Molla et al. 2025 — Heavy metal leaching from construction-and-demolition waste fine residues in a landfill column experiment, Hunter/Central Coast NSW

Molla and colleagues ran a 25-week column-leaching experiment on construction-and-demolition waste (C&DW) fine residues mixed with organic compost and varying gypsum content, simulating the conditions of mixed-stream landfills in the Hunter/Central Coast region of New South Wales, Australia. They measured ten metals (Pb, Hg, As, Cr, Ni, Cu, Zn, Co, Al, Fe) in the leachate over time and at the peak-release week. The findings indicate that higher C&DW and gypsum contents produce substantially higher leachate metal concentrations, with a two-phase release pattern: rapid early mobilisation through weeks 13-15, followed by stabilisation as anaerobic conditions and sulfate-reducing bacteria promote metal-sulfide precipitation. Pb, Hg, As, Cr, Ni, Cu, Zn, and Co stayed within Australian regulatory thresholds for general and restricted solid waste classification, but Al and Fe exceeded Australian and New Zealand Guidelines for Fresh and Marine Water Quality.

Key numbers

Experimental design

Eighteen columns total (Table 1, paper §2.4):

Each column was packed to 75 cm of mixed material with a 30 cm coarse-sand cap, 40 mm pebble base, and three discrete C&DW + gypsum layers between organic-matter sections (paper §2.4, Figure 5). Leaching fluid was tap water acidified with 450 µL of 60/40 w/w% H₂SO₄/HNO₃ in each 15 L feeding tank to a pH of 4.5, matching local precipitation chemistry. The system ran at 10-15 drops/min flow for ~8 h daily, with fortnightly sampling after the initial week-one baseline.

Sieve and physical characterisation (paper §3.1, Figure 7)

Approximately 80% of C&DW fine-residue samples were less than 4.75 mm. The grain-size analysis returned D₁₀ = 0.15 mm, D₃₀ = 0.5 mm, D₆₀ = 3.05 mm, giving a coefficient of curvature Cc = 0.4575 and coefficient of uniformity Cu = 20.33, classifying the residues as poorly graded soil under AS 1289.3.6.1-2009.

pH and conductivity over 25 weeks (paper §3.1, Figure 8)

• Leachate pH range across all columns: 5.05 (100% organic-waste control, week 19 and 21) to 7.47 (100% C&DW fines, week 1).
• Conductivity declined from an initial 3000-7000 µS/cm at week 1 to 1000-4000 µS/cm by week 25, with a notable break around weeks 13-15.

Week-13 peak heavy-metal concentrations (Table 2, paper §3.2)

All values are mean ± standard deviation. Al and Fe are in mg/L; all other metals are in µg/L. The bracketed regulatory limit at each column header is the Australian/New Zealand Fresh and Marine Water Quality guideline (Al, Fe, Cr, Co, Ni, Cu, Zn) [73] or the Australian leachable-concentration / specific-contaminant-concentration limit for general solid waste (As, Pb, Hg) [72]. “<RL” means below reporting limit.

* The Table 2 column header for Zn is printed as “20,0” µg/L; the text §3.2 confirms this is the AU-NZ fresh-water guideline of 20 µg/L (i.e., the value in mg/L is 20 µg/L; the comma is a printing artefact). Zn week-13 concentrations therefore exceed the AU-NZ fresh-water guideline in every column other than the controls and the 5/0 column.

The single highest week-13 concentrations across the test matrix occurred at 20% C&DW + 15% gypsum for Al (102.4 mg/L), Cr (73.7 µg/L), Co (69.1 µg/L), and Pb (25.5 µg/L). The highest Zn (516.0 µg/L) was at 15% C&DW + 15% gypsum; the highest As (187.2 µg/L) and Fe (121.4 mg/L) were at 20% C&DW + 15% gypsum and 15% C&DW + 15% gypsum respectively. Hg week-13 concentrations were either <RL or at sub-µg/L levels, the highest measured value being 0.7 µg/L at 5/5 and 10/10. None of the test-column values exceeded the Australian leachable-concentration / specific-contaminant-concentration limits for general or restricted solid waste classification (As, Pb, Hg, Cr, Co, Ni, Cu, Zn).

Regulatory exceedances

• Al exceeded the AU-NZ fresh-and-marine water quality guideline of 20 mg/L in every test column (range 23.9-102.4 mg/L) and in the 100% organic-waste control (26.4 mg/L).
• Fe exceeded the AU-NZ guideline of 20 mg/L in every test column (range 23.5-121.4 mg/L).
• Zn exceeded the AU-NZ guideline of 20 µg/L in every test column other than the 0% Gyp + 100% C&DW control (1.8 µg/L) and the 100% organic-waste control (0.0 µg/L).

