Removal of Cr(VI) by activated carbon derived from Sida cordifolia stems: kinetics, isotherms, thermodynamics, and mechanisms studies.

The Sida cordifolia is an easily available, low-cost, and nontoxic plant that was introduced to produce Sida cordifolia stem activated carbon (SCSAC) using phosphoric acid (H₃PO₄) activating agent and applied for Cr(VI) removal. This SCSAC has a good surface area of 171.072 m²/g and can remove 99% of Cr(VI) in the optimum conditions (pH 3.0, adsorbent dose 2.5 g, 10 mg/L Cr(VI) initial concentration, 180 min contact time, and temperature of 30 °C). Batch study revealed that the Langmuir isotherm model was fitted to the Cr(VI) removal, showing an adsorption capacity of 38.59 mg/g. The thermodynamics investigation suggested a spontaneous adsorption process, and materials can be significantly used up to five consecutive cycles. X-ray photoelectron spectroscopy (XPS) analysis confirmed that three types of interaction were going on between Cr(VI) ions and the SCSAC surface, i.e., electrostatic attraction, reduction, and complexation. Overall, the noxious plants' plant stems activated carbon applied as an effective adsorbent and showed a huge potential for Cr(VI) removal in polluted water.