Humic Acid Removal Efficiency Research Articles

Effective leachate treatment by a pilot-scale submerged electro-membrane bioreactor.

Most landfill leachates contain organic compounds that cannot be easily separated by conventional biological processes. Recently, integration of membrane bioreactors and electro-oxidation has been proposed as a suitable option for the treatment and separation of organic and inorganic contaminants in leachate. Therefore, in the present study, the performance of submerged electro-membrane bioreactor (SEMBR) along with a conventional membrane bioreactor (MBR) on a pilot scale was evaluated for the treatment of leachate. Both bioreactors were used to compare treatment efficiency under the same conditions. The removal rates of chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), phosphate (PO43--P), color, UV254, and metals were investigated. The results showed that applying electric current to the MBR could approximately increase the COD removal efficiency from 94 to 98.5%; PO43--P removal from 70 to 99%; NH3+-N removal from 91 to 99%; UV254 removal from 80 to 96%; and heavy metals removal from 40 to 95%. Humic acid removal efficiency as another indicator of humic substances was increased from 75% in the MBR to 96% in the SEMBR process. The results also showed that the effluent can be introduced into the wastewater treatment plant for further treatment. The SEMBR process achieved a minimization of fouling of membranes compared to conventional MBR. The consumption of the energy and electrode was in accordance with the previous results, and the required energy of 1.57 kWh/m3 of wastewater was calculated. The sludge volume index (SVI) in SEMBR (105 ml/g) was better than MBR (135 ml/g) due to the electrokinetic effect on the production of denser flocs. Based on the results, it can be concluded that the application of electric current can improve the performance of MBR in removing PO43-, NH4+, and membrane fouling.

Effect of coagulation treatment on antifouling properties of PVC nanocomposite membrane in a submerged membrane system for water treatment

A submerged membrane system was used in this work to investigate the effect of the polyaluminum chloride (PAC) coagulant on the antifouling performance of the polyvinyl chloride/alumina (PVC/Al2O3) nanocomposite membrane. The prepared nanocomposite membranes were characterized with field emission scanning electron microscopy (FE-SEM), atomic force microscopy, contact angle, porosity measurement, and pure water flux. The results revealed that the membrane containing Al2O3 nanoparticles (the mass ratio of PVC to Al2O3 was 98.5/1.5) had a higher hydrophilicity, porosity, and pure water flux than other membranes. The FE-SEM images showed that when Al2O3 nanoparticles were present in the PVC membrane, large pores and macrovoids formed on the surface and cross-section of the membrane. The fouling behavior of membranes was investigated through the filtration of humic acid (HA) solution with and without the PAC coagulant. Without PAC addition, the PVC/Al2O3 membrane significantly decreased the irreversible fouling ratio from 60.7% to 19.4% and showed a high HA removal efficiency of approximately 90.5%. The Hermia model confirmed that the cake formation mechanism best described the experimental data for the neat PVC and nanocomposite membranes with the presence and absence of the PAC coagulant. This confirms that the PAC coagulant can significantly mitigate fouling and improve HA removal in the submerged membrane system.

Enhanced ultrafiltration membrane fouling alleviation by module rotation with Fe-based flocs

The modules of ultrafiltration (UF) membrane always occupy a fixed position during filtration. This results in a gradual aggravation of membrane fouling because of the continuous pollutants deposition. However, it is known that the cake layer is composed of loosely hydrolyzed flocs. Therefore, in this study, the rotation concept of membrane-module was investigated in conjunction with the integrated floc (Fe-based)-UF process. We found that the Fe-based flocs can be easily peeled off the membrane surface during rotation, and the UF membrane fouling was alleviated, particularly with spiral module rotation because of the strong shearing force induced. The fouling behavior after spiral module rotation was similar for different floc dosages because of the looseness cake layer formed. The applied computational fluid dynamics model showed that a greater spiral rotation height corresponded to a larger water velocity distribution and stronger shearing force in the membrane tank, and this resulted in a reduction in the fouling degree due to the thinner cake layer formed. Moreover, in comparison with alkaline conditions, a slightly more severe fouling degree was observed under acidic conditions because of the induced denser cake layer with smaller Fe-based floc sizes; however, a significantly higher humic-acid removal efficiency was observed. In addition, UF membrane performed well with river water samples taken from the South-to-North water diversion project in China (Beijing), thereby indicating the rotating module potential application in membrane-based drinking water treatment.

