· PhD in Chemistry, National University of Singapore
Skills and Expertise
The aim of this project is to evaluate the risks posed by chemical contamination in Mangrove ecosystems in the context of Singapore. Chemical analyses of more than 100 legacy and emerging contaminants were performed in various compartments of the ecosystem (sediment, water/passive samplers and caged/native bivalves) along with a detailed analysis of toxic effects on caged (green mussels) and native (clams) bivalves through the analysis of a series of biomarkers. The results of a series of 2 sampling seasons (monsoon vs dry season) were used to produce a risk assessment of chemical contamination in Singapore mangroves.
Examine the global responses of soybean and Rhizoctonia solani during the early and late Rhizoctonia foliar blight disease stages using transcriptomics and metabolomics approaches and to examine the effect of maple bark biochar amendment on these responses
Research Items (51)
This study investigated the thermal degradation of a veterinary drug, chloramphenicol, in model solutions (water), as well as in spiked and incurred mussel tissues to understand its fate in the food supply chain. Thermal degradation kinetics followed a first-order model in water (e.g. degradation rate: 0.0018–0.0025 min⁻¹ at 100 °C). After 1 h at 100 °C, the percentage degradation in spiked tissues (28.1 ± 7.1%) was significantly different (p <.05) from the values in water (14.2 ± 1.6%) and incurred mussel tissues (19.0 ± 4.1%). Using liquid chromatography coupled to high-resolution accurate mass tandem mass spectrometry, the resulting degradation products of chloramphenicol were identified in water, spiked and incurred tissues, and were all different. Therefore, although the use of water models and spiking food samples may provide some information, our data confirm that “incurred models” should be systematically implemented to provide reliable information about veterinary drug residue stability for food safety risk assessments.
Effect of plantain peel biochar and super absorbent polymer (SAP) Amendment to soil on Fate and Transport of Bisphenol A, F and S in Soil and Potatoes tubers Irrigated with synthetic Wastewater.
Veterinary drug residues and, more recently, pharmaceutically active contaminants have been detected in food. In this research, the thermal stability of five pharmaceutically active compounds commonly detected in food (chloramphenicol, florfenicol, lincomycin hydrochloride, diphenhydramine hydrochloride, and carbamazepine) was investigated using Pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS). A range of degradation products observed with this technique have been reported earlier with more complex approaches. In addition, chloramphenicol, lincomycin and diphenhydramine were shown to degrade into previously unidentified volatile compounds. These results therefore suggest that Py-GC/MS is an effective and simple technique to study the thermal degradation of food contaminants and to identify their volatile degradation products.
Application of biochar, a pyrolyzed biomass from organic sources, to agricultural soils is considered a promising strategy to sustain soil fertility leading to increased plant productivity. It is also known that applications of biochar to soilless potting substrates and to soil increases resistance of plants against diseases, but also bear the potential to have inconsistent and contradictory results depending on the type of biochar feedstock and application rate. The following study examined the effect of biochar produced from maple bark on soybean resistance against Rhizoctonia foliar blight (RFB) disease caused by Rhizoctonia solani, and examined the underlying molecular responses of both soybean and R. solani during interaction with biochar application. Soybean plants were grown in the presence of 1, 3, or 5% (w/w) or absence of maple bark biochar for 2 weeks, and leaves were infected with R. solani AG1-IA. At lower concentrations (1 and 3%), biochar was ineffective against RFB, however at the 5% amendment rate, biochar was conducive to RFB with a significant increase in disease severity. For the first time, soybean and R. solani responsive genes were monitored during the development of RFB on detached leaves of plants grown in the absence and presence of 5% biochar at 0, 6, 12, and 24 h post-inoculation (h.p.i.). Generally, large decreases in soybean transcript abundances of genes associated with primary metabolism such as glycolysis, tricarboxylic acid (TCA) cycle, starch, amino acid and glutathione metabolism together with genes associated with plant defense and immunity such as salicylic acid (SA) and jasmonic acid pathways were observed after exposure of soybean to high concentration of biochar. Such genes are critical for plant protection against biotic and abiotic stresses. The general down-regulation of soybean genes and changes in SA hormonal balance were tightly linked with an increased susceptibility to RFB. In conjunction, R. solani genes associated with carbohydrate metabolism were up-regulated, while genes involved in redox reactions and detoxification had varying effects. In conclusion, this study presents strong evidence that maple bark biochar increased susceptibility of soybean to a foliar disease. This condition is partly mediated by the down-regulation of soybean genes leading to reduced immunity and also affecting R. solani gene expression.
Bisphenols have the ability to disrupt the endocrine system and cause harmful health effects in humans. Crops’ potential uptake and distribution of bisphenol A and bisphenol F in their tissues, along with bisphenol transport through soil need to be closely examined. Therefore, the focus of this study was to (i) investigate the fate and transport of bisphenol A, F and S in potato (Solanum tuberosum L.) tubers grown in soil, irrigated with untreated bisphenol-tainted wastewater; (ii) examine the effect of three amendments, plantain (Musa paradisiaca L.) peel biochar, super absorbent polymer (SAP) as well as both biochar and SAP, in reducing plant uptake and soil pollution. A four-month study in field lysimeters, each housing a potato plant, was conducted to investigate these goals. None of the tested bisphenols was found in the flesh of potato tubers after harvest. However, the concentration of bisphenols in topsoil increased over the growing season, indicating the cumulative effect on the soil. The biochar treatment showed a higher bisphenol removal efficiency (45%) than the other treatments.
Water quality in Singapore's coastal area was evaluated with microbial indicators, pathogenic vibrios, chemical tracers and physico-chemical parameters. Sampling sites were grouped into two clusters (coastal sites at (i) northern and (ii) southern part of Singapore). The coastal sites located at northern part of Singapore along the Johor Straits exhibited greater pollution. Principal component analysis revealed that sampling sites at Johor Straits have greater loading on carbamazepine, while turbidity poses greater influence on sampling sites at Singapore Straits. Detection of pathogenic vibrios was also more prominent at Johor Straits than the Singapore Straits. This study examined the spatial variations in Singapore's coastal water quality and provided the baseline information for health risk assessment and future pollution management.
