,

Chemical Fixation of Heavy Metal-Contaminated Soils

C. Paul Lo, Daniel N. Silverman 111 and Angela M. Porretta Chemfix Technologies, Inc.

INTRODUCTION

The treatment of heavy metal contaminated soil has become an important issue in the past few years. Nearly one-third of the sites on the Superfund National Priorities List (NPL) possess lead concentrations significantly higher than normal background levels. The dif f i- culties in treating heavy metal contamination stems from the fact that they cannot be destroyed or biodegraded.

The U.S. Environmental Protection Agency (EPA) has been working toward the establishment of treatment standards for contaminated soils. The Office of Emergency and Remedial Response is in the process of develop- ing and carrying out a plan to address the data needs for establishing the Pest Demonstrated Available Technologies (BDAT) for Superfund soils. The BDAT program is divided into phases. Phase One, completed in December 1987, evaluated the performance of various treatment technologies utilizing an artificially contaminated soils. The soil composition and contaminant concentrations were designed to reflect those of typical Superfund soils with slight modification for some health and cost concerns. The second phase of the Superfund BDAT program will be to carry but the technology evaluations on real Superfund soils.

The EPA has alsc? established a technelegy research demexs t ra t iGn afid evaluation program to promote the development and use of innovative technologies to treat Superfund wastes. Though this Superfund Innovative Technology Evaluation Program (SITE) addresses all Superfund wastes, a great majority of the sites are partially or entirely con- taminated soils. The program gives developed technologies the opportunity to demonstrate their process on an actual Superfund site. (

Chemfix Technologies, Inc. (CTI) has been chosen as one of the companies to demonstrate their process in the SITE 002 Program. The

204

proprietary CHEMFIXa process is a chemical fixation technology based on a series of complex silicate reactions to render wastes non-hazard- ous. The following study treated the heavy metals contaminated soil commonly found at the Superfund site with the CHEMFIX@ process.

BACKGROUND

Section 3004 of the Resource Conservation Recovery Act (RCRA) pro- hibits the land disposal of certain RCRA wastes. The Hazardous and Solid Waste Amendments (1984) to this act establish treatment standards for certain hazardous wastes prior to landf illing . ( effective dates of the prohibition, wastes may only be land disposed if: 1) they comply with treatment standards promulgated by the EPA that protect human health and environment by minimizing short and long term'threats arising from land disposal or 2 ) the EPA has approved a site-specific petition. (4) hazardous wastes has been expanded to encompass the treatment of contaminated soils.

After the

This concept of treatment standards for

Currently there is a variety of treatment technologies available for heavy-metal contaminated soils (both bench and pilot scales). P. S. Puglionesi et al. ( technologies chosen through extensive literature research and personal contacts.

conducted an evaluation of various treatment

In evaluating the technologies, Puglionesi et al. used the following criteria: effectiveness, residue treatment/disposal, long-term performance, safety, environmental risk and economics. The highest ratings were received by microencapsulation, roasting extraction, stabilization and in situ vitrification. It should be noted, with any waste stream, a variety of factors need to be considered when choosing a treatment technology. No one treatment technology is ideal for all situations. Site specific evaluations need to be conducted in order to make rational decisions as to what technology should be used.

Table One summarizes the treatment technologies evaluated by Puglionesi et al.

TABLE 1

TECHNOLOGIES IDENTIFIED FOR METALS TREATMENT IN SOILS/SLUDGES

TECHNOLOGY TREATMENT TECHNIQUE COMMENTS In situ vitrification Thermal immobilization Electrodes to

heat and glassify ground

Onsite vitrification Thermal immobilization Electrodes to heat and glassify waste stream

Onsite plasma arc (w/metal recovery) Thermal recovery Destroys

organics, gasi- fies and condenses metals

205

High-temperature f Puid wall

Roasting

Chloride volatilization

Onsite precipit.ation

In situ precipitation

In situ precipitation by vapor phase application

Onsite extraction

In situ extraction

Vegetative uptake

Stabilization (admixing)

