Yuk Shan Wong

Spatial variation of heavy metals in surface sediments of Hong Kong mangrove swamps.

The degree of heavy metal contamination in the fine-grained (<63 microm) and sand-sized (2 mm-63 microm) fractions of surface sediments in 18 different mangrove swamps (144 random samples) in Hong Kong was examined. Higher concentrations of heavy metals were found in the fine-grained than the sand-sized fractions of the sediment; however, the differences between these two fractions became less significant when the swamp was more contaminated. The principal component analyses show that the 18 mangrove swamps, according to the median concentrations of total heavy metals, were clustered into four groups. The first group included three mangrove swamps in Deep Bay region which are seriously contaminated, with heavy metal concentrations in sediments around 80 microg g(-1) Cu, 240 microg g(-1) Zn, 40 microg g(-1) Cr, 30 microg g(-1) Ni, 3 microg g(-1) Cd and 80 microg g(-1) Pb. The second cluster, made up of another four swamps distributed in different geographical locations (two in Sai Kung district and two in Tolo region), also had elevated levels of Cu, Pb, Ni and Cr in the sediments. Field observation reveals that these seven stands received industrial, livestock and domestic sewage as well as pollution from mariculture activities, suggesting that anthropogenic input is the main source of heavy metal contamination in Hong Kong mangroves. The sediments from other mangrove swamps were relatively uncontaminated.

Contamination of polycyclic aromatic hydrocarbons in surface sediments of mangrove swamps

The concentrations of total polycyclic aromatic hydrocarbons (sigmaPAHs) and 15 individual PAH compounds in 20 surface sediments collected from four mangrove swamps in Hong Kong were analysed. sigmaPAH concentrations ranged from 356 to 11,098 ng g(-1) dry weight with mean and median values of 1992 and 1,142 ng g(-1), respectively. These values were significantly higher than those of marine bottom sediments of Hong Kong harbours, suggesting that more PAHs were accumulated in mangrove surface sediments. The concentrations of sigmaPAHs as well as individual PAH compound varied significantly among mangrove swamps. The swamps heavily polluted by livestock and industrial sewage, such as Ho Chung and Mai Po, had much higher concentrations of total PAHs and individual PAH than the other swamps. The PAH profiles were similar among four mangrove swamps, and were dominated by naphthalene (two-ring PAH), fluorene and phenanthrene (three-ring PAH). The mangrove sediments had higher percentages of low-molecular-weight PAHs. These indicated that PAHs in mangrove sediments might originate from oil or sewage contamination (petrogenic input). Ratio values of specific PAH compounds such as phenanthrene/anthracene and fluoranthene/ pyrene, were calculated to evaluate the possible source of PAH contamination in mangrove sediments. These ratios varied among samples, suggesting that mangrove sediments might have a mixed pattern of pyrolytic and petrogenic inputs of PAHs. Sediments collected from Ho Chung mangrove swamp appeared to be more dominated by pyrolytic input while those from Tolo showed strong petrogenic contamination.

Effect of algal density on nutrient removal from primary settled wastewater

Laboratory scale batch experiments were employed to examine the effect of initial inoculum sizes of algae on their activities and reduction of nutrients form the primary settled municipal sewage. The microalgae, Chlorella vulgaris, with four initial inoculum sizes, namely ‘superconcentrated’ (1 × 107 cells ml−1) ‘concentrated’ (5 × 106 cells ml−1), ‘medium’ (1 × 106 cells ml−1) and ‘low’ (5 × 105 cells ml−1), were applied to primarily treated sewage. The cell number and chlorophyll content of all algal cultures increased gradually in the first week and the rates of increase were similar in all cultures. During the experimental period, even in the ‘superconcentrated’ cultures, the self-shading problem was not found probably due to good mixing. At the end of the 10 days study, over 90% NH4+ —N and 80% PO43—P were removed from wastewater in all cultures except the ‘low’ one. The residual concentrations of NH4+—N, PO43—P, TKN and total P in wastewater were negatively correlated with the cell numbers and chlorophyll content of the cultures. These results indicated that the efficiency of reducing wastewater-borne nutrients by an algal system was directly related to the physiological activity and growth of the Chlorella cells which in turn were affected by the initial inoculum size. The super-concentrated culture with an initial algal density of 1 × 107 cells ml1 seemed to be more beneficial as this treatment achieved satisfactory nutrient removal within 7 days instead of 10. The sharp initial reduction of COD (>50%) and total organic-N (>60%) was not related to the algal number or chlorophyll content, the performance in the control (without algal cells) was similar to that of the algal cultures. This suggests that the removal of COD and TON was mainly due to the metabolism of the indigenous bacteria. Under the open system, the interaction between algal and bacterial cells was significant which could enhance the simultaneous removal of N, P and organic matter from primary settled sewage.

