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Anaerobic digestion of waste activated sludge (WAS) was one of the directions of sludge treatment, but how to effectively improve the production of methane as a resource product of anaerobic digestion of sludge still needs further research. The study examined how the combination of potassium ferrate (PF) and thermal hydrolysis (TH) pretreatment affected methane production from sludge. The results demonstrated a positive synergistic effect on methane production with PF-TH pretreatment. Specifically, by employing a 0.05 g/g TSS (total suspended solids) PF in conjunction with TH at 80 °C for 30 min, the methane yield increased from 170.66 ± 0.92 to 232.73 ± 2.21 mL/g VSS (volatile suspended solids). The co-pretreatment of PF and TH has been substantiated by mechanism studies to effectively enhance the disintegration and biodegradability of sludge. Additionally, the variation of microbial community revealed an enrichment of active microorganisms associated with anaerobic digestion after treated with PF + TH, resulting in a total abundance increase from 11.87 to 20.45% in the PF + TH group.

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Ferrate (Fe(VI): HFeO4- /FeO42-), a potent oxidant, has been investigated as an alternative chemical disinfectant in water treatment due to its reduced production of disinfection by-products. In this study, we assessed the disinfecting ability of potassium ferrate against a variety of microorganisms, including waterborne pathogens, under varying pH and water temperature conditions. We presented CT values, a metric of ferrate concentrations (C) and contact time (T), to quantify microbial inactivation rates. Among the tested microorganisms, human adenovirus was the least resistant to ferrate, followed by waterborne bacteria such as Escherichia coli and Vibrio cholerae, and finally, the protozoan parasite Giardia duodenalis. We further investigated the impact of two pH values (7 and 8) and two temperatures (5 and 25 °C) on microbial inactivation rates, observing that inactivation rates increased with lower pH and higher temperature. In addition to showcasing ferrate's capacity to effectively inactivate a range of the tested microorganisms, we offer a ferrate CT table to facilitate the comparison of the effectiveness of various disinfection methods.

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The combined process of biochar (BC) and potassium ferrate (PF) offers a fascinating technique for efficient dewatering of digestate. However, the effects of BC/PF treatment on the dewaterability and mechanisms of FWD are still unknown. This study aimed to reveal the impact mechanisms of BC/PF treatment on digestate dewatering performance. Experimental results indicated that BC/PF treatment significantly enhanced the dewaterability of digestate, with the minimum specific resistance to filtration of (1.05 ± 0.02) × 1015 m·kg-1 and water content of 57.52 ± 0.51% being obtained at the concentrations of 0.018 g·g-1 total solid (TS) BC300 and 0.20 g·g-1 TS PF, which were 8.60% and 13.59% lower than PF treatment, respectively. BC/PF treatment proficiently reduced the fractal dimension, bound water content, apparent viscosity, and gel-like network structure strength of digestate, as well as increased the floc size and zeta potential of digestate. BC/PF treatment promoted the conversion of extracellular polymeric substances (EPS) fractions from inner EPS to soluble EPS, increased the fluorescence intensity of the dissolved compounds, and enhanced the hydrophobicity of proteins. Mechanisms investigations showed that BC/PF enhanced dewatering through non-reactive oxygen species pathways, i.e., via strong oxidative intermediate irons species Fe(V)/Fe(IV). BC/PF treatment enhanced the solubilization of nutrients, the inactivation of fecal coliforms, and the mitigation of heavy metal toxicity. The results suggested that BC/PF treatment is an effective digestate dewatering technology which can provide technological supports to the closed-loop treatment of FWD.

