Halloysite/alginate nanocomposite beads for Pb²⁺ removal from aqueous solution

Lead is widely used to manufacture battery, paint, and other products. Massive amount of wastewater laden with lead ions (Pb²⁺) are inevitably generated during these manufacturing processes with clear repercussions on the ecology and human health. Hence, it is mandatory to remove Pb²⁺ from the wastewater before being discharged into the water bodies. Pb²⁺ in aqueous solution can be removed by adsorption, a method which is relatively low cost and efficient. Recently, halloysite (Hal) nanotubes have been studied as potential nanoadsorbent to remove pollutants due to their distinctive features such as large specific surface area, high aspect ratio and mechanical strength. Nevertheless, the submicron size range of Hal could cause large pressure drop and recollection problem during operation. This research was aimed to study the feasibility of immobilizing Hal nanotubes in alginate beads for the removal of Pb²⁺ from aqueous solution. There have been no dedicated studies conducted on Hal/alginate nanocomposite beads for Pb²⁺ removal except this one. The results showed that the immobilization of Hal nanotubes was purely physical without chemical interactions with the calcium alginate matrices. This means the active sites of both Hal nanotubes and alginate remained available for adsorption. The potential of the Hal/alginate nanocomposite beads for the removal of Pb²⁺ was demonstrated through adsorption equilibrium, kinetics, and mechanism studies. The adsorption follows Langmuir isotherm and pseudo second order kinetics. The highest adsorption capacity of the beads was 325 mg/g at 0.2 g Hal loading. Hal nanotubes and alginate contribute to the uptake of Pb²⁺ through physisorption and ion exchange, respectively. In addition, the mechanical strength of the beads was strengthened with the incorporation of Hal nanotubes and the repeated adsorption of Pb²⁺. The spent beads were then regenerated with acids. The performance of the desorption process was dependent on the type and concentration of the acid used. HNO3 at 0.10 M was the most appropriate eluent with desorption efficiency > 90%. The desorption process was rapid and could be completed within two hours. The adsorption capacity of the beads remained high (i.e. approximately 183 mg/g) even after ten consecutive reuse. Also, the beads became more rigid after each cycle of reuse. Their Young’s modulus increased by 6-fold to 2.4 MPa at the tenth cycle of desorption. The beads were then used to treat industrial wastewater generated by a lead-acid battery manufacturer. The wastewater contains 2.3 mg/L of Pb²⁺, 0.093 mg/L of Cu²⁺, and 0.13 mg/L of Zn²⁺. Pb²⁺ was completely removed along with 83% of Cu²⁺ and 5% of Zn²⁺ from the wastewater at pH 5. Finally, the sustainability of the wastewater treatment process using the beads was assessed using Life Cycle Assessment (LCA). The beads production and the adsorption process were found to be environmental friendly. Reusing the beads was found to reduce the environmental impacts of all categories by 72 to 100% compared to replacing the spent beads with fresh beads. In conclusion, Hal/alginate nanocomposite beads are an effective, durable, and sustainable adsorbent for the removal of Pb²⁺ from wastewater. Further research should involve improving the process productivity by converting the adsorption/regeneration process into a continuous operation.