The variation of flocs activity during floc breakage and aging, adsorbing phosphate, humic acid and clay particles

Two-stage injection of polymer and microsand during ballasted flocculation for treating kaolin waters with or without humic acid: Floc evolutional characteristics, performance and mechanisms

Ballasted flocculation is regarded as a most promising water treatment technology in aspects of retrofit and high-rate applications. To deep understand the incorporation behaviors of ballasting agent into ballasted floc growth, two distinct injection modes (namely a two-stage injection of polyacrylamide (PAM) alone, and a two-stage injection of both PAM and microsand) were developed in this study. Then, ballasted flocculation tests of kaolin and kaolin-HA (humic acid) waters were conducted at varying split ratios for fixed total dosages of both PAM and microsand. The experimental results showed that for either two-stage injection mode, the higher the second percentage of each split ratio, the greater the average size of maturated flocs at the second sub-stage of maturation. Meanwhile, the turbidity and UV254 values of settled water became lower at 30 and 180 s of sedimentation, suggesting that varying split ratios significantly affected the kinetics of ballasted floc growth. Moreover, it was suggested that the selection of either two-stage injection mode or corresponding split ratios played a more pronounced role in the HA removal than the total dosage of PAM. This suggestion was supported by SEM, FTIR and XPS analyses for surface morphological details, functional groups and chemical states of maturated flocs eventually formed in the kaolin-HA water through both two-stage injection modes. Accordingly, newly-established conceptual models of ballasted floc growth were proposed to explore the potential influencing mechanisms of varying split ratios on the ballasted flocculation performance. At each sub-stage of maturation, an appropriate dosage ratio between PAM and microsand was of great importance to effectively incorporate microsand particles into ballasted floc formation, besides the hydrolyzed produces of AS coagulant formed at the coagulation stage of ballasted flocculation. This study is expected to provide valuable insights for making ballasted flocculation more effective, economical and sustainable in water treatment engineering.

Effect of surface functional groups of polystyrene micro/nano plastics on the release of NOM from flocs during the aging process

Currently, the hidden risk of microplastics in the coagulation process has attracted much attention. However, previous studies aimed at improving the removal efficiency of microplastics and ignored the importance of interactions between microplastics and natural organic matter (NOM). This study investigated how polystyrene micro/nano particles impact the release of NOM during the aging of flocs formed by aluminum-based coagulants Al13 and AlCl3. The results elucidated that nano-particles with small particle sizes and agglomerative states are more likely to interact with coagulants. After 7 years of floc aging, the DOC content of the nano system decreased by more than 40%, while the micron system did not change significantly. During coagulation, the benzene rings in polystyrene particles form complexes with electrophilic aluminum ions through π-bonding, creating new Al-O bonds. NOM tends to adsorb at micro/nano plastic interfaces due to hydrophobic interactions and conformational entropy. In the aging process, the structure of PS-Al13 or PS-AlCl3 flocs and the functional groups on the surface of micro/nano plastics control the absorption and release of organic matter through hydrophobic, van der Waals forces, hydration, and polymer bridging. In the system with the addition of nano plastics, several DBPs such as TCAA, DCAA, TBM, DBCM and nitrosamines were reduced by more than 50%. The reaction order of different morphological structures and surface functional groups of microplastics to Al13 and AlCl3 systems is aromatic C-H > C-OH > C-O > NH2 > aromatic CC > aliphatic C-H and C-O>H-CO> NH2 >C-OH> aliphatic C-H. The results provided a new sight to explore the effect of micro/nano plastics on the release of NOM during flocs aging.

Selective Modifier-Assisted Humic Acid Extraction: Implications for Soil Quality Enhancement

Efficient use of humic acid (HA) for eco-friendly farming and environmental remediation requires further understanding of how targeted modification of HA affects the chemical structure of HA and thereby its effectiveness in enhancing soil quality. We developed novel selective modifiers (SMs) for extracting HA by codoping sodium and copper elements into the birnessite lattice. The structure of SMs was thoroughly examined, and the HAs extracted using SMs, referred to as SMHs, were subjected to a detailed evaluation of their functional groups, molecular weight, carbon composition, flocculation limits, and effectiveness in saline soil remediation. The results showed that replacing manganese with sodium and copper in SMs alters the valence state and reactive oxygen species. In contrast, SMHs exhibited increased acidic functional groups, a lower molecular weight, and transformed aliphatic carbon. Furthermore, the saline soil was improved through increased salt leaching and an optimized soil aggregate structure by SMHs. This research highlights the importance of targeted modification of HA and demonstrates the potential of these modifiers in improving soil quality for eco-friendly farming and environmental remediation.

Uncovering the neglected role of anions in trivalent cation-based coagulation processes

Coagulation efficiency is heavily contingent upon a profound comprehension of the underlying mechanisms, facilitated by the evolution of coagulation theory. However, the role of anions, prevalent components in raw and wastewaters, has been relatively overlooked in this context. To address this gap, this study has investigated the impact of three common anions (i.e., chloride, sulfate, and phosphate) on Al-based coagulation. The results have shown that the influence of anions on coagulation depends predominantly on their ability to compete with hydroxyl groups throughout the entire coagulation process, encompassing hydrolysis, aggregation, and the growth of large flocs. Moreover, this competition is subject to the dual influence of both anion concentration and hydroxyl concentration (i.e., pH). The results have revealed the intricate interplay between anions and coagulants, their impact on floc structure, and their importance in optimizing coagulation efficiency and ensuring the production of high-quality water.