The rapid loss of mangroves globally has triggered a call for a better understanding of this habitat, including its dynamics and the threats it is exposed to. The present paper reports the study of trace metals at nine mangrove sites in Singapore in 2012/2013, using the simultaneous application of various tools, namely sediment analyses, the technique of diffusive gradients in thin-films (DGT) and caged/native bivalves (for both chemical and biomarker analyses). DGT devices were successfully deployed over 28 days in tropical mangrove waters, and the concentration measured with DGT showed significant correlation with the accumulation for Cu, Zn and Cd in caged mussels, and Cu for native clams, supporting the relevance of DGT to predict metal bioaccumulation. Concentrations in mangrove sediment are reported for As, Cd, Co, Cr, Cu, Ni, Pb and Zn. Sediment levels on a dry weight (dw) basis of Cu (ND-219.5 μg/g dw) and Zn (ND-502 μg/g dw) exceeded general sediment quality criteria at two sites. Most notably for these two metals, investigations based on the four tools (DGT, sediments, caged mussels and clams) were all able to segregate sites above and below the sediment quality guideline. This was further supported by a range of significant linear correlations between the measurements obtained with the various tools. The present findings support that these monitoring tools are comparable in the field to provide a time-integrated assessment of metals such as Cu and Zn.
This study investigated the occurrence of bisphenol A (BPA), atrazine and selected pharmaceutically active compounds (PhACs) in mangrove habitats in Singapore in 2012–2013, using multiple tools (sediment sampling, POCIS and filter feeder molluscs). Using POCIS, the same suite of contaminants (atrazine, BPA and eleven PhACs) was detected in mangrove waters in 28-days deployments in both 2012 and 2013. POCIS concentrations ranged from pg/L to μg/L. Caffeine, BPA, carbamazepine, E1, triclosan, sulfamerazine, sulfamethazine, and lincomycin were also detected in mangrove sediments from the low pg/g dw (e.g. carbamazepine) to ng/g dw (e.g. BPA). The detection of caffeine, carbamazepine, BPA, sulfamethoxazole or lincomycin in bivalve tissues also showed that these chemicals are bioavailable in the mangrove habitat. Since there are some indications that some pharmaceutically active substances may be biologically active in the low ppb range in marine species, further assessment should be completed based on ecotoxicological data specific to mangrove species.
The present study investigated the toxicity of several emerging contaminants: the pharmaceutical drug carbamazepine (CBZ), the plasticizer bisphenol A (BPA) and the herbicide atrazine (ATZ) in a marine bivalve. Green mussels (Perna viridis), were exposed to different concentrations of CBZ, BPA and ATZ, either individually or as mixtures over a 7-day period, and a suite of molecular and cellular biomarkers were analyzed: biomarkers of immunotoxicity (total hemocyte count: THC, phagocytosis, extracellular lysozyme), genotoxicity (Comet assay), neurotoxicity (inhibition of acetylcholinesterase: AChE), endocrine disruption (vitellin-like proteins: Vn) and detoxification enzymes (Cytochrome P4501A: EROD and Glutathione-S-Transferase: GST). Results of the single chemical exposure tests highlighted the relatively low toxicity of CBZ as most biomarker responses observed were recorded at concentrations well above environmental levels. Bisphenol A exposure at environmentally realistic concentrations resulted in clear immunomodulatory, genotoxic and endocrine disruptive effects. Similarly, three of the ten biomarkers tested on green mussels (genotoxicity, inhibition of AChE and EROD) responded after exposure to ATZ at environmentally relevant doses or below and confirmed the potency of this herbicide to marine bivalves. Exposure tests using mixtures of CBZ, BPA and ATZ also revealed that these three substances were generally acting in an additive manner on the selected biomarkers, at environmental doses, with some exceptions (antagonism and/or synergy) at low and high concentrations. This work also confirms that most of the biomarkers used here are suitable for biomonitoring studies with green mussels. This article is protected by copyright. All rights reserved.
The thermal stability of antibiotics commonly detected in food is reviewed. To quantify degradation, two major techniques have been reported: liquid chromatography-based methods and microbiological tests. As the degradation products may also display some antimicrobial activity, microbiological tests may not be considered accurate analytical methods for quantifying antibiotic residues' degradation. Degradation percentages are summarized for different antibiotics and for various media (water, oil, milk and animal tissues). Studies presented in the literature confirm that the thermal degradation of β-lactams, quinolones, sulfonamides and tetracyclines can be described using a first-order kinetic model. Degradation rates, k, derived for this model for liquid matrix (water) at 100°C, followed the general trend amongst antibiotic classes: β-lactams = tetracyclines (most heat-labile) > lincomycin > amphenicols > sulfonamides > oxfendazole > levamisole (most heat-stable). Although thermal processing results in a decrease in the concentration of parent antibiotic residues, degradation by-products have not been properly characterized to date. As some of these products were shown to be hazardous, further investigation is needed to determine their impact on food safety and human health. It is therefore currently difficult to definitively conclude whether or not antibiotic degradation during food processing is necessarily beneficial in terms of food safety.
