Valorization of tea industry waste into boric acid-activated carbon for phenol removal from aqueous solutions

The tea industry generates substantial quantities of solid waste with limited high-value industrial applications. In this study, tea industry waste was converted into boric acid-activated carbon (BA-TAC) through a simple one-step chemical activation process and evaluated as an adsorbent for phenol removal from aqueous solutions. BA-TAC exhibited a mixed micro-mesoporous structure with a BET surface area of 592.59 m² g^-1. The adsorption performance was investigated as a function of solution pH, contact time, and initial phenol concentration. Adsorption kinetics were evaluated using pseudo-first order, pseudo-second order, Elovich, Avrami, and intraparticle diffusion models, whereas equilibrium data were analyzed using Langmuir, Freundlich, and Sips isotherms. Linear, pseudo-linear, and non-linear regression approaches were comparatively evaluated, demonstrating the limitations of conventional linearization and the superiority of direct non-linear regression. Moreover, the so-called linearized forms of multi-parameter models such as Sips are not truly linear because iterative parameter estimation remains necessary. The maximum experimental phenol adsorption capacity reached 101.80 mg g^-1 despite the moderate BET surface area of BA-TAC. The experimentally determined natural pH and pHpzc values, together with the limited desorption efficiency, indicated that adsorption was governed predominantly by surface chemical interactions rather than electrostatic attraction alone. XPS analysis confirmed the incorporation of boron-containing surface functionalities, highlighting their contribution to phenol adsorption. Overall, this study demonstrates the successful valorization of tea industry waste into a functional boron-modified activated carbon while providing methodological guidance for reliable adsorption kinetics and equilibrium modeling, thereby supporting sustainable waste valorization.