The authors note that Al and Fe are not subject to the Toxicity Characteristics Leaching Procedure (TCLP) or SCC general-waste classification because of their natural abundance and lower perceived environmental risk, but exceedance of the AU-NZ water-quality guideline is the load-bearing finding.

Cumulative temporal pattern (Figure 10)

Cumulative leachate concentration rose rapidly through weeks 13-15 and then stabilised for most metals. The peak cumulative Pb concentration over 25 weeks was approximately 110 µg/L at 15% C&DW + 15% gypsum; cumulative Hg rose from ~0 µg/L at week 13 to ~8 µg/L by week 25 with a sharp inflection between weeks 13 and 15 (paper §3.3). Cumulative Zn approached 3,250 µg/L at the highest gypsum/C&DW columns. Cumulative Al rose to ~400 mg/L and cumulative Fe to ~700 mg/L at the highest C&DW columns.

Methods (brief)

Mixed C&DW fine-residue stockpiles were collected from a single Hunter/Central Coast NSW Material Recovery Facility, sub-sampled in proportion, reduced to a 20 L composite from each pile by coning-and-quartering, then homogenised in a cement mixer. Waste construction gypsum board was crushed and oven-dried at 60 °C for 24 h, then ground to ≤2 mm with a pestle and mortar. Commercial organic compost served as the simulated landfill organic fraction. Eighteen UPVC columns of 150 mm diameter were packed to 75 cm with three C&DW + gypsum layers between organic-matter layers (variable layer depths per design; see Figure 5).

Leaching fluid was tap water acidified to pH 4.5 with 60/40 w/w% H₂SO₄/HNO₃ (450 µL per 15 L tank), simulating local precipitation. Flow rate was 10-15 drops/min for ~8 h daily, with bottom outlets closed between sampling intervals to extend contact time. Baseline sampling at week 1, then fortnightly through week 25. Each sample was centrifuged; a 10 mL aliquot was filtered through 0.45 µm syringe filters into 10 mL tubes for trace-metal analysis. A separate 0.5 mL aliquot was diluted 20× for Hg and major-cation analysis. Methods followed the Australian Laboratory Handbook for Soil and Water Chemical Analyses; glassware was prewashed in water, soaked in 3% HNO₃ for ≥6 h, rinsed with tap water then deionised water.

Pb, Hg, As, Cr, Ni, Cu, Zn, and Co were quantified by ICP-MS (PerkinElmer NexION 350X) with a multi-element internal standard mix (Sc-45, Y-89, Rh-103, In-115, Tb-159 at 10 ppm in 2% HNO₃) and Agilent MassHunter 4.3 workstation. Operating conditions: nebuliser Ar 0.9 L/min, auxiliary Ar 0.3 L/min, plasma Ar 15 L/min, reaction gas (He) 4 mL/min, lens voltage 7.25 V, RF 1100 W, CeO⁺/Ce⁺ and Ce⁺²/Ce⁺¹ both maintained at 1%. Al and Fe were quantified by ICP-OES (PerkinElmer Avio 200) with multi-element CertiPUR (Merck) standards in 1% HNO₃ matrix; RF 1500 W, plasma Ar 10 L/min, auxiliary Ar 0.5 L/min, nebuliser Ar 0.7 L/min, uptake ~1 mL/min. Six-point calibration curves achieved r² > 0.999 for all metals; internal-standard recoveries were 90-110%. All samples were analysed in triplicate; triplicate RSDs were < 5% for all elements. Continuous calibration verifications were run every 10 samples with ≤5% deviation accepted. pH and conductivity were measured by a Mettler Toledo SevenCompact Duo S213 coupled meter. Reagents were ACS-grade HCl (37%), HNO₃ (70%), H₂SO₄ (98%) from Thermo Fisher and Sigma-Aldrich; deionised water was 18.2 MΩ Milli-Q.

The paper reports total metals only; no As, Hg, or Cr speciation is performed. Statistical significance is p < 0.05.

Implications

App: Route as construction-and-demolition-waste landfill leachate and as ambient-exposure context for the ten measured metals. Do not route to food or consumer product pages — the matrix is mixed-waste landfill leachate, not a food or personal-care input. The paper supports landfill-leachate-as-environmental-source context for Pb, Hg, As, Cr, Ni, Cu, Zn, Co, Al, and Fe, with the strongest signal for Al, Fe, and Zn (regulatory exceedance) and Pb and Hg (cumulative two-phase release pattern).

