Chemical contamination of drinking water is a major global health issue. While modern water filters treat some major contaminants, not all impurities can be filtered. Moreover, all filters are not cost-efficient and environmentally friendly. The current study aims to explore the effectiveness and efficiency of reduced graphene-oxide-coated sand as a water filter. By using beach sand, sugar, and tap water, the activated graphene-oxide-coated sand was made. After filtering raw well water using the activated graphene-oxide-coated sand, 32 parameters were tested; results showed a notable decrease in most of the major heavy metals, but levels of 8 elements – lithium (Li), silicon (Si), titanium (Ti), uranium (U), chromium (Cr), iron (Fe), rubidium (Rb), and caesium (Cs) – increased. A second phase experiment was conducted after strenuously cleaning the same type of sand using distilled water and thoroughly stirring. After testing the parameters again, even more heavy elements, such as Cr, Fe, Rb and Sb were filtered successfully, but there was still an increase in 4 elements (Li, Si, Ti, and U). The activated graphene-oxide-coated sand was tested under an electron microscope, and it was clearly seen that the sand particles were not completely covered by graphene-oxide, indicating that leaching was possible, since sand particles also contain the same heavy metals. This research suggests it is possible to create a graphene-oxide water filtration system using widely available natural materials such as beach sand and sugar, so this product can be cost-effective and environmentally friendly. However, appropriate technology is needed to envelope the entire sand particles by graphene-oxide.
According to the World Health Organization, almost two billion people in the world do not have access to water that is safe to drink, and approximately one million people die each year from diarrhoea, mostly due to drinking contaminated water. The majority of these cases are from Low Economically Developed Countries (LEDCs) which lack the resources to develop and maintain their water supply infrastructure.1 Traditional public water treatment systems are expensive and technologically complex to run, hence LEDCs can not sustain the public utility.
The quality of drinking water is measured using two factors: microbiological (e.g. as faecal coliforms) and chemicals (eg. as arsenic, mercury, lead, etc.). Microbial agents (E. coli, Campylobacter, Salmonella, Shigella etc.) are the largest cause of waterborne diseases worldwide. However, chemical contaminants, such as arsenic (As), lead (Pb), and chromium (Cr) in drinking water have also been associated with a wide range of adverse health effects such as cancer, hormonal diseases, and neurological disorders.
Filtration of water is essential to remove harmful chemicals before human consumption. Unfortunately, water filters currently available on the market are not always very effective in removing all the chemical contaminants. Therefore, one of the sustainable solutions for this problem is to develop cheap, easily available water filters that can effectively remove all major harmful contaminants without wasting water.
Graphite is a three-dimensional carbon-based material made of millions of single layer tightly packed pure carbon atoms in a honeycomb structure, known as graphene. By using strong oxidizing agents, oxygenated functionalities can be introduced in the graphite structure resulting in graphite oxide, consisting of multiple layers of graphene oxide (GO). GO has a similar hexagonal carbon structure to graphene and contains oxygen-based functional groups such as hydroxyl, alkoxy, carbonyl and carboxylic acid (Figure 1).
Figure 1: Hexagonal carbon structure of graphene oxide.
These oxygenated groups are responsible for many advantages over graphene, including high hydrophilic properties and surface functionalization (altering the surface properties of a material for the desired bio-response) when used in nanocomposite materials, or materials that incorporate nanosized particles into a matrix of standard material. Furthermore, GO can be treated by a number of methods to synthesize activated reduced GO, a flexible free-standing porous carbon film with high specific surface areas (up to 2,400 m2/g) to minimize the number of oxygen groups.[7,8] It has been discovered that activated reduced GO is impermeable to compounds except for water, enabling the material to be a high-quality water filter.[9,10,11] A few experiments have been done in recent years on low-cost GO-based filtration systems (GOFSs) that have shown the removal of heavy metals and minerals from water.[9,10,11] Such studies have demonstrated how to make a GO and sand composite water filter using river sand and sugar.[12,13,14] However, the question is how to make GO filters affordable, high quality, and available to all people.
Therefore, this experiment aims to develop a GOFS using locally available sand and sugar, test its efficiency in terms of the removal of elements present in water, explore its shortcomings in effective filtration of elements, and suggest feasible remedial measures. The experiment followed the validated methods of making GO coated sand.[12,13,14] The experiment intended to test a water sample collected from an artesian well to find any changes in the parameters after filtration.
