Influence of shear force on floc properties and residual aluminum in humic acid treatment by nano-Al₁₃

Suitability of inorganic coagulants for algae-laden water treatment: Trade-off between algae removal and cell viability, aggregate properties and coagulant residue

Inorganic coagulants could effectively precipitate algae cells but might increase the potential risks of cell damage and coagulant residue. This study was conducted to critically investigate the suitability of polyaluminum (PAC), FeCl3 and TiCl4 for algae-laden water treatment in terms of the trade-off between algal substance removal, cell viability, and coagulant residue. The results showed that an appropriate increase in coagulant dosage contributed to better coagulation performance but severe cell damage and a higher risk of intracellular organic matter (IOM) release. TiCl4 was the most destructive, resulting in 60.85% of the algal cells presenting membrane damage after coagulation. Intense hydrolysis reaction of Ti salts was favorable for the formation of larger and more elongated, dendritic structured flocs than Al and Fe coagulants. TiCl4 exhibited the lowest residue level and remained in the effluents mainly in colloidal form. The study also identified charge neutralization, chemisorption, enmeshment, and complexation as the dominant mechanisms for algae water coagulation by metal coagulants. Overall, this study provides the trade-off analyses between maximizing algae substance removal and minimizing potential damage to cell integrity and is practically valuable to develop the most suitable and feasible technique for algae-laden water treatment.

Transformation of Al Species on Carbon Surfaces: Effects of Al Species and Carbon Surface Oxygen Groups

Control of residual Al is critical, owing to its high tendency to accumulate in drinking water distribution systems and its potential risks to human health. Herein, the effects of surface properties of activated carbon (AC) on intercepting different Al species (including monomeric Al and polymeric Al species-Al13) are evaluated. The results showed that Al in the form of monomers was considerably adsorbed by AC; whereas Al in the form of polymeric Al13 was held to a much lower degree by AC, and the effluent Al concentration was even higher than that without AC. By comparing virgin AC and hydrogen thermal treated AC, the surface oxygen functional groups on the AC were proposed to play a critical role in the transformation of Al species. The oxygen functional groups on the AC surface can directly form complexes with monomeric Al, thereby inducing the binding of monomeric Al on the AC surface. However, the AC surface oxygen groups could not bind to polymeric Al13, and the interaction between AC surface oxygen groups and polymeric Al13 partially transforms Al13 into monomeric Al species, which inhibited the self-aggregation of Al13. This study aims to provide new insights into the control of residual Al in water treatment plants to ensure drinking water safety.

Al and Mn speciation changes during the pre-oxidation with potassium permanganate and coagulation removing natural organic matter and its membrane fouling behavior

In this study, we systematically explore coagulation behavior, ultrafiltration membrane fouling behavior and the mechanism involved in during the process of pre-oxidation of potassium permanganate and coagulation of aluminum chloride at different condition to treat model pollutants (humic acid, HA) and natural water. The KMnO4 pre-oxidation significantly enhances flocs formation, and for HA artificial water the flocs size increases from 82 to 122 μm at pH 5.5, from 63 to 185 μm at pH 7.0 and from 0 to 75 μm at pH 8.5, respectively, as for natural water it increases from 72 to 139 μm. The enhanced coagulation at pH 5.5 is attributed to the increased polymeric Al speciation after pre-oxidation along with the generated Mn2+ damaging the electric double layer structure. And for pH 8.5 it is mainly caused by the in-situ MnO2 as combination nuclei during pre-oxidation. Besides, for pH 7.0, the combined effect of in-situ MnO2 and the increased polymeric Al speciation both contribute to improvement of the coagulation. What's more, the enhanced Al coagulation by pre-oxidation of KMnO4 also helps alleviate the membrane fouling for both HA artificial water and natural water, and a much rougher surface with larger flocs forms after KMnO4-aided Al coagulation filtration. This study provides an alternative perspective on the mechanism of pre-oxidation coagulation process.

