Comparative evaluation of ZnCl₂, KOH, and H₃PO₄ activated rice husk–neem leaf adsorbents for phenol adsorption

In the present study, the adsorption capacity of Rice Husk and Neem Leaf Powder (RHNL) activated using three chemical agents, ZnCl₂, KOH and H₃PO₄ with a 1:3-chemical to biomass ratio has been investigated to make biomass-derived activated adsorbents for phenol removal. Adsorption capacity and removal efficiency of phenol were examined from aqueous solutions. The SEM images revealed noticeable morphological differences among the adsorbents. The raw RHNL was compact and inhomogeneous; after ZnCl₂ and KOH activation, the surface became quite porous. In contrast, the H₃PO₄-activated sample exhibited the most developed honeycomb-like surface morphology with visibly open and interconnected surface features. XRD data confirmed that all samples were amorphous, presenting the distinctive broad peak at 2θ of around 20–25° corresponding to (002) Plane in lignocellulosic biomass. Adsorption isotherms were investigated by varying phenol concentration (10–30 mg/L) with a fixed adsorbent dosage (3 g/L). The Langmuir model provides the best fit (R² = 0.9969), which suggests monolayer adsorption. The highest adsorption capacity for activated RHNL was observed for ZnCl₂ (145.86 mg/g) KOH (197.1 mg/g) and H₃PO₄ (284.4 mg/g). A kinetic study exposed that the Pseudo-First-Order (PFO) model was the best fit for H₃PO₄ activation (R² = 0.9869), whereas Pseudo-Second-Order (PSO) kinetics fitted well for KOH activation (R²=0.9668). The excellent properties of H₃PO₄-activated RHNL have great application potential in industrial wastewater treatment, environmental remediation, and pollution control in the circular economy.