavy metal ions, and promoted the aggregation of TNTs. However, Al(3+) and Fe(3+) generally enhanced adsorption because the resulting hydroxyl-Al/Fe intercalated or coated TNTs could also capture metal ions. Furthermore, minor effect of inorganic ions on adsorption of Pb(2+) resulted from its strong affinity to TNTs. Difficult desorption and small inhibiting effect by Na(+), K(+), Mg(2+) and Ca(2+) on adsorption of Cr(3+) was due to the formed stable complex of HOCr(OTi)₂ ≡ with TNTs. Present study indicated potential applications of TNTs in wastewater treatment for heavy metals.

Adsorption and desorption of Cd(II) onto titanate nanotubes and efficient regeneration of tubular structures.

Efficient regeneration of desorbed titanate nanotubes (TNTs) was investigated with cycled Cd(II) adsorption and desorption processes. After desorption of Cd (II) from TNTs using 0.1M HNO3, regeneration could be simply achieved with only 0.2M NaOH at ambient temperature, i.e. 2% of the NaOH needed for virgin TNTs preparation at 130°C. The regenerated TNTs displayed similar adsorption capacity of Cd(II) even after six recycles, while significant reduction could be detected for desorbed TNTs without regeneration. The virgin TNTs, absorbed TNTs, desorbed TNTs and regenerated TNTs were systematically characterized. As results, the ion-exchange mechanism with Na(+) in TNTs was convinced with obvious change of -TiO(ONa)2 by FTIR spectroscopy. The easy recovery of the damaged tubular structures proved by TEM and XRD was ascribed to asymmetric distribution of H(+) and Na(+) on the surface side and interlayer region of TNTs. More importantly, the cost-effective regeneration was found possibly related to complex form of TNTs-OCd(+)OH(-) onto the adsorbed TNTs, which was identified with help of X-ray photoelectron spectroscopy, and further indicated due to high relevance to an unexpected mole ratio of 1:1 between exchanged Na(+) and absorbed Cd(II).

Selective and irreversible adsorption of mercury(II) from aqueous solution by a flower-like titanate nanomaterial

A novel flower-like titanate nanomaterial (titanate nanoflowers, TNFs) was synthesized through a hydrothermal method using nano-anatase and sodium hydroxide, and used for mercury(II) removal from aqueous solution. The large surface area (187.32 m2 g−1) and low point of zero charge (3.04) of TNFs facilitated the adsorption of cations. Adsorption experiments indicated that TNFs could quickly capture 98.2% of Hg(II) from solution within 60 min at pH 5. The maximum adsorption capacity of Hg(II) was as large as 454.55 mg g−1 calculated by the Langmuir isotherm model. Moreover, selective adsorption of Hg(II) by TNFs was observed with the coexistence of other conventional cations (i.e., Na+, K+, Mg2+ and Ca2+) even at 10 times concentration of Hg(II). XRD analysis indicated that the prepared TNFs were a kind of tri-titanate composed of an edge-sharing triple [TiO6] octahedron and interlayered Na+/H+, and ion-exchange between Hg2+ and Na+ was the primary adsorption mechanism. Furthermore, it was interesting that the basic crystal structure of TNFs, tri-titanate (Ti3O72−), transformed into hexa-titanate (Ti6O132−) after adsorption, resulting in the trapping of Hg(II) into the lattice tunnel of this hexa-titanate. Desorption experiments also confirmed the irreversible adsorption due to Hg(II) trapped in TNFs, which achieved safe disposal of this highly toxic metal in practical application.

Simultaneous removal of Cr(VI) and 4-chlorophenol through photocatalysis by a novel anatase/titanate nanosheet composite: Synergetic promotion effect and autosynchronous doping.

