In the last decades, in situ chemical oxidation (ISCO) has become one of the attractive remedial alternatives for treating many organic contaminants. This remediation technique involves injecting oxidants (such as hydrogen peroxide, potassium permanganate or sodium persulfate) into the subsurface to destroy the compounds of concern. Many studies so far have demonstrated the effectiveness of the ISCO processes in the contaminated site remediation. Among the available oxidation processes the Fenton’s one gained an increasing interest due to its ability of treating a wide range of contaminants. However, the persistence of hydrogen peroxide in the subsurface is a key factor to consider since it affects the contact time of the oxidant with the contaminant and ultimately the delivery of H2O2 in the subsurface. With respect to other oxidants, such as potassium permanganate or persulfate, hydrogen peroxide in fact persists in soil and aquifer for relatively short times (from minutes to hours) and hence the radius of influence of the treatment could be relatively limited. To overcome this limitation, various reagents that enhance the lifetime of H2O2 in the aquifer can be used. Namely, the most common H2O2 stabilizers involves various forms of phosphate which reduce the availability of inorganic reactants (e.g. Fe and Mn) via complexation or precipitation reactions. More recently, different studies analysed the performances achievable using also organic acids, such as phytate, citrate and malonate. In this study we evaluate the performance achievable applying a Fenton-like treatment combined with carbon dioxide sparging. The applied CO2 stream in fact could in principle exert a double effect. On the one hand fluxing CO2 in water leads to carbonic acid production with a consequent decrease of pH to acidic values that makes the Fenton’s process more effective. On the other hand the CO2 sparging can enhance the contaminant removal due to a stripping effect. Thus to evaluate the performance of this combined process different lab-scale tests on a soil-water system artificially contaminated by MtBE were performed. In particular a Fenton-like process based on the use of hydrogen peroxide catalyzed by naturally occurring iron and manganese minerals was used. The oxidation process was then applied using either CO2 or KH2PO4 as stabilizing agents and the results, in terms of hydrogen peroxide lifetime and MtBE oxidation, were compared with the ones obtained without the addition of any hydrogen peroxide stabiliser. Furthermore, control tests applying only a carbon dioxide sparging without H2O2 were performed in order to evaluate the stripping effect of CO2. The obtained results showed that the use of either CO2 or KH2PO4 allows to enhance the hydrogen peroxide lifetime. However the stabilizing effect on hydrogen peroxide exerted by carbon dioxide was lower than the one observed when KH2PO4 was used. On the contrary, the removal of MtBE observed at the end of the different tests revealed that the combination of the Fenton-like process with the CO2 sparging was the most effective leading to a reduction of MtBE up to 99%. The application of CO2 sparging alone, instead, led to a MtBE removal in the order of 50-60% whereas the traditional Fenton-like with KH2PO4 allowed to remove up to 80-90% of the initial contaminant concentration. These findings hence suggest that the combination of carbon dioxide sparging with a Fenton-like process could represent a promising remediation option for the treatment of many organic compounds in groundwater.
INTRODUCTION
Polycyclic aromatic hydrocarbons (PAH) are organic compounds consisting of three or more fused benzene rings. PAH have low vapor pressure and negligible solubility in water. Due to their known toxicity, carcinogenic and mutagenic potentials, and their persistence in the environment, it is considered an international priority to take immediate, low cost measures for the removal of this kind of pollutants.
PAHs usually come from the incomplete combustion of organic matter, therefore these pollutants are usually found in coal and oil, coal tar, creosote, industrial areas, even in some cases of burnt food.
Advanced Oxidation Processes (AOPs) have shown good effectiveness in the removal of these contaminants, among all AOPs used for the remediation of PAH contaminated soils, it has become of increasing interest the use of activated persulfate, with a high redox potential of 2.12 V. Furthermore, stability of persulfate in soil is high, which can show activeness for months, making this reagent able to oxidize a wide range of organic contaminants. Besides, unlike hydrogen peroxide, non-productive consumption of persulfate is not strongly influenced by pH. [2], [3].
