Preparation of Bio char-Based Nano composites For Removal of Methylene Blue and Hexavalent Chromium from Water

Abstract

Currently, due to urbanization and rapid development, environmental pollution through industrial effluents and unattended disposal of anthropogenic wastes causes health problems and harms the ecosystem in general. Industrial effluents from tanneries, textiles, breweries, metallurgy, dyeing factories, and agro-processing are polluting environmental waters with heavy metals and synthetic dyes. One of the common organic dyes used for coloring paper, cotton, leather, and plastics is methylene blue (MB). However, consuming high levels of MB can result in various health problems, such as respiratory and gastrointestinal issues, eye irritation, and tissue necrosis. Similarly, heavy metals like arsenic, cadmium, and chromium can be harmful to the kidney, liver, brain, and may even cause cancer. Chromium exists in nature mainly in two forms: Cr(III) and Cr(VI), with Cr(VI) being significantly more hazardous than Cr(III). The main objective of this study was to investigate the effectiveness of manganese oxide coffee husk (CH) and khat leftover (KL) biochar nanocomposites and magnetic CH and KL biochar nanocomposites for the removal of MB and Cr(VI) from water. Each biomass, CH and KL, was pyrolyzed at 300 °C for 1 h to produce the pristine biochars. Similarly, 25 g of each biomass was treated with 12.5 mmol of KMnO4 before being pyrolyzed for 1 h at 300 °C to produce the manganese oxide CH biochar nanocomposite (MnOx-CHBNC) and manganese oxide KL biochar nanocomposite (MnOx-KLBNC). They were characterized by XRD, SEM, FTIR, and BET. The results showed that the adsorbents are amorphous, and the activated biochars are more porous and contain various functional groups such as C-O, C=C, O-H, C-H, and R-MnOx. Additionally, the batch adsorption experiments revealed that MnOx-CHBNC and MnOx KLBNC have higher efficiency in removing MB and Cr(VI) compared to their pristine biochars. Under the optimized conditions, MnOx-CHBNC exhibited a MB removal efficiency of 99.27% with a maximum adsorption capacity (qm) of 39.52 mg g-1. Similarly, MnOx-KLBNC showed a MB removal efficiency of 98.28% with a qm of 43.47 mg g-1. The Cr(VI) removal efficiency of MnOx-CHBNC was 99.63% with a qm of 72.99 mg g-1, while that of MnOx-KLBNC was 99.84% with a qm of 109.84 mg g-1. Similarly, magnetic CH biochar nanocomposites (Fe3O4-CHBNC) and magnetic KL biochar nanocomposites (Fe3O4-KLBNC) were synthesized by pretreating 25 g of biomass with a 12.5 mmol mixture of FeS and FeCl3 at a 1:1 molar ratio, followed by iii pyrolyzing at 300 °C for 1 h. The resulting products were characterized using XRD, FTIR, SEM, and BET. The results showed that the adsorbents are amorphous, and the magnetic biochar nanocomposites are more porous and contain various functional groups such as C-O, C=C, O-H, C-H, and Fe-O. The resulting nanocomposites exhibited a maximum MB removal efficiency of 99.10% and 99.23% with Fe3O4-CHBNC and Fe3O4-KLBNC, respectively, with qm of 51.02 and 78.13 mg g-1. Additionally, the Cr(VI) removal efficiencies were determined to be 99.83% for Fe3O4-CHBNC and 99.86% for Fe3O4-KLBNC, with their Cr(VI) qm values of 109.89 and 151.52 mg g-1, respectively. The study on the regeneration of the adsorbents found that, except for MnOx-CHBNC in the MB removal test, all adsorbents were effective for the removal of MB and Cr(VI) over six successive cycles. However, MnOx-CHBNC can effectively be used for three successive MB removals from water. Therefore, MnOx-CHBNC, MnOx-KLBNC, Fe3O4 CHBNC, and Fe3O4-KLBNC can serve as alternative adsorbents for the removal of MB and Cr(VI) from water.