467482 Disinfection of Water Using Silver and Copper Nanoparticle Impregnated Activated Carbon

Wednesday, November 16, 2016: 2:20 PM
Golden Gate 8 (Hilton San Francisco Union Square)
Pritam Biswas and Rajdip Bandyopadhyaya, Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai, India

Access to safe drinking water is one of the biggest challenges across the globe. In this context, activated carbon is widely used as a water purification media for producing drinking water, which although removing most of the contaminants (responsible for taste, colour, odour, organics, heavy metals etc.), however is incapable of killing pathogens. To mitigate this, antibacterial properties of silver and copper nanoparticles (Ag-NPs and Cu-NPs) have been utilized. Therefore, the objective of this research is to combine both purification and disinfection effects of activated carbon and nanoparticles together and thereby produce drinking water in a continuous, flow-column set-up.

To this end, first we synthesized Ag-NPs by UV reduction of aqueous silver nitrate solution, in presence of tri-sodium citrate. The latter acts both as a reducing and a stabilizing agent. Following this, Ag-NPs were selectively impregnated on the external surface of plasma treated AC granules. This hybrid is referred to as Ag-AC hybrid, having 0.8 wt% of Ag-NPs of 28 nm mean size. Subsequently, microbial cell-killing experiments were performed in two set-ups: (i) shake flask (called batch experiments) and flow-column (called continuous-mode). We have used E. coli K12 (MTCC 1302) as a model organism (as it is typical of contaminated drinking water), always at a concentration of 104 CFU cells/ml in the inlet water. Batch results using 8 mg Ag-AC hybrid/ml of cell suspension showed that, all cells were killed within 25 minutes of contact time. Maintaining same E. coli concentration and Ag-NP loading percentage in Ag-AC (as in the batch experiment), a glass column (8 cm diameter and 25 cm in height) packed with Ag-AC was used for monitoring cell-killing performance over a long duration. Again, inlet water having 104 CFU of E. coli/ml could be completely disinfected to produce treated, outlet water having zero cell count, with a contact time of 23 minutes, very close to the designed value based on the previous batch cell-killing kinetics. Therefore, a continuous throughput of treating 1.62 L of contaminated water per hour could be maintained for at least up to 16 days, beyond which experiments were not conducted presently. Moreover, the Ag concentration in the outlet water reached only up to a maximum of 29.8 µg/L at steady state, which is well within the recommended maximum Ag concentration limit of 100 µg/L for drinking water (EPA). Thus these results demonstrate the viability of such a column-design to produce decontaminated water at a reasonable rate per hour, over a long period and with minimal loss of Ag in the treated water.

Cu-NPs being reported for its antibacterial activity too, are actually cheaper to make than Ag-NPs. For some of the Ag-resistant microorganisms, Cu-NP also shows better activity compared to Ag-NP. Thus, Cu-NPs have been synthesized using the thermal reduction method where L-ascorbic acid reacts with cupric chloride dihydrate to form Cu-NPs. In this green route, L-ascorbic acid works as a reducing agent. Cu-NPs of mean diameter 5.17 nm were synthesized and subsequently impregnated on plasma treated AC by wet impregnation. The Cu-AC showed a copper loading of 2.1 wt. %. Disinfection studies were performed using these Cu-AC, again in both batch and continuous modes, using the same wild type model microorganism E. coli K12 (MTCC 1302), starting with the same initial cell concentration of 104 CFU/ml. Similar to Ag-AC, on using 8 mg/ml of Cu-AC in the batch experiment, zero cell count was achieved in only 10 minutes. Based on this batch mode cell-killing data, the same glass column (but packed with only Cu-AC now) was used for continuous, flow experiments. When 104 CFU/ml of E. colicells were introduced in the inlet water to this column, zero cell count in the outlet water was achieved in a contact time of 9 minutes, giving a maximum treated water flow rate of 3.77 L/h. Therefore, compared to 1.62 L/h of water in case of Ag-AC, a much higher amount of 3.77 L/h of water could be produced in the same column with Cu-AC. Simultaneously, in steady state continuous flow-column experiments, the maximum copper concentration in the outlet water was around 83.4 µg/L, which is far below the upper recommended limit of 1000 µg/L (EPA). Thus, Cu-AC shows better cell-killing performance compared to Ag-AC. This is primarily because the Cu loading (2.1 wt%) is higher in Cu-AC, compared to Ag loading of 0.8 wt.% in Ag-AC. Moreover, the steady state release concentration of Cu was much higher (83.4 µg/L) compared to Ag release concentration (29.8 µg/L) from Ag-AC. Therefore, Cu-AC shows higher antibacterial activity compared to Ag-AC and reaches zero cell count within a shorter contact time of 9 minutes in the continuous flow-column, in contrast to 23 minutes required for Ag-AC column to achieve zero cell count.

As Cu-AC shows better antibacterial activity compared to Ag-AC, further experiments are being pursued now with a mixture of Ag-AC and Cu-AC. Both these samples are mixed at 1:1 wt. ratio, that is, Ag-AC having 0.8 wt% Ag loading and Cu-AC having 2.1 wt% Cu loading. These have also been further used in both batch and continuous, flow-column experiments for cell-killing. Starting with the same 104 CFU/ml E. coli, zero cell count has been achieved in 8 minutes, using 8 mg/ml of the hybrid mixture. This implies that Ag-AC and Cu-AC combination potentially gives a synergistic effect in terms of E. colicell-killing and is hence superior to the usage of only Ag-AC or only Cu-AC. Currently, further continuous column experiments are in progress, using the combination of Ag-AC and Cu-AC to optimize its performance towards the fastest possible cell-killing kinetics, which will maximize the production rate of drinking water.

Hence, water disinfection for potable quality (drinking) water (zero E. coli count with < 100 µg/L Ag and < 1000 µg/L Cu) could be achieved in a continuous manner at a large flow rate of nearly 4 L/h over a long duration, by using mixed packing of Ag-AC and Cu-AC.

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