1. Introduction
Uranium and its compounds are threats to human health and the ecological balance because of their radioactivity and heavy-metal toxicity [1]. Elevated levels of uranium have been found in agricultural irrigation drainage water and industrial wastewater [2] and [3]. The toxic nature of uranium(VI) ions, even at trace levels, has been a public health problem for many years [4]. Therefore, research on uranium removal from wastewater is important.
Uranium in industrial water is usually found in the environment in the quadrivalent uranium [U(IV)] and hexavalent uranium [U(VI)] forms, which coexist with other metal compounds or oxides. Uranium(IV) could be removed in the form of precipitation because it easily forms stable complex-shaped precipitation. Uranium(VI) usually exists in the form of uranium dioxide (UO22+), which has good solubility and is difficult to remove. Therefore, the removal of uranium from wastewater generally refers to the removal of U(VI) and its compounds.
Zero-valent iron was used as the medium in the ultrasonic field to remove uranium from uranium plant wastewater in this study. Iron is an active metal with strong reducibility. It can reduce a variety of pollutants, including uranium. When there is sufficient zero-valent iron and corrosion (i.e., iron hydroxide) in the system, UO22+ is reduced as quadrivalent U(IV) deposited on the iron surface, which could allow complete removal of uranium.
Acoustic cavitation, thermal effect, and chemistry effect have tremendous positive effects; therefore, many researchers have focused much attention on applying ultrasonic technology [5], [6] and [7]. Studies have indicated that ultrasonic mixing is efficient, timesaving, and economically functional, and it offers many advantages over the classical procedure [6] and [7]. Therefore, an ultrasonic field was employed in the uranium removal in uranium plant wastewater research.
In this paper, the effect of pH, ultrasonic reaction time, and dosage of zero-valent iron rates on uranium removal efficiency were evaluated in an ultrasonic field by using response surface methodology (RSM). In addition, the uranium content of the solution, which was treated by zero-valent iron in an ultrasonic field, was detected by inductively coupled plasma mass spectrometry (ICP-MS). The relative standard deviation is less than 5%, and the detection range is between 10−9 ng/mL and 1 mg/L.