Eco-environmental preparation of magnetic activated carbon modified with 3-aminopropyltrimethoxysilane (APTMS) from sawdust waste as a novel efficient adsorbent for humic acid removal: Characterisation, modelling, optimisation and equilibrium studies

ABSTRACT The aim of the present study was to prepare the modified magnetic activated carbon from sawdust (AC/Fe3O4@SiO2-APTMS) as a novel adsorbent for efficient removal of humic acid (HA) from aqueous solutions. The adsorbent was characterised by BET, SEM, FTIR, VSM, and EDS analysis. The optimal values of independent variables were determined using Box-Behnken design based on the response surface methodology (RSM). The maximum HA adsorption of 97.85% was obtained at optimal conditions pH 3, the adsorbent dose 2 g/L, and the contact time 25 min. Analysis of the adsorption data by isothermal, kinetic, and thermodynamic models showed that the adsorption data followed the Langmuir isotherm (R2 = 0.9850) and the pseudo-second-order kinetic model (R2 = 0.9999). The adsorption process was spontaneous and endothermic. The adsorbent regeneration proved high HA removal efficiency after 4 adsorption cycles. The humic acid adsorption behaviour of the adsorbent supports its performance to remove organic contaminants from aqueous solutions.

Enhanced removal of humic acid from aqueous solution by combined alternating current electrocoagulation and sulfate radical.

Application of alternating current in electrocoagulation and activation of persulfate (AEC-PS) for the effective removal of humic acid (HA) from aqueous solution was evaluated. In order to optimize the removal efficiency HA by the AEC-PS process, several influencing parameters such as pH, reaction time, PS dose, current density (CD), concentration of NaCl, initial concentration of HA, and coexisting cations and anions influence were investigated. From the batch experiments, the highest HA removal efficiency obtained was 99.4±0.5% at pH of 5, reaction time of 25min, CD of 4.5mA/cm2, PS dose of 200mg/L, and NaCl concentration of 0.75g/L for an initial HA concentration of 30mg/L. When CD increased from 1.25 to 4.5mA/cm2, the HA removal efficiency was improved from 88.8±4.4% to 96.1±1.5%. In addition, the type of coexisting cations and anions exerted a significant role, leading to a reduction in the removal efficiency of HA. To investigate the dominant free activated radical, radical scavengers such as tert-butyl alcohol and ethanol were employed. It was observed that both OH and SO4- radicals substantially contributed to the removal of HA, and the contribution of SO4- radical was higher than that of OH radical, suggesting that AEC-PS process could serve as a novel and effective treatment technique for the removal of organic matters from aqueous sources.

Polyethersulfone hybrid ultrafiltration membranes fabricated with polydopamine modified ZnFe2O4 nanocomposites: Applications in humic acid removal and oil/water emulsion separation

This study explored the impact of eco-friendly polydopamine (PDA) coated ZnFe2O4 nanocomposites (PDA@ZnFe2O4 NCs) as nanofillers to fabricate a new class of polyethersulfone (PES) hybrid ultrafiltration (UF) membranes for wastewater treatment applications. The hybrid UF membrane was prepared by incorporating PDA@ZnFe2O4 NCs into PES via the non-solvent induced phase separation (NIPS) process. The PDA@ZnFe2O4 NC was characterized by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Additionally, the morphology and performance of the prepared hybrid membranes were characterized by SEM, contact angle, surface charge, and thermogravimetric analysis (TGA). The incorporation of PDA@ZnFe2O4 NC into the PES membrane affects the porosity, mean pore radius, hydrophilicity, and thermal stability of the developed hybrid membranes. The pure water flux of the PES hybrid membrane with 4 wt% PDA@ZnFe2O4 reached ∼687 L/m2 h, which is about 188% higher than that of the pristine PES membrane. The performance of the PES/ PDA@ZnFe2O4ultrafiltration hybrid membrane was also investigated using humic acid (HA) foulant and oil/water emulsion individually. Compared to the pristine PES and PES/ZnFe2O4 membranes, the developed hybrid membranes showed enhanced permeability and HA foulant removal. HA's removal efficiency has improved from ∼65% in the pristine PES membrane to ∼94% in the 4 wt% PDA@ZnFe2O4 hybrid membrane. The abundant functional groups on the PDA@ZnFe2O4 NC surface also enhanced the separation of the oil/water emulsion (96%). In both the HA and oil/water emulsion separation tests, the flux recovery ratio (FRR) and reversible fouling ratio (Rr) were also significantly improved, suggesting that the PES/PDA@ZnFe2O4 membrane was a very promising candidate for HA removal and treatment of oily wastewater.