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Cellulose II nanocrystals (CNC II) possess a higher thermal stability and improved emulsifying capability than cellulose I nanocrystals (CNC I) owing to the higher density of their hydrogen bonds and more larger surface areas. Therefore, CNC II exhibit substantial advantages for value-added nanocomposite materials. Current CNC II preparation methods are mainly based on a two-pot reaction involving acid hydrolysis and crystal transformation. In this study, considering the oxidative nature of potassium ferrate (K2FeO4) in an alkaline environment containing a small amount of sodium hypochlorite (NaClO), a one-step and efficient approach was developed for the preparation of carboxyl-bearing CNC II from cotton pulp, affording a maximum CNC II yield of 45.14 %. Atomic force microscopy analysis revealed that the prepared CNCs exhibited a "rod-like" shape with a width of ~7 nm and a length of ~269 nm. The resulting CNC II also exhibited excellent thermal stability (Tonset = 311.4 °C). Furthermore, high-internal-phase Pickering emulsions (HIPPEs) stabilized by CNC II were prepared to stabilize liquid paraffin in the absence of surfactant. The results revealed that CNC II could be used as an effective emulsifier to fabricate the stable and gel-like HIPPEs, and are promising for the preparation of high value-added nanocomposite materials.

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Potassium ferrate (K2FeO4) has been extensively employed to promote short-chain fatty acids (SCFAs) production from anaerobic fermentation of waste activated sludge (WAS) because of its potent oxidizing property and formation of alkaline hydrolyzed products (potassium hydroxide, KOH and ferric hydroxide, Fe(OH)3). However, whether K2FeO4 actually works as dual functions of both an oxidizing agent and an alkalinity enhancer during the anaerobic fermentation process remains uncertain. This study aims to identify the contributions of hydrolyzed products of K2FeO4 on SCFAs production. The results showed that K2FeO4 did not execute dual functions of oxidization and alkalinity in promoting SCFAs production. The accumulation of SCFAs using K2FeO4 treatment (183 mg COD/g volatile suspended solids, VSS) was less than that using either KOH (192 mg COD/g VSS) or KOH & Fe(OH)3 (210 mg COD/g VSS). The mechanism analysis indicated that the synergistic effects caused by oxidization and alkalinity properties of K2FeO4 did not happen on solubilization, hydrolysis, and acidogenesis stages, and the inhibition effect caused by K2FeO4 on methanogenesis stage at the initial phase was more severe than that of its hydrolyzed products. It was also noted that the inhibition effects of K2FeO4 and its hydrolyzed products on the methanogenesis stage could be relieved during a longer sludge retention time, and the final methane yields using KOH or KOH & Fe(OH)3 treatment were higher than that using K2FeO4, further confirming that dual functions of K2FeO4 were not obtained. Therefore, K2FeO4 may not be an alternative strategy for enhancing the production of SCFAs from WAS compared to its alkaline hydrolyzed products. Regarding the strong oxidization property of K2FeO4, more attention could be turned to the fates of refractory organics in the anaerobic fermentation of WAS.

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The remediation of cadmium (Cd) contaminated soil is challenging for agricultural practices. In this study, a novel vinasse biochar modified by potassium ferrate (K2FeO4) was synthesized to immobilize Cd in agricultural soil. Three biochars [i.e., vinasse biochar (BC), KMnO4 modified vinasse biochar (MnBC), and K2FeO4 modified vinasse biochar (FeBC)] were applied to compare their efficiencies of Cd immobilization. The results showed that the orders of pH, ash content, and functional groups in different biochar were the same following BC < MnBC < FeBC. Scanning electron microscope images showed that the FeBC has more micropores than MnBC and BC. X-ray diffraction identified manganese oxides and iron oxides within MnBC and FeBC, indicating that Mn and Fe were well loaded on the biochar. In the soil-based pot experiment, both MnBC and FeBC significantly reduced soil available Cd by 23-38% and 36-45% compared with the control, respectively (p < 0.05). In addition, the application of BC, MnBC, and FeBC significantly increased the yield, chlorophyll, and vitamin C of Chinese cabbage (p < 0.05), and decreased its Cd uptake compared with the control. Notably, shoot Cd significantly reduced when 2% FeBC was applied (p < 0.05). Overall, using K2FeO4 to modify vinasse biochar enriched the surface functional groups and minerals as well as reduced Cd availability in soil and its uptake by the plant. Our study showed that K2FeO4 modified vinasse biochar could be used as an ideal amendment for the remediation of Cd-contaminated soil.