Rainwater extracting characteristics and its potential impact on DBPs generation: A case study

Frequent extreme precipitation events due to global warming can lead to large amounts of pollutants entering source water bodies via surface runoff and wet deposition, thus posing a threat to water supply security. In order to better understand the source characteristics and leaching mechanisms of rainwater dissolved organic matter (DOM), as well as its disinfection by-products formation potential (DBPsFP) during disinfection processes, rainwater samples were collected and extracting experiments were conducted. Three components were identified in rainwater through Parallel factor (PARAFAC) analysis, which were microbial humic-like component C1 (63.1 %), protein (tryptophan-like) component C2 (28.9 %), marine or terrestrial humic-like component C3 (8.1 %). The average molecular weight of rainwater fractions was ordered: hydrophobic neutral (HON) < hydrophobic bases (HOB) < hydrophobic acidic (HOA) < hydrophilic (HIS). The HOA and HON fractions of rainwater were the dominant precursors of trihalomethanes (THMs), while the rainwater HON fraction and hydrophilic fraction were the main precursor of haloacetic acids (HAAs) and trihloroacetonitrile (TCAN), respectively. Subsoil extracts had a higher concentration of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) than topsoil extracts. Partial least squares path modeling (PLS-PM) demonstrated that the extraction temperature was the dominant factor affecting the abundance of DOM in the topsoil extracts (R2 = 0.28), while the extraction time accounted more for the abundance of fluorescence substance and physicochemical indices in the subsoil extracts (R2 = 0.23 and 0.32, respectively). These results provide key information for controlling the impacts of global warming, in particular the risk of water sources being heavily contaminated by request rainfalls.

Enhancing floc size and strength with a hybrid polymer of zinc oxide, acrylamide, and tannin in textile wastewater

This study involved synthesising new hybrid polymers called ZOPAT, made up of zinc oxide, acrylamide, and tannin, using a blended technique. The effectiveness of ZOPAT in treating textile wastewater was then tested by measuring floc growth rate, flocculation index, strength factor, and recovery factor under optimised conditions. The study also identified the zeta potential, morphology, elemental composition, and functional groups of the polymers. Response surface methodology determines the optimal pH and ZOPAT dose, resulting in 93% colour, 80% chemical oxygen demand (COD), 100% turbidity, and suspended solids (SS) removal at pH 9.22 and 737 mg/L ZOPAT. The study found that ZOPAT was more effective than commercial Polyaluminium chloride in reducing colour and COD, producing larger and stronger flocs, and requiring a shorter coagulation time of 17.5 min. ZOPAT was also easy to homogenise and operate due to its one-unit dosing system. The study attributes the success of ZOPAT to the presence of Zn, N, and K, which create electrostatic attraction with opposite charged particles, and the formation of dye-particle-dye with amide, hydroxyl, and carboxyl groups in ZOPAT, which remove colour, turbidity, COD, and SS. Overall, the study concludes that ZOPAT has significant potential for textile wastewater treatment.

Hydroxyl radical induced Cr flocculation via redox reaction: The extending application of heterogeneous advanced oxidation processes on Cr removal

Flocculation is a traditional and effective method to remove Cr from wastewater, but the addition of flocculants inevitably leads to secondary pollution. In this study, Cr flocculation was induced using hydroxyl radical (•OH) (•OH flocculation) generated in an electro-Fenton-like system, achieving total Cr removal of 98.68% at initial pH = 8 within 40 min. The obtained Cr flocs showed significantly higher Cr content, lower sludge yield, and good settling properties compared to alkali precipitation and polyaluminum chloride flocculation. •OH flocculation behaved like a typical flocculant, introducing electrostatic neutralization and bridging. The mechanism proposed that •OH could overcome the steric hindrance of Cr(H₂O)₆³⁺ and combine with it as an additional ligand. Then Cr(III) was proved to undergo multi-step oxidation to form Cr(IV) and Cr(V). After these oxidation reactions, •OH flocculation took precedence over Cr(VI) generation. As a result, Cr(VI) didn't accumulate in solution until •OH flocculation was completed. This work provided a clean and eco-friendly strategy for Cr flocculation instead of flocculants and extended the application of advanced oxidation processes (AOPs), which is expected to enrich existing strategies of AOPs towards Cr removal.

A review of the application of cerium and lanthanum in phosphorus removal during wastewater treatment: Characteristics, mechanism, and recovery

Owing to their strong bond with anions, rare earth elements (REEs) are prime contenders in wastewater treatment to meet the stringent phosphorus (P) effluent quality requirements. REEs outcompete traditional metals to abate phosphorus. The application of lanthanides in wastewater treatment is mainly through adsorption, where REEs are incorporated into a carrier matrix to improve the adsorption capacity. As coagulants, information on the performance of lanthanides is lacking. In this review, the performance of major water coagulants (iron and aluminum) is discussed and compared to two lanthanides: cerium and lanthanum. The use of lanthanides as adsorbents and as coagulants is elucidated during P treatment. The recovery of P and REEs is also discussed. Where details were lacking in the literature, experiments were conducted to fill these research gaps. Using REEs as adsorbents limits their P precipitation potential; as coagulants, REE capacity is 520.79 mg P/g La³⁺ and 469.96 mg P/g Ce³⁺. In addition, as coagulants, they are not affected by pH (3.0 < pH < 10.0); however, carbonates and sulfate are the major species that can reduce the performance of REEs during P treatment. REE-P precipitation is orchestrated through the formation of an REE-PO₄ bond. Unfortunately, this strong bond between lanthanides and phosphate makes phosphate recovery almost impractical. If the goal is to recover REEs and reuse P in other applications like fertilizers, REEs are not the best candidates. We recommend additional research dedicated to understanding lanthanide coagulants in typical wastewater treatment facilities and their release from phosphate precipitates under different environmental conditions.