Efficiency of Zeolite Coated with Zero-Valent Iron Nanoparticles for Removal of Humic Acid from Aqueous Solutions

The most important effect of natural organic materials in water is reacting with disinfectants and creating disinfectant by-products that are mostly carcinogenic. The aim of this study was to determine the optimum conditions for removal of humic acid (HA) by zeolite coated with nZVI nanoparticles (Zeolite/nZVI) from aqueous solutions. In this study, after synthesis of zeolite/nZVI, its structure and morphology were examined using FTIR, BET, XRF, and FESEM techniques. The effects of HA concentration, composite content, pH, and reaction time were evaluated. The experimental data were analyzed by Langmuir and Freundlich isotherm and pseudo-first-order and second-order kinetic models. Finally, the thermodynamic parameters of enthalpy (ΔΗ°), entropy (ΔS°), and Gibbs free energy (ΔG°) were calculated. The results of the analyses confirmed the accuracy of the composite structure. Its specific surface area by using BET method was 203.43 m2/g. The HA removal efficiency was obtained at 92.98% in optimum conditions of 50 mg/L concentration, 2 g/L composite dose, pH = 3, and reaction time of 60 min. The results of the isotherm and kinetic study showed that the HA adsorption process follows the Langmuir isotherm (R2 = 0.9707) and pseudo-second-order kinetic. The maximum adsorption capacity of the composite was determined at 23.36 mg/g by Langmuir model. Thermodynamic parameters indicate that the adsorption of HA endothermic and the reaction cannot be done spontaneously. Zeolite/nZVI composite had good removal efficiency after five times of recycling. The present study showed that zeolite/nZVI can be used as an effective adsorbent for removal of HA from aqueous solutions.

Ultra-high synergetic intensity for humic acid removal by coupling bubble discharge with activated carbon

Humic acid (HA) removal research focuses on the global water treatment industry. In this work, efficient HA degradation with an ultra-high synergetic intensity is achieved by combined bubble discharge with activated carbon (AC). Adding AC to the discharge greatly improves HA removal efficiency and degradation speed; the synergetic intensity reaches 651.52% in the combined system, and the adsorption residual on AC is 4.52%. After 90 min of treatment, the HA removal efficiency reaches 98.90%, 31.29%, and 7.61% in the plasma-AC combined, solo bubble discharge, and solo AC adsorption systems, respectively. During the plasma process, the number of pore structures and active sites and the amount of oxygen-containing functional groups on the AC surface increase, resulting in a higher adsorption capacity to reactive species (H2O2 and O3) and HA and promoting interactions on the AC surface. For HA mineralization, the presence of AC greatly promotes the destruction of aromatic structures and chromophoric HA functional groups.

Opposite impacts of K+ and Ca2+ on membrane fouling by humic acid and cleaning process: Evaluation and mechanism investigation

Understanding the influences of cations on membrane fouling was important to improve the performance of membrane filtration system, however, opposite conclusions were made in different studies. Meanwhile, although the influences of cation concentration have been studied extensively, few attentions have been paid to the cation valence. To clarify it, the effects of typical cations on membrane fouling and cleaning, as well as the related mechanisms were investigated systemically in this study. K+ and Ca2+ were chosen as the representative cations, and humic acid (HA) was chosen as the membrane foulants. The results demonstrated Ca2+ promoted the formation of reversible fouling, meanwhile higher removal efficiency of HA could also be achieved with the assistance of filtration cake containing HA + Ca2+. However, K+ led to the formation of more recalcitrant irreversible fouling. By comparing the concentration of cations in feed and permeate, analyzing the influence of cations on size of HA flocs, and the detailed SEM, AFM and TEM observation, it could be found that different mechanisms dominated the interaction between cations and HA. The bridging effect induced by Ca2+ attributed to the extension of HA molecules, while the electrostatic shielding effect induced by K+ led to the compression of them. Moreover, the different characteristics of hydrated Ca2+ and K+ also contributed to the different structures of foulant layers formed by HA + Ca2+ and HA + K+. Given the abundance of K+ and Ca2+ in natural water, results of this study can provide valuable advice for practical membrane filtration process.

