Optimized rice husk‐based biochar by response surface methodology for removal of Cu( <scp>II</scp> ) and Mn( <scp>II</scp> ) ions from aqueous solutions

Abstract Rice husk (RH), an abundant agricultural byproduct in rice‐producing countries, has emerged as a renewable and cost‐effective bioadsorbent for pollutant removal. This study developed an optimized approach to synthesize rice husk‐based biochar (RHB) for the adsorption of Cu(II) and Mn(II) ions in aqueous solutions. The obtained adsorbent was characterized using SEM‐EDS and Brunauer–Emmett–Teller (BET) analysis. The effects of three independent variables—RH particle size ( X 1 ), pyrolysis temperature ( X 2 ), and pyrolysis time ( X 3 )—on the physicochemical properties of RHB were indirectly studied through the adsorption capacity of Cu(II) and Mn(II) using response surface methodology (RSM) combined with central composite design (CCD). The optimum synthesis conditions were achieved at X 1 = 0.88 mm, X 2 = 444°C, and X 3 = 140 min, resulting in the highest removal efficiencies for Cu(II) of 73.1% and Mn(II) of 61.3%. The large values of R 2 (≥0.9792), R 2 adjusted (≥0.9605), R 2 predicted (≥0.9035), and small probability values ( p ‐value &lt; 0.0001) demonstrated a strong agreement between the experimental data and the quadratic model predictions. The main effects ranked were pyrolysis temperature ( p ‐value &lt; 0.0001), and pyrolysis time ( p ‐value ≤ 0.0001), followed by RH particle size ( p ‐value ≤ 0.0525).