Oil spill bioremediation on land

A spill on land differs from one at sea in transport rather than chemistry. Oil stays bound to the matrix, oxygen is not delivered by currents, and the washout and evaporation that strip up to half the light fraction at sea act far more weakly. Everything else follows from the composition of the crude itself.

The order in which fractions disappear

Crude is a mixture of thousands of compounds with different enzymatic fates, and they do not fall together. Straight-chain alkanes go first: alkane monooxygenase oxidises a terminal carbon to the alcohol, then the aldehyde, then a fatty acid entering ordinary beta-oxidation. Branched isoprenoids — pristane and phytane — go markedly slower, because a methyl group on the beta carbon blocks the standard cycle and forces a bypass. Cycloalkanes are slower again. Aromatics are opened by ring-hydroxylating dioxygenases, with rates falling as ring number rises. Resins and asphaltenes — high-molecular-weight polyaromatic structures carrying heteroatoms — have no known oxidative route at all.

That ordering explains the field’s central observation. Normalising the residue against a conserved internal standard — usually 17α(H),21β(H)-hopane, a pentacyclic triterpane resistant to biodegradation — separates genuine biodegradation from evaporation and washout. Hopane-normalised field curves all have the same shape: a fast fall over the first weeks, then a plateau. The plateau is not slow kinetics. It is composition: what remains is the fraction with no enzyme for it, plus whatever is physically occluded in aggregate nanopores and sorbed to soil organic matter.

Oxygen and water occupy the same pore

Complete oxidation of hydrocarbon demands on the order of three grams of oxygen per gram of hydrocarbon, a requirement soil cannot meet in dissolved form. Oxygen diffuses roughly four orders of magnitude faster through an air-filled pore than a water-filled one, so air-filled porosity — not microbial numbers — is usually the rate-determining variable. That is what landfarming and biopile turning are for: tillage returns air to the pore space. It also conflicts with moisture, since activity ceases below a water potential of about −1.5 MPa, leaving a narrow optimum.

The second shortage is nitrogen and phosphorus. A spill adds a large excess of carbon, and biomass stoichiometry immediately makes nitrogen limiting; fertiliser is dosed toward a carbon-to-nitrogen-to-phosphorus target in the region of 100:10:1. Overdosing produces ammonium and salinity that are toxic to the same population.

What counts as clean

A fresh spill first suppresses its own degraders: light aromatics at high concentration act as membrane solvents, and the lag lasts until they volatilise. At the other end, hydrophobicity becomes the barrier itself: compounds with octanol–water partition coefficients around five to six log units and above barely enter the aqueous phase, so a cell must contact the oil–water interface directly or secrete a biosurfactant.

Finally, the regulatory endpoint is usually total petroleum hydrocarbons, and that is a method-defined quantity: the chromatographic window covers a carbon range and does not see the asphaltene residue on equal terms. Toxicity tracks it poorly, and a bioassay on the residue says more about the site than the number achieved.