Photocatalytic Degradation of Divalent Metals under Sunlight Irradiation using Nanoparticle TiO2 Modified

Concrete Materials (Recycled Glass )

Mohamed Nageeb Rashed

Chemistry Department, Aswan faculty of Science, Aswan University, Aswan, Egypt.

Aim of the study

• The main target of this work focus on the combination effect of photocatalytic degradation and adsorption processes using the immobilized TiO2 supported on glass powder (TiO2-GB) as adsorbent under the illumination of sun light as a new method for the removal of heavy metals from water .

• In this work nanosized TiO2 has been used as semiconductor material with glass powder as co-adsorbent for the removal of heavy metals from water. The kinetics and equilibrium data were carried out to understand the photocatalytic and adsorption processes. The work extended to study the effect of parameters such as pH, composite dose, contact time and initial metal concentration on the removal efficiency.

INTRODUCTION

Photocatalysis / Adsorption Processes for Metals Removal

Adsorption on glass powder

particle Photocatalysis with nanoTiO2

Photocatalysis / Adsorption on

Composite

Why TiO2 as Nanocatalyst? • In this field, TiO2-based photocatalysts have attracted continuously increasing

attention because of the excellent properties such as high light-conversion efficiency, chemical stability, nontoxic nature, low cost.

• The titanium dioxide most used and studied is “P25” because P25 is small, typically 25 nm in diameter ,highly active. Commercial TiO2 catalysts are typically available in suspension mode (for example, Degussa P25 powder) or immobilized on a substrate.

• The catalysts display high photocatalytic activity because of their large surface area. However, several disadvantages limit the wide application of TiO2 powder.

• The high effectiveness of TiO2 -P25 is related to the inhibition of recombination process on TiO2 (e −CB/h+VB) due to the smaller band gap of rutile section of the particle that absorbs photons and generates electron-hole pairs and the electron transfer from the rutile conductive band to the electron traps occurs in the anatase phase.

Combining the use of nanocatalyst with adsorbent materials (composite)

Adsorption process is one of the effective techniques that have been successfully employed for the removal of various pollutants (organic, inorganic, and heavy metal) from water .

Combining the use of photocatalyst with adsorbent material is an important development in the field of photocatalytic degradation of organic and metal pollutants.

The combination of destructive adsorption properties with photo-activity allows a synergistic “photochemical boosting” in which it increased capacities for destruction of pollutants.

Many researches were focused on the preparation as well as on the modification of TiO2 as composite , for examples composites of SiO2/TiO2 , glass/TiO2, carbon-coated TiO2 , kaolinite –TiO2, Clay-TiO2and TiO2-coated exfoliated graphite. Large surface area and high degree of dispersion of TiO2 powders in the reaction medium were reported to be favorable for high photocatalytic performance.

Many chemical and physical methods have been employed to prepare TiO2 coatings adsorbents, this including sol–gel , chemical vapor deposition, ultrasonic spray pyrolysis, sputtering, pulsed laser deposition mecahnical dispersion (ball mill) , and electrophoretic deposition.

Composite ( Catalyst –adsorpent )

SiO2/TiO2 Activated

carbon-TiO2

Kaolinite –TiO2

Clay-TiO2

TiO2-coated exfoliated graphite

TiO2-Zeolite

TiO2- coconut shell activated

carbon

Mechanism of the processes

Illustration of three different reaction zones on TiO2/glass beads under illumination depicting photoactive zone,

adsorption zone, glass/TiO2 interface.

Each GB/TiO2 particle may possess three different reactions:-

(i) Photoactive zone, which is the outer surface of TiO2 coating receiving UV illumination. (ii) Adsorption zone, which is bulk of GB underneath the photoactive zone and, (iii) GB /TiO2 interface.

The mechanism of metal removal efficiency

As the mainly constituent of GB is SiO2, and so in alkaline media on GB surface new active sites (≡ SiO-) can develop, allowing metals to form complexes 2 (≡ SiO-) + M 2+ → (≡ Si-O)2 M GB/TiO2 catalyst exploits the high adsorbent capacity of SiO2 and the photocatalytic activity of TiO2 toward the metals, resulting in higher removal capacity of heavy metal ions (Pb, Cd, Cu and Zn) than that of GB alone. According to the ‘‘site binding’’ theory, the surface charge on the metal oxides is created by the electrolyte ions adsorption. These processes of TiO2 can be described by the these equations :- TiO2 + hv (UV) → TiO2 (eCB− + hVB+) TiO2 ( hVB+) + H2O → TiO2 + H+ + OH- TiO2 ( hVB+) + OH− → TiO2 + OH• ≡Ti-OH + H+ → ≡Ti-OH2

