Silicon carbide coated alumina tight-ultrafiltration membrane prepared by low-pressure chemical vapor deposition for sulphate ion retention

Sulphate (SO₄^2­) 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 SO₄^2­ 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 SO₄^2­ 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 SO₄^2­ 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 SO₄^2­ rejection, and without a large loss of PWP normally associated with NF membranes.