Shengchun Wu

Concentrations of persistent lipophilic compounds in fish are determined by exchange across the gills, not through the food chain

Uptake of persistent lipophilic toxicants in fish occurs via the food and by transfer across the body surface, notably the gills. Flux rates of most lipid soluble toxicants across the gills is rapid and the animal must eat at very high rates for feeding to have a significant effect on toxicant concentration in the body. The relative rates of uptake via feeding and transfer across the gills are analyzed from a theoretical and experimental standpoint. At the low feeding rates typical of fish, the uptake of toxicants in the food can be ignored when estimating toxicant body concentration.

Bioaccessibility, dietary exposure and human risk assessment of heavy metals from market vegetables in Hong Kong revealed with an in vitro gastrointestinal model.

A systematic survey of heavy metal (HM) concentrations and bioaccessibilities in market vegetables in Hong Kong were carried out for assessing potential health risk to local inhabitants. The average concentrations of Cd, Pb, Cr, Ni, Cu, and Zn in nine major groups of fresh vegetable varied within 0.007-0.053, 0.05-0.17, 0.05-0.24, 0.26-1.1, 0.62-3.0, and 0.96-4.3 mg kg(-1), respectively, and their average bioaccessibilities varied within 21-96%, 20-68%, 24-62%, 29-64%, 30-77%, and 69-94%, respectively. The bioaccessible estimated daily intakes (BEDIs) of Cd, Pb, Cr, Ni, Cu, and Zn from vegetables were far below the tolerable limits. The total bioaccessible target hazard quotient (TBTHQ) of the six HMs was 0.18 and 0.64 for average and high consumers, respectively, with Cd and leafy vegetable being the major risk contributors. Risk assessment of HMs from foods should be modified by taking bioaccessibility into account.

Influence of pyrolysis temperature on properties and environmental safety of heavy metals in biochars derived from municipal sewage sludge.

Dried raw sludge was pyrolyzed at temperatures ranging from 400 to 600°C at the increase of 50°C intervals to investigate the influence of pyrolysis temperature on properties and environmental safety of heavy metals in biochar derived from municipal sewage sludge. The sludge biochar yield decreased significantly with increasing pyrolysis temperature but the pH, ash content and specific surface area increased. Conversion of sludge to biochar markedly decreased the H/C and N/C ratios. FT-IR analysis confirmed a dramatic depletion of H and N and a higher degree of aromatic condensation in process of biochar formation at higher temperatures. The total concentrations of Cu, Zn, Pb, Cr, Mn, and Ni increased with conversion of sludge to biochar and increasing pyrolysis temperature. However, using BCR sequential extraction and analysis, it was found that most of the heavy metals existed in the oxizable and residual forms after pyrolysis, especially at 600°C, resulting in a significant reduction in their bioavailability, leading to a very low environmental risk of the biochar. The present study indicates pyrolysis is a promising sludge treatment method for heavy metals immobilization in biochar, and highlights the potential to minimize the harmful effects of biochar by controlling pyrolysis temperature.

Effects of root anatomy and Fe plaque on arsenic uptake by rice seedlings grown in solution culture

Hydroponic experiments were carried out to investigate the effects of root anatomy, induced by aeration and stagnation, and Fe plaque on arsenic (III&V) uptake and translocation by rice plants. The results showed that As uptake in rice plants (Gui Chao-2) treated by aeration was decreased due to lower root specific surface area. Rice roots with larger specific surface area tended to form more Fe plaque, and Fe plaque affected As uptake kinetics by changing As influx curves from linear to hyperbolic for As(III) and from hyperbolic to S-curve for As(V). Fe plaque increased As(III&V) adsorption and minimized the effects of root anatomy characteristics on As uptake into roots and subsequently translocation to shoots. Fe plaque increased As(III) uptake rate at As(III) concentrations of 0.5 to approximately 8 mg L(-1), reduced As(V) uptake rate at low As(V) concentrations (<2 mg L(-1)), but increased As uptake rate at high As(V) concentrations (>6 mg L(-1)).

