Ancillary Treatment Performance Parameters

In addition to the task of reducing metal solubility, other important features of this technology are considered. These include (1) the nature of hydration and curing; (2) the compactability of the treated material as related to on-site backfilling; (3) the effect of pre- and posttreat-ment particle sizes, which influence metal concentrations in dispersed material; and (4) the long-term viability of the treatment as measured through multiple extraction procedures.

Figure 6 Processing system throughput performance (Case Study II).

A. Hydration and Curing

As previously mentioned, curing the treated material is the final step in the process. In terms of the field application of the technology, this basically amounts to liberating the moisture in the treated material, i.e., allowing it to dry. However, this process has other fundamental and serious impacts on the efficacy of the technology in terms of (1) the proper hydration of ce-mentitious materials, which is related to the transformation of ionic bonds to covalent bonds, and (2) the use of sampling procedures that ensure that the curing process has been completed before the material is subjected to TCLP or other extraction protocols. The following discussion considers the interrelationship between hydration, curing, and sampling, which were extensively evaluated for the soil treated in Case Study II.

The treatment protocol developed for the remediation of the arsenic-contaminated soil of Case Study II used cement/lime ratios of either 20:3 or 15:5 on the basis of weight. The effect of these mixtures on the hydration characteristics of the treated material is illustrated in Figure 7 for a bench-scale test using an initial 500-g sample of soil having a moisture content of 12%. After the addition of reagents and water in the amounts of 85 and 100 mL, respectively, the mixture weight was about 700 g for both cement-lime combinations. Here water essentially behaves as a reaction catalyst. Based on prevailing accepted stoichiometric data, the amount of water bound to cement and lime is on the order of 13% and 30% of their weight, respectively. This would indicate that approximately 17-18 g of water would be retained in the matrix. In

Figure 7 Hydration characteristics of cement-lime mixtures.

Figure 7 Hydration characteristics of cement-lime mixtures.

other words, the dilution factor due to water would be approximately 3.5%. As expected, the mixture with the higher percentage of lime begins to initially lose its free water at a faster rate (Figure 7) due to the increased heat of hydration. After this initial period, a similar linear rate of water loss occurs in both mixtures over a 72-h period. During this time, the samples were subjected to a 70°F, 65% relative humidity environment and were broken at 8-hr intervals to expose new surface. Between 72 and 75 hr into the test period, heat lamps were used to accelerate the water loss. The samples were allowed to equilibrate to room conditions for 1 hr before final weighing.

The data obtained from the above exercise illustrate that (1) the majority of the free water used as a catalyst will be liberated and (2) the retention of water in the matrix appears to be less than that derived from conventional stoichiometry. However, more important in terms of operations, using artificial means such as heat lamps to accelerate the loss of free water does not alter the fundamentals of the treatment. In certain cases, a procedure of this nature, referred to as a lab cure, is used to expedite the turnaround time of obtaining analytical laboratory TCLP data. Operationally, this can allow naturally cured stockpiles to be backfilled at the time the analytical data are available. A comparison of the lab and field cure TCLP arsenic data (Figure 8) illustrates that the two methods yield the same results with proper field curing. It should be noted that in Figure 8, arsenic data are given for both the pilot test and the production phase where the detection limits were 0.01 and 0.2 mg/L, respectively. The acceptable TCLP level of arsenic in the treated material was set at 1.0 mg/L. Basically, the elevated levels in the field data shown in Figure 8 indicate that the material was prematurely sampled. Sufficient free water was still present; that is, the process shown in Figure 7 was not completed. The tendency of the treated material to form monoliths is another ramification of improper field curing. Stockpiled material must be turned, usually on a daily basis, to (1) enhance the rate of curing through accelerated heat and mass transfer to the environment and (2) mechanically overcome the tendency of cementitious mixtures to fuse or solidify into a continuous substance, which would negate the desirable feature of a friable matrix structure.

Number of Samples

Figure 8 Comparison of laboratory and field curing on TCLP arsenic concentrations.

B. Compactability

An important feature of the process is the ability to backfill or recompact the treated material into the excavation. Standard tests of treated material using ASTM Method D-1557 yield a compaction of about 97%. The following procedure is typically used in the field to achieve this compaction.

Stockpiled cured material is loaded into a dump truck and moved to the excavation. Here the dumped pile is spread with a bulldozer into an 8-12-in. lift. The moisture content is adjusted according to the ASTM test data to be consistent with optimum compaction. A drum vibrating compactor is then used to complete the placement of the lift. The volume expansion due to treatment is estimated to be on the order of 20%. Operationally, after the removal of the oversize fraction, the backfilled treated material occupies the volume of the excavation.

Several field observations indicate that the features of the backfilled treated material are suitable for construction. For example, backhoe digging tests on material in place for 28 days showed that it remained friable with sufficient structural stability for trenching; that is, sharp vertical cuts could be made. The equipment operators indicated that the backfilled material was similar to digging hardpan.

C. Effect of Particle Size

An additional feature of the STS polysilicate technology is its ability to indirectly mitigate total metal concentrations through increased posttreatment particle size. The basic mechanism stems from the fact that the emission rate for total respiration particulate matter (E,0) has significant particle size sensitivity. This effect on Em is illustrated through the work of Cowherd et al. [9] and summarized in the California Site Mitigation Decision Tree Process [10]. The basic cal-culational methodology follows.

