Dorthe Lærke Jensen

Characterization of the dissolved organic carbon in landfill leachate-polluted groundwater

Abstract Samples of dissolved organic carbon (DOC) were obtained from landfill leachate-polluted groundwater at Vejen Landfill, Denmark. The humic acids, fulvic acids and the hydrophilic fraction were isolated and purified. Based on DOC measurements, the fulvic acid fraction predominated, accounting for about 60% of the total amount of DOC with an apparent molecular weight of about 1800 Da. The hydrophilic fraction constituted about 30% of the total amount of DOC with an apparent molecular weight of about 2100 Da, and the humic acid fraction made up about 10% of the total amount of DOC with an apparent molecular weight of about 2600 Da. The elemental compositions of the humic acids, fulvic acids and the hydrophilic fraction were in the ranges typical for humic substances from other origins. The O/C ratios for humic acids, fulvic acids and the hydrophilic fraction were similar in the leachate-polluted groundwater. For humic acids the O/C ratios were slightly higher than reported in the literature, indicating a high content of carboxylic groups, phenolic groups or carbohydrates. Acid-base titration indicated that, in the fulvic acids and the hydrophilic fraction, carboxylic acids were the dominating functional group, representing about 6 meq g −1 . The weakly acidic groups in fulvic acids and the hydrophilic fraction represented about 1 and 3 meq g −1 , respectively. The total acidity in fulvic acids and the hydrophilic fraction accounted for 48–57% of the O/C ratio. In the humic acids, carboxylic groups made up about 3 meq g −1 and the weakly acidic groups made up about 1.5 meq g −1 . The total acidity accounted for 29–32% of the O/C ratio. The characterization of DOC in leachate-polluted groundwater in terms of humic acids, fulvic acids and hydrophilic fraction showed that the hydrophilic fraction resembles, in many ways, humic and fulvic acids; thus, a distinction between the fractions may be related to the methods only and be of little practical value. The three fractions constituting the DOC content in a sample should all be considered when evaluating processes such as metal complexation and transport of metals and hydrophobic, organic contaminants.

Effect of dissolved organic carbon on the mobility of cadmium, nickel and zinc in leachate polluted groundwater

Abstract The ability of dissolved organic carbon (DOC) from landfill leachate polluted groundwater to form complexes with the heavy metals cadmium (Cd), nickel (Ni) and zinc (Zn) was investigated. The DOC samples originated from the leachate pollution plume at Vejen Landfill, Denmark and were studied in the original matrix with a minimum of manipulation. The experiments were performed as batch sorption experiments and the metal distribution between the aquifer material and the solution (Kd) was determined in the leachate polluted groundwater samples and in reference solutions of synthetic inorganic leachate. The difference in distribution coefficients was a direct indication of complex formation between DOC and heavy metals. The results showed, that DOC from landfill leachate polluted groundwater has the ability to form complexes with Cd, Ni and Zn, and the distribution coefficients were a factor of 2–6 lower in the presence of DOC. Based on the distribution coefficients, the relative migration velocities of the heavy metals were estimated. The migration velocity of the metals was increased by the presence of DOC but did not exceed 1.2% of the water migration velocity, indicating that the effect of DOC on the mobility of Cd, Ni and Zn may have only minor environmental importance. Conditional complex formation constants (log Kc) were estimated from the Kd-values. The constants for 1:1 complexes increased slightly in the following order Zn

The solubility of rhodochrosite (MnCO3) and siderite (FeCO3) in anaerobic aquatic environments