Effect of immobilized microalgal bead concentrations on wastewater nutrient removal

A unicellular green microalga, Chlorella vulgaris, entrapped in calcium alginate as algal beads were employed to remove nutrients (N and P) from simulated settled domestic wastewater. A significantly higher nutrient reduction was found in bioreactors containing algal beads (at concentrations ranging from 4 to 20 beads ml(-1) wastewater) than the blank alginate beads (without algae). A complete removal of NH(4)(+)-N and around 95% reduction of PO(4)(3-)-P was achieved within 24 h of treatment in bioreactors having the optimal algal bead concentration (12 beads ml(-1), equivalent to 1:3 algal beads:wastewater, v/v). The NH(4)(+)-N removal was significantly lower at low (around 4 beads ml(-1)) and high (>15 beads ml(-1)) algal bead concentrations. On the other hand, the effect of bead concentration on phosphate removal was less obvious, and bead concentrations ranging from 8 to 20 beads ml(-1) showed comparable percentages of phosphate reduction. Algal uptake and adsorption on alginate gels were found to be the major processes involved in the removal of N and phosphate in the present study. In addition, NH(4)(+)-N could be lost via ammonia volatilization while PO(4)(3-)-P was removed by chemical precipitation, as alkaline pH was recorded in the immobilized microalgal treatment system.

Retention and distribution of heavy metals in mangrove soils receiving wastewater

The distribution and chemical fractionation of heavy metals retained in mangrove soils receiving wastewater were examined by soil column leaching experiments. The columns, filled with mangrove soils collected from two swamps in Hong Kong and the Peoples Republic of China, were irrigated three times a week for 150 days with synthetic wastewater containing 4 mg l(-1) Cu, 20 mg l(-1) Zn, 20 mg l(-1) Mn and 0.4 mg l(-1) Cd. Soil columns leached with artificial seawater (without any heavy metals) were used as the control. At the end of the leaching experiments, soil samples from each column were divided into five layers according to its depth viz. 0-1, 1-3, 3-5, 5-10 and > 10 cm, and analyzed for total and extractable heavy metal content. The fractionation of heavy metals in the surface soil samples (0-1 cm) was investigated by the sequential extraction technique. In both types of mangrove soils, the surface layer (0-1 cm) of the columns receiving wastewater had significantly higher concentrations of total Cu, Cd, Mn and Zn than the control. Concentrations declined significantly with soil depth. The proportion of exchangeable heavy metals in soils receiving wastewater was significantly higher than that found in the control, about 30% of the total heavy metals accumulated in the soil masses of the treated columns were extracted by ammonium acetate at pH 4. The sequential extraction results show that in native mangrove soils (the soils without any treatment), the major portion of Cu, Zn, Mn and Cd was associated with the residual and precipitated fractions with very low concentrations in more labile phases. However, in mangrove soils receiving wastewater, a significantly higher percentage of Mn, Zn and Cd was found in the water-soluble and exchangeable fractions. Copper appeared to be more strongly adsorbed in mangrove soils than the other heavy metals. In general, heavy metal accumulation in the surface mangrove soils collected in Hong Kong was higher than those in the PRC, although the metals in the latter soil type were more strongly bound. These findings suggest that whether the heavy metal retained in managrove soils becomes a secondary source or a permanent sink would depend on the kinds of heavy metals and also the types of mangrove soils.

Growth and physiological responses of two mangrove species (Bruguiera gymnorrhiza and Kandelia candel) to waterlogging

Effects of duration of waterlogging on growth and physiological responses of two mangrove species, Bruguiera gymnorrhiza and Kandelia candel, were investigated. The relative growth rate of B. gymnorrhiza decreased significantly with waterlogged time, with the highest value found for drained plants and the lowest in plants under 12 weeks waterlogging. On the contrary, no significant difference was found between waterlogged and drained K. candel plants. The shoot to root biomass ratio of K. candel increased when subjected to 8 or 12 weeks waterlogging but little change was recorded in B. gymnorrhiza, indicating a shift in biomass allocation from roots to shoots in K. candel under prolonged waterlogging but not in B. gymnorrhiza. These different growth responses between the two mangrove species supported the hypothesis that K. candel is more tolerant to waterlogging than B. gymnorrhiza. Under 12 weeks waterlogged treatment, root oxidase activity significantly decreased in B. gymnorrhiza but increased in K. candel. Chlorophyll contents of K. candel increased more rapidly in response to waterlogging than B. gymnorrhiza. Activities of both peroxidase and superoxide dismutase increased significantly in leaves of K. candel when the waterlogging period was longer than 8 weeks, while only the peroxidase activity of B. gymnorrhiza showed a significant increase, indicating that K. candel had stronger resistance to the oxidant damage resulting from waterlogging. These physiological indicators further supported the hypothesis that K. candel is more tolerant to waterlogging than B. gymnorrhiza.

