Calcium-enhanced retention of humic substances by carbon nanotube membranes: Mechanisms and implication

Interaction between microplastics and humic acid and its effect on their properties as revealed by molecular dynamics simulations

The characteristics and fates of microplastics (MPs) and humic acid (HA) in the environment are significantly influenced by their interactions. Thus, the influence of the MP-HA interaction on their dynamic characteristics was explored. Upon MP-HA interaction, the number of hydrogen bonds established in the HA domains decreased significantly, and the water molecules bridging the hydrogen bonds shifted to the exterior regions of the MP-HA aggregates. The distribution intensity of Ca²⁺ located at ∼0.21 nm around HA deceased, indicating that the coordination of Ca²⁺ with the carboxyl on HA was impaired in the presence of MPs. Additionally, the Ca²⁺-HA electrostatic interaction was suppressed because of the steric hindrance of the MPs. However, the MP-HA interaction improved the distribution of water molecules and metal cations around the MPs. The diffusion coefficient of HA decreased from 0.34 × 10^-5 cm²/s to 0.20-0.28 × 10^-5 cm²/s in the presence of MPs, implying that the diffusion of HA was retarded. The diffusion coefficients of polyethylene and polystyrene increased from 0.29 × 10^-5 cm²/s and 0.18 × 10^-5 cm²/s to 0.32 × 10^-5 cm²/s and 0.22 × 10^-5 cm²/s, respectively, indicating that the interaction with HA accelerated the migration of polyethylene and polystyrene. These findings highlight the potential environmental hazards posed by MPs in aquatic environments.

Establishing a high-performance anti-fouling PEI-ZIF-PAA membrane with improved Lewis acid-base interactions and hydrophilicity

Membrane fouling and the trade-off between membrane permeability and selectivity restrict the potential applications of membrane filtration for water treatment. ZIF-8 was found having great permeability and antibiofouling performance, but with issue on particle aggregation makes it difficult to achieve high ZIFs loading and fabricate a defect-free molecular sieving membrane in previous research. In this study, we formed a scalable antibiofouling surface with improved permeability and fouling resistance on a PEI-ZIF-PAA membrane using a layer-by-layer assembly technique. The synergistic effects of being sandwiched between two different polyelectrolyte layers with opposite charges endowed the ZIF nanoparticles with improved stability and scalability for membrane modification. The PEI-ZIF-PAA membrane exhibited a satisfactory water flux of 120.78 LMH, which was 46.97% higher than that of the pristine PES membrane. The normalized water flux loss was serious in the absence of ZIF-8, and the flux increased with the ZIF-8 concentration. Antifouling tests suggested that the PEI-ZIF-PAA membrane possessed good antifouling performance due to the much higher surface hydrophilicity and positive Lewis acid-base interactions with foulants. The HA rejection increased with the ZIF-8 concentration and reached a maximum of 92.1% in the presence of 1.00% (w/v) ZIF-8. The membrane regeneration was tested under physical and chemical cleaning with flux recovery rates of about 85% and 95%. XDLVO analysis showed that the total interaction energy between HA and the PEI-ZIF-8-PAA membrane was 26.45 mJ/m², and the superior antifouling performance was mainly attributed to Lewis acid-base interactions. This study indicates that ZIF-8 nanocrystals are promising materials for fabricating novel membranes for sewage treatment.

