Chemical Characterization of High-Temperature Arc Gasification Slag

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Chemical Characterization of High Temperature Arc Gasification Slag with a Focus on Element Release in the Environment Justin Roessler, Wesley Oehmig, Nawaf Blaisi, and Timothy G. Townsend Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 09 Jun 2014 Downloaded from http://pubs.acs.org on June 20, 2014

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Environmental Science & Technology

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Chemical Characterization of High Temperature Arc

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Gasification Slag with a Focus on Element Release

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in the Environment

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Justin G. Roessler, Wesley N. Oehmig, Nawaf I. Blaisi and Timothy G. Townsend*

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Department of Environmental Engineering Sciences, University of Florida,

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PO Box 116450 Gainesville, FL 32611 – 6450, USA

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ACS Paragon Plus Environment

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KEYWORDS: Leaching, Slag, Plasma, Gasification, Reuse

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TOC ART

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Abstract

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High temperature arc gasification (HTAG) has been proposed as a viable technology for the

25

generation of energy and the production of saleable by-products from municipal solid waste

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(MSW). Total concentrations of elements in HTAG slag were assessed and indicated a high

27

partitioning of trace elements (Pb, Cd, As) into the flue gas, an issue of concern when assessing

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the air pollution control residues (APCR) status as a hazardous waste. Hazardous waste leaching

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tests (TCLP) were performed and confirmed that the slag did not meet U.S criteria for hazardous

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waste. Leaching was assessed using batch and column tests; the results revealed that Sb and Al

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were elevated in respect to risk based regulatory thresholds. Slag samples were carbonated to

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simulate weathering effects and although leachable concentrations of Al did decrease by an order

33

of magnitude, Sb concentrations were found to increase. Low total concentrations of certain trace

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elements (As, Cd, Pb) with respect to municipal solid waste incineration bottom ashes, support

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the potential for reuse of HTAG slag, however leaching of elements (Pb, Al, and Sb) in batch

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and column tests indicate that proper engineering controls would need to be taken to ensure

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protection of water supplies in a reuse application.

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Introduction

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New high temperature, low oxygen gasification technologies for the treatment of municipal

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solid waste (MSW) are now frequently being explored as an alternative to typical MSW

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combustion with energy recovery, or waste to energy (WTE).

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employ plasma, electric arc, or other means to apply large amounts of energy to wastes in an

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effort to gasify organics while melting the residue into a vitreous slag.

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bottom and fly ashes generated from WTE, high temperature-low oxygen treatments produce a

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slag and one or more air pollution control residues (APCR). (1, 5) The vitreous slag is a glass-like

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material composed of the inorganic components of the original waste, chiefly silicates and other

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metal oxides. (3)

(1-4)

These technologies may

(1, 5)

Analogous to the

50

High temperature arc gasification (HTAG) has been reported as a viable technology for the

51

disposal of MSW, generation of energy, and the production of saleable combustion by-products.

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(2, 5)

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thus may be beneficially used in construction applications.

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insoluble nature of the slag matrix, as vitrification has been shown to reduce the leachability of

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certain waste products by encapsulating potentially hazardous constituents in a glass-like matrix.

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(2, 4, 7, 8)

One cited benefit of this technology is that the slag is relatively inert in the environment and (1, 6)

Often, this is attributed to the

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The high temperature, low-oxygen conditions under which HTAG is conducted raise

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questions about the fate of trace metals during HTAG. Partitioning of trace elements into the flue

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gases and ACPRs during thermal treatment may generate residues with enriched concentrations

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of these elements

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HTAG APCR. Previous research has reported high volatilization rates for trace elements in

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similar high temperature waste treatment processes. (3, 5-7, 9) Ecke et al., (7) demonstrated that only

(5, 7)

; this could have implications on the management and disposal options for


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25% of the Pb contained in WTE bottom ash was found in the slag samples generated by high

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temperature vitrificiation. Enriched concentrations of As, Cd, and Pb in WTE APCR (conducted

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at lower temperatures than the technologies of discussion here) have been well established (10, 11)

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and often times influence the management practices of WTE residues, as elevated concentrations

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of these trace metals create the potential for these residues to be classified as a hazardous waste

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under the TCLP. (11, 12)

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When considering the construction of a full scale HTAG facility, understanding the options for

70

management of the generated residues has proven to be a significant hurdle. As a result of the

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limited number of currently operating MSW HTAG facilities and the paucity of research on

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these slags, very little data are available for those attempting to evaluate these options. The

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available data primarily focus on the material’s status as a hazardous waste, through use of the

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Toxicity Characteristic Leaching Procedure (TCLP). (5) Data on the composition and leaching of

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residues from the vitrification of MSW incineration ashes and other industrial waste products has

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shown a reduction in the leaching of the vitrified incineration ashes as well as the volatilization

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of trace metals from the feedstock.

