1. Introduction
Studies have shown that stabilization/solidification technology processes for treatment of contaminated soils/ residues has been successful in stabilizing a wide variety of materials including metals, volatiles, waste oils and solvents creating a hard, soil-like material binding free liquids and chemicals [1-3]. Stabilization/solidification technologies are economical and can include in-situ and ex-situ treatment methods [4,5].
According to Silveira et al., “Cement-based solidification/stabilization is a process in which inorganic reagents react with waste components and/or themselves to form chemically stable solids which are capable of developing mechanical resistance” [6]. Treatment processes using Portland cement, a major inorganic reagent, have been successful in immobilizing constituents of environmental concern [7,8]. Portland cement produces a hardened paste upon addition of water. This paste binds together aggregates and other substances to form concrete and stabilize wastes [9,10] This technology is currently being used to treat a wide variety of wastes and showed to be effective in reducing the mobility of arsenic wastes [11,12].
This research project tested the use of Reactive Powder Concrete (RPC), an ultra high-strength and low porosity composite material with advanced mechanical and superior physical properties [13,14] for encapsulating arsenic contaminated soil.
The main objective was to study water permeability and absorption of RPC during the storage of inorganic material, arsenic, by application of a solidification/stabilization technique. Arsenic contaminated soil was encapsulated in RPC to determine the maximum concentration of arsenic in soil, which allowed formation of acceptable permeability, and leachate concentration less than 10 parts per billion (ppb) when encapsulated in RPC mortar.
This study tested the hypothesis that RPC would provide a better performing containment and disposal solution for solidifying arsenic contaminated soils as compared to solidification with cement mortar.
2. Study of Arsenic-Contaminated Soils
Arsenic is a naturally occurring toxic element in the earth’s crust, which forms inorganic arsenic when combined with oxygen, chlorine and sulfur. Additionally, arsenic combined with carbon and hydrogen forms organic arsenic. The inorganic forms of arsenic are much more toxic than organic forms. The principal valence states of arsenic are +3, +5, and –3. [15] Arsenical pesticides, natural geothermal sources and mine tailings increase arsenic concentrations in soils. The adsorption of arsenicals in soil depends on soil pH, texture, Fe, Al, and organic matter. The amount of arsenic adsorbed on soil increases as clay, Fe, and Al content increases. Toxic amounts of arsenic, greater than 10 parts per million (ppm) in soils will limit the germination of seeds and reduce the viability of seedlings. Organic arsenic is used in catalysts, glass manufacturing, alloys, electronics and weed killer. Inorganic forms of arsenic are also used in pesticides to kill insects or rodents, to preserve wood, and as a component of medicines for asthma and psoriasis. Arsenic levels in municipal sewage are variable from 1 - 18 ppm [16]. An upper limit of 0.2 ppm is recommended for as in livestock drinking water and an upper limit of 0.01 ppm for water intended for human consumption. In soils, the total as concentration normally ranges from 1 - 40 ppm. [17]
Arsenic contamination resulting from natural or xenobiotic sources in ground and surface waters is a major health concern for water designated for agricultural or human consumption uses. A great deal of time and money is being expended to conduct research and development of processes for removal of arsenic from such waters to concentrations as low as 5 ppb.
This study represents the first evaluation of the solidification/stabilization technique using RPC and its capability in containing arsenic. This research determined the maximum concentration of arsenic in soil that resulted in the lowest water permeability, and a leachate concentration less than the current drinking water standard. This study provides operational boundaries for the initiation of a more detailed study of arsenic encapsulation in structural concrete.
A sludge having arsenic concentration of 2000 ppm when treated by incineration or landfill process was reported to lead to volatilization, ecosystem cycling, ground water and air contamination [18,19]. Therefore, application of a solidification/stabilization process using RPC as an additive to effectively encapsulate contaminated material and to produce a stabilized engineered product, which contains toxic products in a less soluble state, would be an important advancement.
3. Materials
The cement used was Type I/II supplied by Dacotah Cement, South Dakota. The fine aggregate used for all the mixes (limestone dust and natural sand) were obtained form Hill City Materials, Rapid City, South Dakota. Commercially prepared topsoil was purchased for this study. Tap water from the Rapid City Municipal water supply system was used for the mixing. The admixtures such as Rheomac SF 100 dry, a densified silica fume, and Glenium 3000 NS, a high range water reducer (HRWR), were both supplied by Master Builders Inc, Cleveland Ohio. The contaminant encapsulated in topsoil was sodium arsenate (Na2HAsO4∙7H2O).
3.1. Tests on Topsoil Absorption Coefficient
Before using the topsoil for mixing, the adsorption coefficient of the topsoil was determined using the standard procedures given by ASTM C128 [20]. The absorption of topsoil was 58.43%. The absorption % of soil was subtracted from the moisture content to calculate the actual proportion of soil to be used for mixing mortar on wet basis.
3.2. Organic Matter and Carbon Content
The soil samples were sent to Soil Testing Laboratory, South Dakota State University for determination of organic matter and carbon content. The results are shown in Table 1.
3.3. Experimental Design
The statistical software package, Minitab, was used to determine a hierarchical design resulting in a series of nine RPC mortar mixes which were tested and the mix having Water-Cement ratio (W/C) of 0.275, limestone/cement of 0.3, silica fume/cement of 0.39 was chosen due to its lowest permeability and highest strength. This mix was then evaluated by holding the amount of cement, limestone, silica fume and W/C constant while soil was substituted at 10, 15, 20, 30, 40, 50 and 100% of
Table 1. Organic matter and organic carbon content for soil samples.
sand and again evaluated for permeability and strength resulting in the choice of 20% soil. Finally, the chosen RPC mortar mix was evaluated with the 20% soil component dosed with arsenic concentrations of 100, 1000, 2000, and 3000 mg/kg.
3.4. Specimens
Specimens of 50.8 mm × 50.8 mm × 50.8 mm were made following the ASTM C 305-94 standard [21]. The cubes were tested for compressive strength in accordance with the Standard test method for compressive strength of mortars given by ASTM C 109-93 [22]. Additionally, cylinders with a diameter of 101.6 mm and length 203.2 mm were cast. The cylinders were cut in 50.8 mm slices and a Rapid Chloride Permeability Test (RCPT) was completed in accordance with ASTM C 1202 along with an absorption test in accordance with ASTM C 497. The Toxicity Characteristic Leaching Procedure (TCLP) test samples were prepared following the EPA Method 1311 [23].
4. Results and Discussion
4.1. TCLP Results on the Raw Materials
The TCLP test results performed on cement, limestone dust, fine sand, uncontaminated soil and silica fume indicated that no arsenic was present in these materials. Therefore, the results of the TCLP test performed for the last phase are representative of the arsenic which leached from the contaminated soil used in the mix preparation.
4.2. Permeability and Absorption Results for Varying Soil Mixes
The 56 and 90 day permeability for varying soil mixes are shown in the Table 2. It was observed that the permeability increased with the increase in the soil content.
Figure 1 shows that the percent absorption increased with the increase in soil content in the mixes and remained approximately constant at 7, 28 and 45 day curing.