STUDIES ON THE REMOVAL OF ARSENICAN ENVIRONMENTAL TOXICANT

Abstract

In recent years, an increasing concern about the effect of heavy metals released from both natural and anthropogenic sources has resulted in more stringent environmental regulations. Arsenic, a commonly occurring toxic metal in the natural ecosystem, can be associated with natural conditions as well as to the industrial practices of mankind. Epidemiological studies have indicated a significant increase in the risks of cancer associated with the high level of arsenic in drinking water. Consequently, arsenic is regulated under the Resource Conservation and Recovery Act (RCRA), which creates a pressing need for developing and regulating efficient remediation steps at sites where a combination of high toxicity and widespread occurrence of arsenic is found. Recent disasters involving cases of human poisoning due to arsenic contamination in drinking water around the globe have spawned a series of worldwide investigations towards arsenic remediation. The episodes in the Asian countries are considered the most severe in terms of the extent of contamination and the number of persons affected. It has been estimated that more than 70 million people have been exposed to water with arsenic concentration > 50 ^g L"1 in West Bengal (India) and Bangladesh. As a result, the World Health Organization (WHO) has lowered the maximum contaminant level (MCL) of arsenic in drinking water systems from 50 to 10 ^g L"1 and ranked this arsenic calamity of the Bengal Delta Plain as "the largest poisoning of a population in history". Arsenic in its most recoverable form is found in various types of metalliferrous deposits, occurring naturally as a major constituent in more than 200 minerals, either as ore minerals or their alteration products. The most abundant arsenic ore is arsenopyrite, which together with other As-sulfide minerals is formed under high temperature conditions in the earth's crust. Pyrite is also formed in low-temperature sedimentary environments, which causes the release of arsenic into the sediments of many rivers, lakes and the oceans as well as many aquifers. The additional factors that cause arsenic pollution are a variety of anthropogenic activities. The major contribution of arsenic pollution is through the industrial operations of sulfide ore metallurgy. The impurities like arsenic, antimony, bismuth and selenium are released by volatilization and slagging during the smelting, conversion, fire-refining and electrorefining process. This drawback can be avoided by removing the impurities prior to electrorefining process as the constant increment in impurities leads to the decrease in the content of intent metal like copper. Thus, the difficulty in removing impurities has thwarted the metallurgist's dream of obtaining highly pure intent metal. The widespread occurrence of arsenic in the environment and its effect on the lifecycle prompted the author to develop innovative, cost-effective, economic and environment friendly arsenic remedial techniques. Thus, the objective of the present research work was set to design the treatment methods which could decontaminate arsenic quantitatively and rapidly from the metallurgical solutions, industrial effluents and from the natural water sources. For the convenience of presentation and to maintain clarity, the present research work carried out to discard arsenic from both natural and anthropogenic sources has been indexed in the thesis in the form of five chapters as, I Introduction II Materials and Equipment III Separation and Removal of Arsenic from Metallurgical Solutions Using Cyanex Extractants IV Removal of Arsenic from Natural Water Using Cyanex 301- Impregnated Adsorbents V Decontamination of Arsenic from Natural Water Using Flat-Sheet Supported Liquid Membrane Containing Cyanex 301 as Carrier Finally, the thesis ends with a brief discussion on the findings of the present investigations in the form of conclusions. Chapter I begins with the general introduction to the occurrence of arsenic in the biosphere and its impact on human health. It is followed by the work plan of the present research work to decontaminate arsenic from natural water and metallurgical solutions. The extraction of arsenic from hydrometallurgical solution was designed and carried out by solvent extraction technique using an efficient extractant. Among the separation techniques, solvent extraction method is preferred due to its unique advantages like easy scaling up of the bench level data to the pilot plant operations and an in-built VI flexibility to tailor the method as per requirements. Further, the distribution data obtained provides a data base for the development of other separation methods like extraction chromatography, adsorption and membrane-based extraction techniques. The literature on the various classes of extractants employed for arsenic removal has been reviewed. The applicability of these extractants namely, chelating agents, carboxylic and sulfonic acids, high molecular weight amines and organophosphorus extractants was found to be invariably limited because of extractant loss, emulsion formation, limited selectivity and other