Removal of hexavalent chromium from contaminated waters by ultrasound-assisted aqueous solution ball milling
Graphical abstract
Introduction
Chromium has been widely used in multiple industries and in agriculture. Among the extensive historical applications, metallurgy, electroplating, leather tanning, fungicides and corrosion inhibitors have been most common. Cr(VI) is generally soluble over a wide pH range and has been shown to exert toxic and carcinogenic effects on humans and experimental animals (Costa, 1997, Flury et al., 2009, Zazo et al., 2008). Leakage, poor storage practices and improper disposal of waste related to chromium uses have released Cr(VI) into the environment, causing contamination of groundwater and surface water (Flury et al., 2009). Therefore, Cr(VI) must be removed from wastewaters before their disposal to natural aquatic environments. Among chromium oxidation states ranging from (− IV) to (+ VI), only the (+ III) and (+ VI) states are stable in the natural environment (Pechova and Pavlata, 2007). Cr(III) is considered to be an essential nutrient for the human body, and the toxicity of Cr(III) is 500–1000 times smaller than Cr(VI) for living cells.
During recent years, various methods have been employed for removing Cr(VI) from contaminated water (Dima et al., 2015, Janoš et al., 2009, Liang et al., 2014, Shirzad-Siboni et al., 2014, Ritter et al., 2002, Lai and Lo, 2008), such as adsorption (Hu et al., 2005, Hu et al., 2007a, Hu et al., 2007b, Shen et al., 2009a, Shen et al., 2009b, Park et al., 2006), separation with reverse-osmosis (Ozaki et al., 2002), and reduction by zero-valent iron (Liu et al., 2009, Lee et al., 2010), in which reducing Cr(VI) to Cr(III) is considered to be a satisfactory solution to eliminate the toxicity of Cr(VI).
Using zero-valent iron for in situ reduction of Cr(VI) has become increasingly popular (Gheju and Iovi, 2006, Chen et al., 2008). The conventional process using zero-valent iron powder in aqueous solution has two disadvantages: low removal efficiency and large iron loss. The reduction rate of Cr(VI) is improved when the pH value is lowered, but the adjustment of pH adds to the complexity of the process (Gheju and Balcu, 2011, Chen et al., 2007). Although nano zero-valent iron powder has high efficiency, it presents some shortcomings, such as tendency to agglomerate, difficulty of preservation, and complex preparation process (Ponder et al., 2000, Geng et al., 2009).
This study investigated the use of ultrasound-assisted aqueous solution ball milling, which is a combination of ultrasound and ball milling techniques. This process cannot only provide fresh surfaces on the iron milling balls by impaction–abrasion, but also generate radicals and H+ ions to participate in the Cr(VI) reduction reactions. Therefore, ultrasound-assisted aqueous solution ball milling can remove Cr(VI) from high concentration contaminated waters in a short time, and form a stable precipitate. Reaction with zero-valent iron is the main principle of the removal of Cr(VI) by ultrasound-assisted aqueous solution ball milling. If other toxic substances can be removed by zero-valent iron, they can be removed by ultrasound-assisted aqueous solution ball milling, too. So the continuous production of new zero-valent iron will make ultrasound-assisted aqueous solution ball milling applicable to not only removal of Cr(VI), but also removal of other toxic substances, such as Pb(II) (Ponder et al., 2000), arsenic(III) (Liu et al., 2016), Methyl Orange (Yuan et al., 2016), and dinitrotoluenes (Patapas et al., 2007). Besides, the iron balls can be reused in the experiment, which can lower iron loss. The resulting precipitate can be removed from the contaminated water, and the treated water can be used again. High efficiency, no adjustment of pH and simple operation process make this method green and practicable.
Section snippets
Devices and materials
A special stainless steel milling pot 165 mm in diameter and 158 mm in height was designed to remove the Cr(VI) by ultrasound-assisted aqueous solution ball milling. The details of this device are shown in Fig. 1. The Cr(VI) solution was added to the milling pot, and air was introduced into the solution for 3–5 min. Then iron milling balls (Φ = 1–1.5 mm) with a total weight of 2 kg were added into the milling pot. Moreover, the stirring rod was adjusted to a height of 5 mm from the bottom of the pot
Cr(VI) concentration and Cr(total) concentration
The Cr(VI) concentration determined in experiment I is shown in Fig. 2a. The results show that the reduction rate of Cr(VI) by ultrasound-assisted aqueous solution ball milling was obviously faster than that by ball milling or ultrasound treatment separately. During the first 30 min, the reduction rate of Cr(VI) was fast in all cases. This is due to the fact that the initial surface of the iron balls, which provided zero-valent iron, reacted with Cr(VI). In addition, there is an initial phase in
Conclusions
Based on the experiments and analyses given above, the following conclusions can be made: Cr(VI) can be removed by ultrasound-assisted aqueous solution ball milling, aqueous solution ball milling, and ultrasonic processing with iron balls, and the Cr(VI) reduction rate for the three methods followed the order: ultrasound-assisted aqueous solution ball milling > ball milling > ultrasound treatment. (2) Ultrasound-assisted aqueous solution ball milling can transform Cr(VI) into Cr(OH)3 and FeCr2O4 in
Acknowledgments
This work was supported by the Hunan Provincial Natural Science Foundation of China (No. 14JJ1013).
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