essay

Sudarshan Tanaji Kurwadkar  

 

Removal of Low Level Saxitoxin from Engineered Water Treatment System using Super Activated Carbon Nanoparticles and Powdered Activated Carbon Statement of problem In recent years, there is a significant increase in threats to drinking water supplies from Chemical Warfare Agents. A terrorist attacks on water systems is real because there are nearly 170,000 public water distribution systems that are often unprotected and readily accessible and the distribution network can deliver the effective dose of toxins to large unsuspected population. Saxitoxin (STX) classified as Agent TZ (chemical weapon designation) and included in the Schedule 1 of the Chemical Weapons Convention can be potentially used as weapon of mass destruction (equatox.net). In the event, the drinking water sources are contaminated by low levels of STX, it is important to evaluate whether slightly modified conventional water treatment process can effectively remove STX such that adverse effect on human health can be prevented. Conventional water treatment processes are not designed to remove low levels of STX. For example, coagulation/flocculation is not effective in removal of STX and also the charcoal based filters have been proved to marginally successful at the recommended drinking water pH level at which STX is mostly cationic (Burrows and Renner, 1999; Wannemacher et al., 1993). The purpose of this study is to evaluate the efficacy of removal of STX from drinking water using commercially available Super Activated Carbon Nanoparticles (SACN) and Powdered Activated Carbon (PAC). Due to their high surface area, both SACN and PAC are known to have high sorption capacity. This study aimed to address the following questions: Does enhancing sorption capacity of the rapid sand filter effectively remove STX from drinking water? What thickness of SACN and PAC produce optimal results? Will the presence of residual free chlorine enhances STX removal? Is SACN a better and economically feasible option than PAC? Background and relevance to previous work The vulnerability of drinking water distribution system to aquaterrorism (terrorism attacks on the water supply) has been definitively identified to be real and imminent but often overlooked and poorly understood by water managers and the public (Hasan et al., 2004; Gleick, 2006;). A conference on early warning monitoring to detect hazardous events in water supplies reaffirmed the vulnerability of drinking water supplies to terrorist attacks. Water infrastructure particularly the drinking water supplies are an attractive target of the terrorist primarily because, both the target are relatively isolated and unprotected and the efforts involving in targeting these systems are minimal. Low production cost of toxins, easy availability and portability and larger targeted population makes public distribution systems a soft target to opportunistic terrorist attacks. Particularly, water storage and distribution network can be used by terrorists to facilitate potentially toxic dose to very large population or lower-level chronic dose below the detection level can expose larger population to chronic exposure to toxins (Foran and Brosnan, 2000). Given this imminent threat of attacks on water distribution system, it is imperative that existing water treatment system should be evaluated for their efficacy to remove STX from drinking water prior to its distribution to the public and options explored to protect the large population that relies on public water distribution system.

Sudarshan Tanaji Kurwadkar  

 

The STX was first isolated during the World War II in the USA under the US biological weapons program (Szinicz, 2005). It is a natural toxin and an important class of chemical warfare agents (CWA) and is much more poisonous than synthetic nerve gas sarin (Burnworth et al., 2007, equatox.net). It is potent neurotoxin and a paralytic poison that attacks the central nervous system. Toxicity of STX depends on the exposure routes; it is highly toxic by ingestion, injection and most toxic by aerosol dispersion because of its potency and toxicity at low dose it is weaponized for covert purposes (Burrows and Renner, 1999). Environmental persistence of STX indicate that it is stable in acidic condition, labile in alkaline and stable in normal environmental conditions (Warner, 1990) Literature review on removal of STX in the existing water treatment processes revealed that typical disinfectants such as chlorine can remove STX from drinking water sources. A study conducted by Rositano et al. (1998) demonstrated that traditionally favored chlorine disinfectant is very effective in removal of STX from drinking water with an effective dose of 20 mg/L and a contact time of 40 minutes. While the effectiveness of chlorine in STX removal is promising, the implementation of this removal method is rather impractical due to longer contact time and higher dose of chlorine. Furthermore, the dose of 20 mg/L will result in residual free chlorine concentration in the treated water well above the recommended maximum residual disinfectant level of 4 mg/L. Similar results were also reported by Ho et al. (2009) regarding effectiveness of chlorine in removal of STX. The authors reported that 90% of destruction could be achieved with CT values of approximately 30 mg. min/L. The authors further reported that the natural organic matter (NOM) affects the removal efficiency of STX with lower efficiency being observed in waters having higher concentration of NOM due to the greater competitive reactions between chlorine and NOM and chlorine and STX. However, if the STX is introduced in the post- disinfection storage and distribution well, then residual chlorine (<4mg/L) level will not remove STX. Removal of STX using various disinfectants and their combination has been used in the past to study the removal of STX from drinking water sources. Other disinfectants such as permanganate, ozone and ozone in combination with hydrogen peroxide were also found to be ineffective in removal of STX from drinking water system (Rositano et al., 1998; Orr et al., 2004). Removal of STX using PAC in drinking water system has been demonstrated to be effective in removal of STX (Ho et al., 2009). Similar results were also reported by Hoeger et al. (2004) in their studies on removal of cynobacterial studies in two Australian drinking water treatment plant. Majority of these studies involves removal of STX from drinking water resources using single of combination of disinfectants, commercially available sorbent materials such as granulated activated carbon and PAC. None of these studies have attempted to retrofit the existing drinking water system for the removal of STX. Within the existing configuration of drinking water treatment system, is it possible to remove STX? The proposed research study aims to address this specific question by simulating the rapid sand filtration system and amending it with a layer of either SACN or PAC and evaluating the removal efficiency at drinking water pH levels. Furthermore, this study will also seek to address if there is enhanced removal in the presence of residual free chlorine and SACN and PAC. No additional disinfectants will be added but the removal efficiency will be evaluated in the presence of allowable residual free chlorine. Since the distribution network has to have a residual chlorine level till the water reaches the consumer, it is important to evaluate its role in net removal efficiency of STX in the distribution network.

