Photocatalytic Decolourization of Rhodamine B by Modified Photo-Fenton Process with Quasicrystals – Preliminary Research

The material for the tests was the Al₆₅Cu₂₀Fe₁₅ alloy produced by casting the liquid alloy into a copper mould cooled with water. This method of producing the material results in the formation of many different intermetallic phases in the alloy, such as Cu₃Al, Al₂Cu, Al₁₃Fe₄, Al₇Cu₂Fe, AlFe, Al₂Fe and the quasicrystal line icosahedral phase of I-AlCuFe. The phase composition of the material produced by this technique is confirmed by X-ray diffraction studies. In addition, the presence of a quasicrystal line icosahedral phase was confirmed by electron diffraction [23, 24]. This phase composition of the alloy causes significant differences in relation to classic aluminium-based alloys. First of all, the very high hardness and brittleness of these alloys are noticeable. The most interesting part of the steppes is the occurrence of the quasi-crystalline phase. Quasicrystals are a relatively new form of crystal. Discovered by Dan Shechtman in 1984, for which he was awarded the Nobel Prize in 2011. Quasicrystals are an ordered structure, but not periodic. A quasicrystal line pattern can continuously fill all available space, but it shows lack of translational symmetry. While the crystals, according to the classical crystallographic restriction theorem, can only possess two-, three-, four-, and six-fold rotational symmetry, the Bragg diffraction pattern for quasicrystals shows sharp peaks with the other orders of symmetry—for example, five-fold. Along with the unusual spatial arrangement of atoms in quasicrystals, there are numerous differences between quasicrystals and traditional crystalline phases. This phase, however, has many unique properties, such as: low electrical and thermal conductivity, high oxidation resistance, low coefficient of friction, high abrasion resistance, high tensile strength and brittleness at room temperature. These unique properties mean that quasicrystals can be used as anti-adhesive materials, protective coatings or composite reinforcements [24,25,26,27].

The studies were aimed at determining the effect of the concentration of quasicrystals and SP on the decolourization efficiency of the RB solution. For this purpose, different amounts of quasicrystals and doses of SP were introduced into the reactor (Fig. 1) containing 60 ml of an aqueous solution of RB at a concentration of 5 mg/l, irradiated with UV light with 4 bulbs (36 W in total; λ= 365 nm) additionally foil reflecting UV light into the solution was placed behind the lamps. The reaction solution was constantly stirred (at 400 RPM). Although the concentration of RB was low, the colour of the solution was intense pink. In addition, the results of studies using similar concentrations of this dye can be found in the literature [28, 29].

Figure 1.

Samples for measuring dye concentration were collected at 0, 5, 10, 15, 20, 30, 45, and 60 minutes. The dye concentration was measured by spectrophotometry using a Shimadzu UV-1800 spectrophotometer.

The initial pH = 7 has not been modified, because the addition of SP no effect on its value [18]. Due to the lack of publications on the degradation of dyes using quasicrystals, the authors used the experience gained while working with high entropy alloys [18] regarding the adoption of the concentration of SP and quasicrystals (8.3; 16.7; 33.3 g/l and 8.3; 16.7; 33.3 g/l, respectively). The UV/Na₂CO₃·1.5H₂O₂ process without the addition of quasicrystals was used as a comparative reaction.

Figures 2 and 3 show the average values (from two experiments) of the decolourization efficiency for different concentrations of the tested quasicrystals and SP, respectively. The difference in the obtained decolourization efficiency values did not exceed 5%. To ascertain the impact of the catalyst on decolourization efficiency, the following quasicrystals concentrations were used: 8.3, 16.7 and 33.3 g/l, at a constant SP concentration of 16.7 g/l (Fig. 2). After 60 minutes of the process, the decolourization efficiency was 84.2%, 78.4% and 84% for quasicrystal doses of 8.3, 16.7 and 33.3 g/l, respectively. At that time, the comparative method had a decolourization degree of 52.7%. For all the used reaction times and doses of quasicrystals, no significant differences in decolourization efficiency were observed. Therefore, for further tests to determine the effect of SP on the decolourization efficiency, the smallest quasicrystal dose of 8.3 g/l was selected.

Figure 2.

Figure 3.

In the next part of the research a series of tests using various SP concentrations (8.3, 16.7 and 33.3 g/l), at a constant quasicrystal dose of 8.3 g/l (Fig. 3), were conducted to examine the effect of the alternative source of hydrogen peroxide in the photo-Fenton process on decolourization degree. After 60 minutes of the process, the decolourization efficiency was 63.3%, 85.1% and 93.6% for SP doses of 8.3, 16.7 and 33.3 g/l, respectively. Practically throughout the entire test, the efficiency of dye degradation for individual doses of SP clearly differed, which proves the significant impact of SP doses on the effects of RB degradation. The greatest increase in the effects of RB degradation was observed during the first 30 minutes of the process.

Even at the lowest doses of catalyst used in the experiments (8.3 g/l), it has an impact on the reaction. The concentration of the oxidant significantly increases the rate of decolourization of the dye relative to the comparative method. The addition of the catalyst affects the rate of degradation very similarly regardless of the dose of the added catalyst Fig. 2. The highest rate of decolourization was observed at the highest concentration of oxidant (33.3 g/l). The decolourization process accelerates with an increase in the dose of the oxidant. However, the difference in decolourization between the highest and the second highest concentration (16.7 g/l) is 10 percentage points (see Fig. 2), which is not significant, taking into account the double dose of the oxidant. The results suggest that the use of a catalyst is essential to achieve a fast and efficient decolourisation of the dye.

Table 1 shows the values of the reaction rate constants for both types of tests. Reaction order and reaction rate constants were estimated from a linear fit of the data matching to characteristic functions for reaction order. For the 0 order reaction C₀-C_t; 1^st order Ln(c0ct) Ln({{{{\rm{c}}_0}} \over {{{\rm{c}}_{\rm{t}}}}}) ; 2^nd order 1ct−1c0 {1 \over {{{\rm{c}}_{\rm{t}}}}} - {1 \over {\rm{c_0}}} [30]. For a constant concentration of quasicrystals (8.3 g/l), a two-fold increase in the concentration of SP from 8.3 to 16.7 g/l caused also an approximately two-fold increase in the value of the reaction rate constant from 0.017 to 0.032 s⁻¹. Another two-fold increase in SP concentration (from 16.7 to 33.3 g/l) resulted in only one and a half times increase in the value of the reaction rate constant (from 0.032 to 0.048 s⁻¹). Generally, the increase in SP concentration significantly increased the degree of degradation of RB (Fig. 2 and 3) and the reaction rate. In the case of tests using a constant concentration of SP (16.7 g/l), similar relationships were not observed. The values of the reaction rate constants when increasing the concentration of quasicrystals from 8.3 to 33.3 g/l did not show an upward trend, remaining at almost the same level. This is also confirmed by the results presented in Fig. 2, which show that the degradation efficiencies of RB for individual quasicrystal concentrations differed to a small extent. On this basis, it can also be concluded that in the presented studies the concentration of SP had a greater impact on the rate and efficiency of dye degradation compared to the concentration of quasicrystals.