Fugacity modelling to predict the distribution of organic contaminants in the soil:oil matrix of constructed biopiles

A review of the occurrence, distribution, and impact of bitumen seeps on soil and groundwater in parts of southwestern Nigeria

The impact of bitumen components on soil and groundwater resources is of environmental importance. Contaminants' influx into the environment from bitumen components through anthropogenic activities such as exploration, mining, transportation, and usage of bitumen in all its forms have been reported globally. However, gaps exist in the geogenic occurrence of bitumen in the shallow subsurface such as in southwest Nigeria, contaminating the soil and groundwater resources. This review presents in situ bitumen seeps as a source of geogenic soil and groundwater contaminants in southwestern Nigeria. We conducted a systematic review of literatures based on defined selection criteria. We derived information on the state of knowledge about bitumen seep occurrences and distribution in southwestern Nigeria. Also, the processes that exacerbate bitumen contaminants' influx into soil and groundwater were enunciated. At the same time, case examples highlighted areas for possible in situ bitumen contamination studies in Nigeria. The results of this review showed that a multidisciplinary approach has been employed to assess and monitor the contaminants resulting from the various activities involving the exploitation and application of bitumen in Nigeria. These studies emphasize bitumen contaminants as emanating from anthropogenic sources. The results also suggested that bitumen studies have been mainly exploratory to improve the understanding of the economic potential of the hydrocarbon reserve. Also, recent advances in bitumen contaminants studies accounted for the heterogeneous nature of the bitumen. This allows for the optimized categorization of the mechanism and processes undergone by the different bitumen components when released as environmental contaminants. However, a knowledge gap exists in characterizing and understanding the effects of in situ bitumen seeps as a geogenic source of soil and groundwater contamination. This review identifies the possibility of geogenic soil and groundwater contamination by in situ bitumen seeps in the coastal plain sand of the Dahomey basin in southwestern Nigeria. The impact of the bitumen contaminants on the environment was discussed, while methods for accessing the occurrence and distribution of the bitumen contaminants were highlighted.

Assessment of the effectiveness of onsite exsitu remediation by enhanced natural attenuation in the Niger Delta region, Nigeria

This study was conducted to quantify and rank the effectiveness of onsite exsitu remediation by enhanced natural attenuation using soil quality index. The investigation was conducted at three oil spill sites in the Niger Delta (5.317°N, 6.467°E), Nigeria with a predominance of Oxisols. Baseline assessment and a two-step post-remediation monitoring of the sites were conducted. Target contaminants including total petroleum hydrocarbon (TPH) and BTEX (benzene, toluene, ethylbenzene, and xylene) were analyzed by gas chromatography-mass spectrometry. Results of the baseline assessment showed that TPH concentrations across the study sites averaged between 5113 and 7640 mg/kg at 0- to 1-m depth, which was higher than the local regulatory value of 5000 mg/kg. The soil quality index across the sites ranged between 68 and 45, suggesting medium to high potential ecological health risks with medium to high priority for remediation. BTEX concentrations followed a similar trend. However, after remediation TPH degraded rapidly initially and then slowly but asymptotically during the post-remediation monitoring period. Then, soil quality index across the study sites ranged between 100 and 58, indicating very low to medium potential ecological health risks. This demonstrates the effectiveness of onsite exsitu remediation by enhanced natural attenuation as a remediation strategy for petroleum-contaminated soils, which holds great promise for the Niger Delta province.

Comparison of landfarming amendments to improve bioremediation of petroleum hydrocarbons in Niger Delta soils

Large scale landfarming experiments, using an extensive range of treatments, were conducted in the Niger-Delta, Nigeria to study the degradation of oil in contaminated soils. In this work the effect of nutrient addition, biosurfactant, Eisenia fetida (earthworm) enzyme extract, bulking and sorption agents and soil neutralization were tested. It was found that these treatments were successful in removing up to 53% of the total petroleum hydrocarbon in the soil within 16 weeks. A comparison between treatments demonstrated that most were no more effective than agricultural fertilizer addition alone. One strategy that did show better performance was a combination of nutrients, biochar and biosurfactant, which was found to remove 23% more Total Petroleum Hydrocarbons (TPH) than fertilizer alone. However, when performance normalized costs were considered, this treatment became less attractive as a remedial option. Based on this same analysis it was concluded that fertilizer only was the most cost effective treatment. As a consequence, it is recommended that fertilizer is used to enhance the landfarming of hydrocarbon contaminated soils in the Niger Delta. The attenuation rates of both bulk TPH and Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG) fractions are also provided. These values represent one of the first large scale and scientifically tested datasets for treatment of contaminated soil in the Niger Delta region. An inverse correlation between attenuation rates and hydrocarbon molecular weight was observed with heavy fractions showing much slower degradation rates than lighter fractions. Despite this difference, the bioremediation process resulted in significant removal of all TPH compounds independent of carbon number.

