Highly efficient Al-Ti gel as a coagulant for surface water treatment: Insights into the hydrolysate transformation and coagulation mechanism

Carbon slow-release and enhanced nitrogen removal performance of plant residue-based composite filler and ecological mechanisms in constructed wetland application

Stable carbon release and coupled microbial efficacy of external carbon source solid fillers are the keys to enhanced nitrogen removal in constructed wetlands. The constructed wetland plant residue Acorus calamus was cross-linked with poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) to create composite solid carbon source fillers (Ac-BDPs). The study demonstrated the slow release of carbon sources from Ac-BDPs with 35.27 mg/g under an average release rate of 0.88 mg/(g·d). Excellent denitrification was also observed in constructed wetlands with Ac-BDPs. Moreover, the average removal rate of nitrate nitrogen (NO3--N) was increased by 1.94 and 3.85 times of the blank groups under initial NO3--N inputs of 5 and 15 mg/L, respectively. Furthermore, the relatively high abundances of nap, narG, nirKS, norB, qnorZ and nosZ guaranteed efficient denitrification performance in constructed wetlands with Ac-BDPs. The study introduced a reliable technique for biological nitrogen removal by using composite carbon source fillers in constructed wetlands.

An innovative exploration to identify and isolate the dominant-flocculated-species from polytitanium chloride synthesized by electrodialysis

Abundance of dominant-flocculated-species is the key to determine coagulation performance of coagulant. Titanium-based coagulants have garnered considerable attention due to their high coagulation efficiency, but with a current challenge of the identification and isolation of the dominant-flocculated-species. Herein, polytitanium chloride (PTC), enriched with dominant-flocculated-species, was successfully synthesized by electrodialysis through accurate micro-interface control of the reaction among Ti-hydrolyzed-species and OH-. Special attention was paid to a feasible and high-effective strategy to isolate the dominant-flocculated-species from PTC through one-step rapid ultrafiltration. Selective preference was the ultrafiltration membranes (made of polyethersulfone) with a molecular weight cut-off of 5 kDa, which enabled the isolation of the dominant-flocculated-species, named PTC-5k. Results from the electrospray time-of-flight mass spectrometry (ESI-TOF-MS) proved a large proportion of the small and medium-sized hydrolyzed products as dominant-flocculated-species in PTC-5k, with the main signals concentrated between m/z 100 and 500. This composition achieved approximately 15.0% higher removal of organic matter with a 33.0% reduction in dosage compared to PTC. Unique snowflake-like branched structure of PTC-5k enhanced the coagulation mechanisms of sweeping and adsorption-bridging flocculation. Worth noting was the more compact flocs formed by PTC-5k than PTC, which was the probable reason for the mitigated fouling of ceramic membrane when PTC-5k was utilized as pre-treatment methodology. Continuous operation of ceramic membrane filtration up to 30 h, demonstrated 30% improvement in stable flux compared to PTC. This study provides the strategy for the isolation of Ti-dominant-flocculated-species, and lays the foundation for practical application.

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.

Coagulation performance and mechanism analysis of humic acid by using covalently bonded coagulants: effect of pH and matching mechanism of humic acid functional groups

Conventional inorganic coagulants (Al, Fe) and Al/Fe-based covalently bonded flocculants (CAFMs) had different hydrolysis species at different pHs, which subsequently led to differences in their binding sites and complexation ability with humic acid (HA). Studying the binding sites and interactions between CAFMs, AlCl3 (Al), and FeCl3 (Fe) hydrolysates and HA molecules is critical to understanding the coagulation mechanism. The results found that CAFM 0.6, Al, and AlCl3 combined FeCl3 (Al/Fe) removed more than 90% of HA at pH 6, and CAFMs showed higher HA removal rate than that of Al, Fe, and Al/Fe under the same reaction conditions. The flocs of CAFMs contained abundant -NH2/OH as well as the large particle size, compact structure, and excellent settling performance. The hydrolyzed species of Al and Fe were predominantly Alb and Feb at pH 6, but the hydrolyzed species of CAFMs were primarily (Al + Fe)c. Moreover, the hydrolyzed species of Al and Al/Fe were found to complex with HA functional groups such as -COOH, C = O, C-H/C-C, C = C, and C-OH to form ligand bonds, while the hydrolyzed species (Al + Fe)c of CAFMs could deeply interact with HA functional groups including C-O, -COOH, C = O, C-H/C-C, C = C, and C-OH by the adsorption and sweeping.

