Advancing mechanistic understanding of cohesive sediment transport: Integrating flume experiments, field measurements, and modelling approaches in a gravel-bed river.

Gravel-bed rivers draining mountainous forested headwater regions are critically important for drinking water supply and ecological integrity. These rivers, however, have been increasingly impacted by intensifying anthropogenic and natural (especially climate change exacerbated) landscape disturbances that commonly increase hillslope/channel connectivity and the delivery of cohesive sediment (<63 µm) and associated pollutants. Despite the known deleterious threats of excess cohesive sediments, there is still limited understanding of their transport and intra-gravel storage due to the complexities of such processes. Accordingly, the objectives of this study were to: i) calibrate and validate a semi-empirical cohesive sediment transport model (RIVFLOC) using the observations from flume experiments; ii) estimate the intra-gravel storage capacity for cohesive sediment with the calibrated model based on the field dataset (collected in two field campaigns between 2019 and 2021), and; iii) investigate mechanisms of cohesive sediment transport dynamics in this gravel-bed river, identifying knowledge gaps and areas for future research. Our results showed that despite the increased floc settling velocity, deposition was hindered by turbulent flow fields. The model predicted that ~60 % of upstream cohesive sediment would ingress within the 10 km study reach due to the flow interaction with the gravel-bed. Despite the agreement between flume and field observations on ingress rates and preferential ingress of coarser (~100 µm) flocs, notable differences were observed between modelled and field datasets, highlighting unknowns regarding cohesive sediment exfiltration without framework mobilization. This study uniquely integrates field measurements, flume experiments, and modelling strategies to evaluate the transport and fate of cohesive sediment in a gravel-bed river. Accordingly, our findings advance current knowledge on the mechanistic understanding of cohesive sediment transport and highlight future research directions.

Novel algae-mediated biosynthesis approach of chitosan nanoparticles using Ulva fasciata extract, process optimization, characterization and their flocculation performance.

Chitosan nanoparticles (CNPs) are promising biopolymeric nanoparticles with excellent physicochemical, antimicrobial, and biological properties. In this investigation, CNPs were produced using Ulva fasciata biomass extract as a reducing agent. The SEM micrograph revealed that the biosynthesized CNPs appeared to be spheres with a mean size of 32.49 nm. The ζ-potential pattern of CNPs has a single peak at +33.1 mV, indicating a positively charged surface. The X-ray diffraction pattern of the biosynthesized CNPs exhibited three different peaks at 2θ = 25.24, 52.96, and 72.28°. The FTIR analysis identifies various functional groups. The thermogravimetric analyses demonstrate that CNPs have high thermal stability. Additionally, the highest biosynthesis of CNPs (8.96 mg CNPs/mL) was obtained via FCCD when the initial pH level was 4, Ulva fasciata extract concentration was 45 %, v/v, and chitosan concentration was 0.9 %. Algae-mediated synthesized CNPs were used as coagulating/flocculating agents. By using the jar test, CNPs exhibited superior flocculation performance compared to commercial coagulants like alum, FeCl3, and chitosan in bulk form. Further, different parameters were screened, and the maximum flocculating activity (FA) recorded was 83.58 ±â€¯0.47 % at 500 mg/L of CNPs, 1-5 % clay suspension at pH and temperature ranges of 6-8 and 10-80 °C, respectively. CNPs displayed eminent performance in water clarification.

Towards a high-rate operation of contact stabilization process: Challenges of flocculation and floc stability.

The high-rate contact stabilization (HiCS) process, a variant of high-rate activated sludge, has gained attention for its superior energy recovery and enhanced biosorption capabilities. The need for efficient energy recovery in HiCS necessitates a high settling efficiency to minimize resource loss due to endogenous sludge consumption. However, the low sludge retention time (SRT) required for HiCS can significantly affect sludge floc stability and flocculation performance, warranting a deeper analysis of the factors influencing these characteristics. This study investigates the impact of SRT reduction on sludge performance, focusing on energy potential, viscoelasticity, and critical pressure. The analysis was conducted using rheological tests, contact angle measurements, zeta potential analysis, Fourier transform infrared spectroscopy, XDLVO theory, and the PARAFAC model. Results indicate that due to the contribution of hydrophobicity, the HiCS system maintained the large flocs morphology of the sludge even when the SRT was maintained for 2d. However, a combination of aerobic microbial activity, high concentrations of loosely bound extracellular polymeric substances, and the presence of the filamentous bacterium Thiothrix contributed to reduced flocculation performance.

