Cyclic Dechroming Process for Chrome Shavings by Coordination Substitution Reaction and Photocatalysis

Abstract

The cyclic dechroming process for chrome shavings by method of coordination substitution reaction and photocatalysis was investigated. Using EDTA (ethylene diamine tetraacetic acid) as dechroming agent and NaOH as recovery agent, the process conditions were optimized through the orthogonal tests. The removal efficiency of chrome from chrome shavings was 98.88% when dechroming was undertaken in 0.03 mol/L EDTA solution at 50Ž and pH 6.0 for 14 days with stirring and then treated in the same solution under ultraviolet (145 W, 254 nm) for 8 days, and the loss of collagen was 5.31%. The chrome was separated from the dechroming solution as Cr(OH)3 precipitate by adding NaOH into the dechroming solution (conc. of NaOH = 1.00 mol/L) and stirring at 70Ž for 1 d. The solution containing EDTA was reused for dechroming again after filtering and adjusting pH to 6.0 by sulfuric acid and the removal efficiencies of chrome were all more than 97% in triplicate trials. The structure of collagen after chrome extraction was characterized by SEM and FT-IR. The results indicated that the triple helix structure of collagen was well preserved. The thermal stability of collagen after chrome extraction was characterized by DSC; the results indicating that it was in conformity with rawhide. The dechroming process without strong acid and alkali can prevent collagen from hydrolysis. A certain amount of Cr(VI) appeared by adding alkali in precipitating Cr and recovering EDTA. However, Cr(VI) was reduced to Cr(III) when pH of the recovered EDTA solution was adjusted to 6.0 by adding sulfuric acid. This kind of closed recycling dechroming technology can reuse dechroming agent, and thus avoid the elution of wastewater during dechroming process.