Leaching as a function of L/S

The liquid to solid ratio was defined in section 2.2.3. For leaching systems under equilibrium or near- equilibrium conditions, the expression of results of leaching tests as concentrations of a substance in the eluate or as accumulated leached amounts of a substance as a function of L/S is very convenient and allows the comparison of results from different test methods, in particular percolation tests and batch leaching tests (see section 3.6.2). This is illustrated in Figure 3.3 which shows accumulated leached amounts of Mo from an aggregate (bottom ash from incineration of hazardous waste) as function of L/S from a percolation test (CEN/TS 14405) and a two-step batch leaching test (EN 12457- 3). The figure also shows the total content of Mo in the aggregate. It can be seen that the batch test results represent two points on the leaching curve described by the percolation test results. Also note the substantial difference between the total content and the leachable amounts of Mo.

Figure 3.3

Results of percolation and batch leaching tests on an aggregate shown as a function of L/S.

In most cases where the difference in pH and redox conditions over the column experiment is limited, the results from batch tests and cumulative results from a column test will most likely be the same. An example of a situation where a difference is likely to develop is where, for instance, the pH changes during the column test, and as a result the leachability of some substances also changes, while the batch result represent only one pH value. This can happen either when the initial condition is acidic and then becomes neutral or alkaline, or when the initial pH is alkaline and then lowered due e.g. to carbonation. In other situations high concentrations of easily soluble salts such as chlorides may influence the leaching results for substances that can form chloride complexes. One such example is the increased leachability of Cd that may occur due to complexation with chloride. Chloride washes out fast and may be present in the first fraction(s) of eluate from the column test in concentrations high enough to form complexes with Cd and cause release of Cd in relatively high concentrations, whereas the average chloride concentration in the eluate from the batch test will be too low to form complexes with Cd, and the release of Cd will therefore remain low in the batch leaching test.

In some cases when sufficient information is available, results from observation of full scale applications of aggregates in the field may also be described as a function of L/S and compared to results of leaching tests performed on the same material in the laboratory. It is therefore sometimes possible – with considerable caution - to predict certain aspects of the leaching behaviour of an

1 10 100 1000 10000 0.01 0.1 1 10 100 A cc u m u la te d le ac h ed a m o u n t (m g /k g ) L/S (l/kg) Mo Total content Percolation test Batch test at L/S = 2 and 10 l/kg

aggregate under field conditions on the basis of laboratory leaching tests, using relatively simple modelling tools, although supplementary hydrogeochemical equilibrium modelling is often required to account for longer term effects. It is, of course, important to validate such prediction methods to the extent possible by appropriate comparisons between lab and field results (see section 3.11). Most laboratory leaching tests on granular materials performed under equilibrium-like conditions are accelerated in time compared to the actual duration of leaching under field conditions.

Under certain conditions, and when the physical layout and hydraulic/water balance situation for a full scale application is known, the L/S scale may be converted to a time scale for that particular utilisation scenario. This can be done by means of the following equation (Hjelmar, 1990):

t = (L/S) x d x H/I (3.2)

where

t is the time since the production leachate from the application started (years) L is the total volume of leachate produced at time t (m3)

S is the total mass of aggregate used in the application (tonnes, dry mass) d is the average dry bulk density of the aggregate in the application (tonnes/m3) H is the average height of the application (m)

I is the annual net rate of infiltration of precipitation (m3/m2)

It is assumed that percolation of the infiltrated precipitation is the sole source of leachate in the application.

The relationship between L/S and time is illustrated in Table 3.1 for an unbound application of an aggregate with a bulk density of 1.5 tonne/m3, heights of 0.5 m and 5 m, and annual rates of infiltration of precipitation of 50 mm and 200 mm, respectively.

Table 3.1

Illustration of the relationship between L/S and time, using equation (3.2).

When using leaching data as input to transport and behaviour models, it is often convenient to be able to quantify the leaching process in terms of simple mathematical formulas. The leaching of several (but not all) inorganic contaminants in an equilibrium controlled system, including a percolation test, may be described as resulting in an initial or early peak concentration of the substance in the leachate followed by an exponential decrease of the concentration with time (or L/S). If it is assumed that a continuously stirred tank reactor (CSTR) model (see e.g. van der Sloot et al., 2003) can be used to

Height Infiltration L/S Time Height Infiltration L/S Time

H I t H I t

m mm/year l/kg Years m mm/year l/kg Years

0.5 50 1 15 0.5 200 1 3.8 0.5 50 2 30 0.5 200 2 7.5 0.5 50 5 75 0.5 200 5 19 0.5 50 10 150 0.5 200 10 38 5 50 1 150 5 200 1 38 5 50 2 300 5 200 2 75 5 50 5 750 5 200 5 188 5 50 10 1500 5 200 10 375

interpret the results of a column leaching test on the granular waste material, the leaching of several components may be expressed by a simple decay function:

C = C0 * e - (L/S)  (3.3)

where C is the concentration of the contaminant in the leachate as a function of L/S (mg/l), the constant C0 is the initial peak concentration of the contaminant in the leachate (mg/l), L/S is the liquid

to solid ratio corresponding to the concentration C (l/kg) and where  is a kinetic constant describing the rate of decrease of the concentration as a function of L/S for a given material and a given substance (kg/l).  values may be estimated from column, lysimeter or serial batch leaching data (see van der Sloot et al., 2003).

By integrating the above expression, the amount of the substance, E (in mg/kg), released over the period of time it takes for L/S to increase from 0 l/kg to the value corresponding to C, can be calculated:

E = (C0/)(1 – e - (L/S)) (3.4)

Even if it is not entirely true, it is assumed that  is independent of the material leached, but specific for each substance. The larger  is, the faster will the concentration in the eluate decrease as a function of L/S. This is illustrated in Figure 3.4 which shows C/C0 as a function of L/S for different

values of .

Figure 3.4

C/C0 as a function of L/S for different values of .

For substances for which the leaching from an aggregate progresses as described by equation (2.3), the equation can be used to “translate” a leaching result (or a limit value associated with a percolation or batch leaching test) from one L/S value to another. If E1 is the amount leached of the substance at (L/S)1, the amount E2 leached at (L/S)2 can be calculated as follows:

E2 = E1 * (1 – e - (L/S)2 _{)/(1 – e} - (L/S)1 _{) (3.5)}

This method was used when setting equal limit values (= values on the same leaching curve) at L/S = 2 l/kg and L/S = 10 l/kg for acceptance of waste for landfilling in Council Decision 2003/33/EC.

Annex 1 shows an example of how equations (3.3) and (3.5) may be used to estimate the pore water concentrations in aggregates of initially released substances from leaching tests performed at higher L/S values.

C/C0 vs L/S for various values of kappa (in kg/l)

0 0.2 0.4 0.6 0.8 1 0.001 0.01 0.1 1 10 100 L/S (l/kg) C /C 0 0.01 0.1 0.2 0.5 1 5