Biodegradation of alkanolamines in batch and packed-bed reactors using free laccase and sol-gel laccase

Abstract

Alkanolamine is commonly used in natural gas processing plant for carbon dioxide removal from natural gas. Alkanolamine may incidentally release and contaminate the surrounding soil and water due to plant operational failure or irresponsible related activities. Thus, the application of laccase for biodegradation of alkanolamine, which has not been reported so far, carried out in batch (shake flask) and continuous (packed-bed) reactors was investigated. Though, biodegradation using laccase may offer many advantages, the free laccase (FL) itself is unstable, cannot be reused and poor of thermal and storage stability. The sol-gel laccase (SGL), i.e. SOLAC04 was therefore synthesized by manipulating triethylamine (TEA) concentration (which was used as a gelating agent), laccase loading (LL), agitation conditions (with or without sonication), and experimental procedures (one-step or two-step) towards obtaining a higher laccase catalytic activity and stability. The SOLAC04 synthesized using two-step procedure, TEA (0.1 mL), laccase loading, LL (5 mg/mL) and without sonication had the highest laccase catalytic activity and stability as compared to other synthesized by SGL samples. This result suggested that the entrapment in silica matrix provided an additional framework for the preservation of an active laccase conformation at higher temperature and long storage duration. The biodegradation of alkanolamines: diethanolamine (DEA), ethanolamine and N-methylethanolamine was carried out in batch reactor using both FL and SGL; and optimized using response surface methodology. The results showed that the biodegradation efficiency (μ) and biodegradation rate of DEA using SOLAC04 was higher than other alkanolamines and observed to be higher compared to FL. The μ of DEA in batch reactor using FL and SOLAC04, respectively reached an optimum value at 50 °C and 40 °C. The μ of DEA increased with increasing dosage (Ds) of FL and SOLAC04. It was also revealed that the SOLAC04 are fairly stable and can be used many times. The μ of DEA remained constant at almost the same level after being reused for 5 times. The biodegradation performance and μ of DEA using FL under optimum pH of 5.8, temperature of 45.71°C, Ds of 37.14 mg, and reaction time of 42.66 minutes were 84.8 % and 0.077 mg-1, while for SGL obtained under the optimum pH of 5, temperature of 41°C, Ds of 34.01 mg, and reaction time of 57.59 minutes were 66 % and 1.11 mg-1, respectively. The μ of DEA in packed-bed reactor using SOLAC04 was optimum at pH 6, 250 mL/h and 500 ppm. The μ of DEA increased with increasing Ds of SOLAC04. These experimental results demonstrated the advantages gained from entrapment of laccase in silica matrix and the biodegradation superiority of the SGL over FL for the removal of alkanolamines. Thus, the potential of laccase especially SGL for biodegradation of the alkanolamine was finally demonstrated.