Degradation of Humic Acid by Ferrous Ion Activated Persulfate

The persulfate anions (S2O8 2™) can be activated by Fe2+ to produce sulfate radicals (·SO4 −) and hydroxyl radicals (·OH), which can oxidize organic compounds rapidly. Humic acid (HA) is widely found in nature and is the main limiting factor for high-concentration wastewater discharge. In this study, HA was treated by Fe2+/S2O8 2™ system, the effects of initial pH, concentrations of Fe2+, persulfate (PS), and HA on the removal efficiencies of HA and E3/E4 were discussed. Besides UV254 and UV280, we found that UV250, UV270, UV300, UV365, UV400, UV436 and UV465 can effectively quantified the concentrations of HA, providing more HA characterization methods. When the initial pH was 3.0, the concentrations of PS and Fe2+ were 100 mmol/L and 20 mmol/L, respectively, the removal efficiency of HA reached 72% after 5 minutes, indicating that PS/Fe2+ effectively degrade HA, and E3/E4 increased from 2.6 to 5.75, suggesting the molecular weight and aromaticity of HA decreased. The removal efficiencies of HA increased as the concentrations of Fe2+ and PS increased, while the removal efficiencies of HA decreased as the initial concentration of HA increased. The addition of hydroxylamine hydrochloride (HAH) in Fe2+/S2O8 2™ system, could effectively promoted the Fe3+/Fe2+ circulation and reduced the accumulation of Fe3+, and strengthened the removal efficiency of HA. The research results indicate that Fe2+/S2O8 2™ oxidation is a promising technique for the treatment of HA in wastewater.

Adsorption of humic acid from aqueous solution by magnetic Zn/Al calcined layered double hydroxides

Magnetic Zn/Al layered double hydroxides (M Zn/Al LDH) were prepared by co-precipitation method and then calcined at 370 °C for 2 h to obtain magnetic Zn/Al calcined layered double hydroxides (M Zn/Al CLDH) used for adsorption of humic acid (HA) from aqueous solution. The morphology and structure of M Zn/Al CLDH were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), specific surface area (SSA), thermogravimetric analysis and differential scanning calorimetry (TG-DSC) and vibrating-sample magnetometer (VSM), respectively. The results showed that the implant of a certain amount of magnetic substrates on Zn/Al LDH didn't damage the layered structure of Zn/Al LDH. After calcination, M Zn/Al CLDH exhibited the characteristics of zinc aluminum oxides, whose specific surface area increased to 80.00m2/g from 49.15 m2/g of M Zn/Al LDH. The magnetism of M Zn/Al LDH originating from Fe3O4 and γ-Fe2O3 kept well under calcination at 370 °C. The influence of solution pH and competitive ions on the HA removal was investigated. M Zn/Al CLDH exhibited high HA removal efficiency which was over 97% from pH 4 to pH 8 for 30 min at M Zn/Al CLDH dosage of 0.2 g/L and HA initial concentration of 20 mg/L. The kinetic, adsorption isotherm and thermodynamic of the adsorption of HA by M Zn/Al CLDH were studied. The results indicated that the adsorption followed pseudo-second-order kinetics model and Langmuir isotherm model. Thermodynamic analysis suggested that the adsorption of humic acid on M Zn/Al CLDH was a spontaneous and endothermic process. The adsorption mechanism is associated with electrostatic interaction and the intercalation of some functional groups of HA during layered structure of M Zn/Al LDH recovery in water due to the memory effect. This study revealed that M Zn/Al CLDH was an effective adsorbent for HA removal in water treatment.