Thermal immobilization Destroys organics, glassifies metals

Thermal immobilization Results in glassification

Thermal recovery Roasting of chloride results in volatilization

Chemical immobilization Use of wastewater precipitation techniques to immobilize metals

Chemical immobilization Use of wastewater precipitation techniques applied directly to the soil in place

Chemical immobilization -------

Chemical mobilization Chelators or surfactants used to mobilize metals, needs associated recovery technique

Chemical mobilization Same as above

Biological Metals accumulate in plants, ultimate fate not addressed

Physical immobilization Chemical fixation in a cementious or pozzolanic mixture

Macroencapsulation Physical isolation

Microencapsulation Physical isolation

Geologic isolation Physical isolation

Secure landfill Physical isolation

Coating with a low permeability mixture

Mixing and extruding in a low-permeability mat e ria 1

Not a treatment

Not a treatment

206

In situ adsorption Chemical immobilization Materials adsorbed and immobilized

In situ ion exchange Chemical immobilization Materials

Source: Adapted from P. S. Puglionesi et al. (5)

adsorbed

CHEMFIX@ PROCESS DESCRIPTION

The CHEMFIX@ process is defined as a chemical fixation/stabilization technology. This proprietary process, patented by Chemfix Tech- nologies, Inc. (CTI), stabilizes mobile constituents of concern within a waste by chemical reactions and physical encapsulation.(6)

The CHEMFIX@ process is based on the use of soluble silicates and silicate setting agents. The combination and proportions of reagents are optimized for each particular waste requiring treatment. The two ( 2 ) part, inorganic chemical system reacts with polyvalent metal ions, other waste components, and also with itself to produce a chemically and physically stable solid material. The cross-linked, three dimensional polymeric matrix displays properties of good stability, high melting point, and a rigid, friable texture similar to that of a clay soil.

Three ( 3 ) classes of interactions can be described. First are the rapid reactions between soluble silicates and the polyvalent metal ions, producing insoluble metal silicates. Second, are reactions between the soluble silicates and the reactive components of the setting agent, producing a gel structure. Third, are hydrolysis, hydration, and neutralization reactions between the setting agent and the waste and/or water.

Most of the heavy metals contained in the waste become part of the complex silicates with some of the heavy metals precipitating as metal hydroxide within the structure of the complex molecules.

There are no side streams or discharges resulting from the CHEMFIX@ process. During processing, all the waste is transferred to the high shear mixer wherein the reagents immediately react to form a gel. This gel is then discharged to the receiving area. stage, the water in the CHEMFIX@ product does not form a separate phase. Some of the water becomes part of the solids, but most is physically bound in the hydrophilic CHEMFIX@ product.

EXPERIMENTAL METHODOLOGY

Even at this early

This investigation was conducted as two separate experiments. One set of experiments was performed on synthetic environmental soil samples created to reflect the composition and concentration of "typical" Superfund site soils. The other set of experiments was performed on contaminated soils from an actual Superfund site. Both experiments were concerned with the ability of the CHEMFIX@ process to solidify and stabilize the heavy metals contaminants in the soils.

A. SYNTHETIC SOIL MATRIX (SSM) EXPERIMENT

The Synthetic Soil Matrix (SSM) composition was prepared as 3 0 % by

207

volume of clay (montmorillinite and Kaolinite), 25% silt, 20% sand, 20% top soil and 5% gravel. The components were assembled, air dried, and mixed together. This Synthetic Soil Matrix sample was prepared to represent a typical eastern U.S. soil samples.

The SSM sample was spiked with seven different heavy metals. The con- centrations of each spike were followed: arsenic - 500 mg/kg, cadmium- 1000 mg/kg, chromium - 1500 mg/kg, copper - 9 5 0 0 mg/kg, lead - 14,000 mg/kg, nickel - 1000 mg/kg, and zinc - 22,500 mg/kg. The concentra- tions used were based on the occurrence, frequency and concentration of contaminants commonly found in Superfund soils.

