Role of soil biofilms in the transport processes of emerging organic contaminants: A laboratory experimental study

Over the last two decades there has been a growing interest in the occurrence and fate of emerging organic contaminants (EOCs) in groundwater globally. Managed aquifer recharge (MAR) has been recognized as one of the possible causes of pollution, because of the use of insufficiently or partially treated waters as infiltration source.

Biofilms are one of the most widely distributed modes of life on Earth, and they drive biogeochemical cycling processes of most elements in water, soil, sediment and subsurface environments. Biofilms are aggregates of microorganisms in which cells are frequently embedded in a self-produced matrix of extracellular polymeric substances (EPS). Biofilms partially cover the underlying material, changing the sorption behavior of the soil surface and giving a proper environment to microorganisms for the degradation of EOCs.

To understand the role of biofilms in the transport processes of EOCs, we performed batches studies and a set of three saturated column experiments where biofilms were established. Two of the columns were filled with a soil having a high organic content (0.6% organic carbon) and the third one contained the same soil but muffled, to characterize the role of organic matter in the growth of biofilms. The feed water consisted of ten-fold diluted synthetic wastewater (SWW) without EOCs, and in the case of one of the organic columns, sodium azide was spiked to have an abiotic control. The columns were equipped with automated sensors (high resolution oxidation-reduction potential, water pressure and soil pH) to link these variables to biofilm development. Several tracer experiments were run during the duration of the experiments and analysis of major ions, organic carbon and trace elements were performed as well. After hydrogeochemical equilibrium was reached in each column, inflow SWW was spiked with a cocktail of five EOCs in environmental concentrations (µg/L), covering different hydrophobicity, speciation and biodegradability parameters: Carbamazepine, Metoprolol, BP3, Ibuprofen and Diclofenac.  Breakthrough curves of the EOCs were measured, and double porosity models were fitted to compare retardation factors and rates of degradation of each system. Post experiments analysis allowed the determination of the biofilms on each column by means of EPS extraction and quantification. Differences in the transport behavior of most of the compounds were observed between the columns, concluding that biofilms and biological structures can be an important factor in the transport of EOCs in soils.