Effort to improve coupled in situ chemical oxidation with bioremediation: a review of optimization strategies

Purpose - In order to provide highly effective yet relatively inexpensive strategies for the remediation of recalcitrant organic contaminants, research has focused on in situ treatment technologies. Recent investigation has shown that coupling two common treatments-in situ chemical oxidation (ISCO) and in situ bioremediation-is not only feasible but in many cases provides more efficient and extensive cleanup of contaminated subsurfaces. However, the combination of aggressive chemical oxidants with delicate microbial activity requires a thorough understanding of the impact of each step on soil geochemistry, biota, and contaminant dynamics. In an attempt to optimize coupled chemical and biological remediation, investigations have focused on elucidating parameters that are necessary to successful treatment. In the case of ISCO, the impacts of chemical oxidant type and quantity on bacterial populations and contaminant biodegradability have been considered. Similarly, biostimulation, that is, the adjustment of redox conditions and amendment with electron donors, acceptors, and nutrients, and bioaugmentation have been used to expedite the regeneration of biodegradation following oxidation. The purpose of this review is to integrate recent results on coupled ISCO and bioremediation with the goal of identifying parameters necessary to an optimized biphasic treatment and areas that require additional focus. Conclusions and recommendations - Although a biphasic treatment consisting of ISCO and bioremediation is a feasible in situ remediation technology, a thorough understanding of the impact of chemical oxidation on subsequent microbial activity is required. Such an understanding is essential as coupled chemical and biological remediation technologies are further optimize

Isolation and characterization of Alicycliphilus denitrificans strain BC, which grows on benzene with chlorate as the electron acceptor

A bacterium, strain BC, was isolated from a benzene-degrading chlorate-reducing enrichment culture. Strain BC degrades benzene in conjunction with chlorate reduction. Cells of strain BC are short rods that are 0.6 microm wide and 1 to 2 microm long, are motile, and stain gram negative. Strain BC grows on benzene and some other aromatic compounds with oxygen or in the absence of oxygen with chlorate as the electron acceptor. Strain BC is a denitrifying bacterium, but it is not able to grow on benzene with nitrate. The closest cultured relative is Alicycliphilus denitrificans type strain K601, a cyclohexanol-degrading nitrate-reducing betaproteobacterium. Chlorate reductase (0.4 U/mg protein) and chlorite dismutase (5.7 U/mg protein) activities in cell extracts of strain BC were determined. Gene sequences encoding a known chlorite dismutase (cld) were not detected in strain BC by using the PCR primers described in previous studies. As physiological and biochemical data indicated that there was oxygenation of benzene during growth with chlorate, a strategy was developed to detect genes encoding monooxygenase and dioxygenase enzymes potentially involved in benzene degradation in strain BC. Using primer sets designed to amplify members of distinct evolutionary branches in the catabolic families involved in benzene biodegradation, two oxygenase genes putatively encoding the enzymes performing the initial successive monooxygenations (BC-BMOa) and the cleavage of catechol (BC-C23O) were detected. Our findings suggest that oxygen formed by dismutation of chlorite can be used to attack organic molecules by means of oxygenases, as exemplified with benzene. Thus, aerobic pathways can be employed under conditions in which no external oxygen is supplie

