chitosan-montmorillonite

Journal of Water Process Engineering 31 (2019) 100843

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Mechanistic of photocatalytic decolorization and mineralization of methyl orange dye by immobilized TiO2/chitosan-montmorillonite

T

⁎

N.N. Bahrudin , M.A. Nawi School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia

A R T I C LE I N FO

A B S T R A C T

Keywords: Chitosan-montmorillonite Degradation mechanism Mineralization pathway Methyl orange Titanium dioxide

The mechanistic of photocatalytic decolorization and mineralization of methyl orange (MO) dye have been successfully studied using immobilized titanium dioxide/chitosan-montmorillonite (TiO2/CS-MT), a combination of TiO2 as the top layer and CS-MT as the sub-layer on a glass plate. The immobilized CS-MT film was selected over the CS film since the former adhered stronger and swelled less than the latter which showed its favorability in the aqueous medium. This bilayer photocatalyst operated via a simultaneous occurrence of adsorption of MO dye from the CS-MT layer and photocatalytic degradation by TiO2 layer. The bilayer photocatalyst could remove the MO dye from the solution 3 times faster than the single TiO2 within 90 min of irradiation under a UV–vis lamp due to strong adsorption of MO by the CS-MT sub-layer. The mechanistic study revealed that the degradation occurred via the e−/%O2- route while the LC–MS analysis suggested the possible degradation pathway of MO dye. It was found that the eluent used for separation of compounds in the LC–MS analysis influenced the intermediates detected for which two degradation mechanisms were proposed.

1. Introduction The synthetic dyes that are largely consumed by the industries can be classified to acid, basic, direct, disperse, mordant, reactive, sulfur, azo and vat dyes whereby azo dyes are the major units and account for 70% of all the dyestuffs produced [1]. The raw materials that are commonly used are hydrocarbons, benzene, toluene, naphthalene and anthracene [2]. One of the synthetic dyes is methyl orange (MO), 4(dimethylamino) phenyl azo benzenesulfonic acid. MO is produced from diazotization of the sulfanilic acid, sodium nitrite and dimethylaniline and used for dyeing in textile, printing, pharmaceutical, paper manufacturing, food industries, leather industries and research laboratories [3,4]. It has a very short excited-state life and is stable in visible and near-UV light. As one of the azo dye family, MO is carcinogenic due to its decomposition to aromatic amines which are hazardous to the aquatic organisms and the food web [5]. In addition, the dye can be transported easily in water due to its solubility and oxidized to form the highly toxic, mutagenic and hazard compounds. Until now, many adsorbents have been used to remove the dyes from wastewater namely silica oxide (SiO2) [6], polyaniline (PANI) [7], montmorillonite (MT) [8], activated carbon (AC) [9] and chitosan (CS) [10]. More advanced works have been done by coupling of two or more adsorbent materials so that these composite type-adsorbents exhibit

⁎

enhanced properties in selectivity, regeneration, surface area, mechanical strength and surface chemistry [11]. In this regard, the combination of CS and MT in various CS-MT composite forms has been proven to improve the pore size, mechanical strength, chemical stability, hydrophilicity and biocompatibility of CS [12]. As generally known, the removal of the dyes by sole adsorption process cannot destroy the dye completely as the process is only a liquid-solid phase transfer process [13,14]. Moreover, some of the adsorbents are not reusable and even saturated after the dye adsorption which make them costly ineffective. Since some dyes are very stable in the environment, non-biodegradable and carcinogenic [15], the CS-MT adsorbent should be combined with a photocatalyst such as TiO2 so that the dyes can be converted into harmless products. The combination can also overcome the TiO2 drawback due to its poor adsorption capacity of some pollutants which influences its degradation performance [16]. This synergistic photocatalysis-adsorption process has shown enhanced photocatalytic performance than the bare TiO2 in the removal of various organic contaminants [9,17–19]. In general, the fundamental of TiO2 mediated photocatalysis is based on the electron excitation from the photocatalyst valence band to its conduction band producing the electron-hole pairs [20]. The electron and hole of TiO2 react with O2 and H2O, respectively to form the oxidative radicals for degrading the organic compounds at ambient

Corresponding author. E-mail address: [email protected] (N.N. Bahrudin).

https://doi.org/10.1016/j.jwpe.2019.100843 Received 21 November 2018; Received in revised form 20 April 2019; Accepted 26 April 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Water Process Engineering 31 (2019) 100843

N.N. Bahrudin and M.A. Nawi

where for this study, the optimum loading of 2.5 mg cm−2 was coated on the dried CS-MT plates.

temperature and atmospheric pressure [21]. However, the band gap energy of TiO2 lies in the UV region making them inactive under visible light irradiation [22,23]. Many efforts have been manifested to enable and enlarge the TiO2 applications under visible light such as doping with metals, nonmetals, semiconductors and adsorbents [24]. The visible light activity is supported by the enhancement in electrochemical and electronic properties as well as adsorption process throughout the entire photocatalytic oxidation process [25,26]. Enhanced separation of the charge carriers of TiO2 will eventually produce more oxidative radicals to oxidize the organic contaminants. Moreover, some dyes can be self-sensitized under visible light irradiation making the dye treatment by TiO2 becomes more favorable [27,28]. This study combined the TiO2 and CS-MT composite as an immobilized bilayer photocatalyst to solve the conventional post-treatment problem of filtration. This type of immobilization assemblage could also overcome the reduction in the exposed surface area after immobilization which frequently slows down the photocatalytic activity. The TiO2 properties would not be discussed in the present study since the details have been extensively reported by Nawi and Zain [29]. Only the physicochemical properties of CS-MT sub-layer were investigated through several selected tests and analyses. The present work hopefully can give an insight into the mechanistic of photocatalytic decolorization by TiO2 combined adsorbent as the adsorption of the dye specifically onto the adsorbent is involved as well as predicting the dye mineralization pathways using different LC–MS eluent mixtures.

