Swee Ngin Tan - Academia.edu (original) (raw)

Papers by Swee Ngin Tan

Environmental Pollution, 2022

Heavy metal or metalloid contamination is a common problem in soils of urban environments. Their ... more Heavy metal or metalloid contamination is a common problem in soils of urban environments. Their introduction can be due to unpremeditated anthropogenic activities like atmospheric deposition produced by diffuse sources, construction activities and landscape maintenance. Phytoremediation is a rapidly evolving, sustainable approach to remediate the contaminated lands where metals and metalloids are highly persistent in the environment. The present work sets out to determine the level of 12 heavy metals and metalloids (As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb and Zn) in soil and their accumulation by plant foliage found in nature parks and industrial sites in Singapore. The latter also involve the investigation of the remediation capacity of selected tropical plant species found at the sampling sites. The study is done using digestion and inductively coupled plasma-optical emission spectrometry. Eleven soil sampling sites across Singapore with 300 sampling points were selected, where soil (0-10 cm) and plant foliage samples were collected. Bioconcentration factors were determined to assess the phytoremediation potential of the collected plant species. Toxicity risk of heavy metals were assessed by comparing the target and intervention values from the soil quality guidelines by the Dutch Standard. Results of the study revealed there were regions where levels of heavy metals and metalloids were relatively high and could affect the environment and the health of flora and fauna in Singapore. Our study discovered that there were available tropical plant species (e.g., wildflowers, ferns and shrubs) which could potentially play a significant role in the remediation of contaminated lands that could open up a huge possibility of developing a sustainable and environmentally-friendly way of managing this emerging urban problem. Results showed that 12 plant species, including hyperaccumulator like Pteris vittata, Centella asiatica, were effective for the accumulation of heavy metals and metalloids.

With the knowledge that removing nitrogen (N) in eutrophic water bodies is important for improvin... more With the knowledge that removing nitrogen (N) in eutrophic water bodies is important for improving water quality, the aim of this paper is to assess the ability of two floating plants, Eichhornia crassipes (water hyacinth) and Pistia stratiotes (water lettuce), to assimilate both inorganic nitrogen (IN) and organic nitrogen (ON) species. The effectiveness of water hyacinth and water lettuce, as plants for phytoremediation, were tested in the greenhouse for their removal capabilities of IN (nitrate, ammonium) and ON, i.e. Soy peptone (SP) and bovine serum albumin (BSA). In terms of IN removal, water lettuce was more effective than water hyacinth. In addition, for both species, the rates of ammonium removal were higher when compared to nitrate removal. For ON removal, both species demonstrated similar removal rates for SP and BSA. It was also observed that IN uptake rates were higher than ON uptake rates for the two plants.

Microbes and environments / JSME, Jan 14, 2015

Styrene is a toxic pollutant commonly found in waste effluents from plastic processing industries... more Styrene is a toxic pollutant commonly found in waste effluents from plastic processing industries. We herein identified and characterized microorganisms for bioconversion of the organic eco-pollutant styrene into a valuable biopolymer medium-chain-length poly(hydroxyalkanoate) (mcl-PHA). Twelve newly-isolated styrene-degrading Pseudomonads were obtained and partial phaC genes were detected by PCR in these isolates. These isolates assimilated styrene to produce mcl-PHA, forming PHA contents between 0.05±0.00 and 23.10±3.25% cell dry mass (% CDM). The best-performing isolate was identified as Pseudomonas putida NBUS12. A genetic analysis of 16S rDNA and phaZ genes revealed P.putida NBUS12 as a genetically-distinct strain from existing phenotypically-similar bacterial strains. This bacterium achieved a final biomass of 1.28±0.10 g L(-1) and PHA content of 32.49±2.40% CDM. The extracted polymer was mainly comprised of 3-hydroxyhexanoate (C6), 3-hydroxyoctanoate (C8), 3-hydroxydecanoate ...

