Monitoring of soil enzyme activity changes in a heavy naphtha-contaminated soil under different bioremediation treatments

Oil contamination can be treated by physical, chemical and biological approaches. The first two methods have limitations such as high costs, inefficacy and altering natural ecosystem. Today, biological treatment is a more interesting process to remove petroleum contamination. Bioremediation is a technique in which biological systems such as microorganisms are applied to degrade or transform harmful chemicals. In recent years, employing hydrocarbon degrading bacteria to cleaning a petroleum contaminated soil has become a prevalent, efficient and cost effective technique that converts toxic wastes to non-toxic end products. Soil contamination with oil compounds such as heavy naphtha can threaten soil, environmental and human health. In bioremediation process, soil microorganisms use these hydrocarbons as a carbon source and, while making a microbial biomass, play a role in its decomposition and conversion to carbon dioxide. Furthermore, this type of contamination can affect soil microbial population and its enzyme activity. Soil microbiome, in turn, has effect on this contamination using relevant enzymes. Application and comparison of various bioremediation methods such as biostimulation, bioaugmentation and integrated treatment were the main aims of this study to bio-remove heavy naphtha from contaminated soil. Moreover, mmonitoring of soil enzyme activity changes (including dehydrogenase and lipase) in this condition under different bioremediation treatments was another goal of this research. For this purpose, in a heavy naphtha-contaminated sandy loam, a variety of bioremediation treatments, including biostimulation (including supply of nitrogen and phosphorus elements, addition of manure and Tween 80), bioaugmentation treatment (using a consortium of efficient bacteria) and integrated treatment (including all biostimulation and bioaugmentation treatments together) were tested.

In this study, a sandy loam soil was used. Heavy naphtha was applied at a rate of 7% V/W to soil samples and various bioremediation treatments were performed as mentioned above. This experiment was carried out in a pot scale (containing 3 kg soil) based on split plot factorial design (pollution factor, bioremediation factor and time) with 3 replications, at room temperature for 120 days. During the experiment, the pots were aerated once a week and the soil moisture content was adjusted to 70-80% (W/W) of the soil water holding capacity twice a week. For bioaugmentation of bacterial isolates, a bacterial suspension (108 cfu mL-1) having a consortium of Arthrobacter sp. COD2-3, Stenotrophomonas nitritireducens COD5-6، Stenotrophomonas asidamainiphila COD1-1 with a ratio of 5% V/W were used. In biostimulation treatment, cow manure with 5% W/W was used. In NP treatment, N and P elements from ammonium nitrate and potassium phosphate (K2HPO4), were used with a ratio of 20:5:1 (C: N: P), considering soil organic carbon. Tween 80 as surfactant in the relevant treatments, was used at a rate of 0.3% V/W. In the integrated treatment, all the mentioned treatments were used together. On days 0, 5, 10, 15, 30, 45, 60, 90 and 120, subsamples were taken from each pot to measure the activity of dehydrogenase and lipase enzymes.

The results showed that during the experiment, bioremediation treatments reduced heavy naphtha contamination and the highest value for removal rate of this compound was 81% in the integrated treatment. Contamination also affected the enzymatic activity of soil so that the activity of dehydrogenase and lipase in all bioremediation treatments showed a decreasing trend. Dehydrogenase enzyme activity in cow manure treatment decreased from 1.67 to 0.59 (μg TPF g-1 h-1) and activity of soil lipase enzyme decreased from 33.82 to 26.24 (mU g-1) in the integrated treatment during the experiment. Among the bioremediation treatments, cow manure and integrated treatment had a greater effect on the elimination of heavy naphtha contamination than other treatments (p <0.01). It seems that the use of the above treatments were able to remove more heavy naphtha by providing optimal nutritional, moisture and aeration conditions while intensifying the activity of soil microorganisms.

In this study, to reduce heavy naphtha contamination in the soil, bioremediation treatments including biostimulation, bioaugmentation and integrated approaches were used. According to the results of changes in dehydrogenase and lipase activity in naphtha-contaminated soil, each treatment was able to reduce contamination individually, while the integrated treatment was both biostimulatory and bioenhancing treatments. However, in the integrated treatment, the efficiency of bioremediation process was higher, due to the simultaneous use of biostimulation and bioaugmentation treatments. The use of integrated treatment in soils contaminated with petroleum compounds, including heavy naphtha, can help to biologically eliminate these contaminants.