© The Minerals, Metals & Materials Society 2018
Boyd R. Davis, Michael S. Moats, Shijie Wang, Dean Gregurek, Joël Kapusta, Thomas P. Battle, Mark E. Schlesinger, Gerardo Raul Alvear Flores, Evgueni Jak, Graeme Goodall, Michael L. Free, Edouard Asselin, Alexandre Chagnes, David Dreisinger, Matthew Jeffrey, Jaeheon Lee, Graeme Miller, Jochen Petersen, Virginia S. T. Ciminelli, Qian Xu, Ronald Molnar, Jeff Adams, Wenying Liu, Niels Verbaan, John Goode, Ian M. London, Gisele Azimi, Alex Forstner, Ronel Kappes and Tarun Bhambhani (eds.)Extraction 2018The Minerals, Metals & Materials Serieshttps://doi.org/10.1007/978-3-319-95022-8_150

Hydrolytic Precipitation of Nanosized TiO2 Phases for Use as Photocatalytic Sorption Media in Effluent Treatment

Konstantina Chalastara1  , Fuqiang Guo1 and George P. Demopoulos1  
(1)
Materials Engineering, McGill University, Montréal, QC, H3A 0C5, Canada
 
 
Konstantina Chalastara (Corresponding author)
Email: konstantina.chalastara@mail.mcgill.ca
 
George P. Demopoulos (Corresponding author)
Email: george.demopoulos@mcgill.ca

Abstract

Mixed phase titanium dioxide (TiO2 ) nanoparticles synthesized through hydrolysis of TiCl4 solution at different conditions, namely concentration, pH, and residence time are characterized and evaluated as photocatalysts. The process is operated in a continuous stirred-tank reactor, CSTR giving blends of anatase, brookite, and rutile nanoparticles that were further evaluated to find the optimum mixture for a photocatalytic sorption material. The titania nanoparticles are first characterized in terms of photocatalytic activity under UV light illumination using an organic model compound (methyl orange) and after that are tested for removal of toxic selenate and selenite species from simulated effluent waters.

Keywords

nanoTiO2 precipitationPhotocatalytic applicationEffluent treatmentSelenium

Introduction

In this work we describe the production of nano titanium dioxide, TiO2 phases via a new scalable process [1] using a continuous stirred-tank reactor, CSTR and evaluate them as photocatalysts. Photocatalytic purification is an environmental technology used extensively for the treatment of contaminated waters [2, 3]. First report of photocatalytic activity goes back to 1921 and states that titania is partially reduced during illumination with sunlight in the presence of an organic compound such as glycerol, where the oxide turns from white to a darker color. Since then, many studies have been reported on the underlying mechanism of these reactions as well as their application to water and air purification for effective decomposition of organic contaminants [4] and other pollutants like selenium [5, 6]. A photocatalytic reaction is a light-driven reaction which is accelerated by the action of a semiconductor catalyst (e.g. TiO2 ) [2]. Upon light absorption the semiconductor generates pairs of electrons (e-) and holes (h+) that are involved in reactions, the former injected into the conduction band (CB) and the latter into the valence band (VB) of the semiconductor. A schematic representation of the two reactions in connection to the band structure of the semiconductor (in this case TiO2 ) is shown in Fig. 1. Pure TiO2 is photoactive only under UV light illumination because its bandgap lies at ~3.2 eV corresponds to 390 nm wavelength. There are at least two reactions which occur at the same time. The first half is the oxidation of surface-adsorbed water molecules by the photogenerated holes and the second half is the reduction of molecular oxygen (in the case of open atmospheric system) by the photogenerated electrons [3, 7]. These reactions, represented by Eqs. 1 and 2 lead to the formation of highly reactive radicals (•OH, O2•-), which are responsible for the oxidative decomposition of organic pollutants.
../images/468727_1_En_150_Chapter/468727_1_En_150_Fig1_HTML.gif
Fig. 1

The mechanism of photocatalysis after UV irradiation on TiO2 particle in an open atmospheric system

$$ H_{2} O_{surf} + h_{VB}^{ + } \to \bullet OH + H^{ + } $$
(1)
$$ O_{2} + e_{CB}^{ - } \to O_{2}^{ \bullet - } $$
(2)

The most commonly used semiconductor photocatalyst is TiO2 which has three main types of crystallographic polymorphs: rutile, anatase and brookite. Each type of TiO2 has different electronic characteristics that influence its photocatalytic properties. In general, anatase has better photocatalytic characteristics than rutile and brookite [8, 9]. The band gap energies for the two main TiO2 photocatalysts, anatase and rutile are 3.2 eV and 3.0 eV respectively. The higher bandgap of anatase has been suggested by Fujishima et al. [7] to be responsible for this phase to be preferred as photocatalyst. As far as brookite is concerned there is a range of band gap energy values reported that vary from 3.1 to 3.4 [10].