A solid-phase extraction/liquid chromatography/electrospray ionization/multi-stage mass spectrometry (SPE-LC-ESI-MS/MS) method was optimized in this study for sensitive and simultaneous detection of multiple antibiotics in urban surface waters and soils. Among the seven classes of tested antibiotics, extraction efficiencies of macrolides, lincosamide, chloramphenicol, and polyether antibiotics were significantly improved under optimized sample extraction pH. Instead of only using acidic extraction in many existing studies, the results indicated that antibiotics with low pK a values (<7) were extracted more efficiently under acidic conditions and antibiotics with high pK a values (>7) were extracted more efficiently under neutral conditions. The effects of pH were more obvious on polar compounds than those on non-polar compounds. Optimization of extraction pH resulted in significantly improved sample recovery and better detection limits. Compared with reported values in the literature, the average reduction of minimal detection limits obtained in this study was 87.6 % in surface waters (0.06–2.28 ng/L) and 67.1 % in soils (0.01–18.16 ng/g dry wt). This method was subsequently applied to detect antibiotics in environmental samples in a heavily populated urban city, and macrolides, sulfonamides, and lincomycin were frequently detected. Antibiotics with highest detected concentrations were sulfamethazine (82.5 ng/L) in surface waters and erythromycin (6.6 ng/g dry wt) in soils. The optimized sample extraction strategy can be used to improve the detection of a variety of antibiotics in environmental surface waters and soils. Graphical Abstract The flowchart illustrating the optimized SPE-LC-ESI-MS/MS method
In the present study, a straightforward approach was validated for the analysis of pharmaceutically active compounds and endocrine-disrupting chemicals in the mollusk tissues, with a focus on two species commonly consumed in Southeast Asia (green mussels: Perna viridis; lokan clams: Polymesoda expansa). This approach relied on a simple solvent extraction (shaker table) followed by direct injection in liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). This "cleanup-free" approach was made possible by the use of isotopically labeled surrogates (to correct for matrix effects) and a post-column switch on the LC-MS/MS system (to remove potential interfering material). Altogether, relative recoveries were satisfactory for 36 out of 44 compounds (26-163 % range) and excellent for 27 out of 44 compounds (79-107 % range). Method detection limits (MDLs) were usually expressed in the nanogram per gram wet weight (ww) range and below. The method was successfully applied to 16 batches of green mussel samples collected in Singapore coastal waters. Trace levels of six compounds were detected in mussel tissues: caffeine (0.22-1.55 ng g(-1) ww), carbamazepine (<MDL-0.10 ng g(-1) ww), diltiazem (<MDL-0.13 ng g(-1) ww), diphenhydramine (<MDL-1.31 ng g(-1) ww), atrazine (<MDL-0.19 ng g(-1) ww), and bisphenol A (<MDL-7.6 ng g(-1) ww). The present method offers the perspective of high sample throughput, allowing for intensive monitoring programs and detailed exposure assessments.
A multi-residue analytical method was developed involving co-extraction and simultaneous determination of 89 hydrophobic organic contaminants (HOCs) in marine sediments and biota using gas chromatography-electron ionization-triple quadrupole mass spectrometry (GC-EI-MS/MS) and liquid chromatography-electrospray ionization-triple quadrupole mass spectrometry (LC-ESI-MS/MS). Target analytes include polychlorinated biphenyls (PCBs), pesticides, chlorobenzenes, brominated and chlorinated flame retardants, nitro-aromatic and polycyclic musks, triclosan and methyl triclosan. Spike-recovery experiments showed relative recoveries of analytes were generally between 70% and 130%. Analyses of a sediment standard reference material (SRM 1944) demonstrated recoveries between 80% and 120% for certified concentrations of individual PCBs and pesticides. Method detection limits of individual compounds ranged from 0.1 to 57.1pg/g dw for sediments and 0.1 to 22.8pg/g ww for biota. A field survey of sediments and biota from Singapore's marine environment demonstrated the occurrence of polycyclic and nitro-aromatic musks (galaxolide, tonalide, musk ketone, etc.), halogenated flame retardants (syn- and anti-dechlorane plus (DP), α, β and γ-hexabromocyclododecane (HBCD), tetrabromobisphenol A (TBBPA), etc.), as well as triclosan and methyl triclosan. Galaxolide exhibited relatively high concentrations, with highest levels in polychaete worms (161.7±72.5ng/g ww) and clams (546.8±220.3ng/g ww) from mangroves. Triclosan and methyl triclosan levels were highly correlated in sediments (r(2)=0.9752), while syn- and anti-DP were strongly correlated in biota (r(2)=0.9279). anti-DP/syn-DP stereoisomer ratios were typically >1 and ranged between 0.94 and 29.2 in sediments and biota samples. γ-HBCD exhibited the highest concentrations among HBCD isomers in sediments. Conversely, α-HBCD was the dominant HBCD isomer in biota. Copyright © 2015 Elsevier B.V. All rights reserved.
A gas chromatography-triple quadrupole mass spectrometry (GC-MS/MS) based method was developed for determination of 86 hydrophobic organic compounds in seawater. Solid-phase extraction (SPE) was employed for sequestration of target analytes in the dissolved phase. Ultrasound assisted extraction (UAE) and florisil chromatography were utilized for determination of concentrations in suspended sediments (particulate phase). The target compounds included multi-class hydrophobic contaminants with a wide range of physical-chemical properties. This list includes several polycyclic and nitro-aromatic musks, brominated and chlorinated flame retardants, methyl triclosan, chlorobenzenes, organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs). Spiked MilliQ water and seawater samples were used to evaluate the method performance. Analyte recoveries were generally good, with the exception of some of the more volatile target analytes (chlorobenzenes and bromobenzenes). The method is very sensitive, with method detection limits typically in the low parts per quadrillion (ppq) range. Analysis of 51 field-collected seawater samples (dissolved and particulate-bound phases) from four distinct coastal sites around Singapore showed trace detection of several polychlorinated biphenyl congeners and other legacy POPs, as well as several current-use emerging organic contaminants (EOCs). Polycyclic and nitro-aromatic musks, bromobenzenes, dechlorane plus isomers (syn-DP, anti-DP) and methyl triclosan were frequently detected at appreciable levels (2-20,000pgL(-1)). The observed concentrations of the monitored contaminants in Singapore's marine environment were generally comparable to previously reported levels in other coastal marine systems. To our knowledge, these are the first measurements of these emerging contaminants of concern in Singapore or Southeast Asia. The developed method may prove beneficial for future environmental monitoring of hydrophobic organic contaminants in marine environments. Further, the study provides novel information regarding several potentially hazardous contaminants of concern in Singapore's marine environment, which will aid future risk assessment initiatives. Copyright © 2015 Elsevier B.V. All rights reserved.
