Patent Application
20070287877
Environmental regulations throughout the world such as those developed by the USEPA under RCRA and CERCLA require heavy metal bearing waste, contaminated soils and material producers to manage such materials and wastes 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 less soluble 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 reducing solubility of 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 chromite mining tailings, wastes such steel smelter slag and ash, chromium plating sludge and chromium contaminated soils are major sources of chromium in our environment. The most water soluble form, and thus most bioavailable form of chromium is hexavalent.
There exists a demand for improved methods of chromium stabilization as a non-hazardous waste and methods that resist the reversion of reduced Cr(3) to oxidize back to the more soluble Cr(6) form. The present invention discloses a chromium hexavalent Cr(6) and chromium trivalent Cr(3) stabilization method through a sequenced contact of chromium bearing material or waste with chromium solution additive, reducing agent, chromium reduction duration, pH adjustment additive for precipitation of Cr(3) and addition of a stabilizing source for Cr(3) precipitate formation into a more stable form such as phosphate apatite mineral.
It is anticipated that the method can be used for RCRA compliance actions such that generated waste does not exceed appropriate TCLP hazardous waste criteria, and under TCLP or CERCLA (Superfund) response where the method is used to treat waste piles or storage vessels previously generated. The preferred method of application of stabilizers would be in-line within the generating source, 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 method chemical additives including: solution agents as water, surfactants; pH precipitating agents as portland cement, cement kiln dust, lime kiln dust, hydrate lime, quicklime, magnesium oxides, dolomitic lime; reducing agents as ferrous sulfate, bisulfite; and stabilizing agents as sulfides, phosphates, silicates, and combinations thereof, with the phosphate group including but not limited to wet process amber phosphoric acid, wet process green phosphoric acid, aluminum finishing Coproduct blends of phosphoric acid and sulfuric acid, technical grade phosphoric acid, monoammonia phosphate (MAP), diammonium phosphate (DAP), single superphosphate (SSP), triple superphosphate (TSP), hexametaphosphate (HMP), tetrapotassium polyphosphate, dicalcium phosphate, tricalcium phosphate, monocalcium phosphate, phosphate rock, pulverized forms of all above dry phosphates, and combinations thereof would be selected through laboratory treatability and/or bench scale testing to provide sufficient control of metals solubility and particle transport potential. In certain cases, such as with the use of amber and green phosphoric acid, phosphates may embody sulfuric acid, vanadium, iron, aluminum and other complexing agents which could also provide for a single-step formation of complexed heavy metal minerals. The stabilizer and agglomeration agent type, size, dose rate, contact duration, and application means would be engineered for each type of chromium bearing material of waste.
Although the exact stabilization formation molecule(s) are undetermined at this time, it is expected that when water borne or surface active trivalent chromium Cr(3) precipitates at pH levels (about or above 8.0 pH units) come into contact with the mineral stabilizing agents at sufficient reaction time, low TCLP/water soluble compounds form such as a chromium substituted calcium mineral phosphate, twinned mineral, mononuclear layers, substitute or surface bonding, which are less soluble than the Cr(3) precipitate and a more stable matrix which resists reversion to soluble Cr(6).
Examples of suitable mineral stabilizing agents include, but are not limited to sulfides, silicates, phosphate fertilizers, phosphate rock, pulverized phosphate rock, calcium orthophosphates, monocalcium phosphate, dicalcium phosphate, tricalcium phosphate, trisodium phosphates, sodium silicates, potassium silicates, natural phosphates, phosphoric acids, wet process green phosphoric acid, wet process amber phosphoric acid, black phosphoric acid, merchant grade phosphoric acid, aluminum finishing phosphoric and sulfuric acid solution, hypophosphoric acid, metaphosphoric acid, hexametaphosphate, tertrapotassium polyphosphate, polyphosphates, trisodium phosphates, pyrophosphoric acid, fishbone phosphate, animal bone phosphate, herring meal, bone meal, phosphorites, and combinations thereof. Salts of phosphoric acid can be used and are preferably alkali metal salts such as, but not limited to, trisodium phosphate, dicalcium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, tripotassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, trilithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate or mixtures thereof.
The amounts of initial solution agents, reducing agents, duration time, precipitating alkali agents and mineral stabilizing agent used, according to the method of invention, depend on various factors including desired solubility reduction potential, desired mineral toxicity, and desired mineral formation relating to toxicological and site environmental control objectives. It has been found that addition of 10% water, 3% ferrous sulfate monohydrate, 3% quicklime, and 0.5% dicalcium phosphate by weight of chromium bearing soil was sufficient for TCLP Cr stabilization of the soil to less than the RCRA 5.0 ppm limit and 0.10 ppm TCLP groundwater leaching limit, and formed a Cr(3) apatite which would not revert after 30 days of open air exposure. However, the foregoing is not intended to preclude yet higher or lower usage of solution, reduction, curing, precipitating or stabilizing agent or combinations.
The examples below are merely illustrative of this invention and are not intended to limit it thereby in any way.
One composite Cr(6) contaminated soil sample was laboratory split into four (4) equal samples as STEP 1; all four subsamples were brought to saturate condition with 10% (wwb soil) DI water as STEP 2; all four subsamples were stirred by hand for 10 seconds with 3% (wwb soil) dry ferrous sulfate monohydrate (FSM) as STEP 3; all four subsamples allowed to cure at STP for 24 hours as STEP 4; two (2) subsamples were combined with 3% (wwb soil) quicklime (CaO) and two (2) subsamples were combined with 5% Portland Cement (PC) as STEP 5; two (2) subsamples were combined with 2.0% (wwb soil) Dicalcium Phosphate DiHydrate (DCP) and two (2) subsamples remained with 0.0% Dicalcium Phosphate DiHydrate as STEP 6; and all four subsamples were aged 30 days in open air before testing for TCLP leachable Cr(6).
The foregoing results in Example 1 readily established the operability of the present process to stabilize Hexavalent Chromium into a non-reverting form thus reducing TCLP leachability, DI availability and bioavailability. Given the effectiveness of the stabilizing method as presented in the Table 1, it is believed that an amount of the reducing and stabilizing agents equivalent to less than 10% by weight of contaminated soil should be effective.
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 | |
|---|---|---|---|
| 60812786 | Jun 2006 | US |