2.3. Characterizations The adherence test to determine the strength of the CS and CS-MT plates was performed following the reported method [30]. The initial weight of the cast plates were measured and they were immersed in a beaker containing water, respectively and sonicated in an ultrasonic bath for 5 s, dried in the oven and weighed. The steps were repeated until 30 s. Meanwhile, the swelling study was performed by peeling off the adsorbent film from the glass plate using a razor blade and the film was soaked in 10 ml of water in a vial for 24 h. After taken out, the wet film was patted slightly with a paper tissue before weighing. The adhesiveness and swelling ratio of the polymer adsorbents are calculated using the following equations:

Adhesiveness (%) =

(W1 − W2) x 100 m

(1)

Swelling ratio (%) =

Ws− Wd x 100 Wd

(2)

Here, W1 is the initial weight of the plate, W2 is the weight of the remaining adsorbent on the plate after 30 s, m is the initial weight of the immobilized adsorbent on the plate, Ws is the weight of swollen sample (g) and Wd is the weight of dried sample. The determination of textural properties of the CS and CS-MT samples were carried out at 77 K under N2 gas flow using a physisorption analyzer (Model ASAP 2010, Micrometrics). The functional groups of CS and CS-MT composites were identified using a Fourier transform infrared (FTIR) spectroscopy (Model Series 2000, Perkin Elmer) at 650-4000 cm−1 with a resolution of 4 cm−1 while the surface morphology, composition and crystallography observations of the samples were done on a high resolution transmission electron microscope (HRTEM) (JEM-2100 F, JEOL).

2. Materials and methods 2.1. Materials Titanium (IV) oxide, Aeroxide® (TiO2; 80% anatase, 20% rutile) was purchased from Jebsen & Jessen Degussa Chemicals (M) Sdn. Bhd., while CS flakes (68.2% degree of deacetylation with a molar mass of 322 g mol−1) and montmorillonite (MT, K-10) were supplied by SigmaAldrich. MO dye in powdered form, manufactured by BDH Ltd. (color index no = C.I 13025, MW: 327.33 g mol−1, molecular formula: C14H14N3NaO3S) was selected as the model pollutant while glacial acetic acid (99.8%) was bought from System®. Epoxidized natural rubber (ENR) and polyvinyl chloride (PVC) powder were supplied from Guthrie Group Sdn. Bhd. and Petrochemical (M) Sdn. Bhd., respectively. Toluene (C7H8) and dichloromethane (CH2Cl2) were the organic solvents for ENR and PVC dilution, respectively. 1,4-benzoquinone (C6H4O2) and disodium hydrogen phosphate (Na2HPO4) were bought from BDH Chemicals Ltd whereas ethylenediaminetetraacetic acid (EDTA) disodium salt was from Ajax Chemicals. Methanol and acetonitrile (HPLC grade) used for eluent preparation were also purchased from Merck. Ultra-pure water (18.2 MΩ cm−1 of conductivity) was used for dilution and solution preparation. All chemicals were used as received without purification.

2.4. Photocatalytic reactor set-up The photocatalytic reactor in Fig. 1 was equipped with a 45-Watt

2.2. Preparation of TiO2/CS-MT plates CS flakes in the range of 0.25 g to 1.25 g of loading were mixed with a fixed amount of MT powder of 0.09 g, respectively. All the mixtures were dispersed in 50 ml of 5% (v/v) of acetic acid solution by grinding with zirconium beads in a ball mill grinder at the rate of 40 rpm for 6 h in a Schott bottle. The solutions were cast directly on the glass plates with 4.7 cm × 7.0 cm × 0.2 cm of dimensions. The wet plates were dried in open-air and an oven at 100 °C for overnight and 2 nights, respectively before the dried weight was measured. The preparation of the TiO2 on the top layer of CS-MT has been explained previously by Nawi and Zain [29] where the TiO2 formulation was prepared based on the reported method. Briefly, 6 g of TiO2 powder in 100 ml of ENR/ C7H8 and PVC/CH2Cl2 adhesive blend mixture was sonicated until a homogeneous formulation was obtained. The amount of TiO2 loading was determined by weighing the plate before and after dip-coating

Fig. 1. Photocatalytic oxidation reactor set up: a) Pasteur pipette, b) glass cell, c) coated plate, (d) PVC tubing, e) aquarium pump, f) compact fluorescent lamp and g) power source. 2

Journal of Water Process Engineering 31 (2019) 100843

N.N. Bahrudin and M.A. Nawi

Table 1 Physicochemical properties and removal efficiency of MO of different ratios of CS-MT composites. CS loading (g)

MT (g)

Strength (%)

Swelling ratio (%)

MO removal (%)

BET surface area (m2 g−1)

Pore volume (x 10−2 cm3 g-1)

Average pore diameter (nm)

Average particle size (μm)

0.25 0.50 0.75 0.75 1.00 1.25

0.09 0.09 0.09 0.00 0.09 0.09

94.5 95.3 95.3 83.1 97.3 97.8

77.2 66.9 56.5 93.6 41.7 33.6

47.7 49.4 91.8 80.2 81.1 64.7

4.38 3.82

0.076 0.081

6.96 8.39

2.21 5.24

2.27

0.027

4.74

4.26

home fluorescent lamp (Philips), an aquarium pump, a Pasteur pipette and a PVC tubing. The UV leakage of the lamp was measured by a radiometer equipped with UV-A and UV-B broadband detectors from Solar Light Co (PMA 2100). The glass cell used in this experiment was 5 cm × 8 cm × 1 cm of dimensions where the prepared plates were put vertically and the coated area of the plates faced the lamp. The directly measured UV leakage of the lamp was 2.78 Wm−2 and 1.78 Wm-2 with glass cell. A total visible light irradiation was conditioned by placing a UV cut-off filter in front of the lamp. Meanwhile, the adsorption study was performed using the similar set-up in which the glass cell was put into a sealed box and the lamp was taken out. The photocatalytic decolorization and adsorption experiments were conducted using 20 ml of 20 mg L-1 of MO solution, ambient pH of 6.5 at room temperature (30 °C) and contact time of 60 min with 15 min of time interval. The absorbance of MO solution between each interval was measured using a direct reading spectrophotometer (HACH DR/2000) at 464 nm. The MO removal (R) and the apparent pseudo-first-order rate constant (k) from the Langmuir-Hinshelwood kinetic model were calculated using the following equations, respectively;