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the thermal plasma treatment of WTE ash had the majority of trace metals reduced to their

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metallic state, and this was hypothesized to support the increased volatilization of these metals.

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Haugsten et al., (13) showed that slag from the vitrification of WTE fly ash met Dutch Category 1

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reuse standards for all elements with the exception of Sb. Differences in feedstocks do not allow

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for a direct comparison to slag generated solely from MSW; and existing data still fall short of

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providing the necessary information to assess the potential for beneficial use of HTAG slag.

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Experiments to assess the behavior of HTAG slag once placed in a reuse scenario have not yet

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been conducted.

(3, 6-8, 13)

Saffarzadeh et al.,

(3)

found that slag generated from


5


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The United States Environmental Protection Agency (US-EPA) has recently adopted a column (14)

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leaching procedure for assessing contaminant release from wastes.

This method attempts to

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better emulate percolation-controlled conditions (e.g use as a granular media) as contaminant

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release may be better understood by examining pore water characteristics (pH, ORP, trace metal

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content) as a function of liquid to solid ratio (L/S). (15) Percolation column testing, similar to the

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procedures described above, has been performed on MSW residues in an attempt to model trace

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element release.(16) With respect to beneficial use, column testing offers a better understanding of

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trace element release in the environment by evaluating leaching as a function of L/S and

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dynamic parameters such as pH.

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Carbonate weathering is one of the driving factors influencing the leaching of WTE residues

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and other combustion residues. Defined as the adsorption of CO2 by a metastable material,

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carbonation is often used as a treatment mechanism for WTE bottom ash.

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leachate pH and creating sorptive secondary mineral forms, carbonation helps to decrease the

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leaching of trace metals (particularly Pb) in WTE bottom ash.

(19)

(17-19)

By lowering

Accelerated carbonation is

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employed in laboratory settings to explore the effects of carbonation in much smaller time scales.

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The effects of accelerated carbonation on WTE ashes have been the focus of several studies, all

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of which used carbonation coupled with leaching tests to assess the effects of carbonation on

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contaminant release.

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tools designed to examine leaching in beneficial use scenarios, as well as evaluate long-term

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trends over shortened time scales. The combination of these concepts, where carbonation (used

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to age a waste artificially) and percolation testing are subsequently conducted, has not been

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reported.

(18, 20-22)

Both column tests and accelerated carbonation experiments are


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A pilot-scale HTAG system was operated by United States Air Force Special Operations

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Command at Hurlburt field in Okaloosa County, Florida from 2010 to 2012; this provided the

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research team with a unique opportunity to sample from an operational facility treating MSW in

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the U.S. The objectives of the research presented herein were to characterize the total and

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leachable concentrations of inorganic elements in HTAG slag, and to provide an assessment of

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this data with regards to potential implications for residue management and beneficial use.

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Leaching tests that are typically used as tools to evaluate a wastes potential for beneficial use

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were conducted, including tests to assess the slag’s long-term element release with respect to

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reuse in a percolation-controlled scenario. Samples of slag were carbonated to simulate natural

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weathering and to evaluate any influences this might have on the slags leachability. Results were

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then compared to risk-based regulatory thresholds, a common technique used by state and federal

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regulatory agencies when assessing beneficial use. The information provided helps fill in data

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gaps and provides an asset to regulators, consultants, municipalities and other interested parties

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considering the construction of a HTAG facility.

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Materials and Methods

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Facility Description, Sample collection, and Preliminary Characterization. Slag samples

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were collected from a HTAG system at Hurlburt Field Air Force base; the unit had a maximum

125

capacity of approximately 11 tons per day, using MSW from the surrounding base as a

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feedstock. A process flow diagram (Scheme-S1) and detailed facility description are found in the

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supplementary information (SI) section.

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In each of two separate sampling events, sub-samples of approximately 150kg were collected

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in sealed containers. These samples were aggregated, size-reduced to pass a 9.5mm sieve by a

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jaw crusher, homogenized through repeated mixing, and placed in polypropylene plastic


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containers that were sealed until the time of analysis. Slag passing a 9.5mm sieve was utilized

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throughout the project in order to meet the maximum particle size requirements outlined in (US-

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EPA)-Methods 1311-1312

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analysis on the generated HTAG sample was conducted using the U.S standard sieves #4 (4.75

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mm), #10 (2.00 mm), #20 (0.850 mm), #50 (0.300 mm), and #100 (0.150 mm).

