Valorization of bottom ash fines by surface functionalization to reduce leaching of harmful contaminants.

This paper focuses on the functionalization of heterogeneous and highly contaminated waste material, namely bottom ashes (BA) with a particle size ≤ 125 µm that cannot be recycled with conventional treatments. The main goal of this study is to modify this waste into a valuable material that can be used in various applications, especially in the building sector. The complex mineralogical nature of this material was investigated with quantitative XRD, which confirms the presence of crystalline and amorphous phases such as silicates, carbonates, metallic oxides and amorphous glass. A hydrophobic modification was performed by using a fluorosilane grafting agent that utilizes the reactive surface sites of these minerals to form silanol bonds. Results showed that the 2.5% (m/m) of silane made the BA hydrophobic. Moreover, a thorough characterization showed that fluorosilane was well-grafted at the surface of the BA, with more than 60% of the fluorosilane chemisorbed on the surface. Additionally, the hydrophobic modification led to a significant decrease of the leaching of the contaminants (Cr, Cu, Mo and Sb) from the BA particles. Following this methodology, fine fraction of BA could be eventually used as a building material, preventing the landfill of this toxic waste.

Chemical speciation, distribution and leaching behavior of chlorides from municipal solid waste incineration bottom ash.

Municipal solid waste incineration (MSWI) bottom ash is an environmentally harmful solid waste that cannot be recycled without pre-treatment. The chloride content in bottom ash (BA) is a major obstacle that restricts its application as secondary building materials. Here, the chemical speciation of the chlorides in BA is systematically studied with multiple analytical techniques, i.e., quantitative XRD, microanalysis and XPS. In addition to halite (NaCl), several chloride-rich minerals are present in BA. These phases are hydrous metal oxides, ettringite, decomposed hydration products (C4A3) and incineration slag with a chloride content of 3.2%, 1.4%, 2.1% and 1.3%, respectively. For the first time, the real-time leaching profiles of chloride (up to 80 h) from BA were obtained with a chloride-ion specific electrode to explain the leaching mechanism. In the initial stage of leaching, highly soluble alkali salts (NaCl) and physisorbed chlorides (especially those adsorbed on hydrous metal oxides) are released, which is controlled by diffusion. Later, the leaching is controlled by the solubility/reactivity of the chloride-containing phases, such as ettringite and incineration slag. The results show that the release of chloride is not only a diffusion-controlled process, as reported in the literature, but also a reaction-controlled phenomenon, during which the chloride-rich phases decompose and release chlorides that are associated with them via sorption/incorporation.

The immobilization of potentially toxic elements due to incineration and weathering of bottom ash fines.

Incineration bottom ash fines (≤ 125 µm) are known to contain potentially toxic elements (PTEs) and inorganic salts. The most abundant PTEs in the fines were Zn (0.5%), Cu (0.25%), Pb (0.12%), Mn (0.08%) and Cr (0.03%). The systematic quantification of the mineral phases and PTEs associated with them was performed with a multimethod approach using quantitative XRD, phase mapping with PhAse Recognition and Characterization (PARC) software and microprobe analysis. The mineral phases in the fines can be categorized as follows: 1) residual phases (e.g., quartz), 2) incineration phases (e.g., melilitic slag and iron oxides) and 3) quenching/weathering phases (e.g., calcite, ettringite, gypsum, hydrous Fe- and Al-oxides). Among the incineration phases, the melilitic slag was observed to contain Cr, Cu and Zn with 0.02%, 0.13% and 0.19%, respectively. In order of predominance, the weathering phases containing the most PTEs were: calcite < ettringite < hydrous Al-oxides < hydrous Fe-oxides. More than 70% of the phases in the BA fines were formed during incineration and weathering processes that explain the enrichment of PTEs in the smaller particles. During the one-batch leaching test, dissolution of weathering phases, especially ettringite, was observed (total mass loss: 7.2%).

In-depth mineralogical quantification of MSWI bottom ash phases and their association with potentially toxic elements.

Municipal solid waste incineration bottom ash fractions ≤4 mm are the most contaminated ones in terms of potentially toxic elements (PTEs). In order to estimate potential environmental impacts, it is important to understand the association of the PTEs with the mineral phases. Large area phase mapping (SEM/EDX) using "PhAse Recognition and Characterization - PARC" software in combination with quantitative X-ray powder diffraction has been used to characterize amorphous and crystalline BA phases for the first time. The results show that one of the main incineration products was melilite and an amorphous phase with a melilitic composition. The ratio of crystalline to amorphous melilite was 1:2. They formed an inhomogeneous layer around BA particles and contained a high percentage of the PTEs, i.e., Cu, Zn, Ni and Cr. Other major sources of PTEs (especially Ni and Cu) were iron oxides produced during incineration and the weathering products, such as calcite and ettringite (Cu and Zn). After extensive characterization of BA, a sequential extraction procedure (SEP) was performed, which exposed bottom ash to different chemical environments designed to dissolve specific phases and release their PTEs into solution. The extracted solutions and solid residues generated from the extraction procedure were analyzed to identify the association between PTEs and dissolved phases of BA. By combining SEP results with information obtained via large area phase mapping it is shown that SEP can be used for studying the association of PTEs with the phase that cannot be investigated with XRD/EDX, such as organic matter and Fe-Mn-hydrous oxides. Furthermore, according to SEP results a high percentage (40-80 wt%) of each investigated PTE can be considered immobile and not susceptible to leaching in the environment.

A two-stage treatment for Municipal Solid Waste Incineration (MSWI) bottom ash to remove agglomerated fine particles and leachable contaminants.

In this lab study, a two-stage treatment was investigated to achieve the valorization of a municipal solid waste incineration (MSWI) bottom ash fraction below 4mm. This fraction of MSWI bottom ash (BA) is the most contaminated one, containing potentially toxic elements (Cu, Cr, Mo and Sb), chlorides and sulfates. The BA was treated for recycling by separating agglomerated fine particles (≤125µm) and soluble contaminants by using a sequence of sieving and washing. Initially, dry sieving was performed to obtain BA-S (≤125µm), BA-M (0.125-1mm) and BA-L (1-4mm) fractions from the original sample. The complete separation of fine particles cannot be achieved by conventional sieving, because they are bound in a cementitious matrix around larger BA grains. Subsequently, a washing treatment was performed to enhance the liberation of the agglomerated fine particles from the BA-M and BA-L fractions. These fine particles were found to be similar to the particles of BA-S fraction in term of chemical composition. Furthermore, the leaching behavior of Cr, Mo Sb, chlorides and sulfates was investigated using various washing parameters. The proposed treatment for the separation of agglomerated fine particles with dry sieving and washing (L/S 3, 60min) was successful in bringing the leaching of contaminants under the legal limit established by the Dutch environmental norms.