Sudarshan Tanaji Kurwadkar  

 

General methodology and procedure to be followed A scaled model of rapid sand filter will be designed to simulate the proposed flow conditions at a typical water treatment plant with appropriate filter media (sand, garnet, anthracite etc.). The scaled version consisting of a feeder tank containing solutions of STX will be applied at the typical water treatment plant flow rate over the constructed rapid sand filter with either SACN or PAC amended as a small layer at the bottom of the column. The peristaltic pump will be used to apply water containing STX solution. The apparatus will be covered in aluminum foil to minimize exposure to sunlight. Analytical grade STX: RM 8642 - FDA Saxitoxin Dihydrochloride Solution (CAS#1981-58-4) will be purchased from National Institute of Standards and Technology. Standards and stock solutions will be prepared in nano-pure water with a resistivity of >18.2 micro-mho-cm using a Milli-Q Simplicity water purification system (Millipore, Bedford, MA). Standard stock solution as well as the feed solution containing STX will be prepared on as required basis and stored in the dark at 4 °C until use. A series of known concentration solutions of STX will be prepared to establish a standard curve and subsequent determination of the concentrations of STX in unknown samples (samples passed through the adsorbent material). The SACN (<100nm; Specific Surface Area >1300 m2/g) (CAS#7440-44-0) will be purchased from US Research Nanomaterials, Inc. Houston, TX. PAC (CAS#:7440-44-0) will be purchased from Fisher Scientific, Hanover Park, IL. Complete characterization of SACN and PAC will be obtained from the manufacturer prior to its use in the laboratory experiment. A scaled model of the rapid sand filter will be amended to include a thin layer of SACN and PAC. A solution containing low levels of STX will be passed over the modified filter media. The filtrate will be analyzed using High Performance Liquid Chromatography (HPLC) system. Explanation of new or unusual techniques The proposed research focuses on effective removal of STX in a sand filtration system using think layers of SACN and PAC. Initial experiments will be carried on with a known concentration of STX and SACN or PAC. Later experiments will be conducted to establish the effective removal efficiency with regard to amount of SACN and concentrations of STX. As explained before, STX exists in predominantly cationic form at drinking water pH level as such the planned experiments will be conducted at pH ranging from pH 6.5-8.5. Two sets of experiments will be conducted, STX removal efficiency in the disinfected water with residual chlorine concentration of <4 mg/L and STX removal in pre-disinfection. The idea is to evaluate the relative contribution to STX removal due to residual free chlorine vis-à-vis removal by SACN and PAC. Concentration of STX in aqueous solution will be measured using reverse- phase High Performance Liquid Chromatography (HPLC). The analytical protocol will be based on the method outlined by Cianca et al. (2007). The method involves pre-column oxidation with fluorescence detector with an excitation wavelength of 330 nm and an emission wavelength of 390 nm. Chromatographic separation protocol involves gradient separation with a mobile phase consisting of 0-100% (v/v) acetonitrile in 0.1 ammonium format, adjusted to pH 6 with 0.1 M HCl. The gradient separation includes 20 minutes run time with a flow rate of 1.5 mL/min that involves separation with 0-5% (first five minutes), 5-70% during the next 4 minutes and 70- 100% during the next 9 minutes, and finally, 100-0% during last two minutes. An injection volume of 20 microliter will be used for analysis. During the analytical quantification of STX in the unknown samples, the standards will also be intermittently run along with the unknown