The migration and microbiological degradation of dissolved organic matter in riparian soils

Riparian zones are important natural means of water purification, by decreasing the aqueous concentration of terrestrial organic matter (OM) through adsorption and microbial degradation of the organic matter within the aquatic ecosystem. Limited studies have been reported so far concerning the migration of dissolved organic matter (DOM) in the horizontal and vertical planes of riparian zones. In this study, the migration of DOM in riparian zones, from forest soil to wetland soil, and with soil depth, were explored, based on a case study reservoir. Results showed that riparian wetlands can absorb the OM from the forest soils and adjacent reservoir, and act as a major OM sink through microbial action. Methylomirabilota and GAL15 bacteria increased with soil depth for the two soil systems, and the wetland soil system also contained microbial sulfates, nitrates and carbonates. These microorganisms successfully utilize the Fe³⁺, SO₄^-, and CO₃^- as electron acceptors in the wetland system, resulting in enhanced OM removal. Although the variation of soil DOM in the vertical direction was the same for both forest and wetland soils, the Chemical structure of the DOM was found to be significantly different. Furthermore, the soil was found to be the main source of DOM in the forest ecosystem, with lignin as the main ingredient. The lignin structure was gradually oxidized and decomposed, with an increase in carboxyl groups, as the lignin diffused down into the soil and the adjacent reservoir. PLS-PM analysis showed that the soil physicochemical properties were the main factors affecting DOM transformation. However, microbial metabolism was still the goes deeper affecting factor. This study will contribute to the analysis that migration and transform of soil organic matter in riparian zone.

Mechanistic insight of weak magnetic field trigger transformation of amorphous Fe(III)-(oxy)hydroxide for enhanced ferrate (VI) towards selective removal of natural organic matter

It was important to regulate the formation of Fe-hydroxyl during ferrate (Fe(VI)) oxidation and hydrolysis which was beneficial for interfacial adsorption of natural organic matter (NOM). Based on the influence of weak magnetic field (WMF) on the physical and chemical characteristics of particles in chemistry. This study investigated the effect of WMF on Fe(VI) oxidation and Fe(III) flocculation performance by regulating iron species during hydrolysis, for NOM removal. Results indicated WMF efficiently accelerate the removal of NOM that the reactions rate constants in magnetization system was twice as much as the control group. With the structure and electrochemical analysis, WMF enhanced Hydrogen-bond and caused much polar hydroxyl groups combined with iron ions, further triggered Fe(III) transformed to amorphous Fe-hydroxide and ferrihydrite with large specific surface area and high surface activity which removed the pollutants by adsorption and co-precipitation, instead of crystalloid Fe₂O₃ and Fe₃O₄. In addition, the nucleation aggregation behavior and interaction energy of Fe-(oxy)hydroxide revealed that the lower free energy obtained in magnetization system, could lead to higher nucleation rate, and promoted the aggregation. WMF increased hydrophobicity of Fe-(oxy)hydroxides, further more easily adsorbed with humic acid (HA) and bovine serum albumin (BSA) with lower interaction energies than in control group. The selective removal mechanism of Fe-(oxy)hydroxide hardly to aggregate with pollutants which caused by the difference of electrostatic interaction, was illustrated that electronegativity HA and SA were liable to electrostatically attract with Fe-(oxy)hydroxide and removed while the low electronegativity BSA was difficult to remove which its attraction was weakened.

Removal of microorganic pollutants in aquatic environment: The utilization of Fe(VI)

Microorganic pollutants (MOPs) in aquatic environment with low levels but high toxicity are harmful to ecosystem and human health. Fe(VI) has a dual-functional role in oxidation and coagulation, and can effectively remove MOPs, heavy metal, phosphate, particulates and colloids. Moreover, Fe(VI) can combine with traditional coagulants, or use as a pretreatment for membrane treatment because of its characters to generate nanoparticles by degradation in water. Based on the relevant toxicity experiments, Fe(VI) had been proved to be safe for the efficient treatment of MOPs. For better utilization of Fe(VI), its oxidation and coagulation mechanisms are summarized, and the knowledge about the control parameters, utilization methods, and toxicity effect for Fe(VI) application are reviewed in this paper. pH, different valences of iron, environmental substances, and other parameters are summarized in this study to clarify the important factors in the treatment of MOPs with Fe(VI). In the future study, aiming at cost reduction in Fe(VI) preparation, transportation and storage, enhancement of oxidation in the intermediate state, and better understanding the mechanism between interface and Fe(VI) oxidation will help promote the application of Fe(VI) in the removal of MOPs. This study offers guidelines for the application and development of Fe(VI) for the treatment of MOPs in aquatic environment.