The sand was bought from a local garden centre (50 lb bag costs C$4) and sugar from a local grocery store (4 kg pack costs C$5). The experiment needed one pound (lb) of sand (8 cents) and 200 gm of sugar (25 cents) for each phase. The entire project had two phases (I and II). In phase I, the GOFS was made by using ordinary tap water to wash sand (as done by other researchers)[12,13,14], sample water was tested and its effectiveness as a filter was assessed. In phase II, an improved version of GOFS was made by using distilled water to wash sand and the sample water was tested to compare its filtration efficiency with phase I.
Beach sand (pre-washed in tap water) was kept in a rice cooker, a saturated sugar solution was poured on top of the sand, and both were left for an hour. The rice cooker was heated up to 94–95°C, and the sugar and sand were stirred thoroughly for 6 hours, to prevent any clumping of sand. After the water evaporated, sugar-coated sand (SCS) was obtained and put in an oven. The temperature in the oven gradually rose to 300°C and was maintained at that point with constant stirring for about 1½ hours. After taking the SCS out and letting it cool, it turned dark brown. For activation, the GO-coated sand was mixed with concentrated (95%) sulfuric acid for 30 minutes and then put in a clay funnel containing concentrated sulfuric acid and a gas pipe supplying heat (120°C) (Figure 2). A filter column was made by hose nozzles filled with the activated GO-coated sand. The GOFS was developed by assembling the filter column with a cut plastic bottle. The terminal end of the filter column was covered with sterile cotton and a metal mesh disc (Figures 3 and 4).
The water sample was collected from an artesian well from a rural community (Flatrock, Newfoundland, Canada) and stored in acid-treated sample containers. Prior to the sampling, the well owner tested the water at an accredited lab and found the water fit for consumption. One of the containers was set aside for testing the original (raw) water, while the other container was filtered. Both samples were analyzed in Perkin Elmer Elan DRC II Inductively Coupled Plasma Mass Spectrometry (ICP–MS). A total of 32 parameters were tested by following all the required protocols for quality control and quality assurance.
Figure 2: Activation of graphene-oxide coated sand in a white clay funnel
Figure 3: Diagram of making the graphene-oxide filtration system. Diagram constructed by the author.
Figure 4: Graphene-oxide filtration system ready to filter water
Phase II was conducted to re-create a GOFS by following the same procedure as Phase I, except the sand was pre-washed 10 times with distilled water to avoid possible contamination with ions in tap water. The same water sample was then filtered by the new GOFS and the filtered water was tested by ICP–MS. Sand particles (plain sand and GO-coated) were examined under an electron microscope to check the nature of the GO coating over sand particles at various magnification levels ranging from 290x to 7600x (Figure 5).
Figure 5: Preparation of samples before checking graphene-oxide coated sand particles in an electron microscope
Phase I: The experiment was expected to show a decrease in the levels of 32 elements. However, the results show that the levels of 16 elements decreased (Table 1, section 1A), and the levels of 8 elements (Li, Si, Ti, U, Cr, Fe, Rb, and Cs) increased (Table 1, section 1C). The changes of levels of 8 elements could not be ascertained (Table 1, section 1B) since their levels (both raw water and filtered) remained below the detection limit of the ICP-MS. The primary interpretation was that the excess amount of 8 elements coming either from the tap water or leaching from surfaces of uncovered sand particles or from both the sources.
Table 1: Change of levels of elements after filtration of raw water (phase I and II). Units – All in parts per billion or ppb (except Mg, Si, S, Cl, Ca, i.e. parts per million or ppm)];
< DL – below detection limit. Green indicates levels of elements lower than those in raw water; yellow shows the levels of elements not detectable and red highlights levels of elements higher than those in raw water.
Phase II: Similar to phase I experiment, the levels of 16 elements (Table 1, section 1A) decreased. The levels of 8 elements of section 1B still remained below detection limits. The levels of Cr, Fe, Rb, and Cs (showed higher in phase I ) decreased. The results show that use of distilled water might have stopped further contamination of Cr, Fe, Rb, and Cs from tap water. However, the levels of Li, Si, Ti, and U still remained higher than the levels in raw water and thus leaching from bare surfaces of sand particles could not be ruled out.
