Abstract
In this work, it was aimed to remove the highly toxic rhodamine B (RhB) dye from wastewater by using a low-cost, renewable, sustainable, and completely natural adsorbent obtained from St. John’s wort ( Hypericum perforatum ) (HP), which is considered an agro-waste. HP was characterized by FTIR-ATR, XRD, FESEM, and pH pzc . Adsorption experiments were conducted by optimizing pH (2–12), dose (0.01–0.30 g), time (0–120 min), and concentration (10–50 mg/L). Equilibrium data were applied to linear isotherms. The results stated that the removal process occurred in a monolayer on the homogeneous surface of HP and the $${q}_{m}$$ q m value was determined as 196.08 mg/g at 298 K. Kinetic results showed that RhB adsorption on HP followed a quasi-second-order. The effect of salt concentration onto RhB adsorption on HP adsorbent and reusability studies were conducted. Thermodynamic tests showed that the process was exothermic ( $${\varDelta H}_{A}^{o}=-13.58 kJ/mol$$ Δ H A o = - 13.58 k J / m o l ) and spontaneous ( $${\varDelta G}_{A}^{o}=-26.61 kJ/mol$$ Δ G A o = - 26.61 k J / m o l ). The results of the study indicated that HP may be a favorable adsorbent for basic dyes in wastewater. Graphical abstract