Variations in NOM during floc aging: Effect of typical Al-based coagulants and different particle sizes

Most studies on the interaction between coagulation and NOM (natural organic matter) currently focus on pollutant removal and coagulant species distribution, while studies on floc aging are lacking. Investigation onto the effects of floc aging could guide further processes that utilize flocs, such as densadeg sludge recirculation, floc predeposition for ultrafiltration, sludge condensation, and other traditional sludge reflux processes. In this study, flocs generated by Al₁₃ and AlCl₃ in microparticle- and nanoparticle-containing water were investigated, and the effect of floc aging on NOM was quantified based on several organic matter characterization techniques. Flocs absorb and release organics during aging. The flocs generated from micro-SiO₂ have a significant absorbing effect for LWM-N (low-molecular-weight neutral substances) and protein-like substances, while the absorption of NOM by flocs generated from nano-SiO₂ is insignificant. HS (humic substances) with high aromaticity are released during floc aging. From the molecular perspective, the molecules released during floc aging are those with higher double bond equivalents and higher aromaticity, while the absorbed molecules are those with lower double bond equivalents and lower aromaticity. 2D-COS (two-dimensional correlation spectroscopy) demonstrated that the flocs generated by Al₁₃ and AlCl₃ had the same organic release patterns but different intensities, while the flocs generated in the micro-SiO₂ and nano-SiO₂ systems had different organics release patterns. Abundant aluminum hydrolysates with low polymerization and amorphous Al(OH)₃ would be produced from AlCl₃ during the coagulation process and then undergo hydroxyl‑bridging reaction and crystallization during floc aging, thus releasing more HS with high aromaticity into the supernatant; in comparison, prehydrolyzed Al₁₃ produces a more stable floc and releases less HS during aging. The flocs produced by nano-SiO₂ and Al-based coagulants release higher aromaticity HS into the water than those produced by micro-SiO₂, which may be related to the formation of more highly polymerized degree hydrolysates and nanocrystalline Al(OH)₃ in the nano-SiO₂ system. The flocs generated in water with micro-SiO₂ may contain a large amount of Al-OH and have a loose structure, thus further absorbing NOM, such as protein-like substances and LWM-N. In contrast, the flocs generated from nano-SiO₂ possess abundant adsorbed water and a denser structure; thus, organic matter cannot be absorbed stably.

Structural Variation of Precipitates Formed by Fe(II) Oxidation and Impact on the Retention of Phosphate

The oxidation-precipitation process of Fe(II) is ubiquitous in the environment and critically affects the fate of contaminants and nutrients in natural systems where Fe(II) is present. Here, we explored the effect of H₂O₂ concentration on the structure of precipitates formed by Fe(II) oxidation and compared the precipitates to those formed by Fe(III) hydrolysis. Additionally, the phosphate retention under different H₂O₂ concentrations was evaluated. XRD, TEM, PDA, XPS, and UV-visible absorbance spectroscopy were used to characterize the structure of the formed precipitates; UV-visible absorbance spectroscopy was also used to determine the residual phosphate and Fe(II) in solution. It was found that the predominant precipitates in Fe(II) solution changed from planar-shaped crystalline lepidocrocite (γ-FeOOH) to poor short-range order (poorly crystalline) spherical-shaped hydrous ferric oxide (HFO) with increasing H₂O₂ concentrations. Although the HFO precipitates formed from Fe(II) resembled those formed from Fe(III) hydrolysis, the former was larger and had clearer lattice fringes. During the formation of γ-FeOOH, both Fe(II)-Fe(III) complexes and ligand-to-metal charge transfer processes were observed, and it was found that Fe(II) was present in the planar-shaped precipitates. Fe(II) might be present in the interior of precipitates as Fe(OH)₂, which could serve as a nucleus for the epitaxial growth of γ-FeOOH. In addition, the extent of phosphate retention increased with the H₂O₂ concentration, indicating the increased reactivity of formed precipitates with H₂O₂ concentration. More phosphate was retained via coprecipitation with Fe than adsorption on the preformed Fe precipitates due to the incorporation of phosphate within the structure of the formed Fe hydroxyphosphate via coprecipitation.

Online evaluation of bacterial cells in sand filter effluents during full-scale treatment of drinking water

Ensuring the microbiological safety of drinking water is critical to protect public health. This study aimed to evaluate the reliability of real-time bacteriological counter coupled with an online dialysis membrane-based pre-treatment system for continuously monitoring bacterial cell counts in sand filter effluents of a full-scale drinking water treatment plant. The pre-treatment system, which included anion exchange resins (porous polymeric microbeads that trap ions for releasing other ions) for dialysate regeneration, successfully achieved the stable attenuation of background interfering substances (humic acids) during the 19-d test. The real-time bacteriological counter equipped with the pre-treatment system provided a continuous profile of bacterial cell counts in the sand filter effluent (0.2-2.5 × 10⁴ counts/mL). The online analysis identified different timing of concentration peaks between particle and bacterial cell counts after backwashing. Bacterial community analysis revealed that Proteobacteria, Planctomycetes, and Cyanobacteria were the dominating phyla. Further, total bacterial cell counts determined by fluorescence microscopy and SYBR® Green I staining, a commonly accepted parameter, was found to be an indicator of online-monitored bacterial cell counts. The results indicated the potential of monitoring the bacterial cell counts in a sand filter process for providing an early warning of filter failures, which can allow plant operators to diagnose the overall system and provide countermeasures.

