AU2017100297A4 - Amendment for treatment of soil to stabilize heavy metals - Google Patents

Abstract

AMENDMENT FOR TREATMENT OF SOIL TO STABILIZE HEAVY METALS The present invention relates generally to a novel reactive stabilizing amendment for treating heavy metals contaminated soil by applying the reactive stabilizing amendment to the soil. The reactive composition in the amendment includes a stabilizing agent to stabilize the heavy metal and a permeability enhancing agent to increase the permeability of the soil, facilitating the reaction of the heavy metal with the stabilizing agent. The treated soil can be left in place or can be disposed in a non-hazardous landfill.

Description

1 2017100297 14 Mar 2017

AMENDMENT FOR TREATMENT OF SOIL TO STABILIZE HEAVY METALS

FIELD

[0001] The present invention relates generally to a novel reactive stabilizing amendment for treating heavy metals contaminated soil by applying the reactive stabilizing amendment to the soil. The reactive composition in the amendment includes a stabilizing agent to stabilize the heavy metal and a permeability enhancing agent to increase the permeability of the soil, facilitating the reaction of the heavy metal with the stabilizing agent. The treated soil can be left in place or can be disposed in a non-hazardous landfill.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a reactive stabilizing amendment for the treatment of heavy metals contaminated soil with the reactive stabilizing composition. Stabilizing agents within the composition can penetrate the soil to stabilize the heavy metals. The stabilizing composition dries and remains adhered to the soil particles to form a treated soil having reduced leachability. The reactive composition can be applied onto soil particle surface to adhere and stabilize heavy metals containing soil particles. The treated soil can be left in place or can be disposed in a non-hazardous landfill.

[0003] Many industrial sites have been contaminated by one or more heavy metals, such as chromium, mercury, lead and cadmium. These heavy metals are recognized as being harmful to animals and humans, even in small quantities. As such, the heavy metals contaminated soil must be treated to minimize the environmental risks. Many amendments have been tested or developed to stabilize heavy metals in soil. For example, lime can effectively reduce the 2 2017100297 14 Mar 2017 mobility of Cu and Pb in contaminated soils by raising the soil pH. The liming materials including CaO, Ca (OH)2, CaMgC03, and CaC03, and the phosphate materials including CaHP03, Ca (HiPCFk, K2HPO4, H3PO4, and (NH4)HP04 were the typical amendments used for the in situ immobilization in order to decrease the metal concentration in soil solution and extractability, to decrease metal mobility and leachability, Alkaline industrial by-products like fly ashes can also neutralize soil acidity arising from acid mine drainage generation in sulfide-rich waste and prevent contaminant spread. Fly ashes have a more sustained neutralizing effect than lime, but increasing soil pH to alkaline region can increase the risk of Cr(III) transformation to Cr(VI) and As(V) to As(III). Both transformed species are much more mobile and toxic, and therefore undesirable. Amorphous Fe(III) hydroxide (ferrihydrite) can be an effective sorbent for both anions and cations. The surface of Fe hydroxide particles can be positively or negatively charged depending on pH making the Fe hydroxides amphoteric. The Fe sulfate application can successfully immobilize As, but due to acid liberation is not recommended in soils containing high concentrations of metal contaminants. Treatment with zerovalent iron significantly reduces concentrations of contaminants accompanied by the immobilization of macro elements, such as Ca, Mg, P. Undesirable reduction of available quantities of nutrients in the Fe amended soil requires additional soil amelioration.

[0004] It is possible to improve the treatment efficiency of multi-element contaminated sites by applying a combination of amendments. For example, acid liberated in iron sulfate amended soils can be neutralized applying appropriate amounts of lime, and leaching of metals could be avoided. Soil acidification due to phosphoric acid can also be managed by the following application of phosphate rock and calcium phosphates as well as lime. Fly ash stabilized sewage sludge can prevent the leaching of trace elements and their uptake by plants. However, there remains a need for an improved stabilizing composition for the treatment of soil including heavy metals. It would be advantageous if such a stabilizing composition had strong adherence to the 3 2017100297 14 Mar 2017 soil particles and did not require the generation of a hazardous waste by removing the soil from the site prior to disposal. It would also be advantageous to provide a stabilizing amendment that can be applied to soil and is capable of reacting with the heavy metals in the soil, reducing their leachability.

SUMMARY

[0005] The present invention is directed to a reactive stabilizing composition that can react with heavy metals contaminated soil to reduce the leachability of the heavy metal to an acceptable level. The reactive stabilizing composition can be formulated such that the treated soil is sufficiently durable to with stand the rigors of exterior weather elements such as repeated rain events, or can be used on the interior of a building and can be safely left in place. In addition, the reactive stabilizing composition is formulated such that it increases the permeability of the soil as it is applied so that the stabilizing agent can react with heavy metals in the soil particles. Without the necessity of additional process steps. In this manner, the available heavy metals in the treated soil are stabilized (i.e., chemically converted) during the application process and remain in the stabilized form in its normal environment or when moved to a disposal environment.