The electron microscopic picture showed that a number of sand particles (phase II) were not fully covered with GO (Figure 6), and some sand particles were barely coated with GO. Fig 5A shows pure uncoated sand for comparison. Figures 6B and 6D show almost fully GO coated sand particles (with small uncoated patches). Figure 6C shows wide uncoated bare surfaces of a sand particle. Its further magnifications (2650x, 7600x) show multiple gaps even within the GO patch (Figures 6E and 6F).
Figure 6: Uncoated and graphene-oxide coated sand particles (phase II) under an electron microscope (the orange arrows show magnification of the same field)
The experiment showed that GOFS developed using locally available sand and sugar can remove the majority of the dissolved elements present in water. Experiments conducted by other researchers, such as Parvathi et al (2015), Gupta et al. (2012) and Rahman et al. (2016), also developed graphene-coated sand filter and tested water filtration efficiency.[11,12,13] However, their target contaminants were different from GOFS. Parvathi et al (2015) tested waste water (textile, sugarcane industrial waste, and domestic waste), Gupta et al. (2012) tested rhodamine 6G as a model dye and chloropyrifos in water and Rahman et al. (2016) tested coke and sewage water.[11,12,13] Since their experiments primarily focused on synthetic organic compounds, they did not have any scope to check leaching of natural elements from the sand particles.
The current experiment of GOFS shows that there are several operational challenges; for example, the manual stirring of sand and sugar may not be an effective method to coat all sand particles with GO. Analyses of different particle-size fractions of sandy sediments found a large distribution and bioavailability of heavy metals.[16,17] Therefore, it is suspected that the uncoated surfaces of sand particles could leach some elements to water during filtration and compromise the quality of the filters. However, further research is needed to establish the possible link between the uncoated surfaces of sand particles and the leaching of elements. It is important to coat the entire surface of each sand particle with sugar, and the coating should remain intact during the drying process and in the hot oven. The minimization or complete elimination of mechanical stirring could ensure the complete envelopment of sand particles with GO. In this regard, the simple technology used in “tablet coating machines” can be used to evenly coat sand particles with sugar. Since sand particles are very small, more appropriate technology is needed. It is recommended to check the sand particles under an electron microscope ensuring quality of sugar coating, followed by testing water to verify the efficiency of GOFS.
Another limitation of the experiment was the small sample size. Therefore, more water samples are needed to explore the efficiency of GOFSs to conduct statistical analysis. Multiple testing of results can give a better interpretation of the filtration efficiency of GOFSs. Moreover, it is important to study filtration capacity (i.e. total volume of water effectively filtered by filter cartridge) of GOFSs and its cost-effective analysis. Studies of the environmental degradation of GOFSs after disposal and the development of proper disposal strategies are also necessary before promoting this novel technology. The open and unregulated disposal of saturated GO-coated sand may naturally degrade and release harmful minerals to terrestrial and aquatic ecosystems. GOFSs have the potential to remove harmful elements from drinking water; however, further research is needed before introducing them to the market.
In LEDCs, contamination of drinking water is a major public health problem, and many people rely on expensive, imported water filters. This research demonstrates that the GOFS is able to remove various harmful elements from water and has the potential to be a sustainable solution for addressing water quality issues, especially in LEDCs. Although more research is needed before promoting the GOFS at the community level, this experiment shows that it is possible to make effective water filters using locally available natural materials, such as beach sand and sugar. A more affordable and environmentally friendly but still effective water filter could have profound impacts on the amount of water-borne disease spread across the world, especially in LEDCs.
I would like to thank Prof. Francesca Kerton and Ms. Jennifer Murphy (Ph.D. candidate), Department of Chemistry at Memorial University for helping me make the activated graphene-oxide. I would also like to thank Ms. Pam King (currently retired) from the CREAIT lab at Memorial University for testing the water samples. The research was presented at the Canada-wide Science Fair in 2017 at the University of Regina (Saskatchewan, Canada) and won the silver medal. I would like to thank Husky Energy for sponsoring my travel to Regina.
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About the author
Aaron Sarkar is a grade 11 student at Holy Heart of Mary High School (St. John’s, Canada) and pursuing full IB diploma program. He is very passionate about science and he enjoys going to the gym, playing flute, reading books and volunteering. His future plan for the project is to advance his research in the field of graphene-based water filter and working for the underprivileged people.