Enhanced nutrient removal from stormwater runoff by a compact on-site treatment system

Efficient and space-saving technologies for on-site treatment of stormwater runoff are required to control water pollution in the urban surface. The intermittent nature of stormwater runoff and extremely limited land available greatly hindered the application of current wastewater treatment technologies, and thus synchronous removal of multiple contaminants (especially for nutrient) efficiently was failed by current processes. In this study, a new compact CFFA treatment system, consisting of coagulation, flocculation, filtration and ammonium ion exchange units, was constructed for on-site treatment of stormwater runoff based on batch test optimization and pilot-scale test verification. The coagulation process effectively aggregated particles and precipitated phosphorus by dosing Al₂(SO₄)₃, while flocculation using anionic polyacrylamide further enlarged particle size for efficient micromesh filtration. The dynamic micromesh filtration obtained turbidity and phosphorus removal efficiencies comparable to 30 min gravity settling with greatly smaller footprint. Ion exchange by zeolite showed higher exchange capacity owing to lower initial ammonium nitrogen concentration in the stormwater runoff. The pilot-scale experiments with treatment capacity of 1 L/s showed that the CFFA treatment system achieved synchronous removal of particles (97.2%), nitrogen (79.7%), phosphorus (95.0%) and organic matters (83.3%) efficiently within short hydraulic retention time of 0.35 h, yielding effluent with chemical oxygen demand, suspended solids, total phosphorus and total nitrogen of 38.7, 7.80, 0.22 and 2.80 mg/L, respectively. The CFFA treatment system had the highest pollutant removal loads compared to reported runoff treatment processes in literatures, and was well suited to on-site treatment of stormwater runoff with high space utilization efficiency.

Removal of disinfection by-product precursors by Al-based coagulants: A comparative study on coagulation performance

Coagulation is well-established for controlling regulated disinfection by-products (DBPs), but its effectiveness for controlling unregulated DBPs remains unclear. The efficiency of coagulation in controlling unregulated DBPs requires clarification owing to their relatively high toxicity. In this study, three Al-based coagulants, aluminum sulfate (Alum), polyaluminum chloride (PAC), and a novel type of covalently bond hybrid coagulant (CBC, synthesized using AlCl₃) were selected, and the coagulation performance of these Al-based coagulants in controlling DBPs and DBP-associated toxicity was compared over 5 classes of DBPs, including trihalomethanes, haloacetic acids, haloacetaldehydes, haloacetonitriles, and halonitromethanes. The results showed that Alum was the least efficient in removing DBP precursors among the three coagulants. The effectiveness of CBC and PAC for DBP control varied with the characteristics of source waters. CBC had an advantage in water with a low content of humic acids, and reduced DBP concentration and DBP-associated toxicity by 47% and 25%, respectively. For water rich in aromatic organics, CBC might serve as DBP precursors at a high-required dosage, suggesting that a trade-off between enhanced DBP control and serving as DBP precursors should be considered for CBC coagulation; PAC achieved the most reduction in DBP concentration and DBP-associated toxicity by 50% and 34%, respectively.

An Extended Entropic Model for Cohesive Sediment Flocculation in a Piecewise Varied Shear Environment

In this study, an extended model for describing the temporal evolution of a characteristic floc size of cohesive sediment particles when the flocculation system is subject to a piecewise varied turbulent shear rate was derived by the probability methods based on the Shannon entropy theory following Zhu (2018). This model only contained three important parameters: initial and steady-state values of floc size, and a parameter characterizing the maximum capacity for floc size increase (or decay), and it can be adopted to capture well a monotonic pattern in which floc size increases (or decays) with flocculation time. Comparison with 13 literature experimental data sets regarding floc size variation to a varied shear rate showed the validity of the entropic model with a high correlation coefficient and few errors. Furthermore, for the case of tapered shear flocculation, it was found that there was a power decay of the capacity parameter with the shear rate, which is similar to the dependence of the steady-state floc size on the shear rate. The entropic model was further parameterized by introducing these two empirical relations into it, and the finally obtained model was found to be more sensitive to two empirical coefficients that have been incorporated into the capacity parameter than those in the steady-state floc size. The proposed entropic model could have the potential, as an addition to existing flocculation models, to be coupled into present mature hydrodynamic models to model the cohesive sediment transport in estuarine and coastal regions.

Deprotonation and aggregation of Al13 under alkaline titration: A simulating study related to coagulation process

Unraveling the transformation of coagulants and their interaction with contaminants at the micro-level is vital to advancing our understanding of the coagulation mechanism. To the best of our knowledge, the coagulation effectiveness of [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ (Al₁₃), regarded as the dominant species in polyaluminum chloride (PACl), is highly related to its aggregation characteristic, but the detailed process of Al₁₃ aggregation in coagulation time scale was not well studies. Here we systematically studied the deprotonation and aggregation processes of Al₁₃ by alkaline titration to simulate the reaction in coagulation case. By reacting with OH^-, Al₁₃ can continuously lose protons regardless of pH until its positive charge was well neutralized. The initial Al₁₃ aggregates (Al₁₃agg) appeared at B of 2.70 and large Al₁₃agg was generated by coalescence of small initial Al₁₃agg. Most Al₁₃ polycations kept their main structure unchanged during aggregation and part was decomposed into monomers or oligomers. Density functional theory (DFT) results reveal that Al₁₃ becomes unstable after deprotonation, but the aggregation of Al₁₃ bridged by Al monomers can stabilize the polycations. Al₁₃ needs to be hydrolyzed before interacting with colloidal particles, but particles can promote the aggregation of Al₁₃ by weakening the repulsion force between the polymers. Strong and compact flocs can be generated induced by in-situ aggregation of Al₁₃ in neutral and alkaline conditions. This study can provide a deep understanding about the role of Al₁₃agg in removing particles and instruct the development of new efficient coagulants against the various water qualities.

