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

Revealing the removal behavior of five neglected microplastics in coagulation-ultrafiltration processes: Insights from experiments and predictive modeling

Typical water treatment processes are essential for mitigating the risk of microplastic contamination in drinking water. The integration of experiments and machine learning offers a promising avenue to elucidate microplastic removal behavior, yet relevant studies are scarce. To address this gap, this study combined experimental and artificial neural network (ANN) modeling to explore the removal behavior and mechanisms of five neglected microplastics in typical coagulation-ultrafiltration processes. Experimental results demonstrated that coagulation achieved an optimal removal rate of 37.0-56.0 % for the five microplastics, and subsequent ultrafiltration almost completely removed all residual microplastics. Five ANN models were constructed and optimized by adjusting activation functions and employing batch normalization, accurately predicting microplastic removal, with high R² values of 0.9972-0.9987. X-ray photoelectron spectroscopy elucidated the involvement of AlIV and AlVI species, hydrogen bonding, and π-π interaction in coagulation. Two-dimensional correlation spectroscopy explored the sequential formation of six chemical bonds (C-H, Al-O-Al, C-O, COO-, C=O, and -OH) and potential mechanisms. Moreover, theoretical calculations clarified the interfacial interactions between microplastics and ultrafiltration membrane, highlighting the roles of hydrophobic attraction and acid-base interaction. This study expands our understanding of microplastic removal in drinking water treatment, providing valuable mechanistic and modeling insights.

Efficient organic removal from hypersaline reverse osmosis concentrates through a hybrid microbubble ozonation-coagulation process: A two-stage removal procedure caused by two-sided effect of salinity

Removal of organic matter in hypersaline reverse osmosis concentrate (ROC) poses significant challenges. In this study, the hybrid microbubble ozonation-coagulation (HOC) process was established for actual hypersaline ROC treatment from energy chemical industry. In this HOC process, the hypersaline environment facilitated the formation of microbubbles, which enhanced ozone mass transfer and ensured an adequate dissolved ozone concentration. Efficient organic removal was achieved through a two-stage procedure: a rapid-removal stage dominated by coagulation (≤ 30 min) and a slow-removal stage dominated by ozone oxidation (> 30 min). Moreover, salinity exhibited two-sided effect on oxidation and coagulation in the HOC process. In the first stage of the treatment process, the alkaline conditions in hypersaline environment promoted oxidation and coagulation through increased •OH production and polymerized Al species generation. However, as the pH decreased owing to coagulant hydrolysis, excessive anions in hypersaline environment inhibited both oxidation and coagulation processes by quenching •OH and promoting large floc generation in the second stage. Furthermore, the two-stage organic removal mechanism was elucidated from the perspectives of oxidative transformation and floc entrapment. In the first stage, high-coagulability organics were directly removed through enhanced coagulation. Meanwhile, low-coagulability organics were oxidized into high-coagulability structures, which were removed via coagulation. In the second stage, organic matter was mainly removed through molecular ozone oxidation, while the coagulation process was inhibited. This study unveiled the two-sided effect of hypersaline environment on oxidation and coagulation, and provided new approaches for enhanced organic removal in the ozone-based process for hypersaline wastewater.

Revealing the removal behavior of polystyrene nanoplastics and natural organic matter by AlTi-based coagulant from the perspective of functional groups

The interactions of nanoplastics (NPs) with natural organic matter (NOM) are influenced by their surface functional groups. In this study, the effects of representative functional groups on the interactions among polystyrene nanoplastics (PS-COOH and PS-NH2), hydrophilic low molecular weight (LMW) substances (salicylic acid (SA), phthalic acid (PA), and gluconic acid (GA)), and a novel AlTi-based coagulant were investigated. We found that PS-NH2 (83.02 % - 93.38 %) was easier to remove over a wider pH range than PS-COOH (6.94 % - 91.07 %). PS-COOH and PS-NH2 were both able to interact with SA (-OH, -COO-, and benzene ring) through hydrogen bonding, π-π conjugation, and n-π electron donor-acceptor interactions. However, the binding of PS-COOH/PS-NH2 with SA has no effect on the interaction strength between SA and PATC due to the preferential occupation of the coagulant binding sites by SA. The lower SA removal in the PS-COOH@SA system was attributed to its stronger electrostatic repulsion and hydrophilicity. PATC could form carboxylate outer and C-O inner complexes with SA and carboxylate inner complexes with PA. In this study, the analysis of the interaction mechanisms among metal-based coagulants, NPs, and LMW substances lays a theoretical foundation for further research and understanding of coagulation theory.

Development and evaluation of a sulfate precipitation method for separating key flocculated-species from titanium-based flocculant

