Coagulation performance and mechanism of different hydrolyzed aluminum species for the removal of composite pollutants of polyethylene and humic acid

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.

Interface adsorption characteristics of microplastics on multiple morphological arsenic compounds

Polystyrene (PS) and polyethylene terephthalate (PET) are commonly used materials that degrade into microplastics in the environment. These microplastics, possessing unique physical properties, can adsorb pollutants and contribute to composite pollution effects. This study examined the loading characteristics and toxic effects of PS and PET on six arsenic compounds, revealing that PS and PET displayed different adsorption capacities for these compounds, with PS demonstrating high adsorption for monomethylarsonic acid (MMA). The adsorption kinetics and isotherm analyses indicated that arsenic compounds quickly reached equilibrium on PS and PET. The kinetics were effectively described by pseudo-first-order models, and the isotherms aligned with the Langmuir and Freundlich models. Furthermore, simulated gastric fluid (SGF) and simulated intestinal fluid (SIF) were used to desorb arsenic compounds bound to PS and PET. The effects of aging, pH, salinity, anions, and humic acid (HA) on the ability of inorganic arsenic (iAs) to bind to PS and PET were analyzed. The results indicated that aging and HA increased the adsorption capacity of the microplastics, while salinity, anions, and elevated pH negatively affected this capacity. Additionally, the influence of microplastics and iAs on the clearance of free radicals by reduced glutathione (GSH) was explored. Microplastics inhibited the clearance of 1,1-diphenyl-2-picryl-hydrazyl (DPPH) by GSH, whereas iAs, especially arsenate, facilitated this process, likely due to synergistic effects with the oxidized form of GSH generated through GSH reactions. This study offers a theoretical foundation for understanding how microplastics transport various forms of arsenic compounds and their potential environmental risks.

Hydrolysis of Al13 and its coagulation mechanism: Role of speciation stability and transformation

Al13 ([AlO4Al12(OH)24(H2O)12]7+) is widely recognized as one of the advantageous species of Al-based coagulants, and its transformation process and micro-interface interaction with pollutants in water treatment continue to attract the attention of researchers. Based on relevant literatures in the past decades, this review summarizes and discusses the characterization methods, stability and aggregation, coagulation performance and mechanism of Al13. The technique development and method establishment such as 27Al nuclear magnetic resonance, electrospray ionization mass spectrometry, and Al-Ferron complexation timed spectrophotometry provide technical support for qualitative and quantitative detection of the species transformation of Al13. Al13 pre-formed by forced hydrolysis is of high structural stability, and solution pH, high Al concentration, and high temperature are important factors affecting its further hydrolysis, aggregation, dissociation or polymerization. Under circumneutral pH conditions in practical water treatment processes, unlike traditional Al salts which undergo extensive hydrolysis to generate Al(OH)3, Al13 transforms into Al13 aggregates. This is the key for Al13 to exert its superior coagulation performance and makes it shows significantly higher efficiency than traditional Al salts in removing particulate matter and organic substances. Specifically, Al13 shows a broader effective dosage range and pH range compared with AlCl3 through the coagulation mechanisms such as electrostatic patch, in-situ aggregation bridging, and complexation adsorption. Furthermore, the flocs formed by Al13 coagulation exhibit a more compact crystalline structure and higher strength, which helps reduce residual Al concentrations. A comprehensive investigation into the hydrolysis characteristics of Al13 and its role in coagulation is crucial for optimizing coagulation processes, while also providing a theoretical foundation for developing novel high-efficiency composite coagulants.

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.

Coagulation performance and mechanism of different novel covalently bonded organic silicon-aluminum/iron composite coagulant for As(V) removal from water: The role of hydrolysate species and the effect of coexisting microplastics

Arsenate [As(V)] pollution is a challenge for water treatment, and the effect of coexisting microplastics (MPs) on As(V) removal is still not clear. In this study, series novel covalently bonded organic silicon-aluminum/iron composite coagulants (CSA/F) with different Al/Fe molar ratios were prepared for enhancing As(V) removal. The effect mechanism of MPs (PS MPs and PS-COOH MPs) on As(V) removal by using CSAF coagulation was analyzed. CSAF and CSF showed significantly better As(V) removal performance than other coagulants under the same conditions, especially CSF, more than 90 % As(V) removal was achieved at dosage of 20 mg/L and pH of 4.0-8.0. Interestingly, the introduction of silane coupling agent and the increase of Fe content in CSA/F changed the Al/Fe species distribution. Charge neutralization dominant in As(V) removal by using CSA, whereas adsorption and net sweeping contributed to As(V) coagulation by using CSAF and CSF with higher iron proportion at neutral pH. 3 µm MPs were removed by net sweeping of amorphous Al/Fe hydroxides, while 26 µm MPs were charge-neutralized or surface adsorbed by coagulant hydrolysates. The aliphatic C-H and -COOH functional groups of MPs were the main sites of hydrogen bonding adsorption with the hydroxyl groups of coagulant hydrolysates. This study is conducive to mitigating the environmental toxicity of arsenic and provides new insights into the interaction mechanism between composite pollutants and coagulants in waters.

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.

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.