Effects of ferric hydroxide on the anaerobic biodegradation kinetics and toxicity of vegetable oil in freshwater sediments

Biodegradation of vegetable oil in freshwater sediments exhibits self-inhibitory characteristics when it occurs under methanogenic conditions but not under iron-reducing conditions. The basis of the protective effect of iron was investigated by comparing its effects on oil biodegradation rate and the toxicity of oil-amended sediments to those of clay and calcium, which reduce the toxicity of oil-derived long-chain fatty acids by adsorption and precipitation, respectively. Kinetic parameters for an integrated mixed-second-order model were estimated by nonlinear regression using cumulative methane production as the response variable and used to compare the effects of the three treatment factors on the rate of oil biodegradation. Ferric hydroxide was the only factor that significantly (P<0.05) increased the rate of methane production from canola oil, whereas calcium significantly reduced the oil biodegradation rate. Measurement of sediment toxicity using the Microtox Solid-Phase Test showed that inhibitory products formed within 5 days of oil addition, but the sediment toxicity decreased over time as the extent of oil mineralization increased. None of the other amendments significantly reduced the toxicity of oil-containing sediments. Since ferric hydroxide stimulated the rate of oil biodegradation without affecting the toxicity of oiled sediments, it must operate through a mechanism that is different from those previously described for clay and calcium.