Use of High-Pressure CO2 for Concentrating CrVI from Electroplating Wastewater by Mg-Al Layered Double Hydroxide

The desorption of Cr-VI from Cr-VI-adsorbed layered double hydroxide (Cr-VI-LDH) and the recycling of LDH adsorbent are the bottlenecks that limit the practical application of LDH in treating Cr-VI-containing industrial wastewater. Given the strong affinity of LDH for CO2, we studied desorption and enrichment of Cr-VI from Cr-VI-LDH as well as recycling of LDH in the presence of high-pressure CO2. Results showed that Cr-VI solution with concentration of 500 mg/L could be enriched more than 20 times in each adsorption-desorption cycle. The regenerated LDH maintained the layer structure and the sheets as revealed by XRD and TEM patterns. FT-IR data showed CO2 formed HCO3- at high pressure. The transformation from CO2 to HCO3- followed by the anion-exchange with CrO42- was the critical factor for Cr-VI desorption and LDH regeneration. A pilot-scale experiment was carried out with 20 L Cr-VI-containing electroplating wastewater. The concentration of the desorbed Cr-VI solution could reach up to 10000 mg/L, which could be used in electroplating after appropriate adjustment. The main advantages of this method are high concentration of Cr-VI, direct reuse of enriched Cr-VI, and efficient regeneration of LDH adsorbent. This method showed promises in recycling Cr-VI and regenerating LDH in treating industrial wastewater.