Science Inventory

REMEDIATION OF SOILS CONTAMINATED WITH WOOD-TREATMENT CHEMICALS (PCP AND CREOSOTE)

Impact/Purpose:

The goal of this project is to develop a slurry biotreatment process for soils contaminated with Pentachlorophenol (PCP) and creosote.

Description:

PCP and creosote PAHs are found in most of the contaminated soils at wood-treatment sites. The treatment methods currently being used for such soils include soil washing, incineration, and biotreatment. Soil washing involves removal of the hazardous chemicals from soils using solvents; the solvent stream must still be treated for destruction of the contaminants. Incineration is an effective tool for destruction of the contaminants. But the cost of incineration is very high; it is also very difficult to install new incineration facilities due to strong public opposition. Bioremediation has been considered and used for treatment of soils contaminated with wood-treatment chemicals. But the present state of bioremediation technology results in rapid mineralization of low molecular weight PAHs only, leaving PCP and high molecular weight PAHs in the spent soil. Unfortunately, these are the chemicals that are the most toxic, carcinogenic, and regulated. Slurry-phase biotreatment of contaminated soils and sediments is an innovative treatment technology. It's advantages include easy manipulation of physicochemical variables and operating conditions to enhance the rates of biodegradation, and ease of containment of exhaust gases and effluent. Use of slurry bioreactors is particularly attractive for fine soils (high silt/clay content) having complex contaminant-matrix, and for systems in which problems of low mass transfer rates and of limited solubility of strongly sorbed contaminants preclude the use of other simpler solid-phase treatment technologies such as land farming and biocells. Bioslurry technology is currently hampered by a lack of information concerning the operating conditions and the strategies for reactor operation that result in mineralization of PCP and high molecular weight PAHs within reasonably short residence times of solids in the reactors, efficient means of keeping the solids in slurry and delivery of oxygen, and reliable cost numbers for operation and maintenance. Relieving these bottlenecks will result in rapid adaptation of this technology and its successful implementation.

Engineering and process development aspects of bioslurry treatment of PCP and creosote contaminated soils from a Superfund site will be studied in this project in shake flasks and in 14 liter well instrumented fermentors. Use of surfactants and cosolvents will be explored in order to enhance the aqueous solubility of hydrophobic and sparingly soluble contaminants. The effect of cosolvent on microbial activity will be studied. Kinetic studies for the biodegradation of PCP and PAHs will be carried out in sealed bioreactors so that accurate material balances can be taken. Experiments are planned to investigate the role of surfactant/cosolvent, temperature, carbon source, and oxygen delivery by sparging of pure oxygen in reduction of concentrations of PAHs and PCP in the contaminated soil slurry.

Reactors with power measurement devices (dynamometer) will be used to investigate several different types of mechanical agitators in order to keep the solids in suspension. The power requirement under aerated and unaerated conditions will be correlated with geometrical and system parameters such as particle size, nature of soil (as determined by plasticity and flow indices), solid density and physical dimensions in the reactor. Oxygen transfer rate and oxygen transfer efficiency in the slurries with sufficient power input for minimal and complete suspension will also be studied in this reactor. Correlations will be developed for these two very important operating parameters for which no information is available from literature.

All of the information will be used to develop a flow diagram of the bioslurry treatment process for cleanup of contaminated sites and to generate cost data that may be used to determine the cost effectiveness of this process for field-scale treatment. This, of course, will require SITE demonstration that will be undertaken in a follow-up stage on the basis of information gathered in this project.

Based on the work conducted in the previous years, Triton X-100 was selected as surfactant of choice for enhancement of solubility of PAHs from contaminated soil and hence to enhance their biodegradation. The concentration of soil in slurry was 30% (w/w) and surfactant concentration in the slurry was 1% (w/w). Enriched culture was developed from the contaminated soil. The medium used for enrichment contained K2HPO4 (3g/l), KH2PO4 (15. g/l), (NH4)2SO4 (1.25 g/l), Yeast Extract (0.5 g/l), NaCl (0.01 g/l), MgSO4 (0.1 g/l), FeSO4.7H2O (1 mg/l), and acetic acid (0.5 g/l). pH of the culture broth was adjusted to 7 before sterilization for 15 minutes at 121 ?C. Contaminated soil (50 g/l) and phenanthrene (1 g/l) was added to 100-ml sterile broth, which was incubated at 25 ?C and 150 rpm. After 72 hours, the aqueous phase was used to inoculate liquid medium containing phenanthrene and the process was repeated three times; the resulting culture was stored at 4 ?C and used in experiments.

