Olive husks biochar application in microbial remediation of pyrene polluted soil: a possible win-win solution

Human activities are negatively affecting ecosystems through the erosion and impoverishment of natural resources. Considering soil, global warming and unsustainable agricultural practices are reducing soil organic matter with consequent loss of fertility. An issue of major concern is also the soil pollution by organic and inorganic compounds affecting soil ability to generate ecosystem services. Polycyclic aromatic hydrocarbons are a large group of pollutants, made of two or more aromatic rings, widespread in the environment, soil included, and extremely toxicity for human and environmental health. Polycyclic aromatic hydrocarbons persistence in soil is an issue great concern that could be effectively faced by microbial remediation techniques.

In this work we studied the remediation of pyrene polluted soil through two parallel tests: bioremediation and biostimulatin techniques. The first involves inoculation into soil of exogenous microorganisms, in our case Trichoderma harzianum, whereas the latter was aimed at promoting the degrading ability of endogenous microorganisms. Moreover, in order to sustain microorganisms growth, we incorporated olive husk biochar in the two tested bioremediation systems.

Biochar is currently considered an excellent soil conditioner and its incorporation into soil seems to promote PAHs adsorption and to interact positively with soil microorganisms. Biochar is a carbonaceous matrix produced through thermal processing of organic biomasses at high temperature and at very low oxygen partial pressure. Here we converted olive husks, that is a potentially toxic by-product of olive oil extraction, by slow pyrolysis process with the aim of deprived this waste biomass of its noxious characteristics.

Soil samples were spiked with 50 ppm of pyrene, biochar was incorporated at a concentration of 13 g·kg^–1 d.w. of soil and endogenous microbial growth, T. harzianum growth and microbial pyrene-degradation activity were measured weekly up to 28 days.

Pyrene concentration was reduced of approximately 70% in 28 days in both bioaugmentation and biostimulation remediation systems. T. harzianum did not display a distinctive ability in degrading pyrene and partially inhibited the endogenous soil microflora from degrading pyrene. Olive husks biochar application did not limited pyrene bioavailability or neither affected microbial pyrene degrading activity. However, our results underline that olive husks biochar increases T. harzianum growth and stimulates soil endogenous microorganisms.