Chromium exchanged insoluble straw xanthate (ISX-Cr³⁺) for removal of free cyanide: combined effect of ligand-displacement reaction and sorption

Abstract

The use of chromium exchanged insoluble straw xanthate (ISX-Cr) for cyanide removal from aqueous solutions is reported in this paper. Using a dose of 0.5 g ISX-Cr per 100 mL, about 75–90% of initial CN⁻ concentration is removed from unbuffered solutions during a contact period of 15 min. Further removal of CN⁻ is possible by extending the contact period to 24 hr. Interestingly, when the solutions were buffered at pH 9.2±0.01, the CN⁻ removal rate is considerably reduced, and a contact period of 4 hr is required to achieve 60–75% CN⁻ removal. The process is found to follow typical pseudo first-order reaction kinetics with respect to initial cyanide concentration. The first order rate constant for CN⁻ removal from unbuffered solutions is an order of high magnitude (k=0.2464 min⁻¹) when compared to that in buffered solutions (k=0.0283 min⁻¹). This difference has been attributed to significant lowering of pH (from pH 9.6 to pH 7.0) found only in the case of unbuffered solutions and to the volatilization of HCN that formed predominantly at pH<8.0. On the other hand, the net removal of CN⁻ from the buffered solutions could be assigned to the combined effect of (i) sorption by ISX and (ii) ligand-displacement reaction involving an aqua-hydroxo-chromium complex on the surface of ISX and CN⁻ as a ligand. The contribution from the ligand-displacement reaction, which is worked out to be 20–25% of net CN⁻ removed, appears to manifest in the beginning of the CN⁻ removal process, while sorption prevailed during the complete experimental period. Finally, a representative experiment conducted using bore-well water spiked with 1.0 mg CN⁻/L and 0.5 g ISX-Cr per 100 mL dose suggested that CN⁻ removal efficiency exceeding 90% can be achieved in 24 hr and demonstrated that ISX-Cr³⁺ material can be used to remove cyanide from real water also.

Keywords:

• Chromium exchanged insoluble straw xanthate
• Cyanide
• Ligand-displacement reaction
• Sorption

Acknowledgments

The authors are thankful to Dr. R. N. Singh, Director, NEERI for granting permission to publish this work.