In spite of these advantages, there is a drawback regarding persulfate activation, which is related to the consumption of activator, as for example, iron. Taking into account that Fe(II) is the specie that activates persulfate for the release of persulfate radicals, when all Fe(II) is oxidized to Fe(III) during activation the release of persulfate radicals is stopped. This can be solved by different ways, as for example by the use of zerovalent iron, which acts as a continuous dose of iron, or the addition of humic acids, which can reduce Fe(III) to Fe(II), increasing the extent of the remediation technique.
The scope of this work is to remediate a PAH contaminated soil by 4 PAHs (anthracene, phenanthrene, pyrene and benzo(a)pyrene) by different kinds of persulfate activation, such as the addition of zerovalent iron (ZVI), either granular or nanoparticle, the addition of ferric sulfate combined with humic acids and the addition of surfactant, in order to increase the pollutant’s solubility. The different species involved in the reaction have been monitored (contaminant, oxidant) and also pH.
EXPERIMENTAL
A sandy loam soil was spiked artificially with 100 mg•kg-1each of 4 PAH, Anthracene (3 rings), Phenanthrene (3 rings), Pyrene (4 rings) and Benzo(a)Pyrene (5 rings), all included in the list of 16 PAHs priority pollutants. Soils were aged for two months before oxidation treatment.
Reactions were conducted without pH adjustement, using PTFE 50 mL centrifuge tubes as reactors, stirred isothermally at 20 ºC in an orbital shaker. Ratio selected for aqueous phase to soil was 2 mL•g-1. It has been studied the removal efficiency of every pollutant (Anthracene, Phenanthrene, Pyrene and Benzo(a) pyrene) with persulfate activated by Fe (II), Fe (III) combined with humic acids, granular ZVI, nanoparticle ZVI. Besides, the evolution of the different species (oxidant, surfactant, total iron in solution and contaminant) as well as pH, was followed during the reaction time and the identification of oxidation products and intermediates was carried out.
RESULTS
Effect of humic acids
It were found, after 5 days, higher removal efficiencies for all PAH when Fe (III) + humic acids were added than Fe (II). However, in both cases phenanthrene was the PAH with showed lower removal efficiencies. Furthermore, given the best results, activation with Fe(III) + humic acids was selected to be compared with ZVI activation for longer, due to the fact that there was an important quantity of remaining persulfate in the media.
Effect of surfactant
The presence of surfactant (sodium dodecyl sulfate) offered better removal efficiencies for all PAH. In this sense, despite being a competitor for the oxidant, as well as the pollutants, the enhancement of the mass transfer to the aqueous phase improved the removal efficiency for each PAH.
Effect of particle size
Comparing the effect of granular ZVI and nanoparticle ZVI, no significant differences were observed regarding removal efficiencies for all PAH. Anyway, after 40 days of reaction, it was observed a complete conversion of the contaminants.
Effect of nanovalent ZVI concentration
In this case, although almost complete removal efficiencies were achieved, when nanoparticle ZVI was used, less time was needed for the achievement of these conversions.
Oxidation intermediates
In some reactions, at intermediate times, it was observed the presence of anthraquinone, typical toxic compound found during oxidation processes of PAH [3].
REFERENCES
[1] Environmental Protection Agency, Polycyclic Aromatic Hydrocarbons (PAHs).
[2] ZHAO et al. Effect and mechanism of persulfate activated by different methods for PAHs removal in soil. Journal of Hazardous Materials. 254– 255 (2013) 228– 235.
[3] Liao et al. Identification of persulfate oxidation products of polycyclic aromatic hydrocarbon during remediation of contaminated soil. Journal of Hazardous Materials 276 (2014) 26–34.
ACKNOWLEDGEMENTS
The authors acknowledge financial support from the Comunidad Autonoma de Madrid provided throughout projects CARESOIL (S2013-MAE-2739) and from Spanish Ministry of Science and Innovation, projects CTM2010-16693 and CTM2013-43794-R.
