Arsenic Contamination - Essay Example

Ceria nanoparticles supported on carbon nanotubes for the removal of arsenate from water. Arsenic contamination in ground water has been reported as natural calamity in the 21st century. It is the twentieth most abundant materials found in earth crust. More than 25 countries are facing its deleterious affect on humans by its ground water contamination which also leads to serious medical symptoms. This paper discusses the dangers of arsenate and arsenic in water and also describes the various technologies employed for arsenic removal including the use of nanoparticles. Carbon nanotubes (CNTs) emerges as one of the most promising agent to be used with other nonmaterials due to its unique mechanical, electrical, optical and thermal properties which takes it one of the most ideal supporting material for nano-coatings. The methods utilized in preparations of ceria nanoparticles and its coating on nanotubes is also reviewed. Introduction History of arsenic in field of chemistry, medicine and technology has been completely overshadowed by its role as poison in various homicides. In today's world extension of its similar role has been widely discussed due to world wide ground water contamination and many times referred as 21st century calamity. High concentration of arsenic has been reported from almost 21 countries and among them Bangladesh and West bengal region of India are considered to be the most affected area and significant amount of population are at risk (1). Arsenic is the 20th most abundant material found in earth crust and its concentration in most of rocks ranges between 0.5 to 2.5 mg/kg. Mobilization of arsenic is contributed by various natural phenomenon which include natural weathering reaction, biochemical mobilization, geochemical and volcanic emission etc. but, at many places excessive mining also contributes for the same. Arsenic exist in various oxidation state in natural environment which includes -3,,0.+3. and +5. Figure 1 indicates the Eh-pH diagram of arsenic at 25C. Figure 1 The Eh-pH diagram of Arsenic at 25C (2) Long term exposure to arsenic contaminated drinking water have many medical manifestation which includes skin, lungs, bladder and kidney cancer, change in pigmentation, hyperkeratosis, neurological disorders, muscular weakness loss of appetite and nausea etc. As per the WHO guidelines the permissible arsenic concentration in ground water is 10ppb (0.01mg/l). World wide problem of ground water contamination with arsenic leads to extensive research in area of arsenic remediation in ground water. There are different methodologies developed arsenic removals, which are mainly classified as 1) Chemical processes, 2) Physical processes, 3) Biological processes, and 4) combination of all. Table 1 characterizes the various methodologies applied for arsenic remediation based on their principles. Arsenic pollution of water occurs due to various reasons like the natural leaching of rocks containing arsenic, mining, processing of mineral deposits and a discharge of industrial pollutants. Many techniques as shown in Table 1 are known for arsenic removal and adsorption is one of the main methods for its treatment. Many adsorbents like carbon, rare earth oxides, lanthanium and yttrium impregnated alumina, ion exchange fiber and lanthanium compounds are being known for the removal of arsenic from water(3). The development of nanotechnology and nanosciences has raised the expectation of its crucial role in environmental issues. A variety of nano-materials have been experimented in treatment of environment pollutants like Photocatalytic TiO2 and ZnS for removal of organics. Zerovalent iron (Fe (0)) and bimetallic Fe (0) as effective redox media for in-situ remediation of organics and inorganic pollutant. Among others Carbon nanotubes (CNTs) emerges as one of the most promising agent to be used with other nanomaterials due to its unique mechanical, electrical, optical and thermal properties which takes it one of the most ideal supporting material for nano coatings. Nano material based fixed bad reactors are consider to be one of the promising technology for water remediation due to two major limiting factor 1) Adsoption factor and 2)mass transport kinetics gets taken care by nanomaterial due to its smaller size and hence larger surface area, Kiril hristovski et al (2) have demonstrated