R (%) =

Ce x 100 Co

3. Results and discussions 3.1. Optimization of CS loading for the fabrication of CS-MT sub-layer The effect of the amount of CS loading within the CS-MT plate is significant since CS can function as bioadhesive and biosorbent [32]. In this study, the CS-MT plates were prepared from the casting solutions consisting of a series of CS flakes amount from 0.25 g to 1.25 g with a fixed MT loading of 0.09 g, respectively. The purpose of adding MT clay to the CS matrix is to enhance the physicochemical properties of the composite film for extended usage in aqueous environment [33]. However, only a little amount of MT was added into the casting solution as an excessive amount of the clay would result in the brittleness of the composite film. The suitability of this polymer composite adsorbent for an aqueous application was analyzed using the adherence and swelling tests. From the test’s result in Table 1, an increase in the CS amount from 0.25 to 1.25 g within the CS-MT plates, has significantly increased the mechanical strength of the plates from 94.5 to 97.8%. That was likely due to the increasing amount of the positively charged amino groups of CS which facilitated the electrostatic attraction with the negatively charged silicate of the glass plates and subsequently increased the adhesion of CS-MT on the plates. Meanwhile, the plate coated with 100% of CS exhibited a mechanical strength of 83.1% after the 30-sec adherence test which shows that 16.9% of CS was lost from the adsorbent initial weight. Here, it was proven that the CS could function as an adhesive and bind very well to the glass plate whereby the strength of the immobilized adsorbent was much better in the presence of MT clay. The data for the swelling test of the similar CS-MT composition series are tabulated in Table 1. The swelling test indicates that for a lower degree of swelling, the corresponding plates are more hydrophobic and less hydrophilic and vice versa. In this case, the increment of the CS amount from 0.25 to 1.25 g within the composite has decreased the swelling ratio of the plates from 77.2 to 33.6% due to the increased hydrogen bonding between the hydroxyl and amino groups of CS with the silicate groups of MT clay. This will reduce the CS-MT composite contact with water molecules to become more hydrophobic [34,35]. Apparently, the presence of MT has resulted in a stronger CS composite on the solid support as compared to CS film since the MT clay hardened when it is in contact with water. Besides, the clay also expands the volume and lengthens the distance of water molecules causing the hydrophobicity of the composite [36]. As for CS content, the plates with higher CS induce stronger CS-MT plates as more of the positively charged amino groups can bind to the negatively charged glass plate. Higher CS can also form less hydrogen bonding with the adsorbed water, decreases the swelling properties of the adsorbent, thus it is more suitable for longer usage. Since some of the amino groups of CS are used for the CS-MT clay bonding as well as binding with the glass plate, an optimum amount of CS flakes is required to obtain the best CSMT plate in term of adherence, swelling properties and MO removal efficiency. It can be observed that increasing the amount of CS flakes from 0.25 to 0.75 g in the CS-MT casting solution has also increased the MO removal by the corresponding CS-MT plates from 47.7 to 91.8%. The

(3)

Co ln ⎛ ⎞ = kt ⎝ Ct ⎠

(4)

Here Co, Ce and Ct is the concentration of MO dye (mg L−1) at initial, equilibrium and time, t, respectively. The k values (min-1) can be obCo tained from the slope of the ln Ct versus t plot.

( )

2.5. Mechanism analyses The role of oxygen in the photocatalytic degradation by TiO2/CSMT was carried out in the absence of O2 by supplying the N2 gas from the gas tank via a PVC tubing which was connected to the Pasteur pipette of the photocatalytic reactor set-up. The top of the glass cell was covered tightly with a parafilm wrapped around the entrance of the cell to prevent the entry of O2 from the air. Meanwhile, the TiO2/MO-saturated plate was used for the detection of radical quenchers where the MO was adsorbed onto the CS-MT plate until fully saturated before coating with TiO2. Then, about 0.001 M of EDTA and 1,4-BQ solutions were spiked into a 20 ml of 20 mg L−1 MO solution, respectively. For the intermediates detection, the plate of TiO2/CS-MT was firstly photoetched for 10 h to eliminate any interferences especially from the degradation of polymer binder within TiO2. The samples for LC–MS analysis were collected after 1 h of irradiation of the MO solution. The analysis was done on a LC–MS instrument from Agilent Technologies under the flow rate of 0.3 ml min-1 using 0.025 M phosphate buffer (pH 6.9): methanol = 40:60 (v/v%) [31] and ultra-pure water: acetonitrile = 50:50 (v/v%) as the eluents, respectively using a reversed phase column (LC-18 Supelcosil) with dimensions; 25 cm × 4.6 cm × 5 μm from SUPELCO™. The obtained chromatograms and mass values were evaluated using the Mass Hunter Qualitative Analysis B.04.00 software. 3

Journal of Water Process Engineering 31 (2019) 100843

N.N. Bahrudin and M.A. Nawi

absorbance at 268 and 464 nm peaks with increasing treatment time corresponds to the breaking of aromatic ring and azo bond of MO, respectively. That means more MO molecules were being degraded by TiO2/CS-MT at the same time as by TiO2 showing enhanced decolorization of the dye by the former photocatalyst due to the contribution of adsorption process by CS-MT sub-layer as compared to TiO2.

increasing CS loading provides more positively amino groups for the negatively charged MO to bind onto the CS-MT composite, thus improves its removal ability. However, increasing the amount of CS over 1 g within the CS-MT casting solution decreased the removal efficiency of MO to 44.7%. This drastic change is related to the sudden reduction of surface area of the composite adsorbent with higher CS content corresponding to the availability of adsorption sites to adequately bind all the MO molecules. Herein, the best amount of CS flakes at the fixed MT loading in the CS-MT casting solution was 0.75 g which is equivalent to 89% of CS within the composite showing the best removal efficiency, with comparable mechanical strength and swelling ratio properties. As a reference, the CS film that was prepared from the 0.75 g CS casting solution only removed 80.2% of MO from the aqueous solution. From the surface area perspective, the incorporation of MT within the CS matrix has increased the BET surface area from 3.82 to 4.38 m2 g−1 but reduced the pore volume and pore diameter from 8.1 to 7.6 × 10-2 cm3 g−1 and 8.39 to 6.96 nm, respectively. However, increasing the CS content at fixed MT loading has essentially reduced the surface area of the CS-MT composite as well as the pore volume and pore diameter to 2.27 m2 g−1, 2.7 × 10-2 cm3 g−1 and 4.74 nm, respectively. Similar trend can be observed for particle size where the size decreased when the clay was present and then increased with high CS content within the composite. From these data, it can be deduced that MT clay is a surface modifier which can affect the characteristics of pores while an excessive CS content exhibits inhibitory effect on the properties of the composite’s pores. Nonetheless, all the pores either with and without MT samples are categorized as mesopores (2–50 nm) according to IUPAC classification [37].