Journal of Chromatography A, 2004

Micellar electrokinetic capillary chromatography (MECC) was developed for the separation of cytok... more Micellar electrokinetic capillary chromatography (MECC) was developed for the separation of cytokinins including trans-zeatin, trans-zeatin-O-glucoside, dihydrozeatin, dihydrozeatin-O-glucoside, meta-topolin riboside, N6-isopentenyladenine and N6-benzylaminopurine. Under the optimum conditions, i.e. a combination of 10 mM phosphate and 10 mM borate as the running buffer containing 50 mM sodium dodecyl sulphate at pH 10.4, the separation of seven cytokinin standards was accomplished within 11 min. The C18 solid-phase extraction (SPE) method was used to pre-concentrate the putative cytokinins present in the coconut water. Following which, the eluate was further purified using mixed mode Oasis MCX SPE columns and this additional step helps to reduce matrix interference during MECC. After the two solid-phase extraction steps, the optimized MECC method was able to screen for certain cytokinins (zeatin-O-glucoside and dihydrozeatin-O-glucoside) present in coconut water. After this screening, the presence of zeatin-O-glucoside and dihydrozeatin-O-glucoside in coconut water was further confirmed by independent high-performance liquid chromatography and liquid chromatography-mass spectrometry experiments.

The development and application of a novel combination of electrochemical techniques and computer... more The development and application of a novel combination of electrochemical techniques and computerized field-based battery-operated instrumentation is investigated. Low-power complementary metal oxide semiconductor technology has been utilized for the development of the field data acquisition systems and the instrumental performance of the complete analytical system is critically evaluated.

RSC Adv., 2014

A one-compartment hydrogen peroxide semi-fuel cell was fabricated using a metallic anode (Mg or A... more A one-compartment hydrogen peroxide semi-fuel cell was fabricated using a metallic anode (Mg or Al) and Prussian blue as the cathode to improve the power density of the device.

Evidence-Based Complementary and Alternative Medicine, 2015

International Journal of Waste Resources

Vandkvalitets Institute (VKI) process is a treatment technology which employs the technique of ex... more Vandkvalitets Institute (VKI) process is a treatment technology which employs the technique of extraction and chemical stabilization on waste, targeting an end pH of 7 to 10 using water, concentrated phosphoric acid (H 3 PO 4) and carbon dioxide (CO 2) as the main stabilizing materials. Different parameters for the ash stabilization were investigated. Based on the acid neutralizing capacity (ANC) and pH-dependent leaching results of Singapore's air pollution control (APC) residues produced from a municipal solid waste (MSW) incineration plant, the main objective of this study was to examine the suitable end pH to be used to treat APC residues. After which, a two-step VKI treatment was carried out on APC residues, using CO 2 and H 3 PO 4 dosing as the two main stabilizing items. ANC results showed that there was a steep drop in pH from 10 to 7 at approximately 20.40 equivalent/kg of acid addition, while pH-dependent leaching results indicated that the most ideal pH range for the suppression of most heavy metal release was between 7 and 10. Subsequently, pH 7 was then selected as the end pH for the VKI treatment process. Under optimum conditions of 0.2 g and 0.5 g of H 3 PO 4 /100 g of APC residues (equivalent to 2 and 5 kg H 3 PO 4 /ton of residues) with a CO 2 flow rate of 1500 cm 3 /min, the treated APC residues had an end pH of around 7 after 100 and 80 minutes of treatment, respectively. Toxicity characteristic leaching procedure (TCLP) was used to evaluate the leaching of heavy metals (Cd, Cr, Cu, Pb, Zn) before and after treatment. The results showed that the leaching of Pb was significantly reduced after treatment, and was able to meet Singapore's acceptance criteria for landfill disposal. However, the leaching of Cd increased by two orders of magnitude after treatment, but was still below Singapore's acceptance criteria for landfill disposal.

International journal of environmental research and public health, Jan 10, 2017

In the present work, thermal treatment was used to stabilize municipal solid waste incineration (... more In the present work, thermal treatment was used to stabilize municipal solid waste incineration (MSWI) fly ash, which was considered hazardous waste. Toxicity characteristic leaching procedure (TCLP) results indicated that, after the thermal process, the leaching concentrations of Pb, Cu, and Zn decreased from 8.08 to 0.16 mg/L, 0.12 to 0.017 mg/L and 0.39 to 0.1 mg/L, respectively, which well met the limits in GB5085.3-2007 and GB16689-2008. Thermal treatment showed a negative effect on the leachability of Cr with concentrations increasing from 0.1 to 1.28 mg/L; nevertheless, it was still under the limitations. XRD analysis suggested that, after thermal treatments, CaO was newly generated. CaO was a main contribution to higher Cr leaching concentrations owing to the formation of Cr (VI)-compounds such as CaCrO₄. SEM/EDS tests revealed that particle adhesion, agglomeration, and grain growth happened during the thermal process and thus diminished the leachability of Pb, Cu, and Zn, b...