hydrometallurgical considerations. Besides this, the potential of a series of alkylphosphine extractants marketed under the trade name of CYANEX by Cytec Canada Inc., Canada has been reviewed. In Cyanex compounds, the alkyl groups are bonded directly to the phosphorus atoms through P-C bonds rather than the P-O-C bonding observed in many other commercial organophosphorus extractants. This tends to make the Cyanex extractants to be more resistant to hydrolysis. In view of these inherent advantages of alkylphosphine compounds, the potential of Cyanex extractants for the extraction of arsenic was explored. The overview of the whole thesis dealing with the removal of arsenic is encapsulated in the form of a conventional solvent extraction method, the innovative adsorption technique using extractant-impregnated adsorbents and the membrane-based extraction technique containing the efficient extractant as carrier. Chapter II gives details of the materials and equipments used during the course of present investigations. Apart from the instruments such as Atomic Absorption Spectrometer (AAS) / Inductively Coupled Plasma Mass Spectrometer (ICP-MS) used to carry out distribution studies, the scanning electron microscope (SEM) was used to view the veins of impregnated adsorbents. The procedures adopted for the dissolution of different matrices are also cited. Chapter III incorporates the extraction behaviour of As(V/lll) from HCI / HN03 / H2S04 media using toluene solution of Cyanex 923, Cyanex 272, Cyanex 302 and Cyanex 301. Except Cyanex 301, no other extractant displayed quantitative extraction of arsenic. As(lll) gets quantitatively extracted into 0.10 mol L"1 Cyanex 301, whereas, As(V) remains unextracted. Thus, in order to achieve complete extraction of arsenic, As(V) was reduced to As(lll) by VII adding 25% sodium thiosulfate to the aqueous phase prior to its extraction. Along with arsenic, the extraction behaviour of associated metal ions like Se(IV), Ga(lll), Sb(lll), Bi(lll), Ni(ll), Cu(ll), Hg(ll) and Pb(ll) from HCI / HN03 / H2S04 media using toluene solution of Cyanex 301 was examined. The effect of various parameters like equilibration time, temperature, nature of diluent and the concentration of mineral acid, arsenic, chloride ion and the extractant on the distribution of As(lll) was studied. The stoichiometry of the probable extracting As(lll)-Cyanex 301 species is suggested. The loading capacity of Cyanex 301, its hydrolytic stability and regeneration power were assessed to suggest the commercial viability of Cyanex 301. The extraction of Cu(ll) is masked using thiourea while the other metal ions such as Se(IV), Sb(lll), Bi(lll), Ni(ll), Hg(ll) and Pb(ll) a-e quantitatively extracted along with As(lll). This permits the use of Cyanex 301 for copper bearing matrices. Since, the limitation generally encountered with Cyanex 301 is that copper is extracted irreversibly and recovery of copper from the loaded Cyanex 301 phase is difficult without disintegrating the extractant. The various metal ions extracted along with As(lll) were recovered quantitatively using selective stripping reagents. The proposed separation and removal conditions have been successfully applied to recover the pure intent metal like lead and copper from the real matrices like galena and copper pyrite. The scheme was also applied on a simulated solution of copper electrolyte bath. The developed procedure can be employed for the removal of impurities from metallurgical effluents before discharging them into the environment. Above all, the removal of impurities from the metallurgical solution makes the complex process of electrolytic purification simpler or optional. Chapter IV embodies studies on arsenic decontamination from the natural water sources using extractant-impregnated adsorbents. In aqueous stream, the predominant species of arsenic are As(V) and As(lll). Of these, the trivalent species is of great environmental concern not only because of its high toxicity but also in view of its high mobility. A variety of remedial techniques explored for the removal of arsenic from aqueous medium were found to be efficient in decontaminating the pentavalent species of arsenic, thereby, requiring a pre-oxidation step to transform As(III) into the less toxic As(V). But the oxidation step is expensive in terms of investment and running costs to be applied in the zones where the major environmental problems concerned with VIII arsenic have been identified. In view of these shortcomings, it was aimed to develop arsenic remedial techniques, which could be both user-friendly and economically feasible. The adsorbents explored range from conventional, cheap and easily available silica gel to the natural oil catcher and synthetic kapok fiber supports. The extractant Cyanex 301 selected on the basis of liquid-liquid extraction study was impregnated onto the adsorbents and the adsorption procedure was developed for the decontamination of arsenic. Batch experiments were performed as a function of process parameters: sorption time, aqueous pH, concentration of As(V/lll) and the amount of Cyanex 301 loaded on the impregnated support. Quantitative adsorption of As(lll) from