Surface engineering of ZnCl2-activated N-doped water hyacinth hydrochar: a mechanistic insight for fast and efficient removal of Cr(vi) and crystal violet dye.

In this study, ZnCl2-activated nitrogen-doped water hyacinth (Eichhornia crassipes) hydrochar (Ac-NWHHC) was synthesized from water hyacinth via a two-step hydrothermal method. The purpose was to develop a high-performance activated hydrochar using a flexible and efficient synthesis strategy for the remediation of toxic organic dye (Crystal Violet) and heavy metal Cr(vi). The XPS analysis of the Ac-NWHHC revealed the successful N incorporation, which existed as pyrrolic, pyridinic, and graphitic-N forms in the hydrochar matrix. BET analysis showed that the hydrothermal activation process led to an enhanced porous structure, expanding the material's overall active surface area. The Ac-NWHHC removed approximately 85% of crystal violet (CV) dye and 82% of Cr(vi) using 0.5 g L-1 adsorbent from their synthetic wastewater. The adsorption processes display better resemblance to the Freundlich isotherm, suggesting multilayer adsorption nature on the heterogeneous surface. The maximum adsorption capacity of Ac-NWHHC was found to be 42.63 mg g-1 for crystal violet and 113.12 mg g-1 for Cr(vi) species. Kinetic analysis indicated both adsorption processes followed the pseudo-second-order kinetic model. Both the adsorption processes were found to be thermodynamically spontaneous and of endothermic nature due to the negative Gibb's free energy (ΔG°) changes and positive values of enthalpy changes (ΔH) Therefore, it can be concluded that Ac-NWHHC can demonstrate substantial adsorption for effluent treatment due to its low-cost precursor, facile synthesis method, and excellent adsorption performance.