Mixed phase titania P25, commercially available photocatalyst by Evonik with a combination of anatase and rutile in an approximately 4:1 ratio, is the benchmark in environmental photocatalysis as it has better photocatalytic properties than phase-pure anatase or rutile [11, 12]. The mixed phase TiO2 nanomaterial has been shown to achieve effective separation of photogenerated holes and electrons at anatase/rutile junctions, i.e. less charge recombination, due to a difference in band gap between the phases giving as result the observed rise in photocatalytic activity [5, 7].

In this work, a number of mixed-phase nano-TiO2 powders were synthesized by a green hydrolysis process that is operating at a steady state in a CSTR (Fig. 3) developed by the HydroMet group at McGill University [1]. The process involves hydrolytic precipitation of TiO2 , see Eq. 3. Taking advantage of the fact that changing process parameters, like temperature , agitation speed, residence time, Ti(IV) concentration and pH, affect the characteristics of the synthesized titania, different mixed phase TiO2 powders were produced and evaluated as photocatalysts. In addition to these products, pure phases “standard” samples were prepared and evaluated as well. It is important to mention, that products mentioned in Table 1 are collected after reaching the steady-state in the CSTR [1].
Table 1

TiO2 phases produced under different conditions; comparison with the standard anatase, brookite, and rutile powders

 

Standard Anatase

Exp 101

Exp 103 (A)

Standard Brookite

Exp 102 (B)

Exp 104

Standard Rutile [20]

Feed solution TiCl4

0.1 M

0.1 M

0.1 M

0.1 M

0.1 M

0.1 M

0.5 M

Temperature

80 °C

80 °C

80 °C

80 °C

80 °C

80 °C

80 °C

Residence time

30 min

30 min

30 min

60 min

60 min

30 min

60 min

Agitation

800 rpm

800 rpm

800 rpm

800 rpm

400 rpm

800 rpm

800 rpm

pH Control (NH4OH)

pH 3

pH 1

pH 2

Noa

Noa

Noa

Nob

Anatase

100%

31%

81%

–

23%

18%

–

Rutile

–

69%

–

8%

–

26%

100%

Brookite

–

–

19%

92%

77%

56%

–

BET, S.S.A. (m2/g)

248.0 [20]

–

122.3

123.9

110.9

–

102.0 [20]

aSteady state pH ~ 0.7–0.8; bSteady-state pH < 0.5

$$ TiCL_{4} (aq) + 2H_{2} O \to TiO_{2} (s) + 4HCl(aq) $$
(3)
A major deleterious contaminant released from various waste waters generated from mining and metallurgical operations is selenium ; such as coal mining, pigment production and non-ferrous metallurgical sulfuric acid production plants [13–15]. Selenium is an important element at low (trace) concentrations, but very toxic at high ones. Environmental regulations stipulate that it should be less than 1 ppm for discharge water and 5 ppb for drinking water [16]. Selenium as an element belongs to the VIA group of the Periodic Table and exists in nature in organic and inorganic forms. The most common oxidation states of selenium are four; elemental selenium (Se0), selenide (Se(-II) or Se2-), selenite (Se(+IV) or SeO32−) and selenate (Se(+VI) or SeO42−), as well as other organic forms. Oxidized inorganic forms of selenium exist as highly soluble oxyanions in aqueous systems. The removal -elimination of selenium from waste water depends on its speciation, i.e. oxidation state and type of complexes. The reduction reactions of the two selenium ion species, selenate and selenite are given by Eqs. 4 and 5 respectively, along with the respective standard potentials (corresponding to pH 0 and activities of soluble species = 1 (~1 M)) [6].
$$ SeO_{4}^{2 - } + 3H^{ + } + 2e^{ - } \rightleftharpoons HSeO_{3}^{ - } + H_{2} O\quad E = 1.060V $$
(4)
$$ SeO_{3}^{2 - } + 6H^{ + } + 4e^{ - } \rightleftharpoons Se^{0} + 3H_{2} O\quad E = 0.903V $$
(5)
The interrelationship of oxidation states (determined by the prevailing) oxidizing-reducing potential and species (determined by pH) can be better appreciated with the aid of the Pourbaix phase diagram shown in Fig. 2. As it can be seen, it is thermodynamically determined that Se(VI) is the predominant oxidation state in strong oxidizing environments, while Se(IV) in moderate potential range. Elemental Se is predominant in reducing environments. Thus at pH 3 (dotted vertical line) according to the thermodynamic diagram, the reduction of Se(VI) to Se(IV), $$ \frac{{SeO_{4}^{2 - } }}{{HSeO_{3}^{ - } }} $$ occurs at ~0.8 V, while the reduction of Se(IV) to Se(0), $$ \frac{{SeO_{3}^{2 - } }}{Se} $$ occurs at ~0.55 V. Of the two reduction steps, the reduction of selenite is more reactive than selenate, i.e. while both steps (Eqs. 4 and 5) are energetically favored, nonetheless the former one occurs slowly [14, 17], which makes the selenite reduction to elemental selenium much easier [13, 18].
../images/468727_1_En_150_Chapter/468727_1_En_150_Fig2_HTML.gif
Fig. 2