Mechanism insight into the role of clay particles on enhancing phosphate removal by ferrate compared with ferric salt

The application of ferrate (Fe(VI)) and ferric chloride as coagulants for treating phosphate wastewater in the presence of kaolin clay particles was comparatively studied. The phosphate removal processes by ferrate and ferric chloride assisted with kaolin clay particles were investigated under different Fe/P molar ratios. At neutral pH, complete removal of phosphates by ferrate and ferric chloride was observed at 2:1 and 6:1 of Fe/P molar ratio, respectively. The effect of kaolin clay particles on the phosphate removal process was discussed by zeta potential, size particle distribution, FTIR and XPS. We showed that with the increase of Fe/P molar ratio, the interaction intensity of kaolin clay particles with Fe flocs was decreased by ferric chloride coagulation while firstly increased and then decreased by ferrate. This depends on the Fe species with positive charge from ferric chloride hydrolysis and ferrate decomposition. Phosphate can inhibit the formation of FeOH²⁺ and Fe(OH)₂⁺ in the ferric chloride hydrolysis but promote the formation of FeOOH and Fe(OH)₂⁺ in the ferrate decomposition. Kaolin clay particles can more remarkably promote phosphate removal by ferrate than by ferric chloride.

Contribution of bacterial extracellular polymeric substances (EPS) in surface water purification

Naturally present aquatic microorganisms play an important role in water purification systems, such as the self-purification of surface waters. In this study, two water sources representing polluted surface water (Olympic Green; OG) and unpolluted surface water (Jingmi river; JM), were used to explore the self-purification of surface water by bacteria under different environmental conditions. The dominant bacterial community of OG and JM waters (both are Firmicutes and Proteobacteria) were isolated, cultured, and then used to carry out flocculation tests. Results showed that the flocculation ability of the dominant bacteria and extracellular polymeric substances (EPS) obtained from OG isolation was significantly greater than that from JM. Further examination illustrated that the main components of EPS were polysaccharides, which played an important role in improving the flocculation ability of bacteria. EPS from dominant cultural bacteria strains (OG1 and JM3) isolated from the two different sources lacked hydrophilic groups (e.g. COOH) and had a networked structure which are the main reasons to enhance the flocculation ability. The bacterial diversity and redundancy analysis (RDA) results also showed that microbial community composition is determined by water quality (SS, TOC, and NH₄⁺), and different Bacteroidetes, Actinobacteria and Proteobacteria community structures can improve the water body's ability to remove environmental pollutants (such as SS, humic acid and fulvic acid). These findings provide new information showing how bacterial communities change with environmental factors while maintaining the purity of surface water.

The influence of various additives on coagulation process at different dosing point: From a perspective of structure properties

Structure properties of flocs (size, fractal dimension (D_f), etc.) have a high impact on coagulation efficiency. In this work, the influences of three different additives (ferric salt (Fe), phosphate (P), and citric acid (CA)) on coagulation process/efficiency were investigated. Results showed that a small amount of extra Fe can facilitate the growth of Al flocs by providing more 'active sites'. Although zeta potential and D_f showed a limited change, the average floc size increased apparently and the increment was more obvious when Fe was added after the formation of the flocs. In contrast, P addition during the rapid mixing period will decrease the final average floc size, while the influence is less significant when P was added after the growth of the flocs. In terms of CA, a more striking negative effect on the growth ability of the flocs was observed compared to P. The strong complexing/coordination interactions between CA and aluminum hydroxide is the main reason behind the influence. CA also significantly decreased the D_f value of the flocs compared to P, and D_f showed a comparatively higher decrease when P or CA was added during the rapid mixing stage compared to the addition after the flocs formation. These results indicated that the addition of CA or P during the rapid mixing stage 'inactivated' or occupied more 'active sites' on the preliminarily formed Al NPs during the hydrolysis process, and therefore presented stronger impact on the morphology/size of the formed flocs.

Iron-nickel bimetallic metal-organic frameworks as bifunctional Fenton-like catalysts for enhanced adsorption and degradation of organic contaminants under visible light: Kinetics and mechanistic studies

Iron-nickel bimetallic organic frameworks (FeNi_X-BDC, H₂BDC: terephthalic acid) were developed as bifunctional materials for adsorption and photo-Fenton degradation of organic dyes with different charge properties. Significantly enhanced adsorption capacity of FeNi_1/15-BDC towards methylene blue (MB) and methyl orange (MO) was achieved, 5.3 and 2.6 times higher than that of pristine Fe-BDC, which was attributed to enlarged specific surface area and pore volume and the decreased surface charges induced by Ni doping. The adsorption kinetics demonstrated that chemisorption was dominant and intra-particle diffusion was the rate-controlling step. Two-stage degradation including slow induction stage and rapid oxidation stage fitted with pseudo-zero-order kinetics well. The increased rate constants (2.472 vs. 1.188 min^-1 for MB; 0.616 vs. 0.421 min^-1 for MO) in the induction stage as well as the superior removal capability by asynchronism relative to synchronism jointly corroborating the improved adsorption performance was favor for subsequent degradation. Notably, this heterogeneous system not only exhibited obvious advantages like wider pH working range (3-9), better stability and reusability of catalysts, but also achieved the dual objectives of in-situ decontamination and adsorbent regeneration. The coupling of adsorption and degradation along with synergism between photocatalysis and Fenton-like process are responsible for the reinforced removal of organic contaminants.