Recent importance has been placed on the ecological and socio-economic aspects of mangroves for adjacent coastal populations, in terms of flood defense, food resources, employment and generation of income. Anthropogenic stressors, such as direct clearance, hydrological alterations, climatic change effects or chemical pollution contribute to mangrove ecosystems degradation. While the relative impact is not well understood, the hydrodynamics specific to mangroves (intertidal, land-marine interface) are undoubtedly influencing those effects. In the present study, a computer-based model was built to understand the hydrodynamic flow characteristic in a mangrove ecosystem, the Sungei Buloh Wetland Reserve in Singapore, with the wider intent to better understand the transport of chemical substances in mangroves. Field surveys in the mangrove and the preliminary development of a two-dimensional hydrodynamic (2DH) model have been carried out. Higher bottom roughness was considered in the vegetated part of the model domain to account for the effect of mangrove roots. Spatial and temporal distributions, as well as minor mean differences between simulated and observed results, suggest that the developed model capture satisfactorily the tidal dynamics within the river, the wetland area covered with mangroves and in the strait. These results indicate that the hydrodynamics are properly understood within the Sungei Buloh mangrove ecosystem and can be used for modeling the fate of chemicals.
The suitability of traditional microbial indicators (i.e. Escherichia coli and enterococci) has been challenged due to the lack of correlation with pathogens and evidence of possible regrowth in the natural environment. In this study, the relationships between alternative microbial indicators of potential human fecal contamination (Bacteroides thetaiotaomicron, Methanobrevibacter smithii, human polyomaviruses, F+ and somatic coliphages) and pathogens (Salmonella spp., Pseudomonas aeruginosa, rotavirus, astrovirus, norovirus GI, norovirus GII and adenovirus) were compared with traditional microbial indicators as well as environmental parameters (temperature, conductivity, salinity, pH, dissolved oxygen, total organic carbon, total suspended solids, turbidity, total nitrogen, and total phosphorus). Water samples were collected from surface waters of urban catchments in Singapore. Salmonella and P. aeruginosa had significant positive correlations with most of the microbial indicators, especially E.coli and enterococci. Norovirus GII showed moderately strong positive correlations with most of the microbial indicators, except for HPyVs and coliphages. In general, high geometric means and significant correlations between human-specific markers and pathogens suggest the possibility of sewage contamination in some areas. The simultaneous detection of human specific markers (i.e. B. theta, M. smithii, and HPyVs) with E.coli and enterococcus, supports the likelihood of recent fecal contamination since the human specific markers are unable to regrow in natural surface waters. Multiple linear regression results further confirm that the inclusion of M. smithii and HPyVs together with traditional ones would better predict the occurrence of pathogens. Further study is needed to determine the applicability of such models to different geographical locations and environmental conditions. Copyright © 2014, American Society for Microbiology. All Rights Reserved.
Tools specifically validated for tropical environments are needed to accurately describe the behavior of chemical contaminants in tropical ecosystems. In the present study, sampling rates (Rs) were determined for the commercial pharmaceutical-type Polar Organic Chemical Integrative Sampler (POCIS) with a 45.8 cm2 exposure surface for 35 Pharmaceutically Active Compounds (PhACs) and Endocrine Disrupting Compounds (EDCs), of which eight compounds (albuterol, atorvastatin, diltiazem, dilantin, enalapril, norfluoxetine, risperidone and warfarin) were reported for the first time. These sampling rates were measured in an outdoor laboratory calibration setup to best capture diurnal tropical temperature variations (29 ± 3 °C). The effect of stirring and salinity was investigated. For all compounds, the sampling rates were higher under stirred conditions as compared to quiescent conditions. Calibration results in the presence of 30 g sodium chloride support that the effects of salinity on POCIS sampling rates are compound-specific. Comparisons between Time-Weight Average (TWA) water concentrations using POCIS and spot sample levels in the field (2 lake and 1 mangrove estuary sites) are presented. Results showed that POCIS TWA concentrations were in agreement with spot sample concentrations for these aquatic systems. Results indicate that POCIS can be used to effectively measure the TWA concentration for a range of PhACs and EDCs in tropical waters. However, based on the results from mass balance and field deployments, POCIS did not appear suitable for compounds with a low mass balance recovery during calibration (e.g. triclosan and linuron in this study).
Emerging contaminants such as antibiotics have received recent attention as they have been detected in natural waters and health concerns over potential antibiotic resistance. With the purpose to investigate fast and high-throughput analysis, and eventually the continuous on-line analysis of emerging contaminants, this study presents results on the analysis of seven selected antibiotics (sulfadiazine, sulfamethazine, sulfamerazine, sulfamethoxazole, chloramphenicol, lincomycin, tylosin) in surface freshwater and seawater using direct injection of a small sample volume (20 μL) in liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). Notably, direct injection of seawater in the LC-ESI-MS/MS was made possible on account of the post-column switch on the system, which allows diversion of salt-containing solutions flushed out of the column to the waste. Mean recoveries based on the isotope dilution method average 95 ± 14% and 96 ± 28% amongst the compounds for spiked freshwater and seawater respectively. Linearity across six spiking levels was assessed and the response was linear (r2 > 0.99) for all compounds. Direct injection concentrations were compared for real samples to those obtained with the conventional SPE-based analysis and both techniques concurs on the presence/absence and levels of the compounds in real samples. These results suggest direct injection is a reliable method to detect antibiotics in both freshwater and seawater. Method detection limits for the direct injection technique (37 pg/L to 226 ng/L in freshwater, and from 16 pg/to 26 ng/L in seawater) are sufficient for a number of environmental applications, for example the fast screening of water samples for ecological risk assessments. In the present study of real samples, this new method allowed for example the positive detection of some compounds (e.g. lincomycin) down to the sub ng/L range. The direct injection method appears to be relatively cheaper and faster, requires a smaller sample size, and is more robust to equipment cross-contamination as compared to the conventional SPE-based method.