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The specific effects of potassium ferrate (PF) on acid and methane production in anaerobic digestion need further exploration. This study comprehensively investigated the role of PF in organic matter conversion in waste activated sludge (WAS) digestion. Due to the high pH produced by PF self-decomposition, the hydrolysis of organic matter was promoted, whereas the methanogenesis was inhibited. PF could further directly oxidize protein and polysaccharides released by hydrolysis to produce volatile fatty acids (VFAs) and involve in the transformation of ammonia nitrogen. PF could induce the enrichment of functional genes related to fermentation pathways and lessen those related to methanogenesis, and the phylum resistant to PF oxidation and the strains capable of producing VFAs were enriched, resulting in VFAs accumulation. This study analyzed the participation way of PF in anaerobic digestion and provided a theoretical basis for the application of PF in promoting VFAs recovery from sludge digestion.

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Although the removal ability of potassium ferrate (K2FeO4) on aqueous heavy metals has been confirmed by many researchers, little information focuses on the difference between the individual and simultaneous treatment of elements from the same family of the periodic table. In this project, two heavy metals, arsenic (As) and antimony (Sb) were chosen as the target pollutants to investigate the removal ability of K2FeO4 and the influence of humic acid (HA) in simulated water and spiked lake water samples. The results showed that the removal efficiencies of both pollutants gradually increased along the Fe/As or Sb mass ratios. The maximum removal rate of As(III) reached 99.5% at a pH of 5.6 and a Fe/As mass ratio of 4.6 when the initial As(III) concentration was 0.5 mg/L; while the maximum was 99.61% for Sb(III) at a pH of 4.5 and Fe/Sb of 22.6 when the initial Sb(III) concentration was 0.5 mg/L. It was found that HA inhibited the removal of individual As or Sb slightly and the removal efficiency of Sb was significantly higher than that of As with or without the addition of K2FeO4. For the co-existence system of As and Sb, the removal of As was improved sharply after the addition of K2FeO4, higher than Sb; while the latter was slightly better than that of As without K2FeO4, probably due to the stronger complexing ability of HA and Sb. X-ray energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the precipitated products to reveal the potential removal mechanisms based on the experimental results.

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This study was carried out to investigate the enhanced removal of arsenite (As(III)) by potassium ferrate (K2FeO4) coupled with three Al-based coagulants, which focused innovatively on the distribution and transformation of hydrolyzed aluminum species as well as the mechanism of K2FeO4 interacted with different aluminum hydrolyzed polymers during As(III) removal. Results demonstrated that As(III) removal efficiency could be substantially elevated by K2FeO4 coupled with three Al-based coagulants treatment and the optimum As(III) removal effect was occurred at pH 6 with more than 97%. K2FeO4 showed a great effect on the distribution and transformation of aluminum hydrolyzed polymers and then coupled with a variety of aluminum species produced by the hydrolysis of aluminum coagulants for arsenic removal. During enhanced coagulation, arsenic removal by AlCl3 was main through the charge neutralization of in situ Al13 and the sweep flocculation of Al(OH)3, while PACl1 mainly depended on the charge neutralization of preformed Al13 and the bridging adsorption of Al13 aggregates, whereas PACl2 mainly relied on the sweep flocculation of Al(OH)3. This study provided a new insight into the distribution and transformation of aluminum species for the mechanism of As(III) removal by K2FeO4 coupled with different Al-based coagulants.

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This work proposed a novel strategy via heat-assisted potassium ferrate (PF) pretreatment to enhance methane production from waste activated sludge (WAS) during anaerobic digestion. In this research, five dosages of PF (i.e., 0, 0.05, 0.1, 0.3 and 0.5 g/g VSS) at two temperatures (i.e., 25 °C and 55 °C) were explored. Biochemical methane potential experiments illustrated that heat-assisted PF pretreatment improved cumulative methane production with the maximum yield up to 163.93 mL CH4/g VSS, 149.0 %, 119.6 % and 121.0 % of that in the control, individual 0.5 g PF/g VSS and individual heat (i.e., 55 °C) pretreatment digesters, respectively. The maximum methane potential (B0) was promoted by 63.2 % with heat-assisted PF pretreatment compared to the control, while the hydrolysis rate (k) changed slightly. Mechanism analysis revealed that heat-assisted PF pretreatment accelerated WAS solubilization and enhanced the biodegradability of released substances, providing more available matrix for bacteria during the following anaerobic digestion processes. Microbial community analysis exhibited that several microbes such as Proteiniclasticum sp., Sedimentibacter sp. and Methanosaeta sp. associated with hydrolysis, acidification and methanogenesis respectively were improved after heat-assisted PF pretreatment. In addition, the relative bioactivities of protease, butyrate kinase and acetate kinase were also increased. Furthermore, variation of dominant genes associated with methane production indicated that acetate-dependent methanogenesis was the main pathway while CO2-dependent methanogenesis pathway was inhibited by heat-assisted PF pretreatment.

