Synthesising novel adsorbent and membranes using Graphene oxide (GO) for removing lead ions from aqueous solutions
Creators: name: Ravishankar, Harish
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400409
subject: Separation technologies
classificationCode: 400410
subject: Wastewater treatment processes
classificationCode: 401103
subject: Global and planetary environmental engineering

description: Since the industrial revolution, there have been profound changes in agriculture, mining and manufacturing sectors, which have in turn laid a tremendous burden on the available water resources. In developing countries, almost 70% of industrial wastes are dumped into the main water streams thereby increasing the levels of toxic elements, such as heavy metals. The continuing growth in industrialization and urbanization has necessitated the re-use of water from different sources to fulfil the demands of the future.<br><br>Lead is a heavy metal which has been extensively used for various application and often ends up in waste streams. Remediation of lead from wastewater is done through different techniques which include precipitation, adsorption, ion-exchange, membrane processes and biological treatment. Recently, carbon based nano-materials such as graphite/graphene-oxide, fullerene and carbon nanotubes have been reported as effective sorbents for heavy metals due to their high surface area and specificity. Despite such properties, drawbacks of nano-material aggregation and separation of heavy metals from nano-materials, hinders their widespread application. Membrane filtration processes can also be effective in removal of heavy metals. However, long term operation can result in membrane fouling and thus requiring frequent cleaning or replacement of membranes.<br><br>Membrane bioreactor (MBR) is a treatment technology which couples both biological treatment and membrane separation processes for the treatment of wastewaters. MBRs are actively employed for municipal and industrial wastewater treatment around the world. The major shortcoming of the MBR system is membrane bio-fouling, which becomes its foremost obstacle for its successful operation. Very limited studies are available on heavy metal treatment using MBR systems.<br>The present study investigated the application of Graphene-oxide (GO) for treatment of lead through three treatment technologies, namely, adsorption, membrane separation process and MBR.<br><br>In the current study, a graphene oxide (GO) based magnetic nano-sorbent was synthesised. This was done by assembling the GO nano-sheets on top of Fe3O4 (magnetite) nanoparticles which were coated on the surface of polystyrene nano-beads (denoted as PS@Fe3O4@GO). Application of external magnetic field was demonstrated to separate the lead ion attached nano-sorbent from the solution, which would overcome the issues of aggregation and separation after heavy metal adsorption. Preliminary lead removal studies were performed and upon successful results, parameters such as pH, temperature and contact time were optimised for effective removal. The kinetics and isotherm for lead adsorption were studied and found to follow pseudo first order kinetics and Langmuir adsorption isotherm model. A maximum lead removal of 94% was demonstrated and the mechanism of adsorption was attributed to electrostatic attraction followed by chelation of lead ions onto the carboxyl and hydroxyl groups of GO. This confirmed the role of GO in removal of lead ions. The re-usability of the nano-sorbent was also investigated and it was found to be stable for four consecutive adsorption-desorption cycles.<br><br>Membrane fouling/bio-fouling are caused due to the hydrophobic nature of the membrane. Studies have proven that improving the hydrophilicity of membranes can improve the anti-fouling properties. In the present study, GO was explored as a hydrophilic additive by preparing GO blended Polysulfone (PSf) microfiltration (MF) and ultrafiltration (UF) membranes through non-solvent induced phase separation process. UF membranes were studied for lead removal using a cross flow filtration setup. MF membranes were studied for their application in an Anaerobic-Anoxic-Oxic (A2O) MBR system for removal of lead ions and other nutrients.<br><br>The effect of different concentrations of GO and PSf on surface morphology, hydrophilicity and mechanical properties were investigated. The porosity and pure water flux were also examined. It was observed that increase in concentration of GO improved the hydrophilicity, porosity and flux of the membranes. The mechanical property i.e., Young’s modulus increased with polymer concentration. Scanning Electron Microscopy (SEM) analysis revealed the presence of circular pores and asymmetric structure of membranes with bigger macro voids for membranes having GO. The pore size for MF and UF membranes were in the range of 179-250 nm and 18-24 nm, respectively. The UF membrane comprised of 15 % PSf - 1% GO under cross flow showed a maximum Pb rejection of 98%. The operating pressure played a role in rejection, which decreased with increase in pressure.<br><br>The MBR system with virgin PSf and GO blended PSf microfiltration membranes was operated and monitored for treatment of nutrients and lead. The system demonstrated complete removal of COD and ammonia. Phosphorus removal reached a maximum of 52%. Lead removal reached a maximum of 98% with majority of it accumulated in the anaerobic tank due to high MLSS (~5 g/L). GO blended PSf membranes were examined for their ability to mitigate fouling (under two operational modes, i.e., intermittent with backwash and continuous) through critical flux study. Membranes with higher GO concentrations showed increased critical flux (Jc), signifying the antifouling property of the membranes. The long term trans-membrane pressure (TMP) profile of the GO blended membrane lasted longer than the membrane devoid of GO, indicating a longer operation time and needing less frequent cleaning. SEM-EDS results on the fouled membranes showed the presence of Pb peaks indicating the association of lead ions with foulants present on the membrane. The permeate water obtained had a turbidity 0.25 NTU, indicating a good filtration performance by both the MF membranes.<br><br>The results indicate the promising role of GO in lead ion removal through nano-sorbent and UF membrane systems. In the MBR system, GO based MF membranes demonstrated fouling mitigation and longer operation for the treatment of lead. However, the feasibility of the treatment systems should be assessed with real wastewater to justify their commercial application.