Do radial oxygen loss and external aeration affect iron plaque formation and arsenic accumulation and speciation in rice

Hydroponic experiments were conducted to investigate the effect of radial oxygen loss (ROL) and external aeration on iron (Fe) plaque formation, and arsenic (As) accumulation and speciation in rice (Oryza sativa L.). The data showed that there were significant correlations between ROL and Fe concentrations in Fe plaque produced on different genotypes of rice. There were also significant differences in the amounts of Fe plaque formed between different genotypes in different positions of roots and under different aeration conditions (aerated, normal, and stagnant treatments). In aerated treatments, rice tended to have a higher Fe plaque formation than in a stagnant solution, with the greatest formation at the root tip decreasing with increasing distances away, in accordance with a trend of spatial ROL. Genotypes with higher rates of ROL induced higher degrees of Fe plaque formation. Plaques sequestered As on rice roots, with arsenate almost double that with arsenite, leading to decreased As accumulation in both roots and shoots. The major As species detected in roots and shoots was arsenite, ranging from 34 to 78% of the total As in the different treatments and genotypes. These results contribute to our understanding of genotypic differences in As uptake by rice and the mechanisms causing rice genotypes with higher ROL to show lower overall As accumulation.

The influence of mariculture on mercury distribution in sediments and fish around Hong Kong and adjacent mainland China waters

To study the influence of mariculture on mercury (Hg) speciation and distribution in sediments and cultured fish around Hong Kong and adjacent mainland China waters, sediment samples were collected from six mariculture sites and the corresponding reference sites, 200-300 m away from the mariculture sites. Mariculture activities increased total mercury, organic matter, carbon, nitrogen and sulfur concentrations in the surface sediments underneath mariculture sites, possibly due to the accumulation of unconsumed fish feed and fish excretion. However, methylmercury (MeHg) concentrations and the ratio of MeHg to THg (% MeHg) in sediments underneath mariculture sites were lower than the corresponding reference sites. The % MeHg in sediments was negatively correlated (r = -0.579, p < 0.05) with organic matter (OM) content among all sites, indicating that OM may have inhibited Hg methylation in surface sediments. Three mariculture fish species were collected from each mariculture site, including red snapper (Lutjanus campechanus), orange-spotted grouper (Epinephelus coioides) and snubnose pompano (Trachinotus blochii). The average MeHg concentration in fish muscle was 75 μg kg⁻¹ (wet weight), and the dietary intake of MeHg through fish consumption for Hong Kong residents was 0.37 μg kg⁻¹ week⁻¹, which was lower than the corresponding WHO limits (500 μg kg⁻¹ and 1.6 μg kg⁻¹ week⁻¹).

Characterization of PAHs in surface sediments of aquaculture farms around the Pearl River Delta.

The distribution and source apportionment of polycyclic aromatic hydrocarbons (PAHs) in surface sediments of freshwater fishponds and mariculture rafts around the Pearl River Delta (PRD) were investigated. Twenty-one sample sites were chosen, consisting of fifteen freshwater fishponds and six mariculture rafts. The total PAH ( summation SigmaPAH) concentrations measured in all the sediment samples ranged from 52.7 to 717 ng g(-1), with mean of 184 ng g(-1). The marine sediment samples contained higher levels of summation SigmaPAH, high molecular weight (HMW) PAHs and carcinogenic PAHs than freshwater sediments (p<0.05). Principal component analysis/multiple linear regression analysis (PCA/MLRA) indicated that the source of PAHs in freshwater sediment was mainly derived from mixed combustion of coal, wood, and vehicle emission (66.2%). Vehicle emissions and coal combustion were the main sources (52.5 and 47.5% of summation SigmaPAH, respectively) of PAHs contaminated in the marine sediments. The present study indicated that surface sediments of freshwater fishponds and mariculture rafts around the PRD were grossly contaminated by PAHs derived from combustion sources.

