Statistical comparison of leaching behavior of incineration bottom ash using seawater and deionized water: Significant findings based on several leaching methods

Highlights

• Metal leaching statistically correlated to the leachant types, i.e., seawater or DI.

• Metals with higher leached amounts with DI were subject to more effects by seawater.

• Seawater tended to enhance most metal leaching in column leaching test.

• Scenario-based leaching tests shall be established for IBA application.

Abstract

Bottom ashes generated from municipal solid waste incineration have gained increasing popularity as alternative construction materials, however, they contains elevated heavy metals posing a challenge for its free usage. Different leaching methods are developed to quantify leaching potential of incineration bottom ashes meanwhile guide its environmentally friendly application. Yet, there are diverse IBA applications while the in situ environment is always complicated, challenging its legislation. In this study, leaching tests were conveyed using batch and column leaching methods with seawater as opposed to deionized water, to unveil the metal leaching potential of IBA subjected to salty environment, which is commonly encountered when using IBA in land reclamation yet not well understood. Statistical analysis for different leaching methods suggested disparate performance between seawater and deionized water primarily ascribed to ionic strength. Impacts of leachant are metal-specific dependent on leaching methods and have a function of intrinsic characteristics of incineration bottom ashes. Leaching performances were further compared on additional perspectives, e.g. leaching approach and liquid to solid ratio, indicating sophisticated leaching potentials dominated by combined geochemistry. It is necessary to develop application-oriented leaching methods with corresponding leaching criteria to preclude discriminations between different applications, e.g., terrestrial applications vs. land reclamation.

Introduction

Incineration may achieve an overall 70–90% of volume reduction of municipal solid waste (MSW), during which a small volume of residue (∼20–25% by mass) namely incineration bottom ashes (IBA) are generated [1]. Recycling of IBA is widely practiced with increasingly adopted waste to energy (WTE) facilities in the globe, while management practices for it vary in different jurisdictions, and there is still need for legislation on recycling of IBA [2].

One challenge in the development of IBA legislation is that, IBA may vary a lot in their quality ascribed to different applied technologies for incineration and the variation of feedstock for burning [3], while reserve most of the metals (>80%) at higher concentrations from the resource [4], [5], [6]. When the total content of elements is taken as a basis, IBA should be considered as hazardous waste from the perspective of eco-toxic hazard properties. Note that, however, substances should be in solution first in order to exert a potential eco-toxic effect, alternative assessments of wastes may consider the leached concentrations instead [7]. European Council Decision 2003/33/EC lists the waste acceptance criteria based on element-leached concentrations for the different categories of waste: inert wastes, nonhazardous wastes, hazardous wastes, pursuant to the Directive of 1999, as showed in Table 1. As such, IBA may not be considered as non-hazardous waste based on leaching data (EU) at pH = 7–12 [7]. National legislation has been implemented to regulate utilization of IBA in the Netherlands and in France, however, in the Netherlands, IBA is placed in a special category, because it does not always meet the regulatory requirement [4].

Another challenge in the development of IBA legislation is that, IBA utilization has to depend on its impact (mainly ascribed to element leaching) to the environment. There are varieties of applications being currently performed, such as substitutes of non-constructive aggregates, sub-base materials for road pavements, embankment fillings, concrete production or coast erosion protection, with involvement of distinct environmental boundaries [1], [8], [9], [10], [11]. Accordingly, many lab-based leaching methods have been developed to simulate the IBA leaching potential when it is applied in the field. In general, these leaching methods may be classified into several categories, such as the batch leaching tests under the natural pH, the pH-static leaching tests, and the column leaching tests, etc. For instance, column leaching tests are considered as simulating the flow of percolating groundwater through a porous bed of the targeting material [12]. On the contrary, batch leaching tests are static leaching method which generates chemical data at equilibrium for mechanistic applications. As compared to batch leaching tests under natural pHs, the pH-static leaching tests intend to establish the complicated leaching profile as a function of pH values, useful to quantify the IBA leachability [13], [14].

Although pH is the most relevant factor on the assessment of the differences between leaching methods due to its strong control on the pollutant release [15], there are many other factors which may significantly affect the leaching results, such as particle size distribution, liquid to solid (L/S) ratio, leaching regents, apparatus and scale, mixing time, leaching procedures, etc. [4], [16], [17], [18], [19], [20]. So far, many of these factors have been specified among various available methods designed for leaching analysis. However, it yields various leached data which is hard to cross-compare among one another and quantify the exact hazard associated with IBA [21]. In this regard, regulations imposed on IBA utilization must be scenario specific to map each other between the laboratory leaching results and the specific application. Furthermore, leaching method development for IBA classification in the laboratory is still at their preliminary stage of development, whereas factors that are considered in their leaching procedures are rather limited. For instance, in the case of IBA application in land reclamation, bottom ashes are in fact exposed to seawater environment. Leaching methods using deionized water (DI) as the basic leachant may not be feasible any longer. According to Schmukat et al. [22], ionic strength (in terms of NaCl) and salinity significantly influenced the release metal(loid)s in the water phase for copper slag. Cetin et al. [23] also mentioned that a raised ionic strength may decrease the surface negativity of the fly ash and released Ba²⁺, Cu²⁺ and Zn²⁺ ions from the solid surface into the aqueous solution by electrostatic interaction. It is hereby of great interest for investigation of seawater-leaching scenario, as IBA for land reclamation may be considered a viable strategy to “close the waste loop” in land-scarce countries/regions, such as Singapore and Japan.

To statistically understand the metal leaching potential caused from seawater, a series of experiments were carried out with several leaching methods in the paper. IBA was collected from 5 source locations across Singapore, subjected to both batch leaching tests (EN 12457-1:2002 at L/S 2 and EN 12457-2:2002 at L/S 10) and the column leaching tests (DD CEN/TS 14405:2004). Two specific leachants i.e. DI and seawater, were intentionally applied to all three leaching methods, simulating the IBA utilization as supplementary construction materials under two scenarios – terrestrial and ocean applications, respectively, for deterministic assessment of the leaching behaviors and the associated impacts on the leaching standards.