(14)

and to maintain sample consistency throughout the tests. A sieve

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Total Elemental Analysis. Both TE and TEA digestions were conducted on the HTAG slag.

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TEA digestions are designed to dissolve all elements with the potential to be released when

138

placed in the environment, and are often used for comparison to risk based regulatory thresholds.

139

(20)

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order to assess total concentrations of trace metals in the HTAG slag a TE digestion was

141

conducted. Prior to both procedures, the HTAG slag was sized reduced to powder in a ceramic

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ball mill; six replicates of each digestion method were carried out. The TE digestion employed

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multiple additions of hydrofluoric acid to ensure full dissolution of the slag, a detailed procedure

144

can be found in the SI section. The TEA digestion was conducted in accordance with the

145

procedures outlined in EPA-Method 3050b. (14)

Elements bound in siliceous matrices are not typically dissolved in TEA digestions (14), and in

146

Batch Leaching Tests. Two batch leaching tests, the Toxicity Characteristic Leaching

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Procedure (TCLP) (EPA Method 1311) and the Synthetic Precipitation Leaching Procedure

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(SPLP) (EPA-Method 1312) were conducted on the HTAG slag. (14) Determination of the TCLP

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extraction fluid was performed in accordance with EPA-Method 1311, and TCLP fluid 1 was

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identified as the appropriate extraction fluid. The results of TCLP fluid 2 testing are also

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included to provide data on TCLP leaching in the circumstance that additional waste processing

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influenced the results of the fluid determination test. The TCLP uses an acetic acid solution that

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is ether buffered with sodium hydroxide at a pH of 4.93 ± 0.05 (Fluid 1) or unbuffered at a pH of


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2.88 ± 0.05;the TCLP is the standard test used in the United States to assess if a waste can be

155

classified as hazardous due to its leachability.

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Column Leaching and Accelerated Carbonation

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Up-flow column leaching was conducted in a manner similar to EPA-Method 1314

(14)

except

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samples were collected in discrete intervals instead of the entire sample volume, the flow rate

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(1.7 L/S per day) was slightly elevated with respect to Method 1314, and the column diameter

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(102mm) was expanded to correspond to the maximum particle size. Each column (control and

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carbonation) test was subjected to five leaching cycles and was conducted in duplicate. Control

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columns had a 24-hour rest between leaching cycles; carbonated columns received 100% v/v

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CO2 at a flow rate of 0.6 L/min for 24 hours. A detailed description of the column and

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carbonation procedures can be found in the SI section.

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Analytical Methods. TE analysis for Ag, As, Ba, Cd, Pb, Sb, Sn, Sr, Ni, V, and Zn were

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conducted using an Inductively Coupled Plasma- Mass Spectrometer (ICP-MS) in accordance

167

with procedures outlined EPA-Method 6020A

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analysis for all elements measured were conducted using Inductively Coupled Plasma-Atomic

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Emission Spectrometry (ICP-AES) following the protocols outlined in EPA-Method 6010C.

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Prior to analysis, leachate samples were digested in accordance with EPA-Method 3015A (14) and

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analyzed for trace metals using ICP-AES.

(14)

. TE analysis for Al, Cu, and Cr and TEA

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Risk Assessment Approach. When attempting to evaluate direct human exposure risk many

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state and federal regulatory agencies compare total and leachable concentrations in wastes to risk

174

based regulatory thresholds. One example of this practice is the State of Florida Soil Cleanup

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Target Levels (SCTLs).

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regularly been compared to risk thresholds, which often reflect both primary and secondary

(21)

With respect to leaching and reuse, SPLP concentrations have


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(22)

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drinking water standards

; in Florida the applicable regulatory standards are the Florida

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Groundwater Cleanup Target Levels (GCTLs). Total and leachable concentrations of trace

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metals were compared to these risk based regulatory thresholds in order to evaluate contaminant

180

release with respect to beneficial use. Although these standards vary between states and

181

countries, the objective was to provide an example of how a beneficial use assessment might be

182

conducted when evaluating the characterization data presented in this study.

