(127c) Nanoparticle-Impregnated Fibrous Adsorbent: A Novel Approach for Pb(II) Adsorption

Rapid industrialization and urbanization have caused an increasing imbalance in the ecosystem because of the release of untreated or inefficiently treated wastewater streams in the water bodies. Amongst different pollutants, heavy metals like lead Pb(II) released from mining activities, electroplating industries, steel, and explosives manufacturing pose high toxicity. Pb(II) has even been detected in the groundwater and other water bodies with concentrations above its permissible limit for drinking water (10 ppb).

Currently, nanoparticle-based adsorbents have demonstrated promising and superior outcomes due to their high surface area-to-volume ratio and better selectivity towards heavy metals. However, they face challenges during scale-up attempts of the process, as their tiny size makes them susceptible to washing away, and their recovery is often tedious and impractical. Hence, the idea of this work is to synthesize a novel adsorbent by impregnation of nanoparticles onto a polymeric fibrous substrate in a manner that controls its leaching. The key advantage of using polymeric fibrous adsorbent is having a lower resistance to liquid flow (lower pressure drop) in fixed bed columns that would further minimize the channelling in contrast to conventional granular counterparts. Hence, the present work aims to apply this approach to the adsorption of Pb(II) from industrial wastewater to mitigate the repercussions caused by the discharge of Pb(II) containing water.

In this work, spherical-shaped hydroxyapatite nanoparticles (SHAP) having high adsorption capacity and selectivity for Pb(II) have been impregnated on nylon-6 fibres by two different in-situ modifications (i) Base treatment of fibres (resulting in SHAP-F), and (ii) KMnO₄ treatment of fibres (resulting in SHAP-M-F). The latter facilitated better loading of SHAP (5.1 wt%) as compared to the former (1 wt%) and subsequently led to MnO₂ nanoparticle formation on the fibre surface. This was confirmed from the SEM images as well as XPS analysis. From the SEM image in the figure, a uniform and dense distribution of nanoparticles on the surface of the fibre is observed.

Further, systematic batch adsorption experiments at varying concentrations of Pb(II) and pH were conducted. By fitting the batch results with isotherm models, the maximum adsorption capacities for SHAP, SHAP-F and SHAP-M-F were calculated to be 288 mg/g, 10 mg/g and 110 mg/g, respectively. These results highlight an enhancement in the maximum adsorption capacity by a factor of 3.63 and 4.73 for SHAP-F and SHAP-M-F, respectively as compared to the pristine SHAP. Additionally, it suggests a synergistic effect between MnO₂ particles and SHAP on the fibre surface, hence proving the potential of SHAP-M-F for Pb(II) removal.

Subsequently, to investigate the efficacy of industrial wastewater treatment, the batch process has been translated to a continuous lab-scale fixed bed process. SHAP-M-F adsorbent demonstrated the capability to treat water initially containing 10000 ppb of Pb(II) to a final concentration of 5 ppb in treated water which is well below the industrial discharge limit of 100 ppb, with a minimal residence time of 1 minute. Furthermore, the release of metal ions was monitored and found to be insignificant and below the discharge limits throughout the continuous operation.

Hence, from the present work, it can be inferred that nanoparticle impregnation on fibres offers a promising solution for leveraging their inherent properties and can effectively remediate Pb(II) from wastewater within a short contact time. This technique, can thus potentially be scaled up for applications towards industrial wastewater.