Hydrogen generation and simultaneous removal of Cr(VI) by hydrolysis of NaBH₄ using Fe-Al-Si composite as accelerator

The present study reports a novel method for hydrogen generation and simultaneous removal of Cr(VI) from synthetic wastewater using NaBH₄ as the reducing agent and Fe-Al-Si composite as accelerator. The results showed that the hydrogen generation yields of NaBH₄ occurred at low pH, high temperature and stirring speed. The presence of Cr(VI) was found to inhibit hydrogen generation at the initial pH 3.0, especially at low temperature conditions. Increasing temperature resulted in the increase of hydrogen generation, whereas a higher reduction rate of Cr(VI) was obtained at low temperature. Therefore, to alleviate the contradiction between hydrogen generation and Cr(VI) removal with respect to temperature, we introduced reduction accelerator by preparing Fe-Al-Si composite using wasted fly ash as raw materials. The resultant hydrogen generation yield could be enhanced from 32.04 to 80.70%, and total Cr removal was increased from 46.72 to 98.96% at 30 °C. First, H⁺ produced by hydrolysis of Fe-Al-Si composite improves the hydrolysis ability of NaBH₄, thus promoting its ability to reduce Cr(VI) and to produce hydrogen by itself. The Cr(VI) reduction and hydrogen generation process are competitive for H⁺, which is particularly evident at insufficient H⁺. Second, at low doses, Fe³⁺ exhibited a lower ability to promote hydrogen generation and simultaneous Cr(VI) removal than Al³⁺. One of the reasons for the low promotion ability of Fe³⁺ in the whole pH range was the formation of Fe⁰ at pH less than 3.0, and the other was the weaker hydrolysis ability of Fe³⁺ itself at pH greater than 3.0.