Green, non-toxic and efficient adsorbent from hazardous ash waste for the recovery of valuable metals and heavy metal removal from waste streams.

As compared to alkali-activated geopolymers with phosphoric acid which may be used in high concentrations resulting in disposal concerns, acid-based geopolymers may have superior properties. A novel green method of converting waste ash to a geopolymer for use in adsorption applications such as water treatment is presented here. We use methanesulfonic acid, a green chemical with high acid strength and biodegradability to form geopolymers from coal and wood fly ashes. The geopolymer is characterized for its physico-chemical properties and tested for heavy metal adsorption. The material specifically adsorbs iron and lead. The geopolymer is coupled to activated carbon forming a composite, which adsorbs silver (precious metal) and manganese (hazardous metal) significantly. The adsorption pattern complies with pseudo-second order kinetics and Langmuir isotherm. Toxicity studies show while activated carbon is highly toxic, the geopolymer and the carbon-geopolymer composite have relatively less toxicity concerns.

Ag@CDs nanohybrid: Fabrication, design of a multi-mode chemosensory probe for selective Fe3+ detection and logic gate operation.

In this study, we propose a unique use of silver-carbon dot nanohybrid (Ag@CDs) with an average size of 16 nm as a multi-mode sensor for the selective detection of Fe3+ and the construction of logic gates based on these unique detection properties. The Ag NPs exhibit colourimetric sensing and fluorescence quenching in response to Fe3+ in the concentration range of 10-100 ppm, with the carbon dots acting as the fluorescent entity. Surprisingly, the nanohybrid was shown to have excellent sensitivity to Fe3+, resulting in aggregation and formation of yellowish-brown clumps. When the reaction mixture was treated with Fe3+, there was a discernible change in the colour of the assay mixture and fluorescence quenching. That is, the Ag@CDs acted as a calorimetric and fluorescence multi-mode sensor. Even in interfering groups in the natural river water sample, the produced nanohybrid displayed good selectivity towards Fe3+, with a minimum LOD of 0.76 ppm. Further, we constructed an advanced logic system, IMP-OR gate, by using additional inputs - ascorbic acid and urea.

An innovative accelerated carbonation process for treatment of incineration bottom ash and biogas upgrading.

Incineration is the main technology used for the management of municipal solid waste, in parallel to various recycling programs. However, incineration should not be considered as the final step for waste management as the ash still needs to be treated and disposed properly. In this work, an innovative accelerated carbonation of incineration bottom ash (IBA) using simulated biogas composition from anaerobic digestion processes (a mixture of CH4 and CO2) has been applied to lower the leaching of heavy metals from the carbonated IBA and its associated toxicity. Various temperatures and reaction times were explored for carbonation optimization and it was found that the carbonation at 25 °C for 8 h was the optimal reaction condition by taking into account the degree of carbonation and time constraint. The mineral content, functional groups, thermal stability, leaching patterns and ecotoxicity of both raw IBA and carbonated IBA were tested. It was found that carbonated IBA leached out less heavy metals than the raw IBA due to the locking of heavy metals in the calcite matrix. Cost-benefit analysis was also conducted on the industrial-scale process with a capacity of processing 10 tons of IBA per day. The results indicated that the proposed process had great economic potential.

Sewage sludge ash-based mortar as construction material: Mechanical studies, macrofouling, and marine toxicity.

Incinerated sewage sludge ash is tested here as a cement and aggregate substitute in mortar blocks. It can be used at various percentages to reduce the overall cost of production and promote ash recycling. The compressive strength of the cast blocks was tested at 28 days to determine the optimal combination of ball milled ash (replacing cement) and sewage sludge ash (replacing sand). This was compared with a control block made of cement and sand only. The cast blocks with the optimal ash formulation were tested for their flexural strength and other properties such as surface functional groups, constituent phases and porosity. The control and ash mortars exhibited similar properties. A potential application of these blocks is to use them as part of seawalls. These blocks were thus suspended in the sea for 6 months. Marine organism attachment was observed over time in both control and ash mortar blocks. There was no significant difference between the mortars after 6 months. The mortar blocks were also subjected to leaching tests (NEN-7345). The leachates did not exhibit toxicity to microalgae. In contrast, mild toxicity was observed in the sea urchin embryo development assay. Overall, the study suggests that sewage sludge ash is a potential material to be used for seawall construction as it has the desirable mechanical properties. However, there remain some residual marine toxicity concerns that need to be further addressed.