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Sludge reuse and utilization is one of important routines of disseminating fecal pollution to surface water and groundwater. However, it remains unclear the spatial distribution of fecal pollution indicators in sludge flocs and their reductions during sludge treatment processes. In this study, the abundances of fecal pollution indicators including cross-assembly phage (crAssphage), JC and BK polyomavirus (JCPyV, BKPyV), human adenovirus (HAdV), the human-specific HF183 Bacteroides (HF183) and Escherichia coli (EC) in soluble extracellular polymeric substances (S-EPS), loosely-bound EPS (LB-EPS), tightly-bound EPS (TB-EPS), and pellets of sludge flocs were determined, and the effect of potassium ferrate (PF) treatment on their removal and inactivation was investigated by using both qPCR and viability-qPCR. Results showed that all investigated indicators were detected in each fraction of sludge flocs. The PF treatment led to a great migration of indicators from sludge pellets to sludge EPS and some extent of their inactivation in each fraction of sludge flocs. The overall reductions of human fecal indicators in sludge determined by qPCR were 0-1.30 logs, which were 0-2 orders of magnitude lower than those of 0.69-2.39 logs detected by viability-qPCR, implying their inactivation by PF treatment to potentially alleviate the associated human health risks.

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It is well known the capacity of potassium ferrate (Fe(VI)) for the oxidation of pollutants or co-precipitation and adsorption of hazardous species. However, little information has been paid on the adsorption and co-precipitation contribution of the Fe(VI) resultant nanoparticles, the in situ hydrolytic ferric iron oxides. Here, the removal of arsenate (As(V)) and arsenite (As(III)) by Fe(VI) was investigated, which focused on the interaction mechanisms of Fe(VI) with arsenic, especially in the contribution of the co-precipitation and adsorption of its hydrolytic ferric iron oxides. pH and Fe(VI) played significant roles on arsenic removal; over 97.8% and 98.1% of As(V) and As(III) removal were observed when Fe(VI):As(V) and Fe(VI):As(III) were 24:1 and 16:1 at pH 4, respectively. The removal of As(V) and As(III) by in situ and ex situ formed hydrolytic ferric iron oxides was examined respectively. The results revealed that As(III) was oxidized by Fe(VI) to As(V), and then was removed though co-precipitation and adsorption by the hydrolytic ferric iron oxides with the contribution content was about 1:3. For As(V), it could be removed directly by the in situ formed particles from Fe(VI) through co-precipitation and adsorption with the contribution content was about 1:1.5. By comparison, As(III) and As(V) were mainly removed through adsorption by the 30-min hydrolytic ferric iron oxides during the ex situ process. The hydrolytic ferric iron oxides size was obviously different in the process of in situ and ex situ, possessing abundant and multiple morphological structures ferric oxides, which was conducive for the efficient removal of arsenic. This study would provide a new perspective for understanding the potential of Fe(VI) treatment on arsenic control.

