(252d) Energy-Efficient Nanoparticle Contamination Control Using Functionalized Fiber Filters

Nanoparticles have always been an important part of environment. Recent advances in nanotechnology have enabled us to utilize engineered nanoparticles (ENPs) in a plethora of applications to improve the performance of products and also as process enhancers. Although the advantages of ENPs are undeniable, there is a growing uncertainty concerning the impact of excessive release of ENPs into environmental systems. In recent years, nanoparticle (NP) contamination has come to the forefront as a critical challenge for environment and human health. The hazards of nanoparticles mostly stem from their small size resulting in highly reactive materials that can penetrate through surfaces effectively. Nano-contaminants released into atmosphere can span a wide variety of materials that make up nanoparticles (NNPs), incidental nanoparticles (INPs), and engineered nanoparticles (ENPs) that includes viruses, nano-plastics, and colloidal metals to name a few. Due to the interrelation of earth and water systems, drinking water treatment plants act as critical nodes for controlling the exposure of humans to NPs as they exist at the interface of natural and man-built systems. Despite ongoing efforts, the understanding about the biological impact and quantification methods available for ENPs are still at primitive stage. In addition, the concentration of ENPs in drinking water systems is much lower compared to natural and incidental nanoparticles (NNPs and INPs) redirecting the focus of scientific community to address the endless list of emerging contaminants in our water streams.

The lack of focus towards developing novel techniques for nanoparticle (NP) removal from drinking water which is also exacerbated by the fact that no clear regulatory (for example US EPA) requirement exists for incidental and engineered NP removal, financial burden to replace legacy equipment, and the multi-barrier approach commonly used in water treatment that apportions the removal of NNPs such as clay particles and enteric viruses to either coagulation and sedimentation or disinfection technologies. However, simple methods to remove NPs from aqueous matrices are energy intensive and challenging to implement in traditional centralized drinking water plants and also point of use applications (household and disaster relief type scenarios). Filtration is actually the most common unit operation used in water purification and adaptable at different scales, scenarios, and resource availabilities but most common media filtration methods do not lead to removal of NPs, necessitating the use of more energy intensive and expensive mode of membrane filtration. In this study, we show the feasibility of using a sustainable filtration technique fabricated from easily-accessible materials that can be deployed with minimal cost to achieve highly efficient removal of both NNPs and ENPs from water.

Human enteric viruses are high-risk NNPs that needs immediate attention as infectious disease spread is a major and continuing health burden faced by humanity. The advent of antibiotics has placated the impact of bacterial infections but viral disease outbreaks continue to cause global epidemics. The recent COVID-19 pandemic underlines the importance of technologies that can limit the sustained transmission of viruses. Due to their small size (~5 to 300 nm), viruses pose unique challenges requiring chemical and energy intensive technologies for their effective removal and inactivation from air and water to mitigate disease transmission. This is particularly evident in the context of water purification, due to the reliance on size-exclusion based virus filtration techniques, which gives rise to a trade-off between the productivity and the level of pathogen removal achieved. This trade-off presents a unique challenge, which is a striking example of the interrelation between clean water production and associated energy consumption (the energy-water nexus). Recent studies in the literature have used chemical functionalization of low-pressure membranes or specialized membrane fabrication techniques such as electrospinning and use of nanofibrous materials to overcome the limitation imposed by the trade-off curve. However, due to the use of advanced fabrication and modification strategies the widespread use of these solutions is limited and energy intensive. In this work, we focused on developing a simple, low-cost, and low (embedded and operational) energy alternative that is applicable for NP removal in a wide range of public health scenarios using viruses as a test surrogate. This strategy of utilizing viruses for devising NP treatment techniques can be useful moving forward as the analytical methods for quantifying virus concentration are robust and advanced compared to ENPs.

Broad application of currently available virus removal technologies across the world is often limited by their cost, complexity, and accessibility. Due to the low efficiency of conventional filtration, disinfection has become the most commonly practiced method to meet drinking water standards imposed by US Environmental Protection Agency (EPA) which mandates 4log₁₀ (99.99%) of virus removal. However, advanced disinfection technologies such as ozone and UV are expensive, require high energy inputs, and chemical disinfection (using chlorine) produces toxic disinfection by-products. In addition, all disinfection strategies include materials or chemicals with high embedded energy which incurs a total energy requirement on par with the filtration techniques and are most commonly used in conjunction with and downstream of filtration technologies. Additionally, widespread disease outbreaks still occur in centralized distribution systems practicing disinfection due to inadequate or interrupted disinfection. Due to the above challenges with conventional filtration and disinfection technologies, the focus has shifted towards application of membrane filtration technology which rely on size-based virus removal. Presently, membrane filtration is the most widely accepted alternative water treatment technology and Reverse Osmosis, Ultrafiltration and Nanofiltration membranes with sub-micron size pores were shown to be effective against viruses. The fundamental challenge with membrane filtration arises from reliance on small pore sizes as a physical barrier for viruses. First, the inherent small pore sizes require high pressure differences to drive flow which increases operation costs limiting their widespread use. In addition, membrane filtration needs near continuous monitoring of their integrity as any microscopic defects can cause the passage of viruses.

In this study, we show that the commonly available natural fibers, upon simple functionalization with water extract from Moringa oleifera (MO) seeds can achieve highly efficient NP removal under practically relevant conditions. The MO tree is prevalent throughout tropic and sub-tropic regions, and its seeds have historically been used as a natural coagulant. The seeds of this tree contain cationic proteins MO coagulant protein (MO2.1) and MO chitin binding protein (MoCBP) with established antifungal and coagulant activities. Natural fibers functionalized with these antimicrobial proteins from the seeds of the MO tree acted as an effective filter media for capture and removal of an array of NPs including MS2 and T7 bacteriophages (~7log₁₀) along with silver and polystyrene latex particles (>4log₁₀). We chose to use unprocessed cotton, wool, and flax for this study to show that our functionalization procedure can be implemented effectively worldwide even in resource-limited areas. Our experiments further show that the proposed filters can effectively remove NPs at superficial velocities of 2 m/hr, which is an order of magnitude faster than slow sand filtration and close to the lower range of rapid sand filtration, two most commonly used filtration techniques in water treatment. Using fundamental characterization techniques, we present an overall description of the filters proposed and compare their permeability and removal efficiency with conventional membrane filtration techniques to show that MO coated cotton is a highly energy-efficient sustainable alternative for effective virus removal.

Caption

Figure 1: Permeability and total energy trade-off curves of traditional virus treatment technologies. The literature review of various filtration and disinfection techniques that are typically used for virus removal from water show that there exists a trade-off curve between the permeability (productivity of a filter water for unit trans-filter pressure) and the efficiency which has been observed in various membrane separations. Here we also show that a new productivity based on the total energy requirement of the given techniques also exhibits a trade-off. In this study the proposed filter was designed to overcome these two trade-off curves in order to achieve a wholistically sustainable filter with high productivity.