Method evaluation for viruses in activated sludge: Concentration, sequencing, and identification.

Activated sludge (AS) in wastewater treatment plants is one of the largest artificial microbial ecosystems on earth and it makes enormous contributions to human societies. Viruses are an important component in AS with a high abundance. However, their communities and functionalities have not been as widely explored as those of other microorganisms, such as bacteria. This gap is mainly due to technical challenges in effective viral concentration, extraction, and sequencing. In this study, we compared four kinds of concentration methods, two sequencing approaches, and four identification bioinformatic tools to evaluate the whole analysis workflow for viruses in AS. Results showed flocculation, filtration, and resuspension (FFR) could get the longest DNA lengths and ultracentrifugation obtained the highest DNA yields for viruses in AS. Based on the results of present study, FFR and tangential flow filtration with the membrane pore size of 100 kDa were most recommended to concentrate viruses in AS samples with huge volumes. Besides, different concentration methods could get different viral catalogs and thus multiple methods should be combined to get the whole picture of viruses in the system. In addition, geNomad was the most recommended identification tool for viruses in the present study and the long-read sequencing could improve the assembly statistics of viruses when compared with the short-read sequencing. For the 8192 viral operational taxonomic units in this study, 95.1 % of them were phages and belonged to the same lineage at the order level of Caudovirales. Virulent phages dominated the AS system and Pseudomonadota were the main host. Taken together, this study provides new insights into methods selection for virus research of AS.

Maximization of Micractinium sp., biomass and use of Dual-Purpose reduced graphene supported vanadium oxide nanoparticles for harvesting and subsequent biodiesel production.

A sustainable media composition comprising of nanourea (NU), groundnut de-oiled cake (GDOC), and seaweed Extract (SE) was formulated using a mixture design for the cultivation of Micractinium sp. Maximum biomass yield and productivity of 5.52 ±â€¯0.09 g/L and 0.72 ±â€¯0.03 g/L d-1 were observed at 1.67 mM NU, 0.134 g/L GDOC and 0.250 mL L-1 SE, respectively. The highest lipid yield of 2.73 ±â€¯0.24 g/L was also observed, respectively. The reduced graphene-supported vanadium oxide nanoparticles (RGO-VNPs) with a net surface charge of + 34.10 mV were developed, which acted as a % as well as a catalyst for transesterification. A maximum flocculation efficiency of 98 % was observed with 200 ppm of RGO-VNPs. The Fatty Acid Methyl Ester (FAME) yield of 96.81 ±â€¯1.61 % was observed. Thus, the present study could provide a plausible solution for one-pot harvesting and synthesis of biodiesel utilizing microalgal lipids.

Integrated processes for olive mill wastewater treatment and its revalorization for microalgae culture.

The olive oil industry generates 30 million cubic meters of olive mill wastewaters (OMWWs) annually. OMWWs are a major environmental concern in the Mediterranean region due to their high organic matter content, suspended solids, unpleasant odor, and dark color. The application of primary treatments such as coagulation-flocculation, adsorption, and hybrid systems combining coagulation-flocculation with adsorption has enabled to remove part of the organic matter, color, turbidity, and growth-inhibiting compounds from OMWWs. Among these methods, the hybrid system combining activated carbon and chitosan has proven to be the best removal efficiency. Subsequently, secondary treatment involving the cultivation of Chlorella sp. on OMWWs pretreated with chitosan achieved the highest maximal specific growth rate (0.513 ± 0.022 day⁻1) and biomass productivity (0.621 ± 0.021 g/L/day). Notably, the fatty acids (FA) profile produced by Chlorella sp. cells grown under these conditions differed, underscoring the potential of OMWWs as a microalgal growth medium. This innovative approach not only addresses environmental issues but also opens new avenues for sustainable bioproducts.

