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• steam conditioning• vapor drying• kiln drying• controlled air seasoning• tunnel drying

• After conditioning, wood is immersed in preservative chemicals, sometimes under pressure, at either ambient or elevated temperatures.

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.

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)

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)

Tetrachlorodibenzo()dioxin (TCDD)

• 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.

• 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.

• 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.

• 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.

• 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.

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

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

• 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)

• 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.

Pyrene concentration in soil (first-order degradation model; Huling et al., 1995

Sims et al., 1999

Total carcinogenic PAHs (TCPAH) concentration in soil (first-order degradation model; Huling et al., 1995

Sims et al., 1999

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