HEAVY METAL ADSORPTION FROM WASTE WATER USING ACTIVATED CARBON

This study investigates the removal of lead (Pb²⁺) and cadmium (Cd²⁺) ions from synthetic wastewater using activated rice husk carbon (ARHC) and unactivated rice husk carbon (URHC) as low‑cost bio‑adsorbents. The research aims to evaluate the adsorption capacities of both adsorbents, establish optimum operating conditions, and model the adsorption mechanisms using isotherm and kinetic studies. Rice husk obtained from a local mill was washed, dried, carbonized at 350°C for 30 minutes, and sieved to produce URHC. ARHC was prepared through chemical activation using ammonium chloride prior to carbonization. Standard solutions of Pb and Cd (1000 mg/L) were prepared, and batch adsorption experiments were conducted to study the effects of pH (3–11), adsorbent dose (0.5–2.5 g), contact time (10–120 minutes), and metal concentration (20–100 mg/L). Residual metal concentrations were analyzed using an atomic absorption spectrophotometer. Results showed that adsorption capacities varied significantly with pH, adsorbent dosage, and contact time. ARHC demonstrated highest Pb and Cd uptake at pH 3, whereas URHC showed optimum adsorption at pH 7 (Pb) and pH 9 (Cd). Maximum adsorption occurred at the lowest adsorbent dose (0.5 g), and equilibrium was attained within 90–120 minutes. Equilibrium data fitted both Langmuir and Freundlich isotherms; however, the Langmuir model provided the best fit, indicating monolayer adsorption, with high coefficients of determination (Pb: 0.954–0.992; Cd: 0.972–0.975). Kinetic data were best described by the pseudo‑second‑order model for both adsorbents and metal ions, with R² values above 0.995, confirming chemisorption as the rate‑limiting mechanism. The study concludes that ARHC exhibits superior adsorption performance compared to URHC and is highly effective for Pb²⁺ and Cd²⁺ removal under acidic conditions. The findings indicate the potential of rice husk—an abundant agricultural waste—as an economical and sustainable adsorbent for wastewater treatment. It is recommended that ARHC be used for heavy‑metal removal at low pH, and future research should explore pH levels below 3, as well as real industrial wastewater applications. The Langmuir isotherm and pseudo‑second‑order kinetic models are also recommended for further adsorption studies involving rice husk–based adsorbents.