A geochemical analysis of municipal solid waste ash

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Virginia Tech

Bulk chemical analysis of a combined municipal solid waste (MSW) bottom and fly ash from one facility showed most elements enriched over average soil abundances. Eleven minerals were identified using powder X-ray diffraction (XRD). Standard additions using XRD gave the following weight % minerals (±2σ): gypsum, 1.8 ± 1.9; hematite, 3.7 ± 1.7; quartz, 2.3 ± 1.0; spinel, approximately 3.5; halite, 0.5 ± 0.4; calcite 3.5 ± 1.9; rutile, 1.1 ± 1.3. Mullite, sylvite, anhydrite, and wüstite were also identified. The ash contained 18% minerals, 9% structural and adsorbed water, and 72% glass.

Chemical sequential extraction showed that most Cr is present in phases resistant to chemical weathering, while significant Cd and Pb are sequestered in acid soluble (carbonate) phases. Little of these toxic trace metals are water soluble or in exchangeable surface sites.

Batch reactors experiments showed that ash-water solutions were dominated by ions released by soluble salts. Three types of reactions are identified. 1) After rapid exhaustion of soluble salts, sodium and potassium exhibited nearly steady state behavior due to slow release of ions from less-soluble minerals and glasses. 2) Calcium and sulfate concentrations are controlled by either gypsum or anhydrite equilibrium after a few hours. Iron, aluminum, and manganese concentrations rapidly equilibrate with respect to hydroxide or oxide solid phases. 3) Silicon clearly shows temperature dependent kinetic behavior, but its rate of release into solution is slowed by back-reaction of a secondary silicate phase.

Calculation of the CIPW normative minerals for MSW ash showed it to be analogous to a tholeiitic basalt. Over the long term, the concentrations of elements in MSW ash will continue to change and can be predicted by Goldschmidt's concept of ionic potential, and mineralogical changes can be predicted based on weathering of basalts.

Comparisons of literature values showed that field MSW ash leachates contained higher concentrations of soluble salts and lower concentrations of magnesium, iron, zinc, copper, and nickel than uncombusted refuse leachates. Comparison of chromium, cadmium, lead, and arsenic concentrations did not clearly delineate which leachate contains more of these elements.