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Results and Discussion

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Material Characterization and Total Elemental Composition. A sieve analysis of the

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sample following size reduction found that the largest mass fraction of the slag (25%) was

186

retained on the 4.75 mm sieve and that more than 50% of the material on the 2 mm.

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Approximately 75% of the HTAG slag was finer than the 4.75mm sieve, therefore 5mm was

188

chosen as the nominal particle size, and used in design of column experiments.

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TE and TEA concentrations of trace elements in HTAG slag were determined and are shown

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in Table 1. To examine elemental concentrations with respect to conventionally produced

191

thermal treatment residues, data on TE concentrations in WTE bottom ash (BA) are also

192

presented. Table 1 shows data adopted from the International Ash Working Group (IAWG);

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included are the minimum and maximum values for a number of elements as well as the median

194

value from the facilities sampled in the U.S.

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abundance, accounting for 11.1% by mass in the total and 7.8% by mass in the TEA digestion.

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This exceeded the data range (21,900-72,800 mg/kg TE) for WTE BA. Conventional mass burn

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WTE systems practice ferrous and non-ferrous metals recovery post combustion

198

HTAG system relies solely on pre-combustion metals recovery, conceivably leading to elevated

199

concentrations of Al in comparison to WTE BA. Concentrations of As, Cd, Pb, V, Sn, and Zn in

(23)

Al was found in the HTAG slag in the highest

(11)

; a MSW


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200

HTAG slag were found to be below or at the low end of the data range presented for WTE BA.

201

Pb, found at concentrations of 23.7 mg/kg in the HTAG slag, was two orders of magnitude lower

202

than the median value (2,400 mg/kg) for WTE BA. As measured at 1.29 mg/kg was an order of

203

magnitude less than the median BA value (25 mg/kg) measured by the IAWG. This supports

204

previous observations that volatilization during high temperature heat treatment processes leads

205

to decreased concentrations of trace metals in slags (5-6, 24). Jakob et al. (24) demonstrated that Pb,

206

Cd, and Zn in WTE APCR were all found to volatilize at 98-100% by mass in an Ar/H2

207

atmosphere at approximately 1100 °C. The total concentration data suggest that the elements

208

which were presumed to volatilize at higher rates than in the WTE combustion processes may be

209

further enriched in the HTAG APCR.

210

Ba, Cr, Sb, Sn, Ni, and Cu were close to the median values reported in the WTE ash data set.

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The speciation of elements formed during the HTAG process and the low oxygen conditions of

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HTAG could be possible explanations for the differences in volatility between elements.

213

Analysis of the mechanisms influencing the volatility of metals within the slag was not the intent

214

of the work presented here; however, examples of potentially similar mechanisms from other

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thermal treatment processes are presented as a basis for discussion. Previous study has shown

216

that the boiling point of Cu in WTE ashes becomes elevated under anoxic conditions as Cu is

217

reduced to its metallic state

218

possible explanation for the presence of Cu in elevated concentrations with respect to WTE BA.

219

The formation of thermally stable calcium antimonates has been demonstrated during heat

220

treatment in N2 environments, and the presence of antimonates in MSW BA has suggested that

221

this process occurs during WTE combustion

222

might occur during HTAG, a potential justification for the lack of volatilization of Sb in the slag.

(24)

. Low oxygen conditions would also be present during HTAG, a

(25)

. It is hypothesized that similar mechanisms


11


(26)

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Kuo et al.

measured the volatilization rates of metals in a mix of medical and laboratory

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waste ashes when vitrified with a plasma torch, and observed that major fractions of As (64%),

225

Cd (100%), Pb (100%), and Zn (98%) were depleted from the ashes during melting; Cu (0.6%)

226

and Al (0.2%) were not significantly depleted. Low TE concentrations of As, Cd, Pb, and Zn in

227

the HTAG slag support that high levels of volatilization of these elements occurred in the HTAG

228

process. TE concentrations of Al (11% by mass) and Cu (3,600 mg/kg) in the HTAG slag agree

229

with the limited volatilization shown by Kuo et al. (26)

230

Table 1 displays the percentage of trace elements that are TEA with respect to the TE

231

digestion. Sn, Cd and Al were the three most environmentally available elements at 104%,

232

73.9% and 71.9% respectively. Prior work on slag, generated from WTE ashes, demonstrated the

233

high mobility of Al in the sequential extraction procedure and suggested that the oxygen atom in

234

the aluminum-sillicate matrix was readily protonated

235

vitritification of WTE ashes did not facilitate an increase in the stability of Cd

236

trends are supported by data shown here.

