Impacts of mono/divalent cations on the lamellar structure of cross-linked GO layers and membrane filtration performance for different DOM fractions

Ionic regulation whey protein composition on membrane fouling during ultrafiltration process analyzed by HPLC

In order to meet the market demand and avoid the increase of operation amount and cleaning cost in the process of ultrafiltration, it is particularly important to find more practical and efficient methods to control and improve membrane fouling. In this study, the ions in the ultrafiltration process were regulated to affect membrane surface proteins composition (lactoferrin, α-lactalbumin, β-lactoglobulin A and β-lactoglobulin B) and delay membrane fouling. It was found that Na+ (21 mmol/L), Zn2+ (0.25 mmol/L) and K+ (44 mmol/L) was added at 4 min, 8 min and 12 min, respectively during ultrafiltration process. The continuous regulation slowed down the decline rate of membrane flux and reduced the content of α-lactalbumin, β-lactoglobulin A and β-lactoglobulin B on the membrane surface analyzed by HPLC. This could reduce the irreversible membrane fouling of proteins cake resistance. Furthermore, the ions concentration was also investigated after filtration. The concentration of K+ was increased significantly and other ions concentration was not significantly changed after continuous regulation such Na+, Mg2+, Zn2+ and Ca2+ compared to control. Therefore, dynamic ionic regulation of whey protein ultrafiltration process is a simple and effective method, which provides technical theoretical basis for optimizing and improving membrane technology.

Effects of dissolved organic matter on the adsorption of norfloxacin on a sandy soil (fraction) from the Yellow River of Northern China

Dissolved organic matter (DOM), which exists widely in the environment, coming from different sources, may greatly affect the adsorption of antibiotics. However, the adsorption mechanisms of antibiotics in a sandy soil and the effects of DOM from different sources on the adsorption remain poorly understood. This study systematically investigated the adsorption characteristics of norfloxacin (NOR) onto a sandy soil obtained from the banks of Xi'an in Yellow River and in the presence of three DOM including HDOM (commercially available humic acids), LDOM (derived from fallen leaves) and MDOM (derived from cattle manure). Elemental analysis, UV-vis spectroscopy, 3D-EEM, XPS, TOC, SEM, and FTIR were used to analyze the adsorption mechanism. It was found that all the DOM sources we used could reduce the adsorption of NOR on sandy soil and prolong the reaction time to reach adsorption equilibrium. The decreasing adsorption capacities of NOR by the three types of DOM (10 mg/L) followed the order as: HDOM < LDOM < MDOM, which was related to their aromaticity, polarity and hydrophobicity. These adsorption processes of NOR on sandy soil in the presence of DOM were well fitted by Double-chamber first-order kinetics, Linear model and Freundlich models. Besides, the adsorption reaction was endothermic and spontaneous. Adsorption competition of DOM molecules with NOR, or formation of DOM-NOR complexes in solution resulted in a decrease of sandy soil adsorption capacity. Correspondingly, co-adsorption and cumulative adsorption were also considered to be the key processes that determined NOR adsorption towards sandy soil after adding DOM. Moreover, the adsorption of NOR onto sandy soil exhibited strong pH-dependent characteristic and NOR might be more easily leached from sandy soil in the aquifer at an alkaline pH. High-ion strength suppressed the adsorption. These results would help to understand the fate and risk of NOR under the action of different DOM.

The effect of ion environment changes on retention protein behavior during whey ultrafiltration process

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The ions environment changes were investigated during whey ultrafiltration process.

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Whey protein surface structure changes were contributed to the changing ions’ concentration.

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The relationship between ions around whey protein and membrane fouling was analyzed.

The factors affecting membrane fouling are very complex. In this study, the membrane fouling process was revealed from the perspective of ion environment changes, which affected the whey protein structure during ultrafiltration. It was found that the concentrations of Ca²⁺ and Na⁺ were overall increased and the concentrations of K⁺, Mg²⁺ and Zn²⁺ were decreased at an ultrafiltration time of 11 min, which made more hydrophilic groups buried inside and increased the content of α-helix, leading to more protein aggregation. The relatively higher K⁺ ratio in retention could lead to an antiparallel β-sheet configuration, aspartic acid, glutamic acid and tryptophan increased, which resulted in more protein aggregation and deposition on the membrane surface at 17 min. When the ion concentration and ratio restored the balance and were close to the initial state in retention, the protein surface tension decreased, and the hydrophilic ability increased at 21–24 min.