The fate and exposure risks of pharmaceutically active compounds (PhACs) and endocrine disrupting chemicals (EDCs) in marine environments are not well-understood. In this study we developed a multi-residue analytical method for quantifying concentrations of forty target compounds in seawater from Singapore. Analyses of samples (n = 24) from eight sites showed the occurrence of several compounds, including gemfibrozil (<0.09-19.8 ng/L), triclosan (<0.55-10.5 ng/L), carbamazepine (<0.28-10.9 ng/L) and ibuprofen (<2.2-9.1 ng/L). A 3D hydrodynamic model for Singapore was used to predict residence time (tR). Principal Components Analysis revealed a strong relationship between tR and contaminant concentrations. While source emissions are undoubtedly important, proximate distance to a wastewater treatment plant had little influence on concentrations. The site with the greatest tR, which exhibited the highest concentrations, is adjacent to Singapore's largest protected wetland reserve. The results highlight an important linkage between hydrodynamic behavior and contaminant exposure risks in complex coastal marine ecosystems.
Although their ecological and socioeconomic importance has received recent attention, mangrove ecosystems are one of the most threatened tropical environments. Besides direct clearance, hydrological alterations, climatic changes or insect infestations, chemical pollution could be a significant contributor of mangrove degradation. The present paper reviews the current knowledge on the occurrence, bioavailability and toxic effects of trace contaminants in mangrove ecosystems. The literature confirmed that trace metals, Polycyclic Aromatic Hydrocarbons (PAHs), Persistent Organic Pollutants (POPs), Pharmaceuticals and Personal Care Products (PPCPs) and Endocrine Disrupters Compounds (EDCs) have been detected in various mangrove compartments (water, sediments and biota). In some cases, these chemicals have associated toxic effects on mangrove ecosystem species, with potential impact on populations and biodiversity in the field. However, nearly all studies about the bioavailability and toxic effects of contaminants in mangrove ecosystems focus on selected trace metals, PAHs or some "conventional" POPs, and virtually no data exist for other contaminant groups. The specificities of mangrove ecosystems (e.g. biology, physico-chemistry and hydrology) support the need for specific ecotoxicological tools. This review highlights the major data and methodological gaps which should be addressed to refine the risk assessment of trace pollutants in mangrove ecosystems.
Background. A well-documented source of mercury contamination is skin-whitening cream. The heavy metal is sometimes added to the cream to block production of melanin. Past studies have shown that about one third of skin whitening creams for sale in Phnom Penh contained mercury levels considerably higher than both United States and Association of Southeast Asian Nations (ASEAN) guidelines. Developing nations such as Cambodia do not have the resources for monitoring and enforcement. Objectives. Evaluate the utility of handheld X-ray fluorescence analyzers for detection of mercury in skin-whiteners. Methods. A handheld XRF unit was used to measure the total mercury content of 676 skin- whitening creams collected from volunteers from a cross-section of Phnom Penh's residents. Results. About 16% of bleaching creams collected contained more than 20 ug/g of mercury. The highest concentration was about 35,000 ug/g. A third of the samples that were custom-made or modified by beauty shops contained more than 20 ug/g of mercury. Conclusions. The handheld XRF analyzer proved an excellent tool for screening mercury in skin creams, and could be particularly useful in developing countries because it is relatively inexpensive, requires no sample preparation or extraction, can be used by staff with little technical background, and can facilitate on-site education.
Background. A well-documented source of mercury contamination is skin-whitening cream. The heavy metal is sometimes added to the cream to block production of melanin. Past studies have shown that about one third of skin whitening creams for sale in Phnom Penh contained mercury levels considerably higher than both United States and Association of Southeast Asian Nations (ASEAN) guidelines. Developing nations such as Cambodia do not have the resources for monitoring and enforcement. Objectives. Evaluate the utility of handheld X-ray fluorescence analyzers for detection of mercury in skin-whiteners. Methods. A handheld XRF unit was used to measure the total mercury content of 676 skin-whitening creams collected from volunteers from a cross-section of Phnom Penh’s residents. Results. About 16% of bleaching creams collected contained more than 20 ug/g of mercury. The highest concentration was about 35,000 ug/g. A third of the samples that were custom-made or modified by beauty shops contained more than 20 ug/g of mercury. Conclusions. The handheld XRF analyzer proved an excellent tool for screening mercury in skin creams, and could be particularly useful in developing countries because it is relatively inexpensive, requires no sample preparation or extraction, can be used by staff with little technical background, and can facilitate on-site education. Competing Interests. The authors declare no competing financial interests.
Under specific conditions (pH, concentrations), trace metals may form, with environmental inorganic ligands, neutral complexes which, in principle, might diffuse passively through biological membranes or influence the response of (bio)analytical sensors for trace metals based on permeation liquid membrane (PLM). In this study, metal (Cu, Cd, Pb) transport through the planar PLM device was evaluated in the presence of major environmental inorganic ligands such as sulfate, carbonate and chloride under conditions where neutral complexes may be formed (up to 73% of neutral metal complex in the solution). In the presence of sulfate, comparison of predicted and experimental PLM fluxes of Cu, Pb and Cd, suggests that passive transport of neutral sulfate-metal complexes does not occur. This was confirmed by comparing fluxes in the presence and absence of carrier. In the presence of carbonate (for Cd, Cu and Pb) and chloride (for Pb and Cd), however, experimental PLM fluxes were greater than predicted (up to 4 and 25 times in the presence of carbonate and chloride, respectively), but experiments in the absence of carrier in the membrane revealed that no passive transport of neutral complexes (MCl(2) or MCO(3)) occurs through PLM. A possible mechanism is discussed. In parallel to the experiments with PLM, the influence of carbonate on the internalization fluxes of Cu(II) and Pb(II) by the freshwater algae, Chlamydomonas reinhardtii, was assessed. Similarly to the results of PLM, the fluxes of these two metals were larger than expected (based on the free metal ion activity model). Thus, even though PLM and bioaccumulation mechanisms are certainly different, similar unexpected behaviours occur for the metal transport through the PLM and biological membrane of C. reinhardtii, in the presence of carbonate.
