Natural Attenuation of Nonionic Surfactants Used in Hydraulic Fracturing Fluids: Degradation Rates, Pathways, and Mechanisms

Hydraulic
fracturing fluids are injected into shales to extend
fracture networks that enhance oil and natural gas production from
unconventional reservoirs. Here we evaluated the biodegradability
of three widely used nonionic polyglycol ether surfactants (alkyl
ethoxylates (AEOs), nonylphenol ethoxylates (NPEOs), and polypropylene
glycols (PPGs)) that function as weatherizers, emulsifiers, wetting
agents, and corrosion inhibitors in injected fluids. Under anaerobic
conditions, we observed complete removal of AEOs and NPEOs from solution
within 3 weeks regardless of whether surfactants were part of a chemical
mixture or amended as individual additives. Microbial enzymatic chain
shortening was responsible for a shift in ethoxymer molecular weight
distributions and the accumulation of the metabolite acetate. PPGs
bioattenuated the slowest, producing sizable concentrations of acetone,
an isomer of propionaldehyde. Surfactant chain shortening was coupled
to an increased abundance of the diol dehydratase gene cluster (pduCDE) in Firmicutes metagenomes predicted
from the 16S rRNA gene. The pduCDE enzymes are responsible
for cleaving ethoxylate chain units into aldehydes before their fermentation
into alcohols and carboxylic acids. These data provide new mechanistic
insight into the environmental fate of hydraulic fracturing surfactants
after accidental release through chain shortening and biotransformation,
emphasizing the importance of compound structure disclosure for predicting
biodegradation products.