Abstract Natural groundwaters are often reported to be highly supersaturated with the carbonate minerals siderite (FeCO 3 ) and rhodochrosite (MnCO 3 ). The kinetics of precipitation and dissolution were determined in the light of new determinations of the solubility products of siderite and rhodochrosite. Laboratory experiments showed that the precipitation kinetics of siderite and rhodochrosite were much slower than that of calcite, and also much slower than their dissolution kinetics. Experiments with supersaturated solutions failed to reach steady state within 474 days in the case of siderite, whereas steady state for rhodochrosite was reached after 140 days. Suspensions of siderite and rhodochrosite crystals reached steady state after 10 and 80 days, respectively. The solubility product of siderite (−log K S0 (FeCO 3 )) was 11.03 ± 0.10 for dried crystals and 10.43 ± 0.15 for wet crystals. For rhodochrosite the solubility product (−log K S0 (MnCO 3 )) was 11.39 ± 0.14 for dried crystals and 12.51 ± 0.07 for wet crystals. The solubility product determined from supersaturated solutions was −log K S0 (MnCO 3 )=11.65 ± 0.14. The observed slow precipitation kinetics of siderite and rhodochrosite might explain the apparent supersaturation that is often reported for anaerobic aquatic environments.

Treatment of waste incinerator air-pollution-control residues with FeSO4: Concept and product characterisation

This paper describes a new concept for treatment of air-pollution-control (APC) residues from waste incineration and characterises the wastewater and stabilised residues generated by the process. The process involves mixing of APC-residues with a ferrous sulphate solution and subsequent oxidation of the suspension (Ferrox-process 1996). The process results in a significant reduction in the leaching of salts and heavy metals from the residue, by washing out most of the salts and by binding the heavy metals in the iron oxides formed. In the laboratory, a semidry gas-cleaning residue and a fly ash were treated by the process. The generated wastewater contained low concentrations of heavy metals (e.g. Pb: 27-39 μg l-1 and Cd: 2.6-4.6 μg l-1), but high concentrations of salts (e.g. Cl, Na, K, and Ca). The treatment process reduced the leaching of Pb from the residues by more than two orders of magnitude at fixed pH as determined by pH-static leaching tests. Likewise, the leaching of Cd, Zn and Cu was significantly reduced. The effect on elements that form oxyanions (e.g. Cr) is marginal and in the current process there is no reduction in the release of Hg.

Organic Halogens in Landfill Leachates

Using a group parameter, totalorganic halogens (TOX), high TOX concentrationswere found in leachates and leachate contaminatedgroundwaters at two Danish mixed sanitary andhazardous waste sites. With commonly used screeningprocedures for organic contaminants, the individualhalogenated organic compounds behind the high TOXcould not be found. Conversely, part of the TOX couldbe attributed to halogenated humic substances that cannot be analysed with conventional organic contaminantanalyses. Even though TOX can still be considered anindicator of leachate contamination in the aquifers,attempts to identify the halogenated contaminantsbehind the TOX using screening methods such as GC-MSwill be unsuccessful in most cases.

High temperature co-treatment of bottom ash and stabilized fly ashes from waste incineration.

Abstract Bottom ashes from two Danish municipal solid waste incineration plants were heated at 900°C with iron oxide stabilized air pollution control residues at actual mass flow ratios (9:1), simulating a treating method for the residues. The two residues were co-treated, producing one combined stream that may be utilized as a secondary road construction material. Scanning electron microscope analysis and grain size distribution analysis indicated that sintering of the particles did not occur. Batch leaching tests at liquid/solid 10 l/kg at a range of pH-values (6–10) quantified with respect to Cd, Cr and Pb revealed significant positive effects of co-heating the ashes, although Pb showed slightly increased leaching. At a liquid/solid ratio of 10 l/kg the leachate concentrations were still low at pH 7–10 and the release of Pb was, thus, not expected to limit the utilization of the mixed ashes. The process, thus, fixates the metals in the solid residues without altering the leaching properties of the bottom ash too significantly.

