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Response surface methodology is a widely used technique for modelling and optimization of the photocatalytic treatment processes of water and wastewater. This methodology not only estimates linear, interaction and quadratic effects of the factors on the response, but also provides a prediction model for the response at the range of the variables studied and the optimum conditions to achieve the highest performance. The present paper reviews the results of application of this innovative methodology in modelling and optimization of the photocatalytic treatment processes. Different experimental designs including 3k factorial, Doehlert, Box-Behnken and central composite designs have been developed to describe the treatment processes of dyeing effluents, pharmaceutical agents and hazardous phenolic compounds. The results showed that response surface methodology can describe the behaviour of complex reaction systems, such as photocatalytic processes, in the range of experimental conditions adopted. Optimization based on response surface methodology can also estimate the conditions of the photocatalytic processes to achieve the highest performance.

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In this study, laboratory batch experiments were conducted on a chromium (Cr)‐spiked soil to evaluate the effectiveness of synthesized starch‐stabilized iron (Fe0) nanoparticles and compared with Fe0 and Fe3O4 with different particle sizes, and also with decreasing water‐extractable Cr(VI). Comparative studies were carried out at a Cr(VI) concentration of 100 mg kg−1 and a Fe materials dosage of 1.5% w/w. Results indicated that stabilized Fe0 nanoparticles had a greater efficiency (100%) to immobilize Cr(VI). The efficiency of the iron materials that we used for immobilization of Cr(VI) was in the following order: starch‐stabilized Fe0 nanoparticles > non‐stabilized Fe0 nanoparticles > Fe3O4 nanoparticles > Fe0 micro‐particles > Fe3O4 micro‐particles. Several factors affecting the immobilization of Cr(VI) by stabilized Fe0 nanoparticles, including reaction time, initial Cr(VI) concentration in soil, Fe0 nanoparticles dosage and soil‐solution suspension pH, were investigated. The overall rate of the Cr(VI) immobilization process was quick and almost 50% of the immobilization was reached within the first 2 minutes of the reaction. Cr(VI) immobilization percentages decreased from 100 to 54% as the initial Cr(VI) concentration increased from 50 to 1650 mg kg−1. Furthermore, increasing Fe0 nanoparticles dosage from 0.5 to 3% w/w caused a 70% increase in the immobilization efficiency. The results indicated that increasing the soil suspension pH from 5 to 9, in both buffered and unbuffered conditions, did not have any significant effect on the extent of water‐extractable Cr(VI).

The response surface methodology involving the five-level central composite design (CCD) was employed to model and optimize the Cr(VI) immobilization process in a Cr-spiked soil using starch-stabilized zerovalent iron nanoparticles (ZVIn). ZVIn were synthesized via a borohydride reduction method and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). All Cr(VI) immobilization experiments were conducted in a batch system. The variables for the CCD optimization were the ZVIn dosage (% w/w), reaction time (min), and initial Cr(VI) concentration in soil (mg/kg). The predicted response values by the second-order polynomial model were found to be in good agreement with experimental values (R 2 ¼ 0.968 and adj-R 2 ¼ 0.940). The optimization result showed that the Cr(VI) immobilization efficiency presented the maximal result (90.63%) at the following optimal conditions: ZVIn dosage of 1.5% w/w, reaction time of 60 min, and an initial Cr(VI) concentration of 400 mg/kg.