Enhanced coagulation process for removing dissolved organic matter, microplastics, and silver nanoparticles

Dissolved organic carbon (DOC), microplastics (MPs), and silver nanoparticles (AgNPs) in water are of major concern because of their direct and indirect toxic effects on aquatic organisms and human exposure via water. This work investigated the effect of poly aluminum chloride (PACl) coagulation for reducing DOC, MPs, and AgNPs. This work used water from a canal in Thailand with a DOC of 5.2 mg/L in the experiment. AgNPs of 5-20 mg/L were added to canal water to create synthetic water for the PACl coagulation. Polyethylene and polypropylene (PP) type MPs were identified in the raw water with Fourier transform infrared spectroscopy. Coagulation with 15 mg/L of PACl performed better in the PP removal. The PACl coagulation at dosages of 15, 40, and 70 mg/L removed DOC by 16-20%, 44-52%, and 46-63% and AgNPs by 34-90%, 53-93%, and 81-95%, respectively. The presence of AgNPs at high levels could inhibit the efficiency of DOC reduction by the PACl coagulation. The FESEM identified the adsorption of silver-containing nanoparticles onto the flocs with increased dosages of PACl. So, PACl is a coagulant in the removal of AgNPs that can reduce health hazards and eco-toxicological risks in water sources due to the release of silver.

Aggregation, settling characteristics and destabilization mechanisms of nano-particles under different conditions

An abundance of nano-particles have been exposed to water environment. Owing to the particle size effects, nano-particles are inclined to absorb harmful substances and increase their levels of toxicity. In this study, the existence state, aggregation and settlement characteristics of nano-particles in the natural water are studied. Influenced by the structural layer repulsion, nano-particles have higher stability in natural water. When coagulants were added, nano-particles could effectively aggregate with slow flocculation speed and relatively small flocs size without hydraulic shearing due to the significant effect of Brownian motion. It is worth noting that the aggregated flocs formed by Brownian motion showed high strength and strong ability to resist hydraulic disturbance, and thus the flocs were harder to break. This is because the combination among nano-particles under hydraulic shearing is the result of a single-point chain-to-chain aggregation mode, while that under the Brownian motion is the result of multi-points face-to-face aggregation mode. Therefore, in the process of re-flocculation, flocs formed by the Brownian motion were more compact. This study provides a new view in nano-particles treatment for both the in-situ treatment process of natural water body and the regular water treatment plants.

Pre-aggregation of Al13 in optimizing coagulation for removal of humic acid

The effective removal of humic acid (HA) by coagulation has been extensively investigated for water treatments. However, the limitations of pH variation and excessive residual aluminum issues were still factors needed to be considered. In this study, to investigate the coagulation mechanism for removing HA by Al₁₃ and optimize Al₁₃ operation for removing HA, Al₁₃ and preformed Al₁₃ aggregates (Al₁₃agg) were applied to remove HA at different pH conditions. The results showed that preformed Al₁₃agg exhibited superior HA removal performance than Al₁₃ due to its wide pH range and low residual Al level. During coagulation, Al₁₃ and Al₁₃agg interacted with HA in their original status, but the DSlope_325-375 difference implied that the complexation capacity between HA and Al₁₃agg was stronger than Al₁₃. The new peaks of HPSEC representing larger molecular weight substances were formed under acidic and neutral conditions, which indicated that HA firstly aggregated into larger complexed molecules by interacting with Al₁₃ or its hydrolysates and was subsequently removed by forming large flocs which was completely different from Al₁₃agg situation. Therefore, the different coagulation mechanisms played the roles in HA removal for Al₁₃ and Al₁₃agg which were studied in this paper. It was believed that the complexation and charge neutralization effects dominated coagulation process for Al₁₃ while sweep flocculation and adsorption coagulation were main driving force for Al₁₃agg in HA removing. This work provides significant understanding of HA removal by Al₁₃ and Al₁₃agg coagulation, which can help to design and optimize the high efficiency coagulant based on Al polycations.

Chitosan Modified Cationic Polyacrylamide Initiated by UV-H2O2 for Sludge Flocculation and New Insight on the Floc Characteristics Study