Strengthen Air Oxidation of Refractory Humic Acid Using Reductively Etched Nickel-Cobalt Spinel Catalyst

Nickel-cobalt spinel catalyst (NCO) is a promising catalyst for air oxidation of humic acid, which is a typical natural refractory organic matter and a precursor of toxic disinfection by-products. In this study, reductive etchers, NaBH4 or Na2SO3, were used to adjust the NCO surface structure to increase the performance. The modified catalyst (NCO-R) was characterized, and the relationship between its intrinsic properties and catalytic paths was discovered. The results of O2-temperature programmed desorption, NH3-temperature programmed desorption, and X-ray photoelectron spectroscopy (XPS) demonstrated that reductant etching introduced oxygen vacancies to the surface of NCO and increased active surface oxygen species and surface acidity. In addition, the modification did not change the raw hollow sphere structure of NCO. The crystallinity and specific surface area of NCO-R increased, and average pore size of NCO-R decreased. XPS results showed that the ratio of Co3+/Co2+ in NCO-R decreased compared with NCO, while the ratio of Ni3+/Ni2+ increased. The results of H2-temperature programmed reduction showed that the H2 reduction ability of NCO-R was stronger. Due to these changes in chemical and physical properties, NCO-R exhibited much better catalytic performance than NCO. In the catalytic air oxidation of humic acid at 25 °C, the total organic carbon (TOC) removal rate increased significantly from 44.4% using NCO to 77.0% using NCO-R. TOC concentration of humic acid decreased by 90.0% after 12 h in the catalytic air oxidation using NCO-R at 90 °C.

Evaluation of membrane tailored with biocompatible halloysite‒polyaniline nanomaterial for efficient removal of carcinogenic disinfection by‒products precursor from water

Humic acid (HA) is the main component of natural organic matter that generates carcinogenic by‒products during disinfection and its removal from water resources is challenging. Biocompatible halloysite (HNTs) nanomaterial decorated with polyaniline (HNTs‒PANI) was synthesized via polymerization technique. HNTs‒PANI was added to prepare polyethersulfone mixed matrix membranes (MMMs). The influence of HNTs‒PANI concentration on HA removal efficiency was studied by varying the HNTs‒PANI (0.5, 1 and 1.5 wt%). The characterization studies of MMMs revealed that the addition of HNTs‒PANI improved the morphology of the membranes, surface properties, chemical stability and thermal property. The amine and hydroxyl groups within the MMMs improved the membrane wettability. The addition of HNTs‒PANI within the MMMs had significantly enhanced the pure water flux and HA filtration. YHP2 MMM with 1 wt% of HNTs‒PANI demonstrated sieving coefficient of 0.10 and the highest HA removal efficiency of 91% greater than the neat PES membrane. Furthermore, the antifouling property of the MMMs was studied using HA as foulant. 1 wt% of HNTs‒PANI added MMM showed a high flux recovery ratio (94.9%) with low total fouling of 12% and low irreversible fouling of 5%, respectively. Thus, HNTs‒PANI was an efficient nanomaterial for enhancing the pure water flux, removal efficiency and antifouling property to treat water contaminated with HA.

Influence of cations on As(III) removal from simulated groundwaters by double potential step chronoamperometry (DPSC) employing polyvinylferrocene (PVF) functionalized electrodes

As(III) removal from groundwaters is challenging because of its neutral charge and low surface affinity under circumneutral pH conditions. In this work, we investigate the influence of Ca²⁺ and Mg²⁺ on the removal of As(III) by a redox active polyvinylferrocene (PVF) functionalized electrode in a modified double potential step chronoamperometry (DPSC) setup. In the absence of divalent cations, nearly 90% As(III) removal is achieved over ten continuous cycles by single-pass DPSC, even in the presence of competing anions, however the presence of divalent cations at concentrations ≥ 1.25 mM significantly inhibits As(III) removal. The divalent cations enhance arsenic removal in the first (removal) step but suppress electrode regeneration in the 2^nd step. Our results suggest that Ca²⁺/Mg²⁺ either acts as a bridge between the electrode surface and As anions or the sorption of Ca²⁺/Mg²⁺ increases the positive charge on the electrode surface thereby facilitating As(V) sorption. We show that effective electrode regeneration can be achieved using an NaOH wash however the overall complexity of the process increases. Overall, we conclude that the influence of divalent cations on As removal by electro-sorption processes needs to be taken into consideration for application of this technology for real groundwater treatment.