The presence of heavy metals is one of the main environmental issues. Heavy metals from industrial activities such as leather tanning, textiles, and electroplating can cause various problems, ranging from a decline in water quality to human health issues. Heavy metals can enter the human body through contaminated food and beverages or through air, leading to negative effects on human health. The leather tanning industry is one of the industries that produce chromium (Cr) heavy metal pollution. Currently, 80–90% of leather tanning industries worldwide still use chromium as a tanning agent. The leather tanning process can produce waste with chromium levels of up to 14.9 mg.L−1, which exceeds the tolerance limit set by the Indonesian government through the Ministry of Environment and Forestry Regulation of 0.6 mg.L−1. If not treated first, heavy metal content in this industrial waste will negatively impact the environmental ecosystem.
Chromium (Cr) is one of the heavy metals with carcinogenic, allergenic, and irritant properties. Chromium has oxidation states, but the most stable forms in nature are trivalent chromium and hexavalent chromium. Trivalent chromium has lower toxicity compared to hexavalent chromium. Cr(III) in wastewater can oxidize into Cr(VI) (hexavalent chromium), a reaction that can occur spontaneously. The oxidation of Cr(III) to Cr(VI) can naturally occur in soil due to photochemical oxidation or through the presence of oxidizers like MnO2 or H2O2 in the soil. Exposure to Cr(VI) heavy metals can lead to perforation, nasal septum damage, skin edema, asthma attacks, rhinitis, sore throat, pulmonary fibrosis, acute gastrointestinal pain, necrotic liver and kidneys, and disturbances in the stomach, urinary system, and bones.
Many methods have been developed to reduce metal ion levels in solutions. The commonly developed methods for reducing chromium levels include adsorption, bioreduction, chemical reduction, photocatalytic reduction, and electrochemical reduction. Although these methods are widely used to remove Cr(VI) in wastewater, they have some drawbacks. These include requiring secondary treatment, high costs for bioremediation methods, the need for additional treatment to remove precipitates resulting from chemical reduction, and concentration polarization that can hinder further Cr(VI) removal. One material that can be used to reduce heavy metal levels is boehmite. Boehmite is an aluminum oxide hydroxide with the chemical formula g-AlOOH.
Boehmite is commonly used as an adsorbent, particularly for separating metal ions in solutions. Its high isoelectric point and point of zero charge (PZC) make boehmite highly affine and effective in adsorbing metal ions, especially Cr(VI). Due to the presence of hydroxyl (–OH) groups on boehmite, it is often combined with polymer chains.
Membrane filtration is another solution for reducing heavy metal concentrations in water bodies. Membrane separation is an effective method due to its advantages, such as continuous particle separation, high permeability, and reusability. Mixed Matrix Membrane (MMM) is often used to improve the function and effectiveness of membranes, especially in separating analytes from their matrix. MMM is a membrane modified by adding inorganic materials. The efficiency of MMM increases due to the combination of adsorption and filtration methods, leading to optimal separation.
The novelty of this study was developed a Mixed Matrix Membrane (MMM) by combining polyethersulfone (PES) membranes with boehmite nanoparticles modified with polyphenols from Samanea saman bark. This combination has been proven to have the ability to remove Cr(VI) heavy metal very well. The PES was chosen because it has good oxidative, chemical properties, mechanical properties, and thermal stability. The addition of boehmite–polyphenol nanofillers (B-polyphenol) is expected to improve the hydrophilicity and permeability of the fabricated membrane. The membrane developed have rich in hydroxyl groups, thus increasing the ability to adsorb Cr(VI). The MMM has been casted in hollow fiber form and applied to remove Cr(VI) heavy metal ions. The hollow fiber form was chosen because it has a very large surface area, thus increasing its ability as an adsorbent. The fabricated membrane will undergo performance testing to study its properties related to water treatment processes. Performance tests include water flux, porosity, hydrophilicity, and Cr(VI) removal capacity.