natural water was reached in few minutes using the impregnated solid supports, whereas, As(V) remains almost unadsorbed. The adsorption of As(lll) onto the 0.50 mol L"1 Cyanex 301-impregnated natural fiber-oil catcher and conventional silica gel showed their correlation with Freundlich adsorption isotherm, whereas in the case of impregnated synthetic-kapok fiber the adsorption equilibrium correlated with Langmuir isotherm model. Kinetic studies revealed that the As(lll) adsorption onto the Cyanex 301-impregnated natural and synthetic fibrous supports were following first and second order kinetics, respectively. While, the adsorption of As(lll) onto the impregnated silica gel followed second-order kinetics. The arsenic loaded impregnated solid supports were regenerated by quantitatively desorbing the arsenic content using 1.0 mol L" NaOH. The efficiency of the impregnated solid supports was also investigated by column studies. The breakthrough capacity of the columns packed with Cyanex 301-impregnated silica gel and fibrous supports was determined. The loading, eluting and regeneration capacities were also assessed. The regeneration power of the developed adsorbents was inferred from the successive loading and elution cycles of As(lll) onto the same columns. The extractant loss from the impregnated adsorbents was examined during the successive cycles of loading and eluting operations. The release of extractant in the aqueous phase was found to be negligible and much below the permissible limit. The developed Cyanex 301-impregnated natural oil catcher and synthetic kapok fiber were found to be more efficient for the rapid and quantitative decontamination of arsenic. The Cyanex 301-impregnated silica gel column has the limitation of low adsorption capacity and flow rate. IX The efficiency o* the developed impregnated supports was demonstrated by applying them onto the groundwater samples collected from the states of West Bengal and Chhattisgarh, India. The decontamination of arsenic using impregnated adsorbents was also applied on the seawater samples collected from the Tsunami affected Bay of Bengal coastal zones of the states Tamil Nadu and Pondicherry, India. The results indicate that the developed Cyanex 301-impregnated supports can decontaminate arsenic from natural water to the level below the limit set by WHO and USEPA. Chapter V presents the studies on the facilitated transport of As(lll) from aqueous solution through the flat-sheet supported liquid membrane containing Cyanex 301 as carrier. The FluoroTrans® PVDF membrane was used for the permeation studies. The permeability of As(lll) was studied as a function of various experimental parameters: hydrodynamic conditions, pH and the concentration of As(lll) in the feed phase and Cyanex 301 in the membrane. The composition of the strippant in the receiving phase has also been investigated. The transport mechanism consists of a diffusion process through an aqueous diffusion film, a fast interfacial chemical reaction and diffusion through the membrane itself. The aqueous mass transfer coefficient and the thickness of the aqueous boundary layer were calculated from the experimental results. The arsenic loaded Cyanex 301-impregnated supported liquid membrane was regenerated by quantitatively desorbing the arsenic using 1.0 mol L"1 NaOH. Quantitative decontamination of As(lll) was attained in 60 minutes using the membrane containing 0.50 mol L"1 Cyanex 301 as carrier. Recycling experiments conducted up to fifteen cycles indicated insignificant change in the permeation capacity. The membrane was found to be stable even after fifteen days of continuous contact with water. The applicability ofthe developed method to decontaminate As(lll) was checked on the groundwater and seawater samples collected from different regions of India. The supported liquid membrane containing Cyanex 301 as carrier was found to be efficient in decontaminating arsenic from natural water below the permissible limit set by various government agencies around the world. Finally, the thesis ends with a discussion on the proposed arsenic remedial methods. Using the solvent extraction data, it has been possible to separate arsenic from its commonly associated metals. The conditions of separation thus developed have been fused together to remove arsenic and other undesirable contaminants from the metallurgical solutions. The distribution data on Cyanex 301 have been utilized to develop extractantimpregnated supports for decontaminating arsenic from natural water. A systematic study of the procured data has resulted in the development of a membrane-based extraction technique to remove arsenic. The author feels that the three remedial techniques proposed in the thesis can be scaled up for the intended application. The proposed chemistry has been worked out on a bench scale and there should not be much difficulty in scaling it up. However, for scaling it up to plant level or field operations some additional inputs may be required. It may be worth mentioning that the proposed methods for decontaminating arsenic from drinking water will require some more stringent tests before finally applying them for field operations.