Oxidative treatment of bisphenol a in municipal wastewater reverse osmosis concentrate (roc) by ferrate(VI)
Creators: name: Widhiastuti, Fitri
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400408
subject: Reaction engineering (excl. nuclear reactions)
classificationCode: 401807
subject: Nanomaterials

description: Reverse osmosis (RO) is often used as the core treatment in the municipal wastewater reclamation processes to produce indirect potable reuse and industrial reuse water. The rejected stream from RO, known as reverse osmosis concentrate (ROC), contains various types of organic matters and nutrients around 4- to 6- fold higher than secondary effluents. The contaminants present in ROC include micropollutants, mainly endocrine-disrupting chemicals (EDCs), pharmaceutical and personal care products (PPCPs), pesticides, herbicides and disinfection by-products, and nutrients and dissolved inorganic salts. The presence of micropollutants and other contaminants in ROC streams poses significant adverse ecological and health effects on the receiving environment upon discharge. Therefore, it is essential to remove harmful contaminants in ROC before it can be safely discharged or reused. 

Bisphenol A (4,4′-dihydroxy-2,2-diphenylpropane, abbreviated as BPA) is one of the micropollutants present in ROC that impose estrogenic potency. Due to its estrogenic activity, BPA is identified as one of the confirmed EDCs by the US EPA. BPA is widely used in the manufacturing industries, for instance, production of epoxy resins and polycarbonates, food packaging, pesticides, dental sealants, and softeners. The extensive use of BPA in the manufacturing industry and human usage contributed to a large number of BPA released in water. Consequently, BPA is one of the most common EDCs detected in various water sources. 

Recently, ferrate(VI) (FeO_4^(2-), Fe(VI)) has been considered a potential water treatment oxidant due to its high reduction potentials in both acidic and basic media. Fe(VI) is categorised as a strong oxidant and can potentially degrade BPA present in municipal wastewater ROC, which is generally considered challenging for the conventional oxidative treatment methods due to the presence of a high level of background organic matters and dissolved inorganic salts. However, no detailed information is available on (1) degradation of BPA present in ROC using Fe(VI) and factors affecting the treatment (pH and Fe(VI) dosage), (2) degradation kinetics of BPA from ROC, (3) comparison of Fe(VI) and combined Fe(VI)/H2O2 on the degradation of BPA in ROC, (4) identification of degradation products of BPA reaction with Fe(VI) and combined Fe(VI)/H2O2 to determine the degradation reaction mechanism. Thus, this study aims to fill the identified research gaps and gaining more understanding for any future technological implementation of Fe(VI) treatment in the water and wastewater industry.

Prior to the degradation of BPA in municipal wastewater ROC, Fe(VI) was successfully synthesised via a wet chemistry method yielding an 85–90 % pure Fe(VI) salt. According to the chemical stability test, freshly prepared Fe(VI) solution must be used immediately within 10 min for pH range of 5.0–9.2. Fe(VI) was found to be relatively more stable at pH 9.2. Freshly prepared K2FeO4 salt can be stored dry and under vacuum for up to 19 days without any significant change in stability. A preliminary degradation experiment confirmed the effectiveness of Fe(VI) to degrade BPA completely with a Fe(VI) to BPA ratio of 10. 
The degradation of BPA using Fe(VI) was conducted in municipal wastewater ROC which has distinct characteristics compared to secondary effluents. The ROC was collected from a wastewater reclamation facility at a local municipal wastewater treatment plant in Victoria, Australia. The results showed that BPA was effectively degraded by Fe(VI) oxidation treatment. The reduction of BPA was also accompanied by a marked reduction in background organics. Among the different combinations of process variables tested, a [Fe(VI)]/[BPA] ratio of 50 and pH 8.0 resulted in 90.1 % of degradation of BPA. In addition, up to 34 % DOC, 95 % UV254 absorbance reduction, and 96.4 % SUVA reduction was achieved. Moreover, the amount of residual concentration of Fe(III) was the lowest at pH 8.0 and a [Fe(VI)]/[BPA] ratio of 50. Fe(VI) selectively reacted with the fluorescence centres in organics. 