Courses: Useful as a worked example of how waste-stream composition (specifically gypsum board content as a sulfate source and organic compost as a microbial substrate) shapes metal mobilisation in landfill environments. The two-phase temporal pattern — rapid surface-bound mobilisation through weeks 13-15, then stabilisation as sulfate-reducing bacteria precipitate metal sulfides — is a clean illustration of how anaerobic biogeochemistry can both release and re-immobilise metals over time. Also a useful case study in how Al and Fe, which are typically excluded from waste-classification thresholds because of their natural abundance, can still exceed water-quality guidelines and pose downgradient risk.

Certification: This paper does not contribute to food or personal-care occurrence data and should not influence HMTc threshold work. It supports environmental-exposure narrative for the ten measured metals when discussing pathways from waste streams into receiving waters and soils.

Wiki pages this source may touch

• lead
• mercury
• arsenic
• chromium
• nickel
• copper
• zinc
• cobalt
• aluminum
• iron

Verification notes

The PDF is the published version-of-record from MDPI Toxics 2025, 13(5), 370, CC BY 4.0; DOI 10.3390/toxics13050370. Title, author roster, affiliations, publication metadata, received/revised/accepted/published dates (6 March / 9 April / 27 April / 2 May 2025), and academic-editor list match the title page. The 18-column experimental design is given in Table 1 and Figure 5 and was verified across §2.4, §3.1, and §3.3. Funding is acknowledged from the NSW Environmental Trust project G G1600460 and the University of Newcastle.

Week-13 leachate concentrations transcribed in the Key numbers table above are taken verbatim from Table 2 of the paper. The Zn column header in Table 2 prints as “20,0” µg/L — this is a typographical artefact: the §3.2 narrative confirms the AU-NZ fresh-water guideline for Zn is 20 µg/L, and the recorded values (e.g., 186.4 µg/L at 5/0) substantially exceed that guideline. The “<RL” (below reporting limit) entries for Hg are reproduced as printed; reporting-limit numerical values are not stated in §2.6 beyond “ICP-MS … parts per trillion (ppt)” trace sensitivity. The single highest concentrations identified in §3.2 (102.4 mg/L Al at 20/15, 516.0 µg/L Zn at 15/15, 187.2 µg/L As at 20/10, 121.4 mg/L Fe at 15/15) match Table 2.

Metals scope: the paper measures ten total metals only (Pb, Hg, As, Cr, Ni, Cu, Zn, Co, Al, Fe) and does not measure cadmium. The metals: frontmatter lists Pb, tHg, tAs, Cr, Ni, Cu, Zn, Co, Al, Fe. tHg and tAs are used because §2.6 describes ICP-MS quantification without species-specific extraction or hydride-generation steps; §3.2 explicitly references arsenic forms only in the discussion (“arsenic forms calcium arsenate hydrates”), not in the measured-species reporting. Cr is used (not Cr-VI) because no hexavalent-chromium speciation is reported.

Matrices: landfill-leachate and construction-demolition-waste are added as new bare-string matrices for this paper. Both are conceptually adjacent to existing _no_route matrices (sediment, wastewater, soil) and should be added to the _no_route list of data/evidence/matrix-to-product-map.json so the routing layer correctly skips them. They are environmental matrices not directly tied to food or consumer-product surfaces; no products: or ingredients: slugs apply.

Brand firewall (Part 12, Exception 2 — scientific-method vendor names): PerkinElmer NexION 350X ICP-MS, PerkinElmer Avio 200 ICP-OES, Agilent MassHunter 4.3 software, Mettler Toledo SevenCompact Duo S213 pH/conductivity meter, CertiPUR (Merck) multi-element standards, and Thermo Fisher / Sigma-Aldrich reagents are retained as scientific-reproducibility identifiers. No branded consumer products are named in the source. The single Material Recovery Facility used as the source of C&DW residues is anonymised in the paper (“a Material Recovery Facility within the Hunter/Central Coast sub-region”) and is not named here. The Conflicts of Interest statement notes co-author Dawit Nega Bekele is employed by Douglas Partners Pty Ltd; that affiliation is reproduced in the authors list and at the affiliation level only, not attached to any contamination value.

No HMTc threshold proposals, consumer-audience translations, or cross-source synthesis claims are made on this page.

Audit subagent (2026-06-02) flagged a row-attribution slip on the highest-As callout: the narrative said “20% C&DW + 10% gypsum” but Table 2 places As 187.2 ± 4.84 µg/L at the 20% C&DW + 15% gypsum row (20/10 has As 174.7 ± 1.60). Verified against the Table 2 transcription above; corrected to 20/15. The paper §2.6.5 prints statistical significance as “p < 0.5” which is a paper-side typo for the conventional p < 0.05; the Methods section above uses p < 0.05 as Molla et al. clearly intended.

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