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The conventional disposal practices of rice stalks (RS) such as burning in situ or incorporating in the soil, contribute to climate change and endanger the long-term soil fertility. This study investigated the production of activated carbon (AC) using rice stalks as a sustainable recycling solution. The rice stalks (RS) were carbonized in a tube furnace at 500 °C and heating rate of 10 °C/min for 2 h. The rice stalks-based activated carbon (RSA) properties were analyzed based on Brunauer-Emmett-Teller (BET) technique to measure the surface area and transform infrared spectroscopy (FTIR) measurements to identify the surface functional groups. RSA was also characterized using X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-Ray spectroscopy (SEM-EDX). Batch experiments were conducted to examine the efficiency of RSA in removing phenol, with varying operation parameters comprising pH (2–9), initial phenol concentration (IPC) (50–200 mg/L), reaction time (5–300 min), and RSA dose (0.2–2 g/100 ml). The surface area (SBET) of RSA was 488.26 m2/g with a total micropore volume of 0.165 cm³/g, and a pore diameter of 6.99 nm. The highest phenol uptake of RSA was 80.37 mg/g. The kinetics of phenol adsorption was found to be accurately described by the pseudo-second-order reaction, while the Langmuir model provided a better match for the isotherm process than the Freundlich model. The thermodynamic study indicated that the adsorption process was exothermic and spontaneous. These results confirm that the adsorption process occurred due to physical forces, rather than involving chemical bonding, providing further insight into the underlying mechanisms.

This study investigated the potential use of sewage sludge-based activated carbon in the removal of copper (Cu) and cadmium (Cd) ions from aqueous solutions. The activated carbon (AC) was prepared by chemical activation using potassium hydroxide KOH and heating in a tube furnace at 600 °C for 2 h. The produced activated carbon was further subject to a sulfurization process to enhance its removal efficiency. The study used Box-Behnken Design (BBD) in response surface methodology (RSM) to optimize several parameters, including initial metal concentration, activated carbon dose, pH, and contact time. Results showed that the sulfurization process introduced sulfur functional groups (SO) to the surface of the activated carbon and decreased its specific area from 190 to 173 m2/g. The quadratic model was suggested as the best model to describe the effect of the adsorption parameters on the removal percentage with high R2 values (≥0.9) and p-value less than 0.05. Second-order polynomial equations, analysis of variance (ANOVA), and three-dimensional surface plots were developed to assess the interaction between the parameters and the optimal conditions to remove copper and cadmium ions. The optimum removal parameters for copper ions were 300 min, 100 ppm initial concentration, 20 g/L activated carbon dose, and pH 6, with a removal efficiency of 83.9%. Cadmium removal of 87.5% was achieved at 11 ppm, 300 minutes contact time, pH 6, and 2.5 g/L adsorbent dose. The activated carbon capacity in removing copper and cadmium was 16 mg/g and 17.6 mg/g respectively. The sulfurization process increased the removal percentage for copper and cadmium ions by 14.5 and 18.7% respectively. Advanced analytical techniques such as FITR, SEM-EDX, BET surface area, and elemental analysis were used to characterize the materials.