An efficient separation for metal-ions from wastewater by ion precipitate flotation: Probing formation and growth evolution of metal-reagent flocs

Hazardous metal pollution became a severe environmental issue in China. An efficient precipitation-flotation process was developed to achieve fast removal for metal-ions from wastewater. Structure and strength of precipitate particles/flocs significantly influence the flotation removal of metal-ions. Formation and growth-evolution of precipitate flocs in precipitate flotation were studied by stage analysis of precipitate particles-formation, flocs-regulation and flotation separation. The results demonstrate that early formed precipitates MHA(humics-metal complexing particles) have small particle size, high fractal dimension, low strength and recovery factor. The addition of Fe³⁺ and CTAB(cetyl trimethyl ammonium bromide) reagents make the precipitate particles aggregated to flocs(MHA-Fe, MHA-Fe-CTAB) much more large, loose, coarse, and small-density. The final generated MHA-Fe-CTAB flocs are hard to be broken up, easy to be recovered and efficient to be separated by flotation process. The flotation removal of MHA-Fe-CTAB flocs is clearly higher than that of MHA or MHA-Fe. The flotation results of MHA-Fe-CTAB are as follows: flotation removal of 98.7 ± 0.40%-99.9 ± 0.10%, residual TOC of 0.96 ± 0.38-1.35 ± 0.41 mg/L and turbidity of 0.44 ± 0.09-0.63 ± 0.16 NTU. Introducing Fe³⁺ and CTAB reagents into flotation solution contributes to the growth-evolution of precipitate flocs, which could intensify the metal-ions removal via precipitate flotation process and result in more ideal purification indexes for metal-containing wastewater.

Iron Molybdate Fe2(MoO4)3 Nanoparticles: Efficient Sorbent for Methylene Blue Dye Removal from Aqueous Solutions

The present study investigated iron molybdate (Fe₂(MoO₄)₃), synthesized via a simple method, as a nanosorbent for methylene blue (MB) dye removal from aqueous solutions. Investigations of the effects of several parameters like contact time, adsorbent dose, initial dye concentration, temperature and pH were carried out. The results showed that MB removal was affected, significantly, by adsorbent dose and pH. Interestingly, lower values of adsorbent dose resulted in the removal of higher amounts of MB. At the optimum pH, the removal efficiency of 99% was gained with an initial MB concentration of ≤60 ppm. The kinetic study specified an excellent correlation of the experimental results with the pseudo-second-order kinetics model. Thermodynamic studies proved a spontaneous, favorable and endothermic removal. The maximum amount of removal capacity of MB dye was 6173 mg/g, which was determined from the Langmuir model. The removal efficiency was shown to be retained after three cycles of reuse, as proven by thermal regeneration tests. The presence and adsorption of the dye onto the Fe₂(MoO₄)₃ nanoparticle surface, as well as the regeneration of the latter, was ascertained by scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). These findings are indicative that the investigated nanosorbent is an excellent candidate for the removal of MB in wastewater.

Impact of water source mixture and population changes on the Al residue in megalopolitan drinking water

This study establishes a new understanding of the contributions of Al residue in a megalopolitan drinking water supply system with mixed water sources. The different influences and contributions of foreign water source, resident migration and season changing to Al residue in drinking water were investigated. Especially, the role of Southern water transferred over 1200 km via the South-to-North Water Diversion Project in the Al residue of drinking water supply system of a northern megalopolitan were revealed for the first time. Comparisons of big data on Al residue in the water supply system with sole and mixed water sources showed that the introduction of Southern water enhanced the Al residue in drinking water by over 35%. The world's largest annual residents' migration during Chinese Lunar New Year and the changes of season affect the water pipework hydrodynamics, which were embodied as the periodic changes of particulate aluminium and the relations with resident's temporal-spatial distribution in the megalopolitan. Because of the differences in water quality, Southern water promotes the release of historically deposited Al and facilitates the cleaning of old pipes.

The formation of planar crystalline flocs of γ-FeOOH in Fe(II) coagulation and the influence of humic acid

Although Fe(II) salts have been widely used as coagulants in water treatment for many years, the underlying mechanisms of their performance remain unclear. Here, we present a detailed study of the coagulation behavior of Fe(II) salts and crystallization of flocs, and investigate the effect of humic acid (HA) under different DO concentrations and pH conditions. The behavior of Fe(II) and Fe(III) coagulants was found to be markedly different with the flocs from Fe(II) consisting of planar-like crystalline γ-FeOOH in contrast to the small amorphous spherical-like flocs from Fe(III) dosing. The effect of HA on Fe(II) coagulation was different under different DO concentrations and pH, where by the growth of γ-FeOOH was inhibited by the presence of HA, but independent of DO concentration and pH. It was found that Fe(II) was present within the internal structure of γ-FeOOH, and a plausible formation mechanism is proposed. Firstly, planar nanoparticles of Fe(OH)₂ were formed via Fe(II) ion hydrolysis which then servedas the nucleus for subsequent crystal growth. With oxidation, Fe(II) on the surface of nanoparticles transformed to Fe(III). Finally, the formation of γ-FeOOH in Fe(II) coagulation was accompanied by a change in solution colour to yellow.

Strong improvement of nanofiltration performance on micropollutant removal and reduction of membrane fouling by hydrolyzed-aluminum nanoparticles

Increasing attention has been focused on the removal of micropollutants from contaminated drinking source water. However, low rejection efficiency and membrane fouling still inhibit further application of nanofiltration membrane in this field. Interesting results were found that the residual hydrolyzed-aluminum nanoparticles from supernatant after coagulation and sedimentation strongly improved the nanofiltration performance for micropollutant removal. A simulated raw water containing humic acids, micropollutants and kaolinite clay was employed to investigate the factors of water matrix affecting the nanoparticle-enhanced nanofiltration for micropollutant removal. Results of experiments showed that these hydrolyzed-aluminum nanoparticles easily induced the aggregation of bisphenol-A (BPA) and humic acids in the supernatant. The enhancement of BPA removal was mainly attributed to the repelling interaction between the Al-BPA-DOC complexity and in situ-modified membrane surface during nanofiltration. 'This in situ surface modification by the hydrolyzed-aluminum nanoparticles improved membrane hydrophilicity, roughness and positively-charging capacity. For the treatment of River Songhua water spiked with micropollutant, the percentage removal of BPA was improved to be 88.5%, much more than the case of single nanofiltration without coagulation (60.7%). Meanwhile, the membrane fouling was reduced by 2.13 times than the case of single nanofiltration without the dynamically deposited-layer of nanoparticles. This in situ modification of nanofiltration membrane by hydrolyzed-aluminum nanoparticles achieved excellent removal efficiency for micropollutants from River Songhua water background.