KEYWORDS: Innovative Tender Procedure, Laboratory Tests, Activated Klozur® Persulfate,
Fully Automated ISCO, Hydrocarbon Fuels
ABSTRACT:
Overview
A marshalling yard for trains has been contaminated as a result of leaks, spills and filling losses of hydrocarbon fuels. An in situ remediation by Biosparging, Bioventing and Nutrient dosing has been carried out in the period from 2004 to 2010. After remediation for an amount of half a million Euro, a large residual TPH contamination remained in the soil.
Innovative Tender Procedure
An additional remediation effort was contracted in an innovative tender procedure under conditions of fixed price, time, removed mass and payment.
There were two selection criteria. For a fixed budget of a quarter of a million Euro, the contractor should indicate how much mass he demonstrably will remove (70% score). The plan of approach has been tested the feasibility (30% score). A number of contractors have put forward a smart solution. But there were also contractors who have decided not to make an offer.
Considerations and choice-based approach to remediation of a sub area
The main part of the pollutant load was located in the range from 14.5 to 18.5 m below ground level and comprises 47.9% of the overall load. This mass was situated in the top of the saturated zone and is more readily available to biological and chemical in-situ remediation techniques then contamination in the unsaturated zone.
Laboratory tests
By linking the results of an aliphatic aromatic TPH split group with substance group properties, a mathematical insight is obtained into possible remediation techniques. It was found that 43% of the oil is soluble in water and that more than 84% of the contamination is moderately aerobically biological or chemically degradable.
A desk study indicated that chemical oxidation would give the best results with activated sodium persulfate. On a laboratory scale, tests were carried out with alkaline activated persulfate and hydrogen peroxide activated persulfate. The hydrogen peroxide activated persulfate showed a decrease of 38-54% in the ground. However, the TPH was almost completely mobilized to the aqeous phase. Treatment with alkaline activated persulfate showed a destruction of 49-54% of TPH in the ground, with 10 times less mobilization of TPH to the water phase.
Full-Scale Application
The Full-scale remediation was carried out using a fully automated remotely controlled ISCO unit. The unit ensures complete telemetric monitoring, operating 24 / 7. Essential parameters, such as the injection pressure, the amount and flow rate of injection as well as the soil temperature were monitored continuously.
During the first injection period, an amount of 6.260 kg alkaline activated Klozur® persulfate, was injected over 15 ISCO injection wells in the most polluted areas.
Application of chemical oxidation with activated Klozur® persulfate leads to an increase in the pH, the redox potential and dissolved oxygen levels.
Comparison of the pollutant load after completion of the chemical oxidation with the load prior to the chemical oxidation, is showing a decrease of 49% of TPH in the ground.
A strong mobilization of product into the aqueous phase was not observed.
A second activation of the remaining Klozur® persulfate will be performed later this year, followed by a phase of enhanced aerobic bioremediation.
Conclusions
The innovative tender procedure has led to a smart solution, according on a ISCO approach with activated Klozur® persulfate based on the results of a bench scale treatability study.
It has been shown that bench scale testing is an indispensable tool in designing ISCO projects.
The bench scale treatability test and full scale field results correspond to each other.
After the first injection period of ISCO treatment was applied the TPH-concentration levels were successfully reduced with 49%.
Barium ferrates for in-situ chemical oxidation of BTEX contaminants
Christine Herrmann, Karin Hauff, Norbert Klaas
University of Stuttgart, VEGAS, Pfaffenwaldring 61, 70569 Stuttgart, Germany
Ferrate(VI) has a high oxidizing capacity - under acidic conditions, its redox potential is even higher than the one of ozone - making ferrate(VI) a very promising agent for water and wastewater treatment processes. It has been shown that various types of organic compounds like for example phenol or thiourea can be oxidised by ferrate(VI) [1].
