Improved degradation of petroleum contaminants in hydraulic fracturing flowback and produced water using laccase immobilized on functionalized biochar

Large quantities of hydraulic fracturing flowback and produced water (HF-FPW) represent an important environmental issue owing to their inclusion of toxic petroleum contaminants. Enzymatic degradation is a promising approach for treating these pollutants. However, the low stability and poor reusability of free enzymes greatly inhibit their applications. This study investigated the effect of biochar-immobilized laccases on the biodegradation of petroleum contaminants in HF-FPW. Acid functionalization of corn straw biochar increased its surface area and pore volume, allowing it to adsorb additional laccase on the surface. Biochar-immobilized laccase exhibited a high tolerance to a low pH and high temperatures and retained >81.0% of its relative activity at 4 °C for 20 d. In addition, the immobilized laccase displayed reusability after six reaction cycles, retaining 40.9% of its activity. Supplementation with immobilized laccase substantially improved microbial growth and fluorescein diacetate hydrolase activity in HF-FPW, which were up to 25.5 times and 4.9 times higher than those of the free laccase treatment, respectively. The immobilized laccase also elevated the transcription levels of degradation genes such as rub and xylE, and greatly enhanced the biodegradation of n-alkanes (reduction from 112.4 mg/L to 41.9 mg/L) and polycyclic aromatic hydrocarbons (reduction from 2606.2μg/L to 1526.6μg/L). Full-length 16S rRNA gene sequencing revealed that Pseudomonas stutzeri, Pseudomonas balearica, and Stenotrophomonas maltophilia dominated the microbial community and were potential functional populations. This study represents the first attempt of utilizing biochar-immobilized laccase to treat HF-FPW and has important implications for efforts to develop strategies for HF-FPW remediation.