Effects of operating conditions on iron (hydr)oxides evolution and ciprofloxacin degradation in potassium ferrate-ozone stepwise oxidation system

Ferrate(VI) assists in reducing cytotoxicity and genotoxicity to mammalian cells and organic bromine formation in ozonated wastewater

Ozonation of wastewater containing bromide (Br-) forms highly toxic organic bromine. The effectiveness of ozonation in mitigating wastewater toxicity is minimal. Simultaneous application of ozone (O3) (5 mg/L) and ferrate(VI) (Fe(VI)) (10 mg-Fe/L) reduced cytotoxicity and genotoxicity towards mammalian cells by 39.8% and 71.1% (pH 7.0), respectively, when the wastewater has low levels of Br-. This enhanced reduction in toxicity can be attributed to increased production of reactive iron species Fe(IV)/Fe(V) and reactive oxygen species (•OH) that possess higher oxidizing ability. When wastewater contains 2 mg/L Br-, ozonation increased cytotoxicity and genotoxicity by 168%-180% and 150%-155%, respectively, primarily due to the formation of organic bromine. However, O3/Fe(VI) significantly (p < 0.05) suppressed both total organic bromine (TOBr), BrO3-, as well as their associated toxicity. Electron donating capacity (EDC) measurement and precursor inference using Orbitrap ultra-high resolution mass spectrometry found that Fe(IV)/Fe(V) and •OH enhanced EDC removal from precursors present in wastewater, inhibiting electrophilic substitution and electrophilic addition reactions that lead to organic bromine formation. Additionally, HOBr quenched by self-decomposition-produced H2O2 from Fe(VI) also inhibits TOBr formation along with its associated toxicity. The adsorption of Fe(III) flocs resulting from Fe(VI) decomposition contributes only minimally to reducing toxicity. Compared to ozonation alone, integration of Fe(VI) with O3 offers improved safety for treating wastewater with varying concentrations of Br-.

Overlooked flocs in electrocoagulation-based ultrafiltration systems: A new understanding of the structural interfacial properties

An integrated ferrate-induced electrocoagulation-ultrafiltration (FECUF) process is proposed to cope with the growing demand for water treatment. Although flocs formed during the electrocoagulation (EC) process are useful for contaminant reduction and mitigation of membrane fouling, few studies have been focused on their structures and properties. Herein, we investigated the formation and structural transformations of flocs and their responses to organic matter, as well as the relationships between their interfacial properties and membrane fouling mitigation. It was found that ferrate contributed to the fast formation of flocs during the ferrate-induced electrocoagulation (FEC) process, which accelerated the FECUF process. Physicochemical analyses indicated that the flocs formed in the FEC process were mainly composed of Fe(III)-(hydr)oxides with abundant hydroxyl groups and poor crystallinity, which allowed complexation with NOM. Therefore, the mobilities of the NOM and the soluble coagulant ions were reduced. The responses of flocs to NOM suggested that the period of 0-20 min resulted in the most efficient NOM removal. In addition, two patterns revealed the relationships between the interfacial properties of the small colloidal particles (SCPs) and the membrane filtration performance: i) the decline in the initial flux was closely related to the composition (gel-type substances or metal-(hydr)oxides) of the SCPs and ii) the steady-state flux was influenced by the energy barrier between the SCPs. However, when the SCPs had the same composition, the interfacial properties influenced both the initial flux and the steady-state flux. This study provides an alternative FECUF process for intensive upgrades of centralized water treatment systems.