[0006] Thus, according to one embodiment of the present invention, a reactive stabilizing amendment is provided for the stabilization of soil containing a heavy metal. The stabilizing composition includes at least about 2 weight percentage of a heavy metal stabilizing agent, at least about 1 weight percent of a permeability enhancing agent that is adapted to increase the permeability of the stabilizing agent into the soil. The stabilizing composition adheres to the soil particles to form a treated soil that can be safely disposed as waste in a non-hazardous landfill or can be left in place on the structure. According to an other embodiment of the present invention, 4 2017100297 14 Mar 2017 a method for making a reactive stabilizing composition is provided. The method can include the steps of providing a water-based amendment composition, adding at least about 2 weight percent of a heavy metal stabilizing agent to the soil composition and adding at least about 1 weight percent of a permeability enhancing agent to the soil composition.

[0007] The present invention provides an economical means for long-term environmental protection by stabilizing heavy metals, such as chromium contained soil. At least a portion of the heavy metals in the soil is converted into a highly stable, heavy metal complex compound having reduced leachability while remaining disposed in the contaminated soil. Mixing and processing maybe required to facilitate formation of the heavy metal complex. The entire soil thereby becomes non-hazardous and capable of passing TCLP (Toxicity Characteristic Leaching Procedure) testing criteria. Solid wastes are considered hazardous and therefore subject to stringent disposal regulations if the waste leaches excessive heavy metals under the TCLP. The TCLP assumes that the soil is exposed to leachate from an uncovered landfill and the test involves leaching a sample of the s with dilute acetic acid, simulating the decay of solid wastes. There active stabilizing amendment of the present invention can be applied using a number of traditional methods including spraying and mixing. The stabilizing agent provides an immediate and long-term treatment of the soil and the stabilizing agent can remain fiixed to the soil particles throughout the disposal process. The mixing can be applied just prior to disposal. No significant chemical set-up time is required.

DETAILED DESCRIPTION OF THE INVENTION

[0008] The reactive stabilizing amendment according to the present invention preferably includes at least two active components that enable the stabilizing composition to penetrate soil particles and react with heavy metals. The first component is a heavy metal stabilizing agent 5 2017100297 14 Mar 2017 adapted to reduce the leachability of the heavy metal, such as by forming a complex compound with the heavy metal. The second component is a permeability enhancing agent that increases the permeability of the soil enabling penetration of the stabilizing agent into the soil particles. The heavy metal stabilizing agent can be selected from among flocculants, coagulants, precipitants, complexing agents, and adsorbents. Preferably, the heavy metal stabilizing agent is a chemical compound either in liquid or solid form that can react with a heavy metal or soluble heavy metal compound to form a heavy metal compound that will not release substantial quantities of the heavy metal when subjected to a wide range of pH conditions such as pH 2.5 to about pH 10.5. Stated another way, the heavy metal will have a reduced leachability. The heavy metal stabilizing agent can be selected from phosphate compounds, silicate compounds, carbonate compounds and sulfide compounds. The heavy metal stabilizing agent can be soluble, insoluble or partially soluble and can be added to the soil composition in the form of a liquid, granular solids or a powder. Particularly preferred are phosphate compounds that can provide a phosphate anion to react with the heavy metal and form a complex compound having reduced leachability. Sources of such phosphate compounds include rock phosphate or fish bones, soluble alkali or alkaline earth phosphate salts, partially soluble salts such as partially soluble calcium phosphate salts and phosphoric acid. Particularly preferred among these are phosphoric acid and triple superphosphate (TSP). A blend of monocalcium phosphate and dicalcium phosphate, can also be used. Water soluble phosphate compounds can be dissolved in water before mixing with the other components of the stabilizing composition. When a solid phosphate compound isutilized, it is preferred that the phosphate compound be in the form of particles having an average particle size of not greater than about 250 Pm.

[0009] In addition to a heavy metal stabilizing agent, the reactive composition also includes a permeability enhancing agent. The permeability enhancing agent is a compound that is adapted to increase the permeability of soil, thereby facilitating the reaction of the heavy metal 6 2017100297 14 Mar 2017 stabilizing agent with the heavy metals contained in the soil particles. Accordingly, the permeability enhancing agent chemically softens the underlying soil. The permeability enhancing agent can be an organic or inorganic compound. For example, the permeability enhancing agent can be selected from alkali or alkaline earth hydroxides or carbonates in liquid or solid form.