(7)

. Furthermore Ecke et al. found that the (7)

. Both of these

237

When comparing TEA values of elements in the slag to the FL commercial/industrial SCTLs,

238

all elements were found to be below their applicable regulatory risk thresholds. Other MSW

239

combustion residues will often have one or more elements in excess of these thresholds, using

240

the data set adopted from the IAWG concentrations of As, Cr, and Pb would likely be elevated

241

above SCTLs for WTE BA. Evaluation of the TEA data demonstrates limited concerns with

242

respect to direct human exposure risk from HTAG slag, something not typically seen from

243

conventional waste combustion residues when utilizing this type of risk assessment approach.

244

Batch Leaching Tests. Leaching concentrations (mg/L) determined from the TCLP (fluids 1

245

and 2) and SPLP tests are shown in the SI section Table S1. TCLP testing was preformed and


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246

similar to data on other slags generated from the high temperature thermal treatment of MSW (5,

247

6)

248

equilibrium pH, ORP, and conductivity (pH, ORP, Cond) of the TCLP fluid 1 and 2 leachates

249

were (8.64, 257, 3.27) and (5.49, 305, 2.55) respectively. Although TCLP fluid 2 did prove to be

250

the more aggressive leaching solution, producing elevated concentrations of Cr, Cd, Ba, and Pb

251

with respect to fluid 1, concentrations of the seven TC metals analyzed were well below TC

252

standards in both cases.

, it did not exceed the U.S toxicity characteristic (TC) criteria for a hazardous waste. The

253

The equilibrium pH, ORP, and conductivity (pH, ORP, Cond) of the SPLP leachates were

254

(10.75, 267, 0.22). Ca (76.2 mg/L) and Al (71.3 mg/L) were found in the highest abundance. The

255

large fraction (11% by mass) of Al in the HTAG slag matrix, and the high level of availability of

256

Al (71.9%) in TEA digestions, support the elevated levels of Al release seen in the SPLP. Of the

257

12 detected elements Al (71.3), Sb (0.023), and Pb (0.017 mg/L) were found to be elevated

258

above their GCTLs of 0.200, 0.006, and 0.015 mg/L respectively. Concentrations of Al exceeded

259

its GCTL by more than two orders of magnitude. Prior research has demonstrated amphoteric

260

leaching behavior for Al in WTE BA, with leaching values elevated in the alkaline pH range (pH

261

10-12).

262

could have contributed to increased Al leaching. The TCLP values for Al support this

263

hypothesis, as TCLP fluid 2 (pH - 8.64) and TCLP fluid 1 (pH - 5.49) were shown to have Al

264

concentrations of 27.1 and 0.1 mg/L respectively. Decreased leaching as a function of pH could

265

be suggested for Pb, which is found to be highly pH dependent in WTE BA.

266

shows elevated concentrations of Pb (0.146 mg/L) with respect to SPLP, this can be explained by

267

the unbuffered acetic acid leaching solution used in TCLP fluid 2. The results of these tests

268

suggest that although the slag was found to be non-hazardous under the TCLP, concentrations in

(19)

If similar mechanisms are present in MSW HTAG slag, the equilibrium pH of 10.75

(19)

TCLP fluid 2


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269

the SPLP (often utilized to assess leaching in a reuse scenario) did indicate some potential for

270

leaching of Pb, Sb, and Al when placed in a beneficial use application.

271

Column Leaching. To better replicate potential reuse scenarios, such as use as a granular fill

272

media, and to examine the long term leaching characteristics of HTAG slag, up-flow column

273

testing was conducted. Figure 1 presents the leachate concentrations from the column testing for

274

Al, Ba, Sb as well as the sample pH. Pb which was detected in SPLP and TCLP leachates was

275

found below detection limits in all of the column tests. The leachate pH was observed to

276

decrease throughout the test, from an initial concentration of 11.34 to a final pH of 10.05. The

277

mean pH of the columns at a L/S of 20 (pH= 10.76) was similar to the final pH of the SPLP test

278

(pH =10.75). Previous study has demonstrated the correlation of batch and column pH values at

279

corresponding L/S ratio for a number of granular wastes.

280

materials in columns at L/S > 20 has not been widely assessed.