Synergistic oxidation - filtration process analysis of catalytic CuFe2O4 - Tailored ceramic membrane filtration via peroxymonosulfate activation for humic acid treatment

This work synthesized catalytic CuFe2O4 tailored ceramic membrane (CuFeCM), and systematically investigated the intercorrelated oxidation - filtration mechanism of peroxymonosulfate (PMS)/CuFeCM catalytic filtration for treating humic acid (HA). PMS/CuFeCM filtration exhibited enhanced HA removal efficiency while reduced the irreversible fouling resistance as compared with the conventional CM filtration. Results from HA characterizations showed that PMS/CuFeCM catalytic filtration oxidized HA into conjugated structures of smaller molecular weight. The unsaturated bonds further caused the re-agglomeration of HA, hence enhancing the size exclusion of CuFeCM. Meanwhile, oxidized HA particles with changing physicochemical properties reduced the total attractive interaction energy between CuFeCM and HA, mainly attributed to the reduced acid-base interaction energy according to the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) analysis. The changing of HA properties and HA-CuFeCM physicochemical interactions rendered more re-agglomerated HA particles retained above membrane with less attachment, which induced decreasing irreversible fouling resistance and facilitated easier external fouling removal by hydraulic cleaning. Overall, the PMS/CuFeCM configuration demonstrated in this study could provide a new insight into the synergistic oxidation - filtration interaction mechanism of hybrid catalytic ceramic membrane filtration process.

Hydrogenated TiO2 membrane with photocatalytically enhanced anti-fouling for ultrafiltration of surface water

A supported Ti3+ self-doped anatase TiO2 membrane is developed by facile partial-sintering and hydrogenation, and its photocatalytically enhanced anti-fouling in ultrafiltration of surface water is explored. The hydrogenated membranes demonstrate much enhanced photocatalytic activity towards humic acid (HA) degradation, and better anti-fouling capability as compared to the pristine TiO2 membrane. Upon UV irradiation, the hydrogenated TiO2 membrane (TiO2_H30, by 30 h of hydrogenation) shows 60% higher HA removal efficiency, compared with pristine TiO2. During subsequent filtration tests, the TiO2_H30 membrane with UV pre-treatment shows a 50% enhancement of residue normalized flux over that of pristine TiO2, and a tripling of residue normalized flux as compared to tests performed without UV pre-treatment. The enhanced fouling resistance of the hydrogenated TiO2 membranes is directly correlated to the boosted photocatalytic activity, which is attributed to the improved carrier trapping, inhibited recombination, and improved conductivity, due to the hydrogenation-induced surface disorder and Ti3+ self-doping.

Enhanced alleviation of ultrafiltration membrane fouling by regulating cake layer thickness with pre-coagulation during drinking water treatment

Ultrafiltration (UF) membrane modules are static in membrane tanks; thus, result in the continuous development of a cake layer and serious membrane fouling. Thickness regulation is the most convenient method to solve this owing to the looseness induced by flocs. Recently, integrated membrane technology is increasingly being applied due to its high pollutant removal efficiency and low space requirements. Herein, with the injection of Fe-based flocs, the UF membrane performance was investigated with module rotation in the presence of humic acid (HA) and south-to-north water in China. The obtained results showed that the outer cake layer was easily shed away owing to the strong flow shear force. The thickness of the cake layer decreased and membrane fouling was significantly alleviated. The faster the rotation speed, the thinner the cake layer and the lower the membrane fouling was. However, the reduction rate of the cake layer thickness decreased as the rotation speed increased owing to the high density of the inner cake layer. Although both the rotation speed and rotation time played an important role in reducing the membrane fouling, the removal efficiency of HA remained constant, and the cake layer tended to be the main fouling mechanism; thus, indicating that module rotation is beneficial to the integrated technology. Owing to the smaller particle size and higher positive charge of flocs formed under an acidic condition, more negatively charged HA molecules were adsorbed, and the UF membrane performance was superior to that in an alkaline condition. Raw water experiments further confirmed the excellent UF membrane performance. Based on this, the proposed technology has great potential for wide applications in rural areas, particularly with the rapid development of clean energy (e.g., solar energy) and intelligent water services.

Influence of floc dynamic protection layer on alleviating ultrafiltration membrane fouling induced by humic substances

Cake layer formation is inevitable over time for ultrafiltration (UF) membrane-based drinking water treatment. Although the cake layer is always considered to cause membrane fouling, it can also act as a “dynamic protection layer”, as it further adsorbs pollutants and dramatically reduces their chance of getting to the membrane surface. Here, the UF membrane fouling performance was investigated with pre-deposited loose flocs in the presence of humic acid (HA). The results showed that the floc dynamic protection layer played an important role in removing HA. The higher the solution pH, the more negative the floc charge, resulting in lower HA removal efficiency due to the electrostatic repulsion and large pore size of the floc layer. With decreasing solution pH, a positively charged floc dynamic protection layer was formed, and more HA molecules were adsorbed. The potential reasons were ascribed to the smaller floc size, greater positive charge, and higher roughness of the floc layer. However, similar membrane fouling performance was also observed for the negative and positive floc dynamic protection layers due to their strong looseness characteristics. In addition, the molecular weight (MW) distribution of HA also played an important role in UF membrane fouling behavior. For the small MW HA molecules, the chance of forming a loose cake layer was high with a negatively charged floc dynamic protection layer, while for the large MW HA molecules it was high with a positively charged floc dynamic protection layer. As a result, slight UF membrane fouling was induced.