An insight into the dynamic aspects of the accumulation process is essential for understanding bioaccumulation as well as effect studies of hydrophobic organic chemicals. This review presents an overview of kinetic studies with organisms (fish, bivalve, crustacean, insect, worm, algae, and protozoan) as well as passive samplers (solid and liquid phase microextraction, semipermeable membrane device, polymer sheet, solid-phase extraction, Chemcatcher, etc.) for the uptake of neutral nonpolar chemicals from the aqueous phase. Information about uptake rates, elimination rates, and 95% equilibration times was collected and analyzed with diffusion based models. The present literature review suggests that the surface to volume ratio appears to be a critical parameter for the uptake rate of the more hydrophobic chemicals both for samplers and organisms. In addition, as a very first approximation, the combination of the first-order kinetic model with the assumption that diffusion through the aqueous boundary layers is rate limiting, gives a reasonable description of the experimental kinetic data. In this way, the presented model might be used to estimate uptake and elimination rate constants of chemicals by organisms or passive samplers.
In the present article, the hollow-fibre liquid-phase microextraction of poly-chlorinated biphenyls (PCBs) was conducted under non-equilibrium conditions, to investigate the bioavailability of PCBs in natural waters. The study was conducted for 12 PCB congeners (log Kow ranging from 5.2 to 8.2) in the ng L range. This appeared as a major challenge since aqueous solutions in this concentration range tend to evolve rapidly due to adsorption of PCBs on glass walls. The average aqueous diffusion layer was measured to be 43 ± 2 µm at 500 rpm. Aqueous diffusion coefficients of PCBs estimated from experimental data were found to be about two times lower than those predicted by the Hayduk-Laurie equation, possibly due to the underestimation of the molar volume of PCBs, the aggregation of PCBs in the aqueous phase, or a decrease of the actual aqueous concentration during the time of extraction. The presence of Aldrich humic acid (AHA) in the solution decreased, as expected, the mass transfer of PCBs to the fibre, but the flux was not linked either to the total or to the free PCB concentration. This suggests a semi-labile behaviour for the AHA–PCB complex, which was confirmed by the effect of stirring speed on the amount of PCBs extracted in the presence and in the absence of AHA. The whole of these observations suggests that diffusion in solution is not only one of the limiting process for the extraction of PCBs but also supports the need for more experimental data to understand in detail the mechanism of extraction of hydrophobic compounds, and their bioavailability in the presence of aquatic complexants.
Singapore is one of the busiest ports in the world and has many shipyards, petroleum refineries and pharmaceutical manufacturing plants are located on its coastline. Data on the prevalence of persistent organic pollutants (POPs) in Singapore's coastal ecosystems are therefore important in order to support research on the potential threats to the local marine environment and human health. This chapter presents data on the prevalence of POPs in the seawater, sediments, biota and mangrove habitats of Singapore. Data are presented for a range of POPs including polycyclic aromatic hydrocarbons (PAHs), organochlorine compounds (OCPs), polychlorinated biphenyls and polybrominated diphenyl ethers (PBDEs). Local studies confirm the ubiquity of POPs in the marine environment of Singapore. The prevailing ocean currents in the region govern the fate and transport of POPs in coastal waters, where the presence of localized high levels of POPs is likely to be a function of local shipping and industrial discharges. Land reclamation activities may also be a factor in the prevalence of POPs in seawater, where compounds are released from sediments and transported to the water column by seabed dredging. The land area under agricultural use in Singapore is negligible, and there is no direct application of OCPs in the country. However, pesticides may be easily transported through the atmosphere following volatilization from soil, and the presence of OCPs in Singapore's marine waters is likely to be a function of their use in neighbouring countries, with subsequent atmospheric transport and deposition. A biomagnification phenomenon was observed amongst the species collected and analysed from mangrove sites, but levels of POPs were not found to exceed relevant food safety standards. However, PCB concentrations in mangrove fish muscles were found to be higher than mean levels of PCBs found in seafood commonly consumed in Singapore. Overall, available data on the prevalence, fate and transfer of POPs in Singapore's marine environment highlights the ubiquity of these pollutants, and supports the need for a greater awareness on their fate, transport and bioaccumulation in local ecosystems.
Persistent organic pollutants (POPs) are man-made chemicals that have an intrinsic resistance to natural degradation processes, and are therefore environmentally persistent. The introduction of POPs into the environment from anthropogenic activities has resulted in their widespread dispersal and accumulation in soils and water bodies, as well as human and ecological food chains where they are known to induce toxic effects. Due to their ubiquity in the environment and lipophilic properties, there is mounting concern over the potential risks of human exposure to POPs. This has led to the establishment of a worldwide research program to determine prevailing levels of POPs in the population and investigate the health risks associated with background exposure. This paper reviews the state of knowledge regarding residual levels of POPs in human adipose tissue worldwide, and provides preliminary data on the levels of key POPs in female adipose tissues collected in Singapore. Organochlorine pesticides (OCPs) were found to be comparable to levels reported for Poland in 2001, with a mean of 0.98μgg−1 (or 0.84μgg−1 on a lipid weight basis). For total polychlorinated biphenyl (PCB) congeners, the mean concentration of 34ngg−1 (lipid weight basis) is lower than values reported from Japan in 1980 and Belgium in 2000. Polybrominated diphenyl ethers (PBDEs) are present at similar levels to Belgium in 2000, at 3.7 (or 4.7ngg−1 lipid weight basis).
There are currently a limited number of techniques to study nickel speciation in aqueous samples. This work reports on the use of the permeation liquid membrane (PLM) for that objective. In this paper, the composition of the organic phase was studied to maximize the Ni flux (thus the sensitivity of the device) over a wide Ni2+ concentration range (50 nM to 100 microM) in order to verify its ability to determine free Ni2+ in the presence of Ni complexes. A mixture containing 1,10-didecyl-1,10-diaza-18-crown-6 ether (22DD) and di(2-ethylhexyl)phosphoric acid (D2EHPA) in toluene/phenylhexane was selected as the optimized organic phase for the PLM. The PLM was shown to be a reliable tool to measure free nickel concentrations down to 10(-7) M. The effect of pH on Ni transport was also studied. Fluxes below pH 6 were reduced significantly, i.e. an order of magnitude smaller than fluxes above pH 7.8. Finally, as part of a broader study examining the ability of trace metals to induce antibiotic resistance in bacteria, we used the PLM to quantify the formation, at pH = 7.2, of a weak complex between Ni and Imipenem (a member of the carbapenem class of beta-lactam antibiotics) or its hydrolysis product(s).