Boron removal by electrocoagulation: Removal mechanism, adsorption models and factors influencing removal

Boron (B), normally present in ground water and sea water, is a vital micronutrient for plants, but is also toxic in excessive amounts. Under typical conditions, aqueous boron is present as boric acid (H₃BO₃), which is uncharged, making B particularly challenging to remove by mechanisms commonly applicable to removal of trace constituents. Adsorption of B onto aluminum hydroxide solids (Al(OH)₃(s)) generated using aluminum-based electrocoagulation (EC) is a promising strategy for B removal. Infrared spectroscopy analysis indicated complexation of B(OH)₃ with aluminum hydroxide solids via surface hydroxyl groups, while X-ray and infrared spectroscopy results indicated that the structure of the Al(OH)₃(s) was influenced both by EC operating conditions and by water quality. A linear adsorption model predicted B removal well when initial concentrations were lower than 50 mg/L, but fit the experimental data poorly at higher initial B concentrations. The Langmuir adsorption model provided a good fit for a broader range of initial B concentrations (5-1000 mg/L). Factors affecting B adsorption during the EC process, including current intensity, Al dissolution rate, boron concentration, pH, and total dissolved solid (TDS), were investigated. Increasing current intensity initially led to a higher Al dissolution rate, and therefore higher B adsorption, but there was a limit, as further increases in current intensity caused rapid formation of Al(OH)₃(s) having a large particle size and a low capacity to complex B. Boron removal decreased as its concentration increased. The best removal of B occurred at pH 8, corresponding to a slightly positive zeta potential for aluminum hydroxide and a small but significant fraction of negatively charged B species. Higher TDS concentrations facilitated the use of higher current intensities, i.e., the limit on the effective Al dissolution rate increased with increasing TDS. Two real water samples (river water and oilfield produced water) spiked with B were treated using EC, resulting in up to 50% B removal from river water (C₀ = 10 mg/L, current = 0.2 A) in 2 h, and 80% B removal from produced water (C₀ = 50 mg/L, current = 1.0 A) in 2 h.

Impact of Al-based coagulants on the formation of aerobic granules: Comparison between poly aluminum chloride (PAC) and aluminum sulfate (AS)

As widely used Al-based coagulants, poly aluminum chloride (PAC) and aluminum sulfate (AS) were adopted in a short term at the start-up stage (from 10th to 16th) to enhance the formation of aerobic granules, and their effects on aerobic granulation were elucidated. The results suggested that both PAC and AS facilitated the granulation by improving the physicochemical properties of sludge. The reactor performance in pollutant removal was also enhanced. Specifically, in terms of extracellular polymeric substances (EPS), PAC dosing mainly stimulated the production of loosely bound EPS (LB-EPS), whereas more tightly bound EPS (TB-EPS) were secreted with the presence of AS. Based on the elemental analysis, polymeric Al hydrolyzed from PAC mainly worked on the exterior of microbial aggregates, and thus the attached aluminum in granules was gradually eliminated by ion exchange and hydraulic shear force. In contrast, the aluminum species in AS hydrolyzed into monomeric and oligomeric Al, and thus could diffuse into the interior of microbial aggregates and eventually created an "Al-core" in the granules. Overall, the present study describes the AGS formation with Al-based coagulants and the mechanisms of PAC- and AS-enhanced aerobic granulation.

Optimized coagulation pathway of Al13: Effect of in-situ Aggregation of Al13

The coagulation mechanism for removing particles by Al₁₃ has been extensively investigated for water treatments. It was widely accepted that Al₁₃ played important roles in coagulation mainly by charge neutralization and electrostatic patch. However, the discovery of Al₁₃ aggregates (Al₁₃agg) in flocs indicated that the real coagulation process should be different from the previous understanding, including when Al₁₃agg were generated and how it interacted with negative particles. The aggregation process of Al₁₃ during coagulation and its micro-interfacial effect on particle coagulation remains to be explored. In this study, to investigate the aggregation of Al₁₃ and its effect on coagulation performance, two parallel coagulation jar tests were conducted on silica suspensions by preformed Al₁₃agg and Al₁₃, respectively. The results showed that optimized coagulation for particle removal by Al₁₃ occurred from pH 7 to pH 9, which was dominated by the in-situ aggregation of Al₁₃. The results confirmed that Al₁₃agg were both present in flocs generated in two tests, however, the morphology and distribution of surface Al of flocs were different for two tests. The in-situ formed Al₁₃agg covered all over the silica particles in flocs, resulting in compact structure with rough surfaces, while the preformed Al₁₃agg mainly distributed on joint sites between particles, generating denser flocs with smooth surfaces. This difference verified that the in-situ aggregation of Al₁₃ was the key factor to optimized particle coagulation. The overall optimized particle coagulation by Al₁₃ should undergo the following pathway: charge neutralization - in-situ aggregation of Al₁₃ - inter-particle bridging.