Microalgae separation in MP-PVC contaminated wastewater using plant-based coagulant over different extraction methods in Bauru, Brazil.

This study investigates the effectiveness of coagulation-flocculation and sedimentation (CFS) for separating microalgae, focusing on the use of various Moringa oleifera extracts as natural coagulants. We examined photobioreactor effluent (PBR) both with and without microplastic PVC (MP-PVC) contamination, referred to as PBR R2 and PBR R1, respectively. Utilising response surface methodology, we identified optimal conditions for the removal of microalgae and MP-PVC. Validation tests demonstrated that the aqueous extract of delipidated Moringa oleifera powder (AEDMOP) achieved high removal efficiencies, with coagulant dosages of 630 mg L-1 for PBR R1 and 625 mg L-1 for PBR R2. Both conditions showed microalgae removal efficiencies exceeding 83% for turbidity, colour, OD540 nm, OD680 nm, and OD750 nm, and 63% for OD254 nm. Interestingly, the optimised conditions for PBR R2 required slightly less coagulant, likely due to the additional particulate matter from MP-PVC. While extracellular polymeric substances (EPS) exhibited a marginal effect on flocculation, further investigation into their role in CFS is necessary. Our findings highlight the potential of AEDMOP for efficient microalgae separation, even in the presence of microplastics.

Microalgae Flocculation: Assessment of Extraction Yields and Biological Activity.

Downstream costs represent one of the main obstacles to enabling microalgae to become widespread. The development of an economical, easily scaled-up strategy could reduce the overall process costs. Here, different flocculants were tested on different microalgae strains and a cyanobacterium. The results indicate that flocculation could be an alternative to centrifugation, as CaCl2 induced a complete flocculation of green and red marine strains (96 ± 4% and 87.0 ± 0.5%, respectively), whereas Chitosan was the only agent able to induce flocculation on the cyanobacterium (46 ± 1%). As for the thermoacidophilic red microalga, 100% flocculation was achieved only by increasing the pH. Carotenoids were extracted from the flocculated biomass, and the strategy improved with the use of the wet biomass. The results indicate that flocculation does not affect carotenoid yield, which is at least the same than that obtained upon centrifugation and extraction from the wet biomass. Then, for the first time, the biological activity of the extracts obtained from the flocculated biomasses was evaluated. The results indicate that only the green microalga extract shows increased antioxidant activity. In conclusion, this work highlights that a general downstream procedure cannot be developed for microalgae strains but should be rationally tailored.

Assessment of Skimmed Milk Flocculation for Bacterial Enrichment from Water Samples, and Benchmarking of DNA Extraction and 16S rRNA Databases for Metagenomics.

Water samples for bacterial microbiome studies undergo biomass concentration, DNA extraction, and taxonomic identification steps. Through benchmarking, we studied the applicability of skimmed milk flocculation (SMF) for bacterial enrichment, an adapted in-house DNA extraction protocol, and six 16S rRNA databases (16S-DBs). Surface water samples from two rivers were treated with SMF and vacuum filtration (VF) and subjected to amplicon or shotgun metagenomics. A microbial community standard underwent five DNA extraction protocols, taxonomical identification with six different 16S-DBs, and evaluation by the Measurement Integrity Quotient (MIQ) score. In SMF samples, the skimmed milk was metabolized by members of lactic acid bacteria or genera such as Polaromonas, Macrococcus, and Agitococcus, resulting in increased relative abundance (p < 0.5) up to 5.0 log fold change compared to VF, rendering SMF inapplicable for bacterial microbiome studies. The best-performing DNA extraction protocols were FastSpin Soil, the in-house method, and EurX. All 16S-DBs yielded comparable MIQ scores within each DNA extraction kit, ranging from 61-66 (ZymoBIOMICs) up to 80-82 (FastSpin). DNA extraction kits exert more bias toward the composition than 16S-DBs. This benchmarking study provided valuable information to inform future water metagenomic study designs.