Biotransformation of toluene, benzene and naphthalene under anaerobic conditions

Aromatic hydrocarbons are widespread in nature, due to increasing industrial activity, and often contribute to polluted soils, sediments, and groundwater. Most of these compounds are toxic at relatively high concentrations, but some are already carcinogenic at very low concentrations, e.g. benzene. A growing awareness of the health risks associated with contamination has directed research to the removal or degradation of such compounds. The use of microorganisms to degrade toxic compounds (bioremediation) is a relatively slow process compared to traditional, chemical methods, but it is a natural process, mostly very specific and low in costs. A review of the available information on the microbial degradation of aromatic compounds is given in chapter 1. The anaerobic degradation is emphasized, since in many polluted environments oxygen is limiting and anaerobic processes will prevail. In the absence of oxygen, compounds like nitrate, metalions (Fe 3+and Mn 4+), sulfate, and carbondioxide, have taken over the function of oxygen as a terminal electron acceptor. In addition, the first transformation reactions differ from those in aerobic processes. Oxygenases are no longer ftinctioning and the degradation of oxygenated aromatic compounds, like benzoate and phenol, is known to occur via e.g. reduction, dehydroxylation and dehydrogenation of the aromatic ring. Information on the anaerobic degradation of mono- and polycyclic aromatic hydrocarbons without functional groups, like toluene, benzene, and naphthalene, is scarse. To gain more insight in the possibilities and limitations of the anaerobic degradation of these aromatic compounds, their behaviour in anaerobic sediment columns was followed. Toluene, benzene, and naphthalene were chosen as model compounds under methanogenic, sulfate-, iron-, manganese-, and nitrate-reducing conditions (Chapter 2). Toluene was transformed readily (within 1 to 2 months), while benzene was recalcitrant over the test period of 375-525 days under all redox conditions tested. Naphthalene was partly transformed in the column with nitrate or manganese as electron acceptor present; the addition of benzoate had a positive effect on the degradation of naphthalene in the column with nitrate. In the column with sulfate, the majority of the added naphthalene disappeared. No effect on the degradation of naphthalene was observed after adding and omitting an easier degradable substrate. [ 14C]naphthalene was used to confirm the disappearance to be the result of degradation; two third of the naphthalene was converted to CO 2 .Numerous attempts have been made for further enrichment of sulfatereducing, naphthalene degrading bacteria (Chapter 3). Unfortunately, the observed degradation of naphthalene in a sediment column could not be obtained in batch cultures, despite the large variety of tested enrichment conditions (different naphthalene concentrations, inoculum. size, medium composition, extra additions etc.). A toxic effect of naphthalene on sulfate- reducing bacteria could not be found.Toluene degradation in the columns was demonstrated under all redox conditions tested. Chapter 4 describes the degradation of toluene in freshly started sediment columns, to which either amorphous or highly crystalline manganese oxide had been added. In batch experiments with material from these columns as inoculum, the degradation of toluene to C0 2 and the formation of biomass under manganese-reducing conditions was demonstrated. The oxidation of toluene was found to be coupled to the reduction of Mn(IV), and the rate of oxidation was found to be lower with the crystalline than with the amorphous manganese oxide. Upon successive transfers of the enrichment cultures, the toluene degrading activity would decrease in time. The activity could only be maintained in the presence of sterilized Rhine river sediment or its supernatant. Without the sediment, but in the presence of solids like teflon beads, glass beads, bentonite, vermiculite and sterilized granular sludge, the toluene degrading activity completely disappeared after 4 to 5 transfers. Furthermore, a direct contact between the bacteria and the manganese oxide was found to be advantageous for a rapid toluene degradation. The degradation rate could further be increased by adding organic ligands such as oxalic acid or nitrilotriacetic acid (NTA).The highly purified enrichment culture LET-13, which degrades toluene with manganese oxide as electron acceptor, was obtained via repeated dilution series, and is described and characterized in chapter 5. LET-13 was able to degrade a variety of substituted monoaromatic compounds like (p-hydroxy) benzylalcohol, (p-hydroxy) benzaldehyde, (p-hydroxy) benzoate, cresol, and phenol. Benzene, ethylbenzene, xylene and naphthalene were not degraded under the experimental conditions used. The degradation of toluene occurred via hydroxylation of the methyl group to benzoate, and a possible side reaction can lead to the formation of cresol.All organisms in the culture look similar; motile rods which are gram negative, oxidase negative and catalase negative. The culture was partly identified with phylogenetic analysis of cloned rDNA sequences. The phylogenetic analysis showed that at least two major groups of bacteria are present. One group of bacteria belongs to the Bacteroides- Cytophaga group, and one group consists of members of the β-subclass of the Proteobacteria.Finally, the results from this research are discussed in relation to their relevance for soil bioremediation technologies

Anaerobic Degradation of Lindane and Other HCH Isomers

Lindane (¿-HCH) is a pesticide that has mainly been used in agriculture. Lindane and the other HCH isomers are highly chlorinated hydrocarbons. The presence of a large number of electron withdrawing chlorine groups makes some of the HCH isomers rather recalcitrant in oxic environments. Especially ß-HCH is poorly degraded by aerobic bacteria. The chlorine groups make HCH isomers more accessible for an initial reductive attack, a common mechanism in anoxic environments. Among the HCH isomers, ¿-HCH is degraded most easily while ß-HCH is most persistent. Little is known about the diversity of the microorganisms involved in anaerobic HCH degradation. Thus far, species within the genera Clostridium and Bacillus, two Desulfovibrio species, and one species each of Desulfococcus, Desulfobacter, Citrobacter and Dehalobacter have been found to metabolize lindane and other HCH isomers. Benzene and monochlorobenzene are the end products of anaerobic degradation, while in some studies pentachlorocyclohexane, tetrachlorocyclohexene, chlorobenzenes and chlorophenols have been detected as intermediates. Enzymes and coding genes involved in the reductive dechlorination of HCH isomers are largely unknown. Recently, a metagenomic analysis has indicated the presence of numerous putative reductive dehalogenase genes in the genome of ß-HCH degrading Dehalobacter sp. High-throughput omics techniques can help to explore the key players and enzymes involved in the reductive dehalogenation of lindane and other HCH isomers

