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 People's 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.     

Analysis of air toxics emission inventory: inhalation toxicity-based ranking

Air toxics emission inventories play an important role in air quality regulatory activities. Recently, Minnesota Pollution Control Agency (MPCA) staff compiled a comprehensive air toxics emission inventory for 1996. While acquiring data on the mass of emissions is a necessary first step, equally important is developing information on the potential toxicity of the emitted pollutants. To account for the toxicity of the pollutants in the emission inventory, inhalation health benchmarks for acute effects, chronic effects, and cancer were used to weight the mass of emissions. The 1996 Minnesota emissions inventory results were ranked by mass of emissions and by an index comprised of emissions divided by health benchmarks. The results show that six of eight pollutants ranked highest by toxicity were also the pollutants of concern indicated in environmental monitoring data and modeling data. Monitoring data and modeling results did not show high impacts of the other two pollutants that were identified by the toxicity-based emission ranking method. The biggest limitation in this method is the lack of health benchmark values for many pollutants. Despite uncertainties and limited information, this analysis provides useful information for further targeting pollutants and source categories for control.     

An analytical method to determine Henry's law constant for selected volatile organic compounds at concentrations and temperatures corresponding to tap water use

Henry's law constants (H) are needed to model human exposure to Volatile Organic Compounds (VOCs) in indoor air resulting from the use of tap water. This paper presents an experimental method to determine Hs for several common tap water pollutants at concentrations and temperatures used in household water. For 5 VOCs Henry's law constants were obtained simultaneously over the 25 degrees C to 45 degrees C temperature range, providing data on H beyond the currently available data (up to 35 degrees C). Henry's law constants were obtained as the ratio of equilibrium concentrations of VOCs in air and water, using simultaneous sampling from sealed bottles kept at constant temperatures. Air and water samples were concentrated by a purge-and-trap method, thermally desorbed from a Tenax trap, and analyzed with a gas chromatograph with an electron capture detector (GC-ECD). Experimental results agreed well with available literature data.     

Role of loosely bound humic substances and humin in the bioavailability of phenanthrene aged in soil

A study was conducted to determine a possible role of loosely bound humic substances (i.e., humic and fulvic acids) in bioavailability of aged phenanthrene with time. In this study, long-term residence of phenanthrene in soil is defined as aging or sequestration, and the effect was determined by the declined bioavailability to bacteria of the polycyclic aromatic hydrocarbon with increased residence time. After 1, 7, and 100 days of aging of phenanthrene in Lima loam, about 90-93% of initial phenanthrene was recovered from the humin-mineral fraction of Lima loam whereas less than 12% was found in humic and fulvic acids of the same soil. Mineralization rates of phenanthrene aged in the humin-mineral fraction significantly decreased with time by the test bacterium P5-2. In terms of extents of mineralization, the difference with time was not appreciable, but still significant at P<0.05. Additional decreases in the rates and extents of mineralization were observed with the whole soil (i.e. Lima loam) to which phenanthrene had been aged. Data suggest that major sequestration sites for phenanthrene may reside in the humin-mineral fraction, and probably humic and fulvic acids may act as a physico-chemical barrier to bacterial degradation so that the compound's bioavailability may be limited.     

Remedial strategies for municipal solid waste management in China

The purpose of this investigation is to evaluate the current status and to identify the problems of municipal solid waste (MSW) management in China to determine appropriate remedial strategies. This is the second of two papers proposed on this topic. Major problems or difficulties identified in MSW management in China include MSW land, air, and water pollution, commingled collection, poor administration, shortage of funds, lack of facilities, and problems of training and public awareness. In order to solve these problems and to improve MSW management in China, remedial strategies in three areas are recommended: institutional reform, technology development, and legislation and administrative improvement. The primary principle involved in institutional reform is unifying legislative responsibilities into one body and developing a market mechanism for handling MSW. Composting, landfills, and incineration should be equally developed in accordance with China's needs. The feasibility of developing technology to handle MSW in China is discussed. Also recommended is the establishment of sound regulatory systems, including a service fee system, a source separation system, and a training program. China is presently undergoing economic and institutional reform at the national and local levels. Results of this study will provide useful information on MSW management in China.     

Removal of arsenic(V) from aqueous solutions using iron-oxide-coated modified activated carbon.

Removal of arsenic(V) from aqueous solutions was evaluated with the following three different sorption materials: coal-based activated carbon 12 x 40 (activated carbon), iron(II) oxide (FeO)/activated carbon-H, and iron oxide. The apparent characteristics and physical chemistry performances of these adsorbents were investigated by X-ray diffraction, nitrogen adsorption, and scanning electronic microscope. Also, batch experiments for arsenic removal were performed, and the effects of pH value on arsenic(V) removal were studied. The results suggest that the main phases of the iron oxide surface are magnetite, maghemite, hematite, and goethite; fine and uniform iron oxide particles can cover activated carbon surfaces and affect the surface area or pore structures of activated carbon; adsorption kinetics obey a pseudo-first-order rate equation; and adsorption capacities of adsorbents are affected by the values of pH. The optimum value of pH for iron oxide lies in a narrow range between 4.0 and 5.5, and arsenic(V) removal by FeO/activated carbon-H is ideal and stable in the pH range 3 to 7, while activated carbon has the lowest adsorption capacity in the entire pH range. Also, the adsorption characteristics of FeO/activated carbon-H composites and virgin activated carbon match well the Langmuir adsorption model, while those of iron oxide fit well the Freundlich adsorption model.     