Sudarshan Tanaji Kurwadkar  

 

sample to make sure that the HPLC system is performing as expected. Since the synthetic samples containing STX will be prepared from the treated water, there is no possibility of matrix interference and as such no internal standards will be tested. Plots of concentration and bed volume will be plotted to see when the adsorbent material need to be replaced. The breakthrough of the STX from the scaled model filter will indicate that the adsorbent material has lost all the sorption sites available and as such need to be replaced or regenerated. An experimental log- book documenting detailed experimental protocol and standard operating procedure including instrumental procedures and sample preparation methodology will be documented. Expected results and their significance and application Vulnerability of drinking water sources to opportunistic terrorist attack and the consequent risk of exposure to STX through ingestion of drinking water is very important from human health perspective. Removal of STX using the existing drinking water treatment processes with amendment of high capacity of sorbents such as SACN and PAC is planned to be investigated in the pre and post-disinfection scenario. At present, not many scientific studies have been conducted to investigate the efficiency of conventional drinking water treatment processes and removal of STX. This research intends to provide a comprehensive, authentic, experiment-based investigation into the removal efficiency of STX in the drinking water treatment plants with slight modification to the rapid sand filter. This original laboratory and simulated research incorporates real life scenarios such as removal efficiency of STX under allowable residual free chlorine level for two of the commercially available sorbents. The findings of this research study will contribute to a new knowledge in the field of removal of STX from drinking water using existing water treatment process configuration. To optimize the cost of treatment a cost comparison will be done between the removal efficiency using PACN and PAC. This will provide the utility operators an opportunity to choose commercially available and economically feasible sorbent material for regular use in the water treatment process. Dose optimization studies will provide an approximation of best removal efficiency for a particular thickness of SACN and PAC. It is expected that the findings of this study will help better address the imminent threat of STX contamination in drinking water treatment plant. Literature citations:

1. Burrows, W. D., Renner, S. E. (1999). Biological Warfare Agents as Threats to Potable Water. Environmental Health Perspectives, 107(12), 975-984

2. Wannemacher, R. W. Jr, Dinterman, R. E., Thompson, W. L., Schmidt,M. O., Burrows, W. D. (1993). Treatment for Removal of Biotoxins from Drinking Water. Rpt no TR9120, AD

3. A275958 Ft. Detrick, MD:US Army Biomedical Research, and Development Laboratory. 4. Hasan, J., States, S., Deininger, R. (2004). Safeguarding the Security Of Public Water

Supplies Using Early Warning Systems: A Brief Review. Journal of contemporary Water Research and Education, 129, 27-33

5. Gleick, P. H. (2006). Water and terrorism. Water policy, 8, 481-503 6. Foran, J. A., Brosnan, T. M. (2000). Early Warning Systems for Hazardous Biological

Agents in Potable Water. Environmental Health Perspectives, 108 (10), 993-996

Sudarshan Tanaji Kurwadkar  

 

7. Szinicz, L. (2005). History of chemical and biological warfare agents, Toxicology 214, 167–181

8. Burnworth, M., Rowan, S. J., Weder, C. (2007). Fluorescent Sensors for the Detection of Chemical Warfare Agents. Chem. Eur. J. 13, 7828 – 7836

9. Equatox.net. Saxitoxin. http://equatox.net/science_of_toxins/saxitoxin/ (Accessed on November 19, 2014)

10. Warner, J. S. (1990). Review of Reactions of Biotoxins in Water. Rpt CBIAC Task 152. Ft. Detrick, MD:U.S Army Medical Research and Development Command

11. Rositano, J., Nicholson, B., Heresztyn, T., Velzeboer, R., 1998. Characterisation and determination of PSP toxins in neurotoxic cyanobacteria and methods for their removal from water. Research Report No. WSAA 148. Urban Water Research Association of Australia, Melbourne.

12. Ho, L., Tanis-Plant, P., Kayal, N., Slyman, N., Newcombe, G. (2009). Optimising water treatment practices for the removal of Anabaena circinalis and its associated metabolites, geosmin and saxitoxins. Journal of Water and Health, 7(4), 544-556.

13. Orr, P. T., Jones, G. J., Hamilton, G. R. (2004). Removal of saxitoxins from drinking water by granular activated carbon, ozone and hydrogen peroxide—implications for compliance with the Australian drinking water guidelines. Water Research, 38, 4455- 4461

14. Hoeger, S. J., Shaw, G., Hitzfeld, B. C., Dietrich, D. R. (2004). Occurrence and elimination of cyanobacterial toxins in two Australian drinking water treatment plants. Toxicon, 43, 639-649

15. Cianca, R. C. C., Pallares, M. A., Barbosa, R. D., Adan, L. V., Martins, J. M. L., Gago- Martinez, A. (2007). Application of precolumn oxidation HPLC method with fluorescence detection to evaluate saxitoxin levels in discrete brain regions of rats. Toxicon, 49, 89-99