METHODSCollection of Soil and Crude Oil

Soil for the remediation study was collected from a depth of 0 – 45 cm from a rice-paddy field situated at an uncontaminated area near the Lakowa oil field (District Sibsagar, Assam, India) run by the Oil and Natural Gas Corporation Limited (ONGCL), India. The soil was air dried and sieved with a 2 mm sieve (2 mm). This soil had an original pH of 5.0 (1:5 soil-water suspension) with a total organic carbon (TOC) load of 0.49 %. A sample of crude oil was collected from a group gathering station (Lakowa GGS1) of ONGCL under Lakowa oil field, Assam.

Collection of N-P-K Fertilizer

A commercial N-P-K mixture (with N, P, and K in the weight ratio of 12:12:12) was used in this study.

Important Chemicals

For controlling soil pH, either sodium hydroxide (GR MERCK), or hydrochloric acid (minimum 35% GR MERCK) were used. Teepol (Intklean-Teepol Grade, International Chemicals, India) was used as de-emulsifier.

The Procedure

The samples for the laboratory degradation experiment were prepared in duplicate as follows. Experiments were carried out in 1000 ml glass beakers containing 100 g soil in distilled water at soil-to-water ratio of 1:5 in each beaker. Soil solutions in required number were adjusted to pH of 3.5, 4.5, 5.5, 6.5, 7.5, 8.5 by addition of either 1N HCl or 1N NaOH. Adjustments to desired pH values were made at every alternate day over a period of one month till the pH values of the soils stabilized. Soil solutions at original pH (pH5.0) in required number were also kept prepared without adjusting pH. After pH adjustment, crude oil (with the help of TEEPOL-used as de-emulsifier) was added to soil solutions including normal soil solutions to yield four different concentrations of TPH viz. 0.3%, 1.5%, 3.0%, and 5.0% for each pH value. Here ‘%’ signifies the amount of oil present, in the unit of gram, per 100g of [soil+oil] system. These initially added TPH concentrations were considered as TPH concentrations at “0 day” without any hydrocarbon degradation due to biological, physical or chemical phenomenon. Just after the addition of crude oil, the above factors may come into immediate action and hence the TPH calculation at later part of the initial day may give TPH concentrations different from initially added TPH concentrations.

For each adjusted pH, 8 (4 x 2) beakers were used. Hence, total number of beakers used was 8 x 6 plus 8 (for normal soil solutions without adjusting pH), i.e., 56 containing 28 types of samples in duplicate. The experimental samples were set up as shown in Table1 and monitored for a period of six months.

Samples were drawn after 30, 60, 90, 120, 150 and 180 days for analysis. Each of the soil samples, after collection, was immediately soaked in dichloromethane to prevent further biodegradation of the hydrocarbons and preserved by refrigeration at 4oC for the calculation of TPH (Janiyani et al. 1992). The experiment on remediation was carried out at room temperature. Similarly, remediation studies using petroleum hydrocarbons contaminated soil (at initial TPH 3.0%) were conducted under pH 7.5, original pH and at different N-P-K environments. Here ‘%’ signifies the amount of oil present, in the unit of gram, per 100g of [soil+N-P-K+oil] system. The experimental samples were set up as shown in Table2.

The petroleum hydrocarbons in the samples (collected as above soaking in dichloromethane) were extracted with dichloromethane by the Soxhlet method, and the concentration of TPH was determined by using the gravimetric estimation as described below. In Soxhlet method, fresh solvent (DCM) is continuously refluxed through the finely divided soil sample (W g) contained in a porous thimble and a siphon system removes the extract back into the refluxing solvent. The process was repeated for many times until the completion of extraction of petroleum hydrocarbons from the sample. The extract containing the petroleum hydrocarbons was transferred in to a weighed glass vial (Wi). After the complete evaporation of the solvent, the final weight (Wf) of the glass vial was recorded. TPH was measured by complete evaporation of the extracting solvent and weighing the residue (Potter, 1999).

The percentage of TPH content is computed using the formula:
% TPH = [(Wf – Wi) / W] x100
Wi = Initial weight of the glass vial (in g)
Wf = Final weight of the glass vial (in g)
W= Weight of the soil sample (in g)

The extent of degradation of crude oil can be determined by estimating the concentration of residual TPH as mentioned above. Also, the total percentage loss in TPH was determined by using the following formula:

Total percentage loss in TPH= [{Conc. of initial TPH – Conc. of residual TPH}
/ Conc. of initial TPH] 100%.
Net percentage loss in TPH due to treatment is defined as:
Net percentage loss in TPH due to treatment = Total percentage loss in TPH due to treatment – Total percentage loss in TPH without treatment