Optimization of photolysis of diclofenac using a response surface methodology

This study investigates the effects of environmental parameters such as UV intensity (X₁, 2.1 ∼ 6.3 mW/cm²), Fe(III) (X₂, 0 ∼ 0.94 mg/L), NO₃⁻ (X₃, 0 ∼ 20 mg/L) and humic acid (X₄, 0 ∼ 30 mg/L) on the removal efficiency of diclofenac (DCF, Y), and optimization using a response surface methodology (RSM) based on Box–Behnken design (BBD). According to analysis of variance and t-test results (p < 0.001), the proposed quadratic BBD model based on a total of 29 experimental runs fitted well to the experimental data. Moreover, the determination coefficient (R² = 0.990) and adjusted determination coefficient (R_a² = 0.981) indicated that this model is adequate with a high goodness-of-fit. Variables of X₁, X₂ and X₃ had significant positive contributions (p < 0.001), while X₄ had significant negative contribution to the DCF removal (p < 0.001). A Pareto analysis showed that X₄ was the most important factor (57.18%) in DCF photolytic removal. The predicted and observed DCF removal were 94.98 and 94.2% under optimal conditions (X₁ = 6.29 mW/cm², X₂ = 0.75 mg/L, X₃ = 15.65 mg/L and X₄ = zero), respectively. The RSM not only gives valuable information on the interactions between these photoreactive species (UV intensity, Fe(III), NO₃⁻, and humic acid) that influence DCF removal, but also identifies the optimal conditions for effective DCF removal in water.