3.3.2. Reusability The reusability of TiO2/CS-MT and its counterpart TiO2 for the removal of MO under UV–vis light irradiation was compared by means of percentage of MO removed as shown in Fig. 4. High percentage of MO removed means more MO molecules were being degraded by TiO2/CSMT leaving less MO remained in the solution. In this case, the dye treatment by TiO2/CS-MT in the presence of light showed the best removal, followed by the reaction in the dark (adsorption) and irradiated TiO2. The TiO2/CS-MT under light irradiation removed almost 95–98% of MO throughout the 10 cycles. The bilayer photocatalyst could maintain its removal throughout the cycles due to the influence of adsorption of MO by the CS-MT sub-layer and photocatalytic reaction by the TiO2 top layer. The adsorbed MO within the adsorption sites of bilayer photocatalyst was removed by the %OH radicals continuously at each cycle and therefore similar sites could be provided for the reaction at the next cycle. During the treatment in the dark (adsorption), a good removal of MO was achieved where about 90% of MO were removed at the first three cycles. However, after the 4th cycle, the removal declined continuously from 88-77% until the 10th cycle. The reason was due to the saturation of adsorption sites within the TiO2/CS-MT after the repetitive cycles, as the adsorption process did not degrade MO to smaller products as compared to the photocatalytic oxidation [41]. Therefore, the MO molecules continuously filled up, remained within and saturated the adsorption sites. This event eventually decreased the removal efficiency significantly. Meanwhile, the single TiO2 operated under the light irradiation could only remove an average of 30% of MO throughout the ten cycles mainly due to the poor adsorption of MO on the photocatalyst surface [18].

3.2. Physical characterizations Fig. 2 shows the FT-IR spectra for the detection of any changes in the functional groups of CS after the MT addition. The peaks of the main functional groups of CS are 3447 cm−1, 1641 cm−1 and 1559 cm−1 which correspond to the overlapping of the eNH and eOH stretching vibrations, amide I and amide II, respectively [38]. After becoming the composite, a broad peak at around 3410–3480 cm−1 is observed that relates to the overlapping of eNH and eOH stretching vibrations of CS and eOH groups of silanol of MT [11,39]. The appearance of a new peak at 1559 cm−1 of CS-MT composite is detected due to the bending vibration of –NH2 of amide II within the CS structure. The morphology of CS and CS-MT surface were then taken under a high-resolution transmission electron microscope (HRTEM) and the corresponding micrographs are shown in Fig. 2. The CS flakes were well dissolved in the acid solution as the CS distribution is homogeneous and no significant particulate can be seen in the micrograph. For the CS-MT micrograph, it can be seen that the MT clay was well-distributed throughout the polymer matrix where no agglomeration and aggregation of CS and MT is observed. The composite surface seems heterogeneous as the clay particles had occupied the CS matrix.

3.4. Identification of the main species in the photocatalysis-adsorption processes A mechanistic study was performed to identify the main active species that are responsible in the photocatalytic-adsorption processes of the TiO2/CS-MT photocatalyst. The study used N2 gas for providing an anaerobic condition, a UV filter for a total visible light condition and − chemical solutions, 1,4-benzoquinone (1,4-BQ) as the superoxide (O2% ) + quenchers and EDTA as the positive holes (h ) quenchers [42] and compared with the normal conditions. The mechanism of photo− catalysis is well established whereby the production of O2% and h+ % which are responsible for the OH radicals formation are depicted in Equation 5–12 [31–33]. On the TiO2 surface, the photocatalytic oxidation is initialized when sufficient or greater energy of light than the band gap energy of TiO2 (Eg =3.2 eV) is applied whereby the electron in the valence band of TiO2 is excited to the conduction band (CB) and leaving the hole (h+) at the valence band (Equation 5). At the CB, the − excited electrons react with the O2 to produce the O2% , HO2%, and H2O2 radical ions (Equation 6–10) which then are converted to the %OH radicals The photo-generated h+ at the VB will oxidize the H2O molecules − (Eq. 11) and OH ions (Eq. 12) at the surface or in the TiO2 bulk to % produce the OH radicals.

3.3. Photocatalysis-adsorption processes 3.3.1. UV–vis spectral changes The optimized CS-MT sub-layer was coated with TiO2 on the top layer to produce the TiO2/CS-MT bilayer photocatalyst. The photocatalyst was used to treat the MO dye solution under the UV–vis light irradiation and was compared with the single TiO2 layer. The disappearance of MO dye as detected via UV–vis spectrophotometer during the photocatalytic decolorization by TiO2/CS-MT and TiO2 photocatalysts are shown in Fig. 3a and b, respectively. The main absorbance peak of MO is at 464 nm in the visible region corresponding to the azo bond of MO which is responsible for the disappearance of its original color while the peak at 268 nm is related to the benzene ring of the dye chromophore [40]. It can be observed that the reduction in

TiO2 + hv → h+ + e −

−

(5)

%−

e + O2 → O2

(6)

−

O2% + H+ → HO2% (hydroperoxyl) %

4

HO2 +

HO2%

H2O2 +

− O2%

→ H2O2 + O2 %

→ OH +

−

OH + O2

(7) (8) (9)

Journal of Water Process Engineering 31 (2019) 100843

N.N. Bahrudin and M.A. Nawi

Fig. 2. FT-IR spectra (top) and HRTEM image (bottom) of (a) CS and (b) CS-MT at 145,000 x magnifications, respectively.

