(242g) Drinking Water Disinfection Using Silver Nanoparticle Impregnated Activated Carbon Hybrid

In most household drinking water purification systems, activated carbon (AC) is extensively used as a filtration medium. However, prolonged use results in biofouling, which can be eliminated by utilizing the biocidal effect of silver (Ag). In the present work, Ag nanoparticle has been impregnated in AC, for disinfection and purification of drinking water.

                     Ag nanoparticles were synthesized by three different methods, namely, UV reduction (A), UV reduction in presence of ammonia (B) and thermal reduction (C). Silver nitrate was used as the precursor, while tri-sodium citrate acts as both a reducing as also a stabilizing agent, to synthesize a stable Ag nanoparticle dispersion in water. Methods A, B and C showed mean primary nanoparticle diameter of 28.1 nm, 44.8 nm and 26.7 nm respectively, with standard deviation of 12.6 nm, 19.4 nm and 11.7 nm respectively. So, method B was not desirable due to the larger particle size. Between the other two, method C does not work at higher precursor concentration and results in aggregation and sedimentation of nanoparticles. So method A was selected, as usage of higher initial precursor concentration was possible, due to the slow rate of reduction, resulting in higher quantity of Ag particles being formed. The AC used in the present work is hydrophobic in nature, with a contact angel with water of 112°. To improve wettability of an aqueous Ag dispersion during the subsequent wet-impregnation step, the AC granules were therefore first plasma treated under a pure oxygen atmosphere. Consequent to this, FTIR spectroscopy showed a definitive increase in polar functional groups like, carboxyl, ketones on the outer surface of AC, which hence resulted in Ag impregnation mostly on the outer surface of AC, instead of the pore-interiors of AC, during the wet-impregnation step. FEG-SEM images confirmed the presence of Ag nanoparticles on the outer surface of the Ag-AC hybrid, with a loading of 0.78 wt. % of Ag in the Ag-AC hybrid.

      Disinfection studies were performed using these hybrid granules, in both batch and continuous mode, using a wild type model microorganism, namely, E. coli K12 (MTCC 1302), starting with an initial cell concentration of 10⁴ CFU/ml, typical of contaminated drinking water. Cells were counted using the plate-count method and the antibacterial activity of Ag-AC was confirmed by the zone of inhibition method. For a typical loading of 8 mg of Ag-AC hybrid per ml of cell suspension, batch experiments showed a first order rate constant of 0.21 min^-1 with respect to E. coli concentration. Subsequently, residence time required for continuous column-mode disinfection experiments were calculated based on batch kinetic data. To this end, a small column with a diameter of 1 cm and a height of 25 cm was used. Inlet water with a flow rate of 0.4 ml/min and having 10⁴ CFU/ml E. coli was successfully treated by Ag-AC, so that no cells in the outlet water stream was detected.

                      In general from such hybrids, there is a slow loss of Ag due to formation of Ag-ions in presence of water. Our batch study however showed only 0.31 wt% of Ag release out of the total Ag loaded in the Ag-AC hybrid, even after a 108 h of contact period with water. The overall release rate was thus significantly low and confirms the potential of a long period of usage for antibacterial activities of Ag-AC. This was possible as Ag was impregnated with a good adhesion on the plasma treated functionalized surface of the hybrid material. Thus, the high biocidal activity coupled with a low rate of Ag release establishes the Ag-AC hybrid of the present study as a good material for drinking water disinfection. Hence, water purification, disinfection and biofouling prevention can be potentially achieved in a single water-filter column, suitable for potable quality water. Further ongoing work is aimed towards this goal.