The water content of the SSMs was determined in order to evaluate the necessity of additional water requirements for ease of handling and/or mixing with the CHEMFIX@ reagents. dried at llO°F overnight. The samples were reweighed the following day every hour until two consecutive readings did not differ by more than 1%. The initial and final weights of each SSM were used in the following formula to determine the water content of the sample.

Known amounts of SSM were oven

The SSM samples were then adjusted so that their water content was approximately 30% by weight by adding distilled-deionized water. This was done to facilitate the reaction in the CHEMFIX@ process. This adjustment also allowed for better product handling capabilities.

The optimization of CHEMFIX@ reagents on the hydrated samples was then conducted. Four samples of SSM were treated with varying ratios of reagents. The soil sample and reagents were thoroughly mixed and consolidated into one large lump and allowed to cure for 48 hours. After curing, the Unconfined Compressive Strength (UCS) of the samples was measured by using a penetrometer. The reagent ratio that resulted in the desired UCS (1.5-2.5 tons/ft2) was utilized for further leach- ability testing.

The solidified SSM sample at the optimum reagent ratio was tested for metal leachability by the Toxic Characteristic Leaching Procedure as per 51 FR 21685-21693.(7) The target metals were analyzed by atomic absorption spectrometer using USEPA SW-846 "Test Methods for Evaluating Solid Wastes," 1986(8) as required by 40 CFR Part 261.(9)

The next phase of the experiment was the determination of the kinetics involved in the metal-binding reactions within the CHEMFIX@ product matrix. Eight (8) samples of each SSM were treated by the CHEMFIX@ process at the optimum reagent ratio. A TCLP extraction was performed 0, 1, 3, 5, 8, 24, 48 and 72 hours after treatment. The TCLP leachate from each sample was analyzed for target metals concentration.

B. SOLIDIFICATION OF SUPERFUND SOILS

Soils from ar, actual Superfund site i n the Nertheast were nced i n this part of the investigation. These soil samples were found to be con- taminated with cobalt, nickel and cadmium ranging from 3000 to 5000 mg/kg

Upon receipt of these samples, the water content was determined and adjusted. For these experiments the desired water content was 55%. These adjustments are done depending on the mixing requirements and material handling requirements.

208

The first screen of reagent optimization testing was then conducted on the diluted material and unconfined compressive strength (UCS) readings were performed at various intervals during the curing process. A total of 10 different reagent ratios were utilized. Once UCS trends were established, three (3) of the best reagent ratios were selected to undergo analysis. New samples of the three ratios were made and UCS readings over time were again recorded.

The three samples with the optimum reagent ratios (Ratio A, B, and C) were subjected to TCLP analysis and the leachate was analyzed for arsenic, barium, cadmium, chromium, cobalt, lead, mercury, nickel, selenium and silver.

In order to determine the long term stability, a Multiple Extraction Procedure (MEP) was conducted on one of the most solidified samples accordin to USEPA SW-846, method 1320 for all eight metals as above. ( 8 7 RESULTS AND DISCUSSION

A. SYNTHETIC SOIL MATRIX (SSM) EXPERIMENT

The spiked synthetic soil sample was analyzed for spiked metals re- covery prior to CHEMFIX@ treatment. each spiked metal. In general, 68-153% recovery was obtained.

Table 2 summarizes the recovery of

Table 2

RECOVERY OF SPIKED HEAVY METALS IN THE

SYNTHETIC SOIL MATRIX (SSM)

Target Actual Re cove ry

(mg/kg 1 (mg/kg 1 ( % I Metal Concentration Concentration -

Arsenic 500 Cadmium 1,000 C hr omi um 1,500 Copper 9,500 Lead 14,000

Zinc 22,500 Nickel 1,000

760 1,070 1,020 7,990 13,940

93 0 24,100

153 106 68 84 99 93 107

In order to facilitate the solidification and aid in material handling, the SSM was adjusted to 70% solids by weight by adding water. This was necessary when treating high solids material to assure sufficient water present to allow the required chemical reactions to go to completion.

