MAECTITE Chemical Treatment Process
Table of contents:
MAECTITE Process Overview
Sevenson Environmental Services, Inc. has conducted all phase of heavy metal chemical fixation, from initial viability study through detailed work plans to full-scale implementation. Sevenson has successfully treated heavy metal contamination on small, relatively uncomplicated sites, as well as large complex sites. This success in treating RCRA metal-contaminated wastes is attributable to Sevenson's patented MAECTITE® chemical treatment process. Metals and compounds that are successfully rendered non-hazardous by RCRA definition with the MAECTITE® technology include, lead, cadmium, arsenic, chromium, selenium, and barium. Additional target species are copper, nickel, zinc, cyanide, and sulfide. Low-level radioactive nuclides have also been rendered non-leachableas determined by Gamma Spectra Analysis in TCLP extract.
To date hundreds of thousands of tons of lead, cadmium, and chromium contaminated soil and waste have been chemically fixed by the MAECTITE® process at nearly 100 sites in 18 states and 8 USEPA Regions. The process may be used to treat metal-contaminated soils, solids, sludges or aqueous wastes from the manufacture and use of batteries, paints, pigments, leaded glass, tetraethyl lead, photographic materials, wastes form primary and secondary lead smelting operations, shooting range soil, lead and cadmium contaminated wastes from foundries, chromium ore, ceramic frit sludge, nickel cadmium battery plant sludge and heavy metal contaminated soil and marshland. Contamination has been remediated in a variety of matrix types, including gravelly sandy soil, clay, red soils, ash, foundry sand, and sediments or sludges. All heavy metal contaminated waste materials and debris that fail TCLP criteria have proven responsive to the MAECTITE® treatment process.
The product of MAECTITE® treatment closely resembles untreated material with no volume increase and minimal increase in mass (i.e.The MAECTITE® process was accepted into the USEPA Superfund Innovative Technology Evaluation (SITE) program in 1992. In 1991 it was also nominated for the President's Environment and Conservation Challenge Award. That same year the MAECTITE® process was selected by USEPA as one of six technologies for inclusion in the US/German Bilateral Agreement as part of the environmental technology demonstration and information exchange program.
As a technology approved under USEPA's Pre-Qualified Offers Procurement System (PQOPS), the MAECTITE® treatment process is available to project coordinators and emergency response teams without the need for technical evaluation on EPA funded projects. The process was patented in March 1993 for lead impacted soil and solid waste, and for chromium contaminated material in 1995. Other related patents have been granted or are pending.
Benefits of Sevenson's MAECTITE® Heavy Metals Treatment Process:
| • Cost-effective | • Field proven |
| • Reacts immediately | • Practical field application |
| • NO volume increase | • Long-term product stability |
| • Minimal mass increase | • Applicable to all matrix types |
| • Applicable to all RCRA metals | • Irreversible mineral products |
| • National and State regulatory acceptance | • Remains soil-like after treatment |
| • Meets OSHA requirements and USEPA ARAR's |
MAECTITE Process Description
The one, two, or three-step MAECTITE® process converts leachable metals into mineral crystal species within the waste matrix, greatly lowering the solubility of the metal in this complexed form. The number of treatment reagent additions is a function of the matrix geochemistry, metal type and valence form. In the first step a proprietary powdered chemical may be blended with the lead-contaminated material. In the second step a proprietary liquid reagent (MAEPRIC®) is blended into this mixture. An additional oxidation reducing step may be required for multi-valent metals. Under standard conditions of temperature and pressure, curing takes 3 to 5 hours.
Treated materials consistently pass the Paint Filter test, and meet TCLP criteria for characteristic and listed hazardous wastes as well as criteria associated with other test procedures. These include USEPA SW 846 methods for TCLP, EP Tox, and Multiple Extraction Procedure (MEP Method 1320), and other procedures, such as the California Wet Test (Citric Acid Leach), Synthetic Precipitate Leaching Procedure (SPLP), the Sonication/Extraction Procedure (exposing the sample to intense ultrasonic energy in the presence of extraction fluid), and recently developed simulated bioavailability extractions.
The principle behind the MAECTITE® process is chemical bonding, which creates substituted mixed mineral forms, stable and resistant to leaching. Traditional and generally accepted stabilization testing procedures focusing on geophysical or geotechnical methods are not applicable to material treated by MAECTITE®, although compliance with engineered properties can be easily attained.
Material treated by MAECTITE® contains the metal species as a mineral within the waste matrix. These minerals cannot be degraded by physical forces or other environmental stressors such as chemical conditions present within landfills or associated with acid rain. MAECTITE®'s stability has been supported by exposing MAECTITE®-treated material (containing metallic-complexed mixed mineral forms) to: (1) intense and prolonged ultrasonic energy as a physical degradation force; (2) TCLP and EP Tox methods; (3) MEP that simulates 1000 year acid rain conditions; and (4) simulated gastric fluids in bioavailability testing. Treated material has also been subjected to electron microscopy mineralogic assay testing.
