Remediation of soils co-contaminated with heavy metals and petroleum hydrocarbons using nano/bio process

Co-contamination of soil and groundwater with inorganic and organic pollutants is one of the challenges facing environmental scientists nowadays. The different physical and chemical properties of these two contaminant groups makes it harder to overcome this problem. The co-contamination of soils with heavy metals and petroleum hydrocarbons is such a problem that requires a different and innovative approach to remediate these kinds of sites. More efficient, cost effective and eco-friendly new processes should be developed to cleanup sites co-contaminated with heavy metals and petroleum hydrocarbons as compared to traditional remediation technologies. Nano/bio process is utilization of nano zero valent iron (nZVI) particles together with bioremediation for remediation of contaminated sites. Recently, nano zero valent iron particles integrated with bioremediation have been used for remediation of halogenated organics. However there are limited number of studies dealing with the use of nano/bio process for the remediation of sites co-contaminated with heavy metals and petroleum hydrocarbons. The aim of this study was to investigate the applicability of the nano/bio process for the treatment of soils co-contaminated with heavy metals and petroleum hydrocarbons. Using nano/bio process, it was aimed to facilitate effective immobilization of heavy metals by nZVI and biodegradation of petroleum hydrocarbons by bioremediation. Laboratory scale experiments were conducted in two stages. In the first stage, maximum nZVI immobilization performances were determined for Pb, Ni, Cr and Cd heavy metals using 24 two level full factorial design. Four parameters (nZVI concentration, temperature, nZVI suspension pH and contaminant concentration) were regarded as factors in the factorial design. Soil heavy metal fractions were analyzed before and after nZVI application and percent decrease in the exchangeable and carbonate bound fractions was used as response variable. The results were used to determine the immobilization performance of nZVI for each heavy metal. The immobilization data of heavy metals was compared and Pb was chosen according to immobilization efficiency for the second stage of the experiments. In the second stage of the experiments, nano/bio process was used on soil which was spiked with Pb and diesel fuel. Four different bioremediation approaches (bioattenuation, biostimulation, bioaugmentation and biostimulation+bioaugmentation) was used as a part of the nano/bio process. Biostimulation was applied by adding N and P and bioaugmentation was applied by adding activated sludge to the soils. Total n-alkane (C10-C25) concentrations were analyzed during the experimental period (90 days) for determination of the process performance. Exchangeable and carbonate-bound fractions of Pb, Ni, Cr and Cd were reduced by 52.4, 43.8, 43.4 and 28.1 %, respectively, by nZVI application. Statistically significant models were obtained for Pb, Ni and Cr (R2 values are 0.87, 0.87 and 0.58, respectively). Under the experimental conditions in this study; the effects of nZVI concentration, nZVI suspension pH, nZVI concentration-suspension pH interaction and suspension pH-contaminant concentration interaction were significant for Pb immobilization. The effects of nZVI concentration, temperature and contaminant concentration were significant for Ni and Cr. Nano/bio processes with four different bioremediation approaches were applied to Pb and diesel fuel spiked soils. Nano/bio process with bioattenuation, biostimulation, bioaugmentation and biostimulation+bioaugmentation resulted in 38.1, 69.3, 74.2 and 91.1 % total n-alkane removal in 90 days, respectively.The results showed that, nZVI application enhanced the condition of the Pb and diesel contaminated soil for the activity of microbial population of the activated sludge. The nZVI application mitigated the toxic effects of Pb which led to effective biodegradation of n-alkanes especially for nano/bio process with the biostimulation+bioaugmentation treatment. Keywords: Nano/bio process, Nano-zero valent iron, Bioremediation, Heavy metal, Petroleum Hydrocarbons.