Nanocrystalline silver supported on activated carbon matrix from hydrosol: antibacterial mechanism under prolonged incubation conditions.

Nanocrystalline Silver-supported activated carbon (AC) was fabricated by directly loading silver nanoparticles into the porous AC matrix from a preformed nanosilver hydrosol. Silver-AC composites were also synthesized using a conventional thermal impregnation method. While XRD calculation indicated the presence of Ag crystallites in nanometer range, silver nanoparticle hydrosol-treated AC having the finest crystallite size CS (< 14.4 nm), SEM images clearly revealed that Ag crystals coalesced significantly with increasing temperature resulting in much larger particle size in thermally impregnated silver-AC composities. To clarify the antibacterial mechanism of silver nanoparticles impregnated into AC under prolonged incubation conditions the antibacterial activity was investigated against Gram-negative Escherichia coli. The kinetics of bacterial inactivation, in presence of hydroxyl radical (*OH) scavengers, and superoxide anion radical (*O2-) inducer suggest the contribution of the reactive oxygen species (ROS) to antibacterial effect. However, these ROS scavengers did not show any inhibition of bactericidal activity after approximately 1 h, suggesting that generated ROS are responsible for E. coli inactivation only during the initial 1 h of the incubation time. This study clearly indicates the plausible implication of eluted Ag+ as major lethal species responsible for the E. coli inactivation over extended process time. The antibacterial process was found to be highly promoted at higher temperature which was ascribed to the enhanced ROS formation and Ag+ elution at higher temperature. SEM images revealed considerable differences in the morphology of E. coli cells contacting with the virgin AC and that contacting with silver-supported AC. The strong antibacterial ability of formaldehyde-modified silver-supported AC further provided the indirect evidences for catalytic oxidation by ROS, and for the synergistic antibacterial effects of nanocrystalline silver and adsorbed formaldehyde. Comparison of the antibacterial activities of the silver-supported materials prepared by silver colloid deposition and by conventional thermal impregnation technique indicates that former is more efficient in controlling microorganism.

Removal of E. coli from water using surface-modified activated carbon filter media and its performance over an extended use.

Modification of activated carbon (AC) by aluminum hydroxychloride (AHC), and diatomaceous earth by zinc hydroxide changed the zeta potentials of these filter media from negative to positive. The modification method is amenable to room temperature, and eliminates the essential requirement of strong base treatment for making metal hydroxide coated filter media. Solid-state MAS 27Al NMR spectra suggested the presence of Al13-mer in the AHC-treated AC. AHC-modified AC samples were further treated with silver halide, and two antibacterial compounds to prevent microbial growth on filter media. In situ precipitation of silver bromide on AC resulted in formation of nanosized AgBr crystals. Bacteria removal performances of the modified media were tested in columns. For the first time, we demonstrated that only 30 g of either AHC-treated AC (60 x 200 mesh) or nano AgBr supported AC could provide >6 log E. coli removal over approximately 1000 L when the input water had a bacterial load of 10(7) CFU/mL. The filter media were robust enough to perform even when water was passed at superficial velocities 3-10 times the typical velocity (6 cm/min) of water treatment processes. Metal leaching from the modified media was found to be less than the USEPA specified Maximum Contaminant Level.

Arsenic contamination in food-chain: transfer of arsenic into food materials through groundwater irrigation.

Arsenic contamination in groundwater in Bangladesh has become an additional concern vis-à-vis its use for irrigation purposes. Even if arsenic-safe drinking-water is assured, the question of irrigating soils with arsenic-laden groundwater will continue for years to come. Immediate attention should be given to assess the possibility of accumulating arsenic in soils through irrigation-water and its subsequent entry into the food-chain through various food crops and fodders. With this possibility in mind, arsenic content of 2,500 water, soil and vegetable samples from arsenic-affected and arsenic-unaffected areas were analyzed during 1999-2004. Other sources of foods and fodders were also analyzed. Irrigating a rice field with groundwater containing 0.55 mg/L of arsenic with a water requirement of 1,000 mm results in an estimated addition of 5.5 kg of arsenic per ha per annum. Concentration of arsenic as high as 80 mg per kg of soil was found in an area receiving arsenic-contaminated irrigation. A comparison of results from affected and unaffected areas revealed that some commonly-grown vegetables, which would usually be suitable as good sources of nourishment, accumulate substantially-elevated amounts of arsenic. For example, more than 150 mg/kg of arsenic has been found to be accumulated in arum (kochu) vegetable. Implications of arsenic ingested in vegetables and other food materials are discussed in the paper.