Sedimentary metals in developing tropical watersheds in relation to their urbanization intensities.

The spatial distribution of seven metals (Pb, Hg, Cd, Cr, Ni, Cu, and Zn) and As in the surface sediments from three major tributaries of a tropical urbanizing river network (i.e., Chao Phraya River, Thachin River, and Pasak River) was investigated. An obvious metal concentration gradient in response to the intensity of urbanization was found at inter-watershed and intra-watershed scales. Sediment Quality Guidelines (SQGs) exceedances of several metals (Pb, Cr, Ni, Cu, and Zn) and high ecological risk were primarily identified at the down streams of Chao Phraya and Thachin watersheds, where the social-economic center of the country with intensive industries is located. Stepwise multiple linear regression revealed significant correlations between studied metals and catchment land use pattern (with p < 0.0001 except for Ni and Cr). Particularly, urban land use showed remarkable effect on sedimentary Pb, Cd, Cu, and Zn loads with high coefficients over 0.65. The results of cluster analysis and principal component analysis indicated the dominated urban/industrial sources for Pb, Cd, Cu, and Zn, mixed natural and industrial sources for Cr and Ni, and diffuse sources for Hg and As in the watersheds, respectively.

Efficient photocatalytic oxidation of gaseous toluene in a bubbling reactor of water.

Conventional gas-solid photocatalytic oxidation (SPCO) of VOCs has drawbacks such as accumulation of intermediates and catalytic deactivation. In this study, gas-liquid photocatalytic oxidation (LPCO) was exploited to improve the catalytic activity and stability by continuously bubbling VOCs into water. Toluene and commercial TiO2 (P25) were chosen as the representative VOC pollutant and photocatalyst, respectively. Toluene removal efficiency in LPCO was about 6 times of that in conventional SPCO, and no intermediates were detected in the exhaust of LPCO probably due to its high degradation and mineralization rates. However, plentiful intermediates were identified by GC-MS and ITMS both in the gas outlet and on the surface of catalyst in SPCO, which may lead to photocatalytic deactivation. Moreover, LPCO exhibited superior catalytic activity towards typical soluble VOCs such as formaldehyde compared to SPCO. The soluble intermediates formed from toluene degradation can be easily removed by sustaining UV irradiation to avoid water pollution and the water after purification can be reused in LPCO. This study provides a novel gas-liquid photocatalytic oxidation to replace conventional gas-solid photocatalytic oxidation for the sake of better catalytic activity and fewer by-products.

Removal of trichloroethylene (TCE) contaminated soil using a two-stage anaerobic-aerobic composting technique.

The effect of organic carbon addition on remediation of trichloroethylene (TCE) contaminated clay soil was investigated using a two stage anaerobic-aerobic composting system. TCE removal rate and processes involved were determined. Uncontaminated clay soil was treated with composting materials (dried cow manure, rice husk and cane molasses) to represent carbon based treatments (5%, 10% and 20% OC). All treatments were spiked with TCE at 1,000 mg TCE/kg DW and incubated under anaerobic and mesophillic condition (35 degrees C) for 8 weeks followed by continuous aerobic condition for another 6 weeks. TCE dissipation, its metabolites and biogas composition were measured throughout the experimental period. Results show that TCE degradation depended upon the amount of organic carbon (OC) contained within the composting treatments/matrices. The highest TCE removal percentage (97%) and rate (75.06 micro Mole/kg DW/day) were obtained from a treatment of 10% OC composting matrices as compared to 87% and 27.75 micro Mole/kg DW/day for 20% OC, and 83% and 38.08 micro Mole/kg DW/day for soil control treatment. TCE removal rate was first order reaction kinetics. Highest degradation rate constant (k(1) = 0.035 day(- 1)) was also obtained from the 10% OC treatment, followed by 20% OC (k(1) = 0.026 day(- 1)) and 5% OC or soil control treatment (k(1) = 0.023 day(- 1)). The half-life was 20, 27 and 30 days, respectively. The overall results suggest that sequential two stages anaerobic-aerobic composting technique has potential for remediation of TCE in heavy texture soil, providing that easily biodegradable source of organic carbon is present.