Sequestration of Reactive Orange 16 Dye by Nitrogen-Impregnated Terminalia arjuna Seed Biochar: Insights into Kinetics, Thermodynamics, Reusability, and Phytotoxicity Assessment

The present investigation deals with feasibility to use available biomass waste, Terminalia arjuna seed as cost-effective sorbent for reactive orange 16 (RO16) dye elimination from wastewater. The present study centers around the modification of Terminalia arjuna seed biochar (TASB) by applying one-step synthesis procedure, by using melamine as a nitrogen doping agent and phosphoric acid (H3PO4) as chemical activator. The effectiveness of the modified nitrogen-doped Terminalia arjuna seed biochar (ND-TASB) as sorbent was assessed on RO16 dye removal from aqueous medium. Surface morphology of synthesized sorbent was identified by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Modified ND-TASB reflected enhancement in adsorption efficacy, at pH of 4, maximum removal of RO16 dye 95% was observed. By utilizing TASB and ND-TASB as an adsorbent, elimination of RO16 dye from aqueous medium was examined under varying experimental conditions like pH, adsorbent amount, and exposure period were reported 1.5 and 1 g at 40, 60 min at pH of 4, respectively with RO 16 dye (200 mg/L concentration). Isotherm and kinetic results were appropriate with Langmuir isotherm and pseudo second-order models. Maximum sorption efficiency of TASB was 2.51 mg g-1, whereas NB-TASB exhibited 4.84 mg g-1, that is, 93% enhancement. Alterations in free energy (Delta G degrees), enthalpy (Delta H degrees), entropy (Delta S degrees) of adsorption process were analyzed to assess nature of adsorption phenomenon. The negative values of Delta G degrees confirms that adsorption phenomenon was spontaneous and feasible. Enthalpy value was -60.39 kJ mol-1 whereas Delta S degrees had 182.72 J K-1 for ND-PKSB. Phytotoxicity study revealed that TASB or ND-TASB treated RO16 dye solution enhanced pearl millet seedlings growth and biochemical variables with better growth with ND-PKSB treated RO16 dye solution. Moreover, the regenerated adsorbents exhibited high efficiency for RO16 removal up to five consecutive cycles. These findings establish that modified ND-TASB acts as durable, multifaceted, and recyclable environmentally benign sorbent with superior properties for its application at industrial scale.