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In this study, a novel in situ iron-loaded activated carbon (AFPAC) was prepared by a FeSO4/K2FeO4 impregnation and oxidation combination two-step supported on activated carbon for enhanced removal of Cr(VI) from aqueous solutions. Cr(VI) removal efficiency greatly increased by AFPAC more than 70% than that of fresh activated carbon (AC), which is due to rich iron oxides formed in situ and the synergistic effect between iron oxides and activated carbon. Cr(VI) adsorption behaviors on AFPAC under different water quality parameters were investigated. The maximum monolayer adsorption capacities for Cr(VI) by AFPAC are as high as 26.24 mg/g, 28.65 mg/g, and 32.05 mg/g at 25 °C, 35 °C and 45 °C at pH 4, respectively. Density functional theory (DFT) results showed that the adsorption energy of K2Cr2O7 on the surface of FeOOH was - 2.52 eV, which was greater than that on the surface of bare AC, and more charge transfer occurred during the adsorption of K2Cr2O7 on the surface of FeOOH, greatly promoting the formation of Cr = O-Fe. Cr(VI) removal by AFPAC included electrostatic attraction, redox reaction, coordinate complexation, and co-precipitation. Cr(VI) adsorption process on AFPAC consisted of the three reaction steps: (1) AFPAC was fast protonation and Cr2O72- would electrostatically attract to the positively charged AFPAC surface. (2) Cr2O72- was reduced into Cr2O3 by the carbons bond to the oxygen functionalities on activated carbon and the redox reaction process of FeSO4 and K2FeO4. (3) The inner-sphere complexes were formed, and adsorbed on AFPAC by iron oxides and then co-precipitation.

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Enhancing anaerobic treatment efficiency of waste activated sludge (WAS) toward preferable resource recovery would be an important requirement for achieving carbon-emission reduction, biosolids minimization, stabilization and security concurrently. This study demonstrated the synergic effect of potassium ferrate (PF) and nitrite on prompting WAS solubilisation and acidogenic fermentation toward harvesting volatile fatty acids (VFAs). The results indicated the PF+NaNO2 co-pretreatment boosted 7.44 times and 1.32 times higher WAS solubilisation [peak soluble chemical oxygen demand (SCOD) of 2680 ± 52 mg/L] than that by the single nitrite- and PF-pretreatment, respectively, while about 2.77 times and 2.11 times higher VFAs production were achieved (maximum VFAs accumulation of 3536.25 ± 115.24 mg COD/L) as compared with the single pretreatment (nitrite and PF)-fermentations. Afterwards the WAS dewaterability was improved simultaneously after acidogenic fermentation. Moreover, a schematic diagram was established for illustrating mechanisms of the co-pretreatment of PF and nitrite for enhancing the VFAs generation via increasing key hydrolytic enzymes, metabolic functional genes expression, shifting microbial biotransformation pathways and elevating abundances of key microbes in acidogenic fermentation. Furthermore, the mechanistic investigations suggested that the PF addition was conducive to form a relatively conductive fermentation environment for enhancing electron transfer (ET) efficiency, which contributed to the VFAs biotransformation positively. This study provided an effective strategy for enhancing the biodegradation/bioconversion efficiency of WAS organic matters with potential profitable economic returns.

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Potassium ferrate has strong oxidation in both acid and alkali environments, which has attracted extensive attention. However, the impact of the pH environment on this coupling process with the goal of resource recovery has not received attention. Under the goal of the efficient recovery of organic acid, the changes of solid-liquid characteristics of sludge after acid and alkaline ferrate pretreatment and during anaerobic digestion were discussed. The results showed that compared with blank control groups, after alkaline ferrate pretreatment, the volatile suspended solids (VSSs) decreased the most, reaching 28.19%. After being pretreated with alkaline ferrate, the sludge showed the maximum VFA accumulation (408.21 COD/g VSS) on the third day of digestion, which was 1.34 times higher than that of the acid ferrate pretreatment. Especially in an alkaline environment, there is no need to add additional alkaline substances to adjust the pH value, and the effect of sludge reduction and acid production is the best.

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In this study, a novel and efficient heterogeneous electro-Fenton (EF) process with a potassium ferrate (K2FeO4) modified carbon felt (Fe-CF) cathode was developed for chloramphenicol (CAP) removal. The catalytic activity was assessed by the comparison of different systems and the effects of multiple operating parameters (K2FeO4 dosage, initial solution pH, applied current) and co-existing constituents. Results indicated that the Fe-CF cathode exhibited excellent performance for CAP degradation (almost 100% removal efficiency within 60 min) over a wide range of pH (pH 3-9) during heterogeneous EF ascribed to the synergistic effect of embedded iron species and porous graphitic carbon structure and effective utilization of the in-situ generated H2O2. Moreover, the Fe-CF cathode possessed good recyclability with low metal leaching (98.2% CAP removal efficiency after reused for 5 times) and outstanding real water application performance. The ∙OH and O2∙- were responsible for CAP degradation, while ∙OH played a main role. Moreover, the toxicity evaluation by E. coli growth experiments demonstrated an efficient toxicity reduction in this system. Overall, a novel heterogeneous EF functional cathode with superior performance was fabricated via a green, low-cost one-step method, which shows promising application potential for actual wastewater treatment.