Performance evaluation of graphene oxide coated on cotton fibers in removal of humic acid from aquatic solutions

We investigated the removal efficiency of humic acid from aqueous solutions by cotton coated with graphene oxide. This research has been conducted as batch on an experimental scale. A self-arrangement approach was introduced in fabrication of the cotton adsorbent coated with graphene oxide. To determine the effect of parameters, including initial concentration, pH, adsorbent dosage and contact time, central composite design (CCD) was employed in response surface method (RSM). The adsorption kinetics were determined based on different times of adsorption of humic acid. Further, the adsorption isotherms were also examined using different concentrations of humic acid. The results obtained showed that with increasing adsorbent dosage and contact time, the removal efficiency increased, while with increasing pH and initial concentration of humic acid, the removal efficiency decreased. The optimal values based on RSM method were obtained as the following: humic acid initial concentration=13.61 mg/L, pH=3.87, adsorbent dosage=0.61 g, and contact time=168.43 min. Langmuir isotherm with R2=0.9987 has been the most suitable model for explaining the adsorption process. Investigation of the adsorption kinetics indicated that humic acid adsorption follows pseudo-second-order model (R2=0.9822). The results indicated that the cotton adsorbent coated with graphene oxide has a good potential for removal of humic acid from aqueous solutions. Mechanical flexibility, availability, and low operational energy costs are among the advantages of this method for fabrication of this adsorbent, which can be developed and used for reducing environmental contaminants.

A sono-photocatalyst for humic acid removal from water: Operational parameters, kinetics and mechanism

As a precursor of disinfection byproducts, humic acid (HA) has adverse effects on aquatic environments and human health. Currently, many advanced oxidation processes (AOPs) have been proposed to remove HA from drinking water, one of which is photocatalysis. However, the long reaction time required for degradation and drawbacks of the photocatalysts limit the large-scale application of photocatalysis. Therefore, two principal objectives were achieved in this work. First regarding the technology, we combined photocatalysis with ultrasonic waves to remove HA. Second regarding the photocatalyst, quaternary Fe3O4/TiO2-N-GO (FTNG) sono-photocatalysts with different amounts of Fe3O4 were first synthesized using a simple hydrothermal method. Characterizations were performed to confirm the successful synthesis of the sono-photocatalyst and to determine some of its properties. The influence of different experimental factors such as Fe3O4 content, ultrasonic power, catalyst dosage and initial HA concentration were studied. The first-order kinetic and second-order kinetic equations were used to simulate the experimental data. The results showed that FTNG-0.2 with 0.2 g of Fe3O4, which was added upon preparation, showed the highest sono-photocatalytic ability. In our experimental setup, greater than 99% removal efficiency (UV254) and 94% mineralization rate (TOC) were achieved within 90 min at the optimum conditions (60 W ultrasound power and 1.0 g/L catalyst dosage for 30 mg/L HA). Compared with the pseudo-first-order kinetic model, pseudo-second-order model fitted better with the experimental data and it had higher R2 values of 0.92, 0.98 and 0.98 for 30, 40 and 50 mg/L of HA, respectively. According to the scavenging tests and the ESR analysis, both of the OH and O2− were produced in the reaction, however, O2− radicals were assumed to be the dominating reactive species for the HA degradation. Moreover, after five repetitive experiments, the removal efficiency of HA can still reach 88.5%, indicating high stability of FTNG-0.2 sono-photocatalyst. The mechanism of degradation of HA by FTNG-0.2 in sono-photocatalytic system was mentioned based on several factors including the ultrasonic cavitation effect, Fenton-like reactions, photocatalytic reactions, etc. In fact, this was the first study to treat HA through sono-photocatalytic process, which showed great potential in drinking water treatment.

