Results and Discussion

Experiments revealed that yield of NCMC(Ti) in the electrolysis process depends on voltage V between electrodes and pH of the solution. The yield dCTi/dt of Ti after 6 min of the electrolysis process, where CTi is the concentration of Ti in the electrolyte, increases with increasing voltage between electrodes up to 12-13 V but then decreases slowly (Fig. 17.1). This behavior is explained as the formation of three valence titanium on the surface of Ti-electrode (electrode becomes of blue color). Figure 17.2 depicts the dependence of dCTi/dt against the concentration of H2SO4 in solution for 6 min after beginning the electrolysis process. When the

Fig. 17.2 Dependence of titanium yield as a function of solution pH

electrolyte pH is close to neutral, three valence titanium is formed on the surface of the Ti electrode.

In the first stage of electrolysis, when the Ti-electrode is an anode, the current between electrodes is about 3-4 mA/cm2. Oxygen is released during the stage onto the titanium anode. Titanium oxides and sulfates are then formed on its surface and titanium ions leaving the anode are either oxidized near the surface of anode in the solution or they react with NCC active carboxyl groups. The thin semiconductor layer formed on the surface of the titanium electrode has a high resistance and small (about 3-4 mA/cm2) electric current between electrodes. At the same time, the negatively charged carbon nanoparticles move away from the graphite cathode and functional groups such as carbonyl (>C=O), hydroxyl (-OH), and carboxyl (-COOH) are formed on the surface of carbon particles.

In the second stage of electrolysis after changing polarity, the electric current increases to 180-200 mA/cm2 in about 0.1-0.2 s. During this stage, oxidation is occurring at the carbon anode. The magnitude of repulsion forces formed between the stacked layers of graphite becomes larger than van der Waals attraction forces

between the layers, initiating formation of carbon nanoparticles when polarity of the electrode is changed. The titanium cathode surface is cleaned from the oxides, and the electric current between electrodes increases up to 180-200 mA/cm2. Titanium ions and charged particles of titanium oxide interact with carbon nanoparticles forming NCMC(Ti). Oxygen adsorbed on the surface of particles forms -Ti(OH)-O-Ti(OH)-, which can help the photo-generated holes h+ to change into Off free radicals. Otherwise, the oxidization activity of Off is the strongest in aqueous solution. Typical TEM micrograph of NCMC(Ti) is given in Fig. 17.3 and shows that nanoparticles have a spherical morphology. Measurements of nanoparticles reveal their size is about 6 ± 2 nm.

Photodegradations of MeO solution, containing NCMC(Ti) as a photocatalyst, were analyzed for pH values of 1.0, 2.0 and 4.4. The relationships between the degradation degree of MeO and pH value of the solution under UV lamp and solar irradiation are shown in Figs. 17.4 and 17.5, respectively. Photodegradations of MeO were not observed in the sample without NCMC(Ti), and in the sample with NCMC(Ti) which was not irradiated by UV and sunlight. These results indicate that a low pH value can facilitate the decolorization reaction. This means that the number of Off radical increases on the surface of NCMC(Ti) particles in solution by trapping electrons and H2O can be absorbed on the NCMC(Ti) surface and reacts with hVB+ in Eq. 17.3, producing larger amounts of Off radicals at a lower pH value. This can promote photogenerated electrons to transfer to the surface of NCMC(Ti) and react with adsorbed oxygen.

The photodegradation of cyanobacteria by NCMC(Ti) was studied under laboratory conditions. A colloidal solution of NCMC(Ti) was added to a Petri dish

Fig. 17.4 MeO degradation degree under UV lamp: pH = 2.2 (1), pH = 4.4 (2), pH = 6.0 (3). Without NCMC(Ti) is curve (4)

Fig. 17.4 MeO degradation degree under UV lamp: pH = 2.2 (1), pH = 4.4 (2), pH = 6.0 (3). Without NCMC(Ti) is curve (4)

Fig. 17.5 MeO degradation degree under solar irradiation with (1) and without NCMC(Ti) (2)
t, min

containing water with cyanobacteria, placed under the sunlight at 8 a.m., and tested over a period of 24 h. Concentration of NCMC(Ti) in the mixed solution was 1 ug/l. Dependences of living cyanobacteria concentration in water samples irradiated by sunlight with and without NCMC(Ti) against a time are presented in Fig. 17.6. Photodegradation of cyanobacteria was not observed in the sample with NCMC(Ti) which was not irradiated by sunlight. These results demonstrate the efficacy of NCMC(Ti) photocatalysts to degrade cyanobacteria.

Field tests of the described technology were conducted on a 4 ha lake. The surface of the lake was treated using a 200 mg/l colloidal solution of NCMC(Ti) to achieve a 10 g/ha consumption of nanocompositions. At 8 am, the solution was dispersed for 30 min over the surface of the lake using a sprayer installed on a motor boat. Because the UV-light penetrates water to a depth of ~10 cm, the formation of OH-radicals was restricted to the upper water layers where cyanobac-teria grow.

Nanocompositions in the water interacted with salt ions, organic molecules, and different organic and inorganic particles resulting in gradual coagulation and

Fig. 17.6 Dependences of living cyanobacteria in water

Fig. 17.6 Dependences of living cyanobacteria in water without NCMC(Ti) (squares) against a time

t, hour samples irradiated by sunlight with (diamonds) and without NCMC(Ti) (squares) against a time

t, hour precipitation in the form of large harmless water-insoluble particles. The rate of their sedimentation depends on water quality but usually is in the range of about 1-3 cm/h. Thus, during daylight hours, photonanocompositions can produce radicals within 3-8 h. Therefore the consumption of nanocompositions is low, about 10 g/ha or 50 l of aqueous solution of the nanocomposition with concentration of 200 mg/l.

During the second day, dead cyanobacterial clumps were found floating on the surface of the lake. After using a motor boat to break up these clumps, this material settled to the bottom over a 3-5 h period. During the next day, the transparency of lake water increased from 10 (initial) to 1,500 mm. These tests demonstrated that the efficacy of this new technology to removed cyanobacteria from water and regulate its concentration in open reservoirs over 2-3 days.

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