All the biodegradation experiments were conducted in triplicate. The matrix of experiments consisted of:

-Control slurry without surfactant
-Control slurry with surfactant
-Inoculated slurry without surfactant
-Inoculated slurry with surfactant

Samples were collected by sacrificing the flasks. The slurry was centrifuged to separate the liquid medium from solids and both the phases were analyzed separately for a number of PAHs. From these data the total PAHs and B(a)P equivalents in the respective phases were calculated. The concentration of Total PAH and BaP equivalents in the slurry have been reported.

Consistent with the observations of scores of other researchers dealing with hydrophobic organic compounds, the concentrations of PAHs (and BaP equivalents) increased with time before decreasing again. Addition of surfactant resulted in a considerable and rapid increase in the solution-phase concentrations of PAHs as well as in the concentration of extractable PAHs present in the solid phase. Addition of surfactant in absence of enriched culture did not result in any significant differences either in the concentrations of PAHs or in the BaP equivalents. Addition of surfactant in presence of enriched culture increased the maximum extractable PAHs by more than 200% over surfactant-free slurry while still resulting in lower total PAH concentration after 60 days. The experiments were stopped on 60th day. BaP equivalents decreased from a high of 30 mg/Kg to 2 mg/Kg in inoculated slurries with surfactant, compared to a final value of 8-10 mg/Kg in other experiments.

Power requirements and oxygen transfer in agitated and aerated soil slurries: These measurements were conducted in a 310 liter working volume agitated reactor. The reactor diameter was 30 inches and it was equipped with a 10-inch hydrofoil agitator mounted on a central shaft. At the top of the agitator shaft, a torque sensor was fitted to measure the shaft torque. Power consumption by the DC motor was also measured as to control the accuracy of power measurements by the torque sensor. The measurements were first conducted with no fluid in the reactor and these values were deducted from any actual reading from rotating agitator in the slurry in order to account to any friction in the bearings. A PC coupled to the instruments automatically logged shaft torque, current and voltage drawn by the motor and any readings from exit gas oxygen analyzer. Any air sparged through the reactor was introduced through a sintered disk sparger (6" diameter located under the agitator) and its flow rate was measured by rotameters installed in line as well by a mass flow controller whose signal was logged by the PC. The experiments were conducted at several different flow rates of air (corresponding to 0, 0.005, 0.01, 0.05, 0.10 and 0.15 vvm), with and without any soil in the reactor. When soil was present, 30% (w/w) clay slurry was used. In the fluid, Triton X-100 was added to study the effect of surfactant on the power consumption and oxygen transfer rate. Sulfite method was used to measure the oxygen transfer rates in the reactor. At a given flow rate of air, concentration of soil and surfactant in solution, the agitation rate was changed in small increments and shaft torque, exit gas oxygen concentration, and other motor power readings were noted under steady condition. The measurements were conducted while increasing the shaft rpm as well while decreasing the shaft rpm. In general, excellent reproducibility was obtained.

The impeller power numbers (calculated from measured shaft torque values) have been plotted against impeller Reynolds numbers. In general, a classical inverse relationship between power number and impeller Reynolds number was observed at low shaft rpms and at high shaft rpms, the power number was constant. The value of power number under the conditions of high rpms was between 0.2 and 0.4.

This project was the subject of two poster presentations at the annual HSRC conference in 1997. Results from the project have been published in one peer-reviewed journal and more articles are planned. Investigators are planning to make presentations at various scientific conferences and to attend conferences to interact with consultants and companies that are interested in bioslurry reactor technology, as funding allows.

URLs/Downloads:

Final Progress Report

Record Details:

Record Type:PROJECT( ABSTRACT )
Start Date:05/18/1995
Completion Date:05/17/1996
Record ID: 57373