Molla et al. 2025 - C&DW fines and landfill leachate metals

Molla and colleagues ran a 25-week laboratory column leaching experiment to test how construction and demolition waste (C&DW) fine residues, gypsum, and organic matter affect landfill leachate chemistry. This is primary landfill and waste-management evidence for metal mobilization in leachate, not food, ingredient, or consumer-product occurrence evidence.

Key numbers

Column design and leachate conditions

Eighteen columns were packed to 75 cm height. Test columns crossed C&DW fine-residue contents of 5%, 10%, 15%, and 20% with gypsum contents of 0%, 5%, 10%, and 15%, with the balance as organic waste. Two controls used 100% organic waste and 100% C&DW.

The leaching fluid was acidified to pH 4.5 to simulate local precipitation. Columns were saturated and then sampled after one week and fortnightly thereafter. The pH of leachates ranged from 5.05 in the 100% organic waste stream at weeks 19 and 21 to 7.47 in the 100% C&DW stream during week 1. Conductivity began around 3000-7000 uS/cm and declined to about 1000-4000 uS/cm by the end of the experiment.

The authors report that more than 80% of the C&DW fine-residue sample was below 4.75 mm. The material was classified as poorly graded soil, with coefficient of curvature 0.4575 and coefficient of uniformity 20.33.

Week 13 peak leaching concentrations

Table 2 reports average metal concentrations at week 13, identified as the maximum leaching week. Al and Fe are mg/L; Cr, Co, Ni, Cu, Zn, As, Pb, and Hg are ug/L. Bracketed limits in the paper are ANZECC freshwater/marine trigger values for Al and Fe and NSW waste-classification leachable concentration/SCC values for the other metals.

The lowest concentrations for most elements occurred at 5% C&DW with no gypsum. The highest concentrations were generally found at 20% C&DW with 15% gypsum. Pb, Hg, As, Cr, Ni, Cu, Zn, and Co remained below NSW general/restricted solid-waste leachable/SCC thresholds. Al and Fe exceeded the Australian and New Zealand water-quality guideline values cited by the authors.

Leaching pattern and gypsum effect

The authors describe a two-phase pattern: an initial rapid release during the first 13-15 weeks, followed by stabilization or lower continuous leaching. They attribute early peaks to soluble, surface-bound contaminants and accessible fractions such as metal oxides, paint residues, corrosion products, treated wood, and galvanized components. Later stabilization is attributed partly to depletion of soluble material and partly to anaerobic sulfate-reducing conditions that form metal sulfides.

Gypsum increased metal mobility by dissolving to release sulfate, lowering pH through coupled organic-matter degradation and bisulfate formation, and promoting soluble metal-sulfate complexes, especially for Zn and As. At 15% gypsum, Zn reached 319.3 ug/L at 5% C&DW, 319.0 ug/L at 10% C&DW, 516.0 ug/L at 15% C&DW, and 514.8 ug/L at 20% C&DW. As reached 165.8, 168.7, 181.0, and 187.2 ug/L in those same 15%-gypsum C&DW mixtures.

The columns with 15% gypsum and 10-20% C&DW had the most consistently elevated leachate concentrations over time. The authors highlight mixtures containing 15% and 20% C&DW combined with 10% and 15% gypsum as higher-risk landfill mixtures.

Methods (brief)

C&DW fine residues were sampled from stockpiles at a material recovery facility in the Hunter/Central Coast region of New South Wales, mixed, and reduced by coning and quartering. Gypsum board was crushed, dried, and partially reduced to 2 mm. Commercial organic compost represented the organic landfill fraction.

Columns were built from 150 mm UPVC pipe, with a bottom outlet and top feed distribution. Each column had a mesh, quartz pebble layer, and washed coarse sand layer beneath the packed waste mixture. Synthetic rainwater feed was made from tap water acidified with sulfuric/nitric acid to pH 4.5. Leachate samples were collected in 50 mL tubes, centrifuged, filtered through 0.45 um filters, and analyzed in triplicate. Pb, Hg, As, Cr, Ni, Cu, Zn, and Co were measured by ICP-MS. Al and Fe were measured by ICP-OES. Reported triplicate RSDs were below 5% for all analyzed elements.

Implications

Certification: Do not use this source in HMTc food, ingredient, or consumer-product occurrence pools. It is environmental waste-leachate evidence.

App: Useful for environmental-pathway context where landfill leachate, groundwater, waste segregation, construction residues, gypsum, and sulfate chemistry may influence metal mobility.

Courses: Useful for teaching why metal concentration in a leachate column, regulatory waste classification, water-quality guidelines, and food/product occurrence are separate evidence types.

Wiki pages this source may touch

• arsenic
• cadmium
• chromium
• copper
• iron
• lead
• mercury
• nickel
• zinc
• wastewater-irrigation
• index

Verification notes

This page was built from the full PDF, including the abstract, study-area selection, column setup, leachate preparation and sampling, ICP-MS/ICP-OES methods, Table 1, Table 2, Figures 7-12, C&DW-content and gypsum-effect discussions, conclusions/recommendations, and data-availability statement. Arsenic and mercury are recorded as total arsenic and total mercury because the source does not report species-specific As or Hg chemistry.

Page history

The five most recent substantive edits to this page. The full version history lives in git; when DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.