Measured and predicted floc size of cohesive sediment in the presence of microalgae.

In aquatic environments, biological factors significantly influence the flocculation process of cohesive sediments, thereby impacting sediment transport dynamics. Due to its complexity, the mechanism of biological flocculation still remains unknown. Here, we conducted laboratory experiments to investigate how living microalgae (Skeletonema costatum) affects the flocculation of mineral clay under various shear rates and suspended sediment concentrations (SSC) in saline water. The microalgae (Skeletonema costatum) and SSC both have positive influences on the increase in floc size. However, the shear rate (G) shows dual effect. Specifically, there exists a critical shear rate, G*, at which the floc size increases with G when G≤G* and decreases with G when G>G*. More importantly, G*is affected by SSC and exhibits no dependence on microalgae content. The microalgae (Skeletonema costatum) has a dominant effect on both floc shape and floc size of microalgae-mineral flocs compared to shear rate under the present experimental conditions (SSC: 700 mg/L, chlorophyll-a concentration: 0∼13.76 µg/L, shear rate: 10∼90 s-1). Additionally, the elongated-rod flocs are more easily formed in microalgae-mineral clay suspensions, whereas the plate-stacked flocs are more abundant in pure mineral clay suspensions. The promoting effect of microalgae is obvious under low shear rate conditions (G≤40 s-1), while at high shear rate (G>40 s-1), this effect is significantly attenuated, with a reduction by nearly half. Finally, a new bioflocculation model was proposed to predict the equilibrium median floc size for both conditions with and without microalgae. The model was well validated through comparisons with laboratory measurements.

Optimization of synthesis of cationic starches for wastewater sludge and microalgae separation.

The implementation of stricter water protection legislation requires the development of novel environmentally friendly water treatment materials. A new method for the preparation of water soluble cationic starch flocculants using potato starch, 3-chloro-2-hydroxypropyltrimethylammonium chloride and CaO additive was developed and surface response methodology was successfully utilized for the optimization of degree of substitution in the cationization of potato starch with and without CaO additive. Based on the results of destabilization studies of model kaolin, wastewater sludge, and microalgae dispersion systems, optimized conditions ware proposed for obtaining an efficient, soluble, and biodegradable cationic starch flocculant with optimal structure. The duration of starch etherification reaction was reduced to <12 h to obtain soluble cationic starch derivatives with a degree of substitution of quaternary ammonium groups of 0.25, which retained the granular structure during synthesis. An efficient flocculation technology using anionic polymer and high content of biodegradable cationic flocculant was proposed. The highly efficient flocculation of microalgae suspensions using developed cationic starch derivative with the degree of substitution of cationic groups of 0.25 has been also achieved. The developed environmentally friendly cationic starches with tailored flocculation properties proofed to have a great potential in various water cleaning and separation technologies with prospects in wastewater treatment, agriculture or energy sectors.

Production and characterization of efficient bioflocculant in high-turbidity drinking water treatment: Identification of flocculation-related genes.

Bioflocculants are eco-friendly water treatment agents produced by bioflocculant-producing strains that are valuable in drinking water turbidity removal. The major challenges in the application of bioflocculants include low flocculation efficiency, high production costs, and unclear flocculation-related genes. In this study, Pseudomonas sp. ZC-41 a highly efficient bioflocculant-producing strain, was isolated from activated sludge to produce polysaccharide-based bioflocculant MBF-ZC with 94.12% flocculation efficiency under more economical culture conditions, which can solve the problem of low flocculation efficiency. Fourier transform infrared spectra (FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed MBF-ZC contained hydroxyl, carboxyl, and amine groups, crucial for flocculation via adsorption bridging effects as the main flocculation mechanism. The 2393 differentially expressed genes (DEGs) in the transcriptome of strain ZC-41 were classified into five co-expression modules, and the turquoise module was associated with flocculation efficiency and bioflocculant yield. Nineteen flocculation-related genes were identified by combining functional pathways related to sugars. In addition, response surface methodology was optimized to achieve the efficiency of 93.57% for turbidity removal from high-turbidity water by bioflocculant. The results not only provide a solid theoretical foundation to solve the challenges of bioflocculants, but also enrich strategies for high-turbidity drinking water treatment.