−

H2O2 + e → %OH +

−

OH

h+ + H2O → %OH + H+ h

+

−

+ OH

%

→ OH

0.051 min−1, respectively by adsorption process as compared with 89.6% and 0.051 min−1, respectively in the presence of O2 gas. Although the N2 gas is inert and does not involve in the photocatalytic reaction, the gas can still provide a driving force for the mass transfer of the dye onto the TiO2/CS-MT and assist in the adsorption process as similar as the removal by O2 that eventually contributes to the high − removal and rate constant achieved. However, no O2% radicals can be generated from the electron excitation process in the presence of N2 gas as it is not an electron scavenger whereby the removal efficiency and rate constant were merely from the adsorption process. This shows that the O2 is an important source for the %OH radicals’ production for a complete photocatalytic decolorization of MO dye.

(10) (11) (12)

The efficiency and rate constant of MO removal by the TiO2/CS-MT photocatalyst under different experimental conditions are presented in Fig. 5a while similar parameters were observed using a TiO2/MO-saturated CS-MT photocatalyst in the presence of radicals’ quenchers as portrayed in Fig. 5b.

3.4.1. Effect of N2 gas During the photocatalytic degradation process, the excited electron − at the CB of TiO2 can reduce the adsorbed O2 to the O2% ions (Eq. 6), + thus inhibit the possibility of recombination with the h at the VB. In order to seek the effect of O2 on the photocatalytic activity of this TiO2CS-MTphotocatalyst, N2 gas was given to the photocatalytic reactor instead of O2 gas. As observed in Fig. 5a, the removal efficiency and rate constant decreased to 91.1% and 0.64 min−1, respectively when N2 gas was present as compared to the control condition (with O2) under the UV–vis light with 98.3% and 0.089 min−1, respectively. The observed percentage of MO removal and rate constant in the presence of N2 gas were still significant due to the contribution of the 87.4% and

3.4.2. Effect of radical quenchers In order to identify the production route of %OH radicals, the TiO2/ MO-saturated CS-MT was used. The CS-MT sub-layer was saturated with MO dye before coating TiO2 on the top layer to eliminate the adsorption process and ensure that only the active radicals contributed to the photocatalytic decolorization of MO dye. In this investigation, the 1,4-BQ (C6H4O2) and EDTA solutions were spiked into the MO solution containing the saturated plate, respectively and irradiated for 60 min. When 1, 4-BQ was present in the dye solution, the photocatalytic decolorization of MO slowed down as portrayed by a drop in the rate and percent of MO removed from 0.045 min−1 to 0.033 min-1 5

Journal of Water Process Engineering 31 (2019) 100843

N.N. Bahrudin and M.A. Nawi

Fig. 3. UV–vis spectral changes of degradation of MO solution by (a) TiO2/CS-MT and (b) TiO2 photocatalysts with increasing irradiation time. (TiO2 loading =2.5 mg cm−2; CS-MT loading =1.3 mg cm−2; [MO]=20 mg L-1; pH = 6.5; aeration flow rate =40 ml min-1).

Fig. 4. Reusability of TiO2/CS-MT (under light irradiation and in the dark) and TiO2 photocatalysts in the removal of MO for 10 consecutive cycles. (TiO2 loading =2.5 mg cm−2; CS-MT loading =1.3 mg cm−2; [MO]=20 mg L-1; pH = 6.5; aeration flow rate =40 ml min-1).

As shown in Fig. 5b, when the EDTA solution was present in the MO solution under light irradiation, the exhibited rate constant was as similar as without its presence which was 0.049 min−1 implying that the h+ was not the main source for the %OH radicals production and therefore, the degradation process did not cease without its presence. As there was no direct adsorption within the photocatalyst, it was proven that the photocatalytic decolorization of MO mainly occurred − via the e-/O2% route %.

and 92.7% to 69.6%, respectively. During the quenching process, 1, 4BQ molecule was reduced to the semiquinone radicals (Eq. 13) and in the presence of H+ ions, the semiquinone radicals underwent a reaction − with the O2% ions to form the hydroquinone (Eq. 14). This would es− sentially reduce the amount of O2% forming into the %OH radicals and eventually slow down the photocatalytic degradation process on the TiO2 top layer. C6H4O2 + e −

−

−

CB

C6H4O2 + O2

→ C6H4O2

%−

+ 2H+ → C6H6O2 + O2

(13) (14)

3.4.3. Effect of light Fig. 5a and b also depict the photocatalytic decolorization of MO dye under UV–vis and visible light conditions by TiO2/CS-MT and TiO2/MO-saturated CS-MT, respectively. A UV filter was used to cut off the UV radiation leakage and condition the lamp to emit total visible light irradiation. The TiO2/MO-saturated CS-MT plates were purposely used so that the only mode of MO decolorization was photocatalysis [42] in order to detect the photosensitization of TiO2 by the MO dye

Meanwhile, EDTA is known as a sacrificial electron donor where it can be reduced by the h+ to become an oxidized EDTA under light irradiation [43] as shown in Eq. 15. The consumption of h+ during the reaction with EDTA would essentially produce less %OH radicals which reduced the photocatalytic efficiency of the photocatalyst. EDTA + h+ → EDTA+

(15) 6

Journal of Water Process Engineering 31 (2019) 100843

N.N. Bahrudin and M.A. Nawi

Fig. 5. (a) Removal efficiency and rate constant after the 60 min-treatment of MO by TiO2/CS-MT under different conditions and (b) MO-saturated TiO2/CS-MT upon the introduction of quenchers’ solution (EDTA and 1, 4-BQ) under UV–vis and total visible light irradiations. (TiO2 loading =2.5 mg cm−2; CS-MT loading =1.3 mg cm−2; [MO]=20 mg L-1; pH = 6.5; aeration flow rate =40 ml min-1).

