Influence of environmental factors on the biological treatment of organic compounds in contaminated lagoon sediments |
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Authors: | Email author" target="_blank">Sabrina?SaponaroEmail author Luca?Bonomo |
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Institution: | (1) DIIAR, Sez. Ambientale, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano, Italy |
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Abstract: | Background and Scope Many technologies available to remediate soils are not cost-effective when applied to marine and lagoon sediments, due to
the physico-chemical characteristics of these matrices (high percentages of small particle size material, high moisture and
organic matter content, many different types of inorganic and organic contamination). For these types of waste, slurry phase
bioreactors can provide versatile processes, with very high removal efficiencies of recalcitrant organic compounds. The biodegradation
of these contaminants strictly depends on the specific structure of the molecules and on environmental factors, such as the
dissolved oxygen concentration, salinity, pH, and macronutrient availability.
This paper presents the results of lab-scale tests performed to investigate the effect of the above-mentioned factors on the
removal efficiency of saturated hydrocarbons, polycyclic aromatic hydrocarbons (PAHs) and polychlorobiphenyls (PCBs) in slurry
phase biological treatment of lagoon sediments.
Methods Sediments were contaminated by saturated hydrocarbons (958 mg/kg d.w.), polyaromatic hydrocarbons (PAHs) (29 mg/kg d.w.) and
polychlorobiphenyls (PCBs) (236 μg/kg d.w.). Carbon to nitrogen to phosphorous ratios in the matrix were not properly balanced
for biological treatment. High concentrations of metals were also present (Cu: 200 mg/kg d.w.; Zn: 1710 mg/kg d.w.; As: 33
mg/kg d.w.; Cd: 13 mg/kg d.w.; Pb: 244 mg/kg d.w.).
Biodegradation studies were carried out at 21 ± 1°C, in completely mixed slurry phase aerobic, anaerobic and sequential anaerobic/aerobic
batch systems (3.5 1), with a solid to liquid ratio of 10% w/w. Where required, anaerobic conditions occurred naturally within
three days of not aerated treatment. The aerated reactors were also equipped with traps for the waste vapors. Twelve tests
were performed, over a period between ten and twenty-two days, mixing water, sediments and different amounts of various additives
(macronutrients to balance C:N:P to 100:10:1 w/w, sodium chloride from 0 to 20 g/1, pH buffer solution) in the reactors, depending
on the test being performed.
Results and Discussion No significant differences among the removal efficiencies of saturated hydrocarbons in tests could be observed. Abatements
between 63% and 93% were obtained.
The dissolved oxygen concentration was the most critical factor affecting the treatment of PAHs. Removal efficiencies below
20% were obtained for two and three-ring PAHs in the non-aerated tests. Higher values, between 45% and 61%, were obtained
for these compounds in the aerated treatments; however, large (16% to 21%) abiotic losses (volatilization) were observed in
these cases. Four-ring PAH removal efficiency was below 5% in all the non-aerated treatments, except for test performed without
additions (30%); in the aerobic processes, removal efficiencies between 40% and 50% were obtained, except for test performed
with nutrient addition and 10 g NaCl/1 (16%). Abatements of five and six-ring PAHs were between 43% and 69% in the aerated
tests, and between 17% and 51% in the non-aerated treatments.
Concerning PCBs, tests evidenced that reductive dehalogenation mechanisms have occurred in the anaerobic reactors with the
most stable pH values, resulting in an increase of 2,4,4′-CB and 2,2′,5,5′-CB concentrations; the aerobic treatments did not
modify the PCB mixture. In both types of systems, no variation of the total PCB concentration could be observed.
Conclusions This study showed that saturated hydrocarbon removal efficiency was nearly insensitive to the environmental factors investigated.
The molecule structure and the dissolved oxygen concentration were the most affecting factors the removal efficiency of PAHs.
PCB degradation under anaerobic conditions could be related to the pH values measured during the tests. Evaluation of results
suggested that the abatement of pollutants investigated was not limited by a high salinity. High concentrations of dissolved
metals and macronutrient addition did not influence the removal efficiency.
Recommendations and Outlook. The slurry phase biological treatment of contaminated sediments could be applied whenever moderate
removal efficiencies were required for remediation from saturated hydrocarbons and PAHs within a few days; further studies
should be performed to improve the PCB abatement. Aerobic conditions should be maintained for the biodegradation of polyaromatic
compounds; seawater or lagoon water could be used in the system. |
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Keywords: | Aerobic treatment anaerobic treatment bioremediation polychlorinated biphenyls polycyclic aromatic hydrocarbons saturated hydrocarbons sediments slurry phase |
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