2.2. Experimental design
To optimize and analyze the effects of solution pH, ultrasonic reaction time, and dosage of zero-valent iron (per 200 mL of uranium plant wastewater) on uranium removal efficiency, a statistically designed experiment with minimal experimental runs is greatly desired. To date, many researchers have tried to enhance the uranium removal efficiency from uranium plant wastewater and underground water through a statistical approach [8] and [9]. Statistical approaches such as the RSM are successful in calculating the complex interaction between the independent process factors [10]. Statistical approaches such as RSM consume minimal resources and time, compared to conventional experimental work, and provide information-rich data and analysis with minimal experimental runs [11].
The RSM is an effective statistical technique for developing, improving, and optimizing complex processes [12] and [13]. The RSM is a collection of statistical and mathematical techniques that are useful for analyzing the effects of several independent variables on a response [14]. This process usually employs a low-order polynomial equation in a predetermined region of the independent variables, which is later analyzed to locate the optimum values of the independent variables for the best response [15]. The RSM defines the effect of the independent variables—alone or in combination—in the processes. In addition to analyzing the effects of the independent variables, this experimental methodology also generates mathematical models [14].
Based on the regulations for radiation protection for uranium processing and fuel fabrication facilities (EJ1056-2005) [16], a uranium concentration of 50 μg/L is acceptable for discharge instead of the 300 μg/L mentioned in the Integrated Wastewater Discharge Standard (GB8798-1996) [17]. Therefore, a novel technology with high uranium removal efficiency is in demand. To date, uranium removal from uranium plant wastewater using zero-valent iron and ultrasonic vibration has not been reported. To obtain high uranium removal efficiency to meet the new national policy of a uranium concentration of 50 μg/L, the current study aimed to illustrate the interaction between the operating conditions of uranium removal using zero-valent iron media by ultrasonic method and uranium removal efficiency in diagnostic analysis using the central composite design (CCD) of the RSM. The final regression models obtained from the CCD may predict the highest uranium removal efficiency operating parameters in the uranium removal process from uranium plant wastewater using zero-valent iron media by the ultrasonic method.