Adsorption of phenol from water using green tea (Camellia sinensis L.) leaf waste

Phenol is one of the most toxic and carcinogenic organic substances. Nonetheless, phenol is used in many important processes as a precursor for the manufacture of paints, pharmaceutical products, rubber and tyres, insecticides and dyes. Untreated wastewater from industrial effluents was recorded as reaching up to 7000 mg/L, which is far above the permissible limit value of 0.3 mg/L. Low-cost methods are necessary to remove phenol from the environment. This study assessed the usefulness of using normal green tea leaves (GTL), carbon from green tea leaves (GTC), carbonized green tea leaves (GTL) and carbon activated with phosphoric acid (GTAC) for remediation of phenol from water. The adsorbents were characterized using SEM, XRD, FTIR and TGA. The adsorption studies showed that the removal of phenol increased considerably as the initial concentration was increased from 20 to 100 mg/L. The effect of contact time showed that the adsorption had two phases: (i) a rapid uptake and (ii) an equilibrium phase. The highest adsorption capacities were measured at a temperature of 298 K and pH 6, with values of 16.24, 27.02, and 30.58 mg/g recorded for GTL, GTC, and GTAC, respectively. The isotherm and kinetics studies matched the Freundlich and pseudo-second order (PSO) models, respectively. Further kinetics analysis suggests that the IPD model was not the rate-determining step. These data indicate that the removal of phenol occurred on a heterogeneous surface. The values for the Freundlich constant (N) were greater than 1, indicating that adsorption occurred spontaneously. This is also consistent with the thermodynamic parameter △G°, which had negative values, suggesting a spontaneous process. △H° had negative values between -4.074 and -19.26 kJ/mol, indicating that uptake was predominated by physisorption. The values obtained for △S° were positive, between 0.0014 and 0.0543 kJ/mol, indicating that increased disorder occurred during the adsorption of phenol.