In the present study, a novel graft modified flocculant CTS-g-PAMD was synthesized and applied to conduct sludge conditioning and dewatering. CTS-g-PAMD was copolymerized with AM, DMC and chitosan (CTS) under UV-H2O2 initiation. In addition, the effects of single factor experiments on the molecular weight (MW) CTS grafting efficiency (GE) of CTS-g-PAMD were determined and the optimal copolymerization conditions were achieved. The GE of CTS-g-PAMD reached 91.1% and the MW was 4.82 × 106 Da. As revealed from the characterized results of Fourier-transform infrared spectra (FT-IR), 1H/ NMR, X-ray diffraction (XRD), scanning electron microscopic (SEM) and X-ray photoelectron spectroscopy (XPS), the successful synthesis of CTS-g-PAMD was confirmed, which is considered to be conducive to explaining sludge dewatering performance. Under the optimal conditions (pH = 7.0, flocculant dosage = 35 mg/L), the best flocculating performance (FCMC: 73.7%; SRF: 4.7 × 1012 m·kg-1, turbidity: 9.4 NTU) and large and dense sludge flocs (floc size d50 = 379.142 µm, floc fractal dimension Df = 1.58) were formed. The DMC and CTS chain segments exhibiting cationic properties significantly improved the positive charge density and enhanced the electrical patching effect of CTS-g-PAMD. The long molecular chain of CTS-g-PAMD exhibited superior extensibility, which enhanced bridging effect on adsorption. Moreover, the sludge floc after undergoing CTS-g-PAMD conditioning exhibited robust shear resistance and regeneration ability. After the sludge floc was crushed and broken, a large and dense sludge floc was formed, helping significantly reduce the sludge specific resistance (SRF), turbidity and cake moisture content (FCMC) and enhance the sludge dewatering effect. The novel CTS-g-PAMD flocculant shows promising practical applications and high market value.

Aluminum distribution heterogeneity and relationship with nitrogen, phosphorus and humic acid content in the eutrophic lake sediment

Increasing amount of aluminum (Al) gets into aquatic ecosystem through anthropogenic activity, but the knowledge about Al migration and relationships with sediments possessing different physico-chemical properties in eutrophic lakes is limited. Here, the Al migration rule and relationships with sediment nutritions in the Hangzhou West Lake, China was investigated, where a certain amount of residual Al-salts can enter because of the pre-treatment of the Qiantang River diversion project every day. Results revealed the obvious spatial distribution heterogeneity of Al in sediment vertical direction and horizontal direction following water flow. The Al content in sediment ranged 0.463-1.154 g kg^-1 in Maojiabu Lake, and ranged 9.862-40.442 g kg^-1 in Xiaonanhu Lake. Higher Al content distributed in upper layer sediment in lake with more disturbance. Total nitrogen (TN) contents were higher 0.917-3.387 mg g^-1 and 0.627-0.786 mg g^-1 in upper layer sediment than that in lower layer in Maojiabu Lake and Xiaonanhu Lake, respectively. Total phosphorus (TP) content ranged 0.779-2.580 mg g^-1, in which IP and Fe/Al-P contributed 24.9-80.8% and 17.0-51.6%, respectively. Correlations between Al content with nutrition, humic acid (HA) etc. of sediment regionally varied in Maojiabu and Xiaonanhu Lake. Spatial distribution of Al-salt in eutrophic lakes closely related with the physico-chemical characteristics of nutrients, humus, human disturbance and water division parameters. Results provides new insight into Al-salts migration and references for Al-risk evaluating in eutrophic lakes.

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.

Mechanism of the hybrid ozonation-coagulation (HOC) process: Comparison of preformed Al13 polymer and in situ formed Al species

Because of the influence of hydrolysed species from Al-based coagulants on coagulation performance, the performance and mechanism of the developed hybrid ozonation-coagulation (HOC) process using AlCl₃·6H₂O and preformed Al₁₃ as coagulants were investigated in this study, in which ozonation and coagulation occurred simultaneously within a single unit. It was found that the HOC process exhibited higher organic matter removal performance compared with coagulation and the pre-ozonation-coagulation process. It was found that the high ibuprofen (IBP) removal efficiency in the HOC process was mainly attributed to OH oxidation promoted by in situ formed hydrolysed aluminium species from AlCl₃·6H₂O and preformed Al₁₃. Furthermore, the surface hydroxyl groups were determined to be the active reaction sites for the HOC process. Due to the higher proportion of surface hydroxyl groups for Al₁₃, the HOC process with preformed Al₁₃ as coagulants (Al₁₃-HOC) exhibited a higher removal performance than that with AlCl₃·6H₂O as the coagulant (AlCl₃-HOC). It was revealed that most of the generated O₂^- in the Al₁₃-HOC was adsorbed on the surface of Al₁₃ at different pH values, while a considerably lower proportion of adsorbed O₂^- was observed on the surface of in situ formed Al species from AlCl₃·6H₂O. Nevertheless, low proportions of adsorbed OH were found on the surfaces of both preformed Al₁₃ and in situ formed Al species, indicating that the OH oxidation for the removal of organic pollutants occurred primarily in the aqueous phase.

