nanotechnology of foam: water filtration

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  • Nanotechnology of Foam: Water Filtration July 2015

    Nanotechnology of Foam: WaterFiltration





    New Jersey Governors School of Engineering & Technology

    I. Abstract

    In an effort to develop an effective and cheapmethod of purifying water, experiments wereperformed to determine the possibility of utiliz-ing sol-gel products as water filtration devices.If sol-gel products were used as water filters, itwas theorized that they would be just as effec-tive as simple water filtration units at removingcontaminants from water. These materials werecompared to common filtering materials in termsof flow rate and extent the amount of contam-inants removed. Alone, different filter compo-nents did not filter contaminated water well buthad much faster flow rates when compared tomixed-media filters.

    When combined, the filters work much bet-ter than they did alone. Also, they work bestwhen they are organized with fine materials atthe top and coarse materials at the bottom.Moreover, alum, a flocculent, purifies the fil-trate even further. Sol-gels and foams were alsotested as filters, as sol-gels are very porous andare very cheap to create. Unfortunately, neitherthe foams nor the gels that were tested seem tobe viable options for filtration systems becausethe foams were unable to allow contaminatedwater to flow through them.

    II. Introduction

    Seven hundred and fifty million people areforced to drink water contaminated by viruses,bacteria, and macrocontaminants every day.1 Incountries, especially developing countries, trou-

    bled by drought and disease, there is a need fora cheap and efficient way to obtain clean waterfrom unsanitary sources without the use of elec-tricity, thermal energy, or other equipment thatis not readily available. Common filters such asthe Lifestraw R and Lifesaver R bottle are tooexpensive for large populations in developingcountries to benefit greatly from.2,3 It is neces-sary that a cheaper filter be made so that themajority of people can benefit.

    Polluted water sources can include contam-inants such as bacteria, viruses, and other wa-terborne pathogens.4 Since all of these contam-inants vary in size and shape, many differenttypes of filter components are often combinedto effectively clean the waters. However, evencomprehensive and cheap filtration techniquescannot wholly purge water of small viruses andbacteria.5 Only a new, alternative filtrationmethod could filter out the smallest viruses andbacteria.

    In theory, reducing the pore diameters offilters would limit the amount of contaminantsremaining in the water. Viruses are the smallestharmful contaminants; they have a minimumsize of 40 nm.6 The sol-gel process, dependingon the specific techniques used, can be a rela-tively inexpensive means of creating foams withpores as small as 2 nm in diameter.7 Watermolecules are only 2.75 Angstroms (0.275 nm)in length.8 This specificity is even more effec-tive than the leading commercial water filter,Lifestraw R, which cannot filter the smallestviruses.9


  • Nanotechnology of Foam: Water Filtration July 2015

    III. Background

    I. The Sol-Gel Process

    Sol-gel, the abbreviated term for "solutiongel", describes a process used to create porousmaterials with novel properties such as super-hydrophobicity and high thermal insulation.9

    This wide range of possibilites allows the prod-ucts of the sol-gel process to be applied in manydifferent fields of technology.

    The sol-gel process involves the reaction ofa precursor, a chemical that contains many ele-ments of the future gel, with a liquid medium.11

    When the two initially react, the molecules cre-ate a network suspended in liquid. After sometime, which varies depending on the procedureperformed, the molecules form an inorganic net-work and the substance turns into a gel. The gelbecomes a continuous, solid shell encapsulatinga liquid, formed by the entanglement of poly-mers within the sol. Finally, when the gel beginsto dry, the liquid contained inside evaporates,causing the substance to shrink and turn into afoam.

    In particular, gels made of collodial silicawere used for this experiment. As they are gen-erally softer and more flexible than foams, theyare easier to create into filters. Moreover, thegels used in this experiment were porous and,therefore, could be used as potential water fil-ters.