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

The mechanism of removal of humic acid, phosphate and kaolin particles by coagulation with alum and PACl or adsorption by their pre-formed precipitates was investigated, and it was found that the coagulation mechanisms for monomeric Al at neutral pH and polymeric Al13 at alkaline pH were very similar. The removal of phosphate and humic acid by coagulation with alum or PACl did not change with stirring time (between 1 min and 15 min), independent of the dose and species of coagulants. However, for adsorption of these impurities by pre-formed precipitates, the results were significantly different. Both Al3+ and nano-sized Al13 could precipitate and form aggregates at pH 7 and pH 9, respectively, and their precipitates became less active (fewer binding sites on the surface of precipitate) with the increase of shear time or shear rates before adsorbing pollutants. Thus, although the total surface area increased (the average size of flocs became smaller) at higher applied shear rates or longer shear time, the removal efficiency of humic acid and phosphate decreased. Also, from the MW distributions, it was confirmed that less humic acid was removed by the adsorption on alum precipitate pre-formed with longer shear time. Chemical groups (OH2and OH) on the surface of precipitate determined the removal efficiency of phosphate and humic acid, and the activity of precipitate become lower as a result of higher applied shear and longer shear time. This is confirmed be due to some crystallization of the amorphous precipitate, forming inactivated hydroxyl. When kaolin was added 10 min after the alum or PACl precipitate formed, the precipitates captured kaolin particles only on their surface, whereas when alum was added to kaolin suspensions particles were trapped within the growing flocs. When alum/kaolin flocs were broken at high shear rate re-growth of flocs decreased with increasing shear time, but after a short breakage period, long aging of broken flocs had little effect on floc regrowth.

Synergistic effects and kinetics of rGO-modified TiO2 nanocomposite on adsorption and photocatalytic degradation of humic acid

Graphene oxide was prepared using the modified Hummers method and reduced graphene oxide (rGO) - titanium dioxide (TiO2) nanocomposite was synthesised using the one-step hydrothermal treatment. The synergistic effects on adsorption and photocatalytic properties of the rGO-TiO2 nanocomposite for the humic acid removal were systematically investigated. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman and infrared (IR) spectroscopy indicate that GO was partially reduced to reduced graphene oxide (rGO) in the hydrothermal synthesis process and anatase TiO2 nanoparticles uniformly grew on the surface of rGO. The photoelectron and photohole generated under visible light irradiation were effectively separated on the surface of rGO-TiO2. The rGO-TiO2 nanocomposite exhibited higher photocatalytic activity as a result of the synergistic effects of surface functional groups for adsorption and the excellent conductivity for photocatalytic reaction. The effect of rGO-TiO2 nanocomposite dosage, light intensity and system temperature on the removal of humic acid solution was investigated. The results show that the removal efficiency of humic acid increased with system temperature and light intensity. When the dosage of rGO-TiO2 nanocomposite was 1.2 g/L, the temperature, the light intensity and the pH of this system was 303 K, 4.37 Wm−2 and 7, respectively, the removal efficiency of humic acid reached 88.7% under visible light irradiation.

Efficient capture of aqueous humic acid using a functionalized stereoscopic porous activated carbon based on poly(acrylic acid)/food-waste hydrogel.

Stereoscopic porous carbons have shown good potential in humic acid (HA) removal. In this work, a novel stereoscopic porous activated carbon (SPAC) was designed and synthesized via the self-assembly of a hydrogel based on food waste during in-situ polymerization, vacuum drying, carbonization, and activation. Then, the SPAC was functionalized with 3-aminopropyltriethoxysilane (APTES) and the adsorption behavior of the modified SPAC (SPAC-NH2) was studied systematically. The effects of pH, contact time, initial concentration of HA, and adsorbent dose were investigated, showing that optimal HA removal efficiency (>98.0%) could be achieved at an initial HA concentration of 100 mg/L. The experimental adsorption isotherm data was fitted to the Langmuir model with a maximum adsorption capacity of 156.0 mg HA/g SPAC-NH2. Analysis of the mechanism indicated that the removal of HA was mainly realized through the amidization reaction between the COOH groups of HA and the NH2 groups of APTES. All of the above results showed that SPAC-NH2 powder is an efficient, eco-friendly, and reusable adsorbent which is suitable for the removal of HA from wastewater.