+ 2≡Ti-OH + M 2+ → (≡Ti-O-)2 M+ 2H+

•The photocatalytic processes on TiO2 involves the formation of active oxidant species as HO- . Considering the experimental conditions on the GB/TiO2, the titanium dioxide component will be slightly negatively charged, while the GB matrix preserves an overall negative surface charge. Under these conditions, cationic heavy metals are supposed to be adsorbed by electrostatic attractive forces. At the same time, the positive charges on the TiO2 surfaces may support an increased amount of HO-. •According to the latest research report, TiO2 grafted inorganic porous materials shows very high efficiency and sensitivity for removing heavy metals from water , which is highly essential for environmental aspects •Synthesis of titanium-functionalzied silica-based materials has been a matter of extensive research due to the great potential of these materials in catalytic oxidation processes and adsorbed with high capacity . Combining the advantages of both synthesis procedures, high dispersion and accessibility of metal sites, is desirable for adsorption

EXPERIMENTAL

Materials • The waste glasses (laboratory glass beakers and conical flasks) were

collected from the waste of our chemical laboratories. All the glasses were made of silicate glass which is a type of glass with the main glass-forming constituent's silica. Typically, the resulting glass (GB) composition is about 57.88% SiO2, 3.63% Al2O3 , 13.97% Na2O, 7.74% K2O, and 16.96% CaO .

• Glass samples were washed with tab water then with deionized water, and crushed by a mechanical crusher in the laboratory. The crushed glass cullets were grinded in agate mortar and further sieved to particle sizes 25μm .

• The chemical composition of GB/TiO2 is 95.76%TiO2, 2.96% SiO2, 0.76% A2O3 oxide, and 0.65% Na2O.

• Stock standard solutions of Cd, Cu, Pb and Zn ions (1000 mg/L) were prepared by dissolving the accurately weighed amounts of their nitrate or sulphate salts (BDH, UK or Merck, Darmstadt, Germany) in 1000 mL deionized water. Working standard solutions were prepared daily by appropriate dilution with deionized water.

Preparation of Composite GB/TiO2

Composite sample GB/TiO2 was prepared by ball milling of nano-TiO2 and GB powder in a ball miller (QM-1F, made in Nanjing University, China). 5.0 g TiO2 powder and three different sizes zirconiam balls were mixed in the miller tank. Amount of GB (0.1–5.0 wt %) and H2O (5 mL) were added. After being milled at the speed of 280 rpm for 120 min, the wet powder was collected and dried at 110◦ C in an oven air. The chemical composition of GB/TiO2 is 95.76%TiO2, 2.96% SiO2, 0.76% Al2O3, and 0.65% Na2O.

Characterizations of prepared composite

Scanning electron microscopy dispersive (SEM-EDX) study was carried out on a JEOL, JSM-5500LV electron microscopy instrument at operating at five kV specimens on which a thin layer of gold or carbon had been evaporated. Crystalline phase, particle size, elemental composition and morphology of GB/TiO2 and GB nanocrystals were investigated.

• The surface characteristics, both for GB/TiO2 and GB were evaluated using a BET surface analyser (Tri Star II 3020 – Micromeritics). The specific surface area of the glass beads reached 162.6 m2/g, The total TiO2 surface area developed on the glass spherules is about 50 cm2.

• X-ray diffraction of GB/TiO2 indicates a well organized crystal structure of titania nanoparticles.

Adsorption and Photocatalysis of Metals by GB adsorbent and GB/TiO2 composite

The photocatalysis of each metal (Cd, Pb, Cu, and Zn) was carried out using 20 ml of 100 mg/L metal solution in a 100 ml closed Pyrex flask over 0.5 g of catalyst (GB/TiO2) with continuous shaking. The pH of the solution was adjusted to 8 (alkaline media). The solutions were exposed to sunlight with constant stirring, average total daily short wave radiation for this period was 734 W/m2, with a 2 h mean sunshine duration from sunrise . The mean temperature was 25oC. After irradiation, the suspension was centrifuged, filtered through a 0.45 µm membrane filter, and the metal content was analyzed using AAS spectrophotometer. Adsorption of each metals (Cd, Pb, Cu, and Zn) was carried out as above with using GB adsorbent ,but with not using sunlight irradiation. The metal removal efficiency was calculated by equation Where C0 and Ce (mg/L) are initial and equilibrium metal concentration , respectively. The amount of adsorbed metal (qe) by GB was calculated using equation

Parameters affect Metal removal

• Effect of metal concentration on metal removal (initial concentrations of 10, 50, 100, 150 mg/L ).

• Effect of irradiation and contact time on metal removal (0.5-6 hrs) .