    II. Nanotechnology of Foam

    Foams synthesized using the sol-gel processare extremely porous and lightweight.7 By usingthe precursor tetraethoxysilane (TEOS), porousfoams are created from the process. In this ex-periment, some of the foams tested for theirfiltration properties were not created using thetraditional precursor method. By replacing theTEOS with methyltriethoxysilane (MTES), thefabrication process is expedited and creates anaerogel.14 Aerogels in particular have very largesurface areas and very low densities because oftheir high porosity, with surface areas typicallyranging from 600-1000 nm.7 They also consistof 95% air and have pores ranging from 2-200

    nm in size.10 One noteworthy feature of thismethod is that it makes superhydrophobic aero-gels without subsequent surface modification.14

    Superhydrophobicity, literally "water fearing",refers to the ability of a substance to be ex-tremely hard to wet.

    Because of its porous nature, an aerogelcan potentially serve as an effective water filter.When contaminated water flows through thefoam, contaminants are prevented from passingthrough, due to their relatively large size.

    III. Mixed-Media Filters

    Most filter setups tested in this project con-taining different materials can be classified as amixed-media filter. Mixed media filters consistof layers of various filtering materials, each withdifferent porosities, densities, and properties.When water flows through a mixed-media filter,each layer removes a certain amount of contami-nants, and the end result is a much cleaner watersample. These filters are effective at removingmacrocontaminants that are easily separatedfrom the water, but are ineffective at removingmicrocontaminants that are either dissolved inthe water or too small to be trapped by thepores. Mixed media filters were used in thisexperiment because many commercial filtrationsystems are mixed-media filters, in addition tothe fact that sol-gel products could easily beincorporated into the filtration system. How-ever, it is unclear as to the most efficient way toorganize the filter components within a mixed-media filter. Therefore, one component of ourexperiment involved testing the layout of thefilter components within mixed-media filters.

    IV. Filtration Procedures

    This experiment involved two major steps:testing the filtering properties of various materi-als, foams, and gels, and then comparing theireffectiveness.


  • Nanotechnology of Foam: Water Filtration July 2015

    I. Filtering Properties

    I.1 Testing Sol-Gel Products

    To test the filtration properties of aerogelsand gels, a number of tests were performed.First, the foams were placed into a plastic fun-nel with a tight seal so that water would notleak through the sides. Water was then droppedonto the foams with a pipette. Then, anotherapproach was taken to get more results. Thespout of a burette was inserted into the topof the foam, and then 1000 mL of water waspoured into the burette in an attempt to forcethe water through the foam.

    When testing gels, plastic funnels were sealedat their narrow ends with layers of the silica gel.Three filters were constructed, with one, two,and five layers of gel. The plastic funnels werefilled with 10 mL of water and were placed ingraduated cylinders.

    Fig 1: Setup of Silica Gel inserted into plasticfunnel

    I.2 Testing Common Filtering Materi-als

    The next part of the experiment involveddetermining the filtering properties of differentfiltering components. The different componentsincluded coffee filters, charcoal, ceramic beads,fine sand with .15 mm grains, and coarse sandwith .37 mm grains. The coffee filters served asthe control for this experiment and were placedinto a glass funnel. All the other componentswere placed into glass funnels along with thecoffee filters so that the fine particles would stay

    elevated. The other filtering components weretested individually during separate runs, duringwhich 100 mL of different contaminant solutionswere poured through the filters.

    The substances used as contaminants werecorn starch, dextrose, and plant protein. Thecontaminated water consisted of 5 mL of cornstarch in 95 mL of distilled water, 7.7 g of dex-trose in 90 mL of distilled water, and 5 mL ofplant protein in 95 mL of distilled water.

    The filters were suspended by ring standsand extension clamps. 10 mL of each contam-inant solution were funneled through the vari-ous water filter components. Graduated cylin-ders were used to measure the amount of con-taminated water that passed through the filtercomponents. Lugols solution, Benedicts solu-tion, and Biuret Reagent solution, which testfor starch, sugar, and protein, respectively, wereused as indicators to observe the quantities ofthe different contaminants remaining in the fil-trate. The equipment used with those methodsincluded test tubes, test tube stands, and a hotwater bath.

    I.3 Constructing Mixed-Media Filters

    Five different filters were created to researchthe filtration properties resulting from differentmixed-media patterns. The container of each fil-ter was an empty plastic water bottle suspendedupside down, with the bottom of the bottle cutoff. Filter Setup 1, as shown in Figure 2, con-tained 45 g of ceramic beads, 50 g of coarsesand, 50 g of fin


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