that most of the commercial nanoparticles are forms aggregates and when used in fixed bed reactor average arsenic removal was found to be more than 90%xtensively reviewed this aspect. While Tio2,Fe2O3 ZrO2 and NiO was found to be best performer having arsenic removal capacity of more than 98%.Similarly it was also established that aggregate formation leads to decrease in efficiency of column compared to nanoparticles in its nano form. Ceria is known to possess good adsorption capacity for the removal of ions and also possess high resistance against attack by acid (3). In addition to that, as a typical earth oxide, ceria is known for its unique properties like the oxygen storage capacity and oxygen ion conductivity. Because of these characters, ceria is also being used as catalyst, fuel cell sensor, UV shielding and luminescence (4). Table 1: Major arsenic removal technologies.(2) No. Major coagulation/co precipitation technologies 1. Coagulation/electro coagulation/ co precipitation Alum coagulation Iron coagulation Lime softening 2. Major sorption and ion-exchange techniques Activated alumina Iron coated sand Ion exchange resin 3. Major membrane techniques Membrane techniques Nano-filteration Reverse osmosis Electrodialysis 4. Others Foam flotation Solvent extraction Bioremediation Preparation of ceria nanoparticles coated on nanotubes Method 1: Carbon nanotubes (CNTs) are fabricated by catalytic pyrolysis of propylene-hydrogen mixture at 750C in a ceramic tube with Ni particles as catalysts. The as-prepared CNTs contain many impurities, thus, needs to be washed several times with nitric acid and hydrofluoric acids to hydrofoluric acid to release catalyst particles. They are then washes several times with distilled water to remove metal particles. The dries samples are ground by ball milling to break up CNTs. The broken CNTs are mixed with solution of nitric acid and sulfuric acid and finally filtered with a ceramic filter.(5) Ceria coating: 1g CNTs were dispersed in D/W, 20 ml and are stirring up using a magnetic stirrer. Cerium chloride solution is added to the mixture slowly. After stirring for 1 h, sodium hydroxide solution is added to a concentration of 0.5wt% and titrated until the pH reaches to 9. The mixture solution is dried and heated in air at 450C for 20 min for the generation of ceria particles coated on CNTs.(5) Method: 2 CNTs can also be prepared by catalytic pyrolysis of the propylene-hydrogen in the ratio of 2:1 mixture at 750 C in a ceramic tube furnace using Ni particles as catalysts. The obtained CNTs prepared by catalytic pyrolysis are then washed with deionized water and grounded. Ceria coating: 1g CNTs were dispersed in D/W, 20 ml and are stirring up using a magnetic stirrer. Cerium chloride solution is added to the mixture slowly. After stirring for 1 h, sodium hydroxide solution is added to a concentration of 0.5wt% and titrated until the pH reaches to 9. The mixture solution is dried and heated in air at 450C for 20 min for the generation of ceria particles coated on CNTs.(5) Results and Discussion: Based on above mention methodology Ceria coated CNTs were successfully synthesized by Xianjia et al. As shown in fig2 XRD pattern of ceria particles synthesized on CNTs clearly indicates good crystallization of both CNTs and Ceria nano- particles (pick 002 and 101) while fig 3 shows TEM image of Ceria on CNTs and it clearly demonstrates the homogeneous distribution of Ceria on CNT. Figure 2: XRD pattern of spectra of Ceo2-CNTs (Xianjia et al) Zeta potentials profiling of CNT and Ceo2-CNTs has indicated that Ceo2-CNTs have higher zeta potential than CNT alone and thus make it more positively charged for given pH range. Arsenic (V) remains negatively charged in aqueous medium and thus Ceo2-CNTs become better adsorptive material compared to CNT alone due to prevalence of attractive forces. Adsorption of As(V) was found to be pH dependent and increase in pH leads to decrease in adsorption for example at pH 3 As(V) adsorption was found to be 19.7mg/gm while at pH 10 it reduced to 8.7mg/gm. Figure 4 indicates As(V) adsorption at different pH. Here oxidized CNT was used as control. Investigation on mechanistic aspect of adsorption was found to be very well fitted to Freunlich adsorption isotherm. This represents relation between amounts of adsorption per unit mass of adsorbent. Figure 3. TEM image of Ceo2-CNTs (Xianjia et al) . Figure 4 Adsorption of As(V) at different pH. (Xianjia et al) There is large number of reports published on the technological innovation in field of Arsenic removal from drinking water. There are certain advantages and disadvantages of each and every technology starting form economy of overall process, quality of water to be treated in terms of other metal ion content, pH of water, volume of water etc. Among others most commonly used technologies are adsorption on activated alumina and adsorption or precipitation by metal oxide particularly Fe(III). Adsorption technology is mainly suited for low level of Arsenic (g/liter) while in case of high arsenic content water precipitation is considered to be the best option. Iron oxide based adsorption of Arsenic in column reactor was widely demonstrated here iron oxide was used in various forms which includes ferrihydrite, ferric oxide impregnated activated carbon,Ce(IV) doped iron oxide, Iron oxide coated sand etc. Ferric hydroxide granules packed fixed bed reactor was shown to be effective means of Arsenic removal. The bigest advantage of this technology is easy operation and elimination of pre-treatment in terms of oxidation and pH adjustment. It has high processing capacity and shown to be effective for almost 30,000-40,000 bed volume. Based on this technology, two commercial product were launched in market namely SORB 33 and Bayoxid. Both these commercial product absorbs arsenic to prescribed limit of 10 g/liter. Similarly manganese oxide, zirconium oxide and alumina are few examples of metal oxide which used for arsenic removal. Comparing all other techniques adsorption have added advantage of not generating any other waste while process including precipitation, flocculation and other chemical processes generated arsenic containing waste in form of sludge and has to be treated separately. Conclusions: . Nanotmaterial based arsenic remediation is one of the fastest immerging technology. Smaller size, higher surface area, catalytically active and superior physico-chemical properties makes it suitable for ground water remediation. The most widely studied nanomaterial for arsenic remediation is ferric nanomaterial where it was shown to be highly influence by its size and pH of the water. Particle size of around 10nm has maximum capacity of arsenic removal. it was clearly demonstrated that crystalline ferric nanoparticles have almost 200 times capacity of arsenic removal compared to its commercial counterpart having size of 300nm(7).Similarly ceria based naoparticle coating on carbon nanotubes are getting attention for arsenic removal due to its larger surface area and many unique properties of carbon nanotubes. The Ceria based adsorption technology mainly influence by pH and concentration of Ca2+ and Mg2+ ion. Increase in pH increases the adsorption and similarly increase in concentration of Ca2+ and Mg2+ in ground water increases adsorption. Another advantage of this system is easy regeneration of column by NaoH which increases the life of column containing ceria nanoparticles. References: 1) Dinesh Mohan, Charles U. Pittman Jr. Arsenic removal from water/wastewater using adsorbents-A critical review Journal of Hazardous Materials (2007),142,1-153 2) Kiril Hristovski, Andrew Baumgarner, Paul Westerhoff. Selecting metal oxide for arsenic removal in fixed bed column: From nanopowder to aggregated nanoparticle media. Journal of Hazardous Materials(2007) 147, 265-274 3) S.Wang, C.N. Mulligan, Occurrence of arsenic contamination in Canada: 3127 sources, behavior and distribution, Sci. Total Environ. (2006), 366,701-721. 4) Xianjia Penga, Zhaokun Luana, , , Jun Dingb, Zechao Dib, Yanhui Lib and Binghui Tian, Ceria nanoparticles supported on carbon nanotubes for the removal of arsenate from water. Materials Letters, Volume 59, Issue 4, February 2005, Pages 399-403. 5) 3D Flowerlike Ceria Micro/Nanocomposite Structure and Its application for Water Treatment and CO Removal, Liang-Shu Zhong, Jin-Song Hu, An-Min Cao, Qiang Liu, Wei-Guo Song, and Li-Jun Wan*, Chem. Mater. 2007, 19, 1648-1655 6) Preparation of ceria particles coated supported on carbon nanotubes, Yanhui Li, Ja Ding, Jungfeng Chen, Cailu Xu, Materailas research bulletin, 37, 2002, 313-318. 7) S.Yean,L.Cong. Effect of magnetite particle size on adsorption and desorption of arsenite and arsenate. J.mater.res.2005 Read More