Acceleration of 13C-labelled photosynthate partitioning from leaves to panicles in rice plants exposed to chronic ozone at the reproductive stage

To investigate the effects of low (0.05 micromol/mol) and relatively low (0.10 micromol/mol) concentrations of ozone on photoassimilate partitioning, rice plants grown in a water culture were fed with (13)C-labelled carbon dioxide at the reproductive stage in an assimilation chamber with constant concentration of (12)CO(2) and (13)CO(2). Rice plants were exposed to ozone 4 weeks before and 3 weeks after (13)CO(2) feeding. The dry weight of whole plants decreased with increasing ozone concentration, whereas net photosynthetic rate (apparent CO(2) uptake per unit leaf area) was unaffected, compared with the control, at the time of (13)CO(2) feeding. Dry matter distribution into leaf sheaths and culms was reduced more than that into leaf blades by ozone exposure. Although panicle dry weight per plant was reduced by ozone, the percentage of panicle dry weight to the whole plant tended to increase considerably. Exposure to ozone accelerated translocation of (13)C from source leaves to other plant parts. Partitioning of (13)C to panicles and roots was higher under ozone treatment than in the control. Respiratory losses of fixed (13)C from plants tended to decrease under treatment with ozone. The increase in photoassimilate partitioning in panicles can be considered to be an acclimation response of rice plants to complete reproductive stage under the restricted biomass production caused by ozone.     

Treatment of atrazine by integrating photocatalytic and biological processes

This research examines the degradation of atrazine by photocatalytic oxidation (PCO) under different experimental conditions. Deisopropylatrazine, deethylatrazine and deethyldeisopropylatrazine were formed as major intermediates based on gas chromatography-mass spectrometry. The reaction mixture was found to be toxic towards two bioassays, i.e. the Microtox and amphipods survival tests even when atrazine was completely degraded by PCO within 2 h under optimized conditions. The results indicate that adding H2O2 could significantly enhance the degradation of atrazine by PCO. Ammeline, ammelide and cyanuric acid (CA) became the major intermediates/products as detected by high performance liquid chromatography from 6th to the 40th h of PCO treatment. After 72 h PCO treatment, only CA was detectable in the reaction mixture. Further degradation of CA was carried out by a newly isolated CA-degrading bacterium, Sphingomonas capsulata. The photochemical pretreatment integrated with microbial degradation lead to the complete degradation and detoxification of atrazine.     

Chemical and microbiological qualities of The East River (Dongjiang) water,with particular reference to drinking water supply in Hong Kong

Currently, about 80% of drinking water in Hong Kong is abstracted from The East River (Dongjiang) that is located in the mainland side of China. Literature records and monitoring results of 2000-2001 confirmed that the lower section of the Dongjiang had been contaminated by organic and inorganic pollutants. Statistical analyses showed that the increases of total cadmium, copper and zinc in the surface layer of sediment of Hong Kong reservoirs from 1994 to 2001 were positively correlated (significant at p<0.05) with those in the surface layer of sediments of the lower Dongjiang. Recent microbiological survey revealed that pathogens such as Salmonella spp., Vibro spp., Giardia lamblia and Cryptosporidium parvum appeared occasionally in water samples of the Dongjiang and Hong Kong reservoirs. While analytical results found that currently most of the heavy metals, trace organics and microbes were removed by the drinking water treatment plants in Hong Kong, the long-term health risk of drinking water contamination should not be overlooked. The Water Supplies Department of Hong Kong is recommended to intensify its water quality monitoring program to cover pathogenic bacteria and parasites in watercourses and reservoirs.     

Microbial activity during composting of anthracene-contaminated soil

Microbial activity of an anthracene-spiked soil mixed with kitchen waste during laboratory composting at 56-59 degrees C was studied using an in-vessel technology. The effect of old compost containing acclimated microorganisms on the composting efficiency was also investigated. Microbial succession, microbial enzyme activity, microbial diversity and anthracene removal rate were analyzed during 42 days of composting. The results demonstrated that inoculating with old compost increased the amounts of thermophilic microorganisms, but did not significantly increase anthracene removal. A microbial succession from mesophilic bacteria to thermophilic bacteria and thermophilic actinomycetes was observed during composting. Polyphenol oxidase activity decreased while catalase activity varied irregularly. Microbial diversity increased drastically when temperature elevated from 35 to 56 degrees C, but decreased when temperature maintained at 56-59 degrees C.