Wastewater Nutrients (N and P) Removal by Carrageenan and Alginate Immobilized Chlorella Vulgaris

Excess nutrients (N and P) discharged from sewage treatment plants has been identified as one of the major pollution point sources leading to eutrophication in the coastal water. Microalgae have been demonstrated as excellent nutrients stripper from the sewage. However, its application is limited by its harvesting problem at the end of the treatment process. In this study, the unicellular green alga Chlorella vulgaris was immobilized in two polysaccharide matrices, namely carrageenan and alginate, in form of spherical beads and was used to treat primary settled domestic wastewater in order to eliminate the harvesting limitation. Growth of the algal cells in the matrices was not inhibited and the exponential growth rates were 0.420, 0.441 and 0.417 day−1 for the free, carrageenan and alginate immobilized cells, respectively. Although algal cells in the carrageenan and alginate matrix exhibited a longer lag period (2 and 3 days, respectively) than the free cells (1 day), the immobilized cells were more meta...

Effect of ammonia concentrations on growth of Chlorella vulgaris and nitrogen removal from media

Abstract The effect of ammonia concentration on growth and physiology of a unicellular green alga, Chlorella vulgaris , was investigated. Growth occurred in all ammonia concentrations examined (10–1000 mg N l −1 ) although less growth was found in cultures containing either very low (10 mg N l −1 ) or very high (750 and 1000 mg N l −1 ) ammonia concentrations. At NH 3 -N concentrations between 20 and 250 mg N l −1 , there were no significant differences in specific growth rates and maximal cell densities attained. Growth in these media was comparable to growth in the commercial Bristol medium which contains nitrate as the nitrogen source. Higher chlorophyll and protein contents were found in cell cultures with higher ammonia concentrations. The algal growth was accompanied by a decrease in nitrogen content in the medium, indicating that nitrogen removal was due to algal uptake and assimilation. In cultures containing nitrogen lower than 40 mg N l −1 , nitrogen was completely removed at the end of the cultivation period. Over 95% ammonium removal was found in cultures containing 40–80 mg N l −1 . The percentage N reduction decreased with the initial N concentrations in cultures containing more than 80 mg N l −1 .

Nickel biosorption by two chlorella species, C. Vulgaris (a commercial species) and C. Miniata (a local isolate)

Abstract The present study compared the efficiency of two unicellular green algae, Chlorella vulgaris (a commercial species from Carolina Biological Supplies Company) and WW1 (an indigenous species isolated from a local sewage treatment works, tentatively identified as Chlorella miniata ) in removing Ni 2+ from nickel solutions with concentration ranges similar to that in electroplating effluents. The Ni 2+ removal efficiency of C. vulgaris (around 33–41%) was significantly lower than that of WW1 (more than 99%) in nickel solutions from 10 to 40 μg ml −1 . The maximum Ni 2+ uptake by C. vulgaris and WW1 under the present batch experiment was 641.76 and 1367.62 μg g −1 , respectively. According to Langmuir adsorption isotherms the nickel adsorption capacity of WW1 (2985.07 μg g −1 ) was two times greater than that of C. vulgaris (1282.05 μg g −1 ). These results demonstrated that WW1 was a more powerful Ni 2+ biosorbent than C. vulgaris . In both species, most Ni 2+ in solution was sequestered by the algal cells within the first few minutes of treatment. The cellular Ni 2+ concentration increased with the concentrations of nickel in solution. After treating Ni-containing wastewater for 24 h, both species were still capable of cell division, but the growth rate was reduced in proportion to the concentrations of nickel in the wastewaters.

Preliminary study on biodegradation of phenanthrene by bacteria isolated from mangrove sediments in Hong Kong.

Elevated concentrations of polycyclic aromatic hydrocarbons (PAHs) have been found in mangrove sediments due to anthropogenic pollution, and microbial degradation has been suggested as the best way to remove PAHs from contaminated sediments. The degradation of phenanthrene, a model PAH compound by bacteria, either the enriched mixed culture or individual isolate isolated from surface mangrove sediments was examined. The effects of salinity, initial phenanthrene concentrations and the addition of glucose on biodegradation potential were also investigated. Results show that surface sediments collected from four mangrove swamps in Hong Kong had different degree of PAH contamination and had different indigenous phenanthrene-degrading bacterial consortia. The enriched bacteria could use phenanthrene as the sole carbon source for growth and degrade this PAH compound accordingly. A significant positive relationship was found between bacterial growth and percentages of phenanthrene degradation. The phenanthrene biodegradation ability of the enriched mixed bacterial culture was not related to the degree of PAH contamination in surface sediments. The growth and biodegradation percentages of the enriched mixed culture were not higher than that of the individual isolate especially at low salinity (0 and 10 ppt). High salinity (35 ppt) inhibited growth and biodegradation of phenanthrene of a bacterial isolate but less inhibitory effect was found on the mixed culture. The inhibitory effects of salinity could be reduced with the addition of glucose.