Application and mechanism of nucleation-induced pelleting coagulation (NPC) in treatment of fracturing wastewater with high concentration of dissolved organic matter

To improve the separation efficiency of fluffy flocs during coagulation of fracturing wastewater with high concentration of dissolved organic matter, a novel nucleation-induced pelleting coagulation (NPC) process is proposed based on pelleting coagulation in this study. In the NPC process, nucleation agents were added to act as pellet seeds to generate high-density pellet flocs. The results implied that the necessary condition for the NPC process was to control the metastable state, i.e. zeta potential around -10 mV after addition of the coagulant, polyaluminium chloride (PAC). Diatomite, which was added after rapid mixing, was used as the nucleation agent, and its optimal dosage was about 100 mg/L with particle size 100-200 μm. In addition, the dosage of the coagulant aid, polyacrylamide (PAM), significantly affected the performance of the NPC process, and the optimal PAM dosage was 10 mg/L in this study. It was found that 60 rpm (G = 55.1 s^-1) was the optimal hydraulic condition for pellet growth during slow mixing. The pellet floc settling velocity reached 14.9 mm/s and the particle size (d₅₀) reached 4.6 mm with an effective density of 0.021 g/cm³ at the optimal condition, which was one order higher than that of conventional aluminium flocs.

Optimization of coagulation pre-treatment for alleviating ultrafiltration membrane fouling: The role of floc properties on Al species

This study investigated membrane fouling in a coagulation/ultrafiltration (C-UF) process by comparing the floc properties and humic acid (HA) removal efficiency of three hydrous Al(III) species (Al_a, Al_b, and Al_c). The results indicated that the coagulation and membrane mechanisms were different for all three Al species because of the differences in floc properties. The HA removal efficiency increased with increasing Al dosage until an equilibrium was reached at the optimal dosage of 6 mg L^-1. In addition, membrane fouling gradually decreased as the Al dosages increased. Regardless of coagulant type, the OH and COOH functional groups of HA reacted with the Al species. Both external and internal membrane fouling were strongly dependent on the porosity of the cake layer and on the size distribution of the floc particulates, respectively. The pore area of the cake layer formed by the Al_a-coagulated effluent was large because of the strong charge neutralization. Moreover, Al_a generated large and loose flocs with a porous cake layer that mitigated external fouling. However, the internal fouling with the Al_c coagulant was significant because the concentration of residual aggregates in the membrane pores was high.

Study on the effects of organic matter characteristics on the residual aluminum and flocs in coagulation processes

Characteristics of organic matter may affect the residual aluminum after the coagulation process. This study reported the results of a survey for one drinking water treatment plant and measured the concentration of residual aluminum species with different molecular weights. Survey results indicated that humic acid or organic matter whose molecular weight was smaller than 1500Da had significant effects on residual aluminum. All the treatment processes were ineffective in removing dissolved organic matter whose molecular weight was smaller than 1500Da. These results also indicated that the addition of sand or polyacrylamide in the coagulation process could greatly decrease the concentration of humic acid, and the concentration of residual aluminum also decreased. These results revealed that for all water samples after filtration, the majority of total residual aluminum existed in the form of total dissolved aluminum, accounting for 70%-90%. The concentration of residual aluminum produced in bovine serum albumin solutions indicated that when the DOC was larger than 4.0mg/L, there were still significant differences when the solution pH value varied from 4.0 to 9.0.

Characterization of re-grown floc size and structure: effect of mixing conditions during floc growth, breakage and re-growth process

The impact of mixing speed in three stages-before breakage, during breakage, and after breakage-on re-grown floc properties was investigated by using a non-intrusive optical sampling and digital image analysis technique, respectively. And then, on the basis of different influence extent of mixing speed during each stage on size and structure of re-grown flocs, coagulation performance with varying mixing speed was analyzed. The results indicated that the broken flocs could not re-grow to the size before breakage in all cases. Furthermore, increasing mixing intensity contributed to the re-formation of smaller flocs with higher degree of compactness. For slow mixing before breakage, an increase in mixing speed had less influence on re-grown floc properties due to the same breakage strength during breakage, resulting in inconspicuous variation of coagulation efficiency. For rapid mixing during breakage, larger mixing speed markedly decreased the coagulation efficiency. This could be attributed that mixing speed during breakage generated greater influence on re-grown floc size. However, as slow mixing after breakage was elevated, the coagulation efficiency presented significant rise, indicating that slow mixing after breakage had more influence on re-grown floc structure upon re-structuring and re-arrangement mechanism.

Evaluation of kaolin floc characteristics during coagulation process: a case study with a continuous flow device

This work investigates the influence of decreasing shear rate on temporal evolution of floc properties in continuous flow device.