Potential-pH diagram of the selenium -water system (Se conc. = 7 * 10 − 3 M) [19]

Experimental Procedure

Synthesis of Titania Powders

The synthesis process involves hydrolytic precipitation of titanium dioxide nanoparticles, operating at a steady state in a CSTR (2 L Applikon jacketed reactor, Fig. 3). Stock solution of 2 M TiCl4 is prepared in an ice-bath and kept refrigerated. Solution Ti(IV) concentration is determined by ICP-OES. pH is controlled in the CSTR through continuous addition of NaOH solution (19 M) at a pre-selected value in the range 1–4. Solution temperature is controlled at 80 °C with a recirculating oil bath. For starting up, the reactor is filled first with 1 L “charge” solution of 0.01 M TiCl4 and then the feed TiCl4 solution, 0.1–0.5 M, is continuously pumped into it at a certain flow rate (33 mL/min or 16.5 mL/min for a residence time of 30 min and 60 min respectively), while being agitated at 800 rpm.
../images/468727_1_En_150_Chapter/468727_1_En_150_Fig3_HTML.jpg
Fig. 3

Synthesis of TiO2 in continuous stirred tank reactor by forced hydrolysis of aqueous solution of TiCl4

Evaluation of Photocatalytic Activity

The synthesized TiO2 nanoparticles are evaluated and compared to commercial Evonik P25 photocatalyst in terms of their photocatalytic activity, initially by measuring the photo-degradation of a commonly used organic model compound, methyl orange by UV irradiation [4] and further the photo-reduction of selenium species (work in progress).

Two different photocatalytic reactor set-ups are used. In the first one, the Ultraviolet light source is placed inside the solution (Atlantic Ultraviolet Corporation, output: 10 W, peak: 254 nm) in a custom-made closed cylindrical chamber and the methyl orange concentration is monitored with the aid of its characteristic absorption peak at 464 nm. The degradation efficiency is calculated using the Eq. 6, where A0 is the absorption peak of methyl orange at 464 nm at time 0; and At is the peak at different irradiation time.
$$ D\% = \left( {\frac{{A_{0} - A_{t} }}{{A_{0} }}} \right)*100 $$
(6)
In the second set-up, four lamps of Ultraviolet light source are placed outside of the Atlas HD, Syrris automated, modular jacketed reactor, see Fig. 4 (Atlantic Ultraviolet Corporation, U-shaped output: 18 W) and the concentration of selenium is monitored and determined by the ICP-OES. The starting selenium solutions for selenate, Se(VI) and selenite, Se(IV) were prepared from sodium selenate, Na2SeO4 and sodium selenite, Na2SeO3 solid respectively. pH is pre-selected by the addition of NaOH and HCl solution at value in the range 1–7 and monitored by the reactor during the experiment. Solution temperature is kept constant at room temperature , while purging nitrogen gas continuously in the reactor. The reactor is connected to two scrubbers; copper (II) sulfate, CuSO4 and sodium hydroxide, NaOH in case of hydrogen selenide gas, H2Se forming in the reactor.
../images/468727_1_En_150_Chapter/468727_1_En_150_Fig4_HTML.jpg
Fig. 4

Custom-adapted photocatalytic reactor (Atlas HD, Syrris) for selenium species reduction

Results and Discussion

Synthesis of Titania Nanoparticles

A series of synthesis experiments were performed using the CSTR reactor by changing the process parameters, like Ti(IV) feed concentration, pH, temperature and agitation speed. Table 1 summarizes the characteristic conditions tried and resultant products produced of different TiO2 nanophases for photocatalytic evaluation. As it can be seen from the results of Table 1, increasing pH (from 1 to 3) favored the production of TiO2 with increasing anatase content. At lower pH, approximately pH~0.8 attained at steady-state without base addition, mixed anatase-rutile-brookite phase TiO2 -brookite was the major phase-was obtained. Lower than that pH, associated with higher Ti(IV) concentration promoted the formation of predominantly rutile TiO2 . In other words, brookite seems to form at intermediate pH values vis-a-vis those for anatase (pH ≥ 2) and rutile (pH ≤ 0.6) for the given set of conditions- a trend in agreement with Yasin et al.’s work [1, 20].

