case study on bioremediation of soils contaminated with wood preservatives
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Case study on bioremediation of soils contaminated with wood preservatives. Wood preservation is a two-stage process: 1. Conditioning the wood to reduce its natural moisture content and to increase permeability, 2. Treating the wood with preservative. Conditioning: seasoning in open yards - PowerPoint PPT PresentationTRANSCRIPT
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Case study on bioremediation of soils contaminated with wood
preservatives
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Wood preservation is a two-stage process:
1. Conditioning the wood to reduce its natural moisture content and to increase permeability,
2. Treating the wood with preservative
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Conditioning:
• seasoning in open yards• steam conditioning• vapor drying• kiln drying• controlled air seasoning• tunnel drying
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• After conditioning, wood is immersed in preservative chemicals, sometimes under pressure, at either ambient or elevated temperatures.
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Wood preservatives include
• creosote• pentachlorophenol• a combination of copper,
chromium, and acetate (CCA)
They are used to prevent decay and to protect against fire and insects.
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Creosote alone or in combination with coal tar or petroleum is the major preservative used in the wood pressure treating industry
• is made by high-temperature carbonization of bituminous coal
• contains a complex mixture of organic compounds consisting mainly of aromatic hydrocarbons, tar acids (phenolic derivative of the aromatic compounds), and tar bases (heterocyclic compounds containing N plus some neutral oxygenated compounds)
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The principle components are shown in Table 25-1 and 25-2 in in your handout
• The major PAHs are 2-, 3-, and 4-ring compounds and their methyl derivatives.
• Commercial PCP contains • 85 to 90% PCP• 3-8% tetraphenols• 2-6% other chlorinated phenols• the remainder: other chlorinated
compounds and inert materials, including toxic hexachlorodibenzo-p-dioxin (HCDD) and heptachlorodibenzo-p-dioxin (HPCDD)
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Tetrachlorodibenzo()dioxin (TCDD)
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• Creosote contains many constituents that are reported to be mutagenic, carcinogenic, teratogenic, fetotoxic, and/or toxic.
• The use of creosote has been restricted by the USEPA to certified applicators to protect applicators and users of the treated wood from unnecessary exposure.
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• PCP toxicity and potential for uptake by organisms are pH-dependent.
• The estimated acute sensitivities of 32 species at pH 6.5 range from 4.355 ppm for larval common carp to greater than 44000 ppm for crayfish.
• Freshwater algae were affected by concentration as low as 7.5 ppm, whereas vascular plants were affected at 296 ppm and above.
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• Acute toxicity tested with 18 species of saltwater animals range from 22.63 ppm to 18,000 ppm.
• In general, fish are more sensitive than invertebrates to PCP.
• Chlorinated dioxin and dibenzofuran impurities in PCP also are of concern. The USEPA has listed PCP manufacturing wastes as acute hazardous wastes because of the presence of hexachlorodibenzo-p-dioxins.
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• PCP, creosote with PAHs, and aqueous solutions of CCA have been introduced into soils and groundwater in the USA.
• In 1989, there were between 415 - 550 creosoting operations within the USA using approximately 454,000 tons of creosote annually.
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• Leaking pipes, leaking tanks, drippings from lumber, and leachate from unlined ponds have caused contamination of soil and/or groundwater at 1397 wood preserving sites.
• In 1990, there were 58 wood preservative sites on the USEPA National Priority List, of which 51 were contaminated with PCP and/or creosote (PAH), and seven were contaminated with CCA.
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Soil bioremediation technologies include:
• In-situ (not well evaluated) and ex-situ (focus of most studies)
• Ex-situ include• Prepared bed (share similarity with
land treatment. Treatments include fertilization, tilling, soil pH adjustment, moisture control, and others)
• Slurry bioreactor• compost-biopile reactors
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Case study
• Location: Champion International Superfund Site in Libby, MT
• Major contaminants: PCP and PAHs (residuals of creosote and PCP wood preservatives)
• Bioremediations technology: prepared bed technology for full-scale remediation
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• Contaminated soils were excavated, screened to remove rocks and placed in an excavated waste pit area.
• Total carcinogenic PAHs: 88 mg/kg based on a site specific risk assessment.
• The prepared bed system consisted of two one-acre lined land treatment units (LTU 1 and LTU 2)
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• Soil from the waste pit area was placed in the prepared beds in approximately 15-cm layers.
• Nutrients were added as• C/N ratio = 12-30:1• N/P ratio = 10:1
• Toxicity of water extracts of soil was evaluated using the Microtox assay.
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Pyrene concentration in soil (first-order degradation model; Huling et al., 1995
Sims et al., 1999
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Total carcinogenic PAHs (TCPAH) concentration in soil (first-order degradation model; Huling et al., 1995
Sims et al., 1999
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Conclusions
• Significant decreased PCP, pyrene, and TCPAH concentrations to targeted remediation levels.
• Mutagenicity testing indicated detoxification to soil background levels in three months.
• Biological processes represented the primary mechanism for the decrease in this system