Materials and Methods
Samples were collected from the Iwate Clean Center landfill site in Iwate Prefecture, Japan. The site is separated into two blocks: block I (landfill capacity, 50,300 m3) and block II (landfill capacity, 75,000 m3). Block I is now closed, whereas more than half of the landfill capacity of block II remains available. Major wastes buried in the landfill site have been combustion residue (26,000 t), glass, concrete, and ceramic (25,000 t), and inorganic sludge (10,000 t).
A flow sheet of the leachate treatment system that indicates sampling points is shown in Fig. 12.1. Leachates from blocks I and II are mixed in the regulating reservoir. Samples were taken after the following steps: regulating reservoir (raw leachate, RL); alkali removal step using sodium carbonate (AR); biological treatment step (BT); membrane filtration step (MF); activation carbon adsorption step (AC); and chelating and zeolite adsorption step (effluent water, EW). Samples were taken in July, September, and December 2012 at all locations, except for the AC, which was only sampled in September and December. Zeolite adsorption for radiocesium
Fig. 12.1 Leachate treatment system with sampling points (filled circles)
adsorption was added to the process from May 2012. There is a pH regulating tank after each step.
The precipitate in the sedimentation tank at the AR step was collected in December to determine the distribution between liquid phase and solid phase (precipitates) for each element.
Determination of Characteristics of Samples
Chemical characteristics of the samples were determined. The pH was measured with a pH meter (HM-25R; TOADKK). Suspended solids (SS) were determined in duplicate by the standard method . Total organic carbon content (TOC) was determined three times using a TOC analyzer (TOC-V; Shimadzu).
Concentration analyses of the target elements (Cs, Sr, Co, Ni, and Mn) and major elements were based on standard methods for examination of wastewater . These analyses were done in triplicate. First, 100 mL of sample was digested with 1 mL concentrated HNO3 and 0.5 mL concentrated HCl, and then evaporated on a hot plate at around 120 °C. Next, deionized water was added to the sample to make a solution volume of 100 mL. The sample solution was filtered through a 0.45-μm membrane filter. Element concentrations in the filtrate were measured using an inductively coupled plasma (ICP) mass spectrometer (iCAP Qs; Thermo) and ICP atomic emission spectrometer (Shimadzu, ICPE-9000). Standard solutions were purchased from Merck Chemicals (CertiPUR series). No contaminations of the analyzed elements were found in a processed blank.
The precipitate collected in the sedimentation tank at the AR step was also analyzed to determine concentrations of each element in the solid phase in the precipitate as already described.
Determination of Physical and Chemical Forms of the Target Elements in Raw Leachate
Filtration and ion-exchange methods were used to determine three fractions of each element in RL sample: particulate, cationic, and anionic fractions. The particulate fraction was the solid phase retained by filtration through a 0.45-μm membrane filter. The filtrate was used for further fractionation.
Two types of ion-exchange resins, an anion-exchange resin (AER) (IRA402BL; Amberlite) and a cation-exchange resin (CER) (IR120B-Na; Amberlite), were used to determine the ion species of each element in the filtrate. A preliminary experiment confirmed that the cations of the target elements were retained by CER but not by AER.
After passing through the 0.45-μm membrane filter, the filtrate was passed through the AER or CER resin column separately. Each column eluent was analyzed for the element concentrations using the ICP mass spectrometer. The cation form fraction or the anion form fraction was the fraction that passed through the AER or CER, respectively.