New insights into the fate and interaction mechanisms of hydrolyzed aluminum-titanium species in the removal of aged polystyrene

Polyaluminum-titanium chloride composite coagulant (PATC) has been demonstrated to be a promising coagulant in microplastics (MPs) treatment. However, the interaction process between the dominant species of PATC and MPs remains unclear, which will hinder our understanding of the coagulation mechanisms. Here, the species transformation of PATC during its interaction with aged polystyrene powder (APSp) was studied. The results showed that the rise of O-containing functional groups in APSp increased the possibility of forming C-O-M coordination bonds and hydrogen bonds between APSp and PATC, which improved the removal of PSp. Furthermore, Al13(OH)53Ti13O17(H2O)204+ (Al13Ti13) was considered to be the most effective species of PATC. At pH 4, electrostatic attraction brought Al13Ti13 approached APSp first, followed by hydrogen bonding and complexation occurred, respectively. However, the Al13Ti13-APSp complexes were easily converted to monomers and dimers during coagulation, which influenced the coagulation efficiency. With the increase of pH, OH- in the solution would further polymerize the depolymerized Al2Ti into oligomers and mesomers. Under weakly acid conditions, the diversity of PATC hydrolysates and the increase in APSp binding sites correspondingly led to the maximum APSp removal of 75%. When the pH further increased to 10, PATC interacted with APSp mainly by hydrogen bonding and sweeping effect.

Dissolved organic material changes during combined treatment of a mixture of landfill leachate and anaerobic digestate using deammonification and chemical coagulation

The current study investigates the combined treatment of wastewater of anaerobic digestate and landfill leachate, using deammonification and coagulation/flocculation processes. The deammonification section studies the performance of a full-scale deammonification plant in nitrogen and chemical oxygen demand (COD) removal, monitored over 2 years. For further COD reduction from the deammonification effluent (DE) to meet the environmental regulatory standards, coagulation/flocculation using three different Al-based coagulants was used to treat the DE. Results revealed that the deammonification plant showed 80% average ammonium removal from the mixed feed over the study period. Additionally, 30% of the feed COD was removed in the deammonification plant. COD analysis after treatment using coagulants revealed that the polyaluminum chloride modified with Fe had the best performance in reducing COD to meet the environmental standards. Excitation-emission matrix-parallel factor analysis (EEM-PARAFAC) of the dissolved organic material (DOM) samples indicated that fluorescents were the compounds mostly affected by the coagulant types. DOM analysis using 2D correlation Fourier-transform infrared spectroscopy revealed that the applied coagulants showed minor differences in removing different functional groups, despite having different COD reduction performance. Wastewater elemental analysis indicated elevated metal concentrations in low pH conditions (<6) due to re-stabilization of the flocs using coagulants.

Preparation of composite coagulant for the removal of microplastics in water

In this work, a composite flocculant (polyferric titanium sulfate-polydimethyldiallylammonium chloride [PFTS-PDMDAAC]) with a rich spatial network structure was prepared for the treatment of simulated wastewater containing polystyrene (PS) micro-nanoparticles. Characterization results showed that the surface of the PFTS-PDMDAAC was a three-dimensional network polymer of chain molecules that exhibited good thermal stability and formed an amorphous polymer containing multiply hydroxyl-bridged titanium and iron. When n(OH- ) : n(Fe) = 1:2, n(PO4 3- ) : n(Fe) = 0.35, n(Ti) : n(Fe) = 1:8, n(DMDAAC) : n(Fe) = 5:100, and the polymerization temperature is 60°C, the prepared composite flocculant has the best effect. The effects of dosage, pH, and agitation intensity on the flocculation properties of PFTS-PDMDAAC were also studied. The optimal removal rates of PS-μm and haze by PFTS-PDMDAAC were 85.60% and 90.10%, respectively, at a stirring intensity of 200 rpm, a pH of 9.0, and a PFTS-PDMDAAC dosage of 20 mg/L. The flocs produced by the PFTS-PDMDAAC flocculation were large and compact in structure, and the flocculation mechanism was mainly based on adsorption bridging. Kaolin played a promoting role in the process of PS-μm removal by PFTS-PDMDAAC floc and accelerated the formation of large and dense flocs. This study provided a reference for the coagulation method to remove micro-nanopollutants in the actual water treatment process. PRACTITIONER POINTS: A composite flocculant with rich spatial network structure (PFTS-PDMDAAC) was prepared. PFTS-PDMDAAC can effectively remove micro-nano polystyrene (PS) in wastewater. The floc produced by PFTS-PDMDAAC is large and compact in structure. The flocculation mechanism of PFTS-PDMDAAC is mainly adsorption bridging.