To test the effectiveness of the CHEMFIX@ process as a treatment option, the solidified samples were subjected to the TCLP test and a comparison made with the TCLP results on a raw sample. This test was designed to determine the mobility of contaminants present in liquid, solid and multiphasic wastes. If the TCLP extract from a representa- tive sample contained any of the listed contaminants above the regulatory levels it would be considered a hazardous waste and should adhere to the strict disposal requirements. (

209

Table 3 lists the TCLP leachable metals in both the untreated and treated SSM samples as well as the TCLP regulatory limits. The un- treated SSM was weight adjusted to compensate for both water and reagent additions during treatment so that the actual amount of SSM would be equal to the CHEMFIXB product. The results showed that the CHEMFIX@ treatment reduced metal leachability by 96-100%. More importantly, though, was the ability of the treated material to pass the TCLP regulatory limits. Examination of the data showed the TCLP leachable metal concentrations were at least one to two orders of magnitude below the regulatory levels. The treated material would no longer be classified as hazardous and could be handled and disposed of like other non-hazardous solid wastes based on the leachable metal concentrations.

Table 3

TCLP LEACHABLE METAL CONCENTRATIONS OF CHEMFIX~

TREATED AND UNTREATED SAMPLES TCLP

Untreated* Treated Percent Limits Metal (mg/l) (mg/l) Improvement (mg/l)

Arsenic 1 0 . 5 0.384 96 Cadmium 28 .0 (0.005 100 Chromium 7.0 0.10 99 Copper 1 7 6 . 0 0.12 99 Lead 38 .0 (0.05 100 Nickel 21.5 0.23 9 9 Zinc 53 0 0.033 1 0 0

5 .0 1 . 0 5 .0 NA 5,O NA NA

*weight - adjusted value NA - Not available A concern raised during full scale operations of the CHEMFIX@ process was the possibility of contaminant leaching from freshly treated, moist material. During normal processing procedures, treated material would be transferred directly onto the solidification cell. It would be this material that exhibited the potential for contaminant leaching in those 48 hours it took to physically solidify. To determine when the metals were actually bound in the product matrix, a TCLP analysis was done on a series of treated samples over a period of curing times. The results are illustrated in Table 4.

Table 4

TREATED SSM 1 TCLP LEACHATE METALS CONCENTRATION, mg/l

AFTER TREATMENT

As Cd Cr cu Ni Pb Zn

Untreated Material* 1 0 . 5 28 .0 7.0 1 7 6 21 .5 38.0 5 3 0 0 hr after treatment 0.424 0.077 0.07 0.07 0.39 (0.05 0.014 1 hr after treatment 0.388 0.064 0.09 0.07 0.33 <0.05 0 . 0 1 1 3 hrs after treatment 0.344 0.035 0.06 0.08 0.20 <0.05 0.039 5 hrs after treatment 0.256 0.005 0.07 0.09 0.22 <0.05 0.028 8 hrs after treatment 0.348 <0.005 0.10 0.08 0.16 <0.05 0.026 2 4 hrs after treatment 0.396 <0.005 0.11 0 .11 0.20 <0.05 0.028

210

48 hrs after treatment 0.384 <0.005 0.10 0.12 0.23 <0.05 0.033 7 4 hrs after treatment 0.300 c0.005 0.12 0.13 0.23 (0.05 0.012

*Weight adjusted

The data shows an immediate decrease (the difference between untreated and 0 hours after treatment) in the TCLP leachable metals concentration for all seven metals. This indicated that the metals were bound immediately upon being treated. There would be'no threat for contami- nant leaching during the period it took for the CHEMFIX@ product to physically solidify. In addition, the TCLP leachable metal concentra- tions of those samples that were not allowed to cure ( 0 hours after treatment) all passed the TCLP regulatory limits. Therefore immediately after treatment the material can be classified as non- hazardous.

