Sustainable Detoxification
of Heavy Metals
Saving landfill space at Semakau island through dissolving
solid waste and detoxifying heavy metals
The daily waste sent to Semakau Landfill comprises incineration ash and non-incinerable waste. At the current rate, the landfill is expected to run out of its capacity by 2035
The Motivation
One environmental issue associated with landfills is the potential contamination from heavy metals. Toxic heavy metal contamination of solid sludge and the surrounding land may lead to subsequent leaching to the groundwater. This is a growing global concern at many waste sites. Toxic metals commonly found in waste streams include lead, cadmium, mercury, chromium and arsenic. Other less toxic metals such as nickel, copper, zinc, barium, manganese, cobalt, tungsten and molybdenum may also cause contamination. Industrial, mining or commercial sites where toxic production residues are improperly stored or buried may also contribute to this issue.
Zero Waste Sustainable Detoxification of Heavy Metals
Chemical remediation (cleanup) is one of the three main categories of remediation techniques for managing solid waste contaminated with heavy metals. The other two categories are physical remediation and biological remediation.
Chemical remediation mainly includes immobilization/stabilization, smelting, and chemical washing techniques. The immobilization/stabilization technique adsorb, chelate, precipitate or encapsulate heavy metals to prevent them from leaching back into the environment. Its cost ranges from low to medium, but this comes with the potential leaching risk of heavy metals. The smelting technique uses the sludge contaminated with heavy metals as one of the components in the production of cement. This effectively traps the heavy metals within the cement. However, this sacrifices the safety of final cement products. The chemical washing technique applies chelating agents or acid to release heavy metal ions on sludge particles. Chemical washing is usually costly due to the large consumption of chemicals, and it carries a risk of secondary contamination.
Figure 1: Our process for sustainable detoxification of heavy metals from sludge and reconstruction of valuable products from waste
“When scaled up, this process can potentially provide an alternative for sludge treatment, which will be cost-effective (reduced waste treatment cost and landfill cost), labor-saving (automated operation), environmentally friendly (zero-waste discharge) and revenue generating (building materials and electrodes to benefit the waste management industry)”
Professor Sam LI
Our approach shows promising results
Our team (Professor Sam LI, A/P Jason YEO, Researchers, Dr LIN Xuanhao, Dr HE Limo, Dr Johannes LIM, Dr LU Di, Dr NG Bao Hui, LOW Kit Meng, PHUA Teng Wei) has started working on a three-year research project supported by the NEA in collaboration with NSL Chemical Ltd and Origgin Venture (Singapore) Pte Ltd, and aims to remove, recover and detoxify heavy metals from non-incinerable wastes and reconstruct valuable materials from it with near-zero waste discharge.
We are developing process for sustainable detoxification of heavy metals from sludge and reconstruction of valuable products out of it. The process comprises several steps: extraction, dissolution, detoxification, and precipitation/ electrodialysis. After the process, the heavy metals may be recovered and electroplated as electrodes for hydrogen generation – a green energy source. The recovered phosphate may be used as fertilizers for agriculture and horticulture use, or as industrial raw materials. The clean sludge could be used as construction materials.
Potential Applications
Through this project, the process developed could potentially reduce the amount of waste sent to Semakau Landfill and extend its useful life beyond 2035. In addition, the proposed process can derive valuable products. The cleaned sludge could now be used to produce construction materials (e.g. ceramic tiles) and recycled heavy metals could be converted to electrodes needed for renewable energy generation. Apart from that, the reconstructed phosphate could be used as fertilizer in agriculture and horticulture [1-3].
Future Plans
We plan to extend our process to other solid wastes to reduce the amount of waste which will eventually end up in a landfill. Through our research efforts, we hope to better manage landfills and minimize environmental impacts for a more sustainable future.
Figure 2: Our display booth in Singapore International
Water Week 2022
Acknowledgement
This research is supported by the National Research Foundation, Singapore, and National Environment Agency, Singapore under its Closing the Waste Loop Funding Initiative (Award No. USS-IF-2021-2), and NUS Resilience & Growth Postdoctoral Fellowship.
Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not reflect the views of National Research Foundation, Singapore, and the National Environment Agency, Singapore.
References
[1] Lin X; Li SFY, Patent application, PCT No: PCT/SG2019/050253, “A method and system for heavy metals immobilization”, 03/05/2019.
[2] Hu Q; et. al. (2021) “Biochar industry to circular economy”, SCIENCE OF THE TOTAL ENVIRONMENT, 757, 143820 [Abstract]
[3] Joshi U; et. al. (2021) “Ruthenium-Tungsten Composite Catalyst for the Efficient and Contamination-Resistant Electrochemical Evolution of Hydrogen” ACS APPLIED MATERIALS & INTERFACES, 10, 7, 6354-6360 [Abstract]