Separations

Simultaneous quantitative analysis of 10 indole compounds, including indole-3-acetic acid (IAA, o... more Simultaneous quantitative analysis of 10 indole compounds, including indole-3-acetic acid (IAA, one of the most important naturally occurring auxins) and some of its metabolites, by high performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS) after solid-phase extraction (SPE) was reported for the first time. The analysis was carried out using a reverse phase HPLC gradient elution, with an aqueous mobile phase (containing 0.1% formic acid) modified by methanol. Furthermore, a novel SPE procedure was developed for the pre-concentration and purification of indole compounds using C 18 SPE cartridges. The combination of SPE, HPLC, and LC-MS was applied to screen for the indole compounds present in sugar cane (Saccharum officinarum L.) juice, a refreshing beverage with various health benefits. Finally, four indole compounds were successfully detected and quantified in sugar cane juice by HPLC, which were further unequivocally confirmed by LC-MS/MS experiments operating in the multiple reaction monitoring (MRM) mode.

Biosensors, Jan 13, 2017

Metallothioneins (MTs) are a family of cysteine-rich proteins whose biological roles include the ... more Metallothioneins (MTs) are a family of cysteine-rich proteins whose biological roles include the regulation of essential metal ions and protection against the harmful effects of toxic metals. Due to its high affinity for many toxic, soft metals, recombinant human MT isoform 1a was incorporated into an electrochemical-based biosensor for the detection of As(3+) and Hg(2+). A simple design was chosen to maximize its potential in environmental monitoring and MT was physically adsorbed onto paper discs placed on screen-printed carbon electrodes (SPCEs). This system was tested with concentrations of arsenic and mercury typical of contaminated water sources ranging from 5 to 1000 ppb. The analytical performance of the MT-adsorbed paper discs on SPCEs demonstrated a greater than three-fold signal enhancement and a lower detection limit compared to blank SPCEs, 13 ppb for As(3+) and 45 ppb for Hg(2+). While not being as low as some of the recommended drinking water limits, the sensitivity o...

Journal of Separation Science, 2016

Gibberellins, as a group of phytohormones, exhibit a wide variety of bio-functions within plant g... more Gibberellins, as a group of phytohormones, exhibit a wide variety of bio-functions within plant growth and development, which have been used to increase crop yields. Many analytical procedures, therefore, have been developed for the determination of the types and levels of endogenous and exogenous gibberellins. As plant tissues contain gibberellins in trace amounts (usually at the level of nanogram per gram fresh weight or even lower), the sample pre-treatment steps (extraction, pre-concentration, and purification) for gibberellins are reviewed in details. The primary focus of this comprehensive review is on the various analytical methods designed to meet the requirements for gibberellins analyses in complex matrices with particular emphasis on high-throughput analytical methods, such as gas chromatography, liquid chromatography, and capillary electrophoresis, mostly combined with mass spectrometry. The advantages and drawbacks of the each described analytical method are discussed. The overall aim of this review is to provide a comprehensive and critical view on the different analytical methods nowadays employed to analyze gibberellins in complex sample matrices and their foreseeable trends.

Talanta

Polyhydroxyalkanoates(PHAs) are commercially-valuable biocompatible and biodegradable polymers wi... more Polyhydroxyalkanoates(PHAs) are commercially-valuable biocompatible and biodegradable polymers with many potential medical, pharmaceutical and other industrial applications.The analysis of PHA monomeric composition is especially challenging due to the broad chemical diversity of PHA monomers and lack of analytical standards to represent the chemically-diverse PHA monomer constituents. In this study, a novel strategy based on on-line liquid chromatography–mass spectrometry (LC–MS) and off-line liquid chromatography–nuclear magnetic resonance (LC–NMR) was established to quantify seven PHA monomers with available standards and used to elucidate the structures of unknown PHA monomers. The strategy was successfully applied for the determination of monomeric composition in bacterial PHAs isolated from Pseudomonads cultivated on different carbon sources after hydrolysis.The results of this work demonstrated that the newly developed strategy was efficient, repeatable, and could have good po...