Comparative growth rates were monitored in the Pacific oyster, Crassostrea gigas, at two sites in Singapore, one uncontaminated and the other contaminated with respect to ambient seawater quality. Growth rates differed significantly at the two sites, revealing that marine water quality can have potentially adverse effects for the oyster aquaculture industry in Singapore. Shell abnormalities (chambering) were observed for juvenile and mature oysters at the contaminated site. Water quality parameters including temperature, salinity, dissolved oxygen, total organic carbon and chlorophyll-a were essentially similar at both sites. Differences in the levels of tributyl tin detected in soft tissues were not observed, but significant differences in the burden of persistent organic pollutants (POPs) existed between the two sites. On a positive note, the effects of pollution on oysters were found to be reversible, where transplantation of individuals to the uncontaminated site resulted in the ability of C. gigas to recover in terms of growth rate and the burden of bioaccumulated POPs.
Chlorinated paraffins (CPs), as technical mixtures of polychlorinated alkanes (PCAs), are ubiquitous in the environment. CPs tend to behave in a similar way to persistent organic pollutants (POPs), leading several countries to impose regulations on the use of CPs. In this article, we review the literature on the properties of CPs, the current analytical tools available to determine CPs in various types of environmental matrices, and concentrations found in the environment. In particular, concentrations of CPs in environmental compartments including air, water, sediments, biota, human food products and human tissues are summarized. Priorities for future research are: improvements in analytical methodologies (reducing the complexity of the analysis, producing reference materials and performing interlaboratory studies); determining background levels of chlorinated paraffins in the environment and human populations (this question should be answered using quality assured analytical tools allowing the intercomparison of data); and investigating the sources of CPs to the environment and to humans.
The permeation liquid membrane (PLM) technique was used to evaluate cadmium speciation in media resembling natural freshwaters. A planar sheet PLM system was characterized by measuring Cd fluxes in the absence and presence of complexing agents such as citrate, malonate, nitrilotriacetate and the Suwannee River standard humic acid. Comparison with theoretical speciation calculations and the results of a Cd2+ selective electrode, showed that free Cd was correctly measured using the planar sheet PLM within the studied concentration range, i.e. 10(-8) to 10(-4) M. The effect of pH and potentially co-transported ions on Cd transport through the PLM was also studied. An example of the ability of the hollow-fiber PLM (HFPLM) to measure free Cd in the nM range is also presented. In order to evaluate the usefulness of the technique as a predictor of bioavailability, Cd PLM measurements (fluxes) were compared to Cd biouptake (internalization flux) for a freshwater alga, Chlorella kesslerii, in the absence and presence of SRHA. The use of PLM measurements is shown to be an attractive tool to better understand Cd biouptake.
In this study, persistent organic pollutants were quantified in sediments, subsurface seawater, sea-surface microlayer and twenty-four biota species collected at two separate mangrove habitats in Singapore. Data confirmed the ubiquity of POPs, including polybrominated diphenyl ethers (PBDEs), in the marine environment of Singapore. A biomagnification phenomenon was observed amongst the species collected and analysed from both mangrove sites studied. Thunder crabs and fish displayed the highest POP levels. Congener profiles of PBDEs varied amongst mangrove biota species and suggested different metabolic pathways exist for flame retardants. Similarly, crab species showed an ability to metabolize chlordane insecticide.
Recent studies have raised concern over the presence of high levels of persistent organic pollutants (POPs) in farmed fish relative to wild specimens of the same species, particularly salmon. Although cooking is known to reduce the burden of POPs in fish, the mechanisms of loss/degradation are not clearly understood. This study investigated the loss of POPs, including polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), p,p'-DDT [2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane] and its related metabolites (sum noted as DDTs), and chlordane congeners, from salmon (Salmo salar) steaks when subjected to baking, boiling, frying, or microwave cooking. Ranges in the raw flesh were 25.1-62.9 ng/g wet weight (ww) for PCBs, 2.5-7.6 ng/g ww for PBDEs, 2.4-5.3 ng/g ww for chlordanes, and 17.5-43.8 ng/g ww for DDTs. Analysis of raw steaks from along the fish body revealed a significant variation of POP concentrations along the fish body, with higher concentrations at the head end than the tail, with a peak in the central section. After cooking, levels of POPs decreased in salmon steak with an average loss of 26 +/- 15% relative to the initial POP load in the raw steak. The removal of the skin from the cooked salmon steak resulted in a further average loss of 9 +/- 3%. The loss of POPs did not differ significantly between cooking methods. Losses of POPs were significantly and linearly correlated with the losses of lipid during cooking, suggesting removal of lipids is the critical factor for POPs reduction in cooked fish. Cooking of raw fish contaminated with POPs can therefore be expected to reduce the consumption exposure risk to human health.
In this study, the levels of several heavy metals and persistent organic pollutants (POPs) were measured in the edible portions of 20 different seafood types consumed in Singapore (2 < n < 12). The mean heavy metal concentrations among the seafood types ranged from below detection limits (BLD) to 14.2 microg/g wet weight (ww) for As (shark), to 0.50 microg/g ww for Cd (kunning), to 25.5 microg/g ww for Cu (gray prawn), to 0.58 microg/g ww for Hg (eel), and to 1.21 microg/g ww for Pb (salmon). Chlordane, polychlorinated biphenyls (PCBs), and p, p'-DDT [2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane] and its related metabolites (sum noted as DDTs) were the main POPs found among the seafood types, with highest concentrations in salmon fillets and green mussels. Polybrominated diphenyl ether (PBDE) concentrations in salmon fillets (2.75 ng/g ww) were one order of magnitude lower than the highest concentration of PCBs (28.5 ng/g ww). The mean daily intake of contaminants from seafood was calculated for the general population of Singapore. Daily intakes of heavy metals and POPs from seafood are below the oral reference dose set by the U.S. Food and Drug Administration (FDA), except for As, although our study did not characterize the species of As present. Daily intake of As, DDTs, heptachlor, and PCBs in seafood exceeded the conservative cancer benchmark concentrations set by the U.S. Environmental Protection Agency (EPA), suggesting that a significant number of people are potentially at risk in Singapore over a lifetime from seafood consumption.
