Cementitious stabilization of chromium, arsenic, and selenium in a cooling tower sludge
Abstract: The Federal Facility Compliance Agreement (FFCA) establishes an aggressive schedule for conducting studies and treatment method development under the treatability exclusion of RCRA for those mixed wastes for which treatment methods and capabilities have yet to be defined. One of these wastes is a radioactive cooling tower sludge. This paper presents some results of a treatability study of the stabilization of this cooling tower sludge in cementitious waste forms. The sample of the cooling tower sludge obtained for this study was found to be not characteristically hazardous in regard to arsenic, barium, chromium, lead, and selenium, despite the waste codes associated with this waste. However, the scope of this study included spiking three RCRA metals to two orders of magnitude above the initial concentration to test the limits of cementitious stabilization. Chromium and arsenic were spiked at concentrations of 200, 2,000, and 20,000 mg/kg, and selenium was spiked at 100, 1,000, and 10,000 mg/kg (concentrations based on the metal in the sludge solids). Portland cement, Class F fly ash, and slag were selected as stabilizing agents in the present study. Perlite, a fine, porous volcanic rock commonly used as a filter aid, was used as a water-sorptive agent in this study in order to control bleed water for high water contents. The highly porous perlite dust absorbs large amounts of water by capillary action and does not present the handling and processing problems exhibited by clays used for bleed water control.
Summary (3 min read)
- Based on prior characterization data of the coolig tower sludge, the metals selected for spiking were chromium, arsenic, and selenium.
- Stabilization of arsenic concentrations as high as 20,000 mgkg in arsenic sludge has been reported using Portland cement.*.
- Perlite, a fine, porous volcanic rock commonly used as a filter aid, was used as a water-sorptive agent in this study in order to control bleed water for high water contents.
- The study scope included controlling the sludge water content and varying this water content over a wide range.
- For this reason, the cooling tower sludge was first oven dried at 105°C.
- The dried sludge was then sieved through 4.75-mm sieve openings and homogenized to provide the feed sludge solids for the experimental design (see Table 1 ).
- Homogeneity was tested by standard total analysis (EPA Method 305 1) of a marker element in five subsamples of the dried-sieved homogenized sludge.
- The percentage relative standard deviation (% RSD, Le., standard deviation divided by the mean times 100) for chromium was 12%.
- The grout preparation consisted of fist mixing the sludge solids w i t h water and the spike compounds and then mixing with the stabilizing agents.
- The treated sludge was cured in a humid environment at room temperature for 28 d to make the cementitious waste form.
- The four dry blend additives consisted of (1) Type I-II Portland fiom the Dixie Cement Co., (2) Class F fly ash (fly ash) fiom the American Fly Ash Co., (3) ground granulated blast hrnace slag (slag) (Blaine fineness of 6220 cm2/g) fiom the Koch Minerals Co., and (4) perlite (Grade H-200) fiom the Harborlite Corp.
- The grout compositions were chosen in a statistical design (mixture experiment), but this statistical approach is not discussed in this paper.
Modified TCLP Measurements
- Both the sludge solids (unspiked and spiked at the three levels) and cementitious waste forms were extracted using a modified TCLP.
- The modifications to the TCLP consisted of (1) extracting a 10-g sample in 200 i d of extraction fluid; (2) size reduction to <4.75-mm particles; and (3) analysis of the extract for arsenic, selenium, and mercury by an inductively coupled argon plasma spectrometer (.
- The bulk density of the as-received sludge and dried-sieved homogenized sludge was determined by weighmg a known volume (using a graduated cylinder) of the granular sludge and calculating the bulk density.
- The bulk density of the cementitious waste forms was measured by packing a 2-in.
- Cube mold with the freshly made grout, determining the net weight of the grout, and calculating the bulk density.
- The corresponding volumes of the as-received and dried-sieved homogenized sludge were calculated along with each grout volume.
- The ratio of each grout volume to the sludge volumes gives an estimate of the volume increase that can be expected fiom cementitious stabilization of the cooling tower sludge.
- Table 1 lists the compositions (including the spike levels of chromium, arsenic, and selenium in the dried-sieved homogenized sludge solids) of the cementitious waste forms made fiom the cooling tower sludge.
- The chromium concentrations had no obvious correlation with pH.
- Figure 2 illustrates the ratio of the TCLP extract concentrations of chromium, arsenic, and selenium before and after treatment as a function of the final extract pH.
- The measured bulk densities (standard deviations of 0.02 kgK, for both) were 0.81 and 1.07 kg/z for the asreceived sludge and the dried-sieved homogenized sludge, respectively.
- The volume ratios listed in Table 4 overestimate the volume increase for high water contents because the water volume is included in the grout and not the waste solids, but the ratios also underestimate for low water contents because of the air voids present in both the as-received and dried sludge.
- The compositions of the grouts in this study were intentionally varied over a wide range, including variations fiom high waste loadings to low waste loadings and fiom high water contents to low water contents.
- The TCLP concentrations of arsenic and selenium were higher for these two grouts than for the untreated sludge spiked to the corresponding level of the RCRA metals .
- Even the behavior of the chromium may be amenable to such an analysis, though the chromium behavior may be complicated by the reduction potential of the waste form and the possible valence conversion of the chromium.
- Hence, the effect 95-RP130.03 of water on the bulk density of the sludge was not measured; however, the volume of the sludge decreases as the air voids are filled and the dry sludge absorbs water and collapses.
- A considerable volume increase can be expected for those grouts with a waste (sludge solids plus water) loading less than 40 wt%, but not as much as Table 4 implies with the volume contribution of the water in the initial waste ignored.
- The most significant hding was that cementitious waste forms can stabilize chromium, arsenic, or selenium to meet TCLP LDR limits, even at concentrations as high as 20,000, 20,000, and 10,000 mgkg, respectively.
- The final extract pH proved to be a good predictor of the TCLP performance for arsenic and selenium, relative to the concentration of these two RCRA metals in the sample.
- The correct blend of cement, slag, fly ash, and perlite can stabilize this cooling tower sludge for a wide range of water contents and concentrations of chromium, arsenic, and selenium.
- The fiee water can be controlled, even ifthe sludge contains high levels of water.
- Therefore, once the composition required to stabilize the RCRA metals is determined, choosing how much blend to use in treating a given waste means balancing the physical strength desired against the volume increase allowed.
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"Cementitious stabilization of chrom..." refers background or methods in this paper
...The modifications to the TCLP consisted of (1) extracting a 10-g sample in 200 id of extraction fluid; (2) size reduction to <4....
...Table 4 lists two ratios for each grout composition: (1) the grout volume to the volume of the asreceived sludge and (2) the grout volume to the volume of dried-sieved homogenized sludge....
...The four dry blend additives consisted of (1) Type I-II Portland cement (cement) fiom the Dixie Cement Co....
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