[0010] Particularly preferred permeability enhancing agents according to the present invention can be selected from N-Methyl-2-Pyrrolidone, ethyllactate, methylethyl ketone, dimethyl sulfoxide and sodium hydroxide. N-Methyl-2-Pyrrolidone is acyclicamine that issoluble in water. Ethyl lactate, sometimes referred to as lactic acid ethyl ester, also has a high solubility in water. Methyl ethyl ketone, sometimes referred to as 2-butanone or methyl acetone, is also highly soluble in water. Sodium hydroxide, sometimes referred to as caustic soda, is a common caustic material that has a high solubility in water. Another preferred permeability enhancing agent is a reused soybean oil, which can be mixed with surfactants. When sodium hydroxide is used as the permeability enhancing agent, it can be added to the stabilizing composition as a solid material. However, it is often more convenient to add the sodium hydroxide in liquid form, such as by adding a fifty weight percent solution of sodium hydroxide. Other permeability enhancing agents are typically added in liquid form. The addition of a permeability enhancing agent such as NMP gives polar solvating capabilities to the stabilizing composition and the solvating characteristics of the permeability enhancing agents, particularly NMP, for oils and hydrocarbons are very good. The phosphate ions make physical contact with heavy metals in soil by diffusion and/or brownian movement and form complex heavy metal compounds having reduced leachability. In addition, the permeability enhancing agent aids in reducing the hardness of the soil. By decreasing the hardness of the soil and increasing the solvating characteristics of the stabilizing, the efficiency of diffusion and brownian movement of the ions responsible for heavy metal complexing is greatly enhanced. Preferably, the reactive stabilizing composition has a low 7 2017100297 14 Mar 2017 volatile organic content (VOC). Volatile organics are harmful to the environment and can also be harmful to the user. It is preferred that the reactive stabilizing compositions of the present invention have a VOC of not greater than about 200 g/1. The stabilizing agent can be in the range of from about 5 to about 60 weight percent and more preferably from about 10 to about 40 weight percent of the stabilizing composition. In one preferred embodiment, the reactive stabilizing composition includes from about 20 to 40 weight percent of the heavy metal stabilizing agent. The amount of permeability enhancing agent utilized in the reactive stabilizing composition is an amount sufficient to increase the permeability of the underlying soil particles enable the penetration of the heavy metal stabilizing agent into the inner layer of soil particles. Preferably, the reactive stabilizing composition includes at least about 1 weight percent of the permeability enhancing agent, more preferably includes at least about 2 weight percent of the permeability enhancing agent and even more preferably includes at least about 4 weight percent of the permeability enhancing agent. Preferably, the amount of permeability enhancing agent does not exceed about 50 weight percent, more preferably does not exceed about 30 weight percent and even more preferably does not exceed about 20 weight percent. For example the amount of permeability enhancing agent can be from about 5 weight percent to about 25 weight percent, more preferably from about 5 weight percent to about 15 weight percent of the reactive stabilizing composition. In addition, it is preferred that the weight ratio of heavy metal stabilizing agent to permeability enhancing agent is in the range of from about 2:1 to 4:1. The stabilizing composition can also include water, either added separately or with the active components. For example, TSP can be added in an aqueous solution and/or the sodium hydroxide can be added as a dilute solution. The additional water can be provided to the stabilizing composition and can also aid dissolution of the heavy metal stabilizing agent. Although the foregoing description refers primarily to chromium, soil can contain other metals that are treated by the stabilizing composition to reduce the leachability of the metals. Examples include barium (Ba), silver (Ag), arsenic (As), cadmium (Cd), mercury (Hg), antimony (Sb), 2017100297 14 Mar 2017 8 selenium (Se) and lead (Pb).

EXAMPLE

[0011] The Examples illustrated in Table 1 demonstrate that for a given lead content in the soil, a sufficient amount of both reactive components should be used. When a permeability enhancing agent (NaOH) content of 5 weight percent was used, a H3PO4 loading of greater than 10 weight percent was needed to reduce the lead leachability to less than 1.5 ppm. When the permeability enhancing agent was increased to 8 weight percent, however, 18 weight percent H3PO4 Was sufficient to reduce the leachability to less than 5 ppm and penetration of the amendment into the soil as visually confirmed. When no sodium hydroxide was used, 18 weight percent H3PO4 Was not sufficient to reduce the leachability of the lead to less than 5 ppm.

Table 1.

Example No. Reactive Components TCLP Pb (ppm) 1 12% H3PO4 + 5.0% NaOH 1.4 2 18% H3PO4 + 8.0% NaOH 4.5 3 18% H3PO4 + 0% NaOH 6.2

Claims (6)

1. A reactive stabilizing amendment for the stabilization of heavy metal contaminated soil, said stabilizing composition comprising: a) at least about 2 weight percent of a heavy metal stabilizing agent; b) at least about 1 weight percent of a permeability enhancing agent adapted to increase the permeability of the heavy metal stabilizing agent into soil, where the permeability enhancing agent is selected from the group consisting of sodium hydroxide, and ethyl lactate.

2. A reactive stabilizing amendment as recited in claim 1, wherein said heavy metal stabilizing agent is selected from the group consisting of flocculants, coagulants, precipitants, complexing agents, epoxy agents and adsorbents.

3. A reactive stabilizing composition as recited in claim 1, wherein said heavy metal stabilizing agent is selected from the group consisting of phosphate compounds, silicate compounds, carbonate compounds and sulfide compounds.

4. A reactive stabilizing composition as recited in claim 1, wherein said heavy metal stabilizing agent comprises a phosphate compound.

5. A reactive stabilizing composition as recited in claim 1, wherein said heavy metal stabilizing agent comprises triple superphosphate.

6. A reactive stabilizing composition as recited in claim 1, wherein said heavy metal stabilizing agent is in the form of a solid dispersed in said amendment composition.