Insights into asphaltene aggregation in the Na-montmorillonite interlayer

This study aimed to provide insights into the diffusion and aggregation of asphaltenes in the Na-montmorillonite (MMT) interlayer with different water saturation, salinity, interlayer space and humic substances. The molecular configuration, density profile, diffusion coefficient and aggregation intensity were determined by molecular dynamic simulation, while the 3D topography and particle size of the aggregates were characterized by atomic force microscopy. Results indicated that the diffusivity of asphaltenes was up to 5-fold higher in the MMT interlayer filled with fresh water than with saline water (salinity: 35‰). However, salinity had little impact on the asphaltene aggregation. This study also showed a marked decrease in the mobility of asphaltenes with decrease in the pore water content and the interlayer space of MMT. This was more pronounced in the organo-MMT where the humic substances were present. The co-aggregation process resulted in the sequestration of asphaltenes in the hollow cone-shaped cavity of humic substances in the MMT interlayer, which decreased the asphaltene diffusion by up to one-order of magnitude and increased the asphaltene aggregation by about 33%. These findings have important ramifications for evaluating the fate and transport of heavy fractions of the residual oil in the contaminated soils.

A study on temporal trends and estimates of fate of Bisphenol A in agricultural soils after sewage sludge amendment

Temporal concentration trends of BPA in soils were investigated following sewage sludge application to pasture (study 1: short term sludge application; study 2: long term multiple applications over 13 years). The background levels of BPA in control soils were similar, ranging between 0.67-10.57 ng g(-1) (mean: 3.02 ng g(-1)) and 0.51-6.58 ng g(-1) (mean: 3.22 ng g(-1)) for studies 1 and 2, respectively. Concentrations in both treated and control plots increased over the earlier sampling times of the study to a maximum and then decreased over later sampling times, suggesting other sources of BPA to both the treated and control soils over the study period. In study 1 there was a significant treatment effect of sludge application in the autumn (p=0.002) although no significant difference was observed between treatment and control soils in the spring. In study 2 treated soils contained considerably higher BPA concentrations than controls ranging between 12.89-167.9 ng g(-1) (mean: 63.15 ng g(-1)). This and earlier studies indicate the long-term accumulation of multiple contaminants by multiple sewage sludge applications over a prolonged period although the effects of the presence of such contaminant mixtures have not yet been elucidated. Fugacity modelling was undertaken to estimate partitioning of Bisphenol A (soil plus sewage: pore water: soil air partitioning) and potential uptake into a range of food crops. While Bisphenol A sorbs strongly to the sewage-amended soil, 4% by mass was predicted to enter soil pore water resulting in significant uptake by crops particularly leafy vegetables (3.12-75.5 ng g(-1)), but also for root crops (1.28-31.0 ng g(-1)) with much lower uptake into cereal grains (0.62-15.0 ng g(-1)). This work forms part of a larger programme of research aimed at assessing the risks associated with the long-term application of sewage sludge to agricultural soils.

Molecular modeling of interactions between heavy crude oil and the soil organic matter coated quartz surface

Molecular dynamic (MD) simulation was applied to evaluate the mobility, diffusivity and partitioning of SARA (saturates, aromatics, resins, asphaltenes) fractions of heavy crude oil on soil organic matter (SOM) coated quartz surface. Four types of SOM were investigated including Leonardite humic acid, Temple-Northeastern-Birmingham humic acid, Chelsea soil humic acid and Suwannee river fulvic acid. The SOM aggregation at oil-quartz interface decreased the adsorption of SARA on the quartz surface by 13-83%. Although the SOM tended to promote asphaltenes aggregation, the overall mobility of SARA was significantly greater on SOM-quartz complex than on pure quartz. Particularly, the diffusion coefficient of asphaltenes and resins increased by up to one-order of magnitude after SOM addition. The SOM increased the overall oil adsorption capacity but also mobilized SARA by driving them from the viscous oil phase and rigid quartz to the elastic SOM. This highlighted the potential of SOM addition for increasing the bioavailability of heavy crude oil without necessarily increasing the environmental risks. The MD simulation was demonstrated to be helpful for interpreting the role of SOM and the host oil phase for the adsorption and partitioning of SARA molecules, which is the key for developing more realistic remediation appraisal for heavy crude oil in soils.