MOads* + TiO2 → MOads%+ + TiO2 (e−)

[44–46]. It was observed in Fig. 5a that the rate constant and removal efficiency of TiO2/CS-MT slightly decreased to 0.083 min−1 and 89.1%, respectively under visible light as opposed to 0.089 min−1 and 98.3% under UV–vis light source. Similarly in Fig. 5b, the activity of the TiO2/ MO-saturated CS-MT photocatalyst shows no much difference when used under the UV–vis and visible light irradiation where the rate constant and removal efficiency of the former was 0.045 min−1 and 92.7%, respectively while it was 0.045 min−1 and 89.7%, respectively for the latter condition. The reason could be due to the adsorbed MO dye that can be self-sensitized under visible light irradiation where the dye absorbs the light photon directly and then excites the electrons from its HOMO to LUMO level (Eq. 16) as seen in Fig. 6a. As the band gap of LUMO level of MO dye (-2.07 eV) is more positive relative to the CB edge potential of TiO2 (-4.21 eV), the electron transfer from the LUMO of the dye to the CB of TiO2 becomes feasible [47–49]. This eventually caused the photocatalytic activity of the bilayer photocatalyst to be also active under visible light irradiation. The electrons − can be also used to produce the O2% and %OH radicals and degrade all the dye molecules (Eq. 17) [50]. MOads + hv (vis) → MOads*

(17)

The results as presented by both figures mean that the O2 gas and − O2% ions were the main species involved in the photocatalytic decolorization of MO by TiO2/CS-MT whereby in their absence, the production of %OH radicals was suppressed and subsequently led to a decrement in the photocatalytic activity of the photocatalyst. 3.5. Mechanism of TiO2/CS-MT photocatalysis-adsorption processes A layer by layer photocatalyst was designed in order to remove the pollutant via dual processes of adsorption by the adsorbent sub-layer while being photocatalytically oxidized by the porous TiO2 on the top layer. It has been accepted that TiO2 plays the main role in the photocatalytic decolorization of pollutants while CS-MT is a good adsorbent of MO. Since TiO2/CS-MT is made up of the photocatalyst and the adsorbent, the photocatalysis and adsorption processes occur simultaneously upon irradiation by light. As observed in Fig. 6b, when light was irradiated over the MO dye, some of the dye molecules would be photocatalytically oxidized on the TiO2 top layer (route I). Some molecules would diffuse into the CS-MT sub-layer through the TiO2 top

(16) 7

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Fig. 6. (a) Mechanistic of a) photosensitization of TiO2 by MO dye and (b) photocatalytic decolorization of MO dye by TiO2/CS-MT.

layer (route II) due to high electrostatic attraction between the SO3− groups of the dye and the CS-MT sub-layer. −

NH3+-R + R-SO3 → R-SO3−- NH3+-R

harmless compounds such as CO2, H2O and mineral acids. This process continued until all the MO molecules were fully oxidized and mineralized by the photocatalyst. Apparently, the presence of CS-MT as the sub-layer increased the concentration of MO near the TiO2 surface and increased the possible chances of the dye molecules to collide and remain in contact with TiO2 nanoparticles for further reaction. Without the CS-MT sub-layer, the process of concentrating the pollutant molecules could be harder since the dye molecules adsorbed poorly on the TiO2 surface [18]. Moreover, the dye molecules would diffuse back into the bulk solution and overcome the mass transfer again to collide with TiO2, which eventually would decrease the decolorization rate and efficiency [53].

(18)

Based on the identified active species in previous section, the degradation of MO by this TiO2/CS-MT would not occur through the h+/%OH route at the VB of TiO2. Instead, the reaction of the excited electrons with the O2 at the CB of TiO2 would produce the oxidative − radical ions (O2% , HO2%, and H2O2) which were converted to the %OH radicals [42]. In addition, the adsorbed MO dye at the TiO2 surface and within the CS-MT sub-layer became a photosensitizer for TiO2 under visible light irradiation. At this point, the excited electrons at the LUMO level of MO dye were transferred to the CB of TiO2 and reacted with the O2 to produce more %OH radicals (route III). The MO%+ radicals could − also react with the O2% and %OH ions to be converted to the mineralization products [51,52]. −

MO%+/MO + O2% /%OH → Mineralization products

3.6. Detection of intermediates and photocatalytic mineralization mechanism The photocatalytic decolorization of the MO dye during the treatment is a result of the oxidative attack on its chromophore (eN]Ne) which cleavages the two aromatic rings on the parent structure [16,17]. However, a colorless solution does not mean a full mineralization has been executed as an amount of suitable energy is required to break down the parent structure from its aromatic ring to the aliphatic chain (small compound) [18,19]. For instance, the energy required to break

(19)

The CS-MT would continuously adsorb the MO dye until the concentration of MO in the bulk solution decreased and ceased. The adsorbed MO molecules within the CS-MT sub-layer would then migrate to the interface of TiO2 (route IV) where they would be oxidized by the % OH radicals at the interface of TiO2 and were mineralized into the 8

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N.N. Bahrudin and M.A. Nawi

Fig. 7. (a) The profile of LC–MS spectra of the identified MO and its intermediates after 1 h of photocatalytic decolorization by the TiO2/CS-MT using phosphate buffer: methanol mobile phase and (b) proposed mineralization pathway.

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Fig. 8. (a)The profile of LC–MS spectra of the identified MO and its intermediates after 1 h of photocatalytic decolorization by the TiO2/CS-MT using acetonitrile/ water eluent and (b) proposed mineralization pathway.

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Table 2 Identified intermediates by various photocatalyst systems, degradation and chromatographic separation conditions. Photocatalyst

Degradation conditions

Separation conditions

Identified intermediates based on m/z values

Ref.