B. SUPERFUND SOIL TREATMENT

Treatability studies were performed on actual soils from a Superfund site located in the northeast United States. The site was contaminated with heavy metals, in particular, cobalt, cadmium and nickel. These contaminants ranged in concentrations from 3000-5000 mg/kg.

Optimization tests for reagent ratios were conducted similarly to those described for the synthetic soils. Figure 1 outlines the UCS with curing time for all three optimum reagent ratios. At the end of the 48 hour curing time, more than 2.5 tons/ft2 UCS was observed. UCS was adequate for the CHEMFIX@ product. A TCLP analysis was also conducted on three of these reagent ratios (Ratios A, B, and C), the results of the leachate analysis are shown in Table 5. All three reagent ratios result in material that passes the TCLP regulatory limits, and could be classified as non-hazardous materials.

This

FIGURE 1 ucs vs TIME OF CURING FOR CHEMFIX~ PRODUCT

I3 0 0 Frr

\ cn z 0 E

0 AATIOA + RATIO B -u- RATIOC

0 1 0 20 30 4 0 5 0

HOURS

The results of the metals concentration of the three reagent ratios did not vary significantly from each other. Ratio B was chosen as optimal when examining all factors, i.e. UCS, material handling and character- istics and economics.

211

In order to test the long term stability of the CHEMFIX@ process, the Multiple Extraction Procedure (MEP) was performed on the soil. samples treated with the optimal, reagent ratio B. These results are illus- trated in Table 6.

The results showed that there was no increase in leachate metals con- centration after ten extractions. Most of the metal concentrations remained the same or decreased during the total MEP run. This proves the long term stability of the CHEMFIX@ product since the MEP was designed to simulate conditions of 1000 years of acid rain exposure. In addition, throughout the entire MEP analysis the leachable metals concentrations were all below the regulatory limits. Thus once the soil is treated by the CHEMFIX@ process it remains non-hazardous for extended periods of time.

TABLE 5

TCLP RESULTS OF CHEMFIX@ PRODUCTS FOR SUPERFUND SOILS

Metals

Ar seni c Barium Cadmium Chromium Cobalt Lead Mer cur y Ni cke 1 Se len i um Silver

Ratio Ratio A

( mg/l)

0.015 0.9 0.194 0.11 0.05

<O. 05 0.0011 0.33

<o. 002 < o . 01

B (mg/l)

0.011 0.3

<O. 005 0.13

(0.05 <O. 05 0.0013 0.41

<o. 002 <o. 01

Ratio TCLP C Limits

(mg/l) (mg/l)

0.004 0.2

<O. 005 0.16 0.06

<O. 05 0.0015 0.41 0.003

<o . 01

5.0 100.0 1.0 5.0 NA 5.0 0.2 NA 1.0 5.0

NA - Not available

TABLE 6

MEP RESULTS OF CHEMFIX@ PRODUCT (RATIO B)

FOR SUPERFUND SOIL

Arsenic Barium Cadmium Chromium Cobalt Lead Mercury Nickel Selenium Silver

0,014 0,012 0.010 <o.002 (0.002 <0-002 <ne002 0,004 <0.002 0.004 0.2 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.021 <0.005 <0.085 <0.005 0.019 0.007 0.021 0.033 0.029 0.010 0.20 0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 0.0020 0.0015 0.0010 0.0013 0.0016 0.0012 0.0018 0.0019 0.0015 0.0018 0.53 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 <0.04 0.020 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <O.O1 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

212

FULL SCALE OPERATIONAL DESIGN

Based on the information generated from these studies, a full scale treatment process was designed. The processing units will be located next to the final solidification cell so that the final treated material can be placed directly into the cell. In the treatment of contaminated soil, the soil will be excavated from the original site prior to processing and stockpiled near the CTI processing unit.

