Selective leaching of valuable metals from waste printed circuit boards

This study was carried out to recover valuable metals from the printed circuit boards (PCBs) of waste computers. PCB samples were crushed to smaller than 1 mm by a shredder and initially separated into 30% conducting and 70% nonconducting materials by an electrostatic separator. The conducting materials, which contained the valuable metals, were then used as the feed material for magnetic separation, where it was found that 42% of the conducting materials were magnetic and 58% were nonmagnetic. Leaching of the nonmagnetic component using 2 M H2SO4 and 0.2 M H2O2 at 85 degrees C for 12 hr resulted in greater than 95% extraction of Cu, Fe, Zn, Ni, and Al. Au and Ag were extracted at 40 degrees C with a leaching solution of 0.2 M (NH4)2S2O3, 0.02 M CuSO4, and 0.4 M NH4OH, which resulted in recovery of more than 95% of the Au within 48 hr and 100% of the Ag within 24 hr. The residues were next reacted with a 2 M NaCl solution to leach out Pb, which took place within 2 hr at room temperature.     

ON TAXATION AND FIRMS CHOICE OF WASTE WATER TREATMENT TECHNOLOGY1

ABSTRACT: The interaction between the level of taxation and the firms accruing choice of treatment technology is discussed. It is shown that there is a risk of overtaxation, that is, simply increasing taxation may fail to improve environmental quality but only increase production costs and thus consumer prices.     

pH-gradient real-time aeration control for nitritation community selection in a non-porous hollow fiber membrane biofilm reactor (MBfR) with dilute wastewater

Nitritation (ammonium to nitrite) as a pre-treatment of Anammox (anaerobic ammonium oxidation) is a key step for an energy-efficient nitrogen-removal alternative from dilute wastewaters, e.g. anaerobically-treated sewage, with which limited study has achieved sustainable nitritation at ambient temperature and short hydraulic retention times. To this end, pH-gradient real-time aeration control in an oxygen-based membrane biofilm reactor was observed at 20 °C in the sequencing batch mode. An optimum oxygen supply via diffusion for ammonium-oxidizing bacteria (AOB) was established, but nitrite-oxidizing bacteria (NOB) could be inhibited. The system achieved nitrite accumulation efficiencies varying from 88% to 94% with the aeration control. Mass balance and rate performance analyses indicate that this aeration control is able to supply an oxygen rate of 1.5 mol O₂ mol^?1 ammonium fed, the benchmark oxygenation rate based on stoichiometry for nitritation community selection. Microbial analyses confirmed AOB prevalence with NOB inhibition under this aeration control.     

Seasonal characteristics of chemically apportioned aerosol optical properties at Seoul and Gosan,Korea

Seasonal variations of aerosol optical properties in Seoul (polluted urban site) and Gosan (coastal background site), Korea, with an emphasis on the relative humidity were investigated using ground-based aerosol measurements and optical model calculations. The mass fraction of elemental carbon was 9–20%, but the optical contribution of these particles to light extinction was higher, up to 33–55% in Seoul. In Gosan, the contribution of non-sea-salt water-soluble aerosols on extinction was 81–93% due to the high mass fraction of these particles. Based on daily MODIS datasets, our analysis showed that the aerosol optical depths at Seoul and Gosan were highest in spring due to the influence of dust particles. The aerosol water content at Gosan, calculated using a thermodynamic equilibrium model, was higher than that at Seoul; this was attributed to the high relative humidity and high fraction of water-soluble aerosols at Gosan. At Seoul, despite abundant water vapors in summer, the possibility of hygroscopic growth of water-soluble aerosols was not more significant than that at Gosan.     

A validated 2-D diffusion–advection model for prediction of drift from ground boom sprayers

Correct field drift prediction is a key element in environmental risk assessment of spraying applications. A reduced order drift prediction model based on the diffusion–advection equation is presented. It allows fast assessment of the drift potential of specific ground boom applications under specific environmental wind conditions that obey the logarithmic wind profile. The model was calibrated based on simulations with a validated Computational Fluid Dynamics (CFD) model. Validation of both models against 38 carefully conducted field experiments is successfully performed for distances up to 20 m from the field edge, for spraying on flat pasture land. The reduced order model succeeded in correct drift predictions for different nozzle types, wind velocities, boom heights and spray pressures. It used 4 parameters representing the physical aspects of the drift cloud; the height of the cloud at the field edge, the mass flux crossing the field edge, the settling velocity of the droplets and the turbulence. For the parameter set and range considered, it is demonstrated for the first time that the effect of the droplet diameter distribution of the different nozzle types on the amount of deposition spray drift can be evaluated by a single parameter, i.e., the volume fraction of droplets with a diameter smaller than 191 μm. The reduced order model can be solved more than 4 orders of magnitude faster than the comprehensive CFD model.     

