Effect of waste-derived soil amendments on mitigating leaching impacts from municipal solid waste incineration (MSWI) ash

This study explores modifying a sandy soil with a low solid to liquid partitioning coefficient (Kd) by adding amendments including iron-rich industrial slag byproducts and biochars, which contain sorption sites for trace metals present in MSWI ash leachate (notably Sb, cited as a concern for reuse applications). Kd values for Sb were determined for the sandy soil to be as low as 1.6 ± 0.1 L/kg. With amendments, Kd values varied from 1.4 ± 0.2 L/kg for combined ash leachate exposed to a blend of sandy soil and 20% iron slag, to 990 L/kg for combined ash leachate exposed to a blend of sandy soil and 20% magnetic solids. A blend of 20% magnetic solids showed orders of magnitude increase beyond 100% sandy soil. The biochars showed limited capacity to reduce leached Sb in the ash-derived leachate, which is likely due to negative surface charges of the biochars and Sb at basic pH. A risk assessment (US EPA IWEM) performed using experimental Kd for each blend suggests that using soil amendments could reduce leached concentrations at points of concern, which could open additional avenues for ash reuse.

Distributions of trace elements within MSWI bottom and combined ash components: Implications for reuse practices

Concentrations of 25 inorganic elements were measured in both bulk ash and individual ash components from residuals at three municipal solid waste incineration (MSWI) facilities in the US (two combined ash (CA) and one bottom ash (BA)). Concentrations were assessed based on particle size and component to understand the contribution from each fraction. The results found that among facilities, the finer size fractions contained elevated concentrations of trace elements of concern (As, Pb, Sb) when compared to the coarse fraction, but concentrations varied among facilities depending on the type of ash and differences in advanced metals recovery processes. This study focused on several constituents of potential concern, As, Ba, Cu, Pb, and Sb, and found that the main components of MSWI ash (glass, ceramic, concrete, and slag) are sources of these elements in the ash streams. For many elements, concentrations were significantly higher in CA bulk and component fractions opposed to BA streams. An acid treatment procedure and scanning electron microscopy/energy-dispersive x-ray spectroscopy analysis revealed that some elements, such as As in concrete, are result of the inherent properties of the component, but other elements, such as Sb, form on the surface during or after incineration and can be removed. Some Pb and Cu concentrations were attributed to inclusions in the glass or slag introduced into the material during the incineration process. Understanding the contributions of each ash component provides critical information for developing strategies to reduce trace element concentrations in ash streams to promote reuse opportunities.

Landfill gas as a source of anthropogenic antimony and arsenic release

Antimony is used extensively in consumer goods, including single use plastic bottles, electronics, textiles and automobile brakes, which are disposed of in landfills at the end of their service lives. As a result, Sb is a constituent of concern in landfill emissions. Previous research has focused on leachate (liquid) and waste incineration flue gas emissions; however, Sb has the potential to volatilize through chemical and microbial processes within a landfill. In this study, iron-amended granular activated carbon was used to adsorb volatile metals directly from gas in a full-scale landfill gas collection system. Metals were quantified using acid digestion and ICP-AES analysis. Antimony concentrations far exceeded those previously reported, at up to 733 μg m^-3 (mean: 254 μg m^-3). In addition to Sb, As was also measured at high levels compared to previous research, as high as 740 μg m^-3 (mean: 178 μg m^-3). Using US EPA landfill and landfill gas databases, total Sb emissions via landfill gas are estimated to be approximately 27.3 kg day^-1 in the US. Based on other estimates of national and global Sb emissions, this corresponds to approximately 4.5% of total US atmospheric emissions of Sb and 0.42% of global atmospheric emissions. Sb mass release via landfill gas is approximately 3.9 times higher than via leachate emissions. Although gas emissions are higher than expected, the vast majority (99.9%) of Sb present in landfilled MSW remains within the waste mass indefinitely. In addition to these mass release estimates, this experiment suggests that iron-amended activated carbon may offer significant metals removal from LFG, especially in the first months of new well operation.

A componential approach for evaluating the sources of trace metals in municipal solid waste

Trace metals concentrations of 25 elements were determined for 22 subcomponents of biodegradable and non-biodegradable waste samples representing the United States municipal solid waste (MSW) stream collected during three separate waste sorts. The subcomponent trace metal concentrations and estimated composition results were used to predict trace metal concentrations present in the overall MSW stream along with MSW compost and waste to energy (WTE) ash, which were compared to health-based standards (i.e., US EPA regional screening levels) and to values previously reported in the literature. These estimates for potentially problematic elements like As and Sb could be attributed to abundant base materials in MSW, while other elements, such as Pb, were calculated at much lower concentrations than other published studies. This suggests that trace metals measured in actual MSW compost and WTE ash could originate not only from MSW base components but also from other sources, such as highly concentrated low-mass wastes (e.g., e-waste). While the removal of small quantity components with high metal concentrations may reduce concentrations of some potentially problematic metals (e.g., Pb), others (e.g., As and Sb) are likely to persist in quantities that impede reuse and recycling since they are present in the more abundant base MSW components (e.g., papers, plastics, organics). Promoting meaningful reductions in potentially problematic trace metals in MSW-derived materials may require reevaluating their presence in higher-volume, lower-concentrated MSW components such as paper, plastics, and organics.

Boron as a contaminant at construction and demolition (C&D) debris landfills

Elevated boron concentrations above the regulatory standard were inadvertently discovered in downgradient groundwater monitoring wells at 22 construction and demolition (C&D) debris landfills in Florida, US. This created a unique opportunity to evaluate whether C&D debris can be considered a plausible source of boron at unlined landfills. Approximately 1200 historical landfill-leachate and groundwater records were surveyed from semi-annual and annual monitoring reports covering a 9-year period. Laboratory leaching experiments were conducted on soils from each of these sites to determine if the source could have been boron mobilized from naturally occurring soils. Historical leachate quality data from lined landfills near four of the unlined C&D debris landfills were examined to determine if leachate from the unlined landfills could be the boron source. The US Environmental Protection Agency (EPA) Method 1312, or Synthetic Precipitation Leaching Procedure (SPLP), and the EPA Method 1316 were performed on materials commonly found in C&D debris to see if these products have the potential to leach appreciable levels of boron. The results of this work indicate leachate from unlined C&D debris landfills as the most plausible source of elevated boron concentrations in downgradient monitoring wells.