Due to the heterogeneity of materials in soil (i-e., gravel, rocks, vegetation) prescreening is necessary to insure the material is uniformly fed to the processing unit. The prescreening would be achieved in several sieving stages to ultimately obtain particles of no more than 1/4 inch in size. The prescreened material would then be stockpiled for future treatment.

Particles that do not pass through the prescreening can be dealt with in one of two ways, 1) crushing/grinding/shredding or 2 ) soil washing. The crushing/grinding/shredding option would result in smaller parti- cles amenable to the CHEMFIX@ process. The soil washing would remove the heavy metals adhering to the surface of the larger particles. After soil washing wash water could be reintroduced into the CHEMFIX@ process later for hydration purposes thus eliminating the need for any post-washing water treatment.

The designed treatment system consists of a dry and liquid reagent storage trailer (or silo) and feeder; a feed hopper; accompanying controls and instrumentation; a CTI high shear mixer; conveyors; and a weigh feeder. The weigh feeder controls the respective feed rates of dry and liquid reagents via a 4 to 2 0 milliamp signal sent to the central control unit.

The contaminated waste materials would be loaded into an input hopper which would feed the CTI processing system. The material would probably have to undergo secondary grinding prior to entering a weigh feeder. The weigh feeder will accurately record the amount of input waste material to the processor, as well as control the addition of subsequent mix water. The waste material would be mixed with the dilution water using a CTI-designed homogenizer. The planned proces- sing range is from 4 0 percent to 60 percent solids. The prepared waste material mixture would then go into the high shear process mixer where the liquid and dry reagents are added. The resultant CHEMFIX@ product would be transferred to the adjacent solidification cells where it will complete stabilization/solidification.

It was estimated that a treatment rate of 250-350 cubic yards/day by the CHEMFIX@ process can be achieved. However, the prescreening rate of the material would vary depending on the nature of the material. The treatment cost was estimated to be $65-$125/cubic yard. Actual costs for other work will vary from lower to hiqher unit costs shown depending on job size.

CONCLUSION

The laboratory data from both parts of this study proved that the CHEMFIX@ process was capable of treating contaminated soils, rendering them non-hazardous. The results from the kinetics experiment on the Synthetic Soil Matrix (SSM) showed that the metal-binding reactions took place almost instantaneously. This should eliminate any concern

213

reqarilinq potential leaching from treated material while in the solidification cell awaiting physical solidification. The conclusions from the SSM experiments were supported hy the work done on the actual Superfund soils. The Multiple Extraction Procedure results demon- strated the long term stability of the CHEMFIX@ product. The informa- tion generated as a result of these experiments together with a viable engineering design for a full scale treatment unit indicated the potential of the CHEMFIX@ process as a treatment option for contamina- ted soils.

214

REFERENCES

"Treating Soils Contaminated With Heavy Metals" The Hazardous Waste Consultant January/February 1988, Pg. 1-20.

"Superfund Innovative Technology Evaluation (SITE) Program" U.S. Environmental Protection Agency HMCRI Conference Publication, 1987.

Federal Register, Volume 51. Pg. 40572. November 7, 1986.

"First-Third Wastes Treatments Proposed; Large Amount Deferred Due to Lack of Capacity" BNA Environmental Reporter Vol. 18, Number 49. April 1, 1988. Pg. 2387.

P. S . Puglionesi et al. 1987, "Treatment Technologies For Heavy Metal Contaminated Soils" American Defense Preparedness Association's 15th Environmental Symposium Proceedings. April 28-30, 1987.

United States Patent No. 3,387,872. "Method of Making Waste. Non-Polluting and Disposable". Inventor: Jesse R. Connor. September 24, 1974.

Federal Register, Volume 51. Pgs. 21685-21693. June 13, 1986.

"Test Methods for Evaluating Solid Waste". SW-846-USEPA. November, 1986.

Code of Federal Regulations, Title 40, Part 261. Pg. 359 of 1986 edition.

2 1 5