XRD, SEM and TEM Characterization

The XRD patterns of two selected products are shown in Fig. 5I and compared to the JCPDS reference data of anatase, brookite and rutile; #01-071-1166, #00-016-0167 and #01-079-5858 respectively to identify their phase content. As it can be seen the A product’s pattern can be matched with the key peaks of anatase. Using the TOPAS software the A material was determined not to be pure anatase, but to have an approximate composition of 80% anatase and 20% brookite. By reference pure anatase (“standard”) was synthesized by Yasin et al. [1] at pH 3. In the meantime, the XRD pattern of B product, as it can be seen in Fig. 5I-b, consists of peaks of both brookite and anatase with the former to dominate (see shoulder at 26°). The same pattern shows a faint peak corresponding to rutile. However, analysis of the data with TOPAS yielded an approximate composition of 80% brookite and 20% anatase; a minor (<10%) rutile component cannot be ruled out.
../images/468727_1_En_150_Chapter/468727_1_En_150_Fig5_HTML.gif
Fig. 5

I XRD patterns and II Electron microscopy images of nanotitania products A and B; (I-a) and (II a–c) anatase, product A and (d)–(f) brookite, product B; (II a,d) SEM images; (II b,e) TEM images; (II c,f) high magnification TEM images

SEM and TEM images of the two powders are shown in Fig. 5II. The two products seem to have differences in their morphology , as product A (Fig. 5II a–c) has mostly spherical nanocrystallites clustered together in larger aggregates [1]. The B titania (Fig. 5II d–f) appears to have very thin nanopetal-like morphology . Using the d-spacing of the crystallites in the high magnification TEM images, we identify both phases, as anatase (101) and brookite (120) gave close values namely 3.5 Å and 3.4 Å, respectively.

Photocatalytic Evaluation

Photo-degradation of methyl orange was monitored using UV–Vis spectroscopy . In Fig. 6a, absorption spectra of methyl orange are shown as function of UV irradiation time, while in Fig. 6b the calibration Beer’s Law curve used to convert absorption data to concentration is shown. nano-TiO2 phases were tested as physical mixtures of pure anatase, brookite and rutile. A sample of results in the form of photo-degradation (following equilibration in the dark) of methyl orange is shown in Fig. 6c in comparison to the benchmark material P25.
../images/468727_1_En_150_Chapter/468727_1_En_150_Fig6_HTML.gif
Fig. 6

a Evolution of UV–Vis absorption spectra of methyl orange degraded by TiO2 under UV irradiation; b Beer’s law calibration curve for methyl orange; c degradation of methyl orange (25 mg/L initial concentration) under UV-light irradiation using different physical TiO2 phase mixtures

Preliminary tests using the Atlas reactor and 25 mg/L Se(VI) solution of pH = 3 resulted in variable degree of reduction (from 20 to 65%) depending on type of nanotitania phase used. Photocatalytic action was promoted with the addition of formic acid, HCOOH at different concentrations serving as hole scavenger. Selenium (VI) reduction occurs on the surface of the nano-TiO2 particles via adsorption and chemical reduction , under UV illumination, as it can be evaluated visually with the pictures of Fig. 7. Further work is underway characterizing the reduction kinetics and TiO2 surfaces, evaluating alternative hole scavengers and strategies for engineering the nanoparticles into porous sorption media along with appropriate regeneration methods.
../images/468727_1_En_150_Chapter/468727_1_En_150_Fig7_HTML.jpg
Fig. 7

Different types of nano-TiO2 phases after adsorption and chemical reduction of Se(VI) under UV illumination with the addition of HCOOH

Conclusion

Mixed phase TiO2 nanoparticles were synthesized by modifying the process parameters like pH, concentration and residence time of a continuous stirred tank hydrolysis reactor operating at 80 °C. Synthesized nanoparticles with brookite, anatase and rutile in different percentages were characterized and tested as photocatalysts for organic and inorganic pollutants (Se(IV) and Se(VI), work in progress). To evaluate the photocatalytic activity, the degradation rate of a model organic compound, methyl orange (MO ), under UV light irradiation was investigated and monitored by UV–Vis spectroscopy analysis. Synthesized TiO2 nanoparticles with mixed phase composition show equivalent photocatalytic activity to commercial powder titania Evonik P25 photocatalyst in terms of organic pollutant degradation but superior performance in terms of selenate reduction .

Funding sources

This work is funded by NSERC through a discovery grant to G. P. D. K. C. would like to acknowledge the Faculty of Engineering for offering her the McGill Engineering Doctoral Award (MEDA) scholarship.