Batch and column leaching tests

Several leaching methods have been applied to study the release of major and minor species from solid wastes, especially fly ash. The leaching tests are broadly divided into two groups; these are batch and column leaching tests.

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2.10.1.1 Batch leaching tests

Batch leaching tests typically involve mechanical mixing of a unit volume of water, or an alternative solution, with a unit mass of hazardous waste with the intent to establish pseudo- equilibrium conditions. The goal of equilibrium batch testing is to represent constituent solubility and release over a range of conditions by varying a single parameter such as final extract pH, liquid-to-solid (L/S) ratio, contact time, etc. Once equilibrium is established, release is dependent on the geochemistry of the solid phase and chemistry of the liquid phase rather than on contact time (Lopez Meza et al., 2008).

Extraction procedure (EP) is a batch test developed to classify a solid waste as hazardous, based on some specific organic and inorganic constituents designated in the US Federal Register (USEPA 1980). Toxicity Characteristic Leaching Procedure (TCLP) is another batch leaching test developed in 1984 under the Hazardous and Solid Waste Amendments for the Resource Conservation and Recovery Act (RCRA) (USEPA, 1990). The TCLP method is designed to simulate the leaching a waste will undergo if disposed in an unlined sanitary landfill. It is an agitated extraction test using a leaching fluid (either a sodium acetate buffer solution having a pH of 4.93 or an acetic acid solution with a pH of 2.88) that is a function of the alkalinity of the solid phase of the waste. This method is used by U.S. EPA regulators for the identification and classification of waste as hazardous or non hazardous based on their toxicity. The classification is based on an extensive list of organic and inorganic compounds and covers a wide range of waste types. TCLP is aggressive towards leaching of silver, arsenic, selenium and chromium (Sorini and Jackson, 1988). American Society for Testing and Materials (ASTM) Method D- 3987 is an agitated extraction method that uses distilled water as the leaching fluid during 18 hours of contact time with the solid waste. The procedure is designed to rapidly generate a leachate from solid waste that can be used to estimate the mobility of inorganic constituents from the waste under the specified test conditions (ASTM 1995a). German Leach Test (DIN 38414 S4) (Institut fűr Normung, 1984) is an extraction test (batch test) that involves agitation of the ash sample with de-ionized water for a period of 24 hours at a liquid-to-solid ratio 10:1. This method is rapid, simple, reproducible, and applicable to solids, pastes, and sludge. This method is used to assess the readily soluble fractions of metals in the fly ash samples. This method has a

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number of shortcomings when the experimental conditions are compared with those occurring in nature under field situation. For instance, the liquid-solid ratio of the laboratory test is much higher than what will commonly be observed in field conditions (Ludwig et al., 2005), and the rate of metal release can be highly influenced by shaking. This method is not suitable for the assessment of short-term and long-term effects of ash disposal on groundwater.

Synthetic Groundwater Leaching Procedure (SGLP) is a generic agitated extraction procedure that was developed to simulate natural groundwater conditions with respect to groundwater chemistry, to be site specific, and be directly comparable to the TCLP (Hassett, 1987). This method is similar to the TCLP except that synthetic groundwater is used in place of acetic acid solution or sodium acetate buffer solution. Acid Neutralization Capacity test (ANC) is a test involving reacting fly ash with acid solutions to attain different predetermined final pH. Leaching of trace elements from combustion residues is a very slow process and the solid and liquid phase equilibrium may not be attained even with long leaching times (Ugurlu, 2004). Since fly ash reacts with acidic solutions like acid rain, the buffering capacity of the residues is consumed by neutralizing reactions. An acidic leaching environment is therefore reasonable for some waste disposal conditions. Acid soluble fractions of fly ash also provide some information about the potential leachability of a given element under acidifying conditions. The difference between the acid soluble fraction and the initial, water-soluble fraction may therefore reflect the dissolution behaviours of various constituents in different leaching stages. Some of these batch leaching tests such as ANC will be used in monitoring the leaching of species from the brine/fly ash interaction in this study. Some modification will be done where necessary for the purpose of this study.

2.10.1.2 Columns tests

Column tests are designed to evaluate the release of constituents under either local equilibrium or advection conditions as a function of time. This method involves the flow of liquid through a fixed bed of solid material and the percolated liquid is collected as a function of L/S ratio, which is used as a surrogate for leaching time. When designing a column for leaching test, the flow rate of the liquid passing through the column is an important parameter that needs to be considered.

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When the flow rate through the column is low, local equilibrium conditions are considered to control constituent release while at higher flow rates, the column testing could be aimed at determining the rates of constituent leaching during advective mass transport (Garrabrants and Kosson, 2005). Column procedures generally include a liquid to solid ratio which more closely represents a field situation. The mechanism of contacting a fixed body of waste with a transient liquid resembles the leaching mechanism imparted by gravity flow of liquid through a waste disposal site (Jackson et al., 1984). Column tests can account for constituent wash out at lower L/S ratios, and the changes in solubility controlling phases that occur as a result (van der Sloot et

al., 2001; Dijkstra et al., 2006). Column leaching testing can be considered a better basis for

assessing field impact data than any other available batch test method because it provides a fundamental basis from which to estimate constituent release under a variety of field conditions. Leachate generated by the column method is reportedly more representative of leachate derived from a disposal site than is the leachate from the batch method.

Studies involving leaching of waste materials using both batch and columns have been investigated by several authors. Jackson et al. (1984) conducted a comparative study on both batch and column leaching tests by comparing the differences in the concentration of leachate constituents and experimental variation. The study was done to evaluate the advantages and the disadvantages of these test methods. They observed that using a column test for relatively impermeable wastes may be problematic and the results from this test could be highly variable. The batch extraction method offers an advantageous approach to extraction of waste constituents through its greater reproducibility and simplistic design. The batch method can be set up and used routinely by laboratory personnel more easily than the column method. Both batch and column extraction methods were considered effective in generating useful leachate profiles for evaluating potentially hazardous wastes. Although useful data can be produced by these methods, the relative accuracy of predicting levels of analytes leached from landfilled wastes remains uncertain based on these tests.

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2.10.1.3 Advantages and disadvantages

Although batch or column testing can be used to evaluate equilibrium-controlled leaching, each of these testing approaches has advantages and disadvantages. Batch testing offers the advantage of simpler design, while column testing provides a closer approximation to leaching processes that occur under field conditions without compromising reproducibility of experiments (Jackson

et al., 1984; Caldwell et al., 1990; Kjeldsen and Christensen, 1990; Sawhney and Frink, 1991;

Wasay, 1992; van der Sloot et al., 1996; Dijkstra et al., 2006). Column tests are more suitable for prediction purposes, but they are often time consuming, ranging in duration from several weeks to years. Alternatively, batch tests can be carried out in shorter periods of time, varying from several hours to a few days. However, batch tests are often subjected to excessive mobilization of dissolved organic carbon (DOC) and/or colloids that are uncharacteristic for field scenarios (van der Sloot et al., 2003).