Diverse interactions between bacteria and microalgae: A review for enhancing harmful algal bloom mitigation and biomass processing efficiency.

The interactions between bacteria and microalgae play pivotal roles in resource allocation, biomass accumulation, nutrient recycling, and species succession in aquatic systems, offering ample opportunities to solve several social problems. The escalating threat of harmful algal blooms (HABs) in the aquatic environment and the lack of cheap and eco-friendly algal-biomass processing methods have been among the main problems, demanding efficient and sustainable solutions. In light of this, the application of algicidal bacteria to control HABs and enhance algal biomass processing has been promoted in the past few decades as potentially suitable mechanisms to solve those problems. Hence, this comprehensive review aims to explore the diverse interaction modes between bacteria and microalgae, ranging from synergistic to antagonistic, and presents up-to-date information and in-depth analysis of their potential biotechnological applications, particularly in controlling HABs and enhancing microalgal biomass processing. For instance, several studies revealed that algicidal bacteria can effectively inhibit the growth of Microcystis aeruginosa, a notorious freshwater HAB species, with an antialgal efficiency of 24.87 %-98.8 %. The review begins with an overview of the mechanisms behind algae-bacteria interactions, including the environmental factors influencing these dynamics and their broader implications for aquatic ecosystems. It then provides a detailed analysis of the role of algicidal bacteria in controlling harmful algal blooms, as well as their role in bioflocculation and the pretreatment of microalgal biomass. Additionally, the review identifies and discusses the constraints and challenges in the biotechnological application of these interactions. By exploring the strategic use of algicidal bacteria, this review not only underscores their importance in maintaining aquatic environmental health but also in enhancing biomass processing efficiency. It offers valuable insights into future research avenues and the potential scalability of these applications, both in situ and at an industrial level.

Utilizing novel Aspergillus species for bio-flocculation: A cost-effective approach to harvest Scenedesmus microalgae for biofuel production.

The present study aimed to isolate a bioflocculating fungal strain from wastewater collected from a local bike garage. The isolate showed maximum similarity to Aspergillus species. The fungus was identified as Aspergillus flavus species F_GTAF1 IU (accession no OP703382). The isolated fungus was evaluated in terms of biomass recovery efficiency in Scenedesmus Sp. GTAF01. The extent of algal fungal co-pelletization was evaluated as a function of the algae-to-fungi ratio, volume of fungal culture in broth, agitation rate, and pH. results showed that at fungal culture volume of 60 ░ %v/v, fungal culture volume of 1:3 ░ %w/w, 100 rpm, and pH 3, 93.6 ░ % biomass was obtained during the initial 5 h. At wavenumbers 1384 and 1024 cm-1 a significant alteration in the transmission percentage was observed in co-pellet compared to algae and fungal cells. This shows the significant role of C-H-H and C-N stretches in co-pellet formation. This study provides deep insight into effective microalgal harvesting along with the simultaneous extraction of lipids that can be used for the sustainable production of biodiesel.

Photochemical Origins of Iron Flocculation in Acid Mine Drainage.

Acid mine drainage (AMD) raises a global environmental concern impacting the iron cycle. Although the formation of Fe(III) minerals in AMD-impacted waters has previously been reported to be regulated by biological processes, the role of abiotic processes remains largely unknown. This study first reported that a photochemical reaction coupled with O2 significantly accelerated the formation of Fe(III) flocculates (i.e., schwertmannite) in the AMD, as evidenced by the comparison of samples from contaminated sites across different natural conditions at latitudes 24-29° N. Combined with experimental and modeling results, it is further discovered that the intramolecular oxidation of photogenerated Fe(II) with a five-coordinative pyramidal configuration (i.e., [(H2O)5Fe]2+) by O2 was the key in enhancing the photooxidation of Fe(II) in the simulated AMD. The in situ attenuated total reflectance-Fourier transform infrared spectrometry (ATR-FTIR), UV-vis spectroscopy, solvent substitution, and quantum yield analyses indicated that, acting as a precursor for flocculation, [(H2O)5Fe]2+ likely originated from both the dissolved and colloidal forms of Fe(III) through homogeneous and surface ligand-to-metal charge transfers. Density functional theory calculations and X-ray absorption spectroscopy results further suggested that the specific oxidation pathways of Fe(II) produced the highly reactive iron species and triggered the hydrolysis and formation of transient dihydroxo dimers. The proposed new pathways of Fe cycle are crucial in controlling the mobility of heavy metal anions in acidic waters and enhance the understanding of complicated iron biochemistry that is related to the fate of contaminants and nutrients.

