Patent Application
20080086022
Environmental regulations throughout the world such as those promulgated by the USEPA under RCRA and CERCLA require heavy metal bearing waste, contaminated soils and material producers to manage such in a manner safe to the environment and protective of human health. In response to these regulations, environmental engineers and scientists have developed numerous means to control heavy metals, mostly through chemical applications which convert the solubility of the material and waste character to a low solubility state and thus low exposure form, thus passing leach tests and allowing the wastes to be either reused on-site or disposed at local landfills without further and more expensive control means such as hazardous waste disposal landfills or facilities designed to provide metals stabilization. The primary focus of scientists has been on singular heavy metals such as lead, cadmium, chromium, arsenic and mercury, as these were and continue to be the most significant mass of metals contamination in soils. Materials such as paints, and cleanup site wastes such as battery acids and slag wastes from smelters are major lead sources. Recently, however, there exists a demand for control methods of heavy metals in combined form as Pb and As in contaminated soils and capable of meeting a combination of test evaluations including TCLP, SPLP and MEP.
The present invention discloses a combined Pb and As bearing material, contaminated soil or waste, stabilization method through contact of material, contaminated soil or waste with stabilizing agents including a single step application combination of water insoluble phosphates, cement kiln dust, and ferric source. The stabilizing agents found effective are available in dry or slurry form, and thus can be contacted with heavy metal bearing material prior to waste generation such as in-stream at wastewater sludge producing plants or in-duct prior to air pollution control and ash collection devices or after waste production in collection devices such as hoppers, dump valves, conveyors, dumpsters, in-situ, in-ground, or in waste piles. The stabilizers are applied in a manner to utilize cement kiln dust as a stabilizing agent and not a cement-like additive thus allowing stabilized material, contaminated soils and waste to remain suitable for fill material or relatively loose handling. The insoluble phosphate agent acts to exchange calcium ions for Pb ions in solution an form Pb substituted calcium phosphate apatite minerals, while not interfering with Ferric Oxide complexing with available As in solution to form insoluble ferric arsenate.
It is anticipated that the stabilizers can be used for both RCRA compliance actions such that generated materials from wastewater facilities, furnaces, incinerators and other facilities do not exceed appropriate SPLP and MEP groundwater criteria and/or TCLP hazardous waste criteria under TCLP or CERCLA (Superfund) response where stabilizers are added to waste piles or storage vessels previously generated and now regulated under RCRA as a hazardous waste pre-disposal. The preferred method of application of stabilizers would be in-situ, in-line within the property and facility generating the heavy metal bearing material, and thus allowed under RCRA as a totally enclosed, in-tank or exempt method of TCLP stabilization without the need for a RCRA Part B hazardous waste treatment and storage facility permit(s).
The use of water insoluble phosphates could include but not be limited to dicalcium phosphate, tricalcium phosphate, monocalcium phosphate, phosphate rock, pulverized forms of all above, and combinations thereof which would, as an example, provide various amount of water insoluble phosphate in combination with cement kiln dust and ferric source with Pb and As bearing material or waste. The water insoluble phosphate, cement kiln dust, ferric source and combination type, size, dose rate, contact duration, and application means could be engineered for each type of heavy metal bearing material, contaminated soils or waste.
Although the exact stabilization formation molecule(s) are unknown at this time, it is expected that when Pb and As come into contact with the stabilizing agent(s), low water and low acid soluble compound(s) begin to form such as a mineral phosphate, twinned mineral or precipitate through substitution or surface bonding, which is less soluble than the heavy metal element or molecule originally in the material or waste. Specifically complexing and/or twinning of Pb and As into pyromorphite amorphous crystals most likely occurs by adding calcium phosphate(s) to the material or waste at standard temperature and pressure. It also remains possible that modifications to temperature and pressure may accelerate of assist formation of minerals, although such methods are not considered optimal for this application given the need to limit cost and provide for optional field based stabilizing operations that would be complicated by the need for pressure and temperature control devices and vessels. Ferric arsenate is likely formed in solution and as a substituted surface reaction and possibly twinned into the calcium phosphate apatite mineral. The kinetics of soluble phosphate causing or contributing to As leaching is not known, but it has been observed that allowing ferric arsenate to form in the presence of water insoluble phosphates achieves a lower soluble As level from the stabilized matrix.
Examples of suitable stabilizing agents include Cement kiln dust, ferric powder, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, water insoluble phosphate fertilizers, phosphate rock, pulverized phosphate rock, calcium orthophosphates, monocalcium phosphate, dicalcium phosphate, tricalcium phosphate, fishbone phosphate, animal bone phosphate, herring meal, bone meal, phosphorites, and combinations thereof. The amounts of stabilizing agent used, according to the method of invention, depend on various factors including desired TCLP, SPLP, and MEP solubility reduction potential, desired mineral toxicity, and desired mineral formation relating to toxicological and site environmental control objectives. It has been found that an amount of certain stabilizing agents such as 8% Cement Kiln Dust, 0.5% Ferric powder, and 1.0% Dicalcium Phosphate by weight of Pb and As bearing contaminated soil both unsaturated and brackish groundwater saturated was sufficient for TCLP, SPLP, and MEP stabilization to less than limits of 10 parts per billion (ppb) Pb and 200 ppb As. However, the foregoing is not intended to preclude yet higher or lower usage of stabilizing agent or combinations if needed since it has been demonstrated that amounts greater than 10% CKD and 1% phosphate by weight also work, but are more costly.
The examples below are merely illustrative of this invention and are not intended to limit it thereby in any way.
In this example Pb and As contaminated soil (waste from Conoco Phillips operations) collected by URS personnel at soil remediation project in Weymouth, Mass., was stabilized with varying amounts of stabilizing agents including Magnesium Oxide (MgO), Cement Kiln Dust (CKD), ferric oxide powder (FP), DiCalcium Phosphate (DCP), Triple Superphosphate (TSP) and combinations thereof. Various stabilized and un-stabilized soil samples were subsequently tested for SPLP, TCLP, and MEP#10 soluble Pb and As and compared to project limitations for TCLP Pb and As of 5 ppm, and SPLP and MEP of 10 ppb Pb and 200 ppb As. Soil samples were extracted according to SPLP, TCLP, and MEP procedure set forth by the USEPA Method 1312, 1311 and 1320 respectively. The extract samples were digested and then analyzed by ICP USEPA Method 200.7.
The foregoing results in Table 1 readily established the operability of the present process to stabilize combined Pb and As thus reducing SPLP, TCLP and MEP leachability and bioavailability. Given the effectiveness of the stabilizing agents in causing combined heavy metals to stabilize as presented in the Table 1, it is believed that an amount of the stabilizing agents total combined equivalent weight to less than 7% of heavy metal bearing material or waste should be effective. It is also apparent from the Table 1 results that certain stabilizing agent combinations are more effective for stabilization of Pb and As than individual stabilizing agent methods.
While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| 60849598 | Oct 2006 | US |