Biochar industry to circular economy.

Biochar, produced as a by-product of pyrolysis/gasification of waste biomass, shows great potential to reduce the environment impact, address the climate change issue, and establish a circular economy model. Despite the promising outlook, the research on the benefits of biochar remains highly debated. This has been attributed to the heterogeneity of biochar itself, with its inherent physical, chemical and biological properties highly influenced by production variables such as feedstock types and treating conditions. Hence, to enable meaningful comparison of results, establishment of an agreed international standard to govern the production of biochar for specific uses is necessary. In this study, we analyzed four key uses of biochar: 1) in agriculture and horticulture, 2) as construction material, 3) as activated carbon, and 4) in anaerobic digestion. Then the guidelines for the properties of biochar, especially for the concentrations of toxic heavy metals, for its environmental friendly application were proposed in the context of Singapore. The international status of the biochar industry code of practice, feedback from Singapore local industry and government agencies, as well as future perspectives for the biochar industry were explained.

Incinerated Sewage Sludge Bottom Ash- Chemical processing, Leaching patterns and Toxicity testing.

Sewage sludge bottom ash, which is the major fraction obtained from the incineration of sewage sludge was treated with various organic and inorganic acids for heavy metal removal, along with a comparative phosphate treatment for heavy metal fixation. Malonic acid, an organic acid, was found to remove heavy metals better as compared to nitric acid, a strong inorganic acid. The acid treated samples were further examined for heavy metal leaching, followed by marine toxicity/abnormality testing of the leachates, where acid treated and phosphate treated ash leachate displayed higher (with malonic acid proving to be most toxic) and similar toxicity profiles as compared to raw ash leachate respectively. Raw ash was tested for its leaching patterns at different liquid/solid ratios(L/S = 5 and 10), salinities and time points (24, 48 and 72 h), where the leaching was found to saturate at L/S = 5 and at 24 h with varied salinity effecting the leaching insignificantly. When raw ash was benchmarked against concrete sand for marine toxicity, a material commonly used for land reclamation, acute toxicity patterns were found to be mostly similar except in case of the sea urchin embryonic assay, where toxicity was detected, indicating the sensitivity of the assay to residual levels of heavy metals. The raw ash was also tested against human cell lines where it displayed size and dose-dependent toxicity. To enable the use of ash for environment applications such as coastal reclamation, appropriate treatments are required to minimize leaching of potential harmful contaminants and this study demonstrates the importance of post-treatment of ash on its subsequent toxicity to organisms.

A factorial experimental analysis of using wood fly ash as an alkaline activator along with coal fly ash for production of geopolymer-cementitious hybrids.

Alkaline activated materials such as geopolymers and cementitious materials derived from pozzolanic reactions offer several advantages over the currently widely used Portland cement, especially in terms of environmental sustainability and physiochemical properties. However due to the need of an alkaline activator, such as NaOH or KOH, which result in high production cost and requires skilled personnel, they have not been deeply explored and put to use. Here in this study, wood fly ash, a by product of wood combustion is used as an alternative source of alkaline activator for producing such alkaline activated materials along with coal fly ash, where the resulting geopolymer-cementitious hybrid (GCH) was characterized physico-chemically through electron microscopy, BET, FTIR, XRF & XRD. However, the leaching of heavy metals from the wood fly ash could potentially pose an environmental concern. Therefore, the focus of this study is to reduce the leachability factor of wood fly ash involved in the alkaline activated process and to understand the effects of various factors (i.e. water-to-ash ratio (w/a), method of curing, type of alkaline activator and ash sieving) on the leaching process, through factorial experimental analysis. The leaching patterns of various elements such as Pb, Zn, Cr, As. Hg, Se were studied along with the contributing factors and results showed that the dominant factor was the type of alkaline activator (i.e. Wood Fly Ash versus Na2SiO3). By comparing the leaching data to Denmark's leaching criteria, the best performing GCH sample was found to be 0.3DI_p_s (0.3 represents the w/a ratio, "p" denotes that samples were precured and "s" denotes sieving).