Microalgae treatment of food processing wastewater for simultaneous biomass resource recycling and water reuse.

Food processing wastewater presents a considerable challenge for treatment owing to its elevated nitrogen and phosphorus levels. Nonetheless, it possesses inherent value attributed to its abundant nutrients and organic content. This study presents an innovative approach for treating food processing wastewater and reusing biomass. Initially, the secondary-treated wastewater undergoes flocculation and sedimentation, followed by reverse osmosis to ensure that the effluent meets reuse standards. Subsequently, reverse osmosis concentrates, generated at varying water recovery rates, are utilized for microalgae cultivation to recover nitrogen and phosphorus. Furthermore, this study highlights the potential of reverse osmosis concentrates in reducing the water demand for microalgae cultivation and in producing commercial-grade nutrients. The findings reveal that reverse osmosis achieves removal rates exceeding 90 % for both nitrogen and phosphorus and effluent meets reuse standards. Following seven days of cultivation, microalgae cultured in reverse osmosis concentrated water with an 80 % water recovery rate demonstrate denitrification and phosphorus removal rates of 73.88 % and 80.92 % respectively, with a biomass concentration of 563 mg/L and a protein yield of 128 mg/L. Moreover, a total volumetric energy yield of 10.08 kJ/L is obtained, facilitating energy valorization. In conclusion, this study offers practical solutions for wastewater treatment and resource recovery, enabling the attainment of zero discharge of pollutants while generating valuable resources through microalgae cultivation.

Toward a better understanding of polymeric aluminum-modified attapulgite for the efficient removal of low phosphorus concentration.

Attapulgite (ATP) is a biocompatible clay mineral that efficiently absorbs water. It is widely used in water treatment due to its environmental friendliness and cost-effectiveness. This study aimed to develop a volume-expansion structure-based attapulgite flocculant (VES-ATP) using aluminum salt and attapulgite (ATP) under alkaline conditions, specifically for the treatment of water containing low levels of phosphorus. The VES-ATP was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The removal of phosphorus by the VES-ATP was conducted by varying the mass ratio of Al to attapulgite (denoted as RmAl/mATP), ATP dosage, and pH. The results showed that the VES-ATP had a good expansion and dispersibility in the presence of alkalized aluminum species. The basicity as the molar ratio of OH to Al (0.8 or 1.6) determined the expansion feasibility, and the coverage degree of Al onto ATP, as indicated by the mass ratio of Al to attapulgite (denoted as RmAl/mATP), determined Al flocculation efficiency. Higher values such as RmAl/mATP = 4:1 and 2:1 may result in a better flocculation. Low phosphorus treatment was successfully achieved through Al flocculation and ATP adsorption, including complexation, hydrogen bonding, and electrostatic attraction. As expected, the VES-ATP generated larger size flocs with a bigger fractal dimension than that with the sole Al flocculation. As a result, the total phosphorus could be reduced to the level below 5 µg/L. It is more efficient in the pH range of 5-9. Overall, the coupling of aluminum and attapulgite has significantly enhanced both purification capabilities of phosphorus. PRACTITIONER POINTS: Polymeric aluminum-modified attapulgite was efficient for removal of low phosphorus concentration. Phosphorus concentrations can be reduced to below 5 µg/L. Polymeric aluminum and attapulgite are both safe, and this technology is suitable for water treatment.

Wastewater Surveillance of SARS-CoV-2: A Comparison of Two Concentration Methods.