Influence of soil and hydrocarbon properties on the solvent extraction of high-concentration weathered petroleum from contaminated soils

Petroleum ether was used to extract petroleum hydrocarbons from soils collected from six oil fields with different history of exploratory and contamination. It was capable of fast removing 76-94 % of the total petroleum hydrocarbons including 25 alkanes (C11-C35) and 16 US EPA priority polycyclic aromatic hydrocarbons from soils at room temperature. The partial least squares analysis indicated that the solvent extraction efficiencies were positively correlated with soil organic matter, cation exchange capacity, moisture, pH, and sand content of soils, while negative effects were observed in the properties reflecting the molecular size (e.g., molecular weight and number of carbon atoms) and hydrophobicity (e.g., water solubility, octanol-water partition coefficient, soil organic carbon partition coefficient) of hydrocarbons. The high concentration of weathered crude oil at the order of 10(5) mg kg(-1) in this study was demonstrated adverse for solvent extraction by providing an obvious nonaqueous phase liquid phase for hydrocarbon sinking and increasing the sequestration of soluble hydrocarbons in the insoluble oil fractions during weathering. A full picture of the mass distribution and transport mechanism of petroleum contaminants in soils will ultimately require a variety of studies to gain insights into the dynamic interactions between environmental indicator hydrocarbons and their host oil matrix.

Sorption and distribution of asphaltene, resin, aromatic and saturate fractions of heavy crude oil on quartz surface: molecular dynamic simulation

The molecular scale sorption, diffusion and distribution of asphaltene, resin, aromatic and saturate fractions of heavy crude oil on quartz surface were studied using molecular dynamic simulation. Sorption of saturates on quartz decreased by 31% when temperature increased from 298 to 398K while opposite trend was observed for resins, but insignificant changes were found in asphaltenes and aromatics. Despite of this variety, the main contribution of interactions was van der Waals energy (>90%) irrespective of molecular components and temperatures. The diffusion coefficient of saturates was predicted as 10.8×10(-10)m(2)s(-1) while that of the remaining fractions was about 4×10(-10)m(2)s(-1) at 298K. The most likely oil distribution on quartz surface was that aromatics and saturates transported randomly into and out of the complex consisting of asphaltenes surrounded by resins, which was influenced by temperature. Overall, the knowledge on quartz-oil and oil-oil interactions gained in this study is essential for future risk assessment and remediation activities as previous studies on soil remediation either limited to light oil fractions with <40 carbons or treated the heavy crude oil as a single pseudo entity ignoring the interactions between oil fractions.

Risk assessment of the use of PAS100 green composts in sheep and cattle production in Scotland

A generalized quantitative risk assessment for the use of source-segregated green waste (SSGW) compost use in livestock production is presented. This assessment focussed on potential risks associated with a specific product, PAS100 compost that meets the UK publicly available specification 100 and represents the majority of compost available for use in extensive Scottish livestock systems. A hazard screening approach was used to identify all potentially hazardous agents present in compost. A total of 497 potentially hazardous agents were screened, with 147 finally put forward for quantitative risk assessment. Scenarios modelled in the assessment included surface application of compost to grazing land and also incorporation into soil and subsequent uptake by fodder crops. Risk estimates were compared to those associated with six comparator materials, including various sludges, slurries and farm yard manures. Overall, five potentially hazardous agents (PCB28, PCB138, PCB153, 1,2,3,4,6,7,8-HpCDD, Clopyralid) returned a hazard quotient >1 but within margins of uncertainty, indicating that further investigation may be required. Within the limitations of available information, SSGW compost was found to pose less risk to grazing livestock, or the environment, than other commonly-used soil amendments. While this assessment relates to a specific product/standard used in the UK, the methodology could easily be applied to other composts/products/situations. Therefore these results have wider applicability.