Photocatalytic bacterial inactivation by polyoxometalates

The photocatalytic inactivation (PCI) of Escherichia coli (Gram-negative) and Bacillus subtilis (Gram-positive) was performed using polyoxometalate (POM) as a homogeneous photocatalyst and compared with that of heterogeneous TiO₂ photocatalyst. Aqueous suspensions of the microorganisms (10⁷–10⁸ cfu ml⁻¹) and POM (or TiO₂) were irradiated with black light lamps. The POM-PCI was faster than (or comparable to) TiO₂-PCI under the experimental conditions employed in this study. The relative efficiency of POM-PCI was species-dependent. Among three POMs (H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, and H₄SiW₁₂O₄₀) tested in this study, the inactivation of E. coli was fastest with H₄SiW₁₂O₄₀ while that of B. subtilis was the most efficient with H₃PW₁₂O₄₀. Although the biocidal action of TiO₂ photocatalyst has been commonly ascribed to the role of photogenerated reactive oxygen species such as hydroxyl radicals and superoxides, the cell death mechanism with POM seems to be different from TiO₂-PCI. While TiO₂ caused the cell membrane disruption, POM did not induce the cell lysis. When methanol was added to the POM solution, not only the PCI of E. coli was enhanced (contrary to the case of TiO₂-PCI) but also the dark inactivation was observed. This was ascribed to the in situ production of formaldehyde from the oxidation of methanol. The interesting biocidal property of POM photocatalyst might be utilized as a potential disinfectant technology.     

Improving the activity of Mn/TiO2 catalysts through control of the pH and valence state of Mn during their preparation

In this study, the authors investigated the influence of the valence state of Mn on the efficacy of selective catalytic reduction using a Mn-based catalyst. The nitrogen oxides (NOx) conversion rate of the catalyst was found to be dependent on the type of TiO2 support employed and on the temperature, as the catalyst showed an excellent conversion of > 80% at a space velocity of 60,000 hr(-1) when the temperature was above 200 degrees C. Brunauer-Emmett-Teller, X-ray diffraction, and X-ray photoelectron spectroscopy analyses confirmed that catalyst displaying the highest activity contained the Mn4+ species and that its valence state was highly dependent on the pH during the catalyst preparation.     

Degradation rates of CFC-11, CFC-12 and CFC-113 in anoxic shallow aquifers of Araihazar, Bangladesh

Chlorofluorocarbons CFC-11 (CCl(3)F), CFC-12 (CCl(2)F(2)), and CFC-113 (CCl(2)F-CClF(2)) are used in hydrology as transient tracers under the assumption of conservative behavior in the unsaturated and saturated soil zones. However, laboratory and field studies have shown that these compounds are not stable under anaerobic conditions. To determine the degradation rates of CFCs in a tropical environment, atmospheric air, unsaturated zone soil gas, and anoxic groundwater samples were collected in Araihazar upazila, Bangladesh. Observed CFC concentrations in both soil gas and groundwater were significantly below those expected from atmospheric levels. The CFC deficits in the unsaturated zone can be explained by gas exchange with groundwater undersaturated in CFCs. The CFC deficits observed in (3)H/(3)He dated groundwater were used to estimate degradation rates in the saturated zone. The results show that CFCs are degraded to the point where practically no (<5%) CFC-11, CFC-12, or CFC-113 remains in groundwater with (3)H/(3)He ages above 10 yr. In groundwater sampled at our site CFC-11 and CFC-12 appear to degrade at similar rates with estimated degradation rates ranging from approximately 0.25 yr(-1) to approximately 6 yr(-1). Degradation rates increased as a function of reducing conditions. This indicates that CFC dating of groundwater in regions of humid tropical climate has to be carried out with great caution.