Wastewater surveillance is crucial for the epidemiological monitoring of SARS-CoV-2. Various concentration techniques, such as skimmed milk flocculation (SMF) and polyethylene glycol (PEG) precipitation, are employed to isolate the virus effectively. This study aims to compare these two methods and determine the one with the superior recovery rates. From February to December 2021, 24-h wastewater samples were collected from the Ioannina Wastewater Treatment Plant's inlet and processed using both techniques. Subsequent viral genome isolation and a real-time RT-qPCR detection of SARS-CoV-2 were performed. The quantitative analysis demonstrated a higher detection sensitivity with a PEG-based concentration than SMF. Moreover, when the samples were positive by both methods, PEG consistently yielded higher viral loads. These findings underscore the need for further research into concentration methodologies and the development of precise protocols to enhance epidemiological surveillance through wastewater analysis.

Long-lasting organics removal via •OH adsorbed transition metal flocs: Electron transfer-mediated H-bond and van der Waals force.

Homogenous advanced oxidation processes (AOPs) based on transition metal catalysts toward the activation of H2O2 to hydroxyl radical (•OH) have been widely applied to organic pollutants removal, such as Fenton and Fenton-like processes. These transition metal catalysts mostly flocculate as the pH increases. It's worth noting that the formed transition metal flocs are complex heterogeneous aggregations with active substances, providing diverse reaction spaces and interfaces. However, it is a challenge to distinguish the roles of transition metal flocs in the organic pollutants removal from homogeneous catalytic reactions. Herein, we unveiled a pathway for the long-lasting removal of organic pollutants via Cr flocs adsorbed with •OH (HO•-Cr flocs) using a stepwise method. First, adsorbed •OH (•OHads) within the HO•-Cr flocs was proved to be the active site forming hydrogen bond (H-bond) and van der Waals force with organic pollutants. Then, the presence of switchable electron transfer between Cr and OH groups within the HO•-Cr flocs was revealed, contributing to the persistent existence of •OHads and consequently ensuring the long-lasting organics removal. Further, this removal pathway of organic pollutants was confirmed during the leather wastewater treatment. These findings will complement a different pathway for organic pollutants removal via transition metal flocs and extend the lifetime of homogeneous AOPs based on transition metal catalysts, providing significant implications for their design and optimization.

Insights into the differential removal of various red tide organisms using modified clay: Influence of biocellular properties and mechanical interactions.

In recent years, red tides have increased worldwide in frequency, intensity, involving a higher number of causative species during the events. As the most commonly used method for control of red tides, modified clay (MC) was found to have differential ability to remove various red tide species. However, the underlying mechanisms have not yet been completely elucidated. In this study, the use of MC to remove three typical disaster-causing species, Aureococcus anophagefferens, Prorocentrum donghaiense and Heterosigma akashiwo, was investigated, and differential removal of these species was probed with insights into their biocellular properties and mechanical interactions. The results showed that removal efficiencies of the three species by MC decreased in the order P. donghaiense > A. anophagefferens > H. akashiwo, while the sedimentation rates decreased in the order H. akashiwo > P. donghaiense > A. anophagefferens. Analyses of the cell surface properties and redundancy analysis (RDA) revealed that the highest surface zeta potential of -5.32±0.39 mV made P. donghaiense the most easily removed species; the smallest cell size of 3.30±0.03 µm facilitated the removal of A. anophagefferens; and the highest hydrophobicity with a H2O surface contact angle of 98.50±4.31° made the removal of H. akashiwo difficult. X-ray photoelectron spectroscopy (XPS) data indicated that the electronegativity of P. donghaiense was caused by carboxyl groups and phosphodiester groups, and the hydrophobicity of H. akashiwo was associated with a high C-(C, H) content on the cell surface. According to the extended Derjaguin, Landau, Verwey, and Overbeek (ex-DLVO) theory calculation, differences in the interaction energies between MC and the three red tide species effectively explained their different sedimentation rates. In addition, the degree of oxidative damage caused by MC to the three red tide species differed, which also affected the removal of red tide organisms.