Phyto-microbial Remediation Of Petroleum Contaminated Soil In The Yellow River Delta

For years, remediation of petroleum-contaminated soil is a hot topic in environmental research. However, few field investigations have demonstrated to be effective for the contaminated soils. How to enhance the petroleum hydrocarbon degradation to its maximum, and establish an efficient, safe, practical field remediation technology has become an international hot topic.The Yellow River Delta (YRD) is an important region of petroleum production in China. Over a long period of oil exploitation, spills, leaks, and other releases of crude oil, the soil is severely contaminated. Composting along with rhizodegradation was used in present study to remediate petroleum-contaminated soils in the Yellow River Delta, China. The mechanisms of petroleum hydrocarbon biological degradation and rhizodegradation were studied systematically. The main contents and conclusions are as follows:Composting of petroleum-contaminated soil was investigated in the lab and field using different amendments and ratios. The best composting system was established preliminarily. After 60d of lab composting, the total petroleum hydrocarbon (TPH) degradation rate reached 80%; the net degradation rate was approximately 40%. After field composting for 150 days, average concentration of TPH in soil was reduced from 7900^-17900 mg kg^-1 to 1400-3700 mg kg^-1. The net degradation rate could reach about 40%.The suitable C/N was 15/1 during the lab and field composting. An optimal C/N could improve the ventilation of the biopile and promote aerobic metabolisms of the microorganisms, then provide nutrients and metabolites for the degraders, which lead the microbial degradation to its maximum potential. Results showed that between 3rd and 18th day of lab composting, the microbial activity was the highest, which resulted in a high removal rate of TPH under temperature of composting pile maintained 40-50 oC, and the degradation rate of TPH per day reached as high as 300 mg kg^-1 d^-1. The number of degrading microorganisms rapidly increased during the first 30 days in field treatment biopiles, especially in biopiles with the C/N ratio of 15, which had more than 7 times of degrading microorganisms than control. In addition, after 150 days of composting, there was a significant decline of the soil pH in all treatment biopiles range between 7.68 and 8.23, compared to the control biopile at pH of 8.74. Decreased pH value of soils close to 7 after composting could be beneficial for mitigating the soil alkalization. Pot experiment was conducted to investigate the biodegrade abilities andrhizodegradation mechanism of 5 plant species, including Sesbania (Sesbania cannabina), Seepweed (Suaeda glauca), Sealavander (Limonium bicolor), and Central Asia Saltbush (Atriplex Centralasiatica), and one conventional phytoremediation plant Ryegrass (Lolium multiflorum), with respect to the petroleum contaminated soil after composting. After 90 days of cultivation, the removal of TPH was more effective in the rhizoshpere, which was between 23.8% and 44.6%. Root morphology analysis indicated that root biomass (r = -0.816, n = 10), root surface area (r = -0.869, n = 10), root volume (r = -0.900, n = 10), the number of hydrocarbon degraders (r = -0.647, n = 12) were all correlated negatively with TPH concentrations (p < 0.05). Consequently, plant with large root biomass, great root surface area, and strong root volume could explore more soil volume and enhanced the possibility for rhizosphere microorganisms to contact with pollutants, then subsequently led to a higher TPH degradation rate.Then 4 local dominant plant species, including Seepweed (Suaeda glauca), Sealavander (Limonium bicolor), Central Asia Saltbush (Atriplex Centralasiatica) and Reed (Phragmites communis), were selected and planted in composted soils for rhizodegradation in the field. After 90 days of cultivation, the highest net TPH degradation rate was over 40% by Seepweed and Central Asia Saltbush with strong root systems and active microbial communities. After 180 days of rhizodegradation, TPH concentration in the rhizospheres of Seepweed and Central Asia Saltbush was reduced to about 400 mg kg^-1. The parameters of root morphology showed obvious differences among the four plants. The root length, surface area and volume of Seepweed were about 4 times larger than Reed. The count of hydrocarbon-degrading microorganisms in the rhizosphere of Seepweed was about 6 times higher than that of Reed.Remediation of petroleum contaminated soil and its efficiency were related to the bioavailability of the pollutants, which was affected by the soil pore structures to a certain degree. The soil micropores in the rhizosphere of Seepweed were measured. The number of soil micropore was significantly degreased compared with the non-rhizosphere soil. The micropore surface and volume in Seepweed rhizosphere soil were significantly reduced from 6.31 m² g^-1 and 2.25 mm³ g^-1 to 5.03 m² g^-1 and 1.72 mm³ g^-1. The reduction of soil micropores in the rhizosphere could sequestrate organic compounds inside, which subsequently accelerated pollutant bioavailability. This could be an important mechanism for the highest degradation rate of organic pollutant in the rhizosphere.Seepweed was most effective in remediating the contaminated soil in the Yellow River Delta. Planting after composting seems promising to remediate heavily petroleum polluted soils in the Yellow River Delta.