Clean-up of wastewaters with coexisting heavy metals and organic contaminants is a huge issue worldwide. In this study, a novel anatase/titanate nanosheet composite material (labeled as TNS) synthesized through a one-step hydrothermal reaction was demonstrated to achieve the goal of simultaneous removal of Cr(VI) and 4-cholophenol (4-CP) from water. TEM and XRD analyses indicated the TNS was a nano-composite of anatase and titanate, with anatase acting as the primary photocatalysis center and titanate as the main adsorption site. Enhanced photocatalytic removal of co-existent Cr(VI) and 4-CP was observed in binary systems, with apparent rate constants (k1) for photocatalytic reactions of Cr(VI) and 4-CP about 3.1 and 2.6 times of that for single systems. In addition, over 99% of Cr(VI) and 4-CP was removed within 120min through photocatalysis by TNS at pH 7 in the binary system. Mechanisms for enhanced photocatalytic efficiency in the binary system are identified as: (1) a synergetic effect on the photo-reduction of Cr(VI) and photo-oxidation of 4-CP due to efficient separation of electron-hole pairs, and (2) autosynchronous doping because of reduced Cr(III) adsorption onto TNS. Furthermore, TNS could be efficiently reused after a simple acid-base treatment.

Absorption of Cr(VI) onto amino-modified titanate nanotubes using 2-bromoethylamine hydrobromide through SN2 reaction.

Unlike the complex reaction of grafting amino groups using harmful organic solvents, we proposed an environmental friendly method for effective amino grafting on titanate nanotubes (TNTs) with 2-Bromoethylamine hydrobromide (2-Bh) through a two-step SN2 reaction in pure water solution. The amino-modified titanate nanotubes (TNTs-RNH2) were characterized by Transmission electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Under optimal conditions, the molar ratio of NaOH/2-Bh was 1.2 for step 1 and 1.3 for step 2 with corresponding reaction time of 12h and 24h, respectively. The adsorption isotherm of Cr(VI) onto TNTs-RNH2 well fits Langmuir model with maximal adsorption capacity of 69.1 mg/g, which is almost five times larger than that of fresh TNTs. The slight decrease in adsorption capacity with NO3(-) concentration was attributed to competition from ions, suppression of electric double layer and changes of Cr(VI) speciation. The wide adsorption pH range was due to much larger point of zero charge (8.2) and electrostatic attraction between positively charged TNTs-RNH2 and Cr(VI) with different speciations. The products could be well reused due to simple desorption at pH 10 with about 20% loss of the adsorption capacity after three recycles without any regeneration treatment.

Synergetic antibacterial activity of reduced graphene oxide and boron doped diamond anode in three dimensional electrochemical oxidation system

A 100% increment of antibacterial ability has been achieved due to significant synergic effects of boron-doped diamond (BDD) anode and reduced graphene oxide (rGO) coupled in a three dimensional electrochemical oxidation system. The rGO, greatly enhanced by BDD driven electric field, demonstrated strong antibacterial ability and even sustained its excellent performance during a reasonable period after complete power cut in the BDD-rGO system. Cell damage experiments and TEM observation confirmed much stronger membrane stress in the BDD-rGO system, due to the faster bacterial migration and charge transfer by the expanded electro field and current-carrying efficiency by quantum tunnel. Reciprocally the hydroxyl-radical production was eminently promoted with expanded area of electrodes and delayed recombination of the electron–hole pairs in presence of the rGO in the system. This implied a huge potential for practical disinfection with integration of the promising rGO and the advanced electrochemical oxidation systems.

Arsenate adsorption onto Fe-TNTs prepared by a novel water-ethanol hydrothermal method: Mechanism and synergistic effect

Arsenate adsorption onto Fe2O3 was highly restricted at acidic condition due to dramatic dissolution. To overcome this difficulty, iron oxide nanoparticle-grafted titanate nanotubes (Fe-TNTs) were synthesized by a facile one-step water-ethanol hydrothermal method and used to remove As(V) from aqueous solutions. This new adsorbent was acid-resistant, and showed a large As(V) adsorption capacity of 90.96 mg/g determined by two-site Langmuir model, which was almost 3 times of the original TNTs. Fe2O3 was proved to bonded to the surface of TNTs by TEM and XRD analysis and synergy of Fe2O3 and TNTs was of great help to excellent As(V) adsorption. Load of Fe2O3 greatly enhanced the point of zero charge. Moreover, tubular TNTs not only inhibited dissolution of Fe2O3 at low pH, but also maintained good sedimentation property. The hydroxyl groups on Fe-TNTs surface played the most important role in As(V) adsorption. Electrostatic interaction followed by complexation was confirmed to be the primary adsorption mechanism by means of XPS analysis. Desorption capability and reuse performance of Fe-TNTs were also investigated, and satisfactory As(V) adsorption was further found with NaOH desorbed even after three reuse cycles.