Suspended TiO2 anatase

Mode = Sonophotocatalytic Volume =250 mL; [MO] =32.7 mg L−1;

Instrument = LC/MS; Column = C18 (50 mm × 4.6 mm); Eluent = methanol/10 mM ammonium acetate (30/ 70 v/v); Flow rate =1 ml min−1 Instrument = GC/MS; Colum = silica capillary (ID 0.25 mm, length 30 m); Extractor = dichloromethane;

322, 320, 308, 306, 292, 290 and 276

[56]

180, 174, 158, 121, 110, 109, 108, 93

[57]

289.2, 260.2, 240.3, 225.0, 224.2 and 156.2

[20]

290 and 276

[58]

320-321, 306-307, 304, 290-292, 276-277, 242-241, 227, 171, 173, 174, 151

[59]

320, 306, 290, 241, 173 and 171

[60]

304, 290, 276, 256, 105

[61]

160, 117, 101

[62]

320, 306, 304, 290, 276, 256, 226, 212, 156

[63]

320, 306, 304, 290, 276, 256, 242, 226, 212, 156

[64]

309, 304, 293, 253, 233, 217, 199, 177, 159, 119, 97

[65]

149, 148, 147, 137, 136, 134, 133, 132, 131, 121, 120, 119, 107, 106, 104, 93, 92, 79, 77, 67, 66, 65, 52, 51, 39

[31]

289, 255, 171, 169, 152, 113, 96, 89, 62, 59

[66]

(1) 309, 304, 293, 290, 277, 276, 261, 253, 233, 217, 199, 159, 97 (2) 304, 290, 276, 233, 173, 157, 119, 97, 62

This study

TiO2/K2S2O8

Suspended TiO2

Lamp = Xenon-arc (cut off at 320 nm) Mode = Microwave assisted UV; Volume =1500 mL; [MO] = 2486 5 mg L−1; Lamp = Microwave discharge electrodeless Mode = Photocatalysis; Volume =5 mL; [MO] =20 mg L−1;

Suspended TiO2

Lamp = Xenon (cut off at 340 nm) Mode = Photocatalysis; Volume =2200 mL; Lamp = Solar

Suspended TiO2 anatase

ZnO/TiO2

CuO-TiO2/rGO

Mode = Photocatalytic binary system; Volume =50 mL; [MO] =5 mg L−1; Lamp = Solar stimulator Mode = Photocatalysis; Volume =200 mL; [MO] =16.4 mg L−1; Lamp = Hg Mode = Sonophotocatalytic Volume =100 mL; [MO] =10 mg L−1;

ZnO-ZnCo2O4

Ag/ZnO

N-NaTaO3

Immobilized TiO2/ PANI

Lamp = Diffused sunlight Mode = Photocatalysis; Volume =20 mL; [MO] =10 mg L−1; Lamp = Tungsten Mode = Photocatalysis; Volume =90 mL; [MO] =16.4 mg L−1; Lamp = Hg Mode = Photocatalysis; Volume =100 mL; [MO] =20 mg L−1; Lamp = Hg Mode = Photocatalysis- adsorption; Volume =20 mL; [MO] =80 mg L−1;

None

None

Immobilized TiO2/ CS-MT

Lamp = Fluorescent Mode = Radiolysis; Volume =100 mL; [MO] =3270 mg L−1; Lamp = 60Co-γ Mode = Ozonation assisted UV Volume = 6 000 mL; Lamp = UV Mode = Photocatalysis- adsorption; Volume =20 mL; [MO] =80 mg L

−1

;

Lamp = Fluorescent

Instrument = HPLC/UV-vis diode array; Column = RP-C18 (250 mm × 4.6 mm; 5 mm particles); Eluent =10 mM acetonitrile/ammonium acetate (pH 6.8, 24/76 (v/v%)); Flow rate =0.8 ml min−1 Instrument = HPLC/UV-vis diode array; Column = RP-C18 (250 mm × 4.6 mm; 5 mm particles); Eluent =10 mM acetonitrile/ammonium acetate (pH 6.8, 24/76 (v/v)); Flow rate =0.13 ml s−1 Instrument = UHPLC-MS; Column = C18 (100 mm × 2.1 mm, 1.7 μm); Eluent = Solvent A: 0.1% formic acid in water, Solvent B: 0.1% formic acid in acetonitrile; Flow rate =0.4 ml min−1 Instrument = HPLC-MS; Column = C18 (2.1 mm x 100 mm i.d., 5 μm); Eluent =10 mM acetonitrile/ammonium formate (pH 6.8, 30/70 v/v%); Flow rate =0.2 ml min−1 Instrument = LC-MS; Column = VP-ODS (150 mm x 2.0 mm ID, 5 μm particle size); Eluent = acetonitrile/0.01 M ammonium acetate (30/ 70%); Flow rate = 200 μL min−1 Instrument = LC-MS; Eluent = water/acetonitrile (95/5 v/v%); Flow rate =0.3 ml min−1 Instrument = LC-MS; Column = C18 (150 × 4.6 mm i.d., 5 μm); Eluent = acetonitrile/10 mM ammonium acetate (pH 6.8, 30/70 v/v%); Flow rate =0.6 ml min−1 Instrument = GC/MS; Column = Capillary; Extractor = Diethyl ether; Carrier gas = helium; Flow rate =1 ml min−1 Instrument = LC-MS; Column = LC-18 Supelcosil (25.0 cm × 4.6 cm ×5.0 μm); Eluent = 0.025 M phosphate buffer (pH 6.9): methanol (40:60 v/v%); Flow rate =0.3 ml min−1 Instrument = GC/MS; Column = Capillary (30 m x 0.25 mm x0.25 μm) Extractor = Redistilled ethyl acetate; Carrier gas = helium; Flow rate =1 ml min−1 Instrument = MS

Instrument = LC-MS; Column = LC-18 Supelcosil (25.0 cm × 4.6 cm ×5.0 μm); Eluent (1) = 0.025 M phosphate buffer (pH 6.9): methanol (40:60 v/v%); Eluent (2) = acetonitrile : water (50:50 v/v%) Flow rate =0.3 ml min−1

