Mechanistic assessment of functionalised biochar derived from customised pyrolyzer for nitrate removal

• Mustard husk char showed a maximum nitrate adsorption capacity of 27 mg/g. • 42 % of desorption efficiency was achieved by varying solution (KCl) pH. • The effects of different operating parameters on nitrate removal have been analysed. • An adsorption mechanism is proposed, and experimental data is statistically analysed by ANOVA. • Surface area reduced from 13.60 to 9.60 m 2 /g on chemical modification due to pores blockage. Nitrate contamination in water is a global concern, necessitating efficient and cost-effective removal techniques. This study investigates the potential of mustard husk biochar (MHB), produced through pyrolysis at 500 °C in a customised auger system, for nitrate adsorption. To enhance its adsorption capacity, MHB was chemically modified with zinc chloride in different weight ratios (1:1, 2:1, and 3:1), with MMHB-2 (2:1) exhibiting optimal performance. Characterisation of biochar using FTIR, SEM-EDS, and BET revealed that functionalized biochar exhibited improved surface properties conducive to nitrate adsorption. The adsorption mechanism was analysed through batch experiments under varying conditions, including nitrate concentration (150–1000 mg/L), temperature (35–55 °C), and pH (2–13), with an optimum biochar dose of 1 g/L at neutral pH (7). Adsorption kinetics followed a pseudo-second-order model, confirming chemisorption, while equilibrium studies indicated that adsorption best fit the Langmuir-Freundlich isotherm. Further, thermodynamic analysis (ΔG° = -7.37 kJ/mol, ΔH° = -22.50 kJ/mol) suggested an exothermic and spontaneous process. The regeneration studies revealed that 42 % of nitrate could be desorbed at high pH, confirming the biochar reusability. The study demonstrates that MMHB-2 can serve as a low-cost and effective adsorbent for nitrate removal, addressing water contamination and biomass waste management. Moreover, the spent biochar, enriched with nitrates, presents a potential application as a soil amendment, promoting sustainable waste-to-resource conversion.