On-site treatment and landfilling of MSWI air pollution control residues

Air pollution control (APC) residues from municipal solid waste incineration (MSWI) are difficult to landfill due to substantial leaching of trace metals. An on-site pretreatment prior to landfilling of APC-residues was investigated in terms of bench-scale experiments with a semidry APC-residue and a fly ash. The treatment involved mixing of the residues with a ferrous sulphate solution and subsequent oxidation of the suspension. Afterwards, the suspension was spread on a dedicated landfill section and allowed to drain by gravity through the drainage system of the landfill. The wastewater from the process, collected through the drainage system, contained large concentrations of salts (Cl: 14-30 g/l, Na: 4-9 g/l, K: 5-11 g/l, Ca: 2-12 g/l) but low concentrations of trace metals (e.g. Pb: 14-100 microg/l, Cd: <2-7 microg/l). The treated residues left in the landfills were later subject to leaching by simulated rainfall. The leachate contained low concentrations of trace metals (Pb: <120 microg/l, Cd: <2 microg/l, Cr: <485 microg/l). The leachate concentrations from the treated APC-residues were substantially reduced compared to concentrations in leachate from untreated APC-residues. Particularly in the early stages of the leaching, concentrations of trace metals were reduced by up to four orders of magnitude.

Stabilization of APC residues from waste incineration with ferrous sulfate on a semi-industrial scale

Abstract A stabilization method for air pollution control (APC) residues from municipal solid waste incineration (MSWI) involving mixing of the residue with water and FeSO4 has been demonstrated on a semi-industrial scale on three types of APC residues: a semidry (SD) APC residue, a fly ash (FA), and an FA mixed with sludge (FAS) from a wet flue gas cleaning system. The process was performed in batches of 165-175 kg residue. It generates a wastewater that is highly saline but has a low content of heavy metals such as Cd, Cr, and Pb. The stabilized and raw residues have been subject to a range of leaching tests: the batch leaching test, the pH-static leaching test, the availability test, and the column test. These tests showed that the stabilized residues have remarkably improved leaching properties, especially with respect to Pb but also with respect to Cd, Cu, and Zn. The release of Pb was reduced by a factor of 250-36,000.

Treatment of waste incinerator air-pollution-control residues with FeSO4: Laboratory investigation of design parameters

The key design parameters of a new process for treatment of air-pollution-control (APC) residues (the Ferrox-process) were investigated in the laboratory. The optimisation involved two different APC-residues from actual incinerator plants. The design parameters considered were: amount of iron oxide supplied, the liquid-to-solid ratio of the process, the separation of solids and wastewater, the sequence of material mixing, the possibilities of reuse of water, the feasibility of using secondary (brackish) water, and simple means to improve the wastewater quality. The investigation showed that an optimum process configuration could be obtained yielding a stabilised solid product with low leaching of heavy metals and a dischargable wastewater with high contents of salts (in order to remove salts from the solid product) and low concentrations of heavy metals. The amount of iron added to the APC-residues must be optimised for each residue. The overall water use can be limited to a L/S-ratio of 3 l kg-1 including water used for washing of the treated products.

Re-use of stabilised flue gas ashes from solid waste incineration in cement-treated base layers for pavements

Fly ash from coal-burning power plants has been used extensively as a pozzolan and fine filler in concrete for many years. Laboratory experiments were performed investigating the effect of substituting the coal-based fly ash with chemically stabilised flue gas ashes (FGA) from waste incineration. Two types of FGA were treated by the Ferrox-process, which removes the majority of the easily soluble salts in the FGA and provides binding sites for heavy metals in terms of ferrihydrite. Cubes of cement treated base layer materials containing 5% stabilised FGA were cast, sealed and cured for two weeks. Cylinders (diameter 100 mm, length 150 mm) were drilled from these cubes for tank leaching experiments. Duplicate specimens were subject to compression strength testing and to tank leaching experiments. The compressive strength of the CTB fulfilled the Danish requirements for CTB, i.e. strength more than 5 MPa after 7 days. The tank leaching tests revealed that leaching of heavy metals was not significantly affected by the use of chemically stabilised flue gas ashes from waste incineration. Assuming that diffusion controls the leaching process it was calculated that less than 1% of the metals would leach during a 100-year period from a 0.5 m thick concrete slab exposed to water on one side. Leaching of the common ions Ca, Cl, Na and SO4 was increased 3-20 times from the specimens with chemically stabilised flue gas ashes from waste incineration. However, the quantities leached were still modest. These experiments suggest that FGA from waste incineration after Ferrox-treatment could be re-used in CTB without compromising the strength and leaching from the base layer.