Precipitation landfill leachates

The harmful liquid that collects at the bottom of a landfill is known as leachate. The generation of leachate is a result of uncontrolled runoff, and percolation of precipitation and irrigation water into the landfill. Leachate can also include the moisture content initially contained in the waste, as well as infiltrating groundwater. Leachate contains a variety of chemical constituents derived from the solubilization of the materials deposited in the landfill and from the products of the chemical and biochemical reactions occurring within the landfill under the anaerobic conditions. 

Siemers, 1995 landfill leachate Bio-O UV-Bio (sequential) (UV not used) 1 BC (each, 3 chemical stages in parallel) (5 barabs) 36 200// 100-400 2.0-3.0 23 Mio. 41.7 pH-control impossible (dirty probe) heavy Ca-oxalate precipitation in reactors, tubes and pumps foaming polluted off-gas 03-catalyser... 

The entry of strongly reduced landfill leachate into a pristine, often oxidized, aquifer, leads to the creation of very complex redox environments. Important processes include organic matter biodegradation, biotic and abiotic redox processes, dissolution/precipitation of minerals, complexa-tion, ion exchange, and sorption. The resulting... 

Heavy metals. The behavior of heavy metals in a landfill leachate plume is simultaneously controlled by sorption, precipitation, and complexation, and proper evaluations of metal attenuation must account for this complex system. Generally, heavy metals do not constitute a groundwater pollution problem at landfills (Arneth et al., 1989), because landfill leachates usually contain only modest heavy metal concentrations, and the metals are subject to strong attenuation by sorption and precipitation in the landfill itself (Kjeldsen et al., 2002). Sulfide-producing conditions result in extremely low solubility of... 

A. Mdller, A. Grahn, and U. Welander, Precipitation of Heavy Metals from Landfill Leachates by Microbially Produced Sulphide, Environmental Technology, 25(1), 69-77 (2004). 

Griffin, R. A., and N. F. Shimp. 1976. Effect of pH in exchange-adsorption or precipitation of Pb from landfill leachates by clay minerals. Envir. Sci. Technol. 10(13) 1256-61. 

A more simple treatment is the pH correction, very often used for regulation compliance (the pH value of the treated effluent must be between 5.5 and 8.5). A pH modification can also be used for metallic compound precipitation usually as hydroxide forms, in alkaline conditions, or for humic substances removal, in acidic conditions. The effect of this last treatment can be shown for landfill leachates treatment in Chapter 10. 

Complex matrices such as soil solutions or landfill leachates usually require clean up where interfering compounds are removed prior to organic acid analysis. Procedures used include removal of humin-like substance by passing through special cartridges or precipitation after... 

Other types of wastes and waste repositories have redox-related performance issues. Figure 5 shows redox potentials associated with (1) reductive precipitation of chromate ion and (2) the progressive reductive dechlorination of tetra-chloromethane to methane. These two contaminants are selected as illustrative compounds to represent components of landfill leachate and hazardous industrial waste. 

The landfill liner, cover, and hydraulic barrier all belong to the subsurface pollutant engineered containment system. The liner is designed at the bottom of a landfill to contain downward leachate. The cover is designed at the top of a landfill to prevent precipitation from infiltrating into the landfill. The hydraulic barrier, or cutoff walls, is a vertical compacted earthen system to contain horizontal flow of plume. The ultimate purpose of these barriers is to isolate contaminants from the environment and, therefore, to protect the soil and groundwater from pollution originating in the landfill or polluted site. 

Many factors influence the production and composition of leachate. One major factor is the climate of the landfill. For example, where the climate is prone to higher levels of precipitation, there will be more water entering the landfill and therefore more leachate generated. Another factor is the site topography of the landfill, which influences the runoff patterns and again the water balance within the site. 

Precipitation of (hydr)oxides and carbonates appears to control the concentrations of some cationic species. In Lysimeter Teuftal Ni(II) and Cu(II) appear to be precipitated as hydroxides, while in leachates of Landfill Lostorf Cd(II) concentrations appear to be controlled by the precipitation of CdC03. In both systems MoO and WO - appear to precipitate as Ca metallates (e.g., Fig. 11). 

Reducing conditions within the leachate plume also cause metal mobility, particularly of manganese and iron. The plume near the landfill has a pH of 6.0-6.5 and is reducing (-50 mV), making Fe2+ stable (Box 5.4). The transition to oxidizing conditions down gradient in the aquifer allows solid iron oxides (e.g. FeOOH) to precipitate, dramatically, reducing the mobility of metals which co-precipitate with iron. 

Mercury entering a landfill will be subjected to a number of reactions. Firstly, part of the mercury would evaporate due to the high volatility of the element coupled with the working face that is the compaction of the wastes. The remaining mercury would be oxidised to ionic mercury (monovalent or divalent), that would either precipitate reacting with sulphate ions, or percolate in the leachates, or undergo the methylation process and be liberated through the form of methyl mercury in the landfill gas. 

Metals often are precipitated within the landfill and are infrequently found at high concentrations in leachate, with the exception of iron. 

---