Base modified activated carbon-nanoparticle hybrid for water disinfection

Highlights

• Surface modification with KOH enhances Cu-NP impregnation on AC.

• Water disinfection achieved with KOH-Cu-AC at lower contact time (25 min).

• KOH-Cu-AC demonstrated for water disinfection in continuous flow-column.

• Continuous disinfection scaled up by keeping same residence time in column.

• Copper content in treated water from column is well-within permissible limit.

Abstract

Selective placement of antibacterial nanoparticles on a carbon filter-medium is crucial to achieve water disinfection. Functionalisation of activated carbon (AC) by aqueous KOH solution introduces polar hydrophilic groups, which facilitates embedding of copper nanoparticles on its external surface. The resulting material, KOH-Cu-AC, provides better contact with bacteria during the disinfection process, as bacteria preferentially remains on carbon surface during water flow. As a result, complete killing of E. coli (10⁴ CFU/ml) could be achieved in just 25 min of contact time, upon using 8 mg/ml of KOH-Cu-AC (0.4 wt% Cu loading on KOH-Cu-AC). Similar performances were achieved in both batch-mode shake flask and pump-driven, continuous flow-column operations. In fact, throughput of bacteria free water increased from 127.5 ml to 1.05 L by scaling up the column diameter (from 1 cm to 2.6 cm), maintaining the same 25 min flow-contact time. Finally, leaching measurement showed copper in the outlet water was (117 ppb), always well-within the US-EPA permissible limit (1000 ppb). Therefore, selective impregnation of Cu-NP on AC is feasible after a simple and inexpensive base treatment, which can be used to achieve E. coli free drinking water. This is implementable in a larger-scale water disinfection process, based on current scaling results.

Introduction

Clean and safe drinking water is one of the essential requirements to maintain a healthy life. As per WHO, by 2025, approximately half of the global population will live under water stress [1]. Moreover, water pollution is a serious concern to mankind in recent days and almost 2 billion human beings across the globe gets affected after consuming unsafe drinking water [2]. Outbreak of diarrheal diseases, gastroenteritis, and viral hepatitis are indications of poor water quality, leading to high morbidity and mortality among the children below five years of age [3,4].

One common approach to address this is by disinfection of drinking water. To this end, different physical and chemical methods including chlorination [5], ozonation [6,7], use of ultraviolet light [8] and reverse osmosis (RO) [9,10] have been tried. However, these have their own concerns. For example, chemical processes like chlorination and ozonation produce carcinogenic disinfection by-products (DBP), which cannot be separated from purified water [11,12]. Similarly, cost of disinfection goes up with the use of UV light and RO. For RO, one more serious drawback after the filtration step is the loss of essential minerals [13]. In addition, fouling of membrane with chemical and microbial contaminants is a significant issue for RO membranes [14], limiting membrane lifetime. Therefore, development of new disinfection methods to produce microorganism-free water at a low cost is still an essential need.

In this context, different inorganic nanoparticles (like, silver, copper, zinc oxide, gold) [15,16] have been reported for their antimicrobial properties. Their use is advantageous in comparison to organic nanoparticles, due to their chemical stability, thermal resistance and long-lasting performance [17]. These biocides can be impregnated on porous or nonporous support materials like activated carbon, zeolite, carbon nanotube and carbon aerogels [15]. The application of silver-based nanocomposite for water disinfection has been already reported [18,19]. However, preparation-cost and toxicity of released silver in treated water remains important concerns. In this regard Cu-NP has gained attention as an alternative biocide to replace silver because of its cost- effectiveness [20].

There are very few reported works where copper and copper-based compounds have been used as a bactericidal agent. For example, the application of metallic copper surface [21], and copper nanoparticle impregnated [22] fibrous material were shown to prevent bacterial growth. Similarly, copper pot [23] and copper mesh [24] were reported to be used for water disinfection. The molecular mechanism of antimicrobial activity of copper nanoparticle has not been described very well in existing literature. Copper nanoparticles induce a contact mediated killing of bacterial cells with adhesion of cell wall of gram negative bacteria by electrostatic interaction [25], with concomitant change in membrane permeability and release of lipopolysaccharides and membrane proteins. It also produces reactive oxygen species which mediates lipid peroxidation and DNA degradation of the bacterial cells [26], [27], [28]. The advantage being, copper nanoparticles are much less toxic (to humans) and cheaper, compared to other metallic nanoparticles having antimicrobial property [29]. In this respect, the selection of proper support material and method of copper nanoparticle impregnation are two critical factors for achieving optimum disinfection performance.

Activated carbon (AC), a well-known low-cost adsorbent [30], is used in regular water filters to remove the odour and organic contaminants and also to maintain the taste of the water [31,32]. However, the attached bacteria on the AC surface can regrow, using it as a preferred energy source [33]. In this regard, placement of copper nanoparticles selectively on the surface of AC would achieve better bacteria to nanoparticle contact, facilitating complete bacterial killing. The reason being, the dimension of bacteria like E. coli (1.5 µm in length and 1 µm in width [34]) is bigger than the pores of AC, thereby remaining mostly on the surface of AC during the disinfection process [31]. As a result, the Cu-NP would not be accessible to the bacteria if the NP goes into the pores of AC granules during the impregnation process. Various methods like oxygen plasma treatment, heat treatment and modification with nitric acid have been reported in the literature for this purpose.

Nevertheless, all the above methods have their own caveats. In the case of heat treatment, reduction of surface area and oxygen functional groups limits its application [31]. Similarly, the generation of nitronium ions by nitric acid causes structural damage on the carbon surface and leads to a reduction in adsorption sites of AC [35]. Plasma treatment of AC, followed by impregnation of silver and copper nanoparticles, has been shown to provide better disinfection performance, but the requirement of expensive plasma cleaner and its use is a major disadvantage of this method. Base modification of activated carbon followed by impregnation with Cu-NP may be an effective, low-cost alternative for the water disinfection process, which so far has not been checked by experiments. The idea of this work is to impregnate Cu-NP selectively on the surface of AC, by using a simple and inexpensive surface modification method and to further evaluate the performance of the nanocomposite for drinking water disinfection ensuring the leaching of copper to be within the safety limit.