The work presented here is especially focusing on the potential applicability of barium ferrate for in-situ groundwater remediation. To the best of our knowledge currently no material, except for oxygen-releasing compounds being applied for in-situ bioremediation, is tested for passive oxidative remediation. Since barium ferrate offers slow-release properties it could be utilized to form zones of strong oxidation potential with the possibility of producing a depot-effect in the aquifer. BTEX contaminants (benzene, toluene, ethyl benzene, and xylenes) represent one major category of contaminants affecting groundwater [2] and hence have been chosen as target pollutants to study the use of barium ferrates for in-situ chemical oxidation.
Ferrates(VI) can either be prepared by dry oxidation, wet oxidation or electrochemically. Here, the electrochemical preparation is used because it offers several advantages like for example a shorter synthesis time and reduced costs [3]. The electrochemical synthesis of ferrate(VI) is based on the oxidation of an iron metal anode in alkaline media [4]. Barium ferrate is obtained by subsequent precipitation and characterised by titrimetric chromite analysis [5] and X-ray diffraction. In order to investigate the reactivity of barium ferrate towards BTEX contaminants batch tests have been conducted using toluene as a model contaminant. The results will be presented along with considerations towards the potential applicability of the material for field application, including aspects of a later remediation technology.
“The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n°309517."
[1] M. Alsheyab, J.-Q. Jiang, C. Stanford, Journal of Environmental Management 2009, 90, 1350.
[2] P. Panagos, M. V. Liedekerke, Y. Yigini, L. Montanarella, Journal of Environmental and Public Health 2013, 2013, Article ID 158764.
[3] X. Yu, S. Licht, Journal of Applied Electrochemistry 2008, 38, 731.
[4] S. Licht, R. Tel-Vered, L. Halperin, Journal of the Electrochemical Society 2004, 151, A31.
[5] S. Licht, V. Naschitz, L. Halperin, N. Halperin, L. Lin, J. Chen, S. Ghosh, B. Liu, Journal of Power Sources 2001, 167.
Background/Objectives
Sodium persulfate is a widely used and accepted remedial strategy, however, the geochemical and physical conditions present in soil and groundwater are often overlooked when selecting an appropriate activator. The persulfate anion alone has the thermodynamic strength to react with organic target compounds but activation either alkaline, iron, hydrogen peroxide, or heat is required to generate the sulfate radical, which is preferred for efficient and kinetically meaningful reactions. Ambient activation methods have been employed at petroleum sites, with the understanding that ferrous iron naturally present in a reduced aquifer is contributing to activation, however, the influence of temperature is often overlooked. At a site in Arizona, sodium persulfate was injected in a groundwater aquifer with very low concentrations of iron (2-3 mg/L), relying on elevated injection fluid and groundwater temperatures (greater than 20 degrees Celsius) as the most prevalent activation method for activation using sodium persulfate.
The impacts at the site are petroleum-related hydrocarbons, with benzene as the primary remedial driver, observed at concentrations up to 2,800 µg/L in the study area. Two injection events were completed at the site to: (1) confirm lateral and vertical reagent distribution and related hydraulic properties, (2) obtain sodium persulfate persistence and consumption rates, and (3) assess the effectiveness of ambient activated in situ chemical oxidation (ISCO) at the site.
Approach/Activities
Two injections of 8,950 and 14,500 gallons of sodium persulfate solution were completed during the summers of 2011 and 2012. Groundwater within the injection area was monitored during and after each injection for approximately eight months.
To assess the transport, persistence, and consumption rates of the sodium persulfate in-situ during the first injection, a groundwater tracer (deuterated water) was used to distinguish between reagent consumption and washout as well as to determine sodium persulfate half-lives. Deuterated water was selected as a tracer for this application as it is very conservative and non-reactive with either sodium persulfate or the aquifer materials and non-toxic at the applied concentrations.
Results/Lessons Learned
The injection solution showed relatively consistent vertical and lateral distribution around the injection well. The kinetic reaction rate was relatively rapid, and as a result of the injections benzene concentrations declined by an average 80% from baseline concentrations. The data shows that at sites where groundwater temperatures are elevated naturally, ambient activation of sodium persulfate is a viable method for oxidation of benzene, and may play a more significant role than activation by background iron in either soil or groundwater.