BPA adsorption tests

BPA adsorption tests were carried out to determine the capability of the SBAs to adsorb this target EDC. As mentioned in Table 1-9, some researchers have carried out experimental tests on the adsorption of BPA onto ACs. Several different UV spectrum peaks have been used for detecting the BPA absorbance, which varies between 221 & 280_nm. Therefore, prior to performing the BPA adsorption kinetics and isotherm tests, the spectrum of UV-VIS was measured to identify the UV-VIS activity of the compound and to identify the λmax that was

most suitable for detecting BPA. After this, BPA adsorption kinetics were performed and followed by the determination of adsorption isotherms for the SBAs, which were performed according to the ASTM test method [54].

3.2.10.1 Ultraviolet-Visible spectroscopy (UV-VIS) and Spectrum test of BPA

A Shimadzu UV2401 pc spectrophotometer was used in this analysis. A matched pair of 100- QS cuvettes, made from Quartz SUPRASIL with a path length of 10 mm were obtained from Hellma. This UV-VIS spectroscopy allowed the auto calibration between the absorbance and

sample concentration, following the equation of the Beer-Lambert law, as shown in Equation 3-2.

𝑨 = ɛ𝒃𝒄 Equation 3-2

where;

A is the absorbance

ε is the molar absorptivity or extinction coefficient (L mol-1 _cm-1₎ b is the path length of the cuvette in which the sample is contained (cm) c is the concentration of the compound in solution (mol L-1_).

For the BPA spectrum tests, different BPA concentration solutions were prepared from 6 to 120 mg/l and each solution was subjected to a wavelength scan over 190-300nm, as shown in Figure 3-6.

Figure 3-6 Absorbance spectrum of BPA (concentration from 6, 18, 30, 54, 78, 102 and 120

mg/l)

The activity of BPA changes following an increase in the alkalinity of the solution. Therefore, for these reasons the effect of pH on BPA activity needed to be established. To achieve this, two BPA solutions were prepared at concentrations of 6 mg/l and 120 mg/l and the absorption photometrics were performed over different pH scales. The pH of each solution was adjusted using concentrated NaOH or HCl. The variation of the absorbance values of BPA at the two

54 mg/l 120 mg/l

λmax values (224 and 275nm, observed in Figure 3-6) over the pH range studied are shown in

Table 3-5. Although both peaks were detected for each concentration, above 54mg/l the 224nm absorbance was obscured and only the 275nm absorbance provided a well-defined peak (see Figure 3-6). Thus the peak detection was only carried out at this wavelength for determining BPA adsorption kinetics and isotherm.

Table 3-5 BPA absorbency at λ = 224 and 275 nm at different pH values

* pH of solution after preparation (no pH adjustment)

Figure 3-7 Calibration graph of BPA absorbance at the UV wavelength of 275 nm

Concentration at 6 mg/l Concentration at 120 mg/l

pH Absorbance 224 nm Absorbance 275 nm pH Absorbance 275 nm

2 5.799 0.082 2.7 1.673 2.4 5.789 0.082 3.3 1.673 3 5.809 0.082 3.9 1.685 3.9 5.82 0.082 4.0 1.682 5.3* 5.848 0.083 4.4* 1.677 6.3 5.695 0.081 6.1 1.678 8.3 5.667 0.080 8.1 1.675 8.4 5.696 0.081 8.6 1.673 8.7 5.803 0.082 8.9 1.676 10.3 5.397 0.077 9.1 1.661 10.4 5.398 0.077 9.7 1.625 10.7 5.01 0.071 10.0 1.594 10.8 4.964 0.071 11.0 1.426 12 4.825 0.069 11.6 1.372

As can be seen in Table 3-5 the BPA absorbance values for both solution concentrations are consistent up to approximately pH 9.0 Consequently, the analyses of the BPA solution after contact with SBAs, need to be undertaken below ≈ pH 8.9. The pKa value of BPA obtained from this experiment is near the literature values which are between pH 9.59 and 11.30 at 25o_{C [90].}

Based on both BPA spectra tests and pH adjustment results, the calibration for the BPA UV absorbance was performed at 275 nm. The data in Figure 3-7 shows that BPA obeys the Beer- Lambert law up to the concentration of 120 mg/l, giving a very good R2 _{of 0.99999.}

3.2.10.2 Adsorption kinetics of BPA

The study of the adsorption kinetics of BPA onto SBAs was performed to determine the adsorption equilibrium time. A few representative samples from carbonized steam activated and chemically activated sludge were used as test samples to determine the BPA adsorption kinetics. For each adsorption kinetics experiment, SBA was crushed and sieved to below 150 μm and then dried at 150o_{C for at least three hours. For each SBA, 5 mg ± 0.1 mg (for}

chemically activated sludge) or 20 _mg ± 0.1 mg (for physically activated sludge) of adsorbent was weighed into 125 ml screw cap glass bottle with Teflon liners. Then, 100 ml of 100 mg/l BPA solution was dispensed into the bottle using a zippette bottle-top dispenser (Jencons Scientific Ltd., UK). The mixtures were shaken using end-to-end rotation with a speed of approximately 45 rpm. After the desired contact time was achieved, three of the sample bottles were taken off and each were filtered through a Whatman WCN 25mm diameter, 0.45 μm membrane filter. The filtrates were then analysed using the UV 2401pc and the calibration curve used to determine the residual BPA concentration, as shown in Figure 3- 7. The filtrate pH was measured to confirm that they were not exceeding the pKa limit.

3.2.10.3 Adsorption isotherms of BPA

To produce an adsorption isotherm for each SBA, six sample bottles were prepared. For each experiment, 20_mg (for carbonized and steam activated sludge) or 5_mg (for chemically activated sludge) was used with 100 ml of BPA solution at various concentrations from 12 to 120 mg/l. The sample bottles were then shaken using end-to-end rotation as described above, for 24 hours, which was the optimized equilibrium time obtained from kinetics tests. The sample bottles were treated using the same procedure that had been performed in the kinetics

tests in section 3.2.10.2. For each SBA, Freundlich, Langmuir and Temkin adsorption models were used to fit each set of data obtained. Three of the CACs (shown in Table 3-3) were also used to determine the BPA adsorption isotherms. An example of the calculation method is provided in Appendix IV.