(16, 27)

The leaching of granular

281

Concentrations of Al, Ba, and Sb decreased throughout the test, from maximum values of 308,

282

4.29, and 0.29 mg/L respectively. This decrease was initially rapid and then continued gradually

283

after L/S ≈ 2. The rapid decrease in concentration observed from L/S = 0 to 3 is likely due to the

284

washing of soluble species from the surfaces of the slag. With respect to the GCTLs, Al

285

concentrations never fell below the regulatory threshold of 0.200 mg/L. Following the initial

286

surface wash off of Ba (L/S = 2) concentrations were observed to fall below GCTLs. Sb

287

concentrations were elevated with respect to the applicable risk threshold initially and after a L/S

288

of 15 were shown to fluctuate around the GCTL (0.006 mg/L) for the remainder of the test.

289

Equilibrium leaching concentrations in batch tests (such as SPLP) are often used in beneficial

290

use determinations as surrogates for more realistic leaching data. The SPLP and TCLP are

291

conducted at a static L/S = 20. To make a comparison between these tests and the column


14

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292

leaching, the cumulative mass of elements released during the SPLP and TCLP tests is shown

293

with the cumulative mass released from a L/S of 0 to 20 in the column test (Figure 2). These data

294

are normalized to the HTAG slag mass used in each test, and carry units of mgreleased/kgslag. The

295

cumulative mass release of Al, Ba, and Sb were shown to be within the same order of magnitude

296

as the concentrations that were observed in the SPLP. Pb, which was observed over the GCTL

297

(0.015 mg/L) in the SPLP, was found to be below instrument detection limits (0.004 mg/L)

298

throughout the column test. The TCLP is not typically considered a useful measure of leaching in

299

beneficial use determinations

300

cumulative mass release of Pb an element with a TC hazardous waste limit. Sb was found to be

301

almost twice as high in the SPLP (0.461 mg/kg) than in the column test (0.257 mg/kg). Though

302

the concentrations of Sb, and Pb were found to be overrepresented by the batch tests, leachate

303

concentrations of Al and Sb in Method 1314 were still shown to be elevated with respect to risk-

304

based thresholds at a L/S > 10. This data suggests that under this type of risk assessment

305

approach, the beneficial use of HTAG slag may require additional scrutiny and controls due to

306

concerns over leaching.

(15)

, but it is noteworthy that this test also overestimated the

307

Effects of Accelerated Carbonation

308

Two of the four leaching columns were subjected to four separate carbonation steps during

309

testing to examine the impacts of carbonate weathering. Weathering has been identified as a

310

treatment process for WTE BA (11, 12), as it has been shown to decrease leachate pH and facilitate

311

the formation of secondary mineral forms which influence leaching in the carbonated residues.

312

(20)

The effect of carbonation on HTAG slag had not previously been explored.

313

Columns that received carbonation are shown in Figure 1 (triangles). The most direct effect of

314

the carbonation treatments was a sudden decrease in pH. The pH of both carbonated columns


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315

dropped approximately 2 pH points immediately following carbonation; as the leaching periods

316

continued the pH rebounded and stabilized at values slightly less than controls. After the first

317

carbonation cycle (during the second leaching cycle) the pH rebounded to approximately 10.0,

318

whereas controls were shown to have pH of ≈ 10.5. By the end of the fifth leaching interval the

319

pH in the carbonated columns was ≈ 9.0 while the controls appeared to stabilize at ≈ 10.2.

320

Release of Al in the carbonated columns was shown to be heavily influenced by the leachate

321

pH (Figure 1); similar to trends seen in WTE BA, Al solubility dropped significantly as pH

322

decreased. Following the first carbonation step, Al values in leachates dropped from ≈ 50 mg/L

323

to ≈ 0.070 mg/L; near the end of the second leaching cycle, the Al concentration had rebounded

324

to ≈ 10 mg/L, roughly 50% of the Al leaching in the controls at that L/S. After each carbonation

325

cycle Al leaching dropped to below the GCTL (0.2 mg/L) and subsequently rebounded to over 1

326

mg/L. At the end of the fifth leaching cycle, Al leaching stabilized at ≈ 2 mg/L, while controls

327

steadied at around 10 mg/L. The precipitation of Al(OH)3 has been suggested as a mechanism

328

controlling Al release in WTE BA

329

decreasing concentrations observed here.

330

(19, 28, 29)

and is hypothesized as the driving factor behind the

Ba concentrations in the carbonated column leachates did not deviate dramatically from

331

the controls. An apparent decrease was observed following the first carbonation; however, small

332

but sudden increases were seen following the third and fourth carbonation cycles. None of these

333

increases produced Ba concentrations in exceedance of regulatory thresholds. Ba concentrations

334

in the leachates of all four columns were initially very consistent with one another, but as L/S

335

increased, variability increased.

