Sulphate (SO42­) is a model ion due to its negative charge and multivalent nature. Its rejection behavior serves as an indicator of the separation performance for other analogous ions in modified membranes. In literature the rejection of the SO42­ by negatively charged polymeric nanofiltration (NF) membranes has been studied extensively with rejection percentages of >90 %. Silicon carbide (SiC) membranes have gained attention for wastewater treatment due to their high hydrophilicity and negative charge. However, no negatively charged ceramic ultrafiltration (UF) membranes have been tested yet for SO42­ retention. In this study, a commercial alumina (Al2O3) UF membrane was converted into a highly negatively charged tight-UF membrane by coating it with SiC. This was achieved by depositing a 5 μm SiC coating in a single-step via low-pressure chemical vapor deposition (LP-CVD). LP-CVD facilitates the preparation of a SiC at much lower temperatures (700–900 °C) compared to the sol-gel methods (ca. 2100 °C), and it does not require multiple coating cycles and sintering steps to achieve the desired selective layer thickness. Subsequently, properties and performance of the as-prepared tight-UF membrane coated with SiC were evaluated. The SiC coated membrane had a highly negative charge of −70 mV at pH of 6, and a pure water permeability (PWP) of 26 L.m−2.h−1.bar−1. The SiC coated membrane furthermore demonstrated a SO42­ rejection of 79 % despite having a large pore size of 7 nm, in comparison with the pore sizes of below 1 nm of NF membranes. These results highlight the potential of singe-step LP-CVD modification of commercial UF ceramic membranes to produce highly negatively charged SiC coated UF membranes with a high SO42­ rejection, and without a large loss of PWP normally associated with NF membranes.

Atomic layer deposition (ALD) is known for its unparalleled control over layer thickness and 3D conformality and could be the future technique of choice to tailor the pore size of ceramic nanofiltration membranes. However, a major challenge in tuning and functionalizing a multichannel ceramic membrane is posed by its large internal pore volume, which needs to be evacuated during ALD cycling. This may require significant energy and processing time. This study presents a new reactor design, operating at atmospheric pressure, that is able to deposit thin layers in the pores of ceramic membranes. In this design, the reactor wall is formed by the industrial tubular ceramic membrane itself, and carrier gas flows are employed to transport the precursor and co-reactant vapors to the reactive surface groups present on the membrane surface. The layer growth for atmospheric-pressure ALD in this case proceeds similarly to that for state-of-the-art vacuum-based ALD. Moreover, for membrane preparation, this new reactor design has three advantages: (i) monolayers are deposited only at the outer pore mouths rather than in the entire bulk of the porous membrane substrate, resulting in reduced flow resistances for liquid permeation; (ii) an in-line gas permeation method was developed to follow the layer growth in the pores during the deposition process, allowing more precise control over the finished membrane; and (iii) expensive vacuum components and cleanroom environment are eliminated. This opens up a new avenue for ceramic membrane development with nano-scale precision using ALD at atmospheric pressure.

Sodium hypochlorite (NaClO) is widely used for the chemical cleaning of fouled ultrafiltration (UF) membranes. Various studies performed on polymeric membranes demonstrate that long-term (>100 h) exposure to NaClO deteriorates the physicochemical properties of the membranes, leading to reduced performance and service life. However, the effect of NaClO cleaning on ceramic membranes, particularly the number of cleaning cycles they can undergo to alleviate irreversible fouling, remains poorly understood. Silicon carbide (SiC) membranes have garnered widespread attention for water and wastewater treatment, but their chemical stability in NaClO has not been studied. Low-pressure chemical vapor deposition (LP-CVD) provides a simple and economical route to prepare/modify ceramic membranes. As such, LP-CVD facilitates the preparation of SiC membranes: (a) in a single step; and (b) at much lower temperatures (700–900 °C) in comparison with sol-gel methods (ca. 2000 °C). In this work, SiC ultrafiltration (UF) membranes were prepared via LP-CVD at two different deposition temperatures and pressures. Subsequently, their chemical stability in NaClO was investigated over 200 h of aging. Afterward, the properties and performance of as-prepared SiC UF membranes were evaluated before and after aging to determine the optimal deposition conditions. Our results indicate that the SiC UF membrane prepared via LP-CVD at 860 °C and 100 mTorr exhibited excellent resistance to NaClO aging, while the membrane prepared at 750 °C and 600 mTorr significantly deteriorated. These findings not only highlight a novel preparation route for SiC membranes in a single step via LP-CVD, but also provide new insights about the careful selection of LP-CVD conditions for SiC membranes to ensure their long-term performance and robustness under harsh chemical cleaning conditions.