The kinetic degradation of BPA present in municipal wastewater using Fe(VI) was evaluated at different reaction temperatures (15, 25, and 35 oC) and pH values (5.0, 6.5, and 8.0). A fixed BPA initial concentration of 50 μg L-1 was employed throughout the studies. The apparent second-order reaction rate constant was calculated, and the results yielded a best-fit line with a correlation coefficient R2&gt;0.98. The values of kapp for the reaction of Fe(VI) with BPA increased as pH decreased. At a fixed [Fe(VI)]/[BPA] ratio of 40, the value of kapp was 3.19×102 M-1 s-1, 2.72×102 M-1 s-1, and 2.33×102 M-1 s-1 at pH 5.0, 6.5, and pH 8.0, respectively. Moreover, the value of kapp increased proportionally as the concentration of Fe(VI) increased. Specifically, the value of kapp at pH 8.0 was found to be 0.38×102 M-1 s-1, 0.87×102 M-1 s-1, and 1.33×102 M-1 s-1 for a [Fe(VI)]/[BPA] ratio of 7, 14, and 21, respectively. The maximum kapp value of 3.19×102 M-1 s-1 can be achieved at pH 5.0 and a [Fe(VI)]/[BPA] ratio of 40. The value of kapp is dependent on solution pH and dosage of Fe(VI). The reaction rate constant for BPA degradation increased from 1.85×102 M-1 s-1 to 3.34×102 M-1 s-1 when the temperature was increased from 15 oC to 35 oC. This indicates that the rate of BPA degradation is dependent on temperature. The degradation kinetics of BPA using combined Fe(VI)/H2O2 was also investigated. Similar to the kinetic degradation using Fe(VI) alone, the value of kapp increased proportionally as the [Fe(VI)]/[H2O2] ratio increased, with kapp of 7×103 M-1 s-1, 42.7×103 M-1 s-1, and 67.9×103 M-1 s-1 at the [Fe(VI)]/[H2O2] ratio of 350:5,000; 700:5,000; and 1,050:5,000; respectively. 
As a continuation of the previous study, the combination of Fe(VI) and H2O2 was used to degrade BPA in ROC and compare it with Fe(VI) alone. Fe(VI) dosage selection is an essential factor for effective treatment using Fe(VI)/H2O2 oxidant. More than 99.0 % of BPA degraded with an initial concentration of 50 µg L-1 using [Fe(VI)]/[H2O2] ratio of 1,050:5,000 at pH 8.0 within 60 min reaction. Under the same condition, 37.2 % DOC was removed, and  94.0 % UV254 was reduced. Overall, the use of combined Fe(VI)/H2O2 showed greater BPA degradation efficiency, DOC removal and UV254 than using Fe(VI) or H2O2 alone. The suggested pH condition for BPA degradation by Fe(VI)/H2O2 in ROC was pH 8.0 based on the environmentally relevant pH range of ROC and chemicals adjustment for the treatment. The BPA containing ROC samples treated with Fe(VI)/H2O2 showed the insignificant formation of toxic by-products based on Microtox assay.

The degradation mechanism of BPA by Fe(VI), H2O2, and Fe(VI)/H2O2 in the aqueous phase, and a preliminary speciation study was carried out by using a GC/MS Triple Quad (QQQ) and a LC/MS Triple Quad (QQQ). Fourteen intermediate products were identified, and three degradation pathways were proposed for the BPA degradation using Fe(VI). Based on the identified intermediates from BPA degradation, two critical steps, namely dihydroxylation and cleavage of C-C bond, were proposed to govern the degradation process. Seven intermediates were identified in the degradation of BPA using H2O2, and one degradation pathway was proposed based on the attack of peroxide on benzene ring and oxidation. Moreover, eleven intermediate products were detected in the BPA degradation using Fe(VI)/H2O2, and three degradation pathways were proposed, including hydroxylation, cleavage of C-C bond, react-couple, and oxidation in the production of ring-opening products. Ecotoxicity test showed a reduction in the toxicity after 60 min of reaction as a result of further degradation of intermediates. 

Overall, this study showed the technical feasibility of Fe(VI) and combined Fe(VI)/H2O2 to degrade BPA in municipal wastewater ROC accompanied by the reduction of DOC and UV254 at the lab scale. Combined Fe(VI)/H2O2 experiments were performed to optimise the degradation, reduce chemical dosage and shorten reaction time. Based on the Microtox assay, the BPA containing ROC after treatment using both Fe(VI) and Fe(VI)/H2O2 showed no toxic formation of by-products. Further studies need to be conducted to gain a better understanding of the treatment process and optimise degradation efficiency. To be specific, (1) degrade multiple micropollutants in ROC, (2) combination studies of combined Fe(VI) (Fe(VI)/O3 or Fe(VI)/UV), (3) biodegradation approaches as pre- or post- treatment, (4) degradation mechanism in complex water matrix, (5) quantitative measurement of generated degradation intermediates, (6) more detailed economic feasibility of Fe(VI) treatment.