Enrichment of polycyclic aromatic hydrocarbons (PAHs) in mariculture sediments of Hong Kong

Surface and core sediments collected from six fish farms in Hong Kong and from reference sites were investigated for the enrichment and sources of polycyclic aromatic hydrocarbons (PAHs). Moderately high SigmaPAH16 levels (123-947 ng g(-1), mean: 450 ng g(-1)) were found in the surface aquaculture sediments. In comparison with the sediments from the reference sites, the average enrichment percentage of total organic carbon (TOC) and PAHs in surface sediments were 21.4 and 43.8%, respectively, and in the core sediments, 24.6 and 73.7%, respectively. Mathematical source apportionment analyses (i.e. isomer ratios, hierarchical cluster analysis, principal components analysis with multiple linear regression analysis) suggested a higher percentage of petrogenic sources in aquaculture sediments. The fish feeds might be the main source of the enriched PAHs in the aquaculture sediments. To our knowledge, this is the first study showing that PAHs in aquaculture sediments could be attributed to human aquaculture activities.

Effects of sulfate reducing bacteria and sulfate concentrations on mercury methylation in freshwater sediments.

Methylmercury (MeHg) is the most poisonous form of mercury (Hg) and it enters the human body primarily through consumption of Hg contaminated fish. Sulfate reducing bacteria (SRB) are major producers of MeHg in anoxic sediments. The dsrAB gene was isolated from freshwater fish pond sediments. Sequence analyses showed that the SRB in sediments was mainly composed of Desulfobulbus propionicus and Desulfovibrio vulgaris. The two species of SRB were cultured from freshwater sediments. The addition of inorganic Hg to these freshwater sediments caused an increase in MeHg concentrations at 30 days incubation. MeHg levels were sensitive to sulfate concentrations; a medium sulfate level (0.11 mg/g) produced higher levels than treatments lacking sulfate addition or when amended with 0.55 mg/g. Assessment of bacterial levels by PCR measurements of microbial DNA indicated that the MeHg levels were correlated with cell growth.

Mercury biomagnification in the aquaculture pond ecosystem in the Pearl River Delta.

This is the first study to investigate the rate of mercury (Hg) biomagnification in the aquaculture pond ecosystem of the Pearl River Delta (PRD), China, by analyzing total mercury (THg) and methyl mercury (MeHg) concentrations in various species of fish at different trophic levels (TLs). Species representing a gradient of trophic positions in the aquaculture pond food chains were chosen for analyzing THg and MeHg concentrations. In this study, there were two kinds of the aquaculture pond food chains: (1) omnivorous (fish feeds, zooplankton, grass carp [Ctenopharyngodon idellus], and bighead carp [Aristichthys nobilis]) and (2) predatory (zooplankton, mud carp [Cirrhina molitorella], and mandarin fish [Siniperca kneri]). Bighead carp and mandarin fish had the highest MeHg and THg concentrations, i.e., an order of magnitude higher than other species, in their respective food chains. More than 90% of the THg concentrations detected in bighead carp, mandarin fish, and mud carp were in the methylated form. In this study, %MeHg increased with TLs and MeHg concentrations, reflecting that MeHg is the dominant chemical species of Hg accumulated in higher concentrations in biota, especially biota associated with higher TLs in the food chains. The trophic magnification factors were 2.32 and 2.60 for MeHg and 1.94 and 2.03 for THg in omnivorous and predatory food chains, respectively, in PRD. Hg concentrations in fish tissue correlated to Hg levels in the ambient environment, and sediment seemed to be the major source for Hg accumulated in fish. In addition, feeding habit also affected Hg accumulation in different fish species. Four significant linear relationships were obtained between log-THg and δ15N and between log-MeHg and δ15N. The slope of the regression equations, as biomagnification power, was smaller in magnitude compared with those reported for temperate and arctic marine and freshwater ecosystems, indicating that THg and MeHg biomagnifications were lower in this PRD subtropical aquaculture pond ecosystem. This was probably due to low Hg bioavailability at lower TLs as well as individual feeding behavior of fish.