• Effect of pH on metal removal (pH values 3, 5, 7 and 9).

• Effect of catalyst dosage on metal removal (0.25, 0.5, 1, 1.5 and 2 g GB/TiO2 composite).

Results and Discussion

Effect of GB adsorbent and GB/TiO2 composite on

metal Removal Efficiency Metal s Removal %

using GB

Removal %

using GB/TiO2

Pb 86.91 96.16

Cd 85.71 91.02

Cu 88.2 97

Zn 92 99

Effect of metal concentration on removal efficiency (initial concentrations of 10, 50, 100, 150 mg/L ).

0

20

40

60

80

100

120

10 30 50 100

Concentration(mg/L)

Rem

oval

%

Cd

Pb

Cu

Zn

Effect of contact time on metal removal (0.5-6 hrs) .

88

90

92

94

96

98

100

0.5 1 2 4 6

Time (h)

Rem

oval

%

PbCdCuZn

Effect of pH on Metal Removal

0

20

40

60

80

100

120

2 3 4 5 6 8 10

pH

Rem

oval

%

Pb

Cd

Cu

Zn

Effect of Adsorbent and Catalyst Dose on Metal removal

0

20

40

60

80

100

120

0.25 0.5 1 1.5 2

Catalyst dose(gm)

Rem

oval

%

Pb

Cd

Cu

Zn

Characterization of the prepared composite ( GB/TiO2 )

Equilibrium metal adsorption measurements and modeling

Adsorption Isotherms

Langmuir and Freundlich isotherms were applied to describe the adsorption equilibrium of metal ions on adsorbent. The Freundlich equation is given as follows: qe = Kf Ce

1/n linearized logarithmic form Log qe = Log kf + (1/n) Log Ce Where qe is the amount adsorbed (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/l), and kf and n are the Freundlich constants related to adsorption capacity and adsorption intensity , respectively. Langmuir equation expressed as follows Ce / qe = Ce/ Qo + 1 /Qo b Where Ce (mg/l) is metal equilibrium concentration, qe (mg/g) the amount adsorbed at equilibrium, b (mg /L) is the Langmuir constant which related to the affinity of the binding site, and Qo (mg/g) the capacity parameter. When Ce / qe is plotted versus Ce, the slop is equal to 1/Qo and the intercept is equal to1/Qo b.

Table 4 . Langmuir and Freundlich constants for the adsorption of Cd, Pb, Cu, and Zn by GB/TiO2

Langmuir constants Freundlich constants Metal ion

Qo b R2 1/n Kf R2

31.44 0.219 0.958 0.618 5.59 0.998 Cd

46.72 0.314 0.859 0.731 10.38 0.997 Pb

21.97 0.489 0.993 0.442 6.511 0.966 Cu

44.44 0.079 0.8562 0.760 3.477 0.988 Zn

Scanning Electron Microscopy (SEM)

The morphology of the adsorbent (GB) and the catalyst ( GB/TiO2 ) samples were examined by SEM. The figures show the SEM micrographs of the two samples, glass waste powder (GB) and after mixing with TiO2 ,composite( GB/TiO2) . Whereas the latter sample is constituted by well-defined micro-size particles of glass particle with titania. Images indicate changes in surface morphology after mixing with TiO2.The TiO2 particle size in GB/TiO2 measured was not more than 10 μm, while the other glass powder (GB) particle consists of agglomerates of particles in the nanometer sizes. The SEM-overview image in Fig. 5 reveals that the sample consists of two kinds of particles: scale-shaped GB, and TiO2 particles with no characteristic morphology. The TiO2 nanoparticles are homogeneously dispersed on the surface of GB..

SEM Micrographs of GB and GB/TiO2

GB GB/TiO2

CONCLUSION

•Synthesis of titanium-functionalzied silica-based materials has been a matter of extensive research due to the great potential of these materials in catalytic oxidation processes and adsorbed with high capacity . •GB/TiO2 composite was successively synthesized via nano-mechanical synthesis. TEM analysis confirmed the presence of crystalline anatase phase with almost uniform diameter and a narrow range distribution of spherical nanoparticles in GB/TiO2 catalyst. •The synthesized catalyst exhibited a good photocatalytic activity for heavy metals (Cd, Cu, Pb and Zn) removal from water under sunlight irradiation and mild conditions in aqueous solutions. •The photocatalytic performance in both cases was markedly dependent on catalyst dose, pH, metal concentrations and contact time. The optimum conditions were catalyst dose 1-2 gm, pH 8, metal concentrations 10 mgl-1 ,and contact time 1 hr. •The present work can be used successfully for treatment of saline water, wastewater and polluted water from divalent cations include heavy and toxic metals.

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