Effect of biochar amendment on PAH dissipation and indigenous degradation bacteria in contaminated soil

Abstract

Biochar amendment has been extensively investigated as a method to remediate polluted soil. However, both increased and decreased pollutant biodegradation has been observed after biochar amendment is applied to soil. This study aims to demonstrate the effect of biochar amendment on 16 polycyclic aromatic hydrocarbon (PAH) dissipation and indigenous degradation bacteria in PAH-contaminated soil. Four biochars derived from dairy manure and rice hull (RH) produced at pyrolysis temperatures of 350 and 500 °C were used to investigate the effect of biochar amendment on PAH dissipation in sterile and non-sterile soil. Different biochars with a rate of 4.0 % dry weight were mixed with the soil and incubated for 90 days. Sixteen PAHs in the USEPA priority-controlled list were determined by gas chromatography–mass spectrometry. Terminal restriction fragment length polymorphism and quantitative polymerase chain reaction were employed to quantify the change in the number of gene copies of the bacterial community 16S rDNA, PAH-RHDα, and nitrogen cycle genes (nirS). The average removal efficiencies of all two-ring to four-ring PAHs in non-sterilized soil with biochar increased by 35 to 37 % compared with those in sterilized soil. Molecular biology analysis results demonstrated that biochar amendment can enhance the number of 16S rDNA, PAH-RHDα, and nirS gene copies in PAH-contaminated soil. Rice hull-derived biochar pyrolyzed at 350 °C most effectively stimulated dissipation of PAHs in contaminated soil. The removal efficiencies of two-ring to four-ring PAHs were higher than those of five- and six-ring PAHs in actual contaminated soil amended with biochar. Biochar amendment can stimulate PAH-metabolizing bacterial activity by enhancing the number of gene copies related to PAH degradation and by changing the structure of soil microbial community. Further studies should be conducted to evaluate the effect of biochar on PAH biodegradation in different soils at different rates of biochar application.