336 337

Sb concentrations saw a substantial increase following carbonation. One to two L/S after a carbonation cycle, the leaching concentrations of Sb decreased and returned to match the


16

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338

values of the controls; this was the same trend which was exhibited by the leachate pH. Sb

339

concentrations did increase sufficiently enough after carbonation to exceed its GCTL (0.006

340

mg/L); a maximum concentration (0.121 mg/L) was reached after the first carbonation step. Each

341

subsequent leaching cycle saw Sb concentrations decrease to the level of controls and increase

342

following carbonation, though each time less than before. This could be attributed to the pH

343

dependent leaching behavior of Sb as an anionic species

344

carbonation, a phenomenon seen in WTE BA (31). Cornelis et al. (31) found Sb leaching in WTE

345

BA to be controlled by Ca mineral phases (ettringite and portlandite) at pH>10; however as the

346

pH decreased below 10 through carbonation, the dissolution of these minerals resulted in an

347

increase in Sb leaching. (31) These tests provide evidence that while weathering did produce a

348

decrease in elemental release for Al and Ba, Sb release may be increased, in some cases above

349

the applicable regulatory risk threshold. Testing through accelerated carbonation demonstrated

350

that when assessing HTAG slags potential for beneficial use, Sb release may be of concern as

351

weathering occurs.

(25, 30)

or the facilitation of Sb release by

352

Management and Reuse Use Implications. TEA digestions of HTAG slag resulted in

353

element concentrations that would not limit the beneficial use of HTAG slag (of the nature tested

354

here) with respect to direct human exposure risk. Al (71.9%) wase seen to have high percentages

355

of its TE content environmentally available, a possible factor facilitating the elevated release of

356

Al seen in leaching tests. Low TE concentrations of trace metals (As, Cd, Pb, V, Sn, and Zn)

357

were found in the HTAG slag in comparison with WTE BA data sets. This indicates that these

358

elements could become enriched in the HTAG APCR. Previous study of the total concentrations

359

of APCR generated from thermal plasma vitrification of MSW yielded Pb, Cd and As mean

360

concentrations of 12,250, 405, and 244 mg/kg respectively. (5) This could have implications as to


17


Page 18 of 29

361

the APCR status in the US as a TC hazardous waste. For example, the measured concentration of

362

Pb in the previous study (5) would require less than 1% of Pb to leach in order for it to exceed the

363

TC standard of 5 mg/L. In the US, WTE ash is often comingled or otherwise treated within the

364

process boundaries to prevent APCR from leaching concentrations elevated above TC standards;

365

similar steps might be required at HTAG facilities to prevent the negative stigma and elevated

366

costs associated with the generation of a hazardous waste.

367

TCLP testing on the HTAG slag demonstrated that the slag was not a TC hazardous waste

368

(regardless of the leaching solution used), indicating that this material would be suitable for

369

disposal in a subtitle D (non-hazardous waste) landfill. When evaluating the slag’s potential for

370

beneficial use, SPLP tests did leach Pb, Sb and Al at concentrations above risk thresholds.

371

Column testing demonstrated that the cumulative mass release of batch tests over predicted what

372

might occur in a percolation-controlled scenario (for Pb and Sb), but concentrations of Al and Sb

373

were still elevated with respect to groundwater risk thresholds. Weathering as a result of

374

carbonation reduced Al concentrations, but Sb concentrations were observed to increase. In all,

375

the data collected support that HTAG slag of the nature tested here does pose less risk compared

376

to conventional WTE ash for many elements. This results from a combination of reduced

377

leaching (because of the vitrified form) and lower initial concentrations (some elements are

378

volatilized and must be addressed as part of managing the APCR). This material is not inert,

379

however, and appropriate risk assessment and engineering controls should be evaluated and

380

incorporated as part of any beneficial use project. The elements of most notable potential

381

concern were Al and Sb.

382 383


18

Page 19 of 29


384 385


19


Page 20 of 29

386 387

388 389

Figure 1. Leachate concentrations (mg/L) and pH in column tests for standard (circles) and

390

carbonated (triangles) HTAG slag.


20

Page 21 of 29


391 392

Figure 2. Cumulative mass release from HTAG slag (mg/kg). Concentrations were measured at

393

a liquid to solid ratio of twenty

394 395


21


396

Page 22 of 29

Table 1. HTAG Total Concentration Data

Elem.