Material treated by the MAECTITE® process resembles untreated material. It is not monolithic, complies with the Paint Filter test free liquid limits, and is easily handled by standard earthmoving equipment. On the rare occasion when the first MAECTITE® application does not achieve treatment criteria, re-treatment is readily accomplished without grinding or shredding to resize previously treated materials as would be the case for competing physical bonding approaches.
MAECTITE Process Geochemistry & Physical Properties
The MAECTITE® chemical process reagents form non-leachable mixed mineral species through induced nucleation form isomorphic reaction-series dynamics using problematic metal ions that are present in soil or waste. As a true chemical process MAECTITE® provides a classical approach to control problem metal and inorganic ions through the manipulation of non-problem inorganic ions.
Traditional stabilization approaches employing silicates, pozzolans, or cement binders create mixtures susceptible to degradation from outside physical forces or pH conditions that overcome buffering capacity. MAECTITE® generated crystal forms cannot be degraded physically or by the most adverse chemical conditions found in environmental settings.
In nature, stability and longevity are largely due to structure and geometric symmetry. The MAECTITE® chemical process is based on this precept. Through the manipulation of soil and solid waste containing problematic metals with mineral dissolution-precipitation reactions, MAECTITE® creates substituted mineral-suite forms in the Barite and Apatite mineral groups.
The Barite Group suit of analogous orthrombic-crystallographic compounds, primarily sulfates, are often present in the matrix to be processed and can be intertwined. From a mineralogist perspective, orthorhombic twinning results in pseudo-hexagonal geometries during crystal nucleation and the MAECTITE® dissolution-precipitation reaction-series.
The Apatite Group represents a suite of hexagonal-crystallographic compounds, primarily as hydoxyapatite, pyromorphite, and other similar forms. Once sulfate ions, either present or supplemented, are consumed from the mother solution or waste matrix, the reaction-series shifts to the post-precipitation stage reverting to supplemental mixed Apatites and/or Apatite/Barite complexes and scavenge the remaining problematic cations. The driving force of the combined MAECTITE® reaction is coincident crystal nucleation, heat loss, and dehydration primarily as a result of stoichiometric geochemical thermodynamics.
Because of the flexibility of the MAECTITE® process, Sevenson can select reagents from a family of reactants ranging from liquids to solids that most efficiently stimulate and induce the desired chemical fixation response. The response is determined one the geochemical properties of a specific material or waste are understood. MAECTITE® process flexibility also allows for the treatment of non-reactive multivalent metal ions such as hexavalent chromium and arsenic. During a cursory treatment step, oxidation-reduction potentials of the material are altered along with the problematic multivalent ions. MAECTITE® then forms minerals with the intermediaries.
The MAECTITE® technology can also be controlled to improve upon geotechnical properties of processed materials. Although longevity and stability (i.e. the ability to resist the leaching of contaminants over prolonged periods of time) of MAECTITE® treated material is not compromised by physical forces as are mixtures and agglomerations, MAECTITE® reactions have significantly enhanced engineering properties of soil and waste. Unconfined compressive strength has been measured in excess of 1500 psi with permeability less than 1x10-8 cm/sec. While these criteria are achievable, the unnecessary use of treatment reagent resources and extended reaction periods must be carefully examined to establish sound, practical, and desired performance objectives.
MAECTITE Process Applications
The MAECTITE® process may be used alone or incorporated into a train of processes that treat organics or other metals. Organics contaminants do not interfere with MAECTITE® ability to form complexed mineral compounds with heavy metal contaminants.
Among the types of material successfully treated by this technology are:
| • Aqueous waste streams | • Battery casings |
| • Construction debris and other oversized material | • Filter and centrifuge cake |
| • Glass (coated & impregnated) | • Matte/Dross |
| • Paint chips and abrasives | • Peat |
| • Sediments | • Shooting/Skeet Range Soils |
| • Sludges | • Smelter slag |
| • Soils-clay, sand, gravel, silt and mixtures | • Wire chop & installation fluff |
The most common application of the MAECTITE® process is ex-situ. Ex-situ treatment can allow for greater control under some conditions. Ex-situ treatment has been successfully applied to wastes amounting to only a few drums up to a project requiring 170,000 tons of treatment at a production rate of 2,000 tons per day. The system may be modified to comply with RCRA regulations on closed/contained and tank treatment systems.
Several in-situ processing techniques are available depending on topography and near surface soil conditions. Projects range from little as 300 yd3 treated to a depth of one foot to over 11,000 yd3 to a depth of 13 feet. Current capabilities now allow treatment to take place at much greater depths and under water tables. In-situ treatment at voluntary remediation and RCRA sites have not required a Part B RCRA treatment permit and have allowed clients to significantly expedite regulator approval for onsite treatment when the alternative was excavation and offsite disposal as hazardous waste.
Cost of MAECTITE® treatment is low to moderate. Cost-effectiveness will depend on a number of site-specific factors. These include:
- Treatability studies and determination of reagent dosage requirements
- Physical handling characteristics of contaminated material
- Treatment system sizing and material variability
- Ease of site access
- Transportation and disposal costs for treated material
- Site support requirements
- Waste quantities (economy of scale)
- Ancillary site tasks additional to treatment