Impact of hydrocarbon type, concentration and weathering on its biodegradability in soil

The objective of this research was to investigate the impact of the hydrocarbon type and concentration, as well as the total effect of the natural weathering process to hydrocarbon biodegradability in sandy soil and the environment. In this experiment, sandy soil was separately contaminated with 0.5%, 1.0%, 2.0% and 3.5% of diesel and crude oils. Oil contaminated soil was taken from the Oil Refinery dumping sites after 9 years of weathering, and its concentration was adjusted to the above-mentioned levels. The biodegradation process was monitored by measuring CO(2), evolution rate, hydrocarbon degradation rate and dehydrogenase activity. The favourable concentration ranges for the soil contaminated with diesel oil were 1.0%, with concentrations at about 2.0% causing slightly adverse effects to CO(2) production which was overcome after 2 weeks, and with 3.5% diesel oil causing significant toxicity. For soil contaminated with crude oil, 2.0% was found to be optimum for effective biodegradation, with 3.5% crude oil also causing adverse effects to CO(2) production, although less so than the same concentration of diesel oil. No adverse effect was obtained for any concentration of the weathered oil, as after the weathering process, the remaining contaminants in the soil were mostly poorly degradable constituents like asphaltenes, resins etc. It has been proposed that such residual material from oil degradation is analogous to, and can even be regarded as, humic material. Due to its inert characteristics, insolubility and similarity to humic materials it is unlikely to be environmentally hazardous.

Multimedia fate of petroleum hydrocarbons in the soil: oil matrix of constructed biopiles

A dynamic multimedia fugacity model was used to evaluate the partitioning and fate of petroleum hydrocarbon fractions and aromatic indicator compounds within the soil: oil matrix of three biopiles. Each biopile was characterised by four compartments: air, water, soil solids and non-aqueous phase liquid (NAPL). Equilibrium partitioning in biopile A and B suggested that most fractions resided in the NAPL, with the exception of the aromatic fraction with an equivalent carbon number from 5 to 7 (EC(5-7)). In Biopile C, which had the highest soil organic carbon content (13%), the soil solids were the most important compartment for both light aliphatic fractions (EC(5-6) and EC(6-8)) and aromatic fractions, excluding the EC(16-21) and EC(21-35). Our starting hypothesis was that hydrocarbons do not degrade within the NAPL. This was supported by the agreement between predicted and measured hydrocarbon concentrations in Biopile B when the degradation rate constant in NAPL was set to zero. In all scenarios, biodegradation in soil was predicted as the dominant removal process for all fractions, except for the aliphatic EC(5-6) which was predominantly lost via volatilization. The absence of an explicit NAPL phase in the model yielded a similar prediction of total petroleum hydrocarbon (TPH) behaviour; however the predicted concentrations in the air and water phases were significantly increased with consequent changes in potential mobility. Further comparisons between predictions and measured data, particularly concentrations in the soil mobile phases, are required to ascertain the true value of including an explicit NAPL in models of this kind.

Biodegradation of semi- and non-volatile petroleum hydrocarbons in aged, contaminated soils from a sub-Arctic site: laboratory pilot-scale experiments at site temperatures

This study evaluates the feasibility of landfarming biotreatment of petroleum-contaminated soils obtained from a sub-Arctic site at Resolution Island, Nunavut, Canada, and evaluates the changes in composition of the semi- and non-volatile petroleum hydrocarbon fractions during the biotreatment. Pilot-scale landfarming experiments were conducted in a laboratory in soil tanks under temperature profiles representative of the 3-year site air temperatures in July and August where temperature varied uniformly between 1 degrees C and 10 degrees C over 10 d. The site soils were acidic and N-deficient, but contained indigenous populations of hydrocarbon-degrading microorganisms. Biostimulation with nitrogen and phosphorus nutrient amendments to achieve C(TPH):N:P molar ratio of 100:9:1, and CaCO(3) amendment at 2000 mg Kg(-1) for maintaining neutral pH, and periodic 10-day tilling, reduced total petroleum hydrocarbon (TPH) concentrations by up to 64% over a 60-day period. The rate and extent of semi-volatile (F2: >C10-C16) and non-volatile (F3: >C16-C34) petroleum hydrocarbon fractions in the landfarms containing higher initial TPH levels ( approximately 2000 mg Kg(-1)) and lower TPH levels ( approximately 1000 mg Kg(-1)) were compared. Significant biodegradation of the F2 and F3 fractions occurred in both of those systems. First-order biodegradation rate constants of up to 0.019+/-0.001 d(-1) were determined for the F3 hydrocarbon fraction and were similar to the F2 fraction biodegradation rate constants of up to 0.024+/-0.005 d(-1). Biodegradation profiles of the C14, C16 and C18 alkanes revealed that at TPH concentrations above 1000 mg Kg(-1) these compounds are degraded concurrently, whereas below 1000 mg Kg(-1) the higher-molecular weight alkanes are preferentially degraded. After the 60-day treatment period, the TPH concentration was approximately 500 mg Kg(-1), and the residual TPH mass was largely associated with particles and aggregated particles with diameters of 0.6-2 mm, rather than the larger or finer particles and aggregates.