Key flocculated-species is the pivotal factor influencing the effectiveness of flocculants, which in turn directly determines the performance of mainstream wastewater treatment processes. Recovery of titanium-coagulated sludge and high-efficiency of titanium-coagulation have made titanium-coagulants an attractive hot point. However, the separation of key titanium-based flocculated-species remains a critical bottleneck limiting the advancement of titanium-based flocculant chemistry in water treatment. This study developed an efficient method for separating the key flocculated-species, named the sulfate precipitation method, which enables the effective purification of the key titanium-based flocculated-species. The electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) results demonstrated the successful separation of medium and large titanium-hydrolyzed-species. Ti-Ferron synchronous analysis was employed to indicate the separation of hydrolyzed products with different degrees of polymerization, including large, medium, and small species. The distribution of [OH-]/[Ti4+] (basicity) molar ratios was verified to confirm the reorganization of hydrolysis products after the reaction, resulting in varied basicities. Zeta potential results showed that species rich in medium and large hydrolyzed products had fewer positive charges, which was likely attributed to their superior flocculation efficiency, possibly due to sweep flocculation. The inductively coupled plasma mass spectrometry (ICP-MS) indicated that sulfur was present in the separated products. It was speculated that, on the one hand, the introduction of sulfate ions might have directly participated in the re-polymerization of titanium-hydrolyzed-species, potentially resulting in sulfate-doped titanium-based hydrolysis products. On the other hand, sulfate ions might have been involved in a substitution reaction with Cl- at active sites. Co-existence of these two pathways was deemed highly probable. The removal efficiency of organic matter was improved by approximately 20%, possibly owing to the rough surface (Ra = 16.2 nm). Additionally, larger flocs were generated, significantly shortening the sedimentation time in practical applications. This research presents a strategy for isolating key Ti-based flocculated species and establishes a base for their practical application.

Efficient tetracycline hydrochloride degradation via peroxymonosulfate activation by N doped coagulated sludge based biochar: Insights on the nonradical pathway

Coagulation could effectively remove microplastics (MPs). However, MPs coagulated sludge was still a hazardous waste that is difficult to degrade. Nitrogen-doped carbon composite (N-PSMPC) was prepared by carbonizing MPs coagulated aluminum sludge (MP-CA) doped with cheap urea in this study. Compared with the carbon material (PSMPC) produced by direct carbonization of MP-CA, N-PSMPC had a higher degree of defects, which could provide more active sites for peroxymonosulfate (PMS) activation. And then, the N-PSMPC was applied to the degradation of tetracycline hydrochloride (TC). The results showed that the N-PSMPC/PMS system exhibited excellent TC degradation performance at the pH range of 3-9, and the coexistence of CO32- and HCO3- inhibited the TC degradation. Moreover, the graphite N, pyridine N and carbonyl group were identified as the primary catalytic active sites. Three TC degradation pathways were speculated based on the intermediates detected by liquid chromatography-mass spectrometry, and the degradation mechanism was dominated by the nonradical pathway. In addition, the analysis of TC and intermediates by toxicity assessment software showed that N-PSMPC/PMS system could mitigate the TC toxicity. This study will provide a novel approach for the resourceful utilization of MP-CA and provide technical support for the removal of MPs and TC in water.

Uncovering the performance and intrinsic mechanism of different hydrolyzed AlTi species in polystyrene nanoplastics coagulation

Hydrolyzed AlTi species are essential metal-based coagulants in a coagulation process to remove nanoplastics (NPs). Understanding the molecular interactions between hydrolyzed AlTi species and NPs is key to promoting coagulation efficiency. In this study, the coagulation performance and intrinsic mechanism of different AlTi species (including monomeric AlTi and polymeric AlTi species-Al13Ti13) for NPs removal were systematically investigated. We found that the polymeric AlTi species exhibited higher turbidity removal (95.0 %) and lower residual Al content (20.67 μg/L) at a low dosage over monomeric AlTi species. Al13 and Al13Ti13 formed by in situ hydrolysis were the dominant species to destabilize and aggregate NPs at pH 6. Main coagulation mechanisms were dominated by charge neutralization, complexation between the aliphatic CH of NPs and Al/Ti-OH, and cation-π interaction between polycations and the aromatic structure of NPs. The preformed Al13Ti13 showed multiple positive charge binding sites assisting its easy adsorption on NPs by electrostatic attraction, and then formed microscale aggregates through charge neutralization or intermolecular interaction. The preformed Al13Ti13 demonstrated a high stability and coagulation performance with respect to pH changes in raw water, whereas the promotion of μ-OH bridges dissociation by OH- and the presence of electrostatic repulsion significantly decreased the NPs removal by monomeric AlTi at high pH. This study provides valuable theoretical insights into the interaction between NPs and various hydrolyzed AlTi species.

Recent advances in microplastic removal from drinking water by coagulation: Removal mechanisms and influencing factors

Microplastics (MPs) are emerging contaminants that are widely detected in drinking water and pose a potential risk to humans. Therefore, the MP removal from drinking water is a critical challenge. Recent studies have shown that MPs can be removed by coagulation. However, the coagulation removal of MPs from drinking water remains inadequately understood. Herein, the efficiency, mechanisms, and influencing factors of coagulation for removing MPs from drinking water are critically reviewed. First, the efficiency of MP removal by coagulation in drinking water treatment plants (DWTPs) and laboratories was comprehensively summarized, which indicated that coagulation plays an important role in MP removal from drinking water. The difference in removal effectiveness between the DWTPs and laboratory was mainly due to variations in treatment conditions and limitations of the detection techniques. Several dominant coagulation mechanisms for removing MPs and their research methods are thoroughly discussed. Charge neutralization is more relevant for small-sized MPs, whereas large-sized MPs are more dependent on adsorption bridging and sweeping. Furthermore, the factors influencing the efficiency of MP removal were jointly analyzed using meta-analysis and a random forest model. The meta-analysis was used to quantify the individual effects of each factor on coagulation removal efficiency by performing subgroup analysis. The random forest model quantified the relative importance of the influencing factors on removal efficiency, the results of which were ordered as follows: MPs shape > Coagulant type > Coagulant dosage > MPs concentration > MPs size > MPs type > pH. Finally, knowledge gaps and potential future directions are proposed. This review assists in the understanding of the coagulation removal of MPs, and provides novel insight into the challenges posed by MPs in drinking water.