Composite adsorbent from sugarcane (Saccharum officinarum) bagasse biochar generated from atmospheric pressure microwave plasma pyrolysis process and nano zero valent iron (nZVI) for rapid and highly efficient Cr(VI) adsorption

Sugarcane bagasse, with a 25–28 % lignin content, can be converted into biochar, serving as a promising adsorbent due to its high surface area. Nano-zero-valent iron (nZVI) is known for its strong magnetization and ability to bind heavy metals. In this research, biochar derived from sugarcane bagasse was composited with nZVI at varying ratios (1:1, 2:1, and 3:1) to improve adsorption efficiency for Cr (VI) removal. The composites were synthesized using the biochar from the plasma pyrolysis method, followed by nZVI incorporation. Batch adsorption tests were conducted with different Cr (VI) concentrations, contact times, pH levels, and adsorbent doses to determine the optimum conditions for each ratio. Characterization of the adsorbent included XRD, BET, FTIR, SEM, VSM, and Zeta potential analysis. XRD analysis of sugarcane bagasse and biochar demonstrated crystallinity and particle size improvements post-pyrolysis. BET results showed that sugarcane bagasse biomass had a surface area of 0.061 m²/g, which increased to 87.50 m²/g after conversion to biochar. However, once composited with nZVI, the surface area decreased to 37.44 m²/g (1:1), 49.26 m²/g (2:1), and 62.37 m²/g (3:1). FTIR and SEM analyses revealed the interactions between biochar and nZVI, as well as the binding of Cr (VI) to the composite surfaces. VSM showed a reduction in magnetization after adsorption, confirming the oxidation of nZVI to various iron oxides (e.g., FeO, Fe₂O₃, Fe₃O₄), which are less magnetic. The adsorption tests indicated that the adsorption capacity increased with a higher SBB/nZVI ratio. The biochar alone had an adsorption capacity of 77.82 mg/g. In comparison, the composites achieved 86.47 mg/g (1:1), 95.12 mg/g (2:1), and 112.41 mg/g (3:1). Optimal removal was achieved at an initial Cr (VI) concentration of 175 mg/L, a contact time of 180 minutes, and a pH of 2. The Langmuir isotherm model best described the adsorption behavior, and the adsorption kinetics followed a pseudo-second-order model, indicating chemisorption as the primary mechanism. The study concluded that the composite's adsorption efficiency increased with a higher nZVI ratio, making the 3:1 ratio the most effective for Cr (VI) removal.