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This study researched the formation of disinfection by-products (DBPs) in sulfamethazine (SMZ) chlorination after preoxidation by K2FeO4 and O3 and the influence of preoxidation on cytotoxicity and biological toxicity. Then, the study emphatically analyzed the influencing factors such as NaClO dosage, pH value, reaction temperature, fulvic acid (FA), and bromide and iodide ions. The results showed that preoxidation by K2FeO4 effectively inhibited the formation of DBPs of haloketones (HKS) and trihalomethanes (THMs), with an average inhibition rate of over 60%. The formation of DBPs after preoxidation by O3 was higher than that by K2FeO4; preoxidation by K2FeO4 reduced the influence of NaClO dosage, temperature, and pH value on the production of DBPs after SMZ chlorination. The cytotoxicity and biological toxicity of SMZ chlorination after preoxidation were evaluated and compared by calculating the LC50 value of DBPs and the luminescent bacteria method. The results of both calculation methods showed that the toxicity of DBPs after preoxidation by K2FeO4 was lower than that by O3 preoxidation. K2FeO4 and O3 preoxidation improved the SMZ removal efficiency by 8.41 and 10.49%, respectively, and inhibited the formation of most DBPs, but the preoxidation promoted the formation of highly toxic DBPs (HANs). The toxicity of DBPs formed in SMZ chlorination after preoxidation by K2FeO4 and O3 was slightly higher than that of chlorination disinfection alone, but it was still within the safe range. This study provides more new details about the formation and toxicity changes of DBPs in the process of SMZ chlorination after preoxidation.

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Iron-loaded porous biochar (FPBC) was synthesized by co-pyrolysis method using sawdust and potassium ferrate at 500 (FPBC500) and 800°C (FPBC800), then characterized and applied to eliminate antimonite (Sb(III)) and antimonate (Sb(V)) in aqueous. Due to alkali erosion on feedstock and K/Fe-oxides attacking carbon, FPBC800 obtained a larger specific surface area (SSA) (515.49 m2·g-1) that was 5.48-fold that of PFBC500, meaning the exposure of more active sites. Fe3O4 was formed on FPBC500, but Fe0 and Fe3C were generated on FPBC800. FPBC800 showed the optimal sorption performance for Sb(III) (144.48 mg·g-1) and Sb(V) (45.29 mg·g-1), which were much higher than that of FPBC500. Noteworthily, Sb(III) anchored on FPBC was oxidized to Sb(V) with less ecotoxicity; moreover, FPBC800 with Fe0 showed stronger oxidization. Although pH-dependent sorption of Sb(III)/Sb(V) on FPBC occurred, the resistance to environmental factors showed a potential for eliminating actual pollution, demonstrating an easy-to-operate construction strategy for modified biochar.

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To improve the dose efficiency of K2FeO4 in waste activated sludge (WAS) treatment, pH regulation on K2FeO4 pretreatment and acidogenic fermentation was investigated. Four pretreatments were compared, i.e. pH3 + 50 g/kg-TS, pH10 + 50 g/kg-TS, neutral pH + 50 g/kg-TS and neutral pH + 100 g/kg-TS (without pH adjustment). The higher short chain fatty acids (SCFAs) yield and phosphorous dissolution rate was found under the condition of pH 10.0. In pH10 + 50 g/kg-TS, the maximum concentration of SCFAs was 5591 mg-COD/L, which yield was 22.6 times higher than that of the neutral pH + 50 g/kg-TS (237 mg COD/L). The acidogenic fermentation period could be shortened to 5 days and acetic acid accounted for 70 % of SCFAs. Furthermore, PO43--P in the hydrolysate (346.5 mg/L) accounted for 47.59 % of TP, which is easier to be recovered by chemical precipitation. Therefore, a more economical and feasible utilization mode of potassium ferrate was proposed.

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