1. Particle Size Distribution

The first step in the procedure involves the determination of the characteristic particle size. This is typically accomplished by subjecting representative samples of pre- and posttreated ma-

Number of Samples

Figure 8 Comparison of laboratory and field curing on TCLP arsenic concentrations.

100908070-

40302010-

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Figure 9 Size distribution of pre- and posttreated soil.

terial to a sieve analysis and arranging the results in terms of percent passing vs. size (Figure 9). It is interesting to note that the characteristics of the treated material (straight line on a semilog plot) are representative of a Rosin Rammler distribution, which will have a characteristic particle size (XD) corresponding to 63.2% passing. Consequently, for illustrative purposes, the value of Xa for the particular treated material shown in Figure 9 would be given by a size of about 6.4 mm. The comparable size for the untreated material can then be taken as 0.8 mm. Thus, as illustrated in Figure 9, the agglomerative features of creating the metasilicate and cementitious matrix during the treatment process has the effect of shifting the size distribution of the treated material to larger sizes. The increase in the characteristic size is usually about an order of magnitude. Here this increase is a factor of 8.

2. Threshold Friction Velocity

As illustrated in the work of Cowherd [9], the threshold friction velocity (Uf) is logarithmically related to the so-called aggregate size distribution mode. Basically, this parameter is intended to be representative of the wind speed at ground level corresponding to the occurrence of soil erosion. In terms of the characteristic particle size, the value of Uf for the 0.8-mm and 6.4-mm particles are 60 cm/sec and 140 cm/sec, respectively. Thus, an eightfold increase in particle size slightly more than doubles the threshold friction velocity.

3. Roughness Height

Roughness height is used to convert the value of Uf at ground level to the wind speed at a typical 7-m-high weather station. Values of roughness height (ZD) range from 0.1 cm for natural snow to 1000 cm for high-rise buildings. Similarly, plowed fields and grasslands are given by Z„ values of 1.0 cm and 2.0-4.0 cm, respectively. A ZQ value of 1 cm is typically used for soil being remediated.

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4. Threshold Wind Velocity

The threshold wind velocity (U,), i.e., the wind speed necessary to initiate soil erosion as determined by 7-m-high weather station data, is given in terms of the value of Uf according to a relation developed by Cowherd,

The corresponding values of U, for the 0.8- and 6.4-mm particles are 7.86 and 18.34 m/ sec, respectively.

5. Respirable Particulate Emission Rate

The following relation developed by Cowherd was used to evaluate the emission rate of respirable particle matter from erodible surfaces:

where El0 is the emission rate for total respirable particular matter [g/(m 2-hr)]; V, the fraction of exposed contaminated area that is vegetated (V = 0 for bare soil); and U, the mean annual wind speed (m/sec).

The parameter F(x) is given by the relation

Assuming a nominal 10 mph wind speed (4.47 m/sec), the value of £10 for the 0.8-mm particle size is 5.15 x 10~3 g/(m 2-hr). The corresponding value for the 6.4-mm particle is 7.3 X 10-8 g/(m 2-hr). Thus, an eightfold increase in particle size reduces the value of Em by 7 X 104. Alternatively, relatively small increases in threshold friction velocity due to increased particle size result in large decreases in respirable emission rates.

The E|0 behavior with wind speed for 0.8-mm and 6.4-mm particles is illustrated in Figure 10. Each particle exhibits a characteristic steep rise in concentration at elevated wind speed. Basically, the larger particles can sustain higher wind speeds for a given concentration. For example, the 0.8-mm particle has an £10 value of 5.15 x 10~3 g/(m2-hr) at 10 mph. A similar value of Ew for the 6.4-mm particle would be reached at a 23 mph wind speed.

6. Downwind Concentrations

The effect of particle size on concentrations of specific constituents reaching a downwind receptor can be obtained from the relation

where X is the concentration of the constituent in ambient air (fig/m); Q is the emission rate (|xg/sec); ov and oz are the standard deviation of horizontal and vertical dispersion, respectively (m); and U is wind speed (m/sec).

The following assumptions are made to illustrate the effect of particle size on downwind concentrations for a working area of 1000 m2 and a length of 100 m to the receptor. Using the data of Turner in the Workbook of Atmospheric Dispersion Estimates [11] and Class C stability, the values of ov and o2 are 13 and 7 m, respectively, for a 100-m distance. Thus, for a 10 mph wind speed, the values of X become 1.12 x 103 and 1.58 x 10~2 ng/m3 for the 0.8 and 6.4mm particles, respectively. Thus, an estimate of the downwind concentration of any specific

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6.4 mm PARTICLE

0.8 mm PARTICLE

1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00

Figure 10 Influence of wind speed on concentration.

constituent can be calculated by multiplying the downwind concentration X by the mass fraction of that constituent. The results of this calculation are summarized in Table 3 for the total metal concentrations given in Table 2 for metal system 5. Basically, these results follow the El0 effect such that the downwind concentrations become negligible for the larger particle size.

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