Removal characteristics of dissolved organic matter and membrane fouling in ultrafiltration and reverse osmosis membrane combined processes treating the secondary effluent of wastewater treatment plant

The widespread implementation of municipal wastewater treatment and reuse must first ensure the safety of reused wastewater. The effluent of the municipal wastewater treatment plant contains a large amount of dissolved organic matter (DOM), which adversely affects the reuse of wastewater. In this study, the ultrafiltration (UF) + reverse osmosis (RO) process was used to treat the secondary effluent from wastewater treatment plants. The relationship between the removal performance, membrane fouling of the UF + RO process, and DOM removal characteristics of influent were studied. The results show that DOM can be removed effectively by UF + RO process. The UF mainly removes DOM with a molecular weight greater than 10 kDa, while RO has a significant removal effect on low-molecular-weight DOM, which mainly causes UF and RO membrane fouling. The UF + RO process has a significant removal rate on fulvic acid, humic acid, tyrosine, and tryptophan, and the order is humic acid > fulvic acid > tyrosine > tryptophan. Fulvic acid contributed the most to the UF membrane fouling, while fulvic acid and protein-like proteins contributed mainly to the RO membrane fouling.

Removal of pharmaceuticals by fouled forward osmosis membranes: Impact of DOM fractions, Ca2+ and real water

This study systematically investigated the impact of dissolved organic matters (DOM) fractions, Ca²⁺, membrane orientation and real water matrix on the membrane fouling and the subsequent pharmaceutical retention in forward osmosis (FO). Ca²⁺ increased the removal of carbamazepine (CBZ) through steric effect, while it reduced sulfamethoxazole (SMZ) removal due to reduced electrostatic repulsion and enhanced external concentration polarization for three organic foulants. The study of operating mode showed that the pharmaceutical removal in pressure retarded osmosis (PRO) mode were lower than those in FO mode for both the baseline and HA fouling, which was attributed to the concentrative internal concentration polarization caused by long-term accumulation of pharmaceuticals or HA in support layer. In terms of the real water tests, the secondary effluent used as feed solution caused higher hydrophilicity and negative charge of fouled FO membrane, leading to increased removal of pharmaceuticals. Seawater used as draw solution also caused severe fouling in the support layer of FO with humic acid-like material as major foulants, increasing the removal of SMZ because of enhanced steric hindrance and electrostatic repulsion. However, the combined effects of increased adsorption and steric effect resulted in little change for the CBZ removal. This study gave implications on the practical application of FO process for pharmaceutical removal.

Impact of monovalent cations on the separation performance of graphene oxide membrane for different organic matters

Graphene oxide (GO) membrane has become a promising membrane due to its advantages of high flux, good anti-pollution performance, and controllable cost. Since wastewater contains inorganic ions and organic substances, the effect of inorganic ions on the removal of organic matters has attracted attention. This study investigated the impact of monovalent cations (Na⁺ and K⁺) on the removal of different organic matters by GO membranes. The results showed that the removal by GO membranes of both macromolecular organic compounds (humic acid, sodium alginate, and bull serum albumin) and small molecular organic substances (new coccine, methyl orange, and methylene blue) were high (>75%). The coexisting Na⁺ caused the increase in flux and the decrease in removal of small molecular organic matters, while the impact on the removal of macromolecular organic matters was weak. The impact of the coexisting K⁺ was weaker than Na⁺. With the crosslinking of ethylenediamine, the structure of GO membrane was more stable. The impacts of Na⁺ on the flux and separation performance of crosslinked membranes were less than that of non-crosslinked membranes. This suggests that crosslinked GO membrane has more potential for practical wastewater treatment.