Bioaccumulation and metabolism of p,p'-DDT was studied in the marine carnivorous fish Lates calcarifer, Asian seabass, in a controlled aquaculture experiment. Over a 42-d period, seabass were fed pellets dosed with p,p'-DDT at environmentally realistic levels. Virtually all p,p'-DDT in pellets bioaccumulated in the fish with an uptake efficiency of 98%. The levels of p,p'-DDT and the metabolites p,p'-DDD and p,p'-DDE were analyzed in muscle, liver, visceral fat, brain, and remaining tissues. Partitioning of p,p'-DDT and its metabolites among the control, low-dose, and high-dose exposed seabass were 14.8% in muscle, 3.5% in liver, 37.1% in visceral fat, 0.11% in brain, and 45.5% in remaining tissues, where partitioning between tissues was a function of tissue lipid content. p,p'-DDT bioaccumulation increased linearly with exposure in visceral fat and muscle tissue. The metabolism of p,p'-DDT, which occurs mainly in the liver, resulted in the degradation of 2.5% of p,p'-DDT into p,p'-DDD. These new findings show that bioaccumulation processes at environmentally realistic ingestion exposure levels (ng/g) differ from previous DDT ingestion studies conducted at unrealistically high DDT levels (microg/g), highlighting the need to revise models on the transfer of persistent organic pollutants in the marine environment and aquaculture systems.
In the last decade, evidence of endocrine disruption in biota exposed to environmental pollutants has raised serious concern. Human cell-based bioassays have been developed to evaluate induced androgenic and estrogenic activities of chemical compounds. However, bioassays have been sparsely applied to environmental samples. In this study we present data on sex hormone activities in the green mussel, Perna viridis, in Singapore's coastal waters. P.viridis is a common bioindicator of marine contamination, and this study is a follow-up to an earlier investigation that reported the presence of sex hormone activities in seawater samples from Singapore's coastal environment. Specimens were collected from eight locations around the Singapore coastline and analyzed for persistent organic pollutants (POPs) and heavy metals. Tissue extracts were then screened for activities on androgen receptors (ARs) and estrogen receptors (ER-alpha and ER-beta) using a reporter gene bioassay based on a HeLa human cell line. Mussel extracts alone did not exhibit AR activity, but in the presence of the reference androgenic hormone dihydrotestosterone (DHT), activities were up to 340% higher than those observed for DHT alone. Peak activities were observed in locations adjacent to industrial and shipping activities. Estrogenic activities of the mussel extract both alone and in the presence of reference hormone were positive. Correlations were statistically investigated between sex hormone activities, levels of pollutants in the mussel tissues, and various biological parameters (specimen size, sex ratio, lipid and moisture content). Significant correlations exist between AR activities, in the presence of DHT, and total concentration of POPs (r= 0.725, p < 0.05).
Growing concern on the environmental impact of polybrominated diphenyl ethers (PBDEs) has created the need for rapid and quality assured analytical methods to quantify PBDEs in a spectrum of matrix types. This study presents the first validated method for the quantification of major PBDE congeners (47, 99 and 100) in marine biological tissues using microwave-assisted extraction (MAE). The recovery of polychlorinated biphenyls and various organochlorine pesticides has also been ascertained. Analytical accuracy, precision, limits of detection and cleanup efficiency were evaluated for PBDE congeners, and empirical data justifies the use of MAE for the extraction and analysis of PBDEs in biological matrices. MAE was also compared to Soxhlet extraction efficiency for PBDEs in the standard reference materials SRM2978 and SRM1588a and gave comparable results (<15% variation).
The green mussel, Perna viridis, was used to measure bioaccumulated levels of organochlorine pesticides (OCPs), together with various heavy metals, in the marine environment of Singapore. Samples were collected from eight different locations in the coastal waters of Singapore between April and May 2002. OCPs (i.e. DDT, Chlordane, Mirex, hexachlorobenzene (HCB), pentachloronitrobenzene (PCNB) and Heptachlor) were quantified by gas chromatography-mass spectrometry. Heavy metals (i.e. As, Cd, Cr, Cu, Ni, Pb and Zn) were quantified by induced coupled plasma spectrometry. The concentration ranged from 2.6 to 54 ng g-1 dw for DDTs (i.e. sum of p,p'-DDT, p,p'-DDD, o,p'-DDE and p,p'-DDE), 3.1 to 15 ng g-1 dw for Chlordanes (i.e. sum of α- and γ-chlordane) and 0.26 to 1.5 ng g-1 dw for Mirex. These levels can be considered low when compared to reported values from similar studies conducted elsewhere in the world. Other pesticides were not detected. All heavy metals analysed were detected in all P. viridis samples, but only Cu, Pb, Zn and As were close to existing safety standards. Samples taken from locations adjacent to the main shipyards on the coastline and busy shipping lanes demonstrated peak levels of heavy metals and pesticides in the mussel tissues.
The green mussel, Perna viridis, was used in this study to measure levels of polychlorinated biphenyls (PCBs) and, for the first time, polybrominated diphenyl ethers (PBDE) in the marine environment. Samples were collected from eight different locations in the coastal waters of Singapore between April and May 2002. Forty-one PCB and 21 PBDE congeners were quantified by gas chromatography-mass spectrometry and were all positively detected in the mussel tissues. Total concentrations in green mussel tissues ranged from 6.1 to 82 ng/g and 2.0 to 38 ng/g on a dry-weight basis for PCBs and PBDEs, respectively. Such levels reflect the ubiquity of these persistent organic pollutants in a tropical marine environment. Principal component analysis was applied to the PCB data and revealed similarities in the congener composition of mussel tissues to that of the commercial PCB mixture, Aroclor 1254. The PBDE levels, to date, were approximately one order of magnitude greater than the upper concentrations reported for blue mussel (Mytilus edulis) tissues in Europe. At some sampling sites, the congener composition of PBDEs in P. viridis tissues indicated recent exposure to a commercial pentabrominated flame retardant.