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the eCeN bond (305 kJ mol−1) is smaller as compared with the eN] Ne bonding (418 kJ mol−1) [54]. After the mechanistic of MO photocatalytic decolorization by TiO2/CS-MT was understood, the mineralization pathway of the dye was further investigated using the LC–MS instrument in the negative mode. The MO solution was obtained after 1 h of treatment under UV–vis light by TiO2/CS-MT. For this experiment, two batches of eluents namely phosphate buffer: methanol and acetonitrile: water were used respectively to detect the intermediates using the same MO treated solution. Fig. 7a shows the LC–MS chromatogram in a 20-min full scan and the main peaks of degradation products present in the treated MO solution when using phosphate buffer: methanol mobile phase whereas the proposed mechanism pathway is shown in Fig. 7b. The peaks of m/ z = 309 and 304 are the parent molecules of MO after losing Na+ due to dissolution in aqueous solution in the form of hydroxylated and nonhydroxylated species, respectively. The main difference of both species is that the eCH3 in the former species was replaced by the Hbonding at both endings of N atom. Furthermore, the substitution of H atom by the hydroxyl groups at the ortho and meta positions to the N atom and SO3− group of the aromatic rings, respectively formed the hydroxylated parent molecule of MO [20]. The initial dissociation after the oxidative attack was the elimination of a hydroxyl and an alkyl group from the parent molecules which gave the m/z = 293 and 290 peaks, respectively. The species with m/z = 293 lost the second hydroxyl group and produced the m/z = 277 species. For the 290 species, the -NCH3 bonding became weaker due to electron withdrawing of the alkyl group, thus, it was easy to lose another eCH3 group by the second attack of the active radicals. At this point, the -NH2 was then taken off by the radicals which produced the m/z = 261 species. The aromatic ring which was bonded with the azo bond of the latter species was broken down to m/z = 253 species and then the m/z = 233 species. It was assumed that more %OH radicals were required to break the aromatic bonding into the aliphatic chain form. The OH groups were taken off from the end of azo bond which essentially produced the m/z = 217 and 199 species. The eN]Ne bond was the most reactive bonding in the m/z = 199 species containing two set of lone pairs and could be easily attacked by the %OH radicals to form the m/z = 159 species. The continuous attack by the %OH radicals on the m/z = 159 species broke down the bonding and obtained the CO2, H2O and HSO4− species with m/z = 97 as the final products at the end of the degradation process. For the acetonitrile/water eluent, the LC–MS spectrum for the intermediates produced and the proposed mechanism pathway can be referred to Fig. 8a and b, respectively. The degradation proceeded in a straightforward pathway since no hydroxylated products were detected in this type of eluent which was in contrast with the intermediates detected using the phosphate/methanol eluent that consist of a mixture of hydroxylated and nonhydroxylated species. Such chromatographic behavior was likely influenced by the structure of the species involved as well as the interactions between the mobile phase and the N-containing moieties together with the SO3− group [20]. Similarly as the phosphate/methanol eluent, the initial oxidative attack on the nonhydroxylated parent molecule of m/z = 304 subsequently lost both of the eCH3 groups at the end of the N atom of one of the aromatic ring to form the m/z = 290 and m/z = 276 species. The ring containing eCH3 group is an electron donor and an ortho directing activators. It also contains the negatively charged molecule from the fragmentation process which induced such type of substitution involving an internal hydrogen bond and the engagement of the lone electron pairs on the NeCH3 groups [20]. The dissociation of the m/z = 233 species by the % OH radicals broke down the eN]Ne bond from the adjacent aromatic rings to form the m/z = 157 and 173 species where the latter species then further dissociated to m/z = 119 species after the elimination of an alkyl group. At this stage, the –N = N bonding would be reduced to form the NO3− ions or N2 gas. During the ring-opening process, the benzene ring and the branched-chains linking to the benzene ring were shorter and more stable. At this point, the radical species reached a high

level in the solution in order to break the benzene ring to the aliphatic chains and finally reached a complete oxidation process to form the harmless compounds [55]. Both mineralization mechanism pathways produced much smaller aliphatic chain compounds to HSO4−, CO2 and H2O as the end-products of MO dye oxidation. Therefore, it can be concluded that the oxidation process proceeded in three stages which are bond breaking by the oxidation process, ring opening process and complete oxidation process to harmless compounds [46]. Table 2 tabulates various photocatalysts and their identified intermediates of MO dye. It was observed that different intermediates of MO were produced by different photocatalysts, degradation conditions and chromatographic separation modes. The degradation process that used very high energy such as microwave-UV and gamma radiation could break the aromatic ring more easily even at high concentration with smaller structures were predominantly observed as the corresponding intermediates. However, due to the cost, environmental and userfriendly concern, the degradation-assisted photocatalyst/catalyst is more favorable than the radiation type process. As shown in Table 2, the m/z = 290 and 276 species frequently appeared during the early stage of MO degradation due to the weak bond energy of CeN which became the easiest target for the %OH radicals attack [37]. Moreover, the difference in the intermediate species detection for each degradation systems suggests that the proposed mineralization pathway of MO is highly dependent on the type of the photocatalysts, degradation conditions as well as the chromatographic separation modes. This can be seen from the results of the present study which suggests that the identified products of MO degradation by separation instrument could be varied depending on the interaction of eluent species and the degradation products. However, further investigations are needed to clarify the possible intermediates produced by the affected parameters. 4. Conclusions The TiO2/CS-MT photocatalyst operated via the synergy process of adsorption by the CS-MT sub-layer and photocatalysis by the TiO2 top layer was studied successfully. The CS-MT composite adsorbent was stronger mechanically and swelled less than the CS adsorbent. Minimal loss of CS-MT on the plates was observed for all the plates due to strong adherence of the positively charged amino group of CS to the negatively charged surface of the glass plate in the presence of MT clay. Faster photocatalytic decolorization of MO dye was observed for TiO2/CS-MT as compared to TiO2 photocatalyst for 90 min of treatment. The maximum photocatalytic decolorization of MO by the TiO2/CS-MT photocatalyst was achieved in the presence of O2 gas under UV–vis light irradiation due to the important role of O2 as the electron scavenger for radicals’ production as well as the need of appropriate light wavelength to initiate the electron excitation of TiO2. The photocatalytic decolorization of MO occurred via the e−/%O2- route with the assist of adsorption process by the CS-MT sub-layer and photosensitization of TiO2 by the adsorbed MO dye. The proposed degradation pathway of MO dye differed with respect to the type of eluent used for separation in LC–MS analysis. The photocatalytic oxidation process involves three stages which are bond breaking by the oxidation process, ring opening process and complete oxidation process to harmless compounds. Acknowledgments The authors would like to thank Universiti Sains Malaysia for all the provided research facilities to conduct this present study. We are also grateful to the Malaysian Ministry of Education for the generous financial support through FRGS: 203/PKIMIA/6711228 grant and My Brain 15 Scholarship. References [1] C. Press, M. Danvers, Editorial board, Crit. Rev. Environ. Sci. Technol. 26 (1996).

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