Molecular insights towards changing behaviors of organic matter in a full-scale water treatment plant using FTICR-MS

The changing behavior of organic matter in a full-scale water treatment process was characterized based on the three-dimensional excitation-emission matrix (3D-EEM) and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Polyaluminum chloride (PAC) as a coagulant can help to effectively remove soluble microbial by-products-like and aromatic protein-like substances during coagulation and sedimentation, corresponding to tannin and coagulated aromatic regions. The leakage of soluble microbial products during sand filtration resulted in an increase in the intensity of biomass-like regions. Nitrogen-containing compounds have higher weighted average value of double bond equivalents (DBE_w) and the modified aromaticity index (AI_mod-w) than nitrogen-free compounds. Water treatment can preferentially remove unsaturated nitrogen-containing compounds with more O atoms and higher-oxidation-state carbon. The dissolved organic carbon (DOC) and UV₂₅₄ were not correlated well with changes in nitrogen-containing compounds due to the preferential removal of nitrogen-containing compounds. This study revealed the specificity of organic matter removal during water treatment, and it was helpful in optimizing treatment processes for various raw water to ensure water quality.

The suitability and mechanism of polyaluminum-titanium chloride composite coagulant (PATC) for polystyrene microplastic removal: Structural characterization and theoretical calculation

Microplastics (MPs) particles bring potential threats to the aqueous environment, and the coexistence of natural organic matter (NOM) enhances their toxicity. Coagulation is an efficient method for particle removal and exploring the binding sites and modes of the coagulant hydrolysates with MPs in the presence of NOM is essential to understand the coagulation mechanism. In this study, a novel polymerized polyaluminum-titanium chloride composite coagulant (PATC) was prepared and used to remove polystyrene (PS). It was found that PATC could compress or even destroy the surface layer of the negatively charged PS. In comparison to PAC and PTC, PATC was more efficient in decreasing the energy barrier of the PS particles and increasing their aggregation rate over a wider pH range. The results of the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) calculation revealed that the interaction between the hydrolysates of PATC and PS was mainly polar interaction (V_AB), such as hydrogen bonding. The peak intensity and peak shift in Fourier-transformed infrared (FTIR) and X-ray photoelectron spectra (XPS) were analyzed to further explore the interaction between the hydrolysates of PATC and PS. It was found that hydrogen bonding existed between the -OH group of PATC and the aliphatic C-H and C=O groups of PS. And the main interaction between HA and PS was the π-π* conjugation and hydrogen bonding between the -COOH, -OH, and C=O groups of HA and the C=O and aliphatic C-H groups of PS. Therefore, in the HA@PS system, the active sites of HA (e.g. -COOH and -OH) and PS (e.g., C=O and aliphatic C-H) binding with the coagulants were occupied, which accordingly led to the dramatic decline in the removal efficiency of both HA and PS. In actual lake water treatment, although the removal efficiency of PS was significantly poor, PATC performed better for PS removal than PAC and PTC. Besides, the effluent pH was maintained at 6.81±0.08, which met the requirements of the subsequent water treatment process. This study provides systematic knowledge for understanding the interaction between PS, NOM, and coagulant hydrolysates, and further confirms the application potential of PATC for MPs removal.

Construction of titanium-aluminum xerogel composite coagulant for removal of tetracycline in water: synergy effects and improvement mechanisms insight

Titanium xerogel coagulant (TXC) is a new type of coagulant that has attracted much attention in recent years. However, the tetracycline removal performance of TXC was not satisfactory because low isoelectric point (pH_iep) inhibited the electrical neutralization efficiency of TXC in an alkaline environment. To overcome this shortcoming, a composite xerogel coagulant (titanium-aluminum xerogel composite coagulant) was prepared. The removal of tetracycline and turbidity was used as evaluation indexes. It was proved that the combination of aluminum (III) and titanium (IV) enhanced the resistance of TXC to pH. The synthesized titanium-aluminum xerogel composite coagulant (TXAC) has an excellent removal ability of tetracycline in a wide pH range (pH = 5-10). At pH 8.8, the dosage required to remove 80% tetracycline from water decreased from 93 (TXC) to 35 mg/L (TXAC). The reason for this improvement could be attributed to (i) aluminum (III) enhanced the electric neutralization of TXC to negatively charged pollutants in an alkaline environment; (ii) the complexing ability of organic matter and aluminum (III) was enhanced. This work provides a feasible scheme for the pretreatment of tetracycline in water to meet the pretreatment requirements of special water.