TE Digestion (Ave ± Std Dev)

TEA Digestion (Ave ± Std Dev)

Percent Environ. Available (%)

IAWG Bottom Ash TE Concentration (23) Min-Max (U.S. median)

SCTL

Al 110,000 ± 2.00 78,200 ± 14,000 71.9 21,900-72,900 (51,100) 80,000* Ca 97,800± 143,000 370-123,000 (77,700) Fe 4,980 ±1,180 4,120-150,000 (79,000) 53,000* K 810 ± 98.9 750-16,000 (9,000) Mg 50,100 ± 8,090 400-26,000 (9,450) Na 6,850 ± 815 2,870-42,000 (34,300) Ag 7.51 ± 0.255 4.71 ± 0.732 62.7 0.29-36.9 (6.75) 8,200 As 1.29 ± 0.287 0.69 ± 0.125 53.3 0.12-189 (25) 2.1 Ba 1,090 ± 16.1 547 ± 101 50.1 400-3,000 (900) 130,000 Cd 0.435 ± 0.026 0.322 ± 0.074 73.9 0.3-70 (13.9) 82 Cr 708 ± 10.4 34.3± 9.95 4.84 23-3,170 (1,072) 470 Pb 23.7 ± 0.540 10.4 ± 2.22 44.0 98-13,700 (2,400) 1,400 V 40.6 ± 1.33 4.02 ± 1.15 9.9 20-122 (49) 10,000 Sn 29.8 ± 6.21 31.1 ± 4.11 104 2-380 (254) 880,000 Sb 85.1 ± 1.42 4.92 ± 0.723 5.8 10-432 (125) 370 Sr 456 ± 8.86 257 ± 38.5 56.3 85-1,000 (500) 52,000* Ni 392 ± 9.46 39.5 ± 18.5 10.1 7-4,280 (470) 35,000 Cu 3,600 ± 83.3 1,930 ± 258 53.6 190-8240 (1,880) 89,000 Ti 596 ± 194 2,600-9,500 (6,500) Zn 83.0 ± 3.00 26.5± 3.74 613-7,770 (3,490) 630,000 Mo 1.43± 0.308 2.5-276 (39.5) 11,000 Be 1.69± 0.225 NR 1,400 Mn 222 ± 38.5 82-2,400 (1,000) 43,000 397 IAWG bottom ash data are presented for comparison to conventional thermal treatment residues (23) 398 . Florida Commercial/Industrial Soil Clean-Up Target Levels are presented as a risk based 399 regulatory threshold for comparison, * designates an element without a corresponding 400 commercial/industrial SCTL therefore the residential threshold is shown. 401 402 403


22

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404

AUTHOR INFORMATION

405

Corresponding Author

406

Corresponding author. Phone: 352-392-0846, Fax: 352-392-3076; email: [email protected]

407

Author Contributions

408

The manuscript was written through contributions of all authors. All authors have given approval

409

to the final version of the manuscript.

410

Funding Sources

411

This research was conducted with funding from the Hinkley Center for Solid and Hazardous

412

Waste Management.

413

ACKNOWLEDGMENTS

414

The authors would like to thank the Hurlburt Field HTAG operators for providing slag

415

samples. Also, the authors would like to thank the University of Florida Department of

416

Geological Sciences for allowing us to instrumentation in their laboratories.

417

SUPPORTING INFORMATION AVALAIBLE

418

The supplementary information section contains a detailed description of the HTAG facility,

419

experimental procedures, and tabulated leaching data. This information is available free of

420

charge via the Internet at http://pubs.acs.org/.

421

ABBREVIATIONS

422

APCR, air pollution control residues; BA, bottom ash; Cond, conductivity; GCTLs, groundwater

423

clean up target levels; HTAG, high temperature arc gasification; IAWG, international ash

424

working group; ICP-AES, inductively couple plasma atomic emission spectrometry; ICP-MS,


23


Page 24 of 29

425

inductively coupled plasma mass spectrometry; L/S, liquid to solid ratio; MSW, municipal solid

426

waste; ORP, oxidation reduction potential; SCTLs, soil clean up target levels; SPLP, synthetic

427

precipitation leaching procedure; TC, toxicity characteristic; TCLP, toxicity characteristic

428

leaching procedure; TE, total elemental; TEA, total environmentally available; US-EPA, United

429

States Environmental Protection Agency; WTE, waste to energy;

430 431 432 433 434 435 436 437 438 439 440


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441

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442

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Chemical Characterization of High-Temperature Arc Gasification Slag

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