In this study, Citrus sinensis (C. sinensis) solid waste was used to catalyze the conversion of free cyanide (F-CN) under alkaline conditions; conditions which represent most industrial wastewater containing F-CN. Acid hydrolysis of the solid waste increased the catalytic conversion of F-CN by 3.86 compared to the unhydrolysed solid waste. The conversion of F-CN using unhydrolysed and hydrolysed solid waste increased linearly with an increase in pH and temperature. The maximum catalytic conversion of a 100 mg F-CN/L solution containing 0.1% (w/v) of unhydrolysed and hydrolysed C. sinensis solid waste was 17.82% and 62.48%, respectively, at a pH of 12 and a temperature of 50 °C. The catalytic process was largely dependent on the availability of activated hydroxyl groups in the solid waste. As most wastewater contains heavy metals, it was determined that the presence of metallic species (Ni, Zn, and Cu) reduced the conversion of F-CN as the metallic ions attached to the hydroxyl groups. The observed reduction was 26.35% when 10 mg/L of heavy metals were present in the F-CN solution containing the hydrolysed solid waste at a pH of 12 and 40 °C.

No abstract

Generation of cyanide-containing wastewater is a growing problem worldwide as numerous cyanide complexes are highly unstable and degrade to form free cyanide (F-CN), the most toxic form of cyanide. Agro-waste materials, such as sweet orange (Citrus sinensis) waste from the citrus industry, are rich in readily metabolisable carbohydrates that can supplement microbial activity and thus support biodegradation of toxic compounds in wastewater. This study reports on optimal operating conditions for the continuous biodegradation of F-CN in wastewater using an Aspergillus awamori isolate in a process supported solely using C. sinensis waste extract. The optimal degradation conditions were pH 8.75 and 37.02 °C with the isolate's F-CN tolerance being observed up to 430 mg F-CN/L. Furthermore, the ammonium produced as a by-product of F-CN degradation was also metabolised by the A. awamori, with negligible residual citric acid and formate being observed in the effluent post treatment. This study demonstrates the feasibility of using agricultural waste as a primary and sole carbon source for the cultivation of a cyanide-degrading A. awamori species for F-CN degradation under alkaline conditions.