Methods
The synthesis of boehmite nanoparticles is carried out by dissolving 6.490 g of NaOH in 50 mL of deionized water, and 20 g of Al(NO3)3.9H2O is dissolved in 30 mL of distilled water. The solutions are then mixed until a white solution is formed. The solution is sonicated for 3 hours at a temperature of 25 °C. The resulting mixture is placed in an oven at 220 °C for 10 hours using the hydrothermal method. The resulting precipitate is filtered, washed with deionized water, and then dried in an oven at 105 °C.
The preparation of B-polyphenol begins with the extraction of polyphenols from Samanea saman bark powder. The extraction process starts by soaking Samanea saman bark powder in ethanol solvent for 2 × 24 hours. The solvent is then removed using a rotary evaporator. A total of 15 g of synthesized boehmite is added to 250 mL of ethanol/water (1 : 4 v/v) and sonicated for 15 minutes. Then, 15 mL of CPTES is added and refluxed at 50 °C for 9 hours. Afterward, 10 g of Samanea saman extract is added and refluxed at 60 °C for 14 hours to obtain boehmite–polyphenol nanoparticles (B-polyphenol). The B-polyphenol is separated by centrifugation, washed with ethanol, and dried in an oven at 60 °C.
The dope of MMM was fabricated by mixing the PES, PVP, NMP, and B-polyphenol. The pellet of PES used was 18 wt%, while the PVP loading used was 1 wt%. The B-polyphenol loading was varied at 0; 0.5; 1.0; 1.5; and 2.0 wt%. The dope solution was heating at 60 °C for 6 h then continuously stirrer for 1 day.
The dry/wet spinning hollow fiber machine was used to fabricate the hollow fiber membrane. The single layer spinneret with diameter size for inner and outer 0.4/0.8 mm was used in this study. The spinneret was placed by distance 40 cm with the coagulant bath. The dope extrusion rate used was 1 mL min−1 , bore fluid (distilled water) pumping speed was 1 mL min−1 , and collection speed is 5 m min−1 . The post-treatment of hollow fiber-MMM (HF-MMM) was washing for 50 h by tap water, thus immersion in 10 wt% glycerol for 24 h.
The B-polyphenol was characterized by X-ray diffractometer (XRD), Fourier-transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Particle size analyzer (PSA). While, the HF-MMM were characterized by Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), Water contact angle (WCA), Porosity, Tensile strength test.
Results
The Cr(VI) removal carried out in this research used a membrane module, so test conditions were obtained that were similar to actual conditions. XRD, FTIR, PSA, tensile strength, and SEM-EDS characterization were carried out to study the characteristics of the membrane and nanofiller. The XRD diffractogram shows a specific peak for boehmite at 2q = 14°. The results of FTIR characterization of B-polyphenols also identified the presence of Al–O functional groups at wave numbers 733, 603, 474 cm−1 for nanofillers and OH, C]C, C–O, and C]O functional groups for PES/B-polyphenol membranes. Based on SEM-EDS and tensile strength tests, it was observed that the Young’s modulus value of the membrane was 56.67 MPa and had a porous surface that was evenly distributed. Mixed matrix hollow fiber membrane PES/B-polyphenols that have been fabricated can reduce Cr(VI) levels up to 92.12%. Mixed matrix hollow fiber membrane PES/B-polyphenols have a water flux 14.1 L m−2 h−1 with porosity is 85.3% and the contact angle formed between the membrane surface and water is 58–32°. Based on the results obtained, this mixed matrix hollow fiber membrane PES/B-polyphenol has potential to be applied on a larger scale regarding its application to reduce Cr(VI) levels.
Author: Yanuardi Raharjo, Ph.D
Detailed information from this research can be seen in our article at:
https://pubs.rsc.org/en/content/articlelanding/2025/ra/d4ra09028d
Yanuardi Raharjo, Rico Ramadhan, Jourdham Nathanael, Mochamad Ifan Nugroho, Amelia Julia Tria Fetty and Ahmad Fauzi Ismail
Application of mixed matrix hollow fiber membrane PES/B-polyphenol nanoparticle for removal of chromium hexavalent, RSC Advances, 2025, 15, 7149-7159.