Stimulation of Hexachlorocyclohexane (HCH) Biodegradation in a Full Scale In Situ Bioscreen

The feasibility of a bioscreen for the in situ biodegradation of HCH and its intermediates is demonstrated at a contaminated site in The Netherlands, via the discontinuous addition of methanol as electron donor. An infiltration system was installed and operated at the site over a length of 150 m and a depth of 8 m, to create an anaerobic infiltration zone in which HCH is converted. The construction of the infiltration system was combined with the redevelopment of the site. During passage through the bioscreen, the concentration of HCH in the groundwater decreased from 600 µg/L to the detection limit of the individual HCH isomers (0.01 µg/L) after one year of operation. The concentration of the intermediate biodegradation products benzene and chlorobenzene increased and achieved steady state values of respectively 800 and 2700 µg/L. Benzene and chlorobenzene were treated aerobically on site in an existing wastewater treatment plant. By changing the infiltration regime, it is conclusively shown that HCH removal is the result of the biological degradation and stimulated by the addition of methanol as electron donor. To our knowledge, this is the first successful field demonstration of the stimulated transformation of HCH to intermediates in a full scale anaerobic in situ bioscreen, combined with an aerobic on site treatment to harmless end products

Degradation of pharmaceuticals in wastewater using immobilized TiO2 photocatalysis under simulated solar irradiation

Pharmaceutically active compounds (PhACs) are not efficiently removed in wastewater treatment plants and are released into surface waters resulting in toxin accumulation. The aims of this study were to investigate the effect of solar irradiation on PhACs in wastewater using immobilized TiO2 present as a catalyst, and to study the potential of this photocatalysis technique as a post-treatment process for wastewater effluent. We treated a mixture of PhACs spiked in wastewater effluent and in deionized water as a control with simulated solar irradiation for 96 h. Experiments were conducted with immobilized TiO2 (photocatalysis) and without (photolysis). First, TiO2 was successfully immobilized on 200–500 µm sand by using a sol–gel method. The photocatalysis resulted in high removal efficiencies for poorly biodegradable PhACs in wastewater effluent: 100% for propranolol, 100% for diclofenac, and 76 ± 3% for carbamazepine. Photodegradation of all four PhACs followed pseudo-first-order kinetics, and the kinetic constant of photocatalysis was much higher than that of photolysis in the absence of a catalyst. Dissolved organic matter (DOM) in wastewater effluent enhanced photodegradation of PhACs by producing reactive radicals. However, at the same time, DOM inhibited photodegradation, possibly because DOM reforms the oxidation intermediates of PhACs into parent compounds. From an application perspective, water depth was confirmed as a key factor in photodegradation of PhACs due to light attenuation by modelling and experimental results. In addition, after photocatalysis, toxicity of PhACs decreased and biodegradability of wastewater effluent increased slightly. In conclusion, the technique is a promising post-treatment process to improve water quality, prior to discharging to natural waters or to polishing water treatment systems such as wetlands and lagoons

Benzene degradation coupled with chlorate reduction in soil column study

Perchlorate and chlorate are electron acceptors that during reduction result in the formation of molecular oxygen. The produced oxygen can be used for activation of anaerobic persistent pollutants, like benzene. In this study chlorate was tested as potential electron acceptor to stimulate benzene degradation in anoxic polluted soil column. A chlorate amended benzene polluted soil column was operated over a period of 500 days. Benzene was immediately degraded in the column after start up, and benzene removal recovered completely after omission of chlorate or a too high influent chlorate concentration (22 mM). Mass balance calculations showed that per mole of benzene five mole of chlorate were reduced. At the end of the experiment higher loading rates were applied to measure the maximal benzene degradation rate in this system; a breakthrough of benzene was not observed. The average benzene degradation rate over this period was 31 ¿mol l¿1 h¿1 with a maximal of 78 ¿mol l¿1 h¿1. The high degradation rate and the necessity of chlorate indicate that oxygen produced during chlorate reduction indeed is used for the activation of benzene. This is the first column study where benzene biodegradation at a high rate coupled with anaerobic chlorate reduction is observe