Photocatalysis of bisphenol A by an easy-settling titania/titanate composite: Effects of water chemistry factors, degradation pathway and theoretical calculation

Bisphenol A (BPA) is a widely concerned endocrine disrupting chemical and hard to be removed through conventional wastewater treatment processes. In this study, we developed a TiO2 decorated titanate nanotubes composite (TiO2/TNTs) and used for photocatalytic degradation of BPA. TEM and XRD analysis show that the TiO2/TNTs is a nano-composite of anatase and titanate, with anatase acting as the primary photocatalytic site and titanate as the skeleton. TiO2/TNTs exhibited excellent photocatalytic reactivity and its easy-settling property leaded to good reusability. After 5 reuse cycles, TiO2/TNTs also could photo-degrade 91.2% of BPA with a high rate constant (k1) of 0.039 min-1, which was much better than TiO2 and TNTs. Higher pH facilitated photocatalysis due to more reactive oxygen species produced and less material aggregation. The presence of NaCl and CaCl2 showed negligible effects on BPA degradation, but NaHCO3 caused an inhibition effect resulting from consumption of ·OH. Humic acid inhibited degradation mainly due to blockage of the active sites of TiO2/TNTs. Degradation pathway was well interpreted through theoretical calculation. Hydroxyl radical played the dominate role in BPA photodegradation, and the atoms of BPA with high Fukui index based on density-functional theory (DFT) calculation are the radical easy-attacking (f0) sites. Considering the good photocatalytic reactivity, reusability, stability and settle property, TiO2/TNTs promises to be an efficient alternative for removal of organic compounds from wastewaters.

Influences of isolated fractions of natural organic matter on adsorption of Cu(II) by titanate nanotubes

With different functional groups and hydrophobic/hydrophilic properties, natural organic matters (NOMs) displayed different combining capacities with metal ions. By using XAD-4 and DAX-8 resins, NOMs in natural lake were isolated into three fractions, i.e., HoB (hydrophobic base), HoA (hydrophobic acid) and HiM (hydrophilic matter). Afterwards, influences on Cu(II) adsorption onto titanate nanotubes (TNTs) were compared with varying NOMs and initial pH. As results, HoB can significantly control Cu(II) adsorption at pHu202f5, with the adsorption capacity increased 15% for 0.5u202fmgu202fL-1 of HoB (ca. 120u202fmgu202fg-1), which could be attributed to the formation of HoB-Cu complexation and electrostatic bridge effect of HoB with optimal concentration. Due to the easier ionization and complexation with Cu(II) at lower pH, HoA showed more obvious impaction on Cu(II) adsorption at pHu202f2. While HiM can influence Cu(II) adsorption at all pH ranges due to its hydrophilic groups and weak affinity to both TNTs and Cu(II). Furthermore, HoB dramatically changed the Langmuir model, with sharp increase of adsorption capacity as equilibrium Cu(II) increased, suggesting its significant involvement in Cu(II) adsorption. X-ray photoelectron spectroscopy (XPS) analysis revealed the absorbed Cu(II) existed in the form of TNTs‑OCu, TNTs‑COOCu and Cu(OH)2, proving Cu(II) adsorption mechanism including both direct adsorption by TNTs and bridging connection with NOMs. Moreover, the CO and OCO groups content ranked as HiMu202f>u202fHoBu202f>u202fHoA, while TNTs‑COOCu content ranked as HoAu202f>u202fHoBu202f>u202fHiM, suggesting HoB had the moderate connection with both TNTs and Cu(II), thus the impact on Cu(II) adsorption was remarkable.